圆偏振激光调制器的数学分析

利用塞曼效应进行激光稳频,无需对激光 器进行调制,且理论上可对增益曲线上任一点进行稳频。现对方法中关键部分圓偏振激光调制器中的有关数学量进行分析,得纠偏信号表达式(13)及检偏振片偏振的最佳取向(π/4),按此设计的稳频器,具有良好的性能。     

Pound-Drever-Hall稳频误差信号线性动态范围及灵敏度分析

在Pound-Drever-Hall激光稳频方法中,误差信号的线性动态范围和灵敏度是影响稳频效果的重要指标。为了对Pound-Drever-Hall激光稳频性能进行优化,本文理论分析了调制频率、FP腔镜反射率和腔长对动态范围及灵敏度的影响,并通过实验仿真进行定量讨论和优化。实验结果表明,调制频率、反射率和腔长都分别与线性动态范围的大小有着紧密的联系,通过对这些参数的正确选取,以及对精度和灵活性要求的综合考虑,可以提升稳频系统的效果;而且对调制深度的优化可以提高误差信号灵敏度。     

双模激光稳频的二阶相干度时谱法

近年来,有关利用内脏He—Ne激光纵模偏振特性实现双模He—Ne激光器稳频稳幅的工作已有不少报道。诸如电热稳频法、水冷稳频法、风冷稳频法和激光等离子体温控稳频法。由于谐振腔的位相各向异性和振荡模式偏振方向的竞争效应及其温度漂移对双模偏振特性的影响,使得内脏He—Ne激光的双模呈现出相当复杂的随机偏振特性,从而使得长时频稳性、再现性和功率稳定性以及抗环境干扰能力不易做得更高,甚至失效。     

基于温度闭环反馈的He-Ne激光器热稳频系统

为了提升激光回馈测量系统中He-Ne激光器的性能, 解决激光回馈镜不断移动时频率无法使用传统方法稳定的技术问题, 采用基于温度反馈控制激光器管体温度的闭环被动稳频的方法, 进行了理论分析和实验验证, 研究了不同管体温度与环境温度差值下频率稳定性。结果表明, 系统最佳温差为25.6℃; 在此温差条件下稳频后, He-Ne激光器波长波动范围达10-4, 即频率稳定性达到1.61×10-7, 功率漂移量低于3.20%。该系统可以根据环境温度的变化调节稳频温度点, 并且稳频结构简单, 满足激光回馈一般应用系统稳定性的要求。     

基于激光回馈效应的纳米计量系统

提出了一种基于激光多重回馈效应的纳米计量系统,系统采用非准直平凹回馈外腔结构,使在回馈外腔中往返多次的回馈光束返回到激光谐振腔中,形成高阶回馈效应,得到了高密度、类正弦和位相正交的双频激光回馈条纹。条纹的密度为传统弱回馈的几×10倍,而且回馈条纹以激光波长为尺子,具有可溯源性,在没有任何电细分的条件下达到了nm级的光学分辨率。特别地,通过采用双折射双频激光器,获得了两偏振正交的激光回馈条纹,而且两正交偏振回馈条纹间还具有位相差,位相差的大小主要由双频激光器的频差、外腔长以及回馈阶次决定。利用两正交偏振回馈条纹间的位相关系,可用于识别物体的运动方向。与激光干涉仪的比对实验表明,系统的线性度优于5.2×10-5,光学分辨率为10.2nm,量程大于500μm。     

全反射棱镜式激光陀螺双纵模稳频技术

针对全反射棱镜式激光陀螺在单纵模稳频条件下纵模调制信号存在盲区的现象,研究了全反射棱镜式激光陀螺双纵模稳频技术。理论分析了功率调谐曲线的特征,获得了双纵模条件下小抖动调制信号的幅度及相位特性,提出了双纵模稳频控制方案,通过软硬件电路设计,搭建了全反射棱镜式激光陀螺双纵模稳频控制系统。在定温和变温环境下,分别对单纵模稳频与双纵模稳频进行了实验测试。实验结果表明,与单纵模稳频控制方法相比,双纵模稳频技术在稳频精度上提高了60%,陀螺精度也得到了相应的提高。     

激光回馈双折射测量系统稳频技术研究

波片、晶体等自然双折射元件广泛应用于各种光学系统中。普通光学元件在加工、镀膜等过程中会引入残余的内应力,形成双折射。双折射会对整个光学系统的性能产生影响,需要对其进行精确测量。基于激光回馈效应,利用偏振跳变中光强调制曲线与双折射的线性关系,构建了光学元件双折射测量系统。通过引入稳频技术,使激光器长期稳定单纵模运转,增强了激光器的抗干扰能力,提高了系统的稳定性。实验结果表明,构建的激光回馈双折射测量系统测量精度优于0.24,重复测量最大偏差0.13,标准差0.06,稳定性好,可靠性高,可实现在线测量。该系统有潜力应用于微小应力的在线测量,如飞机座舱盖、汽车玻璃等。     

