Gas-dynamic laser enters pulse-periodic mode
A group of laser researchers led by Victor Apollonov at the Russian Academy of Sciences (RAS; Moscow) has developed a modification to high-power wide-aperture gas lasers that allows emission in a high-frequency pulse-periodic mode in which very short pulses are produced at a high rate without a sacrifice in average power. The improvement can be made to gas-dynamic lasers, hydrogen fluoride/deuterium fluoride chemical lasers, and chemical oxygen-iodine lasers. Potential uses include launching and propelling spacecraft with ground-based lasers.
At output powers exceeding several kilowatts, producing short pulses based on high-frequency resonator modulation runs into several problems, caused by the wide apertures of the resonator elements. Existing schemes for beam modulation, which include magnetic modulation of gain and physical chopping of the beam, all have problems that greatly reduce average power when compared with continuous-wave (CW) operation. 
FIGURE 1. An experimental 10-kW CW gas-dynamic laser is converted to a high-frequency pulse-periodic mode with pulses 0.1 to 1 µs in length, frequencies of 25 kHz or greater, and peak powers of 100 kW (top). A portion of the laser's beam is passed through a modulator and fed back into the laser, causing the output beam to become pulsed. A scaled-up version of this laser could propel a so-called Lightcraft into space. A small Lightcraft prototype is placed in its launcher by Tregenna Myrabo (bottom).
In the scheme developed by the RAS researchers, a portion of the laser's output is extracted from the resonator, modified spatially and temporally, and then returned to the resonator (see figure). Injecting return light into the paraxial region of the resonator would require that the power of the injected beam be comparable with the output laser power to efficiently control the resonator of a continuously pumped laser – an impractical solution. Instead, the researchers inject the return light into the resonator periphery, resulting in a larger number of beam interactions within the resonator and thus good control with a smaller amount of return light.
Experimental results
To confirm theoretical calculations, an experiment was done on a carbon dioxide (CO2) gas-dynamic laser with a typical optical output of 50 kW. (The gas-dynamic laser is a powerful form of CO2 laser developed in the 1960s that also uses nitrogen and water vapor. It was and is used for military experiments such as the U.S. Air Force's Airborne Laser Laboratory, developed in the 1970s and 1980s to shoot down missiles.) The unstable resonator of the RAS laser consisted of two spherical mirrors with rectangular apertures and a geometrical amplification factor of 1.45. The laser gas flowed perpendicular to the resonator axis. In CW mode, the output was lowered to 10 kW to prevent damage to the mirrors. Because the test-bench components were uncooled, the laser was not operated for more than 3 seconds at a time. Full laser power was achieved after 0.3 seconds.
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About 20% of the laser output was diverted by an inclined metallic mirror to the injection-beam-formation system, which consisted of two spherical mirrors with conjugate focal planes and a modulator placed at the beam waist formed by the mirrors.
The modulator was a rotating metal disk with holes machined along its perimeter. The experiments used disks containing either 150 or 200 holes with respective diameters of 4 and 2 mm and a 0.5 filling factor. The maximum modulation frequency was 33 kHz. To measure temporal characteristics of the laser, the output beam was attenuated and allowed to strike a photodetector hooked up to an oscilloscope. Power measurements were done with a water-cooled calorimeter.
For a modulation frequency of about 27 kHz and a modulation depth (relative to the beam within the system, not the output beam) of 2% to 3%, the laser radiation exhibits intensity fluctuations in time with the modulating signal, with the peak output power departing from the average power value by a factor of three. When the modulation depth was increased to 7% to 8%, the laser shifted to the pulse-periodic operating mode. In this case, lasing took place in the form of a package of five to ten pulses within one cycle of the opened modulator state. The duration of an individual pulse was about 200 ns (recorded pulse durations were limited by the 50-MHz bandwidth of the photodetector electronics). The amplitudes of individual pulses exceeded the average value by factors of 6.5 to 11. Pulse-periodic modulation with a pulse length of 0.1 to 1 ms, a peak output power greater than 100 kW, and an average output power equal to the CW 10-kW power was experimentally obtained for the gas-dynamic laser.
