Text 1. Modern light-wave technologies. Fiber Optics

There has always been a demand for increased capacity of transmission of information, and scientists and engineers continuously pursue technological routes for achieving this goal. The technological advances ever since the invention of the laser in 1960 have indeed revolutionized the area of telecommunication and networking. The availability of the laser, which is coherent source of light waves, presented communication engineers with a suitable carrier wave capable of carrying enormously large amounts of information compared with radio waves and microwaves. Although the dream of carrying millions of telephone (audio) or video channels through a single light beam is yet to be realized, the technology is slowly edging toward making this dream a reality.

A typical lightwave communication system consists of a lightwave transmitter, which is usually a semiconductor laser diode (emitting in the invisible infrared region of the optical spectrum) with associated electronics for modulating it with the signals; a transmission channel – namely, the optical fiber to carry the modulated light beam; and finally, a receiver, which consists of an optical detector and associated electronics for retrieving the signal. The information – that is, the signal to be transmitted – is usually coded into a digital stream of light pulses by modulating the laser diode. These optical pulses then travel through the optical fiber in the form of guided waves and are received by the optical detector from which the signal is then decoded and retrieved.

At the heart of a lightwave communication system is the optical fiber, which acts as the transmission channel carrying the light beam loaded with information. It consists of a dielectric core (usually doped silica) of high refractive index surrounded by a lower refractive index cladding. Incidentally, silica is the primary constituent of sand, which is found in so much abundance on our earth. Guidance of light through the optical fibers takes place by the phenomenon of total internal reflection. Sending the information-loaded light beams through optical fibers instead of through the open atmosphere protects the light beam from atmospheric uncertainties such as rain, fog, pollution, and so forth.

One of the key elements in the fiber optics revolution has been the dramatic improvement in the transmission characteristics of optical fibers. These include the attenuation of the light beam as well as the distortion in the optical signals as they race through the optical fiber. The development of low-loss optical fibers (20 dB/km at the He-Ne laser wavelength of 633 nm) in 1970 made practical the use of optical fibers as a viable transmission medium in lightwave communication systems.

Although a variety of optical fibers are available, the fibers in most use today are the so-called single-mode fibers with a core diameter of about 10 μm and an overall diameter of 125μm. Optical fibers with typical losses in the range of 0.2 dB/km at 1550 nm and capable of transmission at 2-10 Gbit/s (Gb/s) are now commercially available. Most currently installed systems are based on communication at a 1300-nm optical window of transmission. The choice of this wavelength was dictated by the fact that around an operating wavelength of 1300 nm the optical pulses propagate through a conventional single-mode fiber with almost no pulse broadening. Because silica has the lowest loss in the 1550-nm wavelength band, special fibers known as dispersion-shifted fibers have been developed to have negligible dispersion in the 1550-nm band, thus providing us with fibers having the lowest loss and almost negligible dispersion.

In the lightwave communication systems that are in operation today, the signals have to be regenerated every 30-60 km to ensure that information is intelligibly retrieved at the receiving end. This is necessary either because the light pulses have become attenuated, and hence the signal levels have fallen below the detectable level, or because the spreading of the pulses has resulted in an overlapping of adjacent pulses leading to a loss of information. Until now this regeneration had to be achieved by first converting the optical signals into electrical signals, regenerating the signals electrically, and then once again converting the electrical signals into optical signals by modulating another semiconductor laser; such devices are called regenerators.

Recent developments in optical amplifiers based on erbium- (a rare earth element) doped silica optical fibers have opened up the possibilities of amplifying optical signals directly in the optical domain without the need of conversion to electrical signals.

Because of amplification in the optical domain itself, such systems are not limited by the speed of the electronic circuitry and indeed can amplify multiple signals transmitted via different wavelengths simultaneously.

(Ajoy Ghatak and K.Thyagarajan.

Introduction to Fiber Optics)

Vocabulary

6.2 Compile the vocabulary (term) log as shown in the preface (part ofspeech, definition, translation, synonyms and antonyms if possible, example of use). Words (concepts) are given below:

Networking; a coherent source; a carrier wave; to retrieve; beam; a dielectric core; attenuation; loss; single-mode fiber; amplification overlapping circuitry

6.3 Find in the text and suggest English equivalents for the following:

Рефракционный (преломляющий) коэффициент; в диапазоне; диапазон длин волны; одномодовое оптоволокно с минимальным смещением длины волны (за счет ввода легирующих добавок); легированный кремний; среда передачи.

Grammar

6.4 Translate these sentences into Russian, paying attention to the grammar.

1) Tomorrow he will be informed about it. 2) He will be given a new problem to solve. 3) Scientific laws are now being viewed as algorithms. 4) New type of computing equipment is being developed in our research lab. 5) They were asked to repeat the calculations. 6) A digital control system can be thought of as an operator who follows a very complicated set of instructions. 7) Connections can also be made from the new cable via the three carriers’ existing cable networks. 8) These instructions should be followed. 9) The results of the tests should be compared. 10) Then, the dialog may be restarted at an agreed (earlier) synchronization point. 11) These things cannot be compared.

6.5 Choose the correct substitute for each modal verb.

1) We ought to win the race. → We ( are supposed to/are allowed to/are able to) win the race.

2) I can operate the computer. → I (have to/am able to/am supposed to) operate the computer.

3) You must meet my best friend. → You (have to/are able to/ are allowed to) meet my best friend.

4) He should be at home by now. → He (has to/is allowed to/ is supposed to) be at home by now.

5) I must get up early. → I (have to/am able to/ am allowed to) get up early.

6) They may stay up late. → They (have to/are allowed to/are supposed to) stay up late.

7) She needs to see the doctor. → She (has to/is able to/ is allowed to) see the doctor.

8) We need not walk. → We (do not have to/are not able to/are not allowed to) walk.

9) You must not sleep → You (do not have to/ are not able to/are not allowed to) sleep.

10) Should I go to the cinema with them? → Am I (able to/ allowed to/supposed to) go to the cinema with them?