**2. Optical amplifier properties**

The main aim of this topic is to provide a description of optical amplifiers having a device that amplifies an optical signal, without the need to convert it to an electrical signal or source. This can be formed in visible or invisible electromagnetic spectral source such as light or a laser without an optical resonator, or one in which feedback from the cavity is suppressed. A fundamental optical communication link comprises a transmitter and receiver, with an optical fiber cable and connector which connect them. Even though signals propagating in fiber suffer far less energy in terms of absorption and other damped along the media, such conductor media still have a limit of about 140 km on the distance the signal wave can propagate before producing the disturbance like noise. Before going to the market, the optical amplifiers are necessary to regenerate the optical signals every 80–140 km [1] electronically in order to fulfill the transmission value over long distances. This process describes the receiving of the information signal, organizing and multiplying the amplification optically and electronically, and then retransmitting it over the next medium and segment of the circuit and link. It can be feasible if a single low optical signal is transmitted; it will travel fast and be unfeasible, transmitting in tens of high-capacity order of wavelength-division multiplexing (WDM) channel devices. This results in high-cost, power-hungry, and bulky regenerator port. Furthermore, the regeneration hardware and software depend upon bit-rate, protocol, channel numbers, and modulation which are set to each channel. Any upgrade to the link therefore will automatically require upgrades to the regenerator stations. On the other hand, an ideal amplifier is modeled and designed to amplify any input optical signal directly, no need to transform

There are different kinds of processes that can be applied to amplify electromagnetic signals corresponding to the major formation of amplifier optics. For doped fiber ones and bulk lasers, SOA, electron-hole interaction process and recombination will occur. For RA, its scattering of incoming electromagnetic signal with phonons in the lattice of the gain media will produce photons coherent with the incoming photons. Parameters of amplifiers use parametric amplification. **Figure 1** shows amplifier's gain medium causes amplification of incoming light. In semiconductor optical, the block diagram of an amplified signal was optically totally different from an electronic signal regeneration regime, in which channels are usually split, detected, amplified, cleaned electronically, retransmitted, and then recombined. This is a benefit of optical amplifier that can be used to all channels optically and transparently amplified together. Optical transmission media greatly affect the performance of a communication system. Fiber optic is one of the transmission media that is capable of transmitting information with a large capacity, is high speed, and has low attenuation. Although optical fiber provides many advantages, there are also disadvantages that can disrupt the performance of the fiber optics,

**Figure 1.** An optical amplifier, all channels are optically, transparently amplified together.

the signal first to an electronic one.

82 Telecommunication Networks - Trends and Developments

Optical amplifiers have several properties. First, it has a gain that the input optical signal is amplified and that is detected from the output port one. It is typically measured in the range of 5–35 dB. For instance if the gain is 10 dB, meaning the input signal is amplified by a factor of 10 subject to logarithm factor. It is also characterized by the supported input and output powers. Especially, the main specification of the amplifier is the maximum output power, which can be contributed and subjected to as saturated output power. There are two kinds of optical amplifiers. They are single and multichannel. The former is designed to amplify only one channel located within a specified band, such as the C-band (1528–1564 nm). This channel can usually deal with over a wide range of gains and require relatively low output power. In the latter channel, WDM amplifiers are designed to work out if any number of channels are input to the amplifier. The gain flatness is the properties of WDM amplifiers, the variation of the gain for different channels, as depicted in **Figure 2** (right side). When the gain is not flat, different WDM channels will have different gains, which can accumulate along a chain of amplifiers leading to a large mismatch between channels. To maintain flat gain, most low-end WDM amplifiers only support a single gain, or a relatively narrow gain range, supporting

**Figure 2.** Example input (green) and output (blue) spectrums of a single channel amplifier (left) and a WDM multichannel amplifier (right) [1].

both flat gain and a large gain range, providing a large dynamic input power range, to support different input conditions where any number of channels 1 up to 80 may be available. The maximum quantity of WDM channel amplifiers requires a relatively high saturated output power, particularly in the range of 17–23 dBm. Secondly, optical amplifiers have noise during the amplification process. The noise is detected by its noise figure (NF), where it has the ratio between the signal-to-noise ratio (SNR) at the output port and an ideal SNR at the input port. Due to one-to-one connection between the NF and the optical link, the value of NF should be maintained as low as possible. The value of NF depends upon the technology applied and used for it, where higher gain usually has lower NF. Thirdly, amplifiers detection to dynamical conversion at input port source describe that the gain ideally should not convert at all if the source of input power converts it. But, it is impossible if the amplifier deals with at or almost the peak output power source. This has an important reason if the amplifier can respond step by step; hence its gain is determined only by the average input power source, and it does not influence and change fast (for instance, due to data modulation). Amplifiers having responses too quick can result too noisy. It cannot overcome the multiple channels well.

