Analog characterization of low-voltage MQW traveling-wave electroabsorption modulators

In this paper, high-speed traveling-wave electroabsorption modulators (TW-EAMs) with strain-compensated InGaAsP multiple quantum wells as the absorption region for analog optical links have been developed. A record-high slope efficiency of 4/V, which is equivalent to a Mach-Zehnder modulator with a V/sub /spl pi// of 0.37 V and a high extinction ratio of > 30 dB/V have been measured. A detailed study of the nonlinearity and the spurious-free dynamic range (SFDR) is presented. By optimizing the bias voltage and the input optical power, the SFDR can be improved by 10-30 dB. After minimizing the third-order distortion, an SFDR as high as 128 dB-Hz/sup 4/5/ is achieved at 10 GHz. A simple link measurement was made using this EAM and an erbium-doped fiber amplifier and a 50-/spl Omega/ terminated photodetector. At 10 GHz, a link gain of 1 dB is achieved at a detected photocurrent of 7.6 mA with higher gains at lower frequencies. The dependence of link gains on bias voltage, input optical, and radio frequency powers are investigated in detail.     

Peripheral-coupled-waveguide MQW electroabsorption modulator for near transparency and high spurious free dynamic range RF fiber-optic link

Peripheral coupled waveguide (PCW) design has been deployed in InGaAsP multiple quantum-well (MQW) electroabsorption modulator (EAM) at 1.55-/spl mu/m wavelength. PCW enhances the optical saturation power and reduces the optical insertion loss and the equivalent V/sub /spl pi// simultaneously. A radio-frequency link using a 1.3-mm-long lumped-element PCW EAM has achieved experimentally a link gain of -3 dB, at 500 MHz and at input optical power of 80 mW. The corresponding two-tone multioctave spurious-free dynamic range (SFDR) at the same bias is measured at 118 dB/spl middot/Hz/sup 2/3/. The single-octave SFDR at the third-order null bias is 132 dB/spl middot/Hz/sup 4/5/.     

An optical power equalizer based on one Er-doped fiber amplifier

A device which exhibits zero or slightly negative differential optical gain over a broad input power range is demonstrated. The scheme is based upon saturation enhancement by bidirectional signal input in an Er-doped fiber amplifier. Power equalization of better than 1.2 dB over a 14-dB dynamic input power range is achieved at the 1530-nm signal wavelength. The noise figure penalty due to the equalization is below 3 dB as evaluated by a device simulation     

Enhanced linear dynamic range property of Franz-Keldysh effectwaveguide modulator

The spurious-free dynamic range (SFDR) of an InGaAsP-InP Franz-Keldysh effect (FKE) waveguide modulator is studied. Low-biasing the FKE modulator is shown to enhance the SFDR by 22 dB with a 3-dB optical power penalty. Polarization-independent RF operation is also obtained with low bias. At 2 mA photocurrent, the simulated narrowband and broadband SFDR are, respectively, 127 dB and 104 dB in a 1-Hz bandwidth. The measured result projects a narrowband SFDR of 129 dB-Hz ^4/5 at this photocurrent     

Measurement of harmonic distortion in microwave photodetectors

The use of an offset phase-lock heterodyne system with a large dynamic range (>110 dB) for measuring photodetector (PD) harmonic distortion out to microwave frequencies is discussed. Up to 40 harmonics and second-harmonic levels only 17.8 dB below the fundamental were observed in high-speed PDs with only 1.7 mW incident. Measurements of harmonic power versus incident optical power are reported, along with associated PD bandwidth reduction     

Characteristics of regenerative erbium-doped fibre ring amplifier

A study is presented into the dynamic characteristics of a unidirectional regenerative erbium-doped fibre ring amplifier at below laser threshold. A considerable improvement in gain and noise figure performance was observed due to the suppression of amplified spontaneous emission in the regenerative amplification process. It was found that lower feedback power provides higher optical gain. An optical signal of -33.5 dB was found to experience a gain of ⩾35 dB at and near the resonance wavelength using 1% feedback by an optical fibre coupler for an optical pump (1480 nm) power as low as 10 mW     

Improving the Performance of Sampled-Grating DBR Laser-Based Analog Optical Transmitters

In this paper, the available analog link performance of integrated transmitters containing a sampled-grating distributed Bragg reflector laser, a semiconductor optical amplifier, and a modulator is evaluated. It is found that to provide a link gain and a low-noise figure, an RF preamplifier is required, and for this reason, spurious-free dynamic range (SFDR) including a preamplifier has been evaluated. An SFDR of 110-dBHz^2/3, a noise figure of 5.4 dB, and link gain of 6.9 dB at 5 GHz is obtained. It is further investigated how link SFDR can be improved by linearization techniques. Two novel approaches are proposed and demonstrated: first, predistortion by extraction of nonlinear components from an integrated second modulator exposed to the same wavelength, optical power and temperature for matched nonlinear terms; second, a novel linearized modulator configuration balancing electroabsorption and Mach-Zehnder modulation that can reach a null for both second- and third-order intermodulation products at a single bias point.     

