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Ti:sapphire to probe atmosphere

05 Sep 2002

French researchers build a laser tunable in the ultraviolet region that will be used for atmospheric gas detection next year.

A research team in France claims to have built the first Ti:sapphire laser to be frequency-tripled using intracavity optics.

The laser is tunable in the range 275-285 nm, ideal for detecting important molecules in atmospheric and combustion chemistry (Optics Letters 27 1457).

Ajmal Mohamed and colleagues at the Office National d'Etudes et de Recherches Aérospatiales (ONERA) built the laser system. Mohamed told Optics.org that the laser "has many potential applications that few other systems can cover."

Previous efforts to up-convert Ti:sapphire lasers to the ultraviolet range have all relied on extracavity sum-frequency generation. According to Mohamed, these systems have all suffered from three major drawbacks: the laser systems are complex, needing two independent pump sources; critical alignment of the nonlinear optics is required, leading to fragile systems; and the ultraviolet power emitted is very low and barely useful for atmospheric applications.

By placing the frequency-converting optics inside the cavity, all of these problems can be overcome.

Mohamed's scheme uses a combination of lithium triborate and barium borate crystals for intracavity frequency-tripling: "Ultraviolet emission is obtained simply by putting the two conversion crystals in the cavity at roughly the phase-matching angles."

The pump laser is a single, compact diode-pumped Nd:YAG source operating at 532 nm and 2.5 kHz.

Peak power of the tunable ultraviolet emission is around 100 W, corresponding to 3 µJ per 30 ns pulse. According to the team, the average third-harmonic output power is an order of magnitude higher than that obtained using extracavity sum-frequency schemes.

"Far more ultraviolet power could be extracted from the system using a more powerful pump laser, but our objective is to build a compact system to measure hydroxyl radicals (OH molecules)," said Mohamed. He is confident that a ground-based version of the laser will be able to monitor OH concentrations at an altitude of 20 km in the near future.

He points out one current weakness in the intracavity-tripled system - careful alignment is required to tune the laser to the desired wavelength, and this is a problem that the group is still working on.

Despite this, Mohamed says that different test versions of the laser system will be employed next year to monitor concentrations of hydroxyl radicals.

Michael Hatcher is technology editor of Opto and Laser Europe magazine.

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