Page 289 - Fiber Bragg Gratings

266 Chapter 6 Fiber Grating Band-pass Filters

an obvious method for converting the band stop to a band-pass filter, it
is worthwhile to consider the benefits of such a configuration. The first
reported use was as an ASE filter for an erbium amplifier [59]. The
amplified signal is routed to the input of a circulator and reflected by a
narrow-bandwidth grating in the second port. The grating filters the
amplified spontaneous emission from the amplifier and routes the signal
to the output port. The reduction in the out-of-band spontaneous emission
can be considerable, but is determined by the quality of the reflectivity
spectrum of the grating. Optical circulators have an insertion loss of only
~1 dB, turning this very simple device into a superb band-pass filter.
Several gratings with different Bragg wavelengths may be cascaded to
form a multiple band-pass filter. The addition of a second circulator leads
to a simple method of performing an optical circulator based add-drop
multiplexing (OC-ADM) function using gratings and is shown in Fig. 6.32.
Channels injected at the "input" port are reflected by the gratings in
between the two circulators and routed to the "drop" port. All other wave-
lengths continue to the "output" port. If the signals are injected at the
"insert" port on the RHS of Fig. 6.32, the same gratings perform an insert
function, routing the reflected channels to the "output" port, along with
the rest of the channels from the "input" port. The low polarization sensi-

Figure 6.32: An OC-ADM using an all optical circulator. This device allows
several channels to be dropped or added according to the number of fiber gratings
between the circulators. The signal at the Bragg wavelengths are reflected and
appear at the drop port, while the same gratings may be used to insert the same
channels for wavelength reuse from the insert port.