Coherent Optical Receivers Research Articles

IQ imbalance compensation based on simplified GSOP and FPGA implementation in optical coherent QPSK receiver

Abstract In this paper, a simplified Gram-Schmidt orthogonalization procedure (SG) method was proposed for in-phase/quadrature (IQ) imbalance compensation based on the Gram-Schmidt orthogonalization procedure (GSOP) algorithm. The proposed method can compensate IQ imbalance in the optical coherent system by two steps of phase orthogonalization and amplitude balance. In addition, this method avoids square root operation compared to the GSOP in the field programmable gate array (FPGA) hardware implementation, therefore it is easier to implement in terms of resources and complexity. Two steady states are achieved by accumulating scale factor to reduce resource consumption in FPGA hardware implementation of the proposed scheme. Finally, a FPGA design with 32 parallel units is performed to demonstrate the proposed method in a polarization multiplexed quadrature phase shift keying (PM-QPSK) system, the results show the proposed algorithm simplifies FPGA implementation complexity and maintains compensation performance comparable to GSOP algorithms.

Dual Polarization Full-Field Signal Waveform Reconstruction Using Intensity Only Measurements for Coherent Communications

Conventional optical coherent receivers capture the full electrical field, including amplitude and phase, of a signal waveform by measuring its interference against a stable continuous-wave local oscillator (LO). In optical coherent communications, powerful digital signal processing (DSP) techniques operating on the full electrical field can effectively undo transmission impairments such as chromatic dispersion (CD), and polarization mode dispersion (PMD). Simpler direct detection techniques do not have access to the full electrical field and therefore lack the ability to compensate for these impairments. We present a full-field measurement technique using only direct detection that does not require any beating with a strong carrier LO. Rather, phase retrieval algorithms based on alternating projections that makes use of dispersive elements are discussed, allowing to recover the optical phase from intensity-only measurements. In this demonstration, the phase retrieval algorithm is a modified Gerchberg Saxton (GS) algorithm that achieves a simulated optical signal-to-noise ratio (OSNR) penalty of less than 4dB compared to theory at a bit-error rate of 2 times 10-2. Based on the proposed phase retrieval scheme, we experimentally demonstrate signal detection and subsequent standard 2x2 multiple-input-multiple-output (MIMO) equalization of a polarization-multiplexed 30-Gbaud QPSK transmitted over a 520-km standard single-mode fiber (SMF) span.

VLSI Implementations of Carrier Phase Recovery Algorithms for M-QAM Fiber-Optic Systems

We present circuit implementations of blind phase search (BPS) carrier phase recovery (CPR) for M-QAM coherent optical receivers and highlight some BPS algorithm modifications necessary to obtain efficient VLSI circuits. In addition, we show how three key design parameters (input word length, number of test phases, and type and size of averaging window) affect the resulting implementation. To study design tradeoffs, we develop BPS CPR circuit netlists for a 32-GBaud system, using a 22-nm CMOS process technology: our implementations reach energy efficiencies of around 1 pJ/bit for 16QAM up to 3 pJ/bit for 256QAM, at an SNR penalty of approximately 0.25 dB at a BER of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> . Furthermore, we present a circuit implementation of pilot-symbol-aided CPR, reaching 0.38 pJ/bit and 0.34 pJ/bit for 16QAM and 256QAM, respectively, at a slightly higher SNR penalty. The two CPR methods are also evaluated in terms of silicon area and scaling to higher-order modulation formats.

Time and Carrier Frequency Synchronization for Coherent Optical Communication: Implementation Considerations, Measurements, and Analysis

We develop a measurement scheme for the time and frequency offsets, along with their compensation, for a coherent optical receiver. This relies on a modified constant amplitude zero autocorrelation sequence-based method, which is implemented on a field-programmable gate array. Experiments are performed for an optical orthogonal frequency-division multiplexing transmission system. The performance of the proposed scheme is compared to a MATLAB-based implementation of the original scheme. The contributory factors to the error between these two schemes, as well as the implementation complexity, are investigated. Experimental measurements demonstrate that the proposed scheme closely matches the performance of the original scheme while utilizing fewer hardware resources.

