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There are two distinct parts of a fiber optic cable—the optical fiber that carries the signal and the protective covering that keeps the fiber safe from environmental and mechanical damage. This section deals specifically with the optical fiber.
An optical fiber has two concentric layers called core and cladding. The core (inner part) is the light carrying part. The surrounding cladding provides the difference in refractive index that allows total internal reflection of light through the core. The index of refraction of the cladding is less than 1% lower than that of the core. Typical values, for example, are a core index of 1.47 and cladding index of 1.46. Fiber manufacturers must carefully control this difference to obtain the desired fiber characteristics.
Fibers have an additional coating around the cladding. This coating, which is usually one or more layers of polymer, protects the core and cladding from shocks that might affect their optical or physical properties. The coating has no optical properties affecting the propagation of light within the fiber. This coating is just a shock absorber.
Figure 6.10-4 shows the light traveling through a fiber. Light injected into the fiber and striking the core-to-cladding interface at a critical angle reflects back into the core. Since the angles of incident and reflection are equal, the light will again be reflected. The light will continue as expected down the length of the fiber.
Light, however, striking the interface at less than the critical angle passes into the cladding, where it is lost over distance. The cladding is usually inefficient as a light carrier, and light in the cladding becomes attenuated fairly rapidly. The propagation of light is governed by the indices of the core and cladding and by Snell’s Law.
Such total internal reflection forms the basis of light propagation through a simple optical fiber. This analysis considers only meridional rays, the rays that pass through the fiber center axis each time they are reflected. Other rays, called skew rays, travel down the fiber without passing through the axis. The path of the skew ray is typically helical, wrapping around and around the center axis. To simply analyze, skewer rays are ignored in most fiber-optics analysis.
A cone known as the acceptance cone, shown in Figure 6.10-5, defines which light will be accepted and propagated by a total internal reflection. Light that enters the core from within this acceptance cone refracts down the fiber. Light outside the cone will not strike the core-to-cladding interface at the proper angle that allows total internal reflection. This light will not propagate.
The specific characteristics of light propagation through fiber depend on many factors. The factors include the size and composition of the fiber as well as the light source injected into the fiber. An understanding of the interplay between these properties will clarify many aspects of fiber optics.
Fiber itself has a very small diameter. Table 6.10-3 provides the core and cladding diameters of four commonly used fibers.
To realize how small these fibers are, note that human hair has a diameter of about 100 μ. Fiber sizes are usually expressed by first giving the core size, followed by the cladding size. Thus, 50/125 means a core diameter of 50 microns (μm) and a cladding diameter of 125 microns (μm).
Optical fibers are classified in two ways. One way is by the material makeup:
- Glass fiber: Glass fibers have a glass core and glass cladding. They are the most widely used type of fiber. The glass used in an optical fiber is an ultra pure and transparent silicon dioxide or fused quartz. If ocean water was as clear as fiber, one could see to the bottom of the Marianas Trench in the Pacific Ocean, a depth of 36,000 feet. Impurities are purposely added to the pure class to achieve the desired index of refraction. The elements germanium and phosphorus are added to increase the refractive index of the glass. Boron or fluorine is used to decrease the index. There are other impurities that are not removed when the class is purified. These additional impurities also affect the fiber properties by increasing attenuation from scattering or by the absorbing light.
- Plastic-clad silica (PCS): PCS fibers have a glass core and plastic cladding. The performance of PCS fiber is limited compared to a fiber made of all glass.
- Plastic: Plastic fibers have a plastic core and plastic cladding. Plastic fibers are limited by high optical loss and low bandwidth. The very low cost and ease of use make them attractive for applications where low bandwidth or high losses are acceptable. Plastic and PCS fibers do not have the buffer coating surrounding the cladding.
The second way to classify fibers is by the refractive index of the core and the modes that the fiber propagates. Fiber can be categorized into three general types; Figure 6.10-6 shows the three general fiber types and their basic characteristics.
