Page 448 - Fiber Bragg Gratings

9.3 Phase and temporal response of Bragg gratings 425

Figure 9.19: The reflection spectrum of a 2-meter-long grating fabricated at
BT Laboratories. Both radiation and loss due to deuteration reduce the reflected
signal from the shorter wavelength gratings. The combined loss of deuteration
and radiation is 2 dB/m [26].

We remind ourselves that for the dispersive properties of gratings
with a reflectivity of between 10 and 15 dB, the time delay T may be
calculated as

where L g is the length of the grating and c is the speed of light. For a signal
pulse with a transform-limited bandwidth, AA equal to the bandwidth of
the grating chirp, the dispersion can be simply described as

where L is the physical length of the grating, and v g is the group velocity
of the pulse. The effects of dispersion can be considerable even in a short
1
grating since the group delay in a fiber is 5 nsec m" , and in reflection
it is doubled.
We have seen in Chapter 7 how PMD can affect the GD of gratings.
The measurement of PMD, or rather birefringence, can be made simply
by launching light along the two orthogonal birefringent axes of the fiber
grating and observing the wavelength shift. Meltz and Morey [23] reported
a birefringence-induced Bragg wavelength shift of ~0.1 nm, equivalent
5
to an induced birefringence at 1550 nm of —2.3 X 10~ . This value is high