2015-04-01
767
low-divergence beam – луч с низкой дивергенцией (рассеиванием)
well-defined wavelength – точно определенная длина волн
incandescent light bulb – электрическая лампа накаливания
large solid angle – большой пространственный угол
out of step with each other – не соответствующие друг другу
intermittent – прерывающийся, пульсирующий
opaque – светонепроницаемый, непрозрачный
gain medium – среда усиления
optical cavity – оптический резонатор
transparent – прозрачный
to ensure – гарантировать, обеспечивать
pumping – накачка
flash lamp – импульсная лампа
solid state – твердотельный
due to – благодаря, из-за
visible emission – оптическое излучение
junction – переход
storage device – запоминающее устройство
ubiquitous – повсеместный, встречающийся повсюду
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A laser is an optical device that produces coherent radiation. Because the microwave equivalent of the laser, the maser, was developed first, devices that emit microwave and radio frequencies are usually called masers. In early literature, the laser was often called the optical maser. The term “laser” is an acronym: L ight A mplification by S timulated E mission of R adiation. A typical laser emits light in a narrow, low-divergence beam and with a well-defined wavelength (corresponding to a particular color if the laser is operating in the visible spectrum). This is in contrast to a light source such as the incandescent light bulb, which emits into a large solid angle and over a wide spectrum of wavelength.
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We know that light consists of waves. An ordinary light consists of waves all out of phase, out of step with each other. All the waves in a laser beam have the same wavelength. A laser beam has a very definite color. The red color of the ruby is one of the most widely seen colors in it. But the difference between an ordinary beam of ruby red light and a laser beam of ruby red light is that in the laser beam the waves are all in step with each other. This orderly behavior of the laser beam makes a big difference, and there is one to be mentioned. Most beams of light are continuous. But the laser beam is intermittent, and it is off much longer than it is on. Because these switches on and off are very fast an eye doesn’t see them. While the laser beam is off the energy for the next flash is building up, and when this flash comes, it comes very intensively. An ordinary beam of light diverges, but laser beam doesn’t diverge at all. So it carries its energy in a compact form, until it is absorbed when it strikes something opaque.
A laser consists of a gain medium inside an optical cavity, with a means to supply energy to the gain medium. The gain medium is a material (gas, liquid, solid or free electrons) with appropriate optical properties. In its simplest form, a cavity consists of two mirrors arranged such that light bounces back and forth, each time passing through the gain medium. Typically, one of the two mirrors, the output coupler, is partially transparent. The output laser beam is emitted through this mirror.
Light of a specific wavelength that passes through the gain medium is amplified (increases in power); the surrounding mirrors ensure that most of the light makes many passes through the gain medium. Part of the light that is between the mirrors (i.e., is in the cavity) passes through the partially transparent mirror and appears as a beam of light. The process of supplying the energy required for the amplification is called pumping and the energy is typically supplied as an electrical current or as light at a different wavelength. In the latter case, the liquid source can
be a flash lamp or another laser. Most practical lasers contain additional elements that affect properties such as the wavelength of the emitted light and the shape of the beam.
The first working laser was demonstrated in May 1960 by Theodore Maiman. He used a solid-state flash lamp-pumped synthetic ruby crystal to produce red laser light at 694 nanometers wavelength. Maiman’s laser, however, was only capable of pulsed operation due to its three energy level pumping scheme. Later in 1960 the Iranian physicist Ali Javan made the first gas laser using helium and neon. Javan later received the Albert Einstein Award in 1993.The concept of the semiconductor laser diode was proposed by Basov and Javan. The first laser diode was demonstrated by Robert N. Hall in 1962. Hall’s device was made of gallium arsenide and emitted at 850 nm in the near-infrared region of the spectrum. The first semiconductor laser with visible emission was demonstrated later the same year by Nick Holonyak. As with the first gas lasers, these early semiconductor lasers could be used only in pulsed operation, and indeed only when cooled to liquid nitrogen temperatures (77 K). In 1970 Zhores Alferov in the Soviet Union developed laser diodes continuously operating at room temperature, using the heterojunction structure.
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Recently, lasers have become a multi-billion dollar industry. The most widespread use of lasers is in optical storage devices such as compact disc and DVD players, in which the laser (a few millimeters in size) scans the surface of the disc. In industry, lasers are used for cutting steel and other metals and for inscribing patterns (such as the letters on computer keyboards). Lasers are also commonly used in various fields in science, especially spectroscopy, typically because of their well-defined wavelength or short pulse duration in the case of pulsed lasers. They have become ubiquitous, finding utility in thousands of highly varied applications in every section of modern society, including consumer electrons, information technology, science, medicine, industry, law enforcement, entertainment and the military.
Задание I. Выберите правильный ответ на вопрос по тексту:
1. What kind of device is called maser?
a) device that emits radiation
b) device that produces electrical energy
c) device that emits microwave and radio frequency
2. What does a typical laser emit?
a) visible spectrum
b) high-divergence beam
c) light in a narrow, low-divergence beam
3. What do all the waves in a laser beam have?
a) different wavelength
b) the same color
c) the same wavelength
4. What kind of beam is the laser beam?
a) continuous
b) intermittent
c) long
5. What does a laser consist of?
a) semiconductor and isolator
b) high-divergence beams
c) a gain medium inside an optical cavity
6. The material of the gain medium is…, isn’t it?
a) a silicon with chemical properties
b) a glass with physical properties
c) a material with optical properties
7. Where is light of a specific wavelength amplified?
a) in voltage
b) in current
c) in power
8. What is the process of supplying the energy required for the amplification called?
a) multiplication
b) enhancing
c) pumping
9. When did the first working laser demonstrated?
a) In 1962
b) In 1961
c) In 1960
10. Who developed the first laser diodes with heterojunction structure?
a) Nick Holonyak
b) Robert Hall
c) Zhores Alferov
Задание II. Найдите все цепочки слов в тексте и разберите их с преподавателем (последнее слово – определяемое). Выпишите их в свою тетрадь и составьте с ними свои предложения по теме “ Laser”
Задание III. Выполнив все упражнения, будьте готовы побеседовать на английском языке по теме «Laser» с группой и с преподавателем
