Research On Wavelength-Tunable High-Power External Cavity Semiconductor Lasers

Semiconductor lasers are widely used in many fields due to their simple structure,high output power and reliability.Among them,970 nm wavelength semiconductor lasers are not only used for laser pumping and laser processing,but also widely used in military and biomedical applications.The excitation spectrum of ordinary wide strip F-P cavity semiconductor lasers is unstable and has a large linewidth,which is difficult to meet the requirements of narrow linewidth and tunable applications such as space coherent laser communication,environmental gas detection,and spectral analysis.In order to obtain stable output of semiconductor laser with narrow linewidth,high power and tunable output,a wide strip outer cavity semiconductor laser with grating wavelength of 970 nm is constructed by using diffraction grating as feedback element,and the output characteristics are studied and optimized.The main research content and results of this thesis are as follows:(1)Through ZEMAX optical design software,the optical alignment design and simulation of Littrow external cavity semiconductor laser are carried out.Fast axis collimating lens(FAC)and slow axis collimating lens(SAC)are used to collimate the output light of wide strip semiconductor laser.The beam divergence angle in the fast axis direction of the laser is compressed from 28 ° at the free exit to within 0.015 °,and the beam divergence angle in the slow axis direction is compressed from 8 ° at the free exit to within 0.8 °,which optimizes the structure of the external cavity laser and reduces the output loss of the laser.(2)The Littrow external cavity tunable laser is simulated and analyzed.The influence of external cavity structure parameters on laser linewidth,output power and tuning range is analyzed.The simulation results show that the linewidth of the outer cavity laser is related to the cavity length,the reflectance of the output cavity surface of the gain device and the reflectance of the grating.The linear width can be narrowed by increasing the cavity length,decreasing the reflectance of the output cavity surface and increasing the reflectance of the grating.The output power and tuning range of the external cavity laser are related to the output reflectance of the gain chip and the reflectance of the grating.The larger the reflectance of the output cavity surface of the chip,the smaller the output power and tuning range of the external cavity laser.The larger the reflectivity of the grating,the smaller the output power and the larger the tuning range.(3)A wide strip-shaped 970 nm grating external cavity semiconductor laser was constructed using a reflective diffraction grating with diffraction efficiencies of 28 % and55 %,respectively,as the feedback element.The incident polarization of the external cavity laser,the influence of the grating diffraction efficiency on the characteristics of the grating external cavity semiconductor laser,and the temperature characteristics of the grating external cavity semiconductor laser were analyzed.By adjusting the relative position of the laser and the grating,the incident polarization of the grating external cavity laser is changed.The output spectrum of the external cavity laser with P-polarized incidence is wider due to the wide luminescence window,while the external cavity laser with S-polarized incidence can obtain a narrow linewidth tunable laser output.The effects of different diffraction efficiencies and temperatures on the performance parameters of the grating external cavity laser with S-polarization incidence were analyzed.The experimental results show that the tuning range of the external cavity laser can be increased by properly raising the temperature,and the tuning range of the external cavity laser can be increased and the threshold current can be reduced by using a grating with higher diffraction efficiency.Based on the S-polarization incidence grating external cavity laser structure and different operating temperature control of the laser,a wavelength tuning range of 27.87 nm with a narrow spectral linewidth of 0.2 nm and laser output power of 1.11 W was achieved.