4.WDM, Amplifiers

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WDM & Amplifiers


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WDM The fiber optic industry first deployed

single wavelength transmission links. As requirements changed, the industry

responded with wavelength-divisionmultiplexing (WDM), which sends twodistinct signals per fiber, doublingtransmission capacity.

In one way WDM is similar to splitter. Unlike the splitter, however, they have

very little insertion loss.


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WDM contd.., The true potential of optical fibre is fully

exploited when multiple beams of light atdifferent frequencies are transmitted on the

same fibre. Two important considerations in a WDM

device are crosstalk and channelseparation.

Crosstalk, also called directivity, refers to

separation of demultiplexed channels. Each channel should appear only at its

intended port.


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The crosstalk specification expresses how well acoupler maintains this port-to-port seperation.

Channel separation describes a couplers ability

to distinguish wavelengths WDMs allow multiple independent data streams

to be sent over one fiber. The most common WDM system uses two

wavelengths, although four or more-wavelength

systems are available.


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Following figure illustrates two WDMspermitting two streams of data to be carriedon a single fiber.

The type of data does not matter. Forexample, one stream could be a video signaland the other could be an RS-232 data stream.

The configuration shown is unidirectional, butbidirectional configurations are also available.


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WDM can also be used with concave mirrorsand diffraction grating.

Following figure which makes use of a concavemirror.


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Dichroic Filter: An optical filterthat transmits light according to

wavelength. Dichroic filters reflectlight that they do not transmit.


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Constructed from discrete lenses and filters, adichroic filter lies at the center of the WDM.

Dichroic filters, reflect the light that they donot transmit.

In the following figure, Fiber 1 carriers two

wavelengths, 850 nm and 1310 nm


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Here it is designed that dichroic filter passeswavelengths longer than 1100 nm, known aslong-wave pass (LWP) filter

As the light exits Fiber 1 it first passes throughthe lens which focuses the light at a point.

As the light hits the filter, the 1310 nm lightpasses through the filter and is collected by

Fiber 3. The 850 nm light exiting Fiber 1 on the other

hand reflects off of the filter and is collectedby Fiber 2.


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Thus the information on the two effectivelypaired wavelengths can be independentlydecoded.

The dichroic filter can offer a great deal ofisolation in the transmission mode, but haspoor isolation in the reflection mode.

Usually these types of WDMs feature both

short-wave pass (SWP) and LWP filters, andcombining these filters achieves the bestsystem performance.


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Diffraction grating WDM Diffraction grating is An array of fine, parallel,

equally spaced reflecting or transmitting linesthat mutually enhance the effects of diffractionto concentrate the diffracted light in a fewdirections determined by the spacing of thelines and by the wavelength of the light.

This could be of two types as Transmissive

diffraction grating and reflective diffractiongrating.


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In diffraction grating WDM, we usediffraction grating instead of dichrole

mirror which directs incident wave todifferent fibers.

Its operation is also reversible.


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Optical Amplifiers When setting up an optical link, one calculates

a power budget and adds repeaters when thereare more losses.

To amplify an optical signal with conventionalrepeater, it is necessary to do optical toelectrical conversion, electrical amplification,retiming, pulse shaping (reshaping) and thenelectrical to optical conversion.

Although this process works well for moderatespeed single wavelength operation, this will beexpensive for high speed multiwavelengthsystems.


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So a lot of work has been done todevelop all-optical amplifiers.

These devices operate completely inoptical domain to boost the powerlevels of light wave signals.

These amplifiers are called as DFA(doped fiber amplifiers).

Examples are Thulium DFA,Praseodymium DFA, Erbium DFA.

Of them EDFA is most popular.


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Optical Amplifiers With the demand for longer transmission lengths,

optical amplifiers have become an essentialcomponent in long-haul fiber optic systems.

