短相干半导体激光器的射频调制特性及应用

Radio frequency modulation characteristics and application of short coherent semiconductor laser

短相干激光光源在进行高精度的干涉测量时,可以消除被测光学元件前后表面反射形成的杂散光,是低相干干涉仪的理想光源。针对低相干干涉应用对光源的需求,依据速率方程和激光调制特性对射频调制下的短相干半导体激光器光谱特性进行了理论研究。搭建了短相干光源系统,研究了半导体激光器斜率效率η、偏置电流Ib、射频信号频率fm和幅度Am对其相干长度的影响。实验结果表明,斜率效率大的半导体激光器更有助于短相干特性的实现,随着调制信号频率和幅值增加,工作在阈值附近的激光器相干长度随之降低,该系统在I_(b)=1.3I_(th)、f_(m)=950 MHz、A_(m)=19 dBm的条件下获得了相干长度为90μm的短相干光源。并成功应用于斐索干涉仪上,获得了对比度K=0.9318的清晰干涉图像,与现有短相干光源相比,对比度提高了约51.1%,实现了对平行平板玻璃面形的测量。

Objective Short coherent laser light source can eliminate the stray light formed by the reflection of the front and rear surfaces of the optical element to be measured in high-precision interferometry,which is an ideal light source for low-coherence interferometers.There are important applications in optical coherence tomography,refractive index and thickness measurement of organic materials,surface profile detection of optical elements,etc.The imaging quality of the interferometer will be affected by the light source,and the appropriate parameters are very important for the semiconductor laser to obtain high-quality short coherent light source through RF modulation.However,the spectral linewidth of semiconductor lasers is narrow.It is of great significance to reduce coherence length through the coherence control technology.Methods A short coherent light source was obtained by radio frequency modulation using a Fabry-Perot laser diode with central wavelength of 637 nm.The spectral properties of short coherent semiconductor lasers under RF modulation are theoretically studied based on laser rate equations and modulation characteristics.A short coherent light source system(Fig.2)was built to study the effects of laser slope efficiency,bias current,RF signal frequency and amplitude on the coherence length of semiconductor lasers.Compared with the existing short coherent light source with RF modulation under the same conditions,its improvement effect on the interference image quality was verified.Results and Discussions The spectral linewidth of laser was measured by spectrometer.The short coherence characteristics of two Fabry-Perot lasers with different slope efficiency were studied and the results show that the semiconductor laser with high slope efficiency has greater output power variation under the same modulation signal,which is helpful to achieve good modulation effect.The coherence length of a semiconductor laser is the smallest when the bias current is slightly larger than the threshold current(Fig.4).When the bias current is small,the linewidth of the laser is narrow.This is because some RF signals work below the threshold current,which leads to the abnormally low output power of the laser and affects the RF modulation performance.When the bias current is too large,the coherence of the light source is enhanced,and the increased injection current intensifies the mode competition,the number of longitudinal modes output by the semiconductor laser is reduced and the spectral line width is narrowed.The coherence length of the semiconductor laser is negatively correlated with the frequency(Fig.5)and amplitude(Fig.6)of the RF modulation signal.With the increase of the frequency and amplitude of the modulation signal,the emission spectrum of the semiconductor laser shows multi-longitudinal mode output,the spectral line width is broadened,and the coherence length decreases.An experimental setup for measuring the surface profile of transparent parallel plate glass was built(Fig.8).The short coherent light source obtained by using the parameters in this paper makes the interference image have higher image quality.Compared with the existing short coherent light source,the contrast can reach 0.9318,which is increased by about 51.1%.While avoiding background noise,the interference fringes with surface information are displayed more clearly(Fig.9).Conclusions Under the condition of bias current I_(b)=1:3I_(th),a semiconductor laser with higher slope efficiency is selected.With the increase of modulation signal frequency and amplitude,the coherence length of the laser decreases,and the shortest coherence length can reach 90μm at RF signal frequency f_(m)=950 MHz and amplitude A_(m)=19 dBm.It can be used to measure transparent parallel plate optical elements as thin as 0.09 mm,and the interference image contrast is 0.9318,which is higher than the existing short coherent light source.The research improves the performance of short coherent light source and has broad application prospects in the field of low coherent interferometry.