*1.1.1.1 Single mode fiber (based on number of modes)*

Only one kind of light ray can pass through a single-mode fiber. This fiber has a tiny core diameter (5 μm) and a large cladding diameter (70um), with a minimal refractive index difference between the core and the cladding. There is no signal deterioration when travelling through the fiber because there is no dispersion. Using a laser diode, light is sent through it [41] (shown in **Figure 2a**).
