The need, functioning and types.
An optical amplifier is a device which amplifies the
optical signal directly without ever changing it to
electricity. The light itself is amplified.
Reasons to use optical amplifiers:
Wavelength Division Multiplexing (WDM).
Necessity of Optical
Optical amplifiers boost up the power level of
multiple light wave signals.
To Transmit a signals over long distances (>100km),
to compensate attenuation losses.
Initially this was accomplished with an optoelectronic
module consisting of optical RX, regenerator,
equalizer, & an optical TX to send the data.
General applications of
oIn line optical amplifiers:-
• In single mode link, the fiber dispersion may be small
so that repeater can be eliminated.
• Instead of regeneration of signal, simple amplification
can be done.
• It is used to increase the distance between regenerative
o Power amplifier:-
• The device which can be placed after the
transmitter to boost the transmitted power is
called as power amplifier.
• This provides increase in distance depending on
the amplifier gain and fiber loss.
• Optical amplifier being used as a front-end
preamplifier for an optical receiver.
• A weak optical signal is amplified before
photo-detection so that signal to noise ratio
degradation due to noise can be
suppressed in the receiver.
• It provides a larger gain factor and BW.
• Most optical amplifiers use stimulated emission.
• An optical amplifier is basically a laser without
• Optical gain is realized when the amplifier is pumped
optically (or electrically) to achieve population
• Gain depends on wavelength, internal light intensity
and amplifier medium.
• Three types: semiconductor optical amplifiers,
Raman Amplifiers and Er+ doped fibre amplifiers.
o Active medium in an optical fiber amplifier consist 10 to
30 m length of optical fiber that has been lightly doped
with a rare-earth element such as erbium(Er+).
o Erbium's principal involve its pink-colored Er3+ ions,
which have optical fluorescent properties particularly
useful in certain laser applications. silica doped with
erbium is good for long distance communication.
o EDFA operates in the spectral band of 1530 to 1560 nm
o Amplification mechanism:-
• Optical amplifier uses optical pumping.
• Pumping gives energy to electrons to reach the excited
• After reaching its excited state, the electron must
release some energy and drop to the lower level.
• Here a signal photon can then trigger the excited
electron into stimulated emission. And electron
releases its remaining energy in the form of new
• EDFAs include the ability:-
• To pump the devices at several different
• Low coupling loss to the compatible-sized fiber
• Highly transparent to signal format and bit rate.
• Immune from interference effects( crosstalk and
intermodulation distortion) when wavelength
channels are injected simultaneously into amplifier.
Semiconductor Optical Amplifiers (SOA).
• Similar to Laser diodes but the emission is triggered
by input optical signal.
• Works in any wavelength.(+)
• Have high noise resistance, compact and low power
• Cross talk between different wavelengths. (-)
• Two types: Fabry-Perot or Traveling Wave Amp.
• An electrical current passed through
the device that excites the electrons
in the active region.
• When photon(light) travel through
the active region it can cause these
electron to lose some of their extra
energy in the form of more photons
that match the wavelength of the
• Therefore, an optical signal passing
through the active region is
amplified and is said to have
SOA: Amplification Process.
• Semiconductor have valance and
• At thermal equilibrium valance
band has higher population.
• Under population inversion
condition conduction band will
have higher population.
• Population inversion is achieved
by forward biasing the p-n
• Use stimulated Raman effect and pump laser whose
frequency is equal to signal frequency plus frequency of
chemical bond in the material
• Because it is a nonlinear process, requires very high
pump powers (watts)
• A Raman amplifier is a device which takes input 𝜔𝑠 and
amplified in the same direction or opposite direction with
pump laser 𝜔 𝑃.
Optical Amplifiers (in short).
Er‐Doped Fiber Amplifier EDFA
• High gain (40–50 dB),
• Low noise (3–5 dB),
• Low polarization sensitivity,
• EDFAs are fully compatible with the rest of the fiber
optic transmission link.
• Large size,
• High pump power consumption (efficiency ‐ 10dB/1
Raman Amplifier (RA)
• Low noise (3–5 dB).
• Wide gain bandwidth (up to 10 nm).
• Distributed amplification within the transmission fiber.
• Low gain (10 dB).
• Requirement of high pump power.
Semiconductor Optical Amplifier.
• Small size.
• Transmission bidirectional.
• Smaller output power then EDFA.
• Less expensive then EDFA.
• Lower gain (20–30 dB) then EDFA.
• Higher noise (7–12 dB) then EDFA.
• Polarization dependence.
• High nonlinearity.
Optical amplifiers perform a critical
function in modern optical networks,
enabling the transmission of many
terabits of data over long distances of up to
thousands of kilometers.