Research On 1550 Nm Narrow Linewidth Tunable External Cavity Semiconductor Laser

Tunable external cavity semiconductor laser(TECSL)with narrow linewidth has the characteristics of good coherence,low noise,good temperature stability and current stability.It is widely used in coherent optical communication,optical fiber sensing and coherent testing.In order to realize TECSL with narrow linewidth and wide tuning range,a narrow linewidth grating TECSL with 1550 nm Littrow structure was studied and fabricated.The output characteristics of grating TECSL are studied,including current,threshold current,side mode rejection ratio(SMSR),tunable range and linewidth.In order to obtain a narrow linewidth TECSL,the mode selection filter Littrow structure TECSL is designed.Based on the Littrow structure,a mode selection filter is added to the outer cavity composed of the front cavity surface of the gain chip and the diffraction grating to enhance the mode selection effect of the external cavity structure,so that some specific frequency of light transmission filter propagates back and forth in the cavity.The light of other frequencies has a great loss because it cannot pass through the mode-selection filter,which realizes the single-frequency oscillation of the laser and thus narrows the linewidth.The characteristics of the Littrow structure TECSL without mode selection filter are compared,and the results show that the Littrow structure TECSL of the mode selection filter has a narrower linewidth.First,the research background and significance of TECSL are introduced.The progress of TECSL with different structures is studied,including the research progress of F-P confocal external cavity semiconductor laser,the research progress of waveguide external cavity feedback semiconductor laser and the research progress of diffraction grating structure ECTSL.Second,the Littrow structure TECSL device and tuning principle were studied,including the selection of curved waveguide gain chip,the tuning principle of Littrow structure TECSL,the output characteristic analysis of TECSL,the linewidth characteristic analysis of TECSL and the measurement of TECSL spectral linewidth.Besides,Zemax design software was used to simulate the TECSL of the Littrow structure.Third,the effect of current magnitude on the 1550 nm TECSL tuning range and SMSR was studied.At a cavity length of 40 mm,the tuning range of TECSL for a diffraction grating of 600 lines/mm is 85 nm when the current is 100 m A,and the tuning range of the TECSL for a diffraction grating of 600 lines/mm is 140 nm when the current is 200 m A.Secondly,The effect of grating line density on the performance of 1550 nm TECSL was studied.When using a 600 lines/mm grating,the SMSR of the TECSL reaches a maximum of 60 d B at 200 m A,the output power reaches 37.06 m W,a tuning range of 140 nm(1465-1605 nm)is achieved,and a Lorentz linewidth reaches 23.76 k Hz.With a grating of 1200 lines/mm,the maximum SMSR at 200 m A exceeds 63 d B and the output power reaches 43.67 m W,reaching more than60 d B in most of the tuned spectral range of the TECSL,achieving a tuning range of 155 nm(1455-1610 nm)and a Lorentz linewidth of 6.89 k Hz.Finally,the effect of the external cavity length on the performance of the 1550 nm narrow linewidth 1200 lines/mm grating TECSL was studied,When the cavity length was maintained at 45 mm,and the 3 d B Lorentz linewidth of the 1550 nm narrow linewidth 1200 lines/mm grating TECSL was 3.66 k Hz.Fourth,a mode-selecting filter Littrow structure TECSL is designed.In the Littrow structure,a mode-selecting filter is added to the external cavity composed of a single-angle plane(SAF)gain chip rear cavity surface and a diffraction grating,so that light at certain frequencies passes through the mode-selecting filter and propagates back and forth in the cavity.Other frequencies of light have great loss because they cannot pass through the mode selection filter,which realizes the single-frequency oscillation of the laser,and then has the effect of narrowing the linewidth.The cavity length is maintained at 40 mm,the linewidth of the TECSL is 18.4 k Hz for the 600 lines/mm grating with the mode selection filter,and the 3d B Lorentz linewidth for the TECSL for the 1200 lines/mm grating with the mode selection filter is 3.29 k Hz.The TECSL characteristics of the Littrow structure in the case of mode selection without mode selection filter are compared,and it is concluded that the mode selection filter makes the linewidth of TECSL narrower.