Transmission and Control Technology Based on Optical Solitary Subsystem
Abstract: The basic components for optical soliton transmission are introduced, the research progress of optical fiber soliton communication transmission and control technology is analyzed, the problems to be solved at this stage are pointed out, and finally the development prospect of optical soliton communication is prospected.
0 Overview
Optical communication transmission speed and transmission capacity are mainly determined by the pulse broadening caused by the group velocity dispersion of the optical fiber. However, the limitations and disadvantages of ultra-high-speed, long-distance communication using wavelengths with zero group velocity dispersion have become increasingly apparent. The optical soliton technology is one of the means to solve this problem. Once the optical soliton communication technology is proposed, it shows outstanding advantages and huge development potential, and has aroused widespread concern. Although this field is still in the stage of theoretical research and experiment. But it can be expected that it is likely to become the main means of ultra-long-distance information transmission in the future.
1 Optical soliton communication system
The long-distance optical soliton communication system is composed of four basic units: optical soliton source, soliton transmission fiber, soliton energy compensation amplifier and soliton pulse detection receiving unit.
1.1 Optical soliton source
An optical soliton source is a light source that meets certain requirements for optical power and can provide a stable hyperbolic tangent waveform or Gaussian waveform optical pulse sequence with waveform stability and no chirp conversion limitation. It is a key component to realize optical soliton communication. There are many methods for generating optical solitons, such as early color center lasers, modulation unstable lasers, fiber Raman (Raman) soliton lasers and stimulated parametric soliton lasers, and multi-stage compression soliton lasers. In 1980, Bellcore's Mollenauer used a 1.55 μm pin-mode color center laser and a low-loss optical fiber to observe optical solitons for the first time in the laboratory. In 1989, the stimulated Raman distribution amplification of single-mode fiber was used for energy compensation, and the fiber loop structure was used to simulate the long-distance transmission system. The 55ps soliton was stably transmitted 6000km in the 42km fiber ring. Now more popular soliton sources include mode-locked external cavity semiconductor laser (ML-EC-LD), gain switch distributed feedback semiconductor laser (GS-DFB-LD) and so on. The pulse waveform generated by ML-EC-LD is better and the frequency chirp component is lower, but the structure is complex and the stability is poor. Integrated ML-EC-LD is a better soliton source generation scheme; GS-DFB-LD is combined to remove Chirp technology, simple structure, but still have a certain residual frequency chirp, as long as the frequency chirp of the optical pulse is small enough, the pulse can evolve into an optical soliton in the optical fiber, so it is an important light source in the current optical soliton transmission system ML-ER-FRL is a novel ultra-short optical pulse source, which can directly generate solitons, without chirp, can be self-starting and easy to connect with optical fiber, the structure is simpler, and it is also the most used light source at present.
1.2 Optical amplifier for optical soliton transmission
The ideal soliton communication system requires the transmission of ultrashort laser pulses with standard hyperbolic tangent waveforms. It has the characteristics of conformal and stable transmission during the transmission process. The optical relay amplifier only plays a large role. This kind of pulse acts as an information carrier and can realize high-speed and large-capacity communication. However, in actual soliton communication systems, there are often high-order dispersion and nonlinear loss and random changes in system component parameters; the input waveform may deviate from the hyperbolic secant waveform, which contains frequency chirp or light source noise; optical relay In addition to amplifying soliton pulses, the amplifier also introduces spontaneous radiated noise. The stable transmission of optical solitons requires no energy attenuation of the pulse, but the non-ideal state of these light sources, optical relay amplifiers and transmission lines will bring adverse effects to the soliton transmission, so that the energy loss, pulse width expansion and transmission capacity of the soliton are reduced, so it must be The soliton replenishes energy.
Optical amplifiers are used in optical soliton communication systems to compensate for energy loss during soliton transmission. They are another key component to achieve high-speed and long-distance communication. There are four main types of optical amplifiers that can realize optical soliton amplification. They are semiconductor optical amplifiers (SOA ), Erbium-doped fiber amplifier (EDFA), distributed erbium-doped fiber amplifier (D-EDFA) and Raman fiber amplifier.
