Digital adaptive phase noise reduction in coherent optical links

Coherent optical links enable high-density constellations and, consequently, a higher throughput. However, the phase noise associated with the transmitter and the receiver lasers is a challenging issue in coherent lightwave systems. The authors present an approach that relies on using digital signal processing techniques to compensate for the laser phase-noise effects. The proposed approach exploits the digital processing power to address the problems arising from optical imperfections. The authors present an adaptive filtering scheme that reduces the effect of the laser phase noise and, consequently, relaxes the laser linewidth requirement. The proposed approach shows how the signal processing techniques can be exploited to compensate for the optical impairments by utilizing the continuing scale down in size and power in very large scale integration (VLSI) technology.     

Dynamic range of coherent analog fiber-optic links

We investigate the performance of coherent analog optical links employing amplitude modulation (AM), phase modulation (PM), and frequency modulation (FM). The performance of these coherent links is compared to that of AM direct-detection (DD) links. The signal-to-noise ratios, nonlinearities, and-spurious-free dynamic ranges (SFDR's) of the foregoing links are evaluated. We calculate the SFDR for links using DFB and Nd:YAG lasers with typical linewidths of 10 MHz and 5 kHz, respectively. The performance of PM and FM links is dominated by phase noise above a critical value of received optical power. For a linewidth of 10 MHz, and SFDR's of PM and FM links are 30 and 31 dB, respectively, for a received optical power above -27 dBm in a 1 GHz bandwidth. For a linewidth of 5 kHz, the corresponding SFDR's above a received power level of 0 dBm are 51 and 53 dB. The performance of DD and AM links is dominated by RIN above a critical value of received optical power. For a RIN level of -155 dB/Hz, the SFDR's of DD and AM links are 49 and 47 dB, respectively, for a received optical power of 10 dBm in a 1 GHz bandwidth. The SFDR's of the DD and coherent links used for transmission of subcarrier-multiplexed (SCM) signals are also derived. We evaluate target laser parameters needed by a number of different applications. For AM video and antenna remoting applications, linewidths of <1 and <3 kHz are required to use PM and FM links, respectively. For FM video, linewidths of <150 and <350 MHz are required to use PM and FM links. For SCM digital applications, linewidths of <80 and <200 MHz are required to use PM and FM links. The paper concludes with a discussion of system implementation issues, including linearization, optical frequency modulation, balanced receivers, and IF issues     

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,     

RF response of analog optical links employing optical phaseconjugation

The RF transfer function of analog optical links employing optical phase conjugation (OPC) for the compensation of the carrier suppression effect is investigated theoretically. It is well known that the ultimate performance of the OPC technique depends on several system parameters such as fiber span lengths, dispersion characteristics, input optical powers, or optical transmitter chirp. The influence of these parameters on the RF system response is studied by means of simulation, showing that the main degradation of the equalized response is due to the joint action of nonlinear effects and asymmetric optical power change along the fiber spans     

Performance prediction and optimization of a coherent phasemodulated low noise analog optical link operating at microwavefrequencies

Analog optical communication links operating at microwave frequencies are useful for applications like antenna remoting, transceivers, optical signal distribution (CATV), etc. In recent years great progress has been achieved on AM optical links, however, very little has been published on the use of coherent optical links for analog applications. In this paper, we present the analysis, calculated performance, and design guidelines for a coherent phase modulated analog optical link. The performance of this link is compared to that of AM links, and a substantially improved performance is predicted     

Laser mode partition noise in optical wideband transmission links

Laser mode partition noise is an important source of noise in optical transmission systems operating with multimode lasers and dispersive fibres, e.g. single-mode fibres with non-zero material dispersion. The letter describes investigations which confirm the importance of considering laser mode partition noise when practical optical systems are projected. It also gives theoretical estimations and measurement results which are in good agreement, and demonstrates the influence of laser mode partition noise on the transmission quality of an analogue optical system.     

Optical amplifier noise figure in a coherent optical transmissionsystem

Two important aspects of optical amplifier noise figure as measured with a heterodyne detection receiver are investigated. First, differential mode gain will result in polarization-dependent degradation of the perceived amplifier noise figure, even when the noise figures for the TE and TM modes of the amplifier are the same. For a differential mode gain of 7.5 dB the noise figure degradation can be as large as 3 dB, and experimental data is reported in good agreement with theoretical predictions. Secondly, the first experimental demonstration of the use of image-rejection techniques to improve the sensitivity of a heterodyne receiver limited by beat noise between the local oscillator and amplified spontaneous emission is discussed     

