Subhertz linewidth laser by locking to a fiber delay line

An ultralow-noise, subhertz 1.55 mu m erbium-doped fiber laser that is locked on an all-fiber-based Michelson interferometer is presented in this paper. The interferometer uses 500 m SMF-28 optical fiber and an acousto-optic modulator to allow heterodyne detection. By comparing two identical laser systems, a 0.67 (0.21) Hz linewidth beat-note signal is achieved and we obtain fractional frequency instability of 7 x 10(-15) at short timescales (0.1-1 s). The frequency noise power spectral density of two identical lasers is below -1 dB Hz(2)/ Hz at 1 Hz and it reaches -18 dB Hz(2)/Hz from 200 Hz to 1 kHz. (C) 2015 Optical Society of America.