Di Liang

Single-Facet Folded-Cavity Diode Laser With Ultrasmall Bend Radius High-Index-Contrast Oxidized AlGaAs Ridge Waveguide

AlGaAs heterostructure high-index-contrast (HIC) ridge waveguide (RWG) diode lasers incorporating a folded-cavity single-facet resonator with a folding bend radius as small as r=10 mum are demonstrated. Fabricated by a self-aligned deep dry etch (through the active region) plus nonselective O2-enhanced wet thermal oxidization process, the low-index, insulating, and interface-passivating wet thermal oxide grown directly on the etch-exposed AlGaAs waveguide sidewalls yields a high lateral refractive index contrast of Deltan~1.7 and provides strong optical mode confinement. The HIC RWG device geometry also completely eliminates lateral current spreading, which results in an excellent overlap between the optical field and the gain region of the single InAlGaAs quantum-well graded-index separate confinement heterostructure. A threshold current of Ith=65mA is obtained for the r=10 mum device (a half-racetrack ring resonator), giving a threshold current density of Jth=1503 A/cm2, 3.34 times higher than that of same-length straight lasers. At a bend radius of r=150 mum, Ith=16.6 mA, and Jth is comparable to straight cavity values, indicating that at this curvature there is negligible bending and scattering loss for the lowest-order waveguide mode

Reduction of AlGaAs Heterostructure High-Index-Contrast Ridge Waveguide Scattering Loss by Sidewall Smoothing Through Oxygen-Enhanced Wet Thermal Oxidation

We demonstrate the efficacy of oxidation smoothing of sidewall roughness in high-index-contrast AlGaAs heterostructure ridge waveguides via oxygen-enhanced nonselective wet thermal oxidation for reducing scattering loss. Single-mode waveguides of core widths between 1.5 and 2.2 ¿m are fabricated using both the inward growth of a ~ 600-nm sidewall-smoothing native oxide outer cladding and, for comparison, encapsulation of an unoxidized etched ridge with a ~ 600-nm deposited silicon oxide cladding layer. On average, measured loss coefficients are reduced by a factor of 2 with the oxidation smoothing process.

Oxidation smoothing of sidewall roughness in AlGaAs heterostructure waveguides

A 10-100 fold reduction in sidewall roughness of etched AlGaAs ridge structures is demonstrated using a wet thermal oxidation process modified through addition of <1% O2 to the N2 carrier gas, potentially enabling high index contrast waveguides with ultra-low scattering loss for ring resonator and other photonics device applications

Native-oxide-confined high-index-contrast λ=1.15 μm strain-compensated InGaAs single quantum well ridge waveguide lasers

High performance native-oxide-confined high-index-contrast (HIC) ridge waveguide (RWG) diode lasers are fabricated in a strain-compensated In0.4Ga0.6As single quantum well structure by employing a deep dry etch plus nonselective O2-enhanced wet thermal oxidation process. The thermal native oxide grown on the etch-exposed RWG sidewalls of the Al0.74Ga0.26As waveguide cladding layers and GaAs core with GaAsP–InGaAs quantum well provides both strong optical and electrical confinements for the active region. Due to a smoothing of sidewall roughness by the O2-enhanced oxidation, the lasers exhibit a low internal loss in αi=7.2 cm−1 for a w=7.2 μm narrow stripe HIC RWG structure, only 53% larger than that of w=87.2 μm broad-area devices, enabling their room temperature operation at a low 300 A/cm2 threshold current density.

Nonselective oxidation of GaAs-based III-V compound semiconductor heterostructures for in-plane semiconductor lasers

