Confinement loss in solid-core photonic bandgap fibers

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Abstract

It is shown in this paper through simulation that the confinement loss of a down-doped solid-core photonic bandgap (PBG) fiber can be reduced to the level of 0.01 dB/km by only 84 air holes, with an air hole diameter to pitch ratio d/Λ=0.8 and air-filling fraction=35%. The practicality of solid-core PBG fibers is thus enhanced.

Introduction

Light confinement in photonic bandgap (PBG) fibers can be explained by PBG effect in the cladding wherein the average core index could be lower than the average cladding index.

In photonic crystal fibers (PCF) with infinite number of air holes, light is confined to the core region by full two-dimensional (2D) PBG effect and confinement loss (related to the imaginary part of propagation constant β) does not occur. In real PCF, however, there are only finite number of air holes in the cladding, and modes of such fibers are inherently leaky. Consequently, the confinement loss becomes an important issue.

Confinement loss in triangular air-guiding PBG fibers has been discussed in [1], and it is concluded that with d/Λ=0.9 (d and Λ are the air hole diameter and pitch, respectively), at least 20 rings of air holes (totally 1380 air holes, cf. Fig. 2 of [1]) are required to reduce the confinement loss to a level of 0.01 dB/km. Confinement loss in air-core PBG fibers has also been analyzed in [2], which has indicated that the confinement loss can be reduced to lower than 0.01 dB/km with d/Λ=0.97 and 8 rings of (totally 264) circular air holes (cf. Fig. 1(a) of [2]). However, it is very difficult to make air-core PBG fibers with so many air holes in the former case and so high a d/Λ for circular air holes in the latter case, respectively.

Confinement loss can be reduced by increasing the air-filling fraction of PCF, the air-filling fractions in [1], [2] are 73% and 85%, respectively. In order to increase air-filling fraction further, cobweb designs are used to reduce confinement loss in PCF, which can have very high air-filling fractions. Cobweb air-core PBG fiber with air-filling fraction of 94% has been reported in [3], [4], however, large air-filling fraction may cause splicing problems, and Fresnel reflection at the end facet may make it difficult to integrate such air-core PBG fibers with conventional fibers.

Honeycomb PCF can also guide light by a PBG effect [5], where a defect is created by the introduction of an extra air hole in a honeycomb cell. The presence of the central air hole affects the fundamental mode profile to have a ring-like shape, which may cause the problem for efficient coupling to the fiber due to mode mismatch. Confinement loss of this kind of honeycomb fibers has been discussed in [6], where calculations show that, even if there are 8 rings of (totally 456) air holes (cf. Fig. 4(a) of [6]), confinement losses are always higher than 10 dB/m.

In contrast to the above-mentioned PBG fibers, a novel solid-core honeycomb PBG fiber was proposed in [7], where a down-doped region has been used to form a solid-core defect. In this paper, confinement loss of this novel PBG fiber is roundly investigated. It is confirmed from computed results that confinement loss in this solid-core PBG fiber can be reduced to the level of 0.01 dB/km with only 84 air holes and air-filling fraction of 35%, therefore, it is much easier to integrate this solid-core PBG fiber with conventional fibers.

Section snippets

Simulations of confinement loss in solid-core PBG fiber

Air-core PBG fibers can be formed by replacing 7 (or more) air holes in the center of PCF with a large air defect, since the index of air-core is always less than that of cladding, all kinds of photonic crystal lattices are acceptable. If replacing the central air hole with silica, only index-guiding PCF can be formed. To ensure the PBG effect in solid-core PCF, the index of defect core must be less than that of cladding, which can be achieved by down-doping in the original silica area of PCF.

Conclusion

Confinement losses in solid-core PBG honeycomb fibers have been analyzed by multipole method. It is confirmed that down-doped core in honeycomb fibers can guide light by PBG effects, and its confinement loss can be reduced to a negligible level with only 84 air holes in cladding (d/Λ=0.8) and air-filling fraction 35%. Compared with air-core PBG fibers, solid-core PBG fiber has lower air-filling fraction and guides light more easily. These simulations foreshow the potential practicality of

References (12)

  • K. Saitoh et al.

    IEEE Photon. Technol. Lett.

    (2003)
  • Y. Xu et al.

    Opt. Lett.

    (2003)
  • N. Venkataraman, M.T. Gallagher, C.M. Smith, D. Müller, J.A. West, K.W. Koch, J.C. Fajardo, ECOC Proc., 2002,...
  • D.G. Ouzounov et al.

    Science

    (2003)
  • J.C. Knight et al.

    Science

    (1998)
  • D. Ferrarini et al.

    Opt. Exp.

    (2002)
There are more references available in the full text version of this article.

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