T.E. Sale

Reflectance and photomodulated reflectance studies of cavity mode and excitonic transitions in an InGaAs/GaAs/AlAs/AlGaAs VCSEL structure

Angle dependent photomodulated reflectance (PR) and conventional reflectance (R) studies have been performed on an InGaAs/GaAs/AlAs/AlGaAs vertical-cavity surface-emitting laser (VCSEL) structure at room temperature. The PR spectra show three prominent signals: one from the ground-state excitonic transition in the InGaAs quantum well (QW) embedded deep in the structure; Franz-Keldysh oscillations (FKO) from the GaAs layers; and a PR signal attributed to the cavity mode, whereas in the R spectra only the cavity mode is clearly visible. It is found that the ground-state QW transition is not in resonance with the cavity mode. The period of the FKOs shows that the built-in electric field is about 18 kV/cm. The energy positions of the excitonic features are not dependent on the angle of incidence, in contrast to that of the cavity mode, whose angular dependence can be fitted with a very simple model. The full R spectrum of the VCSEL structure has also been calculated using a Jones matrix formalism which not only predicts the actual angular dependence of the cavity mode, but also permits a determination of the layer thicknesses in the VCSEL structure by fitting to the experimental R spectrum. Excellent fits are achieved using only two adjustable parameters : the Ga and Al growth rates, which are found to be 5.3% less, and 2.9% greater, than nominal, respectively. Modelling of the measured PR spectrum of an etched sample shows that the In composition of the QW is 5% higher than the nominal value.

Development of a red VCSEL-to-plastic fiber module for use in parallel optical data links

This paper presents results that have emerged from the European funded ESPRIT Project, Bright Red Surface Emitting Lasers (BREDSELS-23455). The projects main objective has been to develop arrays of Vertical Cavity Surface Emitting Lasers (VCSELs) emitting in the region of 650 nm. These VCSEL arrays, suitably coupled to plastic fiber ribbon, are potentially ideal sources for high-speed plastic optical fiber networks. Linear 1 X 8 VCSEL arrays have been fabricated from wafers grown in multi-wafer MOVPE reactors. Individual VCSELs are shown to generate a peak room temperature power of 2 mW at 674 nm and are capable of operating continuous wave to a temperature of 60 degrees Celsius. The use of selective oxidation in the fabrication process is found to be essential in terms of providing effective heat sinking to the active region, while free carrier absorption is found to be a significant loss mechanism. A detailed description of the device results including modal behavior is presented along with the initial results from the plastic fiber ribbon module.

Photomodulated reflectance as a valuable nondestructive process tool for VCSELs

Optical reflectivity spectra are useful in assessing the structure of vertical-cavity surface-emitting lasers (VCSELs) but show little of the nature of the active quantum well (QW). Here we use photomodulated reflectance to identify a region of an epitaxial wafer with optimal cavity-QW alignment. AlInGaP-AlGaAs visible VCSEL devices fabricated from this region lased well, compared with devices from a nearby control piece of the wafer which failed to lase at all.

Using strained (AlxGa1−x)yIn1−yAszP1−z system materials to improve the performance of 850 nm surface- and edge-emitting lasers

We compute the optical gain of various quantum wells for use in 850 nm lasers. In particular, we investigate compressively strained wells of AlGaInAs, InGaAs, or InGaAsP without any assumptions of the material quality. Reductions of up to 43% in the radiative current and 24% in the sheet carrier density can be expected, compared with GaAs/AlGaAs wells. For the case of vertical-cavity surface-emitting lasers and distributed feedback lasers, compressive strain is particularly attractive in reducing the temperature sensitivity and fabrication tolerances for low threshold and efficient operation.