The characteristics of SOA are small size, frequency bandwidth and high gain. It is easy to integrate with other optoelectronic devices and can work in the low loss window of the entire fiber, but the coupling loss with the fiber is too large, the noise is also large, and the polarization of the gain to the fiber Very sensitive to ambient temperature and poor stability.
EDFA has the characteristics of high gain, low noise, frequency bandwidth, high output power, low pump power, available semiconductor laser as pump source, and polarization insensitivity. It is especially suitable for high-speed long-distance communication applications. Nakazawa and others first successfully implemented 20GHz soliton stable transmission with EDFA in 1989.
D-EDFA uses Er3 + -doped erbium fiber with low Er3 + concentration, low gain factor, long cutoff wavelength, large numerical aperture, and wide negative dispersion, and uses 1480nm bidirectional pumping technology to reduce losses and reduce energy along the line Fluctuations can reach an amplification of about 100km or the distance between pumping stations.
Early Hasegawa et al. Suggested using stimulated Raman amplification of the transmission fiber to compensate for fiber loss. Soon Mollenauer et al. Used this scheme to achieve a stable transmission of 4000km soliton. Raman amplifiers are distributed compensation amplifiers with small fluctuations and good stability, but Raman amplifiers have low pump efficiency, so high pump power is required. At present, it is difficult to use in actual communication systems, but with the development of technology, such amplifiers will also be used in actual information transmission systems, and will greatly improve the performance of optical soliton communication.
2 Analysis of optical soliton transmission technology
The upper limit of the distance code rate product of the optical soliton transmission system is affected by several parameters, including the optical pulse duty cycle, the effective cross section of the fiber, the fiber nonlinear coefficient, fiber loss, fiber dispersion, amplifier spontaneous emission factor, amplifier spacing, etc. . In order to achieve long-distance transmission of optical solitons, it is necessary to reasonably select parameters such as fiber dispersion, amplifier spacing, amplifier gain, transmission distance, soliton pulse width, soliton peak power into the fiber, receiver decision threshold, etc. Gordon-Haus limitation, soliton interaction, system parameter mismatch, non-uniform perturbation and the resulting dispersion wave and instability.
Early soliton transmission experiments used ordinary single-mode fiber as the transmission medium, which had problems such as large dispersion and high threshold power of soliton. In order to solve these problems, a dispersion-shifted fiber (DSF) is used, and a DSF and EDFA periodically cascaded soliton transmission system composition scheme is formed. This transmission scheme has been used until now and has become the basic composition scheme of the optical soliton transmission system.
Shiojiri and Fujii proposed the method of increasing the soliton amplitude to increase the amplifier spacing, called pre-emphasis. In the long-distance soliton transmission system using DSF and EDFA, reasonable selection of schemes and system parameters is the prerequisite for ensuring stable transmission of pulses. However, no matter what kind of system solution, it is inseparable from the use of pre-emphasis measures.
But after adopting pre-emphasis measures, when the transmission distance L is too large, the pulse will still lose its soliton characteristics. To this end, another important parameter should be considered in the transmission system, namely the amplifier spacing La. Amplifier spacing is related to many parameters in the system such as fiber loss, dispersion, pulse width, and pre-emphasis factor. From an application point of view, La should be as large as possible to reduce the cost of the entire system, but from the perspective of soliton transmission performance, the smaller the amplifier spacing, the closer the amplification characteristics are to distributed amplification, which is conducive to the stability of soliton transmission. For the currently considered soliton communication system, the amplifier spacing is generally tens of kilometers. When the La of the orphan system is determined, the dispersion length Ld will change with the change of the pulse width and dispersion. The normalized amplifier spacing Za = La / Ld may appear to be much less than 1, approximately equal to 1 or much greater than 1. Correspondingly, there are different transmission methods such as average soliton transmission, dynamic soliton transmission and adiabatic soliton transmission.
In the design of the pre-emphasis system, several schemes have been proposed as to how large the amplitude emphasis factor should be: 1) As the pulse width changes, the pulse narrowing caused by the pre-emphasis power just compensates for the pulse broadening caused by the loss. This scheme is called dynamic Soliton communication scheme. This scheme is based on the area-invariant theorem that characterizes the characteristics of the soliton when the soliton amplitude changes within a certain range. The advantage is that the amplifier spacing is comparable to the soliton period. 2) Determined with the power change, so that the average path power of the soliton pulse after the weighting is equal to the ground state soliton power without considering the fiber loss, so it is also called the average soliton communication scheme.
