Dynamic-Range Enhancement and Linearization in Electrooptically Modulated Coherent Optical Links

We present a general analysis of various electrooptic-modulation configurations on the system performance of a coherent analog optical link. We evaluate the dependence of the modulation scheme of the optical modulator and its properties on the system dynamic range and linearity. Linearization of a coherent optical link based on extending the length of a directional-coupler modulator is also described, illustrating the unique considerations when linearizing a coherent link. System-level modeling of an analog coherent optical link under different modulation configurations is presented.     

A full-duplex optical access system with hybrid 64/16/ 4QAM-OFDM downlink

A full-duplex optical passive access scheme is proposed and verified by simulation, in which hybrid 64/16/4-quadrature amplitude modulation (64/16/4QAM) orthogonal frequency division multiplexing (OFDM) optical signal is for downstream transmission and non-return-to-zero (NRZ) optical signal is for upstream transmission. In view of the transmitting and receiving process for downlink optical signal, in-phase/quadrature-phase (I/Q) modulation based on Mach-Zehnder modulator (MZM) and homodyne coherent detection technology are employed, respectively. The simulation results show that the bit error ratio (BER) less than hardware decision forward error correction (HD-FEC) threshold is successfully obtained over transmission path with 20-km-long standard single mode fiber (SSMF) for hybrid downlink modulation OFDM optical signal. In addition, by dividing the system bandwidth into several sub-channels consisting of some continuous subcarriers, it is convenient for users to select different channels depending on requirements of communication.     

Optimum operating points for electroabsorption modulators in 10Gb/s transmission systems using nondispersion shifted fiber

The chirp, optical extinction ratio and insertion loss of an electroabsorption modulator (EAM) depend on the properties of the bulk or multiple-quantum-well absorption layer of the device and on the bias voltage and modulating voltage waveform. For 10 Gb/s transmission over nondispersion shifted fiber, joint optimization of the bias and modulation voltages is considered for five different EAM's. To comprehensively explore this issue, the measured dependence of the absorption and α-parameter on applied voltage is used to accurately model an EAM in a system simulator. The effects of group velocity dispersion and self-phase modulation arising from the Kerr nonlinearity are included to permit assessment of the dependence of the optimum bias and modulation voltages on the average transmitted optical power for a given fiber length. The improvement in receiver sensitivity, relative to that obtained with maximum optical extinction ratio, depends quite significantly on the transmitted optical power and the specific properties of the modulator. This makes it difficult to determine optimum operating conditions which apply generally     

Multilevel polarization modulation using a specifically designedLiNbO3 device

Binary and multilevel optical polarization modulation (POLSK) is an innovative transmission technique, suitable for both coherent and direct detection. To prove its feasibility, we have built a flexible polarization modulator based on a specifically designed LiNbO₃ device. We show that it can generate any state of polarization and therefore it is suitable for transmission of any binary or multilevel POLSK constellation. Once electrically equalized, the modulator has a 4 GHz bandwidth. To test the modulator with actual digital modulation, we set up a coherent binary experiment at 800 Mbit/s, and attained a sensitivity 1.5 dB away from the noise limit of the receiver     

A lithium niobate light modulator for fiber optical communications

An electrooptic intensity modulator using lithium niobate has been developed for applications in binary fiber optical digital communications at the wavelength of 1.06 µm. We have shown that many shortcomings generally associated with electro-optic modulators can be surmounted. The modulator was driven by a compact transistor amplifier, temperature dependence of the static birefringence was minimized, and the optical bias was made adjustable by a dc voltage superposed on the signal. The modulator has been operated at 70-Mb/s pulse rate and 100-percent modulation, its extinction ratio is better than 40 to 1 and the optical insertion loss is about 1 dB.     

Linewidth-insensitive coherent AM optical links: design,performance, and potential applications

The use of coherent detection in analog optical links offers several advantages over direct detection: improved receiver sensitivity, inherent frequency translation, and the ability to utilize angle modulation and separate wavelength division multiplexed (WDM) signals. In this paper, we investigate an externally modulated coherent AM optical link. We study the dynamic range of the coherent AM link, considering receiver noise, laser phase noise, laser relative intensity noise (RIN), and system nonlinearities. With proper selection of the receiver's IF bandwidth, the coherent AM link can be made insensitive to the laser linewidth. For optical powers less than 5 mW, RIN of less than -160 dB/Hz reduces the spurious-free dynamic range (SFDR) by less than 3 db with the use of a balanced receiver. The external modulator nonlinearity is the dominant nonideal effect; it reduces the SFDR by 5-19 dB from the theoretical limit for 100% modulation index. We compare the performance of the coherent AM link with that of a conventional direct detection link for two applications: point-to-point links and distribution networks. When the received optical power is less than 1 mW, the coherent link can provide higher SFDR than the direct detection link. Thus, coherent links are well-suited for long distance point-to-point links and FM video distribution systems     

