Direct measurement of loop gain and bandwidth of phase-locked loop for mode-locked laser

A simple and robust technique for measuring the loop gain and bandwidth of a phase-locking loop (PLL) for mode-locked laser is proposed. This technique can be used for the real-time measurement of the PLL's real loop gain and bandwidth in a closed loop without breaking its locking state. The agreement of the experimental result and theoretical calculation proves the validity of the proposed technique for measuring the loop gain and bandwidth. This technique with a simple configuration can be easily expanded to other laser's locking system whose loop gain and bandwidth should be measured in advance.