Coherent Optical Links Research Articles

Demonstration of an 8-Gbit/s quadrature-phase-shift-keying coherent underwater wireless optical communication link using coherent heterodyne detection under scattering conditions.

In this paper, we experimentally demonstrate an 8-Gbit/s quadrature-phase-shift-keying (QPSK) coherent underwater wireless optical communication (UWOC) link under scattering conditions at 532 nm. At the transmitter, we generate the 532-nm QPSK signal using second-harmonic generation (SHG), where the 1064-nm signal modulated with four phase levels of an 8-phase-shift-keying (8-PSK) format is phase doubled to produce the 532-nm QPSK signal. To enhance the receiver sensitivity, we utilize a local oscillator (LO) at the receiver from an independent laser source. The received QPSK data beam is mixed with the independent LO for coherent heterodyne detection. Results show that the bit error rates (BERs) of the received QPSK signal can reach below the 7% forward error correction (FEC) limit under turbid water with attenuation lengths (γL) up to 7.4 and 6.1 for 2- and 8-Gbit/s QPSK, respectively. The corresponding receiver sensitivities are -34.0 and -28.4 dBm for 2- and 8-Gbit/s QPSK, respectively.

Optically synchronized unidirectional optical amplifier-based coherent optical fiber links.

We report on the realization of a unidirectional transmission-based bidirectional erbium-doped fiber amplifier (UTB-EDFA) for the coherent optical fiber links. By applying an optical phase-locked loop (OPLL) between the two unidirectional EDFA (Ui-EDFA) paths, the annoying uncorrelated phase noise between the two paths can be largely suppressed. Promisingly, we can independently optimize the gains of the UTB-EDFAs for bidirectional transmissions, resulting in higher net gain acquired compared with the conventional single-path bidirectional EDFA (SPBA)-based ones. We demonstrate that the fractional frequency instability of the UTB-EDFA-based scheme can be decreased by 26.3% over the most asymmetrical 100 km two-way optical frequency comparison (TWC) system compared with the SPBA-based ones and, more importantly, can acquire higher net gain for unevenly distributed sub-links over ultra-long fiber links, such as 1000 km, by independently optimizing the gains. This technique paves the way for the applications of large-scale fiber networks.

Photon statistics and quantum phase distribution for coherent optical links in the presence of phase noise from both the laser source and the RF oscillator, and dispersion

A model of photon statistics and quantum phase distribution is analyzed for coherent optical communication links in the presence of quantum phase noise, dispersion and phase noise of the radio-frequency (RF) oscillator using an in-line optical amplifier. The quantum analysis of a dual drive Mach–Zehnder modulator and the quantum transfer function for an optical field mode across an in-line amplifier is performed by taking into account both the quantum and the classical phase noises. The resulting photon statistics are compared with the semiclassical model. We analyze the quantum phase distribution by considering the Wigner quasiprobability distribution for a coherent optical link. Here, we investigate the broadening of the quantum phase distribution due to quantum phase noise of the laser, the dispersive optical fiber, the phase noise of the RF oscillator and the inevitable noise coefficient of the optical amplifier for arbitrary amplified spontaneous emission (ASE) field modes. We analytically investigate the quantum analog to the bit error rate (BER) as a function of the laser’s linewidth, the linewidth of the RF oscillator and chromatic dispersion. The results of quantum models are compared with the semiclassical model. Quantum treatment of both the BER and variance is found to agree with prior results for a single-mode ASE field in the absence of phase noise but deviate when the effect of phase noise is considered. Including phase noise effects in the amplitude in the semiclassical treatment of the BER yields a new semiclassical expression that agrees with quantum expressions of machine precision for single-mode ASE fields.

Automatic turbulence mitigation for coherent free-space optical links using crystal-based phase conjugation and fiber-coupled data modulation.

