Coherent Correlator Research Articles

Coherent correlator design analysis for the implementation of deep learning networks.

Optical signal processing can reduce the electrical power consumption required over that required by graphically processing units. There remain a number of challenges to overcome. Noise is potentially much larger in optical systems than in their electronic counterparts, and some of the operations, such as a bias addition, are not easy to implement in free space processors. This paper analyzes a proposed design that utilizes a camera and lightweight electronic processing to perform the convolutional layers. Simulations are performed to compare the expected performance against an ideal system that cannot be physically realized and a proposed architecture. The impact of speckle noise in the system is analyzed and methods to reduce this are proposed.

All-Optical QPSK Pattern Recognition in High-Speed Optoelectronic Firewalls

In the 5G era, more sensitive information will be transmitted on optical networks. Although modern cryptography could support different security mechanisms in different layers, the optical layer has not been paid enough attention but it is also vulnerable to attack by tapping or other methods. The optoelectronic firewall is one of the promising security strategies which can accelerate the operating speed within manageable costs by employing optical signal processing compared with current electronic firewalls. The most significant and challenging component of an optoelectronic firewall is pattern recognition. However, either the latest pattern recognition systems can only process the OOK or BPSK modulation format signals, or the coherent correlators supporting higher order modulation format require advanced components to deal with a higher number of symbols. In this paper, to address the pattern recognition challenges, we propose, analyze and simulate a pattern recognition system of QPSK signals integrated into all-optical high-speed optoelectronic firewalls used for optical layer security. The numerical simulation results first demonstrate the feasibility, and a baud rate of 100GBaud can be achieved. Next, a threshold-setting method is developed which also proves the ability to recognize arbitrary target patterns. Then some noise analysis results demonstrate that the pattern recognition of the I branch can be immune to the noise while the output power of EDFAs in the Q branch needs to be configured carefully. Finally, the matched result of a real Ethernet frame further reveals that our proposed system is promisingly applied in optoelectronic firewalls.

On the use of a multi-raster input of one-dimensional signals in two-dimensional optical correlators

A mathematical description of a coherent optical correlator for the multi-raster input of long signals is given. It is shown that such an input makes it possible to reduce the value of false correlation maxima that are generally found at the output of a correlator with a single-raster input. It is shown that false maxima do not appear when processing signals with a thumbtack ambiguity function, allowing one to do without a multi-raster input. The results of the theoretical analysis are confirmed by experiments with chirp signals and M-sequence type signals.

Coherent correlator and equalizer using a reconfigurable all-optical tapped delay line

We experimentally demonstrate a reconfigurable optical tapped delay line in conjunction with coherent detection to search multiple patterns among quadrature phase shift keying (QPSK) symbols in 20 Gbaud data channel and also to equalize 20 and 31 Gbaud QPSK, 20 Gbaud 8 phase shift keying (PSK), and 16 QAM signals. Multiple patterns are searched successfully on QPSK signals, and correlation peaks are obtained at the matched patterns. QPSK, 8 PSK, and 16 QAM signals are also successfully recovered after 25 km of SMF-28 with average EVMs of 8.3%, 8.9%, and 7.8%. A penalty of <1 dB optical signal to noise penalty is achieved for a 20 Gbaud QPSK signal distorted by up to 400 ps/nm dispersion.

Coherent optical correlator based on combined halftone and position modulation of light phase

Usually only a halftone (grayscale) modulation of light is used to present the processed and reference signals in the input plane of analog coherent optical (ACO) correlators, based on spatially integrating the product of two (processed and reference) signals. The halftone modulation requires two transparencies to record two signals separately, as the desired product of two spatial signals is obtained by sequential location of two spatial halftone records along the light path. Such an optical layout leads to the need for precision mutual alignment of two separate signal recordings. This paper presents a one-stage ACO correlation method based on combined halftone and position modulation of the light phase, which is produced by joint phase recording two signals on a single transparency. The joint phase recording provides the high optical efficiency of informational light modulation, automatically supports the spatial coincidence of corresponding elements of both recorded signals, and provides the same spatial scale for both recordings. The suggested method can also be used for introducing phase weight functions in the schemes of space-time ACO processing of wave signals. Advantages of ACO signal processing methods in comparison with corresponding electronic approaches are briefly noted.

Diffractive optical elements for mode-division multiplexing of temporal signals with the aid of Laguerre–Gaussian modes

Optical aspects of space-division multiplexing with orthogonal modes of coherent light were considered in theory and experiments with the coherent optical correlator. We resorted to the mathematical tool of generating functions and technologies of diffractive optical elements to implement complex spatial filters matched to rotationally symmetrical transverse modes. Successful multiplexing and demultiplexing in free-space transmission of low-frequency temporally modulated signals through different spatial modes was demonstrated. Experimental results show low cross talk between different mode channels and feasibility of further applications in multimode fiber optical communication data links.

