Optical injection locking based amplification in phase coherent transfer of optical frequencies.

We demonstrate use of an optical injection phase locked loop (OIPLL) as a regenerative amplifier for optical frequency transfer applications. The optical injection locking (OIL) provides high gain within a narrow bandwidth (< 100 MHz), and is capable of preserving the fractional frequency stability of the incoming carrier to better than 10^(-18) at 1000 s. The OIPLL was tested in the field as a mid-span amplifier for the transfer of an ultra-stable optical carrier, stabilized to an optical frequency standard, over a 292 km-long installed dark fiber link. The transferred frequency at the remote end reached a fractional frequency instability of less than 1 × 10^(-19) at averaging time of 3200 s.