JP2013061488A - Optical scanner - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical scanner capable of obtaining deviated light flux free from distortion when applying an intense electric field.SOLUTION: An optical scanner comprises an optical waveguide for transmitting incident light. The optical waveguide includes an electro-optical element in which refractive index changes according to the applied electric field. Voltage of a predetermined frequency is applied to the electro-optical element to change the refractive index of the electro-optical element, and the incident light is blinked according to the changed refractive index.

Description

  The present invention relates to an optical scanning device using an electro-optical element.

  As an apparatus for scanning light by controlling the traveling direction of a light beam, an optical scanning apparatus using a polygon mirror, a galvano mirror, or an electro-optical element is known.

  In an optical scanning device using a polygon mirror and a galvanometer mirror, the angle at which light can be deflected (deflection angle) is large. However, since an optical system (such as a mirror) and a movable part are required, downsizing may be difficult. In addition, vibration of the movable part may be a problem.

  On the other hand, in an optical scanning device using an electro-optic element, the deflection angle is small, but the electro-optic effect (the effect that the refractive index of the electro-optic crystal changes depending on the electric field strength) is utilized. High responsiveness can be realized. Further, it is relatively easy to reduce the size.

  Patent Document 1 discloses a technique relating to a high-resolution electro-optic element that enables high-speed random access using a ferroelectric substrate having a plurality of triangular domain-inverted domains, has a large deflection angle, and can be deflected. doing.

  An optical scanning device using a ferroelectric material (electro-optic element) changes the refractive index of the electro-optic crystal in the ferroelectric material by applying a voltage in one direction continuously to the ferroelectric material. It deflects transmitted light. At this time, in order to obtain a large deflection angle, it is necessary to apply a high electric field to the electro-optic crystal. Specifically, in a lithium niobate crystal used as an electro-optic crystal, when deflecting light with a prism structure (prism width D = 2 mm, prism length L = 20 mm) by polarization inversion, the deflection angle is 5 °. In order to achieve the above, a voltage is applied in one direction for a time corresponding to the pulse width of the pulse wave, and an electric field of about 10 kV / mm or more is applied to the lithium niobate crystal.

  In the technique disclosed in Patent Document 1, when a high electric field is applied to the electro-optic crystal, the electric field from the anode is affected by the electrons injected into the crystal due to the space charge effect, and the electric field between the electrodes ( In some cases, the charge distribution was non-uniform. As a result, the refractive index of the electro-optic crystal becomes non-uniform, the output light deflected and emitted is distorted, and the quality of the emitted light may not be maintained.

  An object of the present invention is to provide an optical scanning device capable of obtaining a deflected light beam that is not distorted even when a high electric field is applied.

  In order to solve the above-described problems, the present invention provides an optical scanning device having an optical waveguide through which incident light is transmitted, the optical waveguide including an electro-optic element whose refractive index changes based on an applied electric field. Applying a voltage having a predetermined frequency to the electro-optical element to change a refractive index of the electro-optical element, and flashing the incident light according to the changed refractive index. An optical scanning device is provided.

  According to the present invention, in the optical scanning device, it is possible to obtain a deflected light beam without distortion even when the electric field applied to the electro-optic element is high.

It is a schematic block diagram which shows an example of an optical deflector. It is the schematic which shows an example of the principal part of an optical deflector. FIG. 1A is a side view of the main part of the optical deflector. FIG. 2B is a plan view of the main part of the optical deflector. It is explanatory drawing explaining an example of operation | movement of an optical deflector. FIG. 4A is an explanatory diagram for explaining the irradiation position of the emitted light emitted from the optical deflector. FIG. 4B is an explanatory diagram for explaining the waveform of the voltage applied to the optical waveguide and the on / off operation of the light source. It is explanatory drawing explaining the comparative example of the waveform of the voltage applied to an optical waveguide. FIG. (A) is an explanatory view showing a comparative example in the case of applying a voltage having an arbitrary pulse width. FIG. 7B is an explanatory diagram showing a comparative example when the intermediate value of the applied voltage is a positive voltage level. It is a schematic block diagram which shows an example of an optical scanning device. It is explanatory drawing explaining an example of operation | movement of an optical scanning device. FIG. 4A is an explanatory diagram for explaining an optical scanning position of outgoing light emitted from the optical scanning device. FIG. 4B is an explanatory diagram for explaining the waveform of the voltage applied to the optical waveguide and the on / off operation of the light source.

