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

Physics of Solid-State Lasers

detected in every lasing peak (Fig. 1.27c). However, as a result of a large red displacement of the spectrum during the lasing process, determined by the heating of the crystal as a result of its low heat conductivity, this structure is completely masked in the integral spectrum (Fig. 1.28a and 1.29a), in contrast to lasing in the alexandrite laser. The displacement of the maximum of the lasing spectrum at 200 µs was approximately 5 nm (Fig. 1.27c). This corresponds to heating of the GSGG crystal by 40°C. In a prism dispersion resonator with an angular dispersion of 3 angular min/nm, the lasing spectrum narrowed down and stabilised in the range 1.09 nm (Fig. 1.27d). Lasing with similar parameters was also detected in other REGG crystals. The resultant ranges of rearrangement of the radiation wavelength of REGG lasers were: 740–840 nm (GSGG), 710–790 m (YSGG) and 760–810 nm (GSAG). In the absence of the selection of the longitudinal modes, the width of the integral spectrum of lasing of REGG laser and the position of its maximum depended both on pumping energy and crystal temperature. With doubling of pumping energy, the width of the integral radiation spectrum of the Cr:YSGG increased three times, and its maximum was displaced to the short-wave region of the spectrum (Fig. 1.28). When heating the Cr:YSGG crystal, the maximum of the integral spectrum of lasing and, correspondingly, the maximum of the gain line were displaced to the long-wave region with a mean rate of ~0.13 nm/deg, and its width slightly increased (Fig. 1.29).

Fig. 1.27 Parameters of lasing of TEMmnq modes of a Cr:GSGG laser (diameter 4 mm, length 90 mm) with flat mirrors (L = 1.3 m), Ep = 2Et; a) oscillogram of intensity of radiation, scale 40 µs/div; b) time evolvement of the distribution of radiation intensity in the near-zone; c,d) time evolvement of the lasing spectrum without (c) and with longitudinal mode selection (d).

3 8

Solid-state chromium lasers in free lasing regime

Fig. 1.28 a) spectrograms of the integral spectrum of lasing of TEMmnq modes of Cr:YSGG laser (diameter 4 mm, length 90 mm) with flat mirrors (L = 1.6 m) at constant temperature of the crystal (T = 20 °C) and different pumping energies of Ep = 1.2, 1.4, 1.7, 2.0 and 2.3 Et, Et = 160 J; b) dependence of the width of the integral lasing spectrum ∆λ (nm) and position of its maximum along λ 0 (nm) (2) of the Cr:YSGG laser on pumping energy Ep (kJ).

Fig. 1.29 a) spectrograms of the integral spectrum of lasing of TEMmnq modes of Cr:YSGG laser (diameter 4 mm, length 90 mm) with flat mirrors (L = 1.6 m) at constant pumping energy (Ep = 2Et) and different crystal temperatures (T = 15, 35, 50, 70 and 85 °C); b) dependence of the width of the integral lasing spectrum ∆λ (nm) and position of its maximum along λ 0 (nm) (2) on crystal temperature T.

1.6.2 Energy parameters of lasing

The energy characteristics of the radiation of chromium lasers in the REGG crystals: gadolinium–scandium–gallium garnet (GSGG), yttrium–scandium–gallium garnet (YSGG) and gadolinium–scandium- aluminium garnet (GSAG) with a diameter of 4 mm, the length 50 mm, and with Vg = 0.5 cm3, were investigated in a resonator with flat mirrors in the absence of spurious selection of the modes in the experimental equipment discussed previously. The ultraviolet radiation of pumping was cut-off by a liquid filter.

The energy dependences of the examined lasers are presented in Fig. 1.33: Cr:YSGG (3), Cr:GSGG (4) and Cr:GSAG (6). In con-

3 9

Physics of Solid-State Lasers

trast to the ruby, alexandrite and emerald lasers, the REGG did not show such a large decrease of lasing energy with increasing resonator length (Fig. 1.33a) (3,4,6) in the range of low pumping levels. The heating of the crystals to 90°C did not cause any large losses of radiation energy (Fig. 1.33b) (3,4,6). At a pumping energy of 0.5 kJ, the values of the optimum coefficients of transmission of the output mirrors of the resonators of the lasers on REGG crystals, at which in the maximum lasing energy was obtained, were in the range 30–40% (Fig. 1.33c) (3,4,6). In the examined range of pumping, the lasing energy of the REGG lasers depended in a linear manner on the pumping energy (Fig. 1.33d) (3,4,6).

1.7 LASER ON CHROMIUM IONS IN A CRYSTAL OF POTASSIUMSCANDIUM TUNGSTATE

In order to widen the spectral range of lasing of solid-state lasers, it is interesting to examine a large number of crystals with the impurity of chromium ions, generated on the electronic–vibrational transitions.

