Research On Mode-Hop-Free Tuning Performance Of External Cavity Laser

The ultra-high-resolution(0.1 pm order)spectral analysis system has been widely used in optical fiber communication and optical fiber sensing.The tunable light source with large tuning range,high output power and narrow linewidth is an important part of the system,which directly determines the spectral resolution of the system.In this paper,we focus on the mode-hop-free tuning performance of external cavity lasers:(1)The basic theoretical model of the external cavity tunable laser is given.The rate equations describing the interaction between carriers and photons in the external cavity structure are established.The linewidth model of the Schawlow-Towns is introduced and the main factors affecting the linewidth are analyzed.The mathematical mode of the mode-hop-free tuning performance of the external cavity laser is derived.(2)In order to satisfy the needs of the ultra-high-resolution spectral analysis system based on SBS for the tunable light source without mode hopping in a large range,the structure selection and optimization design of the external cavity laser are carried out.The P-I performance,linewidth performance and mode-hop-free tuning performance of the Littman external cavity laser are simulated and analyzed.The results show that by optimizing the rotation point,the tuning range of the laser can be greatly improved,the optimized Littman external cavity laser has a wavelength tuning range of 1530-1570 nm,a mode-hop-free tuning range of more than 80 GHz,a linewidth of less than 200 k Hz,and an output power of about 23.6 m W when the injection current is 300 m A,which is expected to meet the above application requirements.(3)The experimental setup of Littman external cavity laser is set up.The P-I performance,linewidth performance and mode-hop-free tuning performance of the laser are studied.The results show that the optimized laser has a wavelength tuning range of 1525-1575 nm,a mode hopping free tuning range of 131.3 GHz,a linewidth of 71 k Hz,and an output power of 25 m W when the injection current is 300 m A,which suits the needs of the ultra-high-resolution spectral analysis system based on SBS.(4)A 100 Hz ultra-narrow-linewidth measurement method based on the short fiber delay self-heterodyne structure and dual-parameter-acquisition algorithm of power spectrum is proposed,which has the advantages of simple structure,less influence by 1/f noise and high tolerance of fiber length error.