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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 16, Iss. 21 — Oct. 13, 2008
  • pp: 16489–16495
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Band-rejection fiber filter and fiber sensor based on a Bragg fiber of transversal resonant structure

Daru Chen, Tzong-Jer Yang, Jin-Jei Wu, Linfang Shen, Kun-Lin Liao, and Sailing He  »View Author Affiliations


Optics Express, Vol. 16, Issue 21, pp. 16489-16495 (2008)
http://dx.doi.org/10.1364/OE.16.016489


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Abstract

We propose a novel band-rejection fiber filter based on a Bragg fiber of transversal resonant structure, which can also be used as a fiber sensor. Defect layers are introduced in the periodic high/low index structure in the cladding of the Bragg fiber. Coupling between the core mode and the defect mode results in large confinement loss for some resonant wavelengths inside the band gap of the Bragg fiber. A segment of the Bragg fiber of transversal resonant structure can be used as a band-rejection fiber filter, whose characteristics are mainly determined by the defect layer. The loss peak wavelength of the Bragg fiber is dependent on the refractive index and the thickness of the defect layer which indicates its applications of refractive index and strain sensing.

© 2008 Optical Society of America

1. Introduction

2. Bragg fibers

Figure 1(a) shows the cross section and the refractive index profile of the Bragg fiber of transversal resonant structure. A hollow core (with refractive index unit) of radius r is surrounded by a multilayer cladding which consists of suitable designed alternating layers of high and low refractive indices. The high/low refractive index layers are shown in black/green. Among the N periodic two-layer structures in the radial direction of the Bragg fiber, a defect layer (shown as red part in Fig. 1(a)) with a refractive index n0 and a thickness d0 is introduced in the Mth two-layer structure, which results in resonant operation for some wavelengths. In this paper, we choose high/low refractive index of 3.5/1.45, with thicknesses of d 1=0.2929Λ and d 2=0.7071Λ (forming a quarter wavelength waveguide stack for the wavelength of 1.55µm (within the optical fiber comminication window)), where the thickness of the periodic two-layer structure is Λ=d 1+d 2=0.378µm. Different values of the refractive index n0 and the thickness d0 of the defect layer are chosen in the discussions we concern in this paper. It is well known that the light propagating in the Bragg fiber is confined by the one-dimensional photonic band gap of the multi-layer cladding. To understand the characteristics of the Bragg fiber, band structure of the planar dielectric mirror which consists of suitable designed alternating layers of high and low refractive indices with the same parameters of the periodic structures of the Bragg fiber mentioned above is shown in Fig. 2. The surface-parallel wave-vector component β and the frequency ω are with the unit of 2πc/Λ and 2π/Λ, respectively. The blue regions correspond to the situations where light can propagate in the planar dielectric mirror. The yellow region shows the band gap for the Bragg fiber with multi-layer cladding corresponding to planar dielectric mirror. The dotted line represents the light line (ω=). Note that we only consider the TE mode band gap and the low loss TE01 mode of the Bragg fiber for the ease of discussion in this paper. The red line in Fig. 1(b) show the dispersion characteristics of TE01 mode for the Bragg fiber with 6 periodic two-layer structures (N=6) and a defect layer in the 3rd two-layer structure (M=3) in the cladding. The radius of the Bragg fiber is r=10Λ. The thickness of the defect layer is d 0=1.4142Λ, which means we introduce a π phase shift in the periodic two-layer structures, but the refractive index of the defect layer remains the same as the low refractive index layer. We find the dispersion line almost remain unchanged when the thickness of the defect layer is set to d 0=0.7071Λ, corresponding to a conventional Bragg fiber without a defect layer. However, as we show below, the Bragg fiber with a defect layer show resonant operation for some wavelengths and thus can be used as a fiber filter device.

