Syn-Yem Hu

Lateral carrier diffusion and surface recombination in InGaAs/AlGaAs quantum‐well ridge‐waveguide lasers

We measured the increase in threshold currents due to lateral carrier diffusion in InGaAs/AlGaAs quantum‐well ridge‐waveguide laser diodes. The ridge stripes were fabricated by using both in situ monitored pure Cl2 reactive ion etching and selective wet etching to completely eliminate the spreading current in the conductive upper cladding layer while keeping the ridge sidewalls straight. After comparing the threshold data with a theoretical model, the ambipolar diffusion coefficient is found to be 22 cm2/s in the population‐inverted InGaAs layer. This model is based on the calculated optical gain curve and the ambipolar carrier transport in the quantum‐well and waveguiding layers. The dependence of carrier lifetime on the local carrier concentration is included in the calculation. Moreover, from another set of devices with the portions of the active layer outside the ridge stripes etched away, the surface recombination velocity is found to be around 1–2×105 cm/s.

The effect of lateral leakage current on the experimental gain/current-density curve in quantum-well ridge-waveguide lasers

We stress the importance of considering the effect of lateral leakage current on the material gain/current-density characteristics measured from ridge-waveguide diode lasers. It is found that the inclusion of lateral leakage current is crucial to obtaining a self-consistent result. An experimental demonstration has been performed on an In/sub 0.2/Ga/sub 0.8/As/AlGaAs strained single quantum-well laser sample, from which a gain curve with transparency current density of 53.8 A/cm/sup 2/ was obtained. By using devices of different geometries, the variation of leakage currents is measured and the accuracy of the resultant gain curves is discussed. >

Multimode WDM optical data links with monolithically integrated multiple-channel VCSEL and photodetector arrays

We report on the design and implementation of a novel multiple-wavelength optical data link for low-cost multimode wavelength-division multiplexing (WDM) local-area network applications. This link utilizes a monolithically integrated multiple-wavelength vertical-cavity laser array and a narrow-band resonant-cavity photodetector array to transmit multiple channels of information simultaneously via a single multimode fiber. A detailed analysis on wavelength tuning and threshold characteristics of different laser cavity designs is presented. Theoretical results are compared to our experimental data. On the receiver part, both Schottky and p-i-n photodetectors with a single- or coupled-cavity structure are discussed. A novel p-i-n resonant-cavity photodetector design with a partially oxidized front mirror for linewidth control is proposed. Moreover, we also demonstrate preliminary measurements on the optical link built with our multiple-wavelength vertical-cavity laser array and photodetector array. Finally, the feasibility of constructing a multiwavelength optical data link with a single dual-core multimode fiber and an integrated laser/detector array is evaluated.

High‐efficiency and low‐threshold InGaAs/AlGaAs quantum‐well lasers

High‐efficiency and low‐threshold InGaAs/AlGaAs quantum‐well laser structures have been grown by molecular beam epitaxy. Material characterization was performed on polyimide‐planarized ridge‐waveguide lasers. The measured material gain data are compared to theoretical calculations that include the valence‐band mixing effects. Total injection current densities of 84 and 60 A/cm2 have been measured from 50‐μm‐wide laser diodes with cavity lengths of 2850 μm (from a double‐quantum‐well sample) and 1770 μm (from a single‐well sample), respectively. Moreover, we have also obtained a cw threshold current as low as 2.1 mA from a 1.7‐μm‐wide and 140‐μm‐long as‐cleaved ridge‐waveguide device. In addition, the lateral current leakage for the double‐quantum‐well sample is found to be twice that of the single‐well one.