There are various performance advantages when using temporal phase-based data encoding and coherent detection with a local oscillator (LO) in free-space optical (FSO) links. However, atmospheric turbulence can cause power coupling from the Gaussian mode of the data beam to higher-order modes, resulting in significantly degraded mixing efficiency between the data beam and a Gaussian LO. Photorefractive crystal-based self-pumped phase conjugation has been previously demonstrated to "automatically" mitigate turbulence with limited-rate free-space-coupled data modulation (e.g., <1 Mbit/s). Here, we demonstrate automatic turbulence mitigation in a 2-Gbit/s quadrature-phase-shift-keying (QPSK) coherent FSO link using degenerate four-wave-mixing (DFWM)-based phase conjugation and fiber-coupled data modulation. Specifically, we counter-propagate a Gaussian probe from the receiver (Rx) to the transmitter (Tx) through turbulence. At the Tx, we generate a Gaussian beam carrying QPSK data by a fiber-coupled phase modulator. Subsequently, we create a phase conjugate data beam through a photorefractive crystal-based DFWM involving the Gaussian data beam, the turbulence-distorted probe, and a spatially filtered Gaussian copy of the probe beam. Finally, the phase conjugate beam is transmitted back to the Rx for turbulence mitigation. Compared to a coherent FSO link without mitigation, our approach shows up to ∼14-dB higher LO-data mixing efficiency and achieves error vector magnitude (EVM) performance of <16% under various turbulence realizations.

Enhancing Power Budget and Spectral Efficiency in Amplifier-Less Coherent Optical Links

With the standardization of 400ZR driving the focus on coherent pluggables for short-reach applications, cost-effective links must be deployed. A way to achieve this is by removing the optical amplification; however, this exacerbates the impact of a high peak-to-average power ratio (PAPR). We experimentally demonstrate the impact of pulse shaping and transmitted electrical power in an amplifier-less coherent link by analyzing the PAPR, transmitted optical power and power budget for short-reach applications. Through an extensive experimental campaign, we jointly optimize the spectral and energy efficiency of the system, demonstrating power budget gains of up to 7dB when compared to standard settings of coherent pluggables.

1-to-4 Analog Demultiplexer With up to 128 GS/s for Interleaving of Bandwidth-Limited Digitizers in Wireline and Optical Receivers

Today's ultra-wideband optical equipment enables fiber-based data links with up to 1.6 Tbit/s per wavelength at a symbol rate of 128 Gbaud. However, high input signal path losses need to be overcome in commercial products when the digitizers are integrated with the digital signal processor in a deep-submicrometer FinFET-CMOS transceiver application specific integrated circuit (ASIC). This currently limits the analog-to-digital converter (ADC) sampling rate and the detectable link symbol rate to 97 GS/s and 66 Gbaud, respectively. Four of these bandwidth-limited ADCs can be time-interleaved with the presented analog 1-to-4 demultiplexer front end in a SiGe-BiCMOS technology that allows high bandwidths with transistor cutoff frequencies above 300 GHz. The sampling front end effectively divides the necessary ADC bandwidth by the time-interleaving factor, enabling the detection of ultra-high symbol rates of up to 128 Gbaud with only 16-GHz bandwidth requirement for the four ADCs. At these frequencies, subsequent circuits and chip interconnects exhibit significantly lower signal path losses than at 64 GHz-half the symbol rate-and therefore require only a reduced processing power for channel equalization in the analog and digital domain. We demonstrate sampling operation with a large-signal 3-dB bandwidth of 36 GHz and a 6-dB bandwidth of 50 GHz at 128 GS/s in an experimental testbed, as well as the reception of 128-Gbit/s NRZ/OOK and 256-Gbit/s PAM4 signals. The linearity reaches more than 3-bit effective number of bits (ENOB) with a sinusoidal input signal of 500 mV <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pp</sub> on each of the four output channels. The power consumption is equivalent to a sampling efficiency of 20 pJ/sample, narrowing the gap to slower CMOS solutions and enabling cost-efficient coherent optical links with up to 1 Tbit/s.