Amplification is obtained by processing of emission which is stimulated and producing photons in the dopant ions in the optical fiber which is doped. The source excites ions into a greater energy and will decay via spectral of stimulated emission which has a photon at the signal wavelength back to a lower energy level. The spontaneous emission (decay) can occur to the exited ions or even via non-radiative mechanism involving interactions with phonons of the glass matrix. The last two decay processes compete with stimulated emission, which decreases the efficiency of amplitude or intensity of electromagnetic amplification. The *amplification window* represents the range of wavelengths for which the amplifier results in an applicable gain. This is determined by the measurement of the glass structure of fiber optic or by spectroscopic properties of dopant ions and the wavelength and power of the electromagnetic source. Even though the transitions of electronic or an isolated ion are very well known, the wide band of the energy levels happens if the ions are interacted to the fiber optic. Therefore, the amplification window is also broadened. The broadening will be homogeneous (all ions exhibit the same broadened spectrum) and also it will be inhomogeneous (different ions in different glass locations exhibit different spectra). A relatively high-powered beam of electromagnetic source such as light is combined with the input signal by using a wavelength selective coupler (WSC). This input one and the excitation beam have to be of different wavelengths significantly. The mixed electromagnetics or polychromatics or laser will be guided into a resonator of fiber with erbium ions subject to the fiber core. The highpowered electromagnetics of light beam excites the dopants ions to the higher-energy state. If photons of signal at a particular resonant wavelength from the beam source meet the excited erbium atoms, the erbium atoms will surrender several of their energy to the signal and go back to their lower-energy state. The main point is that the erbium surrenders up its energy in the form of additional photons with the similar phase and direction as the signal being multiplied and amplified. Thus, the signal is amplified along the direction of transmission. This is not unusual—if an atom "lases," it always surrenders its energy in the same direction and phase as the incoming beam source. Therefore, a whole additional signal source is guided in the similar fiber mode as the incoming signal. Usually, an isolator is placed at the output port to overcome reflections going back from the attached optical fiber. As reflections disrupt amplifier operation, in the extreme case, it will cause the amplifier to become a laser. The ED

Optical Amplifiers for Next-Generation Telecommunication

http://dx.doi.org/10.5772/intechopen.79941

85

SOA is an amplifier of small size using a semiconductor to provide the gain medium [4] and pump electronically. It operates as the same as standard semiconductor lasers and is packaged in tiny size as "butterfly" design. In addition to their tiny size, they are low cost. SOA suffers from a quantity drawbacks making it not suitable for wide applications. In special case, it gives relatively low gain (<15 dB), has a low saturated output power (<13 dBm), and has relatively high NF. SOA has quick response time providing the operation to near the saturation level. They suffer from signal distortion for single channel setup and noise as effect of cross-gain modulation such as WDM operation and can suit for single channel booster where they do not require high gain or high output power. SOA has the same formation to Fabry-Pérot laser diodes but with anti-reflection elements at the end faces. Nowadays, the designs include antireflective coatings and tilted waveguide and window regions which can decrease

(erbium-doped) compound has a great gain.

**2.2. Semiconductor optical amplifier**

Optical amplifiers may be used within an optical network as boosters, line amplifiers, or preamplifiers, as shown in **Figure 3**, with slightly different specifications.