Submilliampere-threshold InGaAs-GaAs quantum-well ridge-waveguide lasers with lateral confinement provided by impurity-induced disordering

Significant reduction of threshold currents in InGaAs-GaAs quantum-well ridge-waveguide lasers has been achieved by using silicon-induced disordering to provide lateral confinement. Room-temperature threshold currents as low as 0.7 mA for pulsed operation and 0.9 mA for cw operation have been obtained from an uncoated 137-/spl mu/m-long and 0.3-/spl mu/m-wide device. In addition, the effects of high-temperature annealing on the various device characteristics, such as the gain curve, internal loss, and quantum efficiency, have been investigated.<<ETX>>

Resonant-cavity InGaAs/InAlGaAs/InP photodetector arrays for wavelength demultiplexing applications

Resonant-cavity photodetector arrays are demonstrated in the InGaAs/InAlAs/InP material system grown by solid-source molecular beam epitaxy. To reduce crosstalk between channels, these devices are designed with a high-Q cavity to obtain narrow-band photoresponse. In addition, a novel double-absorber design has been proposed and implemented to avoid position sensitivity related to the cavity standing wave and eliminate the need for in situ cavity-mode adjustment. Both Schottky and P-i-N diode structures are studied and compared. Eight element arrays with linearly distributed resonant wavelengths spanning over 30 nm are fabricated by using a three-level anodic oxidation process. An experimental channel rejection ratio of 14.5 dB at 4 nm away from the resonant peak has been achieved.

Temperature‐dependent threshold and modulation characteristics in InGaAs/GaAs quantum‐well ridge‐waveguide lasers

The observed temperature dependence of threshold currents in InGaAs/GaAs quantum‐well ridge‐waveguide lasers is modeled. This method incorporates both experimental data and theoretical gain calculations. The results show that lateral leakage currents must be included in the ridge‐waveguide laser models. Based on the same model, the temperature‐dependent modulation characteristics for InGaAs/GaAs quantum‐well ridge‐waveguide lasers are also investigated.

Serpentine superlattice nanowire-array lasers

We report characterization and modeling of serpentine superlattice nanowire-array lasers. These samples were grown by molecular beam epitaxy on (100) n/sup +/-GaAs vicinal substrates. In-plane ridge-waveguide lasers with ridge stripes either parallel or perpendicular to the nanowire arrays have been characterized at low temperatures. The measured net gain spectra at 1.4 K showed strong optical gain anisotropy such that the TM mode gain became greater than the TE mode gain when the optical cavity was placed along the nanowire direction. This provides strong evidence that the lateral quantum confinement in the serpentine superlattice is stronger than the vertical quantum confinement. Optical gain spectra in the serpentine superlattice are calculated with consideration of coupling between wires and homogeneous line broadening. A good fit to the measured polarization-dependent gain spectra is achieved when the lateral Al segregation and the homogeneous line broadening are chosen to be 8% and 7 meV, respectively. This small but finite Al segregation in the serpentine superlattice provides lateral quantum confinement for holes, which results in significant anisotropy in the relation between net gain and injection current density,. >

Densely packed pie shaped vertical-cavity surface-emitting laser array incorporating a tapered one-dimensional wet oxidation

Wavelength-division-multiplexing (WDM) photonic integrated emitter (PIE) vertical-cavity surface-emitting laser (VCSEL) arrays are fabricated using a post growth wet oxidation technique. High-density integration of WDM VCSEL arrays is possible by combining the technique of one-dimensional oxidation and large-scale tapered oxidation. Eight channels are integrated into a circle of 60 /spl mu/m in diameter. Seven channels are found to operate as lasers. The lasing wavelengths range from 823 to 836 nm corresponding with the distance between the VCSEL mesa and the tuning trench. The successful demonstration of incorporating wet oxidation into the wavelength control of the PIE VCSEL array opens a new way of fabricating mask-defined densely packed WDM VCSEL arrays.

Coupled-cavity resonant photodetectors for high-performance wavelength demultiplexing applications

A new coupled-cavity resonant photodetector structure is studied to provide a flat-topped passband with steep skirts in the photoresponse. The idea is to add a low-loss cavity that is optically coupled to the absorptive cavity via an intermediate mirror. Experimental reflection spectra confirm this coupling effect. Modeling for an InGaAs/GaAs/AlGaAs design reveals that this new coupled-cavity structure will be able to show a 22 dB channel rejection ratio even when the optical channel spacing is as narrow as 2 nm. In addition, the design and processing tolerances for cavity-mode adjustment are also addressed.