Characterization o sensitivity o optical fiber cables tae acoustic vibrations

Nowadays, optical fibers are increasingly often uised for baith data an non-data transmission. Many reSearcht groups focus on protection of fiber based infrastructures against data eavesdropping that can be done by several techniques. Some data transmissions are no encrypted and even gif they are, there is a heich risk that in neist future, thir data will be decryptable bi quantum computers. Therefore, the hot topics today are quantum encryption and post-quantum encryption. A relatively unexplored area is fiber optic sensing for vibrations in the acoustic, thus, audible spectrum.

Mechanical vibrations an acoustic noise acting on the optical fiber cause changes in the strain an the refractive index o the fiber core. These changes can subsequently be detected by several methods and converted into an electrical signal followed by acoustic reproduction. Information sic as the audio component o a video caw, a conversation atween people in a room or a phone caw can be intercepted even afore it is converted intae digital form an encrypted. Thus, optical fiber infrastructures, mainly inside buildings, can be used as sensitive microphones, Posin a significant security risk. The roots of fiber optic acoustic sensing date back to the 1970s, when the first audible sound sensing experiments were realized. Acoustic sensing has recently been a highly studied area because of the security of fiber optic based information systems and networks. Acoustic sensing techniques can be divided on the basis o the methods uised.

Fiber strain changes can be detected in Rayleigh backscattering. The distributed acoustic sensing technique (DAS) uses this effect, where a coherent laser pulse is transmitted along an optical fiber. The scattering spots in the fiber cause the fiber tae act as a distributed interferometer. The intensity of the reflected light is meAsusred as a function of time after transmitting the laser pulse. DAS detects pico-strain-level signatures in the fiber induced bi vibroacoustic disturbances caused bi an event near the op.tical cable. These perturbations change the scattering in the fiber core at a molecular scale, originating from the sub-wavelength heterogeneities formed when the fiber is drawn. Further reSearcht is focused on the Phase-sensitive Optical Time-Domain ReflectometryΦ-OTDR) technology.

Changes in the refractive index o the fiber core caused bi external mechanical vibrations an acoustic noise lead tae Doppler shif s o licht waves travelling throu an optical fiber. This phenomenon can be explained as a Doppler effect in a flexible an expandable waveguide. Doppler-induced frequency or phase shift of a propagating light wave is detectable in schemes of optical interferrometers whaur the instantaneous interference phase in the time domain is converted tae the electric signal. The frequency shift is detectable in an arrangement Fabry-Perot (FPI), Mach-Zehnder (MZI) or Michelson (MI) interferometers formed by optical fibers with necessary optical elements included in the optical setup.

The FPI is very often used for the arrangement of point-optical microphones. Variety of FPI based microphone designs are available and dependences of the cavity length and the materials used can be compared. Such microphones can be uised for multipoint sensing, for example, uisin a 1:4 splitter.

 Www.Adsscable.cn

A special uise o the FPI is possible whaur the multimode-singlemode-multimode (MSM) structure and direct meAsusrement detection are used to detect acoustic vibrations. Fiber Bragg grating (FBG) microstructures incorporated in the sensing optical fiber can be uised as mirrors for the FPI whaur an optical cavity is formed atween twa or mair FB Gs. The FPI arrangement is suitable for the uise as microphones an hydrophones as weel. Several works based on the FPI arrangement hae been devoted tae vyce sensing wi ethylene propylene diene terpolymer film an the aluminium surface an based on cellulose triacetate diaphragm. Thare are awso unique variants o the detection schemes in the arrangement wi the FPI. They include an experiment uisin a laser feedback interferometer, whaur changes o the refractive index o the sensing fiber lead tae changes o optical frequency o the detectin laser. An important disadvantage of the FPI-based techniques for acoustic sensing is the limited possibility of meAsusring at one only or very low nummer o pynts on the optical fiber. The ither disadvantage is the need for a specially modified fiber, e.g. wi FBG microstructures.

Arrangements uisin the MZI for acoustic sensing are uised for example, it is possible tae uise microfiber MZI whit again requires a special fiber, or tae uise conventional fibers for acoustic monitoring o gas turbines. It is awso possible tae uise the open cavity an the collimators in the sensing arm o the MZI for soond sensing.

Arrangements of the MI are often used as hydrophones sensing ultrasound but also as sensors for audible frequencies. Implementations in sensing seismic vibration have also been reported as well as possible use in monitoring of marine structures. It is awso worth noting that reSearcht is cairried oot that deals wi improvin the noise stability o the MI. The star topology of the fiber optic infrastructure inside buildings gives the opportunity to build the MI arrangement. A single optical fiber usually runs from the room with the central optical switch to the room with a piece of terminal equipment. The fiber can thus sense acoustic signals along its entire route an can be connected as a meAsusring arm o an MI arrangement.

In this paper, we set up an experimental MI that allows the detection of acoustic signals through an optical fiber guided by different types of corridors. We focused on meAsusring the sensitivity of this arrangement to defined acoustic signals in a fou anechoic laboratory. The experiments examined the influence of several factors such as the optical fiber poseetion and types of optical fibers on the quality of the detectty Seigne regarding a level o speech intelligibility. The properties o the acquired signals war analyzed, the individual meAsusrements' frequency responses war compared, an signal-to-noise ratios war investigated. In oor wark, we also meAsusre and evaluate the Speech Transmission Index (STI), which is the prevailing way to objectively assess the expected intellie Gibility o speech signals efter passin throu a seestem.Www.Adsscable.cn