Electrically Pumped Photonic Crystal Distributed Feedback Quantum Cascade Lasers
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Electrically Pumped Photonic Crystal Distributed Feedback Quantum Cascade Lasers Manijeh Razeghi, Yanbo Bai, Steven Slivken, and Wei Zhang Department of Electrical Engineering and Computer Science, Northwestern University, Center for Quantum Devices, 2220 Campus Drive, Evanston, IL, 60208 ABSTRACT In parallel with the effort to improve the efficiency of Quantum cascade lasers (QCL) for high power continuous wave (CW) operations, the peak power in pulsed mode operation can be easily scaled up with larger emitting volumes, i.e., processing QCLs into broad area lasers. However, as the emitter width increases, multi-mode operation happens due to poorer lateral mode distinguishability. By putting a two dimensional photonic crystal distributed feedback (PCDFB) layer evanescently coupled to the main optical mode, both longitudinal and lateral beam coherence can be greatly enhanced, which makes single mode operation possible for broad area devices. For PCDFB laser performance, the linewidth enhancement factor (LEF) plays an important role in controlling the optical coherence. Being intersubband devices, QCLs have an intrinsically small LEF, thus serving as better candidates over interband lasers for PCDFB applications. We demonstrate electrically pumped, room temperature, single mode operation of photonic crystal distributed feedback quantum cascade lasers emitting at λ ~ 4.75 μm. Ridge waveguides of 50 μm and 100 μm width were fabricated with both PCDFB and Fabry-Perot feedback mechanisms. The Fabry-Perot device has a broad emitting spectrum and a broad farfield character. The PCDFB devices have primarily a single spectral mode and a diffraction limited far field characteristic with a full angular width at half-maximum of 4.8 degrees and 2.4 degrees for the 50 μm and 100 μm ridge widths, respectively.
INTRODUCTION High power, single mode, mid-infrared laser sources are highly desirable for a number of applications, such as countermeasures, free space communication and remote chemical detection. Besides the constant effort devoted to improving the light generation mechanism within the emitting core materials, which fundamentally governs the capability of high power operation, novel and superior laser cavities need to be developed to ensure robust single mode operation. Although the wall plug efficiency of the current state-of-art quantum cascade lasers (QCL) reaches close to 10% [1], the single mode maximum output power is limited by the lack of a proper laser cavity. Broad area QCLs are natural candidates to produce multi-watt peak power, however, as the gain width increases, multi-mode operation happens due to poorer lateral mode distinguishability. In addition, nonlinear phenomena like spatial hole burning and filamentation becomes more pronounced. As such, the beam coherence cannot be maintained and divergence far from diffraction limited angles occurs, which prevents the conventional broad area FabryPerot device for single mode operation. The angled-grating DFB (α-DFB) [2] uses a tilted grating with respe
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