Coherent Optical Processing Research Articles

An unsupervised coherent receiver digital signal processing algorithm based on spectral clustering with no data preamble

AbstractA coherent optical receiver digital signal processing (DSP) scheme is proposed based on spectral clustering, which utilises spectral clustering to cluster the signals outputted by the DSP. Compared to existing blind DSP algorithms for coherent optical receivers operating at over 100Gbps with PM‐mQAM, the proposed scheme effectively suppresses various physical impairments, achieving lower bit error rates (BERs) at the same transmission distance, thus enabling longer transmission distances under the same forward error correction threshold. Furthermore, simulations of the proposed scheme on a 14GBaud PM‐16QAM coherent optical transmission system show that, at a BER of 1.9E‐2, the maximum transmission distance increases from 1700 to 2000 km, and at a BER of 3.8E‐3, it increases from 700 to 900 km.

Efficient Superoscillation Measurement for Incoherent Optical Imaging

I propose a superoscillation measurement method for subdiffraction incoherent optical sources, with potential applications in astronomy, remote sensing, fluorescence microscopy, and spectroscopy. The proposal, based on coherent optical processing, can capture all the light on the aperture in principle, perform better than direct imaging on statistical terms, and approach the fundamental quantum limit.

Dynamic power and chirp measurements of a quantum dash semiconductor optical amplifier amplified picosecond pulses using a linear pulse characterization technique

Quantum Dash (QDash) SOAs are of interest as an alternative to quantum dot SOAs, since they have some dot-like properties and are easier to fabricate such as to operate in the 1550 nm region, albeit with longer gain recovery times in the range of hundreds of picoseconds. QDash-SOAs are promising core component is optical subsystems for applications in next generation coherent communications and optical signal processing, which often involve pulse amplification. It is of interest to measure both the dynamic power, phase and associated chirp and thereby the spectrum of typical QDash-SOA amplified pulses. Non-linear measurement techniques are appropriate for pulsewidths less than 5 ps but have low sensitivity for wider pulsewidths often encountered in practice. This paper describes the use of a modified linear pulse characterization technique to measure the dynamic power and phase of the identical pulses of a QDash-SOA amplified 20 GHz repetition rate 19 ps pulsewidth pulse train. The pulse chirp and power from which the pulse chirp and pulse train power spectrum is calculated. The amplified pulse structure is strongly dependent on the amplifier gain and the input pulse shape and energy.

Defect corrections for coherent optical information processing of grayscale images in a DMD-based 4f-system using a collimated light source.

Digital micromirror device (DMD)-based 4f-systems, a type of coherent optical information processing system, have become a powerful tool for optical convolutional neural networks taking advantage of their fast modulation speed and high-resolution capability. However, proper high bit-depth image information processing remains challenging due to the optical diffractions that arise from the binary nature of DMD operation. In this paper, we first characterize the diffraction phenomena that cause irradiance defects, namely the nonlinear grayscale and unintended dark lines. Then to resolve the issues, we propose a DMD operation method and a modified structure of the 4f-system based on blazed diffraction grating theory and numerical calculation of the Rayleigh-Sommerfeld propagation model. As a demonstration, we implement high bit-depth image information processing with an optimized optical 4f-system using DMDs and a collimated coherent light source.

On‐Chip High‐Speed Coherent Optical Signal Detection Based on Photonic Spin‐Hall Effect

AbstractThe use of coherent optical signal processing in long‐distance optical communication systems has dramatically increased data capacity enabling encoding of multiple‐bit information in the phase of a light beam. Direct detection of phase information of a high‐speed modulated light remains challenging and requires an external, local oscillator for referencing, which is expensive for short‐reach optical communications, for example, in data centers. The availability of less‐complex integrated photonics devices for coherent signal detection will alleviate this bottleneck. On the other hand, phase information of coherent, orthogonally polarized light beams can be extracted from their polarization states, and it is, therefore, possible to achieve phase measurements via fast polarization detection. Here, an on‐chip nanodisk device is demonstrated for high‐speed coherent optical signal detection enabled by spin–orbit coupling in Si‐photonics circuitry. In a coherent communication experiment with up to 32 GBaud rate, the high‐speed phase‐shift keying and quadrature amplitude modulation signals detected by a Si nanodisk based polarization measurements at multiple wavelengths in the C‐band are recovered with a bit error rate below the forward error correction threshold. The proposed on‐chip nanodisk device shows promise in high‐speed coherent optical communication applications where phase detection is required at low cost and small footprint.

A novel detection of radio-frequency signal based on an electro-opto-mechanical converter

We study a novel detection of radio frequency signal, which is realized by the coherent microwave-optical conversion and optical processing. An electro-opto-mechanical converter, which enables the bidirectional transduction of the information between microwave and optical domains, is performed to convert the radio frequency signal into optical photons. We demodulate the converted optical signal for the baseband information via homodyne detection. We propose a comprehensive design of radio frequency front end based on the electro-opto-mechanics, and investigate the noise characteristics, which is quantified by the noise figure. The results show that the total noise figure can be significantly suppressed to 3.155 dB. Our scheme can be extended to tunable multichannel multimode radio frequency reception with the latest advances on electro-opto-mechanics. This study opens up prospects for radio frequency sensing with coherent microwave-optical conversion.

