Fiber-optic solutions

Today’s HD, 3-D and 3G environments require wide bandwidth connectivity, and using fiber is a good solution.
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For decades, we have been working with copper connections to our technology. Copper is great, and the available wire technology has evolved to allow us to connect at ever higher bandwidths and over remarkable distances.

The limitations of wire, distance and bandwidth are due to the analog nature of the medium. It is best to think of wire as a transmission medium, for that truly is its nature. We transmit into wire just as we transmit into the ether, launching signals using impedance matched transmission systems designed to transport the content efficiently and with minimum loss.

Wires that are larger sometimes offer better performance electrically, as in large-diameter speaker cables and RG-8 coax. The trade-off is weight, cost and size. Whether coax, paired cable or specially adapted twisted pair for data (CAT5/6 etc), we still rely on copper for much of our infrastructure, nearly 100 years after commercial broadcasting began.

The development of fiber optics

At the limit of the ability of wire to carry digital signals of high bandwidth, or over long distances, we find the natural desire to extend technology using other media for the physical transmission between devices. A long time ago, analog — and then digital — techniques were developed to use optical transmission media. The hugely higher frequency of light as compared to the electromagnetic spectrum used over wire allows modulation with much higher frequency content.

Over short distances, wire works well into the GHz range, but light allows transmission orders of magnitude higher in data rate. Fiber optics date back to John Logie Baird's experiments with transmission of images down light pipes in the 1920s. Early laparoscopic medical imaging followed in the 1930s, and in 1952, physicist Narinder Singh Kapany's work led to led to the invention of optical fiber. Jun-ichi Nishizawa, a scientist at Tohoku University, suggested the use of fiber for communications purposes. A Nobel Prize was not awarded in this field until 2009.

Optical techniques require glass or material with similar optical properties. Generally thought of as fragile and difficult to work with, over the last several decades both the medium (fiber-optic cables) and the connectors and systems used to install fiber optics have made possible simple and reliable systems with extremely high bandwidth and very long distances.

Using fiber

With our industry now routinely sending signals with 3Gb/s and higher data rates, fiber is becoming a critical part of the infrastructure of modern facilities. The physical principal is that of total internal reflection within the fiber due to the difference in the index of refraction between the fiber and the cladding that surrounds it.

Last month, I wrote about some aspects of a new central switching center that uses almost exclusively fiber-optic connections. The reasons were simple. Coax would limit the ability to send 3G video (SMPTE 424) over long distances within the plant. Fiber would extend the available data rate beyond 3Gb/s to perhaps double the standard developed for 1080p60 signals. That extends the life of the facility far into the future.

We are all comfortable with connectors on coax, but one of the points of nervous concern when planning a fiber facility is the use of connectors we have no experience with, in particular termination. This is frankly for good reason when the termination must be of the highest grade with extremely low loss. Often this leads to a decision to use pre-made fiber, or to hire specialize labor with the skills needed. In modest facilities, installing and maintaining fiber optics is well within the capability of skilled labor.

Part of our reluctance to embrace fiber is based in a lack of experience. Just as properly maintaining a high-bandwidth digital video facility on copper interconnections requires the right test equipment, we also need specialized fiber-optic test equipment to adequately install and maintain fiber plants. A well-equipped video facility these days will have a time domain reflectometer to allow troubleshooting coax and twisted pair cabling. Networks wired with copper require the right test instruments for their infrastructure.

With optical systems, we need equipment that can launch known power levels into the fiber, power meters to measure the transmission loss and optical TDRs to find return loss problems with connections, splices, and poorly installed trunks. At one time, these instruments were hugely expensive. While generally more costly than test instruments for copper systems, optical instrumentation has come down in cost to practical levels.

Advantages

One of the advantages of fiber is that it can be used as a bidirectional medium. Launching one wavelength in the forward direction and a second one in the reverse direction allows a single fiber to be used for more than one signal.

Similarly, using wave division multiplexing (WDM) can allow multiple signals on one fiber in one direction. Launching multiple signals requires an optical splitter, which can also allow monitoring test points to be created or passive splits of a signal to be created, allowing delivery to multiple locations without duplicating the electrical-to-optical hardware. You might split a signal into three outputs with 30 percent of the energy launched, reserving the last 10 percent for a test point to an optical patch panel where testing can be done without interruption of the signal.

Types of fiber

Fiber itself comes in a myriad of flavors, of course. For field use, it can be strengthened with kevlar threads and protected by thick rubber jackets into what is called “tactical fiber,” so named for the military use of fiber optic on the battlefield.

SMPTE standardized both fiber cable and fiber connectors for cameras many years ago, and now a substantial portion of HD cameras installed use SMPTE fiber. SMPTE fiber cable has two optical fibers, and twisted pairs suitable to carry power and intercom that is active before power is applied to the camera head. (This is useful when you want to know if there is a camera on the other end!) Mechanically, the connectors can be built with shutters that protect the inside of the connector from dirt and moisture when they are not plugged into a socket (not mandated in the standard).

Fiber camera cable can be used to several kilometers, where triax would require repeaters at much shorter distances. Fiber is generally lighter and need not be any more expensive in bulk, though connections often cost more than copper connections — particularly in terms of labor needed for termination.

For use inside a facility, often it is more practical to use a single jacket that carries many fibers. I have a sample on my desk of a rigid fiber system that carries multiple ribbons of fibers all inside a protective “jacket” not unlike PVC pipe. The total number of fibers in that bundle is more than 800! More practical in facilities might be a jacket with a couple dozen fibers. Twenty such cables could connect the entire backplane of a reasonably large routing system, as I described last month. Elaborate fiber management systems have been developed to allow the individual (tiny) strands to be mechanically protected where splices and other connections are made.

Like copper systems, fiber-optic installations need to be maintained over time. But unlike copper, fiber generally ages much more slowly and exhibits very low error rates for many years longer than a copper installation. Suppliers of fiber systems often will organize training on fiber installation and maintenance.

Conclusion

Over time, we will no doubt see a shift towards fiber as a dominant transmission medium. All of us would do well to spend time learning the details of the technology and looking for places where it solves problems in modern television plant design.

John Luff is a broadcast technology consultant.

Send questions and comments to: john.luff@penton.com