Thomas Earles

Multidimensional Conduction-Band Engineering for Maximizing the Continuous-Wave (CW) Wallplug Efficiencies of Mid-Infrared Quantum Cascade Lasers

By tailoring the active-region quantum wells and barriers of 4.5-5.0-μm-emitting quantum cascade lasers (QCLs), the device performances dramatically improve. Deep-well QCLs significantly suppress carrier leakage, as evidenced by high values for the threshold-current characteristic temperature <i>T</i>₀ (253 K) and the slope-efficiency characteristic temperature <i>T</i>₁ (285 K), but, due to stronger quantum confinement, the global upper-laser-level lifetime τ_4g decreases, resulting in basically the same room-temperature (RT) threshold-current density <i>J</i>_th as conventional QCLs. Tapered active-region (TA) QCLs, devices for which the active-region barrier heights increase in energy from the injection to the exit barriers, lead to recovery of the τ_4g value while further suppressing carrier leakage. As a result, experimental RT <i>J</i>_th values from moderate-taper TA 4.8-μm emitting QCLs are ~14% less than for conventional QCLs and <i>T</i>₁ reaches values as high as 797 K. A step-taper TA (STA) QCL design provides both complete carrier-leakage suppression and an increase in the τ_4g value, due to Stark-effect reduction and strong asymmetry. Then, the RT <i>J</i>_th value decreases by at least 25% compared to conventional QCLs of same geometry. In turn, single-facet, RT pulsed and continuous-wave maximum wallplug-efficiency values of 29% and 27% are projected for 4.6-4.8-μm-emitting QCLs.

5.5 W near-diffraction-limited power from resonant leaky-wave coupled phase-locked arrays of quantum cascade lasers

Five, 8.36 μm-emitting quantum-cascade lasers (QCLs) have been monolithically phase-locked in the in-phase array mode via resonant leaky-wave coupling. The structure is fabricated by etch and regrowth which provides large index steps (Δn = 0.10) between antiguided-array elements and interelement regions. Such high index contrast photonic-crystal (PC) lasers have more than an order of magnitude higher index contrast than PC-distributed feedback lasers previously used for coherent beam combining in QCLs. Absorption loss to metal layers inserted in the interelement regions provides a wide (∼1.0 μm) range in interelement width over which the resonant in-phase mode is strongly favored to lase. Room-temperature, in-phase-mode operation with ∼2.2 kA/cm2 threshold-current density is obtained from 105 μm-wide aperture devices. The far-field beam pattern has lobewidths 1.65× diffraction limit (D.L.) and 82% of the light in the main lobe, up to 1.8× threshold. Peak pulsed near-D.L. power of 5.5 W is obtained, with 4...

HIGH-PERFORMANCE, RELIABLE, 730-NM-EMITTING AL-FREE ACTIVE REGION DIODE LASERS

Compressively-strained InGaAsP QW active (/spl lambda/=732 nm) diode lasers achieve 2.4 W CW front-facet power from 100 /spl mu/m-wide apertures, with reliable operation at 0.5 W CW. Record-high characteristic temperatures for the threshold current and the differential quantum efficiency, T/sub 0/=115 K and T/sub 1/=285 K are obtained by growing on misoriented substrates.

1.1 W continuous-wave, narrow spectral width (<1 Å) emission from broad-stripe, distributed-feedback diode lasers (λ=0.893 μm)

By etching a distributed-feedback grating directly into the Al-free optical confinement region of a 100 μm stripe InGaAs/InGaP/GaAs diode laser, 1.1 W cw front-facet output power has been obtained at 0.893 μm with a spectral full width at half maximum of 0.9 A. These devices have 1 mm long cavities and shallow gratings with a coupling coefficient, κ∼7 cm−1. The combination of long device length and low grating coupling results in both efficient operation as well as a longitudinally uniform field profile. As a result, all excited lateral modes oscillate at the same longitudinal cavity resonance to high power levels. Using shallow gratings etched in an InGaP upper confinement layer permits the growth of a high-quality cladding layer over the grating surface yielding excellent device performance. Facet-coated (5%/95%) devices demonstrate external differential quantum efficiencies of 51% and peak wallplug efficiencies of 32% at 1.1 W cw output power.

Design for high-power, single-lobe, grating-surface-emitting quantum cascade lasers enabled by plasmon-enhanced absorption of antisymmetric modes

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 p...

High-power conversion efficiency Al-free diode lasers for pumping high-power solid-state laser systems

Al-free active diode lasers emitting near 970 nm wavelength have been optimized for high electrical-to-optical power conversion efficiency. There are numerous key contributors such as scattering and absorption losses, band alignment, Joule heating, carrier leakage and below-threshold losses that contribute to power loss mechanisms. We report on improvement from 50% to a record-high 73% power conversion efficiency for a 1 cm bar at 10C, resulting from a multi-pronged approach that has been taken to minimize each of the loss mechanisms as to improve the overall power conversion efficiency.

Magnetic resonance imaging of hyperpolarized 129Xe produced by spin exchange with diode-laser pumped Cs

We report the results of experiments leading to the production of an image of a polarized 129Xe sample prepared by spin exchange with Cs, optically pumped with a spectrally narrowed 894.3 nm diode laser. Representative images of the average electron spin polarization are shown. Appreciable cesium electron polarization values were achieved, and a nuclear polarization of about 2.5% was measured for 129Xe. The absolute nuclear polarization was measured by water-calibrated free induction decay of the nuclear magnetic resonance signal, and the polarized xenon imaged using a 2 T magnetic resonance imaging system.

0.45 W diffraction-limited beam and single-frequency operation from antiguided phase-locked laser array with distributed feedback grating

A second-order diffraction grating placed below the active region of a phase-locked resonant antiguided array selects the in-phase array mode in addition to its role as a single-longitudinal-mode selector. This type of array-mode discrimination relies on the fact that the resonant in-phase array mode has significantly better field overlap with the grating region than nonresonant array modes. Furthermore, it eliminates the need for a conventional array-mode discriminator: interelement loss; which can cause self-pulsations. Diffraction-limited beam and single-frequency operation is obtained to at least 0.45 W peak pulsed power from 20 element, InGaAs/InGaP/GaAs structures (λ=0.97 μm) of 120-μm-wide aperture. Distributed-feedback operation is confirmed over the 20–40 °C temperature range. The results are in good agreement with theory.

High-power, surface-emitting quantum cascade laser operating in a symmetric grating mode

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 devices 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.

Highly temperature insensitive, low threshold-current density (λ = 8.7–8.8 μm) quantum cascade lasers

By stepwise tapering, both the barrier heights and quantum-well depths in the active regions of 8.7–8.8 μm-emitting quantum-cascade-laser (QCL) structures, virtually complete carrier-leakage suppression is achieved. Such step-taper active-region-type QCLs possess, for 3 mm-long devices with high-reflectivity-coated back facets, threshold-current characteristic temperature coefficients, T0, as high as 283 K and slope-efficiency characteristic temperature coefficients, T1, as high as 561 K, over the 20–60 °C heatsink-temperature range. These high T0 and T1 values reflect at least a factor of four reduction in carrier-leakage current compared to conventional 8–9 μm-emitting QCLs. Room temperature, pulsed, threshold-current densities are 1.58 kA/cm2; values comparable to those for 35-period conventional QCLs of similar injector-region doping level. Superlinear behavior of the light-current curves is shown to be the result of the onset of resonant extraction from the lower laser level at a drive level of ∼1.3×...