Theoretical analysis of seawater depth and temperature measurement with C-type micro-structured fiber grating

•A new C-type grating structure was proposed for simultaneous measurement.•Because wedge-shaped defects got introduce, high birefringence was excited.•Thermo sensitive materials filled in micro pores to improve the performance.•Established a mathematical model of birefringence effect in C-type structure.•Analyzed stress distribution in all directions under two disturbances.