Feb. 16, 1998 MURRAY HILL, N.J. – Scientists at Bell Labs, the research and development arm of Lucent Technologies, have demonstrated an experimental device that combines free-space optics and microscopic mirrors to route and switch individual wavelengths, or colors, of light transmitted simultaneously on an optical fiber. The micro-mirrors -- so small that about 100 of them would fit on the head of a pin -- are used to add or drop specific wavelengths, switching them in and out of the transmission fiber, without disturbing the remaining channels.
In an optical communication system, each wavelength serves as a channel that carries information encoded in the ones and zeros of digital data. High-capacity systems use wavelength-division multiplexing (WDM) technology, to transmit multiple wavelengths at once.
In the Bell Labs experiment, an optical-imaging system substitutes for a conventional wavelength-router assembly and an array of micro-mirrors replaces individual switches. In a commercial application, this technology could reduce cost, require less space, and increase efficiency.
A patent covering the device has been filed with the U.S. Patent Office. The Bell Labs research team includes Joseph Ford, James Walker, Vladimir Aksyuk and David Bishop.
The mirrors are micro-electro mechanical systems (MEMS) devices, used in an innovative way.
"This is a substantial achievement," said Cherry Murray, director of the Bell Labs Physical Research Lab. "With this MEMS technology, one can integrate optical switching into a WDM system, allowing for needed components like wavelength-channel add-drops and cross-connects based on standard silicon technology and free-space or integrated waveguide optics."
The system uses a WDM switch in which multi-wavelength light from an optical fiber is imaged through a diffraction grating onto a column of micro-mechanical tilt-mirrors. The mirrors are positioned so that each is illuminated by a single wavelength, and they are tilted so that individual wavelength signals are either passed into the output fiber or reflected directly back into the input fiber.
The switch becomes a four-port add/drop when placed between two optical circulators, in which data traffic is handled in a manner similar to the way vehicles are routed through a traffic circle at a city intersection. Each input wavelength signal passes through the first circulator and enters the WDM switch, in which the active switching device is a column of 16 micro-electro-mechanical tilt-mirrors.
Depending on the mirror angle, the light is either reflected back into the original input port or tilted by about 20 degrees and carried into a second output port. Signals to be passed along without modification are reflected by the switch and are returned to exit through the circulator. Signals to be dropped are transmitted through the switch and pass through a second circulator to the drop output.
If the switch is set to drop a wavelength channel, the reverse path is open to accept an "add" signal, which enters through the second circulator. The WDM add and drop channels can be separated or combined onto individual fibers, using external wavelength routers.
The mirrors, much too small to be made by conventional machine technology, were designed at Bell Labs and fabricated by MCNC, a MEMS foundry in Research Triangle Park, North Carolina, using the Multi-User MEMS Processes (MUMPs). MCNC is supported by the Defense Advanced Research Projects Agency (DARPA).
"MEMS is an exciting technology being investigated by Bell Labs researchers," said Ron Genova, business development manager for Lucent’s Microelectronics Group. "The Microelectronics Group is a leading supplier of a complete portfolio of optoelectronic components for optical networking. It’s also our job to evaluate new technologies from Bell Labs, such as MEMS, and decide how to eventually market that technology in the form of innovative products."
Bell Labs is a global leader in optical technology with more than 1,600 patents in the field. A Bell Labs breakthrough in ultra-dense wavelength division multiplexing led to the recently announced WaveStar OLS 400G, an optical networking system that achieves the record-breaking capacity of 400 gigabits (billion bits) per second over a single fiber.
Lucent Technologies, headquartered in Murray Hill, N.J., designs, builds and delivers a wide range of public and private networks, communications systems and software, data-networking systems, business telephone systems and microelectronic components. More information on Lucent Technologies is available at http://www.lucent.com.
ADDITIONAL TECHNICAL INFORMATION WAVELENGTH-SELECTABLE ADD/DROP WITH TILTING MICROMIRRORS
The Bell Labs add/drop uses a WDM switch that demultiplexes the multi-wavelength input and either transmits or reflects each individual wavelength band. Wavelength multiplexing is performed by free-space imaging through a planar diffraction grating. A single-mode fiber input at port 1 is collimated, then diffracted by a 600 lp/mm grating and imaged onto a micro-mirror array. Depending on the mirror angle, the light is either retroreflected into the original input port or tilted by about 20 degrees and carried into a second output port. A quarter-wave plate between the lens and grating rotates the reflected light polarization to compensate for any grating polarization dependence. The 5x9x19cm packaged component has 5dB total fiber-to-fiber insertion loss and 0.2dB polarization dependence, measured with a gold mirror at the device plane and single-mode fiber. The active switching device is a column of 16 micro-electro-mechanical tilt-mirrors, which uses electrostatic deflection of a gold-coated polysilicon plate. Approximately 20 volts was required to switch between the two mirror positions, which differed by 10 degrees. The micro-mirrors measure 56.8 microns from the center of one to the center of another.
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