Homodyne Coherent Receiver Research Articles

Deep-Learning-Enabled Direct Detection With Reduced Computational Complexity and High Electrical-Spectral-Efficiency

Complex-valued double-sideband (CV-DSB) direct detection (DD) is a promising solution for high capacity and cost-sensitive data center interconnects, since it can reconstruct the optical field as a homodyne coherent receiver while does not require a costly local oscillator laser. In a carrier-assisted CV-DSB DD system, the carrier occupies a large proportion of the total optical signal power but bears no information. Reducing the carrier to signal power ratio (CSPR) can improve the information-bearing signal power and thus maximize the achievable system performance. Recently, we have proposed and demonstrated a deep-learning-enabled DD (DLEDD) scheme to reconstruct the full-field of the CV-DSB signal. In the DLEDD scheme, the optical CV-DSB signal was detected by a dispersion-diversity receiver and then recovered by a deep convolutional neural network (CNN). Nevertheless, the computational complexity of the deep CNN is the main obstacle to the application of the DLEDD scheme. In this paper, we demonstrate a 50-GBaud CV-DSB 32-ary quadrature amplitude modulation (32-QAM) signal transmission over 80-km single-mode fiber with ∼64% computational-budget reduction in the field reconstruction. This is achieved by using 1×1 convolutions to attain a sparse dimensionality (in particular the number of channels) of the deep CNN. To the best of our knowledge, we achieve the highest electrical spectral efficiency of 7.07 b/s/Hz per polarization per wavelength for a CV-DSB DD receiver without requiring a sharp-roll-off optical filter.

Digital mobile fronthaul based on delta-sigma modulation employing a simple self-coherent receiver

The coherent digital radio-over-fiber (DRoF) system is a promising candidate for future mobile fronthaul networks (MFNs) due to its high receiver sensitivity and excellent robustness against nonlinearities. However, conventional coherent receivers with complicated structure and heavy algorithms are too expensive and power-hungry for cost-sensitive MFN applications. In addition, currently deployed digital MFNs based on common public radio interface (CPRI) suffer from low spectral efficiency and high data rate. Towards these issues we propose a novel DRoF downlink scheme employing a simple self-coherent receiver. In baseband unit (BBU), the radio signal is converted to a digital bit stream by a band-pass delta-sigma modulator (BP-DSM), which can be simply recovered with the utilization of a band-pass filter at the receiver. In remote radio unit (RRU), an electro-absorption modulated laser (EML) acts as a low-cost coherent homodyne receiver in virtue of injection locking technique. In the experiment, the injection-locked operation of the DSM signal is successfully achieved, and two modified schemes are proposed for the DSM signal to increase the locking range with a tolerable sensitivity penalty. The experimental results demonstrate the superiority of our approach in two aspects: 1) the EML-based coherent receiver outperforms a PIN photodiode in terms of receiver sensitivity; 2) compared to the analog RoF system, a 5-dB improvement in loss budget is obtained when DSM is employed with the aid of a simple equalizer.

Simplified Coherent Receiver for Analogue Radio Transmission Over High Optical Budgets

The bandwidth and latency demands driven by 5G wireless networks are putting a focus on analog radio-over-fiber techniques as a promising candidate for broadband optical fronthauling. In contrast to intensity modulation / direct detection systems, as they are commonly adopted for mobile fronthaul links due to their simplicity, we are exploring a migration option towards coherent radio-over-fiber transmission. A simplified coherent homodyne receiver based on an electro-absorption modulated laser is co-integrated at the die-level with a transimpedance amplifier and evaluated for analogue coherent radio-over-fiber transmission. This low-complexity homodyne receiver can unlock channel selectivity and high receiver sensitivity inherent to coherent detection, but without the need for digital signal processing. We will demonstrate a sensitivity that allows for an optical budget of 42 dB, and thus eases the fixed-mobile convergence in power-splitting fiber plants. We further show filterless radio signal reception in presence of modulated adjacent channels. Signal integrity is confirmed through transmission of orthogonally frequency multiplexed radio signals with 16-point quadrature amplitude modulated formats, with an error vector magnitude below the corresponding antenna limit.

