CN101217044B - Phase amplitude conversion method and device adaptable for volume hologram memory - Google Patents
Phase amplitude conversion method and device adaptable for volume hologram memory Download PDFInfo
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Abstract
The invention relates to a method and an apparatus suitable for the conversion of phase amplitude of a volume holographic memory. The invention is easy to operate, capable of rectifying phase shifting errors and reference light inclination errors and based on the phase-shifting interferometer. The technique proposal adopted comprises the following steps of: uploading a data-page to a spatial light modulator; conducting holographic recording, interfering on a spectral plane via a Fourier lens and reference wave by object wave, and placing the interference fringe on the photo polymer for recording, thereby forming a volume holographic grating; conducting holographic reading; converting phase-amplitude, separating a coherent plane wave from a light source which arrives at the image plane directly and interferes with the reconstructed object light wave on the CCD surface without passing the optical paths of the holographic recording and the holographic recording and collecting the interference pattern by a CCD camera, and then uniformly changing the phase of the plane wave through a phase shift device, and interfering the plane wave with the object light wave again to form a second interference pattern. The original phase information can be obtained from the two interference patterns through phase shift recovery algorithm. The invention also provides the apparatus applicable for the volume holographic memory to realize the conversion of phase- amplitude.
Description
Technical Field
The invention relates to a phase amplitude conversion method and a phase amplitude conversion device suitable for a volume holographic memory, which can be used for recovering phase modulation data in the volume holographic memory and belong to the field of optical information processing.
Background
Depending on the type of Spatial Light Modulator (SLM) used for volume holographic memory, the data input to volume holographic memory generally takes two forms: amplitude modulation and phase modulation. In the amplitude modulation, the amplitude of the object beam is modulated by a transmissive or reflective spatial light modulator, and input information is loaded on the amplitude of the object beam. In the phase modulation, the phase of the object light wave is modulated by a transmissive or reflective spatial light modulator, and input information is loaded to the phase of the object light wave.
Conventional volume holographic memories use amplitude modulation, such as the volume holographic storage systems DEMONI and DEMON II developed by IBM, the HDSS disk storage system developed in 2000 by stanford university, usa. Since the volume holographic memory requires a very high storage capacity, a 4-f fourier transform optical structure is generally adopted, and the recording is performed on a fourier spectrum plane, as shown in fig. 1. The object light waves with different intensities are subjected to amplitude modulation, and the intensity of the light waves of the reference light is difficult to adjust to achieve the optimal interference effect. More seriously, the object light wave with amplitude modulation can generate extremely strong central light spot on the Fourier spectrum plane of the volume holographic memory, and the simulation result and the optical experiment result are respectively shown in the attached figures 2 and 3. In fig. 2, the left graph is the fourier transform of the random amplitude modulated object light wave, and the right graph is the fourier transform of the random phase modulated object light wave, so that it can be seen that the center of the left graph has an extremely strong peak value, while the right graph is relatively stable. In the attached figure 3, the left graph is the optical fourier transform of the random amplitude modulated object light wave, and the right graph is the fourier transform of the random phase modulated optical object light wave, so that it is obvious that the central light spot of the amplitude modulation can be eliminated through the phase modulation. This phenomenon is an inherent property of fourier transform optical systems, where the central spot represents the zero and low frequency portions of the spatial frequency of the input light wave, which spot does not contribute to holographic recording and reproduction and consumes a lot of the dynamic range of the storage material. To overcome this drawback, researchers have proposed many strategies, such as placing a random phase mask on the surface of the spatial light modulator, or placing an optical element in the fourier plane to block the central spot. However, these methods not only require additional equipment, but also are not effective. In contrast, phase modulation has significant advantages. The phase modulated object light wave is uniform in intensity and interferes with the reference wave to achieve a better interference pattern. Moreover, after fourier transformation, there is no very strong central spot on the spectral plane, as shown in fig. 3. Therefore, the dynamic range of the storage material can be effectively utilized, and the storage density of the volume holographic memory can be improved.
