Discussion

6. Answer the questions to Text A:

1) List the applications of lasers with short pulses of high peak power. 2) What does changing the quality factor result in? 3) What is a Q-switch? 4) Say what happens under normal laser excitation and with the Q-switch open. What is the position of the Q-switch for lasing to stop? 5) What are the best laser candidates for energy storage? 6) Explain why ion laser cannot be Q-switched. 7) Enumerate the types of Q-switches described in the text. Which are the most common types? 8) Describe the operation of rotating-element Q-switches. What are their advantages? What makes these devices difficult to work with?

7. Use exercises 5 and 6 to sum up the contents of Text 11A.

Terminology B:

1. strain waves – волны упругости (упругие волны);

2. laserbeam deflector – лазерный дефлектор;

3. optical signal processor – устройство оптической обработки сигнала;

4. laser frequency shifter – устройство сдвига лазерной частоты;

5. mode-locking – синхронизация мод, mode-locker - синхронизатор мод, mode-locked laser system – лазерная система в режиме синхронизации мод;

6. bulk damage – здесь: разрушение (стойкость к разрушению) кристалла;

7. r.f. drive power - разночастотный источник сигнала.

Vocabulary Practice

1. Match the terms on the left with the Russian equivalents on the right:

1) mechanically induced strain waves	a) плотность на выходной мощности
2) tunable optical filters	b) лазерный дефлектор
3) single-pulse bulk-damage threshold intensity	c) длительность импульса лазера с акустооптической модуляцией добротности
4) laser cavity configuration	d) перестраиваемые оптические фильтры
5) output power density	e) плавленый кварц
6) laserbeam deflector	f) волны упругости, наводимые механическим путем
7) fused quartz	g) инверсная населенность в устойчивом состоянии
8) steady-state population inversion	h) повреждение покрытия
9) coating failure	i) схема лазерного резонатора
10) acousto-optic Q-switched pulse duration	j) пороговая мощность разрушения при моноимпульсном излучении

2. Match the verbs on the left with the definitions on the right:

to mature	to think about something carefully; to ponder; to study; to observe or examine;
to inhibit	to say that something will happen, before it happens; to foretell;
to predict	to become ripe; to reach completion;
to provide	to prevent something from growing or developing well; to restrain; to hinder;
to consider	to bring about or ensure; to arrange for smth or prepare beforehand; to supply smth.

TEXT 11B ACOUSTO-OPTIC Q-SWITCHES

The term “acousto-optic” describes the interaction of optical waves with mechanically induced strain waves, which va-ry the index of refraction in an optical interaction medium. This type of interaction was first predicted by Brillouin in 1922. But it was not until after the development of the laser in the early 1960s that acousto-optic devices began maturing into the products of today. The acousto-optic interaction has led to the development of many useful devices: laser modulators, laserbeam deflectors, optical signal processors, tunable optical filters, laser frequency shifters, modelockers, and Q-switches.

Of particular interest is the acousto-optic Q-switch; con-sider the laser cavity configuration shown in Figure 2. If the laser medium is continually pumped without any optical feed-back mechanism, a steady-state population inversion will be established in the laser transition level. An excess population of this level can be obtained if the radiative lifetime is long and the pumping intensity is high. The addition of the two end mirrors will form a cavity and provide the required feedback for lasing. The properties of the laser cavity are characteri-zed by the quality factor, Q. A high value of Q means that energy is stored in the cavity with only very small losses. A low value of Q, conversely, indicates the presence of large energy losses is the cavity, and the lasing will be inhibited.

Figure 2. Simplified schematic of an acousto-optic Q-switched laser cavity

For the acousto-optic Q-switched laser configuration shown in Figure 2, the Q factor is given by the expression Q = 2d ω/(I – R + Rŋ)c , where R is the reflectivity of the output mirror, d is the dis-tance between the mirrors, ω is the angular frequency of the light, с is the velocity of light, and ŋ is the fraction of light diffracted out of zero order when the Q-switch is on.

