There are currently two main types of optical amplifiers: semiconductor optical amplifiers (SOA) and (OFA). The semiconductor optical amplifier utilizes the stimulated radiation amplification mechanism inherent in the semiconductor material to achieve optical amplification, and its principle and structure are similar to those of the semiconductor laser. A fiber amplifier is different from a semiconductor amplifier in that the active medium (or gain medium) of the fiber amplifier is a special fiber or transmission fiber and is connected to the pump laser; when the signal light passes through the fiber, the signal light is amplified. Fiber amplifiers can be further divided into Rare Earth Ion Doped Fiber Amplifiers and nonlinear fiber amplifiers. Like a semiconductor amplifier, the operating principle of a rare earth doped fiber amplifier is also stimulated radiation; and a nonlinear fiber amplifier amplifies the optical signal by the nonlinear effect of the fiber. Practical fiber amplifiers are erbium doped fiber amplifiers (EDFAs) and Raman fiber amplifiers (Raman Fiber Amplifiers).
The fiber amplifier not only directly amplifies the optical signal, but also has real-time, high-gain, wide-band, online, low-noise, low-loss all-optical amplification, which is an indispensable key component in the new generation of fiber-optic communication systems; The technology not only solves the limitation of attenuation on optical network transmission rate and distance, but more importantly, it creates wavelength division multiplexing in the 1550nm frequency band, which will enable ultra-high speed, ultra-large capacity and ultra-long-distance wavelength division multiplexing (WDM). ), Dense Wavelength Division Multiplexing (DWDM), all-optical transmission, and optical soliton transmission have become a reality, and it is an epoch-making milestone in the history of optical fiber communication. In the current practical fiber amplifiers, there are mainly erbium-doped fiber amplifiers (EDFA), semiconductor optical amplifiers (SOA) and fiber Raman amplifiers (FRA). Among them, erbium-doped fiber amplifiers are widely used for their long-term performance. Distance, high-capacity, high-speed fiber-optic communication systems, access networks, fiber-optic CATV networks, military systems (radar multi-channel data multiplexing, data transmission, guidance, etc.) as power amplifiers, relay amplifiers, and preamplifiers .
Fiber amplifiers are typically composed of a gain medium, pump light, and an input-output coupling structure. At present, fiber amplifiers mainly include three kinds of erbium-doped fiber amplifiers, semiconductor optical amplifiers and fiber Raman amplifiers. According to their applications in fiber-optic networks, fiber amplifiers mainly have three different purposes: as a power amplifier on the transmitter side to improve transmission. The power of the machine; the optical pre-amplifier is used before the receiver to greatly improve the sensitivity of the optical receiver; the relay amplifier is used as a relay amplifier in the optical fiber transmission line to compensate for the transmission loss of the optical fiber and prolong the transmission distance.
Erbium doped fiber amplifier
The erbium-doped fiber amplifier uses an erbium-doped fiber as the active medium. When the pump light is input into the EDF, most of the ground state Er3+ can be pumped to the excited state, and the excited state of Er3+ is quickly and non-radiatively. transferred to the metastable state, since the average residence time of Er3 + in the metastable state is 10ms, it is easy to form a population inversion between the ground state and the metastable state, this time, the signal photon through the erbium doped fiber, by the Under the action of the stimulating radiation, a large number of photons are completely identical to themselves, so that the signal photons are rapidly increased, so that the optical signal that is continuously amplified can be obtained at the output end. Since the development of erbium-doped fiber amplifiers (EDFAs) in the late 1980s and early 1990s, and the application of fiber-optic communication systems in the 1.55mm band, the development of fiber-optic communication to all-optical transmission has been promoted, and the current EDFA technology development and products The most mature C-band EDFA is generally used in the window with the lowest fiber loss of 1530~1565nm, with high output power, high gain, independent of polarization, low noise figure, and amplification characteristics independent of system bit rate and data format. while amplifying wavelength multiplexed series of signals and other characteristics, it has been widely used in long-haul optical communication systems. The disadvantage is that the gain bandwidth of C-Band EDFA is only 35nm, covering only a part of the low-loss window of quartz single-mode fiber, which limits the number of wavelength channels that the fiber can accommodate. However, with the rapid development of Internet technology, the optical fiber transmission system is required. The transmission capacity has to be continuously expanded. In the face of the expansion of transmission capacity, there are currently three main solutions: (1) increasing the transmission rate of each wavelength; (2) reducing the wavelength spacing; and (3) increasing the total transmission bandwidth. For the first method, if the rate is increased to 10 Gbit/s, it will bring about a new dispersion compensation problem. Moreover, the current electronic system still has the problem of the so-called "electronic bottleneck" effect. The second method, if the signal spacing is reduced from 100 GHz to 50 GHz or 25 GHz, will bring nonlinear effects such as four-wave mixing (FWM) to the system, and requires the system to adopt wavelength stabilization technology. Therefore, the research of new fiber amplifiers such as L-band EDFA is one of increasing the total transmission bandwidth. It extends the EDFA operating wavelength from C30 1530~1560nm to L-band 1570~1605nm, which doubles the amplification gain spectrum of EDFA. . Although the wavelength of the L-band EDFA covers the tail of the EDF gain spectrum, it is comparable to the advanced C-band EDFA products: for example, the basic structure of the two is similar, and most C-band EDFA design and manufacturing techniques are still applicable. Developed in L-band EDFA; L-band EDFA has smaller radiation and absorption and lower average inversion factor, the gain fluctuation coefficient is much smaller than C-band EDFA, and there is a longer EDF of L-band EDFA to bring passive fiber The loss is large, and the amplification noise is slightly larger.
