The specifications shown in Table 1 are typical and will vary somewhat between manufacturers. The color codes of the fiber jacket (patch cord) are also defined to differentiate legacy multimode fiber from laser-optimized multimode fiber. The traditional orange colored patch cord jacket material indicates that the fiber is legacy 62.5/125μm or 50/125μm fiber. An aqua colored jacket indicates that the fiber inside is laser-optimized multimode fiber.

Since most optical transport systems will have an optical budget of 12-20 dB, depending on data rate, it is unlikely that any of these systems will be loss-limited. In other words, it is more likely that the system will be signal-quality (bandwidth) limited instead of signal-quantity (attenuation) limited. As such, it’s important to understand how the bandwidth of these fibers affects the maximum transmission distance of high-definition video signals.

As mentioned in a previous application note, the bandwidth of the fiber is inversely proportional to the distance. For example, if the fiber has a bandwidth of 500 MHz-km, the bandwidth of the fiber at a distance of 1 km will be 500 MHz. At 2 km, the bandwidth will have been reduced to 250 MHz and at 5 km, the end-to-end bandwidth of that same fiber will now be 100MHz.

Much of the multimode RGB, DVI, and HDMI fiber transmission equipment operates in the 850nm wavelength region. Using a typical CWDM approach of wavelength multiplexing the RGB colors along with the sync pulses, a typical wavelength will have a data rate in the region of 2 Gbps. This, obviously, depends on the resolution, refresh rate, and compression (if any), and will vary somewhat around this number. One of the unique advantages of wavelength multiplexing in fiber is that each wavelength can utilize the fiber’s full bandwidth capacity. In other words, if the fiber is capable of transmitting 1 Gbps of data over 1 km, each wavelength on that fiber can transmit the same 1 Gbps data rate, thereby significantly increasing the combined data rate of the fiber.

With this as a reference point, if we assume that each wavelength in a wavelength multiplexed DVI channel is transmitting at a maximum data rate of 2 Gbps, the maximum distance that the signal can be transmitted over multimode fiber can be approximated in Table 2. These distances will vary with fiber manufacturer, data rate, video resolution, and other extrinsic factors related to the signal transmission. Notice that the laser-optimized fiber is only optimized in the 850nm region of wavelengths. While laser-optimized fiber can significantly increase the maximum distance of the signal being transmitted, it’s important to know what wavelengths the transmitter is utilizing.

As Table 2 indicates, using a laseroptimized fiber while transmitting at 1300nm will not help to increase the maximum distance. However, using 850nm, the maximum distance can be significantly increased by over a factor of 10 just by changing the type of fiber. Transmitting at 1.485 Gbps, a standard HDSDI signal will have similar distance constraints while a 3G video channel will be limited even further because of the higher signal data rate.

As the data rates of these signals continue to increase, it’s becoming more important to understand both the capabilities and limitations of the fiber infrastructure. Each fiber type has its advantages and disadvantages and, used properly, can yield a high-performance, highreliability system that can support emerging technology trends. It’s incumbent upon all fiber system and equipment designers to understand and properly utilize the capabilities of these fibers.

While laser-optimized fiber can significantly increase the maximum distance of the signal being transmitted, it’s important to know what wavelengths the transmitter is utilizing.