  High-coherence light sources with an ultra-narrow linewidth are of
considerable research interests in numerous fields, such as laser
interferometer gravitational-wave observatory, laser radio, etc. Herein,
commencing from the in-depth analysis for consumption and replenishment law of
the carriers in the gain-band under excitation of the feedback signal, a
compression idea to compress extremely laser linewidth assisted by distributed
feedback has been proposed. Consequentially, a novel laser configuration is
demonstrated. A cavity mode signal matching with the output wavelength is fed
back into the main cavity by the distributed feedback structure to provide an
excitation signal required for laser gain. Moreover, the corresponding
experimental investigation is conducted based on an on-chip laser system with
distributed feedback. Eventually, an ultra-narrow linewidth laser with a side
mode suppuration ratio of 80 dB, a linewidth of 10 Hz, a Lorentzian linewidth
of sub Hz, and a relative intensity noise (RIN) of -150 dB/Hz is obtained under
normal conditions. The proposed idea is valid in any other gain-types lasers
with diverse wavelengths, which also provides a new perspective for other laser
parameters for extreme modulation.
