Implementation of single-beam multiplexing encoding with a dually modulated spatial light modulator

We propose a novel method for signal storage and encryption, called single-beam multiplexing encoding. The single beam is composed of an inside signal beam and an outside reference beam. The signal beam is amplitude modulated, and the reference beam is phase modulated. The dual modulation is implemented by a spatial light modulator (SLM). Multiplexing holography with different reference beams from different directions, called directional multiplexing, is analyzed in detail. With an SLM based on a twisted nematic liquid crystal display, we demonstrate a single-beam directional multiplexing method using a holographic encoding technique, and the retrieved signals are presented. This encoding system is more stable, miniaturized, and flexible. It should be of great interest for applications in signal encryption as well as for high-capacity data storage.     

Homogenized Fourier transform holographic data storage using phase spatial light modulators and methods for recovery of data from the phase image

We present a method for homogenizing the Fourier spectrum for holographic digital data storage by use of a phase spatial light modulator (SLM), and methods for the recovery of data from a phase image are implemented and discussed. Binary digital data are displayed on a phase SLM operating in 0 and pi phase modes to optimally remove the intense dc peak so as to obtain a homogenized Fourier spectrum. Methods based on holographic interferometry have been developed and employed for recovery of the original amplitude data page from the phase-data page. A new edge-detection-based method also has been demonstrated and analyzed for reconstruction of the original data. Experimental results are presented to confirm the feasibility of these novel techniques.     

Holography with Light Beams of Finite Spectral Width

The effects of spectral coherence on holographic images have been analysed in terms of the Fresnel-transform formulation of the process of holography. In this paper the case of very rough object or diffuse illumination has not been considered. The results have been obtained as contrast modulation in the image of a spatial frequency component in the object. The effect of polychromaticity in the recording beam is seen to be more severe than that due to the polychromaticity in the reconstruction beam. Fourier-transform holography seems to yield a better image. With the lensless Fourier-transform hologram recorded on monochromatic light, white light reconstruction seems to be possible if the illuminating beam is suitably dispersed before it falls on the hologram.     

Suppression of the zero-order diffracted beam from a pixelated spatial light modulator by phase compression

Phase compression is used to suppress the on-axis zero-order diffracted (ZOD) beam from a pixelated phase-only spatial light modulator (SLM) by a simple modification to the computer generated hologram (CGH) loaded onto the SLM. After CGH design, the phase of each SLM element is identically compressed by multiplying by a constant scale factor and rotated on the complex unit-circle to produce a cancellation beam that destructively interferes with the ZOD beam. Experiments achieved a factor of 3 reduction of the ZOD beam using two different liquid-crystal SLMs. Numerical simulation analyzed the reconstructed image quality and diffraction efficiency versus degree of phase compression and showed that phase compression resulted in little image degradation or power loss.     

Optical Processing of Bubble Chamber Photographs

Purely optical methods of spatial pulse width coding for coherent optical densitometry are described. The proposed coding scheme correlates the image intensity with the orientation of one edge of triangular or trapezoidal pulses. One encoding technique employs a contact screen similar to those used for half-tone printing. The image to be coded is copied through this contact screen onto hard-limiting film. This method is limited to the encoding of images with density distributions varying only slowly across the area of one pulse. A holographic encoding technique overcomes these drawbacks, utilizing the storage redundancy of holograms recorded with diffuse object beams. An array of identical holograms is reconstructed with a beam spatially modulated by the image transparency to be encoded. The array of reconstructed real images is recorded on hard-limiting film and renders a pulse-modulated version of the original image. The limiting conditions for the pulse size and the size of details in the image imposed by diffraction effects and speckle pattern are derived.     

