Fiber Optic Transmission in AV Comes of Age

By Steve Somers, Vice President of Engineering

Folks, the good ol' days are here right now. More technical solutions are available for audiovisual systems designers in both analog and digital equipment than ever before. One of the most exciting new toys for signal distribution is RGBHV digital fiber optic transmission. Most currently installed fiber transmission systems are analog-based. Our debut into digital fiber transmission is exciting for us because it ushers in an era of higher performance and new features at competitive, or better, prices than existing analog implementations.

VCSEL Cross-Sectional Architecture
Example: VCSEL Cross-Sectional Architecture
Note how the VCSEL emits light from its surface rather than its edge, compared to prior laser diode designs. Such an architecture facilitates on-wafer testing.

Until recently, fiber transmission supporting wideband graphics could only be cost effectively accomplished using analog techniques. Of course, the phrase "cost effectively" is relative. Within what most professionals call a normal AV installation environment, the fiber cable itself is cost competitive; but, historically fiber systems overall were largely cost prohibitive except for those applications where its security and long haul distance were really needed.

It may surprise you that fiber distribution is not entirely new to us. Most people are probably not aware that we used fiber transmission systems to augment "long haul" RGBHV, HD component, and composite video connections in the InfoComm Projection Shoot-Out® event. A small group of fiber equipment vendors supported our needs adequately, but there were challenges associated with the consistent performance of those analog-based systems.

Much of the knowledge base you may have learned for analog-based fiber systems is still applicable. When deploying fiber optic transmission, there are many important component choices to be made. Fiber cable, like coaxial cable, is high on the list of system design concerns; as well as the connectors, transmitters, and receivers. Digital fiber transmission requires us to look at things somewhat differently, but the fundamentals track well with previous discussions on the distribution of digital signals and protocols, such as HD-SDI for example.

In this article I would like to address key concepts associated with fiber cabling in order to set the tone for what is likely to be an ongoing discussion on digital fiber optic transmission.

New Fiber Diet

Certainly, fiber is becoming part of our new product diet and, with the excitement of higher bandwidth performance, new features, and lower cost implementation; it will likely become part of your AV diet too. So, let's start today with fiber cable. Connector and hardware discussions are forthcoming, but whether you're using analog or digital interface components, there's much to understand about fiber cabling and how it impacts fiber optic transmission performance.

Singlemode versus Multimode

At first glance, there would seem to be only two choices for fiber: singlemode or multimode. Prior art paired singlemode fiber with the laser and multimode fiber with the LED. While this distinction is generally appropriate, new fiber cable recipes provide a number of new choices and cable-to-source combinations. A key development driving fiber cable technologies in new directions is the low-cost laser diode. Called VCSEL - sounds like "vek-sel" - for short, these new Vertical Cavity Surface Emission Lasers are quickly replacing the LED in new network installations. VCSELs of 850 nm and 1310 nm are employed with the longer wavelength providing the best bandwidth performance, but at about three times the price of the 850 nm version. Even for modest budget projects, 850 nm VCSELs are rapidly displacing LEDs and providing gigabitplus level performance.

Regardless of the light source, fiber construction is based on the encasement of an ultra-pure glass "conductor" of some diameter, called the "core", within a regular glass cladding layer that adjusts the final conductor diameter to 125 micrometers (µm). Additional outer material layers and jacketing serve to protect the fiber while increasing its ruggedness for handling and installation.

The term "mode" refers to the apparent number of light waves traveling the fiber core's length. The most intense light energy traveling the centerline of the fiber is the primary mode. Light may enter the fiber at an angle and bounce off the fiber walls as it propagates. Light waves randomly bouncing off fiber walls arrive at the receiver detector at slightly different time intervals. Each reflection of the light wave within the fiber is called a mode.

The core of a singlemode cable is very small in diameter; that is about 8 - 9 µm… not a great deal larger than the wavelength of the light passing through it. Therefore, most of the light energy propagates as a single wave creating no significant reflections, or modes, that degrade performance at the receiver detector. For this reason, singlemode fiber is capable of the widest bandwidth and longest transmission distance. Refer to the lower illustration in Figure 1.

