Irnee D'Haenens dies; assisted Maiman in building the first laser
Министерство высшего и среднего специального образования СССР
Московское ордена Ленина, ордена Октябрьской Революции
и ордена Трудового Красного Знамени
высшее техническое училище им. Н. Э. Баумана
Т. И. КУЗНЕЦОВА, Г. В. КИРСАНОВА
Утверждены редсоветом МВТУ
МЕТОДИЧЕСКИЕ УКАЗАНИЯ
ПО ОБУЧЕНИЮ ЧТЕНИЮ ТЕХНИЧЕСКОЙ ЛИТЕРАТУРЫ
НА АНГЛИЙСКОМ ЯЗЫКЕ ПО ОПТИКЕ
Часть 2
( редакция 2008 года,
Только для web -сайта факультета «Лингвистика»)
Москва 1987
Данные методические указания издаются в соответствии с учебным планом.
Рассмотрены и одобрены кафедрой иностранных языков 14.10.86 г., методической комиссией факультета ОТ 22.12.86 г. и учебно-методическим управлением 29.01.87 г.
Рецензент к. т. н. доц. Карасик В. Е.
Методические указания предназначены для обучения чтению и переводу научно-технической литературы. Приведены оригинальные тексты из английской и американской научно-технической литературы по оптике для развития навыков перевода, аннотирования и реферирования. Разработан терминологический словарь. Тематика методических указаний согласуется с курсом лекций, читаемым на факультете. Рекомендуется использовать для обучения студентов IV—V семестров дневного отделения. Способствует интенсификации учебного процесса.
© Московское высшее техническое училище им. Н. Э. Баумана
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| MODULE 4 LASERS Texts: A. Types and Comparisons of Laser Sources: Introduction B. Nd:YAG Laser vs. Ruby Laser C. Free Electron Laser |
Terminology:
1) to irradiate – облучать, излучать, испускать лучи; irradiation - иррадиация, лучеиспускание, излучение;
2) flashlamp – импульсная лампа, лампа накачки;
3) population inversion – инверсная населенность;
4) technique– метод, способ; excitation technique – способ, метод возбуждения;
5) optical pumping – оптическая накачка;
6) nuclear decay – ядерный распад;
7) dilute electron beam – низкоэнергетический электронный пучок;
8) to scatter – разбрасывать, рассеивать, nonlinear scattering – нелинейное рассеяние;
9) spectral tuning range – спектральный диапазон перестройки;
10. output waveform – волновой фронт, фронт волнового излучения;
11. power scalability – диапазон значений (уровень) выходной мощности
12. gain – усиление, коэффициент усиления; gain medium – усиливающая среда;
13. peak power – пиковая (импульсная) мощность; peak power density – плотность пиковой (импульсной) мощности;
14. pulse energy – энергия в импульсе.
Preliminary exercises
- Read and translate without a dictionary:
emission, inversion, chromium, ruby, crystal, xenon, decade, substance, neutral, gas, reaction, generate, periodic, spectral, parameter, neon, helium, unique, ensemble, electronic, dynamical, process, structural, kinetic, coherent, scheme, characteristics, ion.
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- Translate the word-combinations that follow:
pulse duration, peak power density, beam quality, chromium ions energy levels, laser sources types, laser action, electron beam kinetic energy, magnetic field periodicity, laser gain medium, pump excitation energy.
- Find equivalent phrases either in Text 4A or in the right-hand column:
| 1) усиление света в результате вынужденного излучения | a) this spectacular set of characteristics |
| 2) при облучении (когда кристалл облучается) | b) to generate coherent radiation |
| 3) в течение следующих двух десятилетий | c) the upper laser levels |
| 4) во много раз | d) can be varied |
| 5) чтобы получить (создать) когерентное излучение | e) in the ensuing two decades |
| 6) путем правильного выбора значения кинетической энергии | f) rather than with simple laser oscillators |
| 7) можно изменять | g) listed in this table |
| 8) предельные значения выходных параметров | h) light amplification by stimulated emission of radiation |
| 9) а не с простыми лазерными генераторами | i) the extrema of laser output parameters |
| 10) приведенные в данной таблице | j) when irradiated with |
| 11) этот впечатляющий (замечательный) набор характеристик | k) by properly choosing the kinetic energy |
| 12) верхние лазерные уровни | l) manifold |
4. Read Text 4A and answer the following questions:
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1) Какие методы используется для создания инверсной населенности?
