RU2707388C1 - Crystalline material based on fluorite-like systems for beam-lasers - Google Patents

Abstract

FIELD: manufacturing technology.

SUBSTANCE: invention relates to crystals intended for use in solid-state lasers, namely in CPA-lasers (chirp pulse amplification lasers) – short-pulse lasers with high peak power. Crystalline material based on fluorite-like systems for CPA-lasers consists of fluorite-type isostructural analogues, in which as isostructural analogues of fluorite type there are used substances selected from group CaF2, YF3, YbF3 and NdF3.

EFFECT: higher efficiency of laser system and reduced probability of occurrence of "thermal lens".

5 cl, 1 tbl, 8 dwg

Description

Technical field

The invention relates to crystals intended for use in solid-state lasers, namely CPA-lasers (from the English words "chirp pulse amplification") - short-pulse lasers with high peak power. In particular, the invention relates to a method for increasing the emission bandwidth of rare-earth ions used as generating ions in active media of solid-state crystal lasers, especially in fluoride crystal compositions with a fluorite-like cubic structure.

State of the art

One of the most important tasks of laser physics is the development of high-energy laser systems capable of generating laser pulses of the nano, pico, and femtosecond ranges. On crystalline active media as master oscillators, for example, YAG-Nd, YLF-Nd and silicate or phosphate laser glasses with neodymium as amplifiers, powerful laser systems have been created for technological purposes, as well as for use in experiments on controlled thermonuclear fusion (TCF) . The output power of modern petawatt laser systems of ultrashort pulses created on phosphate glass with neodymium for the TCF program (NIF project, USA) reaches 10 ¹⁵ W in a pulsed mode with a pulse duration of up to 100 fs.

The use of crystalline material based on fluorite-like systems due to their spectral-optical and physical (thermal) properties is of interest in the applied aspect, for example, for creating CPA lasers (from the English words "chirp pulse amplification") - lasers generating powerful femtosecond light beams ( D. Stricklend, G. Mouron. Optic Communications. V. 56, p. 219-221, 1985 and Mourou G., Tajima T., Bulanov SV // Review of Modern Physics. 2006. V. 78. P. 309- 371).

A CPA laser consists of four blocks: a generator, a stretcher, an amplifier and a compressor. The principle of operation of a CPA laser is described in (L.M. Gorbunov. Why superpower laser pulses are needed // Nature 2007, No. 4). A weak (~ 10 ^-6 J) femtosecond laser pulse from the generator enters the stretcher, where its length increases thousands of times, and the radiation frequency smoothly varies along the pulse length (the so-called chirped pulse). Then the pulse passes through the amplifier, increasing its energy by many orders of magnitude. From the amplifier, the pulse enters the compressor, where its duration decreases to the initial value.

A known quantum amplifier (patent for the invention of the Russian Federation No. 2411621), where as an optical amplifier it is proposed to use optical amplifiers on neodymium glasses and an oxide crystal of sapphire with titanium.

The most significant drawback of neodymium glass is its low thermal conductivity, leading to thermo-optical disturbances and thermal destruction of the active element. A significant part of the energy of the pump radiation is released inside the active element in the form of heat, which leads to its heating, as a rule - uneven. This heating is associated with optical distortions, which are one of the main sources of aberration of amplification stages. In addition, mechanical stresses and deformations that appear under the influence of an inhomogeneous temperature field lead to the destruction of the active element. Due to the low thermo-optical characteristics, neodymium glass is not used in high-power laser systems with pulse energies from several hundred joules with high repetition rates of more than 1 pulse per hour. . One way to solve this problem is to search for new materials for laser amplifiers, devoid of the disadvantages inherent in laser glasses. Research data show that the thermal conductivity of crystalline media is significantly higher than that of glasses. In addition, iron is part of the glass and, under the influence of UV radiation, 3-valent iron is reduced to divalent iron, which absorbs at a wavelength of 1.06 μm, i.e. in the range of the working wavelength of a neodymium laser. As a result, the generation characteristics decrease (“aging” of the material).

