We present analytical methods to study the physical mechanisms of the optical output stability of blue InGaN laser
diodes in short and long-term stress experiments. Lasers of three ridge widths were stressed under constant current
condition at a constant current density. The output power of a 1.8μm ridge laser decreased down to 40% within 15h
mainly due to an increase in threshold current. Broader ridges showed a more stable output power. The decline in output
power was related to changes in quantum efficiency and longer carrier lifetimes after stress. A simple recombination
model was fitted to the measurements indicating no increase in non-radiative recombination centers. Instead the longer
carrier lifetimes could be well explained with a decrease in carrier density due to an additional current spreading. These
results were confirmed by changes in the sub-threshold wavelength shift before and after aging.
Mobile laser projection is of great commercial interest. Today, a key parameter in embedded mobile applications is the
optical output power and wall plug efficiency. We report improvements of the performance of true blue single mode
InGaN laser at 450nm with output power of more than 200mW in cw operation for temperatures between 20°C and
60°C. We succeeded in temperature independent high wall plug efficiency of 15-18% for stable output power levels
from 100 to 200mW with estimated life times >4000h in cw operation. Furthermore, we present pioneering studies on
long green InGaN laser diodes. The technological challenge is to achieve InGaN-quantum wells with sufficiently high
material quality for lasing. We investigate the density of non radiative defects by electro-optical measurements confirming
that low defect densities are essential for stimulated emission. Laser operation at 516nm with more than 50mW output
power in cw operation is demonstrated in combination with a high wall plug efficiency of 2.7%.
Proc. SPIE. 6997, Semiconductor Lasers and Laser Dynamics III
KEYWORDS: Superposition, Refractive index, Waveguides, Semiconductor lasers, Near field scanning optical microscopy, Thermal effects, Optical simulations, Modes of laser operation, Temperature metrology, Near field optics
For broad ridge (Al,In)GaN laser diodes, which are inevitable for high output power applications in the UV
and blue spectral range, filaments or higher order lateral modes build p, which influence the far-field beam
quality. We investigate the lateral profile of the optical laser mode in the waveguide experimentally by temporal
and spectral resolved scanning near-field optical microscopy measurements on electrically pulsed driven laser
diodes and compare these results with one-dimensional simulations of the lateral laser mode in the waveguide.
We present a model that describes the optical mode profile as a superposition of different lateral modes in a
refractive index profile which is modified by carrier- and
thermal-induced effects. In this way the mode dynamics on a nanosecond to microsecond time scale can be explained by thermal effects.
We investigate two types of 405 nm (In, Al)GaN test laser structures (TLSs), one of them grown on SiC substrates,
the other grown on low dislocation density freestanding GaN substrates. Measuring the lasing spectra of these
structures, we observe an individual behavior depending on the substrate. TLSs on GaN substrates show a
broad longitudinal mode spectrum above threshold, whereas TLSs on SiC are lasing only on one mode with
various jumps of the laser emission at certain currents. Estimating the gain of each longitudinal mode with the
Hakki-Paoli method, we find minute variations of the gain for TLSs on GaN substrate. In contrary, TLSs on
SiC substrate show much larger fluctuations of the gain for individual longitudinal modes. Using a rate equation
model with nonlinear gain effects, we simulate the longitudinal mode spectrum of both types of TLSs. Once we
modify the gain of each longitudinal mode as observed in the gain measurements, the simulated spectra resemble
the SiC or GaN substrate TLS spectra.
Red, green and blue semiconductor lasers are of great interest for full color laser projection. Mobile applications require
low power consumption and very small laser devices. InGaN lasers are the best choice for the blue color in applications
with output power requirements below 100mW: (1) they have much higher wall plug efficiencies than conventional blue
frequency doubled diode pumped solid state lasers and (2) they are more compact than semiconductor IR lasers with
subsequent second harmonic generation.
We present blue InGaN lasers with high efficiency at a power consumption of several 100mW. Excellent epitaxial
quality permits low internal losses. Threshold current densities and slope efficiencies are further optimized by improving
the facet coating. The laser threshold current is as low as 25mA and the slope efficiency reaches 1W/A. We present a
wall plug efficiency of 15% at output power levels of 60mW.
We combine a scanning near-field microscope (SNOM) with a time-resolved detection scheme to measure the mode dynamics of InGaN laser diodes emitting at 405 nm. Observed phenomena are filaments, mode competition, near-field phase dynamics, near-field to far-field propagation, and substrate modes. In this article we describe in detail the self-built SNOM, specialized for these studies. We also provide our recipe for SNOM tip preparation using tube etching. Then we compare the mode dynamics for a 3 μm narrow and a 10 μm wide ridge waveguide laser diode.