Evaluating the continuous-wave performance of AlGaInP-based red (667 nm) vertical-cavity surface-emitting lasers using low-temperature and high-pressure techniques

By measuring visible AlGaAs/AlGaInP vertical-cavity surface-emitting lasers (VCSELs) and similar edge-emitting lasers (EELs) as a function of temperature and pressure, we study the contributions of electron leakage and gain-cavity detuning on the threshold current, Ith, of VCSELs. In the EELs, leakage accounts for ∼20%Ith at room temperature, rising to ∼70%Ith at 80 °C. Similarly, leakage accounts for almost all of the increase in the VCSEL Ith above −100 °C and limits the cw output power. At low temperature or high pressure, however, the VCSEL Ith increases sharply due to misalignment of the gain peak to the high-energy side of the cavity mode. Under normal operating conditions, carrier leakage has the greatest effect on the VCSELs with gain-cavity detuning only becoming important at very low temperatures.

Temperature effects in VCSELs

Vertical cavity surface emitting lasers (VCSELs) have been the subject of intense research in recent years. The compact nature of the devices means that heat generated within is not as readily dissipated as with more conventional stripe geometry lasers. Advances in the design of distributed Bragg reflector (DBR) cavity mirrors and intracavity contact schemes have reduced the threshold voltage from greater than 10 V to little more than the lasing photon potential, in some cases. However, thermal management is still a limiting factor for high power or high efficiency output from VCSELs By analyzing a variety of devices we have devised a simple but powerful model to explain the current-light response of VCSELs which is strongly dependant on the temperature rise in the active layer. Effects of the relative position of the cavity resonance and gain spectrum are also discussed.

Investigation of 1.3-/spl mu/m GaInNAs vertical-cavity surface-emitting lasers (VCSELs) using temperature, high-pressure, and modeling techniques

We have investigated the temperature and pressure dependence of the threshold current (I/sub th/) of 1.3 /spl mu/m emitting GaInNAs vertical-cavity surface-emitting lasers (VCSELs) and the equivalent edge-emitting laser (EEL) devices employing the same active region. Our measurements show that the VCSEL devices have the peak of the gain spectrum on the high-energy side of the cavity mode energy and hence operate over a wide temperature range. They show particularly promising I/sub th/ temperature insensitivity in the 250-350 K range. We have then used a theoretical model based on a 10-band k.P Hamiltonian and experimentally determined recombination coefficients from EELs to calculate the pressure and temperature dependency of I/sub th/. The results show good agreement between the model and the experimental data, supporting both the validity of the model and the recombination rate parameters. We also show that for both device types, the super-exponential temperature dependency of I/sub th/ at 350 K and above is due largely to Auger recombination.

-180 to +80/spl deg/C CW lasing in visible VCSELs

Electron leakage into the p-DBR is the major process limiting the performance of visible VCSELs. It can be reduced by improving electron confinement, or made less important by reducing the mirror reflectivity. By doing this we would then expect to measure even higher output powers at room temperature, comparable to our low temperature results.

Reflectance and photomodulated reflectance studies of an InGaAs/GaAs/AlGaAs vertical-cavity surface-emitting laser structure under hydrostatic pressure

An InGaAs/GaAs/AlGaAs vertical-cavity surface-emitting laser structure has been studied by conventional reflectance (R) and photomodulated reflectance (PR) spectroscopies at room temperature and at different hydrostatic pressures up to 1.24 GPa. A clear cavity-mode signal is observed in both PR and R spectra, whereas the quantum-well ground-state exciton signal is observed only in the PR spectra. The PR spectra also show a strong resonance of the two signals at the highest pressure. The pressure dependences of the energies of the exciton and the cavity mode were determined to be 92 meV/GPa and 15 meV/GPa, respectively.

Ageing studies on red-emitting VCSELs for polymer optical fibre applications

The paper reports on the development of VCSELs emitting near to 650 nm for low cost links employing PMMA based polymer optical fibre (POF). AlGaInP/AlGaAs VCSELs are fabricated on wafers from two different MOCVD sources. Accelerated ageing tests are done for the two types of devices.