Page 413 - Fiber Bragg Gratings

390 Chapter 8 Fiber Grating Lasers and Amplifiers

peak loss of less than 3 dB, resulting in a maximum back-reflection of
<-40dB.
Thus, a concatenation of STGs may be used effectively to flatten the
gain of an optical amplifier, with low back-reflection. The design of the
fiber to alter the bandwidth of the filters has been discussed in Chapter
4. This approach allows finer structures in the gain spectra to be matched
more closely [124].
The application of LPGs for tailoring the gain of optical amplifiers
has also attracted interest. The main feature of the LPG is the coupling
of the guided mode to a forward-propagating cladding mode, one of which
is selected from a large number, to induce the desired loss at the appro-
priate wavelength within the gain spectrum. It should be remembered
that several mode interactions, widely separated in wavelength, occur in
tandem for a given grating period. As such, LPGs have been used to
equalize the gain of erbium amplifiers [125,126] and as ASE-suppressing
filters. The technique used for forming the gain-equalizing filter is identi-
cal to that for the STG and has been already described [122]. There are
major differences between the STG and the LPG. The latter exhibits more
than a single loss peak separated by —30-60 nm, depending on the type
of fiber used for the filter. Written in standard telecommunications fiber,
the temperature sensitivity of the LPG is roughly 4-5 times that of the
STG. However, it has a low back-reflection into the guided mode of —80
dB.
Unless the LPG is fabricated in special temperature-stabilized fiber,
the amplifier gain equalization remains temperature sensitive, leading
to gain tilt. Finally, the wavelength of the peak loss is a function of
the refractive index of the material surrounding the cladding. With an
appropriate low-index polymer coating, the cladding mode resonance is
made insensitive to the ^urrounding material. The LPG has been used
extensively in gain equalization of amplifiers and remains an important
component.
Another method of equalizing the gain of an erbium amplifier is by
the use of two or more apodized reflection gratings in series with an optical
isolator. The resultant spectrum is shown in Fig. 8.29. Gain equalization to
approximately ±0.5 dB may be achieved with this simple arrangement.
Since gain flatness of the optical amplifier with filters is dependent
on the level of inversion, it becomes particularly attractive to combine
the filter with all-optical gain control, which is considered in the next
section.