JP3455775B2 - Optically driven wavefront correction imaging method and apparatus - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention prevents an obtained image from becoming unclear due to phase fluctuations of a medium in an optical path in an image device such as a microscope, a telescope or a camera used for industrial measurement or medical equipment. And a device for implementing the method, and more particularly to an optical drive type wavefront corrected image method and a device for implementing the method, in which the wavefront correction is performed in real time by an optical writing type liquid crystal spatial phase modulator. Regarding

[0002]

2. Description of the Related Art Image devices such as microscopes, telescopes and cameras used for industrial measurement and medical equipment are limited in resolution due to phase fluctuations of a medium in an optical path which is essentially difficult to remove. Are known. For example, in the field of machining technology, when monitoring a workpiece on-site, vibration of the machine tool and air fluctuations caused by generated heat, and in a medical fundus camera, due to the influence of eye aberration (astigmatism). It is known that the obtained image becomes unclear.

That is, in various image systems represented by microscopes, telescopes, cameras, etc., the resolution is remarkably high due to the phase fluctuation of the medium existing in the optical path connecting the object (subject) and the image plane (observation plane). descend. The technology for correcting the effect of the phase fluctuation in real time is called adaptive optics (or adaptive optics), and its effectiveness has already been confirmed by installing it in a large astronomical telescope.

FIG. 6 shows the configuration of a system widely used conventionally. Basically, the wavefront 60 disturbed by the phase variation of the medium in the optical path is reflected by the variable shape mirror 61, and a part of the reflected light is reflected by the beam splitter (B
S) 62, and it is incident on the wavefront sensor 63. Here, the deformable mirror 61 is a thin mirror 64.
A large number of electrostrictive elements 65 are attached to the back surface of the device, and the shape of the mirror can be arbitrarily changed according to the voltage applied to each electrostrictive element 65.

The wavefront sensor 63 measures the disturbance of the wavefront, and the data is guided to the control device 66. The controller 66 calculates the shape of the mirror necessary for correcting the wavefront, that is, the voltage applied to each electrostrictive element 65 based on the data, and changes the shape of the deformable mirror based on the calculated voltage. By performing this series of operations quickly, the wavefront of the reflected light can be corrected in real time. By incorporating the deformable mirror 61 in the image system, it is possible to improve the resolution of the image system which is lowered due to the disturbance of the wavefront.

In order to incorporate this into an astronomical telescope, which is an example of an image system, the reflecting mirror inside the telescope is replaced with the variable shape mirror 61, and a part of the reflected light from the variable shape mirror 61 is introduced into the wavefront sensor 63 to make the mirror. A control signal is generated and the shape of the mirror is controlled based on the control signal. In fact, a bright star (reference star)
By observing, the wavefront of the light disturbed by the fluctuation of the atmosphere is detected, and the deformable mirror is deformed so that the wavefront disturbed in the process of reflection is corrected. If you observe the target star near the reference star in this state, the fluctuation of the atmosphere is corrected and a clear astronomical image is obtained.

[0007]

As described above, the deformable mirror has a large number of electrostrictive elements (several tens to several hundreds) attached to the back surface of a thin mirror, and a high voltage is applied to each electrostrictive element. It is a device that changes the shape of the mirror by applying it. Therefore, the adaptive optics system using this deformable mirror not only makes the entire system expensive, but also consumes a large amount of power and requires a drive power source for the device by the number of electrostrictive elements. It has the drawback of becoming large-scale. In addition, since there is a physical limit to the change in the shape of the mirror, it is difficult to increase the resolution of the wavefront correction, and even if the number of electrostrictive elements is increased to increase the resolution of the wavefront correction, As a result, the number of calculations for generating the signals for driving the respective elements becomes enormous and the wavefront correction in real time becomes impossible.

In the conventional adaptive optics system using the deformable mirror as described above, 1) the device is expensive. 2) Power consumption increases. 3) The equipment becomes large-scale. 4) It is difficult to increase the resolution of wavefront correction. 5) As the resolution of wavefront correction increases, it becomes difficult to correct wavefront in real time. There was a problem such as. Due to these problems, it is extremely difficult to apply this device to various industrial measurement and medical equipment.

