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How to calculate attenuation in optical fiber cable?

AuthorTao

Date12/24/2021

Before laying the optical cable, the engineer will design a detailed transmission scheme of the optical fiber signal. Calculating and measuring the actual loss and signal-to-noise ratio of the optical fiber are the key points in the design. This article will tell you how to calculate the theoretical attenuation of optical cable and briefly explain the concept of signal-to-noise ratio.

Optical fiber loss types

Fiber loss can be called fiber attenuation, which can measure the attenuation of optical signals during transmission. Attenuation in optical fiber is due to many factors, including light absorption, cable bending, connector loss, and so on. The optical signal is absorbed by the optical fiber material in the optical fiber. Therefore, the light absorption in the optical fiber is also called material absorption. Due to wavelength impurities, light energy will be absorbed and converted into other forms of energy, such as heat.

 

We generally divide the loss of optical fibers into two major classifications, the inherent loss of the optical fiber itself and the external loss. The types of losses in optical fiber are divided according to the inherent characteristics of the optical fiber or by external factors. The intrinsic attenuation of optical fiber includes chromatic dispersion loss and transmission power loss. External losses include splicing loss, connector loss, bending loss in optical fiber cable, etc. When we calculate the attenuation, we need to consider the influence of both at the same time. The inherent loss of the optical fiber will increase with the increase of the transmission distance. Let us look at the attenuation standard of the intrinsic loss below.

The industry standard for fiber cable inherent attenuation

The G.65x series of ITU-T Recommendations specify the optical cable's performance and transmission requirements. They are widely accepted and used by the optical fiber industry. The transmission power loss needs to be accurately calculated in the long-distance optical cable link, and the Attenuation coefficient is expressed in dB/km. It is the most crucial parameter in fiber loss measurement. Nowadays, the most widely used optical cable type is the G.652 series optical cable. According to the ITU-T G652 standard, the maximum attenuation of the G.652 series of optical cables is shown in the following table:

 

Description

IEC 60793-2 B1.1, B1.3 SMF, ITUT G.6652, G.652 SMF

Unit

Nominal wavelength

1270

1310

1550

1577

nm

Cable attenuation(max)

0.44

0.4

0.35

0.35

dB/km

Zero dispersion wavelength

1300≤λ≤1324

nm

Dispersion slope(max)

0.093

pcs/nm²·km

 

The Zero dispersion wavelength in the above table means that the dispersion is zero in the 1300~1324 band. Dispersion is also an indicator that needs to be considered in transmission. The dispersion coefficient of G.652D is generally 17ps/(nm*km). Want to know the knowledge about dispersion, we will explain it in detail in the next article.

How to calculate the power loss in optical fiber

Total link loss = fiber attenuation loss + connector loss + splicing loss + equipment insertion loss

Fiber attenuation loss = maximum attenuation coefficient x transmission distance

Connector loss = the number of connector points x the maximum insertion loss of the connector

Splicing loss = the number of splices x loss of splice point is generally calculated at 0.1dB/per.

 

It should be noted that all the above loss calculations are theoretical results. The bending loss caused by the bending of the optical cable and the loss caused by other environmental factors is ignored. Generally, the actual loss will be different from the theoretical calculation result. In order to ensure that the transmission will not be affected, a system margin will be reserved in the calculation. A margin of 3dB will be reserved for the system margin. It is equivalent to if the total attenuation of a link is 10dB, then we will calculate at least according to the standard of 13dB when designing.

 

Let's introduce a practical case to demonstrate the calculation steps. The G.652 optical cable is installed between the two data centers. The length of the optical cable is 45km, and the transmission wavelength is 1550nm. There are 3 fusion points and two LC UPC adapter panels between the links.

 

dci 45km

 

Calculate the fiber attenuation loss. According to the table above, the attenuation coefficient can be 0.35dB/km. 

Attenuation loss of optical fiber=0.35dB/km x 45km=15.75dB

The maximum insertion loss of LC UPC is generally 0.3dB. The connectors and adapter panels at both ends have a total of 4 connector points. 

 

Connector loss = 4 x 0.3dB = 1.2dB

Splice loss = 3 X 0.1dB = 0.3dB

The total link loss=15.75dB+1.2dB+0.3dB=17.25dB

 

The system margin is 3dB. Then we can design this link according to the optical fiber loss of 20.25dB. Please note that all calculations above are theoretical values. If you want to get accurate actual loss, you can use OTDR to measure.

How to calculate the power budget in fiber optic cable?

The power budget is used to evaluate the maximum power attenuation that the entire system can tolerate. The calculation result of the power budget (P Budget) is the difference between the sensitivity of the receiver (P Receive) and the output from the transmitter to the fiber (P Transmit ).

Take the SFP+ 10G 80KM DWDM module as an example. The TX power range of this module is 0~4dBm, and the Receiver Sensitivity is -23dBm. The power budget of this module=0dBm-(-23dBm)=23dBm. Then the total link loss of this module with the optical cable plus the system margin needs to be less than 23dB to be able to transmit normally.

What is OSNR?

OSNR is the ratio of signal power to noise power after an optical channel passes through an optical network. OSNR (Optical Signal-to-Noise Ratio) is a key measure of signal quality in long-distance optical fiber systems. It provides an estimate of the influence of noise power on signal power. In general, the higher the OSNR, the better for the entire system.

 

The common formula for calculating OSNR is: OSNR=10*log(S/N)

The capital S in the formula is the signal power, and N is the noise power, and their units are all expressed in watts or milliwatts. The OSNR value is the most important on the receiver side, a low OSNR value means that the receiver will not detect the signal. Generally, the OSNR at the receiving end should be greater than 15 dB to 18 dB, this value depends on many factors, such as data rate, required BER, and so on. The OSNR measurement method is defined in the IEC 61282-12 / b-IEC 61280-2 -9 standard.

Small test for attenuation calculation in G.652 fiber cable

After reading this, you guys already have a concept about how to calculate fiber attenuation. Now, Let’s make a quiz.

Assume that the parameters of a certain optical module are as follows:

Central Wavelength = 1310nm; Tx power =-4~0dBm; Receiver Sensitivity = -15 dBm;

Please judge whether the module can work normally on the above optical fiber. If you still have a problem with it. Welcome to contact QSPFTEK for help.

 

Part of the content reference: https://www.itu.int/itu-t/recommendations/rec.aspx?rec=13076

 

 

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