2) Каким образом можно изменять длину волны излучения лазера на свободных электронах?
TEXT 4A TYPES AND COMPARISONS OF LASER SOURCES: INTRODUCTION
Light Amplification by Stimulated Emission of Radiation was first demonstrated by Maiman in I960, the result of a population inversion produced between energy levels of chromium ions in a ruby crystal when irradiated with a xenon flashlamp. In the ensuing two decades population inversion and coherent emission have been generated in literally thousands of substances (neutral and ionized gasses, liquids, and solids) using a variety of excitation techniques (optical pumping, electrical discharge, gasdynamic-flow, electron beam, chemical reaction, nuclear decay).
The number and types of laser sources has been further expanded manifold by utilizing one laser source (primary) to generate coherent radiation in a second medium, either by optically producing a population inversion in the second medium or as the result of nonlinear scattering in the second substance. Recently, laser action has even been achieved by passing a dilute electron beam through a periodic magnetic field (free-electron laser, or FEL). By properly choosing the kinetic energy of the electron beam and the periodicity of the magnetic field, the output wavelength of the FEL can be varied, in principle, from the ultraviolet to the far infrared spectral region.
The extrema of laser output parameters which have been demonstrated to date, and the laser media used are summarized in Table I. Note that the extreme power and energy parameters listed in this table were attained with laser systems (such as a master-oscillator-power-amplifier[1], or MOPА system) rather than with simple laser oscillators.
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Table 1 Extrema of Output Parаmеters of Laser Devices and Systems
| Parameter | Value | Laser medium |
| Peak power | 2x1013W (collimated) | Nd:glass |
| Peak power density | 1018 W/cm2 (focused) | Nd:glass |
| Pulse energy | >104J | CO2, Nd:glass |
| Average power | 105W | CO2 |
| Pulse duration | 3x10-13sec, cw[2] | Rh6G dye, various gasses, liquids, solids |
| Wavelength | 60nm ↔385nm | many required |
| Efficiency (nonlaser-pumped) | 70% | CO2 |
| Beam quality | diffraction limited | various gasses, liquids, solids |
| Spectral linewidth | 20Hz (for 10-1sec) | neon-helium |
| Spatial coherence | 10m | ruby |
To be sure[3], no single laser source can simultaneously provide this spectacular set of characteristics. Each laser gain medium possesses a unique ensemble of energy levels (electronic, vibrational, rotational), which are dynamically coupled to each other through various radiative and nonradiative processes. These structural and kinetic features determine laser's nominal operating wavelength(s), its spectral tuning range, its possible output waveforms, and its energy and power scalability. Laser efficiency is determined by the degree to which appropriate pump excitation energy can be generated, fed selectively into the upper laser level(s), and subsequently extracted coherently before deleterious[4] decay processes otherwise remove this excitation energy. It is the very richness of energy level schemes and transition probabilities provided in nature that results in such a large number of lasers with such a wide variety of output characteristics.
Given the considerable diversity in laser properties,it is the purpose of this introductory section to order laser sources into basic classes and to describe the principle characteristics that define the classes and their subdivisions.
3400 п. зн.
Words to be learnt:
to summarize – суммировать, подводить итоги;
to couple – соединять, сцеплять;
recently – недавно;
to date – до сих пор, до настоящего времени;
otherwise – иначе, в противном случае;
to feed (fed - fed) – подавать, питать, снабжать;
to extract – извлекать, удалять.
Exercises
1. In each group find the word that doesn’t belong:
a) minimum, maximum, datum, phenomenon, medium, extrema, spectrum;
b) coherent, neutral, efficiency, gasdynamic, nuclear, single, kinetic;
c) ultraviolet, unique, various, possible, incoherent, spectacular, infrared, optical.
