Comparison Of Different Optical Amplifiers

Optical amplifier is an important technology for optical communication networks. Without the need to first convert it to an electrical signal, the optical amplifiers are now used instead of repeaters. As we know, there are several types of optical amplifiers. Among them, the main amplifier technologies are Doped fiber amplifier (eg. EDFA), Semiconductor optical amplifier (SOA) and Fiber Raman amplifier. Today, we are going to study and compare them in this paper.

 

Before the comparison of the different optical amplifiers, let’s take a closer look at fiber optic amplifer. In general, a repeater includes a receiver and transmitter combined in one package. The receiver converts the incoming optical energy into electrical energy. The electrical output of the receiver drives the electrical input of the transmitter. The optical output of the transmitter represents an amplified version of the optical input signal plus noise. Repeaters do not work for fiber-optic networks, where many transmitters send signals to many receivers at different bit rates and in different formats. However, unlike a repeater, an optical amplifier amplify optical signal directly without electric and electric optical transformation. In addition, an ideal optical amplifier could support multi-channel operation over as wide as possible a wavelength band, provide flat gain over a large dynamic gain range, have a high saturated output power, low noise, and effective transient suppression. Several benefits of optical amplifiers as the following:

 

Support any bit rate and signal format

Support the entire region of wavelengths

Increase the capacity of fiber-optic links by using WDM

Provide the capability of all-optical networks, not just point-to-point links

OK, after a brief introduction of the optical amplifiers, we formally begin today’s main topic. As we talk above, there are three main types of today’s amplifier technology. Each of them has their own working principle, features and applications. We will describe them one by one in the following paragraphs.

 

Doped fiber amplifier (The typical representative: EDFA)

Erbium-doped fiber amplifier (EDFA) is the most widely used fiber-optic amplifiers, mainly made of Erbium-doped fiber (EDF), pump light source, optical couplers, optical isolators, optical filters and other components. Among them, a trace impurity in the form of a trivalent erbium ion is inserted into the optical fiber’s silica core to alter its optical properties and permit signal amplification.

 

Advantages & Disadvantages of EDFA

Advantages

 

EDFA has high pump power utilization (>50%)

Directly and simultaneously amplify a wide wavelength band (>80nm) in the 1550nm region, with a relatively flat gain

Flatness can be improved by gain-flattening optical filters

Gain in excess of 50 dB

Low noise figure suitable for long haul applications

Disadvantages

 

Size of EDFA is not small

It can not be integrated with other semiconductor deviecs

Semiconductor optical amplifier (SOA)

Semiconductor optical amplifier is one type of optical amplifier which use a semiconductor to provide the gain medium. They have a similar structure to Fabry–Perot laser diodes but with anti-reflection design elements at the end faces. Unlike other optical amplifiers SOAs are pumped electronically (i.e. directly via an applied current), and a separate pump laser is not required.

 

Hot to Transport and Aggregate for Optical Amplifiers

by www.fiber-mart.com

Network operators have the common basic target to produce cost-efficient telecommunication services. When considering operators from different nations including carriers operating worldwide, a variety of network architecture designs need to be considered. The suitable network design depends on the individual national properties with respect to the telecommunication services to be provided, such as the local population density distributions, the characteristic local residential consumer behavior, for example, the demand for voice telephony, internet protocol, or broadband TV, or the distribution and service level agreement (SLA) requirements of the business customers. The design of the network is governed by the topology. DWDM network for example, ring, star, mesh, by the purpose (access, aggregation, transport), by the mean and maximum link distance, and by the density and degree of switching or grooming nodes. All this has a direct impact on the choice of amplification in the optical multiplex section (OMS) of DWDM systems and on the local placement of DWDM optical amplifiers.

 

The diameter of networks is one of the most obvious distinctions. Nationwide networks in the United States follow engineering rules different from those applicable to the national backbones in European countries, especially when the design of amplifier maps and the positioning of photonic cross connect (PXC)/ROADM based nodes are considered. The largest diameters within all optical transport is achieved in submarine cable networks that deploy lumped amplifier span designs with very short distance between adjacent DWDM EDFA and eventually supported by additional distributed Raman amplification.

