Design of distributed feedback grating for QCL based on time-domain finite-difference method

Abstract:

Quantum cascade lasers (QCL) have broad application potentials in the fields of infrared countermeasure system, free-space optical communication and trace gas detection due to their high efficiency, compacted size, low power consumption, and wide wavelength tuning range. Compared with traditional Fabry-Pérot (FP) cavity and external cavity, distributed feedback quantum cascade lasers (DFB-QCL) can obtain narrower laser linewidth and higher integration, respectively. In this paper, the structure design, numerical simulation and optimization of the Bragg grating of DFB-QCL are carried out to obtain the transmission spectrum with central wavelength at 4.6 μm. We analyze the relationship among the structure parameters, the central wavelength shift and transmission efficiency using coupled-wave theory and finite-difference time-domain (FDTD) method. It is shown that the increase in the number of grating periods enhances the capabilities of mode selectivity, while the grating length of single period adjustment directly determines the Bragg wavelength. Additionally, variations in etching depth and duty cycle lead to blue and red shifts in the central wavelength, respectively. Based on the numerical simulation results, the optimized design parameters for the upper buffer layer and the upper cladding grating is proposed, which gives an optional scheme for component fabrication and performance improvement in the future.

Key words:Quantum cascade laser, distributed feedback, bragg grating, time-domain finite difference method