Fig. 1. (a). Cross section and the refractive index profile of the Bragg fiber of transversal resonant structure. (b). Band structure of the planar dielectric mirror and dispersion property for TE01 mode of the Bragg fiber of transversal resonant structure.
Fig. 2. Confinement of the different Bragg fibers. In the left figure, the green, red and black lines correspondes to the Bragg fiber of 6 periodic two-layer structrues without defect layer, with a defect layer of d 0=1.4142Λ, and with a defect layer of d 0=2Λ, respectively. In the right figure, the green, red and black lines correspondes to the Bragg fiber of 8, 7, an 6 periodic two-layer structrues with a defect layer of d 0=2Λ, respectively. Inset of the right figure shows the transmission spectrum of a 10-cm Bragg fiber with 8 periodic two-layer structures and one defect layer.

3. Band-rejection fiber filter base on the Bragg fiber

Fig. 3. One eighth-plane of the Bragg fiber’s cross section for calculation (a) and power flow profiles of TE01 modes and defect modes for wavelength of 1223nm (b), 1224nm (c), 1225nm (d), 1226nm (e) and 1227nm (f).

Both the characteristics and the principle of such a band-rejection fiber filter based on the Bragg fiber are found to be similar as a LPG. For a LPG, mode coupling between the core mode and the cladding mode occurs because of the periodic index-modulation structures in the propagating direction of a single mode fiber. The coupling is wavelength-selective, which indicates that the LPG can be used as a band-rejection filter. For a Bragg fiber with defect layer, we can also find coupling between the TE01 mode and the defect mode where most energy concentrates on the defect layer occurs for a certain wavelength. For a Bragg fiber of 6 two-layer structures with a defect layer in 3rd two-layer structure, we show the power flow profiles of TE01 modes and defect modes for wavelength of 1223nm (b), 1224nm (c), 1225nm (d), 1226nm (e) and 1227nm (f) in Fig. 3. When the wavelength is near to 1225nm, part of energy with a relative large fraction of the TE01 mode has released to the defect layer which results in relative large loss. Thus the defect layer results in transversal resonant operation for the resonant wavelength of 1225 nm. When the wavelength (e.g. 1550nm) is far from the resonant wavelength, we can find the TE01 mode has a well confined power flow profile shown in bottom figure of Fig. 3(a). Confinement losses of the Bragg fibers of 7 periodic two-layer structures with a defect layer in 3rd periodic two-layer structure with parameters (d 0=0.7071Λ, n 0=6), a defect layer in 5th two-layer structure with parameters (d 0=0.7071Λ, n 0=5), and two defect layers in 3rd and 5th two-layer structures with parameters (d 0=0.7071Λ, n 0=5) and (d 0=0.7071Λ, n 0=6), are shown in Fig. 4 (a), (b) and (c), respectively, which indicates the similar filter characteristics of two LPGs in the single mode fiber where two different filters can be combined together with combined filter spectrum of each filter. Different from the LPG filter, the characteristics (such as the loss spectrum) of the band-rejection fiber filter based on the Bragg fiber is mainly depend on the defect layer (instead of the periodic structure for LPG) which show its advantage of easy design of a filter. Moreover, the loss level of the fiber filter can be well controlled by adjusting the length of the Bragg fiber.

Fig. 4. Confinement loss of the Bragg fibers of 7 periodic two-layer structures with a defect layer in 3rd periodic two-layer structure (a), a defect layer in 5th two-layer structure (b), and two defect layers in 3rd and 5th two-layer structures (c).

4. Band-rejection fiber filter base on the Bragg fiber

Similar to the LPG, the Bragg fiber of transveral resonant structure can also be used as a fiber sensor. For a Bragg fiber of 6 two-layer structures with a defect layer of thickness of d 0=2Λ, Figure 5(a) shows the loss peak wavelengths of the confinement loss as a function of the refractive index of the defect layer, which indicates the fiber sensor based on the Bragg fiber can achieve refractive index sensing for the medium in defect layer in a large sensing range. The sensitivity is about 7.6×10-4/nm when the refractive index is around 1.45. Comparing with the LPG based refractive index sensor [14

14. H. J. Patrick, A. D. Kersey, and F. Bucholtz, “Analysis of the response of long period fiber gratings to external index of refraction,” J. Lightwave Technol. 16, 1606–1612 (1998). [CrossRef]

], the proposed fiber sensor based on the Bragg fiber have a larger sensing range with high sensitivity. Figure 5(b) shows the peak wavelengths of the confinement loss as a funchtion of the degree of deformation for thickness of the defect layer, which indicates the proposed fiber sensor based on the Bragg fiber can also be used for strain sensing. Different from LPG strain sensing, the proposed fiber sensor is sensitive to the strain in the radial direction of the fiber (instead of propagating direction for LPG strain sensor) which indicates its potential applications for gas/water pressure sensor as LPG does.

