We　propose　and　demonstrate　an　in-band　optical　signal-to-noise　ratio　(OSNR)　monitoring　technique　using　the　output　power　ratio　(PR)　of　the　nonlinear　optical　loop　mirror　(NOLM)　theoretically　and　experimentally.　Operation　point　(OP)　is　investigated　to　be　the　input　average　power　which　corresponds　to　the　first　maximum　of　the　power　ratio　curve.　The　monitoring　performances　using　PR　are　proved　to　be　better　than　those　just　using　one　output　port　of　NOLM　for　on-off　keying　(OOK)　and　differential　phase-shift-keying　(DPSK)　signals　with　different　duty　cycles:　non-return-to-zero　(NRZ),　carrier-suppressed　return-to　zero　(CSRZ),　return-to-zero　(RZ)　with　50%　and　33%　duty　cycle　(RZ　50　and　RZ　33)　at　40　Gb/s.　For　80　Gb/s　RZ-differential　quadrature　phase-shift　keying　(DQPSK)　signal,　we　obtained　12.16　dB　maximum　power　variation　and　13-39.8　dB　monitoring　range　experimentally.　The　power　ratio　scheme　can　improve　the　contrast　ratio　by　7.05　dB　and　the　monitoring　range　by　13.8　dB　compared　with　that　just　using　the　transmission　port.　The　tolerance　to　residual　chromatic　dispersion　of　the　monitoring　performance　is　investigated　to　be　less　than　1.7　ps/nm.　The　experimental　validations　are　consistent　with　the　theoretical　analyses.　The　proposed　scheme　can　be　used　in　ultra-speed　transmission　systems.　©　1983-2012　IEEE.