Recent Developments in Optical Information Processing Using Nonlinearity and Feedback

Recent advances in electro-optics have brought into use communication and information theory to analyze performance in coherent and incoherent optical information processing systems. An optical information processsing system can be analyzed with many of the same concepts of linear system theory (e.g., spatial impulse response, spatial frequency and spatial domain synthesis, etc.), and the photographic images to be processed can be regarded in the same manner as time signals (e.g., spatial frequency content, spatial amplitude and phase modulation, space-bandwidth product, etc.). Both coherent and incoherent optical processing systems can be treated as linear systems, and the processing operation can generally be carried out by communication theory concepts. Although coherent optical information processing operations have been used for performing complex amplitude operations, complex processing can also be performed with incoherent or white-light illumination. The importance of optical information processing operations, either coherent or incoherent, is due to the basic Fourier transform properties of lenses. In this paper, we will discuss mostly the incoherent systems because they are of more recent interest and possess certain advantages, we feel, over the traditional coherent optical processors. Experimental illustrations of the results are provided. In view of the broad area in optical processing of photographic images, we will confine ourselves to a few applications that we consider of general interest. We apologize for the omission of other techniques and applications, and for neglecting the inclusion of their references.

The increasing speed of fibre-optic-based telecommunications has focused attention on high-speed optical processing of digital information. Complex optical processing requires a high-density, high-speed, low-power optical memory that can be integrated with planar semiconductor technology for buffering of decisions and telecommunication data. Recently, ring lasers with extremely small size and low operating power have been made, and we demonstrate here a memory element constructed by interconnecting these microscopic lasers. Our device occupies an area of 18 x 40 microm2 on an InP/InGaAsP photonic integrated circuit, and switches within 20 ps with 5.5 fJ optical switching energy. Simulations show that the element has the potential for much smaller dimensions and switching times. Large numbers of such memory elements can be densely integrated and interconnected on a photonic integrated circuit: fast digital optical information processing systems employing large-scale integration should now be viable.

The advantage of optical signal processing and computing is mainly due to its two-dimensional complex computing ability with the fastest speed. Of computation algorithms, quite a few are iterative or recursive, such as vector or matrix multiplication over a given matrix for many times, and the computation of derivatives of much high order, etc. These algorithms are also suitable for optical computing . To perform iterative algorithms, the output from a computation step reenters the system as the input for the next step, therefore a feedback loop is needed. Many optical signal processing systems have spatial light modulators that are electrically addressed, and the photon-to-electron and electron-to-photon conversions reduce the processing speed and increase the system complexity. In this paper we propose an all-optical signal processing system for performing the iterative algorithms by using a four wave mixing material which introduces a feedback loop in the system. We believe that this system may also be used as an optical two-dimensional dynamic memory device.

Some recent developments in coherent optical processing are described in this paper. Specifically, the use of one or more elementary gratings to perform useful processing operations and computer-generated spatial filters to obtain generalized transforms are discussed. Applications incorporating nonlinear optical elements and optical feedback into the processing system are also presented.

Coherent optical feedback systems are Fabry-Perot interferometers modified to perform optical information processing. Two new systems based on plane parallel and confocal Fabry-Perot interferometers are introduced. The plane parallel system can be used for contrast control, intensity level selection, and image thresholding. The confocal system can be used for image restoration and solving partial differential equations. These devices are simpler and less expensive than previous systems. Experimental results are presented to demonstrate their potential for optical information processing.

We discuss the potential use of nanophotonics in compact optical information processing systems and present recent progress in the development of plasmonics-integrated photodetectors.

The work deals with an influence of aberrations of optical systems that are used in optical information processing systems. A detailed analysis of the influence of aberrations on the image formed using the optical system is carried out. Furthermore, the effect of aberrations of the wave field, which illuminates the object, is also studied. A theoretical analysis is carried out using the third order aberration theory, and several examples are shown for an object of a circular shape.

Miniaturization and performance enhancement of photonic devices remain key challenges in the advancement of optical information processing technologies and photonic neural networks. In this study, an inverted triangular resonator (ITR) based on a metal–insulator–metal (MIM) plasmonic architecture and phase-change material Ge2Sb2Te5 (GST) is introduced. By integrating a GST element with a volume of only 78,000 nm³ into the active region of the ITR, strong electromagnetic field confinement and optimized phase transition control are achieved. This design results in an unprecedentedly small footprint (0.031 μm²) and an outstanding optical contrast of 76% at the telecommunication wavelength of 1598 nm. The switching energy of this structure, estimated by numerical simulations, is about 7 pJ, placing it among the most energy-efficient photonic devices. The unique features of this structure—including the extremely small volume of active material, low energy consumption, high thermal stability, and excellent reliability—make it an ideal candidate for implementing tunable optical synapses in neuromorphic systems. Its fast nonlinear response and stable operation enable precise simulation of synaptic weighting and the implementation of learning algorithms in photonic architectures. These results demonstrate that the intelligent combination of plasmonic technology and phase-change materials in the ITR configuration can open new horizons in the design of ultra-compact, low-power photonic devices for next-generation neural networks and optical information processing.

