title: Subhertz linewidth laser by locking to a fiber delay line.
abstract: An ultralow-noise, subhertz 1.55 μm 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 7×10(-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.
authors.forename: Jing
authors.initials: J
authors.lastname: Dong
authors.forename: Yongqi
authors.initials: Y
authors.lastname: Hu
authors.forename: Junchao
authors.initials: J
authors.lastname: Huang
authors.forename: Meifeng
authors.initials: M
authors.lastname: Ye
authors.forename: Qiuzhi
authors.initials: Q
authors.lastname: Qu
authors.forename: Tang
authors.initials: T
authors.lastname: Li
authors.forename: Liang
authors.initials: L
authors.lastname: Liu