Semiconductor optical amplifier
A semiconductor optical amplifier (SOA) is a traveling wave amplifier fabricated by a similar process using a communication laser. When the bias current is lower than the oscillation threshold, the laser diode can perform optical amplification on the input coherent light. Because the semiconductor amplifier has small size, simple structure, low power consumption, long life, easy integration with other optical devices and circuits, suitable for mass production, low cost, and can realize gain and switching functions, etc., in all-optical wavelength conversion, light The applications in switching, spectral inversion, clock extraction, and demultiplexing have received extensive attention. In particular, the successful development of semiconductor optical amplifiers for strained quantum well materials has attracted widespread interest in SOA. The domestic Wuyouyuan and Huazhong University of Science and Technology have successfully developed a semiconductor optical amplifier, a key component in the optical network, and quickly realized the productization. It has become a mass supply to the international market for optical switches after Alcatel. The supplier of semiconductor optical amplifiers marks a key step in the commercial production of strained quantum well devices developed in China. However, compared with erbium-doped fiber amplifiers, semiconductor optical amplifiers have the disadvantages of high noise, low power, sensitivity to crosstalk and polarization, large loss when coupled with fiber, and poor operational stability. So far, their performance and erbium-doped fiber. There is still a big gap in the amplifier. Since the semiconductor optical amplifier covers the 1300~1600nm band, it can be used for both the optical amplifier of the 1300nm window and the optical amplifier of the 1550nm window. In the DWDM multi-wavelength fiber communication system, without gain locking, it can be used not only as a Optical amplifiers are a beneficial alternative and can also contribute to the implementation of the 1310 nm window DWDM system.
Fiber Raman amplifier
Stimulated Raman scattering (SRS) is a nonlinear phenomenon in optical fibers that transfers a small fraction of the incident optical power to a Stokes wave with a lower frequency; if a weak signal is simultaneously with a strong pumping wave The optical signal is transmitted in the optical fiber, and the weak signal wavelength is placed in the Raman gain bandwidth of the pump light, and the weak signal light can be amplified. The optical amplifier based on the stimulated Raman scattering mechanism is called a fiber Raman amplifier ( FRA). In recent years, fiber Raman amplifier has attracted much attention and has become a hot spot for research and development. It has many advantages: (1) the gain medium is a common transmission fiber, which has good compatibility with the fiber system; (2) the gain wavelength is driven by the pump light. The wavelength is determined, and is not limited by other factors. In theory, as long as the wavelength of the pump source is appropriate, the signal light of any wavelength can be amplified; (3) high gain, low crosstalk, low noise figure, wide spectral range, and good temperature stability. .
Because fiber Raman amplifiers have so many advantages that can be amplified by the erbium doped fiber amplifier not amplifying band, optical amplification can be carried out in the 1292 ~ 1660nm spectral range, a gain of a much wider bandwidth than EDFA; gain again The medium is a common fiber, which can be used to make discrete or distributed FRA. The distributed fiber Raman amplifier can linearize the signal light and increase the transmission distance of the optical amplifier. It is also suitable for 40Gbit/s high-speed optical network. In the submarine cable communication system, and because the amplification is distributed along the fiber rather than the concentration, the optical power of the input fiber is greatly reduced, so that the nonlinear effect, especially the four-wave mixing effect, is greatly reduced, which is for the large-capacity DWDM system. Very suitable. FRA is EDFA supplement, rather than replace, combine both broadband gain flatness can be obtained more than 100nm, which is the benefit of using the distributed Raman optical fiber amplifier.
