SEVEN-TIMES RESOLUTION IMPROVEMENT IN NARROWBAND POLARIZATION-MODE DISPERSION.

Measurements Polarization-mode dispersion (PMD) is the parameter currently limiting higher data rates in optical fiber communications. Therefore, it is critical to have PMD metrology able to characterize these systems. In order to achieve high data rates, wavelength channel spacings are steadily decreasing from 200 GHz to 25 GHz and narrower. Fundamental tradeoffs exist in the measurement of PMD that bring higher measurement uncertainty as the measurement bandwidth is reduced. In 1998, in an effort to address narrow-band PMD measurement needs, NIST demonstrated a Modulation-Phase Shift (MPS) technique able to measure PMD in a narrow 4 GHz bandwidth. That system had a PMD resolution of 150 fs. While the technique was a useful improvement, the PMD resolution was poor compared to resolutions of a few femtoseconds achievable by wider-bandwidth PMD techniques such as Jones Matrix Eigen-analysis.

Now, NIST has assembled a second-generation MPS system that has demonstrated a PMD resolution of about 20 fs in a bandwidth of 5 GHz, an approximate sevenfold improvement over the previous MPS system. We are not aware of any other MPS system with such high PMD resolution. The system is based an RF comb-generator that modulates laser light at 2.46 GHz and mixes the detected power to a 123 MHz frequency measurable by a low-phase-noise RF lock-in amplifier. The PMD resolution of the system appears to be limited by the phase noise of the lock-in amplifier itself. We have also found the technique to be surprisingly susceptible to optical reflections in the component under test. Future work will be done toward reducing optical reflections and their effects.