Analysis of chirp characteristics of DFB lasers and integrated laser-modulators

A new method of analyzing the chirp characteristics of directly modulated lasers and integrated laser-modulators is presented in this paper. Phase-circuit has been introduced into the circuit model of distributed feedback (DFB) lasers in the analysis. Therefore, the chirp characteristics of the device can be obtained by simulating the modified circuit model. The simulation results agree well with the published data. Furthermore, this modified model is combined with the circuit model of electroabsorption (EA) modulators to simulate the chirp characteristics of the monolithic integration of a DFB laser and an EA modulator. The simulation is focused on the dependence of the frequency chirp of the integrated device on the isolation resistance between laser and modulator. Much lower chirp can be seen in the integrated lightwave source compared to the directly modulated laser.     

Integrated Electro-Absorption Modulation DFB Laser Based Optical True Time-Delay Feeder for X-Band Phased Array Antennas

A 4-bit optical true-time-delay feeder incorporating an integrated electro-absorption modulation distributed feedback (DFB) laser for X-band phased array antennas is demonstrated. The integrated electro-absorption modulation DFB laser is an attractive device that provides a very compact, low-cost solution for optical true-time-delay systems. The variable delay line is constructed by cascaded magneto-optic switches, which have fast switching speed. The integrated electro-absorption modulation DFB laser system shows high signal-to-noise ratio up to 58dB, and the transmission performance of the system is relatively good and accredited.     

Novel model of FM-noise conversion in a UMZI using RF external phase modulation of lasers: Theoretical and experimental results

In this paper, we propose and demonstrate a new and original model for theoretical calculation and experimental measurement of the noise power spectral density (NPSD) in phase-modulated optical links. The phase modulation is operated in the RF frequency range by an external phase modulator. The NPSD is derived for the first time in interferometric systems, by considering all effects such as the 1/f FM noise of the laser and white noise applied to light from a 1550 nm DFB laser, with phase modulation. The results show for the first time the influence of the phase modulation index, modulation frequency and interferometric delay in the phase-to-intensity noise conversion. The experimental and simulation results of conversion of FM-noise to intensity noise in an optical link by considering the external RF phase modulation are shown with good agreement.     

Small-signal analysis of OOFDM signal transmission with directly modulated laser and direct detection

This work presents a small-signal analysis for investigating the transmission performance of optical orthogonal frequency division multiplexing signals with a directly modulated DFB laser (DML). The analysis shows the positive chirp of DMLs can intensify power fading after transmission with positive dispersion and provide power gain instead with negative dispersion. The power of subcarrier-to-subcarrier intermixing interference after square-law direct detection, however, is independent on the sign of dispersion.     

Broadband amplitude-modulated laser source with high output power for FM/cw lidar transmitter

We are building a long-range FM/cw nonscanning imaging lidar breadboard. This lidar system achieves ranging based on a frequency modulation/continuous wave (FM/cw) technique, implemented by an amplitude modulated mid-IR diode laser transmitter with a linear frequency modulation (LFM) of the subcarrier. Firstly, various schemes of light beam modulation are analyzed. Secondly, we put forward a laser modulation scheme whose core was formed by a 1.55 μm electro-absorption modulated laser diode (EML) and an erbium-doped optical fiber amplifier (EDFA), then a corresponding experimental system architecture and components for light beam modulation and detection are established. Finally, a corresponding experiment of laser beam modulation is completed for the first time. In our experiment, the EML amplitude is modulated by a 200 MHz to 800 MHz LFM signal, whose amplitude value is 2.05 V. The average output power of the modulated laser obtained in the experiment is 10 W, peak power is 16.4 W, and the average modulation depth is 78%. The results of tests predict that this laser modulation scheme is likely to improve the imaging range of the FM/cw lidar.     

信号光的调制对基于光纤光参量放大光脉冲源的改善

在理论上从四波混频的强度耦合方程出发,给出了基于光纤光参量放大(FOPA)光脉冲的光场表达式,并进一步分析了信号光被相位调制或强度调制后,光脉冲的频率啁啾和强度演化.结论指出：若信号光被强度调制,对所生成脉冲宽度无明显的影响,但可以提高消光比;若信号光被相位调制,所生成的脉冲具有更大的线性正啁啾,可以在相同的抽运功率条件下得到比无相位调制时更窄的脉冲.实验上给出了10 GHz工作速率下的结果,其结果与理论分析符合得很好.通过信号光的相位调制,在05 W平均抽运功率条件下得到了消光比22 dB,脉宽为5 关键词： 光纤参量放大 四波混频 光脉冲源 频率啁啾     

Analysis of linewidth and extinction ratio in directly modulated lasers for performance optimization in 10 Gbit/s CWDM systems

Direct Modulation Lasers (DMLs) have attracted increased attention during the past few years because of their intrinsic simplicity and cost-effectiveness, especially when applied to WDM metro and access networks. However, the output power waveform from a directly modulated laser is not an exact replica of the modulation current and its instantaneous optical frequency varies with time depending on the changes in optical power and the extinction ratio (ER) (an effect also known as frequency chirp). In this work, using an Optical Communication System Design Software, we have studied a directly modulated WDM 10 Gb/s system which transmission performance depends strongly on DML characteristics; simulation results have provided supplementary details about the effects of DML type (adiabatic or transient chirp dominated) as well as the effects of the optical output power. These details can provide useful design guidelines for constructing a WDM metro network.     