With rf drive power applied to the Q-switch, ŋ is high and the value of Q is held low, preventing lasing. In this state the laser medium has time to build up an excessive population in the laser transition level. After pumping the laser rod to the maximum population the rf drive is switched off and all the light is allowed to pass between the mirrors. This allows the laser pulse intensity to build up rapidly to the maximum and then decay as the excited states are depleted. This cycle is repeated at the desired laser pulse rate as limited by the optical and electrical time constants.

Acousto-optic Q-switches are commonly used in continuous-wave, modelocked, and pulsed laser systems to generate intense bursts of laser light. Q-switching usually reduces the energy of the pulses but at the same time the pulse length is shorte-ned so much that the peak power is increased considerably. Acousto-optic Q-switched pulse durations typically range from 40 to 200 nanoseconds, depending on the laser design, Acousto-optic Q-switched ruby and Nd:YAG lasers produce output power densities of 10⁷ to 10⁸ watts per square centimeter; measurements as high as 4 x I0⁹ W/сm² have been reported in simultaneously Q-switched and modelocked systems. By using a variety of opti-cal interaction materials Q-switches have been designed to work in laser systems operating at wavelengths ranging from 300 nanometers to 11 micrometers.

Fused quartz is the most commonly used acousto-optic Q-switch medium for operation between 300nm and 2μm. The single-pulse bulk-damage threshold intensities at wavelengths of 1064 and 532 nm are 110 and 90 GW/cm², respectively. Multi-pulse balk damage occurs at about 80 percent of threshold at 1064 nm and 60 percent of threshold at 532 nm, as reported by Merkle et al. of the University of Southern California. Interestingly, quartz optical elements typically suffer coating failure before bulk damage occurs. For applications requiring low optical po-wer more efficient materials, such as lead molybdate (PbMoO₄) and flint glass are used in some devices operating between 600 and 1200 nm. However, these materials find limited use be-cause of their high internal optical absorption.

Constant improvements in acousto-optic Q-switch design and fabrication techniques are leading to more efficient, reliable and more stable systems. However, the final Q-switch choice will be dictated by the user’s system requirements and economic tradeoffs.

Review

3. Match the parts of sentences in column A with those in column В to produce true statements:

1) The term “acousto-optic” describes	a) the presence of large energy losses in the cavity.
2) It was not until after the development of the laser in the early 1960s	b) energy is stored in the cavity with only very small losses.
3) An excess population of the laser transition level can be obtained	c) the interaction of optical waves with mechanically induced strain waves.
4) A high value of Q means that	d) because of their high internal optical absorption.
5) A low value of Q indicates	e) range from 40 to 200 nanoseconds, depending on the laser design.
6) Acousto-optic Q-switched pulse durations typically	f) that acousto-optic devices began maturing into the products of today.
7) These materials find limited use	g) if the radiative lifetime is long and the pumping intensity is high.

4. Pick out the words that do not belong to the following groups:

1. continually, conversely, rapidly, commonly, reflectivity, considerably, typically, simultaneously, respectively, in-terestingly;

2. shorten, reduce, limit, deplete, build up, decay;

3. transition level, interaction medium, drive power, pulse ra-te, time constant, index of refraction, bulk damage, power density;

4. inhibit, prevent, stop, hinder, provide.

5. Study the items of Text 11В’s plan and reorder them according to the text. Pick out the items that have nothing to do with this text:

1. Lasers using acousto-optic Q-switches.

2. Operation difficulties of dye-cell Q-switching.

3. The expression for the Q factor.

4. Prospects of developing more stable systems.

5. Acousto-optic Q-switch operation. Simplified version.

6. Outstanding electro-optic crystals.

7. Acousto-optic interaction. Background.

8. The most commonly used acousto-optic Q-switch media.

6. Write an abstract: of Text 11B.