Every time the interface demands more bandwidth, the first place people worry about is the transmission line — it is the most volatile area for every high-speed digital transmission. In the early days of digital video the bandwidth was a mere 4.95Gbps (1.65Gbps x 3 channels); it’s since moved to 10.2Gbps and now 18Gbps.
Manufacturers are always wrestling between cable length, gauge and material, a proverbial balancing act to determine performance and market placement. Due to even higher bandwidths, active devices add to the mix and must play a bigger role to reduce wire gauge and increase wire lengths.
Fiber-optic cable, another active cable product, is slowly becoming a bigger player for long-distance applications. For many professionals this is a new science that must be learned and understood for system environments that require going past 10 to 15 meters where current copper active cables fall off. This is a huge subject so here’s a brief introduction to fiber.
Instead of moving electrons down copper as we do with wire we instead move light energy (photons) down glass or plastic material. Optical converters are positioned on both the front and back end of each cable. They can be LED or VCSEL (Vertical Cavity Surface Emitting Laser).
Typically made of glass or Plastic Clad Silica (PCS), different fiber techniques were developed for different applications. Figure 1 illustrates that the construction of a fiber cable can be very similar to shielded cable; however, the functions are very different. The jacket is used for relieving stress cushioning and protecting the clad mate-rial, which effectively is a tube with a mirror finish on its inside in place of the braided shield. The fiber itself carries the light through a core of total reflection allowing the light to carry over very long distances.
There are three popular techniques for glass fiber for data transmission. They all have their good and bad points.
The Single-Mode Fiber has the least amount of loss made of thin glass material carrying one single light ray with no reflections. This is by far the most expensive design but can carry as much as 50Gbps to 100Gbps over 1 kilometer. The input pulse size compared to its output performance is pretty impressive.
Multi Mode Step Index uses the power of reflection to ricochet off the clad at an angle greater than what’s known as its “critical angle.” Too little of an angle and the light will become scattered within the clad creating a loss of light energy, limiting good performance. This has the most attenuation, but is also the lowest in cost.
Then there is Multi Mode Graded Index fiber, which has some distinct advantages. It will usually have a large center core where the light’s reflection angle is gradual, not sharp, reducing discontinuities of refraction between the core and the cladding. It has less attenuation than Multi Mode Step Index, with a slight increased cost.
Like wire, the construction of the fiber cable has a direct influence on the performance. Typical fiber types are:
- OM1 — 850nm/200MHz and 1,300nm/500MHz per kilometer
- OM2 — 500MHz per kilometer
- OM3 — 2,000Mhz (2Gbps) per kilometer (specifically for VCSEL)
- OM4 — 10-40Gbps per kilometer (specifically for VCSEL)
The length and performance of these products rests on a much higher expectation than did their copper-driven cousins. Still the ability to support bandwidth demands beyond 6Gbps has been a challenge, usually found with two construction styles, all glass fiber and Hybrid AOC (Active Optical Cable). Here copper is still used for low speed data and power.
Last year DPL tested several types of products designed to support 18Gbps; none passed DPL’s tough minimums. [UPDATE: DPL has now certified Tributaries Aurora fiber optic HDMI cable for 18 Gbps.] Were expectations too high or were these products just made poorly? Recently that was answered with a series of fiber products, including a new Hybrid AOC fiber product (with 20mm bending radius, 4-channel VCSEL array, 28-AWG copper) that had outstanding performance up to and including 30 meters.