For the application of broad-area laser diodes (BA-LDs) to display or printing, uniform distribution and stability of their near-field pattern (NFP) are the most important demand. In order to control the NFP, we have fabricated BA-LDs with index-guided structure and got better top-hat and stabler NFP than that for gain-guided BA-LDs. However, this mechanism is not understood yet. Therefore, we study the features of BA-LDs' NFP systematically, and set up a new model to understand their behaviors. The NFPs of BA-LDs evolve via three phases with increasing operation current: the first phase is "mode-progressing phase" in which the number of the spatial modes increases orderly, and the second phase is "transition phase" in which the spatial modes become unstable and fluctuate. The third phase has different properties according to the waveguiding structure of BA-LDs: "disordered phase" appears for gain-guided structure in which several specific modes dominate in the whole distribution (filamentation behavior), on the other hand "ordered phase" appears in the case of index-guided structure in which a top-hat distribution is obtained. This top-hat NFP is almost unchanged with increasing output power. With these experimental results, we propose the new model, in which the emitting area of a BA-LD is divided into several parts and they are discussed separately.
We experimentally and theoretically studied degradation phenomena and their mechanism in broad-area semiconductor lasers (BA-LDs) with optical feedback (OFB). We made two types of BA-LDs (one is consisted of AlGaAs emitting at 808nm in TE mode, and another one is consisted of AlGaInP emitting at 642nm in TM mode), and investigated conditions of the degradations caused by an optical feedback. The both types of BA-LDs showed degradations depending on feedback rate and output power. For example, both BA-LDs were damaged with about 20% of intensity feedback rate at half of an output power of a catastrophic optical mirror damage (COMD) levels. To describe a theoretical model for the degradation, the optical power at a front facet of the BA-LDs was calculated and compared with the COMD level of the solitary BA-LDs. In the theoretical model, we included a threshold reduction caused by the OFB. We found that the degradation was explained by a constructive interference between internal and the feedback optical fields. The BA-LDs are damaged when a coherent sum of those fields exceeds the solitary COMD level. We found that the threshold reduction decreases a critical value of the feedback rate corresponding to the damage at low output power regime, and also found that there is an optimum reflectivity of the front facet. The theoretical results show a good agreement with experimental results. According to this model, we can avoid the damages induced by the OFB in the various applications.
High power GaN-based laser diodes (LDs) are very desirable for various applications such as optical storage systems. We have obtained GaN films of low dislocation density using epitaxial lateral overgrowth technique and the raised- pressure metalorganic chemical vapor deposition technique. Dislocation density of the improved GaN is about 107 cm-2. Optimized GaN-based LDs fabricated on the improved GaN films have operated up to 35 mW without any kink. The lifetime is more than 500 hours with a constant power of 20 mW at 25 degree(s)C under continuous wave conditions. Furthermore, we have introduced buried-ridge laser diode structure in order to control the optical transverse mode. The features of the far field patterns of LDs with AlGaN burying layers indicate their controllability.
The longer lifetime is desired for high power AlGaInN based violet lasers. We found that lifetime is strongly dependent on both the initial operating consumption power and the dislocation densities in the laser stripe. Pd/Pt/Au as a metal and AlGaN/GaN superlattice as a p-type cladding layer were incorporated to reduce the operating voltage. The optimization of device parameters as well as the stripe width and the RIE etching device depth led to the lower threshold current of 3.4 kA/cm2. We used the Pendeo epitaxy technique to get lower dislocation density approximately 107 cm-2. The LDs with these technologies showed an output power as high as 35 mW under room temperature CW condition without kink. The lifetime is more than 500 hours under CW operation with a constant power of 20 mW at 25 degree(s)C.