Experimental Investigation of Thin Film InGaAsP Coolers
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Experimental Investigation of Thin Film InGaAsP Coolers
Christopher J. LaBounty, Ali Shakouri1 , Gerry Robinson, Luis Esparza, Patrick Abraham, and John E. Bowers Electrical and Computer Engineering, University of California, Santa Barbara, CA 93106 1 Jack Baskin School of Engineering, University of California, Santa Cruz, CA 95046 ABSTRACT Most optoelectronic devices for long haul optical communications are based on the InP/InGaAsP family of materials. Thin film coolers based on the same material system can be monolithically integrated with optoelectronic devices such as lasers, switches, and photodetectors to control precisely the device characteristics such as wavelength and optical power. Superlattice structures of InGaAs/InP and InGaAs/InGaAsP are used to optimize the thermionic emission resulting in a cooling behavior beyond what is possible with only the Peltier effect. A careful experimental study of these coolers is undertaken. Mesa sizes, superlattice thickness, and ambient temperature are all varied to determine their effect on cooling performance. A three-dimensional, self-consistent thermal-electric simulation and an effective one-dimensional model are used to understand the experimental observations and to predict what will occur for other untested parameters. The packaging of the coolers is also determined to have consequences in the overall device performance. Cooling on the order of 1 to 2.3 degrees over 1-micron thick barriers is reported.
INTRODUCTION Thermoelectric (TE) coolers have encountered widespread use in the temperature stabilization of optoelectronic components (lasers, switches, detectors, etc.) in high speed and wavelength division multiplexed (WDM) fiber optic communication systems. This is even more so in dense WDM systems where the spacing between adjacent wavelengths can be from 0.8nm (100GHz) to as small as 0.2nm (25GHz) [1]. Since typical InGaAsP-based DFB lasers operating around 1.55 µm have a wavelength drift of approximately 0.1 nm/°C, the temperature must be controlled to less than a degree of variance to prevent excessive loss in multiplexers / demultiplexers or crosstalk interference. While TE coolers have successfully met this requirement, they have added greatly to the total cost of components since they are not easily integrated with devices [2]. Another disadvantage to the use of TE coolers is the large mismatch in thermal mass between that of the cooler and the device. The smallest TE coolers are a couple of millimeters squared, whereas a typical optoelectronic device is an order of magnitude smaller. Much work is currently underway in thin film thermoelectric refrigeration for other applications, however the same problems of integration with optoelectronics still exist. The InGaAsP/InP family of materials has poor thermoelectric properties due to the inherently small Seebeck coefficient [3]. However, the use of thermionic emission in heterostructures was recently proposed and has been demonstrated in the InGaAsP system to increase the cooling power [4,5]
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