#### **2.1. Laser amplifier**

Generally, laser active gain medium can be pumped to produce gain for spectral wave of a laser made with the same material as its gain medium to result in very high-power laser systems, such as regenerative and chirped-pulse amplifiers which are applied to amplify ultrashort pulses. In addition, solid-state amplifiers are examples of using a wide range of doped solid-state materials (Yb:YAG, Ti:Sa, Nd:YAG) and other kind of sizes and geometries for instance a disk, a slab, and a rod. The variety of materials allows the amplification of different wavelengths, while the shape of the medium can distinguish between what is more suitable for energy [3]. Doped fiber amplifiers (DFAs) use a doped fiber optics having gain resonator to multiply the signals corresponding to the source of fiber lasers. The signals will be multiplied and amplified, and a pump laser is multiplexed to the resonator. The signal will interact through the doping ions (erbium-doped fiber amplifier, EDFA), where the core of a silica fiber is doped with trivalent erbium ions and can be efficiently pumped with a laser at a wavelength of infrared region.

**Figure 3.** A simple WDM optical network, where a number of transmitted channels are combined using a WDM multiplexer (MUX), amplified using a booster amplifier before being launched into the transmission fiber, re-amplified every 80–120 km using in-line amplifiers, and finally preamplified before being demultiplexed and received [1].

Amplification is obtained by processing of emission which is stimulated and producing photons in the dopant ions in the optical fiber which is doped. The source excites ions into a greater energy and will decay via spectral of stimulated emission which has a photon at the signal wavelength back to a lower energy level. The spontaneous emission (decay) can occur to the exited ions or even via non-radiative mechanism involving interactions with phonons of the glass matrix. The last two decay processes compete with stimulated emission, which decreases the efficiency of amplitude or intensity of electromagnetic amplification. The *amplification window* represents the range of wavelengths for which the amplifier results in an applicable gain. This is determined by the measurement of the glass structure of fiber optic or by spectroscopic properties of dopant ions and the wavelength and power of the electromagnetic source. Even though the transitions of electronic or an isolated ion are very well known, the wide band of the energy levels happens if the ions are interacted to the fiber optic. Therefore, the amplification window is also broadened. The broadening will be homogeneous (all ions exhibit the same broadened spectrum) and also it will be inhomogeneous (different ions in different glass locations exhibit different spectra). A relatively high-powered beam of electromagnetic source such as light is combined with the input signal by using a wavelength selective coupler (WSC). This input one and the excitation beam have to be of different wavelengths significantly. The mixed electromagnetics or polychromatics or laser will be guided into a resonator of fiber with erbium ions subject to the fiber core. The highpowered electromagnetics of light beam excites the dopants ions to the higher-energy state. If photons of signal at a particular resonant wavelength from the beam source meet the excited erbium atoms, the erbium atoms will surrender several of their energy to the signal and go back to their lower-energy state. The main point is that the erbium surrenders up its energy in the form of additional photons with the similar phase and direction as the signal being multiplied and amplified. Thus, the signal is amplified along the direction of transmission. This is not unusual—if an atom "lases," it always surrenders its energy in the same direction and phase as the incoming beam source. Therefore, a whole additional signal source is guided in the similar fiber mode as the incoming signal. Usually, an isolator is placed at the output port to overcome reflections going back from the attached optical fiber. As reflections disrupt amplifier operation, in the extreme case, it will cause the amplifier to become a laser. The ED (erbium-doped) compound has a great gain.

### **2.2. Semiconductor optical amplifier**

both flat gain and a large gain range, providing a large dynamic input power range, to support different input conditions where any number of channels 1 up to 80 may be available. The maximum quantity of WDM channel amplifiers requires a relatively high saturated output power, particularly in the range of 17–23 dBm. Secondly, optical amplifiers have noise during the amplification process. The noise is detected by its noise figure (NF), where it has the ratio between the signal-to-noise ratio (SNR) at the output port and an ideal SNR at the input port. Due to one-to-one connection between the NF and the optical link, the value of NF should be maintained as low as possible. The value of NF depends upon the technology applied and used for it, where higher gain usually has lower NF. Thirdly, amplifiers detection to dynamical conversion at input port source describe that the gain ideally should not convert at all if the source of input power converts it. But, it is impossible if the amplifier deals with at or almost the peak output power source. This has an important reason if the amplifier can respond step by step; hence its gain is determined only by the average input power source, and it does not influence and change fast (for instance, due to data modulation). Amplifiers having responses

too quick can result too noisy. It cannot overcome the multiple channels well.

amplifiers, as shown in **Figure 3**, with slightly different specifications.

