Файл: Antsiferov V.V., Smirnov G.I. Physics of solid-state lasers (ISBN 1898326177) (CISP, 2005)(179s).pdf

Generation of powerful single-frequency giant radiation pulses

the first time for passive Q-modulation in a ruby laser [39], but because of low thermal and beam instability of the dye centres they were not efficient and have not been used widely. Only after developing F2– dye centres stable at room temperature in the crystals of lithium fluoride (F-2:LiF) [40] were they used as non-linear filters [49] and passive laser gates [42]. The availability of the crystals of lithium fluoride, the simple procedure used for radiation-induced colouring using γ –radiation 60Co, high heat conductivity and beam strength have stimulated the extensive use of these crystals as phototropic gates for the production of giant radiation pulses in Nd lasers.

The absorption cross-section of the phototropic F2–:LiF gate at a wavelength of 106 nm is 1.7 × 10 –17 cm–2, the relaxation time is 90 ns

[43].The heat conductivity coefficient is 0.1 W/cm K and the thermal strains do not exceed several per cent of the strains in the Nd:YAG crystals

[44].The measured absorption factor at the concentration of the ac-

tive centres of 2×10 16 cm–3, taking Frenel reflection into account, is 0.41 cm–1 [45]. The threshold of failure of the individual crystals is approximately 2 GW/cm2 at a pulse time of 20 ns [45], but in the majority of crystals the thresholds of failure are usually 4–5 times lower. Consequently, it is necessary to select crystals on the basis of this parameter. In addition, the large scatter of the impurity composition of the initial crystals of lithium fluoride makes it more difficult to obtain reproducible results.

The high heat conductivity of the F2–:LiF crystals makes it possible to obtain stable energy characteristics of the emission of giant pulses of Nd lasers at a repetition frequency of up to 100 Hz [46]. When using the F2–:LiF gates, the linear polarisation of the giant pulses may be achieved in the absence of a polariser in the laser resonator [43]. A review of the earlier studies into the passive laser gates with dye centres was published in [47].

3.4.1 Energy and spectral characteristics of radiation

The maximum energy of lasing of the giant pulses of laser radiation was obtained when optimising the transmission factors of the output mirror T2 (Fig. 3.8a) and the passive filter Tpf (Fig. 3.8a). The radiation energy in this case was measured for a single giant pulse up to the appearance of a second pulse whose lasing threshold was approximately 1.2 of the lasing threshold of the single pulse. Increasing pumping energy increased the number of giant pulses and the radiation energy increased in proportion to the radiation energy of the single pulse. At a high pumping energy, the total energy of the giant radiation pulses was of the order of the free lasing energy.

The dependences of the radiation energy of the giant pulses of the

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Fig. 3.8 Dependence of the density of lasing energy Eg/Vg (J/cm3) on the coefficients of transmission on the output mirror of resonator T2 (a) and transmission of the passive filter Tpf (b) at a constant pumping energy Ep = 12Et of giant radiation pulses for lasers: Nd:YAG (1), Nd:BLN (2) and Nd:Cr:GSGG (3). L = 40 cm. Generated volumes of active media Vg = 1.24 (YAG), 0.56 (BLN) and 0.5 cm3 (Cr:GSGG).

transmission factors of the output mirror of the resonator and the passive filter had maxima. An exception was a Nd:Cr:GSGG laser in which maxima of this type were not detected even at the maximum possible experimental values of the coefficients T2 = 0.96 and Tpf = 0.04. Because of the very low values of the lasing energy of radiation, the graphs do not give the energy dependences for the Nd:LNA lasers. At the optimum parameters of the resonator and the passive filter, the specific powers (Pg/Vg) of the giant radiation pulses in the lasers were: 80 (Nd:Cr:GSGG), 16 (Nd:BLN), 11 (Nd:YAG) and 0.5 MW/cm3 (Nd:LNA).

With increase of the length of the flat resonator, the radiation energy of the giant pulses increased linearly (Fig. 3.9). In the entire range of the variation of the wavelength of the resonator, the duration of the giant pulses, with a smooth symmetric shape, varied from 10 to 15 ns (Fig. 3.10a). When there was no selection of longitudinal modes, several longitudinal modes were included in lasing.

When the output mirror of the resonator was represented by a resonance reflector consisting of two sapphire substrates, stable single-frequency lasing of the giant pulses of radiation (Fig. 3.30b) was obtained in all media. In a complex dispersion resonator single-frequency lasing of giant pulses (Fig. 3.10b) with smooth retuning of the radiation wavelength was observed in the ranges: 0.2 (Nd:YAG), 0.3 (Nd:GSGG), 0.4 (Nd:BLN) and 0.5 nm (Nd:LNA).