Toward Practical Digital Phase Alignment for Coherent Beam Combining in Multi-Aperture Free Space Coherent Optical Receivers

In free space optical communication systems, multi-aperture coherent optical receivers based on digital coherent beam combining (D-CBC) technique can provide exceptionally high sensitivity and are more robust to the atmosphere turbulence compared with the single aperture receivers with the same collection area. D-CBC relies on the digital phase alignment algorithm (PAA) to align the different versions of signals in phase. However, due to the limited working frequency and space of the digital signal processing (DSP) circuits, the main obstacle to realizing real-time phase alignment of multiple high-speed optical signals is the computation complexity. Therefore, we need to minimize the computation complexity while guaranteeing a satisfactory performance. In this paper, we investigate the relationship between the computation complexity and the combining loss (CL) for both maximum ratio combining (MRC) and equal gain combining (EGC) based D-CBC. Universal analytical expressions are deduced that allow easy minimization of the computation complexity for both MRC and EGC based receivers according to the prescribed CL and input optical signal-to-noise ratios (OSNRs). The analytical expressions are validated by extensive numerical simulations. It is demonstrated that the computation complexity is mainly determined by the quality of the signal with a larger OSNR in MRC, while it is determined by the overall quality of the signals in EGC. When EGC is replaced with MRC, the computation complexity can be reduced by more than 55% at the same CL when the OSNR difference between the signals to be combined is above 10dB. The maximum computation complexity increases exponentially with decreasing input OSNR lower limit and the smaller the CL, the steeper the slope. Furthermore, when the prescribed CL is relaxed from 0.1 to 0.5dB, the maximum computation complexity can be reduced by about 80%. The results provide useful guidelines toward practical phase alignment on a real-time platform.

Face-to-Face EML Transceiver Tandem for Full-Duplex Analogue Radio-over-Air

An analogue coherent-optical free-space link for local cloud-based radio access networks is proposed and experimentally demonstrated. The adoption of a single externally modulated laser as optical transmitter and coherent homodyne receiver at either link end guarantees conceptual simplicity for the opto-electronic sub-systems of this optical fronthaul. The realization of full-duplex signal transmission over a single, laser-based transceiver device further allows to off-load the directional split in the optically fronthauled radio signal chain to the radio-frequency domain. We prove this concept for an in-door link and show that bidirectional transmission of orthogonally frequency division multiplexed radio signals with 64-point quadrature amplitude modulated formats is possible over an optical budget of 21.3 dB, or over an estimated reach of ∼100 m in case of bad atmospheric conditions. A small penalty of less than 1% in terms of error vector magnitude compared to a direct-detection receiver confirms the correct operation of the low-complexity coherent homodyne detector, even though no digital signal processing functions are applied for the purpose of signal recovery. Continuous long-term measurements including Ethernet payloads confirm the stability of the free-space optical link architecture, for which a small fraction of less than 1% has shown an excursion in reception penalty.

Digital-analog hybrid optical phase-lock loop for optical quadrature phase-shift keying

We analyze a feasible high-sensitivity homodyne coherent optical receiver for demodulating optical quadrature phase-shift keying (QPSK). A fourth-power phase-lock loop based on a digital look-up table is used. Considering the non-negligible loop delay, we optimize the loop natural frequency. Without error correction coding, a sensitivity of −37 dBm/−35 dBm is achieved, while the bit error rate is below 10−9 at 2.5 Gbaud/5 Gbaud rate. For the QPSK communication system, the bit rate is twice the baud rate. The loop natural frequency is 0.647 Mrad/s, and the minimized steady-state phase-error standard deviation is 3.83°.

A 42-GHz TIA in 28-nm CMOS With Less Than 1.8% THD for Optical Coherent Receivers

This letter shows a fully differential linear transimpedance amplifier designed for the emerging coherent optical communications for high data rate transmissions. The purpose of this letter is to present a possible solution in 28-nm CMOS technology instead of the most frequently used BiCMOS. The idea is to take advantage of the linearity of CMOS to partially offset its inherently lower transconductance over current ratio, which typically limits the bandwidth. The proposed TIA presents a −3-dB bandwidth equal to 42 GHz and its transimpedance gain can be programmable from 78 to 36 dB $\Omega $ with four variable gain stages. At maximum gain, the measured average input noise is equal to 18 pA/√Hz whereas the maximum total harmonic distortion for an output voltage swing of 500-mV peak-to-peak differential is always lower than 1.8% over the entire gain range. The complete transimpedance amplifier including the bias and dc offset cancellation circuits consumes 319 mW from a 2.4-V supply voltage.