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The experimental and theoretical data agreed well for frequencies ranging up to 30 kHz. It may be possible to increase the modulation frequency enough that a once-CW laser can be brought to the Q-switching regime, say the researchers.
Laser-propelled spacecraft
Because they are scalable to higher powers, pulse-periodic lasers may be useful for spacecraft propulsion. The Lightcraft, developed by Leik Myrabo of Lightcraft Technologies (Bennington, VT) and tested at White Sands Missile Range (White Sands, NM), is a craft that receives a ground-based laser beam, focusing it to create a detonating plasma from the air just behind it, propelling it upward (see Laser Focus World, September 2000, p. 29). A 10-kW CO2 laser pulsed at 28 Hz and with a pulse duration of a few microseconds has propelled a small Lightcraft to a height of 128 ft.
"Victor Apollonov's regenerative-amplifier gas-dynamic-laser experiments look very promising, and particularly so for applications that demand rapid scaling into the multimegawatt level, kilohertz pulse-repetition frequencies, and submicrosecond pulse durations—all attractive for the current laser Lightcraft engine design," says Myrabo. "Also, the physics appear to be well in hand with regard to realizing full theoretical efficiency from a 100 kW-class gas-dynamic laser. However, it should be noted that the demonstration of high beam quality with this setup has yet to be accomplished, nor has the power been extended up to the 100-kW level at the present time. This will require further development and funding equal to the task." For ambitious laser-propulsion projects such as this, Myrabo believes that government funding—perhaps by NASA—is the best approach. Apollonov notes that the RAS group is interested in investors in general for its pulse-periodic laser.
(John Wallace, Quantum Electronics 33(9) 753, http://www.laserfocusworld.com).
| MODULE 7 LASER OPERATION Texts: A. Laser Operation B. Stoichiometric Lasers C. Laser Safety |
Terminology:
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1) host crystal –матрица;
2) terminal state нижний рабочий уровень;
3) thermal excitation тепловое возбуждение;
4) phonon-assisted transition – переход с наличием фонона;
5) cavity – полость, резонатор; intracavity – кювет;
6) mirror-folded cavity design – зеркальная конструкция с изломом оси;
7) quasi continuum – квази-непрерывный;
8) coupler – выходной элемент связи;
9) flight time – время пролета;
10) losses – потери;
11) spatial coherence – пространственная когерентность.
Exercises
1. Read and translate the following words :
practice, factor, criteria (pl.) (sing. criterion), ignore, focus, horizontal, vibrational, vibronic, fluorescence, selective, microsecond.
2. Find definitions for the following concepts in Text 7A and translate them:
operation, combination, application, excitation, transition, absorption, condition, suggestion, radiation, inversion, solution, action.
3. Find equivalent phrases either in Text 4A or in the right-hand column:
| 1) ограниченное множество (ряд) | a) long compared to |
| 2) не в состоянии удовлетворить | b) with respect to |
| 3) лазер общего назначения | c) intracavity reflection losses |
| 4) идеально четырехуровневый | d) gain medium for amplification |
| 5) внутри резонаторные потери на отражение | e) continuously operating laser |
| 6) лазер с непрерывным излучением | f) high velocity jet |
| 7) долго по сравнению | g) fail to satisfy |
| 8) полностью определяется требованием | h) strict four - level |
| 9) струя, летящая с большой скоростью | i) limited variety |
| 10) среда для получения усиления | j) general purpose laser |
| 11) относительно чего-то | k) is entirely determined bуthe requirement |
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2. Read Text 7A and answer the following questions:
1) Какое условие является необходимым, чтобы отдельный ион или кристалл (матрица) действовали как активная среда? 3) Каковы рабочие характеристики лазера на красителе?
Text 7 A LASER OPERATION
General. A great many different combinations of transitions, paramagnetic ions, and host exhibited laser operation. In practice, however, only a limited variety of paramagnetic ion lasers are commonly used foreither research or industrial applications. Many factors determine whether or not a given ion and host will operate effectively as a laser, and many of the lasers fail to satisfy all the necessary criteria for practical systems.
