一种多模FP激光器BOTDR系统的建模分析

根据相干检测理论提出了一种基于多模法布里-珀罗激光器的光纤瑞利与布里渊散射自外差检测的布里渊光时域反射计传感系统,分析了该系统提高光纤受激布里渊散射阈值和瑞利与布里渊散射自外差检测的原理,推导出系统信噪比表达式,分析了纵模数与布里渊谱峰值功率、谱宽和系统信噪比、温度及应变测量准确度之间的关系,并进行了计算.结果表明,选用纵模间隔为0.141nm的多模法布里-珀罗激光器时,随着纵模数的增加,在25km光纤末端系统信噪比、温度和应变测量准确度均得到了较大提升,当纵模数为19时,温度和应变测量准确度分别达到最佳值3.81℃和86.69με.     

多模布里渊光时域反射系统的建模分析与计算

提出了一种基于多模法布里-珀罗激光器外调制的移频本地外差检测布里渊光时域反射系统,分析了系统提高光纤受激布里渊散射阈值和移频本地外差检测的原理;推导了系统信噪比的数学表达式,研究了布里渊叠加谱峰功率和谱宽、系统信噪比和频移测量精度与纵模数的关系,以及实现最优测量精度时纵模数与脉冲宽度的关系,得到了相应的拟合公式。结果表明,选用纵模间隔0.141nm的多模激光器和单纵模峰值功率100mW、脉冲宽度50ns的入纤脉冲时,随着纵模数的增加,25km光纤末端的系统信噪比、温度和应变测量精度均得到了明显提高,且纵模数为20时,系统信噪比相对于单纵模增加11.73dB,温度和应变测量精度分别达到最佳值3.2℃和70.8με;最佳频移测量精度随脉冲宽度的增大先快速提高,后趋近于恒定值2.9 MHz。     

宽谱光源光谱相干度的仿真研究

以傅里叶变换为基础 ,研究了具有任意谱形的宽谱光源的相干函数并实现了通用的宽谱光源相干特性仿真算法。计算了应用于干涉式光纤陀螺的各种宽谱光源的相干特性 ,分析了不同谱形以及光谱平滑度对光谱时间相干性的影响。对于宽谱光源 ,此法可以全面准确地仿真其相干特性。     

三阶分布反馈太赫兹量子级联激光器的远场分布特性

研究了三阶分布反馈太赫兹量子级联激光器的设计、制作并分析了其纵模和横模特性.通过建立波导结构模型,利用有限元方法模拟激光器波导结构内的三维模场分布,通过本征模的损耗分析器件的纵模模式选择机理.同时,由本征模的近场通过近-远场傅里叶变换求得激光器的远场分布.采用半导体工艺制作了4.3 THz双面金属波导三阶分布反馈量子级联激光器,测量了不同器件的发射谱和远场光束质量,远场发散角为12?×13?,实验结果和理论模拟符合.     

复自相干度度量超连续谱相干性

引入复自相干度来度量同一脉冲产生的超连续谱的相干性, 并对其进行了实验研究. 采用Mach-Zehnder干涉仪, 测量700 ps脉冲抽运光子晶体光纤产生的超连续谱的相干性. 实验测得超连续谱不同波长成分的相干长度均大于40 μm, 在长波区可达225 μm. 超连续谱的整个光谱区域各谱成分的相干度有差异, 但复自相干度的模平均值为0.461, 相干性较好, 可以满足如光测量、光学传感等很多应用.     

基于时变功率谱的激光器调谐瞬时线宽测量方法研究

半导体激光器的线宽通常采用激光外差测量技术,通过差拍信号的功率谱密度函数来确定,受傅里叶变换方法的限制,得到的均是在一定时间段内的静态平均线宽。为了获得半导体激光器在电流调谐过程中的瞬时线宽特性,提出了利用时变功率谱获知调谐瞬时线宽的相干和非相干测量方法,并分别进行了理论分析和实验验证。首先对半导体激光器输出光信号及差拍信号进行了时间-频率域下的数学描述,确定了时变功率谱与调谐瞬时线宽的关系;其次,针对差拍信号的趋向性特征,提出了趋势局部均值分解方法,并研究了利用分解出的乘积函数建立差拍信号及激光器输出光信号的时变功率谱的方法;最后利用非相干和相干测量法分别获得了分布反馈式半导体激光器在50~51及50~100 mA锯齿波电流调谐过程中的瞬时线宽。     