The lasing of trivalent chromium ions on the electronic–vibrational transitions in tungstate crystals was observed in Zn(WO)4 [57] and Sc2(WO4)3 [58]. The growth of crystals of binary tungstates with trivalent chromium ions is associated with certain difficulties, due to the formation of a sheelite structure of the crystal in the phase the transitions with a decrease of the temperature of the crystal from the melt to room temperature, in which the trivalent chromium ions are implanted only with difficulties. The authors of Ref. 59 observed for the first time the lasing of Cr3+ ions on a crystal of binary potassium tungstatescandium and investigated some energy characteristics of radiation. The results show that these crystals are almost free from the concentration decay of luminescence to the values of the concentration of the activator of 10 wt%.

The absorption spectrum of the Cr3+:KSc(WO4)2 crystal with the content of the chromium ions of 5 wt% is presented in Fig. 1.30. In comparison with the ruby crystal, the maximum of the absorption spectrum of the chromium ions in the crystal of the potassiumscandium tungstate is displaced to the long-wave range of the spectrum. The wide absorption bands in the blue and red regions of the spectrum (V & U-bands) correspond to the permitted transitions 4A2 → 4T1 and 4T2, respectively.

The half width of the luminescence spectrum of the chromium ions in the crystal of the potassium–scandium tungstate in lamp pumping is 78–59 µm with a maximum at a wavelength of 870 nm (Fig. 1.31). In the case of laser pumping by the second harmonics of a Nd laser, the half width of the luminescence spectrum slightly decreases

4 0

Solid-state chromium lasers in free lasing regime

Fig. 1.30 Dependence of absorption coefficient α (cm–1) on the wavelength λ (µm) of Cr3+ ions in a crystal of potassium-scandium tungstate at T = 300 K.

and amounts to 810–940 nm with the centre at a wavelength of 875 nm.

1.7.1 Spectral and energy parameters of lasing

The spectral and energy parameters of the lasing of a Cr laser nn the potassium–scandium tungstate crystal were investigated by the authors of this book in Ref. 30. Investigations were carried out on a crystal of binary potassium–scandium tungstate with an impurity of chromium ions (Cr3+:KSc(WO4)2, Cr: KSW) with a size of 4 × 4 × 5 mm, grown by the method of spontaneous crystallisation, with the concentration of chromium ions of 5 wt%. The measured lifetime of the upper working level at room temperature was 15 µs. Pumping of the active element Cr:KSW was carried out with a ruby laser in the free lasing regime with the pulse time of approximately 1 ms and the radiation energy in the pulse of 2 J.

At room temperature, the lasing of chromium ions was detected on the electronic–vibrational transitions 4A2→ 4T1 in the Cr:KSW laser with a centre at a wavelength of 870 nm and the width of the lasing spectrum of the order of 20 nm with the pumping energy 5 times higher than the threshold energy. The threshold energy of pumping was approximately 25 mJ. With increasing pumping energy the width of the lasing spectrum increased almost linearly in some pumping range, excluding the region of saturation. In a dispersion resonator, the rearrangement of the wavelength of lasing was observed in the range 820–930 nm. The lasing energy of the Cr:KSW laser was characterised by an almost linear dependence (Fig. 1.32) on the pumping energy of the ruby laser.

The lasing of longitudinal and transverse modes in the Cr:KSW

4 1

Physics of Solid-State Lasers

Fig. 1.31 Dependence of the intensity of luminescence I (rel.units) of Cr3+ ions in a crystal of potassium–scandium tungstate on wavelength α (µm) in laser (1) and lamp (2) excitation at T = 300 K.

Fig. 1.32 Dependence of the lasing energy Eg (J) of a Cr:KSW laser on pumping energy Ep (J) of a ruby laser.

laser with flat mirrors under normal conditions with the effect of technical perturbations of the resonator eliminated always took place in the regime of non-attenuating pulsations of radiation intensity, as in other active media with chromium ions.

1.8 OPTIMISATION OF THE ENERGY CHARACTERISTICS OF RADIATION OF CHROMIUM LASERS

The energy parameters of the lasing of chromium lasers in the media examined by the authors were measured in almost all investigations [6–30], and they were compared in Ref. 19, 21, 22–24, 26, 28. The maximum values of the energy characteristics of the radiation of lasers were obtained as a result of optimisation of their parameters: the temperature of the active medium, the length of the resonator and the transmission coefficient of its output mirror.

