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��OPTICAL AMPLIFIERS ��

EDF Amplifier, Raman Amplifier, SOA

Lecture - 3

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Need of Optical Amplifiers

Increase transmission distance

by 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)

Function : Amplification of optical signal without conversion to electrical signal
Ingredients : Pump energy, amplification medium

Optical input

Optical output

amplification

medium

Pumping of energy

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The Need of Optical Amplification

Erbium-Doped Fiber Amplifiers (EDFAs) – application in long haul. Today’s 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

Extend distance light signal can travel

without regeneration

Standard Fiber

Amplifier

Pump Lasers

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General Application of Optical Amplification

In-line amplifier
Preamplifier
Power (booster) amplifier
LAN booster amplifier

Standard Fiber

Amplifier

Pump Lasers

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Erbium Doped Fiber Amplifiers (EDFA)

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Basic EDF Amplifier Design

Erbium-doped fiber amplifier (EDFA) most common

Commercially available since the early 1990’s
Works best in the range 1530 to 1565 nm
Gain up to 30 dB

Input

1480 or 980 nm Pump Laser

Erbium Doped Fiber

Output

Isolator

WDM

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Erbium-doped Fibre Amplifier

When 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.

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Issues of Amplifier Design

Optimization

maximum efficiency
minimum noise figure
maximum gain
maximum gain flatness/gain peak wavelength

Dynamic range, operation wavelength
Gain equalization
Control circuit
Monitoring of amplifier performance

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Erbium-doped Fibre Amplifier

Gain, G (dB)
10log[(P_{Signal_Out} - P_ase)/ P_{Signal_In}]

Noise Figure, F = SNR_out / SNR_in
S-band : 1440 - 1530nm
C-band : 1530 - 1565nm
L-band : 1565 - 1625nm

Spectrum of a typical EDFA

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Optical Gain (G)

G = S _Output / S _Input�S _Output: output signal (without noise from amplifier) � S _Input: input signal�
Input signal dependent

Operating point (saturation) of�EDFA strongly depends on �power and wavelength of�incoming signal

Wavelength (nm)

Gain (dB)

1540

1560

1580

10

1520

20

40

30

-5 dBm

-20 dBm

-10 dBm

P _Input: -30 dBm

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.

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Noise Figure (NF)

NF = P _ASE / (h•ν • G • B _OSA)_�P _ASE: Amplified Spontaneous Emission (ASE) power measured by OSA �h: Plank’s constant �ν: Optical frequency�G: Gain of EDFA�B _OSA: Optical bandwidth [Hz] of OSA
Input signal dependent

In a saturated EDFA, the NF�depends mostly on the�wavelength of the signal
Physical limit: 3.0 dB

Noise Figure (dB)

1540

1560

1580

1520

7.5

10

Wavelength (nm)

5.0

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 (P_ASE / B_OSA) measured at the output but normalized to the input (1/G term) with the “quantum noise” of the incoming photons (h*ν). 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.

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EDFA Categories

In-line amplifiers

Installed every 30 to 70 km along a link
Good noise figure, medium output power

Power boosters

Up to +17 dBm power, amplifies transmitter output
Also used in cable TV systems before a star coupler

Pre-amplifiers

Low noise amplifier in front of receiver

Bi-directional amplifiers

An amplifier which work in both ways

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EDFA Categories

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Typical Design

Figure in the slide shows an EDFA with all the options design

Coupler #1 is for monitoring input light
Coupler #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 EDFA’s.

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Typical Design

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Erbium-Doped Fiber Amplifiers

Figure in the next slide shows two single-stage EDFA’s packaged together
Mid 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 SOA’s and EDFA’s is that amplification is done with photons, as opposed to electronic amplification (requires O/E and E/O conversion)

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Erbium-Doped Fiber Amplifiers

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Typical Packaged EDFA

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EDFAs In DWDM Systems

Gain flatness requirements�
Gain competition �
Nonlinear effects in fibers

Optical amplifiers in DWDM systems require special considerations because of:

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Gain Flatness

Gain versus wavelength

The gain of optical amplifiers depends on wavelength
Signal-to-noise ratios can degrade below acceptable levels (long links with cascaded amplifiers)

Compensation techniques

Signal pre-emphasis
Gain flattening filters
Additional doping of amplifier with Fluorides

G

λ

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Gain Competition

Total output power of a standard EDFA remains almost constant even if input power fluctuates significantly
If one channel fails (or is added) then the remaining ones increase (or decrease) their output power

Output power after channel one failed

Equal power of all four channels

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Output Power Limitations

High power densities in SM fiber can cause

Stimulated Brillouin scattering (SBS)
Stimulated Raman scattering (SRS)
Four wave mixing (FWM)
Self-phase and cross-phase modulation (SPM, CPM)�

Most designs limit total output power to +17 dBm

Available channel power: 50/N mW�(N = number of channels)

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Erbium Doped Waveguide Amplifier (EDWA)

An erbium-doped waveguide amplifier (EDWA) consists of waveguides embedded in an amorphous erbium-doped glass substrate.
The erbium atoms provide the glass with gain in the 1,550-nm fibre-optic window.
The waveguide itself is a localised increase in the glass refractive index.
Today's manufacturers have several methods available to produce erbium-doped glass waveguides: PECVD and flame hydrolysis deposition, sputtering, ion-exchange, or ion implantation.
For the manufacture of waveguide amplifiers, the two most advanced methods are ion-exchange and sputtering.

