Coherent Optical Receivers Research Articles

Laser Linewidth Estimation by Means of Coherent Detection

In this letter, we present a novel approach to measure the Lorentzian linewidth of a laser source. The approach is based on a heterodyne detection scheme, such as may be implemented in a coherent optical receiver for data communications, using both in-phase and quadrature information of the optical field. In contrast to previously published works, this approach does not suffer from modulation effects on the laser output.

DSP for Coherent Single-Carrier Receivers

In this paper, we outline the design of signal processing (DSP) algorithms with blind estimation for 100-G coherent optical polarization-diversity receivers in single-carrier systems. As main degrading optical propagation effects, we considered chromatic dispersion (CD), polarization-mode dispersion (PMD), polarization-dependent loss (PDL), and cross-phase modulation (XPM). In the context of this work, we developed algorithms to increase the robustness of the single DSP receiver modules against the aforesaid propagation effects. In particular, we first present a new and fast algorithm to perform blind adaptive CD compensation through frequency-domain equalization. This low complexity equalizer component inherits a highly precise estimation of residual dispersion independent from previous or subsequent blocks. Next, we introduce an original dispersion-tolerant timing recovery and illustrate the derivation of blind polarization demultiplexing, capable to operate also in condition of high PDL. At last, we propose an XPM-mitigating carrier phase recovery as an extension of the standard Viterbi-Viterbi algorithm.

Impact of Quadrature Imbalance in Optical Coherent QPSK Receiver

In this letter, we present the impact of quadrature imbalance in an optical coherent quadrature phase-shift-keying (QPSK) receiver. Arbitrary conjugate misalignment was realized in all-fiber optical hybrid and two correction algorithms of quadrature imbalance, the ellipse correction method, and the Gram-Schmidt orthogonalization method, were independently applied. Bit-error-rate curves versus optical signal-to-noise ratio (OSNR) were measured for the 20-Gb/s optical QPSK signal. We show that OSNR penalty is reduced from ~6 to 1 dB at ~20deg conjugate misalignment with the help of correction algorithms.

Hardware-Efficient Coherent Digital Receiver Concept With Feedforward Carrier Recovery for $M$-QAM Constellations

This paper presents a novel digital feedforward carrier recovery algorithm for arbitrary <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> -ary quadrature amplitude modulation (M-QAM) constellations in an intradyne coherent optical receiver. The approach does not contain any feedback loop and is therefore highly tolerant against laser phase noise. This is crucial, especially for higher order QAM constellations, which inherently have a smaller phase noise tolerance due to the lower spacing between adjacent constellation points. In addition to the mathematical description of the proposed carrier recovery algorithm also a possible hardware-efficient implementation in a parallelized system is presented and the performance of the algorithm is evaluated by Monte Carlo simulations for square 4-QAM (QPSK), 16-QAM, 64-QAM, and 256-QAM. For the simulations ASE noise and laser phase noise are considered as well as analog-to-digital converter (ADC) and internal resolution effects. For a 1 dB penalty at BER = 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , linewidth times symbol duration products of 4.1 x 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> (4-QAM), 1.4 x 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> (16-QAM), 4.0 x 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> (64-QAM) and 8.0 x 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> (256-QAM) are tolerable.

Optical coherent receiver based on single side sub-carrier phase-locked loop

An analogue optical phase-locked loop technique called single side sub-carrier phase-locked loop was tested in a 10 Gbit/s real-time coherent BPSK experiment. It was experimentally demonstrated that the novel technique allows use of a 6 GHz electrical voltage controlled oscillator to perform a 10 Gbit/s BPSK analogue coherent detection.

Compensation of Quadrature Imbalance in an Optical QPSK Coherent Receiver

This letter explores the Gram-Schmidt orthogonalization procedure (GSOP) for compensation of quadrature imbalance in an optical 90deg hybrid. We present computer simulations for an optical QPSK communication system using a digital coherent receiver and investigate the impact of quadrature imbalance on the required optical signal-to-noise ratio for the receiver and the frequency estimation algorithm. We then demonstrate the improvement which can be achieved using the GSOP, including the impact of quantization in the digital coherent receiver. Finally, we show that the GSOP can equally be applied to polarization-division multiplexed systems, applying the GSOP in conjunction with the constant modulus algorithm to demultiplex a PDM-QPSK signal.

Repeaterless 10.7-Gb/s DPSK Transmission Over 304 km of SSMF Using a Coherent Receiver and Electronic Dispersion Compensation

Record repeaterless transmission of differential phase-shift keying (DPSK) at 10.7 Gb/s over 304 km of standard single-mode fiber (SSMF) is demonstrated using a coherent optical receiver and electronic dispersion compensation. This is the longest repeaterless 10-Gb/s transmission over SSMF in the absence of Raman amplifiers. The high receiver sensitivity and the high tolerance to nonlinearities of DPSK allow us to overcome a total link loss of 58 dB with a 3-dB system margin. Coherent detection enables linear electrical dispersion compensation and avoids the use of optical dispersion compensation.