With increase of the length of the flat resonator from the mini-

4 2

Solid-state chromium lasers in free lasing regime

mum possible to 1.5 m, the radiation energy of a ruby laser was halved, and in the case of alexandrite and emerald lasers it decreased 4–5 times (Fig. 1.33a). This difference is caused by the formation of a thermal lens with a shorter focusing distance in the alexandrite and emerald lasers. This lens transforms the flat resonator to a spherical one with equivalent parameters:

The minimum losses of radiation in the resonator and the highest lasing energy of the lasers were obtained only if the condition of stability of the equivalent spherical resonator was fulfilled:

With increase of the pumping energy the optimum coefficients of trans-

Fig. 1.33 Dependence of the density of lasing energy Eg/Vg (J/cm3) on resonator length L (m) (a); temperature of active medium T (b); the coefficients of transmission of the output mirror at resonance T2 (c) at a constant pumping energy of Ep = 0.5 kJ (a,b,c) and pumping energy Ep (kJ) (d) for ruby lasers (1), aleksandrite lasers (2), Cr:YSGG (3), Cr:GSGG (4), emerald (5) and Cr:GSAG (6) lasers. The generated volumes of the active media Vg = 0.78 (ruby), 1.42 (alexandrite), 0.21 (emerald) and 0.5 cm3 (Cr:YSGG, Cr:GSGG, Cr:GSAG).

4 3

Physics of Solid-State Lasers

mission of the output mirror of the resonator, at which the lasing energy was maximum, increase. At a pumping energy of ~0.5 kJ the values of the optimum transmission factors for the measured active media reached saturation (Fig. 1.33b). When heating the crystals from 20 to 60 °C, the radiation energy of the alexandrite laser was doubled, and the radiation energy of the ruby laser decreased 6 times, whereas the radiation energy of the emerald crystals of the rare-earth–gallium garnets changed only slightly (Fig. 1.33c). The increase of the radiation energy of the alexandrite laser with increasing crystal temperature is determined by the increase of the population of the upper working level 4T2 as a result of its thermal excitation from the metastable level

2E.

In the pumping range to 0.5 kJ, the radiation energy of the chromium lasers in all active media depended linearly on pumping energy (Fig. 1.33d). At higher pumping levels there was a deviation from the linear dependence. This is caused by the fact that increasing pumping energy increases the diffraction of pumping radiation in the ultraviolet region of the spectrum. In the case in which the ultraviolet radiation of pumping was converted by dissolution of the KH-20 dye to the green range of the spectrum, which contains one of the wide bands of absorption of the chromium ions, the dependence of radiation energy on pumping energy remained linear even at high pumping levels.

4 4

Solid-state neodymium lasers in free lasing regime

Chapter 2

Solid-state neodymium lasers in free lasing regime

2.1 SPECTROSCOPIC CHARACTERISTICS OF ACTIVE MEDIA ON NEODYMIUM IONS

Lasing on the Nd ions was produced in a very wide range of active media (crystals and glass) where the majority of these media operates in the four-level system. The absorption and luminescence spectra of the neodymium ions Nd3+ are determined by three electrons of the 4f-shell which is sufficiently screened by 5s and 5p-shells. The energy of interaction with 4f-electrons is only 100 cm–1. This energy is lower in comparison with the energy of spin-orbital interaction. The weak crystalline field does not fracture the bond between the orbital and spin moments of the electrons and the Stark splitting of the energy levels of the ions is smaller in comparison with the splitting of its fine structure. The total angular moment J is retained; (2J + 1) of energy sublevels correspond to different projections of the moment on the direction of the electric field.

The position of the energy levels of the Nd ions is determined by the Coulomb interaction of the electrons with the nucleus of with each other (Fig. 2.1). As a result of the spin-orbital interaction of the electrons, the energy levels of the ions are split into sublevels (multiplets) with a spectral range ~103 cm–1, having the same moments L and S, but differing in the mutual orientation of the moments, i.e. in the total moment J. In the electric field of the matrix of the active media, every multiplet is split into Stark sublevels with the spectral range of approximately 102 cm–1. The Stark splitting of the sublevels changes from matrix to matrix and the position of the levels does not change greatly.

Pumping transfers the Nd ions from the ground state 4I9/2 to several relatively narrow (in comparison with chromium ions) absorption bands.

The two main bands 4F5/2 and 4F7/2 are located on wavelengths of 0.8 and 0.73 µm, respectively, which are linked by a rapid (~10 ns) emissionless

relaxation with the metastable level 4F3/2 being the upper working level.

45

Physics of Solid-State Lasers

Fig.2.1 Diagram of energy levels E (eV) of neodymium ions at Nd3+.

higher than the value kT, and this results in the four-level nature of lasing on this transition.

The spectral–time, spatial, angular and energy parameters of lasing of Nd lasers in different media were investigated by the authors of this book in [1–29]. The main spectroscopic characteristics of the exam-

ined active media with the Nd3+ ions in the transition 4F3/2 → 4I11/2 are presented in Table 2.1.

2.2 NEODYMIUM GLASS LASERS

Lasing on Nd ions in glass was obtained for the first time in Ref. 38. Glass differs from the crystal by the absence of the ordered structure; the frequencies of the Stark components of the different ions do not coincide with each other and the gain line is characterised by the non-uniform broadening. Consequently, the cross-section of the induced transition of the Nd ions in glasses is smaller than in the crystals, and the lasing spectrum of the Nd ions shows fork splitting [39]. The value of the uniform broadening of the individual active centres is approximately 3 nm [40].

46