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

Abstract

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 μm and duty cycles around 0.5, which is attributed to their larger effective duty cycles and phase differences caused by an 18° 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.