## 3 Multi-Mode Fiber (MMF)  
### Theory and Sensing Principle  
MMF is another typical type of telecommunication fiber used in DAS systems. Differing from SMF, MMF has notably larger glass cores, typically ranging from 50 \(\upmu\)m to 62.5 \(\upmu\)m in diameter (Figure 4). These large core sizes facilitate the phenomenon of multipathing, wherein multiple paths of light allow for propagation within the fiber. Hence, multiple lasers propagate within larger glass cores, increasing the dispersion of lasers and light collected back by the DAS interrogator unit. The optical source is essential, with as many as several hundred modes, each accounting for a proportion of the total power being transmitted. Owing to the larger core size of MMF, a significantly greater amount of light can be efficiently introduced into the fiber. Each light pulse experiences temporal spreading due to intermodal distortion or intermodal delay in MMF. This phenomenon restricts the fiber's maximum length or bandwidth product and also makes MMF measure for a wider range of angles of incident and modes in fiber optics. The limitation primarily stems from chromatic dispersion caused by a non-zero-source spectral linewidth. Optical sources are defined by their small spectral bandwidths and are extensively more preferred in long-distance fiber optic sensing systems. This choice is based on their high modulation speed, optical power-coupling efficiency, and minimum wave chromatic dispersion characteristics [7].  
Despite its sensing principle, MMF can accommodate several hundred modes, with the entirety of these modes collectively contributing to its sensing capabilities. This is primarily due to the simpler approach of diminishing intermodal coupling by augmenting the discrepancy between the propagation constant values of individual modes [39]. The sensing principle of MMF that causes a phase difference in Rayleigh backscattering is the same as that of SMF, as mentioned in Equation (1); however, the measured axial strain is different and might affect the data resolution [40].  
In contrast to SMF, MMF has achieved extensive and broad prevalence due to its inherent insensitivity to alignment and cost-effectiveness. Consequently, MMF finds wider  
Figure 4: The sensing principle of MMF cable in DAS.  
use, particularly in short-distance applications [22]. A notable instance is the installation of numerous MMFs in general hydrocarbon wells for diverse applications, including DTS [41]. A study highlighted the utilization of an MMF to measure concurrently distributed sensing of acoustic signals and temperature variations. This exemplified the capability of MMF in easing applications within the scope of DAS [42]. The capabilities of MMF have been utilized in the last few years for DTS applications [40]. Contrary to SMF, MMF stands as the favored platform for DTS measurements. This preference is mostly caused by the substantial specific area and high threshold power of nonlinearity present in standard MMF. These attributes enable the sustenance of requisite high injected pump power without compromising the overall performance of DTS systems that measure differences in backscattered light according to changes in temperature [22]. The work reported in [39] showed a compromise of MMF in performing DTS measurement, resulting a high spatial resolution and the desired temperature variation across extended measurement distances. This configuration involved the development of a fiber optic DAS using phase-sensitive OTDR.