Category Archives: Introduction to Fiber Optics

User’s Guide to Fiber Optic Video Transmission – Optical Windows and Spectrum – Part 3

Optical Windows and Spectrum

Wavelength remains a significant factor in fiber-optic developments. Figure 3 illustrates the wavelength “windows.” Table 1 shows the wavelength of each optical window and the typical application for multimode (MM) or single-mode (SM) operation.

FIGURE 3 Fiber attenuation versus light wave- length characteristics.

The earliest fiber-optic systems were developed at an operating wavelength of about 850 nm. This wavelength corresponded to the so called “first window” in a silica-based optical fiber, as shown in Figure 3. This window refers to the wavelength region that will offer a low optical loss that sits between several large absorption peaks. The absorption peaks are caused primarily by moisture in the fiber and Rayleigh scat- tering, which is the scattering of light due to random variations in the index of refraction caused by irregu- larities in the structure of the glass.

The attraction to the 850 nm region came from its ability to use low-cost infrared LEDs and low-cost sili- con detectors. As technology progressed, the first win- dow lost its appeal due to its relatively high 3 dB/km losses. Most companies began to exploit the “second window” at 1310 nm with a lower attenuation of about 0.5 dB/km. In late 1977, Nippon Telegraph and Telephone developed the “third window” at 1550 nm. The third window offers an optical loss of about 0.2 dB/km.

TABLE 1 De Facto Standard Light Wavelengths

The three optical windows—850 nm, 1310 nm, and 1550 nm—are used in many fiber-optic installations today. The visible wavelength near 660 nm is used in low-end, short-distance systems. Each wavelength has its advantages. Longer wavelengths offer higher performance, but always come with higher cost.

Table 2 provides the typical optic attenuation for each of the common wavelengths versus the fiber- optic cable diameter. A narrower core fiber has less optical attenuation.

The International Telecommunication Union (ITU), an international organization that promotes world- wide telecommunications standards, has specified six transmission bands for fiber-optic transmission. The first is the O band (“original band”), which is from 1260–1310 nm. The second band is the E band (“extended band”), which is 1360–1460 nm. The third band is the S band (“short band”), which is 1460–1530nm. The fourth band in the spectrum is the C band (“conventional band”), which is 1530–1565 nm. The fifth band is the L band (“longer band”), which is 1560–1625 nm. The sixth band is the U band (“ultra band”), which is 1625–1675 nm. There is a seventh band that has not been defined by the ITU that is in the 850 nm region. It is mostly used in private networks. The seventh band is widely used in high-speed computer networking, video distribution, and corporate applications.

Researchers have attempted to develop new fiber optics that could reduce costs or improve performance. Some alternative fiber materials have found specialized usage. Plastic fiber is ideal for short transmission distances that are ideal for home theater installations. Lower cost glass fiber reduces the need to develop longer distance plastic fiber and the higher cost of copper wire has expanded glass fiber-optic cable applications.

TABLE 2 Typical Optical Fiber Loss

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User’s Guide to Fiber Optic Video Transmission – Snell’s Law– Part 2

Snell’s Law

Early fiber optics exhibited high loss that limited transmission distances. To correct this, glass fibers were developed that included a separate glass coating. The innermost region of the fiber, the core, carried the light, while the glass coating or cladding prevented the light from leaking out of the core by refracting the light back into the inner boundaries of the core. Snell’s Law explained this concept. It states that the angle at which a light reflects as it passes from one material to another depends on the refractive indices of the two materials.

FIGURE 2 Light wave refraction principles. The refraction index of the core, n1, is always less than that of the cladding, n2. Light incident on the boundary at less than the critical angle, ϕ1, propagates through the boundary, but is refracted away from the normal to the boundary (a) at the critical angle, ϕC, along the boundary (b). Light incident on the boundary at angles ϕ1 above the critical angle is totally internally reflected (c). (Adapted from Force, Inc., illustration used with permission.)

In the case of fiber optics, this is the refractive index between the core and the cladding. Figure 2 illustrates the equations for Snell’s Law. In this figure, the upper region of the frame, n1, indicates a higher refractive index than the lower region n2. The refractive index of the upper region is designated as n1 while the lower region refractive index is n2. The figure on the top shows the case with the angle of the indices less than the critical angle. Note that the angle of the light changes at the interface between the higher refractive index, in region 1, and the lower refractive index, in region 2. In the center figure, the angle of indices has increased to the critical angle. At this point all the refracted light rays travel parallel to the interface region. In the figure on the bottom, the angle of indices has increased to a value greater than the critical angle. In this case 100% of the light refracts at the interface region.

Advancements in laser technology next elevated the fiber-optics industry. Only the light-emitting diode or its higher powered counterpart, the laser diode, had the potential to generate large amounts of light in a focused beam small enough to be useful for fiber optic transport.

Communications engineers quickly noticed the importance of lasers and their higher modulation frequency capabilities. Light has the capacity to carry 10,000 times more information than radio frequencies. Because environmental conditions, such as rain, snow, and fog, disrupt laser light, a transmission scheme other than free space was needed. In 1966, Charles
Kao and Charles Hockham, working at the standard Telecommunications Laboratory, presented optical fibers as an ideal transmission medium, assuming fiber optic attenuation could be kept under 20 dB per kilometer. Optical fibers of the day exhibited losses of 1,000 dB/km or more. At a loss of 20 dB/km, 99% of the light would be lost over only 1000 meters (3300 ft).

Scientists theorized that the high levels of loss were due to impurities in the glass and not the glass itself. At the time in 1970, an optical loss of 20 dB/km was within the capabilities of electronics and opto- electronic components for short distances (less than 1 km) but not for longer distances (greater than 1 km). Dr. Robert Maurer, Donald Keck, and Peter Schultz of Corning succeeded in developing a glass fiber that exhibited attenuation at less than 20 dB/km, the limit for making fiber optics a usable technology. Other advances of the day, such as semiconductor chips, optical detectors, and optical connectors, initiated the true beginnings of the fiber-optic communications industry.

Click for Part 3 on Optical Windows and Spectrum

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User’s Guide to Fiber Optic Video Transmission – Introduction to Fiber Optics – Part 1

FiberOptic Medium

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.

Click for Part 2 on Snell’s Law

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