D. Botez

8 W continuous wave front‐facet power from broad‐waveguide Al‐free 980 nm diode lasers

Al‐free 980 nm InGaAs/InGaAsP/InGaP laser structures grown by low‐pressure metalorganic chemical vapor deposition (LP‐MOCVD) have been optimized for high cw output power by incorporating a broad waveguide design. Increasing the optical‐confinement layer total thickness from 0.2 to 1.0 μm decreases the internal loss fivefold to 1.0–1.5 cm−1, and doubles the transverse spot size to 0.6 μm (full width half‐maximum). Consequently, 4‐mm long, 100‐μm‐aperture devices emit up to 8.1 W front‐facet cw power. cw power conversion efficiencies as high as 59% are obtained from 0.5‐mm long devices. Catastrophic‐optical‐mirror‐damage (COMD) power‐density levels reach 15.0–15.5 MW/cm2, and are found similar to those for InGaAs/AlGaAs facet‐coated diode lasers.

High-power (>10 W) continuous-wave operation from 100-μm-aperture 0.97-μm-emitting Al-free diode lasers

By incorporating a broad transverse waveguide (1.3 μm) in 0.97-μm-emitting InGaAs(P)/InGaP/GaAs separate-confinement-heterostructure quantum-well diode-laser structures we obtain record-high continuous-wave (cw) output powers for any type of InGaAs-active diode lasers: 10.6–11.0 W from 100-μm-wide-aperture devices at 10 °C heatsink temperature, mounted on either diamond or Cu heatsinks. Built-in discrimination against the second-order transverse mode allows pure fundamental-transverse-mode operation (θ⊥=36°) to at least 20-W-peak pulsed power, at 68×threshold. The internal optical power density at catastrophic optical mirror damage (COMD) PCOMD is found to be 18–18.5 MW/cm2 for these conventionally facet-passivated diodes. The lasers are 2-mm-long with 5%/95% reflectivity for front/back facet coating. A low internal loss coefficient (αi=1 cm−1) allows for high external differential quantum efficiency ηd (85%). The characteristic temperatures for the threshold current T0 and the differential quantum effic...

Phase-locked arrays of antiguides: model content and discrimination

Three classes of array modes of closely spaced antiguides are analyzed: coupled fundamental (element) modes, coupled first-order (element) modes, and modes adjacent to coupled fundamental modes. The behavior of coupled fundamental modes as a function of lateral index step is analyzed and explained from a ray-optics point of view. It is found that at resonance, for both coupled fundamental and first-order modes, the array-mode propagation constant is virtually identical to the propagation constant of the mode of a single, unperturbed antiguide. Several types of mode discrimination mechanisms are discussed. For devices with 3- mu m-wide antiguide cores and 1- mu m interelement spacing, intermodal discrimination values of 15-20 cm/sup -1/ can be achieved. Excellent agreement is found between experimental data and theoretical predictions based on the effective-index method. >

Watt‐range, coherent, uniphase powers from phase‐locked arrays of antiguided diode lasers

Twenty‐element near‐resonant AlGaAs/GaAs arrays of antiguides have been optimized for maximum intermodal discrimination and large Strehl ratio. It is found that 1000‐μm‐long devices with two intracavity Talbot‐type spatial filters, and a 3 to 1 ratio between element core and interelement spacing provide the best results. The intermodal discrimination is discussed for both Talbot and uniform devices. For devices with two Talbot‐type spatial filters, diffraction‐limited‐beam operation is obtained to 1 W pulsed power, and operation in a beam with lobewidth 1.5× diffraction limit is obtained to 2 W and 19× threshold. cw diffraction‐limited‐beam operation is obtained to 0.5 W, limited by thermal considerations. Uniform devices operate in beams with lobewidth ≊3× diffraction limit to 5 W and 45× threshold. At 5 W total output the coherent uniphase power is 1.6 W, and the coherent power in the main lobe is 0.94 W.

Resonant optical transmission and coupling in phase-locked diode laser arrays of antiguides: The resonant optical waveguide array

Uniform linear arrays of antiguides have 100% optical transmission between elements when the interelement spacing is an integer number of leaky wave half‐wavelengths in the lateral direction. Resonant in‐phase‐mode and out‐of‐phase‐mode coupling occurs when the number of half‐wavelengths is odd and even, respectively. Such devices are called resonant optical waveguide (ROW) arrays. The discrimination between the resonant array mode and adjacent array modes reaches a maximum in close proximity to the resonance. An AlGaAs/GaAs ROW diode laser array operating close to resonance is demonstrated. Devices with virtually uniform near‐field intensity profiles operate in stable, diffraction‐limited in‐phase modes to drive levels in excess of three times threshold.

High-power, diffraction-limited-beam operation from phase-locked diode-laser arrays of closely spaced leaky waveguides (antiguides)

A novel type of phase‐locked diode‐laser array is demonstrated: an array of closely spaced index depressions (antiguides). The arrays are ten‐element AlGaAs/GaAs devices fabricated by metalorganic chemical vapor deposition and liquid phase epitaxy over a patterned substrate. The fundamental and highest‐order modes of the array of antiguides have negligible radiation loss: 20–30 times less than that for a single antiguide. The modal‐gain differentials between adjacent array modes are at least an order of magnitude larger than those for evanescently coupled arrays of positive‐index guides. Fundamental‐array‐mode operation in a virtually diffraction‐limited‐beam (1.4°) pattern is obtained to 200 mW. Out‐of‐phase‐mode operation in a virtually diffraction‐limited‐beam (1.2°) pattern is achieved to 110 mW/uncoated facet. The inherent array‐mode stability of antiguided arrays is discussed.

