  In this letter, we propose a modulation classification algorithm which is
based on the received signal's amplitude for coherent optical receivers. The
proposed algorithm classifies the modulation format from several possible
candidates by differentiating the cumulative distribution function (CDF) curves
of their normalized amplitudes. The candidate with the most similar CDF to the
received signal is selected. The measure of similarity is the minimum average
distance between these CDFs. Five commonly used quadrature amplitude modulation
formats in digital coherent optical systems are employed. Optical back-to-back
experiments and extended simulations are carried out to investigate the
performance of the proposed algorithm. Results show that the proposed algorithm
achieves accurate classification at optical signal-to-noise ratios of interest.
Furthermore, it does not require carrier recovery.