This restriction condition makes La not too large, usually La is much smaller than the soliton period, but the transmission is very stable. The adiabatic soliton scheme utilizes the transmission adiabatic characteristic between two amplifiers, and its weighting factor is larger than that required by the average soliton transmission, and is not as stable as the average soliton transmission.
3 Control technology of optical soliton communication system
Common methods for controlling optical soliton communication systems are: frequency domain filter control, time domain synchronous amplitude modulation control and synchronous phase modulation control, nonlinear gain control, dispersion compensation and dispersion configuration control.
The frequency domain filter control is to insert an optical filter, called a pilot filter, after each fiber amplifier (EDFA) in the periodic lumped amplification soliton transmission system to filter out the ASE noise sidebands generated by EDFA and achieve stable transmission. This control scheme is also called a bandwidth limiting amplification control scheme. The optical filter usually uses an air-gap pigtail filter. The basic structure is a Fabry Perot (EP) etalon, which consists of two high anti-wax parallel lenses to form the cavity.
In order to overcome the limitation of ASE noise and Gordon-Haus effect on communication capacity, Nakazawa et al. Proposed another transmission control scheme, called synchronous amplitude modulation control. This scheme is to periodically extract clock pulses on the soliton transmission line to control The electro-optical amplitude modulator connected to the line shapes and times the soliton pulse passing through the modulator to achieve the purpose of suppressing the soliton arrival time jitter. This is a time-domain control technique that not only overcomes the Gordon-Haus effect, but is also very effective in suppressing the interaction of neighboring solitons. The synchronous phase modulation control scheme is to use the clock pulse extracted from the soliton transmission system to control the optical soliton pulse passing through the phase modulator, adjust the center frequency of the optical soliton, and achieve the purpose of suppressing the soliton arrival time jitter. In 1993, Smith conducted a preliminary analysis of the soliton timing jitter in the synchronous phase modulation control soliton transmission system, and found that inserting a single-stage phase modulator at the midpoint of the transmission system can reduce the soliton arrival time jitter variance by 80%.
Adopt frequency domain and time domain control, restrain ASE noise and Gordon-Haus effect, improve the communication capacity of the optical fiber soliton communication system, the two basic methods of soliton transmission control. The bandwidth-limited frequency domain control system formed by the filter can suppress the Gordon-Haus effect, but due to the additional gain that compensates for the insertion loss of the filter, it will cause the accumulation of dispersion waves near the center frequency of the filter, resulting in a stable decline in the system and poor communication capacity. To a great improvement.
The nonlinear gain control scheme is to use the control mechanism that the system gain characteristics vary nonlinearly with the light intensity, so that the strong light transmittance is high and the weak light transmittance is low. The disturbed or distorted soliton pulse can be shaped and the linear dispersion wave can be eliminated to achieve The soliton is transmitted stably.
Dispersion compensation control technology is currently widely used. Dispersion is used to compensate waveform distortion caused by dispersion and non-linearity. The system structure is simple, and ordinary single-mode fiber can be used to implement soliton communication. According to the distribution of optical fiber dispersion along the transmission system, several solutions such as terminal positive dispersion compensation, terminal positive dispersion compensation online filter control hybrid compensation, periodic lumped dispersion compensation, and periodic distributed compensation are proposed.
Dispersion compensation control technology used in optical soliton communication system can control ASE noise, soliton interaction and dispersion wave, etc., to achieve the purpose of improving the system transmission rate, increasing the transmission distance and communication capacity.
4 Status and prospects of optical soliton communication
In recent years, people have continuously expanded the field of research on optical solitons, and made significant progress, such as the application of optical solitons in WDM (Wavelength Division Multiplexing), quasi-soliton theory, the use of optical soliton communication experimental system, the maximum amplification distance is limited, How to extend the amplification distance, reduce the number of amplifiers, and reduce costs is a problem that needs to be solved urgently in optical soliton communication.
At present, the research on optical solitons is in-depth. Optical soliton communication has the advantages of large transmission capacity, long distance, low bit error rate, and strong anti-noise ability. Therefore, it has always been the focus of domestic and foreign scientific and technological workers, and its research prospect is infinitely broad.
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