Objective  Brillouin dynamic grating (BDG) based fiber optic sensing was first proposed in 2008. BDG can effectively separate the pump light, detection light and reflected light to improve the measurement accuracy and spatial resolution while achieving simultaneous sensing of temperature and strain, etc. Both ambient temperature and pressure changes can lead to changes in the birefringence coefficient of the fiber, which in turn can change the birefringence frequency shift of BDG. Therefore, temperature and pressure sensing can be achieved based on the birefringence frequency shift of BDG. The structure of the photonic crystal fiber has a large impact on the birefringence coefficientnd. And its mechanics, thermal, and optical properties can be optimized by designing the shape and arrangement of the air holes. The existing distributed transverse pressure sensors have problems such as low sensitivity and small temperature measurement range. Therefore, a new multilevel structure of photonic crystal fiber is designed in this paper.

  Methods  The core area of the designed new multilevel structure photonic crystal fiber is surrounded by two types of elliptical air holes, and all air holes are stacked along the y-axis and symmetric about the x-axis (Fig.2). The geometric model of the photonic crystal fiber is constructed according to the fiber structure, and the fiber length is set to 1 m. Then, the two-dimensional model of the fiber is numerically analyzed by the finite element method, and the temperature study range is extended to −100-100 ℃ with the applied transverse pressure of 0-40 MPa. The birefringence frequency shift at different temperatures and pressures is calculated at low temperatures and high pressures to study its temperature and pressure sensing characteristics. To improve the reliability of the results, we added a perfect matching layer (PML) outside the fiber for simulating the boundary absorption conditions, and then used extra fine accuracy for meshing. Finally, the model is solved.

  Results and Discussions   The results show that in the temperature range of 0-100 ℃ and pressure range of 0-40 MPa, the pressure sensitivity in the x-axis direction of the photonic crystal fiber is about −1.004 GHz/MPa, and the pressure sensitivity in the y-axis direction of the photonic crystal fiber is about 1.021 GHz/MPa. The temperature sensitivity is about 0.554 3 MHz/℃ when the applied temperature ranges from −100 ℃ to 100 ℃ and applied pressure ranges from 0 MPa to 40 MPa, and it has temperature insensitive characteristics. For every 10° change in force application angle, the change in pressure sensitivity is greater than or equal to 75 MHz/MPa, which is sensitive to the force application angle.

  Conclusions  A new type of photonic crystal fiber with multilevel structure is proposed, which is made of pure silica, and the core area is surrounded by two types of elliptical air holes, all of which are stacked along the y-axis and symmetric about the x-axis. The two-dimensional model of the fiber is numerically analyzed using the finite element method to study its temperature and pressure sensing characteristics at low temperatures and high pressures. It is concluded that the pressure sensitivity in the x-axis direction of the photonic crystal fiber is about −1.004 GHz/MPa, and the pressure sensitivity in the y-axis direction of the photonic crystal fiber is about 1.021 GHz/MPa for pressures ranging from 0 MPa to 40 MPa and temperature ranging from 0 ℃ to 100 ℃. The temperature sensitivity is about 0.554 3 MHz/℃ when the applied temperature ranges from −100 ℃ to 100 ℃ and applied pressure ranges from 0 MPa to 40 MPa, and it has temperature insensitive characteristics. The new photonic crystal fiber proposed in this paper is suitable for high-sensitivity monitoring in high-temperature and high-pressure environments such as subsea and civil engineering.