II. Decide which statement matches the text:

1. The more information can be transmitted through a single channel, the more money the customers of the company make.

2. The T- carrier system allows to send 24 different audio signals through a single channel.

3. It is better to use wires and switches than serial digital data in telecommunications.

4. DSP converts voices into electrical impulses.

III. Decide which statement does not match the text:

1. Before the 1960 s, one pair of wires was used to send one analog voice signal to the receiver.

2. The two main tasks of telecommunication companies are transferring their customer's information and maintaining the channel.

3. DSP has made telecommunications cheaper.

4. DSP technology allows to convert audio signals into electric impulses.

IV. Decide which definitions match the following terms:

1. the elapsed time a) an impulse into which the audio signal turns

during the telephone conversation

2. DSP b) converting sensory signals into digital form

3. the analog voice signal c) to eliminate

4. to cancel d) a period of time during which the echo

travels back to the transmitter

V. Fill in the gaps with the words from the list below:

the analog voice signals a channel, advantage,

telecommunications, the connecting receiver

1. Establishing and maintaining __________________________ for transferring their clients’ information is the main task of ____________________________companies.

2. Before DSP, _______________________ were used in the telephone conversations instead of digital data.

3. __________________________usually returns apart of your voice signal as an echo.

4. DSP technology offers a financial ______________________ to telecom companies.

VI. Match the words in the right and left columns to make up a word expression from the text:

1. telecommunications a) fiber

2. optical b) signal

3. receive c) time

4. establish d) industry

5. audio e) the channel

6. elapsed f) strategy

7. financial g) payment

Laser

A laser is a device that emits light (electromagnetic radiation) through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation.The emitted laser light is notable for its high degree of spatial and temporal coherence, unattainable using other technologies.

Spatial coherence typically is expressed through the output being a narrow beam which is diffraction-limited, often a so-called "pencil beam." Laser beams can be focused to very tiny spots, achieving a very high irradiance. Or they can be launched into a beam of very low divergence in order to concentrate their power at a large distance.

Temporal (or longitudinal) coherence implies a polarized wave at a single frequency whose phase is correlated over a relatively large distance (the coherence length) along the beam. A beam produced by a thermal or other incoherent light source has an instantaneous amplitude and phase which vary randomly with respect to time and position, and thus a very short coherence length.

Most so-called "single wavelength" lasers actually produce radiation in several modes having slightly different frequencies (wavelengths), often not in a single polarization. And although temporal coherence implies monochromaticity, there are even lasers that emit a broad spectrum of light, or emit different wavelengths of light simultaneously. There are some lasers which are not single spatial mode and consequently their light beams diverge more than required by the diffraction limit. However all such devices are classified as "lasers" based on their method of producing that light: stimulated emission. Lasers are employed in applications where light of the required spatial or temporal coherence could not be produced using simpler technologies.