Thermoelectric cooling and heat spreading of high power laser diode modules
Development of thermo-
coolingunits instead of
At wavelengths above 1000 nm the efficiency of laser diodes is reduced significantly so that for optical outputs from 50 - 100 W heat of 250 -500 W needs to be dissipated. Furthermore the temperature drift shall be minimized especially in cases where multiple laser bars are coupled together. In such cases water cooling is an option, however not always readily accepted.
The task was thus to design and build thermoelectric plate-to-air cooling units and prove that temperatures are within limits.
The heat from a 1 by 10 mm laser bar is spread out to its submount footprint of 25 x 25 mm. One or multiple submounts are positioned on a bigger base plate according to optical and electronic design considerations. The hot spots on the base plate, the heat spreading capacity of the base plate and the somewhat varying cooling capacity of the underlying thermoelectric modules form a heat transfer system which could hardly be predicted using analytical methods. Inside a thermoelectric module Joule’s heat generation and Fourier’s heat conduction work against the heat pumping (cooling) peltier effect.
The analysis was performed using a CFD code with an integrated detailed model of the thermoelectric module with its geometry factor, number of couples and material properties Seebeck-coefficient, resistivity and conductivity as a function of temperature. It could be shown that in an array of multiple 40x40 thermoelectric modules even with heat spreading thru a 8 mm copper plate a module positioned directly underneath a laser bar could have 30% higher cooling capacity as compared to an adjacent module. Temperature differences in the base plate can reach 6 K. The laser bars will nevertheless be stabilised in a range < 1K. Care has to be taken with respect to the position of the temperature sensor.
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