AV Signal Transmission Distance in WDM System

In a WDM system, the maximum transmission distance is affected by optical loss, fiber bandwidth, and inter-channel skew. Optical loss equates to attenuation in the fiber, connections, and splices. The loss budget, determined by transmitter output power and receiver input sensitivity, is the maximum amount of allowable optical loss in the fiber link between the transmitter and receiver.

Fiber bandwidth is the maximum frequency or data rate that can be transmitted along a given length of fiber optic cable. Skew is caused by the various wavelengths propagating at different speeds along the fiber as shown in Figure 1. The result is similar to skew created by varying twist ratios in Category cable within a twisted pair system.

Figure 1 Inter-Channel Skew in WDM Applications Limits Transmission Distance

The amount of skew in a WDM system depends upon the range of wavelengths used in the system. In multimode systems using wavelengths around 850 nm, skew can be the dominant effect limiting transmission distance. For example, an HDMI system with a 225 MHz pixel clock has a maximum allowable inter-channel cable skew of 1.78 ns.

A popular device used in multimode WDM HDMI and DVI extenders has a specified maximum distance around 300 meters (984 feet) before the skew exceeds 1.78 ns. The same device can operate up to 400 meters (1,312 feet) for an HDMI or DVI signal with a 165 MHz pixel clock to allow a maximum skew of 2.42 ns.

Video + Data Multiplexer

A TDM system transmits the fiber optic signal at a single wavelength, such that the entire signal propagates at the same speed. With negligible skew, the maximum transmission distance is limited only by the available loss budget and system bandwidth. A well-managed loss budget and use of high-bandwidth, laser-optimized OM4 fiber enables multimode TDM systems to achieve distances up to 2 km (6,561 feet), as shown in Figure 2. As a result, TDM systems achieve much greater transmission distances than WDM systems used for AV signal extension.

Figure 2 TDM Systems Experience Negligible Skew to Achieve Longer Transmission Distance

Single mode systems use long wavelengths around 1310 nm or 1550 nm. At these wavelengths, WDM systems experience less inter-channel skew to achieve longer transmission distances. A typical WDM single mode system for HDMI/DVI signals transmits up to 12 km (7.5 miles), compared to 30 km (18.75 miles) for a TDM single mode system. However, not all WDM systems support OS1 fiber, which is the most common type of installed single mode fiber. WDM systems with wavelengths around 1390 nm suffer high attenuation in OS1 fiber due to water peak absorption as shown in Figure 3.

Figure 3 Attenuation in Optical Fiber

Attenuation severely limits the transmission distance. Around the 1390 nm wavelength, OS1 fiber attenuation can 4 dB/km or more due to water peak absorption. Assuming a loss budget of 10 to 13 dB, the maximum transmission distance is only 2 to 3 km at this wavelength. In order to achieve longer transmission distance, this type of WDM system requires OS2 low water peak single mode fiber. Alternatively, a single mode TDM system uses either a 1310 nm or 1550 nm wavelength, where OS1 fiber attenuation is typically 1 dB/km or less, so it operates at its full distance capability over both OS1 and OS2 fiber types.

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