基于FPGA数字相位调制光外差激光稳频系统设计

为了实现中心波长为1064nm的单频光纤激光器的稳频,采用相位调制光外差（PDH）激光稳频技术,搭建稳频系统光路。分析了相位调制光外差稳频信号以及误差信号特征;设计基于现场可编程门阵列（FPGA）的数字式解调和反馈控制电路,在FPGA中实现对相位调制光外差稳频信号的数字解调,再经数模转换器输出获得误差信号。结果表明,在FPGA中能成功实现对相位调制光外差信号的解调,经Allan方差计算,频率漂移的方差值可达10-11,即所设计的数字系统实现了较高的稳频精度。     

半导体激光器稳频技术

窄线宽稳频激光器在精密干涉测量、光学频率标准、激光通信、激光陀螺、激光雷达、基本物理常数测量和冷原子系统等研究领域有着广泛的应用。自由运转的半导体激光器每天的频率漂移量可以达到GHz,因此研究半导体激光器的稳频具有十分重要的意义。以780 nm的半导体激光器稳频为例,介绍了目前广泛使用的各种半导体激光稳频技术的基本原理及试验方案,如消多普勒饱和吸收光谱稳频技术、消多普勒双色谱稳频技术、调制转移谱稳频技术、调频光谱稳频技术和频率-电压转换稳频技术,并对各种稳频方法的性能和特点进行了分析。     

棱镜式激光陀螺稳频控制系统优化

针对棱镜式激光陀螺稳频精度较低的现象,系统地研究了棱镜式激光陀螺稳频控制系统特性。理论分析了光强调谐曲线及稳频执行机构的特征,建立了棱镜式激光陀螺稳频控制系统数学模型。进一步分析了系统稳态性能及动态性能,分析结果表明,系统在定温及慢变温作用下具有不同程度的稳态误差且调节时间过长,从而导致稳频精度下降。优化控制器参数使二阶系统具有最佳阻尼比及快速性,采用按温度补偿的前馈控制系统,实现了对温度所引起的稳频误差的全补偿,提高了系统的稳频精度。实验结果表明,优化后的稳频控制系统比原稳频控制系统的稳频精度提高一个数量级,陀螺精度提高30%以上。     

Experiments on modulation properties and optical feedback characteristics of laser diodes stabilized by an external cavity or injection locking

In the wavelength range near 1.3-μm, single-mode operation of laser diodes is achieved by means of an external cavity and injection locking. The influence of digital modulation and optical feedback from a directly taper-coupled fiber is investigated. Lasers stabilized by injection locking showed excellent side-mode suppression at CW-operation but strong side modes when the laser was modulated. The frequency of the main mode is neither affected by modulation nor by optical feedback from the fiber. On the contrary the laser stabilized by an external cavity exhibited good side-mode suppression even when it was modulated. However, the frequency stability of the main mode was influenced severely by optical feedback and modulation. A transmission experiment over 21 km at a bit rate of 2.24 Gbit/s showed that this simple stabilization method is sufficient for applications in optical wide-band IM-PCM transmission systems.     

Frequency stabilization of semiconductor lasers for applications in coherent communication systems

The stabilization of the mean frequency of a single-mode semiconductor laser used under modulation in a FSK heterodyne transmission system is achieved by a novel method. The frequency stabilization scheme locks the laser to one of the resonance frequencies of a Fabry-Perot interferometer (FPI). The laser is weakly modulated at a frequency higher than any signal frequencies. Light reflected from the FPI is detected and synchronously demodulated for feedback and current control of the laser. A laser modulated by a pseudorandom sequence with a rate of 140 Mbit/s and an optical frequency deviation of 280 MHz is stabilized with this method against variations in operating temperature and bias current. The mean optical frequency is maintained within ± 10 MHz for hours. Analysis and experiments confirm that the laser frequency remains stable even when the laser linewidth changes under weak optical feedback.     