Differential pairs of PnpN optical thyristors are among the most promising devices for digital parallel optical information processing systems. Very recently it has been shown that these AlGaAs based detector-emitter type of devices can be cascaded at 50 MHz with only 7.2 fJ external optical input energy [1], and that this optical input energy scales with device area resulting in a record sensitivity of 15 aJ/μm2 [2]. The layer structure of these optical thyristors has been designed such that all free carriers can be extracted from the center p- and n-type layers by applying a small negative anode-to-cathode voltage pulse (complete depletion for a turn-off voltage of - 4V), while switching on the device requires a break-over voltage of 2.7 V. To be of use in practical systems the device has also been engineered to be cascadable. In this way, an optical thyristor operating as a detector is sensitive to the light that an identical element, working as an emitter, generates. Moreover, a differential pair configuration has been adopted to increase the optical input sensitivity [3]. Finally, correct operation of 8*8 monolithic arrays (see Fig la) of these completely depleted, cascadable differential pairs has been demonstrated [4].

Fourier methods are widely used in optical information processing for time domain waveform synthesis and detection. In such approaches an input pulse is spectrally decomposed and modulated by an appropriately designed spectral filter. In general, various ultrafast applications may need either generation or characterization of waveforms depending whether their frequency response is or is not known a priori. Ultrafast pulses prove useful in driving or probing such systems because their broad, deterministic complex spectral amplitude “sees” a large continuous portion of the system transfer function. Traditionally, space domain spectral decomposition is used to perform optical Fourier processing, where pairs of gratings and lenses are used to decompose the spectral content of the pulsed, optical waveforms. Liquid crystal and acousto-optic (AO) modulators provide a means for introduction of somewhat arbitrary system transfer functions. More recently, compact waveguide-based approaches have used arrayed waveguide gratings (AWGs) to perform an analogous spectral decomposition to those achieved with traditional, free-space coupled gratings. Information processing arrangements using the space domain for transverse spectral decomposition benefit from the maturity of such techniques, with respect to basic understanding, component quality, and the excellent phase control that free-space manipulations provide. Component insensitivity to optical power permits the shaping and detecting of high peak power pulses used to drive nonlinear optical systems. Furthermore, nonlinear optical elements can be introduced directly into these processors to provide unique ultrafast waveform synthesis capabilities with response times in the range of femtoseconds. Drawbacks to free-space approaches include scaling in volume to achieve large time bandwidth products (TBWPs) and limiting the temporal extent of waveforms coupled back into a single-mode fiber (SMF). The origin of this last restraint is the time∕space interrelation inherent in traditional pulse shaping devices. AWG approaches rely on complex waveguide elements to circumvent large processor volumes and time∕space coupling issues, but they too are practically limited by fabrication requirements. The number of resolvable spots in the output waveform for both Fourier synthesis and direct space-to-time AWG approaches is equal to the number of waveguide channels. An alternative approach to Fourier processing relying on a single transverse spatial mode can better integrate with fiber systems and photonic lightwave circuits and will scale in length only for the achievement of large TBWPs. Fiber-based and fiber-integrated processors can manipulate optical waveforms in the time domain exploiting chromatic dispersion for longitudinal spectral decomposition and applying to it Fourier synthesis techniques. Drawing on the identical mathematical treatments of diffraction in space and dispersion in time, an approximate Fourier transform (FT) of an incident optical signal is achieved via chromatic dispersion after reaching the so-called “far-field approximation.” With such an amount of dispersion, the temporal waveform closely resembles the spectrum. Such a waveform is called a longitudinal spectral decomposition wave (SDW). A time-variant element (e.g., modulator) filters this SDW. And it is possible to recompose the pulse waveform through propagation in a conjugate dispersion source matched (i.e., of opposite sign but equal magnitude) to the first dispersive element. The number of resolvable spots that such a processor offers relies on a suitable modulation scheme and the experimental ability to disperse a sub-picosecond pulse using second-order dispersion.