Therefore, the present invention is an optical drive type wavefront which is a small and inexpensive device, consumes less power, facilitates higher resolution of wavefront correction, and is capable of performing wavefront correction in real time. An object is to provide a corrected image method and apparatus.

[0010]

The present invention is based on the above 1) to.
In order to solve the problem of 4), a high-resolution liquid crystal spatial phase modulator is introduced as a device for correcting the wavefront, and in order to solve 5), an optical operation which does not require any driving of the device is required. An optically driven adaptive optics system based on dynamic feedback interferometer was introduced. That is, by using a high-resolution optical writing type liquid crystal spatial phase modulator as a wavefront correction device and incorporating it into an optical feedback interferometer, an optically driven adaptive optical system is constructed and used as an image system. It is built in. Further, since the main purpose of this device is to be used in various industrial measurement and medical equipment, the light for illuminating the object to be measured and the light for driving this adaptive optics system have high brightness and coherence. Uses excellent laser light.

Based on the above idea, the invention according to claim 1 depends on the intensity of the light applied to the writing surface.
Using a liquid crystal spatial phase modulator that changes the modulation phase of the phase modulation surface on the back side, the object irradiation light that irradiates the measured object
From the outside through the lens to the first beam splitter
On the phase modulation surface by passing the disturbing substance with the reference light of
Refer to the object irradiation light that is reflected and reflected by the phase modulation surface
The light from the second beam splitter through the lens and
The reference light is finely imaged on the imaging surface and the object irradiation light is
It is observable, and an interference fringe reflecting the phase distribution of the disturbing medium is obtained from the reference light separated by the second beam splitter ,
By irradiating the writing surface with this, a phase modulation surface is formed so as to cancel the phase distribution of the disturbing medium .
To slightly tilt the optical axis of the body irradiation light with respect to the reference light
According to another aspect of the present invention, there is provided a light-driven wavefront correction image method, which is characterized in that the object irradiation light is prevented from entering the phase modulation surface .

[0012] The invention according to claim 2, depending on the intensity of light irradiated on the writing surface, and a liquid crystal spatial phase modulation element modulating the phase of the phase modulation surface of the back side thereof is changed, the measured
First beam of light emitted from the object
Guide it to the splitter and remove the disturbing substance with the reference light from the outside.
The phase modulation surface is made to pass and reflected by the phase modulation surface.
The object irradiation light and the reference light reflected by the second beam split
From the shutter through the lens to a small amount of the reference light on the image plane.
An optical system that makes it possible to observe the irradiation light of an object while making an image
Then, an interference fringe reflecting the phase distribution of the disturbing medium is obtained from the reference light separated by the second beam splitter, and the writing surface is irradiated with the interference fringes, so that the phase modulating surface is changed so as to cancel the phase distribution of the disturbing medium. With optical system to form
The optical axis of the object irradiation light is slightly tilted with respect to the reference light.
An optical drive type wavefront correction image device characterized by being eccentric .

According to a third aspect of the present invention, there is provided the optically driven wavefront correction image device according to the second aspect, wherein the measured object is a surface of a workpiece.

According to a fourth aspect of the invention, there is provided the optically driven wavefront correction image device according to the second aspect, wherein the measured object is an object in a shielded space.

According to a fifth aspect of the present invention, there is provided the light-driven wavefront correction image device according to the second aspect, in which a writing surface of the liquid crystal spatial phase modulation element is irradiated with a video image of the interference fringes captured by a projection device. It was done.

[0016]

Embodiments of the present invention will be described with reference to the drawings. The basic structure of the present invention is shown in FIGS. Due to space limitations, one device is divided into two and displayed. FIG. 1 mainly shows a part of an image system, and FIG. 2 shows a part of an adaptive optics system for driving an element that corrects a wavefront in a partially overlapping manner.

In FIG. 1, a plane wave is created from linearly polarized laser light, and the object 1 to be measured is illuminated thereby. The light wave transmitted through the object to be measured 1 passes through the lens L1, the beam splitter BS1, and the lenses L2 and L3, and is reflected by the phase modulation surface 3 of the optical writing type liquid crystal spatial phase modulation element 2, and the reflected light is the beam splitter BS2. Then, the light passes through the lens L4 and an image is formed on the image forming surface 6 as an observation surface.

