JP5320369B2 - Solid wavelength converter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for converting wavelength of light in which information is encoded, while doubling sensitivity. <P>SOLUTION: A method for encoding information encoded during spatial intensity variation of light characterized by a first wavelength into light characterized by a second wavelength, includes: generation of first electronic density distribution homogeneous with spatial variation in intensity of the first wavelength light; generation of second additional electronic density homogeneous with the first electronic density distribution; capture of electrons from the first and the second electronic density distribution in the capture region to generate an electric field homogeneous with the density distribution in a material modulating characteristic of light passed through the material in accordance with the electric field of the material; and encoding of the second wavelength light by the information by making the modulated material transmit the second wavelength light, thereby modulating the second wavelength light in accordance with the electric field. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to a solid-state wavelength converter for encoding light characterized by a first wavelength with data encoded in light characterized by a second wavelength, in particular the data represents an image. Sometimes related to encoding light.

  Solid state wavelength converters that convert an image encoded into light intensity characterized by a first wavelength into an image encoded in light characterized by a second wavelength are known in the art. is there. The light characterized by the first and second wavelengths is hereinafter referred to as “input light” and “output light” respectively, and for clarity and simplicity of display, the encoded image is the image of the object. Suppose that

  One type of wavelength converter comprises a layered body consisting of a plurality of thin continuous layers formed from optical and / or electroactive materials. One of the layers is a photoconductive layer, and one of the layers is a light modulation layer. The photoconductive layer material absorbs energy from the light passing through the photoconductive layer and converts the absorbed energy into electron-hole pairs. The material of the light modulation layer modulates the properties, generally the intensity, of light passing through the light modulation layer by an amount that depends on the strength of the electric field of the modulation layer. In general, the outermost layer of the layered body is formed from a transparent conductive material and functions as an electrode. A suitable electrical power source is connected to the electrodes.

  When the wavelength converter is operating, the power source applies a potential difference between the electrodes, thereby generating an electric field in the layer. Light from the object to be imaged, i.e. input light, is focused onto the photoconductive layer. In the photoconductive layer, the input light has a spatially varying intensity corresponding to the image of the object. The photons of the input light are absorbed by the photoconductive layer and generate electron-hole pairs in that layer. The number of electron-hole pairs generated in the region of the photoconductive layer is substantially proportional to the intensity of input light in that region. Therefore, the fluctuation intensity pattern of the input light in the photoconductive layer, hereinafter referred to as “input image”, is “copied” into the generated electron-hole pair density distribution.

  Under the influence of the electric field generated by the power supply, electrons from the electron-hole pair drift in the direction of the light modulation layer and are trapped near or on the surface of the light modulation layer. The trapped electrons generate an electric field, hereinafter referred to as a “modulation field”, in the light modulation layer. Since the density distribution of electron-hole pairs generated in the photoconductive layer images the object, it will be referred to as the “charge image” from now on, and the density distribution of the captured electrons and the generated electrons. The magnitude of the modulation field also images the object.

  After exposure to input light, the transducer is exposed to light emitted from a suitable light source, ie output light. The output light enters the wavelength converter, passes through the light modulation layer, and then exits the wavelength converter. The light modulation layer modulates the output light according to the magnitude of the modulation field. The output light passing through the region of the modulation layer where the modulation field is strong is strongly modulated. The output light that passes through the region of the modulation layer where the modulation field is weak is weakly modulated. Thus, upon exiting the transducer, the output light is encoded with an image of the object. That is, the input image of the object is copied to the output light, which can then be processed to provide an image of the object.

  U.S. Pat. No. 5,124,545 by Takahashi et al. Describes several different wavelength converters of this type. One wavelength converter described in that patent includes a photoconductive layer formed of cadmium sulfide (CdS) or bismuth silicon oxide (B12SiO20), etc. Adjacent to a light modulation layer, such as a single crystal or nematic liquid crystal. Both input light and output light pass through both layers. Accordingly, the photon energy of the output light is selected to be smaller than the band gap energy of the photoconductive layer. This will eliminate the correspondence between the input image of the object imaged by the wavelength converter and the charge image of the object in the wavelength converter that generates the modulation field. The output light is prevented from being generated in the electron-hole pairs. As a result, this type of prior art wavelength converter is commonly used when the energy of the photons of the input light is greater than the energy of the photons of the output light. This type of wavelength converter is practical for converting the UV input image of an object into an “output” image of the object in the visible spectrum, but the IR input image of the object is visible output of the object. It is not practical to convert to an image.

  Another wavelength converter described in this patent comprises a dielectric mirror or “photo insulating film” sandwiched between a photoconductive layer and a light modulation layer. In this wavelength converter, output light enters the light modulation layer, passes through the light modulation layer, is reflected by the dielectric mirror, passes through the light modulation layer again, and exits the wavelength converter. Due to the mirror, the output light never reaches the photoconductive layer and does not affect the charge image of the object imaged by the transducer. Thus, this type of wavelength converter encodes an image encoded in a relatively “low energy” input light, such as an IR image into a visible light image, into a relatively “high energy” output light. Images can be converted.

  However, in a wavelength converter having a mirror, the distance between the charge image formed on the wavelength converter and the light modulation layer of the wavelength converter tends to increase due to the presence of the mirror. Furthermore, the distance over which the charge image is distributed tends to increase in a direction perpendicular to the surface of the light modulation layer. Both effects of the mirror tend to cause the modulation field generated by the charge image to blur or reduce the sharpness corresponding to the input image of the object being imaged. Thus, the mirror tends to reduce the spatial resolution of the wavelength converter.

  In many conventional wavelength converters, due to the actual intensity of the input light, variations in the density of the trapped electrons are often too small to affect satisfactory modulation of the output light. As a result, the sensitivity of these wavelength converters is not sufficient for many applications. It would be advantageous to have a wavelength converter with increased sensitivity.

  Aspects of the invention are improved so that when the intensity of the input light from the object is relatively low, the output light from the wavelength converter can be used to provide an image of the object with a relatively high resolution. The present invention relates to providing a solid-state wavelength converter having high sensitivity.

  The wavelength converter according to a preferred embodiment of the present invention includes an electron multiplication region disposed between the photoconductive layer and the light modulation layer. The multiplication region preferably comprises a layer of semiconductor material forming a graded-band-gap staircase-multiplier. Graded bandgap step multipliers are described in U.S. Pat. No. 4,476,477 by Capasso et al., And “Nuclear Instruments and Methods in Physics Research: A288 (1990) 99-103 by Ripamonti et al. Described in a paper entitled “Solid State Photomultiplier”, the disclosure of which is incorporated herein by reference.

  Electrons from electron-hole pairs generated in the photoconductive layer by input light incident on the transducer from the object being imaged drift through the multiplication region to the light modulation layer. The drifting electrons ionize the material in the multiplication region and are multiplied by the electron avalanche process. As a result, the number of electrons reaching the light modulation layer is significantly increased with respect to the number reaching the light modulation layer when there is no multiplication region. Thus, the sensitivity of the wavelength converter is significantly increased over prior art wavelength converters.

  An aspect of some preferred embodiments of the invention is that the wavelength of the input light photons has a lower energy than the output light photons, and the output light passes through both the photoconductive layer and the light modulation layer of the converter. It relates to providing a converter. Such a wavelength converter can, for example, convert an IR input image of an object into a visible image of the object without the need to provide a mirror between the photoconductive layer and the light modulation layer. . As a result, the resolution of the visible image is not reduced by the distortion caused by the mirror in the modulation field generated in the wavelength converter in response to the IR input image, as described above.

  However, when the output light passes through the photoconductive layer of the wavelength converter, an electron-hole pair is generated in the photoconductive layer. As described above, electrons from the electron-hole pair may drift to the light modulation layer, and the correspondence between the input image and the charge image generated in the converter in accordance with the input image may disappear. is there. As a result, the modulation field of the modulation layer is distorted, and once distorted, the output light cannot be accurately encoded with the object image.

  An aspect of some preferred embodiments of the present invention is that the electrons formed in the photoconductive layer by the passage of the output light through the photoconductive layer are modulated into the output light and the input of the object encoded in the input light. The present invention relates to preventing disappearance of correlation with an image.

In some preferred embodiments of the present invention, the output light passes through the photoconductive layer before passing through the light modulating layer.
In some preferred embodiments of the invention, the absorption coefficient of the light modulation layer increases as the electric field of the light modulation layer increases. Therefore, the spatial modulation pattern of the output light is a “negative image” of the spatial modulation of the input light. When the intensity of the input light is relatively strong, the intensity of the output light is relatively weak, and the image of the object formed by the modulated output light is a negative image of the object. The light modulation layer whose absorption coefficient increases as the electric field increases is hereinafter referred to as a “negative image modulator”.

