This text introduces optoelectronics, a field merging electronics and light, focusing on its history and fundamentals. Key historical milestones include Henry Joseph Round's 1907 observation of electroluminescence in SiC, Oleg Losev's 1927 light emission from zinc oxide and silicon carbide diodes, the 1961 invention of the helium-neon laser (Javan), the 1962 invention of the visible LED (Holonyak), and the 1983 installation of the first fiber optic trunk line. Optoelectronics, experiencing 30% annual growth since 1992, is crucial in communications (fiber optics, laser systems), computing, entertainment, and defense (infrared imaging, radar). The text then explains fundamental concepts like light as an electromagnetic wave, characterized by frequency (υ) and wavelength (λ), and its speed (v) related to refractive index (n). Wave-particle duality, the Planck-Einstein relation (E=hυ), and the optical spectrum are described. Refraction, reflection, attenuation (power loss due to absorption and scattering), and dispersion (speed variation based on wavelength) are detailed, including Snell's law and total internal reflection (TIR). Finally, the chapter covers light sources (LEDs and laser diodes) and detectors (photodiodes, phototransistors, photoconductive detectors) used in optical communication systems, highlighting their characteristics and advantages over copper wire systems.

Introduction to Optoelectronics: History
and fundamentals
This chapter describes some important highlights in the history of optoelectronics
and also includes a short list of some important applications related to this field of
physics.
Optoelectronics is a part of photonics science related to the study and application
of electronic devices that interact with light, systems where electrons and photons
coexist. Optoelectronic devices operate as electrical-to-optical or optical-toelectrical transducers.
Some highlights in optoelectronics history are the following:
• First observation of electroluminescence from SiC crystals was reported in the
year 1907 by Captain Henry Joseph Round (England).
• Some decades later, in 1927, Oleg Vladimirovich Losev (Imperial Russia)
observed light emission from zinc oxide and silicon carbide crystal rectifier
diodes used in radio receivers when a current was passed through them [1].
• In 1961 Ali Javan (Bell Labs) invented the first gas or helium neon laser. One
year later, Robert Hall invented the semiconductor injection laser.
• Nick Holonyak (USA) invented the first practically useful visible LED (light
emission diode) in 1962.
• The first transmission trunk using glass fibres invented by Corning glass, and
installed by AT&T in 1983, from New York to Washington, D. C., at 45
megabits per second.
Nowadays optoelectronics has become an emerging new technology. The
optoelectronics market is growing every year worldwide, 30% growth every year
since 1992.
Optoelectronics allows generating, transporting and manipulating data at very high
rate. Main applications of optoelectronics are in the field of communications,
including fibre optic communications and laser systems.
However, the applications of optoelectronics extend throughout our everyday lives,
including the fields of computing, communication, entertainment, optical
information systems, education, electronic commerce, environmental monitoring,
health care and transportation.
Optoelectronics is also important in defense applications that include infrared
imaging treatment, radar, aviation sensors, and optically guided weapons
Optical Spectrum. Refraction, reflection,
attenuation and dispersion
In this chapter, some basic equations are presented in the introduction in order to
understand important concepts related with optoelectronics. After that, some
important mechanisms of light transmission as refraction, reflection, attenuation
and dispersion are introduced in this chapter. An important concept: TIR (total
internal reflection), used in optical communications is also defined. Introduction
Light as an electromagnetic wave is characterised by a combinations of timevarying E (electric field) and H (magnetic field) propagating through space
according to the Maxwell equations introduced by James Clerk Maxwell in the late
19th century.
Light can be characterized using several spectral quantities, such as frequency, υ,
2
υ = ω
π
, where ω is the angular frequency or wavelength λ,
λ = υ
c , being c the speed of light in vacuum
c is a universal physical constant and its value is exactly 299 792 458 m/s.
Usually a value of c= 3 108
m/s is used as a good approximation.
In any other medium different of vacuum, the light phase velocity, v (the speed at
which the crests or the phase of the wave moves), depends on the refractive index,
n, of the medium as follows [2] :
v
c
n
= , where n can be defined by the following equation:
n ε r r
μ