However, one of the main disadvantages of fiber Raman amplifiers is that they require extra-power pump lasers. The main ways to solve this problem are as follows: First, research on pump lasers with reduced threshold power, so that ordinary high-power semiconductor lasers can be used as Raman pumps. Pu used; the second is to improve the development level of pump lasers with larger output power; the third is to multiplex the wavelengths of multiple pump source lasers by array and single chip combination to obtain a high power output. The pump laser not only provides a wideband gain spectrum, but also adjusts the gain slope by adjusting the power of a single laser.
Gain flat control technology for fiber amplifiers in WDM transmission systems
In order to ensure the transmission quality of the WDM system, the fiber amplifier used in the WDM system has higher bandwidth, high output power and low noise figure, and also puts higher requirements on the gain flatness control technology. Gain flatness within the fiber amplifier band is the difference between the gain at the maximum gain wavelength point and the gain at the minimum gain wavelength point throughout the available gain passband. Obviously, the gain flatness is as small as possible in the WDM system. Otherwise, if the gain of each channel is uneven, the gain difference will linearly accumulate after multi-stage amplification, and the SNR of the low-gain channel signal will deteriorate, and the gain will be high. The signal of the channel also deteriorates the signal due to the nonlinear effect of the fiber. Therefore, to make the gain deviation on each channel within the allowable range, the gain of the amplifier must be flat, and the gain of the fiber amplifier gain is generally two ways: One is "gain balancing technology"; the second is "fiber technology." The "gain equalization technique" is to use the gain equalizer whose loss characteristic is opposite to the gain wavelength characteristic of the amplifier to cancel the gain non-uniformity. The key of this technology is that the gain curve of the amplifier and the loss characteristic of the equalizer are closely matched to make the comprehensive characteristics. Flat; the fixed gain flat control technology that is practical at this stage mainly includes fiber grating technology and dielectric multilayer thin film filter technology. However, with the development of multi-channel (>80Ch), high-rate (>40Gbit/s), long-distance optical fiber transmission systems, higher requirements are placed on the gain flattening control technology of optical fiber amplifiers, which requires the development of dynamic gain adjustable. The gain flattening filter, the tunable gain dynamic filter technology mainly includes: Faraday rotator type gain tunable filter technology, waveguide Mach-Zehnder gain tunable filter technology, array waveguide type dynamic gain tunable filter Technology and acousto-optic dynamic gain tunable filter technology. As for the "optical fiber technology", the main reason is to further study the characteristics of the erbium-doped fiber, change the fiber material or use a combination of different fibers to change the characteristics of the EDF, thereby changing the gain flatness of the EDFA, mainly with aluminum-doped EDFA. , fluoride-doped EDFA, erbium-doped EDFA, hybrid EDFA and multi-fiber EDFA.
The fiber amplifier not only directly amplifies the optical signal, but also has real-time, high-gain, wide-band, online, low-noise, low-loss all-optical amplification, which is an indispensable key component in the new generation of fiber-optic communication systems; The technology not only solves the limitation of attenuation on optical network transmission rate and distance, but more importantly, it creates wavelength division multiplexing in the 1550nm frequency band, which will enable ultra-high speed, ultra-large capacity and ultra-long-distance wavelength division multiplexing (WDM). ), Dense Wavelength Division Multiplexing (DWDM), all-optical transmission, and optical soliton transmission have become a reality, and it is an epoch-making milestone in the history of optical fiber communication. In the current practical fiber amplifiers, there are mainly erbium-doped fiber amplifiers (EDFA), semiconductor optical amplifiers (SOA) and fiber Raman amplifiers (FRA). Among them, erbium-doped fiber amplifiers are widely used for their long-term performance. Distance, high-capacity, high-speed fiber-optic communication systems, access networks, fiber-optic CATV networks, military systems (radar multi-channel data multiplexing, data transmission, guidance, etc.) as power amplifiers, relay amplifiers, and preamplifiers .
Fiber amplifiers are typically composed of a gain medium, pump light, and an input-output coupling structure. At present, fiber amplifiers mainly include three kinds of erbium-doped fiber amplifiers, semiconductor optical amplifiers and fiber Raman amplifiers. According to their applications in fiber-optic networks, fiber amplifiers mainly have three different purposes: as a power amplifier on the transmitter side to improve transmission. The power of the machine; the optical pre-amplifier is used before the receiver to greatly improve the sensitivity of the optical receiver; the relay amplifier is used as a relay amplifier in the optical fiber transmission line to compensate for the transmission loss of the optical fiber and prolong the transmission distance.
