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C-MEMS filter aims at monitoring

Posted: 16 Dec 2003 ?? ?Print Version ?Bookmark and Share

Keywords:c-mems? mems? d-wdm? wdm? snr?

D-WDM filters can potentially double, triple or even quadruple spectral efficiency by reducing the space separating channels to 50GHz, 25GHz and 12.5GHz, respectively. Thus far, however, the performance of these advances has not delivered a corresponding improvement in cost.

A 25GHz transceiver, for instance, provides twice the channel count of a 50GHz device, but it imposes a 20 percent higher cost per channel. A significant part of the additional per-channel outlay arises from the advanced optical monitoring technology needed to measure more closely spaced channels. Such monitors need higher resolution than the channels they measure and have become increasingly challenged as the prerequisite resolution drops.

Optical monitoring instruments work by drawing a percentage of signal power from a fiber-optic network, demultiplexing it into individual wavelengths and measuring each channel's optical power, spectral accuracy and SNR. The cost of these instruments is linked closely to the complexity of their demultiplexing components, which rely either on diffraction gratings or Fabry-Perot interferometers. Most current instruments rely on a combined diffraction grating and photodetector array. But such devices become problematic as channel counts increase.

Limited tuning range

One approach is to demultiplex signals using Fabry-Perot interferometers, which require only a single-element photodetector to measure multiple channels separated by 25GHz or less, using a resolution bandwidth corresponding to a finesse of around 2,500.

Different filters apply different tuning methods. One approach uses a resonant cavity, filled with an electro-optic material between fixed plates. Applying a voltage to the material alters its refractive index and allows the filter to tune across a wavelength band. Generally, however, the small value of the electro-optic coefficient requires high voltages and provides a relatively small tuning range.

A more versatile approach uses MEMS technology to alter the separation between a fixed and a movable resonator mirror. The mirror spacing establishes a single peak within a given wavelength band. Applying electrostatic forces to the movable mirror alters the spacing, allowing the filter to tune across its free spectral range.

The resonant-cavity structure and detector configuration reduces the footprint and cost of Fabry-Perot filters, but the potential trade-off to this simplicity has been the challenges associated with silicon's high rigidity. That rigidity, and the need to operate the micromirrors at moderate voltages, requires thin structures for the spring hinges. Such designs use a confocal cavity to overcome problems in keeping the mirrors parallel. However, this design causes side modes on the filter profile that vary with wavelength, resulting in power measurement difficulties.

Counterbalance design

An alternative approach, which allows the use of flat mirrors, incorporates compliant elastomeric materials to support the movable mirror, hence the name C-MEMS. These materials are as much as six orders of magnitude less stiff than silicon and can be deposited in a broader range of layer thickness. Unlike carbon-based elastomers, the materials used in C-MEMS have a Si-O-Si backbone, giving them excellent mechanical, chemical and thermal stability.

Compared with silicon-based MEMS, elastomer-based C-MEMS achieve a given mechanical deflection, and their mechanical range of motion is larger. Electrostatic force is enough to drive the mirrors and keep them parallel over the lifetime of the device.

A C-MEMS Fabry-Perot filter consists of a micromachined silicon chip set comprising three or four individual chips. One chip has high-reflectivity and anti-reflective coatings on each side of its optically active area. An elastomeric ring provides the filter's flexible element and provides a passive counterbalance to the electrostatic force.

A second chip incorporates the drive electrodes, spaced several microns away from the movable mirror. Bonding the two pieces together with a reference mirror forms a basic mirror-driving unit with a reasonable voltage budget. An alternate configuration uses four chips to create two identical mirror-drive units.

In either case, C-MEMS requires a low-voltage budget to move the mirror along its normal optical axis and tune over the full range of channels in the C or L-band at scan rates of at least 10Hz. More important, it ensures a high degree of parallelism and a consistent filter shape with peak wavelength insertion losses under 2dB.

- Mike Blake & Linda West

NP Photonics

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