Page 406 - Fiber Bragg Gratings
P. 406

8.7 Fiber grating resonant Raman amplifiers                      383

            A similar scheme for generating solitons was reported using disper-
        sion decreasing fiber [98], based on the earlier predictions [99]. The source
        used for pulse generation is the dual-frequency EDFGL source shown in
        Fig. 8.21 [100]. The narrow line-widths of this source produce an optical
        beat signal. To eliminate stimulated Brillioun scattering, different germa-
        nia concentration fibers are used in the 20-section comblike dispersion
        profiled fiber (CDPF), with a total length of —7.5 km. A beat signal at an
        amplified power of 190 mW and frequency of 59.1 GHz are converted
                              2
        into pedestal-free sech -shaped pulses with a FWHM of 2.2 psec, at a
        wavelength of 1545 nm [93]. Several other configurations based on the
        beat-frequency generation may be found in Ref. [101].
            Mode-locking of fiber lasers has been investigated in a variety of
        "figure-eight" configurations [102] using fiber gratings to generate multi-
        wavelength pulses. Actively mode-locked dark-pulse generation from a
        praseodymium-doped fiber laser with a chirped grating has also been
        reported [103].




        8.7 Fiber grating resonant Raman
                amplifiers

        Raman scattering is a process in which a small fraction of the incident
        light is scattered by the vibrational modes within a material to generate
        a Stokes photon, downshifted in frequency. Stimulated Raman scattering
        (SRS) is a process by which the Stokes photon interacts with the pump
        photon to generate another Stokes photon and is described by the imagi-
                                                3)
        nary part of the nonlinear susceptibility, ^ ( —o> s;o> p, —oip,w s). The Stokes
        field grows exponentially with the length of the medium. Since its discov-
        ery [104], SRS has been a topic of considerable research [105]. SRS can
        be a very efficient process, strongly depleting the pump power. With the
        advent of optical fibers, the observation of SRS has become very easy
        because of the high power densities in the core, low optical loss, and long
        interaction lengths. The Raman gain, which depends on the scattering
        cross-section, has been measured in silica optical fibers [106]; the band-
        width, because of the amorphous nature of the glass, is extremely large,
        extending over some 40 THz with a peak at 13 THz from the pump
        wavelength. Below the threshold for SRS, a signal photon, downshifted
        from the pump frequency, experiences gain if it lies within the gain band-
        width. This is the principle of Raman amplification [107].
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