Enhancing Performance of Coherent Optical OFDM FSO Communication Link Using Cascaded EDFA

Coherent communication aims to support high-speed dynamic networks by making their installation easier along with their monitoring and maintenance. In addition, Orthogonal Frequency Division Multiplexing (OFDM) which has shown gradual emergence in radio frequency (RF) domain during the past years, has now encroached into the optical domain. Combination of these two provides vigorous dispersion transmission as well as enhanced spectral efficiency. Free Space Optical (FSO) communication link performance deteriorates with scintillations, atmospheric turbulences and weather induced attenuations. The optical amplifiers, their arrangement used in the FSO link design has a drastic influence on the performance of the link. This work involves evaluating the proposed Coherent Optical OFDM FSO communication link at 30 Gbps data rate using Cascaded Erbium Doped Fiber Amplifier. The system has been evaluated in clear weather, rainfall and fog (Moderate to Dense) in terms of Q-Factor, BER, SNR and constellation diagram. The system has been designed considering moderate turbulent environment and geometric losses. The FSO communication link shows optimal performance up to 26 km for clear weather, 3.73 km under rainfall, 1.44 km under moderate foggy and 0.66 km under dense foggy environment.

Optimizing Probabilistic Constellation Shaping for Amplifier-Less Coherent Optical Links

Coherent optical systems for short-reach links have seen a boost in their popularity with the standardization of 400ZR. These systems need to be cost and power-efficient, while coping with the ever increase in traffic demand. The use of probabilistic constellation shaping (PCS) can help meet these demands, since it reduces the gap to the channel capacity. However, studies concerning PCS typically include optical amplification; therefore, its gain in amplifier-less systems has been somewhat uncertain. In this work, we focus on an amplifier-less coherent optical link employing PCS, reducing both power consumption and system complexity due to the removal of the amplifier. We employ PCS for different modulation orders and bit rates, while optimizing several key parameters such as the FEC coding overhead, modulation depth and clipping. With this comprehensive study, we demonstrate by system simulation and experimental validation that PCS is beneficial even for amplifier-less systems, since it introduces residual gains while allowing higher flexibility in defining the net bit rate in a system with fixed baud rate.

Pulse interactions in weakly nonlinear coherent optical communication links

The intrachannel interaction of pulses in weakly nonlinear coherent optical fiber lines is theoretically investigated. It is shown that the main contribution to the perturbation of the optical field comes from resonant interactions of ordered triplets of pulses. The structure of triplets is determined. The weight contributions of such interactions are calculated. A classification of interactions using Löschian numbers is proposed. Using computer simulation, the dependence of the average energy of the optical field perturbations on the distance is shown. Based on the performed analysis, an effective algorithm is proposed for assessing the perturbations resulting from intrachannel interaction.

A Dual-Polarization Silicon-Photonic Coherent Transmitter Supporting 552 Gb/s/wavelength

Coherent optical links improve spectral efficiency over their intensity-modulation direct-detect (IM-DD) counterparts using advanced modulation schemes such as quadrature phase shift keying (QPSK) and quadrature amplitude modulation (QAM), and by utilizing dual polarization (DP) of light. This increase in spectral efficiency also leads to stringent requirements for the link components. The transmitter (TX) must simultaneously achieve high bandwidth (BW), linearity, output swing, and reliability. In this article, we present a silicon-photonic Mach–Zehnder modulator-based optical TX on an Silicon-on-Insulator (SOI) process, and linear, high-swing SiGe drivers on 130-nm BiCMOS process. The output stage of the driver uses a voltage breakdown enhancement technique to ensure the reliability of the TX. A resistor-based capacitor splitting technique is introduced, and aided by other methods such as zero-peaking and degeneration, the targeted gain, BW, swing, and linearity for the driver are realized. The driver achieves a differential swing of 6 V peak-to-peak, a total harmonic distortion (THD) of 3.6%, and more than 40 GHz of electrical BW. Co-designed and co-packaged with the silicon-photonic modulators, the TX achieves 272 Gb/s/wavelength with DP-16 QAM at 6-V peak-to-peak driver swing and exceeds 0.5 Tb/s/wavelength data rates with DP-16 QAM at 2.4-V peak-to-peak driver swing. The low-cost, compact, and all-Si/SiGe design matches the required optical SNR performance of LiNbO3 modulators with III–V drivers at 34 Gbaud.