2. Find a synonym for each verb below:
a) produce, enumerate, possess, use, expand, remove, link, achieve, vary, give;
b) extract, enlarge, provide, change, attain, utilize, own, list, generate, couple.
1. Complete the sentences below with the appropriate word or word-combination:
1) The output wavelength of the FEL can be varied from the ultraviolet to the far infrared spectral region by...
a) utilizing one laser source to generate coherent radiation in a second medium;
b) properly choosing the kinetic energy of the electron beam and the periodicity of the magnetic field.
2) The extreme power and energy parameters were attained with...
a) simple laser oscillators;
b) laser systems rather than with simple laser oscillators.
4. Point out the statements which do not correspond to Text 4A:
1) A single laser source can simultaneously provide a spectacular set of characteristics.
2) Light amplification by stimulated emission of radiation was first demonstrated in 1970.
3) Recently laser action has been achieved by passing a dilute electron beam through a periodic magnetic field.
5. Translate the sentences below focusing on the underlined words:
1) As the result of nonlinear scattering in the second substance the number of laser sources has been expanded. 2) The richness of energy level schemes results in a large number of lasers with a wide variety of output characteristics. 3) Light amplification by stimulated emission of radiation was the result of a population inversion produced between energy levels of chromium ions in a ruby crystal. 4) The purpose of this introductory section is to order laser sources into basic classes. 5) One laser source was utilized in order to generate coherent radiation in a second medium. 6) Table I provides the list of the extrema of laser output parameters. 7) The parameters listed in the table were attained with laser systems rather than with simple laser oscillators.
6. In each sentence below find the Subject and Predicate groups. Translate the sentences:
1) Solid state semiconductor laser materials exhibit both high heat capacities and thermal conductivities. 2) To extend the average power output substantially beyond these levels appeal is made to laser – diode arrays (линейки лазерных диодов). 3) Reference 2 is cited in the table as a key literature source dealing with lasers used to illustrate various classes and types of lasers. 4) The costs of lasers and laser systems vary widely and cannot be readily generalized. 5) The major alternative to optical pumping by incoherent sources is pumping by another laser. 6) Excitation into any of these levels decays rapidly down by nonradiative processes because of the relatively small energy gaps (energy gap – энергетическая зона) between various levels. 7) In Table 1 cw stands for continuous wave operation.
7. Answer the questions about the text:
1) Who was the first to demonstrate Light Amplification by Stimulated Emission of Radiation? 2) In what substances were population inversions and coherent emission generated? 3) What excitation techniques are used to generate coherent emission? 4) What method of generating coherent radiation resulted in the expanding of the number and types of laser sources? 5) Which is a better way of attaining the extreme power and energy parameters: using laser systems or simple laser oscillators? 6) What determines laser efficiency?
8. Write an abstract of Text 4A.
9. Read Text 4 B without a dictionary and answer the question:
Каковы преимущества лазера на алюмоиттриевом гранате, активированном неодимом (Nd: YAG laser) перед лазером на рубине?
Text 4B Nd: YAG Laser vs. Ruby Laser
The Cr3+ iron-group ion doped (to dope – добавлять) in Al2O3 is the medium in which laser operation was first demonstrated by Maiman in 1960. Cr: Al2O3 or ruby operates as a three-level system and thus, per unit volume, has a comparatively high threshold (порог). Fortunately, the thermal conductivity and mechanical strength of Al2O3 are both high, superior to any other existing laser host (основа, матрица) crystal, and thus successful operation of the ruby laser is possible. For all but (кроме) a few specialized applications the much-lower-threshold, higher-average-power-output Nd: YAG laser has replaced the ruby laser, however. Efficient frequency-doubling (удвоение частоты) techniques for 1.06ηm radiation have in mаnу cases eliminated the need for 0.69 ηm ruby laser where visible radiation is required.
850 п. зн.