A nonselective wet thermal oxidation technique for AlGaAs-containing heterostructures has been shown to enable the fabrication of a variety of novel high-efficiency, high-power GaAs-based in-plane laser devices. Applied in conjunction with a deep anisotropic dry etch, nonselective oxidation yields a simple, self-aligned high-index-contrast (HIC) ridge waveguide (RWG) structure. The native oxide grown directly on the waveguide ridge simultaneously provides excellent electrical insulation, passivation of the etch-exposed bipolar active region, and a low refractive index cladding, leading to numerous laser performance benefits. The resulting strong lateral optical confinement at the semiconductor/oxide interface (with refractive index contrast &Dgr;n~1.7) enables half-racetrack ring resonator lasers with a record small 6 &mgr;m bend radius. A nearly circularly-symmetric output beam is demonstrated on narrow w=1.4 &mgr;m aperture width straight stripe-geometry lasers with single spatial and longitudinal mode total power output of ~180 mW at 228 mA (9x threshold). With the complete structural elimination of lateral current spreading, the excellent overlap of the optical field with the gain region provides high slope efficiency performance (ranging from >1.0 W/A at w=1.4 &mgr;m to 1.3 W/A for w=150 &mgr;m broad area stripes) for 300 K cw operation of unbonded, p-side up 808 nm InAlGaAs graded-index separate confinement heterostructure (GRINSCH) active region lasers. Using the direct thermal oxidation of a dilute nitride GaAsP/InGaAsN MQW active region, 1.3 &mgr;m emission GaAs-based HIC RWG lasers exhibit a >2X threshold reduction and kink-free operation relative to conventional low-confinement devices. Other recent progress on the application of nonselective oxidation to GaAs-based semiconductor lasers will be reported.

High-Index-Contrast Oxide-Confined GaAsP/InGaAsN Multi-Quantum-Well Ridge Waveguide Lasers

A modified wet thermal process is used to oxidize both GaAs waveguide and GaAsP/InGaAsN MQW layers of deeply-etched ridge waveguide lasers, providing up to a 2.3 times threshold reduction and strong index-guiding for kink-free operation

Oxide-confined high index contrast ridge waveguide curved resonator laser diodes

A simple, novel self-aligned deeply-etched plus wet thermally oxidized ridge waveguide fabrication process is demonstrated which enables high-index-contrast, low loss curved resonator GRINSCH lasers with a bend radius as low as 10 /spl mu/m.

High-Efficiency Oxide-Confined Ridge Waveguide Laser with Nearly Symmetric Output Beam

High-index-contrast InAlGaAs/AlGaAs ridge waveguide lasers exhibiting low threshold current and high efficiency are fabricated by a deep etch plus modified wet thermal oxidation process. A nearly-symmetric output beam is achieved for a 1.4mum wide aperture device

High-Performance Small-Radius Half-Racetrack-Ring-Resonator InAlGaAs Quantum Well Laser Diodes Fabricated via Non-Selective Wet Oxidation

Utilizing the high index contrast of a deep-etched, non-selectively oxidized AlGaAs/InAlGaAs GRINSCH heterostructure, half-racetrack-ring resonator lasers with peak output powers of 239 mW for a r=25 mum bend radius, and 40 mW for r=8 mum, are demonstrated

High-index-contrast ridge waveguide lasers fabricated via oxygen-enhanced wet thermal oxidation

A simple, novel self-aligned deep etch plus wet thermal oxidization process is demonstrated which enables high-index-contrast (HIC) ridge waveguide (RWG) lasers fabricated in a high-efficiency, high-power AlGaAs/InAlGaAs/GaAs graded-index separate confinement heterostructure to operate with a curved half-ring resonator geometry having a bend radius as low as 10 μm. A wet thermal oxidation process modified through addition of <1% O2 to the N2 carrier gas is shown to smooth the sidewall roughness of etched AlGaAs ridge structures 10-100 fold as the oxidation front progresses inward. The reduction of propagation scattering loss due to the reduced sidewall roughness is examined. The thermal oxide grown on the deeply-etched RWG sidewalls and base also provides electrical isolation from the contact metallization, resulting in a simplified, self-aligned process, and yields a RWG structure which effectively prevents current spreading. The thermal oxide appears to be of sufficiently high quality to passivate the etched active region surface based on a comparative analysis of straight RWG lasers of varying stripe widths (w=5 to 150 μm) passivated with native-oxide vs. PECVD-deposited SiO2. For example, at w<15 μm, the SiO2-insulated devices have ~2X higher threshold current densities than the native-oxide devices for comparable bar lengths. The resulting high lateral optical confinement factor at the semiconductor/oxide interface (Δn=1.69) significantly enhances the laser gain and efficiency. A native-oxide-defined straight laser (w=7 μm, L= 452 μm) operates cw (300 K, unbonded, p-side up) with a threshold current of Ith=21.5 mA (Jth=679.5 A/cm2) and slope efficiency of 1.19 A/W (differential quantum efficiency = 78%) at a wavelength of ~813 nm.