Abstract

Coherent detection in lightwave transmission systems can improve receiver sensitivities by 10–20 dB over direct detection sensitivities, in phase-shift-keyed (PSK) systems, the information is impressed on the phase of the optical field while the amplitude is held nominally constant. A phase-modulated (PM) optical wave with no amplitude modulation propagating in an optical fiber will be adversely affected by group velocity dispersion in the fiber. In particular, the various frequency components of the PM wave will arrive at the end of the fiber at slightly different times. This causes amplitude modulation of the received signal which in turn leads to a penalty in coherent transmission systems.^1,2 This induced amplitude modulation has been called self-amplitude modulation.³ In this paper we describe measurements of phase modulation to amplitude modulation (PM-AM) conversion using a 1.53-μm laser sinusoidally phase modulated at rates between 0.7 and 4.0 GHz. The measured PM-AM conversion agrees very well with straightforward theoretical predictions which can be easily extended to other modulation frequencies, fiber lengths, and values of group velocity dispersion (i.e., for different laser wavelengths or for dispersion-shifted fibers).

© 1986 Optical Society of America