The optical polarizer is a crucial component widely used in many optical systems and applications. Fiber-optic polarizers have the merits of excellent compatibility and ease of integration with other fiber components. We report an in-line polarizer enabled by a 45° tilted fiber grating inscribed into a specialty fiber for the next generation fiber-optic gyroscope, i.e., a 40-µm ultra-fine-diameter tiger-type polarization-maintaining fiber with which the size of fiber-optic sensors can be miniaturized. The results show that a 40-mm-long polarizer operates at a center wavelength of around 830 nm with high-performance characteristics, such as a polarization extinction ratio exceeding 30 dB, a low insertion loss of less than 1.5 dB, and a large 3-dB optical bandwidth more than 60 nm. This kind of fiber-optic polarizer may have a broad scope across applications and systems such as fiber lasers and sensors, especially high-precision fiber-optic gyroscopes.