S. Bull

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.

By-emitter degradation analysis of high-power laser bars

The study of degradation process in high-power laser diodes, in particular, high-power laser bars, has become increasingly important as the output power of these devices continues to rise. We present a “by-emitter” degradation analysis technique, which examines degradation processes at both the bar and emitter levels. This technique focuses on understanding the dynamic mechanisms by which packaging-induced strain and operating conditions lead to the formation of defects and subsequent emitter and bar degradations. In the example presented, we examine a highly compressively strained bar, where thermally induced current runaway is found to be an important factor in the bar degradation and eventual device failure.

The impact of temperature and strain-induced band gap variations on current competition and emitter power in laser bars

We report on current competition and emitter power distributions of unaged 650 nm red-emitting and 980 nm infrared tapered high-power laser bars. We observe a correlation between temperature and packaging-induced strain with respect to the emitter output power. A frown-shaped profile of the output power distribution is seen when the temperatures of the emitters near the center of the bar increase compared to those at the edges of the bar. The effect of temperature is found to dominate that of strain and therefore determine the output power distribution across the bar.

Micro-electroluminescence spectroscopy investigation of mounting-induced strain and defects on high power GaAs/AlGaAs laser diodes

The degradation behaviour of AlGaAs high power laser bars has been investigated using micro-electroluminescence (μ-EL) spectroscopy, electroluminescence microscopy (ELM) and micro-photocurrent (μ-PC) spectroscopy. The emitters in each bar have been individually studied in detail. A significant blue shift of the EL spectra was observed for certain emitters. Inspection of these blue-shifted emitters with ELM also showed the presence of extended defects. The EL spectral shifts correlate with the blue shift of the quantum well (QW) absorption edge observed by photocurrent (PC) spectroscopy and is caused by a mounting induced strain. This correlation suggests that μ-EL spectroscopy is a reliable and sensitive technology for the recognition of defective emitters and thus for the reliability assessment of high power laser bars.

The impact of hot-phonons on the performance of 1.3µm dilute nitride edge-emitting quantum well lasers

A robust opto-electronic device simulation tool is extended to model the phonon bottleneck in edge-emitting 1.3µm InGaAsN double quantum well (QW) laser diodes. Both the steady state operation and the transient response of the phonon bottleneck are examined as a function of injection current and heatsink temperature. It is found that the hot phonon population can raise the electron and hole temperatures in the QW active region by up to 7K above the equilibrium lattice temperature at moderate injection currents. At high injection currents, it is found that the phonon bottleneck can significantly decrease the optical power.

A spectroscopically resolved photo- and electroluminescence microscopy technique for the study of high-power and high-brightness laser diodes

A novel and advanced characterization technique is described for performing optical studies of the luminescence properties of materials. It was developed for the investigation of semiconductor materials, including semiconductor laser diodes and photonic integrated circuits. A quantitatively calibrated, spatially and spectrally resolved imaging technique is described, which is based upon the technologies of photoluminescence microscopy and photoluminescence spectroscopy. The principles of the spectroscopically resolved photo- and electroluminescence microscopy techniques are outlined in this paper, and the developed instrument is described in detail. Design calculations used to select and set up the experimental apparatus are presented, and results are found to compare well with this analysis. Various experimental measurements are used to demonstrate the performance of the new instrument. The study of strain and defects in high-power laser diodes is presented as one of the more challenging applications of the new technique. The results presented demonstrate the ability of this technique to image photoluminescence shifts occurring in the substrate of packaged laser bars, enabling the investigation of strain, defects and their evolution with aging. Other applications of the technique include the spectroscopic measurement of near- and far-field patterns and virtual sources of laser diodes, investigations of spectral hole burning and optical scattering processes in lasers and photonic integrated circuits, and studies of organic LEDs. In the future, applications are also envisaged in medicine and the biological sciences.

Separate phase-locking and coherent combining of two laser diodes in a Michelson cavity

We describe a new coherent beam combining architecture based on passive phase-locking of two laser diodes in a Michelson external cavity on their rear facet, and their coherent combination on the front facet. As a proof-of-principle, two ridge lasers have been coherently combined with >90 % efficiency. The phase-locking range, and the resistance of the external cavity to perturbations have been thoroughly investigated. The combined power has been stabilized over more than 15 min with an optical feedback as well as with an automatic adjustment of the driving currents. Furthermore, two high-brightness high-power tapered laser diodes have been coherently combined in a similar arrangement; the combining efficiency is 70% and results in an output power of 4 W. We believe that this new configuration combines the simplicity of passive self-organizing architectures with the optical efficiency of master-oscillator power-amplifier ones.

Design Optimisation of High-Brightness Laser Diodes for External Cavity Operation in the BRIDLE Project

We report on the design aspects of high performance diode lasers for application in high-brightness spectral beam combining and coherent beam combining modules. Key performance trade-offs are identified and potential solutions are explored.

Emulation of the operation and degradation of high-power laser bars using simulation tools

We report on the simulation of high-power laser bars and the emulation of their ageing behaviour, using simulation tools originally developed for single emitter laser diodes. Simulations of a simplified bar indicated that the hotter emitters located at the centre of the bar emit more power than their cooler neighbours, while degradation emulations showed that the centre emitters degrade faster. Although the degraded emitters produced less light, they drew an increasingly large fraction of the total current. These insights into the operation and degradation of the bar motivated new experimental investigations, which confirmed the predicted behaviour.

Fourier transform analysis method for modeling the positions and properties of cavity defects in Fabry–Pérot laser diodes

A simple and fast model is presented, which allows the determination of defect positions within a high-power laser emitter cavity and an estimation of their transmission properties. The model is based upon the calculation of the cavity transmission spectrum below threshold and the analysis of its fast Fourier transform. Modeled and experimental results are compared, showing good correlation. The speed and simplicity of the model means that it is applicable as a screening process for the detection and characterization of defects in manufactured lasers.