(a) Field of the Invention
This patent relates to satellite navigation, and more particularly to a new kind of mechanical buffer based polarization-maintaining photonic crystal fiber (PM-PCF) with low magnetic sensitivity for the fiber optic gyroscope (FOG) application.
(b) Description of the Related Art
Compared with conventional optical fibers, the photonic crystal fiber (PCF) contains many silica-air microstructures in its core region, which determine the fiber's optical characteristics. Therefore, by adjusting the geometric parameters of these microstructures, the PCF could be flexibly designed for the given purpose. FIG. 1 shows the PM1550-PCF 10a, which contains in the core region 11 (at least) three layers of air-holes 111 with diameter d and spacing Λ, while two air-holes 112 are enlarged to diameter D.
The arrangement of air-holes in the core region decides the fiber's optic parameters. During the fabrication process of the fiber optic gyroscope, the fiber could suffer from coiling, bending, twisting and temperature-induced stress, which will change silica's refractive index (elasto-optical effect), and affect the PCF's optical characteristics. FIG. 2 shows two kinds of forces on the fiber. Because of the low mechanical strength of silica, when the fiber is under external force, the refractive index of the silica will vary due to the elasto-optical effect. What's more, the arrangement of air-holes in the core region will be changed as well. Thus the fiber's optical characteristics are no longer reliable.
The factors above will influence the PCF's performance, increasing the fiber's magnetic sensitivity and magnetic errors. So far, the general solution to this problem is provided by adding a magnetic shield, which accounts for more than ⅓ of the weight in some small or medium gyroscopes, thus greatly restricting the application of the fiber optic gyroscope.