正交调制器的校准设计

在当前许多复杂调制射频信号源中,随着数字基带信号越来越多的加入．需要正交凋制器将其调制到需要的载波信号上。正交调制器原理简单：将生成好的I/Q两路基带信号调制到两路正交的载波上,合路后输出。但是在实现时,想获得好的指标就需要考虑载波相位误差,I/Q基带信号幅度不平衡以及载波泄漏等问题。文章给出了一种正交调制器的校准方案,可以减少和模拟以上三种现象造成的调制误差,并给出自动校准和手动校准两种方法。     

Actively mode-locked 1.5-μm 10-GHz picosecond fiber laser usinga monolithic semiconductor optical amplifier/electroabsorption modulator

In this letter, we demonstrate a novel 10-GHz actively modelocked polarization-maintaining fiber ring laser that utilizes a single device for gain, loss modulation, optical isolation, and repetition rate adjustment. This monolithic device incorporates a semiconductor optical amplifier with an electroabsorption modulator and provides net gain of 2.1 dB. The repetition rate of the 1.5-μm picosecond pulsetrains can be tuned by 2 ppm through perturbation of the electroabsorption modulator reverse bias     

外调制宽带外差光相干传输链路的构建

设计了外调制微波外差相干模拟光传输链路.对比分析了外调制外差相干传输链路和外调制直接检测传输链路的性能,对影响链路的关键参数进行了分析,数值分析结果表明,该链路达到了设计要求.

Abstract：

Heterodyne amplitude modulated coherent link is very importmant to produce and transmit high speed microwave signal in wide-band optical transmission system.The performances of wide-band optical coherent link and direct detect link with external modulator are compared and the main parameters are analyzed in detail.Then an amplitude modulation optical coherent transmission system is designed to be used in analog transmission system.     

激光通信中马赫曾德调制器无抖动偏置控制技术

为实现马赫曾德调制器（Mach-Zehnder Modulator,MZM）工作时偏置点的稳定控制,提出了一种基于平均光功率斜率值检测的简单有效的控制方案。首先,分析了MZM工作时偏置点稳定控制技术的重要性,从理论上研究了控制算法的可行性,然后,利用Matlab进行了仿真验证,最后在搭建的实验平台上进行了MZM工作偏置点稳定控制系统的实验验证。结果表明该控制方法是一种适用于多种调制格式的简单有效的无抖动偏置控制技术,并且实验观察结果表明系统误码率性能在72 h内没有下降,能稳定在10-9附近,有效地保证了激光通信调制系统工作的可靠性。     

相位调制微波光传输链路特性分析（特邀）

常规的微波光子系统采用强度调制方式实现微波信号的电光转换,由于调制器采用马赫-曾德尔干涉结构(MZI),系统性能不仅受到自身正弦响应特性的制约,而且需要进行偏置点控制,因此存在动态范围受限、系统控制复杂以及3 dB固有损耗带来的效率不足的问题,而采用相位调制可避免该问题。围绕相位调制光传输链路,为了完成相位调制信号的光电解调,文章提出采用薄膜滤波器通过边带抑制与边带选通两种方式实现相位调制到强度调制的转换,并分析了链路射频性能与器件参数之间的映射关系。实测对比了相位调制与常规强度调制链路之间的传输特性,通过分析可知,在相同链路配置条件下,相位调制链路具有更高的传输效率,而且光滤波带来的均衡作用,使得相位调制链路的3 dB带宽比强度调制链路大两倍。     

Compensation for I/Q Imbalances and Bias Deviation of the Mach–Zehnder Modulators in Direct-Detected Optical OFDM Systems

We demonstrate an equalization technique to simultaneously compensate for the fiber chromatic dispersion and the distortions induced by the in-phase/quadrature-phase (I/Q) imbalances and bias deviation in the Mach-Zehnder modulators (MZMs). With this technique, the requirement for the monitoring circuits at the transmitter could be relaxed or even eliminated, and thus simplifies the system design and reduces the system cost. The system exhibits a good performance with our proposal over a broad range of imbalances and bias deviation in the MZM. With deliberately introducing I/Q imbalances and bias deviation, we have observed a 2- and 4-dB optical signal-to-noise ratio improvement, for the back-to-back and 800 km of uncompensated standard single-mode fiber transmission, respectively, with our proposed scheme.     

Erbium Amplifier Dynamics in Wireless Analog Optical Links With Modulator Bias Optimization

Modulator bias optimization is a technique used to improve the transmitted radio-frequency power of amplified analog optical links. However, when bias optimization is used, the mean optical power at the modulator output is dependent on the amplitude of the modulation signal. In this letter, we show how fluctuations of the modulation signal power, in conjunction with the dynamic behavior of the erbium optical amplifier, can deteriorate the transmission of data frames compliant to the 802.11 a/g IEEE protocol. Optical and electrical compensating methods, based on mean power clamping at the modulator output, are proposed, and their efficiency in restoring a high-quality transmission is demonstrated     

LabVIEW在零差相干光检测系统的应用

基于相干光通信原理设计一套实验装置以搭建零差相干光检测平台,选择NI的高性能数据采集卡PCI6111E,利用LabVIEW虚拟仪器环境,在两台计算机上分别设计发送端和接收端控制软件,验证同步检测,最后进行数据采集和处理并显示测量波形.实验表明,采用光同步方案结合LabVIEW软件控制,解决了发送端的声光调制器、电光调制器、接收端的数据采集和基于压电陶瓷的相位调节器之间的同步过程,实现了相干光信息的发送调制和实时接收解调.     

