All-fiber-based laser with 200 mHz linewidth

We demonstrate the frequency stabilization of a 1.55 μm erbium-doped fiber laser by locking it to a 5-km-long optical fiber delay line (FDL). The stabilized laser is characterized via comparison with a second identical laser system. We obtain a fractional frequency stability of better than 3 × 10−15 over time scales of 1–10 s and a laser linewidth of 0.2 Hz, which is the narrowest linewidth of an FDL-stabilized laser observed to date.