RU2186161C2 - Single-crystal material for lasers of infrared band - Google Patents

Abstract

FIELD: materials for laser equipment, in particular, single-crystal materials designed for production of active components of solid-state lasers. SUBSTANCE: erbium with chemical formula Ca_aY_1-x-yYb_xEr_y(BO₃)₃O, where x>0,001, y>0,001, x+y≤0,57, is additionally introduced as an activator in the single-crystal laser material based on oxyorthoborate of calcium-yttrium with ytterbium. EFFECT: produced active components of the 1.5- micron radiation band. 4 dwg, 1 tbl

Description

 The invention relates to materials for laser technology, namely to single-crystal materials designed to produce active elements of solid-state lasers of the 1.5-micron range of generation.

 Laser radiation with a wavelength of 1.5 microns is extremely important for science and technology. This radiation corresponds to the minimum loss in quartz fibers, which are used today in almost all fields of technology that require the transmission of optical radiation over a distance, for example, in cable optical communication. One and a half micron radiation is the least dangerous to vision (damage threshold 0.8 J). From this point of view, it is promising for applications in ophthalmology.

 For the same reason, it seems possible to replace neodymium lasers (λ = 1.06 μm), which are hazardous to vision, in metal processing technology, ranging, location and other areas of technology.

 The use of 1.5 microns of radiation in areas where light propagates in the Earth’s atmosphere is very beneficial since air has a “transparency window” in the region of 1.5 microns.

Active media based on alkali halide crystals: MgO and MgF ₂ with an admixture of Co ²⁺ Ni ²⁺ , V ^{2+ ions} , which allow one to obtain stimulated radiation in the region of 1.2-1.8 μm, are known [1]. However, the use of such media is difficult, since the operating temperature is 77 K.

Active media based on yttrium aluminum garnet [2] with chromium generate light in the region of 1.36-1.45 nm. An active medium based on yttrium aluminum garnet with activators - ytterbium and erbium Y ₃ Al ₅ O ₁₂ : Yb, Er generates in the region of 1.54 μm [3], but is characterized by a low efficiency (less than 0.01%) and an extremely high generation threshold ( 200 J). The low efficiency of such media is due to the so-called “reverse transfer” effect, as well as cooperative processes (up-conversion and cross-relaxation), which arise due to the high lifetime of the pre-laser level ⁴ I _11/2 .

 In the well-known scientific literature, there is no data on the creation of a highly efficient erbium 1.5 micron laser on a crystal, i.e. Despite the abundance of crystalline matrices and realized generation channels [4,5], in practice, erbium glasses remain the main source of radiation with a wavelength of 1.5 μm [6-11]. But the low thermophysical characteristics of the latter force scientists to continue the search for crystalline matrices for the erbium ion.

Monocrystalline laser materials based on scandium orthoborate are known, where erbium or erbium and ytterbium are used as an activator, in accordance with the chemical formula Sc _1-x M _x BO ₃ , where 0 <x≤0.4, and M is an activator [12] and Yb, Er: LaSc ₃ (BO ₃ ) ₄ [13], which are the closest in the achieved effect. The radiation wavelength of such materials lies in the region of 1.5 μm. In these materials, due to the developed phonon spectrum, the luminescence of the level ⁴ I _11/2 of the Er ³⁺ ion is completely quenched. This determines the high energy transfer efficiency Yb-> Er. The development of the phonon spectrum is explained by the presence of borate groups — VO ₃ . The main disadvantages of these materials are the low isomorphic capacitance of the matrix with respect to the activators, as well as the short lifetime of the upper laser level ⁴ I _13/2 ~ 390 μs for Sc _1-x M _x BO ₃ and ~ 680 μs for Yb, Er: LаSс ₃ (VO ₃ ) ₄ , which does not allow one to obtain efficient radiation generation due to weak energy storage.

The closest to the claimed material analogue in technical essence is a single-crystal laser material corresponding to the chemical formula Ca ₄ Y _1-x Yb _x (BO ₃ ) ₃ O [14, 15], this material has a wide phonon spectrum, self-doubling of the generation frequency and a fairly high isomorphic capacity. But the wavelength of the laser radiation based on such a material is about 1 μm, which falls into the range that is hazardous to vision. In addition, radiation with such a wavelength is strongly absorbed in quartz fibers when transmitting an optical signal.

