Narrow linewidth ytterbium-doped fiber laser with external distributed feedback.

We demonstrate a sub-kilohertz linewidth ytterbium-doped fiber laser at 1064 nm by importing Rayleigh scattering based external distributed feedback into the linear lasing cavity. The lasing cavity consists of two fiber Bragg gratings with a full width at half maximum of 0.16 nm. An additional bandpass filter with a full width at half maximum of 84.21 MHz is inserted into the lasing cavity to maintain a stable single longitudinal mode operation. As the external distributed feedback based on Rayleigh scattering from a piece of high numerical aperture fiber is imported into the lasing cavity, the Lorentzian linewidth of the laser is narrowed from 3.36 kHz to 232 Hz. The frequency and relative intensity noises are reduced to levels of 10.84 Hz2/Hz @1 MHz and -140.62 dB/Hz @1 MHz, which are 16.78 dB and 8.94 dB lower than those of the fiber laser in its free-running state. Unlike the self-injection locking based noise reduction techniques with a shift to the lower frequency, Rayleigh scattering based external distributed feedback can reduce the relaxation oscillation peak from -100.68 dB/Hz to -126.03 dB/Hz without any frequency shift, which is of crucial importance for protecting the laser from the external disturbances in the low frequency range, such as vibration and acoustic noise.