# High Birefringence D-Shaped Germanium-Doped Photonic Crystal Fiber Sensor

## TL;DR

This paper proposes a high-performance sensor using a D-shaped germanium-doped photonic crystal fiber for detecting changes in refractive index with high sensitivity and accuracy.

## Contribution

The novel design introduces a D-shaped germanium-doped PCF sensor with high birefringence and improved sensitivity for surface plasmon resonance.

## Key findings

- The sensor achieves a maximum birefringence of 1.06 × 10−2 and a resolution of 1.78 × 10−5 RIU.
- Optimal wavelength sensitivity of 5600 nm/RIU is observed at a refractive index of 1.37.
- Two distinct surface plasmon polariton coupling modes are produced on the polished surface.

## Abstract

In this work, a surface plasmon resonance (SPR) sensor based on a D-shaped germanium-doped photonic crystal fiber (PCF) is proposed. The finite element method (FEM) is introduced to analyze the structure parameters, such as germanium-doped concentration, lattice pitch, and air hole size. In addition, the coupling properties and birefringence properties of PCF are also studied. The computer simulation results indicate that two different surface plasmon polariton (SPP) coupling modes are produced on the polished surface, covered with metal film, when the analyte refractive index (RI) is 1.34. Then, with the increase of the RI, the incompleteness of one of the coupling modes will be transformed into the complete coupling. The effect of germanium concentration on the birefringence is also analyzed. It has an optimal wavelength sensitivity of 5600 nm/RIU when the RI is 1.37. This sensor exhibits a maximum birefringence of 1.06 × 10−2 and a resolution of 1.78 × 10−5 RIU with high linearity.