Rayleigh Backscattering Rejection in Single-Laser Homodyne Transceiver for UD-WDM Using λ-Shifting

With the aim of using a single laser per transceiver in single fiber splitter-based passive optical networks (PON) with coherent homodyne receivers, we apply wavelength shifting in an ultra-dense wavelength division multiplexing (UD-WDM) scheme. The studied architecture is based on a dual-parallel Mach-Zehnder Modulator (DP-MZM) operating in carrier-suppressed single-sideband (CS-SSB). In this configuration, a single laser feeds both the coherent homodyne receiver, and the transmitter, simplifying the wavelength management in the PON. The modulator carries out both data modulation and wavelength shifting. In this study, a bit rate of 4 Gb/s in QPSK is used to test several roll-off factors and channel spacing configurations. The results show that power budget gains of 18 dB are achievable with a channel spacing equal to only the symbol rate.

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°.

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.

Analog Coherent TDMA Receiver With Fast Locking to Free-Running Optical Emitters

As coherent reception technology continues to move downstream the optical telecommunication infrastructure, the complexity of the involved transceiver technology can quickly introduce a techno-economic roadblock. Under this umbrella, we experimentally demonstrate a conceptually simple, single-polarization and analogue coherent homodyne receiver that builds on no more than an optically locked externally modulated laser. We evaluate this coherent homodyne receiver in the context of analogue radio-over-fiber transmission – a demanding application setting, where a small degradation in signal integrity is leading to large reception penalties. We conduct a continuous-mode characterization of the locking methodology, which enables homodyne detection and hence the transparent translation of electrical signals from the optical to the electrical domain during the coherent reception process. Furthermore, the locking dynamics are being investigated for packet-level reception at a 1 MHz frame rate and two time division multiplexed channels, which are sourced by two optical emitters with free-running laser sources. The radio-over-fiber transmission performance is evaluated for 64-ary quadrature amplitude modulated, orthogonal frequency division multiplexed radio with a short guard interval of 2.7 μs between the packet radio signals. A data rate of 0.5 Gb/s over 100 MHz radio bandwidth is obtained at an optical loss budget of >35 dB between transmitter and receiver, without resorting to digital signal processing resources for the purpose of signal recovery. Moreover, a small ∼0.3% penalty in error vector magnitude between continuous- and packet-mode confirms the compatibility of the analogue coherent receiver in networks with fast locking requirements.

掺铒光纤放大器作为光学预放的高灵敏度零差相干接收机

掺铒光纤放大器作为光学预放的高灵敏度零差相干接收机

Homodyne coherent optical receiver for intersatellite communication.

For increasing the data rate and reducing the power consumption in satellite communication systems, free-space optical communication technology has been verified as an attractive alternative to traditional radiofrequency communication systems. In this study, a high-sensitivity homodyne coherent optical receiver was demonstrated. A sensitivity of -59.2 dBm was achieved with an unencoded bit-error rate below 10-3 at a bit rate of 1Gbps (9.4photons/bit does not include any overhead for forward error correction). To achieve homodyne detection, a modified decision-driven Costas optical phase-locked loop and a digital control algorithm were used to suppress the phase error and track the Doppler shift. Test experiments show that the receiver sensitivity penalty caused by the phase error is approximately 1dB and that the receiver can track a Doppler shift at the speed of 100MHz/s. Test experiments demonstrate that the receiver has a robust adaptive capacity to received-power fluctuation. The theoretical analysis and experimental results are presented in this paper.

BER Performance of PDM 4-QAM Optical Transmission System Considering the Effects of PMD and GVD Using Exact Probability Density Function

A noble theoretical approach is presented to evaluate the bit error rate (BER) performance of an optical polarization division multiplexed (PDM) 4-multilevel quadrature amplitude modulation (4-QAM) transmission system under the combined influence of polarization mode dispersion (PMD) and group velocity dispersion (GVD) in a single mode fiber (SMF). The analysis is carried out considering a coherent homodyne receiver. The exact probability density function (pdf) fluctuation due to PMD and GVD is evaluated from its moments using a Monte-Carlo simulation technique. Average BER is evaluated by averaging the conditional BER over the pdf of the random phase fluctuation. BER performance results are evaluated for different system parameters. It is found that PDM 4-QAM coherent homodyne system doubles the data rate but suffers more power penalty than the 4-QAM system. Results show that for a BER of 10^-9 at DGD of 0.5T and GVD value of 1700 ps/nm the PDM 4-QAM system needs 2.21 dB more power than 4-QAM systems giving the leverage of doubling the data rate.