An important role is played by the values of nonlinear refractive indices n ₂ , the magnitude of which determines the evolution of the laser beam passing through the active medium of the amplifier. The smaller the value of this parameter, the less is the probability of a defocusing of the laser beam as a result of the formation of a “induced thermal lens", the focal length of which is determined by the value of n ₂ and the radiation power. Studies show that the n ₂ index for glasses is up to 4 times higher than for fluoride crystals.

A patent for the invention of the Russian Federation No. 2403661 is known, where the use of a Ti: Al ₂ O ₃ (Ti: sapphire) crystal is proposed as a laser amplifier.

The use of both neodymium glasses and Ti: Al ₂ O ₃ crystals (Ti: sapphire) is the subject of RF patent No. 2629499.

Titanium - sapphire crystals have a wide emission band, as well as large cross sections for laser transition radiation. These properties, in combination with the characteristic thermal, physical, and optical properties of a sapphire crystal, satisfy the requirements for choosing this material as promising for active elements of solid-state ultrashort pulse lasers. Short time however

The lifetime of the initial level of the luminescent state necessitates the pumping of a Ti: sapphire laser by other lasers. Titanium-activated sapphire short-pulse lasers are usually pumped by lasers with a wavelength of 514 to 532 nm, for example, an argon-ion laser (514.5 nm) or a second harmonic Nd: YAG, Nd: YLF, and Nd: YVO (527- 532 nm), or Nd-glass lasers, which, in turn, are pumped by flash lamps. This type of laser is used mainly in laboratory experiments, since it requires a stable and powerful pump laser.

The technical problem solved by the invention is the use of powerful short-pulse femtosecond laser systems of crystalline material of the same chemical class and structural type — simple and disordered fluoride compounds with fluorite structure as active media of the master oscillator and quantum amplifier.

Disclosure of the invention

The technical result of the invention is to increase the efficiency of the laser system and reduce the likelihood of the formation of a "thermal lens" in it.

To achieve a technical result, a crystalline material based on fluorite-like systems for CPA lasers is proposed, consisting of fluorite-type isostructural analogs, in which substances selected from the CaF ₂ , YF ₃ , YbF _3, and NdF ₃ groups are used.

In addition, the crystalline material of the master oscillator of the femtosecond pulses of a CPA laser consists of calcium fluoride activated by trivalent ytterbium ions CaF ₂ : Yb ³⁺ , and the concentration of ytterbium ions in the crystal is from 3% to 6%.

In addition, the crystalline material of the optical laser pulse amplifier of the CPA laser is a solid solution of calcium fluoride and yttrium fluoride activated by trivalent ytterbium ions CaF ₂ -YF ₃ : Yb ³⁺ , the yttrium fluoride concentration being from 2% to 7%, and the concentration ytterbium ions ranges from 2% to 5%.

In addition, the crystalline material of the optical laser pulse amplifier of the CPA laser is a solid solution of calcium fluoride and yttrium fluoride activated by trivalent neodymium ions CaF ₂ -YF ₃ : Nd ³⁺ , the yttrium fluoride concentration being from 1% to 4%, and the concentration neodymium ions ranges from 1.5% to 3%.

In addition, the crystalline material of the optical laser pulse amplifier of the CPA laser is a solid solution of calcium fluoride and yttrium fluoride activated by trivalent ytterbium ions and additionally activated as trivalent neodymium ions CaF ₂ -YF ₃ : Yb ³⁺ + Nd ³⁺ , moreover the concentration of yttrium fluoride is from 3% to 8%, the concentration of ytterbium ions is from 2% to 5%, and the concentration of neodymium ions is from 1% to 3%.

Brief Description of the Drawings

In FIG. Figure 1 shows the dependence of the laser pulse duration on the spectral half-width.

In FIG. Figure 2 shows the luminescence spectrum of Nd in phosphate glass (Δν ~ 22.5 nm).

In FIG. Figure 3 shows the absorption and luminescence spectrum (dashed line) of Yb in silicate glass (Δν ~ 55 nm).

