Optical amplifiers.The need, functioning and types.

Introduction :-

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:

Reliability.

Flexibility.

Wavelength Division Multiplexing (WDM).

Low Cost.

Necessity of Optical amplifiers?

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.

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BASIC OPERATION OF OPTICAL AMPLIFIER.

General applications ofoptical amplifiers.

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

repeaters.

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.

o Preamplifier:-

• 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.

Basic Concepts• Most optical amplifiers use stimulated emission.• An optical amplifier is basically a laser without

feedback.• Optical gain is realized when the amplifier is pumped

optically (or electrically) to achieve population inversion.

• Gain depends on wavelength, internal light intensity and amplifier medium.

• Three types: semiconductor optical amplifiers, Raman Amplifiers and Er+ doped fibre amplifiers.

ERBIUM-DOPED FIBER 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 region .

A figure of EDFA Device.

Inside an EDFA.

o Amplification mechanism:-

• Optical amplifier uses optical pumping.

• Pumping gives energy to electrons to reach the excited state.

• 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 photon.

• EDFAs include the ability:-

• To pump the devices at several different wavelengths.• Low coupling loss to the compatible-sized fiber

transmission medium.• Highly transparent to signal format and bit rate.• Immune from interference effects( crosstalk and

intermodulation distortion) when wavelength channels are injected simultaneously into amplifier.

Schematic diagram of EDFA.

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

consumption.• Cross talk between different wavelengths. (-)• Two types: Fabry-Perot or Traveling Wave Amp.

SOA .

Semiconductor Amplifier.

• 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 initial ones.

• Therefore, an optical signal passing through the active region is amplified and is said to have experienced “gain”.

SOA: Amplification Process.

• Semiconductor have valance and conduction band.

• 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 junction.

SOA Design.

Raman amplifiers.• 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 and amplified in the same direction or opposite direction with pump laser .

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Optical Amplifiers (in short).

Advantages

And

Disadvantages.

Er‐Doped Fiber Amplifier EDFA

Advantages:• 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.

Limitations:• Large size, • High pump power consumption (efficiency ‐ 10dB/

1mW). 

Raman Amplifier (RA)

Advantages:• Low noise (3–5 dB). • Wide gain bandwidth (up to 10 nm).• Distributed amplification within the transmission fiber.

Limitations:• Low gain (10 dB).• Requirement of high pump power.  

Semiconductor Optical Amplifier.

Advantages:• Small size.• Transmission bidirectional.• Smaller output power then EDFA.• Less expensive then EDFA.

Limitations:• Lower gain (20–30 dB) then EDFA.• Higher noise (7–12 dB) then EDFA. • Polarization dependence.• High nonlinearity.

Conclusion

Optical amplifiers perform a critical function in modern optical networks, enabling the transmission of manyterabits of data over long distances of up to thousands of kilometers.