Digital filters for coherent optical receivers

Digital filters underpin the performance of coherent optical receivers which exploit digital signal processing (DSP) to mitigate transmission impairments. We outline the principles of such receivers and review our experimental investigations into compensation of polarization mode dispersion. We then consider the details of the digital filtering employed and present an analytical solution to the design of a chromatic dispersion compensating filter. Using the analytical solution an upper bound on the number of taps required to compensate chromatic dispersion is obtained, with simulation revealing an improved bound of 2.2 taps per 1000ps/nm for 10.7GBaud data. Finally the principles of digital polarization tracking are outlined and through simulation, it is demonstrated that 100krad/s polarization rotations could be tracked using DSP with a clock frequency of less than 500MHz.

Exact Moments of Filtered Laser Phase Noise

In this paper, we derive an exact finite power series expression of the nth-order moment of a complex filtered phase noise random variable. This random variable is usually encountered in the error probability analysis of coherent heterodyne optical receivers. The result is then used to derive an infinite power series expression for the moment generating function of the same random variable. The two expressions represent a novel full statistical characterization of filtered phase noise. They also constitute an important step toward deriving optimal heterodyne receiver designs in the presence of phase noise. In a previous work Banat (J. Opt. Commun., vol. 5, 267-271, Dec. 2004), the author presented an approximate finite power series moment expression for filtered laser phase noise. The new results will be compared to those of Banat (J. Opt. Commun., vol. 5, 267-271, Dec. 2004)

Coherent optical array receiver for PPM signals under atmospheric turbulence

Adaptive combining of experimentally obtained heterodyned pulse position modulated (PPM) signals with pulse-to-pulse coherence, in the presence of simulated spatial distortions resembling atmospheric turbulence, is demonstrated. The adaptively combined PPM signals are phased up via an least-mean-square algorithm suitably optimized to operate with PPM in the presence of additive shot noise. A convergence analysis of the algorithm is presented, and results with both computer simulated and experimentally obtained PPM signals are analyzed.

Coherent detection of optical quadrature phase-shift keying signals with carrier phase estimation

This paper describes a coherent optical receiver for demodulating optical quadrature phase-shift keying (QPSK) signals. At the receiver, a phase-diversity homodyne detection scheme is employed without locking the phase of the local oscillator (LO). To handle the carrier phase drift, the carrier phase is estimated with digital signal processing (DSP) on the homodyne-detected signal. Such a scheme presents the following major advantages over the conventional optical differential detection. First, its bit error rate (BER) performance is better than that of differential detection. This higher sensitivity can extend the reach of unrepeated transmission systems and reduce crosstalk between multiwavelength channels. Second, the optoelectronic conversion process is linear, so that the whole optical signal information can be postprocessed in the electrical domain. Third, this scheme is applicable to multilevel modulation formats such as M-array PSK and quadrature amplitude modulation (QAM). The performance of the receiver is evaluated through various simulations and experiments. As a result, an unrepeated transmission over 210 km with a 20-Gb/s optical QPSK signal is achieved. Moreover, in wavelength-division multiplexing (WDM) environment, coherent detection allows the filtering of a desired wavelength channel to reside entirely in the electrical domain, taking advantage of the sharp cutoff characteristics of electrical filters. The experiments show the feasibility to transmit polarization-multiplexed 40-Gb/s QPSK signals over 200 km with channel spacing of 16 GHz, leading to a spectral efficiency as high as 2.5 b/s/Hz.

Optical phase-locked loop for coherent detection optical receiver

A novel subcarrier-based optical phase-locked loop (SC-OPLL/sup /spl reg//), with off-the-shelf optical components, is presented and demonstrated. The method, based on a continuous-wave laser and optical subcarrier modulation using a standard LiNbO/sub 3/ Mach-Zehnder modulator, allows easier practical implementation than the previously proposed OPLL circuits based on laser direct modulation.

The channel capacity of a multispan DWDM system employing dispersive nonlinear optical fibers and an ideal coherent optical receiver

We present channel capacity calculations for a multispan dense wavelength division multiplexed (DWDM) system that employs an ideal coherent optical receiver. Both dispersive and dispersion-free single-mode nonlinear optical fibers are considered. Degradation due to interference among Kerr nonlinear noise and optical amplifier noise accumulated along many spans is included in our model calculations. We will show that in the low-power quasi--linear regime, a multispan system can be approximated by an "equivalent" single-span system. The "equivalent" Kerr coefficient for most dispersive fibers is shown to increase with the square root of the number of spans, in contrast to the linear scaling dependence for a dispersion-free fiber. For a conventional fiber with β = -20 ps <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /km, our calculated capacity of 10 (2 x 80 km), 8 (8 x 80 km), and 6 (32 x 80 km) bps/Hz indicates that today's technologies with 0.4 bps/Hz have only realized 5% of the theoretical total capacity. We have shown an increased (or decreased) capacity by about 1 bps/Hz per ten-fold increase (decrease) in dispersion.