Space-Ground Coherent Optical Links: Ground Receiver Performance With Adaptive Optics and Digital Phase-Locked Loop

In the framework of optical high data rate satellite-to-ground links, we investigate the performance of a coherent receiver which combines an adaptive optics system and a digital carrier synchronization technique. We propose a digital carrier synchronization system based on a phase-locked loop and use end-to-end simulations to characterize the impact of the atmospheric turbulence in the case of a representative low Earth orbit satellite-to-ground transmission link. The results show that classical digital phase-locked loop techniques can be a reliable solution to accurately compensate for the Doppler frequency shift between the incoming signal and the local oscillator at the ground station, in the presence of a preliminary coarse frequency estimation. Adaptive optics compensation enables the loop to lock after a few milliseconds and to accurately track the phase fluctuations. The bit-error-rate performance is not degraded by the digital loop but suffers from the residual signal amplitude fluctuations.

Highly Efficient Full-Duplex Coherent Optical System Enabled by Combined Use of Optical Injection Locking and Frequency Comb

We propose a novel point-to-point (P2P) coherent optical link based on optical frequency comb and injection-locking optical process, featuring single wavelength and wavelength division multiplexing (WDM) full-duplex coherent transmission over a fiber link up to 100-km distance. With light sources and local oscillators that are injection locked to an optical frequency comb, simultaneous down-stream and up-stream operations at the same wavelength using full-duplex coherent optical transceivers have been experimentally demonstrated. 32-GBd dual-polarization quadrature phase shift keying (DP-QPSK) and dual-polarization 16 quadrature amplitude (DP-16QAM) modulation formats have been investigated in the setup. When using QPSK, full-duplex operation up to 100 km has been achieved, which introduces 2.2 dB penalty at the hard-decision forward-error-correction (HD-FEC) and 1.3 dB penalty at the soft-decision forward-error-correction (SD-FEC) threshold. When using 16QAM, full-duplex operation up to 80 km has been successfully achieved and as large as 2 dB penalty is observed at the HD-FEC threshold, and 1 dB penalty at the SD-FEC threshold. The penalty mainly comes from continuous and discrete reflection impairments.

Analog I/Q FIR Filter in 55-nm SiGe BiCMOS for 16-QAM Optical Communications at 112 Gb/s

We propose a novel implementation of a complex analog equalization filter for the compensation of frequency-dependent variations in coherent optical links. The analog compensation filter can be used in coherent-lite optical communication links where digital signal processing (DSP) is removed to limit the complexity and power consumption. In these links, the filter can compensate for electrical bandwidth limitations and distortion introduced by chromatic dispersion in the fiber. The complex filter is implemented by combining four distributed analog finite-impulse response (FIR) filters to obtain the necessary response. The filter delays are implemented using active delay cell structures to create a compact solution. The analog filter is implemented in a 55-nm BiCMOS technology and consumes 185-mW core power for five complex filter taps. Performance is evaluated using the S-parameter measurements, noise and linearity measurements, and real-time system experiments using 112-Gb/s 16-QAM-modulated signals.

All-Analog Adaptive Equalizer for Coherent Data Center Interconnects

In a high-speed coherent optical transmission system, typically the signals obtained at the receiver front-end are digitized using very high-speed ADCs and then processed in the digital domain to remove optical channel impairments. In this work, we show that these signals can instead be processed in the analog domain itself, which can significantly reduce the power consumption as well as the complexity of the receiver. The first all-analog adaptive equalizer for receivers of coherent dual-polarization optical links has been presented with its detailed architecture and measurement results. The proof-of-concept equalizer uses the constant modulus algorithm for blind adaptation of its weight coefficients to implement a 4x4 2-tap FIR filter in 130 nm SiGe BiCMOS technology. Its functionality is evaluated experimentally for 40 Gb /s data rate and 10 km standard single-mode fiber channel. This demonstration shows that the use of all-analog processing for short-reach data-center interconnects is feasible and is a much simpler solution than the use of the high-speed ADC+DSP based approach. Moreover, when implemented in advanced CMOS or FinFET technologies, the power consumption of the equalizer is expected to be significantly lower than the DSP based implementations in similar process technologies.

