Phase noise in coherent analog AM-WIRNA optical links

Coherent analog amplitude modulated-wideband rectifier narrowband (AM-WIRNA) systems have been the focus of many recent studies because of their high performance and relative immunity to phase noise compared to angle modulated systems. Despite their natural advantages over angle modulated systems, AM-WIRNA receivers are still vulnerable to phase noise because of distortion of their phase broadened signals in a finite bandwidth system. We present the first numerical analysis of the effects of this distortion on the performance of AM-WIRNA systems. The analysis accurately models the power spectral density of the phase-to-intensity noise with a root-mean-square deviation from the averaged experimental noise spectrum of 1.2 dB and a maximum deviation of 3.8 dB in the modulation range of <2 GHz. The accuracy of the analysis is limited primarily by nonidealities in the AM-WIRNA receiver and the accuracy of the analytical intermediate frequency (IF) filter model. Optimal link designs are presented which minimize the impact of phase-to-distortion noise in AM-WIRNA systems. We present experimental data from AM-WIRNA links which use both external cavity and distributed feedback lasers for the signal and local oscillator sources. The numerical analysis predicts the link signal-to-noise ratio (SNR) for different signal laser powers to within 1.4 dB of experiment. We find that systems requiring high SNR such as phased array antennas and AM-CATV are significantly affected by this noise     

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     

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。     

一种自适应低相位噪声相参时钟源的设计

通过锁相环电路(PLL),不仅将外部系统提供的具有高频率准确度但相位噪声较差的主时钟信号转化为高频率准确度、低相位噪声的内部时钟信号,同时也满足了内外部系统的相参要求。通过仿真和测试,重点分析了锁相环电路中环路滤波器的环路带宽对输出信号相位噪声的影响。测试结果显示,当环路带宽为100 Hz时,锁相环的输出信号在偏离载波1 kHz处的相位噪声与其内部振荡器在此处的相位噪声基本一致;而当环路带宽为500 Hz时,输出信号在偏离载波1 kHz处的相位噪声会由于环路影响,相比内部振荡器产生8 dB左右的恶化。设计所得时钟源在输出100 MHz信号时,其相位噪声优于-147 dBc/Hz@1 kHz,相比外部参考时钟信号改善了12 dB,并且其频率准确度可达1×10-9。     

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。     

Performance of coherent optical CPFSK-DD with intersymbolinterference, noise correlation, and laser phase noise

An exact probability of error expression is presented for a narrow-deviation binary CPFSK coherent optical receiver utilizing differential detection (CPFSK-DD). The result is given in terms of the Marcum Q-function and takes into account the non-Gaussian noise statistics at the decision moment, intersymbol interference, noise correlation, and laser phase noise. Numerical results indicate a local oscillator shot-noise-limited receiver sensitivity of 23.4 photons/b when using a modulation index of 0.67 in combination with an IF filter having a sixth-order Butterworth magnitude response and a normalized 3-dB bandwidth of 1.09. For a given IF filter and IF linewidth, it is found that the modulation index and the IF filter bandwidth should be jointly optimized in order to achieve the best overall receiver sensitivity. When the IF linewidth times the differential delay product is 0.34 %, modulation index and the optimum normalized 3-dB bandwidth are found to be 0.72 and 1.1, respectively. By using Monte Carlo simulation, it is demonstrated that adding laser phase noise at the demodulation stage of the analysis is a valid simplifying assumption for a wide range of practical design parameters     

相对相位噪声对相干光通信系统性能影响

在使用前向抽运喇曼放大器的相干光正交频分复用传输系统中,由于喇曼抽运的强度噪声和交叉相位调制之间相互作用产生相对相位噪声,导致系统性能大幅下降。为了研究此问题,采用数值分析方法进行了理论分析与实验验证,分析了不同的调制格式下,相对相位噪声对相干光正交频分复用系统的影响,并且比较了正交频分复用多载波系统与单载波系统在相同条件下的系统性能,取得了由相对相位噪声所导致的系统损伤程度数据。结果表明,抽运和信号之间相对较大的离散系数有助于抑制相对相位噪声引起的损伤,高阶调制信号比低阶信号对相对相位噪声耐受性低;频谱相同效率时,正交幅度调制格式比相移键控耐受性更好。     

Effects of laser phase noise on the performance of optical coherent receivers

Laser phase noise (LPN) plays an important role in optical coherent systems. Based on the algorithm of Viterbi-Viterbi carrier phase estimation (CPE), the effects of LPN imposed on the coherent receivers are investigated for quadrature phase shift keying (QPSK), 8 phase shift keying (8PSK) and 16-quadrature amplitude modulation (16-QAM) optical coherent systems, respectively. The simulation results show that the optimal block length in the phase estimation algorithm is a tradeoff between LPN and additive white Gaussian noise (AWGN), and depends on the level of modulation formats. The resolution requirements of analog to digital converter (ADC) in the coherent receivers are independent of LPN or the level of modulation formats. For the bit error rate (BER) of 10-3, the required bit number of ADC is 6, and the gain is marginal for the higher resolution.     