Optical amplifiers have made WDM practical

The erbium doped fiber amplifier has beenparticularly important in the growth of WDMtechnology


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Repeater vs Amplifier Repeater (or Regenerator)

Generates fresh copy of signal (+) Requires O-E-O conversion (-)

Tied to bit rate and signal format (-) Operates on single wavelength (-)

Amplifier Amplifies input and all (-) Adds noise (-) No O-E-O conversion (+) Independent of bit rate and signal format (+) Amplifies multiple wavelengths in a range (+)


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Two Types of OpticalAmplifiers

Semiconductor optical amplifiers Variation of a laser diode

Fiber optical amplifiers Special type of fiber spliced between two

sections of transmission fiber Pump laser excites electrons in this fiber

Incoming photons stimulate additionalphotons in the special fiber, resulting in gain Primary example: erbium-doped fiber

amplifier (EDFA)


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Optical Amplifier Functions

Optical amplifier can serve any of the followingpurposes as

Booster

In-line Amplifier

Preamplifier

Many tunable laser designs output low optical

power levels and must be immediatelyfollowed by an optical amplifier.


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Booster Amplifier: Many tunable laser designs output low

optical power levels and must beimmediately followed by an opticalamplifier.

A power amplifier or booster amplifiermay be employed to boost the signalimmediately following the transmitter.


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Line Amplifier: This can be used in the signal path to compensate

for transmission loss and increase the distancebetween regenerative repeaters.Pre Amplifier:

An optical preamplifier is placed immediatelybefore a receiver to improve its sensitivity.

By doing so a weak optical signal is amplifiedbefore photodetection.


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Booster

In-line amplifier

Preamplifier

Tx

Tx

Tx

Rx

Rx

Rx

OA

OA

OA


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Semiconductor OpticalAmplifiers

Semiconductor optical amplifiers (SOAs) areessentially laser diodes, without end mirrors,which have fiber attached to both ends.

They amplify any optical signal that comes fromeither fiber and transmit an amplified version ofthe signal out of the second fiber.

SOAs are typically constructed in a small

package, and they work for 1310 nm and 1550nm systems.


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SOA contd..,

In addition, they transmit bidirectionally,making the reduced size of the device.

However SOA produces more coupling lossesand are polarization dependent.

Polarization dependent means gain of theamplifier is dependent on polarization ofincoming wave which in not maintainable. Due

to this gain can vary over time due to shift inpolarization.


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EDFA (Erbium Doped FiberAmplifier)

EDFAs allow information to be transmittedover longer distances without the need forconventional repeaters.

The fiber is doped with erbium, a rare earthelement, that has the appropriate energylevels in their atomic structures foramplifying light.

EDFAs are designed to amplify light ataround 1500 nm to 1600 nm range.(Practically 1530-1560 nm region)


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An Erbium Doped Fibre Amplifier consists of a short(typically ten metres or so) section of fibre which hasa small controlled amount of the rare earth element

erbium added to the glass in the form of an ion. The principle involved here is same as the principle of

a laser.

Erbium ions are able to exist in several energy states


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When an erbium ion is in a high-energystate, a photon of light will stimulate it

to give up some of its energy (also inthe form of light) and return to a lower-energy (more stable) state.

This is called stimulated emission


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The basic principle of the EDFA isillustrated in following figure

Coupler CouplerIsolator

AndFilter

Pumplaser

Input Output

Pumping light

Erbium-dopedfiber

Residual pumpinglight


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EDFA Operation

Operation is as follows: 1. A (relatively) high-powered beam of light is

mixed with the input signal using a wavelengthselective coupler. (The input signal and theexcitation light must of course be at significantlydifferent wavelengths.)

2. The mixed light is guided into a section of fibrewith erbium ions included in the core.

3. This high-powered light beam excites the erbium

ions to their higher-energy state.


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EDFA Operation contd..,

4. When the photons belonging to the signal (at adifferent wavelength from the pump light) meet theexcited erbium atoms, the erbium atoms give upsome of their energy to the signal and return totheir lower-energy state.

5. A significant point is that the erbium gives up itsenergy in the form of additional photons which areexactly in the same phase and direction as thesignal being amplified. So the signal is amplified

along its direction of travel only.


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EDFA Operation contd..,

6. There is usually an isolator placed at theoutput to prevent reflections returning fromthe attached fibre.

Since the pump photon must have higherenergy than the signal photon, the pumpwavelength is shorter than signal wavelength.

Pump laser will work at 980 nm or 1480 nmwavelengths.


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Benefits of EDFA

Minimal polarization sensitivity

Low insertion loss

High output power

Low noise

High sensitivity Minimal crosstalk

4.WDM, Amplifiers

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