Fast all-fiber system compensates for PMD
Researchers at Corning Incorporated have developed a polarization-mode-dispersion (PMD) compensator for 10-Gbit/s systems that has a response time of 200 ms. Polarization-mode dispersion is the statistical ensemble average of differential-group dispersion delay between the two principal states of polarization. It is not usually a problem for systems with low bit rates, but as bit rates rise PMD becomes more prevalent.
Donald J. Sobiski and others at Corning's Science and Technology Division (Corning, NY) recently described the all-fiber compensator.1 The device's response speed to external PMD and internal principal state-of-polarization changes is faster than 200 ms, and is limited by the digital-to-analog converter board used in the experiment.
The compensator consists of four parts: a polarization transformer, variable differential-group delay component, RF electrical detector, and microprocessor-driven controller.
Polarization-maintaining (PM) fiber acts as three variable phase plates (two parallel ones sandwiching the middle one oriented at 45° to them) inside the transformer. Three piezoelectric transducers are installed along the length of the fiber and serve to vary operation of the phase plates.
The variable differential-group delay component consists of successively longer lengths of PM fiber that are spliced together with short sections of additional fiber, which have their axes rotated 45° from the longer lengths. Short-section fibers are equipped with transducers as well, which allow them to operate as half- or full-wave plates. Depending on the position of the transducer, this allows sections of fiber to be added or subtracted to compensate for the dispersion-group delay in the signal. A Pentium III-based PC controls the entire process.
The device contains 13 splices of PM-to-PM fiber. Developers were able to keep the total insertion loss (including the splices, connectors, and coupler) to 2.8 dB.
Because the speed of the system is limited by the electronics, custom electronics are in development. Researchers believe this will increase the response speed of the system by at least an order of magnitude. For more information, contact Donald J. Sobiski at email@example.com.
- D. J. Sobiski et al., Elec. Lett. 37, 46 (Jan. 4, 2001).
The R&D note, "SOA speed-up uses optical assist," (WDM Solutions, February 2001, p. 12) gave the incorrect location of the researchers. The researchers are from Ecole Polytechnique Federale de Lausanne (EPFL).