Single-beam data encoding using a holographic angular multiplexing technique

We propose a novel configuration for angular multiplexing holographic encoding in which the signal beam and the reference beam are combined into a single beam. By using a spatial light modulator based on twisted nematic liquid crystals, the signal and the reference beams are modulated in amplitude mode and phase mode, respectively. The multiplexed interference patterns with the reference beams of different incident angles are recorded near the Fourier transform plane, and then the signals are selectively reconstructed by the corresponding reference beam. Both the simulation and the experiment of single-beam angular multiplexed holography are performed with consistent results. Compared with the traditional angular multiplexing holographic recording system, the single-beam configuration is more compact, easier to adjust, and less sensitive to the vibration of the environment. Therefore, it will be more attractive for potential applications in many fields, such as high-density signal recording and data encryption.     

Holographic display with tilted spatial light modulator

In this paper, we analyze a holographic display system utilizing a phase-only spatial light modulator (SLM) based on liquid crystal on silicon (LCoS). An LCoS SLM works in reflection, and, in some applications, it is convenient to use with an inclined illumination. Even with a highly inclined illumination, the holographic display is capable of good-quality image generation. We show that the key to obtain high-quality reconstructions is the tilt-dependent calibration and algorithms. Typically, an LCoS SLM is illuminated with a plane wave with normal wave vector. We use inclined illumination, which requires development of new algorithms and display characterization. In this paper we introduce two algorithms. The first one is designed to process a digital hologram captured in CCD normal configuration, so it can be displayed in SLM tilted geometry, while the second one is capable of synthetic hologram generation for tilted SLM configuration. The inclined geometry asymmetrically changes the field of view of a holographic display. The presented theoretical analysis of the aliasing effect provides a formula for the field of view as a function of SLM tilt. The incidence angle affects SLM performance. Both elements of SLM calibration, i.e., pixel phase response and wavefront aberrations, strongly depend on SLM tilt angle. The effect is discussed in this paper. All of the discussions are accompanied with experimental results.     

Complex Fresnel hologram display using a single SLM

We propose a novel optical method to display a complex Fresnel hologram using a single spatial light modulator (SLM). The method consists of a standard coherent image processing system with a sinusoidal grating at the Fourier plane. Two or three position-shifted amplitude holograms displayed at the input plane of the processing system can be coupled via the grating and will be precisely overlapped at the system's output plane. As a result, we can synthesize a complex hologram that is free of the twin image and the zero-order light using a single SLM. Because the twin image is not removed via filtering, the full bandwidth of the SLM can be utilized for displaying on-axis holograms. In addition, the degree of freedom of the synthesized complex hologram display can be extended by involving more than three amplitude holograms.     

Comparison of diffraction efficiencies for single-exposure and unity-contrast multiple-exposure holograms

The diffraction efficiency for a single-exposure holographic recording is compared with that of a multiple-exposure hologram recorded with maximum signal-to-bias ratio. For the multiple-exposure recording the object is illuminated point by point, and the reference wave amplitude for each subexposure is adjusted to produce a unity-contrast interferogram in the holographic recording plane. It is shown that, for the same M-point object and the same average exposure, the ratio of the single-exposure diffraction efficiency to the mulitple-exposure diffraction efficiency is proportional to M/R, where R is the reference-to-object beam ratio in the single-exposure case. This result is compared with that obtained in the multiple-exposure case when the reference wave amplitude assumes a constant value. Application areas include optical interconnects, holographic data storage, and volumetric multiplexed holograms for display.     

Thermal diffusivity imaging with the thermal lens microscope

A coaxial thermal lens microscope was used to generate images based on both the absorbance and thermal diffusivity of histological samples. A pump beam was modulated at frequencies ranging from 50 kHz to 5 MHz using an acousto-optic modulator. The pump and a CW probe beam were combined with a dichroic mirror, directed into an inverted microscope, and focused onto the specimen. The change in the transmitted probe beam's center intensity was detected with a photodiode. The photodiode's signal and a reference signal from the modulator were sent to a high-speed lock-in amplifier. The in-phase and quadrature signals were recorded as a sample was translated through the focused beams and used to generate images based on the amplitude and phase of the lock-in amplifier's signal. The amplitude is related to the absorbance and the phase is related to the thermal diffusivity of the sample. Thin sections of stained liver and bone tissues were imaged; the contrast and signal-to-noise ratio of the phase image was highest at frequencies from 0.1-1 MHz and dropped at higher frequencies. The spatial resolution was 2.5 μm for both amplitude and phase images, limited by the pump beam spot size.     

