- Diamond Heat Spreader Facilitates VECSEL’s Room-Temperature Operation
Output of 10 W achieved with optical efficiency of 44 percent.
The circular Gaussian beam of a vertical-external-cavity surface-emitting laser (VECSEL) is more useful in numerous applications than the elliptical, nonsymmetrical beam emitted by edge-emitting lasers. The external-cavity design — with the output coupler physically separated from the laser chip — allows access to the intracavity space for nonlinear optics, mode-lockers and/or spectrum-controlling elements. These features make improvements in VECSEL technology of interest to a great many users.
Figure 1. The nearly hemispheric resonator was formed by the external mirror and the distributed Bragg reflector of the laser chip.
Investigators at Samsung Advanced Institute of Technology in Yongin, South Korea, recently demonstrated some improvements to the technology that resulted in high output power, yielding what they believe is the highest optical-to-optical efficiency for an optically pumped VECSEL. The improvements involve the careful epitaxial growth of the chip — to ensure that the peaks of the intracavity standing wave lie precisely on the quantum wells — and the addition of a diamond heat spreader.
The laser chip comprised a distributed Bragg reflector (DBR) fabricated from 35 pairs of AlAs/AlGaAs epitaxial layers and 15 7-nm thick, compressively strained In0.28Ga0.72As quantum wells. The barrier regions between the quantum wells were made of strain-compensating GaAs0.9P0.1 and pump-photon absorbing GaAs. The scientists capillary bonded the 330-µm-thick, single-crystal-diamond heat spreader to the surface of the VECSEL chip.
Figure 2. Input/output curves show increasing output for increasing output coupling, indicating that optimal coupling may not have been achieved. The operating temperature for these data was 20 °C.
They placed an external mirror approximately 15 cm from the laser chip and aligned it with the DBR to create a nearly hemispheric laser resonator (Figure 1). They pumped the chip with the 808-nm output of a laser diode incident on the chip at a 30° angle and observed lasing at 1060 nm. They found that the alignment between the peaks of the 1060nm intracavity standing wave and the 7-nm-thick quantum wells was crucial to obtaining optimal laser performance. A slight misalignment could increase the laser threshold by up to 35 percent and diminish the output power by 16 percent.
Figure 3. Output at 20 °C was nearly identical to that at 0 °C, indicating a great deal of design tolerance. The difference between these data and those in Figure 2 results primarily from a different focus of the pump laser onto the vertical-externalcavity surfaceemitting laser chip.
The available pump power limited the VECSEL’s output, and the device showed no thermal rollover at the upper ends (Figure 2). Its spectrum consisted of a half-dozen or so spectral peaks separated by ~0.7 nm, the free spectral range of the 330-µm-thick etalon formed by the surfaces of the diamond heat spreader (Figure 2, inset). The best results, achieved with a 94 percent reflecting output coupler, were more than 10 W of output from 24 W of pump power, for an efficiency of 44 percent.
Figure 4. An alternate setup enabled intracavity second-harmonic generation.
To evaluate the effectiveness of the diamond heat spreader, the investigators varied the operating temperature from 0 to 80 °C (Figure 3). They observed no thermal rollover at temperatures of 60 °C and below and nearly identical output at 0 and 20 °C. The wide range in operating temperature allows a significant tolerance in practical applications.
Figure 5. With a 13-mm-long LBO crystal inside the resonator, the laser generated 5 W of 530-nm output.
By tilting the external mirror and adding a flat dichroic one, the researchers created a folded, three-mirror resonator suitable for intracavity second-harmonic generation. The dichroic mirror was highly reflective at both the fundamental and second-harmonic wavelengths and combined the green light generated in both directions by the 13-mm-long LBO crystal into a single beam that exited through the folding mirror (Figure 4). The laser generated 5 W of output power in the green (Figure 5).
Applied Physics Letters, Feb. 27, 2006, 091107.
- gaussian beam
- A beam of light whose electrical field amplitude distribution is Gaussian. When such a beam is circular in cross section, the amplitude is E(r) = E(0) exp [-(r/w)2], where r is the distance from beam center and w is the radius at which the amplitude is 1/e of its value on the axis; w is called the beamwidth.
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