3.5 SINGLE-FREQUENCY TUNABLE ALEXANDRITE LASER WITH PASSIVE Q-MODULATION

In alexandrite lasers, giant radiation pulses were produced in the regimes of active [48,49] and passive Q-modulation of the resonator [50–53].

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Fig. 3.9 Dependence of the relative radiation energy Eg (1) of the giant pulse on its duration τ (ns) (2) for neodymium lasers on resonator length L (m). Ep = 1.2Et.

Fig. 3.10 Parameters of the lasing of powerful single frequency neodymium lasers

20 ns/division; b) interferogram of the lasing spectrum, the range of dispersion of the interferometer 20 pm.

In the active Q-modulation regime, the efficiency of the selection of the longitudinal modes rapidly decreases and it is not possible to obtaine single-frequency lasing under the normal conditions. In Ref. 50, the passive Q-modulation of the resonator was carried out using a gate on Cr4+:Y2SiO2 crystal whose maximum of the absorption bands coincides with the maximum of gain of the alexandrite crystals. In the regime of the giant pulse with a duration of 70 ns, an energy of 20 mJ was obtained. Without the selection of the longitudinal modes of the flat resonator with a length of 30 cm, the width of the lasing spectrum of the laser on alexandrite in the Q- modulation regime was 2 nm.

For the passive duration of the resonator of the alexandrite laser, the authors of Ref. 51 used a gate based on the crystal lithium fluoride with thermally transformed F–3 dye centres (LiF:F–3). Narrow-band lasing with the spectrum 10 nm wide was obtained, the pulse time was 100 ns, retuning of the wavelength of lasing took place in the range 710–780 nm. The selection of the longitudinal modes and the retuning of the radiation wavelength in the laser with flat mirrors were carried

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out using dispersion prisms and a Fabry–Perot selector-etalon. In Ref. 52, similar passive gates were used to produce giant radiation pulses in an alexandrite laser with a duration of 800 ns, energy 1.5 mJ, the width of the lasing spectrum 5 × 10 –3 cm–1 . The selection of the modes in the laser with a hemispherical resonator was carried out in self-in- jection conditions using an additional passive resonator containing a diffraction grating and a Fabry–Perot etalon.

The F–3 dye centres in the LiF crystals were generated at high doses of γ –radiation 60Co (2 × 10 8 R) without special cooling. At room temperature, they have two wide absorption bands with maxima at 710 and 800 nm. The effect of powerful optical radiation on the short-wave absorption bands causes failure of the F–3 dye centres. They remain stable when the wavelength of laser radiation coincides with their long-wave absorption band. The low thermal stability of the F3– centres complicates their use as modulators of the Q-factor of laser resonators.

In thermal annealing lithium fluoride crystals with F–3 centres at a temperature 425 K new, thermally transformed (TT) F–3 centres appear with a wide absorption band with the maximum at a wavelength of 780 nm and a different vibron structure [54] in comparison with the conventional F–3 centres in cooling. The temperature of failure of the TT F–3 centres is 100 degrees higher than the temperature of thermal transformation of the F–3 centres (425 K), indicating the high thermal stability of the TT F–3 centres. The contrast between the optical density in the initial and clarified conditions in the case of the TT F–3 centres was almost an order of magnitude. Relatively high values of the concentration of TT F–3 centres in the LiF crystal and the transverse the cross-section of absorption result in a high absorption factor of ~2.5 cm–1. Consequently, it is possible to use passive gates because of their small thickness (3 ÷ 10 mm). The TT F–3 centres are characterised by high photoresistance. In the radiation of a passive gate with the laser radiation with the power density of 100 MW/cm2 at a wavelength of 800 nm, there were no significant changes in the concentration of the TT centres [54]. However, as in the case of conventional F–3 centres, the TT centres failed under the effect of laser radiation of a ruby laser (λ = 694 nm), indicating the presence in the centres of a second absorption band in this spectral range, as in the case of th F–3 centres.

3.5.1 Experimental equipment

The circuit of the alexandrite laser [53] is shown in Fig. 3.11. An alexandrite crystal (6) with a diameter of 5.5 and 85 mm long, with the ends cut under the Brewster angle, was used. The laser was pumped with a IFP– 800 lamp, the pumping pulse time was 0.25 ms, the crystal temperature 70 °C; this temperature was maintained using a thermostat pumping

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the solution of the KN-120 dye and cutting off the ultraviolet radiation of pumping below 300 nm. Part of this pumping radiation was converted by the dye into a rule absorption band of the alexandrite, increasing the efficiency of pumping [55].