Frequency-Stabilized Links for Coherent WDM Fiber Interconnects in the Datacenter

The continued growth in Hyperscale data center (HDC) deployment is expected to drive world-wide Internet traffic to an astounding 21 zettabytes by 2021. This growth will place increased demands on data center interconnects (DCIs) and drive the capacity of the underlying electronic application specific integrated circuit (ASIC) switch chips that route DCI ethernet traffic, from 12.8 Tbps per chip today to 100 Tbps and beyond in the future. This astounding growth will push the limits of today's incoherent fiber link technologies that connect switches, including power dissipation, density, and practical engineering solutions. To overcome these limits, high capacity coherent WDM, traditionally relegated to the metro and long-haul networks, will need to move into the DCI. However, migrating coherent WDM into the DCI, particularly for link distances less than 2 km, will require elimination of power consuming and costly technologies like the digital signal processor (DSP). Additionally, new photonic integration technologies will be needed to co-locate the coherent optical interfaces directly with switch ASICs to alleviate the bandwidth, power, and density limits. In this article, we introduce a new approach to DSP-free coherent WDM for the DCI called FRESCO: FREquency Stabilized Coherent Optical Links for Low Energy DCIs. FRESCO utilizes spectrally pure, ultra-stable light source technology, normally associated with high-end scientific applications like atomic clocks, to enable high capacity high-order WDM QAM with low bandwidth, low power electronics normally associated with RF links. Terabits per second FRESCO links based on shared, stabilized sources and high-density coherent WDM silicon photonic coherent transceivers that are co-located with the switch ASIC will pave the way to a DSP-free coherent WDM scalable DCI solution.

Impact of laser flicker noise and linewidth on 64 to 96 Gbaud/DP-nQAM metro coherent optical links.

We experimentally investigate the impact of laser flicker noise and linewidth on 64 Gbaud/DP-64QAM, 96 Gbaud/DP-32QAM and 64 or 96 Gbaud/DP-16QAM links. To give a more practical viewpoint, the examined flicker noise closely follows that of an industry forum (OIF 400ZR). We have found that higher modulation order (e.g., 64QAM) is sensitive to phase noise from the linewidth and flicker noise, even in the back to back case. Significant optical signal to noise ratio (OSNR) and cycle slip rate penalties can also be observed with a transmission distance $ {\gt} {200}\;{\rm km}$>200km for both 64QAM and 32QAM signals, which mainly comes from equalization-enhanced phase noise. Moreover, with the increasing of transmission distances, the effective linewidth of a tunable laser with a higher flicker noise and higher linewidth (210 KHz) increases significantly, while it remains unchanged for an external cavity laser (ECL) with 47-kHz linewidth. The result indicates the importance of more stringent flicker noise and linewidth requirement for future ultrabaud rate transmissions.

Numerical simulation and experimental demonstration of accurate machine learning aided IQ time-skew and power-imbalance identification for coherent transmitters.

