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We propose an asynchronous-amplitude-histogram-based optical signal-to-noise ratio monitoring technique for higher order modulation formats. Simulation results show that the proposed technique enables a wide monitoring range for 25-GSym/s nonreturn-to-zero/return-to-zero 16- and 64-quadrature amplitude modulation systems with high monitoring sensitivity. The tolerance of the monitoring technique to other signal impairments such as chromatic dispersion and polarization-mode dispersion is also investigated. The proposed technique can be used along transmission links with simple low-cost hardware and/or in digital signal processing (DSP)-based coherent receivers without additional hardware, thus enabling next-generation optical networks using higher order modulation formats to achieve transmission rates of 100 Gb/s per channel and beyond.