  The present invention will be described in detail using an optical deflector that deflects the traveling direction of a light beam by an electro-optic effect. In addition to the optical deflector, the present invention deflects incident light used in a printer, a scanner, an in-vehicle laser radar, a wavelength tunable laser, a medical laser, and the like by using an electro-optic effect, and deflects the luminous flux (incident light ) Can be used as long as it emits as emitted light.

(Configuration of optical deflector)
FIG. 1 is a schematic configuration diagram illustrating an example of an optical deflector according to the present embodiment.

  In FIG. 1, the optical deflector 10 includes a control unit 11, an incident light unit 12, an optical waveguide 13, an electrode 14, and a substrate 15. In this embodiment, the optical deflector 10 deflects the incident light incident from the incident light means 12 by the optical waveguide 13 and emits the deflected light beam (incident light) as outgoing light. Further, the optical deflector 10 controls the voltage applied to the electrode 14 by the control means 11, changes the refractive index of the optical waveguide 13 by the electro-optic effect, and deflects the light beam transmitted through the optical waveguide 13. Here, the electro-optic effect refers to an effect in which the refractive index of the electro-optic crystal changes depending on the electric field strength.

  The control means 11 is a means for controlling the entire optical deflector 10. The control unit 11 controls the operation of the incident light unit 12 and the like. The control unit 11 controls the refractive index of the optical waveguide 13 by controlling the voltage applied to the electrode 14.

  The incident light means 12 emits light and is incident on the optical waveguide 13 as incident light. In the present embodiment, the incident light means 12 includes a light source and an optical system. The incident means 12 condenses the light beam (semiconductor laser or the like) emitted from the light source by an optical system (lens and mirror or the like), and enters the optical waveguide 13 (core layer to be described later).

  The optical waveguide 13 deflects the traveling direction of the incident light incident by the incident light means 12, and emits the deflected light beam (incident light) as outgoing light. In the present embodiment, the optical waveguide 13 includes a core layer 13c and a cladding layer (13up and 13dw).

  In the present embodiment, the core layer 13c is made of a ferroelectric material as an electro-optic element. In addition, the core layer 13c has a region (a polarization inversion region to be described later) in which the direction of spontaneous polarization (the direction of the polarization axis of the electro-optic crystal) is reversed in a part of the ferroelectric material. The cladding layer is a coating layer that reflects light leaking from the core layer 13c. In this embodiment, the clad layer has an upper clad layer 13up and a lower clad layer 13dw. The upper clad layer 13up and the lower clad layer 13dw are formed on the surface of the core layer 13c on the direction side that intersects the incident direction of incident light in order to cover the core layer 13c.

  The electrode 14 applies a voltage to the optical waveguide 13 (core layer 13c). In the present embodiment, the electrode 14 has a pair of electrodes (an upper electrode layer and a lower electrode layer) on the surface of the upper clad layer 13up and the lower clad layer 13dw opposite to the side in contact with the core layer 13c (hereinafter referred to as the outer side). ). Details will be described later (configuration of optical waveguide and electrode).

  The substrate 15 supports the optical waveguide 13 and the electrode 14. Details will be described later (configuration of optical waveguide and electrode).

(Configuration of optical waveguide and electrode)
The configuration of the optical waveguide and electrodes of the optical deflector will be described with reference to FIG. Fig.2 (a) is a side view of the principal part of an optical deflector.

  In FIG. 2A, the optical deflector includes, as optical waveguides, a core layer 13c that is a thin film waveguide that transmits light, and cladding layers (13up and 13dw) that reflect light leaking from the core layer. . The core layer 13c has a positive region 13cp in which the direction of spontaneous polarization is the positive direction of the z axis in the drawing, and a region in which the direction of spontaneous polarization is reversed (in the negative direction of the z axis) in order to deflect light (hereinafter referred to as the z axis). It is referred to as a domain-inverted region.) 13cr. The clad layer has an upper clad layer 13up and a lower clad layer 13dw coated on both end faces of the core layer 13c on the z-axis direction side in the drawing. The clad layer reflects light leaking from the core layer 13c.

