JPH0549083B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention This invention relates to optical information processing, and more particularly to iterative filter devices for use in optical spatial filter devices.

[Conventional technology]

Techniques for achieving selective spatial frequency filtering using optical spatial filters and Fourier transform devices are known. Such devices usually have a parallel coherent light source, modulated in the light path from the source by a suitable converter, behind which a convex lens, an optical spatial filter (which a second convex lens, which may be a programmable spatial filter (PSF);
and detection means are arranged. In such devices, light from a light source passing through the converter is modulated, thus producing a diffraction pattern (Fourier transform) in the focal plane behind the first lens. A second lens provides an inverse Fourier transform of the diffraction pattern passing through the spatial filter so that it is a filtered representation of the spatial frequency of the modulated input optical signal. This optical signal exiting the second lens contains all spatial frequencies appearing in the Fourier plane, minus those selectively filtered out by the PSF. Therefore, according to the teachings of the prior art, in the Fourier plane
Changing the spatial frequency distribution of the PSF results in an inversely transformed image with corresponding resolution/higher frequency, contrast/lower spatial frequency and phase response.

It has previously been proposed to adapt or electronically program filters according to time-varying criteria. The concept of a filter matched to the spectrum of the desired signal provides theoretically optimal filtering. In real-world situations, especially when a wide spectrum signal containing higher level narrowband signals is detected, a narrowband (notch) filter within the passband of the wideband spread spectrum signal is used. Therefore, it is desirable to reduce or eliminate this high level signal.

There is a significant advantage in achieving this filtering optically, rather than using relatively conventional electronic filters, especially when a certain number of filters are required. The use of optical techniques for fast adaptive filtering and regeneration of broadband RF signals has been taught in the prior art. See, for example, the paper "Fast Adaptive Filtering and Regeneration of Broadband Signals Using Acousto-Optical Techniques" in the IEEE Ultrasonics Conference (1981) (John N. R. et al.) and the references listed below. .

T. M. Turpin, "Spectral Analysis Using Optical Processing," IEEE Bulletin Vol. 69, No. 1,
Published in January 1981.

D.T. Rose, "Convolution and Correlation in Acoustic-Optical Signal Processing," IEEE Bulletin Vol. 69, No. 1, January 1981.

A. Kopel, “Report on the Fundamentals of Acoustics-Optics,” IEEE Bulletin Vol. 69, No. 1, January 1981. All of these prior art teachings of optical spatial filtering appear to focus on single stage filter devices. For these conventional devices, especially those using PSF, the maximum filter attenuation is approximately 25 dB.
, which is the highest attenuation achievable with currently available single-stage PSF devices.

[Problem that the invention seeks to solve]

Thus, the prior art does not teach anything more than a single stage spatial filter device.

The present invention has been made in view of the above circumstances, and provides a repeating filter device in which each stage of the PSF is efficiently connected in cascade by the repeating means, so that attenuation can be increased as a function of the number of stages (number of repeats). It is something.

[Means for solving the invention]

The device according to the invention is an optical filtering device in which a beam of coherent, parallel-directed optical radiation is passed through an optical filtering device multiple times before being removed for use. The optical filter device includes a device for forming an optical Fourier transform of an incident beam, an optical filter for passing the transformed beam, a device for forming an inverse optical Fourier transform of the filtered beam, and a device for forming an optical Fourier transform of an incident beam. and a device for repeatedly passing the filtered beam back to the filter device. A suitable optical conversion forming device is a lens system, and the filter is a spatial filter, preferably a programmable type. The array of reflectors or refractors is configured to cause the beam emerging from the optical filter device to pass back through the device to obtain the desired number of filtering stages. If necessary, a light inversion device can be incorporated into the system to invert the beam.

Iterative filtering improves the attenuation of unwanted parts of the processed signal. An electronic filter stage where a single filter stage is provided for each resolution element m in a total of n×m electronic filters, where n is the number of filter stages per resolution element, e.g. The number is from 10 to 10, and m is the resolution (1≦m≦200). ], up to 2000 electronic filters can be obtained in a single optical filter device with a resolution of 200 using the present invention. In the present invention, the attenuation of unnecessary frequencies is multiplied by a coefficient determined by the number of repetition stages n. The results obtained from a single spatial filter and a pair of conversion lenses are similar to those obtained by cascading filter devices into n stages, provided that allowances are made for diffraction effects and other device equipment. It is effectively the same thing.