Figure 1 - Optical fiber cores compared
Figure 1 - Optical fiber cores compared

The multimode fiber core is characteristically several times larger in diameter than singlemode core while the cladding and packaging compensate to perpetuate a competitive overall cable diameter. Typically available in 50 µm or 62.5 µm core diameters, multimode alignment with the transmitter is more forgiving; however light reflections create modes all along its length. Refer to the upper illustrations in Figure 1. The cumulative effect of multiple modes is called "dispersion". The receiver detector interprets dispersion as signal jitter. Just as with other serial digital signal formats, the eye pattern is used to analyze transmission quality. Refer to Figure 2a. The presence of dispersion decreases the width of the eye pattern. In addition, the length of the fiber along with all connector interfaces and cable bends affects final signal amplitude seen by the receiver detector. See Figure 2b. These cumulative effects attenuate the signal, thus accounting for a reduction in eye amplitude. Amplitude and dispersion, in combination, are the two key parameters limiting frequency bandwidth and transmission distance.

Figure 2a - Normal digital signal eye pattern
Figure 2a - Normal digital signal eye pattern

Regardless of whether the fiber is singlemode or multimode, it is clad in a glass shell designed to contain a high percentage of the light wave within the fiber structure. The total glass diameter for all current commercial fiber cable is 125 µm and is the second numerical identifier for the cable; e.g. 8/125, 50/125, or 62.5/125. The cladding has a different index of refraction than the fiber core. It reflects off-axis light back into the fiber. While the fiber conductor has a uniform index of refraction, the cladding typically uses a lower index of refraction. The sudden change in index of refraction is called a "step index" fiber. See Figure 3.

Most all singlemode and some multimode fiber cables use step-index construction. Since light waves pass through a singlemode fiber with minimal bounce along the cladding interface, there is generally insignificant reflective loss associated with step-index singlemode fiber. Multimode cable is a different matter. Step-index multimode cables exhibit higher dispersion due to the multiple modes created via reflections from the sudden refractive change between the fiber core and its cladding. Like looking at repeating images within mirrors of a funhouse, multiple reflections degrade system performance; that is, transmission distance.

Figure 2b - Effects of dispersion and attenuation
Figure 2b - Effects of dispersion and attenuation

To improve performance of multimode fiber cable, the core may be encased within a cladding constructed using five to six layers of deposited glass having increasingly lower indices of refraction. Near the core, the cladding's refractive index is highest. As light passes through each layer of the cladding, the refractive index becomes lower. This changing refractive index "steers" the light into a very shallow approach angle at the cladding outer boundary. The shallow angle results in the light glancing off the outer boundary and traveling back toward the core. The repeat path through the increasing refractive indices directs the light back into the main path within the core. The light is essentially bent into an arc that reunites with the main wave, thus minimizing the number of modes seen by the receiver detector. The result is lower dispersion and greater effective bandwidth. This style multimode fiber construction is called "graded index fiber". See Figure 4.

Key Fiber Parameters

Fiber transmission is length and data rate dependent. The characterization of modes and dispersion in multimode fiber transmission is summarized by the modal bandwidth parameter, expressed as frequency in MHz times the distance in kilometers, or MHz*km. A larger MHz*km product specification means better performance capability. Much like coaxial cable, where we look for a specific attenuation in dB at a reference frequency - say 100 MHz - for a specific length - usually 100 feet or 100 meters, the modal bandwidth parameter used with multimode and singlemode fiber provides a quick snapshot of performance expectations less the effects of other intrusions such as connectors, splits, and splices.

For example, a fiber rated at 500 MHz*km is capable of conveying a data rate of 500 MHz for one kilometer. As the frequency or data rate increases, the transmission distance decreases. At 1 GHz, the transmission distance is one half kilometer, etc.

What about attenuation? We're used to that concept in copper cables. How does it apply in fiber cable? Table 1 lists both internal and external parameters affecting fiber attenuation. Popular light wavelengths are 850 nm and 1310 nm - both in the infrared - with the longer wavelength yielding less attenuation. Attenuation due to cable loss, connectivity, and so forth is measured in dB just as with copper cables. In typical AV applications, fiber attenuation is likely to be a lesser factor of concern than system bandwidth. Unless your application calls for multi-kilometer transmission distances, fiber cable attenuation will be fairly insignificant within the whole installation picture compared to transmitter and receiver selection, as well as the quality of cable installation and termination.