10. Translate Text 4C in writing using a dictionary (time limit 30 min.):
TEXT 4С FREE ELEGTHON LASER
In the Free Electron Laser (FEL) gain is generated by the interaction of photons with an electron beam. A freely propagating electron does not interact with an electromagnetic field. To obtain gain the electrons and photons must interact within a perturbing environment that permits the simultaneous conservation of energy and momentum; spontaneous emission from the electron is then possible. The synchrotron radiation that occurs when the trajectory of a high energy electron is bent by a magnetic field is an example of one such process.
The process that generates gain may be viewed as stimulated scattering, as stimulated “free-free” transitions between continuous states of the perturbed electron-photon system, or as the inverse of the interaction that accelerates electrons in an accelerator. If the velocity distribution of the electrons in the beam is carefully selected, the radiation emitted by each electron adds coherently to the radiation from other electrons in the beam. The wavelength of maximum gain is primarily a function of the energy of the beam. With a minimum of constraints, the operation of an FEL should be possible at any wavelength from millimeter wavelengths into the visible and near ultraviolet.
1300 п. зн.
SUPPLEMENTARY READING TASKS
Nm Fiber Laser Source
With the continued interest in development of solid-state blue laser sources we would like to show thatfiber lasers and nonlinear frequency conversion are an attractive approach. Fiber sources are a good choice for nonlinearfrequency conversion because of their good beam quality and high brightness. Using non-critical phase matchingeliminates the problems of spatial walk off allowing for longer interaction lengths and this leads to higher conversionefficiency.
Our fiber amplifier uses the 4F3/2 - 4I9/2 transition in neodymium and because of the 3-level nature of the transition there is strong competition from the 4F 3/2 - 4I11/2 4-level transition. Optical fiber hosts have the advantage of wavelength selective loss dependent on bend diameter allowing the user to choose a fiber coil diameter to act as a variable short pass filter. In our case we were able to choose a coil diameter that will generate ~10dB of loss for the competing 4 level 1088 nm parasitic transition while generating very little loss at 938 nm.
High power levels have been achieved for this Neodymium transition in crystal hosts; however to our knowledge this is the highest power achieved for this transition in a silica fiber host. The silica host offers a broader absorption spectrum reducing the precision requirements of the pump and a broader emission spectrum (900nm to 950nm) enabling more applications. We have previously reported multi-watt operation on this transition and continue investigating power scalability.
While the idea of quasi-phase matching has been around for a long time engineered nonlinear materials are starting to gain maturity and are commonly used for nonlinear frequency conversion. A lot of progress has been made in both materials and periodic structure fabrication in recent years. Fabricating the short periods required for first order frequency doubling into the blue still remains challenging. Because of its anisotropic lattice structure KTiOPO4 (KTP) exhibits very limited domain wall spreading during the poling process leading to the ability to pole very short domain periods. Also the KTP has a coercive voltage about 10 times lower than congruent LiNbO3 enabling electric field poling of thicker materials.
(Alex Drobshoff, Jay W. Dawson, Deanna M. Pennington, Stephen A. Payne, Raymond Beach,
Lawrence Livermore National Laboratory, PO Box 808, Livermore, CA 94551; Luke Taylor, European Southern Observatory, Karlschwartzchild Strasse 2, 85748 Garching-bei-Muenchen; http://www.osti.gov/energycitations/index.jsp)
Good Fundamentals
Stephen Matthews
For most applications, the size to which a laser beam can be focused is as important a consideration as the laser output power. Frequency doubling, for example, depends on the square of the intensity of the primary laser. The depth of a hole drilled by an industrial laser depends on the laser intensity and the hole diameter is proportional to the spot size.
A beam profile composed entirely of five higher-order modes can look like a TEM00 beam to instruments that measure beam diameter. The figure on the left is the highest order mode in the beam, TEM21. The figure on the right is the apparent profile of the composite beam, which contains no TEM00 component.
Maintaining a consistent beam profile is usually important whether the beam is focused or not. Ophthalmic surgery uses a beam with a flat cross section (a "top hat" profile) that must remain constant during the procedure. All of these applications require a laser designed to produce a consistent and well-characterized beam.To be propagated over a long distance, a laser beam needs to have the lowest divergence possible. Telecommunications combines this requirement with a need to control the spectral content of the beam to ensure data quality. Whenever low divergence or small spot size is required, a laser with TEM00 output is specified.