In a preferred embodiment, a repeating filter device is disclosed that is incorporated into a radio frequency signal optical filter device. In that embodiment, the radio frequency signal is provided to an acousto-optic modulator to modulate the laser beam. The modulated output beam passes through an optical Fourier transform lens to produce a spatial frequency distribution at the back focal plane of the lens. This signal contains a one-to-one spatial and temporal correspondence to the radio frequency distribution. The transformed beam is then directed through a spatial filter, which is also placed at the back focal plane of the transforming lens (if a PSF is used for filtering)
The transmission of light from one point to another may be controlled by the PSF to block the passage of certain spatial frequencies and pass other frequencies corresponding to the programmed notch frequency). The spatial frequency of the light passing through the spatial filter consists of the carrier frequency of the laser light modulated with radio frequency. The inverse Fourier transform lens of light images the filtered beam and outputs it.
This output is directed back through a repeating reflector array and an inverting prism, passing through a transformation lens, a spatial filter, and an inversion device multiple times. This repeatedly filtered beam then reaches a light mixing device. In the mixing device, the beam is mixed with a local oscillator reference beam. When the modulated laser beams are combined by a local oscillator beam and the sum is inputted to a photodetector having a square law detection function, different frequencies are observed due to the heterodyne effect. The electrical output of this photodetector is amplified and first filtered before being subjected to normal post-processing.

Accordingly, a first object of the present invention is to provide an optical device in which a signal beam is passed through a single optical spatial filter in a large number of repetitions so that the attenuation of the unwanted signal frequency is multiplied by that number of times. The purpose is to provide equipment for

It is a further object of the present invention to provide an optical arrangement that effectively cascades spatial filter stages in a single compact closed loop repeating stage.

Another object of the invention is to provide an optical device for a nozzle filter that is compatible with the radio frequency spectrum.

〔Example〕

Other objects and advantages will be described in more detail below with reference to the accompanying drawings, which illustrate embodiments of the invention.

FIG. 1 of the drawings shows a repeating optical filter device 10 that is incorporated into a one-dimensional optical notch filter device with heterodimatic action. Of course, although the repeating device of the invention is described in this embodiment and the subsequent embodiment for use with a filter device with heterodyne action, such use is limited to the above-described embodiment of the invention. It will be understood that this does not mean that The iterative filter device 10 includes an optical Fourier transform device 12, a spatial filter 14, and an optical inverse Fourier transform device 16.
The reflectors 20 to 2 cooperate with the inverting prisms 70 to 74 arranged in parallel on each side of the filter 14 so as to face the filter 14.
8 array 18.

Double convex lens sets 12 and 16 are shown that effect the conversion and inverse conversion of optical signals. However, other known transform generating means, such as holographic lenses, etc., may optionally be used as appropriate. Also, known devices such as programmable spatial filters (PSFs), code wave plates, optical transparencies can be used as spatial filters. If the spatial filter 14 is programmable, filter initiation electronics 15, such as that taught in the aforementioned IEEE Ultrasonics Conference Papers by John N.R. et al., may be utilized to program the spatial filter. Although a reflector array 18 consisting of retroreflective cubes 20-28 is shown, square reflectors could be used instead.

