Page 251 - Fiber Bragg Gratings

228 Chapter 6 Fiber Grating Band-pass Filters

the question lies in their ability to invert the function to the desired one
with a minimum of engineering and expense. The sales volume of gratings
will crucially depend on how well and easily they fit this task. A problem
needing a solution is ideal for creativity. To this end a number of options
have appeared. None is ideal, but within the context of a wider technology,
there are appropriate solutions for many applications, albeit at a cost.
What are the options? These may be categorized into two types. First
are those that work in reflection, as is normally the case with Bragg
gratings. These are principally the following:

1. The optical circulator with grating
2. The single grating in one arm of a coupler
3. (Possibly the most attractive) The in-coupler reflection band-pass
filter
4. The dual grating Michelson interferometer
5. The dual grating Mach-Zehnder interferometer
6. The super-structure grating
Those that work in transmission include most notably:

7. The distributed feedback (DFB) grating
8. The Fabry-Perot interferometer
9. The composite moire resonator
10. The chirped grating, or radiation loss with transmission window
11. The side-tap filter
12. The long-period copropagating radiation mode coupler
13. The polarization rocking coupler
14. The intermodal coupler
15. The in-coupler Bragg grating transmission filter
The above list may be subdivided into interferometric, which include
devices 4-9, and noninterferometric. It is worth noting that although
interferometric devices conjure up the image of sensitivity to external
stimuli, it is not necessarily true of all in that category (devices 6, 7, and
9). By suitable design, devices 4, 5, and 8 have been rendered insensitive
and demonstrated to be stable. All gratings are temperature and strain
sensitive; however, the temperature sensitivity is low, <0.02 nm/°C, so
that over a working temperature range of 100°C, the change in the op-