Optical Fiber Sensing Technologies

9Optical Fiber Sensors Based on the SMS Structure

9.1 Theory of SMS Fiber Structure

A typical structure of an single‐mode/multimode/single‐mode (SMS) fiber structure is depicted in Figure 9.1. It is fabricated by connecting a section of multimode fiber (MMF) between two single‐mode fibers (SMFs). Under the linearly polarized (LP) mode approximation, the input mode field of the MMF, i.e. the LP₀₁ mode of the SMF, can be approximated by a Gaussian beam as

(9.1)

where r is the radial coordinate and ω is the mode field radius that can be determined by [1]

(9.2)

where a₀ is the radius of the SMF core and

(9.3)

is the normalized frequency of ...