The heterodyne device disclosed in FIG. 1 includes a light source 30, such as a laser, that produces a radiation beam 32. The heterodyne device disclosed in FIG. The beam is directed and passed through a modulator to impart spectral and/or temporal signal information onto the radiation beam passing through the modulator. Preferably, light source 30 is collimated and provides a substantially coherent beam of radiation. An acousto-optic modulator 34, such as a known Bragg cell, is used to impart signal information to the radiation beam 32. A suitable input 36, such as a radio frequency in a passband of interest, is applied by a transducer portion 38 of a modulator 34 to the modulation medium of that modulator. The modulated output beam 40 from modulator 34 is
It is then introduced into the filter device 10. As shown, beam 40 is transformed by lens 12 and the transformed signal 42 is passed through spatial filter 14.
pass through. In this case, unwanted frequencies are filtered and spatially distributed, resulting in a filtered radio frequency spectrum modulated light output. The light output passes through lens 16 to produce an inverse Fourier transformed beam 46. The beam 46 is reflected in the opposite direction by the retroreflector 24, passes through an inverting prism 70, and then forms an inverted output beam 4.
8 is lens 16, spatial filter 14, lens 1
2, is reflected by the retroreflector 28, passes through the inverting prism 71, and returns to the Fourier optical device again. After being successively retroreflected by retroreflectors 22, 26 and 20 and thus passing through a filter optic (where n is the number of repetitions for n-1 retroreflectors), the output from the filter optic is Output beam 50 is directed by reflector 52 into an optical summer, such as a beam combining cube 54 . The attenuation of unwanted frequencies due to repeated filtering is multiplied by a factor determined by the optical resolution of the system. The result for a single spatial filter and a lens set using the repeater of the present invention is effectively much the same as cascading n stages of filter devices. In the embodiment of FIG. 1, the signal beam is passed through six stages of filtering in the iterative filter arrangement 10. In the embodiment of FIG. The filtered signal beam undergoes only a one-dimensional transformation, which is an iterative filter, and the optical system of the FIG. 1 embodiment of the invention directs the beam through a spatial filter (FIG. ), a one-dimensional spatial filter is used.
After the signal beam passes through the spatial filter, the spatial frequency of the light is reduced by the component removed by the filter. The beam spectrum therefore consists of an optical carrier at the laser frequency with modulation at the radio frequency. .

The output beam 50 from the repeating filter device 10 is combined in an optical coupling device 54 with a local oscillator reference beam 56 and its output 58 is introduced into a photodetector 60 with suitable square law detection. In this case, a frequency different from the radio frequency occurs. The electrical output 62 from the detector is filtered or otherwise conventionally processed for use.

Referring now to FIGS. 3 and 4, there is disclosed another embodiment ¹⁰¹ of a repeating filter of the present invention incorporated into an optical two-dimensional notch filter device having heterodyne action. The iterative filter device 10 ¹ is an optical Fourier transform device 11
2, a spatial filter 114, an optical inverse Fourier transform device 116, and inverse reflectors 120 to 128 arranged in parallel and opposite each other on one side of the Fourier.
18. In this embodiment, cylindrical lenses can be used for conversion devices 112 and 116. Element 1
20-128 together with a two-dimensional spatial filter (see FIG. 5) to provide the operative displacement, the cylindrical lens performs a one-dimensional transformation during the repeated filtering period. It should be noted that since the inversion is achieved by alternating adjacent elements 170 to 175 of the two-dimensional spatial filter, no inverting prism is used as in FIG. 1 of the device 1. .

In operation of the heterodyne device of FIGS. 3 and 4, the light source 130 is connected to the acousto-optic modulator
4, resulting in a collimated coherent radiation beam 132 that is directed through. A radio frequency input signal 136 drives a transducer portion 138 of the modulator to provide radio frequency modulation to an output beam 140 emerging from the modulator. The beam 140 is introduced into the iterative filter device 10 ¹ where it is transformed by the lens 112 and, after passing through the element array 170 of the spatial filter 114, passes directed through the lens 116, where it is subjected to an inverse Fourier transform. receive. Output beam 146 exiting lens 116 is reflected in the opposite direction by a retroreflector 120 from which an inverted output beam 148 is directed to transformation lens 1
16 again to another inverted element array 171 of the spatial filter 114 and the transformation lens 11
2 and is further reflected by a retroreflector 128 to pass back through the filter optics.

Reverse reflectors 122, 126 and 124 (n-
After successive retroreflections (n repetitions for one retroreflector) have thus passed through the filter optics, the filter output beam 150 from the optics is reflected by the reflector 152. and is directed into optical coupler 154. The output beam is then combined with a local oscillator reference beam 156. As is well known, the reference beam 156 may utilize a beam separated from the input beam 132 by a suitable beam splitter 158 but directed into the combiner 154 by a reflector 160. Ru. Output beam 162 from the combiner
is introduced into a square law photodetector 164 which produces an electrical output 166 by heterodyne. The electrical output is filtered or otherwise processed as would normally be done for use.

