Optical Amplifiers

OpticalAmplifiers.ppt (3.11.2004) Copyright © Jouko Kurki, 2004

Jouko Kurki:

Optical amplifiers in single channel and WDM systems; nonlinear effects in fiber


Optical Fiber Amplifier Optical Fiber Amplifier --Operation PrincipleOperation Principle

980 or 1480 nm Pump Laser Residual Pump Power

1550 nm Signal Amplified 1550 nm Signal

Erbium Doped Fiber

Level 2

Level 1

Level 0

Upconversion (pumping) usingpump energy

Electron energy

Spontaneous emission(noise)

Stimulated emission(signal amplification)

Amplification is based on stmulated emission (as in laser)


Optical Amplifier classification by their application

Booster ( or Power ) Amplifier• In most cases the best solution to increase a hoplength. It is used in the transmitting end

of a optical link, right after the transmitter. Typical output powers range from +10...+16dBm, with input -6..+2 dBm. Booster is operated in saturated mode, so max. output power depends mostly on amplifier model, not the input power.

(In-)Line Amplifier• Located somewhere between Regenerator stations. Combination of a preamplifier (high

gain) and booster amplifier (high power output). Rises the optical level of the signal typically by 30 dB. In most cases requires some kind of additional optical service channel to regenerator stations to carry the management traffic of the amplifier equipment.

Preamplifier• Amplifies the optical signal arriving to receiver. The achieved amplification advantage

rises with the transmission speed. At 2.5 Gbit/s a sensitivity of about -39 dBm is achieved (normal STM-16 receiver by it self gives about -30 dBm). Unlike booster, preamplifier needs a narrowband filter in its output (typically around 0.5 nm) to filter the amplified spontaneous emission band generated by the OFA. In some cases a WDM muxcan serve as a fixed filter. This requires a fixed wavelength signal laser source


Amplifier types and typ. distances for STM-16 (2488 Mb/s)

OT OR

STM-16

-2 dBm -27 dBm

24 dB(80 km)

1 dB Penalty

OT OR

Booster

STM-16

+12 dBm -27 dBm

37dB(120 km)

2 dB Penalty

No amplifier

Booster (power) amplifier


Amplifier types, cont.

OT OR

Booster Preamp.STM-16

+12 dBm+16 dBm

-38 dBm

47/51 dB(160 km)

3 dB Penalty

120 kmTx Rx

Booster

WDM WDM

λ1

λ2

λ4

λ3 λ1,λ2,λ3,λ4

λ1

λ2

λ4

λ3

Rx

Rx

Rx

Tx

Tx

Tx

Booster and Preamplifier

4 Channel WDMWith Booster. ~40 km without amplifier, 80-120 km with amplifier.

Wavelength Division Multiplexing (WDM) allows several fixed wavelengths to be transmitted over one fiber. One optical amplifier amplifies all wavelengthssimultaneously ! – but power / wavelength decreases with number of wavelengths.


Optical Amplifier DefinitionsOptical amplifier• A device which amplifies optical signal and rises optical power level. The amplification is purely

optical, no conversion to electricity occurs (opposite to an conventional regenerator).• Advantages: Independent of transferred data speed or format, so the same OFA can be used when the

transmitting line equipment is upgraded. Increases hoplengths.• Main types:

– Optical Fiber Amplifiers (OFA), amplification happens in a fiber. Erbium Doped Fiber Amplifier (EDFA) is the most popular type today. Amplification happens in Erbium doped silica fiber at 1550 nm wavelength range. Another type using Prasedymium as active material amplifies at 1310 nm wavelength range (not commercialized).

– Raman amplifiers. High power pumping causes amplification in a fiber. Raman amplifiers are used sometimes together with EDFA’s to get very long spans.

– Semiconductor Optical Amplifiers (SOA), a device that has similarity with a laser. SOA’s cab be used as no-loss switches.

Optical Fiber Amplifier (OFA)Erbium Doped Fiber Amplifier (EDFA)

• The most common OFA type. Pump light of 980nm or 1480 nm converts into signal amplification in erbium fiber. The operational region of this type of amplifier is typically 1530-1565 nm (C-band) of 1570-1610 nm (L-band).


Erbium doped fiber enrgy levels for EDFA

EDFA levels allow pumping at 1480 nm or at 980 nm.

1480 nm pumping is at the same wavelength as downward transitions. Thus pumping itself causes stimulated emissions and very high population inversion is difficult to achieve. Result is more noise.

