We present results and analysis of 5 Gb/s On-Off-Keyed (OOK) data transmission at ~ 4.6-micron wavelength using, at room temperature, a directly modulated, single-mode DFB-QCL transmitter and a Resonant-Cavity Infrared Detector (RCID) receiver. The DFB-QCL design enables relatively high 165-mW CW output power. We used a 3-mm device with ~ 4-micron aperture for stable single-spatial-mode operation, operating at 65 mW. The RCID detector suppresses background radiation while providing enhanced quantum efficiency, ~ 60%, with low polarization dependence and low dark current at room temperature. The data transmission was achieved with no transmitter pulse shaping, consistent with lower-cost transceiver implementation.
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.
Grating-coupled, surface-emitting (GCSE) quantum-cascade lasers (QCLs) offer a pathway towards realizing watt-range, surface-emitted output powers in the mid-infrared spectral region with high beam quality. Previously we have reported wide-ridge GCSE QCLs which employed metal/semiconductor, 2nd-order distributed feedback (DFB) gratings with distributed Bragg reflector (DBR) terminations. We report here on the lasing characteristics of narrow-ridge (~7 μm-wide) GCSE devices, which employ the STA-RE-type active-region design, for obtaining single-spatial-mode both laterally and longitudinally. The QCL structure was grown using Metalorganic Chemical Vapor Deposition (MOCVD) and the grating was defined using a combination of e-beam lithography patterning and wet-chemical etching, and the ridge (~7 μm) was dry-etched. The total length of the DFB + DBR regions is 5.1 mm, and was electrically isolated in the DBR regions by employing AlOx. Due to resonant coupling of the guided light to the antisymmetric surface-plasmon modes of the 2nd-order grating, the antisymmetric (A) modes are strongly absorbed; thus, allowing for the symmetric (S) mode to be favored to lase. Initial devices have demonstrated maximum pulse output power from the surface of ~150 mW at 4.88 μm, with only ~10% power emitted from the edge facets. An anti-reflective (AR) coating of a quarter-wavelength Y2O3 layer was applied on the emission window, drastically improving the far-field beam pattern, that resulting in a central, near-diffraction-limited single-lobe beam pattern. COMSOL simulations were performed to further optimize the SE-base design for high CW performance. Parameter sweeps of cladding-layer thickness, grating height, and grating duty cycle were performed, which identified design tradeoffs for the various structural parameters.
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