KEYWORDS: Semiconductor lasers, Near field, Picosecond phenomena, Streak cameras, Broad area laser diodes, Optical damage, Quantum wells, Near field optics, Optical microscopes, Temperature metrology
One of the failure mechanisms preventing diode lasers in reaching ultra high optical output powers is the catastrophic
optical damage (COD). It is a sudden degradation mechanism which impairs the device functionality completely. COD is
caused by a positive feedback loop of absorbing laser light and increasing temperature at a small portion of the active
material, leading to a thermal runaway on a nanosecond timescale. We analyze commercial gain-guided AlGaAs/GaAs
quantum well broad area diode lasers in single pulse step tests. The near-field emission on the way to and at the COD is
resolved on a picosecond time scale by a streak-camera combined with a microscope. In the final phase of the step tests
the COD is occurring at ~50 times threshold current. The growth of the COD defect site is monitored and defect
propagation velocities between 30 and 190 μm/μs are determined. The final shape of the damage is verified by opening
the device and taking a micro-photoluminescence map of the active layer.
Many applications such as pumping of solid state lasers or ignition of explosives require high optical output powers
during a short period. Pulsed operated diode lasers meet these requirements. They can be driven at elevated power
levels, well above the ones specified for continuous wave (cw) operation. The optical near-field intensity of a diode laser
in this operation regime is a key parameter since it determines the beam properties of the device. High power
AlGaAs/GaAs quantum well broad area diode lasers are subjected to single pulse step tests carried out up to and beyond
their ultimate limits of operation. Laser near-fields are monitored on a picosecond time scale using a streak-camera setup
during pulse currents of up to ~50 times the threshold current. A transition from gain guiding to thermally-induced index
guiding of the near-field is shown. A further power increase is prevented by catastrophic optical damage (COD). This
sudden failure mechanism is studied in conjunction with filamentary properties of the near-field. The defect growth
dynamics resolved on the picosecond time scale is used to gather inside into the physics behind COD.
To analyze the problem of modal filamentation and beam-instabilities in wide-aperture semiconductor lasers, we have developed a sophisticated opto-electro-thermal model based on Maxwell-Bloch formalism, to describe
frequency-, carrier- and temperature-dependent gain and dispersion. Effects of both homogeneous and inhomogeneous gain broadening are analyzed. It is shown, via linear stability analysis and high resolution space-time adaptive FEM simulations that inhomogeneous gain broadening in quantum dot (QD) lasers enhances spatial coherence and leads to suppressed filamentation and stable far-fields in both thermal and non-thermal regimes even when the phase-amplitude coupling is comparable to that in quantum well (QW) gain medium.
For novel devices such as quantum dot lasers, the usual thermal characterization using temperature induced wavelength
shift is ineffective due to weak thermal shift of the inhomogeneously broadened gain-peak. This calls for new thermal
characterization techniques for such devices. To this end we have analyzed bulk thermal properties of broad area
quantum dot lasers theoretically, and have experimentally verified these calculations using the novel technique of microthermography.
InGaAs/GaAs 950 nm emitting, 50 μm wide and 1.5 mm long, large optical cavity quantum dot lasers
were used for the study. Our two-dimensional steady-state model self-consistently includes current spreading and
distributed heat sources in the device and using finite element method reproduces high resolution temperature maps in
the transverse cross section of the diode laser. A HgCdTe based thermocamera with detection spectral range 3.5-6.0 μm
was employed for micro-thermography measurements. Its microscope with 6x magnification has a nominal spatial
resolution of 4 μm/pixel for full frame images of 384×288 pixels. A ray tracing technique was used to model the
propagation of thermal radiation inside the transparent laser die which in turn links calculated and experimentally
derived temperature distributions. Excellent agreement was achieved which verifies the model-calculation and the
thermal radiation propagation scheme inherent in the experimental approach. This result provides a novel means for
determining reliable bulk temperature data from quantum dot lasers.
Spatial mode dynamics in continuous-wave high-power wide-aperture quantum dot lasers is studied within the
framework of Maxwell-Bloch equations. The effects of phase-amplitude coupling, junction temperature profile, carrier
diffusion - capture - escape phenomena on the lateral modal dynamics is included in the model. It is shown that for low
values of alpha factor and at normal operating points, there is an improvement in spatial coherence of the emitted light.
It is found that active layer temperature and carrier-diffusion plays a key role in deciding the spatial mode structure in
the device.
KEYWORDS: Broad area laser diodes, Refractive index, Modeling and simulation, Temperature metrology, Physics, Optoelectronic devices, Continuous wave operation, High power lasers, Profiling, Optical simulations
We propose and demonstrate theoretically a method of achieving single lateral mode field patterns in CW broad area laser diodes at high power. The method relies on laterally profiling the thermal conductivity of the bonding solder layer and thereby thermally controlling the lateral index. Various configurations are simulated using a split step FFT beam propagation method including thermal, carrier (alpha-parameter) and optical (Kerr effect) index variations. Thermal effects are described using a 2-D finite element electro-thermal model including current spreading.
In this work we propose a new approach to model video data. To interpret the video semantic, we propose to model the video on the basis of the underlying dynamics contained in the video. Thus, the video is seen as a measurement of properties of objects embedded in the video and of their behaviors over time. The objects' behaviors are described by states and state transitions using statechart diagram. Then, this diagram is used to partition the video into meaningful segments. For efficient retrieval of information, we propose to use indexes based on the states of objects. The proposed model thus helps to store information about similar types of video data in a single database schema and supports content-based querying from a repository of video data.
Image representation schemes are of importance in fields of computer vision, graphics, image processing, CAD/CAM, etc. Various representation schemes are presented in the literature for both 2D and 3D. In this paper, we present a scheme of representation using the concept of octagonal distances. They are called Medial circle representation (MCR) and Medial sphere representation (MSR) in 2-D and 3-D, respectively. Storage requirements, computational complexity, merits and demerits of the representation schemes are discussed.
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