Development of a laser photothermoacoustic frequency-swept system for subsurface imaging: theory and experiment

In conventional biomedical photoacoustic imaging systems, a pulsed laser is used to generate time-of-flight acoustic information of the subsurface features. This paper reports the theoretical and experimental development of a new frequency-domain (FD) photo-thermo-acoustic (PTA) principle featuring frequency sweep (chirp) and heterodyne modulation and lock-in detection of a continuous-wave laser source at 1064 nm wavelength. PTA imaging is a promising new technique which is being developed to detect tumor masses in turbid biological tissue. Owing to the linear relationship between the depth of acoustic signal generation and the delay time of signal arrival to the transducer, information specific to a particular depth can be associated with a particular frequency in the chirp signal. Scanning laser modulation with a linear frequency sweep method preserves the depth-to-delay time linearity and recovers FD-PTA signals from a range of depths. Preliminary results performed on rubber samples and solid tissue phantoms indicate that the FD-PTA technique has the potential to be a reliable tool for biomedical depth-profilometric imaging.     

Optical coherence pulsed interferometry: shaping probe pulses in time-domain interferometry

The autocorrelation of a modulated coherent light source is used as a probe pulse in a time-domain interferometry scheme. With respect to conventional techniques, higher flexibility in selecting the shape of the probe pulse can be achieved by simply acting on the modulation parameters. The complex amplitude of short pulses propagating through a generic optical device can be directly measured, with no need for fast sampling and time synchronization. The potentialities of the technique are shown by reporting measurements of amplitude distortion, group delay, and frequency chirp of optical pulses transmitted through integrated ring resonators.     

高斯白噪声相位调制的激光光谱展宽

激光器是现代光学中一种常用的光源,分析其谱宽特性对于激光的研究具有重要意义.由于高性能窄线宽激光的半高全宽通常在几十M Hz以内,难以直接满足宽光谱领域的应用需求,因而无法发挥其在成本和性能方面的优势,这在一定程度上限制了激光的发展.如果在激光内部直接进行调制实现线宽展宽,又会导致频率的严重漂移,破坏稳频特性.为了在激...     

Experimental and Theoretical Study of High Temperature-Stability and Low-Chirp 1.55 µm Semiconductor Laser with an External Fiber Grating

An experimental and theoretical study of the effect of temperature on the static and dynamic characteristics of packaged external fiber grating semiconductor lasers (FGL) is reported on. Operating in single frequency mode, the laser exhibits high output power (> 8 mW), high temperature stability of operating frequency (-3.4 GHz/K), and low static chirp (-60 MHz/mA). The observed hysteresis in wavelength versus temperature dependence is explained in the frame of a time-domain FGL model accounting for asymmetric nonlinear gain. The laser has low dynamic chirp (~16 MHz/mA) under 2.5 GB/s direct modulation, which is the key factor determining low penalty transmission over 312 km of SSM fiber. Dense WDM transmission performed at 2.6 Gbit/s over 117 km of SSM fiber shows that an FGL-based transmitter is a factor of 7 more tolerant to temperature variations than externally modulated DFB lasers.     

基于激光二极管光反馈效应的肌肉震颤测量系统

介绍一种测量肌肉震颤频率和幅度的无损伤、非接触光学方法。分析了在直接频率调制下 ,激光二极管中的光反馈效应 ,探讨了振动微小位移和速度的理论模型。设计并研制了一套基于光反馈效应的直接频率调制激光二极管光电探测系统。利用该系统对微弱机械振动信号进行了探测 ,表明该系统能用于肌肉震颤的测量。最后介绍了人体前臂内侧屈肌群紧张收缩时 ,肌肉震颤的光学测量结果     

Directly modulated lasers in negative dispersion fiber links

In the paper the impact of frequency chirp of directly modulated lasers is considered in context of high-speed data transmission over negative dispersion fiber. Chirp/dispersion induced signal distortions are described, and detailed investigations of their influence on 10 Gb/s transmission system performance are presented. The desired laser chirp characteristics and optimal driving conditions are determined. It is also demonstrated that directly modulated laser with low chirp offers similar or even better dispersion tolerance than unchirped (externally modulated) source. Experimental verification of described investigations is presented at the end.     

Extended Koch's model for chirp assessment in semiconductor singlemode lasers operating at high currents

Semiconductor singlemode laser chirp models that discriminate the adiabatic and the transient frequency chirp components are very relevant and useful, as they allow evaluating per se the effects of both chirp components on directly modulated (DM) laser communication system performance. For this purpose, Koch's model has been extensively used, showing high accuracy for low and moderate laser current regimes.This paper aims at proposing an extended Koch's (e-Koch's) model that allows its accurate use in high current regimes for an arbitrary functional form of the laser nonlinear gain compression factor. High current regimes are commonly found in high bit-rate optical communication systems, in order to achieve sufficient laser intensity modulation bandwidth. Frequency chirping predictions by the proposed model, assessed by numerical simulation, show good agreement with the ones estimated by the laser rate equations, for both linear and logarithmic carrier density dependent laser gain models. It is also shown that e-Koch's model provides good estimates of the performance of 40 Gbit/s DM laser multi-span system.     