Optical preamplifier using optical modulation of amplified spontaneous emission in saturated semiconductor optical amplifier

This paper demonstrates a novel optical preamplifier using optical modulation of amplified spontaneous emission (ASE) emitted from a saturated semiconductor optical amplifier (SOA). Requirements on optical alignments and antireflection coating for SOAs can be relaxed and the elimination of an optical filter gives us a large tolerance of an input light wavelength in the proposed optical preamplifier. A small-signal gain of a fabricated preamplifier was over 13.5 dB for an input power of below -20 dBm. An optical gain bandwidth was over 60 nm. We measured the small-signal response of the optically modulated ASE. The 3 dB bandwidths at SOA bias currents of 200, 300, and 400 mA were 5.8, 12.6, and 16.5 GHz, respectively. We also investigated improvements in receiver sensitivities with the proposed optical preamplifier. Our calculation shows a possibility of 10 dB improvement in receiver sensitivities by using the optical preamplifier at 10 Gb/s. The measured receiver sensitivity was -22.7 dBm at 10 Gb/s with the optical preamplifier, which is corresponding to an improvement of 2.5 dB in the receiver sensitivity. Further improvements of the receiver sensitivity can be expected by optimizing the structure of SOAs for saturating ASE.     

Broad-band GaAs monolithic equalizing amplifiers formultigigabit-per-second optical receivers

The authors discuss the development of ICs (integrated circuits) for a preamplifier, a gain-controllable amplifier, and main amplifiers with and without a three-way divider for multigigabit-per-second optical receivers using a single-ended parallel feedback circuit, two (inductor and capacitor) peaking techniques, and advanced GaAs process technology. An optical front-end circuit consisting of a GaAs preamplifier and an InGaAs p-i-n photodiode achieves a 3-dB bandwidth of 7 GHz and -12-dBm sensitivity at 10 Gb/s. Moreover, a gain-controllable amplifier obtains a maximum gain of 15 dB, a gain dynamic range of 25 dB, and a 3-dB bandwidth of 6.1 GHz by controlling the source bias of the common-source circuit. Gain, 3-dB bandwidth, and output power of the main amplifier with the three-way divider are 17.4 dB, 5.2 GHz, and 5 dBm, respectively. These ICs can be applied to optical receivers transmitting NRZ signals in excess of 7 Gb/s     

Analog performance of an ultrafast sampled-time all-optical fiber XPM wavelength converter

Analog performance of an all-optical ultrafast wavelength converter is measured and reported for the first time. The wavelength-conversion process is based on nonlinear cross-phase modulation in an optical fiber combined with an optical filter to convert phase modulation to amplitude modulation. The spurious-free dynamic range (SFDR) of the converter is measured to be 82 dB/spl middot/Hz/sup 2/3/. We define a new metric called the SFDR power penalty, which measures the degradation in SFDR relative to baseline the back-to-back analog optical link. The SFDR power penalty was measured to be 5 dB/spl middot/Hz/sup 2/3/ and is shown to be a function of the input optical power. This metric is used to characterize the linear region of the optical wavelength converter.     

两段级联内插差异损耗功率箝制掺铒光纤放大器

针对全光网中信道路径不同所引起的功率失衡问题 ,设计了一种利用商用光纤波分复用器实现两段级联内插差异损耗环的功率箝制掺铒光纤放大器 (EDFA)光路结构 ,并从理论和实验上研究了光路结构参数对放大器动态范围和噪声特性的影响。得出损耗插入最佳位置在 6 0 %附近 ,最佳插入损耗大小 5～ 8dB依抽运功率和光纤长度而定。所研制的功率箝制EDFA在信号波长 15 5 3nm处 ,动态范围大于 2 0dB ,动态范围内平均输出功率超过2 90 μW。     

L-band fiber laser with wide tuning range based on dual-wavelength optical bistability in linear overlapping grating cavities

A new mechanism to tune an erbium-doped fiber laser in the L-band is presented. The technique is based on the dual-wavelength bistability in overlapping linear cavities, which allows two lasing lines to be switched from one to another by a triggering pulse superimposed to the dc bias current applied to the pump source. The two wavelengths can be independently tuned to give the laser a total tuning range of 33 nm that covers the whole L-band, while the tuning range of either of the tunable fiber Bragg gratings (FBGs) used is only half of that range. An output dynamic range of 7 dB, and a maximum output power of up to 11 dBm with high optical signal-to-noise ratios are obtained. A study of the switching transient responses shows that the typical switching time is about 4 to 12 ms, depending on the dc bias current applied to the pump source. Moreover, our wide tuning technique is implemented by only applying ordinary commercial tunable FBGs which normally would have given rise to only about half of the tuning range; no additional filtering mechanisms are required to achieve almost uniform outputs.     