What is TEM00?
It is useful to think of the light inside of a laser as formed of standing waves with distinct vibrational modes. Only a small number of modes will exist in the transverse direction. The fundamental transverse mode is designated as TEM00, where the "00" indicates no nodes appear in the beam profile. "TEM" stands for "transverse electromagnetic" and refers to the form of the standing waves. The TEM00 mode is mathematically described by the familiar bell-shaped Gaussian curve.
Higher-order modes are formed by multiplying the Gaussian by a polynomial with an exponent that corresponds to the order of the laser mode. These higher-order modes describe the number of nodes that appear in the beam—the TEM11 mode of a rectangular resonator, for example, will appear to have a dark cross in the middle of the profile. Higher-order modes add frequency components to the fundamental mode.
The Gaussian function extends to infinity in the radial direction, leaving open the question of the beam diameter. Measuring a laser beam diameter has been compared to using calipers to measure the width of a cotton ball. The accepted definition is the diameter at which the intensity has fallen to 1/e2 (13.5%) of its peak value in the center.
The 1/e2 definition works well for Gaussian modes, but is not useful for other profiles. In these circumstances the diameter is calculated using the "second moment" algorithm, a combination of integrals similar to a formula for calculating an rms (root-mean-square) value. The second-moment calculation should be used cautiously because it gives heavy weight to the edges of the beam.
Measuring beam size
Early means of determining a profile were essentially visual, such as examining the pattern of a continuous-wave (CW) beam on a lab wall or the burn marks made by a pulsed infrared beam on photographic film. It is an indication of the difficulty in measuring high-power pulses that visual techniques are still used. Instruments that measure beam profiles (profilometers) either use CCD cameras, or else scan a slit or knife-edge through the beam.
A CCD camera is a user-friendly system capable of instantly displaying the entire beam profile. It can be used with both CW and pulsed beams. The intensity distribution of the profile can be displayed as either a two-dimensional (2-D) or three-dimensional (3-D) contour plot. Charge-coupled-device cameras are superior for measuring elliptical beams, and their real-time capability is useful in production control (see Fig. 1).
FIGURE 1. Profilometers based on CCD arrays can provide real-time displays of 3-D beam profiles.
The limitation of this instrument is its resolution, set by the pixel size of the CCD array. Currently this can be as small as 10 µm, but a pixel size closer to 20 µm is more typical. In addition, most beams must be attenuated to avoid saturating the array, and the attenuating element introduces some degree of distortion, although a new CCD array using a diamond substrate appears robust enough to measure short-wavelength pulses directly (see Laser Focus World, May 2000, p. 265). Finally, a CCD camera is not the first choice if second-moment calculations are important—the signal-to-noise ratio of the CCD array decreases at its edges.
Scanning profilometers
These instruments scan a slit or a knife-edge through the beam and correlate the measurement from a detector behind the aperture with the aperture position. Different detectors can be positioned to allow these instruments to work at almost all wavelengths. Resolution, which is limited by diffraction from the scanning edge, is on the order of the wavelength of the beam.
A scanning slit masks most of the beam from the detector, eliminating the need to attenuate the beam. It is important to choose the correct slit size for the beam diameter—a slit too wide will make the measurement appear smaller than the beam itself. The slit should be no wider than one-third of the beam, and preferably narrower.
Knife-edge profilometers have a resolution as fine as 100 nm. As the blade moves across the beam, the detected signal decreases to zero and the measurement is differentiated to obtain the profile. Some systems use the same algorithm as that used in medical tomography for MRI and CAT scans to calculate the profile. However, scanning systems are not useful for pulsed measurements.
Whatever instrument is used, the beam should be measured at a distance from the laser sufficient to allow spontaneous emission and other light noise to diverge and not pollute the measurement. Lasers that produce the profile for which they are designed, free of aberration and the like, are said to be "diffraction limited." This does not mean, however, that their output is TEM00.