激光器纵模数与模竞争

本文用激光器半经典理论,特别是模竞争的观点,系统地阐述了纵模数同激光参量的关系。     

长周期光纤光栅(LPFGs)的谱结构研究

利用相位匹配条件,从理论和实验上研究了均匀LPFGs的谱结构特点.结果表明,导模和不同包层模耦合产生的光谱特性有很大差别.在模序较小时,谱宽较小;模序增加,谱宽也增加;在某一模序附近,谱宽达到最大;以后随模序增加,谱宽迅速减小.给出了LPFGs中透射谱的最大损耗率及3 dB带宽与包层模序、光栅周期、耦合系数和栅长的关系.这些关系对设计满足特定最大损耗率和带宽的带阻滤波器或增益均衡器提供一种理论参考.选用不同的包层模、耦合系数和栅长,可以设计出谱宽很大或很小的LPFGs.     

用于激光稳频系统的控温F-P标准具特性研究

法布里-珀罗标准具由于其结构简单、性能稳定,不仅可以实现不同频率光束的分离、高精度的长度测量、分析激光的纵模结构等,还可用于激光器的稳频系统。本文对F-P标准具的透射率、半高宽度、自由光谱区以及透射率与温度的关系等特性进行了研究,并将其应用在激光器的稳频装置中。     

F－P滤波窗在半导体光孤子源中的作用

利用光谱窗滤波法对增益开关半导体激光器输出光脉冲消啁啾进行了理论分析，在线性啁啾近似下得到了啁啾光脉冲经Ｆ－Ｐ后的谱宽表达式，给出了经Ｆ－Ｐ后光脉冲的宽度、波形以及自相关测量时的波形因子与Ｆ－Ｐ参数的关系曲线．实验结果与理论分析基本一致．     

Characterization of 60 GHz Multi Quantum well passively mode-locked laser under optical self-injection locking

The quality and pulse compression of the 60 GHz millimeter wave signals generated by 750 μm long InAlGaAs Multi Quantum Well (MQW) passively mode locked laser under free running and optical self-injection locked conditions are experimentally characterized in terms of longitudinal modes under certain bias currents that range from 24 mA to 90 mA. Initially, the MQW laser is characterized in free running condition with no external injection. The measurements reflect that the free spectral range of laser under test is around 61 GHz and exhibit more than 22 lasing modes. The laser is then integrated into low phase noise self-injection locking oscillator by feeding a part of output RF signal back into the laser cavity to enhance passive mode locking. By doing so the microwave line width of our laser is reduced from 900 kHz to 24 kHz with significant increase in output of resultant beat tones which exhibits strong passive mode locking. This is the first time that the free running microwave line width of MQW laser is reduced up to this level. It is evident from our experimental investigation that as we increase the power and phase correlation between different longitudinal modes inside laser cavity through optical self-injection, the strength of the passively mode locked mechanism is significantly increased and the phase noise of radio frequency signal is drastically reduced.     

四能级原子相干系统的探讨

对N型四能级原子与光场相互作用的原子相干效应进行了理论计算,讨论了能级衰减率、激光线宽以及基态间的驰豫对原子相干效应的影响。原子第四能级的衰减率、激光线宽以及基态间驰豫增大,原子相干性将会减弱。     

多模气体激光纵模线型函数及其频宽的实验观测

本文根据多模激光时间相干性ｇ（１）（τ）的准周期性特点，提出了一种观测多模激光纵模线型函数及其频宽的新方法－程差２ｋＬ法。文章介绍了测量原理与方法，并以双模Ｈｅ－Ｎｅ激光器为例，给出了相应的实验结果，研究表明，多模气体激光的纵模线型函数为洛仑兹（Ｌｏｒｅｎｔｚ）线型函数，相应的纵模频宽约为１０＾７Ｈｚ。     

1.3μm InGaAsP／InP脊形波导激光器偏振输出的测量

本文对发射波长为１．３μｍ，具有不同脊形波导宽度ＩｎＧａＡｓＰ激光器的ＴＥ，ＴＭ模偏振输出特性进行了测量，实验观察到脊形波导比较窄的器件具有ＴＥ，ＴＭ模竞争现象，而脊形波导比较宽的器件在阈值电流以上，只有ＴＥ模激射，没有模式竞争。利用偏振度概念计算了阈值电流以上和阈值电流以下的偏振度，并对其进行了讨论。本文指出，只要严格选择激光器的参数，并控制环境温度，合理使用器件，就可获得稳定的偏振输出，而不必     

