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Optical Sources: LED

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There are three possible reasons for loss of photons (a) Fresnel reflection (b) total internal
reflection (c) absorption.
Loss due to Fresnel Reflection
Consider the emission from a surface-emitting structure, where we assume that the photon is
emitted only from one surface. Let the refractive index of the medium be n. When an electromagnetic wave is an incident at an interface, it experiences a reflection because of the
the difference in the constitutive parameters ( and ) between the two media. The amount of
light reflected can be quantified through the Fresnel coefficients. For light propagating from
a medium of the refractive index of n1 to another medium with refractive index of n2, this
reflection coefficient (ratio of reflected electric field to incident electric field) is given by
௡భି௡మ
௡భା௡మ
, for the case of normal incidence. The ratio of reflected power to incident power is
given by ቀ
௡భି௡మ
௡భା௡మ


= ቀ
௡ିଵ
௡ାଵቁ

=
4n
(n+1)
2
. Thus, only a fraction of the incident light will get
transmitted and it undergoes the loss due to reflection. The fraction of incident power
available at the output is quantified through the efficiency, ηଵ = 1 − 4n
(n+1)
2
.

NPTEL-Fiber Optic Communication Technology – Lecture 11 Page 2
Consider a typical semiconductor material which is Gallium Nitride or Indium Gallium
Arsenide Phosphide, with refractive index 3.4 - 3.6 depending on the actual bandgap of the
material. For n=3.5, ηଵ= 69.1 %. Thus, there is a significant loss of power because of Fresnel
reflection. Note that, we have calculated this for normal incidence, the value of reflection coefficient changes with angle.