Thermal phase noise in coherent optical-fibre systems

Coherent optical-fibre systems are sensitive to optical phase changes along the length of the fibre link. Thermal energy in the optical fibre can give rise to acoustic waves propagating along it and these result in optical phase changes. Thus, there is the possibility of a noise signal induced by thermal acoustic energy propagating along the fibre. This letter postulates a theoretical model for this interaction and estimates the magnitude of the effect. Conditions under which it may be a significant noise source are discussed.     

相干光OFDM通信系统中的IQ补偿和相位估计

文章首先研究了相干光OFDM(正交频分复用)通信系统中的线性损耗,包括IQ(同相/正交相)不平衡以及线性相位噪声;提出了一种DA(基于决策)的不需要周期性地插入训练数据的方法。计算机仿真结果表明,所提出的方法可以有效估计并补偿这两类线性损耗;在此基础上,提出了一种在考虑信道色散噪声的情况下,联合补偿IQ不平衡和线性相位噪声的新算法。     

The performance of coherent BPSK in the presence of oscillator phase noise for ka-band pico-terminal inbound links

In microwave systems, the phase noise is well known to be a significant source of degradation especially for low data rate transmission, giving rise to a significant irreducible error rate. In an earlier study [1,2] based on the European Space Agency's Olympus Satellite 30/20 GHz payload parameters, it was found that several thousands of simultaneous transmissions of 50 to 200 bits/sec each could be supported from hand-held pico-terminal earth stations. It is important to use the most appropriate modulation scheme in the presence of oscillator phase noise for such low data rate systems. It is generally assumed that noncoherent or differentially coherent modulation schemes will perform best in applications where the coherence time of the oscillators is short compared to the bit period. However, our analysis of these schemes has shown that they are subject to irreducible error rates and minimum useable data rates. This has led us to reconsider coherent BPSK (CPSK). It is shown that the choice of the optimum noise bandwidth of the phase locked loop (PLL) in the carrier recovery circuit is very important when considering the coherent schemes. The PLL bandwidth must be wide enough to track the oscillator phase noise at its input, but, on the other hand, it must be as narrow as possible to exclude the thermal noise. Results show that coherent BPSK in the phase noise contaminated channel is also subject to an irreducible error rate, which increases as the noise bandwidth is reduced. For the optimum loop bandwidth, however, this appears to be lower than the corresponding figure for DPSK [3]. There is a minimum useable data rate for any given bit error rate (BER) requirement. Results of a complete analysis are presented in the form of BER versusE _b/N _o plots.     

Characteristics and reduction of coherent fading noise in Rayleighbackscattering measurement for optical fibers and components

The characteristics of fading noise in Rayleigh backscattering measurements made with coherent lightwaves such as in coherent-OTDR (optical time-domain reflectometry) and coherent-OFDR (optical frequency-domain reflectometry) are studied. The effects of frequency shift averaging on fading noise reduction are clarified theoretically, and the relationships between measurement accuracy and other parameters, such as spatial resolution and frequency variation range are derived. The calculated results of loss measurement accuracy are in good agreement with experimental data. The formula can also be applied to low-coherence interferometric OTDR     

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     

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     

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.     

Digital optical phase control in ridge-waveguide phase modulators

The authors report a digital optical phase modulation concept based on depletion-edge-translation p-n junction GaAs/AlGaAs ridge-waveguide modulators. Digital modulation is achieved by integrating in series several discrete waveguide modulators with lengths related by successive factors of two. To illustrate the concept, the authors fabricated and demonstrated a three-bit digital phase modulator with 45° resolution. This structure represents the first photonic integrated circuit that performs direct digital-electronic to analog-optical conversion     

Interferometric noise reduction in fiber-optic links bysuperposition of high frequency modulation

The reduction of interferometric noise by superposition of high frequency modulation is analyzed. It is shown that the nature of this reduction is due to a redistribution of noise energy from baseband to higher frequencies where it can be discarded by low-pass filtering. Detailed analysis revealed the dependence of the noise reduction factor on the product f_0τ, and the modulation index of the high frequency superimposed modulation. The proper choice of parameters can lead to complete elimination of the converted phase noise from the system     

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     

An experimental and theoretical study of the suppression ofinterferometric noise and distortion in AM optical links by phase dither

The effects of phase dither on the magnitude and spectral distribution of interferometric fiber optic noise and intermodulation products is considered. A theory is presented as well as experimental data which show major reduction of AM noise brought about by intentional narrow band and wide band phase dither of the laser output