Coherent Receiver Research Articles

Free Space Ground to Satellite Optical Communications Using Kramers-Kronig Transceiver in the Presence of Atmospheric Turbulence.

Coherent detection provides the optimum performance for free space optical (FSO) communication systems. However, such detection systems are expensive and require digital phase noise compensation. In this paper, the transmission performance of long-haul FSO system for ground-to-satellite communication based on a Kramers–Kronig (KK) transceiver is evaluated. KK transceivers utilize inexpensive direct detection receivers and the signal phase is retrieved from the received current using the well-known KK relations. KK transceivers are not sensitive to the laser phase noise and, hence, inexpensive lasers with large linewidths can be used at the transmitter. The transmission performance of coherent and KK transceivers is compared in various scenarios such as satellite-to-ground, satellite-to-satellite, and ground-to-satellite for weak, moderate, and strong turbulence. The results show that the transmission performance of a system based on the KK transceiver is comparable to that based on a coherent transceiver, but at a significantly lower system cost and complexity. It is shown that in the absence of turbulence, the coherent receiver has a ~3 dB performance advantage over the KK receiver. However, in the presence of strong turbulence, this performance advantage becomes negligible.

Multiplication-free timing phase error detector for Nyquist and non-Nyquist optical signals.

A versatile digital coherent receiver capable of handling optical signals with different kinds of pulse shaping schemes (PSSs) is indispensable for future flexible and heterogeneous coherent optical communication networks. Therefore, a low-complexity timing phase error detector (TPED) versatile for all PSSs is of particular interest. In this paper, we propose a TPED suitable for both Nyquist signals with different roll-off factors and non-Nyquist signals. It requires two samples per symbol and involves no multiplications. As far as we know, it has the lowest computation complexity compared with the existing TPEDs used in coherent systems, while incurring no receiver sensitivity penalty. Numerical simulations and experiments are carried out to demonstrate the superior timing performance and PSS versatility of the proposed TPED in both open-loop and closed-loop working conditions. We also implement the novel TPED in a field programmable gate array (FPGA) and verify its real-time clock recovery performance using the 10 Gbaud very low roll-off Nyquist and non-Nyquist quadrature phase shift keying (QPSK) signals.

Laser linewidth characterization via self-homodyne measurement under nearly-coherent conditions.

In this work we propose a novel and efficient characterization scheme for a narrow linewidth laser using a nearly-coherent delayed self-homodyne (NC-DSH) technique. The modulated signal of an analog coherent optics (ACO) transceiver, configured in optical loop-back, and the local oscillator (LO) are mixed after a very short optical path difference (OPD), corresponding to an interferometer operating in its nearly-coherent regime. The phase noise is extracted from a digital signal processing algorithm of carrier phase estimation (CPE), while data is transmitted. The interferometric pattern's E-field power spectral density (PSD) enables the extraction of the OPD and the linewidth of the transceiver's laser source in high accuracy. The proposed technique is demonstrated using a commercial integrated coherent transmitter and receiver optical sub-assembly (IC-TROSA).

Carrier Phase Recovery and Compensation in Analog Signal Processing Based Coherent Receivers

Analog coherent receiver (ACR) is an attractive alternative to ADC+DSP based coherent receiver for reducing the complexity and power consumption in near-future high-capacity coherent data center interconnects (DCIs). Carrier phase recovery and compensation (CPRC), which is one of the major operations in the coherent DCIs, is essential to compensate for the phase and frequency offsets between the lasers and phase noise of lasers. In this work, we present a CPRC architecture for ACR based DCIs. The proposed CPRC approach comprises a dual loop architecture for frequency and phase offset corrections. Detailed design, analysis, effects of non-idealities, and simulation results of the dual loop CPRC are presented. For our analyses, circuit-realizable parameters have been used and the CPRC can be implemented in advanced CMOS/FinFET/SiGe technology nodes. The presented analog CPRC technique, along with lowering the power consumption, reduces the complexity in 200 Gbps/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\lambda$</tex-math></inline-formula> coherent DCIs.