Raleigh range and divergence
Gaussian wavefronts start out as planes at a location called the "beam waist" (sometimes located inside of the resonator). The wavefronts become increasingly curved as they propagate from the waist until they reach their smallest radius, after which they flatten out. The distance from the waist to the location at which the wavefront is most curved is called the Rayleigh range.
The region between the beam waist and the Rayleigh range is the near field. In the far field the beam diverges in a cone with (nearly) straight sides. Divergence is always specified in the far field, which is usually chosen to begin around 10 times the Rayleigh range.
The distance from the laser to the far field can be meters, an inconvenient distance for measurement, so a lens is often used to focus the beam, thereby forming a new beam waist. The divergence is then the beam size at the lens divided by the distance from the lens to the focus. It is important that divergence is measured in the far field, or calculations for beam parameters will be incorrect.
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.
| MODULE 5 CLASSES OF LASER SOURCES Texts: A. Classes of Laser Sources B. Semiconductor Lasers C. Glass Lasers D. X-Ray Lasers |
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 5 A or in the right-hand column:
| 1) состояние активной среды | a) electron beam excitation |
| 2) различающиеся по лазерному действию | b) specific types of lattice |
| 3) разнообразные способы возбуждение | c) differentiated by laser action |
| 4) возбуждение пучка электронов | d) solid state lasers have been developed |
| 5) вид используемого твердого вещества | e) a wide variety of excitation methods |
| 6) были созданы твердотельные лазеры | f) state of the active medium |
| 7) особые виды дефектов решетки | g) the type of solid used |
3. 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, dc[5] or rf[6], 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.
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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
| class | types of laser medium | method of pumping |
| 1. _________ | 1. _________ 2. _________ 3. _________ 4. _________ 5. _________ 6. _________ | 1. _____________ a) ____b)____c)____d)____e)____ 2. _____________ 3. ____________ 4. _____________ 5. ____________ |
| 2. _________ | 1. _________________ 2. _________________ 3. _________________ | 1. _________________________ 2. _________________________ 3. _________________________ |
|
3. _________ | 1. ___________________ a)_______b)_______c)_______ | 1. ___________________________ 2. ___________________________ 3. ___________________________ 4. |
| 2. ___________________ | 1. ___________________________ 2. ___________________________ 3. |
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 уровняна 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:
practical, photon, absorption, phonon, valence, vector, schematic, diagram, variation, technological, interest, radiative, coefficient, potential.
1. Combine the appropriate words from the two columns to obtain terms. Translate them:
| 1) radiative | 4) valence | a) gap | d) lifetime |
| 2) quantum | 5) energy | b) transition | e) carrier |
| 3) excess | 6) band-to-band | c) efficiency | f) band |
TEXT 5B SEMICONDUCTOR LASERS
Introduction. Semiconductor lasers consists of injection lasers, where a p-n junction or heterojunction is used to inject excess carriers into the active region, optically pumped lasers, where an external light source produces excess carriers, and electron-beam pumped lasers, which use high energy electrons to produce excess carriers. Injection lasers, which are the most practical devices, are discussed at length[7] in this review.
Operating principles. In this section, we review a few of the key concepts concerning laser action in semiconductors. Extensive theoretical treatments of this subject can be found elsewhere[8].
Direct and indirect bandgap semiconductors. In direct bandgap semiconductors (the only ones in which stimulated emission has been observed), both photon emission and absorption can occur without the need for a phonon to conserve momentum. This is because the lowest conduction band minimum and the highest valence band maximum are at the same vector (k) in the Brillouin zone[9]. Figure I shows the schematic diagram of electron energy vs. k in a semiconductor, such as GaAs, where the smallest bandgap energy Eg = Ec – Ev at k = [000].
In indirect bandgap semiconductors the conduction band minimum and valence band maximum are not at the same k value. Hence, photon emission and absorption require the participation of phonons to conserve momentum. A schematic diagram of an indirect bandgap semiconductor such as GaP or AlAs is shown in Figure 2. In these semiconductors the lowest-lying conduction band minima are along k= [100].