  The optical deflector includes an upper electrode layer 14up and a lower electrode layer 14dw formed outside the upper cladding layer 13up and the lower cladding layer 13dw as electrodes for applying an electric field to the optical waveguide. Furthermore, the optical deflector includes a support substrate 15s that supports the optical waveguide and the electrode, and an adhesive layer 15g that bonds the optical waveguide and the support substrate 15s.

In this embodiment, as the material of the core layer 13c, a ferroelectric lithium niobate material (LiNbO ₃ ) having an electro-optic effect can be used as the electro-optic element. Also, the support substrate 15s can be made of a lithium niobate material, and damage due to thermal expansion of the optical deflector can be reduced.

As the upper cladding layer 13up and the lower cladding layer 13dw, a Ta ₂ O ₅ film or SiO ₂ film having a film thickness of about 1 μm can be used. For the upper electrode layer 14up and the lower electrode layer 14dw, a Ti film or a Cr film having a film thickness of about 200 nm can be used.

  The core layer 13c is formed by forming the lower clad layer 13dw and the lower electrode layer 14dw on the core layer 13c, and then inverting the core layer 13c upside down (inverted in the z-axis direction) so that the surface on which the core layer 13c is not formed is formed. Can be polished. Thereby, the film thickness of the core layer 13c can be adjusted. In the present embodiment, the thickness of the core layer 13c is about 20 μm. After polishing, the core layer 13c is formed by depositing the upper cladding layer 13up and the like on the polished surface of the core layer 13c.

(Operation to deflect the luminous flux)
An operation in which the optical deflector deflects the traveling direction of the light beam will be described with reference to FIGS. 2A and 2B. FIG. 2B is a plan view of the main part of the optical deflector of FIG.

  From FIG. 2B, the shape of the domain-inverted region 13cr in the core layer 13c is a triangular cross-sectional shape (hereinafter referred to as a prism shape). A plurality of polarization inversion regions 13cr are arranged in the traveling direction of incident light (the y-axis direction in the figure). In this embodiment, the prism shape is such that the height of the triangular section (the length in the x-axis direction in the figure) is 2 mm, and the total length of the domain-inverted regions having a plurality of prism shapes (the length in the y-axis direction in the figure). Is 20 mm.

  Next, the operation of the light deflector for deflecting the light beam will be specifically described.

  2 (a) and 2 (b), light emitted from a light source (incident light means) (not shown) is collected by a lens (not shown) or the like and is incident on the core layer 13c of the optical waveguide as incident light B1. Is done. At this time, the incident light B1 propagates through the positive region 13cp and the polarization inversion region 13cr in the core layer 13c as a light beam B2.

  Here, when a voltage is applied between the upper electrode layer 14up and the lower electrode layer 14dw above and below the core layer 13c, an electric field is applied to the core layer 13c, and due to the electro-optic effect of the core layer 13c, The refractive indexes of the positive region 13cp and the polarization inversion region 13cr change. At this time, the positive region 13cp and the polarization inversion region 13cr are different in the amount of change in refractive index because the direction of spontaneous polarization is different by 180 degrees. As a result, the light beam B2 in the core layer 13c passes through the positive region 13cp and the domain-inverted region 13cr in the core layer 13c, and is at the interface between the positive region 13cp and the domain-inverted region 13cr (interface having a different refractive index). Refracts and deflects in the XY plane in the figure.

  Thereafter, the deflected light beam B2 is emitted from the end face of the core layer 13c as the outgoing light B3.

  Here, the refractive index difference Δn between the positive region 13cp and the polarization inversion region 13cr in the core layer 13c is expressed by the following equation.

Δn = −1 / 2 × r × n ³ × V / d
In the above formula, r is an electro-optic constant (Pockels constant), n is the refractive index of the core layer 13c, d is the thickness of the core layer 13c, and V is a voltage applied to the core layer 13c.

  From the above, the optical deflector controls the refractive index of the core layer by the electro-optic effect by controlling the voltage applied to the core layer (optical waveguide), and deflects the traveling direction of the light beam passing through the core layer. Can do.

  In addition, by forming the region of the core layer to which the voltage is applied into a region having the same shape as the prism shape of the polarization inversion region, a region where the voltage is applied and a region where the voltage is not applied are formed in the core layer. The refractive index can be locally changed. As a result, it is possible to refract the light beam that passes through the interface between the region to which the voltage is applied and the region to which the voltage is not applied (interface having a different refractive index), and to deflect the traveling direction of the light beam.