Low-Complexity Chromatic Dispersion Equalization FIR Digital Filter for Coherent Receiver

This paper proposes a novel and efficient low-complexity chromatic dispersion equalizer (CDE) based on finite impulse response (FIR) filter architecture for polarization-multiplexed coherent optical communication systems. The FIR filter coefficients are optimized by weights to reduce the energy leakage caused by the truncation effect, and then quantization is used uniformly to reduce the number of real number additions and real number multiplications by utilizing the diversity of the quantized coefficients. Using Optisystem 15 to build a coherent optical communication system for simulation and experimental demonstration, the results show that after the filter coefficients are optimized by weights. Compared with the time-domain chromatic dispersion equalizer (TD-CDE), the proposed design has a lower bit error rate (BER) and better equalization effect. When the transmission distance is 4000 km and the system quantization stages M = 16, the multiplication operation and addition operations reduce computing resources by 99% and 43%, and the BER only increases by 5%. Compared with frequency-domain chromatic dispersion equalizer (FD-CDE), widely used in long-distance communication, the multiplication operation reduces computing resources by 30%. The proposed method provides a new idea for high-performance CDE in long-distance coherent optical communication systems.

Mode-Group Division Multiplexing: Transmission, Node Architecture, and Provisioning

Few Mode Fibers (FMF) and Space DivisionMultiplexing (SDM) are an attractive solution to offer high capacity in optical networks. Although transmission along FMF presents several issues mainly due to the cross-talk among modes, the use of Multiple Input Multiple Output (MIMO) coherent receivers permits to limit the impact of such physical impairment. However, the complexity of MIMO is not negligible (especially with a large number of modes) and the spatial modes must cover the same path, thus limiting network flexibility, e.g., routing modes along different paths is not admitted. In this paper we exploit the concept of Mode-Group Division Multiplexing (MGDM) and we investigate the network architecture and provisioning supporting MGDM. Modes are divided in groups: the modes within a group are co-routed and received with a reduced-complexity MIMO receiver, while the different groups can be routed along different paths. Different node architectures supporting MGDM are presented taking in account state-of-the-art components and devices, even commercially available on the market. A quality of transmission (QoT) model is also presented accounting to the inter mode group crosstalk, that cannot be neglected in the reach evaluation. QoT is exploited by a proposed connection provisioning scheme for MGDM. Simulations are carried out in metro&#x002F;rural network scenarios with different link span lengths. Simulations show high throughput increase while limiting the complexity of receivers.

The observations of localize ionospheric scintillation structure by FORMOSAT-7/COSMIC-2 Tri-band Beacon network

The preliminary results of FORMOSAT-7/COSMIC-2 beacon data are presented in this paper, and the localized scintillation structure where the radio signals are affected by irregularities is recognized. Irregularities in the ionosphere will affect a radio transmission traveling through it. This study looked at a case of beacon satellite scintillation measurement from three stations in Chung-li (25.136N, 121.539E), Tsao-tun (23.981N, 120.697E), and Che-cheng (25.136N, 121.539E) (22.725N, 120.544E). The open-source coherent beacon receiver may receive beacon signals from beacon satellite systems such as the FORMOSAT-3/COSMIC Tri-band Beacon Transmitter (150, 400, 1066.7 MHz) or the FORMOSAT-7/COSMIC-2 Tri-band Beacon Transmitter (150, 400, 1066.7 MHz) (400, 965, and 2200 MHz). In Taiwan, the three stations were located around 360 km apart in a north–south direction. We can recognize the localize scintillation position where the radio signals are affected by abnormalities by projecting the course of the satellite’s orbit. Because the maximum potential height in the F2 layer is 350 km, the height was previously believed to be 350 km. We apply a project method to approximate the realistic height and length from the 3 stations by a scintillation case near midnight in order to improve on this assumption. As a result, the height of scintillation would be confirmed using the project approach, whether in the F layer or the E layer.

Rayleigh Fading Channel Capacity for Coherent Signaling With Asymmetric Constellations

This paper investigates the channel capacity of coherent reception with antenna diversity in Rayleigh fading using asymmetric constellations, such as nonbinary two-sided amplitude-shift keying (ASK) and quadrature amplitude modulation (QAM) with &#x2265; 8 levels. Due to the asymmetries, the capacity achieving input or a priori probability distributions deviate from uniformity. To perform the investigation, an analytical expression for the mutual information, in terms of an integral over a single variable, between the input (transmitted symbol) and the output (received signal vector comprising signals at receive antennas), is derived. For two-sided ASK, 8-QAM with cross constellation, and 16-QAM, analytical closed form expressions for the high and low signal-to-noise (SNR) approximations, respectively, of the capacity achieving input probability distribution, the capacity, and the relative gain in mutual information (between using the uniform and the capacity achieving input probability distributions) are obtained. Numerical results show that at low SNRs, the use of the capacity achieving nonuniform input probability distribution offers significant advantage over the use of the uniform input probability distribution, while at high SNRs, this advantage is significant only for configurations with one receive antenna. Furthermore, the high and low SNR approximations are shown to be reasonably accurate.

