OL2.DEC–pm
Liquid crystals tune light waves
Small, inexpensive device could lower the cost of installing fiber in the
local loop
Researchers at the Bellcore Research Facility in Red Bank, NJ have
devised an efficient, low-cost method for selectively filtering light
waves transmitted along a fiber optic medium. Using liquid crystals
similar to those in a typical watch display, the experimental tunable
optical filter separates information-carrying light waves into different
colors of light. This allows present fiber optic networks to send much
more information on a single fiber by using high-density
wavelength-division-multiplexing (HD WDM). The tunable optical tuned
filter is based on a Fabry-Perot cavity, see Fig. 1. This principle is
used widely in lasers where the lasing depends on the length of the
cavity. The device comprises a mirror, a transparent electrode, and a
glass plate on either side of a 1 microns, liquid-crystal-filled cavity.
The fiber optic beam, comprising many wavelengths, is passed through the
combination as shown. To select (or tune to) a wavelength, a voltage is
applied across the cavity via the electrodes. The crystals, which are
initially aligned perpendicular to the incoming polarized light, are
tilted toward the propagation direction, thus changing the refractive
index of the cavity. This in turn changes the cavity's optical length
which is a product of its refractive index and physical length. By
increasing or decreasing the applied voltage, the degree of tilt can be altered and the desired wavelength selected.
The Bellcore filter represents a significant stride in the
commercialization of optical technology. “While we have been able to
create experimental devices capable of sorting different wavelengths,
until now these devices have been too large, expensive, or energy
inefficient to be considered practical for mass production.” said Jay
Patel, a member of Bellcore's research team. The new filter is expected to
be a much-needed improvement over available technologies, which include
spectrometers, prisms, and gratings.
The complete device comes in a 1- x 1- x 1-cm package, much smaller than
any other devised for this purpose, see Fig. 2. It also has the potential
to be relatively inexpensive as it can be mass-produced using processes
similar to those for ICs. The voltage needed to adjust the crystals is 0
to 5 V and the overall power requirements are in the pW range. Parameters
under continuous development include the selectivity range and accuracy,
temperature stability, polarization insensitivity, and switching speed.
Presently, the range is 200 nm with an accuracy of less than 0.5 nm, the
temperature stability is specified at a bit error rate of 10
-9> from 0 degrees to 70 degreesC, and the switching speed is in the tens
of milliseconds.
Polarization insensitivity is necessary because of changes in polarity
of the incoming light due to external factors such as cable stress. One
method under consideration for improving this requires splitting the beam
into two orthogonal polarizations and processing each beam separately. For
more information, contact Jay Patel, at Bell Communications Research, Red
Bank, NJ 908-758-3313
Patrick Mannion
caption…
Fig. 1 Using an applied voltage, the angle of tilt of the liquid
crystals can be adjusted to select the required wavelength from up to 100
incoming wavelengths.
Fig. 2 The small size, low cost, and low power consumption of the filter
could provide a boost in continuous efforts to bring fiber to the home.
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