Coherent Optical Signal 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.

Real-Time Detection of Fiber Bending and/or Optical Filter Shift by Machine-Learning of Tapped Raw Digital Coherent Optical Signals

We have proposed autonomous network diagnosis platform for operation of future large capacity and virtualized network, including 5G and beyond 5G services. As for the one candidate of information collection and analyzing function blocks in the platform, we proposed novel optical sensing techniques that utilized tapped raw signal data acquired from digital coherent optical receivers. The raw signal data is captured before various digital signal processing for demodulation. Therefore, it contains various waveform deformation and/or noise as it experiences through transmission fibers. In this paper, we examined to detect two possible failures in transmission lines including fiber bending and optical filter shift by analyzing the above-mentioned raw signal data with the help of machine learning. For the purpose, we have implemented Docker container applications in White Box Cassini to acquire real-time raw signal data. We generated CNN model for the detections in off-line processing and used them for real-time detections. We have confirmed successful detection of optical fiber bend and/or optical filter shift in real-time with high accuracy. Also, we evaluated their tolerance against ASE noise and invented novel approach to improve detection accuracy. In addition to that, we succeeded to detect them even in the situation of simultaneous occurrence of those failures.

DBN-based online identification of porosity regions during laser welding of aluminum alloys using coherent optical diagnosis

As a typical serious defect during laser welding of aluminum alloys, porosity can significantly impair the performance of the weld seam. Real-time monitoring of porosity defect can provide guidance for feedback adjustment of welding quality to prevent the subsequent continued formation of porosity, which has attracted increasing attention. This paper implements the deep learning-based online identification of porosity regions using coherent optical diagnosis. Considering the formation mechanism of porosity, a monitoring platform including a coherent optical sensor is established to measure the keyhole 3D morphology and extract the keyhole depth as the coherent optical signal. The acquired keyhole depth signal is processed by ensemble empirical mode decomposition, and it is discovered that there is a segmentation threshold in the reconstructed signal to separate the porosity regions and the non-porosity regions. A deep belief network is constructed to complete the reception of the reconstructed keyhole depth signal and the response of the prediction state, realizing the online identification of the porosity regions. The results show that the identification model has high accuracy, and the validity verification demonstrates the reliability of the proposed method.

Ultra-long-distance distribution of low-phase-noise reference lightwave for optical communications.

The remote delivery of optical reference with highly stable oscillation frequency and carrier phase can eliminate the need of digital signal processing for the estimation of these parameters in optical communication. The distribution distance of the optical reference has been limited, however. In this paper, an optical reference distribution over 12,600 km is achieved while maintaining low-noise characteristics, using an ultra-narrow-linewidth laser as a reference source and a fiber Bragg grating filter for noise removal. The distributed optical reference enables 10 GBaud, 5 wavelength-division-multiplexed dual-polarization 64QAM data transmission without using carrier phase estimation, which significantly reduces off-line signal processing time. In the future, this method can enable all coherent optical signals in the network to be synchronized to a common reference ideally, thereby improving overall energy efficiency and cost.

Experimental realization of multimode nonlinear parametric amplification from cascading four-wave mixing of dressed atoms.

The nonlinear parametric process is of great significance for achieving high-quality coherent optical signals and quantum correlated photons. With the development of classical and quantum information processing, the study of the properties of parametric processes is evolving in complex scenarios of multimode, which is limited in conventional nonlinear media due to strict phase matching, e.g. nonlinear crystals. Here we study the dressing-energy-level-cascaded four-wave mixing process to generate multimode optical parametric signals. Via cascading double-Λ type configuration of 85Rb D1 line, the non-degenerate energy-level-cascaded FWM is constructed to generate multimode self-parametric amplification. Moreover, with the dressing effects based on atomic coherence, the spatial and frequency multimode characteristics of energy-level-cascaded FWM parametric amplification, i.e., the modes number and pattern, are actively modulated by the pump fields detuning. Also, the spatial modes from the coupling of two coexisting spontaneous parametric FWMs can be controlled to reach tremendous scalability via the atomic coherence and Kerr non-linearity. The atomic coherence effects and unique phase-matching symmetry nature allow flexible modulation of the multimode property of the generated parametric signals within a nonlinear device, which paves a way for multimode classical and quantum information processing.

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.