  In both preferred embodiments, the electron density in the photoconductive layer generated from electron-hole pairs generated by the output light passing through the light modulation layer is not homologous to the input image. Electrons from the heterogeneous electron density drift to the light modulation layer, and the correspondence between the input image and the modulation field of the light modulation layer disappears.

  In order to prevent the lack of correspondence between the modulation field and the input image from affecting the modulation of the output light, according to a preferred embodiment of the invention, the output light illuminating the transducer is in the form of a short pulse of light. Provided in. Because the pulse width of the output light pulse is short, the pulse is almost completely out of the converter before the electrons generated by the pulse in the photoconductive layer reach the light modulation layer and affect the modulation field of the light modulation layer. It passes through the light modulation layer. Thus, when a charge image is generated in the wavelength converter by input light from the object, the output light that subsequently illuminates the converter is modulated almost accurately by the charge image. The modulation of the output light pulse is substantially affected by the passage of the output light pulse through the wavelength converter, even if the output light pulse generates electron-hole pairs in the photoconductive layer as it passes through the photoconductive layer. Absent.

  The modulation of the output light pulse is not affected by the passage through the photoconductive layer, but the light of the output light pulse is partially absorbed by the photoconductive layer. Thus, according to a preferred embodiment of the present invention, the output light is provided with sufficient intensity to compensate for absorption in the photoconductive layer.

  In some preferred embodiments of the invention, the absorption coefficient in the light modulation layer decreases as the electric field of the light modulation layer increases. In these preferred embodiments, the pattern of spatial modulation of the output light is a positive image of the spatial modulation of the input light. When the intensity of the input light is relatively strong, the intensity of the output light is also relatively strong, and the image of the object formed by the modulated output light is a positive image of the object. The light modulation layer whose absorption coefficient decreases as the electric field increases is hereinafter referred to as a “positive image modulator”.

  In some preferred embodiments of the invention, the light modulating layer is a positive modulator and the output light passes through the light modulating layer before passing through the photoconductive layer. In these preferred embodiments of the present invention, the electron density of the photoconductive layer generated from the electron-hole pairs generated by the output light passing through the light modulation layer is homologous to the input image. Electrons from this electron density drifting to the light modulation layer do not eliminate the correspondence between the input image and the modulation field of the light modulation layer. Instead, the electrons amplify the modulation field while maintaining the correspondence between the modulation field and the input image.

  According to some preferred embodiment aspects of the invention, the output light is used to amplify a modulation field that modulates the output light.

  In some preferred embodiments of the invention in which the light modulation layer is a positive modulator and the output light passes through the light modulation layer before passing through the photoconductive layer, the first pulse of output light is followed by Is used to amplify the modulation field for the second pulse of the output light. In some preferred embodiments of the present invention, electrons generated by the passage of the light pulse through the photoconductive layer pass through the wavelength converter such that the light pulse amplifies the modulation field before leaving the light modulation layer. The output light pulse has a sufficiently long pulse length. As a result, the output light pulse is modulated by an amplified modulation field that is itself amplified.

  An aspect of some preferred embodiments of the present invention relates to providing a wavelength converter comprising a shutter. The shutter is made on one of the conductive layers of the wavelength converter using methods known in the art. In some wavelength converters according to preferred embodiments of the present invention, a shutter is used to block the input light. In some wavelength converters according to preferred embodiments of the present invention, a shutter is used to block the output light. In some wavelength converters according to a preferred embodiment of the present invention, the wavelength converter comprises two shutters. One of the shutters is used to block the input light and one of the shutters is used to block the output light. In a preferred embodiment of the invention, the shutter is made on one or both of the two conductive layers of the wavelength converter.

  An aspect of some preferred embodiments of the present invention relates to providing a camera with a wavelength converter, according to a preferred embodiment of the present invention. The camera collects light characterized by a first wavelength from the object being imaged and the object with light characterized by the second wavelength on a suitable photosensitive surface, such as a CCD. Image. The light characterized by the first wavelength is the input light of the wavelength converter, and the light characterized by the second wavelength is the output light of the wavelength converter. In some preferred embodiments of the present invention, the camera is a 3D camera, which is used to measure the distance to an object in the scene imaged by the camera.

  Accordingly, a preferred embodiment of the present invention provides information encoded during the spatial intensity variation of light characterized by the first wavelength in the light characterized by the second wavelength. A method for encoding, wherein the method generates a first electron density distribution that is homologous to a spatial intensity variation of the first wavelength light, and is homologous to the first electron density distribution. To generate a second additional electron density and to generate an electric field that is homologous to the density distribution in the material that modulates the properties of light passing through the material in response to the electric field in the material. Capturing electrons from the first and second electron density distributions, and transmitting the second wavelength light through the modulation material, thereby causing the second wavelength in response to the electric field. Modulates light and uses the information to convert the two wavelengths of light And a be-coding.

  Preferably, generating the second additional electron density passes electrons from the first density distribution to a structure comprising a plurality of graded band gap layers where the electrons are multiplied in an avalanche process. Including.

  Alternatively or in addition, generating the first electron density may cause the first wavelength in a photoconductive material in which electron-hole pairs are generated by absorbing photons from the first wavelength light. It preferably includes transmitting light.

  Preferably, the generation of the second additional electron density is performed in the photoconductive material after an electric field substantially homologous to the intensity variation of the first wavelength light is established in the modulating material. Transmitting the light pulse having sufficient energy to generate electron-hole pairs into the modulating material, and then generating the electron-hole pairs in the photoconductive material; Transmitting a pulse into the photoconductive layer.

  The pulse length of the pulse is sufficiently long so that after the electrons generated in response to the pulse are captured and the electric field in the modulation material is changed, a portion of the pulse is in the modulation material Preferably there is.

  Alternatively, before the second wavelength light pulse captures electrons generated in the photoconductive layer and changes the electric field in the modulation material, the second wavelength light pulse is substantially Completely through the modulation material.

  In some preferred embodiments of the present invention, transmitting the second wavelength light into the modulating material transmits the second wavelength light pulse into the modulating material and the photoconductive Including transmitting in the material.

  It is preferable that the second wavelength light has sufficient energy to generate electron-hole pairs in the photoconductive material.

  Preferably, generating the second additional electron density causes the second wavelength light after an electric field substantially homologous to the intensity variation of the first wavelength light is established in the modulating material. And then transmitting the light pulse into the photoconductive layer to generate the second electron density distribution.

  In some preferred embodiments of the present invention, the pulse length of the pulse is sufficiently long so that the electrons generated in response to the pulse are captured and after changing the electric field in the modulation material, the Part of the pulse is in the modulation material.

  In some preferred embodiments of the invention, electrons generated in the photoconductive layer by the second wavelength pulse are captured and the second wavelength before the electric field in the modulation material is changed. A pulse of light passes through the modulation material substantially completely.

  In some preferred embodiments of the invention, generating the first and second electron densities is in the train of first wavelength light pulses, all of which have substantially the same spatial intensity variation. For each light pulse, including generating first and second electron densities, capturing the electrons removes electrons from the density generated for the light pulses in the light pulse train in the same capture region. Including capturing.

  Further provided by a preferred embodiment of the present invention is that information encoded during the spatial intensity variation of the light characterized by the first wavelength is encoded in the light characterized by the second wavelength. A method of transmitting at least one pulse of light of a first wavelength into a photoconductive material, whereby a density of electrons homologous to a spatial intensity variation of the light of the first wavelength. In order to generate a distribution and to generate an electric field that is homologous to the spatial variation in a material that modulates the characteristics of light passing through the material according to the electric field in the material, Transmitting a pulse of second wavelength light having sufficient energy to trap electrons and generate electron-hole pairs in the photoconductive layer into the modulating material and the photoconductive material; Depending on the electric field Modulating the pulses, including, encoding the pulse with the information.

  Preferably, before the second wavelength pulse captures electrons generated in the photoconductive layer and changes the electric field in the modulation material, the second wavelength light pulse is substantially Pass completely through the modulation material.

  Alternatively, the method preferably comprises first transmitting a pulse of the second wavelength light into the light modulation layer and then into the photoconductive layer, wherein the pulse length of the pulse is: After the electrons generated in response to the pulse are captured and the electric field in the modulation material is changed, a portion of the pulse is in the modulation material.

