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.
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.
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.
There are two common dispersion compensation modes: using dispersion compensation fibers or using the fiber Bragg grating (FBG).
|DCM||Based on DCF||
The insertion loss increases with distance.
|DCU||Based on FBG||
The insertion loss is irrelevant with distance.
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
Working principle of Raman amplifiers
Comparison between EDFA and 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.
Notes VOA: variable optical attenuator BA: booster amplifier FRA: forward Raman amplifier
FOA: fixed optical attenuator PA: power amplifier BRA: backward Raman amplifier