Semiconductor optical amplifiers: recent advances and applications

Owing to advances in fabrication technology and device design, semiconductor optical amplifiers (SOAs) are evolving as a promising candidate for future optical coherent communication links. This review article focuses on the fundamentals and broad applications of SOAs, specifically for optical channels with advanced modulation formats, as an integrable broadband amplifier in commercial transponders and as a nonlinear medium for optical signal processing. We discuss the basic functioning of an SOA and distortions of coherent signals when SOAs are used as amplifiers. We first focus on the techniques used for low-distortion amplification of phase-modulated signals using SOAs. Then we discuss optical signal processing techniques enabled by SOAs with an emphasis on all-optical wavelength conversion, optical phase conjugation, and phase quantization of coherent optical signals.

Iterative detection of CO‐OFDM signals with phase errors using circular random variables

SummaryWe propose a low‐complexity graph‐based iterative algorithm for the detection of coherent optical OFDM signals in the presence of residual carrier frequency offset and phase noise. We exploit the circular properties of the frequency offset and phase noise and derive graphical messages using the Gaussian probability density function. The proposed algorithm shows similar error rate performance as the existing algorithm at a much lower implementation complexity.

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.

Separation of interface and substrate carrier dynamics at a heterointerface based on coherent phonons

Transient reflectivity spectroscopy is widely used to study ultrafast carrier- and phonon-dynamics in semiconductors. In their heterostructures, it is often not straightforward to distinguish contributions to the signal from the various layers. In this work, we perform transient reflectivity measurements on lattice-matched GaP/Si(001) using a near infrared pulse, to which GaP is transparent. The pump laser pulse can generate coherent longitudinal optical (LO) phonons both in the GaP overlayer as well as in the Si substrate which have distinct frequencies. This enables us to track the amplitude of the respective signal contributions as a function of GaP layer thickness $d$. The Si phonon amplitude in the signal exhibits an oscillatory behavior with increasing $d$. This can be quantitatively explained by the interference of the probe light reflected at the air/GaP/Si heterointerface. Based on this knowledge, we can then separate the interface- and the substrate-contributions in the carrier-induced non-oscillatory transient reflectivity signal. The obtained interface signal provides evidence for ultrafast carrier injection from the Si substrate into the GaP overlayer. This is also corroborated by examining the deviation of the polarization-dependence of the GaP coherent optical phonon signal from that of the bulk semiconductor.

Hyperfine optical vector analysis of ultra- narrowband optical bandpass filters based on coherent two-tone sweeping and fixed low-frequency detection

A hyperfine optical vector analyzer (OVNA) is proposed and experimentally demonstrated to characterize ultra-narrowband optical bandpass filters (OBPFs). In this scheme, a coherent two-tone optical signal (TTOS) with a small frequency interval is generated to act as a probe light. Through finely sweeping the TTOS across the passband of the OBPF via electro-optic modulation, the magnitude and phase responses of the OBPF under test can be measured with a high signal-to-noise ratio based on fixed low-frequency detection. The frequency response measurement is immune to external disturbance. In the experiment, the magnitude and group delay responses of a fiber-based Fabry-Perot tunable filter (FFP-TF) and a fiber-based Fabry-Perot interferometer (FFP-I) with 3-dB bandwidths of 1.5 GHz and 60 MHz, respectively, are successfully measured. In addition, the measurement uncertainty is theoretically and experimentally analyzed. This method paves a way to characterize ultra-narrowband OBPFs with high out-of-band rejection ratios.

An Ab Initio Multiple Cloning Method for Non-Adiabatic Excited-State Molecular Dynamics in NWChem.

The recently developed ab initio multiple cloning (AIMC) approach based on the multiconfigurational Ehrenfest (MCE) method provides a powerful and accurate way of describing the excited-state dynamics of molecular systems. The AIMC method is a controlled approximation to nonadiabatic dynamics with a particular strength in the proper description of decoherence effects because of the branching of vibrational wavepackets at a level crossing. Here, we report a new implementation of the AIMC algorithm in the open source NWChem computational chemistry program. The framework combines linear-response time-dependent density functional theory with Ehrenfest mean-field theory to determine the equations of motion for classical trajectories. The multidimensional wave function is decomposed into a superposition of Gaussian coherent states guided by Ehrenfest trajectories (i.e., MCE approach), which can clone with fully quantum mechanical amplitudes and phases. By using an efficient time-derivative based nonadiabatic coupling approach within the AIMC method, all observables are calculated on-the-fly in the nonadiabatic molecular dynamics process. As a representative example, we apply our implementation to study the ultrafast photoinduced electronic and vibrational energy transfer in a pyridine molecule. The effects of the cloning procedure on electronic and vibrational coherence, relaxation and unidirectional energy transfer are discussed. This new AIMC implementation provides a high-level nonadiabatic molecular dynamics framework for simulating photoexcited dynamics in complex molecular systems and experimentally relevant ultrafast spectroscopic probes, such as nonlinear coherent optical and X-ray signals.

