This work shows a study of pressure gas sensing using an optical fiber interferometer inside a gas cell; here, a thin-core fiber modal interferometer (TCFMI) is fabricated using two distinct types of fiber, a single-mode fiber (SMF model 1060XP) and thin-core fiber (TCF model 460 HP). This interferometric structure is set into a ring fiber laser with an ytterbium-doped fiber (YDF) pumped with a 980 nm laser diode. The TCFMI interference spectrum shows wavelength shifting and amplitude variations as the chamber pressure is altered in the range of 5 to 40 PSI; these changes control the ring fiber laser cavity response linearly and lead to good stability in its wavelength operation in a range of 30 nm. The proposed interferometer operates as a wavelength-selective filter in the fiber ring laser cavity. The single laser emission shows a side mode suppression ratio of 34.17 dB. The fiber ring laser is a reliable alternative for air pressure sensing applications.