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21 Jun 2007
A European project known as NATAL is striving to produce high-power optically pumped semiconductor disk lasers emitting in the visible for projection applications. Mircea Guina from the Optoelectronics Research Centre at Tampere in Finland updates OLE on progress.
Optically pumped semiconductor disk lasers (OP-SDLs) combine many of the advantages of solid-state lasers with the versatility of semiconductor gain materials. Also known as vertical external-cavity surface-emitting lasers (VECSELs), these sources have the ability to deliver watt-level diffraction-limited output beams over a broad spectral range.
Exploiting the potential of OP-SDL technology is the foundation of the European FP6 nano-photonics materials and technologies for multicolour high-power sources (NATAL) consortium. The main goal of NATAL is to develop compact and practical OP-SDLs at new red and infrared wavelengths that are suitable for applications with a big market potential such as laser displays. The Optoelectronics Research Centre at Tampere University of Technology coordinates NATAL.
What is an OP-SDL?
The key block of an OP-SDL is the semiconductor gain mirror – see figure 1. This typically consists of a quantum-well (QW) based gain section and a distributed Bragg reflector (DBR) grown monolithically on a semiconductor substrate such as GaAs, InP or GaSb. The laser cavity is formed between the gain mirror and one or more external cavity mirrors.
The laser operation and power scaling rely to a large extent on how quickly and efficiently heat can be removed from the gain region. One common way to remove heat is to place high thermal conductance transparent materials, such as diamond, SiC or sapphire, between the semiconductor sample and a metallic heat sink.
An alternative method of dissipating heat is to grow the gain mirror in reverse order (first the QWs and then the DBR) and flip-chip bond the sample to the heat sink. The substrate is then removed, exposing the QW gain region to the pump. Although more complex, this approach has the advantage that it does not require the use of high optical-quality transparent heat spreaders.
Red-emitting OP-SDLs
The use of an external cavity allows non-linear elements and optical filters to be used for efficient frequency conversion. Frequency-converted SDLs are particularly important for accessing the visible spectral range where using directly emitting OP-SDLs is not an option due to the lack of compact pump sources (i.e. high-power edge-emitting semiconductor lasers) and gain materials.
High-power, good beam quality, compact visible lasers are in demand by many applications seeking a replacement for large solid-state, dye or gas lasers. One market where visible SDLs are predicted to have a big impact is laser projection.
The most impressive results for OP-SDLs emitting in the visible have been obtained by frequency-doubling 940–1040 nm lasers into the blue–green region. The rapid progress in this area has been fuelled by the availability of high-power 808 nm pump diodes and mature GaAs-based semiconductor technology for fabricating high-quality gain regions and DBRs.
On the other hand, the development of frequency-converted OP-SDLs emitting in the red region has been hindered by the availability of suitable semiconductor materials for gain regions and DBRs operating at 1200–1250 nm. Owing to their high refractive index difference, Ga(Al)As compound semiconductors are preferred for DBR fabrication. This in turn limits the range of semiconductor materials for the gain region to either GaAsSb/GaAs or GaInNAs/GaAs.
Unfortunately, due to weak electron confinement, OP-SDLs based on GaAsSb/GaAs gain regions have strong temperature sensitivity that prevents high-power operation at room temperature. On the contrary, GaInNAs/GaAs-based compound semiconductors have a larger conduction band offset, which leads to improved electron confinement.
Despite the advantages, the use of GaInNAs/GaAs QWs has been mainly confined to the development of edge-emitting semiconductor lasers operating in the 1.3 µm wavelength range.
One downside of using dilute nitride materials is that incorporating N into an InGaAs lattice leads to a significant amount of non-radiative recombination centres. In turn, this has a detrimental effect on the efficiency of the lasers and ultimately limits the maximum output power.
The aim of our research was to demonstrate high-quality GaInNAs/GaAs gain regions that allow high-power operation. The main outcome was a 1230 nm OP-SDL emitting around 1.2 W of optical power at room temperature. The laser was intra-cavity frequency doubled to 615 nm using a nonlinear BBO crystal (see figure 2).
Using this configuration, we obtained 300 mW of converted continuous-wave power at a wavelength of 615 nm in a narrow linewidth. The emission wavelength could be tuned by 4.5 nm using a 25 µm-thick Fabry–Perot glass etalon. Our aim in NATAL is to scale the power to 500 mW in a TEM00 mode. This should be possible by optimizing the gain region and employing advanced designs that would, for example, ensure a better absorption of the pump beam. The results of the current 615 nm experiments were published in Optics Express in March 2007 (Optics Express 15 3224).
2000 nm OP-SDLs
We have also worked with Alfred Forchel's group at the University of Würzburg in Germany to develop Sb-based OP-SDLs with an output power of 1 W emitting around 2030 nm. This wavelength range is particularly important for applications in gas spectroscopy and environmental monitoring. Here, the gain mirror consisted of a DBR with 18 pairs of quarter-wave thick AlAsSb-GaSb layers and five groups of three Ga0.78In0.22Sb QWs. Detailed results were published in Optics Express in July 2006 (Optics Express 14 6479).
Power scaling
Despite the good power handling capabilities shown by OP-SDLs, their emission is ultimately limited by thermal rollover. In order to cope with this limitation, we have implemented a power-scaling scheme using dual 1.05 µm gain elements in a single OP-SDL cavity.
By dividing the heat load between multiple-gain elements, negative effects such as overheating and thermal lensing can be reduced and the power can be increased without significant deterioration of the beam quality. Using the laser set-up depicted in figure 3, we achieved about 8 W of output power at 1050 nm, while the maximum output power from individual gain elements was approximately 4 W.
The advantage of this implementation over externally combining the beams delivered by individual gain chips is that the output is coherent and the quality of the combined beams do not deteriorate. Also, the intracavity power is increased proportionally – a feature that is advantageous for intracavity frequency conversion applications. We published the results of our dual gain experiment in Optics Express in December 2006 (Optics Express 14 12868).
The NATAL project
For more information on the NATAL project, see www.orc.tut.fi/natal. Other partners involved in NATAL are the Institute of Photonics at University of Strathclyde, UK; Technical University of Berlin, Germany; Chalmers University, Sweden; OSRAM Opto Semiconductors, Germany; TOPTICA Photonics, Germany; OPTOCAP, UK; and EpiCrystals, Finland.
• This article originally appeared in the June 2007 issue of Optics & Laser Europe magazine.
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