However, the phase modulation input method brings inconvenience to the holographic data reading process. The CCD or CMOS detector can only detect intensity and cannot directly detect phase. For this purpose, a phase-amplitude conversion is necessary for detection. The existing phase-amplitude conversion methods mainly include: real-time interferometry; secondary exposure interferometry; coaxial interferometry. The real-time interference method is that the object light wave and the reference light are irradiated simultaneously in the reading process, the reference light irradiates the recording material to reproduce the recorded object light wave, and the object light wave passes through the blank spatial light modulator without loading any information. The object light wave passes through the recording material and interferes with the reproduced object light wave to achieve the conversion from phase to amplitude. In the method, two light waves pass through different light paths, so that a precise optical instrument and higher resetting precision are required. The second exposure method is to record the phase modulation information and a blank page at the same position of the recording material. The two object light waves can be reproduced by one reference wave, so that the phase-amplitude conversion is carried out. The disadvantage of this method is obvious, since an extra blank page is stored for phase-amplitude conversion, the dynamic range of the material is consumed by half, and the storage density of the volume holographic memory is correspondingly reduced. The coaxial interference method ingeniously utilizes the central light spot on the spectrum plane as reference light to interfere with the reproduced object light wave, and achieves the purpose of phase-amplitude conversion. However, this method requires a stronger central spot as the reference beam, which is contrary to the requirement of eliminating the central spot on the spectral plane to increase the density of the volume holographic memory. In addition, these methods have a common problem: the beams used for phase-amplitude conversion all pass through the recording material, with a certain consumption of the recording material. These phase-amplitude conversion methods are not strictly suitable for volume holographic memories.
Disclosure of Invention
The invention aims to provide a phase amplitude conversion method and a phase amplitude conversion device for a volume holographic memory based on a phase shift interference method. Aiming at the structure that the reference light passes through the recording material in the prior method, the invention introduces a third light wave which does not pass through the recording material as a reference wave, and introduces a phase shift interference method to avoid precise optical path adjustment, extracts original phase information from a plurality of recorded interference patterns, and can correct the phase shift error and the tilt error of the reference wave. Although the method needs phase-amplitude conversion in the data reading process, the phase-amplitude conversion is only carried out in the holographic reproduction process, the recording process is not influenced, and the storage density of the system is improved.
The technical scheme of the invention is as follows: the phase amplitude conversion method suitable for the volume holographic memory is characterized by comprising the following steps of:
in a first step, a page of data is uploaded to a spatial light modulator. Compared with amplitude adjustment, phase modulation has the advantages of eliminating central light spots, being uniform in spectrum plane, facilitating improvement of density of a volume holographic memory and the like, so that the phase type spatial light modulator is used for carrying out phase modulation on object light waves.
And secondly, carrying out holographic recording. The object light wave is interfered with the reference wave on the spectrum plane through the front Fourier transform lens, and the interference fringe is placed on the photopolymer material on the 4-f system spectrum plane for recording to form the holographic grating.
And thirdly, reading the hologram. In the input reading process, the shutter is used for shielding object light waves, only reference waves are used for irradiating recording materials to reproduce phase-modulated object light waves, and the object light waves reach the image surface of a 4-f system through the post-Fourier transform lens. A CCD camera is placed on the image plane for data reading. Since the data input process employs phase modulation, the directly acquired image will discard phase information with uniform intensity.
And fourthly, converting the phase to amplitude. A beam of coherent plane wave is separated from a light source, does not pass through a light path of a holographic recording and holographic reading part, and directly reaches an image plane to interfere with a reproduced object light wave on the surface of the CCD. The interference pattern is collected by a CCD camera to obtain a first interference pattern. Then the phase of the beam of plane wave is uniformly changed through a phase shifting device, and the beam of plane wave interferes with the object light wave again to obtain a second interference pattern.
By a phase shift recovery algorithm, original phase information of the object light wave is calculated from the two interferograms. The phase shift recovery algorithm is simple and easy to implement, and has good correction capability for phase shift errors and reference light tilt errors. Is very suitable for volume holographic memory.
Specifically, the following are adopted:
let two interference patterns collected be I1I2Then both are expressed as:
wherein AArThe amplitudes of the object wave and the reference wave are shown, a (x, y) is the phase of the object wave, b is the phase of the reference wave, and c is the phase shift amount. The method can be obtained from the two formulas,
wherein,
for the case where the phase shift amount c in the above formula is pi, special treatment is required:
a (x, y) is the result of the phase amplitude conversion, i.e. the stored data page.
The actual volume holographic memory also comprises a preprocessing step and a post-processing step in the using process. A pretreatment step: the original data to be stored is encoded to form a two-dimensional data page, with the pixel groupings or distributions on the data page appearing in a series of fixed patterns. As with amplitude modulation, the original data is encoded in order to ensure less cross-talk in holographic storage and to obtain a reconstructed hologram with a high signal-to-noise ratio. The principle of encoding is to impose certain restrictions on the pixel combinations or distributions on the page, i.e. to allow only certain fixed patterns to appear, and to prohibit other patterns from appearing. This mode is disabled, for example, if 1 pixel is present again in the four neighborhood of a 1 pixel, it is easy to create a disturbance; also, more than one 1 pixel is forbidden to appear in the eight neighborhood of 1 pixel.
