### Transmission characteristics of optical fiber

• Thursday, July 14 at10:43AM
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Fiber Loss

The loss of power during transmission is one of the basic and important parameters of optical fibers. Due to the existence of loss, the optical power transmitted in fibers attenuates by index with the increase of transmission distance.

1.    Cause of optical fiber loss and low-loss window

The loss of optical fiber mainly comes from the following two causes:

1)    Loss coming from the optical fiber itself, including the inherent absorption loss of fiber materials, absorption loss of material impurity (especially the loss caused by the remained OH- component in the optical fiber), Rayleigh scattering loss, as well as the scattering loss caused by inperfect fiber structure.

2)     Since optical cables are made of a bundle of clustered optical fibers, the layout

of optical cables, connection of optical fibers, and coupling and connection of the transmission system may all cause the additional loss of fibers, including bending loss, microbending loss, coupling loss in the optical fiber line, and coupling loss between optical components.

The fiber attenuation spectrum is the average loss of window I is 2 dB/km, that of window II is 0.3dB/km to 0.4dB/km, and that of window III is 0.19dB/km to 0.25dB/km. The 1380 nm point in window V is an OH- absorption peak.

Relationship between loss and Optical Signal-to-Noise Ratio (OSNR)

OSNR is the ratio between optical signal power and noise power. It is a very important parameter for estimating and measuring the system bit error performance, engineering design and maintenance.

Take the OSNR at the receiving end of a DWDM system for example. The calculation formula is:

OSNR= Pout - 10 log M - L + 58 - NF - 10 log N

Where,

Pout: the input optical power (dBm)

M: Number of multiplexing channels of the DWDM system

L: Loss between any two optical amplifiers, that is, section loss (dB)

NF: Noise figure of EDFA (dB)

N: Number of optical amplifiers between optical multiplexer and optical demultiplexer of the DWDM system.

The formula shows that when the other parameters keep unchanged, greater line loss leads to lower OSNR, which means decreased transmission quality of the optical line