Model of an invariant correlator with liquid-crystal spatial light modulators

Invariant pattern recognition using a model of a coherent holographic correlator with liquid-crystal spatial light modulators as input devices has been experimentally analysed.

Coherent optical three-dimensional spectrum-correlation processing of wave signals based on space–time integration

The architectures of classical analog coherent optical (ACO) spectrum analyzers and correlators are not designed to process the wave signal as a whole, i.e., simultaneously in three dimensions. In this paper, the theory of ACO three-dimensional direct spectrum-correlation processing of spatial-temporal optical replicas (copies) of wave signals is discussed. In the single-stage and two-stage ACO systems, the spatial power spectrum and spatial correlation function of the wave signal (envelope) are obtained on the basis of space-time integration. The geometry of the final compressed signal in the output plane of either optical system allows one to evaluate the angle of wave arrival. The wave signal to be processed can theoretically have any form (due to autocorrelation properties of the systems) and an unlimited duration (due to time integration of wave energy and possibility of electronic subtraction of the intermediate bias terms of the time integration).

Analysis of coherent optical filter and correlator systems using pixellated spatial light modulators

The authors analyse the optical performance of 4f coherent optical filtering systems using pixellated spatial light modulator devices for the input and filter planes. The performance of such systems is frequently analysed using oversampling fast Fourier transform (FFT) techniques which we show to require large oversampling factors (and hence complexity) to fully take into account the optical field distribution across each individual pixel of the filter plane and thus provide improved accuracy. The authors proposed two methods by which they obtain a piecewise series expansion of the optical field across each pixel using FFT techniques and demonstrate that these methods show faster convergence with increasing complexity and require less memory storage to compute than the oversampling FFT techniques usually used. If the output plane intensity is only required at a few points, such as when optimisation techniques are being used to design the filter, the authors present a third method which also results in improved accuracy but with no penalty in speed.

Amplitude filter and Zernike polynomial expansion method for quality control of microlens arrays

This paper deals with a computer simulation and an experimental realization of an optical setup for automatic quality control of microlens arrays. The method is based on a 4f coherent light correlator setup with an amplitude filter placed in the Fourier plane. The output intensity signal is proportional to the first derivative of the distortion of the input wavefront. An analysis can be carried out with the use of the Zernike polynomial expansion method. It must be carried out separately for each lens, but it allows for a more precise, quantitative assessment of their quality. What is important is that the analysis is computer-based and performed on the basis of the initial single optical measurement.

LPCC filter realization in 4-F correlator of images with application of purely amplitude binary spatial modulation

In this paper the results of numeric modeling and experimental realization of correlation filter with linear phase coefficient (LPCC-filter) in scheme of optical coherent correlator of images are represented. Represented results prove the possibility of synthesized complex correlation filter realization as digitally synthesized hologram in scheme of optical correlator using modern fast-operating amplitude spatial light modulators (SLM) with limited dynamical range of the hologram transmission gradation levels representation.

Volume holograms for optical correlation

Pattern recognition of objects within highly cluttered images is a demanding task, although it has many applications in both the military and civilian sectors. The techniques are extremely computationally intensive, making real-time processing using standard computer equipment difficult if not impossible. One possible solution to overcome the computational limitations employs a coherent optical correlator, i.e., an optical processor that allows objects to be identified and tracked at the speed of light. The coherent optical correlator has had a long and successful history, starting from Vander Lugt’s original design. The basic arrangement is still in use today: an input signal or image is coherently encoded on a beam of light, it is subsequently Fourier transformed, and correlated with a holographically stored Fourier signal or filter. The correlated output signal is again Fourier transformed and a peak is produced in the output plane when the filter matches. One of the nice features of this approach is that it is shift invariant, implying that the peak position corresponds to the previously unknown position of the input object. This means that we can identify the object and track its position in a 2D plane. In modern systems, the filter is implemented on a phase-only spatial-light modulator (SLM) that can be dynamically updated, thus allowing correlation of many templates for real-time target identification. However, the overall bandwidth is limited to the data-bus speed, the frame rate of the SLM, or the frame rate of the camera used for capturing the output. This is a significant weakness. If one wanted to test 36 filter templates for every video frame, each correlation must take less than 926μs or occur at a frequency higher than 1080Hz. If the filter consisted of 1024×1024 pixels, one would need a data bus of over one gigahertz. If the number of templates were increased further, the speed would become even more problematic. Figure 1. Schematic layout of the correlator. MQW SLM: Multiplequantum-well-based spatial-light modulator.

Realisation of invariant holographic filters with the linear phase coefficient in a Van der Lugt correlator

Invariant correlation filters with the linear phase coefficient are synthesised and pattern recognition experiments are performed by using these correlation filters in a coherent holographic correlator.

