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The output energy and/or power obtainable from a given laser medium are determined both by the microscopic properties of the gain medium and by its associated “scaling laws”¹⁷.

In general terms, a laser medium is said to be a "three-level laser system" when the lower laser level is the ground state of the system, the other two levels being the upper laser level and a higher-lying pump level; it is said to be a "four-level system" when the lower laser level is a level lying above the ground level of the system (usually with sufficient energy, so that it is thermally unoccupied).

The relaxation times of the upper and lower laser levels determine the basic modes of operation possible for the laser itself. If the relaxation time of the lower laser level is much
shorter than the upper laser level relaxation time (due to stimu­lated as well as spontaneous processes) then the laser may be operated in the steady state with a cw output. When the inverse relation between level relaxation times is obtained, cw opera­tion is precluded and self-terminated pulsed operation may occur.

A pulse-pumped laser medium is said to be an energy-storage medium when the lifetime of the upper laser level is much longer than the desired pulse duration of the output pulse. In this situation the upper laser level is able to integrate the power supplied by the pumping source. Stored energy can then be released in an output pulse using mode-locking, Q-switching, or cavity dumping techniques described above; alternatively, pump energy stored in a laser power amplifier can be released in an intense short pulse upon passing a weak short pulse from a master-oscillator through the power amplifier (MOPA).

The key microscopic (intrinsic) laser parameters of the gain medium are: nominal wavelength; stimulated emission cross-section; spectral gain-bandwidth and type of saturation (homogeneous/inhomogeneous); saturation fluence or flux; radiative and kinetic lifetimes of upper and lower laser levels; and the characteristic specific excitation parameters are population inversion density; small signal gain coeffici­ent; input and output power (energy) densities.

Spectral tunability is a particularly useful property of many laser sources. Semiconductor diode lasers, organic dye lasers and colour center lasers are particularly known for this property. The nominal spectral regions these types of lasers operate in are shown below. Using several different dye types and various solvents, the spectral region from 350 to 1000 nm can be spanned with tunable dye lasers. A single dye-solvent combination typically can be tuned several hundred wave numbers (cm^-1) away from the spectral peak of the gain curve. Best dye laser performance is currently achieved with the yellow – orange rhodamine dye. Power and energy availability tend to roll-off to¹⁸ the blue and to the red, also useful amounts of energy and power can be achieved in these spectral regions.

Semiconductor lasers of various types collectively span the spectral region from 330 nm to beyond 15nm. Depending on the type of diode tuning can be accomplished using an applied magnetic field, by changing the current passing through the diode, or by applying pressure to the diode .

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Words to be learnt:

to preclude – устранять, предотвращать;

to occur – иметь место, случаться;

to release – выпускать освобождать;

to span – охватывать; покрывать (пространство, промежуток времени).

Exercises

1. In each group find the word that doesn’t belong:

1. lower, upper_, smaller, power, bigger, hotter, shorter, higher;

2. associated, terminated, stored, precluded, released, unoccupied, described, tuned;

3. sufficient, different, coefficient, magnificent, fluorescent, efficient.

2. Find an antonym for each verb below in Text 6A:

lower level, longer than, output power, direct relation, occupied level, released energy, weak pulse, homogeneous satura­tion, above.

3. Complete the sentences below with the appropriate word or word-combination from Text 6A:

1) When the lower level is above the ground level of the system, a laser medium is said to be...

2) When the lifetime of the upper laser is much longer than the desired pulse duration of the output pulse, a laser me­dium is said to be...

1. When the relaxation time of the lower laser level is much longer than that of the upper level...

2. Tuning of semiconductor lasers can be accomplished by...

3. Semiconductor lasers, organic dye lasers and colour center lasers are famous for the property of...

4. Translate the sentences below focusing on the underlined words:

1) The term laser stands for light amplification stimulated emission of radiation. 2) There are many phenomena of the interaction of light with matter, which are readily described in terms of photon. 3) Modeling of continuous systems should be analyzed in terms of modified curves. 4) In broad terms it is found that optical threshold depends on the wavelength of the incident radiation. 5) Semiconductor lasers are usually diffe­rentiated in terms of the means by which the hole-electron pair population inversion is produced. 6) Laser sources are commonly classified in terms of the state of matter of the active medium. 7) Laser oscillation is marked by dramatic narrowing of the spectral and angular distribution of the spontaneous emission radiation. This statement was first made by Maiman in 1960. 8) The United States of America is a Federal Republic of 50 states done together by the pact of 1787. 9) As the engine has a mechanical compression it is capable of operating under static conditions.

5. Answer the following questions:

1) What determines the output energy? 2) What is the difference between a three-level laser system and a four-level laser one? 3) How does laser operation depend on the relaxation time? 4) In what case can self-terminated pulsed operation occur? 5) What laser property allows spanning the spectral region? 6) What types of lasers are known for this property? 7) What can a semiconductor laser be tuned by?

6. Write an abstract of Text 6 A

7. Read Text 6В (time limit 3-4 min.) and answer the following questions: Почему излучение лазера имеет высокую направленность?

TEXT 6B SOLDIERS IN LOCKSTEP¹⁹

То understand why light from the laser is so concentrated, you must recall that light travels in waves_, like ripples on a pond. The distance from the crest of one wave to the crest of the next is the wavelength. Ordinary white light is made up of many wavelengths travelling in every direction. This is known as incoherent light. Laser light, on the other hand, is coherent. It is essentially of one wavelength, with all the waves moving in one direction. Because the laser wavelengths reinforce each other, like soldiers marching in lockstep, they can remain in an unbelievably straight narrow beam for long distances instead of fanning out like a flashlight beam. Almost any substance can be forced to “lase” if you work hard with it. Gas lasers give off continuous beams of laser light, in contrast to the sharp pul­ses of the ruby laser. Tiny semiconductor lasers made of bits of such materials as gallium arsenide work best at ultra-cold tem­peratures. Many lasers give off invisible radiation, either in­frared or ultraviolet. The carbon-dioxide laser, one of the most powerful yet invented, shoots a continuous beam of intensely hot but invisible infrared light.

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8. Translate Text 6C in writing using a dictionary (time limit 50 min.):

TEXT 6C AVERAGE POWER SCALING

Increasing the average power output of a laser as it is made bigger is determined primarily by the rate at which waste heat generated in the laser process can be removed from the laser medium and/or the active volume enclosed by the optical resonator. In average power producing lasers of practical in­terest, removal of waste heat is accomplished by either convection or conduction, the choice depending on the class of laser medium involved. For both gaseous and liquid laser media, scaling to high average power is achieved using convective flow of the waste heat (and spent laser medium) out of the ac­tive volume defined by the laser resonator. In the case of gas lasers, the flow may be supersonic (as in the C0₂ gasdynamic laser which has resulted in the highest average output power yet achieved) or it may be subsonic. In the case of liquid dye lasers, significant average power has been obtained using a con­fined transverse flow of the organic dye laser medium through the optically pumped laser volume, as well as by using a free - flowing transverse jet stream.

In the case of solid state lasers, the laser medium it­self cannot be rapidly and continuously moved through the vo­lume of space defined by the laser resonator and the cooling of the laser medium must be accomplished by conduction of waste heat to an exterior surface. This surface can then be cooled using a gaseous or liquid cooling fluid flowing across it. Crystal­line materials generally exhibit relatively high thermal con­ductivities which are strongly temperature dependent compared to those of amorphous glasses which are essentially tempera­ture independent.

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SUPPLEMENTARY READING

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.

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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