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.

Figure 1. L-I-V Curves for laser diode power output
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.

Figure 2. Spectra Comparison of Cooling Effect
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.

Thin-Film Thermoelectrics

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.

Cooling Demonstration

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.

Figure 3: Temperature profiles through the cross section of a package included thermal interface materials (TIMs) without (a) and with (b) an eTEC. The temperature inversion created by the eTEC lowers the junction temperature relative to the no-eTEC case.
A test bed consisting of a power meter, temperature controller, laser diode controller and optical spectrum analyzer was assembled to measure the effects of cooling on laser output and spectra.

A photodetector was positioned in front of the infrared window in the package and connected to the power meter. The drive current for the laser diode was increased in 10mA steps up to 100mA with the TEC turned off and on. Figure 1 shows the resulting light-current-voltage curves plotted in 10mA steps. With the TEC turned on, the output level of the laser diode nearly doubled from 0.416 mW to 0.755 mW at 100 mA. What should also be noted, although it’s not shown here, is the ability of the TEC to hold output levels steady at higher currents.

To illustrate the effect of cooling on wavelength, a fiber optic cable was positioned in front of the laser diode package to act as a light pipe into the optical spectrum analyzer (OSA). With the drive current set at 100mA and the temperature of the diode at 42°C, the OSA displays the spectral gain curve for the Fabry-Perot 1310 nm laser diode. When the TEC is turned on, the temperature of the diode quickly cools to ~21°C and the wavelength shifts to the left (blue) as illustrated in Figure 2.

The eTEC, being an active thermal device creates a thermal inversion that dramatically changes the thermal profile inside the package. Figure 3 shows a comparison of the thermal profile through the cross section of the module in two cases, a) with no eTEC, or in other words, a passive solution only, and b) with an eTEC actively cooling the junction. It can be clearly seen that the introduction of the eTEC provides a substantial benefit.

System Level Considerations

The heat that is pumped by the device and the additional heat created by the eTEC in the course of pumping that heat will need to be rejected into the system. Since the performance of the module can be improved by providing a good thermal path for the rejected heat, it is beneficial to provide high thermally conductive pathways. For small TO packages, this is typically accomplished through the electrical connections themselves, and depending on the operating characteristics, this level of thermal management might be sufficient. For packages with higher heat densities, thermally- conductive feed-throughs or posts need to be employed to remove the heat.

Summary

Cooling laser diodes inside a package using eTECs provides several key benefits:

  1. The eTEC enables a quicker response time.
  2. When placed close to the chip, the eTEC reduces temperature drops that would occur through any intervening material.
  3. The eTEC's thin form factor allows for integration into the package and doesn’t drive the device to a larger package size.

By taking advantage of the smaller, thinner form-factor of an eTEC, a new approach has been enabled for electronic thermal management that focuses on providing appropriate cooling when and where it is needed. This solution involves the integration of thinfilm thermoelectric modules into the package and as close to the heat source as possible.

This article was written by Dr. Paul Magill, VP of Marketing and Business Development, Nextreme Thermal Solutions (Durham, NC). For more information, contact Dr. Magill at This email address is being protected from spambots. You need JavaScript enabled to view it., or visit http://info.hotims.com/28055-200 .