Transparent Optical-THz-Optical Link at 240/192 Gbit/s Over 5/115 m Enabled by Plasmonics

A transparentOptical-subTHz-Optical link providing record-high single line rates of 240 Gbit&#x002F;s and 192 Gbit&#x002F;s on a single optical carrier over distances from 5 to 115 m is demonstrated. Besides a direct mapping of the optical to a 230 GHz subTHz-carrier frequency by means of a uni-traveling carrier (UTC) photodiode, we demonstrate direct conversion of data from the subTHz domain back to the optical domain by a plasmonic modulator. It is shown that the subTHz-to-optical upconversion can even be performed at good quality without any electrical amplifiers. Finally, at the receiver, the local oscillator is employed to directly map the optical signal back to the electrical baseband within a coherent receiver.

Optical Arbitrary Waveform Measurement (OAWM) Using Silicon Photonic Slicing Filters

We demonstrate an optical arbitrary waveform measurement (OAWM) system that exploits a bank of silicon photonic (SiP) frequency-tunable coupled-resonator optical waveguide (CROW) filters for gapless spectral slicing of broadband optical signals. The spectral slices are coherently detected using a frequency comb as a multi-wavelength local oscillator (LO) and stitched together by digital signal processing (DSP). For high-quality signal reconstruction, we have implemented a maximum-ratio combining (MRC) technique based on precise calibration of the complex-valued opto-electronic transfer functions of all detection paths. In a proof-of-concept experiment, we demonstrate the viability of the scheme by implementing a four-channel system that offers an overall detection bandwidth of 140 GHz. Exploiting a femtosecond laser with precisely known pulse shape for calibration along with dynamic amplitude and phase estimation, we reconstruct 100 GBd QPSK, 16QAM and 64QAM optical data signals. The reconstructed signals show improved quality compared to that obtained with a single high-speed intradyne receiver, while the electronic bandwidth requirements of the individual coherent receivers are greatly reduced.

Single-Ended Coherent Receiver

Commercial coherent receivers utilize balanced photodetectors (PDs) with high single-port rejection ratio (SPRR) to mitigate the signal-signal beat interference (SSBI) due to the square-law detection process. As the symbol rates of coherent transponders are increased to 100 Gbaud and beyond, maintaining a high SPRR in a cost-effective manner becomes more and more challenging. One potential approach for solving this problem is to leverage the concept of single-ended coherent receiver (SER) where single-ended PDs are used instead of the balanced PDs. In this case, the resulting SSBI should be mitigated in the digital domain. In this paper, we show that SSBI can be effectively mitigated using various low-complexity techniques, such as the direct filed reconstruction (DFR), clipped iterative SSBI cancellation (CIC) and gradient descent (GD). In addition, we present a self-calibration technique for SERs which can be extended for characterizing the optical-to-electrical (O/E) response of a conventional balanced coherent receiver (BR). Using the developed techniques, we then experimentally demonstrate a 90 Gbaud probabilistically constellation shaped 64-QAM (PCS-64QAM) transmission using a SER, achieving a net data rate of 882 Gb/s over 100 km of standard single mode fiber (SSMF). The sensitivity penalty compared to the BR is below 0.5 dB. We expect that when the symbol rate is increased further, a SER can potentially outperform a BR, especially when applied to cost-sensitive commercial pluggable coherent transceivers.

