Readership Developed as an introductory course, this up-to-date text discusses the major building blocks of present-day fiber-optic systems and presents their use in communications and sensing. Starting with easy-to-understand ray propagation in optical fibers, the book progresses towards the more complex topics of wave propagation in planar and cylindrical waveguides. Special emphasis has been given to the treatment of single-mode fibers the backbone of present-day optical communication systems. It also offers a detailed treatment of the theory behind optoelectronic sources LEDs and injection laser diodes , detectors, modulators, and optical amplifiers. Building upon these fundamental principles, the book introduces the reader to system design considerations for analog and digital fiber-optic communications.
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Generalized configuration of a fiber-optic communication system Information Input The information input may be in any of the several physical forms, e. Therefore an input transducer is required for converting the non-electrical input into an electrical input. For example, a microphone converts a sound signal into an electrical current, a video camera converts an image into an electric current or voltage, and so on.
In situations where the fiber-optic link forms a part of a larger system, the information input is normally in electrical form. Examples of this type include data transfer between different computers or that between different parts of the same computer.
In either case, the information input must be in the electrical form for onward transmission through the fiber-optic link. The modulation of an optical carrier may be achieved by employing either an analog or a digital signal.
An analog signal varies continuously and reproduces the form of the original information input, whereas digital modulation involves obtaining information in the discrete form. In the latter, the signal is either on or off, with the on state representing a digital 1 and the off state representing a digital 0. These are called binary digits or bits of the digital system. The number of bits per second bps transmitted is called the data rate.
If the information input is in the analog form, it may be obtained in the digital form by employing an analog-to-digital converter. Analog modulation is much simpler to implement but requires higher signal-tonoise ratio at the receiver end as compared to digital modulation. Further, the linearity needed for analog modulation is not always provided by the optical source, particularly at high modulation frequencies.
Therefore, analog fiber-optic systems are limited to shorter distances and lower bandwidths. Ideally, an optoelectronic source should generate a stable single-frequency electromagnetic wave with enough power for longhaul transmission. However, in practice, LEDs and even laser diodes emit a range of frequencies and limited power. The favourable properties of these sources are that they are compact, lightweight, consume moderate amounts of power, and are relatively easy to modulate. Furthermore, LEDs and laser diodes which emit frequencies that are less attenuated while propagating through optical fibers are available.
It collects the signal from the transmitter and directs this to the atmospheric channel. At the receiver end again the antenna collects the signal and routes it to the receiver. In the case of guided channel transmission, e. In fiber-optic systems, the function of a coupler is to collect the light signal from the optoelectronic source and send it efficiently to the optical fiber cable. Several Introduction designs are possible. However, the coupling losses are large owing to Fresnel reflection and limited light-gathering capacity of such couplers.
At the end of the link again a coupler is required to collect the signal and direct it onto the photodetector. In fiber-optic systems, the optical signal traverses along the cable consisting of a single fiber or a bundle of optical fibers. An optical fiber is an extremely thin strand of ultra-pure glass designed to transmit optical signals from the optoelectronic source to the optoelectronic detector.
In its simplest form, it consists of two main regions: i a solid cylindrical region of diameter mm called the core and ii a coaxial cylindrical region of diameter normally mm called the cladding.
The refractive index of the core is kept greater than that of the cladding. This feature makes light travel through this structure by the phenomenon of total internal reflection.
In order to give strength to the optical fiber, it is given a primary or buffer coating of plastic, and then a cable is made of several such fibers.
This optical fiber cable serves as an information channel. For clarity of the transmitted information, it is required that the information channel should have low attenuation for the frequencies being transmitted through it and a large light-gathering capacity. Furthermore, the cable should have low dispersion in both the time and frequency domains, because high dispersion results in the distortion of the propagating signal.
As the optical signals propagate along the length of the fiber, they get attenuated due to absorption, scattering, etc. After a certain length, the cumulative effect of attenuation and dispersion causes the signals to become weak and indistinguishable. Therefore, before this happens, the strength and shape of the signal must be restored. This can be done by using either a regenerator or an optical amplifier, e.
