  In this letter, we derive a fully-stabilized narrow-linewidth optical
frequency comb (OFC) reference to a kilometer-long fiber delay line for the
first time, to the best of our knowledge. The 1537-nm comb modes and 1566-nm
comb modes in the OFC are phase-locked to the fiber delay line with 40-kHz
locking bandwidth. From out-of-loop measurement, the 1542-nm comb mode has
residual phase noise of 925 mrad (integrated from 10 MHz to 1 kHz), fractional
frequency stability of 9.13*10(-13) at 12.8 ms average time and 580 Hz
linewidth. The linewidth has been compressed by a factor of ~ 170 compared to
the free-running condition. Short-term stability of presented OFC exceeds most
commercial microwave oscillators. The entire phase-locking system is compact
and highly-integrated benefiting from absence of optical amplifiers, f-2f
interferometers and optical/radio references. The presented OFC shows
significant potential of being reliable laser source in low-noise-OFC-based
precise metrology, microwave generation and dual-comb spectroscopic
applications outside the laboratory.