 

Besides the distance, many other parameters influence decisions for special network layouts, such as the local distribution of population and industry to be connected, the traffic patterns and capacity evolution, the telecommunication service kinds and classes, and much more. Also, the deployment choice of lumped inline amplifiers . distributed Raman amplification or hybrid schemes, gain equalizing devices, electrical or optical inline regenerators, and electrical grooming nodes or optically amplified multi degree ROADM nodes is strongly dependent on these multiple factors.

 

The research shows that some network options with consequences for optical amplifier applications will be described against the background of European national network. Here a variety of requirements force operators to select many different network architectures for different local domains with suitable primary foci to meet optimum transport efficiency and operational performance. The present trend is to consolidate different network domains into a converged platform to simplify the overall network management process.

 

European networks cover many scenarios of possible architectures, for ultra long-haul (ULH) pan-European backbone to national European backbone, metro, and access networks. The typical distance characteristics of link lengths between major backbone nodes for North America and pan-European networks, but the distance are significantly shorter. The backbone links of national networks of the different European states like Germany reference network. Here the mean fiber link distance between major between major cities and thus backbone nodes is about 400 km which could be still called “metro”. However, as for the next generation architecture it is intended to intensively apply optically transparent transmit nodes （ROADM/PXC）, future national networks will also demand systems with a longer reach. In the following sub-sections we will focus on typical modern intranational European network architectures.

 

Future converged telecommunication platforms will comprise access, aggregation, and transport networks. Their design rules depend on their primary purpose: either traffic aggregation or distribution from and to customers, or the transport and routing of large amounts of combined capacity.

Transport and Aggregation Networks Solutions for Optical Amplifiers

by www.fiber-mart.com

Network operators have the common basic target to produce cost-efficient telecommunication services. When considering operators from different nations including carriers operating worldwide, a variety of network architecture designs need to be considered. The suitable network design depends on the individual national properties with respect to the telecommunication services to be provided, such as the local population density distributions, the characteristic local residential consumer behavior, for example, the demand for voice telephony, internet protocol, or broadband TV, or the distribution and service level agreement (SLA) requirements of the business customers. The design of the network is governed by the topology. DWDM network for example, ring, star, mesh, by the purpose (access, aggregation, transport), by the mean and maximum link distance, and by the density and degree of switching or grooming nodes. All this has a direct impact on the choice of amplification in the optical multiplex section (OMS) of DWDM systems and on the local placement of DWDM optical amplifiers.

 

 

The diameter of networks is one of the most obvious distinctions. Nationwide networks in the United States follow engineering rules different from those applicable to the national backbones in European countries, especially when the design of amplifier maps and the positioning of photonic cross connect (PXC)/ROADM based nodes are considered. The largest diameters within all optical transport is achieved in submarine cable networks that deploy lumped amplifier span designs with very short distance between adjacent DWDM EDFA and eventually supported by additional distributed Raman amplification.

 

Besides the distance, many other parameters influence decisions for special network layouts, such as the local distribution of population and industry to be connected, the traffic patterns and capacity evolution, the telecommunication service kinds and classes, and much more. Also, the deployment choice of lumped inline amplifiers . distributed Raman amplification or hybrid schemes, gain equalizing devices, electrical or optical inline regenerators, and electrical grooming nodes or optically amplified multi degree ROADM nodes is strongly dependent on these multiple factors.

 

The research shows that some network options with consequences for optical amplifier applications will be described against the background of European national network. Here a variety of requirements force operators to select many different network architectures for different local domains with suitable primary foci to meet optimum transport efficiency and operational performance. The present trend is to consolidate different network domains into a converged platform to simplify the overall network management process.

 

European networks cover many scenarios of possible architectures, for ultra long-haul (ULH) pan-European backbone to national European backbone, metro, and access networks. The typical distance characteristics of link lengths between major backbone nodes for North America and pan-European networks, but the distance are significantly shorter. The backbone links of national networks of the different European states like Germany reference network. Here the mean fiber link distance between major between major cities and thus backbone nodes is about 400 km which could be still called “metro”. However, as for the next generation architecture it is intended to intensively apply optically transparent transmit nodes （ROADM/PXC）, future national networks will also demand systems with a longer reach. In the following sub-sections we will focus on typical modern intranational European network architectures.

