Tunable Dispersion Compensator Uses Micromirrors

Breck Hitz

At Sumitomo Electric Industries Ltd. in Yokoha-ma, Japan, engineers have developed a device based on microelectromechanical systems (MEMS) technology that provides tunable dispersion compensation across the spectrum of channels of a wavelength division multiplexing (WDM) telecommunications system. It is the first demonstration of such a device that offers control over each channel.

Chromatic dispersion in optical fiber distorts a signal by stretching the duration of the pulses, causing them to overlap, and increases the bit-error rate of the system. "Fixed" dispersion compensators, such as chirped fiber Bragg gratings or dispersion-compensating fiber, can remedy the steady-state dispersion in a system but cannot compensate for temporally varying effects caused by temperature changes and other perturbations. Mechanically stressed fiber Bragg gratings and other technologies have been investigated for tuning the dispersion compensation of individual channels of a WDM system, but the Sumitomo group's device achieves this all in one package.

Figure 1. A new multichannel, tunable dispersion compensator utilizes a bank of deformable microelectromechanical mirrors and a bulk diffraction grating.

In the tunable dispersion compensator, the input beam passes through an optical circulator and is focused in free space onto a bulk diffraction grating (Figure 1). (An optical circulator is a three-port device in which light entering port one emerges from port two, and light entering port two emerges from port three.) The grating separates the WDM channels and spreads out the frequency components of each. The frequency components then are incident on the MEMS mirrors.

The behavior of an individual MEMS mirror element is shown in Figure 2. Depending on the voltage applied to the electrostatic actuator, a different phase correction is applied to the frequency component. Although the drawing shows a significant change to the curvature of the mirror surface, in practice it moves only a small fraction of a wavelength, so the angular displacement of the reflected radiation is negligible.

Figure 2. Electrostatic force distorts the surface of the flexible mirror elements, imposing a different phase shift on each part of the reflected beam.

The reflected frequency components then are recombined at the diffraction grating. This single beam is coupled back into the optical fiber and emerges from the third port of the optical circulator to continue on its way.

The engineers built a three-channel device to demonstrate the concept. Although the experimental apparatus was designed for fairly wide, 3.0-nm channel spacing, the optical layout can be modified to provide greater separation between the MEMS mirrors and, therefore, an arbitrarily small channel spacing. The dispersion could be individually controlled for each of the three channels and varied from 4.6 to 16.7 ps/nm.

The device's zero-voltage insertion loss was flat at 10 dB across each channel and only slightly increased when the full voltage was applied. Improved optical assembly and alignment could reduce the insertion loss to 6 dB, the engineers believe.