Erbium doped fiber amplifier
The erbium-doped fiber amplifier uses an erbium-doped fiber as the active medium. When the pump light is input into the EDF, most of the ground state Er3+ can be pumped to the excited state, and the excited state of Er3+ is quickly and non-radiatively. transferred to the metastable state, since the average residence time of Er3 + in the metastable state is 10ms, it is easy to form a population inversion between the ground state and the metastable state, this time, the signal photon through the erbium doped fiber, by the Under the action of the stimulating radiation, a large number of photons are completely identical to themselves, so that the signal photons are rapidly increased, so that the optical signal that is continuously amplified can be obtained at the output end. Since the development of erbium-doped fiber amplifiers (EDFAs) in the late 1980s and early 1990s, and the application of fiber-optic communication systems in the 1.55mm band, the development of fiber-optic communication to all-optical transmission has been promoted, and the current EDFA technology development and products The most mature C-band EDFA is generally used in the window with the lowest fiber loss of 1530~1565nm, with high output power, high gain, independent of polarization, low noise figure, and amplification characteristics independent of system bit rate and data format. while amplifying wavelength multiplexed series of signals and other characteristics, it has been widely used in long-haul optical communication systems. The disadvantage is that the gain bandwidth of C-Band EDFA is only 35nm, covering only a part of the low-loss window of quartz single-mode fiber, which limits the number of wavelength channels that the fiber can accommodate. However, with the rapid development of Internet technology, the optical fiber transmission system is required. The transmission capacity has to be continuously expanded. In the face of the expansion of transmission capacity, there are currently three main solutions: (1) increasing the transmission rate of each wavelength; (2) reducing the wavelength spacing; and (3) increasing the total transmission bandwidth. For the first method, if the rate is increased to 10 Gbit/s, it will bring about a new dispersion compensation problem. Moreover, the current electronic system still has the problem of the so-called "electronic bottleneck" effect. The second method, if the signal spacing is reduced from 100 GHz to 50 GHz or 25 GHz, will bring nonlinear effects such as four-wave mixing (FWM) to the system, and requires the system to adopt wavelength stabilization technology. Therefore, the research of new fiber amplifiers such as L-band EDFA is one of increasing the total transmission bandwidth. It extends the EDFA operating wavelength from C30 1530~1560nm to L-band 1570~1605nm, which doubles the amplification gain spectrum of EDFA. . Although the wavelength of the L-band EDFA covers the tail of the EDF gain spectrum, it is comparable to the advanced C-band EDFA products: for example, the basic structure of the two is similar, and most C-band EDFA design and manufacturing techniques are still applicable. Developed in L-band EDFA; L-band EDFA has smaller radiation and absorption and lower average inversion factor, the gain fluctuation coefficient is much smaller than C-band EDFA, and there is a longer EDF of L-band EDFA to bring passive fiber The loss is large, and the amplification noise is slightly larger.
Semiconductor optical amplifier
A semiconductor optical amplifier (SOA) is a traveling wave amplifier fabricated by a similar process using a communication laser. When the bias current is lower than the oscillation threshold, the laser diode can perform optical amplification on the input coherent light. Because the semiconductor amplifier has small size, simple structure, low power consumption, long life, easy integration with other optical devices and circuits, suitable for mass production, low cost, and can realize gain and switching functions, etc., in all-optical wavelength conversion, light The applications in switching, spectral inversion, clock extraction, and demultiplexing have received extensive attention. In particular, the successful development of semiconductor optical amplifiers for strained quantum well materials has attracted widespread interest in SOA. The domestic Wuyouyuan and Huazhong University of Science and Technology have successfully developed a semiconductor optical amplifier, a key component in the optical network, and quickly realized the productization. It has become a mass supply to the international market for optical switches after Alcatel. The supplier of semiconductor optical amplifiers marks a key step in the commercial production of strained quantum well devices developed in China. However, compared with erbium-doped fiber amplifiers, semiconductor optical amplifiers have the disadvantages of high noise, low power, sensitivity to crosstalk and polarization, large loss when coupled with fiber, and poor operational stability. So far, their performance and erbium-doped fiber. There is still a big gap in the amplifier. Since the semiconductor optical amplifier covers the 1300~1600nm band, it can be used for both the optical amplifier of the 1300nm window and the optical amplifier of the 1550nm window. In the DWDM multi-wavelength fiber communication system, without gain locking, it can be used not only as a Optical amplifiers are a beneficial alternative and can also contribute to the implementation of the 1310 nm window DWDM system.