(Operation to apply voltage to the optical waveguide)
The operation in which the optical deflector applies a voltage to the optical waveguide will be described with reference to FIG. FIG. 3A shows the irradiation position of the outgoing light emitted from the optical deflector. FIG. 3B shows the waveform of the voltage applied to the optical deflector and the blinking operation of the light source.

In FIG. 3A, the optical deflector can irradiate the emitted light to an arbitrary irradiation position every arbitrary time. Specifically, the control means of the optical deflector can be any time period _{(t =} 0,0~t ₁ in _{FIG, t 1 ~t 2, t 2} ~t 3, _{and, t} 3 ~) at, The emitted light can be emitted to arbitrary irradiation positions (x ₁ , x ₂ , 0, −x ₃ , and −x ₁ in the drawing). Here, the x-axis in FIG. 3A corresponds to the x-axis in FIG.

  Next, FIG. 3B shows a voltage applied to the optical waveguide by the control unit of the optical deflector during the operation of FIG. The elapsed time t on the horizontal axis in FIG. 3B corresponds to the elapsed time t on the horizontal axis in FIG. Further, the applied voltage V on the vertical axis in FIG. 3B is a voltage applied to the optical waveguide (core layer).

  From FIG. 3B, the control means applies the voltage to the optical waveguide while changing the voltage at a predetermined frequency or higher. Here, the predetermined frequency is equal to or higher than a frequency at which the distribution (charge distribution) of electrons injected into the optical waveguide (electro-optic crystal) is not uniform due to the space charge effect in the optical waveguide to which an electric field is applied. Can be a value. Further, the predetermined frequency can be set to a value determined by numerical calculation, experiment, or the like.

3 (b), the control means 3 each time period on the horizontal axis in _{(a) (t = 0,} t = 0~t 1, t 1 ~t 2, t 2 ~t 3 and,, t ₃ ~) corresponding to the irradiated position _{x 1,} x 2, 0 (FIG. 3 (a), -x _3, and, for irradiating the emitted light to the position) of -x _1, flashing light source of the optical deflector (Lit and extinguished). Specifically, the control means turns on the light source at a timing when the applied voltage is 0 V in order to irradiate the outgoing light that is not deflected at t = 0. Next, the control means turns on the light source only at the timing when the applied voltage is V ₁ in order to irradiate the emitted light to the irradiation position x ₁ in the time zone of t = 0 to t ₁ , and the other voltage. Turn off the light source. Hereinafter, similarly, in order to irradiate the emitted light to the irradiation position −x ₁ or the like in the time zone such as t = t _{1 to} t ₂ , the light source is turned on only at the timing when the applied voltage is −V ₁ or the like.

  In this case, by changing the applied voltage at a predetermined frequency or more, the direction of the voltage gradient in the electro-optic crystal is alternately changed, so that the electrons are prevented from being unevenly distributed in the electro-optic crystal. The space charge effect can be suppressed from occurring. For this reason, the charge distribution does not become non-uniform and the applied electric field does not become non-uniform, so that the refractive index becomes uniform and outgoing light without distortion can be emitted. In addition, by increasing the frequency of the voltage to be applied, the timing at which the emitted light can be turned on increases, and smooth light beam irradiation is possible.

  As described above, a voltage is applied to the optical waveguide at a predetermined frequency or more, and the light source is blinked in response to the applied voltage, thereby emitting undistorted emission light and irradiating the emission light at an arbitrary irradiation position. Can do.

  Next, FIG. 4 shows a comparative example of the voltage applied to the optical deflector. FIG. 4A shows a comparative example when a voltage having an arbitrary pulse width is applied. FIG. 4B shows a comparative example when the intermediate value of the applied voltage is a positive voltage level.

  In FIG. 4A, when the emitted light is irradiated to an arbitrary irradiation position, a voltage is continuously applied to the optical waveguide for a time corresponding to the pulse width. For this reason, an electric field in one direction is applied to the optical waveguide at a time corresponding to the pulse width, and the space charge effect may cause non-uniform charge distribution in the electro-optic crystal, resulting in non-uniform refractive index. . At this time, distortion occurs in the emitted light.