Generation of faster-than-Nyquist coherent optical DFT-spread OFDM signals with high-baud and high-order modulations

In this paper, we successfully demonstrate the generation and reception of faster-than-Nyquist (FTN) orthogonal frequency-division multiplexing (OFDM) signals with coherent detection. We can improve the spectrum efficiency by using higher order QAMs. However, we believe that the realization of super-Nyquist channel has the following advantages to improve spectral efficiency: Firstly, when we use higher order modulation, the requirements for equipment will be increased, such as the linearity of the modulator. Secondly, flexibility. The signal compression ratio can be arbitrarily set within the maximum compression ratio in our experiment. However, if we increase the modulation order, it can only be an integral multiple of 2, such as 4QAM to 8QAM, which can’t fully and effectively use the channel resources. In the FTN OFDM transmission system, frequency shaping is realized by a delay-and-add filter (DAF) at the transmitter to ensure the energy of signal is concentrated at low frequencies, which will make the OFDM much more robust to strong filtering effect to some extent. We generate a quadrature phase-shift keying OFDM (QPSK-OFDM) signal with equivalent baud rate (In this paper, the generation of FTN signal is in the frequency domain since we used M-point DFT and N-point IDFT while M is greater than N. In other words, the high-frequency parts are discarded to increase spectrum efficiency. Hence, we use “equivalent baud rate” instead of a simple “baud rate”) of 100–120 Gbaud, which is much higher than the sampling rate (80 GSa/s) of digital-to-analog converter (DAC). With this FTN Discrete-Fourier transform spread (DFT-spread) OFDM signal generation technique, we achieve the highest equivalent baud rate (120 GBaud) QPSK-OFDM signal generation and transmission over 80 km single mode fiber (SMF) with bit error rate (BER) under soft-decision forward-error-correction (SD-FEC) limitation of 2.4 × 10−2. We also investigate the generation and reception of 32 Gbaud FTN 16-ary quadrature-amplitude modulation OFDM (16QAM-OFDM) with coherent detection to achieve higher spectrum efficiency in this paper. The experimental results show that there is negligible optical signal to noise ratio (OSNR) penalty when 32 Gbaud FTN DFT-spread 16QAM-OFDM is compressed and transmitted within 28 GHz bandwidth with the BER at 2.4 × 10−2 in optical back to back (OBTB).

All-optical carrier recovery for self-homodyne detection via injection locked Brillouin laser in whispering-gallery-mode microcavity

We demonstrate comprehensive investigation of the injection locking dynamics of a backscattered Brillouin laser in silica whispering-gallery-mode microcavity. Via injection locking, the Brillouin laser acquires highly correlated phase with the seed laser, enabling ultra-narrow bandwidth, high gain, and coherent optical amplification. Also, for the first time, to the best of our knowledge, the injection locked Brillouin laser is utilized to implement all-optical carrier recovery from coherent optical data signals. We show that by using the injection locked Brillouin laser as a local oscillator for self-homodyne detection, high-quality data receiving can be realized, even without traditional electrical compensations for carrier frequency and phase drifts.

Noise processing of superconducting kinetic inductance single photon detector

Noise is one of the main factors affecting the performance index of weak signal detection devices, and the optimal filtering algorithm is an effective method to adaptively extract various useful weak signals from the white noise background. In order to improve the performance of single photon detector (especially the photon number resolution ability), one mainly focuses on the optimization of detector hardware such as the optimization of photosensitive materials and the technology of device fabrication. However, in this paper the performance of microwave kinetic Inductance detector (MKID) in the way of data processing is improved. Considering the fact that the template of light pulse signal in the optimal filtering algorithm is obtained by taking the average, we replace the noise model in the original optimal filtering algorithm with the white noise model and the whitening noise model. Then we process the photon response data that are detected by the MKID in an extremely low temperature environment. The results show that the energy resolution (one of the main performance indexes of single photon detector) of MKID is improved by about 15%, and we achieve an infrared single photon energy resolution of 0.26 eV. In this paper, the application and development trends of superconducting single photon detector are briefed. Then, how the MKID responds to weak coherent optical signal in low temperature environment, and the process of signal conversion, acquisition and output are explained in detail. According to the optimal filtering algorithm, we use different noise models to analyze the results of the signals detected by MKID. After that, we count the optimal amplitude multiple, perform the Gaussian fitting analysis on the statistical graph, and compare the energy resolution with the photon number resolution of the optimal filtering algorithm under different noise models. As a result, we find that under the white noise model, the optimal filtering algorithm is used to obtain the best result for MKID processing, and high energy resolution can be achieved.