  Further provided by a preferred embodiment of the present invention is that information encoded during the spatial intensity variation of the light characterized by the first wavelength is encoded in the light characterized by the second wavelength. A plurality of pulses of a first wavelength light, all having substantially the same spatial intensity variation pattern, and each received first wavelength light In the material that generates an electron density distribution that is homologous to the spatial intensity fluctuation of the first wavelength light pulse according to the pulse, and modulates the characteristics of the light that passes through the material according to the electric field in the material In order to generate an electric field that is homologous to the density distribution in FIG. 2, accumulating electrons from the generated electron density distribution, and the second wavelength light is modulated according to the electric field therein, thereby Modulation encoded In the material, including, and thereby transmit the second wavelength light.

  Preferably, transmitting the second wavelength light includes transmitting a pulse of the second wavelength light.

  Further provided by a preferred embodiment of the present invention is to image information encoded during the spatial intensity variation of light characterized by a first wavelength with light characterized by a second wavelength. A) encoding a second wavelength pulse in accordance with a preferred embodiment of the present invention for encoding a second wavelength light pulse; and b) Imaging the encoded second wavelength light pulse on a photosensitive surface and c) repeating a) and b) at least twice, wherein at least received One first wavelength light pulse has substantially the same spatial intensity variation in all iterations.

  Further provided by a preferred embodiment of the present invention is a method for imaging an object, collecting a first wavelength light reflected or emitted by said object, According to a preferred embodiment, the intensity variation in the first wavelength light is encoded in the light characterized by the second wavelength, and the encoded second wavelength light is transmitted on the photosensitive surface. Imaging.

  Further provided by a preferred embodiment of the present invention is a method for determining a distance to an object, wherein the object is illuminated with at least one pulse train of light pulses characterized by a first wavelength. Receiving light from the light pulses in the pulse train reflected by the object on a shutter that is opened and closed at a time that is linked to the time at which the light pulses in the pulse train are emitted, and a first wavelength According to a preferred embodiment of the present invention for encoding a pulse train of light into a second wavelength light, the intensity variation of the reflected first wavelength light transmitted through the shutter is characterized by the second wavelength. Encoding during fluctuations in the intensity of the applied light, imaging the encoded second wavelength light on the photosensitive surface, and recording in a region of the photosensitive surface that images the surface elements The according to the intensity of the second light wavelength, including, and determining the distance to the surface elements of the object.

  It is preferable that the at least one pulse train includes a plurality of pulse trains. In some preferred embodiments of the present invention, the wavelength of the first wavelength light is longer than the characteristic wavelength of the second wavelength light. In some preferred embodiments of the present invention, the wavelength of the first wavelength light is shorter than the characteristic wavelength of the second wavelength light. In some preferred embodiments of the present invention, the wavelength of the first wavelength light is substantially equal to the characteristic wavelength of the second wavelength light.

  Further provided by a preferred embodiment of the present invention is a wavelength converter, wherein light characterized by a first wavelength and having spatial variations in its intensity passes through the wavelength converter. A first port, a photoconductive layer through which the first wavelength light entering the converter passes and in which an electron density distribution is generated according to the spatial variation; and within the photoconductive layer An electron multiplier that receives the generated electrons and generates a larger number of electrons in response to the received electrons; a capture region that captures the electrons generated in the multiplication layer; and a second wavelength. A second port through which the characterized light enters the wavelength converter; and a light modulation region through which the second wavelength light passes, wherein the second wavelength light is modulated in accordance with an electric field therein And within that, within the capture region Electric field generated by the electrons is, and an optical modulator region is homologous place the spatial variation in said first wavelength light.

  The multiplication region preferably includes a plurality of graded band gap layers. Additionally or alternatively, the light modulation region preferably comprises an MQW structure having layers with alternating narrow and wide band gaps.

  In some preferred embodiments of the present invention, the capture region comprises a reflector that reflects both first and second wavelength light, and the modulated second wavelength light passes through the second port. Through the wavelength converter. The reflective layer is preferably a dielectric mirror.

  In some preferred embodiments of the present invention, the wavelength converter comprises a reflector that reflects the first wavelength light and transmits the second wavelength light, the reflector having a second number than the photoconductive layer. The first wavelength light that is arranged away from one port and enters the wavelength converter and passes through the photoconductive layer reflects the first wavelength light so that it passes through the photoconductive layer twice. . Preferably, the reflector comprises a dielectric mirror.

  In some preferred embodiments of the present invention, the wavelength converter comprises a third port through which the modulated second wavelength light exits the wavelength converter.

  Preferably, the capture region includes a reflector that transmits the first wavelength light and reflects the second wavelength light. It is preferable that the reflector is a dielectric mirror.

  Some wavelength converters according to preferred embodiments of the present invention comprise a light source that emits a pulse of second wavelength light modulated by the wavelength converter, wherein c is the speed of light and d is the thickness of the photoconductive layer. , V is the drift velocity of electrons in the photoconductive layer, and the pulse width of the second wavelength light pulse entering the wavelength converter is shorter than (cd) / v.

  In some preferred embodiments of the invention, the first port and the second port are the same.

  Some wavelength converters according to preferred embodiments of the present invention comprise a perforated metal layer coupled to the photoconductive layer, the photoconductive layer having a hole aligned with the perforations in the metal layer. Formed therewith, the hole penetrates or substantially penetrates the entire width of the photoconductive layer. Preferably, the metal layer is closer to the second port than the photoconductive layer.

  In some wavelength converters according to a preferred embodiment of the present invention, the first port and the third port are the same.

  Some wavelength converters according to preferred embodiments of the present invention comprise a first wavelength shutter operable to allow and prevent the first wavelength light from passing through the photoconductive layer. The first wavelength shutter preferably includes an MQW structure having layers in which narrow band gaps and wide band gaps are alternately arranged.

  Some wavelength converters according to preferred embodiments of the present invention comprise a second wavelength shutter operable to allow and prevent the second wavelength light from entering the wavelength converter. . The first wavelength shutter preferably includes an MQW structure having layers in which narrow band gaps and wide band gaps are alternately arranged.

  Further provided by a preferred embodiment of the present invention is a camera that images information encoded during light intensity variations characterized by a first wavelength with light characterized by a second wavelength. The camera comprises a wavelength converter according to a preferred embodiment of the present invention, a photosensitive surface, and an optical component that receives the second wavelength light exiting the wavelength converter and images it onto the photosensitive surface. .

  The camera preferably includes a light source that emits light of a second wavelength. The camera preferably includes a shutter that blocks the first wavelength light from reaching the wavelength converter. It is preferable that the shutter is provided in the wavelength converter.

  Further provided by a preferred embodiment of the present invention is a 3D camera for determining a distance to an object, the camera according to a preferred embodiment of the present invention comprising a light source emitting a second wavelength light, A pulse light source that irradiates the object with at least one pulse train of one wavelength light pulse, and a shutter that is gated and opened at a time that is linked to a time at which the light pulse in the at least one pulse train is emitted; A controller that controls the second wavelength light source to irradiate the wavelength converter with a pulse of the second wavelength light following the last light pulse of each of the two pulse trains.

FIG. 2 schematically shows a cross-sectional view of a solid state wavelength converter that encodes output light with an image encoded in the input light incident on the wavelength converter, according to a preferred embodiment of the present invention. Fig. 4 schematically shows the dependence of the absorption coefficient of output light on the electric field and the wavelength of output light for a light modulation layer according to a preferred embodiment of the present invention. FIG. 6 schematically shows a cross-sectional view of another solid-state wavelength converter that encodes output light with an image encoded in the input light incident on the wavelength converter, according to a preferred embodiment of the present invention. FIG. 6 schematically shows a cross-sectional view of another solid-state wavelength converter that encodes output light with an image encoded in the input light incident on the wavelength converter, according to a preferred embodiment of the present invention. 1 schematically shows a camera with a wavelength converter according to a preferred embodiment of the present invention. 1 schematically illustrates a wavelength converter with a shutter for blocking input light, according to a preferred embodiment of the present invention. Fig. 6 schematically shows another camera with a wavelength converter according to a preferred embodiment of the present invention.

  FIG. 1 is a cross-sectional schematic diagram of a solid-state wavelength converter 20 according to a preferred embodiment of the present invention that encodes output light represented by a straight arrow 22 with an image encoded in the input light represented by a wavy arrow 24. It is. The input light 24 has a spatially varying intensity that is schematically represented by clustering the wavy arrows 24 into groups. The variation intensity encodes the image of the object.

  The wavelength converter 20 includes a heavily p-doped layer 40, a photoconductive layer 30, an electron multiplication region 32, a first dielectric mirror 34, a light modulation region 36, and a heavily n-doped layer 42. And a layered body 26 composed of the second dielectric mirror 38. A power supply 44 reverse biases the p and n doped conductive layers 40 and 42 and generates an electric field in the layer sandwiched between the two layers, represented by a double arrow 46, having the direction indicated. The electric field 46 has a substantially constant direction, but has a magnitude that varies from layer to layer depending on the properties of the material in the layer.