 In this regard, the technical task is to obtain a single-crystal laser material capable of efficiently emitting at wavelengths near 1.5 μm and having a high isomorphic capacity with respect to activators and a high lifetime of the upper laser level.

To solve the technical problem proposed in the single crystal laser material based oksiortoborata-yttrium calcium ytterbium additionally administered as an activator trivalent erbium in accordance with the formula: Ca ₄ Y _1-x-y Yb _x Er _y (BO _{3) 3} O where x> 0.001, y> 0.001, x + y≤0.57.

The composition of the proposed material, as well as the composition of the prototype, includes borate groups (VO ₃ ). The proposed material, like the prototype, is coactivated with ytterbium, but in contrast to it additionally contains trivalent erbium ions as an activator.

The choice of yttrium oxyorthoborate crystal as the matrix base allows one to obtain a single-crystal laser material with a low lifetime of the pre-laser level ⁴ I _11/2 and a sufficiently high lifetime of the upper laser level ⁴ I _13/2 of the erbium ion - of the order of 1.3 ms. The low lifetime of the prelaser level ⁴ I _11/2 is explained by the developed phonon spectrum due to the presence of borate groups (BO ₃ ) in the single crystal. It is the low lifetime of the ⁴ I _11/2 level of the Er ³⁺ ion in the Yb, Er: LSB crystal that is the basis for the efficiency of generation of one and a half micron radiation.

In FIG. Figure 1 shows the luminescence decay kinetics of crystals of Ca ₄ Y _0.8 Yb _0.2 (BO ₃ ) ₃ O (YCOB: Yb) and Ca ₄ Y _0.775 Yb _0.2 Er _0.025 (BO ₃ ) ₃ O (YCOB: Yb, Er ), measured at a wavelength of 970 nm (level ² F _5/2 of a Yb ³⁺ ion) after excitation by the first harmonic of a YAG: Nd laser, in FIG. 2 - kinetics of luminescence decay of a Ca ₄ Y crystal _0.775 Yb _0.2 Er _0.025 (VO ₃ ) ₃ O measured at a wavelength of 1530 nm (laser and a half micron channel ⁴ I _13/2 -> ⁴ I _15/2 of the Er ³⁺ ion) after excitation by the second harmonic of a YAG: Nd laser, Fig. 3 shows the luminescence spectrum of a Ca ₄ Y crystal of _0.7 Yb _0.2 Er _0.1 (BO ₃ ) ₃ O in the wavelength range of 1420-1698 nm, measured upon InAsGa excitation laser diode with a wavelength of 970 nm, figure 4 - spectrum of the gain cross section, calculated taking into account the reabsorption of radiation.

 The crystals were grown by the Czochralski method from iridium crucibles with a diameter of 30-50 mm at a speed of 1-3 mm / h.

Example 1
A mixture of finely dispersed high-purity (grade OSCH) materials:
Calcium oxide (CaO) - 48.7175 g
Yttrium oxide (Y ₂ O ₃ ) - 19.0036 g
Ytterbium (III) oxide (Yb ₂ O ₃ ) - 8.5588 g
Erbium (III) oxide (Er ₂ O ₃ ) - 1.0385 g
Boron oxide (B ₂ O ₃ ) - 22.6804 g
thoroughly mixed, pressed into tablets and placed in a muffle furnace, where at a temperature of 1300 ^o With conducted synthesis in the solid phase for 8 hours. The synthesized substance was placed in a crucible and melted ( _Tmelt = 1550 ^o C). The crystal was grown by the Czochralski method with a drawing speed of 3 mm / h. As a result, a transparent pink-tinted crystal of high optical quality with a height of 17 mm and a diameter of 9 mm of the chemical formula Ca ₄ Y _0.775 Yb _0.2 Er _0.025 (BO ₃ ) ₃ O was obtained. The crystal density determined by hydrostatic weighing was 3.39 g / cm ³ .

