基于时变功率谱的激光器调谐瞬时线宽测量方法研究

摘要

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.%Linewidth measurement of semiconductor laser always uses the heterodyne technology and the linewidth is confirmed with the power spectral density function of the beat signal.According this method,linewidth depends on the power spectral density function confirmed with Fourier transform,but the power spectral density function only defining for the wide-sense stationary signals can not apply to the unstable signals such as the beat signals.Fourier transform is a global transformation losing temporal information,so the dynamic or instantaneous linewidth getting through the power spectral density function is a stable value or a dynamic point in a period of time but not a instantaneous one.In order to contain the instantaneous linewidth of the semiconductor laser in the tuning process,the coherent and the incoherent measuring methods based on the time-varying power spectrum is proposed.The instantaneous linewidth measuring method based on time varying power spectrum has two ways according the time delay,the tuning instantaneous linewidth can be learned by building time varying power spectrum of the beat signals or the laser through doing time-frequency analysis to the beat signals.When doing time-frequency analysis,the beat signals must be decomposed to several mono-component signals firstly because the beat signals are typical non-stationary multi-component signals,and only when the signal is a mono-component signal,has the instantaneous frequency a clear and justifiable physical interpretation.However,the precision of decomposition algorithms is affected seriously by a main slow-varying tendency which changes as a sine or a frequency-modulated wave superimposing the beat signals,and the trend is so legible that it can be obtained in advance,so we propose the Tendency Local Mean Decomposition (TLMD) algorithm combining trend pre-extraction and Local Mean Decomposition (LMD) method which brings more excellence to decompose the beat signals.The time-varying power spectrums of the beat signal and the laser by using the product functions come from decomposition have been built,and the instantaneous linewidth of the distributed feedback semiconductor laser tuned by a sawtooth wave current of 50～51 and 50～100 mA has been obtained finally.