Laser diodes for telecommunications have traditionally used thermoelectric coolers (TECs) for precision temperature control to improve diode output levels and maintain wavelength integrity. A major trend for photonics in telecommunications has been the move to more integrated packaging that is smaller and simpler in structure in order to lower costs. This, in turn, has opened the door for higher volume manufacturing. In the course of this transition, conventional TEC solutions have become increasingly difficult to implement as conventional bulk thermoelectric technology has not kept pace with the size and power density requirements for next generation devices.
In addition to smaller packaging and higher heat densities, the junction temperature of laser diodes can directly affect the performance and longevity of devices. As the junction temperature rises, a significant loss of power output (luminosity) will occur. The forward voltage of the diode is also dependent on the junction temperature. As the temperature rises, the forward voltage decreases causing excessive current drain on other diodes in the array.
In some instances, designers choose to place the cooling device outside the package if it is too large to be placed inside. Of course this means you are now cooling the entire package. Cooling the device by cooling the entire package is at best an inefficient method for thermal management.
If it is our desire to continue to shrink the overall size of our devices while maintaining an efficient thermal management system, we must shrink the size of the TEC. Thin film thermoelectric devices have demonstrated heat pumping capacities up to 150 W/cm2 and can be embedded within the package itself. Embedded Thermoelectric Coolers (eTECs) actively cool the diode to reduce the diode’s junction temperature, improving performance, increasing reliability and decreasing cost.
Thermoelectric cooling makes use of the Peltier effect to create a heat flux between the junctions of two different types of materials. A Peltier thermoelectric cooler is a solid-state active heat pump that transfers heat from one side of the device to the other side against the temperature gradient (from cold to hot) with the consumption of electrical energy.
Thin-film thermoelectric coolers (eTECs) are smaller and thinner than conventional TECs and show promise for direct integration using industry standard manufacturing methods. In addition, thin-film TECs have a low mass and therefore have little self heating or cooling. By placing the eTEC in the package, the cooling is closer to the heat source providing more rapid thermal response. This is particularly important when the amplifier is run at a reduced duty cycle. All integration is done inside the package to get the cooling as close to the junction as possible.
To illustrate the benefits of cooling a laser diode with an eTEC, a 1310 nm Fabry-Perot laser diode was mounted on the active side of an embedded thermoelectric module in a TO-8 package. The eTEC is thermally coupled to the chip and TO base. A thermistor was installed to measure the temperature of the active side of the TEC.
At 85°C, the HV14 module operates at a maximum of 2.7V and can pump 1.5 watts of heat in a footprint of 3 mm2. The module can create a temperature differential (ΔT) of up to 50°C between its hot and cold sides, making it suited for the cooling and temperature control of optoelectronic devices such as laser diodes.