MEMS Based Variable Optical Attenuators

It is commonly known that fiber optic attenuators are used in fiber optic communications, as fiber optic tester tools to test power level margins by temporarily adding a calibrated amount of signal loss, or installed permanently to properly match transmitter and receiver levels. According to its stability, it divided into fixed fiber optic attenuators and variable optical attenuators. Variable fiber optic attenuators generally use a variable neutral density filter, with advantages of being stable, wavelength insensitive, mode insensitive, it offers a large dynamic range.

With the rapid increases in traffic on optical telecommunications systems, there is an active program for developing transmission devices for use in wavelength division multiplexing (WDM), which is becoming mainstream technology for providing higher transmission speeds and a larger number of signal channels. It has been suggested that in the WDM systems of the future, variation in power due to the wavelength could be reduced a the quality of transmission improved by adjusting the power after demultiplexing into individual signals wavelengths. It is envisaged that the current method, in which the power of all the multiplexed optical signals is adjusted by a single variable optical attenuators (VOA) would give way to a method in which one VOA is used for each wavelength. Given the number of multiplexed wavelengths, this change will require VOAs that are considerably more compact. Against this background, There have developed a VOA using micro-electromechanical system (MEMS) technology with loss characteristics that have low wavelength dependence.

Single-mode fiber was used as the input and output of the VOA developed here, with a graded index fiber having the same diameter, 125um, as the SMF fusion spliced for a specified length, to form an optical coupling with a lens function. An anti-reflection coating is applied to the tip of the GIF (graded index fiber). GIF tip is polished at an angle so that the light beam emitted from the end of the GIF is not aligned with the optical axis of the fiber, but is at an angle to it. This angled optical beam is interrupted by means of a shutter that has been formed by inductively-coupled plasma deep reactive ion etching. The MEMS chip uses a silicon-on-insulator wafer, with the shutter, actuator and fiber grooves formed simultaneously on the chip by ICP-DRIE, followed by metal vapor deposition over the whole chip.

The actuator of the MEMS chip is of the comb type, and the GIF is held in the fiber grooves by means of adhesive. The MEMS chip with this GIF optical coupling system is fixed by adhesive within a casing, which is hermetically sealed.

MEMS variable optical attenuators are variable in three different configurations. The VA series works in transmission, whereas the VP series uses reflection to modulate the attenuation. The VX series is the VP or the VA series in mint plastic packing. In terms of performance, the VP series achieves lower insertion loss and better Polarization dependent loss characteristics. Whereas the VA series allows for an easier array integration and is the lower cost.

FiberStore offers a full line of optical attenuator variable testers, they are often combined with an active system component to maintain optical power on a network even if the power changes in the input signals. Our automatical variable optical attenuators are specifically designed for use in DWDM networks with individual channel source elements such as add/drop transmitters. The cost and performance characteristics of our automatically variable optical attenuators are specifically targeted to allow for the use of these devices in volume as principal DWDM channel stabilization components.