Page 177 - Optical Communications Essentials

Active Optical Components

Active Optical Components 167

Figure 10.4. Fiber Bragg gratings are wave-
length-selective reflective filters with steep spec-
tral profiles.

grating inside of it, the spacing of the index perturbations and the refractive
index will change. This process induces a change in the Bragg wavelength,
thereby changing the center wavelength of the filter. Such optical filters can be
made for the S-, C-, and L-bands and for operation in the 1310-nm region.

Tunable Grating Technology Before it is stretched, the center wavelength λ c of a
fiber Bragg grating filter is given by λ c 2n eff Λ, where n eff is the effective index of the
fiber containing the grating and Λ (lambda) is the period of the index variation of the
grating. When the fiber grating is elongated by a distance ∆Λ, the corresponding
change in the center wavelength is ∆λ c 2n eff ∆Λ.
The stretching can be done by thermomechanical, piezoelectric, or stepper-motor
means, as shown in Fig. 10.5. The thermomechanical methods might use a bimetal
differential-expansion element which changes its shape as its temperature varies.
In the figure the high-expansion bar changes its length more with temperature than
the low-expansion frame, thereby leading to temperature-induced length variations in
the fiber grating. This method is inexpensive, but it is slow, takes time to stabilize, and
has a limited tuning range. The piezoelectric technique uses a material that changes
its length when a voltage is applied (the piezoelectric effect). This method provides pre-
cise wavelength resolution but is more expensive, complex to implement, and has a
limited tuning range. The stepper-motor method changes the length of the fiber grat-
ing by pulling or relaxing one end of the structure. It has a moderate cost, is reliable,
and has a reasonable tuning speed.
One tunable filter version is a tunable variation on the classical structure
that has been used widely for interferometer applications. The device consists
of two sets of epitaxial layers that form a single Fabry-Perot cavity. Its operation
is based on allowing one of the two mirrors to be moved precisely by an actua-
tor. This enables a variation of the distance between the two cavity mirrors,
thereby resulting in the selection of different wavelengths to be filtered. This

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.