All-optical pattern recognition for digital real-time information processing.

To recognize digital streams of digital data, all-optical and passive techniques able to discriminate optical bit words in real time are presented. Discrimination capability of different correlators, both in free space architectures and in delay lines structures, is theoretically and experimentally analyzed. Experimental performances in word recognition are shown in the case of a volume holographic correlator, in the case of a lithographic phase-only-filter correlator, and in the case of a novel coherent delay lines correlator operating at the wavelength 1550 nm and at the bit rate of 2.5 Gbit/s.

Quadriphase DS-CDMA with pulse shaping and the accuracy of the Gaussian approximation for matched filter receiver performance analysis

Probability of bit-error (P/sub e/) performance of asynchronous direct-sequence code-division multiple-access (DS-CDMA) systems is analyzed. In particular, the effects of pulse shaping, quadriphase (or direct-sequence quadriphase shift keying (DS-QPSK)) spreading, aperiodic spreading sequences and the coherent correlator or, equivalently, the matched filter (MF) receiver are considered. An exact P/sub e/ expression and several approximations: one using the characteristic function (CF) method, a simplified expression for the improved Gaussian approximation (IGA) and the simplified improved Gaussian approximation (SIGA) are derived. Two main results are presented. Under conditions typically satisfied in practice and even with a small number of interferers, the standard Gaussian approximation (SGA) for the multiple-access interference component of the MF statistic and for MF P/sub e/ performance is shown to be accurate. Moreover, the IGA is shown to reduce to the SGA for pulses with zero excess bandwidth. Second, the P/sub e/ performance of quadriphase DS-CDMA is shown to be superior or equivalent to that of biphase DS-CDMA. Numerical examples with Monte Carlo simulation are presented to illustrate P/sub e/ performance for square-root raised-cosine (Sqrt-RC) pulses and spreading factors of moderate to large values.

Signal correlator with programmable variable time delay based on optical coherent transients

An optical coherent transient signal correlator with variable true time delay is experimentally demonstrated. Storage of a pattern waveform in a spectral-spatial population grating is achieved by interfering it with two chirped pulses instead of a single brief reference pulse. Correlative signal processing results when a data waveform probes that grating, stimulating an emitted coherent output signal after the appropriate time delay. The signal is the result of a six-wave mixing process that can be as efficient as the traditional four-wave mixing process using a single brief reference pulse. Frequency shifting of the two chirped pulses enables programming of a variable time delay in the grating without changing the timing of the programming pulses. This concept is experimentally demonstrated with variable time delays over 448 ns (offset by greater than 1 μs) for the auto-correlation of a code with an experimentally limited bandwidth of 40 MHz. Processing bandwidths exceeding 10 GHz are achievable in materials such as Tm 3+:YAG (0.1 at%).

Optical implementation of segmented composite filtering

We investigate possible performance improvements of coherent optical correlators by using an appropriate filter design. Multidecision strategies are often required in high-level image-processing tasks. For an optical system characterized by a given space-bandwidth product we show that the filter design plays a crucial role in satisfying both system and processing requirements, with respect to the optimization of the encoding capacity. This leads us to the definition of segmented composite filtering, which is discussed in terms of processing performance. This filtering is assessed experimentally in the case of a face-recognition problem.

Optical–digital methods of fingerprint identification

We compare optical–digital systems for fingerprint identification that use the spatial spectrum and the direction field. For the construction of the direction field, we propose an optical configuration of a coherent correlator with a spatial filter consisting of segmentized diffraction gratings. The analysis using the direction field is shown to be more advantageous. It has also been shown that in analysing fingerprints using the Fourier spectrum, it is only the information related to the angular spectrum distribution that appears to be essential, while data on the radial spectrum distribution are redundant.

Coherent recognition of colored patterns

A properly preprocessed image of a colored scene can be used with conventional coherent correlators to recognize and locate colored patterns. Color information is encoded by means of gratings with different orientation, frequency and amplitudes. This codification permits to process polychromatic images in parallel with a monochromatic correlator.

Multichannel single-output color pattern recognition by use of a joint-transform correlator

A novel method for performing color image recognition by the use of the coherent joint-transform correlator is introduced. The input plane of the proposed method is a spatial rearrangement of the separation into color channels of both the color input scene and the color target. This input plane is gray scaled and monochromatic, thus it can be displayed by the use of amplitude spatial light modulators to achieve real-time operation. The system provides a single output-plane result of the optical coherent addition of the separate channels' correlation outputs. At the output plane no electronic postprocessing is needed, and the detection decision is achieved simply by the application of threshold detection. Experimental results and computer simulations are presented to demonstrate the abilities of this system.