Heat Spreader Boosts Laser Power
Optically pumped vertical-external-cavity surface-emitting lasers are attractive in a number of applications but have suffered from thermal loading because the pump beam is focused to such a small spot -- typically 100 µm -- in the lasers' gain region. Scientists at the University of Strathclyde's Institute of Photonics in Glasgow, UK, have enhanced the performance of one of these GaAs-based lasers by bonding a silicon carbide (SiC) heat spreader to its front surface.
Figure 1. The use of a silicon carbide heat spreader with a vertical-external-cavity surface-emitting laser facilitates lateral heat flow across the face of the device, improving performance.
The heat spreader facilitates lateral heat flow across the face of the laser (Figure 1). The researchers bonded a 6 X 6-mm, 360-µm-thick piece of SiC to the 5 X 5-mm surface of the vertical-external-cavity surface-emitting laser using liquid capillary bonding. They moistened the clean surfaces with distilled water and lightly pressed them together. As the water evaporated, surface tension pulled the surfaces together until they were bonded by van der Waals forces.
Two commercial 660-nm modules pumped the vertical-external-cavity surface-emitting laser chip with a maximum power of 2.3 W, yielding 523 mW of 850-nm output (Figure 2). The fact that the output power was linear with input, with no thermally induced rollover at higher power, indicates that the laser could produce greater outputs if more pump power were available.
Figure 2. In the setup, 660-nm light from two multidiode pump modules was focused onto the bonded vertical-external-cavity surface-emitting laser. The output from the water-cooled chip was linear with th einput, indicating that powers greater t han that obtained in the experiments should be possible with higher input powers. The path of the 850-nm output beam is indicated. The inset shows the copper mount.
A chip without the SiC heat spreader produced 100 mW, and another with a sapphire heat spreader produced 150 mW. Sapphire heat spreaders have been investigated elsewhere, but their thermal conductivity is much lower than that of SiC.
An optical spectrum analyzer revealed the laser's output to be a set of longitudinal modes consistent with the 360-µm-thick etalon inside the longer, folded resonator. Another indication of efficient heat removal was the low spectral shift as a function of pump power: The shift was only 0.6 nm/W with the SiC heat spreader in place, but increased to 10 nm/W when it was removed, or to 8 nm/W when replaced with a sapphire heat spreader.
The researchers did not perform M2 measurements on the laser's output beam, but an image captured at the maximum output power showed it to be circularly symmetric with a Gaussian, TEM00 profile. The radiation was linearly polarized, with the polarization direction determined by the orientation of the birefringent SiC heat spreader.
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