Space missions are probably the most demanding environment for laser diodes. A comprehensive study on the reliability
of commercially available laser diodes arrays (LDA), with the objective of bar stacks for ESA's BepiColombo Laser
Altimeter mission to the planet Mercury was performed. We report the best results of lifetime tests performed on SCD
808 nm QCW stacks at different levels of current load in a unique combination with operational temperature cycles in
the range of -10°C to 60 °C. Based on a field-proven design that includes Al-free wafer material and a robust packaging
solution, these arrays exhibit long operational lifetime of up to 20 billion pulses monitored in the course of several years.
Zero failures and stable performance of these QCW arrays were demonstrated in severe environmental conditions
reflecting both, military and space applications. In order to achieve maximum device efficiency at different operational
conditions of the base temperature and current, an optimum combination of the wafer structure and bar design is
required. We demonstrate different types of QCW stacks delivering peak power of up to 1 kW with a usable range of
50-55% wall plug efficiency at base temperatures up to 60 °C.
High power diode laser bars require packages with a high cooling efficiency and long-term stability. Due to the increasing output power of the diode laser bars the thermal resistance of the packaging becomes even more important. It is the key information about the cooling efficiency of a package and in particular of the heat sink. Besides the heat sink the thermal resistance depends also on the solder interface, packaging process, and bar structure such as fill factor and resonator length.
This work presents a thermal comparison of different packaging types and laser bar designs. Different package types are experimentally measured and analyzed by numerical calculations to obtain information about the influence of the different parameters: Conductively cooled and water cooled copper heat sinks as well as a new type of expansion matched micro-channel heat sink made out of Cu-AlN sandwich are investigated. In addition to the different packages, laser bars with different resonator lengths are mounted and analyzed regarding their thermal behavior; the dependency of the thermal resistance on the resonator length is a particular interest of the investigation. In parallel to the experiments thermal simulations of the same packages and laser bar geometries are performed. The boundary conditions chosen in the simulations are comparable to the experimental values and the same parameters are varied.
The relations between theoretical and experimental results are presented. The analysis shows the influencing factors, so that the optimum package can be chosen for a specific application.
We present a detailed study of oxide-confined, vertical-cavity surface-emitting lasers (VCSELs), where the reflectivity of the top mirror has been patterned by means of a metal grid, which at the same time acts also as an electrode. Owing to their features, these kind of devices are commonly referred to as phase-coupled VCSEL arrays. The anlaysis is based on a joint experimental and theoretical effort: the former is devoted to a complete characterization of the emission properites, while the latter is based on a comprehensive fully vectorial model for the structure eigenmodes with the details of their complex structure. The detected characteristics make them quite attractive for various applications and the comparison of their modal properties with the model is proven to be essential for a deep understanding of these lasers. In particular we explain for the first time, a characteristic behavior of the lasing array, which displays spatially inhomogeneous polarization characteristics with symmetry properties with respect to the array diagonals. The good matching between theory and experiment opens new perspectives for optimized devices.
We present an accurate experimental characterization of the dynamical properties of polarization switching (PS) in single transverse and longitudinal mode vertical-cavity surface-emitting lasers (VCSELs). When a VCSEL is driven with a constant current at its polarization switching point, it makes random jumps between its two linear polarization states. This phenomenon is called mode-hopping. The permanence times in the two polarization states show an exponentially decreasing distribution, according to Arrhenius? law. The average permanence time varies over several orders of magnitude depending on the relative difference between threshold and switching current. We have performed a statistical experimental characterization of the residence times of mode hopping VCSELs for both proton implanted and oxide confined samples, and find our results to be in excellent agreement with the theoretical predictions from a novel intensity rate equation model.
Coupled arrays of vertical-cavity surface-emitting lasers were realized by patterning the reflectivity of the top-distributed Bragg reflector using a phase-matching layer and a metal grid. For improved current injection and better heat dissipation the devices were selectively oxidized. Continuous-wave room-temperature operation of these anays has been achieved at 960 urn. Polarization resolved measurements revealed a stable behavior without any polarization flips in the fundamental lasingmode regime. Spectrally resolved measurements of the near- and far-field showed an abrupt transition from the fundamental super-mode with a TEM00-shaped near-field in every individual pixel with a beam divergence of 5.5°in both arraydirections to a TEM01-like super-mode with 5.5° beam divergence in one direction and 1 1° in the other direction. Measurements with high angular resolution showed full-width at half-maximum of the far-field intensity lobes as small as 0.89° for a lOxlO array.
In this contribution, we bring forward and compare the polarization switching (PS) dynamics and the polarization modulation characteristics of gain- and index-guided VCSELs. We then discuss the steady-state and dynamic characteristics of both types of VCSELs. Finally we focus on the polarization modulation limit and the average mode hopping frequency, which both scale over 8 orders of magnitude when the switching current is varied from just above threshold up to 2 times the threshold current.
The use of VCSELs in spectrally-demanding applications is considered. Vertical cavity lasers with emission in the (lambda) equals 763 nm, 780 nm and 852 nm wavelength ranges have been developed and optimized for spectrally-pure emission. Measurements of output spectra, linewidth, noise, polarization and spectral aging properties are considered in detail. Typical linewidth values below 5 MHz are seen, which makes these lasers very attractive for a variety of spectroscopic applications. Two of these optically-demanding applications are considered, namely oxygen sensing and pump sources for atomic clocks; performance results for the former are presented.