The above optical writing type liquid crystal spatial phase modulator 2
Is an element in which the modulation phase of the phase modulation surface 3 on the back side of the writing surface 4 changes depending on the intensity of light applied to the writing surface 4.
However, the phase modulation has polarization dependency, and modulates only the phase of the light wave of the polarization component parallel to the alignment direction of the liquid crystal molecules. Further, since this element can arbitrarily perform phase modulation according to the intensity pattern irradiated on the writing surface 4,
Has high resolution wavefront correction capability. Disturbing medium 5 that disturbs the wavefront
Are arranged between the beam splitter BS1 and the lens L2 and so as to serve as an image plane formed by the lenses L2 and L3 with respect to the phase modulation plane 3. The focal lengths of the lenses are the same for the lenses L1 and L4 and the lenses L2 and L3, respectively. Regarding these arrangements, an afocal system is used in which the focal points of the respective lenses match.

In this optical system, when the phase of the phase modulation surface of the liquid crystal spatial phase modulation element 2 is opposite in direction to the phase of the disturbing medium 5 and has a distribution of half its size, the two cancel each other out in the process of reflection. , The effects of disturbing media in this video system are eliminated. At this time, the image on the image plane 6 (which is directly looked into by looking at this portion or where a camera, a microscope, or the like is installed) which has been blurred due to the influence of the disturbing medium changes into a clear image. However, since the phase modulation characteristic of the liquid crystal spatial phase modulator 2 has polarization dependency, the polarization direction of incident light is such that the polarization direction of incident light is sufficiently modulated so that the phase of the reflected light wave is sufficiently modulated. Vertical direction)
Should be parallel to.

In order to make the phase distribution of the phase modulation surface 3 of the liquid crystal spatial phase modulator 2 a distribution that cancels the phase fluctuation of the disturbing medium 5, the adaptive optical system shown in FIG. 2 is used. First,
As shown in FIG. 1, the disturbing medium is illuminated through a beam splitter BS1 with a plane wave (corresponding to the “reference star” in the above-mentioned astronomical observation) created from a linearly polarized laser beam, and the lenses L2 and L3 are illuminated with it. An image is formed on the phase modulation surface 3 of the phase modulation element via the light. The reflected light from the phase modulation surface 3 is
It is guided to the lens L5 via the beam splitter BS3 (see FIG. 2). The distribution of the light wave on the phase modulation surface 3 is incident on the virtual plane F1 via the lenses L5 and L6, and further incident on the polarization beam splitter 7. This virtual plane F1 is the entrance of the Mach-Zehnder interferometer.

In this interferometer, the vertically polarized component of the light wave is reflected by the polarization beam splitter 7, and lenses L7 and L7
8, the half-wave plate 8, and the beam splitter BS4 to enter another virtual plane F2. On the other hand, the horizontal polarization component passes through the polarization beam splitter 7, and the lens L9
Then, a reference plane wave is pseudo-created from the disturbed wavefront whose size is enlarged via L10 and L10 and is imaged on the virtual plane F2. As a result, interference fringes that faithfully reflect the phase distribution of the disturbing medium existing in the video system are formed on F2.

Since it is necessary to align the polarization directions of the two light waves to be superimposed in order to form the interference fringes, a half-wave plate is introduced to rotate the polarization direction of one light wave by 90 degrees and The polarization directions are aligned. In addition, the polarization direction of the laser for driving this system is adjusted so that the intensities of two light waves to be superposed are equal to each other.

The interference fringes formed on the imaginary plane F2 surface as described above are imaged on the writing surface 4 (the back side of the phase modulation surface 3) of the optical writing type liquid crystal spatial phase modulation element 2 by the lenses L11 and L12. To be done. However, since the image is inverted as it is, the image rotating prism 9 is used to compensate for it.
Have been inserted. For the focal length of each lens,
The lenses L5 and L12, L6 and L11, and L7 and L8 are made equal to each other, and they are arranged so as to form an afocal system in which the focal points of the respective lenses are matched.

