There’s a lot of emphasis in the government sector of the AV industry on using optical fiber due to its ability to prevent, or at least deter, security intrusions. Optical fiber also eliminates some other problems inherent in twisted-pair cable, such as near-end crosstalk (NEXT) and electromagnetic interference (EMI).
While fiber optic cable itself is cheaper than an equivalent length of copper cable, fiber optic cable connectors and the equipment needed to install them have typically been more expensive than their copper counterparts. With an increased emphasis on protecting digital information, however, optical fiber has become more cost-competitive over the last few years.
The ability of fiber optic cable to meet the evolving needs of government AV/IT end users is a good reason for integrators to know a little more about it. Here’s a look at the anatomy of a fiber optic cable.
Basic Construction of a Fiber Optic Cable
A fiber optic cable consists of five main components: core, cladding, coating, strengthening fibers, and cable jacket.
Core: This is the physical medium that transports optical signals from an attached light source to a receiving device. The core is a single continuous strand of high-purity glass or plastic whose diameter is measured in microns (less than the diameter of a human hair). The larger the core, the more light the cable can carry, which correlates to a higher data transfer rate.
Cladding: This is a thin layer that is extruded over the core and serves as the boundary that contains the light waves (more on this later), enabling data to travel through the length of the fiber.
Coating: This is a plastic coating over the cladding to reinforce the fiber core, help absorb shocks, and provide extra protection against excessive cable bends. It does not have any effect on the optical waveguide properties, though.
Strengthening fibers: These components help protect the core against crushing forces and excessive tension during installation. The materials can range from Kevlar®, to wire strands, to gel-filled sleeves. Sometimes light-absorbing (“dark”) glass is added between the fibers, to prevent light that leaks out of one fiber from entering another. This reduces cross-talk between the fibers.
Cable jacket: This is the outer layer, or sheathing, of the cable. Its purpose is to protect the cable from environmental hazards, such as construction work, fishing gear, and even sharks, which are often attracted to the electrical fields created by signal conductors to repeaters.
Multi-Mode vs. Single-Mode Fiber
The main difference between multi-mode and single-mode optical fiber is that multi-mode has a larger core diameter, typically 50–100 micrometers; which is much larger than the wavelength of the light carried within it. The larger core allows multiple light rays or modes (modalities) to be transmitted concurrently, each at a slightly different angle of reflection within the optical fiber core. In practical terms, the larger core size simplifies connections and also allows the use of lower-cost electronics such as light-emitting diodes (LEDs) and vertical-cavity surface-emitting lasers (VCSELs).
However, multi-mode fiber transmission is used for relatively short distances because the modes tend to disperse/distort over longer lengths, resulting in an unclear and incomplete data transmission. For longer distances, single-mode fiber (sometimes called monomode) is used because it is not limited by modal dispersion.
Due to its high capacity and reliability, multi-mode optical fiber is mostly used for communication over short distances, such as within a building or on a campus. Typical transmission speed and distance limits are 100 Mbit/s for distances up to 2 km, 1 Gbit/s up to 1000 m, and 10 Gbit/s up to 500 m.
Bandwidth can be further increased by using wavelength-division multiplexing (WDM), a technology which multiplexes (“muxes”) a number of optical carrier signals onto a single optical fiber by using different wavelengths (i.e. colors) of laser light. This technique enables bidirectional communications over one strand of fiber, as well as increasing capacity.
A WDM system uses a multiplexer at the transmitter to join the signals together, and a demultiplexer at the receiver to split them apart. With the right type of fiber it is possible to have a device that does both simultaneously. Modern WDM systems can handle up to 160 signals or more and can expand a 10 Gbit/s system over a single fiber pair to over 1.6 Tbit/s.
How Fiber Transmission Works
Light rays are modulated into digital pulses with a laser or LED and move along the core without penetrating the cladding. The light stays confined to the core because the cladding has a lower refractive index, which is the measure of a material’s ability to bend light.
This results in the phenomenon of total internal reflection (TIR), which happens when a propagating wave strikes a boundary between two mediums (in this case, the core and the cladding) at an angle larger than the fiber’s critical angle. If the refractive index is lower on the other side of the boundary (the cladding) and the incident angle is greater than the critical angle, the wave cannot pass through and is entirely reflected.