Example 2
A mixture of finely dispersed high-purity (grade OSH) substances:
Calcium oxide (CaO) - 48.1045 g
Yttrium oxide (Y ₂ O ₃ ) - 16.9483 g
Ytterbium (III) oxide (Yb ₂ O ₃ ) - 8.4510 g
Erbium (III) oxide (Er ₂ O ₃ ) - 4.1015 g
Boron oxide (B ₂ O ₃ ) - 22.3946 g
thoroughly mixed, pressed into tablets and placed in a muffle furnace, where at a temperature of 1200 ^o With conducted synthesis in the solid phase for 10 hours. Then the synthesized substance was placed in a crucible and melted (T _melt = 1540 ^o C). The crystal was grown by the Czochralski method with a drawing speed of 2 mm / h. The result was a transparent pink crystal of high optical quality with a height of 20 mm and a diameter of 12 mm of the chemical formula Ca ₄ Y _0.7 Yb _0.2 Er _0.1 (BO ₃ ) ₃ O. The crystal density determined by hydrostatic weighing was 3.44 g / cm ³ .

 Similarly, crystals were grown, the chemical formulas of which are given in the table. If in the proposed material we take trivalent ytterbium with a stoichiometric coefficient x <0.001, then the low absorption coefficient of such a material will not allow absorbing energy sufficient to exceed the generation threshold. If we take trivalent erbium with a stoichiometric coefficient y <0.001, then the low density of excitations in the medium, due to the low concentration of active ions, will not allow to exceed the parasitic absorption losses of the base matrix, and it makes no sense to talk about such material as laser. On the other hand, as x + y values approach 0.57, the optical quality of the crystal deteriorates - samples 4, 8 (see table), and with x + y values> 0.57 it is impossible to obtain single-crystal material.

Freshly grown samples were boules with a diameter of 8-12 mm and a length of 10-21 mm, transparent, with a smooth shiny surface. For spectral-luminescent measurements, 4x5 mm ² and 0.1 to 3 mm thick plates were cut out.

 The absorption and luminescence spectra were measured using an MDR-23 diffraction monochromator (with a grating of 600 lines / mm) with an inverse linear dispersion of 2.6 nm / mm and a slit width of not more than 0.15 mm. Absorption spectra were measured using a single-beam scheme [16].

 The luminescence spectra were corrected for the spectral sensitivity of the photodetector.

The quantum energy transfer efficiency of Yb ³⁺ -> Er ^{3+ was} determined by the formula
η = (W + γ ² ) / (1 / τ + W + γ ² ),
where τ is the lifetime of the excited state of yttibium ion, γ is the transfer microparameter, and W is the energy migration rate over Yb ³⁺ ions. The values of τ, γ, and W were determined by analyzing the shape of the luminescence decay kinetics of the ytterbium ion shown in FIG. 1 [17].

The lifetime of the ⁴ I _13/2 ion Er ³⁺ determined by taumeter, i.e. at the point of intensity drop by e times [18].

The spectra of the effective gain cross section of single crystals σ _eff (λ) taking into account reabsorption were calculated by the formula
σ _eff (λ) = βσ _lum (λ) - (1-β) σ _diff (λ),
where β = n / N is the ratio of the populations of the upper and lower levels, σ _lum (λ) is the luminescence cross section, and _σm (λ) is the absorption cross section [18].

The matrix Ca ₄ Y _1- xy Yb _x Er _y (BO ₃ ) ₃ O contains borate groups (BO ₃ ) and, accordingly, has an extended phonon spectrum (ω _max = 1400 cm ^-1 ). As a result, the nonradiative multiphonon relaxation of electronic excitation energy quickly, in less than 0.1 microseconds, relaxes the laser level ⁴ I _13/2 ion Er ^3+. This determines the high quantum efficiency of energy transfer Yb ³⁺ -> Er ³⁺ (see figure 1). In the Ca ₄ Y crystal, _0.775 Yb _0.2 Er _0.025 (BO ₃ ) ₃ O, the quantum transfer efficiency is η = 96%. Fast multiphonon relaxation of ⁴ I _11/2 - ⁴ I _13/2 almost completely prevents the reverse energy transfer of Er ³⁺ -> Yb ³⁺ and various cooperative processes involving the ⁴ I _11/2 level. The lifetime of the laser level ⁴ I _13/2 , despite the developed phonon spectrum, remains quite high for both semiconductor and lamp pumping, and is 1.26 ms (see Fig. 2).

 From FIG. 3 shows that the proposed material emits in the region of 1.5 μm. Positive amplification occurs at a wavelength of 1597 nm with a small level of inversion (Fig. 4), and when the population ratio β = 0.6, the luminescent frequency tuning region is ~ 90 nm.