In the apparatus of the invention as implemented in FIGS. 3 and 4, the signal beam to be processed passes through six filter stages in a repeating filter device ¹⁰¹ .
However, in various embodiments of the invention, the number of filter passes made will vary depending on the number of retroreflectors used to obtain a signal beam pass through the filter, and may be varied from that shown. Recognize that you can do it. In this embodiment, a two-dimensional spatial filter is used to advantageously achieve a filter response also by alternating inversions by element sequences in the transmission, as occurs during repeated filtering.

The filtering of the present invention provides high resolution and very high notch attenuation when passed through a repeating filter with a small spatial filter (or a similar filter shape if the spatial filter is programmable). It can be said that high recording accuracy can be achieved. In the iterative filter device of the present invention, the transition time for each iteration stage n is expressed as: transition time=4f·(1/V _L )·n. However, f = lens focal length, V _L
= The speed of light.

If the iterative filter device of the present invention is used in conjunction with a programmable spatial filter, and the filters are reprogrammed for successive light paths, the reprogramming is done in a manner that does not affect the light path currently in use. If the program is desired,
The embodiment illustrated in FIGS. 3 and 4 is particularly advantageous for very high speed signal processing (on the order of 10 ^-9 seconds), such as that achieved with Kerr Cell type optical switches. This also corresponds to cases where a predetermined continuous filter action is desired, since a flexible transition from one filter configuration to another is obtained. This also corresponds to cases where it is desired to program advantageous filter modifications in different filter paths.

Having shown and described what is believed to be the most practical and preferred embodiment, it will be apparent to those skilled in the art that variations from the specific methods and configurations described will be apparent to those skilled in the art. and it is clear that this may be done without exceeding the scope.
Therefore, we do not intend to be limited to the specific arrangements described and illustrated, but wish to cover all modifications that come within the scope of the claims.

[Brief explanation of the drawing]

FIG. 1 is a schematic side view of a repeating optical filter of the present invention incorporated in a heterodyne optical notch filter device, and FIG. 2 is a schematic diagram of a one-dimensional spatial filter implemented in the device of FIG. , FIG. 3 is a schematic side view of another embodiment of the repeating optical filter of the present invention incorporated in an optical notch filter device that performs a hederodyne action, and FIG.
Schematic plan view of the repeating optical filter of FIG.
The figure is a schematic diagram of a two-dimensional spatial filter implemented with the apparatus of FIGS. ³ and 4.
12... Fourier transform generator, 14, 114...
...optical filter, 16,116...optical Fourier inverse transform generator, 30,130...coherent collimated radiation light source, 40,14
0...Modulated beam, 62,166...Electrical signal.

Claims (1)

