Taking a Laser’s Temperature Remotely

Hank Hogan

It is not just people who get hot and bothered; the same is true for high-power laser diodes, where the buildup of heat leads to thermally induced changes in the refractive index. Such changes can significantly alter the shape and position of the laser’s mode, greatly affecting the efficiency of coupling into an optical fiber. If the mode moves into a high-loss region, the laser’s efficiency suffers. However, that is not the worst that can happen.

“Facet heating is the main cause of failure in high-power laser diodes,” said graduate student Kwok-Leung “Paddy” Chan of the University of Michigan in Ann Arbor.

A problem facing researchers has been how to map thermal effects. Now Chan and his colleagues at the university and at MIT’s Lincoln Laboratory in Lexington, Mass., have developed thermoreflectance-based high-resolution thermal imaging of a high-power laser.

The Michigan group was led by assistant mechanical engineering professor Kevin P. Pipe, while research scientist Paul W. Juodawlkis led the Lincoln Laboratory team. The imaging technique, which is described in the Nov. 13 issue of Applied Physics Letters, could result in a better understanding of what is going on inside high-power laser diodes and, eventually, in improved devices.

Thermoreflectance measurements are not new. It has long been known that the reflectance of a material can change with temperature, making it possible to measure temperature difference by detecting reflectance change given the thermoreflectance coefficient. However, this method has traditionally taken the temperature only at a point.

Investigators used thermoreflectance imaging to measure the effects of internal heating on a diode laser. A slab-coupled optical waveguide laser is shown being profiled under two operating conditions: 1 A (left) and 5 A (right) of bias current. The top shows the temperature, as measured by thermoreflectance; the center, the simulated beam profile; the bottom, the directly measured beam profile. Image courtesy of Kwok-Leung Chan.

The researchers overcame this problem by using a 12-bit, 652 × 494-pixel CCD camera from Opteon Corp. of Cambridge, Mass., to capture the entire thermal map at once. In their setup, they used a blue LED operating at 472 nm as a light source. They sent the light through a microscope to illuminate the 1-cm-long InGaAsP/InP watt-class laser that they studied, collecting the reflected light with the camera. Filters removed the 1.55-μm light from the laser during operation.

Thermal imaging

The researchers cycled the laser using a sine wave, snapping four images during each cycle. They averaged the readings from the CCD over many cycles, achieving 500-nm spatial and 450-mK temperature resolutions, respectively. The spatial resolution is a function of the light source, the optics and the pixel size of the camera, while the thermal resolution depends in part on how much data is averaged. “By increasing the number of periods — perhaps by integrating over several hours — this technique can reach a spatial resolution of 250 nm and a temperature resolution of 10 mK,” Chan said.

With this setup, the researchers captured thermal images, which indicated large temperature variations across the laser’s facet. They used this information to determine mode changes resulting from thermal effects and found that these agreed well with direct beam-pattern measurements.

They plan to study other photonic devices using the thermoreflectance technique. According to Chan, the method also could be used to investigate transistor circuits — looking, for example, for current hogging. Eventually, it could be used to study the physics behind device failures.

The scientists are looking to improve the temperature-measurement technology as well. “We are still working on improvements to the setup — for example, by using a higher-speed CCD camera,” Chan said.

Contact: Kwok-Leung “Paddy” Chan, University of Michigan, Ann Arbor;
e-mail: klchan@umich.edu.