FMCW Lidar Using Phase-Diversity Coherent Detection to Avoid Signal Aliasing

A frequency-modulated continuous-wave (FMCW) lidar is proposed to avoid signal aliasing in the measurement of distance and velocity. In the transmitter, a lightwave is amplitude-modulated in a Mach-Zehnder modulator by a linear-frequency modulated (LFM) waveform. The modulated light is used as a probe signal. The reflected optical signal beats with a local optical (LO) signal in a phase-diversity coherent optical receiver. The in-phase and quadrature components of the output signal are simultaneously recorded. The distance is extracted from the sum of the squares of the two components, while the velocity is calculated from the ratio of them. An experiment is carried out, in which the real-time distance and velocity of a spinning disk are measured. The proposed method can be used to solve the problems of signal aliasing which may lead to the ambiguity of distance and velocity measurements in traditional FMCW lidars. Moreover, the impacts of optical phase fluctuations are removed from the distance measurement in the proposed method.

Adaptive Filtering in Optical Coherent Flexible Bit-Rate Receivers in the Presence of State-of-Polarization Transients and Colored Noise

In this article, we analyze the performance of adaptive filtering in the context of dual-polarization coherent optical flexible bit-rate transceivers. We investigate the ability of different adaptive algorithms to track fast state-of-polarization (SOP) transients in the presence of colored noise. Colored noise exists due to the concatenation of Wavelength Selective Switches (WSSs) and polarization dependent loss (PDL) which can be considered as spatially dependent noise. We consider the use of different modulation formats, and the practical limitation of error signal feedback delay in decision-directed adaptive filters is also taken into account. The back-to-back required signal-to-noise ratio (RSNR) penalty that can be tolerated determines the maximum SOP rate of change that can be tracked by the adaptive filters as well as the filter’s adaptive step size. We show that the recursive least squares algorithm, using the covariance matrix as an aggressive “step size,” has a much better convergence speed compared to the least mean squares (LMS) and normalized LMS (NLMS) algorithms in the presence of colored noise in the fiber. However, the three algorithms have similar tracking capabilities in the absence of colored noise.

BER Performance Analysis of an Orthogonal FDM Free Space Optical Communication System with Homodyne Optical Receiver over Turbulent Atmospheric Channel

Abstract Atmospheric turbulence induced fading may severely impair free-space optical (FSO) communication systems, affecting the quality of the propagated laser beam and lead to significant performance degradation. Recent research works reveal that performance can be improved by using orthogonal FDM. In this paper, an analytical approach is presented to evaluate the bit error rate performance of an OFDM FSO link with optical intensity modulation and coherent homodyne receiver taking into consideration the effect of strong atmospheric turbulence. The turbulence effect is modeled as gamma-gamma distribution and the performance results are evaluated in terms of average CNR and BER. Power penalty suffered by the system due to the effect of turbulence at a given BER is evaluated for several system parameters viz. link distance, turbulence parameter, local oscillator power etc. It is noticed that effect of turbulence can be significantly reduced by increasing the number of OFDM subcarrier. For example, power penalty for BER of 10–9 at a link distance of 3,600 m is 6 dB when number of subcarrier is 4 and can be reduced to 0.5 dB by increasing the number of subcarrier to 64. In addition, by utilizing coherent optical receiver and synchronous demodulation at the receiving end, we have introduced local oscillator (LO) for both Optical and RF state which have ability to track the signal’s phase changes over time relative to the LO’s phase and helps the system to remain stable.

A Low-Power Low-Distortion 20-GS/s Flash Analog-to-Digital Converter for Coherent Optical Receiver in 0.13-μm SiGe BiCMOS

High-speed, low-power analog-to-digital converter (ADC) is a critical element for 5-GBd, 20-Gb/s digital signal processing (DSP)-based coherent optical communication receiver. To satisfy high data transmission rate requirement of optical receiver, a single-core and open-loop flash ADC with a new proposed multiplexer-based encoder is presented in this paper. Compared with conventional encoder topology, the new proposed topology achieves the fastest encoding speed and lowest power consumption. The optimized distortion is achieved by utilizing a leakage current compensation technique and a local negative feedback method in switched-buffer track-and-hold amplifier (THA). Strict synchronization is obtained for clock signals by careful designing of layout in tree-based clock networks. Furthermore, a master–slave comparator incorporated with a preamplifier reduces signal-dependent kickback noise as well as offset voltage. By using master–slave comparators and proposed encoders, the sampling rate is up to 21.12[Formula: see text]GS/s. The 4-bit, 20-GS/s flash ADC is realized in 0.13-[Formula: see text]m SiGe BiCMOS technology and it only occupies 1.05[Formula: see text]mm[Formula: see text][Formula: see text][Formula: see text]1.46[Formula: see text]mm chip area. With a power consumption of 1.831[Formula: see text]W from 4-V supply, the ADC achieves an effective number of bits (ENOB) of 2.5 at 15[Formula: see text]GS/s.

