Record-breaking short single-frequency fiber lasers based on a composite highly doped erbium fiber

Two compact fiber lasers generating in the range of 1535 nm, which are based on a specially developed composite highly doped erbium fiber, are presented. A DFB laser based on a 4.1-mm-long phase-shifted fiber Bragg grating (FBG) generates single-frequency radiation with single polarization; the linewidth is ∼3 kHz and the output power is 170 μW at a pump power of 450 mW. The laser with a linear cavity configuration formed by the FBG and a dielectric mirror, which has a length of the active medium of 1.3 cm, also demonstrates single-frequency radiation with a linewidth of ∼1 kHz and an output power of 1.08 mW at a pump power of 400 mW. As far as we know, these lasers have record-breaking short cavity lengths in their classes with the spectral and power characteristics suitable for a wide range of applications.