Cost-effective, distributed Bragg reflector and distributed feedback quantum cascade lasers with high-order, top-cladding, and sidewall gratings and a waveguide pullback from the output facet

Distributed-feedback (DFB) and Distributed Bragg Reflector (DBR) Quantum Cascade Lasers (QCLs) are routinely used in Longwave-Infrared (LWIR) spectroscopy and free space optical communication. First-order gratings have sub-micron features, which are expensive to fabricate. To reduce the fabrication cost, we investigate high-order, surface and sidewall gratings: Gratings of 3 rd -higher orders have larger features that are easier to fabricate, while both top-cladding and sidewall gratings are defined after epi-growth. DBR/DFB QCLs often need an Anti-Reflection (AR) coating deposited on the front facet to improve the wavelength selection of the chip as the facet reflectivity may interfere with the grating. It is difficult to entirely suppress Fabry-Perot feedback in the LWIR range with AR coatings, especially within 8-12µm. By adding a waveguide pullback between the front facet and the DBR/DFB grating, optical feedback is significantly reduced due to beam divergence in the unguided section. Therefore, this waveguide pullback effectively acts as an integrated AR coating into the laser chips, which ensures single-mode operation. Experimental data from fabricated devices and numerical models (COMSOL and custom Floquet-Bloch models) for the DBR/DFB-QCLs of 3rd -6 th order and waveguide pullback is presented in this work. Measured spectral width and central wavelength for these devices are in good agreement with numerical projections. The results presented here demonstrate DBR QCLs based on high-order gratings with waveguide pullback promise reduction in cost of infrared narrow spectrum devices. All experimental data reported were collected with low duty cycle (pulsed width ~300ns and 10kHz repetition rate) with average optical power less than 10mW.