带调谐光纤F-P腔的光纤环腔半导体激光器的频偏特性研究

本文由速率方程出发分析了带调谐光纤F-P腔的光纤环形腔半导体激光器的频偏特性,结果表明在低频时的频偏功率比自由运转激光器低,而在高频时则趋于一致。改变调制频率和F-P腔的腔长可以改变激光器的频偏功率比,选择适当的调制频率和F-P腔参数就能获得不同的频偏特性。     

Dynamic analysis of directly modulated Fabry–Perot laser diodes using time-domain modeling with random noise fluctuations

Based on time-domain laser modeling, we have systematically evaluated multi-mode excitation and mode competition in directly modulated Fabry–Perot laser diodes biased with different current levels. In our simulation, random noise fluctuations depending on the bias current are included since they play an important role in determining the number of lasing modes. Multi-mode excitation and mode competition occurred at the biased current below the threshold current can be explained with the aid of frequency chirp at turn-on and gain difference in between '0' and '1' bits. The positive frequency chirp (25 GHz for lasers biased below the threshold current and 15 GHz for lasers biased above the threshold current) affects to enhance the probability of multi-mode excitation in low-frequency region during turn-on delay.     

激光回馈引起的微片Nd:YAG激光器频差调制

微片激光器由于自身的内应力会输出频率分裂的正交偏振的两个模式.研究了回馈对微片激光器频差的影响.研究发现:在回馈的作用下微片激光器的频差大小呈现出以初始频差为中心的类正弦的频差调制,周期为λ/2;频差调制曲线的幅度与回馈水平成正比,但是当回馈水平过强时会出现偏振态跳变,从而使频差消失;在一定的回馈外腔范围内,频差调制曲线的幅度与初始频差亦成正比.并利用复合腔模型和激光器的自洽理论进行了理论分析和模拟,所得结果与实验结果相符.此调制现象在精密测量领域有潜在应用.     

All-Fiber Self-Injection Seeding for Optical Timing Jitter Reduction in a Chirp Compensated Gain-Switched DFB Laser

We demonstrate an original all-fiber self-injection seeding technique based on a noninterferometric reflector for the optical timing jitter reduction in a chirp compen sated gain-switched Distributed FeedBack (DFB) laser. Two experiments, at 2.5 and 5 GHz, show standard deviations of the optical timing jitter of 0.28 ps and a reduction factor of 17 in the best case. The self-injection seeding sensitivity to polarization effects and to modulating frequency changes was measured in terms of standard deviation. A final experiment at 10 GHz, with no external feedback, shows an optical timing jitter down to 0.4 ps due to a time dynamics of spontaneous emission fluctuations slower than the pulse period.     

基于光纤色散相位补偿的高分辨率激光频率扫描干涉测量研究

为了实现更高分辨率的激光频率扫描干涉测量, 增大光源的扫频范围以及减小扫描频率的非线性成为关键. 采用外腔式大带宽扫频光源结合光纤辅助干涉仪构建的外部时钟频率采样非线性校正是目前较为常用的方法. 本研究发现随着扫频带宽和测量范围的增加, 光纤辅助干涉仪与测量光路中存在的色散失配导致频谱出现严重展宽, 极大的降低了测量的分辨率. 本文建立了辅助干涉仪和测量干涉仪色散失配影响的理论模型, 利用该模型分析了扫频带宽和测量范围与测量分辨率的变化关系, 与实验结果相一致, 并进而提出了基于峰值演化消畸变的色散相位补偿方法, 有效地提高了测量的分辨率, 在2.53 m 处实现了接近理论值的64.5 μm的测量分辨率. 该色散失配模型及补偿方法为提高大尺寸激光频率扫描干涉仪的测量分辨率及测距范围提供了参考.     

Influence of laser linewidth on performance of Brillouin optical time domain reflectometry

The effects of optical sources with different laser linewidths on Brillouin optical time domain reflectometry (BOTDR) are investigated numerically and experimentally. Simulation results show that the spectral linewidth of spontaneous Brillouin scattering remains almost constant when the laser linewidth is less than 1 MHz at the same pulse width; otherwise, it increases sharply. A comparison between a fiber laser (FL) with 4-kHz linewidth at 3 dB and a distributed feedback (DFB) laser with 3-MHz linewidth is made experimentally. When a constant laser power is launched into the sensing fiber, the fitting linewidths of the beat signals (backscattered Brillouin light and local oscillator (LO)) is about 5 MHz wider for the DFB laser than for the FL and the intensity of the beat signal is about a half. Furthermore, the frequency fluctuation in the long sensing fiber is lower for the FL source, yielding about 2 MHz less than that of the DFB laser, indicating higher temperature/strain resolution. The experimental results are in good agreement with the numerical simulations.