Experimental 1.5 μm coherent optical fibre transmission systems

The authors consider the design requirements for 1.52 μm coherent optical fibre transmission systems. Some of the essential requirements are: narrow linewidth sources, polarisation stable transmission media, low insertion loss optical mixers and frequency tracking or phase locked local oscillators. Having been successful in reducing the linewidth of semiconductor lasers to less than 30 kHz, by using injection locking techniques, and in fabricating optical mixers with insertion losses of the order of dB, they have been able to experiment with coherent optical fibre transmission systems to assess their practicability. In this paper it will be shown that coherent systems can be operated over long lengths of conventional low loss monomode fibre without any polarisation penalty and give far better performance than direct detection systems.     

Why FSK and CPFSK have identical linewidth requirements

CPFSK is the most widely used modulation scheme in coherent optical transmission. Theoretical analyses so far predicted a much more stringent linewidth requirement compared to ASK and FSK systems with noncoherent IF-detection. In this paper we present a unified linewidth analysis of ASK, FSK, CPFSK, and DPSK receivers, with leads to the conclusion that the linewidth performance of these different schemes is to a large extent identical. The inclusion of post-detection filtering in the analysis is critical for obtaining these results, which are supported by practical system measurements on a 1 Gbit/s phase diversity CPFSK receiver     

千赫兹量级窄线宽半导体激光器研究进展

窄线宽半导体激光器由于其高单色性、低频率噪声、高可调谐性等优点,广泛应用于高速相干光通信、分布式传感、激光雷达等领域.随着高品质因子(Q)光学谐振腔、硅光异构集成芯片等技术的发展,窄线宽半导体激光器近十年经历了革命式发展,线宽压缩至千赫兹(kHz)量级,甚至到亚千赫兹量级.文章阐述了千赫兹量级窄线宽半导体激光器的最新进展,针对不同压缩线宽机制的窄线宽激光器进行了分类介绍,深入讨论了优化耦合系数、减少外腔损耗等对窄线宽激光器性能的影响,并针对未来应用需求展望了千赫兹量级窄线宽激光器在进一步压缩线宽、提升输出光功率方面的发展方向.     

The effect of laser linewidth on coherent optical receivers with nonsynchronous demodulation

Recent theoretical analysis of the effect of using lasers with significant linewidths in coherent optical fiber transmission systems has shown how the design of the optical receiver, particularly the IF stage, affects the sensitivity. This paper reviews this theory for two classes of coherent systems-those making a differential phase measurement, and those not using phase information-where the requirements on laser linewidth are not stringent. We discuss the factors that affect the performance of systems with significant laser phase noise. Wherever possible, we compare the theoretical results with published system experiments. We show that our theory fits the experimental data well and indicates why experimental results have fallen short of the theoretical limits.     

大气信道下窄线宽激光器对相干通信系统性能影响研究

在空间下行相干激光通信系统中,激光器的线宽大小会影响通信系统的性能,而窄线宽激光器能有效降低激光器线宽引起的激光器相位噪声,目前已经成为相干激光通信系统的首选。光信号在大气信道传输时,大气湍流会引起光信号强度和相位的波动,从而进一步影响系统的通信性能。针对上述问题,基于四相移键控(Quadri Phase Shift Keying, QPSK)通信系统的工作原理,进一步考虑窄线宽激光器线宽和大气湍流引起的光信号强度和相位的波动,给出了空间下行QPSK通信系统误码率模型。并基于该模型,数值仿真分析了窄线宽激光器对空间下行相干激光通信系统性能的影响。结果表明：大气湍流不仅严重影响系统性能,也会弱化激光器线宽对系统性能的影响。而对于大气湍流影响而言,其引起的相位波动要大于光强波动的影响。此外,随着通信速率增大,激光器线宽对于系统性能影响也会随之降低。文中对于空间下行相干激光通信系统的优化设计和调试具有一定的实际参考意义。     

Experimental linewidth-insensitive coherent analog optical link

We constructed an experimental linewidth-insensitive coherent analog optical link. The transmitter utilizes an external electro-optic amplitude modulator and a semiconductor laser. The receiver consists of a heterodyne front-end, a wideband filter, square law detector and narrowband lowpass filter. We performed experimental measurements and theoretical analyses of the spurious-free dynamic range (SFDR), link gain and noise figure for both the coherent AM and the direct detection links; we investigated the dependencies of the foregoing parameters on the received optical signal power, laser linewidth, IF bandwidth, and the laser relative intensity noise (RIN). By selecting a wide enough bandpass filter, we made the coherent AM link insensitive to laser linewidth. The coherent AM link exhibits a higher SFDR than the corresponding direct detection link when the received optical signal power is less than 85 μW. The noise figure for the coherent link is greater than that for the direct detection link under all conditions investigated. For received optical signal powers greater than 4 μW, the link gain for the direct detection link is greater than that for the coherent AM link. The following are the link parameters that have been achieved for the coherent AM link investigated: SFDR=88 dB·Hz^2/3, link gain=-25 dB and noise figure=78 dB; this performance has been obtained with a received optical signal power of 85 μW, and a local oscillator power at the photodetector of 228 μW. The link performance can be further improved by auxiliary subsystems such as a balanced receiver and impedance matched transmitter and receiver ends; and/or by using better optical and electrical devices like higher power lasers, linearized optical modulators, low-noise and high gain RF amplifiers, and optical amplifiers,     