, being εr and μr the relative electrical permittivity and magnetic
permeability of the medium respectively [3]. The refractive index is a function of
the wavelength.
The relationships among electricity, magnetism, and the speed of light in a medium
are summarized by the following equation:
r r
c
v
ε
μ

Wave-particle duality: Every elementary particle or quantic entity exhibits the
properties of not only particles, but also waves. Electromagnetic radiation
propagates following linear wave equations, but can only be emitted or absorbed as
discrete elements: Photons, thus acting as a wave and a particle simultaneously.
The energy of a photon, E, is proportional to its frequency,υ, and can be calculated
by using the Planck–Einstein relation also known as Planck equation [4] :
c
E h h
υ
λ
= =
where h is the Planck’s constant, h = 6.62·10–34 Js or 4.1356·10–15 eVs.
2.2 Optical Spectrum
The optical spectrum is a small part of the electromagnetic spectrum. Human eyes
can detect lights of wavelength in the range of 450 nm to 650 nm. This part of the
electromagnetic spectrum is called optical spectrum or visible light. Figure 1 shows
the electromagnetic spectrum and the colours associated to the optical spectrum.
Fig. 1. Optical spectrum.
Refraction, reflection, attenuation and
dispersion
When light reaches the plane boundary between two media, a transmitted light in
medium 1 and a reflected light in medium 1 appear. The transmitted light is the
refracted light. The angles associated to the directions of the transmitted, refracted
and reflected light are shown in Fig.2.
The angle of incidence, φ1, is equal to the angle of reflection angle φ3.
Refraction is the changing direction of light when it goes into a material of different
refractive index, n.
Fig. 2. Refraction and reflection angles.
Snell’s law gives the relationship between the sine’s of the incident and refraction
angles and the refractive indexes of the media as follows:
1 2
2
1
sin( )
sin( )
n
n
ϕ
ϕ

For angles larger than the critical angle we have TIR (total internal reflection) [5].
light hits the interface between two media at any angle larger than this critical
angle, it will not pass through to the second medium at all. Instead, all of it will be
reflected back into the first medium, a process known TIR. This principle is applied
by traditional waveguides as optical fibres and is shown by Fig.3.
Fig. 3. TIR effect.
Light of different frequencies propagate at different speeds through the medium.
Moreover, the refractive index depends on the wavelength. Due to these effects,
some dispersion appears in the medium.
Fig. 4. Dispersion effect.
Attenuation is the loss of the optical power. Attenuation is mainly due to absorption
and scattering that give rise to a loss of energy in the direction of propagation. The
specific attenuation: Power loss in dB per unit length, depends on the wavelength
of the radiation travelling along the medium. The attenuation coefficient, α, is given
by the following equation
10 ( )
log
(0)
P L
L P
α
 

 
 
where P(0) is the initial power or incident power, P(L) is the power at a distance L
from the initial point.
Consider a ray of light traveling in a medium of refractive index n1 = 1.44 becomes
incident on a second medium of refractive index n2 = 1.4. The wavelength of the
light is 1.1 µm.
Calculate the incident angle to have TIR.
SOLUTION
Snell’s Law:
1 2
2
1
sin( )
sin( )
n
n
ϕ
ϕ

