Sensitivity of fiber‐based modalmetric devices intended for optical detection of acoustic signals

• Published in: Microwave and optical technology letters 2020-03, Vol.62 (3), p.999-1008
• Main Authors: Freitas, Taiane A. M. G., Silva, Viviane H., Ribeiro, Ricardo M.
• Format: Article
• Language: English
• Subjects: acoustic communications; acoustic monitoring; Bragg gratings; Carrier frequencies; Devices; Diameters; modal interferometer; modalmetric; Optical communication; optoacoustic receiver; Sensitivity; ultrasound
• ISSN: 0895-2477; 1098-2760
• DOI: 10.1002/mop.32136

The basic structure of fiber‐based modalmetric devices consists of a single‐mode fiber (SMF) spliced to a sensitive multimode fiber (s‐MMF) strand. The aim of this article, is to show and compare the sensitivity measurement results amongst of distinct implementations of lumped multimode interference fiber‐optic reflecting (R‐MMI) and transmitting (T‐MMI) modalmetric detectors, and a fiber Bragg grating (FBG) device, each one working as uncalibrated detector of ultrasound acoustic‐signals. All experiments were performed at 42.9 kHz carrier frequency and most of them were at 1551.5 nm wavelength. In all cases, the interaction length of the s‐MMF for a single traveling was kept at 50 mm and the fiber ends of R‐MMI samples were not mirrored. The sensitivity was observed to increase when: the modalmetric device was used instead of the FBG; the core diameter of s‐MMF was increased from 50 μm to 62.5 μm; the SMF/s‐MMF connection was angularly misaligned from 0° to 8°; the feeding optical power level was increased; the interaction length of s‐MMF was doubled and the wavelength probe was reduced from 1551.5 nm to 1520 nm. The dependence of the modalmetric devices' transmission with the length of the s‐MMF and wavelength was numerically calculated. It was concluded that modalmetric devices are highly sensitive and can be further improved yet, but have a stochastic nature in their present implementation.