Fig. 5. (a). Peak wavelengths of the confinement loss as a function of the refractive index of the defect layer. (b) Peak wavelengths of the confinement loss as a funchtion of the degree of deformation for thickness of the defect layer.

5. Conclusion

In conclusion, by employing a full-vector finite-element method, Bragg fibers of transversal resonant structure have been investigated. Mode coupling between the normal guiding mode and the defect mode occurs for some resonant wavelengths, which results in relatively large loss in the band gap of the Bragg fiber and thus a segment of the Bragg fiber can be used as a band-rejection fiber filter. Simulation results have shown that the characteristics of fiber filter based on the Bragg fiber are mainly dependent on the parameters of the defect layer. Complex loss spectrum of the fiber filter can been achieved by employing multiple defect layers in the cladding of the Bragg fiber. We have also shown that the Bragg fiber of transversal resonant structure can be used as a fiber sensor. The relationship between the loss peak wavelengths of the Bragg fiber and the refractive index (or the degree of deformation for thickness) of the defect layer has been investigated which indicates that the Bragg fiber has potential applications for refractive index and strain sensing.

Acknowledgments

This work has been partially supported by National Science Council (NSC 97-2112-M-216-001), Chiness Development Fund, a grant from the Ministry of the Education (MOE) in Taiwan under the ATU Progrom at National Chiao Tung University, the Technology Department of Zhejiang Province (grant No. 2007C21159) and the National Natural Science Foundation of China (grant No. 60707020).

Daru Chen would like to thank the hospitality of Department of Electro-physics in NCTU during his visit to NCTU.

References and links

1.

J. C. Knight and P. S. J. Russell, “Photonic crystal fibers: New way to guide light,” Science 296, 276–277 (2002). [CrossRef] [PubMed]

2.

J. C. Knight, J. Broeng, T. A. Birks, and P. S. J. Russell, “Photonic band gap guidance in optical fibers,” Science 282, 1476–1478 (1998). [CrossRef] [PubMed]

3.

T. A. Birks, J. C. Knight, and P. St.J. Russell, “Photonic crystal fibers: New way to guide light,” Opt. Lett. 22, 961–963 (1997). [CrossRef] [PubMed]

4.

M. Ibanescu, Y. Fink, S. Fan, E. L. Thomas, and L. D. Joannopoulos, “An all-dielectric coaxial waveguide,” Science 289, 415–419 (2000). [CrossRef] [PubMed]

5.

G. Ouyang, Y. Xu, and A. Yariv, “Theoretical study on dispersion compensation in air-core Bragg fibres,” Opt. Express 10, 889–908 (2002).

6.

S. D. Hartet al., “External reflection from omnidirectional dielectric mirror fibers,” Science 296, 510–513 (2002). [CrossRef] [PubMed]

7.

D. H. Kim and J. U. Kang, “Sagnac loop interferometer based on polarization maintaining photonic crystal fiber with reduced temperature sensitivity,” Opt. Express 12, 4490–4495 (2004). [CrossRef] [PubMed]

8.

S. O. Konorov and A. M. Zheltikov, “Photonic-crystal fiber as a multifunctional optical sensor and sample collector,” Opt. Express 13, 3454–3459 (2005). [CrossRef] [PubMed]

9.

G. Ouyang, Y. Xu, and A. Yariv, “Comparative study of air-core and coaxial Bragg fibres: single-mode transmission and dispersion characteristics,” Opt. Express 9, 733–747 (2001). [CrossRef] [PubMed]

10.

B. W. Liuet al., “Tunable bandpass filter with solid-core photonic bandgap fiber and Bragg fiber,” IEEE Photon. Technol. Lett. 20, 518–520 (2008). [CrossRef]

11.

K. O. Hill and G. Meltz, “Fiber Bragg grating technology fundamentals and overview,” J. Lightwave Technol. 15, 1263–1276 (1997). [CrossRef]

12.

