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Power and, especially, brightness are important characteristics of quantum-cascade lasers (QCLs) for many applications. Brightness is a measure of the intensity of a laser’s emission that can be delivered by an optical system. Applications based on vibrational-mode gas spectroscopy or mid-infrared illumination benefit from a higher brightness beam providing a better signal-to-noise ratio, external-cavity tunable QCL systems gain efficiency when more of the beam power can be re-focused into the active region, and, of course, infrared countermeasures depend on optical power delivered by a lens system to a distant target.
For a given wavelength, brightness requires both optical power and beam quality. The optical power emitted by a QCL depends on its efficiency and its active region volume. The efficiency can be optimized through design features, quality of epitaxy, thermal management, and facet reflectance. The active region volume is governed by the number of cascades and the width and length of the emitter. For a given efficiency – and QCL efficiency is close to optimized today – the route to power is either through a larger number (20–40) of cascades combined with a narrow stripe or a small number (5–15) of cascades using a broad-area emitter. For a given input power – which means for a given laser volume – a broad-area laser will have the same output power and better thermal conductance than a narrow-stripe buried-heterostructure laser. Further, because both emission power and thermal conductance of a broad-area QCL with around 10 cascades scales with area, such broad-area QCLs cascade number can be scaled to higher emission power [1,2,3]. But, wide stripes are notorious for poor beam quality and the reduced cascade number reduces efficiency.
The realization of good beam quality from broad-area QCLs has been achieved through several approaches. Angled facets to suppress non-fundamental modes, tapered stripes that combine large area with a section that favors the fundamental mode, photonic crystal facets, constricted stripes to filter high-order modes, and engineered waveguide loss that suppress high-order modes have been demonstrated. Further, the reduced efficiency has been demonstrated to be acceptable down to about 10 cascades.
This talk will discuss the paradigm shift to broad-area, reduced-cascade-number QCLs for high brightness applications, including the optimization of efficiency and beam quality.
[1] W.T. Masselink, M.P. Semtsiv, A. Aleksandrova, and S. Kurlov, “Power scaling in quantum cascade lasers using broad-area stripes with reduced cascade number,”
Optical Engineering 57(1), 011015 (2017); http://dx.doi.org/10.1117/1.OE.57.1.011015.
[2] W.T. Masselink, M.P. Semtsiv, Y. V. Flores, A. Aleksandrova, and J. Kischkat, “Design issues and physics for power scaling of quantum-cascade lasers,” Proc. SPIE 9989, 99890B (2016).
[3] P. Figueiredo, M. Suttinger, R. Go, A. Todi, H. Shu, E. Tsvid, C.K.N. Patel, and A. Lyakh, “Continuous wave quantum cascade lasers with reduced number of stages,” IEEE Photonics Tech. Lett. 29, 1328–1331 (2017).
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