  First and second dielectric mirrors 34 and 38 are each provided with a layer of suitable semiconductor material 48 and 49 and are designed to reflect output light 22 and input light 24, respectively. It is preferable that the first dielectric mirror 34 is substantially transparent to the input light 24 and the second dielectric mirror 38 is substantially transparent to the output light 22. The multiplication region 32 preferably comprises a plurality of graded band gap layers 50 forming a graded band gap step multiplier. The light modulation region 36 preferably comprises a plurality of alternating layers of low and high band gaps, which layers are of the type Multiple Quantum Well (MQW) modulation of the type described in PCT Publication WO 99/40478. Forming a vessel. This disclosure is incorporated herein by reference.

  The input light 24 enters the wavelength converter 20 through the conductive layer 40 and enters the photoconductive layer 30. In the photoconductive layer 30, the photons of the input light 24 are absorbed, and electron-hole pairs are generated in the photoconductive layer 30. This is represented by a code pair of “+” and “−”. The unabsorbed photons of the input light 24 travel through the other layers of the wavelength converter 20, enter the second dielectric mirror 38 and are reflected, pass through the photoconductive layer 30 again, and further Generate electron-hole pairs. The photons of the input light 24 reflected by the second dielectric mirror 38 are represented by the wavy arrow 24 ′ of “reflected input light” originating from the second dielectric mirror 38. The number of electron-hole pairs generated per unit volume at a certain point of the photoconductive layer 30 is substantially proportional to the intensity of the input light 24 at that point. As a result, the density of the generated electron-hole pairs images the object encoded in the intensity variation of the input light 24.

Electrons from the electron-hole pair drift in the electric field 46 toward the multiplication region 32. In the multiplication region 32, electrons are multiplied by an avalanche process. The avalanche process for the “original” electrons represented by the minus sign 54 is schematically illustrated in FIG. Initial electrons enter the multiplication region 32 from each region of the photoconductive layer 30 where electron-hole pairs are generated by the input light 24. Each time electrons passing through the multiplication region 32 under the influence of the electric field 46 pass from the first graded bandgap layer 50 to the second graded bandgap layer 50, the electrons almost always pass through the second graded bandgap. The material of layer 50 is ionized to generate additional electrons. That is, each time electrons pass from one layer of graded band gap layer 50 to another, they “double”. An ideal graded bandgap step multiplier with an “n” graded bandgap layer provides an electron multiplication factor approximately equal to 2 ⁽ⁿ⁻¹⁾ .

  In FIG. 1, the multiplication region 32 includes, as an example, three graded band gap layers 50. For each initial electron 54 entering the multiplication region 32, four electrons exiting the multiplication region 32 are present. It is shown. Theoretically, an “ideal” multiplication region 32 with three graded bandgap layers provides an “electron multiplication factor” equal to four. According to a preferred embodiment of the present invention, the number of graded bandgap layers used in multiplication region 32 can be different from three, with different numbers of graded bandgap layers providing different electron multiplication factors.

  Electrons exiting the multiplication region 32 are captured by the first dielectric mirror 34 and generate a spatially varying electronic charge density, or “charge image”, at the first dielectric mirror 34. The electrons captured by the first dielectric mirror 34 are represented by a minus sign, and the clustering schematically shows a varying density that is a charge image. The density of the trapped electrons is substantially homologous to the density of electron-hole pairs generated in the photoconductive layer 30 by the input light 24. Thus, the charge image images the object imaged by the variation in intensity of the input light 24 in the photoconductive layer 30.

  The charge image creates a spatially varying electric field or “modulation field” in the light modulation region 36. This modulation field has a magnitude that is approximately proportional to the charge density of the charge image at a point in the light modulation region 36 in the region of the dielectric mirror 34 just opposite that point. Thus, variations in the magnitude of the modulation field also image the object imaged by the input light 24 in the photoconductive layer 30.

  The absorption coefficient of the material of the light modulation region 36 for the output light 22 is a function of the electric field of the light modulation region 36. In some preferred embodiments of the present invention, the absorption coefficient increases with increasing electric field magnitude, and the light modulation region 36 is a negative modulator. In some preferred embodiments of the present invention, the absorption coefficient decreases with increasing electric field and the light modulation region 36 is a positive image modulator.

  In some preferred embodiments of the invention, for some wavelengths of output light 22, the absorption coefficient increases with increasing electric field, and for other wavelengths of output light 22, the absorption coefficient increases. Decreases with electric field. In this case, the light modulation region 36 is a positive image modulator for some wavelengths of the output light 22 and is a negative image modulator for other wavelengths of the output light 22.

  FIG. 2 shows the dependence of the absorption coefficient on the wavelength and electric field for an optical modulation region 36 comprising an MQW modulator of the type described in PCT Publication WO 99/40478 referred to above according to a preferred embodiment of the present invention. The graph 90 which represents typically is shown. The light modulation layer is a positive image modulator for some wavelengths of light and a negative image modulator for other wavelengths of light.

  In the graph 90, the wavelength is on the horizontal axis and the absorption coefficient “β” is on the vertical axis. A curve 92 represents the relationship between the wavelength and the absorption coefficient when there is no electric field in the light modulation region 36. A broken line 94 represents the relationship between the wavelength and the absorption coefficient when an arbitrary electric field is present in the light modulation region 36. For the wavelength λ1, as shown in the graph, the light modulation region 36 is a positive image modulator, and the absorption coefficient β decreases as the electric field increases. For the wavelength λ2, the light modulation region 36 is a negative image modulator, and β increases as the electric field increases.

  The light modulation region 36 normally absorbs the output light 22 partially, and under the influence of only a relatively uniform electric field 46, the absorption coefficient is relatively spatial in the light modulation region 36. Homologous. The modulation field is substantially parallel to the electric field 46. Accordingly, the magnitude of the electric field in the light modulation region 36 increases everywhere the modulation field in the light modulation region 36 is not zero. Therefore, the modulation field generates a spatial variation of the absorption coefficient in the light modulation region 36 for the output light 22 having a pattern that is similar to the variation pattern of the intensity of the input light 24. When the light modulation layer is a negative image modulator, the magnitude of the electric field increases the absorption coefficient in a state of a pattern that is homologous to the intensity variation pattern of the input light 24. When the light modulation layer is a positive image modulator, the electric field decreases the absorption coefficient in a state of a pattern homologous to the intensity fluctuation pattern of the input light 24. Therefore, the spatial variation pattern of the absorption coefficient in the light modulation region 36 images the object imaged with the input light 24.

  Referring again to FIG. 1, after the modulation field is established in the light modulation region 36, the output light 22 emitted from a suitable light source (not shown) is directed in a direction substantially perpendicular to the layers of the layered body 26. The light is incident on the second dielectric mirror 38. The arrow lines schematically representing the output light 22 follow the path of the rays in the output light 22 as they enter and exit the wavelength converter 20. The output light 22 incident on the second dielectric mirror 38 from the outside of the layered body 26 has a uniform intensity on the surface of the second dielectric mirror 38. It passes through the conductive layer 42 and the light modulation region 36 until it reaches the first dielectric mirror 34. The output light 22 is reflected by the first dielectric mirror 34, passes through the light modulation region 36 and the conductive layer 42 again, and exits the wavelength converter 20 through the second dielectric mirror 38.

  In the light modulation region 36, the intensity of the output light 22 is spatially modulated in a plane parallel to the layer of the wavelength converter 20 due to a change in the absorption coefficient of the light modulation region 36. In FIG. 1, as an example, the light modulation region 36 is assumed to be a positive image modulator. The length of the arrow line representing the output light 22 that exits the wavelength converter 20 schematically represents the intensity of the output light 22 that exits the wavelength converter 20. Accordingly, the outgoing arrow line representing the output light 22 is relatively long when the density of electrons captured by the first dielectric mirror 34 is large, and relatively large when the density of captured electrons is not present. short. The spatial modulation pattern of the output light 22 is a positive image of the intensity of the input light 24 and the spatial variation of the electron density distribution captured by the dielectric mirror 34.

  The presence of multiplication region 32 significantly increases the sensitivity of wavelength converter 20 relative to the sensitivity of prior art wavelength converters. For the same intensity input light, the wavelength converter 20 generates more trapped electrons than the prior art wavelength converter, thereby providing a stronger modulation field than the prior art wavelength converter. Thus, for input light of the same intensity, the wavelength converter 20 modulates the output light to a higher degree than prior art wavelength converters. As a result, the wavelength converter 20 can provide an image of the object from input light having an intensity lower than that required by prior art converters.