Table 1
Table 1

Installation practices for fiber cable must be closely adhered to for optimum performance. Like UTP cable, fiber cable manufacturers clearly specify handling and installation methods and practices. Inadvertent pinching of fiber may lead to micro-bending and cracking. Remember the fiber is glass! Cable bend radius directly impacts attenuation. The TIA 568 standard allows for 30 mm (1.18 in) bend radius minimum for two and four fiber multimode cables.

The numerical aperture, NA, of a fiber cable is its maximum angle of acceptance for incoming light. This angle varies with the indices of refraction for the fiber core and its cladding. It would appear that having a large NA fiber is desirable; however, too high a numerical aperture allows for high order modes or more drastic reflections from the core into the cladding. Too high an NA results in light energy passing through the cladding; in other words, increased dispersion, energy loss, and lower bandwidth.

Fibers with lower NA have increased bandwidth. Smaller NA requires more precise source-to-fiber alignment and precise termination. Historically, use of LED transmitters required the larger NA of the 62.5 µm fiber. The power output and design of newer LEDs and VCSELs increase bandwidth performance using the smaller NA of the 50 µm fiber.

Mismatched Fibers

What about mixed fiber installations? Let's say you've encountered older 62.5 µm fiber in an application where the new portion of the install calls for 50 µm fiber. What is the impact of splicing two differing fibers? Would it work?

Mixed singlemode to multimode connections are not recommended. However, the use of 50 µm multimode fiber is increasing and displacing 62.5 µm cables in the U.S. While 50 µm is the standard in Europe, it is now recommended worldwide for all new installations. The 50 µm fiber typically has the higher bandwidth - perhaps 3X that of 62.5 multimode - and is now deployed using VCSEL sources for Gigabit Ethernet and 10 GE networks.

Figure 3 - Step-index fiber light propagation characteristics
Figure 3 - Step-index fiber light propagation characteristics

Back to the question of mixed fiber installations: if a laser source is used with multimode cable, there is minimal insertion loss impact regardless of transmission direction; i.e. from larger core to smaller, and vice versa. In LED-based systems, mismatch transmission from a larger to smaller core has a larger insertion loss impact since the LED light wave uses all the core cross-section. Transmissions from 50 µm cable into 62.5 µm cable is less troublesome than the opposite. 50 µm cable is considered compatible with 62.5 µm systems. Pay attention to your insertion loss budget and consult the cable manufacturer for best results.

Figure 4 - Graded-index fiber light propagation
Figure 4 - Graded-index fiber light propagation

Why the migration away from 62.5 µm in favor of 50 µm? The larger diameter core became popular early-on to support first generation Ethernet due to its larger numerical aperture and compatibility with the large spot size of LEDs. In addition, 62.5 µm cable provides a higher tolerance for connectorization and alignment. Adoption by IBM and AT&T led to wide use in an era when termination systems and LEDs were less advanced. Whereas today, characteristics of both are improved and standards bodies have evolved to 50 µm multimode cable for most all network applications save long haul high-speed backbones.

Fiber Termination

Well, this has been just a little splice of fiber heaven. Today's singlemode and multimode fiber cable menus list more flavors than Ben and Jerry's®. Remember that fiber cable bandwidth is characterized in large part by the parameter MHz*km and that many grades of fiber are available depending on your data rate and transmission distance requirements. Fiber cable price is proportional to the MHz*km rating.

We, as AV system and equipment designers, all owe a debt of thanks to technical advances in fiber optic transmission components derived from the mother of inventive necessity: increased Ethernet data network speeds. While attempting to quell our hunger for higher speed networks, blazing speed serial digital chips and solid-state lasers are lowering the entry requirements to a new serial digital RGB graphics paradigm. The current network system installation trend is toward 850 nm VCSELs operating with 50 µm fiber for the cost conscious and 1310 nm VCSELs with 50 µm multimode or singlemode fiber for those needing the extra distance.

Our position on considering cable first in a project is no exception where fiber is concerned. The vast choices in fiber will help or hinder the performance of the best optical hardware. In upcoming installments, I'll address connectivity, transmitters, and receivers. As always, each piece of the system contributes significantly to the overall system bandwidth and ultimately, performance.