  When a lithium niobate crystal added with magnesium is used as a material for an optical waveguide with a thickness of 20 μm, when a voltage of ± 250 V is applied with a triangular wave waveform of 1 kHz or more, the shape of the deflected outgoing light is not distorted. . On the other hand, when the frequency is less than 1 kHz, the shape of the deflected outgoing light is distorted. This is because when the frequency is decreased, the time for the electrons in the electro-optic crystal to stay is secured, and the space charge effect is likely to occur. Therefore, when using a lithium niobate crystal to which magnesium is added, the frequency of the waveform of the voltage applied to the optical waveguide is preferably 1 kHz or more.

  In FIG. 4B, when the intermediate voltage level of the applied voltage is not zero (in the case of the voltage level Vm on the plus side in the drawing), as in FIG. Charge distribution occurs, the refractive index becomes nonuniform, and the emitted light may be distorted. This is because when a voltage is applied to the optical waveguide, a voltage gradient corresponding to an intermediate voltage level exists in the electro-optic crystal, and thus electrons are unnecessarily retained in the electro-optic crystal. Thereby, a non-uniform charge distribution is formed in the electro-optic crystal and the refractive index becomes non-uniform.

  When using a lithium niobate crystal to which magnesium is added as the material of the optical waveguide, the distortion of the emitted light becomes significant when the voltage level in the middle of the applied voltage is not zero. This is because the addition of magnesium facilitates the generation of space charge effects. Therefore, the waveform of the voltage applied by the optical deflector is preferably a waveform in which the intermediate voltage level is zero, as shown in FIG.

(Example)
The present invention will be described using an embodiment of an optical scanning device. The present invention is not limited to an optical scanning device, but in a printer, a scanner, an in-vehicle laser radar, a wavelength tunable laser, a medical laser, or the like, incident light is deflected by using an electro-optic effect, and the deflected light beam (incident light) Can be used for any object as long as it irradiates the object as emitted light.

  The present invention will be described using the optical scanning device of the first embodiment.

(Configuration of optical scanning device)
FIG. 5 shows the configuration of the optical scanning device 100 of this embodiment.

  In FIG. 5, the optical scanning device 100 includes an optical deflector 110, a control circuit 121, a light source driving circuit 122, and an optical deflector driving circuit 123. In this embodiment, the optical scanning device 100 makes incident light from the light source 111 enter the optical waveguide 112 under the control of the light source driving circuit 122. Further, the optical scanning device 100 controls the voltage applied to the electrode A (113a) or the like by the control of the control circuit 121 or the like, and changes the refractive index of the optical waveguide 112 by the electro-optic effect. At this time, the light beam (incident light) transmitted through the optical waveguide 112 deflects the traveling direction, and the deflected light beam is emitted as outgoing light (hereinafter referred to as optical scanning).

  Since the configuration of the optical deflector 110 is the same as that of the optical deflector 10 (FIG. 1), description thereof is omitted.

  The control circuit 121 generates a signal (hereinafter referred to as a lighting signal) for controlling the lighting and extinguishing timing of the light source 111 based on the optical scanning signal for starting the optical scanning operation, and the light source driving circuit. It outputs to 122. The control circuit 121 generates a signal for controlling the voltage applied to the electrode A (113a) or the like (hereinafter referred to as an applied voltage signal) based on the optical scanning signal, and supplies it to the optical deflector driving circuit 123. Output.

  The light source driving circuit 122 controls the timing for turning on and off the light source 111 based on the lighting signal. The optical deflector drive circuit 123 controls the voltage applied to the electrode A and the electrode B (113a and 113b) based on the applied voltage signal.

(Optical scanning operation)
The operation of the optical scanning device for optical scanning will be described with reference to FIG. FIG. 6A shows an irradiation position (hereinafter referred to as an optical scanning position) of outgoing light emitted from the optical scanning device. FIG. 6B shows the waveform of the voltage applied to the optical waveguide and the blinking operation of the light source.