Holographic projection of arbitrary light patterns with a suppressed zero-order beam

We present what is to our knowledge a novel technique for efficient suppression of the zero-order beam inherent in light patterns projected via phase-only computer-generated holograms (CGHs). Encoding a CGH on a spatial light modulator (SLM) with a limited fill factor produces a disturbing zero-order beam at the optical axis. Here, we propose to derive a CGH, which includes holographic information to project a corrective beam that destructively interferes with the zero-order beam. The CGH for projecting arbitrary light patterns plus a corrective beam are derived using the Gerchberg-Saxton algorithm where the iterations impose both amplitude and phase constraints for the target field pattern at the Fourier plane. As proof of principle, we analyze the viability of the technique by simulating the performance when applied on a practical SLM with a limited fill factor, fixed number of phase-shifting pixels, and wavefront distortion associated with the surface roughness of the SLM.     

Long-term reading experiment on a photorefractive holographic memory with the hologram sustainment technique by optical feedback

We demonstrate a long-term continuous readout of a two-dimensional image in a photorefractive holographic memory with a BaTiO₃ crystal. A considerable extension of reading time is achieved by use of a hologram sustainment technique with an optical feedback circuit. Hologram rewritings by the simultaneous illumination of the reading beam and the feedback beam, which is incident on a crystal from the opposite direction to the reading beam, contribute to all-optical hologram sustainment without any fixing technique. In this paper, the effectiveness of the hologram sustainment technique is explained by the analysis of the temporal property of the amplitude of the index grating in a crystal. By calculating the temporal properties of the reconstructed beam intensity, we reveal the feedback rate and the coupling strength for high output efficiency. We perform an experiment on two-dimensional image reading and writing with a 45° cut BaTiO₃ crystal and show that a recorded image can be reconstructed over 20?min without critical image degradation whereas a reconstructed image fades away within 20?s in the conventional readout technique.     

Secure holographic memory by double-random polarization encryption

A novel optical encryption based on polarization is proposed and applied to a holographic memory system. Original binary data are described as two orthogonal linear polarization states. These input polarization states can be modulated by use of two polarization-modulation masks located at the input and the Fourier planes. Each modulation mask can convert an input polarization state into a random polarization state. Once encrypted, the polarization state is recorded as a hologram. For the decryption, the hologram can generate a vector phase-conjugate beam. When the same polarization-modulation masks are used, the vector phase-conjugate readout can cancel the polarization modulation at each mask, and the original polarization state can be recovered. The encryption of the proposed method is evaluated numerically. We also present experimental results by demonstrating holographic recording in a bacteriorhodopsin film.     

Optical compensation of hologram distortion avoiding interpage crosstalk on reconstructed image in angle-multiplexed holograms

We are studying a form of holographic data storage with phase conjugation, and we compensated for hologram distortion due to shrinkage of photopolymer materials in the holographic medium by controlling the wavefront of the reference beam. When a high NA lens and narrow angle interval of angle multiplexing are employed to obtain a high data recording density, some wavefronts cause interpage crosstalk on the reconstructed image. We tried to determine the moving range of actuators in a deformable mirror for controlling the wavefront. As a result, we found that the distortion in the hologram could be compensated while avoiding interpage crosstalk and that the bit error rates of the reproduced data could be decreased. We also found that the optimized wavefront could compensate for distortions in several neighboring data pages. This method can ensure a high data recording density in holographic data storage.     

