I do not understand the operating principle of a subwavelength-diameter optical fiber? I would love to know the difference in the transmission principle of light between a subwavelength-diamater optical fiber and a single-mode optical fiber. Please explain me the path of light in the subwavelength-diameter optical fiber ?
thank you so much!
 A: So the general transmission principle is the same, but the subwavelength diameter fiber is designed with different applications in mind.
As with any optical fiber, the light is held in the core due to total internal reflection. But remember that there is also an evanescent field leaking outside the core. The smaller the core gets, the more evanescent field leaks out.  This evanescent field ideally stays with the propagating mode, but it can interact with things outside the core, and a large evanescent field generally indicates a loosely-bound, lossy mode (light can escape more easily in this case).
Single-mode fibers have a small core to reject higher-order modes from propagating, but they also have a cladding, which is a second layer of glass (lower refractive index, typically) around the core to protect the mode. The cladding totally encompasses the evanescent field to prevent it from interacting with outside particles and to prevent radiative loss.
For subwavelength diameter fibers, the outside interaction is the point. Once again, there is total internal reflection resulting in a propagating mode with an evanescent field extending outside the core. But in this case, no effort is made to contain this field. The fiber becomes more lossy, but now it is sensitive to the outside environment. This could be used for sensing applications, for example, or to design a device to modulate the mode. 