A novel method assisted by feed-forward artificial neural network model for joint identification of in-phase/quadrature (IQ) time-skews and power-imbalances for coherent optical transmitters is proposed in this paper. This method not only reduces the complexity of hardware design but also significantly improves the accuracy of time-skew/power-imbalance estimation, therefore largely enhances the efficiency of coherent transmitter offline calibration. The proposed method works in the heterodyne detection manner to detect both the time- and power- imperfections. A modified 2 × 2 real-valued channel equalizer for QPSK and 16-QAM signals is applied to extract the channel coefficients, which can fully reconstruct the signals. A key tool using artificial neural networks is used to establish the explicit numerical relationships between the values of IQ time-skew/power-imbalance and the channel coefficients. Both simulation and experimental tests are carried out to verify the capability of the proposed method. Simulation results show that the mean square errors (MSE) of IQ time-skew and power-imbalance estimation can reach below 0.03% of the symbol period and 5.72 × 10-5 at the optical signal-to-noise ratio value of 20 dB. Experiment tests based on 100-Gb/s and 200-Gb/s coherent optical modules show that the mean absolute errors (MAEs) of estimated IQ time-skew and power-imbalance are 0.145 ps and 0.01 for 32-GBaud QPSK signal and 0.162ps and 0.006 for 32-GBaud 16-QAM signal. A demonstrative calibration process is applied to a coherent optical module with pre-set IQ time-skew and power-imbalance by improving the Q2 factor of 32-Gbaud 16-QAM signals from 12.9 dB to 20.3 dB in a coherent optical transmission link at back-to-back case.

On Performance Limits for Spectrally Efficient Optical Transmission Techniques in Short-Haul Metro/Access Links

The demand for transmitting optical signals with high spectral efficiency (SE) by exploiting cost-effective systems and techniques is raising an increasing interest in the area of wide or metro area networks (WAN/MAN). Recently, many different solutions, e.g., amplitude modulation formats, direct-detection schemes, line coding, and simple signal processing techniques have been investigated for wavelength division multiplexing (WDM) short haul optical links, where coherent systems turn out to be economically unfavourable. In this article the Time-Frequency Packing technique, already successfully proposed for long-haul coherent optical links, is extended to direct-detection systems, based on pulse amplitude modulation (PAM) and differential phase-shift keying (DPSK), with the purpose of trading between detection complexity and SE, and of providing performance limits for advanced digital signal processing (DSP) at the receive side.

Performance enhancement of high-capacity coherent DWDM free-space optical communication link using digital signal processing

In this paper, 1.28 Tbps (32 × 40 Gbps) high-capacity DWDM-FSO link has been investigated for performance enhancement using coherent detection and digital signal processing (DSP). The DP-16QAM-modulated proposed DWDM-FSO link has been analyzed for both adverse weather and turbulent atmospheric conditions. It is observed that when link is subjected to strong turbulence along with adverse weather conditions, the DSP-aided coherent DWDM-FSO receiver achieves target bit error rate (BER) of 10−4 at signal-to-noise ratio (SNR) of 36.4 dB, while for similar conditions, the SNR requirements for IM/DD-based DWDM-FSO link shoots by 12.8 dB to 49.2 dB. Also, in terms of operational link range, the proposed link even under strong turbulent conditions serves 1.88 km, whereas IM/DD link was restricted to mere 1.12 km for target of BER of 10−4, thus producing a decent range increment of 760 m. The proposed link has been designed and investigated using OptiSystem™ 14.2.

Advanced technologies for satellite navigation and geodesy

This manuscript reviews recent progress in optical frequency references and optical communication systems and discusses their utilizations in global satellite navigation systems and satellite geodesy. Lasers stabilized with optical cavities or spectroscopy of molecular iodine are analyzed, and a hybrid architecture is proposed to combine both forms of stabilization with the aim of achieving a target frequency stability of 10-15 [s/s] over a wide range of sampling intervals.The synchronization between two optical frequency references in real-time is realized by means of time and frequency transfer on optical carriers. The technologies enabling coherent optical links are reviewed, and the development of an optical communication system for synchronization, ranging and data communication in space is described. An infrastructure exploiting the capabilities of both optical technologies for the realization of a modernized constellation of navigation satellites emitting highly synchronized signals is reviewed. Such infrastructure, named Kepler system, improves satellite navigation in terms intra-system synchronization, orbit determination accuracy, as well as system monitoring and integrity. The potential impact on geodetic key parameters is addressed.