The study on application of high-order tilted asymmetric Bragg gratings in quantum cascade lasers

High -order gratings eliminate the need for high -resolution lithography in the definition process. However, they are not widely adopted due to inability to provide sufficient coupling coefficients in long -wave mid -infrared quantum cascade lasers (QCLs). Fifth order tilted asymmetric gratings (TAS-gratings) distributed feedback (DFB) QCLs with stable single longitudinal mode are firstly reported in this paper. TAS-gratings exhibit longitudinal mode selection capability across a broad range of asymmetric distances around 1.18 mu m and duty cycles around 0.5, which is attributed to their larger effective duty cycles and phase differences caused by an 18 degrees tilt angle of the grating in the middle and two asymmetrical distributions of gratings on both sides. Further experimental results indicate that DFB QCLs with TAS-gratings exhibit stable excitation of single longitudinal modes within the duty cycle range of 0.48-0.54 due to the large coupling coefficients, the temperature drift coefficient of 0.29 nm/ K of TAS-grating device is reduced by 64% compared with symmetric gratings (S -gratings) devices, the maximum side -mode suppression ratio (SMSR) of the devices exceeds 25 dB. TAS-gratings offer greater process tolerance, design flexibility and lower preparation difficulty. This study provides a solution to achieve sufficiently large coupling coefficients for high -order gratings applied to long -wave mid -infrared QCLs.