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Thulium extends fiber laser emission

15 Jul 2004

A Danish team unveils a single-frequency DFB fiber laser that uses thulium as the amplifying medium.

Researchers in Denmark claim to have developed the first single-frequency thulium-doped DFB fiber laser. The laser emits 1 mW at 1735 nm from a cavity that is just 4.7 cm long. (Optics Letters 29 1503)

According to the team, the lasing wavelength is the longest demonstrated with a DFB fiber laser and is among the shortest obtained for thulium-doped silica fiber lasers.

"The laser source is compact and stable and requires no active heat dissipation," report the team from the Technical University of Denmark and Danish fiber laser specialist Koheras. "It has potential applications as a source for high-resolution spectroscopy, coherent lidar and optical frequency mixing."

To date, single-frequency DFB fiber lasers based on ytterbium and erbium-doped fibers have been shown to emit around 1080 nm and 1550 nm respectively. But as thulium's emission spectrum spans more than 400 nm, the team believes its laser could emit anywhere within the 1.7 to 2.1 micron wavelength range.

The laser cavity is a 4.7 cm long piece of silica fiber. Bragg gratings are written into the core of the thulium-doped fiber using a phase-mask technique. A grating phase shift is created in the center of the cavity to allow resonance at a single frequency.

A Ti:sapphire tuned to 790 nm pumps the fiber. The authors report that their laser had a threshold of 59 mW and a maximum output power of 1 mW at a pump power of 590 mW. The slope efficiency was 0.20%.

The researchers also tested the tunability of their laser by compressing and straining the gratings. "With this fiber, we have demonstrated piezo strain tuning over more than 1 nm from 1734 to 1736 nm without changing the single-frequency properties of the emission," report the authors.

Jacqueline Hewett is technology editor on Optics.org and Opto & Laser Europe magazine.

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