Design of an ultrabroadband and compact filter based on square-lattice photonic crystal fiber with two large gold-coated air holes

https://doi.org/10.1016/j.photonics.2020.100816Get rights and content

Highlights

  • A compact, ultrabroadband, and single-mode single-polarization photonic crystal fiber (PCF) filter based on surface plasmon resonance (SPR) is proposed.

  • Compared with some reported SPR-based PCF polarization filters, this filter can operate in the 1.31 and 1.55 μm communication windows, and achieve a wider bandwidth over a shorter fiber transmission distance.

  • When the fiber length is only 700 μm, the bandwidth can reach 730 nm, from 0.97 to 1.70 μm, covering all communication bands from 0 to U.

  • With the increase of wavelength, the birefringence of the PCF filter and the confinement loss difference of x- polarized and y-polarized core modes increase, which is of great importance for the study of a broadband and single-mode single-polarization PCF filter.

  • The designed PCF is deformed from a square lattice, which is easy to stack and fabricate.

  • The fiber exhibits excellent tolerance to the changes of structural parameters and thickness of the gold film, which further reduces the difficulty of fabrication and also facilitates its application in experiments and practice.

Abstract

An ultrabroadband and compact square-lattice photonic crystal fiber polarization filter based on surface plasmon resonance is proposed. Four large air holes are introduced near the core to destroy the symmetry of the original fiber structure so that the resonance wavelengths of x- and y-polarized core modes can be well separated. The inner walls of two large air holes along the x polarization direction are coated with gold film to generate the surface plasmon mode. In addition, to further enhance the coupling resonance strength between the core mode and the surface plasmon polariton mode, the six air holes near the gold-coated air holes are replaced by solid glass rods. Then the dispersion characteristics and loss spectrum of the proposed polarization filter are studied by the finite-element method. The results show that when the fiber length of the proposed filter is only 700 μm, ultrawideband y-polarized light transmission with a bandwidth of up to 730 nm in the wavelength range from 0.97 to 1.70 μm can be achieved. The effective filtering range covers all optical fiber communication bands from O to U. Moreover, the filter has excellent structural tolerances. These excellent properties are expected to make it a good candidate for a broadband polarization filter.

Introduction

Photonic crystal fiber (PCF) [1] is also known as microstructure fiber, and its concept was first proposed by St. J. Russell et al. PCF is a two-dimensional photonic crystal with line defects. The cladding is composed of regularly distributed air holes. The fiber core consists of silica or air holes to form line defects, thereby confining the transmission of light in the core by utilizing its local light capability. This silica-air composite PCF has a large refractive index contrast and a highly controllable periodic refractive index change, which provides a new mechanism for light transmission and control. According to the light guiding mechanism, PCF can be divided into two types: (1) photonic bandgap fiber [2], which relies on a photonic bandgap effect to propagate light in the fiber core; (2) total internal reflection photonic crystal fiber [3], whose core refractive index is greater than the average refractive index of the cladding, making it dependent on total internal reflection to confine the propagation of light in the core. Compared with conventional optical fibers, PCF has many unique properties, such as flexible structural design, endlessly single mode, controllable nonlinearity, excellent birefringence, and adjustable singular dispersion. These amazing characteristics make PCF widely used in optical communication, optical devices, optical sensors, and advanced lasers.

Through in-depth theoretical and experimental research, researchers have been surprised to find that if the air holes of PCF are filled with some functional materials, the transmission performance of the fiber can be further improved, thus effectively expanding the application scope of PCF. PCF polarization filters based on metal-filled or metal-coated holes have been reported. Li et al. [4] designed a triangular-lattice PCF polarization filter with two small holes in the core. The fiber has a length of 1 mm and a bandwidth of 235 nm near the communication window of 1.55 μm. Liu et al. [5] produced a polarized filter with a length of 3 mm and a wide bandwidth of 430 nm by selectively coating the inner walls of the two cladding air holes with gold film. Shi et al. [6] designed a single-polarization PCF filter with two large gold-coated holes, achieving a bandwidth of 150 nm with a fiber length of 4 mm. Li et al. [7] proposed a dual-wavelength single-polarization PCF filter by changing the size of the air holes near gold-coated air holes. When the transmission length of the optical fiber was 10 mm, a bandwidth of 440 nm was obtained. However, to the best of our knowledge, most of the reported PCF polarization filters operate only in a single communication window of 1.31 or 1.55 μm [4], [6], [8]. Although some can work simultaneously in the two aforementioned communication windows, the fiber length is relatively long and the bandwidth needs to be further increased to meet the demand for large communication capacity and highly integrated optical devices [5], [7].

In this article, a compact and ultrabroadband PCF polarization filter based on two gold-coated cladding holes is proposed. The electric field distribution and polarization performance of the polarization filter are studied by the full-vector finite-element method. The structure of the PCF is derived from the deformation of a square lattice. When the incident photon frequency is equal to the frequency of the surface plasmon, resonance occurs. As the wavelength increases, the difference in refractive index and resonance loss between the x and y polarization modes becomes larger and larger, which is of great significance for obtaining a wide bandwidth and high-performance single-mode single-polarization PCF filter. The designed filter also has excellent structural tolerance.

Section snippets

The structure and basic theory

The cross section and stacked preform of the proposed ultrawideband single-polarization PCF filter are shown in Fig. 1. From Fig. 1a, the lattice constant Λ between two adjacent holes is 2 μm. The diameters of the air holes from small to large are denoted by d, d1, and dm, with values of 1.2, 1.4, and 2.4 μm, respectively. According to the theory of waveguide optics, air holes near the core of PCF play an important role in controlling its mode field. To generate the surface plasmon resonance

Simulation results and analysis

Fig. 2 shows the dispersion relationship between the core mode and the SPP mode and the confinement loss of the core modes as a function of wavelength. As is clear from Fig. 2a, the effective refractive index of the x-confinement loss of the core modes as a function of wavelength. As is clear from Fig. 2a, the effective refractive index of the x- and y-polarized core mode and y-polarized core mode decreases with the increase of wavelength. At long wavelengths, the effective refractive index of

Effect of structural parameter changes on the characteristics of the proposed PCF

The lattice structure and structural parameters of PCF have a very important influence on the mode field distribution of the fiber. Therefore, in Fig. 5, Fig. 6, Fig. 7, we show the effects of air hole diameter d1, outer diameter dm of gold-coated holes, and thickness of the gold film t on the dispersion and mode loss of the proposed PCF filter. In addition, we also studied the effect of the number of gold-coated air holes on the confinement loss of the PCF in comparison with other reported

Conclusion

A compact, broadband, and single-mode single-polarization PCF filter based on two large gold-coated air holes was proposed, and its polarization filtering performance was studied by the full-vector finite-element method. The results show that x- and y-polarized light can be well separated. Moreover, the 700 μm long fiber has a bandwidth of up to 730 nm, from 0.97 to 1.70 μm, covering all the optical fiber communication bands from O to U. Furthermore, the proposed polarization filter has high

Acknowledgments

This work was supported by the National Key Research and Development Project (Grant No. 2019YFB2204001), the Program of the Natural Science Foundation of Hebei Province (Grant Nos. F2017203193 and F2017203110), and the Postdoctoral Preferred Funding Research Project of Hebei Province (Grant No. B2018003008).

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