Ultra-Long-Haul Transmission

Overview

The ultra-long-haul optical transmission technology increases the span between optical sites. This will reduce the number of optical relay sites, which can lower engineering cost and OPEX, 

and improve system reliability.

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Key Technologies
1.Optical Power Budget
In optical network design, the total loss of optical power must be calculated to ensure that performance of the optical transmission span meets requirements.

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Optical power budget (dB) + Design margin (3 dB) ≤ Pout (dBm) – Pin (dBm)
L (km) x a (dB/km) + Design margin (3 dB) ≤ Pout (dBm) – Pin (dBm)
a indicates the optical fiber loss coefficient. For a G.652/G.655 optical fiber, a takes the value of 0.275 dB/km (generally, the value ranges from 0.2 to 0.3 dB/km).
2.Code Error Correction Technology
FEC is a forward error correction technology widely applied to communication systems.

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Basic principle: At the transmit end, one k bits signals and (n-k) bits redundancy verification bits are added into a group for encoding, which will generate a code word with the length of n bits. After the code word is transmitted to the receive end through the channel,the receive end verifies the code word. If errors are correctable,error bits can be 

detected and corrected. This will defend against interference on the channel and improve system reliability.

3.Dispersion Compensation
There are two common dispersion compensation modes: using dispersion compensation fibers or using the fiber Bragg grating (FBG).
ModuleDescriptionDifference
DCMBased on DCFThe insertion loss increases with distance.
Passive
Subrack
DCUBased on FBGThe insertion loss is irrelevant with distance.
Active
Board

4.Optical Amplifier
An optical amplifier can amplify optical signals without involving the conversion between optical and electrical signals.
Common optical amplifiers include erbium-doped fiber amplifiers (EDFA) and Raman amplifiers.
Working principle of EDFA

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Working principle of Raman amplifiers

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Comparison between EDFA and Raman

ItemEDFARaman
Wavelength gainFixed rangeDetermined by wavelength of the pump light.
Theoretically, a signal can be amplified to anywavelength if the wavelength of the pump allows.
Power conversion efficiencyHighLow
CostMediumHigh
Example

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NotesVOA: variable optical attenuatorBA: booster amplifierFRA: forward Raman amplifier
FOA: fixed optical attenuatorPA: power amplifierBRA: backward Raman amplifier

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Ultra-Long-Haul Transmission

OverviewThe ultra-long-haul optical transmission technology increases the span between opt..