980 nm uses 3rd level with fast decay to the metastable level 2. Very high population inversion and low noise performance can be achieved. 980 nm pumping is used in preamplifiers and first stage of line amplifers.


EDFA gain block components

Pumplaser

WDM

CouplerIsolator

Er-fiber

Input-monitor

Output-monitor

IN OUT

Electrical interface

Coupler

Isolator

Main components of the EDFA are:• Pump laser – provides pumping power. Power several hundred mW or more. Very

expensive.• Erbium doped fiber (1…20 m depending on type ) amplification occurs in Er-

doped fiber.• Other components for coupling monitoring, In power amplifiers additionally

isolators are used in input /output.

• Typical size of gain block: 1 cm thick (to fit on PCB), w x d =(5..10) x (5..10) cm• Typical price range a few thousand Euros… 20.000 Eur depending on power and

other parameters


Amplifier variations and componentsIsolators prevent reflections from causing noise to the EDFA. Normally they are used with high power levels (above around 15 dBm).

In In-Line amplifiers and high power amplifiers, two gain stages are often used:1.First amplifier uses 980 nm pumping and works as high gain preamplifier. A gain equalizing filter or dispersion compensating fiber could be inserted after first stage.2.Second stage uses 1480 nm pumping and boosts power (not so much gain needed).

For high output power two pumps can be used (forward / backward pumping). Adding two polarizations for both; four pumps are possible for very high power.


WDM Wavelengths - Optical Amplifier bands

[dB]

Wavelength [nm]1500 16001550

30-50 nm optical band

WDM channels

• Gain 10 - 30 dB in 1530 - 1565 nm band

• Amplifier noise an Gain tilt sets limit for cascading amplifiers

• High power can cause non-linear effects

• Width of flat part of amplifying band sets limit to no. channels

• Narrow channel spacing 3.2, 1.6 or 0.8 nm (0.8 nm =100 GHz)

• C-band: 1530-1565 nm, 30 nm => 32 ch @100 GHz spacing ad 64 @50 GHz spacing

• L-band: 1570-1625 nm, e.g. additional 64 ch.

• For WDM: Possible to do 50 GHz (0.4 nm -> ~ 80 wavelengths at C-bad) channel spacing at 2.5 Gb/s and 100 GHz at 10 Gb/s. 50 GHz spacing at 10Gb/s also available.

Fiber attenuation

Amplifier gain


Optical fiber amplifier parameters (OFA)

• Output power; strongly affects link budget• Wavelength ranges and flatness of amplification and output power; affects number of

channels and maximum number of cascaded amplifiers• Noise performance in preamplifiers and line amplifiers

Typical values:• OFA provides a 10-30 dB amplification at 1530-1565 nm band; transparent to signal,

bitrate etc.• Output power, typ. 12-25 dBm • Amplifier noise (NF typ. 4-7 dB) sets limitations for cascaded chains; optical filter

needed after preamplifier to filter Amplified Spontaneous Emission (ASE) noise• OFA gain tilt can cause problems in WDM-systems (increased WDM-isolation

requirement)


EDFA parameters, cont.Gain flatness is important when cascading many amplifiers to avoid power of some channels to be far lower than others (this could cause crosstalk).

Typical gain flatness for good EDFA is shown here. Note wavelength dependence especially at high gain.


Power level calculation in WDM system

• In WDM system the total power is divided by N wavelengths.

• Thus if Booster output power is P, the output power / wavelength is P/N, where N is number of wavelengths. E.g, if P = 100 mW and N= 8 then P/λ = 12.5 mW. This power is used in link power budget calculations (each λ has to get through).

• When calculationg with dBm, each dubling of the number of wavelenghts reduces the power / wavelength by 3 dB. So for 2 wavelengths –3 dB, for 4 wavelengths –dB, for 8 wavelengths –9 dB and so on.

• For very big number of wavelengths total power gets very high. This makes amplifiers difficult to make and very expensive.


ADVANCED READING –OPTICAL AMPLIFIER


Noise in EDFAThe signal gets amplified by the stimulated emission process producing photons having same wavelength as incoming photons.In addition spontaneous emissions occur over a broad wavelength spectrum. These spontaneous emissions get further amplified causing noise called Amplified Spontaneous Emission (ASE) noise. The effect can be seen in fig b)The noise is modeled with Noise Figure F defined as ratio of signal to noise ration S/N at input (i) dived by output (o) a)

b)

For high gain amplifiers:

Here N1 and N2 are the populations of higher and lower energy levels. The minimum F is obtained when N2>>N1, I.e strong population inversion. In EDFA’s this is achieved by 980 nm pumping and resulting noise figure with adequate pumping in good amplifiers is nearly 3 dB (factor 2 degradation in noise figure).


