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Why needed 1490 nm testing in PON?

Why needed 1490 nm testing in PON? : In this article, I will discuss the same topic why 1490 nm testing is required in the PON. In other words we can say that why do not test on 1550 nm in PON? We will also try know that what is the differences between 1490 nm and 1550 nm when analyzing the intrinsic characteristics of optical fiber.

When we test the ODB(Customer end) of FTTH network, we do it on 1490 nm. To properly detect an event and measure reflection, technicians must enter fiber specifications into the OTDR setup. The optical fiber type G.652 standard does not require specification at 1490 nm and most fiber manufacturers provide fiber specifications for the 1310 and 1550 nm wavelengths,then now question is arises that Why needed 1490 nm testing in PON.

Why needed 1490 nm testing in PON?
1490 nm testing in PON

Optical testing tools used for FTTH networks

This article will be incomplete if to go ahead without mentioning the optical testing tools of the FTTH network. It is only through the following image it is mentioned that the optical testing tools used for FTTH networks. Which will be easy for you to understand.
Optical tools used for FTTH network testing
Optical testing tools for FTTH network

PON System and OTDR Testing

This is the most interesting point that the index of refraction and backscatter coefficient settings for 1490 nm is usually unknown, assuring users about the location and accuracy of reflection results. Also, hope you all know that splice / connector losses are not wavelength-dependent. A 0.2 dB splitting loss at 1550 nm would show a loss of 1310 and a loss of 0.2 dB at 1490 nm.

Another important element of the PON system is the splitter (from 1x4 to 1x64). Most commonly used is 1x32 and loss variation between 1490 and 1550 nm wavelength is less than 0.3 dB. Once again we get a chance to understand that, both shows the minimum or marginal value-added information for testing at these two wavelengths.

In addition we can say that OTDR can detect and localize macro bends by comparing two OTDR shots made at different wavelengths, typically 1310 and 1550 nm. Longer wavelengths are more sensitive to macro turns, meaning that the bend-dependent loss is higher and the location is easier to determine.

Optical Sensitivity at 1490 nm and 1550 nm

More sensitive optical fiber is the single mode optical fiber to bending losses at longer wavelengths, therefore, 1550 nm wavelengths are more sensitive to macro bends than 1490 nm, a macro bend with a radius of 25 mm at 1550 nm. Will exhibit high losses but nothing at 1490 or 1310 nm.

If you work on FTTH network, then you know very well that the optical bends in the FTTH network is more than NLD and other networks.

According to OTDR measurements, additional value of bend detection at 1490 nm wavelength does not provide and may give incorrect location and reflection results on PON systems.

Insertion Loss Testing

Anyone who works in the field of optical network definitely measure the insertion loss of the fiber and high attenuation. Attenuation characteristics of a fiber according to the wavelength you will found very little difference in terms of attenuation between the S-band (1460-1530 nm) and C-band (1530-1565 nm).
You can understand from the below image-
Fiber attenuation as a function of wavelength
Attenuation Vs Wavelength
If we look at the value of the attenuation coefficient of G.652C fiber, then there will be a very little difference between 1490 nm and 1550 nm.

Wavelength # Typical Attenuation Coefficient for G.652.C

1310 nm       
#0.32 dB/km
1490 nm       
#0.21 dB/km
1550 nm       
#0.19 dB/km

We see that the difference between the attenuation coefficient of 1490 nm and 1550 nm is only 0.02 dB / km. So we can say that maximum limit for fiber length in PONs is 10 km.

For PON fiber link using the traditional 1310/1550 nm wavelength equal value as 1490 nm. PON testing at 1490 nm with a PON selective power meter is essential for measuring absolute power levels during turn-up or troubleshooting.

Larger telecom operators invested in OTDRs and loss test sets with 1550 nm and 1310 nm wavelength and they also arranged to used 1490 nm test solutions with PON power meters for turn-up and troubleshooting.

Friends, how did you like this article " Why needed 1490 nm testing in PON", tell me by commenting. I have tried my best to give you complete information about why 1940 nm is used in PON. Share this if you want to help a telecom engineer.

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