Moment characterization of phase noise in coherent optical systems

The statistical characterization of the phase noise introduced by a semiconductor laser in a coherent optical transmission system is a key problem in the system performance evaluation. The authors consider the moment characterization, of the complex random process. Starting from the implicit representation of the probability density function through the Fokker-Planck equation, the authors obtain closed form analytical expressions for the moments of the filtered phase noise both in stationary and nonstationary conditions. Then the use of the moments for the computation of probability densities through orthogonal polynomial series expansion and maximum entropy approach is considered in application examples     

Phase diversity techniques for coherent optical receivers

In the present state of the art, coherent optical receivers most often operate in the heterodyne mode. Here a photodiode-amplifier combination having bandwidth greater than twice the bit rate (B) is needed: indeed bandwidths considerably greater than2Bare preferably employed to ease design of the bandpass filter needed for noise limitation, and to avoid demodulator penalties in some modulation schemes. For the high bit rate systems now coming into service (560 Mbit/s-2.4 Gbit/s), the optical receiver design requirements become more stringent for coherent heterodyne operation. The various modes of "zero IF" operation, however, require only baseband receiver module bandwidth. The options available are either homodyne (phase locked) operation, or phase diversity (multiport) techniques. In this paper, we compare these options, and show that phase diversity techniques are capable of good performance for high bit rate coherent receivers. In phase diversity operation, not only is phase locking avoided, but also the necessary frequency locking does not have high stability requirements. Furthermore, there are advantages in operating with a small frequency offset from zero (of the order of 1 percent of the bit rate). An experimental receiver has been operated at 320 and 680 Mbit/s, demodulating both amplitude shift keying (ASK) and differential phase shift keying (DPSK). Operation with FSK is also possible. Sensitivities so far achieved of -47.5 dBm (320-Mbit/s ASK) and -42 dBm (680- Mbit/s ASK) with limited local oscillator power are capable of substantial improvement when higher power local oscillators and lower noise receive modules become available. Demodulation of DPSK at 320 Mbit/s has also been achieved and shows a measured receiver sensitivity improvement of over 4 dB over ASK at the same bit rate and local oscillator power. These practical results show clearly that phase diversity is a very realistic option for high bit rate systems.     

Equalization and mode partition noise in all-plastic optical fiberdata links

High-speed, short-distance data transmission over all-plastic step-index fiber (plastic optical fiber, or POF) is normally considered to be limited by intermodal dispersion. Theoretical calculations in this letter show that the baseband frequency response can be significantly improved using simple linear equalization. However, as the fiber length is increased, the sharp absorption attenuation peaks of PMMA fiber can potentially introduce mode partition noise. The bounds on practical laser characteristics and fiber lengths are explored; these results indicate that 530-Mb/s transmission over 100 m of 1-mm-diameter POF is feasible, thus potentially satisfying a significant segment of future computer interconnect applications     

Phase noise in coherent systems: basic model for simulations

For modelling filtered phase noise more efficiently, the number of numerical samples can be reduced by the method of Foschini et al., making use of the Radon-Nikodym theorem. In contrast to this abstract formalism, the author reviews the underlying principles using only very basic techniques of probability theory. As a consequence, a transparent, more intuitive understanding is developed, which allows a simple error estimation. The deduced formulas depend only on the modelling of phase noise as Brownian motion, but not on the details of the filtering process.<>     

Capacity enhancement in coherent optical MIMO (COMIMO) multimode fiber links

In this paper, we study a coherent optical MIMO (COMIEMO) multi-mode fiber link proposed for enhancing the fiber information capacity. We examine the statistical characterization of the equivalent MIMO channel and the improvement in the fiber capacity due to MIMO transmission. It is shown that the equivalent channel behaves similarly to a complex Gaussian MIMO channel, suggesting that the available results on wireless MIMO communication systems can be applied to optical fiber links for capacity enhancement.     

Realization of an analog predistortion circuit for RF optical fiber links

This paper presents an analog predistortion circuit for RF optical fiber links. The circuit consists of two source-coupled differential transconductance amplifiers which could provide linear and nonlinear transfer charac-teristics by adjusting the bias voltage and the transistor sizes. The circuit was designed and realized in a standard 0.18-μm CMOS technology of SMIC. The chip occupies 0.48 × 0.24 mm~2. The DC supply is 3.3 V. Using this circuit, the third-order intermodulation distortion (IMD) suppression of a directly modulated RF optical fiber link can be improved by 9-16 dBc at relatively low cost.     

射频光传输模拟预失真电路实现

This paper presents an analog predistortion circuit for RF optical fiber links. The circuit consists of two source-coupled differential transconductance amplifiers which could provide linear and nonlinear transfer characteristics by adjusting the bias voltage and the transistor sizes. The circuit was designed and realized in a standard 0,18-μm CMOS technology of SMIC. The chip occupies 0.48 × 0.24 mm^2. The DC supply is 3.3 V. Using this circuit, the third-order intermodulation distortion （IMD） suppression of a directly modulated RF optical fiber link can be improved by 9-16 dBc at relatively low cost.     

