Abstract

The capability offered by the low-coherence interferometry in the measurement of extremely small-optical path changes is well recognized.¹ This technique is at the basis of attosecond chromatic-dispersion measurements. Figure 1 reports the realized optical circuit.² Light from a superluminescent source is coupled to the fiber under test (FUT). The FUT is finished by two reflective splices and acts as a Fabry-Perot cavity. This cavity presents an optical-path unbalance that is fully compensated by the fiber-loop insert into the Mach-Zehnder cavity set in the receiver apparatus. When the compensating loop is kept in quiet conditions, any optical-path change in the FUT causes a fringe-shift that is measurable after C2. The double-cavity structure guarantees that any other optical-path change caused by the leading fiber is not revealed. In other words, the FUT is completely isolated from the unwanted perturbations. To increase the phase-reading precision, a carrier at frequency f_c is provided by an acousto-optic modulator. The dispersion is measured by using four bandpass optical filters whose center wavelength spans the source spectrum and by reading the interference through the filters. As an example, consider the first two filters, F1 and F2. The same interference signal, coming from the coupler C2, is read with a different phase after the two filters because of the different center wavelengths. This difference in phase gives the measurement of the group velocity in the considered spectrum region. A symmetric measurement is performed in the source complementary spectrum region by the other two filters, F3 and F4. The difference between the two readings is, by definition, the measurement of the dispersion (in units of phase) in the considered spectrum zone.

© 1995 Optical Society of America