Of major importance for any system is the need for a low pump-power or energy threshold for stimulated emission. It is desirable to have a minimum initial population of ions in the desired laser level or terminal state. In strict four-level laser this population is sufficiently small such that the effect on threshold of absorption from the lower state can be ignored. At the other extreme, in a three-level laser, the terminal level is the ground state of the ion and the threshold is almost entirely determined by the requirement that more ions be in the upper laser level than the ground state. Many paramagnetic-ion lasers fall in the region between the two extremes; in some cases laser operation may be four-level in nature at low temperature, where thermal excitation of the terminal level is negligible, but tend toward three-level operation at higher temperatures. For vibronic or phonon-assisted transitions, four-level operation is obtained between two electronic levels when vibronic absorption is negligible in the vibronic emission region. This condition is achieved when the difference between zero-phonon-line energy[20] and the desired emission energy is large compared to the thermal energy.
The first suggestion that organic materials might be useful in laser applications was made by Rautian and Sobelman. After the discovery of the dye laser by Sorokin and Landkard, numerous reports followed which detailed the study of various classes of fluorescent organic materials. These lasers provide maximum user flexibility, producing cw or pulsed output across the visible spectrum and into the infrared. They are designed for convenient and reliable operation over a broad range of wavelengths and are found to be reliable sources of intense tunable laser light.
Let’s follow the excitation and relaxation processes associated with dye lasers on the General Purpose dye laser. It is a linear laser based on three mirror-folded cavity design. A fourth mirror focuses the incoming pump laser beam into a high velocity horizontal dye jet. This jet is placed at Brewster’s angle with respect to the dye laser beam to minimize intracavity reflection losses. Upon excitation with the intense pump beam, a population inversion between ground and first excited state of the complex, organic dye molecules is achieved. The dye solution then can act as a gain medium for amplification of the spontaneous emission (fluorescence) of dye molecules returning to the ground state. Since this relaxation occurs into the quasi continuum of ground state vibrational levels, the fluorescence has a continuous character and laser action can be obtained over a broad wavelength range, often 100 nm or more. By inserting a wavelength selective element in the large intracavity space near the output coupler the dye laser can be tuned over this wavelength range with linewidths down to a few GHz. The high velocity jet restricts the flight time of dye molecules through the active area to less than a microsecond, which is long compared to the fluorescence process but is short compared to other processes like phosphorescence that would reduce the dye laser efficiency.
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Words to be learnt:
desirable –желательный;
to insert – вставлять, включать, помещать;
initial – начальный, исходный;
negligible – незначительный, не принимаемый в расчет;
to restrict – ограничивать;
strict – строгий, точный;
to reduce – уменьшать, сокращать.
Exercises
1. In each group find the word that doesn’t belong:
a) tunability, mobility, availability, variety, impurity, probability, cavity, velocity;
b) effectively, entirely, sufficiently, relatively, efficiency, generally, commonly, practically;
c) initial, usual, electrical, negligible, horizontal, thermal, removal.
2. Match synonyms: act, gain, occur, large, rate, general, amplification, take place, broad, velocity, common;
and antonyms: low, simple, general, excitation, short, specific, initial, relaxation, high, long, complex, terminal.
3. Complete the sentences below with the appropriate word or word-combination from Text 7A:
1) There are a great number of substances, operating as a laser but only some of them are effectively used for ...
2) Therefore it is necessary for any system to have...
3) As for the four-level lasers the effect on the threshold of absorption from the lower state can be ignored because...
4) For vibronic transitions, 4-level operation is obtained when…
5) A population inversion in the dye laser is excited with...
6) Rautian and Sobelman were the first to suggest that...
7) The dye laser can be tuned over a wavelength range of I00nm or more by…
8) The flight time of dye molecules is restricted with...
