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VECSEL premiers in microscopy system

02 Sep 2008

A compact and reliable frequency doubled VECSEL could be an ideal replacement for the gas lasers used in confocal microscopy systems.

A vertical external-cavity surface-emitting laser (VECSEL) has been used in a confocal laser scanning microscope (CLSM) for the first time. The researchers from the UK's Strathclyde University say that their solid-state laser approach offers the convenience of wavelength tunability for optimized sample excitation in a more compact and reliable package compared with conventional gas-based systems (Review of Scientific Instruments 79 083702).

"The new part of our research is the use of a wavelength-tunable, semiconductor laser for confocal imaging," Gail McConnell, a researcher at Strathclyde's Centre for Biophotonics, told optics.org. "Most commercial confocal systems use a gas laser, but this solid-state system is compact, low-noise, offers some wavelength tunability, has a long operational lifetime and, importantly, has lower cost and maintenance implications than the gas-based approach."

To date, argon-ion lasers have been the laser of choice in CLSMs thanks to their availability, reliability and emission wavelength of 488 nm, which matches the absorption peaks of many dyes. However, an argon-ion's lack of tunability restricts the fluorescent dyes that can be used and the samples that can be imaged. The argon-ion is also power inefficient and its beam quality is dependent on operating conditions.

In a bid to overcome these limitations, the Strathclyde team engineered a frequency doubled infrared emitting VECSEL. "VECSELs combine the flexibility of precise wavelength control, with the advantages of inherent near-diffraction limited output from an external cavity," explained McConnell. "Frequency doubling a 980 nm emitting VECSEL provides an emission at 490 nm, which enables direct comparison with an argon laser."

The VECSEL is grown on a GaAs wafer and comprimes a Bragg mirror (200 mm radius of curvature), a gain layer containing 12 InGaAs quantum wells and a cap layer. The structure was pumped using a 808 nm fibre-coupled semiconductor source.

"The laser was tuned around the peak emission wavelength of 980 nm by varying a birefringent filter and was then frequency doubled to 490 nm using a 10 mm long KNbO3 crystal," explained McConnell.

The team says that the gain medium supported a tuning range of 20 nm, and that the laser produced an output-power in excess of 500 mW over 15 nm. Approximately 1.8 mW was measured at 490 nm.

Although the group has no immediate plans to commercialise the technology, it is following a programme of work to improve the capability of the system, including novel geometries to improve the nonlinear conversion efficiency and spectral coverage. "We are also interested in alternative spectral regions and further applications of this technology, both within the life sciences and in other fields," concluded McConnell.

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