STATISTICAL STUDY OF THE RELIABILITY OF OXIDE-DEFINED STRIPE CW LASERS OF (ALGA)AS

In this report, we describe a statistical study of the reliability of oxide-defined stripe-contact cw injection lasers of (AlGa)As. These devices have one facet coated with Al2O3 and one facet coated with an Al2O3/Si dichroic reflector; the lasers are optimized for cw low-threshold currents at room temperature, with values typically about 50 mA. Lifetests were carried out at 70 °C ambient, in the cw mode of operation with about 5 mW output. Previous lifetests showed that the degradation rate followed a 0.95-eV activation energy so the 70 °C environment provides a degradation acceleration factor of 190 over that at room temperature. We have found that the device failures follow a log-normal distribution, characterized by a mean time before failure of 4200 h and a standard deviation of 1.3. This corresponds to a mean time to failure (MTTF) of 106 h at room temperature. Failure is defined here as the inability of the device to emit 1 mW of stimulated cw output at 70 °C, and assumes that optical feedback will be employed to adjust the laser current during operation. If a constant-current drive is envisioned, the failures for a 3-dB drop in light output also follow a log-normal distribution with a similar slope (standard deviation=1.1) and a MTTF of 2000 h at 70 °C (500 000 h at room temperature). The failures were found to be mainly due to bulk gradual degradation and not facet or contact failure. Careful study of lasers before and after lifetest showed a significant increase in contact thermal resistance. However, this increase accounts for only a small portion of the nearly 70% increase in room-temperature cw threshold after failure at 70 °C. After failure at 70 °C, we also noted a degradation in the near-field and associated far-field pattern of the laser.