JP2850263B2 - Optical image processing device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a logical operation on a one-dimensional or two-dimensional image, in particular, a photo-induced refractive index effect (Photorefractive effect, PR
The present invention relates to an optical image processing apparatus that performs logical operations such as NAND, NOT, and NOR, threshold processing, and the like on a pure optical basis using a PR crystal having the above-described effect.

[Prior Art] Conventionally, optical bistability (Optical Bistability) has been used for the purpose of performing a logical operation on a two-dimensional image at a higher speed than processing by an electronic computer.
Optical bistable device (Bistable O using Bistability)
ptical devices) in a two-dimensional array.

In such an optical bistable element, a configuration including a pair of parallel-plate-shaped Fabry-Perot resonators and a medium having a nonlinear refractive index effect therein provides a nonlinear optical output response characteristic with respect to optical input. I have.

Therefore, in order to perform a logical operation between images using the optical bistable element, an exciton resonance absorption line of a semiconductor having a multiple quantum well structure (for example, GaAs (well) / AlGaAs (barrier)) having a large third-order nonlinear optical effect is used. It is a necessary condition to combine such a medium with a Fabry-Perot resonator having a high finesse (finesse) by using the effect of refractive index change due to heat such as ZnS.

[Problems to be Solved by the Invention] However, the above configuration has the following problems.

(1) When the resonance wavelength is sufficiently close to the used wavelength, the light intensity in the resonator rapidly increases, jumps to the resonance state, and the transmitted light rapidly increases. Therefore, the temporally high coherence of the input light is increased. And the variation of the wavelength used must be minimized.

(2) An element having two-dimensionally uniform characteristics is required, but its production is difficult.

(3) It is easily affected by disturbance (particularly temperature).

(4) Since a change in the refractive index of the medium in the resonator due to an increase in the intensity of the input light is used, it is necessary to sufficiently increase the intensity of the input light.

In view of the above problems, it is an object of the present invention to minimize the requirement for the temporally high coherence of input light and the limitation of the fluctuation range of the used wavelength, to be less susceptible to disturbance, and to reduce the An object of the present invention is to provide an optical image operation device capable of performing a logical operation of an input image satisfactorily even in intensity.

[Means for Solving the Problems] In the present invention for achieving the above object, a resonator having a photo-induced refractive index result (PR crystal) and a phase conjugate mirror provided opposite thereto is provided, By changing the optical characteristics of the PR crystal according to the light intensity of the image incident on the PR crystal, the threshold value of the resonator is changed to perform the logical operation of the image.

In the present invention, the PR between two light beams incident opposite to the PR crystal
The non-reciprocal reflection and transmission characteristics of the PR crystal due to energy coupling in the crystal were used, and more specifically, the incident light from the outside into the PR crystal and transmitted through the PR crystal and reflected from the phase conjugate mirror The resonator is oscillated by using non-reciprocal reflection / transmission characteristics by two-wave coupling with the light beam in the PR crystal. At this time, by controlling the intensity of the two-wave coupling in the PR crystal by a light beam having image information which is independently incident on the PR crystal, the oscillation state is controlled and the logical operation of the input image is performed.

By using the PR effect, a non-linear optical effect such as the two-wave coupling effect can be efficiently obtained nonresonantly at a low power of several mW (for the PR effect, for example,
GCValley and MBKlein, Opt. Eng. 22, 704 (1983)
Is described in).

Embodiment FIG. 1 is a schematic view showing one embodiment of the present invention. In the same figure, BaTiO ₃ , SBN, BSO, GaAs having one-wave PR effect
PR crystals, such as photo-induced refractive index crystals (Photorefractiv
e crystal), 2 is a phase conjugate mirror, which is designed so that the reflectivity of the phase conjugate mirror 2 is 100% or more by supplying pump light 3, 3 'from outside to realize four-wave mixing. I have. The phase conjugate mirror 2 is made of a non-linear optical material, but the material is not limited to a PR crystal and may be another non-linear optical material (for example, an organic material). Further, 4 is a probe light beam incident on the PR crystal 1, 5, 8, 11, and 13 are light splitting means such as a beam splitter and a half mirror.
One-dimensional or two-dimensional input image of PR crystal 1 irradiated with sensitive incoherent light, 9 is PR of input images 6 and 7
A lens 10 for forming an image on the crystal 1, and a light beam 10 is a phase conjugate mirror 2.
Reference numerals 12 and 14 denote output surfaces, and 15 denotes a deflecting plate. At this time, it is assumed that the incident light beam 4 has such a wavelength that the PR crystal 1 induces a PR effect and the phase conjugate mirror 2 generates a phase conjugate wave. The crystal orientation of the PR crystal 1 is arranged such that an energy coupling effect between two non-reciprocal light beams due to two-wave coupling in the PR crystal can be obtained.