**2.1. Laser amplifier**

84 Telecommunication Networks - Trends and Developments

a wavelength of infrared region.

Optical amplifiers may be used within an optical network as boosters, line amplifiers, or pre-

Generally, laser active gain medium can be pumped to produce gain for spectral wave of a laser made with the same material as its gain medium to result in very high-power laser systems, such as regenerative and chirped-pulse amplifiers which are applied to amplify ultrashort pulses. In addition, solid-state amplifiers are examples of using a wide range of doped solid-state materials (Yb:YAG, Ti:Sa, Nd:YAG) and other kind of sizes and geometries for instance a disk, a slab, and a rod. The variety of materials allows the amplification of different wavelengths, while the shape of the medium can distinguish between what is more suitable for energy [3]. Doped fiber amplifiers (DFAs) use a doped fiber optics having gain resonator to multiply the signals corresponding to the source of fiber lasers. The signals will be multiplied and amplified, and a pump laser is multiplexed to the resonator. The signal will interact through the doping ions (erbium-doped fiber amplifier, EDFA), where the core of a silica fiber is doped with trivalent erbium ions and can be efficiently pumped with a laser at

**Figure 3.** A simple WDM optical network, where a number of transmitted channels are combined using a WDM multiplexer (MUX), amplified using a booster amplifier before being launched into the transmission fiber, re-amplified every 80–120 km using in-line amplifiers, and finally preamplified before being demultiplexed and received [1].

SOA is an amplifier of small size using a semiconductor to provide the gain medium [4] and pump electronically. It operates as the same as standard semiconductor lasers and is packaged in tiny size as "butterfly" design. In addition to their tiny size, they are low cost. SOA suffers from a quantity drawbacks making it not suitable for wide applications. In special case, it gives relatively low gain (<15 dB), has a low saturated output power (<13 dBm), and has relatively high NF. SOA has quick response time providing the operation to near the saturation level. They suffer from signal distortion for single channel setup and noise as effect of cross-gain modulation such as WDM operation and can suit for single channel booster where they do not require high gain or high output power. SOA has the same formation to Fabry-Pérot laser diodes but with anti-reflection elements at the end faces. Nowadays, the designs include antireflective coatings and tilted waveguide and window regions which can decrease end-face reflection <0.001%. Because it produces power losses from the resonator which can be higher than the gain, it prevents the amplifier from the source of laser. There are two kinds of SOA. One region is a laser diode having a Fabry-Pérot, and the second one is a tapered geometry to decrease the value power density on the output facet. SOA is particularly made from III-V compound periodic system such as InGaAs/ InP, AlGaAs/GaAs, InAlGaAs/ InP, and InGaAsP/InP, though any direct band gap semiconductors such as II-VI will conceivably be applied. These components are usually used for amplifier in telecommunication systems and technology such a fiber-pigtailed components, operating at signal wavelengths between 0.85 and 1.6 μm and generating gains of up to 30 dB [5].

NF-combined amplifiers. These are beneficial to many usages in communication, for example, ultra-long links spanning by order of 10<sup>3</sup> km, the long links with no in-line amplifiers, or very

Optical Amplifiers for Next-Generation Telecommunication

http://dx.doi.org/10.5772/intechopen.79941

87

It is not like the EDFA and SOA; the effect of amplification is achieved by nonlinear factors between the optical signal and a laser source within the optical waveguide. There are two kinds of RA, i.e., distributed and lumped one. The former is the transmission fiber used as the gain medium by multiplexing a source wavelength with signal wavelength, while latter one utilizes a dedicated, shorter length of optical waveguide to provide amplification. Particularly, a lumped RA with highly nonlinear fiber having a small core is applied to enhance the interaction to signals and source wavelengths and thereby decreases the length of optical fiber required. The laser source may be combined to the fiber of transmission signal with the same direction (codirectional pumping), on the other direction (contra-directional pumping), or both. Contra-directional pumping source is often used as the noise transfer to the source pump to the signal decreased. Source power of RA is greater than that of the EDFA, >500 mW being required to achieve useful levels of gain in a distributed amplifier. In lumped amplifiers, the pump light can be safely contained to avoid safety implications of high optical powers, may use over 1 W. The principal advantage of Raman amplification is its ability to amplify and distribute the signal within the waveguide, by increasing the length of spans between amplifier and regeneration sites. The amplification bandwidth represents the source wavelengths used so that the amplification can be provided over wider, and different, regions than it is possible with other amplifier which depends upon dopants and optical component design to introduce the