All-Fiber Hyperparametric Generation Based on a Monolithic Fiber Fabry–Pérot Microresonator

Hyperparametric oscillation is essential for coherent optical signal generation in a broad wavelength range. Integration in a compact system, such a broadband light source, is of special interest for practical applications requiring field-deployable spectroscopy devices. Here we demonstrate an all-fiber hyperparametric oscillation source based on four-wave mixing in a high-Q fiber Fabry–Pérot (FFP) microresonator. Assisted by the Raman effect, the generated optical signal spans over 400 nm with fine line-to-line spacing of 667 MHz. The compatibility of this FFP microresonator enables a robust and reliable all-fiber system through a splicing technique and fiber connectors. Such a plug-and-play platform is convenient and efficient for broad applications in optical communications and spectroscopy.

Annihilation Dynamics of Molecular Excitons Measured at a Single Perturbative Excitation Energy.

Exciton-exciton annihilation (EEA) is a ubiquitous phenomenon, which may limit the efficiency of photovoltaic devices. Conventional methods of determining EEA time scales rely on measuring the intensity dependence of third-order signals. In this work, we directly extract the annihilation rate of molecular excitons in a covalently joined molecular trimer without the need to perform and analyze intensity dependent data by employing fifth-order coherent optical spectroscopy signals emitted into ±2k⃗1 ∓ 2k⃗2 + k⃗3 phase matching directions. Measured two-dimensional line shapes and their time traces are analyzed in the framework of the many-body version of the Frenkel exciton model, extended to incorporate annihilation dynamics. Combining double-sided Feynman diagrams with explicit simulations of the fifth-order response, we identify a single peak as a direct reporter of EEA. We retrieve an annihilation time of 30 fs for the investigated squaraine trimer.

Common-clock very long baseline interferometry using a coherent optical fiber link

Among the most powerful techniques for the exploration of the Universe is very long baseline interferometry (VLBI), which is based on the simultaneous observation of radio sources in the sky with arrays of distant ground-based antennas. One of the effects currently limiting its ultimate sensitivity is the phase-instability of the reference clocks adopted at each antenna. This term can be made negligible delivering the same clock signal to multiple telescope sites using optical fibers. We realized such an infrastructure by disseminating a coherent optical frequency signal to two distant radio telescopes using a 1739-km-long fiber. We performed a 24 h geodetic VLBI campaign in which the same clock reference was used at both telescopes and analyzed it using standard VLBI procedures. The results were consistent with the expectations, confirming that the proposed approach is feasible and configures as a novel tool for studying the role of clocks, troposphere, and systematic effects in the ultimate VLBI resolution.

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.

Remote broadband RF signal down-conversion with stable phase and high efficiency using a sideband optical phase-locked loop.

We propose a phase-stable high-efficiency down-conversion approach for broadband radio frequency signals transmitted from a remote site. A high power coherent optical local oscillator signal is used at the local site to increase the conversion gain and the spurious-free dynamic range (SFDR) at the same time. A sideband optical phase-locked loop ensures the suppression of the phase noise induced by the fiber transmission and the relative frequency drift of the remote and local lasers, which are essential for the signal transmission and the down-conversion. We first experimentally demonstrate the down-conversion of a single frequency signal at 16.45 GHz to a 250 MHz intermediate frequency (IF) signal with 3 dB gain and 103 dB/Hz2/3 SFDR after 10 km fiber transmission. Then we show the broadband down-conversion capability by down-converting a 1 GHz wide linear frequency modulated pulse signal centered at 11 GHz to 1 GHz with 3 dB gain. Along with a positive gain, the SFDR of the IF signal down-converted from 5 GHz to 40 GHz has reached 97.6 dB/Hz2/3 on average. This approach is suitable for weak broadband remote signal down-conversion with a simple-structured remote end.