Multiport Spectral Measurements
Peter T. Johnson, J.E. Carroll · Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE · 1987
There is a need to determine the suitability of laser sources for use in coherent transmission systems and to study the spectral characteristics and processes that determine phase and amplitude noise of these sources. A novel method for determining the laser lineshape, linewidth, and phase noise is presented. An optical-fibre, 4 x 4 multiport is used to derive amplitude and phase information of an optical signal by mixing it with a reference optical signal. Signal processing then yields the complex cross-correlation function of the two signals, (or for a single source, the complex auto-correlation function) in the frequency domain. It is intended that information, concerning the amplitude and phase noise of the source or sources, may then be derived or inferred by comparison. The initial system has been designed to operate at 1.55 μm wavelength with a distributed feedback laser.