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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.