Here, the optical action of the two-wave coupling will be described with reference to FIG.

In the figure, 21 and 22 are mutually coherent input lights, 23 and 24 are their reflected lights, 25 is a pattern of interference fringes,
26 is a refractive index grating pattern formed by the PR effect. Now, when the light beams 21 and 22 having the same light intensity enter the PR crystal 1, the interference fringes 25 formed in the PR crystal 1 by the incident light beams 21 and 22 cause the PR effect to cause a refractive index grating 26 (a change in the refractive index). Pattern). At this time, when the crystal axis direction depending on the PR crystal 1 and the deflection azimuths of the input light beams 21 and 22 are appropriately arranged, the incident light beams 21 and 22 are subjected to Bragg reflection by the refractive index grating 26. At this time, interference fringe 25 and P
The spatial position (or phase) with the refractive index grating 26 due to the R effect generally does not match. For this purpose, for example, the Bragg reflected light of the incident light beam 21 (or 22) and the incident light beam 22 (or
Since the transmitted light of 1) overlaps with the same phase (or opposite phase), the energy exchange between the two light fluxes 21 is performed. As a result, the reflected light flux 23 is amplified to become amplified light, and the reflected light flux 24 is attenuated to become attenuated light.

The direction of the amplified light in this case depends on the crystal axis orientation of the PR crystal 1. For example, when BaTiO ₃ is used, the incident light flux 2
Assuming that the c-axis of the PR crystal 1 is arranged parallel to the directions 1 and 22 (the left-right direction in FIG. 2), the light beam to be amplified is a light beam reflected in the + c-axis direction, and a light beam reflected in the −c-axis direction is It becomes an attenuated light.

The above phenomenon is two-wave coupling, and this phenomenon is described in detail in the papers Optics Communications 50 146 (1984) and Optics Letters 10 187 (1985) together with experimental results.

Returning to FIG. 1, the operation of the present embodiment will be described. The linearly polarized incident light beam 4 is incident on the PR crystal 1 via the light splitting means 13 and 15 and transmitted therethrough. After that, the phase conjugate mirror passes through the polarizing plate 15 and the lens 10 having the same orientation as the polarization direction of the incident light beam 4. 2
Incident on. At this time, a time-reversed wave (phase conjugate wave) of the incident light beam 4 is generated by four-wave mixing in the phase conjugate mirror 2. This phase conjugate wave is incident on the PR crystal 1 again. However, if the crystal orientation of the PR crystal 1 is arranged such that the phase conjugate wave is strongly reflected by the two-wave coupling (at this time, the first The incident light beam 4 incident from the right side of the figure corresponds to one of the mutually coherent input lights 21 and 22 in FIG. 2, and this phase conjugate wave corresponds to the other of the input lights 21 and 22). The phase conjugate light strongly reflected from 1 returns to the phase conjugate mirror 2 again.

This process is repeated, and in the steady state, the PR crystal 1
Oscillation occurs due to the resonator formed between the phase conjugate mirror 2 and the phase conjugate mirror 2. In this case, there are an amplitude condition and a phase condition as conditions necessary for oscillation. The amplitude condition is such that the phase conjugate reflectance of the phase conjugate mirror 2 becomes larger than 1 (100%). It is necessary that the gain exceeds the loss in the surrounding optical path.

On the other hand, as a phase condition, the phase in one optical path of the resonator must be an integral multiple of 2π, but in the present invention, since the phase conjugate mirror 2 is used as one mirror of the resonator, the resonator length Regardless, the phase condition is automatically satisfied. Further, in the present invention, since the phase conjugate mirror 2 is used as one of the mirrors of the resonator, an arbitrary spatial pattern can oscillate as a resonator transverse mode.

As for the amplitude condition in the above-described resonator, the loss around the resonator is L, the interaction length of the two-wave coupling in the PR crystal 1 is z, the gain coefficient due to the two-wave coupling in the PR crystal 1 is Γ, Assuming that the phase conjugate reflectance at the phase conjugate mirror 2 is R, the following relationship is established.

Γz ≧ −1 nR (1−L) (1) In the equation (1), when the PR crystal 1 is irradiated with an incoherent light beam at the same time, the gain coefficient Γ is expressed by the following equation.

Γ = Γ _o / (1 + η I _in / I _o ) (2) where I _in is the intensity of the incoherent light flux independently irradiated, and I _o is the laser light involved in two-wave coupling in the PR crystal 1. the intensity of the (incident light beam 4 of FIG. 1), eta is the sensitivity to PR effect of the light flux of the light beam and the intensity I _o of the intensity I _in a ratio, gamma _o is I _in
= 0.

