First multi-channel tunable chromatic dispersion compensator based on MEMS

First multi-channel tunable chromatic dispersion compensator based on MEMS
Chromatic dispersion management is one of the most important technologies for high-bit-rate DWDM systems. In particular, the transmission quality in 40Gbit/second systems is highly sensitive to chromatic dispersion. For compensating chromatic dispersion fixed values, a dispersion compensating fiber (DCF) is one of the most practical solutions.

However, DCF cannot fully meet the tight dispersion tolerances required by 40Gbit/s systems. At those speeds chromatic dispersion should be managed by taking account of chromatic dispersion variation in the transmission line induced by daily or seasonal temperature fluctuations. For that reason, tunable dispersion compensators are the solution for precise control of chromatic dispersion.

While various kinds of chromatic dispersion compensators have been reported, none of these are multi-channel chromatic dispersion compensators that can independently control each channel. We have proposed and experimentally demonstrated a novel multi-channel tunable dispersion compensator based on MEMS and diffraction grating technologies.

The multi-channel tunable chromatic dispersion compensator is composed of an optical circulator, a bulk diffraction grating, lenses and MEMS mirrors. The basic operation is to decompose the distorted optical signal into angular frequency components, and give each of them a phase shift by a MEMS mirror in such a way that the signal waveform is recovered.

The input beam from a single-mode optical fiber is first collimated through a lens and goes to a bulk diffraction grating. WDM signals are separated into single channels and these signals are separated to angular frequency components simultaneously by the bulk diffraction grating. Each beam is focused by a second lens onto the MEMS mirrors. The optical phase of each signal is tailored by one of the MEMS mirrors to give appropriate chromatic dispersion. The beams travel back to the lens and the bulk diffraction grating, and are coupled to the optical fiber. The output is separated by the optical circulator.

The channel spacing can be adjusted by the MEMS mirror spacing and the optical circuits. In this design, the channel spacing is 3.0 nanometers, but this can easily be adjusted to the ITU-T optical standard.

Each mirror in the array is equipped with a MEMS actuator and can be individually deformed. The surface roughness of the mirror is very important for group delay characteristics. We have optimized the mirror fabrication process and succeeded in realizing less than 50nm surface roughness including Au coating. By employing MEMS technologies, it will be possible to realize multi-channel compensators with more than 40 channels.

In the multi-channel tunable chromatic dispersion compensator, the input beam is collimated through the first lens and to the bulk diffraction grating. Source: Sumitomo Electric, Industries, Ltd.

Our experiments confirmed the variable chromatic dispersion characteristics of each channel. This is, to the best of our knowledge, the first demonstration of a truly multi-channel tunable chromatic dispersion compensator.

This article will be presented in full at the Optical Fiber Communications conference in a paper titled "Multi-channel tunable chromatic dispersion compensator based on MEMS and diffraction g rating."