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Fiber-optic transport

Oct 1, 2008 12:00 PM, By Jim Jachetta

As the demand for the transport of high-resolution video at higher bandwidths increases, the use of fiber in facilities will be more and more prevalent.

Figure 2. Self-healing ring topology for real-time uncompressed fiber transport
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The equipment complexity and size is also reduced at the destination point from 3RU to 4RU down to 1RU to 2RU. Signal quality is maintained by digitizing once as opposed to digitizing twice. The video, audio and data signals are digitized once at the fiber head and then are decoded back to an analog signal at the fiber receiver hub.

The technology is ideal for a head to hub or star configuration. Another configuration is a self-healing ring. The system can be designed using a combination of the head to hub and self-healing ring topologies. (See Figure 2.)

Optical repeaters and distribution amplifiers

There are applications in fiber-optic communications where a signal requires regeneration and replication. The function required is similar to that of a distribution amplifier or digital signal reclocker. A passive splitter can be used to split an optical signal, but each signal is significantly weaker after the split. A device called an optical repeater or distribution amplifier can be used to repeat or regenerate a weak optical signal. This is helpful on long fiber-optic runs where a fiber signal is reaching its limit, because the repeater can be used to regenerate the signal for further distribution.

The same device can be used to replicate an optical signal. One optical signal can be replicated up to 16 times with one device. Unlike the passive split where the optical output is diminished, the output optical signals are regenerated to full optical power.

The device can also be used as a mode converter or wavelength re-mapper. The device can be configured with a single-mode input and multimode outputs. This gives the ability to convert from multimode to single-mode or from one wavelength to another wavelength. The device can convert an optical signal to CWDM wavelengths.

Fiber-optic routing switchers

Most broadcast and audiovisual systems today have a video and audio routing switcher. The switcher gives the user the ability to control the source and destination of a given video and audio signal. As more and more video and communications migrate from copper to fiber, there will be a need for an optical routing switcher. The optical routing switcher is a new concept for the video market, but it has been used for many years in the telecommunications industry to route and control telephone traffic.

Optical switching starts to make more sense as the complexity increases with dozens of different video and encoding formats. If all or most of our video information is in the optical domain, why not switch in the optical domain?

In broadcast or video applications, there may be analog video, component video, SDI and HD-SDI. To switch all these signals, we would need a different switcher for each type or format of video. If we transport signals in the optical domain, we will have to convert back to electrical to switch, and then back to fiber after the output. If we switch optically, only one switch is required because an optical switch can switch virtually any format signal in the optical domain. There are two basic types of optical switching: photonic fiber-optic and electro-optical.

Photonic fiber-optic switching is 100 percent optical switching using 3-D MEMMS technology, which uses electronically controlled mirrors to route optical signals. This type of switch has an optical input, an optical crosspoint and an optical output. The abbreviation for this technology is OOO. An OOO switch provides only point-to-point switching. One input cannot be multicast to many outputs, because the mirrors cannot point to more than one output at a time. The use of mirrors does permit multiple wavelengths in both directions.

Switches are available in sizes from 8 × 8 to 256 × 256. Pure optical switching is available for multimode and single-mode applications and supports both analog and digital optical signals. Photonics switching has virtually infinite bandwidth.

An electro-optical switch uses a hybrid approach. The input is optical, the crosspoint is electrical, and the output is optical. The abbreviation for this technology is OEO. An OEO switch supports point to multipoint or multicast switching. Any input can be switched to every output, if necessary. Because the optical signal is converted to electrical, only one wavelength can be switched at a time. Also, an electrical crosspoint only operates in one direction; therefore, only one wavelength in one direction is supported.

Electro-optical switchers can be configured for 4.25Gb/s and 10Gb/s bandwidth, which will future-proof system designs to 10Gb/s. Broadcasters that purchased an optical switch a few years ago for SDI or HD-SDI can easily upgrade to 3Gb/s HD-SDI today.

An optical switch supports a wide range of formats, from 19.4Mb/s ATSC through 3Gb/s HDTV, as well as NTSC, PAL, SECAM, SMPTE 259M Serial Digital (SDI) Video and many more. Optical switcher technology can be used in the field to support applications requiring reliable, high-quality video distribution, such as mobile production trucks, sports venues and professional video facilities. Optical layer protection and fault-tolerant switching can be configured for mission-critical, nonstop applications.

Optical switching is extremely cost-effective for any application requiring 32 or more switched optical ports. It eliminates the need for expensive video transceivers to convert signals between electrical and optical formats. Switching the signals in optical format can save thousands of dollars per port in fiber-optic transport equipment costs.

The future

Systems are currently in development for the transport of high-resolution video at bit rates exceeding 40Gb/s. Digital cinema and the proliferation of HD television will demand fiber-optic transport systems with high-bandwidth capabilities.

Fiber transport to the home for video, telephone and Internet traffic is slowly becoming a reality in many North American communities. This will fuel the demand for high-speed content delivery and distribution throughout the globe.

Jim Jachetta is senior vice president of engineering and product development at MultiDyne Video and Fiber Optic Systems.