InP-based Long Wavelength Sources for Solid State Lasers Pumping

摘要

For GaAs-based diode lasers (DL) operating in the 790 - 980 nm range, major power limitations are associated with catastrophic optical degradation (COD) of mirror facets and, measurably, other reliability issues. Reliability concerns do not limit the performance of InP-based diode lasers to such a large extent. The maximum power for these devices is determined by the "initial", near-threshold differential efficiency η_(d-max) and the rate of η_(d-max) reduction with increasing pump current (P-I characteristic rollover). Rollover effect is negligible in GaAs-based lasers since the T_0 and T_1 parameters characterizing the temperature dependencies of the threshold and η_d are much higher than those for InP-based emitters. Rollover power limitation can be reduced by using devices with long cavity length, provided the internal optical losses (α_(int)) are low and η_(d-max) does not decrease considerably with increasing cavity length. Therefore, α_(int) reduction is a key to achieving high-power InP-based diode lasers. The absorption loss analysis in InP lasers shows that the absorption by free holes in the p-InP cladding layer and in Quantum Wells (QW) are the major loss mechanisms. So far, three approaches have been used to reduce absorption losses: (ⅰ) the thickness of undoped waveguide was increased up to 1300 nm to prevent the lasing mode penetration into the p-doped cladding layer (Broad Waveguide design); (ⅱ) stepped acceptor doping profile in p-InP cladding was used in structures with narrow waveguide; (ⅲ) the number of QW was reduced from 3 to 2 in the last version of the narrow wavesuide structures.