4. Translate the sentences below focusing on the underlined words:
variety – разнообразие, ряд, множество; variation – изменение, перемена; to vary – изменять(ся), менять(ся); variable – переменная (величина), изменчивый; various (varied) – различный, разнообразный
1) Laser oscillation has been observed in a wide variety of gas systems. 2) Holes of various shapes can be cut by focused radiation of pulsed or cw lasers. 3) The ability to vary the lasing ion concentration over a wide range allows further optimization of a particular laser design. 4) Variations in the dye flow rate can cause variations in the output power. 5) The output from a radar receiver can be viewed as a random (случайный) process and the models of this process as random variables.
as - как, т.к., в качестве, по мере того, как; as well as - так же как, as well - также, тоже as for (to) - что касается
6) As for increasing the peak-power output of a laser, it is limited by the optical damage properties of the laser medium itself. 7) As shown in Figure 1 different tuning elements may be utilized in the general purpose dye laser. 8) In the dye laser both tuning elements are available as separate modules as well and can be installed in the laser at any time. 9) In order to achieve flashlamp pumping of organic dye lasers, the strictest attention must be paid to the attainment of very short pulses as well as determination of the actual limit.
5. Answer the questions about Text 7A:
1) Do many lasers satisfy all the necessary criteria for practical systems? 2) What condition is considered to be of great importance for any system? 3) Is the initial population of ions in the 4 and 3 - level lasers equal? 4) When is 4 - level operation for vibronic transitions obtained? 5) How is it possible to achieve such condition? 6) When was the study of organic materials useful for laser application begun? 7) What could you say about the dye laser performance?
6. Speak about dye laser structure and operation.
7. Write an abstract of Text 7A.
8. Read Text 7 В (time limit 2 min.) and answer the following question: Чем отличаются стоихиометрические лазеры от обычных твердотельных?
TEXT 7B STOICHIOMETRIC LASERS
A stoichiometric crystal laser is by definition a laser whose gain medium contains the lasing (активный) ion as an intrinsic constituent (неотъемлемая составная часть) of the insulating crystal lattice. In such laser crystals the active ion may be partially replaced by other ions; however the pure or truly stoichiometric form of such mixed crystal must have demonstrated laser action. Although not exactly synonymous, the term ‘high-concentration’ is often used to describe such lasers. In scientific literature these lasers are frequently referred to as ‘self-activated’. The major distinction to be made between this type of laser material and the more common solid state laser crystals developed earlier is that the active ions in the latter case occur in the lattice as imparities with concentrations generally less than a few percent. The first reported stoichiometric laser was HoF3, (Гольмий Фтор3). Interest in this field was stimulated in 1972-73 by the achievement of lasing in NdP3O14, (neodymium pentaphosphate). The significance of this development lies in the utilization of Nd, a lasing ion of great practical importance but whose concentration in earlier hosts had been severely limited, since then manу other stoichiometric laser crystals have been synthesized and the potential for future development seems very promising.
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9. Translate Text 6C in writing using a dictionary ( time limit 40 min.):
Text 7С LASER SAFETY
The health and safety hazards associated with the use of lasers are often broken into three general categories: laser radiation hazards, electrical hazards, and hazards from associated contaminants. This chapter is therefore divided into three sections which emphasize these three types of hazards.
The hazards from laser radiation are confined largely to the eye and, to a smaller extent, the skin. Few serious eye injuries due to lasers have been reported in the 18 years since the appearance of commercial devices. The accident rate is not that low because the ocular exposure limits are overly conservative; they are not. Instead, the possibility of accidental exposure of the eye to a collimated beam is extremely remote if a few rudimentary commonsense precautions are followed.
Electrical hazards so far have proven more serious. At least five laser workers have been electrocuted. Procedures for handling high voltages safely are to be found elsewhere.
Hazards from airborne contaminants, such as vaporized target materials, cryogenic fluids, noise and explosive mixtures are also of concern in some specialized applications and in some research laboratories. Some of the solvents used in dye solutions have the ability to carry their solutes through the skin and into the body chemistry.
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