布里渊光时域反射计中电光调制器的调制特性与控制

在布里渊光时域反射计（BOTDR）中,电光调制器（EOM）的工作点会随着外部环境变化而发生漂移,这将影响BOTDR系统的测量精度及测量稳定性。本文在分析BOTDR系统中EOM调制特性的基础上,提出了一种控制EOM工作点的方法,使用数字信号处理器作为控制核心,采用＂步进跟随＂的算法。实验结果表明,本文方法可以使EOM长期...     

Millimeter-wave asymmetric Fabry-Perot modulators

We have designed, fabricated, and characterized GaAs-AlGaAs (λ=864 nm) asymmetric Fabry-Perot modulators with ≈37 GHz modulation frequency response, comparable to the fastest waveguide modulators. The modulation response saturates at high optical powers due to saturation of the excitonic absorption and heating effects, but the frequency response is independent of the incident optical intensity, since it depends only on the RC time constant, and not on the carrier transit time. The device design takes advantage of the fact that the quantum-confined Stark effect is more pronounced at some distance from the absorption edge to achieve a modulator with ⩾20 dB contrast and ≈3 dB insertion loss for ±2 V operating voltage, but only 21 fF capacitance. The DC bias used to move the operating point off the absorption edge has the additional benefits of improving the linearity and chirp of the device, as well as the saturation intensity. Here we present measurements of the modulation and photocurrent responses of the modulators, calculate the RC and transit times for the device, analyze the saturation mechanisms, and discuss the linearity and chirp of the device from the perspective of a high-speed digital optical communications system     

外调制微波光子链路性能研究

针对外调制方式工作的微波光子链路,建立了链路的小信号分析模型,理论仿真了调制器输入光功率及调制器直流偏置点对链路增益、噪声系数和线性动态范围的影响。理论仿真与实验结果表明,适当增大调制器输入光功率以及使调制器工作在最佳线性偏置点,可提高链路增益和线性动态范围,同时降低链路噪声系数。该研究为优化外调制微波光子链路性能提供了有益参考。     

基于相位调制器的宽带窄线宽的线性调频激光源的产生

提出了一种基于相位调制器(PM)和可调谐光滤波器产生线性调频激光信号的方法。该方法利用带有基频的微波线性调频信号作为相位调制器的驱动信号,窄线宽的激光种子源经相位调制器调制后产生一系列的宽带线性调频激光信号。通过可调谐光滤波器抑制其他边带保留所需阶次的线性调频激光信号。实验结果表明:当光滤波器保留正二阶调频激光信号时,获得了调频带宽为2 GHz、调频速率为6 THz/s的线性调频激光信号。在观测时间为1 ms时,测得的线性调频激光信号的瞬时线宽为3.2 kHz。该方法结构简单,易于实现,并且对调频连续波激光雷达、相干光谱分析等测量应用有重要意义。     

High-Speed Low-Loss Schottky-i-n InP-Based Optical Modulator for RF Photonics

An InP-based high-speed optical modulator is presented. The Schottky-i-n waveguide structure on InP-based material was used to reduce the switching voltage V_pi and the excess loss, while maintaining high-modulation efficiencies. To minimize residual amplitude modulation and to improve power handling capability, the bulk electrooptic effect in InGaAlAs was utilized for phase shifting. As a result, a simple structure InAlAs-InGaAlAs Mach-Zehnder optical modulator with traveling-wave electrodes was fabricated and characterized. This device achieved a switching voltage V_pi of 3.6 V, extinction ratio (>23 dB) and high-speed operation at 1.55-mum wavelength     

Optimizing the bias and modulation voltages of MQW Mach-Zehndermodulators for 10 Gb/s transmission on nondispersion shifted fiber

The chirp and optical extinction ratio of a multiple quantum-well (MQW) Mach-Zehnder modulator depend on the device design and on the voltage waveforms applied to the arm electrodes. For 10 Gb/s transmission over nondispersion shifted fiber, joint optimization of the bias and modulation voltages is considered for a conventional modulator and a π-phase-shift modulator. Measured attenuation and phase constants for an optical signal propagating in the modulator waveguide are used to accurately model the Mach-Zehnder modulators. The influence of asymmetric Y-branch waveguides in the modulators is examined taking into consideration group velocity dispersion and self-phase modulation arising from the Kerr nonlinearity. When the modulators are operated with maximum optical extinction ratio, the dispersion limited transmission distance depends on the device design (phase-shift and Y-branch splitting ratio) and modulation format (dual drive or single drive). Optimization of the bias and modulation voltages reduces this dependence significantly, while also increasing the dispersion limited transmission distance