Edge-emitting laser diodes emit very high optical powers from small emission areas, therefore, the optical intensity at the front facet is extremely high. A lot of heat is created in the front facet, where in addition to normal operation, a lot of photons are absorbed by different defects including dangling bonds, threading dislocations, amorphous native oxides, or in a short high level of surface states defects.
EFFECTIVE PASSIVATION OF LASER FACETS IS THE KEY TO OBTAINING THE HIGHEST POWER OUTPUT OF EDGE-EMITTING LASERS.
This absorption leads to heating of the semiconductor which shrinks the LD bandgap, which results in an ever increased absorption and a further shrinkage of the bandgap. Such a thermal runaway process ultimately melts the semiconductor in what is known as Catastrophic Optical Damage, or COD. Increasing the power threshold at which the COD happens is an important target for almost all laser manufacturers.
Existing methods include cleaving of the laser bars in UHV conditions followed by passivating protection, i.e. ZnSe layer, prior to the deposition of the mirror layers. One of the disadvantages of this approach is the difficult and very costly implementation of the cleaving step performed in the UHV.
At Comptek, we first treat laser bars (that are generally cleaved under air exposure) with an in-situ cleaning process optimized for commonly used materials, such as AlGaAs or InGaAsP. Once a good clean facet is obtained, the passivation step happens in a very controlled atmosphere before being transferred to the mirror coatings step. This way, we achieve results similar to existing methods at significantly reduced costs.