The laser resonator was formed by flat dielectric mirrors: a nontransmitting mirror (2) (R = 99.5% at λ = 750 nm), and the output mirror

(8) (T = 50%), sprayed on wedge-shaped substrates. The Q-modulation of the laser was carried out by the passive gate (3) of the lithium fluoride crystal with thermally transformed F–3 centres. The longitudinal modes were separated by the diaphragm (7). The selection and smooth retuning of the lasing wavelength were carried out with an interference–polarisation filter (IPF) (4) with a thickness of 8 mm and a scanning Fabry–Perot interferometer (SFPI) (5), with a base of 3 mm, controlled in the automatic regime by the electronic unit (14).

The electronic unit (14) was controlled by a step motor, rotating the IPF around its axis with the accuracy to 1 angular minute, adjusted the mirror of the SFPI with respect to the angle using two piezoceramic columns with the accuracy to 1 angular second and maintained with the high accuracy with respect to time the base of the SFPI using a piezoceramic ring to which one of the mirrors of the interferometer of a He–Ne laser

(1) and feedback circuits (9–13) were bonded. The parasitic selection of the longitudinal modes in the resonator was completely eliminated: all the optical elements in the resonator were slightly wedge-shaped, the elements (3,4) were positioned under the Brewster angle, and the optical axis of the SFPI (5) deviated from the axis of the resonator by

Fig. 3.11 Diagram of a single-frequency tuneable aleksandrite laser with passive modulation of the key factor: 1) He–Ne-laser; 2,8) the mirrors of the resonator, dense and output, respectively; 3) passive gate on LiF crystal with TT F–3 dye centres; 4) interference–polarisation filter (IPF); 5) scanning Fabry–Perot interferometer (SFPI); 6) aleksandrite crystal, diameter 5.5 mm, length 80 mm; 7) diaphragm; 9,10) dense mirrors, λ = 6.33 nm; 11) interference filter, λ = 633 nm; 12) long focus lens; 13) photodiode with a crossed photocathode; 14) electronic unit of controlling SFPI and IPF.

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an angle of the order of 1 degree.

The spectrum of the giant pulse of radiation was recorded with a Fabry–Perot interferometer with the dispersion range of 8 pm and with a photographic camera. The region of retuning of the lasing wavelength was controlled using a spectrograph. The time characteristics of the giant pulse were investigated using a coaxial photodiode and a high-speed oscilloscope.

3.5.2 Spectral–time and energy parameters of lasing

The lasing wavelength of the alexandrite laser and, consequently, the base of the SFPI were stabilised on the wavelength of the single-fre- quency He–Ne laser, whose radiation passed through the resonator of the laser, the SFPI and, using two mirrors (9,10), the interference pattern from the SFPI was projected with a long-focus lens (12) on the photodiode (13) with a cross-shaped photocathode. A small part of the arc of one of the interference rings was focused in the cross of the photocathode. Smooth retuning of the lasing wavelength within in the limits of the transmission band of the SFPI was carried out by supplying stabilised voltage to the piezoceramic ring of the SFPI from the electronic unit (14). The wide-band retuning of the lasing wavelength of the alexandrite laser (Fig. 3.12a) was carried out using the electronic unit (14) by synchronous rotation of the IPF by the step motor and the smooth change of the base of the SFPI when supplying high-stability voltage.

The single-frequency lasing of the alexandrite laser (Fig. 3.12c) was obtained as a result of numerical optimisation (in a computer) of the transmission band of the IPF and the region of dispersion of the SFPI taking into account the length of the laser resonator and the absorption band of the passive gate at a wavelength of 760 nm.

Laser radiation at this wavelength was used previously by the authors of this book [51] for the two-photon spectroscopy of the beam of helium atoms at the 23S → 33D transition. Laser beam diagnostic makes it possible to take local measurements of the magnetic field and the spatial distribution of the field in high-temperature plasma utilising the Zeeman effect. The measurement accuracy is increased by eliminating the Doppler broadening of the spectral line of radiation of the beam atoms when using the method of two-photon spectroscopy [56].

The duration and energy of the giant pulse of the alexandrite laser depend greatly on the transmission factor of the passive gate and the resonator length. As shown previously (Fig. 1.33a) [55], at high pumping energy (≥ 500 J) the increase of the length of the flat resonator of the alexandrite laser from 0.4 to 1.5 m decreases the lasing energy by a factor of 4 or more times with increasing pumping energy. Therefore, in the experiments, the length of the flat resonator was selected mini-

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10 nm

Fig. 3.12 Parameters of lasing of the single-frequency tunable alexandrite laser with passive modulation of the Q-factor of the resonator by the LiF:F–3 crystal gate: a) sequence of spectrograms of the integration radiation spectrum, produced in STE- 1 spectrograph, illustrating the tuning range of the lasing wavelength; b) oscillogram of the intensite of radiation of the giant radiation pulse, scale 150 ns/div; c) interferogram of the spectrum of the lasing pulse, the dispersion range of the Fabry–Perot interferometer 8 pm.

mum possible (0.5 m) so that it was possible to install all the necessary elements of the dispersion resonator.

