Core-cladding interface

Fig. 9.1 Cross section of a simple three layer slab waveguide, and a plot of the fundamental mode intensity profile. Light rays (dashed line) in the waveguide are confined by total internal reflection at the core cladding interfaces...

Step Index Fiber—A fiber having a uniform refractive index within the core and a sharp decrease in refractive index at the core/cladding interface. 

The total reflection of light at the core/cladding interface is... 

Figure 7.1 A light beam undergoing total internal reflection at the core/cladding interface. The bottom part shows a perspective view consisting of the Ught conducting core, the cladding that has a lower refractive index than the core, and the protective coating.

The SIM technique uses the intensity infomiation in two dimensions (2D), which can enhance the sensor detection sensitivity. The intensity distribution in 2D is the function of the optical excitation and the boundary conditions of the optical fiber, whereby changing the boundary conditions results in intensity modulation in 2D [36]. It is known that in SIM technique applications, higher-order modes are excited by off-axis illumination of the optical fiber [16, 38]. Those modes have more interactions with the core/cladding interface therefore, they are more sensitive to changes in the refractive index of the cladding material. 

Consider the situation when the core region of the capillary is filled with a fluid that has a refractive index which is smaller than that of the wall ( i < n-. Snell s law now stipulates that all rays entering the core region will be radiated into the wall, that is, there is no critical-angle condition for total internal reflection at core/ cladding interface. As depicted in Fig. 5, rays that enter the liquid core at an angle will exit the capillary structure at an angle 03, or be reflected at point C or be partially reflected at point B. 

Schematic of an optical fiber. All light rays that enter the fiber end face and strike the core-cladding Interface at an angle (b > (the critical angle), where sin (b(. = /n. , j.g, are totally internally...

Because the integrity of the core-cladding interface is critical to beam guiding, the clad fiber is encased typically in a tough but flexible protective fluoropolymer buffer coat. 

The condition for total internal reflection at the core-cladding interface (Fig. 2) is given by... 

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