Reversible logic is emerging as an important research area having its application in diverse fields such as low power CMOS design, digital signal processing, cryptography, quantum computing and optical information processing. This paper presents a new 4*4 parity preserving reversible logic gate, IG. The proposed parity preserving reversible gate can be used to synthesize any arbitrary Boolean function. It allows any fault that affects no more than a single signal readily detectable at the circuit's primary outputs. It is shown that a fault tolerant reversible full adder circuit can be realized using only two IGs. The proposed fault tolerant full adder (FTFA) is used to design other arithmetic logic circuits for which it is used as the fundamental building block. It has also been demonstrated that the proposed design offers less hardware complexity and is efficient in terms of gate count, garbage outputs and constant inputs than the existing counterparts. Keywords: Reversible Logic, Parity Preserving Reversible Gate, IG Gate, FTFA and Carry Skip Logic. doi: 10.3329/jbas.v32i2.2431 Journal of Bangladesh Academy of Sciences Vol.32(2) 2008 234-250

A new design for obtaining optical bistability in two dimensions is reported in this paper. This design uses optical feedback on a photoconductor-twisted nematic liquid crystal (PC-TNLC) device with a new structure. The PC-TNLC device is essentially a 2-D array of optically controlled light values. The device structure with a new arrangement of the photoconductor, the liquid crystal film, and a patterned transparent electrode gives better control over its input-output (I-O) response, which is important in obtaining bistable operation. A theoretical analysis is carried out to determine the conditions under which bistability occurs. A sharp transition from the OFF to ON state in the I-O response suggests that the thresholding operation on gray scale input images can be performed. Experimentally, an 8 x 8 PC-TNLC device with optical feedback was demonstrated to have bistable behavior. The ON:OFF ratio of 50:1 was obtained in the output. This device was also used to perform thresholding operation on a gray scale image with eight levels. The observed value of gamma for the thresholding operation was 8. Such a bistable device was shown to have potential applications in digital optical processing.

Liquid crystal spatial light modulators are used in a wide range of modern problems in science and technology. These modulators are used to control the amplitude, phase, and direction of propagation of coherent optical radiation in optical information processing systems. However, the influence of the characteristics of the temporal dynamics of liquid crystal spatial light modulators on the performance of information optical systems, including diffractive neural networks, has not been sufficiently studied. The article presents the results of a study of the temporal dynamics of phase modulation of the liquid crystal spatial light modulator SLM-200 (Santec, Japan). Computer-synthesized binary phase diffractive optical elements were used in the experiments, and the characteristics of the temporal dynamics of the optical modulator were measured: 125 ms is the rise time of the diffraction efficiency when displaying diffractive optical elements on the screen; 61.9 ms is the decay time when switching frames. With these characteristics, it is possible to ensure the formation of a variable optical field at a frame display frequency of 2 Hz with an interference level of –17.1 dB. Increasing the frame display frequency leads to the appearance of unavoidable interframe interference, which in turn limits the effective performance of the information system. The results obtained can be useful in designing high-performance optical information processing systems and diffraction neural networks

This work presents an integrated framework combining optical signal processing and VLSI system design for the performance evaluation and optimization of 5G waveform candidates under different modulation schemes. The analysis focuses on four major waveforms – OFDM, FBMC, NOMA, and OTFS – and examines their performance based on bit error rate (BER), peak-to-average power ratio (PAPR), and power spectral density (PSD) metrics. Simulation results demonstrate that OTFS outperforms the other waveforms, achieving a BER of 10 −3 at an SNR range of 7–10 dB, corresponding to an SNR gain of up to 8 dB over OFDM. Furthermore, OTFS exhibits a PAPR reduction of approximately 3.5 dB and the lowest out-of-band emissions with a PSD of around −310 dBW/MHz, indicating superior spectral efficiency and reduced interference. These characteristics make OTFS a promising waveform for high-mobility, high-capacity 5G and future 6G communication environments. The integration of optical signal processing techniques enhances system bandwidth and energy efficiency, while VLSI-based architectures support low-latency, high-throughput implementation. Future advancements will explore optical-VLSI codesign and machine learning–assisted frameworks for real-time adaptive processing, improving scalability, power optimization, and intelligent signal handling in next-generation wireless and optical communication systems.

Optical correlators are the key components in various analog and digital optical free-space processing systems. The disadvantages of classical optomechanically assembled setups with respect to packaging density, reliability and production costs can be overcome with integration techniques. In planar optics all components are integrated on the surfaces of a single substrate using lithographic fabrication technologies. High light efficiency and design flexibility in the passive optical system is achieved by using multiple phase level diffractive optical elements. In this paper we present an optical Vander Lugt correlator integrated in planar optics.