Narrow beam (3.2 x D.L. FWHM) is demonstrated up to 5.85 W pulsed output power from a five-element phase-locked array of 4.7 μm-emitting quantum cascade lasers. Devices are fabricated by a two-step MOCVD process and operate predominately in an in-phase array mode, in agreement with design simulation studies.
Grating-coupled surface-emitting (GCSE) lasers generally operate with a double-lobed far-field beam pattern along the cavity-length direction, which is a result of lasing being favored in the antisymmetric grating mode. We experimentally demonstrate a GCSE quantum-cascade laser design allowing high-power, nearly single-lobed surface emission parallel to the longitudinal cavity. A 2nd-order Au-semiconductor distributed-feedback (DFB)/distributed-Bragg-reflector (DBR) grating is used for feedback and out-coupling. The DFB and DBR grating regions are 2.55 mm- and 1.28 mm-long, respectively, for a total grating length of 5.1 mm. The lasers are designed to operate in a symmetric (longitudinal) grating mode by causing resonant coupling of the guided optical mode to the antisymmetric surface-plasmon modes of the 2nd-order metal/semiconductor grating. Then, the antisymmetric modes are strongly absorbed by the metal in the grating, causing the symmetric mode to be favored to lase, which, in turn, produces a single-lobed beam over a range of grating duty-cycle values of 36%–41%. Simulations indicate that the symmetric mode is always favored to lase, independent of the random phase of reflections from the device's cleaved ends. Peak pulsed output powers of ∼0.4 W were measured with nearly single-lobe beam-pattern (in the longitudinal direction), single-spatial-mode operation near 4.75 μm wavelength. Far-field measurements confirm a diffraction-limited beam pattern, in agreement with simulations, for a source-to-detector separation of 2 m.
Resonant coupling of the transverse-magnetic polarized (guided) optical mode of a quantum-cascade laser (QCL) to the antisymmetric surface-plasmon modes of 2nd-order distributed-feedback (DFB) metal/semiconductor gratings results in strong antisymmetric-mode absorption. In turn, lasing in the symmetric mode, that is, surface emission in a single-lobe far-field beam pattern, is strongly favored over controllable ranges in grating duty cycle and tooth height. By using core-region characteristics of a published 4.6 μm-emitting QCL, grating-coupled surface-emitting (SE) QCLs are analyzed and optimized for highly efficient single-lobe operation. For infinite-length devices, it is found that when the antisymmetric mode is resonantly absorbed, the symmetric mode has negligible absorption loss (∼0.1 cm−1) while still being efficiently outcoupled, through the substrate, by the DFB grating. For finite-length devices, 2nd-order distributed Bragg reflector (DBR) gratings are used on both sides of the DFB grating to prevent uncontrolled reflections from cleaved facets. Equations for the threshold-current density and the differential quantum efficiency of SE DFB/DBR QCLs are derived. For 7 mm-long, 8.0 μm-wide, 4.6 μm-emitting devices, with an Ag/InP grating of ∼39% duty cycle, and ∼0.22 μm tooth height, threshold currents as low as 0.45 A are projected. Based on experimentally obtained internal efficiency values from high-performance QCLs, slope efficiencies as high as 3.4 W/A are projected; thus, offering a solution for watt-range, single-lobe CW operation from SE, mid-infrared QCLs.
On-chip resonant leaky-wave coupling of quantum cascade lasers (QCLs) emitting at 8.36 μm has been realized by selective regrowth of interelement layers in curved trenches, defined by dry and wet etching. The fabricated structure provides large index steps (Δn = 0.10) between antiguided-array element and interelement regions. In-phase-mode operation to 5.5 W front-facet emitted power in a near-diffraction-limited far-field beam pattern, with 4.5 W in the main lobe, is demonstrated. A refined fabrication process has been developed to produce phased-locked antiguided arrays of QCLs with planar geometry. The main fabrication steps in this process include non-selective regrowth of Fe:InP in interelement trenches, defined by inductive-coupled plasma (ICP) etching, a chemical polishing (CP) step to planarize the surface, non-selective regrowth of interelement layers, ICP selective etching of interelement layers, and non-selective regrowth of InP cladding layer followed by another CP step to form the element regions. This new process results in planar InGaAs/InP interelement regions, which allows for significantly improved control over the array geometry and the dimensions of element and interelement regions. Such a planar process is highly desirable to realize shorter emitting wavelength (4.6 μm) arrays, where fabrication tolerance for single-mode operation are tighter compared to 8 μm-emitting devices.
We report on catastrophic degradation in 8.4 µm InGaAs-InAlAs quantum cascade lasers using focused ion beam (FIB) and high-resolution transmission electron microscope (HR-TEM) techniques.
Phase-locking, via leaky-wave coupling, of five 4.7 µm-emitting quantum cascade lasers is demonstrated for coherent-power scaling. Non-resonant devices fabricated by two-step MOCVD operate in a mixture of in-phase and out-of-phase modes to 3.85 W peak pulsed output power. Design analysis shows pure in-phase-mode operation under resonant-coupling occurs for optimized devices.
Summary form only given. We present here the first demonstration of a phase-locked array for which a DFB grating acts as an array-mode selector. Single-frequency and single-spatial mode operation are achieved to 0.45 W peak power. Thus, resonant antiguided optical waveguide (ROW) DFB laser diode arrays have potential for stable-beam, reliable operation to watt range coherent powers.
A numerical model is developed for a phase-locked array of quantum cascade lasers. The population density is derived from rate equations. The temperature distribution for stationary generation is produced by ohmic heating. It is shown that the results of above-threshold operation modeling by the semi-vectorial beam propagation method are in good agreement with the modal analysis provided by the vectorial-COMSOL solver supplemented with the Rigrod's model estimations. The wall-plug efficiency and the limits of the single-mode lasing are found. Thermal lensing is shown to be the main reason limiting the single-spatial-mode power under CW-operating conditions.
Tunable IR filters with integral electromagnetic drive actuators have been fabricated using the MEMS LIGA process at the University of Wisconsin.Two types of actuators with IR filter loads have been demonstrated: a single-pole solenoid-type actuator and a three-phase electromagnetic stepper motor drive.The IR filters were designed for a tunable range from 8 to 32 µm.The filter and actuator structures are fabricated from electroplated permalloy.Measurements of the magnetic properties of the permalloy show it to be a soft magnetic material with low coercive force (0.3 Oe) and high permeability (-65,000).An IR filter structure with a large displacement stepper motor has been demonstrated.The bi-directional stepper motor had a total travel of 1.7 mm with an average force per step of 2.4 mN (basic step size of 25 µm) at a drive current of 45 mA per phase.This is the first demonstration of a MEMS linear electromagnetic stepping motor.