Enabling S-C-L-Band Systems With Standard C-Band Modulator and Coherent Receiver Using Coherent System Identification and Nonlinear Predistortion

One promising and competitive solution to keep up with the rapid growth in data traffic while at the same time addressing increasing network cost, is the efficient reuse of legacy optical fiber infrastructure. This is highly desirable as deployed single mode fibers represent a valuable asset in the network while new installations would require high additional investments. Multiband (MB) or ultra-wideband (UWB) systems, combined with high symbol rates and higher-order modulation formats, are promising solutions to capitalize the already existing fiber plants. In this contribution, we experimentally demonstrate S-C-L-band reception with 64 GBd dual-polarization (DP) 64-ary and 32-ary quadrature-amplitude modulation (QAM) while using C-band components off-the-shelf (COTS) such as DP-IQ modulators and coherent receivers. To achieve such broadband operation with components that are not optimized for an out-of-band use, mitigation of the associated penalties is decisive. To this end, we apply an end-to-end electro-optical Volterra-based coherent system identification followed by nonlinear digital predistortion of the transmitter. We achieve 150-nm operation bandwidth of the transmission system by performing only a single identification and predistortion at a reference wavelength of 1500 nm.

An Ultraefficient Broadband Photonic Channelizer Based on Polarization-Division Multiplexing and Integrated Dual-Polarization Coherent Detection Receiver

We propose and experimentally demonstrate an ultraefficient broadband photonic channelizer. By polarization-division multiplexing the received signal and using dual-output Hartley image-reject receiving topology, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$4N$ </tex-math></inline-formula> channels can be generated with only ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$N +1$ </tex-math></inline-formula> )-line signal optical frequency comb (OFC) and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$N$ </tex-math></inline-formula> -line local oscillator (LO) OFC, and the frequency interval of the OFC is reduced to half of the typical value. With the use of the integrated dual-polarization coherent detection receiver (DP-CDR), the required quadrature optical hybrid (QOH) and the balanced photodetectors (BPDs) are reduced to half. The system size is reduced and stability is improved. Besides, assisted with a digital signal processor (DSP) algorithm, the IQ imbalance can be compensated, and approximately, ideal image rejection can be realized. A proof-of-concept experiment is carried out. A 2–14-GHz wideband signal is successfully divided into 12 channels with 1-GHz bandwidth by applying four-line signal OFC and three-line LO OFC. The image rejection ratio (IRR), channel isolation (CI), and spurious-free dynamic range (SFDR) are also measured and discussed.

Avalanche Photodiodes With Composite Charge-Layers for Low Dark Current, High-Speed, and High-Power Performance

In this work, a novel In_0.52Al_0.48As based top-illuminated avalanche photodiode (APD) is demonstrated. By combining the composite charge-layer design with a special p-side up etched mesa structure to zero the electric (E)-field at the periphery of this APD&#x0027;s multiplication (M-) layer, the edge breakdown phenomenon can be eliminated. This in turn leads to the simultaneous high-speed, high-saturation-power, high responsivity, and low-dark current performance characteristics of our APDs, which are essential for high-performance coherent receiver applications. The demonstrated device, with its simple top-illuminated structure exhibits a wide optical-to-electrical (O-E) bandwidth (21 GHz), high responsivity (5.5 A&#x002F;W at 0.9 V_br), and saturation current as high as 8 mA with a large active diameter of 24 &#x03BC;m for easy optical alignment. Furthermore, the nonlinear driving of a wavelength sweeping laser in the self-heterodyne beating setup can generate an optical pulse train like waveform, providing an effective optical modulation depth of up to 158&#x0025;, which leads to a maximum photo-generated RF power (at 10 GHz) from our APD as high as &#x002B;5.5 dBm. The excellent performance of our demonstrated APDs opens up new possibilities for the next generation of coherent receivers.

Performance Oriented DSP for Flexible Long Haul Coherent Transmission

In long haul optical fiber communication networks, whichcan span thousands of kilometers, bandwidth is at a premium due to the relatively low availability of optical fibers when compared with network traffic demands. Therefore, these networks require the highest performance in the physical layer, with transceivers that are capable of extracting all the available capacity from each optical fiber. Digital coherent transmitters and receivers, which enhance optical transmission systems by using digital signal processing, are essential for achieving this goal. This tutorial discusses the digital signal processing techniques that are used in the design of high performance coherent modems to compensate for adverse channel effects such as fiber impairments and optoelectronic device non-idealities.

