Interferometric Measurement of Dispersion in Optical Components

As optical communications systems begin to fill the available bandwidth in an optical fiber by transmitting either 40 Gb/s on 200 GHz channel spacings or 10 Gb/s on 50 GHz channel spacings, the optical phase response of the components deployed in the systems will need to be carefully controlled. The dominant method of obtaining this optical phase measurement is commonly called the modulation phase-shift method[1], and uses an amplitude modulation of a narrow linewidth tunable laser to measure dispersive properties of optical devices. If the bit rate is a small fraction of the channel spacing, the dispersion caused by the transmission system, optical fiber and amplifiers, will dominate the phase characteristics of the entire system. If one wishes to measure the dispersive qualities of a long (>1km) broadband (10’s of nm) device, such as a length of optical fiber, the modulation phase-shift method is ideal and produces excellent results. The success of the modulation phase-shift measurement technique for fiber-link characterization has lead to its acceptance as the preferred method of characterizing the optical phase response of optical components, and in particular DWDM (Dense Wave Division Multiplexing) components. When the modulation phase-shift method is applied to DWDM components, however, some problems can arise. In particular, DWDM devices are not spectrally broad by design, and as a result, both the phase and amplitude can vary rapidly with wavelength. Since the modulation phase-shift method measures the derivative of the phase with respect to frequency (i.e. group delay), the optical phase response of the device must be recovered through integration. This additional step of integrating the measurement introduces further error into the measurement, thus tightening the tolerance on the underlying group-delay measurement.