RADIO PHYSICS AND RADIO ASTRONOMY

Subject and Purpose. The use of the two-mode operation of an optical fi ber in fiber-optic sensors simplifies the interferometerdesign and enhances the reliability of sensing.
 

Methods and Methodology. The mechanism behind the appearance of intermode phase shifts of the light modes Lp₀₁ and Lp₁₁ under various types of optical fiber deformation is examined. A mathematical model of a hydroacoustic sensor based on a two-mode optical fiber is proposed and experimentally validated. The intermode phase-acoustic sensitivity of the optical fiber to hydroacoustic pressure is evaluated for the Lp₀₁ and Lp₁₁ modes. A segment of a multimode optical fiber is fusion-spliced to the output of the two-mode fiber. This method creates conditions for the interference of the Lp₀₁ and Lp₁₁ modes and increases the modulation depth of the light flux at its output.

Results. The heart of a hydroacoustic pressure-sensitive element in this research is an optical fiber with a core diameter of 18 μm and a numerical aperture of 0.0592. With the use of a 0.86 μm laser, propagation conditions are created for the two modes, Lp₀₁ and Lp₁₁ . The alteration of the mode content when a mode mixer is installed at the input of the optical fiber has been studied. For the fiber-optic sensor with a signal-to-noise ratio of 1:1 at the photodetector output and a frequency bandwidth of 100 Hz, the minimum detectable acoustic pressure inside the sensitive element is 55 Pa, and the intermode phase-acoustic sensitivity is 5.2*10^-9 Pa.

Conclusions. It has been shown that the two-mode operation of the optical fiber enables the development of interferometric fiber-optic sensors that are sensitive to mechanical deformations of the fiber. These sensors are characterized by a simpler design compared to traditional fiber-optic sensors based on Mach–Zehnder interferometers. It has been demonstratez that using a single-mode optical fiber with a reduced optical radiation wavelength ensures propagation conditions for the two lowest-order modes of the fiber.

Keywords: optical fiber, fiber-optic sensor, intermode interference, phase-acoustic sensitivity, pressure

Manuscript submitted  25.07.2025

Radio phys. radio astron. 2025, 30(4): 276-284

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