Spatial-phase code-division multiple-access system with multiplexed Fourier holography switching for reconfigurable optical interconnection

A new, to our knowledge, space-variant optical interconnection system based on a spatial-phase code-division multiple-access technique with multiplexed Fourier holography is described. In this technique a signal beam is spread over wide spatial frequencies by an M-sequence pseudorandom phase code. At a receiver side a selected signal beam is properly decoded, and at the same time its spatial pattern is shaped with a Fourier hologram, which is recorded by light that is encoded with the same M-sequence phase mask as the desired signal beam and by light whose spatial beam pattern is shaped to a signal routing pattern. Using the multiplexed holography, we can simultaneously route multisignal flows into individually specified receiver elements. The routing pattern can also be varied by means of switching the encoding phase code or replacing the hologram. We demonstrated a proof-of-principle experiment with a doubly multiplexed hologram that enables simultaneous routing of two signal beams. Using a numerical model, we showed that the proposed scheme can manage more than 250 routing patterns for one signal flow with one multiplexed hologram at a signal-to-noise ratio of ~5.     

Digital recording and numerical reconstruction of holograms: an optical diagnostic for combustion

Holographic interferometry (HI) has proved to be a useful tool for nonintrusive temperature measurements in flames (and thereafter for inference of the local composition based on the state relationship approach) with high spatial and temporal resolution. Digital holographic interferometry (DHI) is a relatively new imaging and measurement technique that electronically records a hologram (e.g., on a CCD) and reconstructs it by a numerical method. Cumbersome chemical processing of the hologram is avoided in DHI, which thereby provides greater flexibility, speed, and the potential for real-time processing. In conventional holography, fringes that are neither bright nor dark on a hologram cannot be accurately resolved. The DHI technique has not yet to our knowledge been used for combustion applications. Herein we evaluate its efficacy for making temperature measurements in flames and assess its applicability through a simulation. Each part of a double exposure associated with the holographic technique is considered to be recorded by a hypothetical CCD sensor at a separate time from the other part. We applied the principles of Fourier optics to develop two numerical methods for hologram reconstruction, and we show that both methods provide an accurate reconstruction of the phase image associated with a flame. Because of the periodic nature of the wave function, the reconstructed phase values are limited to the interval [-pi/2, pi/2]. Thus an unwrapping algorithm is provided that produces a continuous phase distribution based on the condition that the reconstructed phase is jumped by a value of -pi or pi. We have also developed an iterative calculation method to adjust the value of the digital reference wave parameters that determines the phase imaging reconstruction in DHI.     

Modulation transfer function measurement method for electrically addressed spatial light modulators

The modulation transfer function (MTF), when used with amplitude modulation (m(A)) data, is a vital coherent optical performance measure for a spatial light modulator (SLM). A new image plane amplitude MTF (MTF(A)) measurement method is presented for electrically addressed SLMs. It involves digital analysis of the output image of a square-wave pattern written onto the SLM. Modulation-level effects are also addressed. Optical laboratory results are presented for two liquid-crystal SLMs. The need to consider amplitude rather than intensity modulation (when coherent optical processing applications are considered) is noted in terms of SLM biasing.     

Time-multiplexed real-time one-way image compensation for high-spatial-frequency aberration correction

A new self-aligning geometry for real-time holographic image reconstruction for one-way imaging through a phase aberrator is demonstrated. The input beams are time multiplexed to isolate the diffracted image from the reference beams after the image beams propagate through the hologram. This geometry permits the image-bearing beam and the reference beams to copropagate through the holographic plane.     

Compensation for time fluctuations of phase modulation in a liquid-crystal-on-silicon display by process synchronization in laser materials processing

We demonstrate the adverse influence of temporal fluctuations of the phase modulation of a spatial light modulator (SLM) display device on nanosecond laser micromachining. We show that active cooling of the display reduces the amplitude of these fluctuations, and we demonstrate a process synchronization technique developed to compensate for these fluctuations when applying the SLM to laser materials processing. For alternative SLM devices developed specifically for laser wavefront control (which do not exhibit such flickering problems), we show that our process synchronization approach is also beneficial to avoid machining glitches when switching quickly between different phase profiles (and hence beam patterns).