We have studied the Stark structure of levels ⁴ I _15/2 , ⁴ I _13/2 , ⁴ I _11/2 , ⁴ I _9/2 of the Er ³⁺ ion in the proposed material. The results of studies and calculations indicate the absence of excited absorption channels during transitions from the ⁴ I _13/2 level to the ⁴ I _9/2 level overlapping with the ⁴ I _{15 / 2-} -> ⁴ I _13/2 laser channels.

 Thus, the proposed single-crystal laser material in its characteristics is of interest for creating crystal lasers with a wavelength of about 1.5 μm, which is more preferable than glass lasers and lasers with other wavelengths for a wide variety of applications.

Literature
1. Moulton PF IEEE J. Quantum Electron. 1982. V. 18. P. 1185.

 2. Angert N.B., Borodin N.I., Garmash V.M. and other quantum electronics. 1988.Vol. 15.1.S. 113.

 3. White K.O., Scleusener S.A. Appl. Phys. Lett. 1972.V. 21. 9.P. 419.

 4. Kaminsky A. A., Antipenko B. M. Multilevel multifunctional schemes of crystal lasers. M .: Nauka, 1989.270 s.

 5. Kaminsky A.A., Aminov L.K., Ermolaev V.L. and others. Physics and spectroscopy of laser crystals M.: Nauka, 1986. 272 p.

 6. Anton D. W., Pier T.J., Leilabody P.A., Digest of Conference on Optical Fiber Communications, 1991, paper FB6, p. 206.

 7. Laporta P., De Silvestri S., Magni V., Pallaro L., Svelto 0., Digest of Conference on Lasers and Electro-Optics, 1991, paper CthR l.

 8. Laporta P., De Silvestri S., Magni V., Svelto 0., Opt. Lett. 16 (1991) 1952.

 9. Hutchinson J. A., Caffey D.P., Schans C.F., Trussel C.W., Digest of Conference on Lasers and Electro-Optics, 1990, paper CPDP - 19.

 10. Hutchinson J. A., Allik, T. N., Appl. Phys. Lett. 60 (1992) 1424.

 11. Labranche B., Mailloux A., Levesque M., Taillo Y., Morin M., Mathieu P., OSA Proceedings on Advanced Solid-State Lasers 24 (1993) 379.

 12. RF patent 2084997, MKI (6) H 01 S 3/16, Single-crystal material for infrared lasers. / Lebedev V.A., Pisarenko V.F., Chuev Yu.M., Fateev V.M., Shestakov A.V. - 93052869/25; declared 11/22/93; Publ. 07/20/97, Bull. 20 - 7 s.

 13. Perfilin A., Nesynov E., Podsepko M., Lebedev V., Chuev Yu. Spectral-luminescent studies of single crystals of borates and silicates with impurities of ytterbium and erbium. Nature. Society. Man, Bulletin of the South-Russian Branch of the International Academy of Sciences of Higher Education, 4-5 (7-8) / 1996, p. 31-33.

14. V.N.T. Chai et al. "Lasing, Second Harmonic Conversion and Self-frequency Doubling of Yb: YCOB (Yb: USA ₄ V ₃ O ₁₀ )," OSA Trends in Optics and Photonics on Advanced Solid State Lasers 19 (OSA, Washington, DC, 1998), pp . 59-61.

15. Ye Q., Chai BHT Crystal growth of YCa ₄ O (BO ₃ ) ₃ and its orientation, Journal of Crystal Growth 197 (1999) 228-235.

 16. Zaydel A.N., Ostrovskaya GV, Ostrovsky Yu.I. Technique and practice of spectroscopy. M .: Nauka, 1976, p. 108-112.

17. Sigachev VB, Doroshenko ME, Basiev TT, Lutz GB, Teas B. Sensitization of luminescence of Er ³⁺ and Ho ³⁺ ions with Cr ⁴⁺ ions in a Y ₂ SiO ₅ crystal. Quantum Electronics, 22, 1, 1995.

 18. Kaminsky A.A. Laser crystals, Moscow: Nauka, 1975, p. 16-24.

Claims (1)

Monocrystalline material for infrared lasers based on yttrium-activated calcium yttrium oxyorthoborate, characterized in that it additionally contains trivalent erbium as an activator in accordance with the chemical formula
Ca ₄ Y _1-xy Yb _x Er _y (BO ₃ ) ₃ O,
where x> 0.001, y> 0.001, x + y≤0.57.

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