Aug 9, 2020

Is there a need for 1490 nm testing in PONs?

This paper explains the difference between 1490 nm optical time domain reflectometer (OTDR) and insertion loss testing as well as physical layer and equipment/transmission signal testing. It also describes the technical and economical differences between 1490 and 1550 nm when analyzing the intrinsic characteristics of the fiber. 
Point-to-multipoint passive optical networks (PONs), such as Ethernet PON (EPON), Gigabit PON (GPON) or Gigabit Ethernet PON (GEPON) technologies, bring imminent testing challenges, especially at the construction stage of the fiber link when using splitters. The most recurring question concerns the need for qualifying the fiber plant at 1490 nm, the wavelength used to transmit data from the optical line terminal (OLT) to the optical network terminal (ONT), making it legitimate to consider testing at this particular wavelength. But is it worth testing at this wavelength? OTDR Testing The OTDR helps technicians characterize fibers and optical networks. 

Primarily it provides location information regarding localized loss and reflective events, offering a pictorial and permanent record of the characteristics of a fiber. Secondly it measures the total loss of the link, which is discussed later. When characterizing a fiber link, field technicians also measure the insertion loss of the fiber and investigate possible issues that could occur, such as high connector loss, splice loss, high attenuation, or possible bends. To properly locate an event and measure the reflectance, technicians must enter the fiber specifications into the OTDR setup. 

Today the International Telecommunications Union-Telecommunications Sector (ITU-T) G.652 standard does not require specification at 1490 nm and most fiber manufacturers provide fiber specifications for the common 1310 and 1550 nm wavelengths. The index of refraction and backscatter coefficient settings for 1490 nm are typically unknown, leaving users unassured of the accuracy of location and reflectance results. 

Furthermore, splice/connector losses are not wavelength-dependent. A 0.2 dB splice loss at 1550 nm will exhibit a 0.2 dB loss at 1310 and 1490 nm. Another key element of a PON system is the splitter (from a 1x4 to a 1x64). The most commonly used is the 1x32 and the loss variation between the 1490 and 1550 nm wavelengths is as low as 0.3 dB. Once again, it shows minimal or marginal value-added information for testing at these two wavelengths. Also the 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 bends, meaning the bend-dependent loss is higher and the location easier.

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