CHAPTER 7

OPTICAL AMPLIFIERS

Increase transmission distanceby increasing optical power coupled to transmission fiber(power booster)by compensating optical fiber losses(in-line amplifier, remote pump amplifier)by improving receiver sensitivity (optical preamplifier)

Why Optical Amplifiers?Function : Amplification of optical signal without conversion to electrical signal Ingredients : Pump energy, amplification mediumOptical inputOptical outputamplification mediumPumping of energyl l l l l l l l l

The Need of Optical Amplification Erbium-Doped Fiber Amplifiers (EDFAs) application in long haul. Todays amplifier of choice. Erbium-Doped Waveguide Amplifiers (EDWAs) application in metro and access networks Raman Amplifiers application in DWDM Semiconductor Optical Amplifiers (SOA) not fiber based type, application in metro and access networks

Why? Extend distance light signal can travel without regeneration

General Application of Optical AmplificationIn-line amplifierPreamplifierPower (booster) amplifierLAN booster amplifierStandard FiberAmplifierPump Lasers

60 mW45 mW30 mWOutput power[dBm]in-lineamplifierBoosteramplifierpreamplifierAmplifier Operation Points

Improvement of System Gain

Erbium Doped Fiber Amplifiers (EDFA)

Basic EDF Amplifier DesignErbium-doped fiber amplifier (EDFA) most commonCommercially available since the early 1990sWorks best in the range 1530 to 1565 nmGain up to 30 dB

Erbium-doped Fibre AmplifierWhen you actually look at an EDFA block diagram you see the input on the left the output on the right, photo diodes to measure input and output power levels, pump lasers which are used to excite this piece of erbium doped fiber to a higher energy state, and 1550 light is allowed to pass through. The isolators are used to keep the 980 and 1480 pump wavelengths contained in the fiber but allow 1550nm light to pass through.

Optimization maximum efficiencyminimum noise figuremaximum gainmaximum gain flatness/gain peak wavelengthDynamic range, operation wavelengthGain equalizationControl circuitMonitoring of amplifier performanceIssues of Amplifier Design

Erbium-doped Fibre AmplifierGain, G (dB)10log[(PSignal_Out - Pase) / PSignal_In]

Noise Figure, F = SNRout / SNRinS-band : 1440 - 1530nmC-band : 1530 - 1565nmL-band : 1565 - 1625nm

Optical Gain (G)G = S Output / S Input S Output: output signal (without noise from amplifier) S Input :input signal Input signal dependentOperating point (saturation) of EDFA strongly depends on power and wavelength of incoming signalWavelength (nm)

Noise Figure (NF)NF = P ASE / (h G B OSA) P ASE: Amplified Spontaneous Emission (ASE) power measured by OSA h: Planks constant : Optical frequency G: Gain of EDFA B OSA: Optical bandwidth [Hz] of OSA Input signal dependentIn a saturated EDFA, the NF depends mostly on the wavelength of the signalPhysical limit: 3.0 dBNoise Figure (dB)15401560158015207.510Wavelength (nm)5.0

EDFA CategoriesIn-line amplifiersInstalled every 30 to 70 km along a linkGood noise figure, medium output powerPower boostersUp to +17 dBm power, amplifies transmitter outputAlso used in cable TV systems before a star coupler Pre-amplifiersLow noise amplifier in front of receiverBi-directional amplifiersAn amplifier which work in both ways

EDFA Categories

Typical DesignFigure in the slide shows an EDFA with all the options design Coupler #1 is for monitoring input lightCoupler #2 is optional and is used for monitoring backreflections. The microcontroller can be set to disable the pump lasers in case the connector on the output has been disconnected. This provides a measure of safety for technicians working with EDFAs.

Typical Design

Erbium-Doped Fiber AmplifiersFigure in the next slide shows two single-stage EDFAs packaged togetherMid stage access is important for high performance fiber optic systems requiring a section of dispersion-compensating fiber (DCF)The high loss of DCF (10dB+) is compensated by the EDFA\The main advantage of SOAs and EDFAs is that amplification is done with photons, as opposed to electronic amplification (requires O/E and E/O conversion)

Erbium-Doped Fiber Amplifiers

Typical Packaged EDFA

EDFAs In DWDM SystemsGain flatness requirements Gain competition Nonlinear effects in fibersOptical amplifiers in DWDM systems require special considerations because of:

Gain FlatnessGain versus wavelengthThe gain of optical amplifiers depends on wavelengthSignal-to-noise ratios can degrade below acceptable levels (long links with cascaded amplifiers)

