English for Specific Purposes

Electronic Engineering 43

SPECIALIST READING A: Laser Working Principles

9. Scan the text and match the headings (a – e) with the paragraphs (1 – 4). There is one heading you will not need to use.

n) Laser design;

o) Acronym origin;

p) Applications;

q) Essential features;

r) Laser variations;

10. Read the text and answer the questions.

1. What does the acronym LASER stand for? Why has there appeared the humorous acronym LOSER?

2. How can you explain the term MASER?

3. What is “pencil beam”?

4. Which qualities does a beam produced by a thermal light source have?

5. What are the modes of producing radiation, mentioned in the text?

6. What does optical cavity serve for?

7. How are the mirrors arranged and what is their assignment?

8. What is gain medium?

9. How is energy typically supplied in a laser?

11. Look at the picture and mark all the principal components of laser: high reflector, outputcoupler, laser pumping energy, gain medium, laser beam.

1) ________________________

2) ________________________

3) ________________________

4) ________________________

5) ________________________

1. ______________________________

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. In modern usage "light" broadly denotes electromagnetic radiation of any frequency, not only visible light, hence infrared laser, ultraviolet laser, X-ray laser, and so on. Because the microwave predecessor of the laser, the maser, was developed first, devices of this sort operating at microwave and radio frequencies are referred to as "masers" rather than "microwave lasers" or "radio lasers". A laser which produces light by itself is technically an optical oscillator rather than an optical amplifier as suggested by the acronym. It has been humorously noted that the acronym LOSER, for "light oscillation by stimulated emission of radiation," would have been more correct. With the widespread use of the original acronym as a common noun, actual optical amplifiers have come to be referred to as "laser amplifiers", notwithstanding the apparent redundancy in that designation.

2. ______________________________

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.