Reliability assurance of laser-fiber coupling in an undersea lightwave package

Abstract

Reliability assurance of a laser package for undersea lightwave communiation systems is discussed. The difficulty in assuring long Lie on new technology components such as a laser package is the lack of extensive experience with failure modes. For this reason, an extensive testing program was conducted to detect and assess the probability of failure of the package in the lightwave system. The reliability methodology consists of four park 1) Direct low temperatare aging 2) High temperature accelerated aging 3) Acmmulation of device-hours 4) Environmentalmecharxkal overstress testing to destruction. Our "Its show more than adequate margin against failure over the 27 year Service life.A laser package, designed and developed for use in 280 Mb/s undersea lightwave systems such as TAT-8, has undergone extensive reliability testing over the past 5 years. The package consists of (1) a stud mounted 1 . 3~ semiconductor laser, (2) a lensed single mode fiber aligned and soldered to achieve maximum laser to fiber coupling, and (3) an extemal PIN photodetector coupled to the laser backface for power control.The package assembly has undergone reliability qualification tests to assure 27 year reliability in the undersea repeater environment. In addition to comprehensive quality control and individual certification of the manufactured product, a formal qualification program consisting of 1200 packages was used to assess the integrity of the laser package design. The qualification program entailed testing samples under high temperature stress to end-oflife, aging at ocean temperature, and environmental stress conditions such as shock, vibration, pressure, humidity, and temperature cycling.Detailed failure mode analysis was used to identify package weaknesses or instability. One of the primary reliability concems was mechanical stability of the laser to fiber alignment [']. Any non-reversible, submicron movement would permanently modify the power coupled to the fiberr2] [31 and degrade transmission. For example, at peak alignment, a radial motion of the fiber relative to the laser in the range of f 0 . 5~ will result in 1 dB decrease in fiber power. The factors that control fiber power and the rate of change are: (1) the initial alignment position of the fiber to the laser gaussian mode/41 (2) the direction of motion, and (3) the velocity of the fiber relative to the laser. If the package optical components have some residual mechanical stress, the initial coupling and fiber output changes as a result of stress relaxation induced fiber movement.Reliability assurance against laser-fiber decoupling is achieved in the following principal ways in our qualification and certification programs:Continuous isothermal aging of laser packages for over 4 years at the ocean temperature of 12'C shows the linear change rates of the coupling efficiency to be essentidy 1. 42limited by the measurement capability of the aging system at M.2% per kilohour. Figure 1 shows the experimental aging rates measured at 32 Khr for a representative population o...