Design of 1×16 phase-locked array of quantum cascade laser in mid-infrared band

  Objective  Quantum cascade laser (QCL) in the mid-infrared (MIR) band suffer from the problem of limited output power, and many important applications require high power outputs above the watt level and high beam quality. Simply increasing the width of the active region can obtain higher power output, but it often directly affects the beam quality and generates a large amount of heat in the core region that cannot be exported, resulting in the device not being able to operate continuously. If a set of narrow-ridge QCL phase-locked arrays, can greatly improve the thermal efficiency, but also avoid the different phases between the units QCL brought about by the problem of poor beam quality, to facilitate the realization of high-power continuous output.

  Methods  The seed laser is designed with a resonator composed of a 50% Bragg mirror on the front and a 100% Bragg mirror on the back. A stable laser beam with a central wavelength of 4.6 μm is achieved; a thermal simulation analysis was established to determine the array distance at the amplified end; in order to avoid the problem of poor beam quality caused by phase inconsistency between the laser array elements, the 1×16 beam splitter consisting of a multimode interference coupler (MMI) and a bend waveguide is designed, so that the seed optical passing through the beam splitter can maintain the phase consistency and realize the function of uniform beam splitting; Al₂O₃ anti-reflection coating is plated on the array output port, and optical amplification is completed through the end amplification part to achieve coherent beam enhancement of laser output power.

  Results and Discussions  The simulation results show that the reflectivity of the total and semi-reflector mirrors is 100% and 50% for the beam with a central wavelength of 4.6 μm, indicating that optical with a wavelength of 4.6 μm can be used to generate stable oscillations using the mirrors (Fig.8). The total transmittance of the 1×2 MMI output port with the taper is 99.8%, and the optical field distribution is clear and stable, indicating that uniform beam splitting with low loss is achieved (Fig.9 and Fig.10). Due to the high symmetry of the beam splitter, each beam passes through waveguides of equal length, resulting in consistent phase at the amplification end. A comparison is made between different amplifier array spacings in terms of temperature and far-field distribution (Fig.11 and Fig.14). The output port of the array is coated with Al₂O₃ coating, and its high transmittance can further increase the output power of the laser (Fig.13).

  Conclusions  Aiming at the low output power of the current single QCL, a QCL phase-locked array with an operating wavelength of 4.6 μm is designed to improve the output power of the laser. The epitaxial thickness of each layer of the device is determined by mode analysis, combining the calculation of optical limiting factor and waveguide transmission loss, and the single-mode waveguide is designed. The waveguide loss is 0.055dB·cm^-1, and the optical limiting factor is 0.733. A seed laser with a laser wavelength of 4.6 μm is designed by using a resonator composed of 100% and 50% Bragg mirrors. The 1×16 low loss splitter is designed by using MMI and bending waveguide, and its loss is 0.254 dB. The output port is plated with 0.7 μm thick Al₂O₃ anti-reflection coating, and the transmission rate can reach 0.975, which further improves the output power of the laser.