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Transmission property and evanescent wave absorption of cladded multimode fiber tapers

Shangping Guo and Sacharia Albin

Open Access Open Access

Abstract

Cladded multimode fiber optic tapers are proposed as chemical sensors using evanescent wave absorption. There is no need to strip the cladding; therefore, fabrication is easy and the taper is mechanically stronger than the taper without cladding. The transmission property and evanescent wave absorption are modeled using ray theory and wave theory, respectively. Effects of some parameters on the absorption sensitivity are analyzed numerically. Due to the presence of the cladding, the taper core is not in direct contact with the external medium, leading to some significant differences from the uncladded one, especially when the index of the external medium approaches the index of cladding or core. Tapers are fabricated and absorption experiments are conducted to show the feasibility of such a chemical sensor.

©2003 Optical Society of America

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Figures (10)
Fig. 1.
Fig. 1. Schematic structure of a biconic multimode fiber taper. The parameters are: r0, fiber radius, rw, taper waist radius, L, taper length, nco, ncl and nex are the indices for core, cladding and external medium, respectively.

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Fig. 2. (a).
Fig. 2. (a). Cladded taper: the incident light before entering the tapered region will divide into 5 parts: Part 1, bound rays in the core; part 2, tunneling rays of the core; part 3, bound rays in the cladding; part 4, tunneling rays of the cladding; part 5, refractive rays. (b). Uncladded taper: parts 3, 4 do not exist.

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Fig. 3.
Fig. 3. Transmission properties of cladded multimode fiber tapers with different taper ratios vs. the refractive index of the external medium, nco=1.469 and ncl=1.459.

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Fig. 4.
Fig. 4. Evanescent wave energy fraction distribution along z. The power is normalized to the power in the core and cladding at each point. nco=1.469, ncl=1.445, nex=1.330, λ=1µm, L=4mm, r0=62.5µm.

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Fig. 5.
Fig. 5. The absorbance vs. the taper ratio for cladded and uncladded fiber tapers. nex=1.330, nco=1.469, ncl=1.445, λ=1.0µm, L=4mm, r0=62.5µm for cladded tapers and r0=31.25µm for uncladded tapers. C is assumed to be 1 in the calculation.

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Fig. 6.
Fig. 6. Absorbance vs. the taper length. λ=1.0µm, r0=62.5µm, nco=1.469, ncl=1.445, nex=1.33, R=0.5 and taper profile is assumed to be parabolic. C is assumed to be 1 in the calculation.

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Fig. 7.
Fig. 7. The absorbance vs. the refractive index of the external medium. Due to the power loss when the refractive index of the external medium is close to the cladding, the sensitivity drops. C is assumed to be 1 in the calculation.

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Fig. 8.
Fig. 8. Multimode fiber taper fabrication system. The LD (Laser diode) and the PD (photo detector)laser diode (LD) and the photo detector (PD) are used to monitor the tapering process.

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Fig. 9.
Fig. 9. Setup of experiment, L1, L2, L3, L4 are lenses, the light source is a white light Halogen lamp, which covers the wavelength region from 400nm to 1200nm.

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Fig. 10.
Fig. 10. Measured absorption spectrum for IR-140 in a solution of 20%DMSO+80%DH2O using a multimode fiber taper. The spectrum is superimposed by a 5-order best-fit function.

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Equations (14)

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0 ≤ θ ≤ θ 1 , 0 ≤ ϕ ≤ π 2 , θ 1 = sin − 1 ( R sin ⁡ θ f )
θ 1 ≤ θ ≤ θ f , 0 ≤ ϕ ≤ ϕ 1 ( θ ) , ϕ 1 ( θ ) = sin − 1 ( R sin ⁡ θ f sin ⁡ θ )
θ 1 ≤ θ ≤ θ 2 , ϕ 1 ( θ ) ≤ ϕ ≤ π 2 , θ 2 = sin − 1 ( R sin ⁡ θ c )
θ 2 ≤ θ ≤ θ f , ϕ 1 ( θ ) ≤ ϕ ≤ ϕ 2 ( θ ) , ϕ 2 ( θ ) = sin − 1 ( R sin ⁡ θ c sin ⁡ θ )
θ f ← min ( θ f , sin -1 R )
θ 2 ← min { max ( θ 1 , sin -1 ( R sin ⁡ θ c ) ) , θ f }
I taper = π 8 sin 2 θ f − ∫ θ 2 θ f ∫ ϕ 2 ( θ ) π 2 sin ⁡ θ cos ⁡ θ sin 2 ϕ d θ d ϕ
η ( θ , ϕ ) = λ n cl tan ⁡ θ sin ⁡ θ 2 π n co 2 r 0 sin 2 θ c sin 2 θ c − sin 2 θ sin 2 ϕ
η ( z ) = ∫ θ 1 ( z ) θ f ( z ) ∫ ϕ 1 ( θ , z ) ϕ 2 ( θ , z ) η ( θ , ϕ , z ) sin ⁡ θ cos ⁡ θ sin 2 ϕ d θ d ϕ ∫ 0 θ f ( z ) ∫ 0 ϕ 2 ( θ , z ) sin ⁡ θ cos ⁡ θ sin 2 ϕ d θ d ϕ
θ 1 ( z ) = sin − 1 r 0 sin ⁡ θ 1 r ( z ) ,
θ 2 ( z ) = sin − 1 r 0 sin ⁡ θ 2 r ( z ) ,
ϕ 1 ( θ , z ) = sin − 1 R r 0 sin ⁡ θ f r ( z ) sin ⁡ θ ( z ) ,
ϕ 2 ( θ , z ) = sin − 1 R r 0 sin ⁡ θ c r ( z ) sin ⁡ θ ( z )
A = C ∫ 0 L η ( z ) d z .
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