In FIG. Figure 4 shows the luminescence spectra of Yb ³⁺ ions in CaF ₂ and CaF ₂ -YbF ₃ crystals.

In FIG. Figure 5 shows the luminescence spectra of Nd ³⁺ ions in CaF ₂ (A), CaF ₂ -YF ₃ (B) crystals and KGSS-7 (C) glass.

In FIG. Figure 6 shows the luminescence spectra of the concentration series of Yb ³⁺ ions in CaF ₂ crystals at 77K: a — 0.3; b - 1.0 and s - 10 weight. %

In FIG. 7 shows a fragment of the combined luminescence spectrum of Yb ³⁺ and Nd ³⁺ ions in a disordered CaF ₂ -YF ₃ crystal.

In FIG. 8 is a schematic diagram of the operation of a CPA laser.

The implementation of the invention

Ytterbium-doped crystalline material has the highest quantum efficiency (> 90%) among commercial laser products. CaF ₂ : Yb ³⁺ crystals have a wider emission spectrum than YAG: Yb. This property is most important for the generation and amplification of ultrashort pulses. The CaF ₂ crystal matrix has a higher thermal conductivity and a lower n ₂ value than glass, which makes the effect of the appearance of a “thermal lens” very small. These crystals can also be doped with ytterbium ions to high concentrations.

When solving the problem of creating powerful technological laser systems generating ultrashort light pulses, crystals of the present invention activated by ytterbium ions, such as simple ones with an ordered structure CaF ₂ : Yb ³⁺ as a master oscillator and mixed with a disordered structure CaF ₂ -YF, are of particular promise. ₃ : Yb ³⁺ as amplifiers. In contrast to neodymium laser media on crystals and glasses, ytterbium media, irrespective of the structural ordering of the crystal matrix, exceed the latter in radiation line widths and are better suited for creating ultrashort pulses. As an example of ordered crystalline media with uniform broadening of the spectral lines, Table 1 shows the spectral-laser characteristics of yttrium-aluminum garnet crystals with an admixture of Nd ³⁺ and Yb ³⁺ ions (AA Kaminsky, S.E. Sarkisov. “Physics and spectroscopy of laser crystals / M., Nauka, 1986, p. 282). The widths of the Yb ³⁺ lines are up to 10 times higher than the same value for Nd ³⁺ .

For media with inhomogeneous broadening of spectral bands, in FIG. Figures 2 and 3 show the luminescence spectra of Nd ³⁺ ions in phosphate glass and Yb ³⁺ ions with a maximum at 1035 nm in silicate glass. Analysis of the spectra shows that the emission bandwidth of Yb ³⁺ in FIG. 3 to 2 times more than Nd ³⁺ (Fig. 2).

In FIG. Figure 4 shows the luminescence spectra of a simple CaF ₂ crystal with an ordered structure: at. 3% Yb ³⁺ (curve A) and at. 3% Yb ³⁺ in disordered crystals of CaF ₂ -YF ₃ solid solutions with a concentration of YF ₃ = at. 3% (curve B) and YF ₃ = at.6% (curve C). The effect of disordering of the crystal structure on the inhomogeneity of the broadening of spectral lines is shown in FIG. 5, which shows the luminescence spectra of Nd ³⁺ ions on the main ⁴ F _3/2 → ⁴ I _11/2 laser transition at 300 K in a simple CaF ₂ crystal (a), crystalline CaF ₂ -YF ₃ (b) solid solution and neodymium laser KGSS-7 brand glass (s). The inhomogeneously broadened luminescence band of the mixed crystal of yttrofluorite CaF ₂ -YF ₃ : Nd ³⁺ is equal to ~ 35 nm, and for glass it is less than ~ 25 nm.