Claims: 1. A substantially coherent collimated radiation light source 30, 130, an optical Fourier transform generator 12, 112, an optical filter 14, 11
4, as well as an optical Fourier inverse transform generator 16,
an optical filtering device 10 consisting of 116;
10 ¹ , repeating means 20 - 28 , 120 - 128 for directing the radiation beam from said radiation light source 30 , 130 and causing it to pass through said optical filtering device 10 , 10 ¹ a plurality of times; means 52, 152 for taking out for use
and wherein the radiation beam is repeatedly passed through a filtering device to sufficiently attenuate unwanted spectral components in the beam, and the repeating means for directing the radiation beam to an optical filtering device comprises a first set of Multiple reflectors 20-24, 120-
A plurality of reflectors 26-28, 126-128 consisting of a second set parallel to and facing 124;
These first and second sets of reflectors sandwich the optical filtering devices 10, ¹⁰¹ in between, and the incident beams 40, 140 transmitted through the optical filtering devices are directed to the first set. a second set of reflectors, said incident beam passing through said optical filtering device a plurality of times; and each reflector of each set is arranged such that each successively reflected reflected beam is offset from the previous beam, and each reflected beam is parallel. type optical filter device. 2 the radiation beam 40 passes through the optical filter 14 in the optical filtering device 10;
said first and second sets of reflectors 2
2. The repeating type optical filter device according to claim 1, wherein the repeating type optical filter device is made to pass through the linear region of the optical filter 14 continuously from both directions. 3. The repeating optical filter device according to claim 1 or 2, wherein the optical Fourier transform generator and the optical inverse Fourier transform generators 12, 16 are lens systems. . 4. A repeating optical filter device according to any one of claims 1 to 3, characterized in that the reflectors 20-28, 120-128 reflect-direct the incident beam by 180°. 5. A repeating optical filter device according to any one of claims 1 to 4, characterized in that the reflector is a reverse reflector cube. 6. The reflector is a pyramidal prism having an elongated rectangular shape with an apex and is parallel to each other, and the bottom surface of the prism is connected to the optical filter device 14,1.
14, the bottom surface is transparent to the radiation beam, and the inner surfaces on both sides of the prism reflect the beam, and both longitudinal edges of the prism bottom surface are transparent to the radiation beam. The beam incident near one of the prisms is reflected by the inner surfaces on both sides of the prism, and is emitted from near the other of the longitudinally opposite edges of the bottom surface of the prism. A repeating optical filter device according to any one of the above. 7 The surface of the pyramidal prism is relative to its bottom surface.
7. A repeating optical filter device according to claim 6, characterized in that it forms an angle of 45°. 8. The optical Fourier transform generating device 112 is a cylindrical convex-plano lens, the optical Fourier inverse transform generating device 116 is a cylindrical plano-convex lens, and each plane side of these lenses is the optical filter 11.
A repeating optical filter device according to claim 1 or 2, characterized in that it faces 4. 9. The repeating optical filter device according to claim 1, wherein the optical filter 114 of the optical filtering device 10 ¹ is a two-dimensional filter. 10. The repeating optical filter device according to claim 2, characterized in that the optical filter 14 of the optical filtering device 10 is a one-dimensional filter. 11. The repeating optical filter device according to claim 2, wherein the optical Fourier transform generator and the optical inverse Fourier transform generators 12 and 16 are biconvex cylindrical lenses. . 12. The radiation beam 32 is modulated in the radio frequency spectrum, and the optical filter 14 is a programmable spatial filter that filters out unwanted frequency components. A repeating optical filter device according to any one of the above. 13. A repeating optical filter device according to any one of claims 1 to 12, characterized in that spectrum information is added to the radiation beam from the radiation light source 30. 14 substantially coherent collimated radiation light source 30, 130, optical Fourier transform generator 12, 112, optical filter 14, 1
14, and optical Fourier inverse transform generator 1
Optical filtering device 1 consisting of 6,116
0,10 ¹ and repeating means 20-28, 120-128 for directing the radiation beam from said radiation light source 30, 130 to pass through said optical filtering device 10, 10 ¹ a plurality of times; means 52,1 for removing said beam for use;
52, a radio frequency signal generator, and an acousto-optic modulator 34 driven by the radio frequency signal 36 from the radio frequency signal generator, the radiation beam being repeatedly passed through a filtering device to filter the radiation. Unwanted spectral components in the beam are sufficiently attenuated, and the repeating means for directing the radiation beam onto an optical filter arrangement is parallel to and opposite a first set of reflectors 20-24, 120-124. It consists of a plurality of reflectors 26-28, 126-128 constituting a second set, and the reflectors of the first and second sets sandwich the optical filtering devices 10, ¹⁰¹ in between, and the optical Incident beam 4 transmitted through the filtering device
0,140 is reflected by one reflector of said first set, passed through an optical filtering device, and reflected by one reflector of a second set, said radio frequency signal generating A radio frequency signal from the device drives the acousto-optic modulator 34 to impart its radio frequency spectrum to an optical radiation beam 32 directed from the radiation source 30 to the acousto-optic modulator; A modulated beam emerging from optical filtering device 10 carries a spatially filtered radio frequency modulation, said incident beam passing through said optical filtering device multiple times, each successive reflector. 1. A repeating optical filter device characterized in that each reflector of each set is arranged so that each reflected beam is reflected off-set from the previous beam and each reflected beam is parallel. 15. The optical filter further includes an optical local oscillator, a light beam combiner 54, a detection device 60 for converting an optical signal into an electrical signal, and an electronic bandpass filter, and the optical filter An output beam 50 from the effector 10 is combined by the beam combiner 54 with a beam 56 from the local oscillator to produce a summed output whose electrical output is in the band. 15. A repeating optical filter arrangement as claimed in claim 14, characterized in that it is incident on said detector 60 producing a down-converted radio frequency spectrum which is filtered by a pass filter.