# Switchable Fiber Ring Laser Sensor for Air Pressure Based on Mach-Zehnder Interferometer
A gas pressure sensor based on a TCFMI has not been reported in the literature, having as advantages the dimension of the configuration, a pressure range of 45 PSI, and the capability of implementation as a sensor and as a switchable fiber ring laser sensor with a selection of focus areas. Another experimental work presents a robust optical fiber modal Mach-Zehnder (MZ) interferometer, which is embedded in a fiber ring laser system, as a tunable filter to improve the detection limit (DL) due to its narrow full-width at half-maximum (FWHM) and high signal-to-noise ratio (SNR) [41]. The principal difference is based on the sensing element, comprising two cascaded up-taper joints fabricated via showing fusion. Likewise, there are works in which the authors use fiber ring laser demodulation technology [42]. Alternatively, some papers propose a fiber laser sensor based on a ring cavity configuration. The cavity is operated by a core-offset Mach-Zehnder interferometer, which is used as a wave selection filter and temperature detection device. Here, the proposed fiber laser sensor exhibits sensing properties, such as a sensitivity of 0.03119 pm/\({}^{\circ}\)C, a dynamic range of 90 \({}^{\circ}\)C, and a signal-to-noise ratio of 52 dB [43], as well as a highly stable and switchable dual-wavelength laser using a coupled microfiber Mach-Zehnder interferometer as an optical filter [44]. Moreover, ring fiber laser sensors have been employed to measure various parameters. These include gas pressure using a Fabry-Perot interferometer and a Sagnac Interferometer [45], temperature measurements utilizing fiber Bragg gratings (FBG) [46], gas concentration sensing based on a hollow-core photonic crystal fiber [47], and distributed pressure sensing employing a mode-locked laser [48]. Based on comparisons with previous research described in the Introduction section, we present a switchable fiber ring laser sensor for air pressure using an MZI, where the MZI implemented is made with a section of thin-core optical fiber spliced between two sections of standard single-mode fiber, using as a broad spectrum light source the amplification spontaneous emission (ASE) of an ytterbium-doped fiber pumped with a 980 nm laser diode, and obtaining an emission spectrum of 1020 to 1200 nm; this range of wavelengths helped us to characterize our interferometer observing the interference fringes. A ring laser was implemented in the region close to one micron of operation; inside the loop, the Mach-Zehnder interferometer was added and fixed inside the gas cell, thus achieving an air pressure laser sensor. This laser sensor obtained relevant results by varying the air pressure inside the cell, obtaining different emissions of wavelengths. The basic principle of operation of our sensor is that by introducing the interferometer inside the cell and applying air inside it, the interferometer undergoes stress due to pressure; this stress causes the refractive index of the interferometer fiber to change. The power spectrum responds to fluctuations in air pressure, and the device can measure within a range of 5 to 40 PSI while maintaining wavelength stability. Finally, this paper's novelty is based on investigating pressure gas sensing using a thin-core fiber modal interferometer (TCFMI) composed of a single-mode fiber and a thin-core fiber. Integrated into an ytterbium-doped fiber (YDF) ring fiber laser, it responds effectively to chamber pressure changes, controlling the laser cavity response with wavelength shifts and amplitude variations. The interferometer also functions as a wavelength-selective filter, achieving a remarkable side mode suppression ratio (SMSR) of 34.17 dB.