Ultra-Long-Haul Transmission



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.



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).




 Description  Difference
 DCM  Based on DCF


 The insertion loss increases with distance.





 DCU  Based on FBG


 The insertion loss is irrelevant with distance.






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




 EDFA  Raman
 Wavelength gain  Fixed range


 Determined by wavelength of the pump light.


 Theoretically, a signal can be amplified to anywavelength if the wavelength of the pump allows.



 Power conversion   




 High  Low




 Medium  High



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Notes      VOA: variable optical attenuator      BA: booster amplifier     FRA: forward Raman amplifier

                FOA: fixed optical attenuator           PA: power amplifier       BRA: backward Raman amplifier


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