Crystalline CaF ₂ -YF ₃ : Yb ³⁺ solid solutions also have wider emission and absorption bands (provides more efficient optical pumping) and higher τ _lum values compared to simple ordered CaF ₂ : Yb ³⁺ crystals. The spectra of fluorite crystals activated by rare-earth ions, and in particular, Yb ³⁺ ions, are characterized by the presence of optical centers, the composition and relative number of which vary depending on the concentration of activator ions. At low concentrations of rare-earth impurities, a small number of centers are formed that differ significantly in structure, so that a large difference arises in their optical properties. With increasing concentration, the number of centers with differing structures increases, but the difference between the structures decreases significantly. As a result, at high concentrations of activator ions, the absorption and luminescence lines of various rare earth centers are grouped around energy levels, which are determined by the average values of the crystal field acting on impurity ions. The structures of the optical spectra do not change with concentration anymore and the number of Stark components corresponds to a possible maximum for a given average value of the crystal field. By way of illustration, FIG. Figure 6 shows the luminescence spectra at the ⁴ F _5/2 → ⁴ F _7/2 transition at 77K of a concentration series of CaF ₂ crystals with Yb ³⁺ ions: a-0.3% Yb; b-1.0% Yb; c-10% Yb. It can be seen that, in contrast to the concentration of 0.3%, at 10% Yb ^{3+, the} spectral bands lose resolution for individual Stark components.

The addition of YF ₃ to the composition of a simple CaF ₂ : Yb ³⁺ crystal introduces disorder in the crystal lattice of the latter, modifies the local crystal field around the Yb ³⁺ ions in such a way that, with an increase in their concentration, the probability of the formation of luminescence quenching centers of Yb ³⁺ ions decreases sharply compared to with simple ordered crystals of structural analogues. The latter is important for obtaining effective power output laser parameters. Therefore, disordered crystals of CaF ₂ -YF ₃ : Yb ³⁺ solid solutions are more promising in comparison with CaF ₂ : Yb ³⁺ as an optical amplifier material. CaF ₂ : Yb ³⁺ crystals have all the necessary properties and spectroscopic parameters for use as a short-pulse laser oscillator in systems with amplifiers based on CaF ₂ -YF ₃ : Yb ³⁺ and CaF ₂ -YF ₃ : Nd ³⁺ crystals.

The active elements of amplifiers based on disordered crystals with the fluorite structure CaF ₂ -YF ₃ : Yb ³⁺ and CaF ₂ -YF ₃ : Nd ^{3+ with} large volumes and apertures can provide a high level of stored energy and will allow working at laser radiation intensity below the optical breakdown threshold up to the energies of the petawatt level in pulses of a femtosecond duration. The possibility of obtaining femtosecond pulses was demonstrated in (ZP Qin, GQ Xie, J. Ma, WY Ge, P. Yuan, LJ Qian, LB Su, DP Jiang, FK Ma, Q. Zhang, YX Cao, J. Xu Opt. Lett ., 39 (2014), p. 1737) for a CaYF ₅ : Nd crystal laser with a pulse duration of 103 fs when operating in the mode of passive synchronization. This result proves that disordered crystals with fluorite structure are excellent matrices for creating short-pulse high-power laser systems. The main question is related to the possibility of producing large single crystals of yttrofluorite with an optimal content of YF ₃ , YbF _3, and NdF ₃ components, which should provide sufficiently high values of thermal conductivity, amplification characteristics, and stored energy for amplifying elements. The most promising for these purposes may be CaYF ₅ : Nd ³⁺ crystals synthesized in the solid solution system with the composition CaF ₂ - 1.5-2% YF ₃ - 2-3% NdF ₃ and CaYF ₅ : Yb ³⁺ in the CaF ₂ system -3-6% YF ₃ -2-5% YbF ₃ . This will allow the thermal element to retain the value of thermal conductivity more than 6 times higher and up to 4 times lower in n ₂ than for phosphate neodymium glass and at the same time create an active medium with non-inferior spectral-laser characteristics. High-power laser systems with amplifiers based on disordered yttrofluorite crystals with neodymium and ytterbium can operate using CaF ₂ : Yb ³⁺ or YLF: Nd crystals, which are used in laser systems with phosphate glass amplifiers, as master oscillators.

