Microsecond time scale lateral-mode dynamics in a narrow stripe InGaN laser

Abstract

Time-resolved measurements of the spectrum and the far field of InGaN-based laser diodes show lateral-mode changes and gradual tilting of the far field on a microsecond time scale. Numerical simulations based on a microscopic theory are in good agreement with the measurements. The observed effects are attributed to lateral carrier diffusion in combination with thermal lensing.For violet-blue diode lasers, there exist several important applications, such as optical storage and laser printing, requiring lateral mode stability. While the design of singlemode lasers is an ongoing problem, the underlying physical mechanisms leading to stable fundamental-lateral mode operation in conventional semiconductor lasers is, in principle, well understood. However, this understanding may not apply to the wide-band-gap group-III nitride ͑III-N͒ material system because of its drastically different physical properties. The investigated laser diodes were fabricated by OSRAM Opto Semiconductors. They were grown on a SiC substrate without ELO techniques by metalorganic chemical vapor deposition and consist of a 560 nm thick AlGaN:Si cladding, followed by a 120 nm GaN:Si lower waveguide layer, three 2 nm In 0.1 Ga 0.9 N/GaN quantum wells with GaN:Si barriers, an Al 0.2 Ga 0.8 N electron blocking layer, a 100 nm thick GaN:Mg upper waveguide layer, and a 400 nm thick AlGaN:Mg upper cladding layer. Contacts are deposited on a p-GaN cap layer on top of the ridge and on the n-SiC backside. The cleaved facets are coated with high reflectivity coating (Rϳ98%/70%).2 The devices are operated in junction side up configuration, which is possible due to the SiC substrate. Since we use short pulses in the microsecond range, the back contact of the device does not see a temperature increase during the pulse, as shown in Ref. 3. After each pulse, the laser has enough time to cool down again due to the very low duty cycle. Thus, the kind of mounting the device onto the heatsink has only negligible influence on the effects described in this letter.For measurements of the time-resolved optical spectrum, the light of the temperature stabilized laser diode is collimated and then focused on the entrance slit of a grating monochromator. At the output slit, the light of the actually selected wavelength is collected by a fast photomultiplier tube. Thus, for this particular wavelength the intensity distribution versus time can be observed on an oscilloscope connected to the photomultiplier tube. Scanning the selected wavelength of the monochromator over the range of the emission spectrum of the laser diode yields therefore a threedimensional graphics with time and wavelength as the x and y axis, respectively, and optical intensity as the z axis ͑color encoded͒. This plot, which is shown in Fig. 1 ͑top͒, allows to observe the evolution of the laser spectrum during a current pulse. The corresponding time-resolved far field is measured in the same manner. The only difference to the previous measurement is that the monochromator is now replaced by a step motor c...

Time-resolved scanning near-field microscopy of InGaN laser diode dynamics

“…Exceptions are some studies on relaxation oscillations in InGaN laser diodes, 2 on relative intensity noise (RIN), 1, 3, 4 and on far-field dynamics on a microsecond time scale. 5 The important dynamical effect of filamentation of laser modes has been treated in theoretical simulations of Chow et al. 6,7 They also predict values of the antiguiding factor derived from a microscopic theory.…”

Characterization of Lateral and Longitudinal Mode Competition in Blue InGaN Broad‐Ridge Laser Diodes

“…The maximum optical power is limited by catastrophic optical damage [9,10] occuring at a critical power density at the facet, so broader ridge waveguides are used for power scaling in one emitter. If the ridge width is increased beyond 2 μm, lateral modes of higher order arise, [11][12][13] limiting the beam quality.…”

“…The maximum optical power is limited by catastrophic optical damage [ 9,10 ] occuring at a critical power density at the facet, so broader ridge waveguides are used for power scaling in one emitter. If the ridge width is increased beyond 2 μm, lateral modes of higher order arise, [ 11–13 ] limiting the beam quality. In earlier devices, filamentation dominated the lateral dynamics, which was shown to be thermally driven and influenced by material composition fluctuations.…”

Observation of temperature-independent longitudinal-mode patterns in violet-blue InGaN-based laser diodes

“…Nevertheless, many of the fundamental properties of the nitride material system are not well known and a further understanding is required for future progress in light-emitting diode efficiency or laser reliability [2]. Especially material-and growth-related problems and their influence on device performance are not yet understood very well [3]- [5].…”