Post-treatment: and (4) binarization processing, namely converting the gray-scale image obtained by the phase shift recovery algorithm into a binary image.
The device for realizing the phase amplitude conversion method suitable for the volume holographic memory is characterized by comprising a laser, a beam filtering and collimating device, a polarization beam splitter prism, a reflector, a front Fourier transform lens and a rear Fourier transform lens of a 4-f system, a reference light path condenser lens, a diaphragm, a phase shift generating device, a photopolymer material optical disc, a reflective phase type spatial light modulator, a CCD detector and a computer, wherein monochromatic linear polarization plane waves emitted by the laser are collimated by a micro objective pinhole filtering and collimating lens of the beam filtering and collimating device, one beam is refracted by a first polarization beam splitter prism to be used as a phase-amplitude conversion beam, the transmitted beam is divided into an object light wave and a reference light wave by a second polarization beam splitter prism, and the object light wave irradiates the reflective phase type spatial light modulator through a third polarization beam splitter prism, the object light wave after phase modulation is focused on the surface of the optical disk made of the photopolymer material through the front Fourier transform lens, the reference wave irradiates the surface of the optical disk made of the photopolymer material through the diaphragm and the reflecting mirror and the reference light path condenser lens to interfere with the object light wave, the interference light field is recorded by the photopolymer material, when the holography is read, closing object light wave, only opening reference light wave, irradiating the reference light wave to the surface of the optical disk made of photopolymer material, making the diffracted reproduced object light wave pass through the back Fourier transform lens and reach the surface of CCD detector, meanwhile, the converted light beam reaches the surface of the CCD detector and the reproduced object light wave through the reflector, the diaphragm, the phase shift generating device, the reflector and the beam splitting prism, so that phase-amplitude conversion is realized, and the CCD detector is connected with a computer for controlling input and output and interference pattern processing.
The invention is further described below with reference to the accompanying drawings and examples.
Drawings
FIG. 1 is a simplified schematic diagram of a prior art volume holographic memory;
FIG. 2 is a comparison graph of simulation experiments for amplitude modulation and phase modulation;
FIG. 3 is a graph comparing amplitude modulation and phase modulation in an optical experiment;
FIG. 4 is a diagram of the optical path of a phase-amplitude conversion device in the volume holographic memory according to the present invention;
FIG. 5 is a diagram of an input phase information code;
FIG. 6 is an image of example one;
fig. 7 is an image of example two.
Description of the symbols: 400 is a laser; 401 is a beam filtering collimating device; 402. 403 and 404 are polarization beam splitting prisms; 405 is a beam splitter prism; 406. 407, 408 and 409 are reflectors; 410 is the front Fourier transform lens of 4-f system; 411 is a rear Fourier transform lens of a 4-f system; 412 is a reference optical path condenser lens; 413. 414 is a diaphragm; 415 is a phase shift generating device; 416 is an optical disc of photopolymer material; 417 is a reflective phase-type spatial light modulator; 418 is a CCD detector; 419 is a computer that controls input and output and interferogram processing.
Detailed Description
The following describes in detail two embodiments and accompanying drawings of the method and apparatus for phase-amplitude conversion in a volume holographic memory.
Example 1:
this embodiment is an example of phase-amplitude conversion applied in the case where an uncoded binary image is input to a volume hologram memory. The input image is for example a black and white square grid as shown in fig. 5, with a grid size of 16 pixels, i.e. 211.2 microns.
The phase-amplitude conversion process of the present embodiment is as follows:
1. the input image is uploaded to a phase type spatial light modulator. The spatial light modulator used was a reflective ferroelectric liquid crystal spatial light modulator from Displaytech, usa, Model number Model LDP-0983-HS1 lightcase, resolution 1280 × 768 pixels, pixel size 13.2 μm, fill ratio 90%. The spatial light modulator is originally amplitude modulation, and when the spatial light modulator is rotated by 22.5 degrees along an optical axis relative to an incident polaroid, binary phase modulation can be realized, namely, the output phases are 0 and pi.
2. And (4) carrying out holographic recording. Opening the object light wave and the reference light to let them interfere on the Fourier spectrum plane of the volume holographic memory, and recording by the photopolymer material disc. The cationic ring-opening polymeric material used in this example was HMD-120-G-1206-D-400, model number, available from DCE Aprilis, USA. The thickness is 400 microns.
3. And (4) holographic reading. The object light wave is turned off and only the reference wave is turned on to irradiate the storage material. The reconstructed object light wave is transmitted to an image surface through a rear Fourier transform lens of the volume holographic memory.
