Transmission Techniques: Fiber Optics 455the relationship between incident and reflected light as
shown in Fig. 15-6.
Snell’s Law equation is(15-7)
where,
n 1 is the refractive index of the core,
n 2 is the refractive index of the cladding,
T 1 is the angle of incidence,
T 2 is the angle of reflection.The critical angle of incidence, Tc, (where T 2 = 90°) is(15-8)At angles greater than Tc, the light is reflected.
Because reflected light means that n 1 and n 2 are equal
(since they are in the same material), T 1 and T 2 , the
angles of incidence and reflection are equal. These
simple principles of refraction and reflection form the
basis of light propagation through an optical fiber.
Fibers also support skew rays, which travel down the
core without passing through the fiber axis. In a straight
fiber, the patch of a skew ray is typically helical.
Figure 15-4. Refraction and reflection. Because skew rays are very complex to analyze, they
InterfaceAngle of refractionRefracted rayAngle of
incidenceIncident ray Normal
Reflected rayAngle of incidenceAngle of incidenceAngle of refractionAngle of reflectionCritical angleMediumn 1
Mediumn 2Q 1Q 2Q 1MediumnQcQ Q 2Q 2Mediumn 1Mediumn 1Mediumn 1
Mediumn 2Mediumn 2n 1 is greater than n 2n 1 is less than n 2n 1 is greater than n 2n 1 n 2 are the refractive indices.A. Light is bent toward the normal.B. Light is bent away from the normal.C. Light does not enter the second material.When the angle of incidence is more than
the critical angle, light is reflected.D.n 1 is greater than n 2Q 2Figure 15-5. Optical fiber cross section.Figure 15-6. Light guided through an optical fiber.Buffer (jacket)CladdingCoreExample: A 50/125 fiber nomenclature indicates
both the outside diameter of the core (50 microns)
and the cladding (125 microns)Cladding n 2Cladding n 2Core n 2Angle of
incidenceAngle of
returnAxisLight
rayBufferQ 2 oQ 1 Q 2n 1 sinT 1 =n 2 sinT 2Tc sin1– n 2
n 1---- -
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