Page 301 - Fiber Bragg Gratings

278 Chapter 6 Fiber Grating Band-pass Filters

the two modes. This device can made with either two gratings in polished
couplers [76,77] or a single one written into a fused coupler [78], since
the perturbation has to be present in the entire cross-section of the cou-
pling region. The dropped channel is reflected and routed to port 2 of the
fiber coupler. In the following sections, the characteristics of the latter
two devices are presented.

6.7.1 Bragg reflecting coupler OADM
The BRC-OADM [76-78] is probably the most promising of all the OADM
devices that rely on interference. Essentially, this is a new twist to a range
of generic devices based on the grating-assisted coupling action. Apart from
being a simple device and having very low insertion loss, the BRC has the
potential of fulfilling the requirements for a high extinction at the "dropped"
as well as the "through" ports, and low back-reflection into the input port.
Schematics of the BRC in the assembled and fused forms are shown in Fig.
6.41 (iii, iv). In its fused form, it comprises two fibers tapered down to form
a long coupling region in which the fibers are kept parallel, followed by a
short grating and another long coupling region before the fibers separate.
The principle of operation is probably the cleanest of all the different
types of grating couplers and may be understood in the following phenome-
nological way: The light input into port 1 propagates adiabatically in the
tapered region to excite the supermodes of the coupler. In this region, the
two fibers merge into a single strand and become a glass rod without a core,
surrounded by air. In a normal coupler without the grating, 100% of the
light is transferred from one set of modes to the other to exit at port 4. If,
however, a point reflector is placed at exactly half the coupling length, then
the divided power between the modes travels backward toward ports 1 and
2, and the coupling process continues uninterrupted, apart from the 77-phase
change induced by the reflection in all the supermodes. Thus, instead of
propagation in the positive z-direction, the supermodes travel in the nega-
tive 2-direction and interfere at the exit of the coupler and are routed by
symmetry into port 2. In the BRC, the Bragg grating replaces the point
reflector, which is wavelength selective, and routes light only near the band-
gap into port 2. This simple picture is surprisingly accurate, despite the fact
that coupling continues within the grating region, due to light penetration.
It is immediately apparent that a strong grating would be preferable, al-
though complications arise since the presence of the grating detunes the
coupling action. The BRC can also be considered to be a close analog to the