 

Future converged telecommunication platforms will comprise access, aggregation, and transport networks. Their design rules depend on their primary purpose: either traffic aggregation or distribution from and to customers, or the transport and routing of large amounts of combined capacity.

Comparison of Different Types of Optical Amplifiers

by www.fiber-mart.com

Optical amplifier is an important technology for optical communication networks. Without the need to first convert it to an electrical signal, the optical amplifiers are now used instead of repeaters. As we know, there are several types of optical amplifiers. Among them, the main amplifier technologies are Doped fiber amplifier (eg. EDFA), Semiconductor optical amplifier (SOA) and Fiber Raman amplifier. Today, we are going to study and compare different types of optical amplifiers in this paper.

 

Before the comparison of the different types of optical amplifiers, let’s take a closer look at fiber optic amplifier. In general, a repeater includes a receiver and transmitter combined in one package. The receiver converts the incoming optical energy into electrical energy. The electrical output of the receiver drives the electrical input of the transmitter. The optical output of the transmitter represents an amplified version of the optical input signal plus noise. Repeaters do not work for fiber-optic networks, where many transmitters send signals to many receivers at different bit rates and in different formats. However, unlike a repeater, an optical amplifier amplify optical signal directly without electric and electric optical transformation. In addition, an ideal optical amplifier could support multi-channel operation over as wide as possible a wavelength band, provide flat gain over a large dynamic gain range, have a high saturated output power, low noise, and effective transient suppression. Several benefits of optical amplifiers as the following:

 

Support any bit rate and signal format

Support the entire region of wavelengths

Increase the capacity of fiber-optic links by using WDM

Provide the capability of all-optical networks, not just point-to-point links

OK, after a brief introduction of the optical amplifiers, we formally begin today’s main topic. As we talk above, there are three main types of today’s amplifier technology. Each of them has their own working principle, features and applications. We will describe them one by one in the following paragraphs.

 

Doped fiber amplifier (The typical representative: EDFA)

Erbium-doped fiber amplifier (EDFA) is the most widely used fiber-optic amplifiers, mainly made of Erbium-doped fiber (EDF), pump light source, optical couplers, optical isolators, optical filters and other components. Among them, a trace impurity in the form of a trivalent erbium ion is inserted into the optical fiber’s silica core to alter its optical properties and permit signal amplification.

 

Working Principle

The working principle of the EDFA is to use the pump light sources, which most often has a wavelength around 980 nm and sometimes around 1450 nm, excites the erbium ions (Er3+) into the 4I13/2 state (in the case of 980-nm pumping via 4I11/2), from where they can amplify light in the 1.5-μm wavelength region via stimulated emission back to the ground-state manifold 4I15/2.

 

Advantages & Disadvantages of EDFA

Advantages

 

EDFA has high pump power utilization (>50%)

Directly and simultaneously amplify a wide wavelength band (>80nm) in the 1550nm region, with a relatively flat gain

Flatness can be improved by gain-flattening optical filters

Gain in excess of 50 dB

Low noise figure suitable for long haul applications

Disadvantages

 

Size of EDFA is not small

It can not be integrated with other semiconductor deviecs

Semiconductor optical amplifier (SOA)

Semiconductor optical amplifier is one type of optical amplifier which use a semiconductor to provide the gain medium. They have a similar structure to Fabry–Perot laser diodes but with anti-reflection design elements at the end faces. Unlike other optical amplifiers SOAs are pumped electronically (i.e. directly via an applied current), and a separate pump laser is not required.

 

A Primer on Optical Amplifiers

by www.fiber-mart.com

Attenuation is major concern in every fiber optic system. The further a digital signal travels, the more its strength diminishes. Every splice, every connector, and even the fiber itself will contribute a tiny bit more to the overall system loss. Fortunately, for spans that are too long or have losses that are too great, a solution exists in the form of optical amplifiers.

When there is too much attenuation, the bit-error rate rises rapidly. Ideally, optical signals should be transmitted at the maximum possible data rate that offers a tolerable bit error rate, typically between 10–9 and 10–12. Bit-error rate issues can be avoided by either amplifying or regenerating the signals during transmission. Regeneration converts a digital optical signal into an electrical one, transmits it, and then restores it to an optical signal at its destination. While in its electrical form, the signal is re-shaped and re-timed to correct for signal dispersion. This approach can be costly. However, in some instances, optical amplification will achieve the same result.
 