Temperature dependence of the key electro-optical characteristics for midinfrared emitting quantum cascade lasers

The equations for threshold-current density Jth, differential quantum efficiency ηd, and maximum wallplug efficiency ηwp,max for quantum-cascade lasers (QCLs) are modified for electron leakage and backfilling. A thermal-excitation model of “hot” injected electrons from the upper laser state to upper active-region states is used to calculate leakage currents. The calculated characteristic temperature T0 for Jth is found to agree well with experiment for both conventional and deep-well (DW) QCLs. For conventional QCLs ηwp,max is found to be strongly temperature dependent; explaining experimental data. At 300 K for optimized DW-QCLs, front-facet, continuous-wave ηwp,max values >20% are projected.

Analysis of surface-emitting second-order distributed feedback lasers with central grating phaseshift

An analysis of second-order distributed feedback lasers (DFB) with central grating phaseshift is performed. The devices have an active grating (i.e., DFB) section, passive grating sections (i.e., DBRs); and the active grating is formed at a metal-semiconductor interface. Coupled-mode theory and the transfer matrix method are employed. It is found that a central grating phaseshift, /spl Delta//spl phi/, of 180/spl deg/ causes the laser to radiate in a beam of symmetric near-field amplitude profile, in sharp contrast to conventional second-order DFB lasers which radiate in beams of asymmetric near-field amplitude profile. In turn the far-field profile becomes a single-lobe beam pattern. Thus, a means to fundamentally obtain surface emission in an orthonormal single-lobe beam from a second-order DFB/DBR device has been found. The orthornomal-beam emission is achieved at no penalty in device efficiency. External differential quantum efficiencies, /spl eta//sub d/, in excess of 70% can be obtained, and the guided-field intensity profile is substantially uniform. The effects of the lengths of the DFB section (L/sub DFB/) and of each of the DBR sections (L/sub DBR/) on device performance are analyzed and optimal values are found to occur for L/sub DFB/ in the 500-700 /spl mu/m range and for L/sub DBR/ in the 600-700 /spl mu/m range. One can obtain /spl eta//sub D/ values as high as 76% from devices with 80% of the energy in the central lobe, and moderate threshold gains (i.e., 40 cm/sup -1/). Threshold gains as low as 25 cm/sup -1/ can also be obtained from highly efficient devices (i.e., /spl eta//sub D//spl cong/70%), at some penalty in guided-field uniformity. In either case the intermodal discrimination is quite high (70-75 cm/sup -1/). Gratings with half-wave (i.e., /spl pi/) phaseshifts have been fabricated by using the dual-tone photoresist method, and the concept has been experimentally proven: orthonormal, single-lobe emission in a diffraction-limited beam from 1500 /spl mu/m-long devices. Extension to two-dimensional (2-D) large-aperture: 200 /spl mu/m/spl times/1500 /spl mu/m; surface emitters is quite possible, which should allow for the emission of watts of coherent CW power in a stable, single mode. The 2-D structure represents a defect-free, second-order active photonic lattice.

Design optimization of ARROW-type diode lasers

Antiresonant reflecting optical wavelength (ARROW)-type diode lasers have been optimized for high-power, single-spatial-mode operation. Calculated modal behavior predicts strong intermodal discrimination with low loss for the fundamental ARROW mode. Single-lobe far-field operation is obtained only when the high-index reflecting (antiresonant) cladding layers correspond to an optical thickness of lambda /sub 1/ (m+3/4), where lambda is the lateral (projected) wavelength of the leaky wave in the high-index layers, and m is an integer (m=0, 1,. . .). Experimental results include stable, single-spatial mode operation to 500-mW peak pulsed power and 300-mW CW power at an emission wavelength of 0.98 mu m.<<ETX>>

X-valley leakage in GaAs-based midinfrared quantum cascade lasers: A Monte Carlo study

We present a detailed Monte Carlo simulation of electron transport incorporating both Γ- and X-valley states in GaAs-based quantum cascade lasers (QCLs). Γ states are calculated using the K⋅p method, while X states are obtained within the effective mass framework. All the relevant electron-phonon, electron-electron, and intervalley scattering mechanisms are included. We investigate the X-valley leakage in two equivalent-design GaAs/AlGaAs QCLs with 33% and 45% Al-barrier compositions. We find that the dominant X-valley leakage path in both laser structures is through interstage X→X intervalley scattering, leading to a parallel leakage current JX. The magnitude of JX depends on the temperature and occupation of the X subbands, which are populated primarily by the same-stage scattering from the Γ-continuum (Γc) states. At 77 K, JX is small up to very high fields in both QCLs. However, at room temperature the 33% QCL shows a much higher JX than the 45% QCL even at low fields. The reason is that in the 33% QC...