Semiconductor p-i-n or avalanche photodiodes are employed for this purpose. The photocurrent developed by these detectors is normally proportional to the incident optical power and hence to the information input. The desirable characteristics of a detector include small size, low power consumption, linearity, flat spectral response, fast response to optical signals, and long operating life. After filtering, the photocurrent is amplified if needed. These two functions are performed by the receiver module.
For digital transmission, in addition to the filter and amplifier, the receiver may include decision circuits. If the original information is in analog form, a digital-toanalog converter may also be required. The design of the receiver is aimed at achieving high sensitivity and low distortion.
The signal-to-noise ratio SNR and bit-error rate BER for digital transmission are important factors for quality communication. For example, it may be required to transform the electrical output into a sound wave or a visual image.
Suitable output transducers are required for achieving this transformation. In some cases, the electrical output of the receiver is directly usable. This situation arises when a fiber-optic system forms the link between different computers or other machines. For communication purposes, the transmission bandwidth and hence the information-carrying capacity of a fiber-optic system is much greater than that of coaxial copper cables, wide-band radio, or microwave systems.
The concepts of bandwidth and channel capacity are explained in Appendix A1. Small size and light weight of optical fibers coupled with low transmission loss typically around 0. Optical fibers are insulators, as they are made up of glass or plastic. This property is useful for many applications. Particularly, it makes the optical signal traversing through the fiber free from any radio-frequency interference RFI and electromagnetic interference EMI.
RFI is caused by radio or television broadcasting stations, radars, and other signals originating in electronic equipment. EMI may be caused by these sources of radiation as well as from industrial machinery, or from naturally occurring phenomena such as lightning or unintentional sparking.
Optical fibers do not pick up Introduction or propagate electromagnetic pulses EMPs. Thus, fiber-optic systems may be employed for reliable monitoring and telemetry in industrial environments, where EMI and EMPs cause problems for metallic cables. In fact, in recent years, a variety of fiber-optic sensor systems have been developed for accurate measurement of parameters such as position, displacement, liquid level, temperature, pressure, refractive index, and so on.
In contrast to copper cables, the signal being transmitted through an optical fiber cannot be obtained from it without physically intruding the fiber.
Further, the optical signal propagating along the fiber is well protected from interference and coupling with other communication channels electrical or optical. Thus, optical fibers offer a high degree of signal security. This feature is particularly suitable for military and banking applications and also for computer networks. For large-scale exploitation, the systems cost and the availability of raw material are two important considerations.
The starting material for the production of glass fibers is silica, which is easily available. Regarding the cost, it has been shown that for long-distance communication, fiber cables are cheaper to transport and easier to install than metallic cables. Despite the fragile nature of glass fiber, these cables are surprisingly strong and flexible.
The firstgeneration fiber-optic systems employed a wavelength of 0. This wavelength corresponds to the first low-loss window in a silica-based optical fiber. This is shown in Fig. The region around 0. Therefore, as technology progressed, the first window become less attractive. In fact from the point of view of long-haul communication applications, it is not only the attenuation but also the dispersion of pulses by a fiber that plays a key role in selecting the wavelength and the type of fiber.
Therefore second-generation systems used a second window at 1. The corresponding loss was about 0.
Fiber Optics and Optoelectronics
Khare Description Developed as an introductory course, this up-to-date text discusses the major building blocks of present-day fiber-optic systems and presents their use in communications and sensing. Starting with easy-to-understand ray propagation in optical fibers, the book progresses towards the more complex topics of wave propagation in planar and cylindrical waveguides. Special emphasis has been given to the treatment of single-mode fibers the backbone of present-day optical communication systems. It also offers a detailed treatment of the theory behind optoelectronic sources LEDs and injection laser diodes , detectors, modulators, and optical amplifiers. Building upon these fundamental principles, the book introduces the reader to system design considerations for analog and digital fiber-optic communications.
Optoelectronics: An Introduction (3rd Edition)