4. A first interferogram is acquired. Coherent plane waves (hereinafter, simply referred to as converted waves) extracted from a light source directly propagate to an image plane to interfere with reproduced object light waves, and a CCD camera placed on the image plane acquires a first interference pattern (shown in FIG. 6. a).
5. A second interferogram is acquired. The phase of the converted wave is changed by the phase shifting device, the converted wave interferes with the reproduced object light wave again, and the CCD camera acquires a second interference pattern (shown in figure 6. b).
6. And subtracting the first interference pattern from the second interference pattern, dividing by the amplitudes of the object light wave and the reference wave and the sine value of half of the phase shift amount, and calculating the sine inversion plus the initial phase and half of the phase shift amount of the reference wave to obtain the phase pattern of the object light wave, as shown in figure 6. c.
7. And (6) carrying out binarization processing. Since the system noise of the volume holographic memory is unavoidable, the calculated phase pattern of the incoming object light wave is not perfect. In addition, since the interference pattern collected by the CCD has 256-step gray scale, the resulting phase pattern also has 256-step gray scale. Noise on the phase map can be removed by median filtering and thresholding can result in a binary map, as shown in fig. 6. d. As the 4-f optical transformation system framework is used in the volume holographic memory, the obtained image plane phase diagram is the phase of the input plane.
Example 2:
this embodiment is an example of phase-amplitude conversion applied in the case where an uncoded binary image is input to a volume hologram memory. The input image is for example a black and white square of fig. 6, with a grid size of 8 pixels, i.e. 105.6 microns.
The phase-amplitude conversion process of the present embodiment is as follows:
1. the input image is uploaded to a phase type spatial light modulator. As in the previous example, the white squares on the input map are modulated to phase pi and the black squares are modulated to phase 0.
2. And (4) carrying out holographic recording. Opening the object light wave and the reference light to let them interfere on the Fourier spectrum plane of the volume holographic memory, and recording by the photopolymer material disc.
3. And (4) holographic reading. The object light wave is turned off and only the reference wave is turned on to irradiate the storage material. The reconstructed object light wave is transmitted to an image surface through a rear Fourier transform lens of the volume holographic memory.
4. A first interferogram is acquired. The converted wave directly propagates to the image surface and interferes with the reproduced object light wave, and a CCD camera placed on the image surface acquires a first interference image (shown in the attached figure 7. a).
5. A second interferogram is acquired. The phase of the converted wave is changed by the phase shifting device, and the converted wave interferes with the reproduced object light wave again, and a second interference pattern (shown in the attached figure 7. b) is collected by the CCD camera.
6. And subtracting the first interference pattern from the second interference pattern, dividing by the amplitudes of the object light wave and the reference wave and the sine value of half of the phase shift amount, and calculating the sine inversion plus the initial phase and half of the phase shift amount of the reference wave to obtain the phase pattern of the object light wave, as shown in the attached figure 7. c.
7. And (6) carrying out binarization processing. Noise on the phase map can be removed by median filtering and thresholding can result in a binary map, as shown in fig. 7. d. As the 4-f optical transformation system framework is used in the volume holographic memory, the obtained image plane phase diagram is the phase of the input plane.
Example 3: as shown in fig. 4, 400 is a laser; 401 is a beam filtering collimating device; 402. 403 and 404 are polarization beam splitting prisms; 405 is a beam splitter prism; 406. 407, 408 and 409 are reflectors; 410 is the front Fourier transform lens of 4-f system; 411 is a rear Fourier transform lens of a 4-f system; 412 is a reference optical path condenser lens; 413. 414 is a diaphragm; 415 is a phase shift generating device; 416 is an optical disc of photopolymer material; 417 is a reflective phase-type spatial light modulator; 418 is a CCD detector; 419 is a computer that controls input and output and interferogram processing.
Monochromatic linear polarization plane waves emitted by a laser (400) are collimated by a filter collimating lens (401) of a micro-objective pinhole (401), and then refracted by a polarization beam splitter prism (402) to form a path of light beam as a phase-amplitude conversion light beam. The transmitted light beam is divided into an object light wave and a reference light wave by a polarization beam splitter prism (403), the object light wave irradiates a reflective phase type spatial light modulator (417) through the polarization beam splitter prism (404), and the phase modulated object light wave is focused on the surface of a photopolymer material optical disc (416) through a front Fourier transform lens (410). The reference wave is reflected by a diaphragm (413) through mirrors (408) and (409) and is irradiated to the surface of a photopolymer material optical disc (416) through a lens (412) to interfere with the object light wave, and the interference light field is recorded by the photopolymer material. When the hologram is read, the object light wave is closed, and only the reference light wave is opened. The reference light wave is directed to the surface of an optical disc (416) of photopolymer material and the diffracted reconstructed object light wave is directed through a post-Fourier transform lens (411) to the surface of a CCD detector (418). At the same time, the converted light beam reaches the surface of the CCD detector (418) and reproduces the object light wave through the mirror (406) diaphragm (414) phase shift generating device (415) mirror (407) beam splitting prism (405), and phase-amplitude conversion is realized. The CCD detector is connected to a computer (419) for controlling the input and output and processing of the interferograms.
