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.
Cooling laser diodes inside a package using eTECs provides several key benefits:
- The eTEC enables a quicker response time.
- When placed close to the chip, the eTEC reduces temperature drops that would occur through any intervening material.
- 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.