Page 225 - Fiber Bragg Gratings
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202                              Chapter 5 Apodization of Fiber Gratings

















        Figure 5.4: The self-apodized fringe profile of a two-wavelength source. The
        parameters have been chosen for illustration purposes only [9]. The grating length
        must be chosen to exactly match a single period of the envelope. Since this process
        has not modified the beam intensity profile, the change induced in the effective
        index of the mode by the UV dose is the same all along the fiber.




                                                       2
            There are other methods of generating moire  patterns for apodiza-
        tion. An optical wedge placed in the path of one of the beams of the UV
        interferometer will change the Bragg wavelength of the grating being
        written. Removal changes the Bragg wavelength; if the wedge angle is
        chosen such that the wavelength difference between the two Bragg periods
        is exactly one period more or less over the length of the grating, then
        apodization will occur. In this case, the difference in the Bragg grating
        period is given by





            The wedge required for this purpose must impart a TT phase change
        from one end of the grating to the other. Thus, for ~10-mm-long gratings,
        a wedge angle of the order of 5' to 10' of arc is required at a Bragg
        wavelength of ~1550 nm [9].
            Now that the principles have been set out whereby apodization can
        be performed without altering the intensity of illumination across the
        length of the grating, other practical implementations are discussed in
        the following sections.


            2
             Etymology: Fr., watered silk; referring to pattern formed on it.
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