Fiber optics is a way to provide high bandwidth, high-speed communications for 5G networks. Fiber optics have low attenuation and high security since there is no radiated electromagnetic field. This type of communication has resistance to interference and humidity and can provide a reliable means of high transmission communications for the upcoming 5G deployments around the globe.
Fiber is basically a waveguide that is used for the propagation of light through a medium. This medium is a fiber cable that consists of two layers, the Core and the Cladding. The refractive index of the cladding layer is higher than the refractive index of the core. In this way, when light tries to pass from a lower to a higher refractive index and at a specific, correct angle, it is reflected back. This phenomenon causes the light to be reflected back into the core, allowing it to travel to the other end of the fiber.
Although fiber is a reliable means of communication, it is usually costly to deploy since there should be fiber digging all the way to the end site. Furthermore, when there is damage to the fiber, it is also not easy to replace/restore, so usually, protection paths should also be available, especially in aggregation sites.
When the fiber has a large core diameter, usually higher than 10 micrometers, it is known as multi-mode fiber. Fiber with a smaller than 10 micrometers core diameter is known as single-mode fiber. Single-mode fibers can typically cover greater distances compared to multi-mode ones. However, multi-mode fibers are less expensive.
WDM principles
Legacy communications use Time-division multiplexing (TDM) to offer PDH, SDH, and ISDN services. Frequency-division multiplexing (FDM) was introduced as a technology, and many services such as radio broadcasting, Cable TV, etc., were using this form of communication. Wavelength-division multiplexing (WDM) uses the same concept of multiplexing like FDM. However, this term is used in fiber optics since optical carriers are usually described by wavelengths compared to the frequency, which generally applies to radio signals.
Dense wavelength division multiplexing (DWDM) is used to multiplex different wavelengths of optical signals to one single optical fiber. This type of communication offers very high capacity and is also used for Long-haul transmission. DWDM uses 50 or 100 GHz channel spacing, and according to the ITU-T grid, it can support up to 96 channels. It offers an optical supervisory channel (OSC) for management and OSC out of C-band (not amplified).
CWDM stands for coarse WDM. These systems cost typically less than DWDM and offer a limited number of channels (up to 18). They use 20 nm channel spacing and are generally used for short-haul communication. These systems are typically not amplified like the DWDM ones.
WDM Signal Modulation
WDM transmission may use Intensity Modulation with Direct Detection (IM-DD), also known as On/Off Keying (OOK) and Non-Return to Zero (NRZ). This modulation is typically not efficient in spectrum utilization beyond 10G.
In DPSK (Differential Phase Shift Keying), the data is differentially encoded so that the bits are represented by phase changes. This modulation scheme is used in 40G applications.
Differential Quadrature Phase Shift Keying (DQPSK) carries 2 bits in each symbol, which means a lower baud rate. This configuration is also typically used for 40G transmission.
Polarization Multiplexing: By selectively transmitting modulated signals using polarization multiplexed (PM) carriers, the spectral efficiency of a given modulation technique can effectively be doubled. This modulation scheme is used for 100G transmission.
WDM network elements
A WDM network typically consists of terminal nodes where all DWDM channels are terminated, and none of the channels are passing through. Client (grey) signals are connected to transponders, which make O-E-O (optical-electrical-optical) conversion to colored (WDM) signals. All colored signals are multiplexed in the multiplexer to composite (“trunk”) signal. In systems with a high number of channels, addition coupler can be used to combine odd and even channels.
Optical line amplifiers (OLAs) are used only to re-amplify the signal, and none of the DWDM channels can be dropped. In OLA nodes, no wavelength is terminated, added, or dropped. The composite signal is amplified by the Erbium-Doped Fiber Amplifier and covers the whole C-band. Only OSC dropped/added, and only re-amplification of data pulse is carried out. No reshaping or retiming takes place.
Optical add/drop multiplexers (OAM) are used so that some of the DWDM channels are dropped or added at that spot. Other DWDM channels are just passing through.
In FOADM (Fixed optical add/drop multiplexer) nodes, some wavelengths are dropped from the composite signal, and some wavelengths are added to the composite signal. Wavelengths dropped/added by filtering, are defined by hardware. Typically 2, 4, and 8-channel filters are used. All other wavelengths are passing through.
In ROADM (Reconfigurable optical add/drop multiplexer) nodes, the selection of which wavelength is passing through and which is locally dropped/added is made by the wavelength selective switch (WSS).
4-degree ROADM nodes use 4 lines (directions) where traffic (wavelengths) can be routed from any source to any destination. Routing of traffic is done by WSSs per wavelength.
In Directionless ROADM nodes, locally added/dropped wavelengths are treated as another direction. Thus, these local wavelengths can be routed in any direction.
In Colorless RADM nodes, transponders are connected to WSSs instead of mux/demux. On the receiver side, the WSS is used in reverse setup and deals out wavelengths on desired ports. In the transmit direction, the Tx WSS couples the individual wavelengths from transponders.
DWDM limitations
Attenuation: Attenuation occurs when the signal propagates through the fiber, and there is a reduction in signal strength or light power.
Types of attenuation are Rayleigh scattering, which is caused by the reflection of light rays on rough and irregular surfaces, even at the molecular level of the glass. Rays are reflected in many random directions, however, not changing the wavelength.
UV, IR absorption is the process of conversion of electromagnetic wave energy into other forms of energy (i. e. lattice vibration). Intrinsic silica glass absorption occurs in both ultraviolet and infrared bands. Infrared absorption tail causes attenuation for the wavelengths longer than 1,6 mm.
OH absorption is caused by absorption on impurities in optical fibers, mainly hydroxyl ions (OH-). The most significant OH losses occur at 950, 1250, and 1380 nm.
Dispersion: Dispersion is the phenomenon that occurs when the light pulses are spread as they travel down the fiber. Chromatic dispersion is caused by different propagation velocity of particular frequency components of the signal. The typical value of chromatic dispersion is 17 ps/nm/km for fiber, according to G.652 at 1550 nm.
Polarization mode dispersion (PMD) is caused by different propagation velocity of two polarizations. It is induced by the asymmetry of fiber, such as geometrical inaccuracy, impurities, fiber strain, etc. Modal dispersion is caused by different propagation velocity of particular modes and occurs in multi-mode fibers only.
Non-Linear Effects
These effects take place when the light interacts with the material as it travels. Non-linear effects may cause a change in the light wave or various interactions between light waves.
In SPM (Self-phase modulation), the Refractive index is also a function of the intensity of signals propagating in fiber. It is caused by changes in the intensity of the optical signal. SPM phenomenon results in changes in the signal phase, and it usually influences systems based on PSK coding. It also causes signal frequency changes XPM (Cross-phase modulation), which has a similar effect to SPM. Phase changes are caused by the change of intensity of optical signal propagating with different wavelengths.
FWM (Four-wave mixing) is the Intermodulation effect caused by the Kerr effect. When three frequencies (wi , wj , and wk ) interact in a nonlinear medium, they give rise to a fourth wavelength (wijk). FWM effect gets stronger when chromatic dispersion gets closer to zero. Other limiting effects are Brillouin scattering, Mie scattering, Raman scattering, etc.