Optical amplifiers increase the amplitude of a weak optical signal without the costly process of regeneration. An amplifier has fewer parts than an electronic regenerator and is protocol independent, thereby eliminating the need for optical-to-electronic conversion. Their operation is based on lasing principles, in which photons passing by an atom with an electron are pumped into a higher energy orbit. The photon will stimulate the electron to drop to a lower energy orbit and the energy difference between the two orbits will be the energy of an identical photon. Doubling the number of photons will result in a power gain of 3 dB. These two photons also may stimulate other photons, which will further increase the gain.
 
There are three types of optical amplifiers that are commonly used:

• Erbium-doped fiber amplifiers (EDFAs)
• Raman fiber amplifiers
• Semiconductor optical amplifiers (SOAs)

 
The most commonly implemented is the EDFA. An EDFA contains an optical fiber that has been doped with the rare earth element erbium, which can be pumped using either a 980-nm or 1480-nm pump laser for optical gain. Most EDFA amplifiers operate in the low loss C-band (1530 to 1565 nm) and typically have a signal gain in the range of 15 to 25 dB, depending on the power of the pump laser and the length of the erbium doped fiber. The gain can be adjusted by varying the output power emitted by the pumping laser. This pumping optical power in the EDFA will cause the erbium electrons to be pumped into a greater energy higher orbit. Once in the higher orbit, the electrons can be stimulated by a photon to emit an identical wavelength photon in the same direction.
 
EDFAs were originally developed for oceanic long-haul and dense wavelength division multiplexing applications. However, their highly desirable operating characteristics quickly forced their migration towards terrestrial long-haul, metropolitan area networks, cable television, and fiber to the home—where analog transmission mandates the low attenuation of the 1550-nm wavelength.
 
The second type of amplifier, the Raman amplifier, involves a non-linear interaction called Raman scattering that occurs between light and the atoms of the glass transmission fiber itself. Stimulated Raman scattering is a similar process where photons of an applied optical signal stimulate the scattering process. This interaction has the effect of shifting energy from a strong pump beam to a weaker signal beam as both pass along the length of a fiber, resulting in amplification of the signal.
 
The energy shift depends on the type of glass used, but the wavelength depends on the pump beam, so Raman amplification can be used across a wide range of wavelengths by changing the pump wavelength. Use a different pump wavelength, and you can amplify a different set of signal wavelengths. Furthermore, multiple pump beams can be used to create any desired gain profile, thus allowing broadband amplification across multiple bands.
 
Unlike EDFAs, where the gain spectrum is constant and determined by the erbium atoms, the Raman amplification gain-spectrum depends on the pump wavelength. The maximum gain occurs when the pump wavelength is approximately 100 nm shorter than the signal wavelength. For example, in silica glass fibers, a 1550-nm signal is amplified through absorption of pump energy at approximately 1450 nm.
 
While conventional EDFA technology can be used at the terminal sites for links shorter than about 160 kilometers, longer spans will require Raman amplification. For extreme distances, both EFDAs and Raman amplification may be used.
 
Other applications for Raman amplifiers include high-loss spans in regional, long haul, and ultra long-haul systems with span lengths greater than 1000 kilometers. They are also ideal for applications where commercial, legal, or security constraints make intermediate amplification sites impractical or impossible.
 
The third common type of amplifier is the semiconductor optical amplifier (SOA). An SOA is simply a laser diode that is operated without a resonant cavity to perform stimulated emission for signal amplification. Fiber pigtails are bonded to each end of the amplifier chip. When electrically pumped with a forward bias current, incoming photons stimulate the emission of identical photons, providing signal amplification. The device is operated below threshold to prevent the cleaved facets at each end of the chip from forming a resonant cavity resulting in laser oscillation. This amplification process is bidirectional and independent of the type of signal being carried.
 
SOAs typically do not provide the performance that EDFA and Raman amplifiers do. They have lower gain (between 10 to 15 dB), low output power (less than 13 dBm), and a higher noise figure (typically about 6-9 dB). But unlike EDFAs and Raman amplifiers, SOAs are available in small 14-pin dual-inline packages and have much lower power requirements, making them a good low-cost choice for some applications. They also may be used as a pre-amp to increase the sensitivity of receivers.