24 April 2012
The first vertical external cavity surface emitting lasers (VECSELs) that have a continuous-wave power output over 100 W, even up to 3 °C, have been reported in joint work from Philipps-Universität Marburg in Germany and the University of Arizona in the US. Key changes to the design, growth procedure, chip mounting and thermal management allowed the researchers to reach 106 W at 1028 nm, which is over 30 W higher than their previous record set at -15 °C.
VECSELs are currently unrivalled in their potential to develop multi-Watt and kW class high-brightness, high-power semiconductor lasers. Epitaxial design enables flexibility in the choice of wavelength between 600 nm to over 2 μm, and the high intra-cavity powers and use of frequency conversion can extend the range to create high-power high-brightness visible and UV sources.
With such design flexibility, there are a large number of possible applications for VECSELs, many of which need a high output power. They are currently being employed or are anticipated to replace many commercial bulky laser sources such as Argon ion lasers. There have also been recent demonstrations of VECSELs as record room-temperature performing CW terahertz sources.
One of the biggest challenges with increasing VECSEL performance is overcoming the heating effects. ‘Thermal roll-over’, in which the heating leads to a rapid decrease of output power with increasing pump power, is a particular issue, and thermal management to overcome this is critical.
There have been two general heat extraction techniques employed by different groups using high thermal conductivity diamond heat spreaders. The previous highest output powers were all obtained employing the ‘upside-down’ growth described below. Significantly lower powers have been obtained using an intra-cavity single crystal diamond in contact with the active region to spread the heat, and it is thought that defects over the contact area limit the VECSEL performance in this case.
The Marburg and Arizona researchers have been closely collaborating on the development of high-power VECSELs, with the Arizona group funded by a US Joint Technology Multidisciplinary Research Initiative project to scale up the power of VECSELs. Also involved are Nonlinear Control Strategies, based in Arizona, who provide the quantum epitaxial designs, and NAsP III/V GmbH who established the epitaxial growth on a production machine to increase yield and reproducibility.
The breakthrough for the researchers in surpassing the 100 W barrier came from “the seamless integration of sophisticated microscopic quantum design of optimised resonant periodic gain structures coupled with unsurpassed growth accuracy of the full resonant periodic gain on a Bragg mirror,” according to Bernd Heinen and Jerome Moloney from the Marburg and Arizona groups, respectively.
“The wafer growth methodology delivers fully qualified wafers where the growth accuracy can be immediately validated at wafer level via high resolved X-ray diffraction in line with surface photoluminescence and reflection spectra guaranteeing unsurpassed performance of the operational device,” they said.
Also contributing to the record-high output power were improvements in the chip mounting and thermal management.
“Our mounting technique involves bonding a VECSEL chip grown upside-down (the top DBR region) to a CVD diamond heat spreader, completely removing the entire GaAs substrate via a chemical etch to expose the active RPG structure,” said Heinen and Moloney. “Even though heat has to flow down through the DBR in this setting, the heat spreading and sinking proves much more efficient. Ideally one could additionally side-cool the chip for further improvement in heat removal.”
The researchers believe that the versatility of VECSELs will open up a broad applications space. They plan to extend their joint studies to explore ultrafast pulsed VECSELs, and NAsP III/V is exploiting the VECSEL growth for further emission wavelengths.
“The remarkably accurate growth of the quantum designs by the Marburg group is essential to moving this field forward,” said Heinen and Moloney. “Best effort growths typically offered by commercial growers are simply not sufficient. The new paradigm will require iterative and seamless interaction between sophisticated epitaxial design, growth calibration and final wafer growth.
“One of the biggest future challenges will be to better control highly multimode passive pump profiles to better match the generated VECSEL signal and significantly improve thermal management by replacing metallisation and soldering with novel direct contact phononic crystal interfaces to reduce the thermal impedance between semiconductor and diamond.”
The Letter presenting the results on which this article is based can be found on the IET Digital Library.
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