In the basic discussions on total internal reflection, it was indicated that under certain conditions between the core and cladding indexes, light is reflected at the interface between the two surfaces. When the light wave meets the boundary between the two indexes the light standing wave actually penetrates about a quarter wavelength into the second media while being reflected. This penetrating wave is called an evanescent wave. It actually decays exponentially with distance from the interface, with a characteristic penetration depth of 50 to 100nm.

Since the field cannot go to zero immediately, it decays exponentially into the lower cladding according to the relation,

where d is the distance from the interface. This exponentially decaying wave is the evanescent wave. The decay of this wave at an angle a, between the incidence ray and the normal to the surface, is characterized by a penetration depth, d_p, the distance from the interface at which the wave amplitude falls to 1/e of its initial value at the interface.³

The referenced article gives an excellent application for using the evanescent wave. Care should be taken, however, when specifying tapers and small fibers to make sure that the cladding is thick enough that evanescent wave does not create unwanted losses. As a rule of thumb, the cladding thickness should be at least 10 times the operational wavelength.

An optical fiber that has a core to clad ratio of 1:1.1, i.e., a core of 50mm and an OD of 55mm has a cladding thickness of only 2.5mm. This situation is barely 10 times the wavelength if used in the UV and only about 4 times if used at 600nm. In this situation, it is advisable to use a larger fiber (thicker cladding and/or larger core) if possible. Since the loss in fused silica is higher in the UV, some additional cladding losses may be contributed if the cladding is too thin, but they will be even more noticeable in the red part of the spectrum and NIR. The FVP100110125 fiber has a 5mm cladding thickness and the FV-data is typical of high -OH silica/silica fibers with cladding of 10mm or greater. The effect of having too thin a cladding can be seen in the chart below.