We　propose　an　optical　signal-to-noise　ratio　(OSNR)　monitoring　technique　based　on　the　beat　noise　detection.　The　signal　under　test　(SUT)　is　filtered　by　a　band　pass　filter　(BPF)　at　the　signal　center　frequency.　The　incident　power　of　the　monitor　module　is　adjusted　to　keep　constant.　Through　measuring　the　noise　power　at　low　radio　frequencies　(RF),　OSNR　can　be　monitored.　The　monitoring　performance　for　80　Gb/s　non-return-to-zero　differential　quadrature　phase　shift　keying　(NRZ-DQPSK)　signal　is　experimentally　demonstrated.　OSNR　of　8~33.5　dB　can　be　monitored　with　a　25.4　dB　dynamic　range　(DR)　at　100　MHz　RF.　The　impacts　of　the　BPF　bandwidth,　received　power,　RF　position,　chromatic　dispersion　(CD),　and　first　order　polarization　mode　dispersion　(PMD)　on　the　monitoring　performance　are　experimentally　investigated.　The　bandwidths　of　the　receivers　can　be　reduced　to　tens　of　MHz.　©　2014　Elsevier　B.V.