Concept of EDFA ( Erbium-doped Optical Fiber Amplifier ) - Technopediasite


Tuesday, June 23, 2020

Concept of EDFA ( Erbium-doped Optical Fiber Amplifier )

Concept of EDFA ( Erbium-doped Optical Fiber Amplifier ) : You all must have heard this word EDFA a lot but few people will know its concept. What does EDFA work in DWDM, why use it etc. In this article, we will discuss about EDFA. To get complete information, full article has to be read.

As a key component of new generation optical communication systems, erbium doped fiber amplifiers ( EDFA ) have several advantages, We can also call it the god of optical communication systems, which has many advantages, such as high gain, large output power, wide operating optical bandwidth, polarization independence, low noise factor and system bit rate. Increase independent attribute. Data format. It is an indispensable core component of high-capacity DWDM systems.

Operating Theory of EDFA

To increase optical power, some passive optical components, pump sources and erbium-doped fibers are added together according to the specific optical structure. The EDFA optical amplifier is then formed. I will show below a typical optical structure of the dual pumping source erbium-doped optical fiber amplifier.
Typical internal path of EDFA
Path of EDFA in DWDM

In the image above, the signal light and pump light from the pumping laser are connected via a DWDM multiplexer, then sent to Erbium-doped fiber (EDF). Two pumping lasers have a two-stage pump. Excited by the pumping light, EDF produces the amplification function. Therefore, the task of amplifying the optical signal is implemented.

Erbium-doped optical fiber ( EDF )

Erbium-doped optical fiber (EDF), doped with Er3 + of a given density, is the kernel of the optical fiber amplifier. To illustrate its amplification principle, we have to start with an energy level diagram of Er3 +.

The outer-shell electrons of Er3 + in the figure below have a three-tier structure (E1, E2 and E3), where E1 is the ground state, E2 is the metastable state, and E3 is the higher level, as shown in the figure below.
Diagram of EDFA energy level
EDFA Energy level

When high energy pumping lasers are used to excite the EDF, lots of erbium ions are excited from ground state to high levels E3. However, the high level is not stable and the erbium ions are soon degraded to the metastable state E2 via a radiation-free decay process (ie no photons are free).

The E2 level is a metastable energy band at which the particles have a relatively long survival period. Particles stimulated by the pumping light gather at this stage through noninvasive transition. Thus, the population inversion distribution is implemented.

When an optical signal of wavelength 1550nm passes through this erbium-doped fiber, particles in the metastable state are transferred to the ground state through excited radiation and produce photons similar to photons of the incident signal light.

This greatly increases the amount of photons in the signal light, that is, by implementing the function of continuously increasing the transmitted signal light in the EDF.

Optical coupler (WDM)

The optical coupler, as its name implies, is the function of coupling. It connects the signal light and pumping light and sends them to the erbium-doped fiber. It is also called optical multiplexer, which typically employs optical fiber fusible cone multiplexes.

Optical Isolator (ISO)

Optical isolator (ISO), a type of component utilizing the Faraday magnetooptical effect, only allows unidirectional light transmission. Through the light path, the functions of the two Isolators are as follows: Input isolators can block the backward ASE in the EDF, preventing it from interfering with the system's transmitters and causing large noise when it is reflected in the input end and re-entering the EDF.

The output isolator prevents the amplified optical signal, when the output is finally reflected, from reproducing the EDF, consuming particles, and affecting the amplification characteristics of the EDF.

Pumping laser (PUMP)

The pumping laser, the energy source of EDFA, provides energy to amplify the optical signal. Generally, it is a semiconductor laser with an output wavelength of 980nm or 1480nm.

While passing through the EDF, the pumping light pumps erbium ions from low levels to high levels. Thus the antonyms of the population are formed. When the signal passes through the light, the energy will be transferred to it. Therefore, optical amplification is implemented.

Optical Splitter (TAP)

The optical splitter used in EDFA is one by two components. Its function is to tap a small portion of the optical signal to monitor the optical power of the main channel.

Optical detector (PD)

PD is an optical power detector. Its function is to convert the optical power obtained through photoelectric conversion into photocurrent. Therefore, it monitors the input and output optical power of the EDFA module.

Applications of EDFA

As per its location in the DWDM optical transmission network, EDFA can be classified into three types 1. Booster amplifier (BA), 2. line amplifier (LA) and 3. Preamplifier (PA). Now I will discuss all three in detail.

Booster amplifier (BA)

The booster amplifier is installed behind the transmitters of the terminal equipment or regeneration equipment, as shown below. The major function of the booster amplifier is to boost the launched power and longer transmission distance so that the power injected into the fiber can be increased (typically above 10dBm).

Therefore in some documents, it is also designated as a power booster amplifier. Here, its noise characteristic is not necessarily high. The major requirement is linear power amplification characteristic.

Typically the booster amplifier operates in the saturation range of the gain source or input power to increase the conversion efficiency from pumping source power to optical power power.

Location of the amplifier in the regenerator section
Location of Amplifier

Line amplifier (LA)

The line amplifier is located in the middle of the entire regeneration section, as shown in the figure below. It is an application for inserting EDFA into optical fiber transmission links and amplifying the signal directly.

A regenerative segment can be configured according to demands with multiple line amplifiers. The line amplifier is mainly implemented in long haul communications or CATV distribution networks. Here, EDFA requires high short-signal gain and low noise factor.
Location of the line amplifier in EDFA
Line- Amplifier 

Pre-amplifier (PA)

The pre-amplifier is located at the end of the regeneration section but in front of the optical receiving device, as shown in the figure below. The main function of this amplifier is to increase the small signal along the link and to increase the received sensitivity of the optical receiver.

