Coupling length measurement in active directional couplers

The authors describe a simple experimental technique which enables one to assess the coupling length of a test device. The technique is based on measuring the transmission spectrum of an uncoated directional coupler of arbitrary length in both the through and cross states. The shapes of the spectra are characteristic of the coupling state of the test device. The authors outline the theoretical basis for the technique and present results of such measurements of InP-based active directional coupler devices demonstrating fabrication of half beat-length couplers with good crosstalk