Page 173 - Fiber Bragg Gratings

150 Chapter 4 Theory of Fiber Bragg Gratings

where L b is the beat length, so that the rocking period A r is

N is the order of the grating, and the detuning parameter is

We assume that the dielectric constants of the principal axes are e^
and e y A UV beam incident at an angle 6 degrees to the jc-axis and
orthogonal to the propagation direction induces a new set of orthogonal
birefringent axes with a change Aa,. and Aey in the dielectric constants.
Figure 4.10 shows the incident UV beam on the cross-section of the fiber.
The major and minor axes of the ellipse are the birefringent axes of the
fiber. The beam is incident at an angle 6. As a result, the birefringence
changes locally, inducing a rotation in the birefringent axes of the fiber.
The rotation angle 0 is related to the change in the birefringence as [35]

where <5Ae = Ae~< — Ae v< and Ae = a, — e v For the case when the induced
•*• y * y
birefringence is much less than the intrinsic birefringence, then the rota-
tion angle </> is small, and it follows that

Figure 4.10: Birefringence induced by the incident UV beam in a birefringent
fiber for the formation of a rocking filter.