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Mode transition in high refractive index coated long period gratings

A. Cusano, A. Iadicicco, P. Pilla, L. Contessa, S. Campopiano, A. Cutolo, and M. Giordano

Open Access Open Access

Abstract

In this work, the numerical and experimental investigation of the cladding modes re-organization in high refractive index (HRI) coated Long Period Gratings (LPGs) is reported. Moreover, the effects of the cladding modes re-organization on the sensitivity to the surrounding medium refractive index (SRI) have been outlined. When azimuthally symmetric nano-scale HRI coatings are deposited along LPGs devices, a significant modification of the cladding modes distribution occurs, depending on the layer features (refractive index and thickness) and on the SRI. In particular, if layer parameters are properly chosen, the transition of the lowest order cladding mode into an overlay mode occurs. As a consequence, a cladding modes re-organization can be observed leading to relevant improvements in the SRI sensitivity in terms of wavelength shift and amplitude variations of the LPGs attenuation bands.

©2006 Optical Society of America

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Figures (13)
Fig. 1.
Fig. 1. Transversal section of the investiated structure (not in scale).

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Fig. 2.
Fig. 2. Effective refractive index of the LP02-LP08 cladding modes versus the surrounding refractive index in HRI coated fiber with: (a) 150nm thin film; (b) 200nm; (c) 250nm and (d) 300nm thin film.

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Fig. 3.
Fig. 3. Coupling coefficients of the LP02-LP08 cladding modes versus the surrounding refractive index in HRI coated fiber with: (a) 150nm thin film; (b) 200nm; (c) 250nm and (d) 300nm thin film.

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Fig. 4.
Fig. 4. SRI corresponding to the maximum sensitivity versus the overlay thickness for first 7 cladding modes.

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Fig. 5.
Fig. 5. Cladding mode field in a 200nm coated LPG, before transition (SRI=1), in transition (SRI=1.40) and after transition (SRI=1.45): (a) LP02; (b) LP03; and (c) LP04.

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Fig. 6.
Fig. 6. SEM Photogram reveals an overlay thickness of about 150nm.

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Fig. 7.
Fig. 7. Bare and 150nm sPS coated LPG transmission spectra for the LP06 cladding mode.

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Fig. 8.
Fig. 8. Transmission spectra of a 150nm sPS coated LPG for different values of SRI in the range 1.33-1.472

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Fig. 9.
Fig. 9. Transmission spectra of a 150nm sPS coated LPG zoomed on LP07 and LP08 cladding modes

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Fig. 10.
Fig. 10. Wavelength shift of different cladding modes for the LPG coated with a 150nm sPS overlay versus SRI.

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Fig. 11.
Fig. 11. Peak Loss of different cladding modes versus SRI for the LPG coated with a 150nm sPS overlay.

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Fig. 12.
Fig. 12. Wavelength shift of different cladding modes for the LPG coated with a 140nm sPS overlay versus SRI.

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Fig. 13.
Fig. 13. Wavelength shift of a higher cladding mode for the LPG coated with a 180nm sPS overlay versus SRI.

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Equations (12)

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2 π λ ( n eff , 01 − n eff , 0 i ) + s 0 ( ζ 01,01 ( λ ) − ζ 0 i , 0 i ( λ ) ) = 2 π Λ
T = cos 2 ( k i L )
K vj , μi = ω 4 P 0 × ∫ ϕ = 0 2 π ∫ r = 0 ∞ Δε ( r , ϕ , z ) ψ vj ( r , ϕ ) ψ μi ( r , ϕ ) rdrdϕ
K 0 j , 0 i = [ s 0 + s 1 cos ( ( 2 π Λ ) z ) ] ζ 0 j , 0 i
ζ 0 j , 0 i = 2 πω 2 P 0 n 1 ∫ r = 0 r 1 R 0 j ( r ) R 0 i ( r ) rdr
R v ( r ) = { A 0 Z v , 1 ( u 1 r r 1 ) for r ≤ r 1 A 1 Z v , 2 ( u 2 r r 2 ) + A 2 T v , 2 ( u 2 r r 2 ) for r 1 < r ≤ r 2 A 3 Z v , 3 ( u 3 r r 3 ) + A 4 T v , 3 ( u 3 r r 3 ) for r 2 < r ≤ r 3 A 5 K v ( v r r 3 ) for r > r 3
Z v , i ( x ) = { J v ( x ) if n eff < n i I v ( x ) if n eff > n i
T v , i ( x ) = { Y v ( x ) if n eff < n i K v ( x ) if n eff > n i
u i = r i k 0 ∣ n i 2 − n eff 2 ∣ for i = 1,2,3
v = r 3 k 0 n eff 2 − n out 2
n 2 − 1 n 2 + 1 = N 3 Mε ρβ
t = k ( U ) 2 3 ( ρ ) 1 2
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