Analytical evaluation of the combined influence of polarization mode dispersion and group velocity dispersion on the bit error rate performance of optical homodyne quadrature phase-shift keying systems

A theoretical approach is presented to evaluate the bit error rate (BER) performance of an optical fiber transmission system with quadrature phase-shift keying (QPSK) modulation under the combined influence of polarization mode dispersion (PMD) and group velocity dispersion (GVD) in a single-mode fiber (SMF). The analysis is carried out without and with polarization division multiplexed (PDM) transmission considering a coherent homodyne receiver. The probability density function (pdf) of the random phase fluctuations due to PMD and GVD at the output of the receiver is determined analytically, considering the pdf of differential group delay (DGD) to be Maxwellian distribution and that of GVD to be Gaussian approximation. The exact pdf of the phase fluctuation due to PMD and GVD is also evaluated from its moments using a Monte Carlo simulation technique. Average BER is evaluated by averaging the conditional BER over the pdf of the random phase fluctuation. The BER performance results are evaluated for different system parameters. It is found that PDM-QPSK coherent homodyne system suffers more power penalty than the homodyne QPSK system without PDM. A PDM-QPSK system suffers a penalty of 4.3 dB whereas power penalty of QPSK system is 3.0 dB at a BER of 10−9 for DGD of 0.8 Tb and GVD of 1700 ps/nm. Analytical results are compared with the experimental results reported earlier and found to have good conformity.

Design of Low-Power DSP-Free Coherent Receivers for Data Center Links

Coherent detection offers high spectral efficiency and receiver sensitivity, but digital signal processing (DSP)-based coherent receivers may be prohibitively power hungry for data centers even when optimized for short-reach applications, where fiber propagation impairments are less severe. We propose and evaluate low-power DSP-free homodyne coherent receiver architectures for dual-polarization quadrature phase shift keying (DP-QPSK) for inter- and intradata center links. We propose a novel optical polarization demultiplexing technique, for DP-QPSK and higher-order modulation formats, with three cascaded phase shifters driven by marker tone detection circuitry. We consider carrier recovery based on either optical or electrical phase-locked loops (PLLs). We propose a novel multiplier-free phase detector based on XOR gates, which exhibits less than 0.5 dB power penalty relative to a conventional Costas loop phase detector. We also study the relative performance of homodyne DP-differential QPSK, for which carrier phase recovery is unnecessary. Our proposed DSP-free architectures exhibit ∼1 dB power penalty at small chromatic dispersion compared to their DSP-based counterparts. We estimate conservatively that the high-speed analog electronics of an electrical PLL-based coherent receiver consume nearly 4 W for 200 Gbit/s DP-QPSK, assuming a 90-nm complementary metal-oxide semiconductor process.

Multiple-Input Multiple-Output Enabled Large Bandwidth Density On-Chip Optical Interconnect

We propose to apply the multiple-input multiple-output (MIMO) from wireless communication to high density on-chip optical interconnect. MIMO makes it possible to reduce the waveguide pitch to subwavelength range and uses the crosstalk to improve system performance. The proposed N×N on-chip MIMO system consists of transmitter, high-density waveguides, homodyne coherent receivers, and electrical signal processing components. As an example, a 10×10 MIMO system with waveguide spacing of 250 nm is simulated. The possibility of data transmission at 10 Gb/s/channel from high-density waveguide array is numerically investigated. The minimum input optical power for the BER of 10−12 can reach −18.1 dBm. The BER is better than 10−12 when there is a phase shift of 73.5°. Compared to the conventional parallel waveguides with 2-μm pitch, the bandwidth density can be enhanced from 5 to 13.33 Gbit/μm/s at 10 Gb/s by using the MIMO techniques.

