Laser based displays, such as 10 lm to 70K lm laser projectors and laser diode (LD) backlight liquid crystal display (LCD) TVs, have attracted much attention because of the large gamut, low power consumption, and so on. Laser light sources for the displays are operated mainly under CW, and requested to be highly reliable. In this work, we will present the latest reliability study on high-power 638 nm broad stripe (BS) LD with a window-mirror structure, which is formed by using Zn diffusion into a quantum well active layer. Although the LD showed no catastrophic optical mirror degradation (COMD) even above the output of 1.6 W initially, the LDs aged at output power around 1.0 to 1.5 W showed sudden degradation during 1,000 to 4,000 hours. The duration to the failure became shorter as the power increased. Electro luminescence (EL) imaging revealed that the root cause of the sudden degradation was COMD at the front facet even though the LD had a measure to COMD. The LDs aged at 0.42 W output showed no COMD up to 6500 hours with extremely stable operation. The result also revealed that the mean time to failure due to COMD was proportional to optical density to the power of -3.2. The LDs, which had 60% small power density compared to the former, showed stable one up to 4,000 hours without COMD at 1.25 W. It is clarified that maintaining a low optical output power density is essential to develop high-power and highly reliable red BS-LDs.