CENTRAL AND LOGARITHMIC CENTRAL IMAGE CHORD TRANSFORMATIONS FOR INVARIANT OBJECT RECOGNITION

Pattern descriptors invariant to rotation, scaling, and translation represents an important direction in the elaboration of the real time object recognition systems. In this article, the new kinds of object descriptors based on chord transformation are presented. There are described new methods of image presentation - Central and Logarithmic Central Image Chord Transformations (CICT and LCICT). It is shown that the CICToperation makes it possible to achieve invariance to object rotation. In the case of implementation of the LCICT transformation, invariance to changes in the rotation and scale of the object is achieved. The possibilities of implementing the CICTand LCICToperations are discussed. The algorithms of these operations for contour images are presented. The possibilities of integrated implementation of CICT and LCICT operations are considered. A generalized CICT operation for a full (halftone) image is defined. The structures of the coherent optical processors that implement operations of basic and integral image chord transformations are presented.

A Novel Method of Measuring Instantaneous Frequency of an Ultrafast Frequency Modulated Continuous-Wave Laser.

Ultrafast linear frequency modulated continuous-wave (FMCW) lasers are a special category of CW lasers. The linear FMCW laser is the light source for many sensing applications, especially for light detection and ranging (LiDAR). However, systems for the generation of high quality linear FMCW light are limited and diverse in terms of technical approaches and mechanisms. Due to a lack of characterization methods for linear FMCW lasers, it is difficult to compare and judge the generation systems in the same category. We propose a novel scheme for measuring the mapping relationship between instantaneous frequency and time of a FMCW laser based on a modified coherent optical spectrum analyzer (COSA) and digital signal processing (DSP) method. Our method has the potential to measure the instantaneous frequency of a FMCW laser at an unlimited sweep rate. In this paper, we demonstrate how to use this new method to precisely measure a FMCW laser at a large fast sweep rate of 5000 THz/s by both simulation and experiments. We find experimentally that the uncertainty of this method is less than 100 kHz and can be improved further if a frequency feedback servo system is introduced to stabilize the local CW laser.

Compressive <italic>k</italic>-Space Tomography

The coherent optical processing technique, referred to as k -space tomography, enables simultaneous detection of radio waves’ frequency and angle of arrival. As with the majority of signals, the spatial and spectral distribution of RF signals in the electromagnetic environment is sparse in nature. This inherent characteristic provides an ideal opportunity to merge this imaging modality with compressive sensing techniques. Compressive sensing has set a foundation for data acquisition of sparse signals from a small amount of data. By adapting specific aspects from this methodology, and applying them to k -space tomography, compressive k -space tomography is developed. Fundamental performance limits of this compressive sensing application are defined through numerical simulations, and these responses are subsequently utilized in experimental spatial-spectrum utilization mapping for impending 5G bands between 28 and 40 GHz. As a result, reconstruction data size and processing time are reduced consistently by a factor of over 100, without loss of information, for sufficiently sparse RF scenes.

Fast and cost-effective in-process defect inspection for printed electronics based on coherent optical processing.

This paper presents an all-optical difference engine (AODE) sensor for detecting the defects in printed electronics produced with roll-to-roll processes. The sensor is based on the principle of coherent optical subtraction and is able to achieve high-speed inspection by minimising data post-processing. A self-comparison inspection strategy is introduced to allow defect detection by comparing the printed features and patterns that have the same nominal dimensions. In addition, potential applications of the AODE sensor in an on-the-fly pass-or-reject production control scenario are presented. A prototype AODE sensor using a digital camera is developed and demonstrated by detecting defects on several industrial printed electrical circuitry samples. The camera can be easily replaced by a low-cost photodiode to realise high-speed all-optical information processing and inspection. The developed sensor is capable of inspecting areas of 4 mm width with a resolution of the order of several micrometres, and can be duplicated in parallel to inspect larger areas without significant cost.

Passive Three-Dimensional Spatial-Spectral Analysis Based on k-Space Tomography

A 2D distributed-aperture imaging system paired with a newly developed imaging modality, k-space tomography, is presented for detection of radio waves’ frequency and angle of arrival in azimuthal and elevation directions through coherent optical processing. This technique uses fiber-length dispersion in conjunction with a distributed antenna array to provide unique spatio-temporally encoded charge-coupled device (CCD)-captured interferograms, from which a computational tomographic reconstruction of the RF signal environment at newly allocated 5G bands can be obtained.

Photonic probing of radio waves for k-space tomography.

We harness coherent optical processing to simultaneously sense the angle of arrival and the frequency of radio waves. Signals captured by a distributed antenna array are up-converted to optical domain using electro-optic modulators coupled to individual antennas. Employing a common laser source to feed all the modulators ensures spatially coherent up-conversion of radio-frequency (RF) waves to optical beams carried by optical fibers. Fiber-length dispersion extends the spatial aperture of the distributed antenna array into the temporal dimension. The interference of beams emanating from the fibers is captured by a CCD and used to computationally reconstruct RF waves in k-space.