  In FIG. 6A, the optical scanning device first determines a predetermined frequency of the applied voltage corresponding to the object to be scanned by the control circuit based on the optical scanning signal, and applies the applied voltage related to the predetermined frequency. The signal is output to the optical deflector drive circuit. At this time, the optical deflector drive circuit applies a voltage to the electrode A and the electrode B based on the applied voltage signal. Further, the optical scanning device outputs a lighting signal relating to a predetermined frequency to the light source driving circuit. At this time, based on the lighting signal, the light source driving circuit irradiates the light source with an arbitrary light scanning position (vertical axis in the figure) at an arbitrary elapsed time (horizontal axis in the figure). Turns off.

  Here, the predetermined frequency can be a frequency value at which the distribution (charge distribution) of electrons injected into the optical waveguide does not become non-uniform due to the space charge effect. Further, the value of the predetermined frequency can be set to a value corresponding to an object to be optically scanned, which is determined by numerical calculation or experiment.

  Next, FIG. 6B shows the waveform of the voltage applied to the optical waveguide by the optical scanning device and the operation of turning on and off the light source during the operation of FIG. Here, the elapsed time t on the horizontal axis in FIG. 6B corresponds to the elapsed time t on the horizontal axis in FIG. Further, the applied voltage V on the vertical axis in FIG. 6B is a voltage applied to the optical waveguide (core layer).

  In FIG. 6B, when changing the optical scanning position corresponding to the elapsed time, the optical scanning device first applies a voltage at a predetermined frequency to the optical waveguide. Next, the optical scanning device turns on and off the light source of the optical deflector in order to irradiate the emitted light to the optical scanning position corresponding to the elapsed time. Specifically, the optical scanning device turns on the light source at the timing when the applied voltage is 0 V in order to irradiate the emitted light that is not deflected at the elapsed time t = 0 under the control of the light source driving circuit. Next, the optical scanning device turns on the light source at the timing when the applied voltage is V1 (FIG. 6B) in order to irradiate the emitted light to the optical scanning position X1 (FIG. 6A). Hereinafter, similarly, the light source is turned on in order to irradiate the emitted light at the applied voltage corresponding to the optical scanning position.

  In this case, by changing the applied voltage at a predetermined frequency, the direction of the voltage gradient in the electro-optic crystal is alternately changed, so that electrons are prevented from being unevenly distributed in the electro-optic crystal, Generation of the space charge effect can be suppressed. For this reason, the charge distribution does not become non-uniform and the applied electric field does not become non-uniform, so that the refractive index becomes uniform and outgoing light without distortion can be emitted. In addition, by increasing the frequency of the applied voltage, the timing at which the emitted light can be turned on increases, and smooth optical scanning is possible.

  In addition, when optical scanning is always performed at high speed and at a constant frequency, the optical scanning device can also perform optical scanning by applying a voltage at a constant frequency to the optical waveguide and turning on the light source at all times. is there. Here, the constant frequency can be a triangular wave of 100 kHz or more.

  As described above, a voltage is applied to the optical waveguide at a predetermined frequency or more, and the light source is turned on and off according to the applied voltage, thereby emitting outgoing light with no distortion and outgoing light at an arbitrary optical scanning position. Irradiation light scanning can be performed.

10: Optical deflector 11: Control means 12: Incident light means (light source)
13: Optical waveguide 13c: Core layer (electro-optic element)
14: Electrode 15: Substrate 100: Optical scanning device

JP-A-9-146128

Claims (6)

An optical scanning device having an optical waveguide through which incident light is transmitted,
The optical waveguide includes an electro-optic element whose refractive index changes based on an applied electric field,
By applying a voltage of a predetermined frequency to the electro-optic element, the refractive index of the electro-optic element is changed,
Flashing the incident light in response to the changed refractive index;
An optical scanning device. A light source that emits the incident light;
The optical scanning device according to claim 1, wherein the light source blinks the incident light.   The light according to claim 2, wherein the light source blinks the incident light at a timing when the incident light transmitted through the optical waveguide changes to the refractive index when irradiating a desired optical scanning position. Scanning device. Having a pair of electrodes facing the optical waveguide;
The electro-optic element is disposed at a position corresponding to the pair of electrodes,
The refractive index is changed by applying a voltage of the predetermined frequency to the pair of electrodes.
The optical scanning device according to claim 1, wherein the optical scanning device is an optical scanning device.   5. The optical scanning device according to claim 1, wherein an intermediate voltage level of the voltage is zero. 6.   6. The optical scanning device according to claim 1, wherein the predetermined frequency is 1 kHz or more.