D. Z. Tsang

Q switching of low-threshold buried-heterostructure diode lasers at 10 GHz

Buried‐heterostructure actively Q‐switched diode lasers have been made with threshold currents as low as 14 mA. The lasers operate continuously at room temperature. Modulation has been observed at rates up to 10.5 GHz. Evidence of several modes of Q switching has been obtained.

Intracavity loss modulation of GaInAsP diode lasers

Diode lasers with an intracavity electroabsorption modulator have been fabricated in the GaInAsP/InP material system. The additional absorption loss produced by operating the modulator near maximum reverse bias increased the laser threshold by a factor of as much as 2.9 relative to the threshold with the modulator open circuited. Large depth of modulation of the laser output at frequencies up to 2.5 GHz, the measurement system limit, has been achieved by application of a microwave signal to the modulator.

Low threshold GaInAsP/InP buried‐heterostructure lasers with a chemically etched and mass‐transported mirror

The mass‐transport technique has been used to improve chemically etched mirrors on GaInAsP/InP double heterostructure wafers. Vertical and flat mirror facets have been obtained. Buried‐heterostructure lasers fabricated with one such mirror and the other mirror cleaved show high device yield with threshold currents as low as 6 mA and differential quantum efficiency as high as 33%.

Characterization of mass-transported p-substrate GaInAsP/InP buried-heterostructure lasers with analytical solutions for electrical and thermal resistances

Studies have been carried out to evaluate mass-transported p-substrate GaInAsP/InP buried-heterostructure lasers, which have a number of potential advantages over the more conventional n-substrate lasers. Devices have been fabricated with series resistances as low as 3 Omega , in good agreement with the p-substrate spreading resistance calculated using conformal mapping. A further development of this theory yields simple formulas of thermal resistances of heat generated both in the active region and in the p-InP. The presently fabricated p-substrate lasers also showed CW threshold currents as low as 4.5 mA, differential quantum efficiencies as high as 34% per facet, output powers as high as 33 mW per facet, and a maximum total electrical-to-optical power conversion efficiency of 36%. >

High‐performance InGaAsP/InP buried‐heterostructure lasers and arrays defined by ion‐beam‐assisted etching

Ion‐beam‐assisted etching has been used to fabricate mass‐transported InGaAsP/InP buried‐ heterostructure lasers. These lasers have a novel, deeply etched rectangular mesa that results in reduced current leakage. Both single‐stripe lasers and Y‐junction‐coupled multiple‐stripe laser arrays have been demonstrated. The single‐stripe lasers have 12 mA cw threshold currents, differential quantum efficiencies of 32%–34% per facet, and smooth single‐lobe far‐field patterns. The multiple‐stripe arrays lase in‐phase with strong phase coherence.

Multistable mode locking of InGaAsP semiconductor lasers

We have investigated the pulsation behavior of InGaAsP semiconductor lasers with a proton‐bombarded segment. These lasers emit picosecond (30–70 ps) pulses at gigahertz (0.6–3.0 GHz) rates. An antireflection‐coated diode in an external cavity is passively mode locked and multistable; as many as four co‐existing states are observed. Interlocking hysteresis loops are observed in the pulsation frequency, pulse width, and output power as functions of the bias current. A delayed feedback model explains qualitative features of the multistable mode locking. To our knowledge this is the first report of multistability of laser pulsation.

Simple compact diode-laser/microlens packaging

A technique has been demonstrated in which the precisely polished substrate of a microlens is directly attached to the front face of a diode laser and heatsink to form a compact stable package. An experimental package using mass-transported GaP spherical microlenses and InGaAs-AlGaAs ridge-waveguide lasers (0.98-/spl mu/m wavelength) showed a well-collimated beam with a near diffraction-limited 0.7/spl deg/ divergence. The high-index microlens and the favorable lens configuration showed high tolerance for alignment errors in the optical-path-difference and Strehl-ratio calculations.

IVB-7 continuous multi-gigahertz modulation of Q-switched GaInAsP diode lasers

Diode lasers capable of producing shorl optical pulses at high rates are of interest fait applications in communications and optical signal processing. Previously, we have reporteti pulsed operation of an intracavity-lossmodulated laser [ l l . A cw diode laser with two electrically isolated buried-heterostructure sections has been operated with full on/ofl modulation at 3 GHz. The device operation is; based on a combination of gain switching andl Q-switching in which both gain and loss are actively varied in a modulator section while an amplifier section is driven with a constant dc: current.

Uniform linear arrays of strained-layer InGaAs-AlGaAs quantum-well ridge-waveguide diode lasers fabricated by ECR-IBAE

Uniform linear arrays of strained-layer multiple-quantum-well InGaAs-AlGaAs ridge-waveguide diode lasers have been fabricated that operate near 980 nm and have low threshold currents I/sub th/ and high differential quantum efficiencies /spl eta//sub d/. Uniformity was achieved by a combination of uniform ion-beam-assisted etching with an electron cyclotron resonance ion source and uniform organometallic vapor-phase epitaxial (OMVPE) growth. We investigated the effects of device geometry, namely, ridge width, cavity length, and remaining cladding thickness outside the ridge t, on I/sub th/ and /spl eta//sub d/. For uncoated lasers with 500-/spl mu/m-long cavities, 2- to 3-/spl mu/m-wide ridges, and t=165/spl plusmn/75 nm fabricated in double-quantum-well OMVPE material, I/sub th/ was typically in the range 6-7 mA and /spl eta//sub d/ was >40% per facet. A 24-element array of 2-/spl mu/m-wide, 200-/spl mu/m-long ridge-waveguide lasers with a high reflection coating on the back facet exhibited excellent uniformity, with threshold currents and single-ended differential quantum efficiencies that averaged 3.4 mA and 72%, respectively. Similar arrays with high-reflectivity coatings on both facets exhibited threshold currents as low as 2 mA. >

TP-B6 intracavity-loss-modulated GaInAsP diode lasers

A reactive-ion-etching (RIE) process capable of controllably providing vertical or positively sloped-wall profiles with no mask undercutting has been developed for the InP-GaAs system. With this process, we have been able to create stripe geometry GaInAsP/InP DH lasers using RIE to form mirror facets, as has been done previously1-5 with wet chemical etching (WCE). We have also integrated such lasers with monitoring detectors by etching relatively narrow slots using both WCE and RIE so that transmitted light can be collected in the adjacent region. Separate experiments have demonstrated the usefulness of RIE in forming accurately defined channels for transverse current or optical confinement. Previous work with GaA1As/GaAs1>2 has recognized the desirability of providing lasers on relatively large-area substrates which can hold other optical or electronic components, and initial results have been reported with G ~ I ~ A S P / I ~ P ~ ? ~ using WCE. Although WCE can provide smooth arld vertical facets, mask undercutting and reproducibility are problems that may be overcome with RIE.5 In this work, we show that appropriate pre;ssures of C12 and O 2 gases in a conventional parallel-plate spu.tter-etching system can provide an attractive combination o’f the advantages of wet chemical and ion-beam etching, while avoiding some of the shortcomings of each, in the etching of GaInAsP/InP. High-aspect ratio slots can be formed independent of substrate orientation, for example. With this new capability, a new class of integrated electronic and optical device configurations will be possible. Initial results with RIE mirror facets in stripe-geometry DH lasers at 1.3 pm demonstrate that reasonable threshold currents (-340 mA for 20-pm stripe width) and quasi-single mode operation are possible. Expected improvements in facet smoothness and more optimum geometries should provide better performance. Updated results will be presented for the work in progress. Monitoring detectors integrated with RIE or WCE facet lasers have given outputs comparable to an external Ge photodetector.