A. M. Vengsarkaret al., “Long-period fiber gratings as band-rejection filters,” J. Lightwave Technol. 14, 58–65 (1996). [CrossRef]

13.

K. Saitoh and M. Koshiba, “Single-polarization single-mode photonic crystal fibers,” IEEE Photon. Technol. Lett. 15, 1384–1340 (2003). [CrossRef]

14.

H. J. Patrick, A. D. Kersey, and F. Bucholtz, “Analysis of the response of long period fiber gratings to external index of refraction,” J. Lightwave Technol. 16, 1606–1612 (1998). [CrossRef]

OCIS Codes
(060.2310) Fiber optics and optical communications : Fiber optics
(060.2370) Fiber optics and optical communications : Fiber optics sensors

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: August 18, 2008
Revised Manuscript: September 4, 2008
Manuscript Accepted: September 5, 2008
Published: October 1, 2008

Citation
Daru Chen, Tzong-Jer Yang, Jin-Jei Wu, Linfang Shen, Kun-Lin Liao, and Sailing He, "Band-rejection fiber filter and fiber sensor based on a Bragg fiber of transversal resonant structure," Opt. Express 16, 16489-16495 (2008)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-16-21-16489


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References

  1. J. C. Knight and P. S. J. Russell, "Photonic crystal fibers: New way to guide light," Science 296, 276-277 (2002). [CrossRef] [PubMed]
  2. J. C. Knight, J. Broeng, T. A. Birks, and P. S. J. Russell, "Photonic band gap guidance in optical fibers," Science 282, 1476-1478 (1998). [CrossRef] [PubMed]
  3. T. A. Birks, J. C. Knight, and P. St.J. Russell, "Photonic crystal fibers: New way to guide light," Opt. Lett. 22, 961-963 (1997). [CrossRef] [PubMed]
  4. M. Ibanescu, Y. Fink, S. Fan, E. L. Thomas, and L. D. Joannopoulos, "An all-dielectric coaxial waveguide," Science 289, 415-419 (2000). [CrossRef] [PubMed]
  5. G. Ouyang, Y. Xu, and A. Yariv, "Theoretical study on dispersion compensation in air-core Bragg fibres," Opt. Express 10, 889-908 (2002).
  6. S. D. Hart et al., "External reflection from omnidirectional dielectric mirror fibers," Science 296, 510-513 (2002). [CrossRef] [PubMed]
  7. D. H. Kim and J. U. Kang, "Sagnac loop interferometer based on polarization maintaining photonic crystal fiber with reduced temperature sensitivity," Opt. Express 12, 4490-4495 (2004). [CrossRef] [PubMed]
  8. S. O. Konorov and A. M. Zheltikov, "Photonic-crystal fiber as a multifunctional optical sensor and sample collector," Opt. Express 13, 3454-3459 (2005). [CrossRef] [PubMed]
  9. G. Ouyang, Y. Xu, and A. Yariv, "Comparative study of air-core and coaxial Bragg fibres: single-mode transmission and dispersion characteristics," Opt. Express 9, 733-747 (2001). [CrossRef] [PubMed]
  10. B. W. Liu et al., "Tunable bandpass filter with solid-core photonic bandgap fiber and Bragg fiber," IEEE Photon. Technol. Lett. 20, 518-520 (2008). [CrossRef]
  11. K. O. Hill and G. Meltz, "Fiber Bragg grating technology fundamentals and overview," J. Lightwave Technol. 15, 1263-1276 (1997). [CrossRef]
  12. A. M. Vengsarkar et al., "Long-period fiber gratings as band-rejection filters," J. Lightwave Technol. 14, 58-65 (1996). [CrossRef]
  13. K. Saitoh and M. Koshiba, "Single-polarization single-mode photonic crystal fibers," IEEE Photon. Technol. Lett. 15, 1384-1386 (2003). [CrossRef]
  14. H. J. Patrick, A. D. Kersey, and F. Bucholtz, "Analysis of the response of long period fiber gratings to external index of refraction," J. Lightwave Technol. 16, 1606-1612 (1998). [CrossRef]

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