Abstract

Recent work on the mode locking of rare-earth-doped fiber lasers has resulted in the demonstration of ultrashort pulses at a number of different wavelengths. Neodymium-doped silica-fiber lasers have produced pulses as short as 32 fs at 1.06 µm,¹ and 620-fs pulses at 1.3 µm have been produced by praseodymium-doped fluoride fiber lasers.² There has been significant interest in erbium-doped fiber lasers operating at 1.55 µm because of their potential as sources in fiber-based communications systems. Pulses as short as 63 fs have been generated from erbium-doped fiber lasers, corresponding to spectral widths greater than 60 nm and close to the gain bandwidth of erbium.³ Fibers doped with thulium have extended the attainable wavelengths of fiber lasers to the 1.8-2.1 µm range,⁴ in which there are possible applications in medicine, eyesafe laser radar, and spectroscopy. The gain spectrum of thulium-doped silica fiber is smooth and may be as broad as 150 nm, indicating that tunable ultrashort pulse generation should be possible. Thulium-doped fiber also has the advantage of strong absorption at 785 nm, at which AlGaAs laser diodes may eventually provide sufficient pump power. In this paper we report the first demonstration, to our knowledge, of a self-starting, passively mode- locked thulium-doped fiber laser operating between 1.8 and 1.9 µm.

© 1995 Optical Society of America