WDM transmission with amplifiers and

dispersion compensation

• At very high bitrates and long distances dispersion can be compensated by dispersion compensating fiber.

• However, due to variation of dispersion over the WDM frequencies cumulated dispersion finally limits the length x bitrate product.

• Dispersion compensation can be achieved with negative dispersion dispersion compensating fibers (Disco) for case when λ0less than operating wavelength or with standard SMF when λ0 > operating wavelength.

• Gain tilt of OFA (Different wavelengths are differently amplified) limits the number of cascaded amplifiers.


Non-linear effects• High power at booster output can cause non-linear effects in fiber causing

problems. The effects can be grouped to: a) ones causing signal distortion:Self Phase Modulation (SPM) and Cross Phase Modulation (XPM) due to Kerr-effect and b) Effects causing crosstalk in WDM-systems: Four Wave mixing (FWM), Stimulated Raman Scattering (SRS). Stimulated BrillouinScattering (SBS) sets limitation to power level and laser linewidth(Problem at 10 Gbit/s speed and beyond).


Four Wave mixing (FWM)• Four Wave mixing (FWM) causes intermodulation products. If ω1, ω2, ω3 are three signal frequencies

propagating in a (non-linear) fiber, then FWM results in generation of frequencies given by:ω1 +/- ω2, +/-ω3

• If any of these frequencies hit an existing WDM-channel crosstalks causes transmission impairments if power level exceeds 15-20 dBm.

• FWM became a problem in dispersion shifted fiber, and therefore a fiber with slight dispersion was developed (NZ-DS, ITU-T G.655). In this fiber slight dispersion prevents phase matching of signal frequencies over long periods and FWM is suppressed. Still power levels need to be maintained moderate.


Self Phase Modulation (SPM)

In silica fiber, when power is high the refractive index increases with light intensity (I) due to Kerr effect

Here intensity I = P/Aeff, where P is optical power and Aeff is effective area of the SM-fiber core. In typical SM-fiber

Thus intensity of the signal causes variation to the refractive index: The effect is called Self Phase Modulation (SPM) . The primary effect of SPM is to broaden the spectrumwhile pulse shape stays constant. The leading edge (beginning) of the pulse will have longer wavelengths and the trailing edge shorter wavelengths. Normally this together with fiber dispersion leads to pulse broadening.

λ gets longer Time

λ gets shorter


SPM and solitonsIn silica fiber above zero dispersion wavelength the longer wavelengths travel slowlier, which in absence of SPM causes chirping so that during the pulse the instantaneous wavelength gets longer (slowlier λ’s come later) .Due to SPM the pulse wavelength gets shorter during the pulse ( frequency spectrum of pulse gets broader). Dispersion causes temporal broadening of the pulse. With positive dispersion the shorter wavelengths travel faster and compensate for pulse broadening due to SPM. With proper choice of pulse shape and power level the effects can compensate each other.Such a pulse does not broaden in time domain (As in case of linear dispersion) nor in frequency domain (as in case of SPM). This kind of pulse is called a soliton.Soliton does not broaden during transmission and has therefore a high potential for super high bandwidth systems.The practical difficulty is to keep the required high power level all the way during propagation in ling distance systems.

λ gets longer

λ gets shorterSPM:

λ 1550 nm, positive dispersion

During time dispersed pulse wavelength gets longer (slowlier λ come later)

Time


Solitons


Soliton power

For a soliton at 1550 nm wavelength, the peak power (P) in mW and FWHM pulse duration τf in picoseconds (τf ~1.18 t0 for a Gaussian pulse) with fiber dispersion D in ps/nm.km, are related as

E.g. for a 10 ps soliton in dispersion shifted fiber with D = 1 ps/nm.km, P ~15 mW (note that power for conventional SMF would be 270 mW)


Demonstration of soliton transmission, Ref. Ghatak

Pulse power for solitionsin SM-fiber 19 dBm in the experiment


Cross Phase Modulation (XPM)

• Cross Phase Modulation influences WDM systems.

• If power of on WDM channels is high enough to cause nonlinear change of refractive index, then this index changes is seen by other wavelengths. Net effect is crosstalk

Optical Amplifiers

Documents

booster power amplifier

type of amplifier

erbium fiber

amplifier types

line amplifier

optical power level

optical level

amplifier equipment