Long term in-vacuum reliability testing of 980nm laser diode pump modules for space applications

Abstract

Pump laser diodes emitting at 980 nm are typically used in distribution network transmission systems in order to amplify C+L band signals for long distance optical telecommunication systems and are now implemented in space missions especially for Fiber Optic Gyroscopes (FOGs) and foreseen for intra-satellite communication links and calibration systems. However as the pump source is potentially a critical point for such systems, there is a huge need to assess their reliability in space environment, especially in terms of operation under vacuum conditions. Some published studies have been already reported on qualification results of high power 808nm pump laser diodes arrays in vacuum environment especially for NASA future space missions. Because no catastrophic failures were observed, the authors concluded that the laser diodes are robust enough to survive in the harsh space environment. Nevertheless, regarding both complexity of space environment and variety of Laser diode technologies, a good understanding of the physics of failure and an appropriate testing method are crucial to scope high levels of reliability. However up to now, long term in-vacuum reliability testing of 980nm laser diodes was never clearly addressed. Because the chip is embedded into a sealed package, this last is not directly exposed to vacuum environment but depending on the package leak rate, the internal pressure may vary from its initial value to the external pressure one. Our paper deals with these aspects with a special focus on long term continuous wave aging of 980nm pump Laser modules under vacuum. Our results take part of a full evaluation program managed by the French Space Agency (CNES). A vacuum lifetest during 5000 hours has been conducted on eight 400mW operating power uncooled pump modules. The chip is a spatially single mode Laser diode housed in a hermetically sealed butterfly package with dual-lens coupling optics. The package incorporates the latest improvements aiming at optimizing mechanical and thermal behavior allowing to reach high power levels and TeIcordia GR 468-CORE qualified. An ultra-high reliability is actually reached with a failure rate below 25FITs (UCL 90%) for submarine applications. Four of them were specifically punctured before the aging test to accelerate the out-diffusion of gaseous contents and to simulate the behavior of the laser diode after a long high vacuum exposition at 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-7</sup> Torr. After the test, the non-punctured components are fully operational whereas three out of four modules, of the punctured group, suddenly failed. Analysis revealed no evidence of organic contamination discarding the PIF assumption and leading to the conclusion that the vacuum test system and the laser diode package are unlikely at the origin of the failure. Even if the root failure cause was not clearly established, we emphasize that such a test reveals a specific failure mode in relation with vacuum operation. We conclude that a dual-lens optical alignment architecture, as included in such pump Laser modules and featuring a measured leak rate close to 2×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-10</sup> atm.cc/s (He), are completely suitable to withstand reliable operation in vacuum during at least ten years. We believe that other lower hermetic package construction could lead to a significant drop of pressure during the operational lifetime of the module with a possible activation of failure mechanisms reported in this paper. The validity of failure criteria in relation with operating conditions could be evaluated representing a significant part of the general PDfR effort.