Page 410 - Fiber Bragg Gratings

8.8 Gain-flattening and clamping in fiber amplifiers 387

flattening and gain stabilization and a combination of the two, using fiber
gratings.

8.8.1 Amplifier gain equalization with fiber gratings

In Chapter 4 the properties of side-tap gratings (STG) and long-period
gratings (LPG) were described. Both types of gratings may be used as
narrowband, wavelength-specific, loss-inducing components. In particu-
lar, the STG, which is a tilted Bragg grating, couples a narrowband at
the short-wavelength side of the Bragg reflection wavelength to a contin-
uum of the radiation field in the case of the unbounded cladding. This
requires matching the cladding with an appropriate oil/polymer to destroy
the well-defined boundary. For an appropriate blaze angle, which also
minimizes Bragg reflection into the guided mode, the radiated light has
an angular and spectral bandwidth. At this blaze angle, the period of the
grating determines the wavelength of peak loss. Adjustment of the grating
period at the same tilt angle enables the loss to be placed at any position
within the gain spectrum of the amplifier. It should be noted that coupling
to the radiation field for the first-order grating interaction is restricted
to a local loss spectrum, close to the Bragg reflection wavelength, and
hence a combination of several such spectra allows the fabrication of
complex spectral loss features. By appropriate choice of filters, the filter
loss may be matched to the inverse of the gain variation in the erbium
amplifier to flatten the gain spectrum. Both single [121] and multiple [122]
STGs have been used to tailor the gain spectrum of erbium amplifiers. In
the first instance, a single, 4-dB peak-loss, 10-nm bandwidth (full-width)
grating was placed at the 1533-nm peak of the erbium amplifier, eliminat-
ing the gain variation. By appropriate choice of the grating period (with
the use of a phase mask) the peak loss is moved anywhere within the
gain bandwidth. The inversion and therefore the gain shape is dependent
on the pump power. The filter is thus appropriate for a given inversion
(gain).
Figure 8.26 shows eight such loss spectra with approximately identi-
cal shapes, but with different peak-loss wavelengths and insertion loss.
Each filter is ~8 mm long with an external blaze angle of —8°, chosen to
minimize back reflection, written in a boron-germanium codoped fiber
with a core diameter of ~12 microns (see Chapter 3).