fiber optics introduction 2

dogear · 发表于 2004-7-13 04:12:00

Advantages of Fiber Optic Systems

Fiber optic transmission systems – a fiber optic transmitter and receiver, connected by fiber optic cable – offer a wide range of benefits not offered by traditional copper wire or coaxial cable. These include:

1. The ability to carry much more information and deliver it with greater fidelity than either copper wire or coaxial cable.

2. Fiber optic cable can support much higher data rates, and at greater distances, than coaxial cable, making it ideal for transmission of serial digital data.

3. The fiber is totally immune to virtually all kinds of interference, including lightning, and will not conduct electricity. It can therefore come in direct contact with high voltage electrical equipment and power lines. It will also not create ground loops of any kind.

4. As the basic fiber is made of glass, it will not corrode and is unaffected by most chemicals. It can be buried directly in most kinds of soil or exposed to most corrosive atmospheres in chemical plants without significant concern.

5. Since the only carrier in the fiber is light, there is no possibility of a spark from a broken fiber. Even in the most explosive of atmospheres, there is no fire hazard, and no danger of electrical shock to personnel repairing broken fibers.

6. Fiber optic cables are virtually unaffected by outdoor atmospheric conditions, allowing them to be lashed directly to telephone poles or existing electrical cables without concern for extraneous signal pickup.

7. A fiber optic cable, even one that contains many fibers, is usually much smaller and lighter in weight than a wire or coaxial cable with similar information carrying capacity. It is easier to handle and install, and uses less duct space. (It can frequently be installed without ducts.)

8. Fiber optic cable is ideal for secure communications systems because it is very difficult to tap but very easy to monitor. In addition, there is absolutely no electrical radiation from a fiber.

How are fiber optic cables able to provide all of these advantages? This guide will provide an overview of fiber optic technology – with sections devoted to each of the three system components – transmitters, receivers, and the fiber cable itself. An appreciation of the underlying technology will provide a useful framework for understanding the reasons behind its many benefits.

Optical Transmitters

The basic optical transmitter converts electrical input signals into modulated light for transmission over an optical fiber. Depending on the nature of this signal, the resulting modulated light may be turned on and off or may be linearly varied in intensity between two predetermined levels. Figure 2 shows a graphic representation of these two basic schemes.

The most common devices used as the light source in optical transmitters are the light emitting diode (LED) and the laser diode (LD). In a fiber optic system, these devices are mounted in a package that enables an optical fiber to be placed in very close proximity to the light emitting region in order to couple as much light as possible into the fiber. In some cases, the emitter is even fitted with a tiny spherical lens to collect and focus "every last drop" of light onto the fiber and in other cases, a fiber is "pigtailed" directly onto the actual surface of the emitter.

LEDs have relatively large emitting areas and as a result are not as good light sources as LDs. However, they are widely used for short to moderate transmission distances because they are much more economical, quite linear in terms of light output versus electrical current input and stable in terms of light output versus ambient operating temperature. LDs, on the other hand, have very small light emitting surfaces and can couple many times more power to the fiber than LEDs. LDs are also linear in terms of light output versus electrical current input, but unlike LEDs, they are not stable over wide operating temperature ranges and require more elaborate circuitry to achieve acceptable stability. In addition, their added cost makes them primarily useful for applications that require the transmission of signals over long distances.

LEDs and LDs operate in the infrared portion of the electromagnetic spectrum so that their light output is usually invisible to the human eye. Their operating wavelengths are chosen to be compatible with the lowest transmission loss wavelengths of glass fibers and highest sensitivity ranges of photodiodes. The most common wavelengths in use today are 850 nanometers, 1300 nanometers, and 1550 nanometers. Both LEDs and LDs are available in all three wavelengths.

LEDs and LDs, as previously stated, are modulated in one of two ways; on and off, or linearly. Figure 3 shows simplified circuitry to achieve either method with an LED or LD. As can be seen from

Figure 3A, a transistor is used to switch the LED or LD on and off in step with an input digital signal. This signal can be converted from almost any digital format by the appropriate circuitry, into the correct base drive for the transistor

Overall speed is then determined by the circuitry and the inherent speed of the LED or LD. Used in this manner, speeds of several hundred megahertz are readily achieved for LEDs and thousands of megahertz for LDs. Temperature stabilization circuitry for the LD has been omitted from this example for simplicity. LEDs do not normally require any temperature stabilization.

Linear modulation of an LED or LD is accomplished by the operational amplifier circuit of figure 3B. The inverting input is used to supply the modulating drive to the LED or LD while the non-inverting input supplies a DC bias reference. Once again temperature stabilization circuitry for the LD has been omitted from this example for simplicity.

Digital on/off modulation of an LED or LD can take a number of forms. The simplest, as we have already seen, is light-on for a logic "1", and light-off for a logic "0". Two other common forms are pulse width modulation and pulse rate modulation. In the former, a constant stream of pulses is produced with one width signifying a logic "1" and another width, a logic "0". In the latter, the pulses are all of the same width but the pulse rate changes to differentiate between logic "1" and logic "0".

Analog modulation can also take a number of forms. The simplest is intensity modulation where the brightness of an LED is varied in direct step with the variations of the transmitted signal.

In other methods, an RF carrier is first frequency modulated with another signal or, in some cases, several RF carriers are separately modulated with separate signals, then all are combined and transmitted as one complex waveform. Figure 4 shows all of the above modulation methods as a function of light output.

The equivalent operating frequency of light, which is, after all, electromagnetic radiation, is extremely high – on the order of 1,000,000 GHz. The output bandwidth of the light produced by LEDs and Laser diodes is quite wide. Unfortunately today’s technology does not allow this bandwidth to be selectively used in the way that conventional radio frequency transmissions are utilized. Rather, the entire optical bandwidth is turned on and off in the same way that early "spark transmitters" (in the infancy of radio), turned wide portions of the RF spectrum on and off. However, with time, researchers will overcome this obstacle and "coherent transmissions", as they are called, will become the direction in which the fiber optic field progresses.

fifa · 发表于 2004-7-13 23:47:00

看了

fiber optics introduction 2

Advantages of Fiber Optic Systems

发表回复

浏览过的版块


热搜: 防雾膜棒料红外反射膜光电二极管镀膜机反射膜光谱仪光电探头真空镀银滤光片光器件应用光学 zemax激光远心镜头 sio2 增透膜离子源栅网 zemax 论文高反膜半反半透膜