Presence Of Laser Phase Noise Research Articles

Diode laser phase noise influence on the ultimate performance of its frequency stabilization to a mach-zehnder interferometer fringe

We report a detailed investigation on the effect of semiconductor laser phase noise on the achievable frequency stability when locked to a Mach-Zehnder interferometer fringe. We show that the modulation-demodulation operation produces in the presence of laser phase noise two kinds of excess noise, which could be much above the shot noise limit, namely: conversion noise (PM-to-AM) and intermodulation noise. We show that in typical stabilization conditions, the frequency stability of the locked laser is limited by the intermodulation excess noise. This effect, reported initially in the microwave domain, can be considerably reduced by a convenient choice of the modulation frequency. To our knowledge, this is the first time such a phenomenon is reported in the optical domain.

Evaluation of optimal bandwidth in optical FSK system influenced by laser phase noise

In this paper, an optical frequency shift keying (FSK) system with TFS (two-filter structure), also known as dual-filter FSK, is observed in the presence of laser phase noise. Sensitivity of the receiver is shown as a function of intermediate frequency (IF) bandwidth and a bandwidth of the low-pass postdetection filter. It is shown that there is an optimal pair of bandwidths which minimizes required sensitivity; a few figures for different amounts of phase noise are given. The results in this paper are based on truncated Taylor expansion method and a method based on complex analysis is used to evaluate the integrals involved.

A novel procedure for jones' parameters estimation for M‐4QAM optical systems

AbstractA new procedure is proposed for estimating the State‐of‐Polarisation of the received optical signal in Four‐Quadrature Amplitude Modulation systems. The proposed solution, which is obtained on the basis of the Maximum Likelihood criterion, is merely electronic and operates in the presence of laser phase noise, photodetection noise and cross‐polarisation interference. It employs a suitably designed training sequence to estimate Jones' parameters in the data‐aided training mode. A proper decision‐directed procedure is then activated to lock‐in the estimated parameters in the operative mode. The variances of the resulting estimation errors are evaluated and the overall system performance is given in terms of Bit Error Probability versus signal‐to‐noise ratio. The theoretical analysis and the computer simulations show the superiority of the proposed recovery procedure with respect to alternative solutions recently proposed in literature.

Performance of coherent ASK lightwave systems with finite intermediate frequency

The impact of finite intermediate frequency (IF) on the performance of heterodyne ASK lightwave systems is examined and quantified in the presence of laser phase noise and shot noise. For negligible linewidths, it is shown that certain finite choices of IF (R/sub b/,3R/sub b//2,2R/sub b/,5R/sub b//2, etc.) lead to the same ideal bit-error-rate (BER) performance as infinite choices of IF. Results indicate that for negligible linewidths the worst case sensitivity penalty is 0.9 dB for proper heterodyne detection and occurs when f/sub IF/=1.25 R/sub b/. For nonnegligible linewidths (e.g., when /spl Delta//spl nu/T/spl ges/0.04) the sensitivity penalty is always less than 0.9 dB for finite choices of IF. The analysis presented does lead to a closed-form signal-to-noise ratio (SNR) expression at the decision gate of the receiver which can readily be used for BER and sensitivity penalty computations. The SNR expression provided includes all the key system parameters of interest such as system bit rate (R/sub b/), the peak IF SNR (/spl xi/), laser linewidth (/spl Delta//spl nu/), and the IF filter expansion factor (/spl alpha/). The findings of this work suggest that the number of channels in a multichannel heterodyne ASK lightwave system can be increased substantially by properly choosing a small value for the IF at the expense of a small penalty <1 dB. On the negative side, IF frequency stabilization becomes a more critical requirement in multichannel systems employing small values of IF.

Performances of the ASK System with Optical PLL in Presence of Laser Phase Noise

In this paper performances of the ASK system with optical PLL in presence of laser phase noise are considered. Probability density function for phase error and system error probability are determined considering stochastically frequency fluctuations instead of stochastically phase fluctuations, in presence of laser phase noise. Calculation results are given graphically. All shown results correspond to the case when PLL is within lock-in range.

