D.B. Young

Enhanced performance of offset-gain high-barrier vertical-cavity surface-emitting lasers

The temperature dependence and power output of vertical-cavity surface-emitting lasers (VCSELs) are addressed. The peak wavelength of the quantum well has been offset from the wavelength of the device cavity mode so that they are aligned at elevated temperatures. The result of this design change is to produce an 8- mu m-diameter VCSEL capable of operation to 145 degrees C, as well as CW operation of broad-area (70- mu m diameter) heat-sunk devices to record power levels. Fiber coupling experiments were also carried out, and a record 33-mW CW power was coupled to a multimode fiber. >

Low threshold planarized vertical-cavity surface-emitting lasers

Vertical-cavity surface-emitting lasers fabricated utilizing a self-aligned process to provide planarized contacts are discussed. A single 80-AA In/sub 0.2/Ga/sub 0.8/As strained quantum well was used in the active region. Emission was at 963 nm. Threshold currents under continuous-wave room temperature operation of 1.1 mA, at 4.0-V bias, were measured for numerous 12- mu m*12- mu m devices. Corresponding threshold current densities were 800 A/cm/sup 2/ (600 A/cm/sup 2/ for broad area devices). These are the lowest figures yet reported for this type of device. It was found that grading of the mirror had a marked effect on mirror resistance.<<ETX>>

Band-gap engineered digital alloy interfaces for lower resistance vertical-cavity surface-emitting lasers

We report on a technique of grading the heterobarrier interfaces of a p‐type distributed Bragg reflector mirror to reduce the operating voltages of vertical‐cavity surface‐emitting lasers (VCSELs). We report VCSELs with lower operating voltages (2–3 V) and record continuous‐wave room‐temperature power‐conversion efficiencies (17.3%). We experimentally demonstrate that by using a parabolic grading and modulating the doping correctly, a flat valence band is generated that provides low voltage hole transport. The low resistance mirrors are achieved using low Be doping, digital‐alloy grading and 600 °C growth temperatures.

High efficiency submilliamp vertical cavity lasers with intracavity contacts

Contacts have been made to p- and n-type layers on opposite sides of the active region within the cavity of an InGaAs vertical cavity surface emitting laser. The two concentric ring contacts allow all electrical connections on and emission from the top surface of a semi-insulating GaAs substrate. The design includes a novel current leveling layer to minimize current crowding effects. A high external quantum efficiency of 46% has been measured with maximum output powers up to 6 mW for a 15 /spl mu/m diameter device and threshold currents of 0.72 mA for a 7 /spl mu/m diameter laser.<<ETX>>

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.

17.3% peak wall plug efficiency vertical-cavity surface-emitting lasers using lower barrier mirrors

Modifications to the epitaxial growth of vertical-cavity surface-emitting laser (VCSEL) material have recently led to improved characteristics. By offsetting the quantum-well gain peak from the cavity mode, and implementing lower barrier p-type Al/sub 0.67/Ga/sub 0.33/As/GaAs DBR mirrors with parabolic interface gradings, better high-temperature operation and lower voltages have been achieved. These effects combine to yield a peak wall plug efficiency of 17.3% for room temperature, CW operation.<<ETX>>

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. >

Evaluating the effects of optical and carrier losses in etched‐post vertical cavity lasers

We demonstrate the combined effects of optical scattering loss and surface recombination (or carrier diffusion) on the performance and scalability of etched‐post vertical cavity lasers (VCLs). The size dependence of optical losses and threshold gain are determined from pulsed measurements of external quantum efficiency. Deeper etch depths result in a stronger radial dependence of the threshold gain, which quickly increases the threshold current density. With optical loss accounted for, pulsed threshold current density measurements give the extra information needed for evaluating carrier loss. Surface recombination or carrier diffusion also results in threshold current density increases, but scalability is ultimately limited by the ability of the active region to provide enough gain for smaller size, higher optical loss devices. Even with these losses, three‐quantum‐well VCLs with shallow etches have threshold currents as low as 420 μA.

High-power temperature-insensitive gain-offset InGaAs/GaAs vertical-cavity surface-emitting lasers

The authors have grown 997 nm vertical-cavity surface-emitting lasers with an offset between the wavelength of the cavity mode and the quantum well gain peak to improve high temperature operation, and with higher aluminum-content barriers around the active region to improve the carrier confinement. They fabricated lasers of 8-15 and 20- mu m diameters. The 8- mu m-diameter devices exhibited CW operation up to 140 degrees C with little change in threshold current from 15 degrees C to 100 degrees C, and the 20- mu m-diameter devices showed CW output power of 11 mW at 25 degrees C without significant heat sinking.<<ETX>>

Thermal crosstalk in 4 x 4 vertical-cavity surface-emitting laser arrays

We measured thermal crosstalk in 4/spl times/4 VCSEL arrays with a 30-/spl mu/m pitch between devices. The effective thermal resistance of laser diodes in two-dimensional (2-D) arrays is about 50% higher than that of single elements. The output power of the lasers is fairly temperature insensitive under constant voltage operation. From experiments we inferred values for the average thermal conductivity of AlAs-GaAs Bragg reflectors. We found anisotropy in the effective thermal conductivity with numbers of 0.28 W/(cmK) and 0.35 W/(cmK) for the transverse and lateral direction, respectively.