We demonstrate a large improvement in the efficiency of the method proposed by Krug et al. [Opt. Lett. 20, 1767 (1995)] to measure the amplitude of the refractive-index modulation along a fiber Bragg grating. The basic idea consists of using what to our knowledge is a new modulation scheme for the probe beam that not only allows the user to get a better discrimination of the probe light incoherently scattered by the fiber from that scattered by the grating but also facilitates alignment of the setup.

© 2001 Optical Society of America

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Canning, J.

Garside, B. K.

Glenn, W. H.

Janos, M.

Krug, P. A.

Lam, D. K. W.

Meltz, G.

Morey, W. W.

Ouellette, F.

Sceats, M. G.

Solte, R.

Ulrich, R.

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Figures (5)

Fig. 1
Fig. 1

Experimental setup.

Fig. 2
Fig. 2

(a) Signal from the PMT when the beam crosses the core. The probe power is equal to 5 mW. The refractive-index modulation amplitude is 7 × 10-4. (b) 270-Hz sawtooth sweep voltage driving the optical scanner.

Fig. 3
Fig. 3

Scattered optical power from a 10-mm-long apodized grating.

Fig. 4
Fig. 4

Photomultiplier signal from the light scattered by a pristine fiber (SMF 28), obtained (a) by use of a chopper for modulating the probe beam, (b) by means of the transverse core scanning method.

Fig. 5
Fig. 5

Scattered optical power from a low-strength Bragg grating (Δn mod = 5.8 × 10-5).

Equations (3)

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sin(θext)=n0 sin(θ0)=λ/2Λ.
S=S12+S221/2=S cos ϕ2+S sin ϕ21/2,  tang ϕ=S2/S1.