Other advantages of RA are as follows. Firstly, its gain is available in fiber, providing a cost-effective means of upgrading of the terminal ends. Secondly, the Raman gain is nonresonant, that gain is available over the whole transparency area of the fiber approximately 0.3–2 μm. Thirdly, by organizing the source of wavelengths, the gain may be tailored such as the multiple source lines can be utilized in order to enhance the bandwidth, and also the source distribution describes the gain flatness. The benefit to Raman amplifier is a broadband amplifier with a bandwidth relatively >5 THz, this result gain is reasonably flat over a wide wavelength range. But, the challenges of Raman amplifiers prevent their earlier adoption. Firstly, if one compares to the EDFAs, RA has relatively less pumping efficiency at lower-level signal power. Even though it has a disadvantage, this lack of pump efficiency becomes gain clamping readily in RA. Secondly, RA requires a longer gain of optical fiber. On the other hand, this disadvantage can be mitigated by mixing gain and the dispersion compensation in a single fiber. Thirdly, it has a fast response time, which gives rise to new sources of noise. Finally, there are concerns of nonlinear penalty in the amplifier for the WDM signal channels. Amplifier parameters will allow the amplification of a weak signal impulse in a non-centrosymmetric nonlinear medium. On the other hand, the amplifiers are mostly used in telecommunication application and technology. This kind finds its main application in expanding the frequency tunability of ultrafast solid-state lasers. For a noncollinear interaction geometry, its optical parameters are suitable for extremely wide

high bit-rate (40/100 Gb/s) links.

amplification "window."

bandwidths for amplification.

### **2.3. Raman amplifier**

Raman amplifier (RA) amplifies signal by stimulated Raman scattering (SRS). SRS is a device having a process of electromagnetic wave scattered by ions or molecule compound from a lower state to a higher state of wavelength source. Sufficient great power source at a lower state which stimulated scattering may happen if data signal with a higher wavelength state is multiplied and amplified by Raman's from the source. SRS actually represents a nonlinear interaction between higher and lower wavelength. It can take place in optical waveguide. The efficiency of SRS is low for most fibers having high pump power particularly 1 W to obtain useful signal gain. Generally, RA cannot compete to EDFAs as depicted in **Figure 4**.

Raman amplification provides two unique benefits to other amplification telecommunication and technologies. This amplification wavelength band can be tailored by changing the source of wavelengths. It can be obtained at wavelengths that are not supported by competing technologies. The Raman amplification can be also achieved within the propagation wave in the optical fiber itself, enabling a distributed Raman amplification (DRA). In this mechanism, a high source power is launched into the optical fiber (from the output end) to amplify the wave signal to the fiber optics. Because the gain happens along the optical fiber cable, DRA prevents the wave signal from being damped or attenuated to very low powers, improving the SNR of information signal. RA is also always used with EDFAs to deal with the ultra-low

**Figure 4.** Signal power for Raman and EDFA.

NF-combined amplifiers. These are beneficial to many usages in communication, for example, ultra-long links spanning by order of 10<sup>3</sup> km, the long links with no in-line amplifiers, or very high bit-rate (40/100 Gb/s) links.

end-face reflection <0.001%. Because it produces power losses from the resonator which can be higher than the gain, it prevents the amplifier from the source of laser. There are two kinds of SOA. One region is a laser diode having a Fabry-Pérot, and the second one is a tapered geometry to decrease the value power density on the output facet. SOA is particularly made from III-V compound periodic system such as InGaAs/ InP, AlGaAs/GaAs, InAlGaAs/ InP, and InGaAsP/InP, though any direct band gap semiconductors such as II-VI will conceivably be applied. These components are usually used for amplifier in telecommunication systems and technology such a fiber-pigtailed components, operating at signal wavelengths between