In contrast to the case of F–2 dye centres, used widely for the passive Q-modulation of the lasers on Nd ions (see previously), the passive gate on the F–3 centres proved to be highly critical to the value of the transmission factor. The range of the transmission factors at which it was possible to generate a single giant pulse per pumping pulse was very narrow and this greatly complicated practical operation with these gates.

For every pumping level it is necessary to select very carefully the optimum transmission factor of the passive gate. For this purpose, it is necessary to have a relatively wide set of such passive gates or repolish the existing passive gates. At a transmission factor of the passive gate of 65%, the duration of the giant pulse is of the order of 11 ns (Fig. 3.12b), the energy is 80 mJ.

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Chapter 4

Lasing of stable supershort radiation pulses in solid-state lasers

The active synchronisation of longitudinal modes and the lasing of supershort radiation pulses were obtained for the first time in a helium–neon laser [1]. The passive synchronisation of the longitudinal modes was observed for the first time in a ruby laser [2] and in Nd-doped glass lasers [3,4]. In Ref. 55, the authors proposed a fluctuation model of the formation of supershort radiation pulses (SRP).

The model is based on the assumption that from a large number of fluctuation intensity transients, existing in the resonator at the start of the amplification process, as a result of the non-linear effect of the saturating absorber, the maximum transient is separated and amplified, and others are suppressed. The quantitative theory, describing the process of lasing of the laser with an antireflection filter, was developed in Ref. 6, in which it was shown that regular pulsations of radiation intensity can form in the laser with a saturating absorber without external regular effects. In Ref. 7 it was reported that in the Sixties the minimum duration of the SRP was reduced by an order of magnitude every two years, whereas in the Seventies, seven years was already required for reducing the duration of the SRP by an order of magnitude. The approximate duration of the light pulses to the fundamental limit in a single light period can be expected when using the lasers on active media with very wide gain lines.

The supershort radiation pulses are used widely in various areas of science and technology: non-linear optics, the development of superstrong fields, examination of high-rate processes, optical information processing, communications, location, etc. [8, 9]. At present, extensive investigations are being carried out into various types of SRP generators, directed to: 1) improvement of the energy characteristics and peak power of the generated light pulses; 2) retention of their duration; 3) increase of the reproducibility of the parameters of the produced pulses; 4) lasing of

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SRP in a wide spectrum range; 5) improvement of the quality of pulses (absence of the time structure, photon emission and phase modulation); 6) operation with the repetition frequency of pulses in a wider frequency range; 7) improvement of the reliability and ease of operation of the SRP generators.

In comparison with other types of generators, the solid-state lasers have a number of important special features and advantages. They have good energy characteristics, wide gain bands, required for the formation of the SRP of picosecond and femtosecond duration, they are compact, reliable and easy to operate. In the solid-state lasers, it is possible to greatly amplify the produced SRP in the giant pulse regime. On the whole, the solid-state lasers represent a suitable basis for the development of advanced SRP generators. The examination of the lasing dynamics of solid-state lasers in the regime of passive synchronisation of the modes has been recently the subject of a large number of investigations (for example, [11–17]). A significant shortcoming of the solid-state lasers is the instability of the regime of passive synchronisation of the modes in which the shortest duration of the produced light pulses is obtained. The resultant instability is linked basically with the self-modulation of the Q-factor of the resonator, caused by the illumination of the saturating absorber.

The positive feedback, determined by clearing up, in addition to the formation of the SRP, leads to an avalanche-like increase of the intensity, averaged for the axial period of intensity, with the duration of approximately 10–100 ns. This results in the formation of a giant pulse and lasing is interrupted until the completion of the process of formation of the SRP. The duration and shape of the SRP depend greatly on the initial lasing conditions and can not be reproduced. The process of separation of the single pulse from the initial fluctuation field of the laser with a saturating absorber has been examined in Ref. 5.

In dye lasers, the instability of this type does not form because the positive feedback, determined by the illumination of the saturating absorber, is compensated by a negative feedback formed as a result of the saturation of the gain of the active medium. The negative feedback in this case operates efficiently because of the large cross-section of the working transition of the dyes.

The solid-state lasers have high energy characteristics of the generated pulses. The long lifetime of the inverse population of the upper working level of the solid-state active media and the possibility of operation in the giant pulse regime make it possible to enhance the formation of the SRP by a simpler method in comparison with that used in other types of lasers. The energy of the SRP of the solid-state lasers reaches the value of the order of a milliJoule, whereas in the dye lasers this energy

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