Therefore, the expression (2) means that the oscillation threshold given by the expression (1) can be controlled by the incoherent light flux. The oscillation condition at this time is given by the following equation from equations (1) and (2).

I _in ≦ −I _o / η · {1 + z} _o / 1nR (1-L)} (3) Therefore, an incoherent light beam having a large light intensity that does not satisfy the expression (3) is incident on the PR crystal 1. In this case, the oscillation of the resonator stops.

Utilizing the above, the input image 6 as shown in FIG.
Is irradiated with an incoherent white light of polarized light orthogonal to the incident light beam 4 (therefore, this light is transmitted through the light splitting means 5 but not transmitted through the polarizing plate 15), and is irradiated onto the PR crystal 1 by the lens 9. When the image is formed, the gain coefficient ２ of the two-wave combination of the portion corresponding to the bright portion of the input image 6 decreases according to the equation (2). In that case, in the portion of the PR crystal 1 where the irradiated incoherent light intensity does not satisfy the condition of the expression (3), two-wave coupling in the PR crystal 1 is suppressed and oscillation in the resonator is stopped. The light intensity in the resonator decreases. On this occasion,
When the reflected light beam from the light splitting means 11 is observed in the output image 12, an inverted (negative) image binarized by a certain threshold with respect to the input image 6 is obtained on the output surface 12.

Further, in the portion where the light intensity in the resonator decreases, the light flux passes through the PR crystal 1 from left to right in FIG.
When an image of the surface of the PR crystal 1 is formed by an appropriate optical system (not shown) via the light splitting means 5 and the light splitting means 13, an inverted image of the output surface 12 is obtained on the other output surface 14.

At this time, in order to remove the reflected light from the input image 6, the light splitting means 5 transmits the polarized light from the input image 6 but reflects the polarized light of the incident light beam 4 as described above. It is preferable to use a certain one.

3A and 3B show the (analog) input image 6 described above.
It is explanatory drawing which shows the threshold value calculation of. FIG. 2A shows a specific example of the input image 6 showing an image divided into two by a high-contrast light and dark portion, and FIG. 2B shows the result of the input image 6 on the output surface 12. In the figure, 29 is the input image 6
, A dark portion 30 of the input image 6, a bright portion 31 due to a high optical electric field accumulated in the resonator, a dark portion 32 due to a low optical electric field equal to or lower than the oscillation threshold, and 33 a formula (3) This is the boundary by the threshold.

This calculation result means that a logical operation of NOT having been subjected to a certain threshold value processing can be performed on the input image 6.

Similarly, on the other output surface 14, an inverted result of the result on the output surface 12 is obtained. This means that a certain threshold-processed image of the input image 6 is converted from an incoherent light beam to a coherent laser light beam of the incident light beam 4 and output.

The above result is of course substantially the same even when the input image 6 is a binary digital image of light and dark.

The result of the above threshold processing of the analog image is (2)
This is caused by the nonlinearity of the white light intensity _Iin dependence of the gain coefficient Γ as shown in the equation and the nonlinearity due to the oscillation condition of equation (3). Thus, the output surfaces 12, 14
Then, an image inverted or similar to the input image 6 is output in a state where the contrast is enhanced or reduced (depending on the value of η, etc.).

In the above case, the degree of the threshold value may be controlled by controlling the bias point of the I _in dependency of the gain coefficient の in the equation (2). For this purpose, an independent incoherent light is further PR from the outside.
It is sufficient to uniformly irradiate one surface of the crystal, and the bias point can be raised and lowered by controlling the intensity of this uniform irradiation.

Next, with reference to FIGS. 4 (A), (B), (C) and (D), a binarization operation using inputs from the two input images 6 and 7 of FIG. 1 will be described. 3A shows an example of the input image 6 in FIG. 3, and FIG. 3B shows one example of the input image 7.
FIGS. 7C and 7D show output images on the output surface for these two input images 6 and 7.

As in the case of FIG. 3, the light intensity of any of the bright parts of the two input images 6 and 7 shown in FIGS. 4 (A) and 4 (B)
_Assuming that I _in is set to exceed the threshold value of the equation (3), an output image shown in FIG. 2C, that is, an image corresponding to the result of the NOR operation appears on the output surface 12. on the other hand,
When only (3) is set so as exceeding the formula threshold are relative to the sum I _in the light intensity of the bright portion of the input image 6,7, the output image shown in the Graph 1 (D), causing the words NAND An image corresponding to the calculation result of appears on the output surface 12.