A Study of Power Distributions in Photonic Lantern for Coherent Optical Receiver

We study the power distributions at single-mode fiber end of the non-mode-selective photonic lantern for the coherent optical receiver using multivariate Gaussian distribution and uniform distribution. Equal-gain combining is used to combine the electrical signals at the outputs of the balanced photodetectors. The signal-to-noise ratio is studied for the coherent optical receiver. It is found that, when the number of single-mode fibers of the photonic lantern is equal to the number of guided modes at multimode end of the photonic lantern, the average signal-to-noise ratio attains its maximum value.

Joint Polarization Tracking and Equalization in Real-Time Coherent Optical Receiver

We experimentally demonstrate a real-time polarization tracking and equalization method based on the state of polarization detection from the adaptive finite impulse response (FIR) filter coefficients in a polarization division multiplexed (PDM) coherent optical communication system. By calculating the variation of the polarization within a time interval, the polarization rotation speed can be evaluated. In a field-programmable gate array (FPGA), a feedback structure for polarization rotation speed detection and FIR coefficients update can be built up. In this feedback structure, the update coefficient of the filter will be adjusted to an optimal value with the change of detection result. The experimental results show that the establishment of this feedback structure improves the polarization tracking ability in the real-time coherent optical receiver.

Highly efficient passive InP polarization rotator-splitter.

Monolithically integrated polarization beam splitters (PBSs) are needed to reduce the form-factor and assembly cost of optical coherent receivers. A highly efficient passive polarization rotator and splitter based on mode-evolution is demonstrated. The device is fabricated on InP substrate with a single etch-step and uses an adiabatic mode-converter and an asymmetric Y-coupler. Despite its simple fabrication process, the device shows a polarization extinction ratio (PER) better than 19 dB over 1520 nm to 1620 nm, thus covering both C- and L-band. The peak value of 24 dB is obtained for TE and TM polarizations. Its fabrication tolerance is large, so that even under a width variation of +/- 200 nm the PER remains above 17 dB over the entire C- and L-band.

Binarized Coherent Optical Receiver Based on Opto-Electronic Neural Network

The resolution of analog to digital converter (ADC) and the power consumption of digital signal processing (DSP) are becoming the bottleneck of optical communication system. To acquire and recover signals with low resolution ADCs, we propose a binarized coherent optical receiver based on opto-electronic neural network, which consists of three layers, the broadcast layer, the optical neural layer, and the electronic neural layer. The broadcast layer that composed of Mach–Zehnder interferometers (MZIs) is to duplicate and select the input optical signal into the opto-electronic neural network. The optical neural layer is constituted by specially designed structures to map the binarized weights into optical domain, which can be integrated on silicon together with the broadcast layer. The one-bit vertical resolution ADCs act as the binarized activation after the optical neural layer. This binarized coherent optical receiver is demonstrated in simulation of 50-Gb/s single polarization (SP) QPSK system and 100-Gb/s polarization multiplexing (PDM) QPSK system. The one-bit vertical resolution ADCs can be utilized to recover the complex modulation format signal successfully. The broadcast layer and the optical neural layer can be integrated based on silicon, which could increase the calculation speed and reduce the energy consumption of the total system.

Adaptive Compensation of Bandwidth Narrowing Effect for Coherent In-Phase Quadrature Transponder through Finite Impulse Response Filter

Coherent in-phase quadrature (IQ) transponders are ubiquitous in the long-haul and the metro optical networks. During the transmission, the coherent signal experiences a bandwidth narrowing effect after passing through multiple reconfigurable optical add-drop multiplexers (ROADMs). The coherent signal also experiences a bandwidth narrowing effect when electrical or optical components of the coherent IQ transponder experience aging. A dynamic method to compensate the bandwidth narrowing effect is thus required. In the coherent optical receiver, signal bandwidth is estimated from the raw analog-to-digital converter (ADC) outputs. By adaptively adjusting the tap coefficients of the finite impulse response (FIR) filter, simple post-ADC FIR filters can increase the resiliency of the coherent signal to the bandwidth narrowing effect. The influence of chromatic dispersion, polarization mode dispersion, and polarization dependent loss are studied comprehensively. Furthermore, the bandwidth information of the transmitted analog signal is fed back to the coherent optical transmitter for signal optimization, and the transmitter-side FIR filter thus changes accordingly.