The lenses L9 and L10 are determined according to the magnifying power for creating the reference plane wave (for example, to obtain a magnifying power of 5 times, the lenses L9 and L10 are determined).
The ratio of the focal lengths is 1 to 5). When the above is realized,
Due to the polarization dependence of the optical writing type liquid crystal spatial phase modulator 2, the phase of only the light wave of the vertically polarized component of the incident light is modulated on the phase modulation surface of the element. A feedback interferometer whose principle was created by the group is realized. As a result, the phase distribution of the phase modulation surface 3 of the liquid crystal spatial phase modulation element 2 becomes a distribution that cancels the phase fluctuation of the disturbing medium 5, and operates as an adaptive optical system.

If this optical system is assembled correctly, the light used for the image (laser for object illumination) enters the writing surface of the liquid crystal spatial phase modulator through BS3 and the adaptive optical system shown in FIG. , Will interfere with its normal operation. This phenomenon is avoided by slightly tilting the incident direction of the light that illuminates the object. On the other hand, the light that drives the adaptive optics system (laser for driving the adaptive optics system)
Also enters the image plane of the image system via the beam splitter BS2, but since the light becomes a small spot on the image plane, it does not significantly affect the operation of the image system.

Since the above-mentioned embodiment shows an example in which it is used for various industrial measurement and medical equipment, the laser light having high brightness and good coherence is used as the reference light and the illumination light of the object to be measured. .

Optically driven adaptive optics systems have been used since
Although the proposal of its own concept and the report on the verification experiment of the operation have been made, no specific proposal has been made for the whole device incorporating it into a concrete video system. It is considered that this is because the light used for driving the adaptive optics system and the light used for the image system influence each other, and the entire apparatus does not operate normally. In that respect, in the device of the present invention, the light wave used for the image and the light wave used for driving the adaptive optical system do not affect each other. Therefore, when the function of the optically driven adaptive optical system is incorporated in the video system. It is something that can now be fully used.

The present invention based on the above-mentioned basic principle can be applied to various techniques. A typical example among them will be described. FIG. 3 shows, as an embodiment of the present invention, a system for monitoring a workpiece in a factory on the spot. Since it is difficult for many of the workpieces to transmit light for observation, the beam splitter BS0 is used here to irradiate the workpiece 10, which is the object to be measured, with a laser from the front. In general, when the workpiece 10 is observed in this state, the air between the object to be measured and the observation surface fluctuates due to the vibration of the operating machine and the generated heat, and the image on the observation surface deteriorates. The system of FIG. 3 is used to compensate for it. This system functions basically the same as that shown in FIG. However, in the arrangement of the devices in this system, the air fluctuation 11 to be corrected is arranged between the beam splitter BS1 and the lens L2. The system shown in FIG. 2 is used as the adaptive optics system.

As another embodiment of the present invention, FIG. 4 shows a system for monitoring an object in the shielded space 15. Since microfabricated products such as high-density integrated circuits are easily affected by minute dust, work in a clean room is often required. Therefore, in order to observe the processed object, it is necessary to take an image through the observation window. In addition, there is also a situation in which a precision instrument such as a camera cannot be brought inside such as a nuclear reactor, and observation of the internal state is performed through a window from the outside of a shielded space. At this time, the thickness of the window,
Disturbance of light waves at the interface caused by phase fluctuations due to non-uniformity of density and environmental differences between inside and outside (temperature difference, pressure difference, etc.) induces image disturbance on the observation surface. . In the arrangement of FIG. 4, the position of the observation window 16 in the shielded space 15 is set exactly at the beam splitter BS1 and the lens L.
By using the adaptive optics system described above, the phase disturbance generated in the vicinity of the window is corrected and the image disturbance is restored.

The basic structure of the optically driven adaptive optics system is shown in FIG. 2, but the structure can be modified and used. One of them is shown in FIG. In the figure, the interference fringes formed on the virtual plane F2 are photographed by the CCD camera 20 or the like. The image is formed on the writing surface 4 of the phase modulation element 2 in real time using a projection device such as the video projector 21. At this time, it is necessary to enlarge or reduce the interference fringes on the projection device side so that the light waves of the phase modulation surface 3 and the light waves of the writing surface 4 have a one-to-one correspondence. Basically, since the interference fringes formed on the virtual plane F2 are imaged on the writing surface 4 of the phase modulation element 3, this optical system operates exactly the same as the optical system of FIG.

