The catastrophic degradation of high power lasers depends on both external factors, associated with the technological processes followed to fabricate the laser, and also on intrinsic aspects related to the materials forming the laser structure, more specifically the active zone composed by the QW, guide layers and claddings. The materials properties: optical, thermal and mechanical, play a paramount role in the degradation of the laser. We analyse here how these properties have an impact on the mechanisms responsible for the catastrophic degradation.
Cathodoluminescence (CL) analysis of high power laser diodes permits to reveal the main defects issued from the catastrophic optical degradation (COD). These defects are revealed as discontinuous dark lines along the ridge. The different levels of damage are analysed, and a thermomechanical model taking account of the thermal and mechanical properties of the laser structure is settled up. In this model the COD is described as a local temperature enhancement, which generates thermal stresses leading to the generation of dislocations, which are responsible for the degradation of the thermal conductivity of the of the active zone of the laser.
The catastrophic optical damage (COD) of laser diodes consists of the sudden drop off of the optical power. COD is
generally associated with a thermal runaway mechanism in which the active zone of the laser is molten in a positive
feedback process. The full sequence of the degradation follows different phases: in the first phase, a weak zone of the laser
is incubated and the temperature is locally increased there; when a critical temperature is reached the thermal runaway
process takes place. Usually, the positive feedback leading to COD is circumscribed to the sequential enhancement of the
optical absorption in a process driven by the increase of the temperature. However, the meaning of the critical temperature
has not been unambiguously established. Herein, we will discuss about the critical temperature, and the physical
mechanisms involved in this process. The influence of the progressive deterioration of the thermal conductivity of the laser
structure as a result of the degradation during the laser operation will be addressed.
Internal degradation of 980 nm emitting single-spatial-mode diode lasers during ultrahigh power operation is investigated for pulsed operation (2 μJ, 20 W). Analysis of the evolution of the emission nearfield with picosecond time resolution enables the observation of the transition from single- to multi-spatial-mode operation at elevated emission powers. Moreover, internal degradation events and subsequent defect propagation processes are in situ monitored by thermal imaging. Subsequently, these devices are opened and defect pattern are inspected by cathodo- and photoluminescence spectroscopy. The results complete earlier findings obtained with broad-area lasers and help to establish models covering defect generation and propagation in edge-emitting devices in general.
There is an intense interest on integration of III-V materials on silicon and silicon-on-insulator for realisation of optical
interconnects, optical networking, imaging and disposable photonics for medical applications. Advances in photonic
materials, structures and technologies are the main ingredients of this pursuit. We investigate nano epitaxial lateral
overgrowth (NELOG) of InP material from the nano openings on a seed layer on the silicon wafer, by hydride vapour
phase epitaxy (HVPE). The grown layers were analysed by cathodoluminescence (CL) in situ a scanning electron
microscope, time-resolved photoluminescence (TR-PL), and atomic force microscope (AFM). The quality of the layers
depends on the growth parameters such as the V/III ratio, growth temperature, and layer thickness. CL measurements
reveal that the dislocation density can be as low as 2 - 3·107 cm-2 for a layer thickness of ~6 μm. For comparison, the
seed layer had a dislocation density of ~1·109 cm-2. Since the dislocation density estimated on theoretical grounds from
TRPL measurements is of the same order of magnitude both for NELOG InP on Si and on InP substrate, the dislocation
generation appears to be process related or coalescence related. Pertinent issues for improving the quality of the grown
InP on silicon are avoiding damage in the openings due to plasma etching, pattern design to facilitate coalescence with
minimum defects and choice of mask material compatible with InP to reduce thermal mismatch.
Polycrystalline SiGe layers have been oxidized in either dry or wet atmospheres in order to form Ge nanoparticles embedded in a dielectric matrix. The evolution of the growing oxides and the SiGe layer during the oxidation processes have been characterized using Raman spectroscopy, X-ray diffraction, Rutherford backscattering and Fourier transform infrared spectroscopy. Ge nanocrystals have been formed in both oxidation atmospheres. Violet luminescence emission (3.1 eV) has been observed and its relation to the oxidation processes has been studied. For dry oxidation, the luminescence intensity appears suddenly when the pure segregated Ge layer starts to be oxidized forming Ge nanocrystals. It remains as long as Ge nanoparticles are present. For wet oxidation, an initial luminescence appears, that depends on the oxide thickness, which is related to the formation of Ge-rich nanoclusters trapped in the SiGeO growing oxide. A sharp increase of the luminescence for long oxidation times is then observed, which is related to the formation of Ge- nanoparticles by the oxidation of the segregated Ge. In both processes the luminescence is quenched for long enough oxidation time. The intensity of the luminescence in the dry oxidized samples, for equal initial thickness of the polycrystalline SiGe layer, is 10 times higher than in the wet oxidized ones. The violet luminescence is neither related to the recombination of excitons inside the Ge nanocrystals nor to defects in the germanium oxide. Ge oxygen deficient centers, located at the interface between the nanoparticles and the dielectric matrix, are proposed as the origin of the violet luminescence.