E.C. Larkins

Control of differential gain, nonlinear gain and damping factor for high-speed application of GaAs-based MQW lasers

Utilizing small-signal direct modulation and relative intensity noise measurements, the authors investigate changes in the modulation response, the differential gain delta g/ delta n, the nonlinear gain coefficient in , and the damping factor K, which result from three structural modifications to GaAs-based multiple quantum well lasers: the addition of strain in the quantum wells; and increase in the number of quantum wells; and the addition of p-doping in the quantum wells. These modifications are assessed in terms of their potential for reducing the drive current required to achieve a given modulation bandwidth, for increasing the maximum intrinsic modulation bandwidth of the laser, and for improving the prospects for monolithic layer/transistor integration. It has been possible to simultaneously increase delta g/ delta n and decrease K, yielding very efficient high-speed modulation (20 GHz at a DC bias current of 50 mA) and the first semiconductor lasers to achieve a direct modulation bandwidth of 30 GHz under DC bias. >

Improved refractive index formulas for the AlxGa1−xN and InyGa1−yN alloys

A detailed understanding of the nitride refractive indices is essential for the modeling and design of III–N laser structures. In this article, we report on the assessment of the refractive index data available for the nitride alloys and present formulas for evaluating the refractive indices for variations in both composition and photon energy. For AlxGa1−xN, an expression is given which fits well to experimental data below x<0.38, sufficient for the molefractions found in the cladding layers of III–N lasers. Due to the almost complete lack of experimental refractive index data for InyGa1−yN, we propose an expression to give a first-order approximation for the refractive index.

Damping-limited modulation bandwidths up to 40 GHz in undoped short-cavity In/sub 0.35/Ga/sub 0.65/As-GaAs multiple-quantum-well lasers

We demonstrate record direct modulation bandwidths from MBE-grown In/sub 0.35/Ga/sub 0.65/As-GaAs multiple-quantum-well lasers with undoped active regions and with the upper and lower cladding layers grown at different growth temperatures. Short-cavity ridge waveguide lasers achieve CW direct modulation bandwidths up to 40 GHz for 6/spl times/130 /spl mu/m/sup 2/ devices at a bias current of 155 mA, which is the damping limit for this structure. We further demonstrate large-signal digital modulation up to 20 Gb/s (limited by the measurement setup) and linewidth enhancement factors of 1.4 at the lasing wavelength at threshold of /spl sim/1.1 /spl mu/m for these devices.

Nonlinear properties of tapered laser cavities

The nonlinear phenomena accompanying the process of light generation in high-power tapered semiconductor lasers are studied using a combination of simulation and experiment. Optical pumping, electrical overpumping, filamentation, and spatial hole burning are shown to be the key nonlinear phenomena influencing the operation of tapered lasers at high output powers. In the particular tapered laser studied, the optical pumping effect is found to have the largest impact on the output beam quality. The simulation model used in this study employs the wide-angle finite-difference beam propagation method for the analysis of the optical propagation within the cavity. Quasi-three-dimensional (3-D) thermal and electrical models are used for the calculation of the 3-D distributions of the temperature, electrons, holes, and electrical potential. The simulation results reproduce key features and the experimental trends.

Process parameter dependence of impurity-free interdiffusion in GaAs/Al x Ga 1–x As and In y Ga 1–y As/GaAs multiple quantum wells

The dependence of the impurity-free interdiffusion process on the properties of the dielectric cap layer has been studied, for both unstrained GaAs/AlxGa1−xAs and pseudomorphic Iny Ga1−yAs/GaAs MQW structures grown by molecular beam epitaxy. The influence of the cap layer thickness, composition, and deposition technique on the degree of interdiffusion were all systematically investigated. Electron-beam evaporated SiO2 films of varying thickness, chemical-vapor-deposited SiOxNy films of varying composition, and spin-on SiO2 films were used as cap layers during rapid thermal annealing (850-950°C). Photoluminescence at 10K has been employed to determine the interdiffusioninduced bandgap shifts and to calculate the corresponding Al-Ga and In-Ga interdiffusion coefficients. The latter were found to increase with the cap layer thickness (e-beam SiO2) up to a limit determined by saturation of the outdiffused Ga concentration in the SiO2 caps. A maximum concentration of [Ga] = 4–7 ×1019 cm−3 in the SiO2 caps was determined using secondary ion mass spectroscopy profiling. Larger band-edge shifts are also obtained when the oxygen content of SiOxNy cap layers is increased, although the differences are not sufficiently large for a laterally selective interdiffusion process based on variations in cap layer composition alone. Much larger differences are obtained by using different deposition techniques for the cap layers, indicating that the porosity of the cap layer is a much more important parameter than the film composition for the realization of a laterally selective interdiffusion process. For the calculated In0.2Ga0.8As/GaAs interdiffusion coefficients, activation energies EA and prefactors Do were estimated to ranging from 3.04 to 4.74 eV and 5 × Kh−3 to 2 × 105 cm2/s, respectively, dependent on the cap layer deposition technique and the depth of the MQW from the sample surface.