Feasibility study on QPSK optical-heterodyne detection system

The feasibility of a QPSK (quadrature-phase-shift-keying) optical heterodyne detection system for a high-speed and long-span coherent transmission is investigated. The key factors, which are the modulation method and the spectral linewidth requirement for optical light sources, are discussed. Through theoretical analysis and the simulation experiment, it is confirmed that the required beat spectral linewidth for QPSK coherent detection is less than 7.5×10^-5×bit rate. It is also shown that the required spectral linewidth for QPSK differentially coherent detection is less than 2.5×10^-5×bit rate. These results show that the gigabits-per-second range QPSK optical heterodyne detection can be realized with conventional external cavity laser diodes     

Design of asymmetric quarter-wave-shifted DFB semiconductor lasers

Quarter-wave-shifted distributed-feedback (QWS-DFB) semiconductor lasers with asymmetric structure are theoretically analysed in terms of wavelength selectivity, differential efficiency, linewidth and sensitivity to external optical feedback. The analysis is used to design narrow-linewidth lasers with low sensitivity to optical feedback for coherent transmission.     

Theory of signal degradation in semiconductor laser amplifiers withfinite facet reflectivities

An analytic expression is derived for the output signal spectrum from a semiconductor laser amplifier (SCLA) using an approach analogous to the Langevin equations of laser noise analysis. The authors present an analysis of the line broadening effects of SCLAs taking into account the effect of facet reflectivities. The signal linewidth is shown to increase with input linewidth and amplifier gain. There is an optimum facet reflectivity which gives the smallest linewidth increase. The results have significant implications in weakly coherent systems utilizing cascades of optical amplifiers     

分布反馈激光器的线宽及其与相干光通信的关系

本文详细地讨论了分布反馈激光器的线宽及其影响因素。同时还讨论了线宽与相干光通信的关系。     

Signal linewidth broadening due to nonlinear Kerr effect inlong-haul coherent systems using cascaded optical amplifiers

Signal spectral linewidth broadening due to the interaction between nonlinear Kerr effect and spontaneous emission from optical amplifiers in long-haul coherent optical fiber communication systems using optical amplifiers is examined theoretically and experimentally. By theoretical consideration and recirculating-loop experiments, it is shown that this effect becomes an essential limitation on maximum transmission length in transoceanic coherent optical fiber submarine cable systems     

BER impairment due to laser linewidth in OFDM-OOK systems usingdouble-cavity Fabry-Perot demultiplexers

We report on the impact of finite laser linewidth on the bit error rate (BER) performance in directly detected optical frequency-division multiple-access (OFDMA) employing double-cavity Fabry-Perot (FP) filters. Three different double-cavity FP demultiplexers have been considered in this study. Also the laser linewidth impairment on the crosstalk probability mass functions, the crosstalk power penalties and the OFDMA network capacity are calculated. The results demonstrate that, even though these demultiplexing schemes are much superior than single-cavity FP filters for highly coherent sources (laser linewidth lower than a few megahertz), when laser linewidth increases this feature appears to be much less significant due to the severe impairment of the laser linewidth on the BER performance, specially for structures with higher optical filter finesse     

Extremely narrow spectral linewidth and low chirping of theMQW-DFB-PPIBH laser diode

The low threshold current of 9 mA, the high side-mode suppression ratio of more than 45 dB, the extremely narrow spectral linewidth of 1.1 MHz, and the low chirping of 2.8 Å at -20 dB at 2 Gb/s nonreturn to zero (NRZ) modulation have been achieved in the multiple quantum well (MQW) distributed feedback (DFB) p-substrate partially inverted buried heterostructure (PPIBH) laser diode. The spectral linewidth of 1.1 MHz is the narrowest value among 300-μm-length solitary laser diodes. These results suggest that the MQW-DFB laser diodes are a promising light source for longer distance and higher bit-rate optical communication systems and coherent optical communication systems     