By way of example, according to a preferred embodiment of the present invention, the wavelength converter 20 receives input light 24 having a wavelength of 1500 nanometers and provides modulated output light 22 having a wavelength of 850 nanometers. Designed. The highly doped conductive layers 40 and 42 are each preferably 400 nanometers thick and formed of AlGaAs. The photoconductive layer 30 is preferably made of InGaAs and has a thickness of about 500 nanometers. The graded band gap layer 50 in the multiplication region 32 is formed of AlGaAs, and in each graded band gap layer 50, the concentration of Al is from zero to the desired maximum in the direction of electron drift caused by the electric field 46 of the layer. The rating is preferably up to the value. The thickness of graded band gap layer 50 is preferably about 150 nanometers. The layer 48 in the first dielectric mirror 34 that also captures electrons emerging from the multiplication region 32 is preferably an alternating layer of low temperature GaAs and AlGaAs, and these layers are on the order of 150 nanometers. It is preferable that it is the thickness of this. The narrow band gap and the wide band gap layer 52 of the light modulation region 36 are preferably made of GaAs and AlxGa _(1-X) and have a thickness of about 7 to 10 nm. Layer 49 of second dielectric mirror 38 is preferably an alternating layer of GaAs and AlGaAs, and these layers are preferably about 200 nanometers thick. Layer 49 can also be formed of a common dielectric material. In some preferred embodiments of the present invention, the positions of the dielectric mirror 38 and the conductive layer 42 are interchanged. In these preferred embodiments of the present invention, the dielectric mirror layer 49 is preferably formed of a semiconductor material. In operation, a potential difference of 50 to 100 volts is applied to the conductive layers 40 and 42 by the power supply 44 to generate a field in the wavelength converter 20 of about 10 ⁵ vol / cm.

  The material and thickness of the layers described above and their degrees are given by way of illustration, and those skilled in the art will be able to conceive other materials and thicknesses of the layers, and different degrees thereof. It may also be advantageous. The material and thickness and the extent of such layers depend on, among other things, the wavelength of the input and output light, the operating voltage, and the desired sensitivity of the wavelength converter.

FIG. 3 schematically shows a cross-sectional view of another solid-state wavelength converter 70 that encodes the output light 22 with an image encoded in the input light 24.
The wavelength converter 70 includes the same layer included in the wavelength converter 20, except that the first dielectric mirror 34 of the wavelength converter 20 is replaced with a capture layer 72 in the wavelength converter 70. The trapping layer 72 is preferably a 200 nm layer formed of low temperature AlGaAs. Unlike the wavelength converter 20, the wavelength converter 70 does not include a layer designed to reflect the output light 22. The power supply 44 generates an electric field 46 in the layer of the wavelength converter 70. In the wavelength converter 70, as an example, the light modulation region 36 is a negative image modulator, or a light modulation region operable as a positive image or negative image modulator operated as a negative image modulator.

  The electrons are generated and multiplied in the photoconductive layer 30 and the multiplication region 32 of the wavelength converter 70 in the same manner as the electron is generated and multiplied by the wavelength converter 20. Input light 24 is incident on layer 40 and passes to photoconductive layer 30 where it generates electron-hole pairs. Photons of the input light 24 not absorbed by the photoconductive layer 30 travel through other layers of the wavelength converter 70, reach the dielectric mirror 38, are reflected, pass through the photoconductive layer 30 again, and Exit the wavelength converter. Photons from the input light 24 reflected by the dielectric mirror are represented by dashed arrows 24 '. Electrons generated in the photoconductive layer 30 are multiplied in the multiplication region 32.

  However, in the wavelength converter 70, after the electrons are multiplied and exit the multiplication region 32, the electrons of the wavelength converter 70 are not captured in the dielectric mirror as in the case of the wavelength converter 20, but instead. Is captured by the capture layer 72. (Electrons captured by the capture layer 72 are represented by a collection of minus signs.) The capture layer 72 is thinner than the dielectric mirror 34 that captures electrons by the wavelength converter 20 and is captured in a volume smaller than the dielectric mirror 34. It is preferable to condense the electrons. As a result, the modulation field generated by the charge trapped in the wavelength converter 70 “tracks” the input image encoded in the input light 24 with better fidelity than the modulation field of the wavelength converter 20. .

  The output light 22 can enter the wavelength converter 70 through the dielectric mirror 38 or the conductive layer 40. In FIG. 3, the output light 22 enters the wavelength converter 70 through the dielectric mirror 38 as an example. (However, the input light 24 must enter the wavelength converter 70 through the conductive layer 40 due to the presence of the dielectric mirror 38. The entry through the dielectric mirror 38 is This is not possible because it reflects the input light 24. In some preferred embodiments of the present invention, there is no dielectric mirror 38 and the input light 24 is wavelength converted through the conductive layer 40 or conductive layer 42. 70. In some preferred embodiments of the present invention, the dielectric mirror 38 is positioned to the left of the photoconductive layer 30 and the input light 24 passes through the conductive layer 42 from the right side through the wavelength converter. Enter 70 and go out.)

  After entering the wavelength converter 70, the output light 22 passes through the conductive layer 42 and passes through the light modulation region 36 where it is modulated by the modulation field generated by the electrons trapped in the capture layer 72. After modulation, the output light 22 continues to pass through the trapping layer 72, the multiplication region 32, and the photoconductive layer 30 and exits the wavelength converter 70 through the conductive layer 40.

  The intensity of the output light 22 exiting through the conductive layer 40 is schematically indicated by the length of the arrow representing the output light 22 exiting the wavelength converter 70. As described above, the light modulation region 36 operates as a negative image modulator. Therefore, the output light passing through the region of the light modulation region 36 having a relatively large modulation field (the region of the light modulation region 36 is adjacent to the region of the capture layer 72 having a relatively high concentration of trapped electrons). 22 is absorbed relatively strongly. The output light 22 that passes through the region of the light modulation region 36 where the modulation field is relatively small (that is, the light modulation region adjacent to the relatively small concentration of trapped electrons in the capture layer 72) is absorbed relatively weakly. . Accordingly, the short straight arrow 22 showing the modulated output light 22 of FIG. 3 having a relatively weak intensity is a wavy arrow 24 representing the input light 24 entering the wavelength converter 70 where the input light 24 is relatively strong. In the position. The long straight arrow 22 representing the modulated output light 22 having a relatively strong intensity is in a region where the input light 24 is relatively weak or absent, ie where there is no wavy input arrow 24 entering the wavelength converter 70. . The spatial modulation pattern of the output light 22 after the modulation region 36 is added is a negative image of the spatial modulation of the input light 24. (However, by operating the light modulation 36 in the wavelength range operating as a positive image modulator, a positive image of spatial modulation of the input light 24 is generated, and the positions of the short straight arrow 22 and the long straight arrow 22 are exchanged.)

  When passing through the photoconductive layer 30, if the energy of the photons of the output light 22 is greater than the energy of the photons of the input light 24, the output light 22 generates electron-hole pairs in the photoconductive layer 30. As in the case of electrons from the electron-hole pair generated by the input light 24, the electrons from the electron-hole pair generated by the output light 22 are multiplied in the multiplication region 32, and the trapping layer 72. Captured at. “The captured electrons of the output light 22” change the modulation field of the light modulation region 36 and weaken the correspondence between the modulation field and the image encoded in the input light 24. If the modified modulation field is weakened and used to modulate the output light 22, the output light 22 will not be correctly encoded with the image encoded in the input light 24.

  Since the light modulation area 36 is a negative image modulator and the modulation pattern of the output light is a “negative image” of an image encoded in the input light, the electrons of the output light weaken the correspondence. However, even if the output light 22 enters the modulator 70 through the conductive layer 40, passes through the photoconductive layer 30, and then the output light 22 is modulated "negatively", the electrons generated by the output light are Note that it still weakens the correspondence between the input image and the modulation field. This is because the output light 22 will generate a uniform electron density in the photoconductive layer 30 assuming that the output light 22 has a uniform spatial intensity before being modulated. This electron density then generates a uniform trapped electron density in the capture layer 72, which distorts the contrast of the captured charge image in the capture layer 72 and generally reduces its contrast.

