A novel in-line Fabry-Pérot interferometer is directly fabricated in a single mode fiber by the use of femtosecond lasers. The spectrum fringe of this interferometer has the visibility of ～5 dB due to the rough and slopped cavity surfaces, making it more difficult for post signal processing and reducing the measurement accuracy and reliability. By choosing the optimal parameters, improving machining environments and methods, ideal Fabry-Pérot cavities are fabricated, which are so smooth and parallel that the fringe visibility is as high as ～18 dB. In addition, with an accessible cavity and all-fiber structure, this sensor can work in high temperatures up to 800 ℃, which is attractive for sensing applications in harsh environments.