Although the present invention has been described with reference to two embodiments, it is not intended to be limited thereto. Any person skilled in the art can obtain similar results without departing from the scope of the invention. The scope of the invention is defined by the appended claims.
Claims (4)
1. The phase amplitude conversion method suitable for the volume holographic memory is characterized by comprising the following steps of:
-uploading the data page to a phase type spatial light modulator;
-holographic recording, the object wave interferes with the reference wave on the spectral plane through a front fourier transform lens, the interference fringes are placed on the photopolymer material recording on the 4-f system spectral plane, forming a volume holographic grating;
in the process of inputting and reading, a shutter is used for shielding object light waves, only reference waves are used for irradiating recording materials to reproduce phase-modulated object light waves, and the object light waves reach the image plane of a 4-f system through a rear Fourier transform lens; a CCD camera is placed on the image surface and used for data reading;
-phase-amplitude conversion, splitting a coherent plane wave from a light source, directly reaching an image plane to interfere with a reproduced object light wave on the surface of the CCD, and acquiring an interference pattern by a CCD camera to obtain a first interference pattern; then uniformly changing the phase of the plane wave through a phase shifting device, and interfering with the object light wave again to obtain a second interference pattern;
calculating the original phase information of the object light wave from the two interferograms by a phase shift recovery algorithm, wherein the specific calculation formula is that the two interferograms are respectively I1I2Then both are expressed as:
wherein AArThe amplitudes of the object wave and the reference wave are shown, a (x, y) is the phase of the object wave, b is the phase of the reference wave, and c is the phase shift amount. The method can be obtained from the two formulas,
wherein,
in the above formula, the phase shift amount c ≠ pi, and for the case where c ═ pi, special processing is required:
a (x, y) is the result of the phase amplitude conversion, i.e. the stored data page.
2. The phase-amplitude transforming method for volume holographic memory according to claim 1, further comprising the steps of: and a preprocessing step, namely encoding the original data to be stored to form a two-dimensional data page, wherein the pixel combination or distribution on the data page appears in a series of fixed modes.
3. The phase-amplitude conversion method for volume holographic memory according to claim 1 or 2, further comprising the steps of: and (4) binarization processing, namely converting the gray-scale image obtained by the phase shift recovery algorithm into a binary image.
4. The device for realizing the phase-amplitude conversion method of the volume holographic memory as claimed in the above claims is characterized by comprising a laser, a beam filtering and collimating device, a first polarization beam splitter prism, a second polarization beam splitter prism, a third polarization beam splitter prism, a first reflector, a second reflector, a third reflector, a fourth reflector, a front-back Fourier transform lens of a 4-f system, a reference light path condenser lens, a first diaphragm, a second diaphragm, a phase shift generator, a photopolymer material optical disc, a reflective phase type spatial light modulator, a CCD detector and a computer, wherein a monochromatic line polarization plane wave emitted by the laser is collimated by a micro objective lens pinhole filtering and collimating lens of the beam filtering and collimating device, and then is refracted by the first polarization beam splitter prism to form a beam as a phase-amplitude conversion beam, the transmitted light beam is divided into object light wave and reference light wave by the second polarizing beam splitter prism, the object light wave irradiates to the reflecting phase type spatial light modulator through the third polarizing beam splitter prism, the object light wave after phase modulation is focused to the surface of the optical disc made of the photopolymer material through the front Fourier transform lens, the reference wave irradiates to the surface of the optical disc made of the photopolymer material through the first diaphragm, the second reflector and the reference light path condenser lens to interfere with the object light wave, the interference light field is recorded by the photopolymer material, the object light wave is closed during holographic reading, the reference light wave irradiates to the surface of the optical disc made of the photopolymer material, the diffracted reproduced object light wave reaches to the surface of the CCD detector through the rear Fourier transform lens, and meanwhile, the converted light beam passes through the third reflector, the second diaphragm, the phase shift generating device, the fourth reflector, And the beam splitter prism reaches the surface of the CCD detector and reproduces object light waves to realize phase-amplitude conversion, and the CCD detector is connected with a computer for controlling input and output and interference pattern processing.
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