The main problem here is noise. The main noise in EDFA is spontaneous emission (ASE). This noise makes the optoelectronic detector output three noise components, namely optical power, signal-ASE beat noise and additional shot noise due to the increase of ASE-ASE beat noise.

By using a narrow-band optical filter (1nm bandwidth), most ASE-ASE beat noise can be filtered and additional shot noise can be reduced. But the signal-ASE beat noise cannot be filtered. Despite this, the adoption of optical filters greatly improves the noise characteristic of EDFA.

The pre-amplifier greatly improves the sensitivity of the direct detecting receiver. For example, an EDFA receiver sensitivity of 2.5Gbit / s can be up to -43.3dBm. An improvement of about 10dB is achieved compared to direct detection receivers without EDFA.
Location of the pre-amplifier in the regenerator section

BA, PA and LA differ from each other in that they have different locations in the DWDM network. There are most important difference lies in their input optical power and gain:

BA: Comparatively high input optical power and low gain;
PA: Comparatively low optical power and low gain similar to BA;
LA: Comparatively low input optical power similar to PA, but its advantage is larger than BA.

Gain Control of EDFA

EDFA gain flatness control

In DWDM systems, the more optical channels multiplexed, the more optical amplifiers require cascading. EDFA gain flatness control requires that a single amplifier occupy a wider and wider bandwidth.

However, EDFA 1549 and 1561nm, based on ordinary pure silicon optical fiber, have a very narrow flat range between the range of about 12nm. And the gain fluctuation between 1530 and 1542nm is huge, up to about 8dB.

When the channel arrangement of the DWDM system exceeds the flat gain limit, channels near 1540nm will suffer severe signal-to-noise degradation and normal signal output cannot be guaranteed.

To solve the above problem and adapt to the development of DWDM systems, EDFA is flattened on the basis of aluminum-doped silicon optical fiber. This improves the operating wavelength bandwidth of EDFA and suppresses fluctuations. Up-to-date maturing techniques can achieve a 1dB gain flattened limit that spans almost the entire aerobium pass-band (1525nm ~ 1560nm).

Fundamentally, it has solved the problem of simple EDFA's profitability. The figure below compares the gain curves of non-aluminum-doped EDFA and aluminum-doped EDFA.
Technically, the range of 1525nm ~ 1540nm in EDFA gain curve is called blue band area and 1540nm ~ 1565nm range is called red band area. When the transmission capacity is less than 40Gbit / s, the red band region is usually preferred.
Amplifier gain fitness
Amplifier Gain Fitness 

EDFA Gain-Locking

EDFA gain-locking is a significant problem because the WDM system is a multi-wavelength working system. When certain wavelengths are dropped, their energy will be transferred to those uncontrolled signals due to the competition for gain.

Thus the power of other wavelengths increases. At the receiving end, a sudden increase in the electrical level causes the error to be possible. In the case of limiting, if seven wavelengths of eight wavelengths are dropped, all the energy will be centered one wavelength to the left. This will result in achieving strong nonlinear effects or power overload of the receiver, and will also cause lots of errors.

There are several benefit-locking techniques for EDFA. A specific method is to control the gain of the pumping laser. Internal monitoring of the EDFA controls the output of the pumping source by monitoring the electric circuit input-output power ratio.

When some signals of input wavelength are dropped, the input power will decrease and the output-input power ratio will increase. Through the feedback circuit, the output power of the pumping source will be reduced to maintain the gain (output / input) of EDFA. Therefore, the total output power of EDFA decreases and the output signal power is kept constant. The process is shown in the figure below.
Gain-locking technology of controlling the pumping laser
Gain-locking technology in EDFA

Another method is the saturation wavelength. At the transmitted end, the system sends another wavelength as the saturation wavelength, leaving eight operating wavelengths. In normal cases, the output power of this wavelength is very low.

When some line signals are dropped, the output power of the saturation wavelength will automatically increase to compensate for the energy of the lost wavelength and to maintain the output power and gain of the EDFA.

When the multi-wavelength line signal is restored, there will be a steady decrease in the output power of the saturation wavelength. This method directly controls the output of the saturation wavelength laser, so its speed is faster than that of controlling the pumping source.

Performance comparison of EDFA gain unlocking and locking
Performance EDFA gain

Limitations of EDFA

EDFA solves the problem of line attenuation in DWDM systems but it also create some new problems. I am going to discuss about the problems brings by EDFA.

Non-linearity problem

Although enhanced by adopting EDFA, optical power is not better. When it reaches a certain level, the optical fiber will produce the Nonlinear effect (including Raman scattering and Brillouin scattering).

In particular, EDFA has been shown to be more influenced to affect Brooklyn scattering (SBS). The nonlinear effect greatly limits the amplification performance of EDFA and the implementation of long-range repeater transmission.

Optical surge problem

The EDFA input can rapidly increase optical power. However, since its dynamic gain variation is slow, optical buoyancy will occur when the input signal power jumps, namely a peak for the output optical power.

The optical surge phenomenon is particularly evident in the case of EDFA cascading. The peak power can be up to a few watts and it is possible to damage the end surface of the O / E converter and optical connector.

Dispersion problem

Although the problem of attenuation-limited repeaterless long-haul transmission is resolved after the adoption of EDFA, the total dispersion becomes longer. Thus the pre-attenuation limiting system changes to the dispersion limiting system.

Last Word

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  1. To work with this system, we must know how to minimize stress and this is vital. Thank you for the concept!

  2. A good description of the components in EDFA and the principle of operation. Thanks a lot.

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  4. Brooklyn Scattering? There's nothing like that. You may need to check SBS again.
    However, that's a good effort at describing EDFA..

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