High fidelity detection of the orbital angular momentum of light by time mapping

We demonstrate high-fidelity detection of the orbital angular momentum (OAM) of light using a compact and practical OAM spectrometer that maps the OAM spectrum to time. The spectrometer consists of a single optical delay loop to achieve timing mapping, a vortex phase plate that iteratively decreases the OAM value and a single mode fiber to distinguish zero from non-zero OAM states. Light with arbitrarily OAM compositions can be measured. For light with OAM up to 4 (h) over bar, we measured an average crosstalk of -21.3 dB, which is mainly limited by the purity of the input states and optical alignment.