  According to a preferred embodiment of the present invention, the output light 22 is provided in short pulses of light to ensure that the output light 22 is accurately encoded in the image encoded in the input light 24. Is done. The length of the pulse of output light 22 is such that the pulse passes through the light modulation region 36 and exits before the electrons that the pulse of output light 22 generates in the photoconductive layer 30 significantly alters the modulation field. To be determined. For example, assume that multiplication region 32 has a thickness “d” and electrons pass through multiplication region 32 at a drift velocity “v”. Electrons generated in the photoconductive layer 30 by the pulse of output light 22 cannot reach the capture layer 72 earlier than time t = d / v after the light pulse first enters the photoconductive layer 30. According to a preferred embodiment of the present invention, when the pulse length of the pulse of output light 22 is shorter than “t” (assuming that the speed of light is much faster than v), it is generated by the pulse of output light 22 in the photoconductive layer 30. The electrons do not affect the modulation of the output pulse. In some preferred embodiments of the present invention, the output light pulse length can be longer than t without compromising the modulation accuracy of the output light 22. This is possible because the modulation field is generally not significantly “impaired” until a significant number of electrons generated by the pulse of output light 22 in the photoconductive layer 30 reach the capture layer 72.

For example, it is assumed that the modulation field is not substantially changed until electrons from the photoconductive layer 30 generated at a depth δ inside the photoconductive layer 30 by the pulse of the output light 22 reach the trapping layer 72. When the pulse length of the pulse of the output light 22 is shorter than or equal to (d + δ) / v, the modulation of the pulse in the light modulation region 36 is not significantly impaired by the electrons generated in the photoconductive layer 30. As an example, if d is equal to 1000 nm (ie, seven 150 nm graded band gap layers), δ is equal to 5000 nm, and v is equal to about 10 ⁷ cm / sec, t = 6 × 10 ⁻¹¹ sec. . In this example, when the pulse length of the output light 22 is less than 60 picoseconds, the electrons that generate a pulse in the photoconductive layer 30 do not impair the modulation of the pulse.

  After the first pulse of the output light 22 has passed through the converter 70, subsequent times of the output light 22 until a sufficient time has passed for the electrons captured from the first pulse to escape the capture layer 72. The two pulses preferably do not pass through the transducer 70. This ensures that the second pulse of output light 22 is not encoded by the modulation field generated by the charge generated by passing the first pulse of output light through the wavelength converter 70. Normally, the trapped electrons escape from the trap layer formed of AlGaAs with a “relaxation” of about 1 msec. Thus, the wavelength converter 70 (and also the wavelength converter 20) can be used to convert an input image encoded in the input light 24 into an output image about once per millisecond.

  For video applications, assuming that all output light pulses 22 have substantially the same intensity, the video frame rate determines the minimum intensity required for the output light pulse 22. For example, assume that an amount of energy E is required to form a video image on a suitable photosensitive surface. If this energy is to be provided by the encoded output light pulse 22, the wavelength converter 70 can image a scene at a rate of about 1 image per millisecond, up to about 30. The encoded light pulse 22 can be used to provide the required video image. Let τ be the pulse width of the output light pulse 22, and let α be the intensity portion of the output light pulse 22 that remains after the output light pulse has passed through the photoconductive layer 30. According to a preferred embodiment of the present invention, the peak intensity of the light pulse 22 should be about E / (30ατ).

  The above discussion is based on the assumption that the pulse of the output light 22 passes through the wavelength converter 70, thereby eliminating the correspondence between the input image and the captured charge image. The disappearance of the correspondence is the result of the output light pulse passing through the photoconductive layer 30 prior to passing through the light modulation region 36, or because the light modulation region 36 is a negative image modulator. .

  In some preferred embodiments of the present invention, the light modulation region 36 is a positive image modulator and the output light 22 passes through the light modulation region 36 before passing through the photoconductive region 36. In these preferred embodiments of the present invention, the density of electron-hole pairs generated by the output light 22 in the photoconductive layer 30 tends to be homologous to the initial density of electron-hole pairs generated by the input light 24. It is. This is because the modulation pattern of the output light 22 is a positive image of intensity fluctuation of the input light 24, that is, a positive image of the input image, as described above. In these preferred embodiments of the present invention, the electrons generated in the output light pulse are multiplied and trapped in the capture layer 72 in a pattern that is approximately the same as the pattern of electron density in the charge image generated by the input light 24. . The effect of the modulated output light passing through the photoconductive layer 30 is to amplify the captured charge image while maintaining its shape and thereby maintaining a correspondence with the input image.

  However, the intensity of the output light 22 is such that the amount of electrons generated by the output light 22 in the photoconductive layer 30 can easily saturate the wavelength converter 70. When the output light 22 saturates the wavelength converter 70, the effect of the output light 22 passing through the photoconductive layer 30 is not to maintain the shape of the charge image, but at the same time to multiply the charge image, rather than to modulate the field 24, weakening the correspondence of the input image encoded in 24. Thus, in some preferred embodiments of the invention, the pulse intensity and pulse length of the output light 22 are controlled so that the output pulse does not saturate the wavelength converter 70 by passing through the photoconductive layer 22. .

  According to a preferred embodiment of the present invention, when the modulation region 36 of the wavelength converter 70 is a positive image modulator, the pulses of the output light 22 are transmitted substantially continuously through the wavelength converter 70, and the wavelength converter “Query” the captured charge image of. As long as the output light pulse does not saturate the wavelength converter 70, the charge image is not distorted and each interrogation pulse of the output light 22 is appropriately modulated by the charge image. The rate at which the pulses of the output light 22 can be transmitted through the wavelength converter 70 without damaging the correspondence between the charge image and the input image is the pulse length of the output light pulse, their intensity, and the output light pulse Determined by conditions that do not saturate. However, if the image to be provided by the wavelength converter 70 fluctuates significantly in a time shorter than the relaxation time of the wavelength converter, the periodicity between the series of output light pulses 22 that travel through the wavelength converter 70 A time delay is required. The series of output light pulses should last only for a period of time during which the acquired image does not vary substantially. A time delay is then required to allow the “old image” to be fully collapsed so that it does not overlap the new image or break the new image. Of course, the time delay should be approximately equal to the relaxation time of the wavelength converter.

  Further, as described above, the pulse of output light 22 that does not saturate the wavelength converter 70 inquires about the charge image and amplifies the charge image in the wavelength converter. Thus, in some preferred embodiments of the present invention, at least one pulse of output light 22 is used to multiply electrons generated by input light 24. For example, after a charge image is established in the wavelength converter 70, at least one pulse of output light 22 that does not saturate the wavelength converter is transmitted to amplify the number of electrons captured in the charge image. (Of course, this pulse can also function as an interrogation pulse.) A short and strong pulse of output light 22 (eg, saturates wavelength converter 70 and weakens the charge image) is then wavelength converted. The device 70 is transmitted to “read” the charge image. In some preferred embodiments of the present invention, a pulse of output light 22 generates electrons in photoconductive layer 30 that multiply the charge image while part of the pulse is still in the light modulation layer. To be long enough. Thereby, the modulation of a part of the pulse in the optical modulation is amplified. “The part before the pulse amplifies the modulation of the part after the pulse.”

  By using the output light 22 to multiply electrons, some preferred embodiments of the present invention can reduce the number of layers in the multiplication region 32 or eliminate the multiplication region 32. it can.

  According to a preferred embodiment of the present invention, light other than the output light 22 can be used to multiply electrons, and it is advantageous to use light other than the output light 22 to multiply electrons. Note that this can be the case. According to a preferred embodiment of the present invention, sufficient energy is generated to generate electron-hole pairs in the photoconductive layer 30 in the same manner as described above for multiplying electrons using the output light 22. The light of any wavelength can be used to multiply the electrons.

  In some preferred embodiments of the invention, input light 24 is incident on wavelength converter 70 to establish a steady state charge image on the wavelength converter. In the wavelength converter, the trapped charge generated in response to the input light is balanced with the leakage of the trapped charge from the trap layer 72. The charge image is then formally “queried” with a pulse of output light 24 depending on the saturation condition described above. This query destabilizes the charge image. After waiting for enough time to reduce the impact of the query, additional queries are made.

  Although “steady state” operation has been described for wavelength converter 70, steady state operation is also possible and advantageous for wavelength converter 20 and other wavelength converters according to a preferred embodiment of the present invention. .

  FIG. 4 schematically shows another wavelength converter 80 according to a preferred embodiment of the present invention. The wavelength converter 80 is similar to the wavelength converter 70 and includes the same many layers as the wavelength converter 70. However, the wavelength converter 80 does not include the dielectric mirror 38 provided in the wavelength converter 70. In addition, a thin metal layer 82 is coupled to the conductive layer 40 provided in the wavelength converter 70. The metal layer 82 extends to the photoconductive layer 30 and is drilled through the width of the photoconductive layer almost completely, or through the photoconductive layer 30 completely. The holes in metal layer 82 and photoconductive layer 30 are formed using methods known in the art, such as ion beam drilling.

