# Comparison of the Sensitivity of Various Fibers in Distributed Acoustic Sensing
## 1 Introduction  
Fiber optic distributed acoustic sensors (DASs) were introduced more than 40 years ago [1]. Nevertheless, these systems are still relatively expensive. The optical and optoelectronic industry offer numerous devices with either enhanced or cost-optimized characteristics every year [2, 3, 4, 5], including specialty optical fibers. The standard single-mode telecommunication optical fiber is one of the most popular sensing elements for DASs. A lot of specialty fibers were invented recently and are commercially available. They may vary in their methods of production, the material and structure of their core, their cladding and coating, and so on [6]. A limited number of them have undergone performance tests in DAS setups and for acoustic event sensitivity [7]. On the contrary, there are many studies devoted to DAS interrogation system and data processing software modification [8, 9, 10, 11]. Since 1977, DASs have become a well-known, efficient instrument in oil, gas and other fossils fuels' exploration, transportation and processing industries [12, 13, 14], as well as in structural health monitoring [15] and perimeter security [16]. Along with DASs, common customers and manufacturers in other industries have continued to develop uses for DASs. This resulted in a rise in new potential applications for DASs. For instance, DASs are believed to be highly perceptive in performing nondestructive testing of details and structures as an inherently distributed sensor [17, 18, 19] that is also immune to electromagnetic interference, corrosive and explosive environments. DASs are reported to be indispensablefor red palm weeevil monitoring [20], as no trained dogs, x-ray setups or means of visual control can provide such efficiency. DASs seem to be perfectly suitable for the health monitoring of each individual plant in a large plantation [21], the tracking of animals and their groups [22] and environmental event monitoring [23; 24]. Moreover, such sensors can be successfully applied for voice commands and acoustic signal recognition in smart homes and manufacturing concepts [25; 26] and even in sound design to record music and concerts, which is especially convenient when the fiber covers all of the concert hall perimeter, as it allows us to focus on one or another acoustic feature by choosing a certain spatial point along the fiber at the stage of the recording or postprocessing [27]. A common feature of these industries--biology, ecology, agriculture, flaw detection, voice recognition and sound design--is having a budget that is usually smaller than that in oil, gas, defense and engineering areas. Moreover, the specifications of DASs commonly used by the latter can be slightly or even drastically different from the ones required by new potential applications, for example, the acceptable acoustic frequency range of the events to be detected, the sensitivity, the sensing element length, etc. This is where a need for DAS optimization arises. By improving the software and choosing the most suitable combination of hardware for a certain perspective application, we can make this sensor system more affordable and increase the number of its users. An extremely simple DAS model consists of a sensing fiber (cable) and interrogator (Figure 1). The latter includes a radiation source, one or more optical amplifiers (if required), an optical circulator or splitter, a photodetector and a data acquisition device. It is no surprise that lots of scientists have devoted their research to the improvement of interrogators and data processing [28; 29; 30; 31], because light generating and amplifying devices along with analog-to-digital converters are usually the most expensive parts of DAS setups. Nevertheless, a study of the sensing element optimization possibilities seems to be quite interesting and a perspective that is insufficiently explored.