Self-heating in mid-infrared QCLs leads to beam instabilities and facet related failures. Single-element 4.6 μm-emitting BH QCLs were fabricated, where a tapered region scales the output power and, ahead of the emitting aperture, a narrow section provides mode filtering for suppressing high-order spatial modes. Beam-stability measurements indicate a small degree of collimated-beam centroid motion (< 0.25 mrad) can be achieved at >1.5W QCW output powers. Comparisons between short-pulse current and QCW operation reveal the impact of thermal lensing on the beam properties, while full 3D modeling provides insights into influence of device geometry on mode selection.
Buried heterostructure quantum cascade lasers (BH-QCLs) operating at high temperature in mid-infrared (MIR) to THz spectral range are desired for chemical sensing and free-space optical communication (FOC). In this work, Fe doped semi-insulating InP (SI-InP) regrowth is demonstrated in a hydride vapor phase epitaxy (HVPE) reactor for advanced MIR and THz BH-QCLs grown by MBE and MOCVD. SI-InP regrowth is implemented in THz QCL pillar arrays and narrow width and reverse-taper MIR BH-QCLs for efficient heat dissipation. By exploiting SI-InP regrowth, the parasitic capacitance in MIR distributed feedback BH-QCL can be suppressed, which is exploited for high speed FOC application.
Scaling the coherent power of mid-infrared (IR)-emitting quantum cascade lasers (QCLs) to the multi-watt range remains an important objective for applications where the laser beam needs to travel through air to remote targets, such as freespace communication links. For such applications requiring long-range pointing accuracy, measurements of beam stability are also important. We present beam-quality measurement results of narrow-ridge (4-5 μm), 4.6 μm-emitting buriedheterostructure (BH) QCLs. A 40-stage, step-tapered active-region (STA) structure was grown by MOCVD, and ICP etching was used to make deep ridges. InP:Fe was preferentially regrown in the field regions by using an SiO2 mask for ridge etching and Hydride Vapor Phase Epitaxy (HVPE). The HVPE process is attractive for selective regrowth, since high growth rates (0.2-0.3 μm/min) can be utilized, and highly planar top surfaces can readily be obtained. HVPE regrowth has been previously employed for BH devices of MBE-grown QCL ridges, but beam-stability measurements were not reported. HR-coated, 7.5 mm-long devices were measured under QCW operation (100 μsec pulse width, 0.5%-10% duty cycle) – very good beam quality factors, M2 < 1.2, were observed for both 4 μm and 5 μm ridge widths, but the narrower ridge exhibited better pointing stability. Collimated 5 μm-wide BH devices displayed some small degree of centroid motion with increasing power (< 0.125 mrad). This corresponds to a targeting error of ~1.25 cm over a distance of 100 m. Significantly improved lateral-beam stability was observed for narrower ridge width, although at the expense of reduced output power.
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