Lasing in indirect bandgap semiconductors is improbable because the lowest-energy band-to-band transition probabilities are much smaller than in direct semiconductors. Thus, the radiative lifetime is long. Because of the relatively long lifetime of electrons in the indirect minima, there is time for nonradiative recombination processes to occur, thus yielding low internal quantum efficiency. Furthermore, the stimulated recombination rate is related to the band-to-band absorption coefficient. Since the coefficient is lower for indirect than direct transitions, the potential laser gain is correspondingly reduced.
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Words to be learnt:
key concepts – основные понятия;
to treat – трактовать, рассматривать;
treatment – трактовка;
to yield – производить, приносить результаты, давать.
Exercises
- Read the following nouns. Say which verbs they are derived from:
conductor, semiconductor, conduction, conductivity; injection, emission, absorption, participation, composition, treatment, transition, recombination.
- Match synonyms:
absorb, conserve, combine, occur, consume, watch, mix, observe, preserve, happen.
- Translate the adjectives below paying attention to the negative prefixes:
direct – indirect, efficient – inefficient, convenient – inconvenient; probable – improbable, practical – impractical, possible – impossible; radiative – nonradiative, nuclear – nonnuclear, conducting – nonconducting; continuous – disсontinuous; fortunate – unfortunate; comfortable – uncomfortable.
- Translate the following word combinations avoiding prepositions:
внешний источник света, лазеры с накачкой электронным пучком, излучение и поглощение фотонов, зона проводимости, вероятность перехода, переход с уровня на уровень, вероятность перехода с уровня на уровень, коэффициент поглощения.
- Which of the statements below are true according to Text 5B:
1) In injection lasers high energy electrons are used to produce excess carriers. 2) In indirect band gap semiconductors both photon emission and absorption can occur without the need for a phonon to conserve momentum. 3) Lasing in indirect bandgap semiconductors is improbable because the lowest energy band-to-band transition probabilities are much smaller than in direct semiconductors. 4) In direct bandgap semiconductors the conduction band minimum and valence band maximum are not at the same k value.
- Translate the sentences below paying attention to the functions of the words which are underlined:
1) The total amount of radiation absorbed from broadband pump sources clearly increases with ion concentration in a given size host crystal. 2) The growth of the density modulation gives increasing coherence to the scattering process resulting in a growing scattered wave, which in turn increases the density modulation still further. 3) Laser diodes’ degradation manifests itself primarily in an increase in threshold current although other parameters may also change. 4) Increasing the peak-power output of a laser isconstrained by the optical damage[10] properties of the laser medium itself or of the optical materials required to make the laser operate. 5)Laser - pumped glass oscillators provided wavelength versatility[11] because of their wide fluorescence bandwidth. 6) Provided the velocity distribution of the electrons in the beam is carefully selected, the radiation emitted by each electron adds coherently to the radiation from other electrons in the beam.
- Translate the sentences below with a special attention to the Verbals:
1) If the electron velocity is close to the speed of light, long wavelength imposed fields can be used to build FELs operating in the visible region of the spectrum. 2) As large-scale commercial applications of lasers become more numerous and mature[12], additional cost scaling models and data bases are sure to become available in the field. 3) By varying the composition of a semiconductor diode it is possible to adjust the wavelength of its spectral gain peak. 4) The purpose of these dye absorption curves is to assist the user in selecting the laser pump source which will most effectively pump the dye laser. 5) Several molecular lasers should be mentioned when discussing tunable lasers. 6) When placed in a suitable cavity[13], the device (FEL) will radiate coherently. 7) After the discovery of the dye laser by Sorokin and Landkard, numerous reports followed, most of them detailing the study of various classes of fluorescent organic materials.
- Answer the questions about Texts 5A and 5B:
1) What are the classes of laser sources? 2) What are the types of gas lasers? 3) What excitation methods are used to pump gas lasers? 4) What are the types and methods of pumping liquid lasers? 5) What excitation techniques are used to pump lasers utilizing dielectric insulators? 6) In what way are excess carriers produced in semiconductor lasers (injection lasers, optically pumped lasers, electron - beam pumped lasers)? 7) What is the difference between direct bandgap semiconductors and indirect bandgap semiconductors? 8) In what type of semiconductors do photon emission and absorption require the participation of phonons to conserve momentum? Why? 9) Why is lasing in indirect bandgap semiconductors improbable?
