New VECSEL for spectroscopy

Dr. Jörg Schwartz, joerg.schwartz@photonics.com

Vertical external cavity surface-emitting lasers (VECSELs) have attractive features but have been hard to make for wavelengths above 2.5 μm – at least until now. Scientists at ETH Zürich (the Swiss Federal Institute of Technology) have realized the first one, a monomode continuously tunable mid-infrared laser that operates at ~5 μm and that is expected to be a powerful tool, especially for spectroscopic applications.

Compared with VCSELs (vertical-cavity surface-emitting lasers), their more commonly known siblings, VECSELs – also known as semiconductor disc lasers – come with an additional “E” in their name, referring to the external resonator that distinguishes them from all-semiconductor lasers.

External resonators can be used to shape the output beam, not only spatially, but also spectrally. For VECSELs, the resonator helps to generate low-divergence, circular, near-diffraction-limited output beams. Most semiconductor lasers are edge emitters – with huge aperture angles in one axis – and their output is highly astigmatic and requires extensive beam conditioning for all applications that demand good transverse beam quality.

The researchers have extended the benefits of an external resonator into the spectral domain by building a VECSEL with monomode frequency output that even offers mode-hop-free tuning.

Mode hopping is likely to happen, for example, when a laser that can operate in more than one longitudinal mode is tuned via a wavelength-selective element in the resonator. The switching between the modes leads to short “blips” in the output power, obviously disadvantageous for many applications. One way to avoid them is to make the resonator very short, so that only one mode – a standing wave in the resonator – can develop.

That is exactly what ETH’s thin-film physics group, led by Dr. Hans Zogg, did. For the VECSEL structure, the investigators used a silicon wafer as the substrate and molecular beam epitaxy to grow a multilayer structure. The first part is a Bragg mirror, consisting of four pairs of alternating layers with a low and high refractive index. This highly reflective “front” mirror is then overgrown by a very thin (~1.2 μm) layer of active material, lead telluride.

The external cavity of the tunable vertical-cavity semiconductor laser is formed by two Bragg mirrors. The one at the bottom of the structure extracts beams, whereas the movable top mirror injects the pump radiation. Courtesy of ETH Zürich.

“This IV-VI semiconductor is particularly attractive for mid-infrared applications, despite being a bit out of fashion in recent years,” Zogg said.

The laser is completed by an external top mirror, also on a silicon substrate with a Bragg reflector that, rather than being flat, has a machined spherical surface. The sphere is beneficial for the transverse beam quality, and the distance between the active medium and the top mirror is kept small (50 to 100 μm) and controlled by a piezoelectric controller. The low cavity length is important to ensure single-mode operation – avoiding mode hops – and the controller allows wavelength tuning.

The output of the compact device offers continuously tunable radiation at a wavelength of ~5 μm, with a tuning range of almost 1 μm and output power of up to ~100 mW in a near-diffraction-limited beam with M² of 1.14.

Although these are impressive features, the researchers say there is still room for improvement.

One challenge is that the unit is optically pumped using widely available 1.55-μm lasers. “The optical pumping itself is not an issue, as users don’t care whether they connect their drive voltage to the VECSEL itself or an attached semiconductor pump laser,” Zogg said. However, the large energy difference between the pump and emission photons means that a lot of excess heat is produced and must be removed. This means that the device currently operates at only —130 °C.

Nevertheless, “room temperature operation is not a fundamental problem at all,” Zogg said. “It just requires some further engineering. We already achieved room temperature operation with other setups in pulsed mode, and even continuous wave at room temperature is possible.”

The investigators’ confidence in the device’s potential is underlined by the fact that they are planning to launch a startup company to commercialize it.

“Our VECSELs are particularly attractive for absorption spectroscopy applications,” said Dr. Ferdinand Felder, a member of the group and co-founder of Phocone, a company expecting to start operation early this year. “Our potential customers are looking at detecting greenhouse gases, but, also, various biomedical applications such as breath analysis offer huge potential.”