The critical angle θc is determined by Snell’s Law, which states that “the ratio of the sines of the angles of incidence and refraction is equivalent to the reciprocal of the ratio of the indices of refraction”.
Rearranging Snell’s Law, the angle of incidence can be calculated as:
To find the critical angle, we calculate the value for θi when θt = 90°, which means that sin θt = 1 (90° is the absolute maximum angle of transmission). Solving for θi, we arrive at the following equation:
For example, consider a ray of light moving from water to air. The refractive indices of water and air are approximately 1.333 and 1, respectively, so calculating for the critical angle we get:
Therefore, the angle of incidence must be greater than 48.6° in order for total internal reflection to occur in our example.
Signal attenuation in an optical fiber is measured in decibels (dB). Fiber optic cable specifications express loss as attenuation per 1 km length (dB/km). This value is multiplied by the total length of the optical fiber in kilometers to determine the fiber’s total loss in dB.
Light traveling in an optical fiber is not 100% efficient; there are several causes of signal attenuation. The loss of power also depends on the wavelength of the light and on the propagating material. For silica glass, the shorter wavelengths are attenuated the most. The lowest loss occurs at the 1550 nm wavelength, which is commonly used for long-distance transmissions.
Loss Inherent to Fiber: Light loss in a fiber that cannot be eliminated during the fabrication process is due to impurities in the glass and the absorption of light at the molecular level. Loss of light due to variations in optical density, composition, and molecular structure is called Rayleigh scattering. Rays of light encountering these variations and impurities are scattered in many directions and lost.
The absorption of light at the molecular level in fiber is mainly due to contaminants in glass such as water molecules. The ingress of water molecules into an optical fiber is one of the main factors contributing to the fiber’s increased attenuation as it ages. Silica glass’s (Si02) molecular resonance absorption also contributes to some light loss.
Loss Resulting from Fiber Fabrication: Inconsistencies in the fiber manufacturing process will result in the loss of light. For example, a 0.1% change in the core diameter can result in a 10 dB loss per kilometer. Precise tolerances must be maintained throughout the manufacturing of the fiber to minimize loss.
Splice Loss: Splice loss occurs at all splice locations. Mechanical splices usually have the highest loss, commonly ranging from 0.2 to over 1.0 dB, depending on the type of splice. Fusion splices have lower losses, usually less than 0.1 dB. A loss of 0.05 dB or less is usually achieved with good equipment and experienced personnel. High loss can be attributed to a number of factors, including:
- Poor cleave
- Misaligned fiber cores
- Air gap
- Index-of-refraction mismatch
- Core diameter mismatch
Connector Loss: Losses at fiber optic connectors commonly range from 0.25 to over 1.5 dB and depend greatly on the type of connector used. Other factors that contribute to the connection loss include:
- Dirt or contaminants on the connector (very common)
- Improper connector installation
- Damaged connector faces
- Poor cleave
- Misaligned fiber cores
- Index-of-refraction mismatch
Bend Loss: Bend loss occurs at fiber cable bends that are tighter than the cable’s minimum bend radius. Bend loss can also occur on a smaller scale from such factors as:
- Sharp curves of the fiber core
- Displacements of a few millimeters or less, caused by buffer or jacket imperfections
- Poor installation practice
This light power loss, called microbending, can add up to a significant amount over a long distance, as much as 2dB/km for a multi-mode fiber. For example, with this level of attenuation, if light travelled over 10km of cable (without amplification), only 10% of the signal would arrive at the receiving end.
Fresnel Reflection: Fresnel reflection occurs at any light boundary where the refractive index changes, causing a portion of the incident light ray to be reflected back into the first medium. For instance, if the end of a fiber has any kind of air gap, then some of the light traveling from the air to the core, about 4%, is reflected back into the air instead of transmitting/refracting into the core. The amount being reflected can be estimated using the following formula:
An index-matching material may be used in conjunction with mating connectors or with mechanical splices to reduce the reflected signal at the boundaries. The material is usually a liquid, cement (adhesive), or gel, which has an index of refraction that closely approximates that of the fiber’s core. Without the use of an index-matching material, Fresnel reflections will occur at the ends of a fiber unless there is no fiber-air interface or other significant mismatch in refractive index.
We’ll take a look at the anatomy of other signal conductors in future blog posts.