Detailed explanation of common fiber types
Optical fiber broadband is to convert the data to be transmitted from electrical signals to optical signals for communication. Both ends of the optical fiber are equipped with "light cats" for signal conversion. The difference between optical fiber broadband and ADSL access methods is: ADSL is the transmission of electrical signals, and optical fiber broadband is the transmission of optical signals.
Optical fiber is the most ideal one among various transmission media in broadband network. It is characterized by large transmission capacity, good transmission quality, low loss, and long relay distance. Optical fiber transmission uses wavelength division multiplexing, that is, the data of multiple users in the community is concatenated into high-speed signals using PON technology, and then modulated to optical signals of different wavelengths for transmission in one optical fiber.
1. Optical fiber for transmission
The application of optical fiber technology in transmission systems is first realized through various optical networks. The topological structures of various optical fiber transmission networks constructed so far can basically be divided into three categories: star, line and ring. Further, in terms of the layered model of the network, the network can be divided into several layers from top to bottom, and each layer can be divided into several subnets.
That is to say, the network and network composed of each switching center and its transmission system can be further divided into several smaller subnets, so that the entire digital network can effectively communicate and serve, and the fully digital integrated services digital network (ISON ) is the general goal of the communication network. The popularization of ADSL and CATV, the continuous increase of the capacity of the metro access system, and the expansion of the backbone network all require different types of optical fibers to play the important role of transmission.
2. Dispersion Compensation Fiber (DCF)
Fiber dispersion can broaden the pulse, which can lead to bit errors. This is a problem that must be avoided in the communication network, and also a problem that needs to be solved in the long-distance transmission system. Generally speaking, fiber dispersion includes material dispersion and waveguide structure dispersion. Material dispersion depends on the dispersion of silica masterbatch and dopant used to manufacture fiber, while waveguide dispersion is usually a mode of effective refractive index with wavelength. the tendency to change.
Dispersion compensating fiber is a technology used to address dispersion management in transmission systems. Non-dispersion-shifted fiber (USF) is dominated by positive material dispersion, which, after combining with small waveguide dispersion, produces zero dispersion near 1310 nm. Dispersion-shifted fiber (DSF) and non-zero dispersion-shifted fiber (NZDSF) are deliberately designed to produce a waveguide dispersion comparable to material dispersion after using technical means. After that, the zero-dispersion wavelength of DSF is shifted to around 1550nm. The 1550nm wavelength is the most widely used wavelength in today's communication networks.
In the submarine optical cable transmission system, the dispersion management is performed by combining two optical fibers with positive dispersion and negative dispersion respectively to form a transmission system. With the increase in the distance and capacity of the transmission system, a large number of WDM and DWDM systems are put into use. In these systems, DCFs with double-cladding and triple-cladding refractive index profiles that can operate in the C-band and L-band have been developed for dispersion compensation. The dispersion value of the SMF that can perform dispersion compensation in the C-band is 60-65 Ps/nm/km, its effective area (Apff) reaches 23-28m2, and the loss is 0.225-0.265 dB/km.
3. Optical fiber for amplification
Amplifying fibers can be made by doping rare earth elements in the core layer of silica fibers, such as erbium-doped fiber (EDF), thulium-doped fiber (TOF), and so on. Amplifying fiber has good integration performance with traditional silica fiber, and also has many advantages such as high output, wide bandwidth, and low noise. Fiber amplifiers (such as EDFAs) made of amplifying fibers are the most widely used key components in today's transmission systems.
The amplification bandwidth of EDF has been expanded from C-band (1530-1560nm) to L-band (1570-1610nm, and the amplification bandwidth is 80nm. The latest research results show that EDF can also be amplified in S-band (1460-1530nm). Mann fiber amplifier for amplification in the S-band.
For L-band (1530-1560nm) amplifying fibers, double-clad fibers have been developed in the field of high output. The first cladding layer transmits the pump light in multi-mode, transmits the signal light in the single-mode cladding layer of the fiber core and doped nail (Yd) as a sensitizer to increase the absorption coefficient.
In solving the nonlinearity of the fiber, the EYDF fiber is made by co-doping rare earth elements such as Yb or La (lanthanum). Almost no FWM occurs with this fiber. This is because the agglomeration of Yb ions and Er ions increases the distance between Er ions after the agglomeration of Er ions, which solves the concentration extinction caused by the excessive concentration and agglomeration of Ev ions, and also increases the doping amount of Er ions. The gain factor is increased, which reduces nonlinearity.
For the L-band (1570-1610nm) amplifying fiber, it has been reported that the C-band EDF developed by Japan's Sumitomo Electric requires 1/3 of the length of the short-size EDF to expand to the L-band. The L-band three-stage structure fiber amplifier with zero total dispersion and suitable for 40Gb/s high-speed transmission was successfully produced. The first stage of the amplifier is a conventional EDF with negative dispersion, while the second and third stages are short-dimension EDFs with positive chromatic dispersion.
For the S-band (1460-1530nm) amplifying fiber, NEC Corporation of Japan used dual-wavelength pump GS-TD FA to conduct a long-distance transmission test of 10.92 Tb/S, and used 1440nm and 1560nm dual-wavelength lasers (LD) to achieve 29%. Conversion rate; NTT achieved 42% conversion rate with single-wave and 1440nm dual-channel pump lasers (thulium-doped concentration of 6000ppm); Alcatei achieved 48% conversion rate with 1240 and 1400nm multi-wave Raman lasers, while utilizing 800nm The dual-wavelength pumping of titanium sapphire laser and 1400-level Raman laser has achieved a conversion rate of 50%. The latest report is that Japan's Asahi Glass Company has proposed an S-band pump amplification using bismuth (Bi) group oxide glass as the host material. Program. In short, the main technical problem to be solved is how to reduce the entanglement of phonon energy components and improve the quantum efficiency.