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Erbium Doped Waveguide Amplifier (EDWA)

EDWA advantages

EDWAs are inherently compact.
One of the smallest gain block amplifiers to date, featuring 15-dB gain at 1,535 nm, fits in a 130x11x6-mm package.
EDWAs also offer a better price/performance ratio than comparable EDFAs for access and metro network applications.

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Raman Amplifier

Raman Amplifier was demonstrated in the 1980s
Unavailability of high-power diode laser pump source
Why do you need it :

amplify signals from 1270 to 1670 nm

any optical fiber can serve as the amplifying medium

Raman process itself provides high-power laser
Disadvantage: Cross-talk

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Raman Amplifier

Amplifier Gain

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Raman Amplifier

Wide bandwidth Raman amplifier can be realized using multiple pumps

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Raman Amplifier

Multistage Amplifier
Counter propagating pump

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EDFA+Raman

Combined Raman and EDFA transmission Experiment: DWDM (Lucent)

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Raman Amplifier

Advantages

Variable wavelength amplification possible
Compatible with installed SM fibre
Can be used to "extend" EDFAs
Can result in a lower average power over a span, good for lower crosstalk
Very broadband operation may be possible

Disadvantages

High pump power requirements, high pump power lasers have only recently arrived
Sophisticated gain control needed
Noise is also an issue

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Semiconductor Optical Amplifier (SOA)

Basically a laser chip without any mirrors
Metastable state has nanoseconds lifetime�(-> nonlinearity and crosstalk problems)
Potential for switches and wavelength converters

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Semiconductor optical amplifiers, like their semiconductor-laser, consist of gain and passive regions. Layers of antireflective coatings prevent light from reflecting back into the circuit while the incoming signal stimulates electrons in the gain region

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Semiconductor Optical Amplifier (SOA)

Semiconductor optical amplifiers are similar in construction to semiconductor lasers. They consist of a gain (active) section and a passive section constructed of a semiconductor material such as indium phosphide. The main difference is that SOAs are made with layers of antireflection coatings to prevent light from reflecting back into the circuit. Optical gain occurs as excited electrons in the semiconductor material are stimulated by incoming light signals; when current is applied across the p-n junction the process causes the photons to replicate, producing signal gain. The gain medium can be either a bulk or a multiple-quantum-well active layer.

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Semiconductor Optical Amplifiers

Semiconductor Optical Amplifiers (SOA’s)

Laser diodes without end mirrors
Fiber attached to both ends
Amplified version of the optical signal is produced
Advantage: bidirectional
Drawbacks

High-coupling losses
High noise figure

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Semiconductor

By adjusting the chemical composition of III-V semiconductors (typically GaInAsP) the band gap can be adjusted to give optical gain in the telecommunications windows of interest. The devices are typically 250mm long although devices of up to 1mm have been made. In general the longer devices can achieve higher gain and wider bandwidths.
The optical bandwidth of a semiconductor optical amplifier (SOA) can be as large as 100 nm and the fact that the energy source is a DC electrical current makes these devices look promising as optical amplifiers.

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Limitations/Advantages/Applications

SOAs have severe limitations:

Insufficient power (only a few mW). This is usually sufficient for single channel operation but in a WDM system you usually want up to a few mW per channel.
Coupling the input fibre into the chip tends to be very lossy. The amplifier must have additional gain to overcome the loss on the input facet.
SOAs tend to be noisy.
They are highly polarisation sensitive.
They can produce severe crosstalk when multiple optical channels are amplified.

This latter characteristic makes them unusable as amplifiers in WDM systems but gives them the ability to act as wavelength changers and as simple logic gates in optical network systems.

A major advantage of SOAs is that they can be integrated with other components on a single planar substrate. For example, a WDM transmitter device may be constructed including perhaps 10 lasers and a coupler all on the same substrate. In this case an SOA could be integrated into the output to overcome some of the coupling losses.

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SOA operation

Conduction band

Valence band

E

k

Optical transition

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SOA operation

The DC current applied to the device results in electrons being pumped into the (normally empty) conduction band and removed from the (normally full) valence band. This creates the population inversion which is a pre-cursor to optical gain. When signal photons travel through the device they cause stimulated emission to occur when an electron and hole recombine.
The phase effects in these devices can be quite strong with a gain change of 3dB corresponding to roughly a phase change of p.