Compensation techniquesSignal pre-emphasisGain flattening filtersAdditional doping of amplifier with Fluorides

Gain CompetitionOutput power after channel one failed Equal power of all four channels Total output power of a standard EDFA remains almost constant even if input power fluctuates significantlyIf one channel fails (or is added) then the remaining ones increase (or decrease) their output power

*Any discussion of lightwave transmission systems must include, at a minimum, a brief description of EDFAs, erbium doped fiber amplifiers. When coupled with DWDM filters, EDFAs are the enabling technology behind the huge growth in lightwave transmission capacity. *This diagram shows a very basic amplifier that possibly has 5 to 15 dB gain and less than 10 dB noise figure. High performance commercial designs provide output powers from 10 to 23 dBm (10 mW to 200 mW) and noise figures between 3.5 and 5 dB (the physical limit is 3.01 dB).

EDFAs have been deployed in terrestrial and submarine links and now are considered as standard components using a well understood technology.

EDFAs are self-regulating amplifiers. When all metastable electrons are consumed then no further amplification occurs. Therefore a system stabilizes itself because the output power of the amplifier remains more or less constant even if the input power fluctuates significantly.

*The gain versus wavelength curve of the EDFA (as well as the ASE versus wavelength plot) can vary with input signal wavelength and power.

Carefully watch how the gain decreases with increasing input power. If the input is -20 dBm then the gain is about 30 dB at 1550 nm, resulting in +10 dBm output. If the input is -10 dBm then the gain is about 25 dB and the output about +15 dBm. In other words, when the input changes by a factor of ten then the output changes only by a factor of three in this power range.

Above -10 dBm input the amplifier is in full compression: -5 dBm input power has 20 dB gain, therefore the 5 dB increase in input power has no effect on the output power (but it may have improved the noise figure).

You can also recognize the saturation by the fact that the traces become more flat when the input power increases. Saturation is a preferred point of operation because it stabilizes the system and reduces noise without causing nonlinear effects (like clipping) inside the amplifier for high speed modulation.

*Noise figure describes how close an amplifier comes to an ideal amplifier that amplifies the input spectrum including noise but does not add any noise. According to quantum physics, it is impossible to build an optical amplifier with better than 3.0 dB noise figure. The noise figure formula compares noise density (PASE / BOSA) measured at the output but normalized to the input (1/G term) with the quantum noise of the incoming photons (h*). Note that this formula is based on some assumptions usually present in fiberoptic communication systems (for a detailed discussion, see Fiber Optic Test And Measurement by Dennis Derickson, ISBN 0-13-534330-5).

As we can see from the measurements shown here, the noise figure becomes better with increasing wavelength. The traces overlap significantly because even at -30 dBm input power the amplifier is already saturated sufficiently. You have to drop input power considerably before the noise figure becomes noticeably worse.

*The output gain, output power and noise figure of EDFAs can be tweaked by various design modifications.

Using 980 nm pumps usually produce EDFAs with lower noise figures. EDFAs of this type make better preamps. It is thought that the reason for this NF improvement is that the pump wavelength is farther out of the emission band and, for this reason, reduces ASE.

Remotely pumped EDFAs allow system designers to extend medium range submarine links, such as those between islands. Their main advantage is that there are no electronics and therefore no power needs along the link, a fact that improves reliability and reduces cost.*

DWDM systems can cause additional complications for designers. Let us look at the key issues one at a time.

Note that most of these aspects become irrelevant on DWDM systems over short distances. For example, many municipal links do not need any amplifier at all (the average cable length between network terminals in big cities can be as short as seven miles).

*The gain and noise figure of the first generations of erbium-doped fiber amplifiers (EDFAs) were not flat over wavelength. The amplitude or signal-to- noise ratio (SNR) can degrade quicker than desired if amplifiers are cascaded. Therefore they may not be well suited for DWDM applications.

More recent designs compensate this effect with different fiber doping, compensation filters or simply by pre-emphasizing the channels at the transmitter side in order to achieve a good signal-to-noise ratio for all channels.

*In a simple model, an amplifier has a pool of optical energy available for amplification. If a channel drops in a DWDM system (or is added) then fewer (more) signals compete for this energy. As a result, the output power for the other channels increases (decreases) accordingly. These unwanted fluctuations must be compensated by either a high tolerance of the receiver for power fluctuations or by adjusting the power of the pump lasers inside the amplifier.