Characterization of Electrostriction Nonlinearity in a Standard Single-Mode Fiber Based on Coherent Detection and Cross-Phase Modulation

The electrostriction nonlinearity in a standard single mode fiber is measured by cross-phase modulation (XPM) between pump and probe. Novel use of a coherent homodyne receiver allows measurement of in-phase and quadrature components of the probe field, eliminating the need for an optical interferometer. The Kerr effect ( $\gamma _{k}$ ) and frequency-dependent electrostriction $(\gamma _{e})$ nonlinear parameters are characterized experimentally, and $\gamma _{e}$ is found to be approximately one third of $\gamma _{k}$ at the highest peak resonance frequency. The joint action of $\gamma _{k}$ and $\gamma _{e}$ also introduces a polarization dependence of the electrostrictive signal when the pump and the probe are not copolarized.

Adaptive optics for the free-space coherent optical communications

We present the results that the performances of the atmospheric coherent optical communications improved by a real 127-element adaptive optics system operating in 1550nm wavelength on a 1.8m telescope. The mixing efficiency and the BER of the homodyne free-space optical coherent communications have been analyzed with the correction of the adaptive optics system. It is shown that, the AO can expand the scope of the atmospheric turbulence condition D/r0(1550) from 1 to around 6.5 in which the coherent optical communications can function well. In order to acquire favorable results, the mixing efficiency should be larger than 0.4 and the residual wavefront error after the AO correction should be smaller than λ/6. We also find that the mixing efficiency of the coherent homodyne receiver is approximate to the Strehl ratio of the far field of the signal light. In conclusion, we have shown that the adaptive optics is a very promising technique that can largely improve the performances of the free-space coherent optical communications degraded by the atmospheric turbulence.

Numerical analysis of 50 Gbaud homodyne coherent receivers relying on line-coding and injection locking in lasers

We present a numerical analysis of 50Gbaud coherent detection enabled by injection locked lasers and line coding. The coherent receiver was tested with respect to an ideal receiver for two higher order modulation formats (16-QAM, QPSK) and under diverse operating regimes relating to the slave laser linewidth properties, the injection level and the frequency detuning between the incoming signal and the slave laser. The impact of the slave laser properties and line coding techniques on the receiver performance is highlighted showing that the technique could be used as a practical solution in order to enable low-cost and short reach n×100Gb/s Ethernet communication systems with the potential of flexibility in terms of the data rate.

Parametric coherent receiver.

We present the parametric coherent receiver based on a two-mode pump-degenerate fiber optical parametric amplifier (FOPA). The receiver is inherently single-ended and offers a simultaneous gain and coherent mixing of the received signal and a reference wave (known as a local oscillator signal) with, in principle, arbitrary wavelength separation. We analyze the receiver theoretically and in a proof-of-concept experiment. As a reference we compare the performance to a standard single-ended homodyne coherent receiver.

Influences of hybrid phase-shift error on the performance of optical coherent receivers

The influences of phase-shift error induced by a 90° space-optical hybrid on the performance of homodyne coherent receivers are analyzed. Phase-shift errors bring a fixed phase difference between the signal and local oscillation (LO) light. For an optical phase-locked loop OPLL, it becomes the third source of phase noise, further increasing loop phase noise. Under the joint action of phase difference and increasing phase noise, the sensitivity of the receiver deteriorates considerably. For a bit error rate (BER) of 10−9, in order to make the additional received power penalty less than 0.5dB, the phase shift error of each branch must be less than 2°. Further, if the BER is 10−10, the limiting value becomes 1°. This presents a constraint for the design of the 90° space-optical hybrid.

BPSK homodyne receivers based on modified balanced optical phase-locked loop

Simulation analysis of a modified balanced PSK homodyne coherent optical receiver has been investigated taking into account laser phase noise, photo detectors shot noise and loop propagation delay. This modified loop contains all the components of a standard balanced optical phase-locked loop in conjunction of an additional phase modulator. This modified homodyne receiver offers a much improved tracking performance and also shows good improvements in the reduction performance over previously reported methods, like, improved receiver stability, relaxed line-width requirement, increased power sensitivity and improved lock-in range.