When the above operation is performed on the other output surface 14, the result is NOT in FIGS. 4 (C) and 4 (D), that is, the OR operation result in FIG. 4 (C), and FIG. For, an AND operation result is obtained.

In the above calculation, as described with reference to FIG. 3, the bias point of the threshold light intensity is controlled by irradiating the PR crystal 1 with another uniform incoherent light beam from the outside. Contrast can be controlled.

In the above description, the case where the input images 6 and 7 are irradiated with incoherent white light has been described. However, an LED or a similar laser beam having a polarization state orthogonal to the polarization of the incident light beam 4 may be used. Further, a laser beam having a wavelength different from that of the incident light beam 4 in a wavelength range sensitive to the PR crystal 1 may be used. In this case, the light splitting means 5 has the above-mentioned polarization characteristic (in the case of the former laser light having the same orthogonal polarization state) or has the wavelength selection characteristic (the latter in the case of the laser light having a different wavelength). ), The reflectance of the light splitting means 5 is maximized with respect to the polarization state or wavelength of the incident light beam 4 and the S / N of the calculation result on the output surfaces 12 and 14 is obtained.
May be increased.

In addition, it is desirable to laminate an anti-reflection film on two opposing incident surfaces of the PR crystal 1 in order to minimize Fresnel reflection of the incident light beam 4.

[Effects of the Invention] As described above, according to the present invention, a resonator having a PR crystal and a phase conjugate mirror is provided, and an analog or digital input image is irradiated on the PR crystal surface to set the oscillator threshold to PR. By spatially modulating over the crystal plane, NOT operation of the input image, or logical operation such as NOR, NAND, OR, AND between two input images, and threshold processing are performed purely optically. I can do it.

Further, by appropriately irradiating the PR crystal with control light from the outside and controlling the bias point of the threshold value of the oscillator, an output image or the like appropriately binarized in the logical operation can be obtained.

Furthermore, according to the pure optical image processing method of the present invention,
Compared to the conventional Fabry-Perot type resonator, it is less susceptible to coherence and wavelength fluctuation of the laser light used, has heat resistance and stability, and it is necessary to increase the intensity of the input light to the left. An optical image calculation device having a simple configuration can be realized.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing one embodiment of the present invention, FIG. 2 is a diagram for explaining two-wave coupling, FIGS. 3 (A) and 3 (B) are explanatory diagrams showing calculation of an input image. 4 (A), (B),
(C), (D) is explanatory drawing which shows the calculation at the time of two image input. 1 ... PR crystal, 2 ... Phase conjugate mirror, 3,3 '... External pump light, 4 ... Incident light beam, 5, 8, 11, 13 ... Light splitting means, 9, 10, ... Lens, 6 , 7 ... input image, 12, 14 ...
… Output surface, 15 …… Polarized plate

Claims (9)

(57) [Claims] 1. A phase conjugate mirror and a photo-induced refractive index crystal (PR crystal)
Is provided, and the threshold value of the resonator is changed by changing the optical characteristics of the PR crystal in accordance with the light intensity of the image incident on the PR crystal to perform a logical operation of the image. An optical image calculation device characterized by performing. 2. An optical image processing apparatus according to claim 1, wherein non-reciprocal transmission / reflection characteristics of said PR crystal due to energy coupling in said PR crystal between two light beams incident opposite to said PR crystal are used. . 3. The two luminous fluxes are incident light from the outside into the PR crystal and a luminous flux transmitted through the PR crystal and reflected from the phase conjugate mirror, and are formed by two-wave coupling in the PR crystal. 3. The optical image processing device according to claim 2, wherein the resonator is oscillated using the non-reciprocal transmission / reflection characteristics of the PR. 4. The optical image processing apparatus according to claim 1, wherein a light beam having a plurality of pieces of image information is incident on said PR crystal. 5. The optical image processing apparatus according to claim 2, wherein a light beam that spatially uniformly controls the intensity of energy coupling in the PR crystal is incident on the PR crystal. 6. The optical image processing apparatus according to claim 2, wherein a dielectric film for adjusting the reflectance is laminated on a crystal surface of the PR crystal on which a light beam is incident. 7. The optical image processing device according to claim 3, wherein an output image surface is provided on at least one of a reflection side and a transmission side of the incident light on the PR crystal that causes energy coupling. 8. The optical image processing device according to claim 1, wherein a lens for condensing light on the phase conjugate mirror is provided between the PR crystal and the phase conjugate mirror. 9. An optical image processing apparatus according to claim 1, wherein said phase conjugate mirror is made of a non-linear optical material, and said phase conjugate mirror is supplied with pump light from outside to realize four-wave mixing. apparatus.