True blue lasers with wavelengths of ~450 nm are of great interest for full color laser projection. These kind of
applications usually require high output power and, in particular, an excellent wall plug efficiency within a wide
temperature range. In this paper we therefore present experimental and theoretical investigations of the temperature
behavior of 60mW InGaN lasers in a range of -10 °C to 100 °C.
The laser parameters threshold current density, slope efficiency and operating voltage describe the wall plug efficiency
of the device. The slope efficiency does not show any significant temperature dependence which is due to an almost
temperature independent injection efficiency in the temperature range that is of interest for most commercial
applications. In contrast, the laser threshold current density increases with temperature and we determine a characteristic
temperature T0 of about 141K for our devices emitting at 445nm. This increasing threshold current density can be
explained by lower gain of the quantum wells at higher temperature. Furthermore, Auger recombination influences the
threshold as verified by simulations. The second electro-optical parameter is the electrical voltage, which is dominated
by electrical barriers. The voltage decreases with increasing temperature and compensates the increasing threshold
current resulting in a nearly constant high wall plug efficiency of 13% between -10°C and 100°C.
Proc. SPIE. 7230, Novel In-Plane Semiconductor Lasers VIII
KEYWORDS: Superposition, Refractive index, Gaussian beams, Waveguides, Laser beam propagation, Semiconductor lasers, Near field scanning optical microscopy, Near field, Modes of laser operation, Near field optics
For broad ridge (Al,In)GaN laser diodes, which are inevitable for high output power applications in the near-UV
to blue spectral region, filaments appear, which influence the far-field beam quality. We present an extensive
study of the optical mode profile of conventional c-plane LD test structures with ridge widths from 1.5 to 10
micrometers. The broad ridge samples are optimized to reach several hundred milliwatt of cw output power.
Spectral and spatial resolved near- and far-field measurements show, that the characteristic lateral multi-lobed
far-field pattern can be interpreted as superposition of interfering phase-locked filaments in the ridge waveguide.
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.
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.
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.