Raman amplifier (RA) amplifies signal by stimulated Raman scattering (SRS). SRS is a device having a process of electromagnetic wave scattered by ions or molecule compound from a lower state to a higher state of wavelength source. Sufficient great power source at a lower state which stimulated scattering may happen if data signal with a higher wavelength state is multiplied and amplified by Raman's from the source. SRS actually represents a nonlinear interaction between higher and lower wavelength. It can take place in optical waveguide. The efficiency of SRS is low for most fibers having high pump power particularly 1 W to obtain

Raman amplification provides two unique benefits to other amplification telecommunication and technologies. This amplification wavelength band can be tailored by changing the source of wavelengths. It can be obtained at wavelengths that are not supported by competing technologies. The Raman amplification can be also achieved within the propagation wave in the optical fiber itself, enabling a distributed Raman amplification (DRA). In this mechanism, a high source power is launched into the optical fiber (from the output end) to amplify the wave signal to the fiber optics. Because the gain happens along the optical fiber cable, DRA prevents the wave signal from being damped or attenuated to very low powers, improving the SNR of information signal. RA is also always used with EDFAs to deal with the ultra-low

useful signal gain. Generally, RA cannot compete to EDFAs as depicted in **Figure 4**.

0.85 and 1.6 μm and generating gains of up to 30 dB [5].

86 Telecommunication Networks - Trends and Developments

**2.3. Raman amplifier**

**Figure 4.** Signal power for Raman and EDFA.

It is not like the EDFA and SOA; the effect of amplification is achieved by nonlinear factors between the optical signal and a laser source within the optical waveguide. There are two kinds of RA, i.e., distributed and lumped one. The former is the transmission fiber used as the gain medium by multiplexing a source wavelength with signal wavelength, while latter one utilizes a dedicated, shorter length of optical waveguide to provide amplification. Particularly, a lumped RA with highly nonlinear fiber having a small core is applied to enhance the interaction to signals and source wavelengths and thereby decreases the length of optical fiber required. The laser source may be combined to the fiber of transmission signal with the same direction (codirectional pumping), on the other direction (contra-directional pumping), or both. Contra-directional pumping source is often used as the noise transfer to the source pump to the signal decreased. Source power of RA is greater than that of the EDFA, >500 mW being required to achieve useful levels of gain in a distributed amplifier. In lumped amplifiers, the pump light can be safely contained to avoid safety implications of high optical powers, may use over 1 W. The principal advantage of Raman amplification is its ability to amplify and distribute the signal within the waveguide, by increasing the length of spans between amplifier and regeneration sites. The amplification bandwidth represents the source wavelengths used so that the amplification can be provided over wider, and different, regions than it is possible with other amplifier which depends upon dopants and optical component design to introduce the amplification "window."

Other advantages of RA are as follows. Firstly, its gain is available in fiber, providing a cost-effective means of upgrading of the terminal ends. Secondly, the Raman gain is nonresonant, that gain is available over the whole transparency area of the fiber approximately 0.3–2 μm. Thirdly, by organizing the source of wavelengths, the gain may be tailored such as the multiple source lines can be utilized in order to enhance the bandwidth, and also the source distribution describes the gain flatness. The benefit to Raman amplifier is a broadband amplifier with a bandwidth relatively >5 THz, this result gain is reasonably flat over a wide wavelength range. But, the challenges of Raman amplifiers prevent their earlier adoption. Firstly, if one compares to the EDFAs, RA has relatively less pumping efficiency at lower-level signal power. Even though it has a disadvantage, this lack of pump efficiency becomes gain clamping readily in RA. Secondly, RA requires a longer gain of optical fiber. On the other hand, this disadvantage can be mitigated by mixing gain and the dispersion compensation in a single fiber. Thirdly, it has a fast response time, which gives rise to new sources of noise. Finally, there are concerns of nonlinear penalty in the amplifier for the WDM signal channels. Amplifier parameters will allow the amplification of a weak signal impulse in a non-centrosymmetric nonlinear medium. On the other hand, the amplifiers are mostly used in telecommunication application and technology. This kind finds its main application in expanding the frequency tunability of ultrafast solid-state lasers. For a noncollinear interaction geometry, its optical parameters are suitable for extremely wide bandwidths for amplification.
