Page 433 - Fiber Bragg Gratings

410 Chapter 9 Measurement and Characterization of Gratings

tance and techniques that have been developed for characterization. These
include reflectivity and transmission spectrum, bandwidth, average re-
fractive index change and refractive index modulation coefficient, grating
uniformity and quality of apodization, insertion loss, radiation loss, and
group delay of chirped gratings, drift due to out-diffusion of hydrogen,
temperature effects during measurements, PMD, and stress changes.
Methods that have been reported for the measurement of thermal decay
of gratings will be covered in the final section. The aim of this chapter is
to provide an overview of the properties of optical fibers used for grating
fabrication, including thermal annealing and characterization of fiber
gratings and mechanical strength.

9.1 Measurement of reflection and
transmission spectra of Bragg gratings

The nice thing about gratings is that their growth can be monitored during
the inscription process. Since the fabrication is noninvasive, apart from
stripping part of the coating, the input and output ends of the fiber are
often accessible. Usually the source at the wavelength of interest is an
edge-emitting diode, which provides sufficient output power for a variety
of measurements. Alternatively, white light may be coupled into the fiber,
although the dynamic range is limited. The amplified spontaneous emis-
sion from a fiber amplifier is a very good broadband source, and the choice
is available to cover the 900-1700 nm wavelength band. It is normal to
have either a circulator or a 50:50 fiber splitter between the source and
the grating to be measured. The most sensitive method for detecting
gratings is in reflection, and for this reason it is best to measure gratings
in reflection for diagnostic purposes and display the signal on an optical
spectrum analyzer. The basic apparatus for measuring Bragg grating
reflection and transmission is shown in Fig. 9.1. The inset shows an
alternative arrangement using the coupler.
Reflections just above the noise floor of the spectrum analyzer are
easily displayed. When a grating is written into the fiber, a reflection
peak appears which may be checked for the wavelength. At the same
time, the transmission spectrum shows no change until the grating re-
flectivity is a few percent. A useful guide is the 3.5% reflection from the
cleaved far end of the fiber to calibrate the actual reflection from the