Analysis of optical amplifier noise in coherent opticalcommunication systems with optical image rejection receivers

A detailed theoretical analysis of optical amplifier noise in coherent optical communication systems with heterodyne receivers is presented. The analysis quantifies in particular how optical image rejection receiver configurations reduce the influence of optical amplifier noise on system performance. Two types of optical image rejection receivers are investigated: a novel, all-optical configuration and the conventional, microwave-based configuration. The analysis shows that local oscillator-spontaneous emission beat noise (LO-SP), signal-spontaneous emission beat noise (S-SP), and spontaneous-spontaneous beat noise (SP-SP) can all be reduced by 3 dB, thereby doubling the dynamic range of the optical amplifier. A 2.5-dB improvement in dynamic range has been demonstrated experimentally with the all-optical image rejection configuration. The implications of the increased dynamic range thus obtained are also discussed from a systems point of view     

基于延时自零差光相干接收机的激光器相位噪声测试系统

在相干光通信系统中,激光器相位噪声是影响接收机灵敏度的重要因素。针对相干光通信中的激光光源相位噪声测试提出并研究了表征激光器相位噪声的3个关键指标,分别是电场的功率谱密度、相位误差方差和FM噪声谱线,建立了基于延时自零差相干接收技术的窄线宽激光器相位噪声测试系统,实现了系统仿真,并对一窄线宽激光器进行了相位噪声测试,相比传统的自外差线宽测量技术,此方法在满足测试分辨率要求的同时能够更全面表征激光器相位噪声特性。     

Low-loss 1.3-μm MQW electroabsorption modulators forhigh-linearity analog optical links

Low-loss InAsP-GaInP multiquantum-well electroabsorption waveguide modulators have been developed for transmitting microwaves as subcarriers over optical fibers. The fiber-to-fiber insertion loss is only 5 dB at 1.32-μm wavelength. The electrooptic slope efficiency of an 185-μm-long 11-GHz bandwidth device is equivalent to a Mach-Zehnder modulator with a V_π of 2.2 V. The linearity performance was characterized for a test link without any form of amplification. A RF-to-RF link efficiency of -25.5 dB, noise figure of 27 dB and suboctave spurious-free dynamic range of 114 dB.Hz^4/5 have been achieved with 16 mW input optical carrier power. The measured 3-dB electrical bandwidth exceeds 20 GHz for a 90-μm-long device     

Measurement of semiconductor laser amplifier noise figure in coherent optical transmission system

Reports the first detailed investigation of the effect of semiconductor laser amplifier noise figure on the sensitivity of a heterodyne receiver. Independent measurement of the amplifier spontaneous noise spectral density confirms that local oscillator-spontaneous emission beat noise accounts for all of the observed penalty.<>     

Phase noise cancellation in a carrier recovered optical heterodynereceiver

The performance and application of an optical heterodyne receiver which uses a carrier recovery demodulator are described. Phase sensitive demodulators used in coherent optical transmission are compared, and the suppression of both phase noise and frequency instability of light sources by a carrier recovery (CR-) demodulator is described. A carrier recovered PSK (CR-PSK) demodulator and a phase noise canceling circuit (PNC) for a coherent SCM receiver are introduced as examples of CR-demodulators. The relationship between laser diode spectral linewidths and the delay time difference between the two paths in the CR-PSK demodulator necessary to keep the system performance within a certain power penalty is then derived. In a preliminary experiment using 560-Mb/s CR-PSK transmission, a receiver sensitivity of -51.6 dBm was obtained, and a laser phase noise suppression of about ¹/₂ that of DPSK was confirmed. The results suggest the possibility of constructing a heterodyne receiver which has no AFC-loop. Applications of a CR-demodulator to an optical frequency division multiplexing (OFDM) system and to a multivalue modulation scheme are discussed     

微波光链路和光电探测器残余相位噪声的测量

提出了一种测量长光纤微波光链路以及链路中某一器件残余相位噪声的方法。与以前传统的残余相位噪声测量方法相比,新提出的基于互相关的双音法抑制了不相关噪声源引入的噪声,而保留了待测器件的噪声。通过这种方法,测量了外调制长光纤微波光链路的残余相位噪声,一个6 km微波光纤链路的残余相位噪声在1 kHz频偏处被测得为-130 dBc/Hz,在10 kHz频偏处被测得为-140 dBc/Hz,相较于光纤长度为1 m的短微波光链路,残余相位噪声恶化了接近10 dB。另外,为了找出光电探测器的残余相位噪声与其非线性引起的射频功率压缩度之间的关系,还用这种方法测量了工作在不同条件下的光电探测器的残余相位噪声,实验结果表明:光电探测器的非线性会恶化它的残余相位噪声。