Abstract

A new technique for single-mode fibers chromatic dispersion measurements using phase-shift measurements is presented. The poor dynamic range and the perturbation of the rf interferences are the main drawbacks of the well-known method using a LED filtered by a monochromator as a light source and a vector voltmeter for the phase mea surements.¹ These problems are overcome by our technique, which also uses a high-frequency modulated LED whose filtered light is launched into the fiber. This HF modulation gives rise to sensitive phase shifts. But between the fiber and detector the modulation frequency is downshifted to the kilohertz range, which enables a low-frequency ultrasensitive detection scheme to be used. Actual downshifting is performed by an acoustooptic intensity modulator. The acoustooptic modulation frequency is very close but different from the LED frequency, so that sum- and difference-frequency signals are generated which conserve the phase of the initial signal. The LF detector only detects the difference-frequency signal whose phase is measured by a lock-in amplifier.

© 1987 Optical Society of America