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  • January 2012

Optics InfoBase > Spotlight on Optics > Operation of Brillouin dynamic grating in single-mode optical fibers

Operation of Brillouin dynamic grating in single-mode optical fibers

Published in Optics Letters, Vol. 36 Issue 23, pp.4686-4688 (2011)
by Kwang Yong Song

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Spotlight summary: Fiber Bragg gratings are used widely as the basic components in both optical communication systems and strain sensors. In optical communications they can be used to filter specific wavelengths in a wavelength-division multiplexing scheme for adding or dropping individual channels. The periodic modulation of the refractive index in the fiber results in Bragg reflection of light around a central wavelength determined by the grating pitch. For sensing of strain one can detect a shift in the reflected spectrum as the pitch is slightly changed by stretching the fiber. This is typically used in areas ranging from seismology to overseeing structural health of buildings. However, the refractive index modulation is written permanently into the fiber, and can only be modified slightly by stretching it. Furthermore, the grating is typically written into the fiber from the side using a holographic phase mask and UV light, making it difficult to write the grating over more than a few cm of fiber length. Longer grating lengths could be used for distributed sensing and more spectrally narrow filtering.

To address these limitations, dynamic gratings have previously been demonstrated based on counter-propagating beams forming standing waves in a rare-earth doped fiber; the standing waves build a gain or absorption grating along the length of the fiber. The grating could then be tuned simply by tuning the optical pumping wavelength. Unfortunately, in this scheme it is not straightforward to separate the probe beam reflected off the grating from the beams used to write the grating, and amplified spontaneous noise can also limit practical applications.

In 2008, Dr. Song and co-workers demonstrated an alternative technique based on Brillouin scattering in polarization maintaining fibers (PMFs). As light propagates in the fiber, some of the photon energy may be transferred to acoustic phonons (longitudinal vibrations). Phase-matching and energy-conservation considerations show that the photon is scattered backwards and downshifted in frequency by the Brillouin frequency vB determined by the acoustic properties of the fiber material. The process can be stimulated by having two counter-propagating pump beams, differing in frequency by vB. Even though the acoustic phonons can be excited by counter-propagating beams both being in the same polarization axis of the PMF (e.g. along the x-axis), the longitudinal nature of the phonons ensure that the grating they form will also scatter a probe beam polarized along the y-axis. This means that one can simply use polarization filtering to separate the two x-polarized pump beams from the y-polarized probe beam. Additionally, due to the differing phase velocities of the x- and y-polarized beams in the PMF, the peak reflection wavelength of the probe beam experiences a shift (in addition to the vB -shift) determined by the degree of birefringence of the PMF; this further helps spectrally separating the probe from the pump beams. However, the use of PMF also poses some problems. Since the scheme is highly sensitive to the local birefringence of the fiber, and therefore requires high uniformity along the fiber, it was previously necessary to strip a PMF of its protective jacket and stretch it to make a sufficiently long and uniform piece of fiber for obtaining a narrow reflection bandwidth. This limits practical applications.

Now, Dr. Song has demonstrated that the Brillouin dynamic grating scheme can also be realized using single-mode fibers (SMFs). Compared to PMFs, SMFs typically show much greater uniformity, and it should therefore be possible to obtain narrower bandwidth and higher reflectance of the grating, without having to strip the fiber of its protective jacket. The main disadvantages of using a SMF instead of a PMF are that the probe and reflected probe beams are no longer significantly spectrally separated from the pump beams, and the input polarization of the beams needs to be carefully controlled. According to Dr. Song, the potential advantages of using SMF are high enough to overcome these disadvantages. Indeed, the experimental results using SMF show a higher reflectance (8%) and a narrower bandwidth (2.4 MHz) than when using PMF. Dr. Song notes that this seems to be the narrowest bandwidth ever reported for a Bragg reflector in a fiber. The demonstration of Brillouin dynamic gratings in SMF can therefore lead to exciting new investigations and applications of optical fiber gratings.

--Michael Henoch Frosz

Technical Division: Optical Design and Instrumentation
ToC Category: Diffraction and Gratings
OCIS Codes: (050.2770) Diffraction and gratings : Gratings
(060.2310) Fiber optics and optical communications : Fiber optics
(190.4370) Nonlinear optics : Nonlinear optics, fibers
(290.5900) Scattering : Scattering, stimulated Brillouin

Posted on January 31, 2012

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