# Comparison of the Sensitivity of Various Fibers in Distributed Acoustic Sensing
## 2 Approach
### Setup  
Testing conditions are crucial when studying the acoustic sensitivity of various fibers. To enable a fair comparison, the testing parameters must be as consistent as possible across all FUTs, with fiber tension and distance to the sound source being the main considerations. Initial methods, such as laying the fiber in a groove beneath the acoustic source (Figure 3a) or attaching the fiber directly to it (Figure 3b), were deemed unreliable. Therefore, a setup with two pulleys, two fiber spools and a load (Figure 3c) was selected to transmit sound waves reproducibly from the source to the fiber. A 2 m segment of the FUT was fusion-spliced to a 440 m coil 1 of Corning SMF-28 fiber to ensure that acoustic perturbations occurred far from any reflective distortions at the input face. This configuration also ensured minimal transmission losses, enabling equal probing for all FUTs. The segment was then passed through half-pulley 2, with a flat face firmly attached to sound source 3, ensuring a uniform distance between the FUT and the sound source across samples, and minimizing acoustic wave losses by using a solid medium rather than air. Next, the FUT segment was placed under pulley 4 with a 60 g load 5. The load was selected to apply sufficient tension to the FUT without introducing additional probing  
Figure 3: Variants of test setups considered for the DAS experiment. (**a**) Fiber laid in a groove beneath the acoustic source; (**b**) fiber directly attached to the acoustic source; (**c**) a complex setup with the fiber loaded using pulleys and weights to ensure consistent tension and sound transmission. light losses, thereby providing consistent and reproducible tension across all FUTs. Finally, the segment was fusion-spliced to another coil 6 of 412 m of Corning SMF-28, ensuring a sufficient distance between the end-face reflection disturbance and the acoustic event. The interrogation system used Rayleigh scattering and phase-sensitive OTDR technology [14] to detect acoustic events, utilizing a \(\varphi\)-DAS variant with direct detection (no phase demodulation). The setup included a Connet CoSF-D-ER-M (Connet Laser Technology Co., Ltd., Shanghai, China) laser source with 15 mW of output power, a 1550 nm wavelength and 1 kHz of linewidth; an SSP-1550-200-PM-FC (Special Systems. Photonics, LLC, Saint-Petersburg, Russia) acousto-optical modulator (AOM); an Amonics AEDFA-23-M-FA Erbium-doped fiber optic amplifier (Amonics Ltd., Hong Kong, China); an optical circulator (Advanced Fiber Resources, Ltd., Zhuhai, China); an Amonics Ltd., Hong Kong, China) AEDFA-PA-35-M-FA Erbium-doped fiber optic booster (pre-amplifier); a Thorlabs PDA 10D-EC photodetector (Thorlabs Inc., Newton, NJ, USA) and a La-n1 usb (Rudnev-Shiyaev, LLC, Moscow, Russia) analog-to-digital converter (Figure 4). Probing light pulses had a duration of 40 ns and a repetition time of 10 \(\upmu\)s. The amplifier output light power was 30.5 mW, while the preamplifier increased the power of the backscattered signal to 1.99 mW. The ADC had the sampling frequency of 500 MHz, yielding a spatial resolution of approximately 0.2 m, and a buffer capacity allowing for acquisition of up to 838 DAS traces in series. The sound source was a 30 mm, 8 Ohm resonant speaker. As the acoustic sensitivity of optical fibers can vary with the frequency of the incident acoustic wave, the continuous laser radiation was converted into light pulses with the AOM and amplified by the EDFA. The light was then sent through the circulator to the FUT, where it underwent Rayleigh scattering due to small inhomogeneities within the fiber that cause intrinsic light loss. A narrow laser linewidth enabled interference within the pulse, as scattering components interacted. The resulting light intensity depended on the spacing between scattering centers and the refractive index of the medium between them. An acoustic signal modulated these parameters, so the scattered light from that position was modulated at the same frequency.