John Grady
Experimental results from a polarization-state rotator demonstrate the potential of microelectromechanical systems (MEMS) technology to control polarization and bring cost and size advantages to WDM optical-transport systems. Polarization-mode dispersion (PMD) limits a large portion of the installed base of single-mode fiber to per-channel bit rates of 2.5 Gbit/s or less. As WDM systems increase per-channel bit rates from 10 to 40 Gbit/s, the need to compensate PMD becomes critical.
Advantages of MEMS technology may be realized not only through reduced cost and size in PMD compensators, but in responding quickly to erratic PMD fluctuation rates because MEMS devices usually have response times on the order of milliseconds.
The rotator was designed at AT&T Laboratories-Research (Red Bank, NJ) and fabricated by a commercial MEMS foundry. Research team members Chuan Pu, Lih Lin, and Evan Goldstein are now with Tellium (Oceanport, NJ).
The demonstrated device uses MEMS devices to rotate an input state-of-polarization (SOP) along its principal axis on the Poincaré sphere. A polarization beamsplitter (PBS), fixed micromirror, and phase shifter are all integrated on a single silicon chip and stand perpendicular to the substrate with the aid of established surface-micromachining technology (see Fig. 1). The micro-optical elements are prealigned to each other photolithographically. The device occupies a compact area of 3 mm x 1.6 mm.
The PBS consists of a polysilicon plate that is oriented so that the optical beams are incident at the Brewster angle (74° for polysilicon). The fixed mirror and the phase shifter are both parallel to the PBS (see Fig. 2). The phase shifter consists of a micromirror and an electrode plate. Phase shifting is achieved by fine-tuning the angle of the micromirror to create a phase difference between the two optical paths that is realized by applying a bias voltage to the electrode plate. Moving the micromirror changes the phase difference between the optical paths. Imposing a one-wavelength difference in the optical path moves the SOP in a full circle of constant latitude about the TE/TM axis on the Poincaré sphere.
The polarization rotator was tested using a tunable laser generating light at 1550 nm coupled into and out of the device with fiber collimators. Output light was collected by another fiber collimator and fed into a polarization analyzer. Trajectory of the SOP on the Poincaré sphere depends on the power ratio between TE and TM modes. When a nearly equal power-splitting ratio is approached, the SOP would traverse on the equator of the Poincaré sphere. Actuating the micromirror creates a three-wavelength phase shift. Departures from closed circle trajectories are believed chiefly to be due to the mirror-angle variation of the phase shifter. Numerous controls and techniques were used to eliminate extraneous polarizing factors.
Results suggest the promise of MEMS for incorporating compact, low-cost forms of polarization control into optical-transport systems. Using the polarization rotator as a building block, researchers believe a full-polarization controller can be made by cascading several such rotators, so that any input SOP on the Poincaré sphere can be transformed into any other SOP as an output.
"Subsequent experiments use MEMS polarization controllers to gain full control of any arbitrary input polarization states and combine with MEMS tunable delay lines to make fully functional first-order PMD compensators," said Pu. For more information, contact Chuan Pu at [email protected].FIGURE 2. In the MEMS polarization rotator, moving the micromirror changes the phase difference between the optical paths. Imposing a one-wavelength difference in the optical path moves the SOP in a full circle of constant latitude about the TE/TM axis on the Poincaré sphere.