  The input light 24 enters the wavelength converter 80 through the conductive layer 42, passes through the layers of the wavelength converter 80 and then reaches the photoconductive layer 30, where most of the input light 24 is absorbed and electron − Generate hole pairs. Light from the input light 24 that is not absorbed by the photoconductive layer 30 then travels to the metal layer 82. Some of the light incident on the metal layer 82 is reflected by the metal layer 82 and again passes through the photoconductive layer 30 to generate further electron-hole pairs. As in the other described wavelength converters according to the preferred embodiment of the present invention, electrons generated by the input light 24 in the photoconductive layer 30 are multiplied in the multiplication region 32 and captured in the capture region 72. It is preferred that The trapped electrons generate a modulation field in the light modulation region 36 that modulates the output light 22. As an example, the light modulation region 36 is assumed to operate as a positive image modulator.

  Output light 22 enters wavelength converter 80 through holes 84 in the metal layer and photoconductive layer 30, passes through all layers of wavelength converter 80, and exits wavelength converter 80 from conductive layer 42. Thus, the output light 22 does not pass through or substantially interact with the material of the photoconductive layer 30. Therefore, the output light 22 does not generate electrons in the photoconductive layer 30 that may reduce the correspondence between the charge image captured by the capture layer 72 and the input image encoded in the input light 24.

  It can be predicted that hole 84 will result in output light 22 that is unmodulated in accordance with the input image encoded in input light 24. Not only does the output light 22 generate electrons in the holes 84, but the input light 24 does not generate electron-hole pairs in the holes 84. Therefore, the charge image and the modulation electric field in the light modulation region 36 generated according to the input image are predicted to have “holes” corresponding to the holes 84 in the metal layer 82 and the photoconductive layer 30. Since the output light 22 substantially passes through the wavelength converter 80 only if there is a hole, the output light 22 will also generate a captured charge image in the modulation region 36, which is also generated in the modulation region 36. There is no “seeing”. However, according to a preferred embodiment of the present invention, the hole 84 is formed with a sufficiently small diameter so that the lateral spread of the charge image and its modulation field fills the expected hole in the charge image and modulation field. The As a result, the charge image and its modulation field are relatively “intact” and smooth in the lateral direction with their integrity substantially uncompromised. Therefore, the output light 22 is appropriately “modulated” corresponding to the input image without “seeing” the “hole” in the modulation field. The intensity of the unmodulated output light 22 is such that the amount of output light 22 that enters and is modulated by the wavelength converter 80 is sufficient to provide an image of acceptable intensity variations in the input light 24. To be controlled.

  FIG. 5 schematically shows a camera 100 comprising the wavelength converter 20 shown in FIG. 1 for imaging an object 102 according to a preferred embodiment of the present invention. FIG. 5 shows only the features and elements of the wavelength converter 20 necessary for the following description.

  Camera 100 preferably includes collection optics 104, shutter 106, light source 108 that provides output light to wavelength converter 20, and photosensitive surface 110 such as a CCD. The light source 108 is preferably a laser diode. The shutter 106 is preferably a large aperture shutter of the type described in PCT Publication WO 99/40478 referenced above. The collection optic 104 is preferably covered with a suitable filter (not shown) that substantially transmits only light that is to be imaged by the camera 100. A control device (not shown) controls the opening and closing of the shutter 106, switching the light source 108 on and off, and / or the intensity of light emitted by the light source 108.

  To image the object 102, the controller collects light from the object 102, represented by the wavy arrow 24, by the collection optics 104 so as to image the object 102 on the wavelength converter 20. The shutter 106 is opened for an appropriate period while being transmitted through the shutter 106. The light 24 from the object 102 focused on the wavelength converter 20 is input light to the wavelength converter 20, and the object 102 is applied to the first dielectric mirror 34 of the wavelength converter 20 as described above. Generate a charge image of the trapped electrons. The charge image is schematically represented by a “trapped” minus sign in the first dielectric mirror 34.

  Following the formation of the charge image, the controller switches the light source 108 so that the light, ie the output light to the wavelength converter 20 represented by the arrow line 22, is emitted by the light source 108. The output light 22 is preferably collimated by a collimating lens 112 and transmitted toward a beam splitter 114 that directs the output light 22 to enter the wavelength converter 20.

  The incident output light 22 is modulated by the light modulation region 36 of the wavelength converter 20, is reflected by the wavelength converter 20 by the dielectric mirror 34, and travels toward the beam splitter 114 again. Output light 22 from wavelength converter 20 incident on beam splitter 114 is focused onto photosensitive surface 110 by suitable imaging optics 116 to produce an image of object 102. The controller may determine the length of time that the light source 108 emits the output light 22 and / or the output light so that the amount of output light 22 incident on the photosensitive surface 110 is sufficient to provide an image of the object 102. 22 intensity is controlled.

  In some preferred embodiments of the present invention, the camera 100 comprises a light source that illuminates the object 102 with light having the wavelength characteristics of the input light of the wavelength converter 20. Input light from the object 102 is light from the light source reflected by the object 102. The controller controls the light source to switch the light source and / or determines the intensity of light emitted by the light source. In some cameras with an input light source as described above according to a preferred embodiment of the present invention, it is not necessary to block the input light, instead the light source is pulsed.

  In camera 100, shutter 106 and wavelength converter 20 are illustrated as separate optical elements, but in some preferred embodiments of the invention, shutter 106 and wavelength converter 20 are combined into one optical element. Form.

  FIG. 6 schematically shows a wavelength converter 150 including the wavelength converter 20 that is integrated with the shutter 106 to form one optical element. The shutter 106 and the wavelength converter 20 are made in an integrated manner using methods known in the art. The shutter 106 is preferably a shutter of the type described in PCT Publication WO 99/40478 referenced above. The shutter comprises an epitaxial MQW structure 152 made up of alternating narrow and wide band gap layers 154 made on the conductive layer 40 of the wavelength converter 20. Conductive layer 155 is formed on the end of MQW structure 152 opposite the conductive layer 40. The conductive layer 40 functions as a common electrode for the wavelength converter 20 and the shutter 106. The power source 156 provides a voltage to the wavelength converter 20, and the power source 158 generates a potential difference between the layer 40 and the layer 155 to control the shutter 106.

  FIG. 7 schematically illustrates a camera 170 with the wavelength converter 70 shown in FIG. 3 that images the object 102 according to a preferred embodiment of the present invention. FIG. 7 shows only the features and elements of the wavelength converter 70 necessary for the following description. Although the camera 170 is illustrated with a wavelength converter 70, the camera 170 may also be used with a wavelength converter 80 that replaces the wavelength converter 70 according to a preferred embodiment of the present invention. Is possible.

  The camera 170 includes a source 172 of output light 22, a photosensitive surface 174, first and second mirrors 176 and 178, preferably an input light shutter 180 and an output light shutter 182. The mirrors 176 and 178 are almost transparent to the input light and reflect almost completely to the output light. Shutters 180 and 182 are preferably large aperture shutters of the type described in PCT Publication WO 99/40478. A control device (not shown) controls the shutters 180 and 182 and the light source 172.

  In forming an image of the object 102, the controller opens the shutter 180 for an appropriate amount of time and the input light represented by the wavy arrow 24 is collected by the collection optics 104. The collected input light 24 passes through the input light shutter 180 and the second mirror 178, and in the capture layer 72 of the wavelength converter 70, is represented by the charge image (“captured” minus sign of the object 102). Generated).

  After the charge image is formed, the control device controls the light source 172 to emit the output light 22. The output light 22 is collimated by the collimating lens 184 and directed to enter the first mirror 176. The first mirror 176 reflects the output light 22 and directs it toward the shutter 182. The controller controls the shutter 182 to turn the output light 22 into a pulse of light having a pulse length determined as described in the discussion of FIG. The pulse of the output light 22 passes through the wavelength converter 70 and is incident on the second mirror, and the mirror reflects the incident pulse of the output light 22 to the imaging optical component 186. Imaging optics 186 images the pulse of output light 22 onto photosensitive surface 174.

  In camera 170, the pulse of output light 22 is formed using shutter 182; however, in some preferred embodiments of the invention, the pulse of output light 22 is generated using methods known in the art. , By controlling the power input to the light source 172. Furthermore, as in the case of the camera 100 shown in FIG. 5, according to some preferred embodiments of the present invention, the camera 170 comprises a light source that illuminates the object 102 with the input light 24.

  Further, in camera 170, wavelength converter 70 is shown separated from shutters 180 and 182, but in some preferred embodiments of the present invention, wavelength converter 70 is one of shutters 180 and 182. Or combined with both and combined into one optical element. The method of combining the wavelength converter 70 with one or both of the shutters 180 and 182 to form one optical element is similar to the method of combining the wavelength converter 20 with the shutter 106 described in the discussion of FIG.

