Valerie C. Coffey
Equalization of power levels of different wavelength channels in wavelength-division multiplexed (WDM) optical networks is becoming increasingly important as the number of wavelengths increases. Simple linear spectral power equalization is sometimes all that is required in optical networks, especially in optical amplifiers, but this can adversely affect attenuation. Researchers at Bell Laboratories (Holmdel, NJ) have produced a micromechanical, optical interference device that produces changes in the slope (in dB) of its transmitted spectra via an applied voltage, without changes in attenuation level. The spectrally linear optical power equalizer (SLOPE) is less costly and easier to control than current multiple-element equalizers.
Slope equalization is important for optical amplifiers in which the complex spectral features are removed by a static filter but gain slope is controlled in erbium-doped fiber amplifiers with variable attenuators to change power levels or by pre-emphasis at the transmitter. Better noise margins can be obtained with the SLOPE device because slope can be controlled without changing power levels. For this reason the device produces changes in the slope of its transmitted spectrum without adding attenuation. That is, the transmitted spectra of the device at different bias levels should be lines of different slope that cross at nearly the same wavelength.
The device element is a vertically moving drumhead membrane, similar to the mechanical anti-reflection switch (MARS) modulator (see figure). The MARS modulator can produce changes in spectral slope with no reduction in performance and no damage to the device. However, the standard MARS device produces changes in attenuation. The SLOPE device achieves changes in reflectivity slope without altering attenuation. Both devices consist of a silicon nitride membrane with holes for etch processing and air flow, and support beams. The reflectivity of the element is varied by the adjustment of the air gap via a bias applied to the element`s electrode. The element can be thought of as a thin-film optical stack where one of the films (the air gap) is variable. Its reflectivity is transmitted via a dual fiber ferrule arrangement, in which the light comes into the device in one fiber, is reflected off the device, and focuses onto the outgoing fiber.
The SLOPE device achieved a resulting change in slope of 4 dB/50 nm of spectrum, exceeding theoretical predictions based on plane wave analysis. For more details, contact Keith W. Goossen at email@example.com.
The MARS device produces changes in spectral slope using MEMS technology (top). The SLOPE device is similar, but with a different layer structure, as shown.