Ultra low frequency noise laser by locking to an all-fibered interferometer

Very low frequency noise lasers are important tools for many applications such as high-resolution spectroscopy, optical atomic clock local oscillator, interferometric sensor (including gravitational waves detection), and coherent optical communications systems. Laser linewidth is usually reduced by locking to an ultra-stable optical cavity, using the Pound-Drever-Hall method. It led to fractional frequency instability lower than 10^-15 for 1 s averaging time, and subhertz linewidth. However this scheme requires fine alignment of free space optical components, tight polarization adjustment and spatial mode matching. Moreover, high-finesse cavities are relatively expensive, bulky and fragile devices. This paper reports on the frequency stabilization of an erbium-doped fibre distributed-feedback laser using an all-fibre based Michelson interferometer of large arm imbalance. The interferometer uses a 1 km SMF-28 optical fibre spool in the delay arm and an acousto-optic modulator AOM2 allowing RF heterodyne detection. The frequency noise power spectral density is reduced by more than 40 dB for for Fourier frequencies ranging from 1 Hz to 10 kHz, leading to a level well below 1 Hz²/Hz over the whole range. Between 40 Hz and 100 kHz, the frequency noise is shown to be comparable to the one obtained by Pound-Drever-Hall locking to a high finesse Fabry-Perot cavity used as reference laser for frequency noise measurement. This method can consequently constitute an interesting alternative to cavity locking for applications where this frequency range is relevant.