Substrate-emitting quantum cascade lasers with a distributed Bragg reflector and a spectrally detuned second-order outcoupler

Demonstration of substrate-emitting quantum cascade lasers (QCLs) with a distributed Bragg reflector (DBR) used for wavelength selection and a second-order outcoupler spectrally detuned from the reflector is reported. A 40-stage QCL structure with the gain spectrum centered at 4.1 μm was processed into 20 μm-wide ridge-waveguide devices with a 4 mm-long gain section, a 0.5 mm-long first-order DBR section with 644 nm period, and a 1.5 mm-long second-order outcoupler with 1100 nm period. Etch depth and duty cycle were 900 nm and 45%, respectively, for both gratings. The three sections shared the same electrical contact and were, therefore, pumped at the same current density. A device with a high-reflection coated back facet and anti-reflection coated front facet and substrate side delivered a peak power of 0.6 W from the substrate into a single-lobed beam with ∼1° × 18° angular full width at half maximum (FWHM). Due to the spectral detuning, the beam was extracted from the device at an angle of ∼−34° relative to the growth direction. The measured spectrum had a FWHM of 3 cm−1, and its peak was centered at 4.07 μm, close to the design value of 4.10 μm. The detuning between the two gratings allows for independent optimization of grating profiles to significantly improve the overall laser performance, which will be the focus of future work.