OPTICAL AMPLIFIERS

Crosstalk in SLA

Crosstalk is any distortion of a channel caused by the presence of another channel.There are two types of crosstalk in SLA:

Interchannel crosstalk - when two wavelengths (channels) enter an SLA, their nonlinear interference produces new signals at the beat (combination of sums and differences) frequencies.

Cross saturation - For a SLA works in the saturated mode (I > IS), the gain undergoes an opposite change when one channel changes from ON to OFF. This gain change results in variations in the amplification of another signal because all signals share the same gain produced by one active medium.

SLA12

21-212

22-1

Input 1

Input 2

Output 2

Gain

Gain reduction due to cross saturation

OPTICAL AMPLIFIERS

Polarization-dependent Gain in SLAsThe gain of SLAs depends on the state of polarization of the input signal - the amplification of transverse-electric (TE) and transverse-magnetic (TM) modes is different (5 to 7dB).For nonpolarization-preserving fiber, the state of polarization of an optical transmission signal changes with propagation along the fiber and causes variation in signal gain.

b) Twin amplifiers in parallela)Twin amplifiers in series

c) double pass through a SLA

Three configurations used to reduce the polarization sensitivity of SLAs:

OPTICAL AMPLIFIERS

Noise in SLAs• SLA magnifies the signal noise along with the signal itself• A SLA generates its own noise primarily caused by Amplified Spontaneous Emission - the spontaneously emitted photons in optical amplifier are in the same frequency range as the information signal and those photons follow in the direction of the information signal are amplified by the active medium. • The spontaneously emitted photons are random in phase and they do not contribute to the information signal but generate noise within the signal’s bandwidth• The spontaneous-emission factor 122 NNNnsp N2 and N1 are populations

of excited and lower levels• For ideal amplifier where • The actual value of nsp ~ 1.4 to 4.• The average total power of amplified spontaneous emission,

1122 NNN12 NN

hfGBnP spASE 2 G = amplifier gainB = optical bandwidth of the amplifier

• The noise performance of an optical amplifier is quantified by the noise figure, outinn SNRSNRF

• Typical noise figures for commercially available optical amplifiers range from 6 to 9 dB for an SLA.

OPTICAL AMPLIFIERS

Advantages of SLAs able to operate at the 1300nm and 1550nm wavelengths (simultaneously)• wide bandwidth, up to 100nm• can be readily integrated along with other semiconductors and photonic devices into one monolithic chip called an opto-electronic integrated circuit (OEIC)

Drawbacks of SLAs a relatively high crosstalk level• polarization sensitivity• large coupling loss• difficult to produce an active medium with reflectances as low as 10-4 (TWA)• optical noise

Merits of TWAsTWAs have been used more widely than FPAs (particularly for linear application) because they have - a large optical bandwidth,

- high saturation power, and - low polarization sensitivity.

In particular, TWAs are used as amplifiers in the 1300nm window and as wavelength converters in the 1550nm region.

OPTICAL AMPLIFIERS

Generation Applications of Optical Amplifier

1. In-line Amplifier - use to compensate for transmission loss and increase the distance between regenerative repeaters.2. Preamplifier - used as a front-end preamplifier for an optical receiver.

3. Power Amplifier - to boost transmitted power and increase the transmission distance - as booster of signal level in the local area network

OPTICAL AMPLIFIERS

Doped-fiber Amplifiers• Single mode fiber doped with rare earth materials such as erbium, holmium, neodymium, samarium, thulium, and ytterbium can be used as an optical amplifier with large spectral bandwidth and sufficient gain.• The major type of fiber amplifier use in WDM fiber-optic communication system is Erbium-doped fiber amplifiers (EDFAs) which provide gain near 1.55m wavelength.• In EDFA, an information signal is transmitted in the vicinity of 1550nm but the pump lasers radiate either at 980 or 1480nm, or both.• The two beams propagate together along the doped section of fiber, where information signal is amplified while pumping signal loses its power.• A pumping signal can copropagate with an information signal or it can counterpropagate.

copropagate pump

counterpropagate pump

bidirectionalpump

OPTICAL AMPLIFIERSAmplification in Erbium-doped Fiber Amplifiers• Amplification in an EDFA occurs through the mechanism of stimulated emission.• Two ways to attain population inversion in EDF: Indirect pumping at 980nm wavelength - Er ions are excited to upper level (3) and they nonradiatively fast decay to the intermediate energy (metastable) level (2). Direct pumping at 1480nm wavelength -Er ions are excited directly to the level (2)

• The energy bands of Er ions in silica fiber gives the EDFA the ability to amplify the range of wavelengths from 1500 to 1600nm and eliminates the need of fine-tune a pumping wavelength.

• The information signal stimulates transition of the excited Er ions from level 2 to level 1 and results in radiation of photons with same wavelength, direction, and phase.

OPTICAL AMPLIFIERS

Gain in an EDFA

in

ASEout

PPPdBGain 10log10

• the gain of an EDFA is restricted by the width of the radiating energy bands• gain saturation - gain is proportional to the population inversion, a high input power will stimulate a vast number of transitions per unit time from level 2 to 1 and results in depleting level 2 means decreasing gain.• optimal length (20 to 50m) - the power of pumping signal damps along an active fiber, thus an information signal experience less and less gain and eventually begin to undergo loss.

~ 20 to 40dB

OPTICAL AMPLIFIERS

• Fn varies from 3.5 to 9 dB for EDFAs.

Noise in EDFA• The dominant noise generated in an EDFA is amplified spontaneous emission (ASE)• Signal degradation comes from beating signals generated at noise-noise and noise-signal interference.• Each “slice of noise can interfere with another “slice” to generate a beating signal at frequencies that are combinations of the sum and difference of the input frequencies.• Noise-noise beating can be easily removed by a narrowband filter.• Noise-signal beatings cannot be filtered because it is within a signal’s bandwidth.

• The noise figure based on signal-noise beating is SSASEn hfGBPF

spn2

OPTICAL AMPLIFIERS

Gain and noise as a function of the length of an active EDFA fiber

• Both gain and noise are higher for a counterpropagating pump and almost constant for a copropagating pump.

OPTICAL AMPLIFIERS

Gain and noise as a function of input power

• With an increase in input power, gain decreases because of gain saturation, while noise rises.• There is a specific input power where noise is minimal

Gain and noise as a function of signal wavelength

•Gain and noise is almost wavelength independent within the spectrum of an EDFA’s stimulated emission

OPTICAL AMPLIFIERS

Advantages of EDFA over SLA

The doped-fiber amplifiers have some advantages over semiconductor laser amplifiers :• wider spectral bandwidth which allows more number of signal channels to be amplified simultaneously• flat gain characteristic over the practical range of wavelengths appropriate for optical fiber links• compatibility for in-line interconnection within optical fiber links• suitable for use in dense wavelength division multiplexed transmission