Coherent Optical Heterodyne PSK Receiver Performance in Multichannel Environment

This paper presents the analysis of coherent optical heterodyne phase-shift keying (PSK) receiver performances in the presence of the laser phase noise, photodiode shot noise and adjacent channel interference. The reference carrier is extracted by the nonlinear phase locked loop (PLL). Due to the finite laser linewidth, both the useful signal phase and adjacent channel interference phase have the Gaussian distribution. Taking the previously mentioned influences into consideration, the steady state phase error probability density function (PDF) is determined as well as the average error probability (P c ) dependence on signal to noise ratio (SNR).

Endlos-Polarisationsregelung für den optischen Phasendiversitätsempfang

Polarization fluctuations at the end of a single-mode fibre can severely reduce the sensitivity of a coherent optical receiver. This can be avoided by an active polarization control unit. A new control algorithm is presented that is designed for a phase diversity receiver. In each operation step the distance between the actual retardation and the physical range limits is maximized. Thus reset procedures of the utilized finite-range polarization control devices can be avoided. Simulations of the new scheme showed that the sensitivity degradation is less than 0.8 dB.

Coherent optical array receivers for the mitigation of atmospheric turbulence and speckle effects

A description is given of the design, operation, and test over a 2-km path (roundtrip) of a continuous wave, coherent laser array receiver that uses two independent aperture-receivers whose intermediate frequencies are electro-optically co-phased in real time and then added as a proposed way to overcome effective aperture limitations imposed by atmospheric turbulence and to mitigate signal fading associated with atmospheric turbulence and speckle effects. The experiment resulted in a mean carrier-to-noise ratio increase of 1.8, which is within 1% of the theoretical predictions, when the two signals were phase locked, versus no increase without phase locking. Further, the carrier fading strength, or normalized carrier-to-noise ratio variance, was reduced by a factor of 0.53, which is within 2% of the theoretical prediction. The bandwidth of the electro-optic phase-locked loop was measured to be of the order of 600 Hz, which is adequate to compensate for atmospheric refractive turbulence fluctuations.

A new automatic frequency-control method for fast frequency switching of a coherent optical receiver

The authors propose a new automatic optical frequency-control method for fast frequency switching. Simulation and experimental results for the new method are presented. In the experiment, the frequency of an optical coherent receiver was changed freely and accurately and was not affected by the thermal effect of the laser diode. By this method, switching time was reduced to less than one tenth that of an uncontrolled laser diode.

Effect of phase noise on the performance of 10-Gbits/s coherent optical synchronous receivers

Effect of phase noise on the performance of 10-Gbits/s coherent optical synchronous receivers

The effect of error control coding in multichannel FSK coherent lightwave communication systems influenced by laser phase noise

It has been shown that coherent optical fiber receivers with a two-filter structure (TFS) consisting of a wide-band IF filter and a narrow-band postdetection filter are less susceptible to the influence of phase noise. However, the expanded IF bandwidth required to achieve optimum sensitivity performance is large, particularly in multichannel FSK systems. Forward error control coding can relax the laser linewidth requirement and improve receiver sensitivity. In this paper a multichannel asynchronous FSK scheme equipped with (31, k) Reed-Solomon codes is used to verify the coding benefit. A systematic error probability analysis is developed and a stable and accurate performance evaluation procedure is provided. The sensitivity penalties due to the combined phase noise and interchannel crosstalk for both coded and uncoded systems are calculated for comparison. The results show that the performance reduction due to phase noise can be largely alleviated by choosing a proper code rate and an optimum value of the expanded IF bandwidth.

I.F. Filter bandwidth optimization of coherent heterodyne analog FM optical receiver

AbstractThe performance of the coherent heterodyne optical FM receiver employing a discriminator has been simulated under different i.f. filter bandwidths to find the optimal bandwidth. The value of the signal‐to‐noise ratio has been obtained under different values of i.f. filter bandwidth, modulation index, and signal power. The phase noise linewidth is fixed at Δv = 10 kHz. Even for very small linewidth, the performance of the coherent optical FM receiver is mainly dominated by phase noise. The advantage of FM modulation exists only when received signal power is small. © 1995 John Wiley &amp; Sons, Inc.