1. 8.2.2 Fiber Optic Communications
Transmission of optical signals from source to detector can be greatly enhanced is an optical fiber is used
Multimode fibers (~25 m) for LEDs, and single-mode fibers (~5 m) for LASER
relatively flexible, and can be used to guide signal over distances of kilometers
light is transmitted along the fiber by internal reflection
large amounts of data can be transferred over long distances using bundles of many fibers
repeater stations may be required periodically along the data path
losses in fiber at a given wavelength are described by an attenuation coefficient 
the attenuation is not the same for all wavelengths
dips in  around 1.3 m and 1.55 m provide “windows” in the attenuation
chromatic dispersion is less at 1.3 m than at 1.55

2. 8.2.2 Fiber Optic Communications

4. 8.2.3 Multilayer Heterojunctions for LEDs

5. 8.3 Light Amplification by Stimulated Emission of Radiation (LASER)
LASERs are highly directional, monochromatic, coherent light sources
LASER light is either a continuous beam of low or medium power (cw), or a short burst of intense light (pulsed) delivering millions of Watts
particularly useful for fiber optic communication systems
we have already discussed spontaneous emission of a photon when an electron falls from an excited state to a lower energy state, this is a random process
for a LASER, we wish to stimulate the emission

6. the rate of spontaneous emissions at any time is proportional to the instantaneous population (number of electrons n2) of the excited state, E2
where sp is the mean spontaneous decay time (ie: the average time an electron spends in the excited state)
in stimulated emission, we encourage excited electrons to drop to a lower energy level in a time st much shorter than its mean spontaneous decay time.
st < sp
the stimulus required for this to happen can be provided by the presence of photons of the proper wavelength
if the electrons in the excited state are immersed in an intense photon field
where each photon has an energy hν = E2 – E1, and
is in phase with all of the other photons
then the excited electron may be induced to drop in energy from E2 to E1
thereby contributing a photon that is in phase with the radiation field
if this process continues a large radiation field can build up
the resulting radiation field will be both monochromatic and coherent

7. We must describe the stimulated emission process quantum mechanically in order to relate the probability of emission to the intensity of the radiation field
at thermal equilibrium we know that:
the negative exponent indicates that n2 « n1 at equilibrium, ie: most electrons are at the lower energy level
if the atoms exist in a radiation field of photons of energy hν12 having an energy density ρ(ν12), then stimulated emission can occur along with absorption and spontaneous emission
Absorption
Spontaneous Emission
Stimulated Emission = +
(8-7)

8. Absorption
Spontaneous Emission
Stimulated Emission = +
(8-7)
the coefficients B12, A21 and B21 are called the Einstein Coefficients
for steady-state the two emission rates must balance the absorption rate in order to maintain constant populations n1 and n2
at equilibrium:
a large photon field energy density is required
an optical resonant cavity is used to allow the photon density to build up to a large value
to obtain more stimulated emission than absorption we require n2 > n1:

9. the condition n2 > n1is called a Population Inversion, it is quite unnatural
we require a method to maintain a population inversion
this is covered in Section 8.4 Semiconductor LASERs
before we go into this let us discuss the optical resonant cavity
the optical resonant cavity may be obtained reflecting mirrors (one, or both, of the mirrors must be only partially reflecting so that the light can “leak out”
the gain in photons per pass between the mirrors must be larger than the transmission at the ends, scattering from impurities, absorption, and other losses
light of a particular frequency can be reflected back and forth within the resonant cavity in a reinforcing manner
the length of the cavity must satisfy this for the photon wavelength within the LASER material
the output light of the LASER wil have λ0 = nλ in air

10. 8.4 Semiconductor LASERs
the first semiconductor LASERs were built in 1962 using GaAs p-n junctions
junction LASERs:
are very small (typically about 0.1 mm  0.1 mm  0.3 mm)
are highly efficient
are easily modulated by controlling the junction current
operate at low power compared to ruby or CO2 LASERs
output power comparable with that of He-Ne LASERs
semiconductor LASERs provide a portable and easiily controlled source of low-power coherent, monochromatic radiation
particularly well suited for use in fiber optic communication systems