The Optical Fiber: Optical Connectors
Optical connectors are the means by which fiber optic cable is usually connected to peripheral equipment and to other fibers. These connectors are similar to their electrical counterparts in function and outward appearance but are actually high precision devices. In operation, the connector centers the small fiber so that its light gathering core lies directly over and in line with the light source (or other fiber) to tolerances of a few ten thousandths of an inch. Since the core size of common 50 micron fiber is only 0.002 inches, the need for such extreme tolerances is obvious.
There are many different types of optical connectors in use today. The SMA connector, which was first developed before the invention of single-mode fiber, was the most popular type of connector until recently. Figure 8 shows an exploded view of the parts of this connector.
The most popular type of multimode connector in use today is the ST connector. Initially developed by AT&T for telecommunications purposes, this connector uses a twist lock type of design. A typical mated pair of ST connectors will exhibit less than 1 dB (20%) of loss and does not require alignment sleeves or other similar devices. The inclusion of an “anti-rotation tab” assures that every time the connectors are mated, the fibers always return to the same rotational position assuring constant, uniform performance.
ST connectors are available for both multimode and single-mode fibers, the primary difference being the overall tolerances. Note that multimode ST connectors will only perform properly with multimode fibers. More expensive single-mode ST connectors will perform properly with both single-mode and multimode fibers.
The installation procedure for the ST connector is not difficult and can be easily mastered by any system installer. Figure 9 shows some of the major features of the typical ST connector.
The Optical Fiber: Optical Splices
While optical connectors can be used to connect fiber optic cables together, there are other methods that result in much lower loss splices. Two of the most common and popular are the mechanical splice and the fusion splice. Both are capable of splice losses in the range of 0.15 dB (3%) to 0.1 dB (2%).
In a mechanical splice, the ends of two pieces of fiber are cleaned and stripped, then carefully butted together and aligned using a mechanical assembly. A gel is used at the point of contact to reduce light reflection and keep the splice loss at a minimum. The ends of the fiber are held together by friction or compression, and the splice assembly features a locking mechanism so that the fibers remained aligned.
A fusion splice, by contrast, involves actually melting (fusing) together the ends of two pieces of fiber. The result is a continuous fiber without a break. Fusion splices require special expensive splicing equipment but can be performed very quickly, so the cost becomes reasonable if done in quantity. As fusion splices are fragile, mechanical devices are usually employed to protect them.
Optical Receivers
The basic optical receiver converts the modulated light coming from the optical fiber back into a replica of the original signal applied to the transmitter.
The detector of this modulated light is usually a photodiode of either the PIN or the Avalanche type. This detector is mounted in a connector similar to the one used for the LED or LD. Photodiodes usually have a large sensitive detecting area that can be several hundred microns in diameter. This relaxes the need for special precautions in centering the fiber in the receiving connector and makes the “alignment” concern much less critical than it is in optical transmitters.
Since the amount of light that exits a fiber is quite small, optical receivers usually employ high gain internal amplifiers. Because of this, optical receivers can be easily overloaded. For this reason, it is important only to the size fiber specified for use with a given system. If, for example, a transmitter/receiver pair designed for use with single-mode fiber were used with multimode fiber, the large amount of light present at the output of the fiber (due to over-coupling at the light source) would overload the receiver and cause a severely distorted output signal. Similarly, if a transmitter/receiver pair designed for use with multimode fiber were used with single-mode fiber, not enough light would reach the receiver, resulting in either an excessively noisy output signal or no signal at all. The only time any sort of receiver “mismatching” might be considered is when there is so much excessive loss in the fiber that the extra 5 to 15 dB of light coupled into a multimode fiber by a single-mode light source is the only chance to achieve proper operation. However, this is an extreme case and is not normally recommended.
As in the case of transmitters, optical receivers are available in both analog and digital versions. Both types usually employ an analog preamplifier stage, followed by either an analog or digital output stage (depending on the type of receiver). Figure 10 is a functional diagram of a simple analog optical receiver.
The first stage is an operational amplifier connected as a current-to-voltage converter. This stage takes the tiny current from the photodiode and converts it into a voltage, usually in the millivolt range. The next stage is a simple operational voltage amplifier. Here the signal is raised to the desired output level.
Figure 11 is a functional diagram of a simple digital optical receiver. As in the case of the analog receiver, the first stage is a current-to-voltage converter. The output of this stage, however, is fed to a voltage comparator, which produces a clean, fast rise-time digital output signal. The trigger level adjustment, when it is present, is used to “touch up” the point on the analog signal where the comparator switches. This allows the symmetry of the recovered digital signal to be trimmed as accurately as desired.
Additional stages are often added to both analog and digital receivers to provide drivers for coaxial cables, protocol converters or a host of other functions in efforts to reproduce the original signal as accurately as possible.
It is important to note that while fiber optic cable is immune to all forms of interference, the electronic receiver is not. Because of this, normal precautions, such as shielding and grounding, should be taken when using fiber optic electronic components.
Designing A Fiber Optic System
When designing a fiber optic system, there are many factors that must be considered – all of which contribute to the final goal of ensuring that enough light reaches the receiver. Without the right amount of light, the entire system will not operate properly. Figure 12 identifies many of these factors and considerations.
The following step-by-step procedure should be followed when designing any system.
1. Determine the correct optical transmitter and receiver combination based upon the signal to be transmitted (Analog, Digital, Audio, Video, RS-232, RS-422, RS-485, etc.).
2. Determine the operating power available (AC, DC, etc.).
3. Determine the special modifications (if any) necessary (Impedances, Bandwidths, Special Connectors, Special Fiber Size, etc.).
4. Calculate the total optical loss (in dB) in the system by adding the cable loss, splice loss, and connector loss. These parameters should be available from the manufacturer of the electronics and fiber.
5. Compare the loss figure obtained with the allowable optical loss budget of the receiver. Be certain to add a safety margin factor of at least 3 dB to the entire system.
6. Check that the fiber bandwidth is adequate to pass the signal desired.
If, after performing the above calculations, it is discovered that the fiber bandwidth is inadequate for transmitting the required signal the necessary distance, it will be necessary either to select a different transmitter/receiver (wavelength) combination, or consider the use of a lower loss premium fiber.
Good Luck!
We hope this guide has helped you to better understand the basics of fiber optic technology and system design. The specification check sheet below and on the following pages can be used to help collect and organize the necessary information when actually designing a system.
Remember, if you ever have any questions about how to proceed, please contact Communications Specialties at
(631) 273-0404 for additional guidance
Bibliography
Marvin Freeman