Nonlinear dynamics of a laser diode with optical feedback systemssubject to modulation

The nonlinear dynamic behavior of a direct frequency-modulated diode laser with strong optical feedback is examined and compared to a laser diode subject to electro-optically modulated, strong optical feedback. Direct modulation is achieved by sinusoidal modulation of the diode laser injection current. Electro-optic modulation is achieved by applying a sinusoidal voltage to an intracavity phase modulating element. The output state (characterized by the output power versus time, the intensity noise spectrum and the optical frequency spectrum) for both types of modulation is dependent on the ratio of the modulation frequency to the external cavity resonant frequency, and the modulation power. A number of distinct states are observed: conventional amplitude modulation (with FM spectra); multimode, low-noise amplitude modulation; multimode, high-noise amplitude modulation; periodic limit-cycle operation; quasi-periodicity; chaos; low-frequency fluctuations; and mode-locking. There are significant differences between the direct and electro-optic frequency-modulation cases. The onset of the dynamic instability is characterized as a noisy period-one oscillation for direct modulation and a low-frequency fluctuation for intracavity electro-optic modulation. Phase portraits produced experimentally with the use of a digital phosphor oscilloscope are shown to agree well with those constructed from output power versus time data. This represents an experimental method for examining the dynamics phase portraits in real-time     

相位调制光外差稳频信号检测技术

在高功率条件下,由于受到多种因素的影响,单频光纤激光器输出激光的谱线宽度大幅展宽,输出激光的稳定性也不高。相位调制光外差稳频（PDH）技术在高功率条件下可以实现高频率稳定性。为实现对中心波长为1 064 nm的单频光纤激光器的稳频,理论分析了PDH稳频系统的原理并搭建PDH稳频系统。实验发现100 MHz相位调制光外差信号的检测是稳频系统的关键。实验中首先利用自行设计的探测器前置放大电路,基于Si探测器,实现了信号的探测和放大;其次,设计解调电路,通过将光电转换后的信号与参考信号进行混频实现解调,得到鉴频曲线,实现对光外差信号的检测。     

空间光通信中高帧频相机动态调光技术研究

为了满足空间光通信精密跟踪的需求,在信标光路中保证光电探测器接收到的激光能量稳定不变,研制了满足大气环境下动态光强稳定装置。对该装置所采用的基于液晶的高帧频相机动态调光技术进行研究。通过与传统机械调光、电子快门调光相比较,验证基于液晶的高帧频相机动态调光技术的可行性。结合空间光通信系统的特点,阐述基于液晶的动态调光原理。介绍了基于液晶的动态光强稳定装置系统组成,给出实验数据和结果。结果表明:采用液晶光束衰减器对高帧频CCD相机进行动态调光,可以实现在无机械运动部件条件下对信标激光能量进行自动控制,从而保证探测器接收到的激光能量稳定。因此在空间光通信中,基于液晶的高帧频相机动态调光技术是可行的,而且有着广阔的应用前景。     

Optical-feedback-induced chaos and its control in multimodesemiconductor lasers

The effects of optical feedback in multilongitudinal mode semiconductor lasers are studied through computer simulations. Two separate regimes are found based on the length of the external cavity. For long external cavities (external-cavity mode spacing larger than the relaxation-oscillation frequency), the laser follows a quasi-periodic route to chaos as feedback is increased. For short external cavities, the laser can undergo both quasi-periodic and period doubling routes to chaos. When the laser output becomes chaotic, the relative-intensity noise is greatly increased (by more than 20 dB) from its solitary-laser value. Considerable attention is paid to the effects of optical feedback on the longitudinal-mode spectrum. The stabilization of the mode spectrum and the reduction of the feedback-induced noise through current modulation are studied and compared with experimental results. Current modulation eliminates feedback-induced chaos when the modulation frequency and depth are suitably optimized. This technique of chaos control has applications in optical data recording     

S/N and error rate evaluation for an optical FSK-heterodyne detection system using semiconductor lasers

Receiving performance is evaluated for an optical FSK-heterodyne detection system in which semiconductor lasers are used as both an FSK transmitter and an independent local oscillator. Noise and error rate are measured under feedback stabilization of IF signal frequency and electrical equalization of semiconductor laser FM modulation frequency characteristics. The minimum received signal power of -44 dBm, which is about 2 dB better than that in IM direct detection, is achieved at a 10^-9error rate for a 200 Mbit/s signal. Excess errors for FSK signals result from frequency broadening of the laser spectrum. Both AM and FM quantum noises in the lasers are primary factors which determine system performance. Error rate characteristics in an optical FSK direct detection system, in which a Michelson interferometer is employed as an optical frequency discriminator, are compared with the above results.     

Optical feedback on linearity performance of 1.3-μm DFB andmultimode lasers under microwave intensity modulation

Under large signal modulation and quantitatively controlled optical feedback (from about -50 dB to -25 dB, from the end of ~2-m pigtails), the linearity performance of both a 1.3-μm distributed-feedback (DFB) and a multimode laser is examined. Under proper bias levels and modulation frequencies, both lasers exhibit good linearity even with strong optical feedback. The DFB laser exhibits good linearity because of its coherence reduction, resulting from modulation-induced frequency chirping