Laser phase noise impact on optical DP-MQAM: experimental investigation

Advances in optical technologies, modulation formats, and symbol transmission rates became paramount for today’s agile high spectral efficiency optical networks. Nonetheless, laser phase noise (PN) has played a major impact on the performance of these systems due to the employment of moderate-quality lasers that significantly affect the implementation of high-order modulation schemes starting from existing QPSK and growing to MQAM, \(M=16\), 32, and 64. In this paper, we report an optical dual-polarization (DP) transmitter prototype that digitally emulates the effect of the laser phase noise, according to Wiener process, applied to a different types narrow-linewidth laser sources. This enables the generation of an arbitrary laser linewidth ranging from kilohertz up to tens of megahertz. Our experimental study evaluates the effect of PN on the performance of tracking algorithms commonly implemented on practical coherent receivers. The experimental results show that, for laser linewidth \(\le \)1 MHz, practical filter recovery algorithm succeeds to make appropriate tracking for both DP-QPSK and 16 QAM at 16- and 32-Gbaud system rates. However, for 10 MHz linewidth, correct recovery is possible only for DP-QPSK.     

Theoretical analysis of heterodyne optical receivers for transmission systems using (semiconductor) lasers with nonnegligible linewidth

We present a general theoretical model of receivers for coherent optical communication systems where transmitters and local oscillators having nonzero linewidth are used. Key issues in the model are the concept of single realization measurements of a stochastic intermediate frequency, and development of the probability density function for this stochastic process. Analytical results are derived for heterodyne ASK and dual filter FSK receivers and include the shot-noise limit, the asymptotic error-probability limits in ASK and FSK receivers, the influence of the IF on receiver noise, and the effective local oscillator strength. Detailed numerical results for typical p-i-n-FET wide-band receivers illustrate the influence on receiver sensitivity of IF filter bandwidth and relative threshold setting in ASK systems and of modulation index and IF filter bandwidth in FSK systems. A receiver sensitivity penalty for nonzero linewidth is found to be, for IF linewidths of 0.1 to 0.3 of the bit-rate, 3 to 9 dB in optimum ASK receivers, and 2 to 8 dB in optimum FSK receivers. Thus DFB lasers of linewidth 5 to 20 MHz could be used without external cavities in simple systems with near-ideal performance, which could find application wherever the great multiplexing advantage of coherent systems is a prime advantage. We present some guidelines for system design based on the results of this work.     

Effect of phase shift position on spectral linewidth of the π/2distributed feedback laser diode

The dependence of the spectral linewidth on the phase shift position (PSP) of a phase-shifted distributed-feedback (DFB) laser diode is investigated theoretically, using the coupled wave model. In particular, the analysis is performed to find an optimum PSP so as to minimize the spectral linewidth of the laser diode. This optimum PSP is compared to that obtained by maximizing the stability of the laser oscillation. Since both narrow spectral linewidth and good stability are essential for the light source in coherent optical communications, the analysis revealed the suitability of using phase-shifted DFB laser diodes in coherent transmissions     

A new method for high-resolution measurement of semiconductor laserlinewidth in coherent optical systems

A method for the observation of laser linewidth during operation in a coherent optical system is presented. The measurements agree well with the theoretical calculations and show that the power density spectrum of the backscattered intensity from a single-mode fiber corresponds to the laser spectrum with twice the source linewidth, shifted to baseband     

基于相干包络解调的窄线宽测量算法研究

激光线宽对光学相干探测系统的探测范围、精度和噪声特性都有重要影响。为精确测量半导体激光器的线宽值,提出了通过解调短延时自外差的干涉谱来实现高精度窄激光线宽测量的新方法。搭建短延时的自外差干涉测量系统,得到半导体激光器的干涉谱,通过设计算法对干涉谱的相干包络进行解调,最终获得了严格的洛伦兹线型谱和相应的谱线宽度,通过理论分析、仿真和实验验证了该方法的可行性。该方法可以忽略因1/f频率噪声产生高斯展宽带来的影响,与传统线宽测量方法相比,该方法的测量结果更加精确。     

LDPC码在相干光通信下的性能分析与研究

对于相干光通信系统,相位噪声成为影响系统性能的重要因素之一,并严重影响LDPC码(低密度奇偶校验码)软译码的性能。仿真结果表明,在16QAM调制方式下,500kHz线宽的激光器相位噪声将导致1.5dB的性能损失。为了降低相位噪声的影响,提出了一种基于相位噪声估计值修正LLR(对数似然比信息)的方法。经仿真验证,该LLR修正方法能够挽回0.5dB的译码性能损失。     

Signal linewidth broadening due to fibre nonlinearities in long-haul coherent optical fibre communication systems

Signal spectral linewidth broadening due to the Kerr effect in long-haul coherent optical fibre communication systems is examined theoretically and experimentally. By recirculating loop experiments, it is shown that linewidth broadening becomes serious in a system of more than a few thousand kilometres.<>