Quasi-3-D simulation of high-brightness tapered lasers

We present a simulation tool useful to optimize the design of semiconductor tapered lasers and to study the physical processes inside of them. This is achieved by using a state-of-the-art quasi-three-dimensional (quasi-3-D) electrical and thermal model, coupled to a two-dimensional (2-D) wide-angle beam propagation method optical model. A calibration procedure of model parameters is proposed to contribute to the development of reliable simulation tools. Different laser diodes with a tapered gain section, emitting at 735 and 975 nm, are used to validate the model through the extensive comparison of experimental and simulated results. The suitability of 2-D and 3-D electrical, thermal, and optical models is discussed in terms accuracy and computational effort.

Design of wide-emitter single-mode laser diodes

An accurate laser simulator is developed to investigate the influences of various device parameters on modal discrimination and beam quality to optimize the brightness of semiconductor lasers. Previous work using the Prony method is extended, both by using a more robust laser simulator to solve for the optical fields and by extending the Prony method to operating conditions above threshold. The semiconductor laser model is used to investigate the effects of various device parameters on the characteristics of a gain-guided 980-nm Al-free stripe geometry laser. The device parameters investigated are the device geometry, the thickness and doping level of the waveguiding layer and the front facet reflectivity. Together with practical and economic considerations, the device parameters of a wide emitter gain-guided laser are optimized. The optimized gain-guided laser is predicted to operate in the fundamental mode up to 1.15 W. At this maximum fundamental mode power, the slow axis beam divergence at 1/e/sup 2/ points is 5.4/spl deg/, the beam quality factor M/sup 2/ is 1.62, the astigmatism is 27 /spl mu/m, and the brightness is 61 MW/cm/sup 2/sr.

Design and Simulation of Next-Generation High-Power, High-Brightness Laser Diodes

High-brightness laser diode technology is progressing rapidly in response to competitive and evolving markets. The large volume resonators required for high-power, high-brightness operation makes their beam parameters and brightness sensitive to thermal- and carrier-induced lensing and also to multimode operation. Power and beam quality are no longer the only concerns for the design of high-brightness lasers. The increased demand for these technologies is accompanied by new performance requirements, including a wider range of wavelengths, direct electrical modulation, spectral purity and stability, and phase-locking techniques for coherent beam combining. This paper explores some of the next-generation technologies being pursued, while illustrating the growing importance of simulation and design tools. The paper begins by investigating the brightness limitations of broad-area laser diodes, including the use of asymmetric feedback to improve the modal discrimination. Next, tapered lasers are considered, with an emphasis on emerging device technologies for applications requiring electrical modulation and high spectral brightness.

Narrow-line coherently combined tapered laser diodes in a Talbot external cavity with a volume Bragg grating

We present the phase locking of an array of index-guided tapered laser diodes. An external cavity based on the self-imaging Talbot effect has been built. A volume Bragg grating is used as the output coupler to stabilize and narrow the spectrum at 976nm. A power of 1.7W is achieved in the in-phase single main lobe mode with a high visibility. We have checked that each emitter is locked to the Bragg wavelength with a 100pm spectrum linewidth. The experimental results compare well with numerical simulations performed with two-dimensional wide-angle finite difference beam propagation method.

Low-bias-current direct modulation up to 33 GHz in InGaAs/GaAs/AlGaAs pseudomorphic MQW ridge-waveguide lasers

Modulation bandwidths of 24 GHz (I/sub bias/=25 mA) and 33 GHz (I/sub bias/=65 mA) are demonstrated for 3/spl times/100 /spl mu/m/sup 2/ In/sub 0.35/Ga/sub 0.65/As/GaAs multiple quantum well ridge-waveguide lasers with undoped and p-doped active regions, respectively. These performance enhancements have been achieved both by lowering the growth temperature of the high-Al-mole-fraction cladding layers and by utilizing short-cavity devices, fabricated with dry-etched facets using chemically-assisted ion-beam etching. Both the undoped and p-doped lasers also demonstrate modulation current efficiency factors exceeding 5 GHz/mA/sup 1/2/, the best reported results for any semiconductor laser.<<ETX>>