Reliability of Peltier Coolers in Fiber-optic Laser Packages

Abstract

Laser modules of various types are used as the transmitters in fiber-optic telecommunications networks. Peltier coolers are employed in some of these laser modules to maintain the laser chip at a constant temperature, typically 25°C. The Peltier cooler removes both the heat generated by the laser chip itself (up to 200 mW), and that received from its surroundings. Many laser modules are intended for operation within telephone exchange buildings, where the maximum temperature is 45°C. In other locations, the temperature may be higher, for example 65 or 85°C.' The Peltier cooler is needed for two main purposes. The first is to ensure that the optical output power of the laser does not change if the outside temperature fluctuate-because its power output is temperature sensitive. Second, tight control of temperature is necessary for lasers where emission wavelength is critical. A typical telecoms systems will be required to have an operating life of between 10 and 25 years. In order to perform adequately in such applications, therefore, the Peltier coolers are required to exhibit a high level of reliability. Hence, the telecom-grade coolers need to be designed for longterm reliability. Specifically, this means a very low rate of infant mortality, and very low wear-out and random failure rates over a period of years. Individual burn-in and screening may be necessary, and suggested methods have been des~ribed.~ These may include thermal cycling. Over several years BT Laboratories (BTL) has gained considerable experience of the reliability of laser modules, many of which include a Peltier cooler. BTL3 and otherd have found that Peltier coolers have shown reliability problems. Further, from test laser modules assembled at BTL which have contained Peltier coolers, it has been found that all the subsequent assembly methods are critical in determining cooler reliability. The bismuth telluride (BiTe) elements within the Peltier cooler are relatively fragile, and can be damaged when being assembled into the laser module package unless the process is carefully controlled. In this chapter results are presented which are typical of this experience at BTL. Failure analysis is included, and areas for improvements are discussed.