Figure 6.10-6 shows the difference between the input pulse injected into a fiber and the output pulses exiting the fiber. The decrease in the height of the pulse shows the loss of optical signal power. The broadening of the pulse shows the bandwidth limiting effects of the fibers. It also shows the different paths of rays of light traveling down the fiber. And, it shows the relative index of refraction of the core and clad- ding for each type of fiber.
One of my favorite subjects is the transmission of video over fiber optic cable. I have had the pleasure of working on fiber optic implementations with Broadcasters to cover Presidential Elections and with Integrators on projects like the Las Vegas City Center.
Because of this passion, I am starting a multi-part series on the subject of Fiber Optic Video Transmission. My goal in writing this is to speak from my experience to make a topic that is scary to many, easy to understand and accessible so you can implement your own systems. I hope to do it in a humorous way relating my successes and challenges implementing many of these systems.
Anyone who knows me has also seen my passion for problem solving and doing the “impossible” and “never been done before”. I enjoy troubleshooting multi-million dollar fiber optic systems to discover a bad $20 patch cord or dirty fiber optic connector. The good news is that once a fiber optic system is up and running I know you will get many years of reliable operation.
In this series I will start with the basics and work my way up to the bleeding edge with 4K video fiber optic transmission. The series is perfect for the beginner and a good review for the expert. Clink these links to go to my first posts:
I’ve had a great time putting this series on Fiber Optic Video Transmission together for you and I hope you get great insight and some practical tips for your particular situation. From time to time I may also show you some interesting fiber optic products you can take a look at like these two below. If you have any questions about any of the content you can reply to this email or contact me at 949-777-5435 x 1001.
|FVT/FVR-5400-3G, VidOptic Series, 4 Channel 3G HDSDI Fiber Optic Transport Card with 4×4 Matrix for openGear||SilverBack 4K & HD SDI Fiber Optic Camera Back Camera Mount System|
Watch for my next installment in about 4 weeks. Please click to download additional white papers and presentations on wireless, webcasting, streaming and fiber optics. Thank you.
All the best,
President and CEO
Fiber optics is a method of carrying information using optical fibers. An optical fiber is a thin strand of glass or plastic that serves as the transmission medium over which information is sent. It thus fills the same basic function as a copper cable carrying a telephone conversation, computer data, or video. Unlike the copper cable, however, the optical fiber carries light instead of electrons. In so doing, it offers many distinct advantages that make it the transmission medium of choice for applications ranging from telephone calls, television, and machine control.
The basic fiber-optic system is a link connecting two electronic circuits. Figure 1 shows a simple fiber-optic link.
There are three basic parts to a fiber-optic system:
• Transmitter: The transmitter unit converts an electrical signal to an optical signal. The light source is typically a light-emitting diode, LED, or a laser diode. The light source performs the actual conversion from an electrical signal to an optical signal. The driving circuit for the light source changes the electrical signal into the driving current.
• Fiber-optic cable: The fiber-optic cable is the trans- mission medium for carrying the light. The cable includes the optical fibers in their protective jacket.
• Receiver: The receiver accepts the light or photons and converts them back into an electrical signal. In most cases, the resulting electrical signal is identical to the original signal fed into the transmitter. There are two basic sections of a receiver. First is the detector that converts the optical signal back into an electrical signal. The second section is the output circuit, which reshapes and rebuilds the original signal before passing it to the output.
Depending on the application, the transmitter and receiver circuitry can be very simple or quite complex. Other components that make up a fiber-optic trans- mission system, such as couplers, multiplexers, optical amplifiers, and optical switches, provide the means for building more complex links and communications networks. The transmitter, fiber, and receiver, how- ever, are the basic elements in every fiber-optic system.
Beyond the simple link, the fiber-optic medium is the fundamental building block for optical communications. Most electrical signals can be transported optically. Many optical components have been invented to permit signals to be processed optically without electrical conversion. Indeed, one goal of optical communications is to be able to operate entirely in the optical domain from system end to end.
FIGURE 1 Basic building blocks of a fiber-optic system.