Analysis and design of a DPSK optical heterodyne receiver in the presence of laser phase noise and frequency detuning

AbstractThis paper is concerned with the analysis and design of an improved differential heterodyne optical receiver for differentially‐encoded binary PSK (briefly, DPSK) signals in a coherent lightwave communication system. In the first part of the paper, we discuss the relevant design criteria to be employed when dealing with asynchronous heterodyne receivers for ASK, FSK or DPSK optical signals. In particular, we introduce a convenient definition of the signal‐to‐noise ratio at the data detector input to be assumed as the system performance measure when the non‐negligible linewidth of the transmit/receive laser sources are to be taken into account. Following this design approach, we show that by properly modifying the traditional delay‐and‐multiply DPSK receiver, i. e. by allowing the delay to be a fraction of the symbol interval, we can considerably reduce the performance degradation caused by laser phase noise. We show thus that the superior power‐efficiency of DPSK can be traded in favour of a decreased sensitivity to phase noise through a proper choice of the differential detector delay. In this respect, our results reveal that DPSK may still be competitive with other modulation formats even with non‐negligible linewidth sources. In the last part of the paper, the behaviour of the optimized DPSK, ASK and large‐deviation FSK data demodulators in the presence of a quasistationary frequency detuning of the local laser is also discussed under the same set of conditions as in the previous analysis. The results can be employed to derive accurate design requirements for the AFC loop of the receiver.

Performance of coherent optical polarization shift keying modulation in the presence of phase noise

The performance of optical coherent modulation schemes using the state of polarization of light as the modulating parameter (POLSK, POLarization Shift Keying) is analyzed, in the presence of laser phase noise. First, we compare three different analysis methods to account for the effects of phase noise in the binary case, and choose the one offering the best trade-off in terms of accuracy/complexity. Successively, we derive a tight upper bound to the performance of multilevel POLSK systems with postdetection filtering. Then, we include phase noise using the best method as tested in the binary case and extensively discuss the performance of binary and various multilevel POLSK constellations, optimized with respect to the IF filter bandwidth. The results show that binary POLSK has a very low sensitivity to phase noise and that multilevel POLSK is even less sensitive than the binary scheme.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Recursive carrier phase tracking for synchronous multilevel 4QAM receivers

Four-quadrature amplitude modulation (4QAM) allows high spectral efficiency in coherent optical data transmission by exploiting the two phases and the two polarizations of the propagating wave. In this paper the performance of 4QAM is analyzed in the presence of laser phase noise and photodetection noise. A decision feedback phase tracking scheme based on the Kalman filter and leading to a novel hybrid PLL structure is employed and its accuracy evaluated. The computational complexity of the proposed phase estimator is also discussed. The performance of the resulting 4QAM system is evaluated in terms of bit error probability versus the signal-to-noise ratio E/sub b//N/sub 0/, for different ratios of the symbol rate R/sub s/ over the laser linewidth B/sub L/.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Spectral Density Function of Optical CPFSK Signals in the Presence of Laser Phase Noise

Spectral Density Function of Optical CPFSK Signals in the Presence of Laser Phase Noise

Bit Error Rate Evaluation of a Dual-Filter Heterodyne FSK Optical System

The frequency separation between the two channels of a dual-filter heterodyne FSK optical system is a very important parameter to evaluate in the presence of laser phase noise. In this work the impairment due to the corsstalk interference between the two transmitted tones of a weakly coherent FSK optical system is investigated. In order to make accurate computations, the method of moments seems to be the most suitable: to this aim, in this work an algorithm for the moments evaluation is proposed. The numerical results are then compared with a Monte Carlo simulation appraoch

Impact of laser linewidth on performance of heterodyne single filter FSK system with optical preamplifier

A quantitative analysis is provided which shows a significant reduction in the sensitivity of the coherent FSK receiver with a semiconductor optical preamplifier in the presence of laser phase noise. The reduction in sensitivity is about 7.0 dB when the linewidth of the laser is only 4% of the bit rate and is higher for larger values of linewidth.