Novel Photodetector Chip for Polarization Diverse Detection

A new photodetector chip for polarization diverse detection is presented. The presented chip allows for smaller footprints and design simplifications by utilizing a waveguide-integrated multiple-quantum well photodiode (MQW-PD) serially coupled to a bulk photodiode (PD) in InP technology. The MQW-PDs with their large polarization-dependent responsivities (PDR) detect the transverse electric (TE) polarized light, while the transverse magnetic (TM) polarized light is predominantly absorbed in the succeeding bulk detector with much lower PDR. Especially polarization diverse coherent optical receivers (PD-CRx) will benefit from this monolithic photodetector concept because neither polarization beam splitters nor rotators are needed. The separated detection of TE- and TM-polarized light will improve the fabrication tolerances and will reduce the footprint of a conventional, fully integrated PD-CRx to approximately 25%. The external TE and TM responsivities of the serially connected MQW- and bulk-PDs were measured up to 0.65 A/W and 0.45 A/W, respectively. Detection of 2 × 25 GBaud for dual polarization signals with a bit error rate of 10−6 is demonstrated without any post-compensation algorithms.

34-GBd Linear Transimpedance Amplifier for 200-Gb/s DP-16-QAM Optical Coherent Receivers

High spectral efficiency offered by the coherent optical communication links makes them attractive for the next-generation optical communication links. Using advanced modulation schemes such as dual-polarization quadrature-amplitude modulation (DP-QAM) data rates beyond 200 Gb/s can be achieved. A key component of such links is the wide-bandwidth and high-linearity coherent optical receiver. In this paper, we present a fully differential (FD) optical receiver architecture consisting of a variable-gain transimpedance amplifier (VG-TIA) followed by a VG amplifier (VGA). The proposed optical receiver employs a dual-feedback automatic gain control (AGC) loop that controls both the front-end VG-TIA and the following VGA to achieve both low-noise and high-linearity operation. A new photodiode (PD) dc current cancellation scheme is developed and implemented for the full differential front-end TIA. A prototype dual-TIA chip is fabricated in a 0.13- $\mu \text{m}$ SiGe BiCMOS process. The presented TIA achieves 20-pA/ $\sqrt {\text {Hz}}$ input-referred noise (IRN) density, 27-GHz, 3-dB bandwidth, and 1.5% total harmonic distortion (THD) at 1-mApp input PD current and 500-mVpp output voltage swing. This enables the 34-GBd operation with the bit error rate (BER) of $10^{-10}$ and $5.4\times 10^{-4}$ using DP-QPSK and DP-16-QAM formats at optical signal-to-noise ratios (OSNRs) of 25 and 30 dB, respectively, demonstrating the 100- and 200-Gb/s single wavelength optical coherent receiver operation. The dual-TIA chip occupies an area of $1.4\,\,\text {mm}\times 1.6$ mm and consumes 313 mW per channel at 34 GBd from a 3.3-V supply.

Green photonics integrated circuit for NGOA coherent receivers

The recent advances in coherent optical systems enable the use of efficient modulation formats, such as quadrature phase shift keying (QPSK). In this framework, we present an integrated 90° hybrid 2 × 4 coupler based on self-patternable organic–inorganic hybrid thin films fabricated using direct laser writing. The device was coupled to balanced photodiodes, resulting in an optical coherent receiver with capacity to extract the modulated data. The device performance was demonstrated for a transmission over 40 km (G.652 fiber) with a 20 Gb/s QPSK, yielding a 2.5 dB power penalty (BER = 3.8 × 10−3), being the best figures of merit for waveguides processed by low-cost patterning techniques. Furthermore, the hybrid materials are thermally and mechanically stable than polymers, with lower fiber-to-device insertion losses. Therefore, the proposed optical coherent receiver is an easy processed and cost-effective approach for the next generation of optical access networks.