1. Introduction
From 1991 to 1994, I engaged in the research and development of AlGaInP visible laser diodes at Fujitsu Laboratories Ltd. The laser diodes were put into practical use and installed in the magneto-optical disk drives sold by Fujitsu Ltd. [1,2]. Here, I introduce our research for improving the reliability of the AlGaInP laser diodes.

2. Proposal of a method to evaluate degradation of the laser diodes [3]
The degradation of AlGaInP laser diodes is mainly caused by degradation of the semiconductor crystal (bulk degradation) and end mirror facets (facet degradation). We proposed a unique method for quantitatively evaluating the effects of the bulk degradation and the facet degradation on the lasing characteristics, which at that time had been difficult.
First, we prepared two laser diodes with long cavities on the same epitaxial wafer, as illustrated in Fig. 1. The front end-facet had an anti-reflection (AR) coating, and the rear end-facet had a high-reflection (HR) coating. Then, one of the laser diodes was degraded through an automatic power control (APC) aging test. The laser cavity was divided into three cavities by cleaving, forming three laser diodes. Lasers A and C had degraded AR-coated and degraded HR-coated facets, respectively, while laser B had no degraded facets. Accordingly, the effects of the bulk and facet degradation could be quantitatively evaluated by comparing the lasing characteristics of the degraded laser diode and the virgin laser diode for lasers A, B, and C.

Fig. 1 Laser diodes prepared to evaluate degradation.

Fig. 2 Light output power vs. injection current characteristics for lasers A, B, and C.

3. Method of lifetime estimation in the case of sudden failure caused by COD[4]
AlGaInP laser diodes exhibit two kinds of degradation modes under an APC aging test. One is gradual degradation, in which the injection current increases gradually, and the other is the sudden failure, which is often caused by catastrophic optical damage (COD) at the end-facet mirror. The lifetime in the case of gradual degradation can be estimated from the rate of increase of the injection current; however, it is difficult to estimate the lifetime in the case of sudden failure. We proposed an estimation method for the lifetime in the case of a sudden failure by measuring the dependence of the COD level on the operation time.
Figure 3 shows the dependence of the COD level on the operation time under APC conditions for AlGaInP laser diodes with AR-HR facet coatings. The COD level decreases as the operation time increases and can be expressed as a logarithmic function of the operation time. Moreover, the rate of decrease of the COD level significantly depends on the output power of the laser diodes. Sudden failure occurs when the COD level decreases to the operation power of the laser diode. Accordingly, one can estimate the lifetime of a sudden failure by extrapolating the time dependence of the COD level.

Figure 4 shows optical microscope images of the degraded end-facet. Facet erosion was observed at the light-emitting region. We also confirmed mutual diffusion at the interface between the coating material and semiconductor material using micro-Auger sputter profiling.

Fig. 4 Optical microscope images of the degraded end-facet.

4. Dependence of facet degradation on the coating film [5]
The gradual degradation of the laser diode depends on the coating film formed on the end-facet. We evaluated the degradation rate for three types of AlGaInP laser diodes with different AR coating films (an e-beam-evaporated SiO₂ film with an O₂ ion-assisted coating, an e-beam-evaporated Al₂O₃ film, and an electron cyclotron resonance (ECR)-deposited SiN film) under APC conditions of 20 mW and 50 ^oC. Figure 5 shows the normalized rates of increase of the injection currents for the three types of laser diodes. We found that the laser diodes with an ECR-deposited SiN film exhibited the lowest rate of increase of the injection current.

Fig. 5 Dependence of the degradation on the coating film.

By improving the lasing characteristics of the AlGaInP laser diode and the facet coating film, we achieved stable operation for over 2,000 hours under APC conditions of 35 mW and 50 ^oC, which were required for practical use at that time [6].

Acknowledgements
I would like to thank Dr. T. Misugi, Dr. H. Ishikawa, Dr. T. Sakurai, Dr. S. Yamakoshi, and Mr. T. Tanahashi for giving me the opportunity to engage in the research and development of AlGaInP visible laser diodes at Fujitsu Laboratories Ltd. I also want to thank my collaborators, Mr. A. Furuya, Mr. Y. Kito, Ms. M. Sugano, Mr. H. Sudo, Mr. M. Kondo, Dr. C. Anayama, and Ms. K. Domen.

References
[1] Denpa-Shinbun (Japanese newspaper), Fujitsu Ltd. Average seek time 23 ms, data transfer rate 4.9 MB/s, 3.5-inch MO disk drives, July 16, 1999.
[2] Fujitsu Ltd. press release, 640 MB magneto-optical disk drives (https://pr.fujitsu.com/jp/news/1999/Jul/15.html) in Japanese.
[3] T. Fukushima, A. Furuya, Y. Kito, H. Sudo, M. Sugano, and T. Tanahashi, “Method of evaluating facet and bulk degradation in semiconductor lasers, ” IEICE C-I, vol. J78-C-I, no. 2，pp. 73-79, 1995 (in Japanese).
(Translated paper) T. Fukushima, A. Furuya, Y. Kito, H. Sudo, M. Sugano, and T. Tanahashi, “Method of evaluating facet and bulk degradation in semiconductor lasers, ” Electronics and Communications in Japan, vol. 78, no. 6, part 2, pp. 65-73, 1995.
[4] T. Fukushima, A. Furuya, Y. Kito, H. Sudo, M. Sugano, and T. Tanahashi, “Catastrophic optical damage of AlGaInP visible laser diodes under high-power operation, ” IEICE C-I, vol. J78-C-I, no. 3，pp. 143-149, 1995 (in Japanese).
(Translated paper) T. Fukushima, A. Furuya, Y. Kito, H. Sudo, M. Sugano, and T. Tanahashi, “Catastrophic optical damage of AlGaInP visible laser diodes under high-power operation, ” Electronics and Communications in Japan, vol. 78, no. 7, part 2, pp. 11-19, 1995.
[5] T. Fukushima, A. Furuya, Y. Kito, M. Sugano, H. Sudo, and T. Tanahashi, Facet degradation of AlGaInP visible semiconductor lasers with facet passivation, Jpn. J. Appl. Phys., vol. 33, Part 2, no. 7B, pp. L1007-L1009, 1994.
[6] T. Fukushima, “Study on reliability of 680 nm AlGaInP visible laser diodes,” doctoral dissertion, Osaka Prefecture University, 1995. (in Japanese).