Frequency Synchronization and Coherent Combining of Two Fiber Ring Cavities

Low noise synchronization of the frequency and phase of a DFB fiber laser and two fiber ring cavities was achieved based on a two-channel optical injection lock and phase lock loop. Two-channel modulation signals of different frequencies were adopted to extract the phase noise signals of the two fiber ring cavities. Finally, the lasers from the two fiber ring cavities were coherently combined in the closed loop of the control system owing to the synchronous frequency and phase. The power stability and polarization stability of the combined laser was improved under the optical injection lock and phase lock loop. The phase noise of the two cavities was
$6.56 \times 10^{-7}$
rad/Hz
^1/2
and
$1.34 \times 10^{-6}$
rad/Hz
^1/2
at 10 Hz; and
$7.77 \times 10^{-8}$
rad/Hz
^1/2
and
$1.35 \times 10^{-7}$
rad/Hz
^1/2
at 10 kHz. The total combined power was ~580 mW in the closed loop, and the entire system oscillated under a single frequency, providing excellent power scaling ability.