Previous attempts to expand the emission bands in combined high-power neodymium laser systems using a master oscillator on a chip and an amplifier on glass have focused on modifying the structure of the original glass. The present invention is based on the use of rare-earth activating additives in active amplification media, the interactions between which as a result of energy transfer mechanisms affect the radiation bandwidth. In accordance with this invention, the initial fluorine-containing crystalline substance of the CaF ₂ -YF ₃ laser amplifier is activated by rare-earth compounds, typically NdF ₃ in combination with YbF ₃ . The resulting emission bands turn out to be much wider than those currently achieved with a single activating additive in the initial simple or disordered crystals. In FIG. Figure 7 shows a fragment of the combined luminescence spectrum (Yb + Nd) of a disordered crystal of a solid solution of the composition CaF ₂ -5% YF ₃ -4% YbF ₃ -2% NdF ₃ at 300 K with an effective emission bandwidth of ~ 80 nm.

Thus, the use of fluoride crystals such as CaF ₂ : Yb ³⁺ as a master oscillator and CaF ₂ -YF ₃ : Yb ³⁺ , CaF ₂ -YF ₃ : Nd ³⁺ or CaF ₂ -YF ₃ : Nd ³⁺ + Yb ³⁺ as amplifiers, allows for much simpler design decisions for lasers, since these laser crystals can be directly pumped by flash tubes. Unlike lasers using Ti: sapphire crystals, the technology with Yb: fluoride crystals does not require additional use of lasers for pumping. Due to the large values of the luminescent lifetime of the metastable levels and the half-widths of the absorption and emission bands, fluorides with ytterbium can provide a high level of stored energy and are of practical interest in the generation of superpower optical pulses.

In addition, the use of crystalline materials based on fluorite-like systems with improved thermophysical and mechanical characteristics due to wide nonuniformly broadened absorption and luminescence spectra in CPA lasers gives a number of advantages compared to single crystals with a regular crystal lattice, namely:

makes it possible to tune the generation frequency within an inhomogeneously broadened line, the width of which can reach tens of nanometers;

makes it possible to obtain ultrashort pulses.

Claims (5)

1. A crystalline material based on fluorite-like systems for CPA lasers, consisting of isostructural analogues of the fluorite type, in which substances selected from the group CaF ₂ , YF ₃ , YbF ₃ and NdF ₃ are used as isostructural analogues of the fluorite type. 2. The crystalline material according to claim 1, characterized in that the crystalline material of the master oscillator of the femtosecond pulses of the CPA laser consists of calcium fluoride activated by trivalent ions of ytterbium CaF ₂ : Yb ³⁺ , and the concentration of ytterbium ions in the crystal is from 3% to 6 % 3. The crystalline material according to claim 1, characterized in that the crystalline material of the optical laser pulse amplifier CPA laser is a solid solution of calcium fluoride and yttrium fluoride activated by trivalent ytterbium ions CaF ₂ -YF ₃ : Yb ³⁺ , and the concentration of yttrium fluoride is from 2% to 7%, and the concentration of ytterbium ions is from 2% to 5%. 4. The crystalline material according to claim 1, characterized in that the crystalline material of the optical laser pulse amplifier CPA laser is a solid solution of calcium fluoride and yttrium fluoride activated by trivalent neodymium ions CaF ₂ -YF ₃ : Nd ³⁺ , and the concentration of yttrium fluoride ranges from 1% to 4%, and the concentration of neodymium ions is from 1.5% to 3%. 5. The crystalline material according to claim 1, characterized in that the crystalline material of the optical laser pulse amplifier CPA laser is a solid solution of calcium fluoride and yttrium fluoride activated by trivalent ytterbium ions and additionally activated as trivalent neodymium ions CaF ₂ -YF ₃ : Yb ³⁺ + Nd ³⁺ , whereby the concentration of yttrium fluoride is from 3% to 8%, the concentration of ytterbium ions is from 2% to 5%, and the concentration of neodymium ions is from 1% to 3%.

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