Propagation of partially coherent partially phase-locked laser array in turbulent atmosphere

The mechanism for beam quality degradation in recently developed phase locking of high power solid state lasers, which is caused mainly by partially coherent property of element beam and partially phase locking of the laser array, is analyzed in detail. Analytical expression for propagation of partially coherent partially phase-locked laser array in turbulent atmosphere is obtained based on extended Huygens-Fresnel principle. The effect of coherence width, phase error and intensity of turbulence on the beam quality in the target-plane is studied in detail. The tolerance on phase error for laser array with different coherence property is analyzed. It is concluded that the laser array with better coherence is more sensitive to turbulence, and phase control can improve beam quality in the receiving plane only in the case when the element beam has good beam quality and propagates in weak turbulence.     

Experimental measurement of the four-dimensional coherence function for an undulator x-ray source

A full measurement of the four-dimensional coherence function from an undulator beam line is reported. The analysis is based on the observation that the data are consistent with a coherence function that is mathematically separable. The effective source size can be altered by changing the width of the exit slit, and the complete coherence function is presented for two settings. We find, to within experimental error, that the four-dimensional complex degree of coherence can be described as a real Gaussian function that depends only on the difference of the spatial coordinates.     

Effect of the laser axial modes on the coherence length of the laser beam

The effect of the presence of the axial modes within the spectral halfwidth of a laser spectral line on the visibility of its interference fringes is represented. It is found in case of axial modes with equal spectral halfwidths that the dominant factor in determining the coherence length of the laser beam is the spectral halfwidth of its axial mode and not the spectral halfwidth of the laser line. For the case of a laser beam with axial modes of different spectral halfwidths, the calculation shows a coherence length, closer to the mode with the smaller halfwidth.     

Effect of the mode composition of laser radiation on the intracavity formation of periodic surface structures

We investigated the effect of the mode composition (longitudinal and transverse modes) of laser radiation on the basic parameters of the stroke pattern formed on a treated surface placed inside a laser cavity. We found that the contrast of the stroke pattern formed by a laser beam with a complex set of longitudinal modes depends, in the first place, on the width of the laser radiation spectrum and the laser cavity length and is virtually independent of the angle of rotation of the beam. In the region of small rotation angles the broadening of strokes caused by the effect of transverse modes becomes predominant. B. I. Stepanov Institute of Physics, Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikaladnoi Spektroskopii, Vol. 64, No. 1, pp. 50–54, January–February, 1997.     

Propagation of partially coherent controllable dark hollow beams with various symmetries in turbulent atmosphere

Normalized intensity distribution, the complex degree of coherence and power in the bucket for partially coherent controllable dark hollow beams (DHBs) with various symmetries propagating in atmospheric turbulence are derived using tensor method and investigated in detail. Analytical results show that, after sufficient propagation distance, partially coherent DHBs with various symmetries eventually become circular Gaussian beam (without dark hollow) in turbulent atmosphere, which is different from its propagation properties in free space. The partially coherent DHBs return to a circular Gaussian beam rapidly for stronger turbulence, higher coherence, lower beam order, smaller p or smaller beam waist width. Another interesting observation is that the profile of the complex degree of coherence attains a similar profile to that of the average intensity of the related beam propagating in a turbulent atmosphere. Besides the laser power focusablity of DHBs are better than that of Gaussian beam propagating in turbulent atmosphere.     

Longitudinal coherence and spectral profile of a nickel-like silver transient soft X-ray laser

The spectral width of a Ni-like silver transient soft X-ray laser (4d-4p λ= 13.9 nm) was experimentally inferred from the measurement of its temporal coherence, performed with a novel wavefront division interferometer. The measured coherence time of 3 ps corresponds to a spectral linewidth of 0.7 m? which is narrow. We compare this experimental result to numerical simulations of the amplified line profile, performed along two different approaches. Both calculations predict a spectral linewidth that is about 3 times larger than the experimental value. We discuss several effects that might explain this discrepancy. We show that, due to the short duration of the X-ray laser pulse, the assumption of a stationary electromagnetic field used in both the experimental analysis and in the calculations need to be reconsidered.