Page 28 - Fiber Bragg Gratings

1.4 Overview of chapters 9

including that of rare-earth doped fibers are compared in a summary
at the end of Chapter 2.
Chapter 3 is on fabrication of Bragg gratings. It deals with the princi-
ples of holographic, point-by-point replication and the technologies in-
volved in the process: various arrangements of the Lloyd and mirror
interferometers, phase-mask, along with the fabrication of different type
of Bragg and long-period gratings, chirped gratings, and ultralong grat-
ings. The attributes of some of the laser sources commonly used for fabrica-
tion are introduced in the concluding section of the chapter.
Chapter 4 begins with wave propagation in optical fibers, from the
polarization response of a dielectric to coupled mode theory, and formu-
lates the basic equations for calculating the response of uniform gratings.
A section follows on the side-tap gratings, which have special applications
as lossy filters. Antenna theory is used to arrive at a good approximation
to the filter response for the design of optical filters. Long-period gratings
and their design follow, as well as the physics of rocking filters. The
last section deals with grating simulation. Here two methods for the
simulation of gratings of arbitrary profile and chirp based on the transfer-
matrix approach and Rouard's method of thin films are described.
Chapter 5 looks in detail at the different methods available for apodi-
zation of Bragg gratings and its effect on the transfer characteristics.
These include the use of the phase mask, double exposure, stretching
methods, moire gratings, and novel schemes that use the coherence prop-
erties of lasers to self-apodize gratings.
Chapter 6 introduces the very large area of band-pass filtering to
correct for the "errant" property of the Bragg grating: as the band-stop
filter! We begin with the distributed-feedback (DFB) structure as the
simplest transmission Bragg grating, followed by the multisection grating
design for the multiple band-pass function, chirped grating DFB band-
pass filters widening the gap to address the Fabry-Perot structure, and
moving on to the superstructure grating. Other schemes include the Mi-
chelson-interferometer-based filter, Mach-Zehnder interferometer, prop-
erties, tolerances requirements for fabrication, and a new device based
on the highly detuned interferometer, which allows multiple band-pass
filters to be formed, using chirped and unchirped gratings. An important
area in applications is the optical add-drop multiplexer (OADM), and
different configurations of these are considered, along with their advan-
tages and disadvantages. The special filter based on the in-coupler Bragg
grating as a family of filters is presented. Simple equations are suggested