Fiber Raman amplifier
Stimulated Raman scattering (SRS) is a nonlinear phenomenon in optical fibers that transfers a small fraction of the incident optical power to a Stokes wave with a lower frequency; if a weak signal is simultaneously with a strong pumping wave The optical signal is transmitted in the optical fiber, and the weak signal wavelength is placed in the Raman gain bandwidth of the pump light, and the weak signal light can be amplified. The optical amplifier based on the stimulated Raman scattering mechanism is called a fiber Raman amplifier ( FRA). In recent years, fiber Raman amplifier has attracted much attention and has become a hot spot for research and development. It has many advantages: (1) the gain medium is a common transmission fiber, which has good compatibility with the fiber system; (2) the gain wavelength is driven by the pump light. The wavelength is determined, and is not limited by other factors. In theory, as long as the wavelength of the pump source is appropriate, the signal light of any wavelength can be amplified; (3) high gain, low crosstalk, low noise figure, wide spectral range, and good temperature stability. .
Because fiber Raman amplifiers have so many advantages that can be amplified by the erbium doped fiber amplifier not amplifying band, optical amplification can be carried out in the 1292 ~ 1660nm spectral range, a gain of a much wider bandwidth than EDFA; gain again The medium is a common fiber, which can be used to make discrete or distributed FRA. The distributed fiber Raman amplifier can linearize the signal light and increase the transmission distance of the optical amplifier. It is also suitable for 40Gbit/s high-speed optical network. In the submarine cable communication system, and because the amplification is distributed along the fiber rather than the concentration, the optical power of the input fiber is greatly reduced, so that the nonlinear effect, especially the four-wave mixing effect, is greatly reduced, which is for the large-capacity DWDM system. Very suitable. FRA is EDFA supplement, rather than replace, combine both broadband gain flatness can be obtained more than 100nm, which is the benefit of using the distributed Raman optical fiber amplifier.
However, one of the main disadvantages of fiber Raman amplifiers is that they require extra-power pump lasers. The main ways to solve this problem are as follows: First, research on pump lasers with reduced threshold power, so that ordinary high-power semiconductor lasers can be used as Raman pumps. Pu used; the second is to improve the development level of pump lasers with larger output power; the third is to multiplex the wavelengths of multiple pump source lasers by array and single chip combination to obtain a high power output. The pump laser not only provides a wideband gain spectrum, but also adjusts the gain slope by adjusting the power of a single laser.
Gain flat control technology for fiber amplifiers in WDM transmission systems
In order to ensure the transmission quality of the WDM system, the fiber amplifier used in the WDM system has higher bandwidth, high output power and low noise figure, and also puts higher requirements on the gain flatness control technology. Gain flatness within the fiber amplifier band is the difference between the gain at the maximum gain wavelength point and the gain at the minimum gain wavelength point throughout the available gain passband. Obviously, the gain flatness is as small as possible in the WDM system. Otherwise, if the gain of each channel is uneven, the gain difference will linearly accumulate after multi-stage amplification, and the SNR of the low-gain channel signal will deteriorate, and the gain will be high. The signal of the channel also deteriorates the signal due to the nonlinear effect of the fiber. Therefore, to make the gain deviation on each channel within the allowable range, the gain of the amplifier must be flat, and the gain of the fiber amplifier gain is generally two ways: One is "gain balancing technology"; the second is "fiber technology." The "gain equalization technique" is to use the gain equalizer whose loss characteristic is opposite to the gain wavelength characteristic of the amplifier to cancel the gain non-uniformity. The key of this technology is that the gain curve of the amplifier and the loss characteristic of the equalizer are closely matched to make the comprehensive characteristics. Flat; the fixed gain flat control technology that is practical at this stage mainly includes fiber grating technology and dielectric multilayer thin film filter technology. However, with the development of multi-channel (>80Ch), high-rate (>40Gbit/s), long-distance optical fiber transmission systems, higher requirements are placed on the gain flattening control technology of optical fiber amplifiers, which requires the development of dynamic gain adjustable. The gain flattening filter, the tunable gain dynamic filter technology mainly includes: Faraday rotator type gain tunable filter technology, waveguide Mach-Zehnder gain tunable filter technology, array waveguide type dynamic gain tunable filter Technology and acousto-optic dynamic gain tunable filter technology. As for the "optical fiber technology", the main reason is to further study the characteristics of the erbium-doped fiber, change the fiber material or use a combination of different fibers to change the characteristics of the EDF, thereby changing the gain flatness of the EDFA, mainly with aluminum-doped EDFA. , fluoride-doped EDFA, erbium-doped EDFA, hybrid EDFA and multi-fiber EDFA.
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