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Figure 2 Profile of standard single-mode optical fiber
Standard outer diameter optical fiber
According to international standards, the outer diameter of the glass (cladding) of optical fibers is 0.125 ± 0.0007 mm, and the outer diameter of the coating layer is 0.235 to 0.265 mm. The optical fiber widely used in optical communication systems is a single-core single-mode fiber with an outer diameter of 0.125 mm, and the capacity limit is considered to be about 100 terabits per second in the conventional C and L-bands and 200-300 terabits per second if adopting additional bands.
Wavelength division multiplexing (WDM) technology
WDM is a method of transmitting optical signals of different wavelengths within a single optical fiber. WDM is a widely used technology to increase the transmission capacity in proportion to the number of wavelengths. However, the available bandwidth suitable for efficient optical transmission is limited and the current number of wavelengths used in current long-distance optical transmission systems is typically around 90.
Optical fiber transmission windows
Various wavelength bands for optical fiber transmission are defined, distinguished by regions with different transmission characteristics arising from physical properties of the fiber, as summarized in Figure 3. The C-band (wavelength 1,530 - 1,565 nm) and L-band (1,565 - 1,625 nm) are most commonly used for long-distance transmission, with O-band (1,260 - 1,360 nm), currently used only for short-range communications. Although the U-band (1,625 - 1,675 nm) is rarely used due to lack of suitable amplification, new amplifier technologies have recently enabled research into the use of E-band (1,360 - 1,460 nm), S-band (1,460 - 1,530 nm) windows. In this experiment we utilize the majority of the S-band for the first time.
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Figure 3 Optical communication wavelength band
Optical amplification system
Optical fibers have a very small transmission loss compared to coaxial and other electrical cables, but since data is often transmitted over long-distances, it is necessary to compensate for attenuation periodically, typically after several tens of kilometers. This is usually done in an optical amplifier which may amplify many wavelength (WDM) channels simultaneously. A common practical amplification method uses rare-earth doped fibers. By adding a small amount of rare-earth ions such as erbium (Er3+) and thulium (Tm3+) to the base material of an optical fiber, amplification can be achieved by exciting these ions with lower wavelength pump lasers and then amplifying signal photons through stimulated emission. Such amplifiers have significantly increased the transmission range of optical fiber communication and allowed amplification of many wavelength channels simultaneously. For recent wide-band transmission systems other amplification schemes such as Raman amplification and semiconductor optical amplifiers, have been also employed.
Raman amplification
Raman amplification is based on stimulated Raman scattering (SRS), when signal photons induce the inelastic scattering of a lower wavelength 'pump' photon in a non-linear optical medium. When this occurs, additional signal photons are produced, with the surplus energy resonantly passed to the vibrational states of molecules in the fiber core. This process, as with other stimulated emission processes, allows all-optical amplification in optical fibers with the gain depending on material of the fiber core.
MIMO(Multiple-input-multiple-output) digital signal processing
In multi-mode transmission in which the arrival time of an optical signal differs depending on the mode, MIMO processing is almost always required in the receiver processing. MIMO is a signal processing technique used to eliminate multipath interference in wireless communication and is used to eliminate interference between different optical signals propagating in the same optical fiber in optical communication. The load of MIMO processing increases according to the difference in propagation speed of each mode, and the mode separation, meaning it becomes progressively more difficult as the transmission distance increases.
Advanced optical fiber
The capacity of the standard single-core single-mode fiber currently used for short and long-distance optical communication systems is considered to be limited to about 250-300 terabits per second. In order to solve this problem, research has been advanced on multi-core fibers with increased cores (optical paths) and multi-mode, multi-core fibers. A comparison of high data-rate transmission demonstrations using such fibers is shown in Figure 4 and recent achievements by NICT with standard diameter fibers are shown in Table 2.
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Figure 4 Recent high data-rate demonstrations in 0.125 mm diameter optical fibers – Here 1 Pb/s achieved with only 4 spatial channels compared to previous 15-mode fiber
Table 2 Comparison table of standard diameter fiber transmission demonstrations performed at NICT
256QAM
QAM is a technique for modulating information data on optical signals using multiple levels of both phase and amplitude of the optical wave, that can enable very high spectral information density. 256 QAM uses 256 different signal symbols and can therefore encode 8 bits of information (28 = 256 bits) in each symbol. The spectral density of 256 QAM is therefore 8 times higher than for simple modulation formats such as on-off keying. 256 QAM symbols may also be transmitted in both polarizations simultaneously, increasing the number of bits transmitted in each dual polarization symbol to 16.
