Кузнецова Т.И., Кирсанова Г.В. - Чтение технической литературы на английском языке по оптике (1058939), страница 3

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The propagation of a Gaussian beam is fully specified by its beam waist and its divergence. For an ideal TEM₀₀ beam, the product of the beam waist ω₀ times the divergence angle θ₀ can be expressed as

ω₀θ₀ = λ/π . This implies that a Gaussian beam can be characterized by measuring its beam waist and its diameter at one other location. In practice it can be difficult to locate the beam waist. Modern instruments determine beam parameters from measurements taken at multiple locations.

Non-Gaussian beams

All lasers deviate to some extent from the Gaussian ideal. Many high-power carbon dioxide (CO₂) lasers emit beams with rectangular profiles; diode laser arrays produce a beam that does not appear to come from a laser at all. Even lasers operating in TEM₀₀ mode truncate the beam because of the limiting aperture in the cavity, which results in fringes in the near field.

There are limitations in choosing a beam for an application based on its correlation to a Gaussian profile. In fact, a high correlation to a Gaussian fit can be achieved by a beam that contains only higher-mode components (see figure, p. 82). The spot size to which such a beam can be focused differs significantly from what one might expect.

When the beam deviates from Gaussian, the product of the beam waist times the divergence must be increased by the "quality factor" of the beam, M². The product of beam waist and divergence becomes ωθ = M²λ/π. M² represents how many times wider the focused spot is than the theoretical minimum. An M² of 2, for example, indicates that the focused beam will be twice the ideal minimum spot size, and so this beam will have only 25% of the intensity of a fundamental beam of the same power. M² values for beams of the highest quality are <1.1, while values of M² for multimode lasers might be around 4.

Measuring M²

The wide applicability of M² has led the ISO to adopt it as the standard for beam quality. M2 profilometers form a new beam waist with a lens and take measurements before, within, and after the waist. According to the ISO standard, the lens must be stationary and the detector move to take the measurements. In addition, the calculations must be based on the second-moment algorithm.

For well-collimated beams, an instrument with a fixed detector and a variable lens will provide a reliable approximation of M² (see Fig. 2). This simplifies the design of a scanning mechanism. Such instruments can provide precision measurements in applications that employ well-controlled laser sources.

FIGURE 2. Although not designed strictly according to ISO standards for M², a profilometer using a rotating knife-edge with an adjustable lens can make precise measurements of well-controlled beams.

Astigmatism

Another parameter that relates to diode lasers deserves mention. Most diode lasers have rectangular output facets that produce elliptical beams. In addition, the cross section of the beam in the plane vertical to the direction of propagation has a waist and divergence different from that in the horizontal plane—that is, the beam is astigmatic.

The astigmatism in a focused beam must be corrected for the beam to be useful—a cylindrical lens tilted in the direction of propagation can do the trick. The "astigmatic distance" is the distance between the two different foci, which must be eliminated in the correction. Instruments based on CCD cameras are well suited to determining the astigmatic distance.

(Stephen J. Matthews, Contributing Editor, Laser Focus World, 2002)

OMISSION

In "Back to Basics: Semiconductor Lasers" (May, p. 145), Fig. 1 on p. 149 represents technology patented by Alcatel. The author wishes to acknowledge the assistance of Alcatel in preparing the illustration.

Text 5A terminology:

1. transition – переход, electronic transition – электронный переход, vibrational transi­tion – колебательный переход, rotational transition – вращательный переход;

2. species (sing. + pl.) – вид, разновидность, active species – активная среда, активатор, активная частица;

3. gas dynamic expansion – газодинамическое расширение;

4. dye laser – лазер на красителе;

5. solid state laser – твердотельный лазер;

6. glass laser – лазер на стекле;

7. solvent – растворитель;

8. rare earth ion – ион редкоземельного элемента;

9. гаге earth chelate laser – лазер на редкоземельных халатах;

10. spectral tunability – спектральная перестройка;

11. insulator – изолятор;

12. impurity – примесь, impurity-doped crystal – кристалл о примесями;

13. discharge – разряд, arc discharge – дуговой разряд; glow discharge – тлеющий разряд;

14. lattice – кристаллическая решетка;

15. junction – переход.

Preliminary exercises

1. Read and translate without a dictionary:

classify, classification, basic, atomic, ionic, molecular, expansion, spontaneously, organic, inorganic, chelate, trivalent, dielectric, amorphous, stoichiometry, specific, defect, differentiate, electron, injection.

1. Combine the appropriate words from the two columns to obtain terms. Translate them:

   1) spectral	5) gasdynamic	a. expansion	e. laser
   2) dielectric	6) electrical	b. inversion	f. tunability
   3) dye	7) impurity-doped	c. discharge	g. transition
   4) vibrational	8) population	d. crystal	h. insulator

2. Find equivalent phrases either in Text 5A or in the right-hand column:

1. Read Text 5A and answer the following question:

Какие методы используются для накачки газовых жидкостных и твердотельных лазеров?