Critical Angle, φ1c , occurs at φ2=90 º, then
2
1
1
1.4
arcsin arcsin 76.5 º
1.
Light transmission, sources and detectors
In this chapter, main sources of light and detectors commonly used in optical
communication systems are presented. Optical communications offer important
advantages respect to conventional communications supported by copper wires.
Some of these advantages are introduced in this chapter.
Introduction
Optical communication systems transmit information by means of light. Compared
to copper wire used in electrical communications, optical fibres have lower cost,
weigh less, have less attenuation and dispersion and provide more bandwidth.
Optical fibre can support ultra-high data rates: Terabits per second and can be used
to transmit light and thus information over long distances. Moreover, there are no
problems associated to EMC (Electromagnetic Compatibility) interference
immunity and there is no fire hazard because of the pass of electricity through the
communication channel is eliminated.
Figure 5 shows the typical block diagram of an optical communication system. The
electrical signal (information) controls the source of light; the light emitted by the
source is coupled to the transmission channel: Optical fibre, waveguide or free
space. The light is transmitted through the transmission channel up to the light
detector that is coupled with the channel. The light detector transforms the light into
electrical signal and the information is received.
Fig. 5. Optical communication system bloc 2 Sources and detectors of light
LED : Light-emitting diode
Light sources are used to generate input signals of the optical communications
systems. Optical communication systems often use semiconductor optical sources
such as LEDs (light emitting diodes) and semiconductor LDs (laser diodes).
LASER : Light Amplification by Stimulated Emission and Radiation
These kinds of semiconductor optical devices offer high efficiency and reliability.
Moreover, they allow an accurate selection of the wavelength range and emissive
areas compatible with optical fibre core dimensions. The following table
summarizes main characteristics and structures of LEDs and LDs used in optical
communication systems through optical fibres.
Semiconductor optical
sources
Characteristics
Structures
LEDs
LEDs used in optical
communications must
have a high radiance
(light intensity), fast
response time and high
QE (quantum
efficiency).
Planar, dome, edgeemitting led or surfaceemitting led.
LDs
LDs used in optical
communications should
have coherent light,
narrow beam width and
high output power.
Spontaneous emission.
Stimulated emission
At the end of the optical communication systems optical sensors (detectors of light)
are used in order to recover the transmitted information and convert it again into an
electrical signal through the photoelectric effect. The role of a photodetector is to
recover the data transmitted through the optical fibre communication system.
Photodetectors are optoelectronic devices that convert an incident radiation (light)
to an electrical signal, such as voltage or current.
Light detectors or photodetectors are usually based on PDs (photodiodes),
photoconductive detectors and phototransistors. Photoconductive detectors have
the simplest structure of this family of light detectors and can be obtained by
attaching two metal electrodes to a semiconductor material. The conductivity of the
semiconductor increases when some incident photons are absorbed in the
semiconductor. As result, an increase of the external current appears when a voltage bias is applied to the electrodes. Solar cells are a specific type of photodetectors
used in photovoltaic solar energy generation systems, not in communication
systems.
A photodiode is a semiconductor diode that functions as a photodetector. It is a pn junction or p-i-n structure. When a photon of sufficient energy strikes the diode,
it excites an electron thereby creating a mobile electron and a positively charged
electron hole.
Phototransistors are BJTs (bipolar junction transistors) that operates as
photodetectors and offer as well photo-current gain. These devices are
semiconductor light sensors formed from a basic transistor with a transparent cover.
Semiconductor optical
detectors
Characteristics
Examples of structures
Photodiodes Based on pn junctions.
pn or p-i-n diodes. APDs
(Avalanche
photodiodes).
Heterojunction
photodiodes.
Schottky junction
Junction formed by an ntype semiconductor in
contact with a metal.
Schottky contacts.
Solar cells
Solar cells convert the
incident radiation energy
into electrical energy.
cSi ( crystalline silicon)
aSi:H (amporphous
silicon). HiT
(heterojunction intrinsic
layer thin film solar cell
). GaAs
Phototransistors
Light-sensitive
transistors.
Phototransistors amplify
variations in the light
striking it.
npn BJTs
pnp BJTs
Photoconductive
detectors
Conductivity variation
due to absorption of
light.
LDR (light-dependent
resistor). PbS ( lead
sulfide) IR ( infrared
detector). Lead selenide
(PbSe) IR detectors.