Measurement of index modulation along an optical fiber Bragg grating

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Abstract
We analyze a fiber Bragg grating by focusing the beam of a probe laser (λ = 633 nm) through the side of the fiber onto its core. Designed for Bragg retroreflection at some longer wavelength (λB = 1290 nm), the grating causes Bragg reflection of the probe beam if the input angle is chosen suitably. The reflected power is proportional to the square of the amplitude of the refractive-index modulation at the probed position. By scanning the probe beam along the fiber we measure the axial profile of the modulation amplitude with a spatial resolution of 10 μm. The measured profile is consistent with a single-photon UV writing process.