Coherent receivers for fiber optic communications

Coherent receivers for fiber optic communications

Phase Retrieval-Based Coherent Receivers: Signal Design and Degrees of Freedom

Kramers-Kronig (KK) detection exploits properties of minimum-phase, single-sideband signals to recover the phase of a signal from its intensity. It offers many of the benefits of standard coherent detection, while not requiring a local oscillator, quadrature hybrid, or balanced photodiodes. Adding a carrier to a transmitted signal to render it minimum-phase, however, causes a power penalty for KK detection relative to standard coherent detection. We study the mutual information (MI) achieved by KK detection and the impact of signal and system design parameters, including excess bandwidth, carrier-to-signal power ratio, modulation order, probabilistic shaping, and chromatic dispersion, making comparisons to standard coherent detection and standard direct detection. We study the effective degrees of freedom for KK detection, confirming that it tends towards unity at high signal-to-noise ratio (SNR), supporting the classification of KK detection as coherent detection. At low SNR, however, KK detection is outperformed by standard direct detection in terms of MI per symbol. This study focuses on the fundamental performance of the detection methods under comparison, by numerically approximating ideal continuous-time signals and signal processing operations. The results and conclusions reported here will be constrained, in practice, by limitations of practical implementation, including those addressed in the Discussion section.

Vibration Detection and Localization Using Modified Digital Coherent Telecom Transponders

We demonstrate a vibration detection and localization scheme based on bidirectional transmission of telecom signals with digital coherent detection at the receivers. Optical phase is extracted from the digital signal processing blocks of the coherent receiver, from which the vibration component is extracted by bandpass filtering, and the position along the cable closest to the vibration's epicenter is recovered by correlation. We demonstrate our scheme first using offline experiment with 200-Gb/s DP-16QAM, and we report field trial results over installed fiber to detect real-world vibration events.

Two-Dimensional Transmission of Four-Dimensional LDPC-Coded Modulation with Slepian Sequences for DSP-Free 40 km Metro Network Applications.

The growing data demands are pushing researchers to pay more attention to spectrally efficient modulation formats. The four-dimensional (4D) signal constellation modulation format has been investigated for metro networks’ applications to achieve better power efficiency. To cope with such modulation formats, the requirement of better digital signal processing (DSP) is also increasing rapidly. More complicated DSPs bring us extra costs; thus, the DSP-free coherent receivers are also investigated because of the high-power consumption of conventional DSP-based receivers, but the transceivers upgrading also results in extra costs. In this invited paper we implement a 4-dimentional modulation format based on Slepian sequences. We applied LDPC coding and experimentally investigated the BER performance in a two-dimensional (2D) 40 km fiber link transmission and demonstrate that being error free is possible without employing the complicated DSP. We compared our proposed modulation scheme with regular 16QAM and found it outperforms 16QAM with DSP over back-to-back transmission by 3.8 dB improvement in OSNR when BER = 10−5, while over 40 km metro network communication link our proposed 4D modulation signals are still successfully transmitted, and the LDPC-coding still works properly with such a new transmission strategy. On the other hand, DSP-free transmission of LDPC-coded 16-QAM exhibits an early error floor phenomenon.

Microwave photonic link to transmit four microwave vector signals on a single optical carrier based on coherent detection and digital signal processing.

A microwave photonic link to transmit four independent microwave vector signals modulated on a single optical carrier based on coherent detection and digital signal processing (DSP) is proposed and experimentally demonstrated. At the transmitter, a continuous-wave (CW) light wave is modulated by four independent microwave vector signals with an identical center microwave frequency at a dual-polarization dual-drive Mach-Zehnder modulator (DP-DDMZM), to generate four intensity-modulated optical signals, with two signals sharing one of the two orthogonal polarization states. After transmission over a single-mode fiber (SMF), the optical signals are applied to a polarization- and phase- diversity coherent receiver to which a light wave from a second laser source as a local oscillator (LO) is also applied. To eliminate the joint phase noise and the unstable offset frequency from the transmitter and the LO laser sources and to perform polarization demultiplexing, a digital noise cancellation algorithm and a polarization demultiplexing algorithm are developed. The proposed approach is evaluated experimentally. For four independent 16 quadrature amplitude modulation (16-QAM) microwave vector signals at 4 GHz with a symbol rate of 0.5 GSymbol/s, error-free transmission over a 9-km SMF is achieved when the received optical power at the coherent receiver is higher than -21 dBm with forward error correction (FEC).