On the difference between single- and double-sided bandpass filtering of spatial frequencies

It is well-known that bandpass filtering will lead to edge extraction in image processing. However, the difference between single- and double-sided bandpass filtering has never been compared and investigated in the literature. We investigate the difference between single- and double-sided bandpass spatial filtering in a 4-f coherent optical image processing system. We find that single-sided filtering can approximate the operation of a first-order derivative, while double-sided filtering gives a second-order derivative. Simulations and optical experiments confirm our findings.

Analytical Investigations on Carrier Phase Recovery in Dispersion-Unmanaged n-PSK Coherent Optical Communication Systems

Using coherent optical detection and digital signal processing, laser phase noise and equalization enhanced phase noise can be effectively mitigated using the feed-forward and feed-back carrier phase recovery approaches. In this paper, theoretical analyses of feed-back and feed-forward carrier phase recovery methods have been carried out in the long-haul high-speed n-level phase shift keying (n-PSK) optical fiber communication systems, involving a one-tap normalized least-mean-square (LMS) algorithm, a block-wise average algorithm, and a Viterbi-Viterbi algorithm. The analytical expressions for evaluating the estimated carrier phase and for predicting the bit-error-rate (BER) performance (such as the BER floors) have been presented and discussed in the n-PSK coherent optical transmission systems by considering both the laser phase noise and the equalization enhanced phase noise. The results indicate that the Viterbi-Viterbi carrier phase recovery algorithm outperforms the one-tap normalized LMS and the block-wise average algorithms for small phase noise variance (or effective phase noise variance), while the one-tap normalized LMS algorithm shows a better performance than the other two algorithms for large phase noise variance (or effective phase noise variance). In addition, the one-tap normalized LMS algorithm is more sensitive to the level of modulation formats.

Wavefront sensing based on phase contrast theory and coherent optical processing

A novel wavefront sensing method based on phase contrast theory and coherent optical processing is proposed. The wavefront gradient field in the object plane is modulated into intensity distribution in a gang of patterns, making high-density detection available. By applying the method, we have also designed a wavefront sensor. It consists of a classical coherent optical processing system, a CCD detector array, two pieces of orthogonal composite sinusoidal gratings, and a mechanical structure that can perform real-time linear positioning. The simulation results prove and demonstrate the validity of the method and the sensor in high-precision measurement of the wavefront gradient field.

X-y curvature wavefront sensor.

In this Letter, we propose a new curvature wavefront sensor based on the principles of optical differentiation. The theoretically modeled setup consists of a diffractive optical mask placed at the intermediate plane of a classical two-lens coherent optical processor. The resulting image is composed of a number of local derivatives of the entrance pupil function whose proper combination provides the wavefront curvature. In contrast to the common radial curvature sensors, this one is able to provide the x and y wavefront curvature maps simultaneously. The sensor offers other additional advantages like having high spatial resolution, adjustable dynamic range, and not being sensitive to misalignment.

Pattern recognition based on the correlated intensity fluctuations of thermal light.

Here we present a pattern recognition scheme based on the intensity correlation of thermal light. We prove theoretically that under spatially incoherent illumination the matched filtering technique can be realized in the ghost imaging field. Using the matched filtering technique, it is possible to distinguish an object from a preestablished set of objects through their ghost images, which are extracted by means of intensity correlation measurement. According to the pattern recognition scheme, we present a numerical simulation in which we can easily identify the character inserted into the object arm from a set of two characters through the position of the autocorrelation peak. This pattern recognition scheme opens up the possibility of performing coherent optical processing under spatially incoherent illumination.

Spatial light modulator based on liquid-crystal video projector matrix for information processing systems

Experimental results of the optical properties investigation of the liquid crystal video projector matrix are given as well as its application for the input of images into the coherent optical Fourier processor and into the joint Fourier transform correlator. It is shown that such modulators are perspective for the application in the coherent optical processing systems

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).

Partially coherent optical processor for enhancement of partial second order derivatives of an image: Analysis and implementation

We present an optical processor capable to perform partial second derivatives of an image working with partially coherent illumination. The proposed system utilizes a liquid crystal display, onto which images to be derived are displayed. The method is based on the capacity of these devices to generate simultaneously a contrast reverted replica of the image displayed on it. A positive replica of original image is obtained when the LCD is between crossed polarizers, while the negative one is obtained with the LCD is between parallel polarizers. Since the LCD is a diffraction element, both polarizer arrangements may be simultaneously implemented, for example, in a 4 f optical processor using polarization filtering of the different diffraction orders. When three images (two replicas of original image displaced in opposite direction and the other one with contrast reverted and centered at the origin.) are imagined across a slightly defocused plane, one obtains an image with enhanced second derivatives in a given direction. No other incoherent (or partially coherent) optical system is presently known which can perform partial second-order derivatives of an image. Experimental results obtained are presented.