Fiber Bragg grating temperature insensitive filter based on bimetal structure

  Objective   Fiber optic gyroscopes (FOG) are a new class of instrument capable of accurately determining the orientation of moving objects, which have been widely used in tactical missile guidance, land traffic navigation and aerospace attitude adjustment because of their long service life, easy integration and small size, etc. However, the changing operating temperature in these application fields seriously reduces the average wavelength stability of the broad spectrum light source in the fiber optic gyroscope, thus hindering the further application of the fiber optic gyroscope. To improve the average wavelength stability of the fiber optic gyro wide spectrum light source, this study proposes a method to make a temperature-insensitive filter with a bandwidth of 11.77 nm using a 60 μm ultrashort fiber grating.

  Methods   The bimetallic temperature compensation structure (Fig.1) is designed based on the thermal expansion coefficient difference between two metal materials. This structure will compress/stretch the fiber grating with the increase/decrease of temperature and effectively compensate for the wavelength change of fiber grating caused by the thermo-optic effect. The filter is fabricated by combining metal material with fiber grating in two-point packaging, and the effects of the thermal expansion coefficient of the material and the geometric parameters of the filter on the temperature sensitivity of the filter are systematically studied.

  Results and Discussions   The results show that the temperature sensitivity of the filter is mainly affected by the thermal expansion coefficient and length of the substrate and the strain transfer beam. The largest adjustable range of L₁/L₂ (the ratio of the length between two fixed points on the substrate and the length of the filter fiber grating) is achieved as the thermal expansion coefficient difference is −5.8. Besides, the temperature sensitivity of the filter exhibits a negative linear relationship with the value of L₁/L₂. According to the comparison of the control variable method, it is found that the adjustable range of L1 is larger, and the value range of L₁ is 7 times that of L₂, which is more conducive to the packaging operation. The base of the bimetal structure is made of brass with a high coefficient of thermal expansion, and the transfer beam is made of aluminum with a low coefficient of thermal expansion. At last, the temperature-insensitive filter with a size of 74 mm×6 mm×4 mm was prepared (Fig.4). When the L₁/L₂ is 8.39, the base length is 67.1 mm and the filter grating length is 8 mm, the temperature sensitivity coefficient of the fiber grating is 0.15 pm/℃ and the wavelength change is only 4.5 pm in the range of 30-60 ℃, which is more than 60 times lower than that before compensation (Tab.6).

  Conclusions   In this study, a bimetallic temperature compensation structure has been successfully designed, and a temperature-insensitive filter with a bandwidth of 11.77 nm has been fabricated using a 60 μm ultrashort fiber grating (Fig.3). The temperature insensitivity filter can effectively improve the temperature insensitivity of the fiber grating, which can be used as a light source filter to improve the average wavelength stability of the light source and is expected to be used in high-precision fiber optic gyroscopes.