TEXT 5A CLASSES OF LASER SOURCES

Laser sources are commonly classified in terms of the state of the active medium: gas, liquid, and solid. Each of these classes is further subdivided into one or more types.

Gas Lasers. Gas lasers are conveniently described in terms of six basic types, two involving electronic transition in atomic active species (neutral and ionic), three based on neutral mole­cular active species (differentiated by laser action occurring on electronic, vibrational, and rotational transitions), and one based on molecular-ion active species. Gas lasers are pum­ped using a wide variety of excitation methods, including seve­ral types of electrical discharges (cw, pulsed, dc⁵ or rf⁶, glow or arc), electron beam excitation, gasdynamic expansion, electrically or spontaneously induced chemical reactions, and optical pumping using primary lasers.

Liquid Lasers. Liquid lasers are commonly described in terms of three distinct types: organic dye lasers which are most well-known for their spectral tunability, rare-earth chelate­
lasers which utilize organic molecules, and lasers utili­zing inorganic solvents and trivalent rare earth ion active cen­ters. Liquid lasers are optically pumped using three basic methods: flashlamps, pulsed primary lasers, or cw primary lasers.

Solid State Lasers. Solid state lasers are subdivided by the type of the solid used - a dielectric insulator or a semi­conductor. Dielectric insulators may take the form of an impurity-doped crystal or an impurity-doped amorphous material such as glass. Recently, solid state lasers have been developed using insulating crystals in which the active species has bean fully substituted into the lattice (stoichiometric materials) and using insulator crystals in which color centers (specific types of lattice defects) serve as the active centers. Lasers utilizing dielectric insulators are almost exclusively pumped optically, either with flashlamps, cw arc -lamps, or with other laser sources.

Semiconductor lasers are usually differentiated in terms of the means by which the hole-electron pair population inver­sion is produced. Semiconductor lasers can be pumped optically (usually with other laser sources), by electron-beams, or more commonly by injection of electrons in a p-n junction.

2300 п.эн.

Words to be learnt:

in terms of – в смысле, с точки зрения, на основании;

to involve – затрагивать, включать в себя, подразумевать;

distinct – отдельный, особый, ясный, отчетливый;

to substitute – заменять, подставлять, использо­вать вместо;

to exclude – исключать;

exclusive – исключительный;

exclusively – исключительно.

Exercises

1. Match synonyms:

distinct, exclusive, usual, specific, common, different, clear, convenient, particular, exceptional, comfortable, differing.

2. Complete the sentences below with the appropriate word or word-combination according to text 5A:

1) Gas lasers are conveniently described in terms of...
a) six basic types;

b) three distinct types: organic dye lasers...;
c) the solid used.

2) Laser sources are commonly classified in terms of...
a) the type of solid used;

b) the state-of-matter of the active medium;

c) six basic types.

3) Liquid lasers are pumped...

a) using a wide variety of excitation methods;

b) optically by electron beam, or by in­jection of electrons in a p-n junction;

c) optically by three basic methods: flashlamps, pulsed primary lasers, or cw primary lasers.

3. Bring the sentences below under the following headings:

A. Gas Lasers

B. Liquid Lasers

C. Solid State Lasers

1) These lasers are subdivided by the type of solid used – a dielectric insulator or a semiconductor. 2) They are described in terms of six basic types. 3) They are optically pumped using three basic methods: flashlamps, pulsed primary lasers, or cw primary lasers. 4) Organic dye lasers are most well-known for their spectral tunability. 5) These are pumped using a wide va­riety of excitation methods. 6) Dielectric insulators may take the form of an impurity-doped crystal or an impurity-doped amorphous material such as glass.

4. Complete the table below to match Text 5A:

Table 2 Classes of Laser Source

Text 5B Terminology:

1. carrier – носитель заряда, электрон проводимости; excess carrier – возбужденный электрон проводимости/электрон с избыточной энергией;

2. band – полоса, зона (уровней энергии); conduction band – зона проводимости; valence band – валентная зона; band-to-band transition – переход с уровня на уровень;

3. gap – зазор, промежуток, интервал; energy gap – запрещенная зона (в полупроводниках), энергетическая зона;

4. bandgap – запрещенная зона, ширина запрещенной зоны; bandgap semiconductor – полупроводник с запрещенной зоной; direct bandgap semiconductor – собственный, беспримесный полу­проводник; indirect bandgap semiconductor – примесный, несобст­венный проводник;

5. momentum – количество движения, импульс, импульсная сила; to conserve momentum – сохранять количество движения;

6. lifetime – время жизни; radiative lifetime – излучательное время жизни;

7. internal quantum efficiency - внутренняя квантовая эффективность.

Preliminary exercises

1. Read and translate without a dictionary:

Характеристики

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