[0031]

Since the present invention is configured as described above, by using a high resolution liquid crystal spatial phase modulator,
Compared to adaptive optics systems that use deformable mirrors,
The advantages are small size, low cost, low power consumption, and high resolution correction capability. Furthermore, since electronic calculation processing is not required to control the phase modulation element, there is an advantage that the real-time property of control is not lost even if the phase modulation element has a dramatically higher resolution in the future.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an optical device mainly showing a video system portion in a basic configuration of the present invention.

FIG. 2 is a configuration diagram of an optical device mainly showing an adaptive optics system portion in the basic configuration of the present invention.

FIG. 3 is an optical device configuration diagram of a part of a video system showing an example in which the present invention is used to monitor a work piece in a factory on the spot.

FIG. 4 is a configuration diagram of optical equipment of a portion of a video system, showing an example in which the present invention is used for observing an object to be measured in a shielded space.

FIG. 5 is a configuration diagram of an optical device showing another embodiment of the optically driven adaptive optics system.

FIG. 6 is a conceptual diagram of a conventional adaptive optics system using a deformable mirror.

[Explanation of symbols]

1 Object to be measured 2 Optical writing type liquid crystal spatial phase modulator 3 Phase modulation plane 4 Writing surface 5 Disturbing medium 6 Image plane 7 Polarizing beam splitter 8 Half-wave plate 9 Image rotation prism

Continuation of the front page (56) Reference T. Shirai et al, AD ACTIVE WAVE-FRONT CORRECTION BY MEANS OF ALL-OPTICAL F EEDBACK INTERFROM ETRY, Optics Letters, June 1, 2000, vol. 25, no. 11, pp. 773-775 T.I. Shirai et al, SU RFACE-PROFILE MEAS UREMENT BY MEANS OF AF A POLARIZATION S AGNAC INTERFROMETER WITH PARALLEL OPTICAL. ． , Optics Letters, March 1, 1999, vol. 24, no. 5, pp. 297-299 (58) Fields investigated (Int.Cl. ⁷ , DB name) G02B 27/46

Claims (5)

(57) [Claims] 1. A liquid crystal spatial phase modulation element, in which the modulation phase of a phase modulation surface on the back side of the writing surface changes depending on the intensity of the light irradiated onto the writing surface, and the object irradiation light irradiated onto the object to be measured is lensed. Through first
It is guided to the beam splitter and disturbed with the reference light from the outside.
The substance irradiating light and the reference light which have passed through the substance and are reflected by the phase modulation surface and reflected by the phase modulation surface are
From the 2 beam splitter through the lens,
Able to observe the object irradiation light while forming a minute image on the image plane
Then, an interference fringe that reflects the phase distribution of the disturbing medium is obtained from the reference light separated by the second beam splitter, and the writing surface is irradiated with the interference fringes so that the phase modulating surface is formed so as to cancel the phase distribution of the disturbing medium. And tilt the optical axis of the object irradiation light with respect to the reference light slightly
This allows the object irradiation light to enter the phase modulation surface.
An optical drive type wavefront correction image method characterized by preventing . 2. A liquid crystal spatial phase modulation element in which a modulation phase of a phase modulation surface on the back side of the writing surface changes depending on the intensity of light applied to the writing surface, and an object irradiation light that irradiates an object to be measured through a lens. First
It is guided to the beam splitter and disturbed with the reference light from the outside.
The substance passes through and is reflected by the phase modulation surface,
The object irradiation light reflected by the modulation surface and the reference light are used as a second beam.
From the splitter, through the lens, the reference beam to the image plane
Optical that makes it possible to observe the irradiation light of an object while forming a minute image
An interference fringe reflecting the phase distribution of the disturbing medium is obtained from the system and the reference light separated by the second beam splitter, and the writing surface is irradiated with the interference fringes to cancel the phase distribution of the disturbing medium. And an optical system for forming the optical system, wherein the optical axis of the object irradiation light is slightly inclined with respect to the reference light . 3. The light-driven wavefront correction image device according to claim 2, wherein the measured object is a surface of a workpiece. 4. The optically driven wavefront correction image apparatus according to claim 2, wherein the measured object is an object in a shielded space. 5. The light-driven wavefront correction image device according to claim 2, wherein the writing surface of the liquid crystal spatial phase modulation element is irradiated with an image obtained by imaging the interference fringes by a projection device.