  We report on a fiber-stabilized agile laser with ultra-low frequency noise.
The frequency noise power spectral density is comparable to that of an
ultra-stable cavity stabilized laser at Fourier frequencies higher than 30 Hz.
When it is chirped at a constant rate of ~ 40 MHz/s, the max non-linearity
frequency error is about 50 Hz peak-to-peak over more than 600 MHz tuning
range. The Rayleigh backscattering is found to be a significant frequency noise
source dependent on fiber length, chirping rate and the power imbalance of the
interferometer arms. We analyze this effect both theoretically and
experimentally and put forward techniques to reduce this noise contribution.
