S.D. McDougall

A UNIVERSAL DAMAGE INDUCED TECHNIQUE FOR QUANTUM WELL INTERMIXING

We report a novel technique for quantum well intermixing which is simple, reliable and low cost, and appears universally applicable to a wide range of material systems. The technique involves the deposition of a thin layer of sputtered SiO2 and a subsequent high temperature anneal. The deposition process appears to generate point defects at the sample surface, leading to an enhanced intermixing rate and a commensurate reduction in the required anneal temperature. Using appropriate masking it is possible to completely suppress the intermixing process, enabling large differential band gap shifts (over 100 meV) to be obtained across a single wafer.

Monolithic integration via a universal damage enhanced quantum-well intermixing technique

A novel technique for quantum-well intermixing is demonstrated, which has proven a reliable means for obtaining postgrowth shifts in the band edge of a wide range of III-V material systems. The technique relies upon the generation of point defects via plasma induced damage during the deposition of sputtered SiO/sub 2/, and provides a simple and reliable process for the fabrication of both wavelength tuned lasers and monolithically integrated devices. Wavelength tuned broad area oxide stripe lasers are demonstrated in InGaAs-InAlGaAs, InGaAs-InGaAsP, and GaInP-AlGaInP quantum well systems, and it is shown that low absorption losses are obtained after intermixing. Oxide stripe lasers with integrated slab waveguides have also enabled the production of a narrow single lobed far field (3/spl deg/) pattern in both InGaAs-InAlGaAs, and GaInP-AlGaInP devices. Extended cavity ridge waveguide lasers operating at 1.5 /spl mu/m are demonstrated with low loss (/spl alpha/=4.1 cm/sup -1/) waveguides, and it is shown that this loss is limited only by free carrier absorption in waveguide cladding layers. In addition, the operation of intermixed multimode interference couplers is demonstrated, where four GaAs-AlGaAs laser amplifiers are monolithically integrated to produce high output powers of 180 mW in a single fundamental mode. The results illustrate that the technique can routinely be used to fabricate low-loss optical interconnects and offers a very promising route toward photonic integration.

Design and fabrication of low beam divergence and high kink-free power lasers

We report the design and fabrication of high performance high power lasers with emission wavelength from 800 to 1000 nm using a novel wafer structure, in which a graded V-shape layer was incorporated, to reduce the vertical far field (wafer growth direction) and to suppress higher order mode lasing. The structure offers the freedom to independently design the vertical far field and optical overlap with the quantum wells. An extremely low far field can be achieved, which still retains high optical overlap, allowing a low threshold current to be maintained. In addition, the structure can greatly enhance the laser kink-free power by suppressing or even completely eliminating higher order mode lasing, an extremely desirable property for high power single mode lasers.

Quantum well intermixing in material systems for 1.5 μm (invited)

Precise control over local optical and electrical characteristics across a semiconductor wafer is a fundamental requirement for the fabrication of photonic integrated circuits. Quantum well intermixing is one approach, where the band gap of a quantum well structure is modified by intermixing the well and barrier layers. Here we report recent progress in the development of intermixing techniques for long wavelength applications, discussing two basic techniques. The first is a class of laser disordering techniques which take place in the solid state. The second is a novel intermixing technique involving plasma induced damage. Both techniques enable large band gap shifts to be achieved in standard GaInAsP multiple quantum well laser structures. The potential of both techniques for photonic integration is further demonstrated by the fabrication and characterisation of extended cavity lasers.

Terahertz repetition frequencies from harmonic mode-locked monolithic compound-cavity laser diodes

Compound-cavity laser diodes are mode locked at a harmonic of the fundamental round-trip frequency to achieve repetition rates of up to 2.1 THz. The devices are fabricated from GaAs/AlGaAs material at a wavelength of 860 nm and incorporate two gain sections with an etched slot reflector between them, and a saturable absorber section. Autocorrelation studies are used to investigate device behavior for different reflector types and reflectivity. These lasers may find applications in terahertz imaging, medicine, ultrafast optical links, and atmospheric sensing.

Broad optical bandwidth InGaAs-InAlGaAs light-emitting diodes fabricated using a laser annealing process

The use of a laser-induced quantum-well intermixing technique in the InGaAs-InAlGaAs material system is presented. We report blue shifts of up to 240 nm in the 1.55-/spl mu/m emission wavelength, generated by exposure to a Nd:YAG laser. Variations in the optical intensity across the irradiating beam were used to laterally grade the bandgap along a sample. We used this technique to fabricate broad optical bandwidth, light-emitting diodes. The devices showed an increase in the full width half maximum of the emission spectrum from 125 nm in undisordered devices to over 260 nm in intermixed material. The output spectrum was also observed to be flat-topped (within 5%) across a wavelength range of 140 nm.

High d/gamma values in diode laser structures for very high power

Record values for the rollover power and rollover linear power densities of 9xx nm devices, obtained by simultaneous scaling of length and d/Γ, are reported. The values for d/Γ lay in the range 0.8 μm to 1.2 μm with corresponding cavity lengths from 3.5 mm to 5 mm. The transversal structures were asymmetric, with a higher refractive index on the n side. An optical trap was helpful in reducing the radiation extension on the p side and the overall thickness. The highest rollover linear power densities were 244 mW/μm for structures without an optical trap and 290 mW/μm for those that included an optical trap

Design and fabrication of low beam divergence and high kink-free power 980 nm lasers

We report on the design and fabrication of high power 980 nm single mode lasers using a novel wafer structure, which leads to a greatly enhanced kink-free power and low beam divergence.

Three band-gap QW intermixing in InP/InGaAs/InGaAsP system for monolithically integrated optical switch

We have demonstrated that independent control of three band-gaps across an InP-InGaAs-InGaAsP QW wafer can be achieved by a two-stage sputtered silica intermixing processes. This will be used for optimisation of the performance of optical switches which consist of passive components, modulators and amplifiers.

Integrated RGB laser light module for autostereoscopic outdoor displays

We have developed highly compact RGB laser light modules to be used as light sources in multi-view autostereoscopic outdoor displays and projection devices. Each light module consists of an AlGaInP red laser diode, a GaInN blue laser diode, a GaInN green laser diode, as well as a common cylindrical microlens. The plano-convex microlens is a so-called “fast axis collimator”, which is widely used for collimating light beams emitted from high-power laser diode bars, and has been optimized for polychromatic RGB laser diodes. The three light beams emitted from the red, green, and blue laser diodes are collimated in only one transverse direction, the so-called “fast axis”, and in the orthogonal direction, the so-called “slow axis”, the beams pass the microlens uncollimated. In the far field of the integrated RGB light module this produces Gaussian beams with a large ellipticity which are required, e.g., for the application in autostereoscopic outdoor displays. For this application only very low optical output powers of a few milliwatts per laser diode are required and therefore we have developed tailored low-power laser diode chips with short cavity lengths of 250 μm for red and 300 μm for blue. Our RGB laser light module including the three laser diode chips, associated monitor photodiodes, the common microlens, as well as the hermetically sealed package has a total volume of only 0.45 cm³, which to our knowledge is the smallest RGB laser light source to date.