- Write an abstract of Texts 5A and 5B.
- Use Table 2 and Figures 1 and 2 to talk about:
a) Types of lasers; b) Direct bandgap semiconductors vs. indirect bandgap semiconductors.
- Read Text 5C without a dictionary (time limit – 4 minutes) and answer the questions that follow:
a) Каковы преимущества стекла перед кристаллическими материалами?
b) О каком недостатке стекла упоминается в этом тексте?
TEXT 5C GLASS LASERS
Lasers made from vitreous[14] and crystalline materials comprise the two classes of solid state lasers. Their different material properties are complementary for use in lasers. Because of their lower cross sections, glass lasers store energy well and thus make good short pulse lasers and amplifiers. On the other hand, crystalline materials are better for cw oscillators and amplifiers because of their higher gain and good thermal conductivity.
Glass has advantages over crystalline materials. It can be саst[15] in a variety of forms and sizes, from small fibers to meter–sized pieces. Tremendous flexibility in choosing glass and laser properties is afforded by the ability to vary the glass composition over very large ranges. Glass is also relatively inexpensive because of the shorter time required for its manufacture and the use of inexpensive chemical components. Further, large pieces of laser glass can be made with excellent homogeneity, uniformly distributed rare earth concentrations, low birefringence, and can be finished[16] easily, even in large sizes. The only major drawback of glass is its low thermal conductivity, which limits its applicability in high average power systems.
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- Translate Text 5D in writing using a dictionary (time limit – 40 minutes):
TEXT 5D X - RAY LASERS
Research toward advancing lasing to the X-ray spectral regions is in an early and progressive state.
The challenge of inventing and developing X-ray lasers may be approached by a) adapting familiar X-ray sources to lasing action; b) extending proven ion laser processes progressively toward shorter wavelengths, perhaps through isoelectronic extrapolation; c) discovering new pumping and emission processes more appropriate to the task.
With potential applications in the vacuum-ultraviolet spectral region seemingly limited as compared to those for the penetrating X-ray region, early thoughts were directed toward making the big leap to the X-ray and perhaps γ-ray regions. Formidable pumping problems were projected. Meanwhile advancements into the ultraviolet regions, accompanied by rising uses and interests as specific devices have emerged, seem to indicate that the more reasonable approach is the continued systematic advance toward shorter wavelengths. Indeed, over the past 12 years the so-called short-wavelength “barrier” has been pushed from 200 nm into the vacuum region - first near 100 nm, and presently it appears that 60 nm has been reached. These advances have been achieved both with cavities and in the amplified spontaneous emission (ASE) single pass mode, where the latter requires considerably higher gain.
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SUPPLEMENTARY READING
Irnee D'Haenens dies; assisted Maiman in building the first laser
January 4, 2008, Los Angeles, CA--Irnee D'Haenens, a physicist who assisted Ted Maiman in making the first laser at Hughes Research Laboratory (Malibu, CA) in 1960, died December 24; he was 73. The two were the only people present when a little ruby rod emitted the world's first pulse of laser light on May 16, 1960. Later, D'Haenens called the laser "a solution looking for a problem," a joke that became common in the early years of the laser era as developers sought laser applications.
Born in Mishawaka, Indiana, the son of a service-station operator, D'Haenens spent his entire professional career at Hughes, starting while he was earning a masters degree from the University of Southern California. He received a Hughes doctoral fellowship and earned his PhD from the University of Notre Dame in 1966. As a member of the technical staff at Hughes, he worked on semiconductor physics, microwave technology, and spectroscopy as well as lasers before retiring in 1989. A long-time Hughes colleague, David Pepper, recalled D'Haenens as "as a wise and learned uncle who helped me travel along my path in life," whose first priority was always his family. He is survived by his wife Shirley, four children, 19 grandchildren, and three great-grandchildren.
(http://www.laserfocusworld.com)
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