Amorphous silicon phototransistor as nonlinear optical device forhigh dynamic range imagers

An amorphous silicon bulk barrier phototransistor is studied as a basic element for the pixel of high dynamic photosensor arrays. The device is an n-i-δp-i-n structure whose optical gain shows a nonlinear dependence on the illumination intensity. For each applied bias voltage, a quasi-hyperbolic decrease of the optical gain as a function of the incident power is found. This behavior can be explained taking into account both the material characteristics (defect distribution, dopant concentration), and the structure properties. Our measurements lead to a minimum detectable signal of about 0.7 nW/cm² independently on the applied voltage, making the device suitable for low illumination conditions. On the other hand, by increasing the input power up to 35 mW/cm², we did not find saturation of output photocurrent leading to a dynamic range of at least of 120 dB. This value can be further increased by using as basic cell of the pixel the phototransistor and a resistor connected between the voltage supply and the collector electrode     

Noise figure monitoring of optical amplifiers via backwardamplified spontaneous emission

A new configuration of an optical amplifier is proposed that provide in-service noise figure (NF) monitoring. It is difficult to determine the NF of traditional optical amplifiers in operation because the amplified spontaneous emission (ASE) power cannot be measured separately from the output signal power. The proposed technique measures the NF from the backward ASE power. The discrepancy between the NF measured from forward ASE (using the traditional technique) and backward ASE (proposed technique) is found to be less than 1.0 dB at input powers below -6.2 dBm. A prototype optical amplifier is successfully used as a linear repeater, in a 10 Gbit/s 200 km transmission experiment     

Sampled-grating DBR laser-based analog optical transmitters

Sampled-grating distributed Bragg grating (SGDBR) laser-based widely tunable optical transmitters are investigated for application in high-performance analog links. More than 45 nm tuning range, 40 dB sidemode suppression ratio, and peak relative intensity noise below -153 dB/Hz is measured. SGDBR lasers integrated with semiconductor optical amplifiers and electroabsorption modulators (EAMs) are characterized with spurious free dynamic range of 125-127 dB/spl middot/Hz/sup 4/5/ over the wavelength tuning range. It is also shown how the modulation response of the EAM is affected by the optical power to limit the performance of the analog transmitter.     

中频通信信号信噪比的快速盲估计

信噪比是通信信号处理中的一个重要参数,许多算法都以它作为先验信息来获取最佳性能。该论文通过确定中频通信信号的频谱范围来计算信号能量,进而根据定义估计信噪比,由此提出了一种中频通信信号的信噪比快速盲估计方法。对BPSK,QPSK,16QAM等常用线性调制通信信号的仿真表明:该方法性能稳定,在数据样点长度为4096且信噪比变化范围为-5 dB到25 dB时,估计值的偏差和均方根误差基本都小于0.5 dB。与基于特征值分解的方法相比,在得到精确估计结果的同时具有计算复杂度小、适应动态范围广等优点。     

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LFM-OTDR的超长跨距光纤电力通信链路监测

为了降低电力通信系统中白噪声和随机多径噪声,提出了2种基于线性调频-光时域反射仪(LFM-0TDR)技术的监测方案,即单频LFM-0TDR方案和频分复用LFM-0TDR(FDM-LFM-0TDR)方案.该监测方案采用LFM光脉冲信号作为探测信号,利用分数阶傅里叶变换将后向散射回来的探测信号变换到分数阶傅里叶域进行信号处...     

Optical waveguide electric field sensor with controllable operating point using asymmetric Mach-Zehnder structure

For an integrated electro-optical sensor, the operating point has a significant effect on the performance of the sensor. In this paper, an optical waveguide electric field sensor with controllable operating point is designed using LiNbO3 materials, which has an asymmetric Mach-Zehnder interferometer (MZI) structure. Theoretical results show that the optimal operating point can be obtained and controlled by tuning the output wavelength of the tunable laser used in the sensing system. The simulation results show that the sensitivity about 83 dB·μV/m can be obtained, and the linear dynamic range as large as 60 dB can be achieved. And the fabrication tolerance of the center-to-center distance for the 3 dB coupler used in the asymmetric MZI is ~0.5 μm, while the power splitting ratio of the Y branch is with more tolerance.     

一款适用于WCDMA标准的大动态范围SiGe HBT可变增益放大器设计

第三代移动通信标准WCDMA要求放大器增益可调,并且增益动态范围较大.根据这一要求给出了一种基于SiGe HBT具有高动态范围的可变增益放大器(VGA)设计.放大器为三级级联结构,第一级为输入缓冲级,第二级为增益控制级,最后为放大级.VGA的增益控制通过调整第二级的偏置实现.VGA在1.95 GHz频率下,在0～2.7 V增益控制电压变化下,具有44 dB增益变化范围,最大增益49 dB.在最大增益处最小噪声系数为2.584 dB,输入输出电压驻波比低于2,性能良好.