  In some preferred embodiments of the invention, camera 170 operates as a gated 3D camera. A gated 3D camera and how it operates is described in PCT Publications WO 97/01111, WO 97/01112, and WO 97/01113, the disclosure of which is incorporated herein by reference.

  In the gated 3D mode, the camera 170 is moved with a pulsed light source (not shown) to provide a 3D image of the scene. The pulsed light source emits a train of light pulses that illuminate the scene. After each light pulse in the column is emitted, following a predetermined time delay, shutter 180 gates camera 170 on and off. During the time that the shutter 180 is gated on, the camera 170 receives light from a light pulse reflected by the reflective surface of the scene. The amount of light in each light pulse that reaches the camera 170 from the reflective surface while the shutter 180 is open is a function of the distance from the camera to the reflective surface. The reflected light passes through the shutter 180 and enters the wavelength converter 70, and the capture layer 72 generates an image charge for the scene. The electron density in the area of image charge is a function of the distance to the reflective surface imaged in the area of image charge.

  All light pulses in the pulse train may be emitted within a period sufficiently shorter than the relaxation time of the capture layer 72 (ie, the time it takes for the captured electrons to escape the capture layer 72 and erase the charge image). preferable. As a result, after collecting light from the last light pulse in the light pulse train, the total amount of charge captured in layer 72 is the sum of the trapped charges generated by the reflected light from each light pulse in the light pulse train. It is. The capture layer 72 accumulates and effectively integrates the electrons generated in the photoconductive layer 30 by light from the light pulses reflected by the scene object.

  After collecting the reflected light from the last light pulse of the light pulse train, a pulse of output light 22 is emitted, passes through the wavelength converter 70 and strikes the photosensitive surface 174. The next “illumination” cycle, which includes illuminating the scene with a train of light pulses and emitting the output light pulse 22, is preferably at the relaxation time of the capture layer 72 after the output light pulse 22 is emitted. Following an equal or longer delay, it can be repeated. The illumination cycle is repeated as many times as necessary to form an image of the scene on the photosensitive surface 174. The amount of light used to provide the image is proportional to the product of the number of illumination cycles and the number of light pulses in the light pulse train that illuminates the scene in each cycle. The light intensity of the region of the image is used to determine the distance to the reflective surface in the scene imaged on the region. A method for determining the distance from the intensity of the image is described in PCT Publication referred to above.

  To illustrate the operation of the camera 170 in 3D mode, in accordance with a preferred embodiment of the present invention, assume that the camera 170 is used to provide a 3D video image of the object 102 and that the object 102 is assumed to be located at a position 6 m to 9 m from the camera 170. A suitable pulsed light source illuminates the object 102 with, for example, a light pulse train having a pulse width equal to 10 nsec and a time delay between light pulses equal to 100 nsec. Following the time delay of preferably 40 nsec, after each light pulse in the pulse train is emitted, the shutter 180 is preferably gated open for 10 nsec. (40nsec time delay and 10nsec pulse width and gate width range from 6m to 9m, set the camera to measure the distance from the camera. Placing 100nsec interval between light pulses Provides enough time in between, so when the shutter 180 is open, substantially the reflected light from only one light pulse reaches the shutter.)

  The number of optical pulses in the optical pulse train is preferably such that the duration of the train is sufficiently shorter than the relaxation time of the capture layer 72. For example, in a preferred embodiment of the present invention, the train of light pulses includes 1000 light pulses. As a result, the reflected light from all the optical pulses in the optical pulse train enters the wavelength converter 70 within a period of 0.1 msec. This period is sufficiently shorter than the 1 msec relaxation time of the trapping layer 72 so that the trapped charge generated by the light reflected during this period will hardly leak.

  After irradiating the object 102 with a train of light pulses, the pulse of output light 22 is emitted by the laser device 172, passes through the wavelength converter 70, and strikes the photosensitive surface 174. Following the 1 msec delay, the next row of light pulses from the pulsed light source is emitted to irradiate the object 102 and the irradiation cycle begins again. The illumination cycle is preferably repeated 30 times, so the total number of light pulses used to form a 3D video image of the object 102 is 30,000. Variations in the pulse and gate width, timing sequence, and number of pulses in the pulse train in the imaging scenario described above are possible and may be advantageous according to preferred embodiments of the present invention. Such variations will occur to those skilled in the art.

  While the preferred embodiment according to the present invention described above uses an illumination light pulse train that is sufficiently shorter than the relaxation time of the wavelength converter 70, in other preferred embodiments of the present invention, the object 102 is a light pulse 24. Irradiation is substantially continuous. Accumulation and leakage of trapped charge reaches a steady state, and the pulse of output light 22 is continuously transmitted as a condition to avoid the saturation of the wavelength converter 70 described above.

  Although only camera 170 has been described operating in 3D mode, it should be understood that camera 100 operates in 3D mode as well.

  In the description and claims of this application, the words “comprising”, “including”, “having” and the words of the same origin refer to the object, the object of the verb, the component, the element, or the subject or verb. It is not necessarily a complete list of subject elements.

  Although the present invention has been described using non-limiting detailed descriptions of preferred embodiments, these descriptions are provided by way of illustration and are not intended to limit the scope of the invention. The described preferred embodiments comprise different features, not all of which are required in all embodiments of the invention. Some embodiments of the invention use only some features or possible combinations of features. Those skilled in the art will envision embodiments that include variations of the described embodiments and different combinations of features noted in the described embodiments. The scope of the invention is limited only by the appended claims.

20 wavelength converter 22 output light 24 input light 30 photoconductive layer 32 multiplication region 34 dielectric mirror 36 light modulation region 38 dielectric mirror 40 conductive layer 42 conductive layer 44 power supply 46 electric field 50 graded band gap layer

Claims (7)

A method for encoding information encoded during a spatial intensity variation of light characterized by a first wavelength into light characterized by a second wavelength comprising the first wavelength In order to generate a first electron density distribution that is homologous to the spatial intensity variation of light, the first wavelength light is absorbed by photons from the first wavelength light so that an electron-hole pair is generated. Transmitting in the generated photoconductive material;
Capture electrons from the first electron density distribution inside the capture region to generate an electric field that is homologous to the density distribution in the material that modulates the properties of light passing through the material in response to the electric field in the material. To do
Transmitting a pulsed light having sufficient energy to generate electron-hole pairs in the photoconductive material into the modulating material, and then a second additional homologous to the first electron density distribution. Transmitting to the photoconductive material to generate electron density;
Capturing electrons from the second electron density distribution in the capture region;
Transmitting the second wavelength light into the modulating material, thereby modulating the second wavelength light in response to the electric field, and encoding the second wavelength light with the information; Including methods. 2. The method of claim 1, comprising passing electrons generated in the conductive material through a structure comprising a plurality of graded band gap layers in which the electrons are multiplied in an avalanche process. The pulse length of the pulse is sufficiently long so that after the electrons generated in response to the pulse are captured and the electric field in the modulation material is changed, a portion of the pulse is in the modulation material 3. A method according to claim 1 or claim 2, wherein: Electrons generated in the photoconductive within the material are captured by the second wavelength, before changing the electric field in the material of the modulation, the second wavelength light, the substantially completely the modulation 3. A method according to claim 1 or claim 2, wherein the material is passed through. The transmitting of the second wavelength light into the modulating material comprises transmitting the second wavelength light into the modulating material and into the photoconductive material. The method according to claim 4. Generating the second additional electron density;
Transmitting the second wavelength light into the modulation material after an electric field substantially similar to the intensity variation of the first wavelength light is established in the modulation material;
6. The method of claim 5 , further comprising: transmitting the pulsed light into the photoconductive material to generate the second electron density distribution. A method of encoding information encoded during a spatial intensity variation of light characterized by a first wavelength into light characterized by a second wavelength comprising:
Transmitting at least one pulse of light of a first wavelength into a photoconductive material, thereby generating an electron density distribution that is homologous to a spatial intensity variation of the light of the first wavelength;
Capturing electrons from the electron density distribution in order to generate an electric field that is homologous to the spatial variation in the material that modulates the properties of light passing through the material in response to the electric field in the material;
The second wavelength light pulses having sufficient energy to generate electron-hole pairs in the photoconductive within the material, is transmitted to the material and in the photoconductive material of the modulation, thereby depending on the field Modulating the pulse and encoding the pulse with the information; and
Including transmitting a pulse of the second wavelength light first into the light modulation layer and then into the photoconductive material , wherein the pulse length of the pulse is an electron generated in response to the pulse After being captured and changing the electric field in the modulation material, a portion of the pulse is in the modulation material,
Method.

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