RU2106050C1 - Laser material - Google Patents

Abstract

FIELD: quantum electronics; solid-state lasers. SUBSTANCE: solid-state laser of wavelength between 1.9 and 2.0 mcm uses laser material of chemical formula A_1-0,4-xB_0,4Tm_xVO₄ and incorporates, in addition, at least one of group elements: Li, B, Be, Na, Ca, Mg, Si, K, Ti, Mu, Fe, Co, Ni, Cu, Zn, Rb, Sr, Nb, Mo, W, Ta, Hf, Bi, in the amount of 1•10¹⁷ to 5•10²⁰cm^-3. EFFECT: improved quality of laser material. 4 dwg, 2 tbl

Description

 The invention relates to quantum electronics, namely to materials for laser technology and is intended for use in solid-state lasers with a wavelength of stimulated radiation in the range from 1.9 μm to 2.0 μm.

A known laser substance is a crystal of orthovanadate Nd: Gd _0.5 La _0.5 VO ₄ , which can be used as an active medium for lasers with a generation wavelength of 1.06 μm [1]
The disadvantages of this crystal include the laser-hazardous range for the eyes.

Also known crystals with the structure of garnet, the composition of which is expressed by the chemical formula:
A ₃ B ₂ C ₃ O ₁₂
Where
AY and at least one of the elements of the group: Tm, Ho, Er,
B - one of the elements of the group: Cr, Sc, Al, Ga;
C-Ga or Al,
moreover, the concentration of Cr, Tm, Ho, and Er, expressed in formula units (f.units), ranges from: 0.022 f.ed. up to 0.062 fed units for Cr; from 0,087 fed up to 0.199 f. for Tm; from 0.011 fu up to 0.025 f.ed. for Ho; from 0,022 f.ed. up to 0.249 f.ed. for Er.

On these crystals, in laser-pumped generation schemes, lasing at Tm (1.860 μm), Ho (2.090 kmk) and Er (1.650 μm, 2.710 μm) was obtained at room temperature. In addition, the regime of effective Q-switching at room temperature with diode pumping (G. Huber, EWDuczynski, K.Petermann, "Laser pumping of Ho" was implemented on a sodium-aluminum crystal with thulium and holmium (Y, Tm, Ho) ₃ Al ₅ O ₁₂ , Tm-, Er-doped garnet lfsers at room temperature ", IEEE J. Quantum Electronics, v.24, No.6 (1998), pp. 920-923).

Unlike other garnets doped with thulium ions, on crystals (Y, Tm) ₃ Al ₅ O ₁₂ , (Y, Lu, Tm) ₃ Al ₅ O ₁₂ and (Lu, Tm) ₃ Al ₅ O ₁₂ was obtained the maximum efficiency diode-pumped generation (JDKmetec, TS Kubo, and Thomas J. Kane "Laser performance of diodepumped thulium-doped Y ₃ Al ₅ O ₁₂ and Lu ₃ Al ₅ O ₁₂ crystals". OPTICS LETTERS, Vol. 19, N 3, ( 1994), pp. 186-188).

However, laser matrices with thulium based on crystals with a guarantor structure have several disadvantages. Firstly, the absorption maximum in these crystals is located at a wavelength of λ 785 nm, therefore, diode-pumped lasers require the use of either expensive non-standard diodes or complex systems based on standard (radiation at l 808 nm) diodes with cooling below 0 ^o C. Secondly, miniature gallium crystal lasers with thulium have a high generation threshold (more than 1.7 W) at a command temperature. Moreover, efficient generation of such lasers was obtained only at temperatures below 0 ^o C, but in real laser systems at an operating temperature of 30-40 ^o C the laser efficiency is less than 5%. Thirdly, these crystals have a low absorption coefficient at the pump wavelength, since For large Tullium ions Tm ³⁺ , intense concentration quenching of luminescence is observed in crystals with a garnet structure. Therefore, a significant drawback of 2 micron lasers based on garnets with thulium is the need to use relatively long (3-7 mm) active elements, which requires the use of an expensive complex optical system with cylindrical and spherical lenses to focus the radiation of a semiconductor ALGaAs diode.

 All of the disadvantages listed above complicate and make costly lasers based on crystals with a garnet structure.

As a prototype of the invention, it is possible to choose a camp substance based on yttrium orthovanadate containing thallium ions (Y, Tm) ₁ VO ₄ [2] This crystal was used to obtain stimulated radiation with a wavelength l = 1.94μm at a ³ H ₄ laser transition → ³ H ₆ . The differential efficiency of a laser operating at room temperature with a Ti-sapphire laser pumped at a wavelength of 800 nm was 25% at a generation threshold of 360 mW.

 The main disadvantages of this material include the low generation efficiency under laser pumping and the high generation threshold at room temperature, the absence of generation under diode pumping, and the low thermal conductivity of this material, which leads to strong local heating in the crystal pump region and cracking of the laser element at medium powers pump (1 W). The combination of these disadvantages does not allow the use of this laser substance in efficient miniature diode-pumped lasers.

 An object of the invention is to increase the efficiency of laser generation under diode and laser pumping, lower the generation threshold, increase the thermal conductivity of the laser substance, increase the temperature range of the laser and expand the arsenal of technical means that implement generation of thulium ions, including those operating in the continuous generation mode, and in the modes of generation and amplification of short pulses in the radiation range from 1.9 μm to 2.0 μm. In specific embodiments, it is also an object of the invention to increase the crystal growth rate and prevent crystal cracking during the manufacture and operation of laser elements, to reduce passive radiation losses at scattering centers and small-angle boundaries in the range from 0.35 μm to 2 μm.

The technical result is achieved both due to a change in the average ionic radius of the dodecahedron in the crystalline matrix, and as a consequence, an increase in the difference between the mass of vanadium ions and the average mass of ions replacing yttrium in the dodecahedron positions in the structure of YVO ₄ orthovanadate. In this case, due to a change in the strength of the crystalline field, it is possible to shift the wavelength of the laser radiation, in comparison with the known garnet and YVO ₄ crystals doped with thulium ions. In addition, when the crystal field strength changes, the probability of a ³ H ₄ → ³ H ₆ laser transition increases due to weaker splitting of the ³ H ₄ and ³ H ₆ levels for Tm ions and the participation of several shutters in the generation process. In turn, a change in the degree of splitting of the laser levels makes it possible to increase both the width of the luminescence spectrum and the cross section of the lasing transition. Additionally, due to the increase in the ion mass difference, the mean free path of thermal phonons along the layers of vanadium ions in the <110> direction increases, as a result of which the thermal conductivity of crystals increases by replacing the “light” Y ³⁺ ions with “heavy” ones (La ³⁺ , Gd ³⁺ , Lu ³⁺ , Er ³⁺ , etc.) ions. An increase in the cross section of the lasing transition and thermal conductivity of the laser substance makes it possible to reduce the thickness of the laser element (up to 1 mm), which leads to a high pump energy density along the entire length of the laser element and, as a result, to a sharp increase in the efficiency of a laser operating according to a 3-level scheme, and also allows the use of standard AlGaAs diodes for pumping, the radiation of which can be effectively focused only in thin (less than 2 mm) laser elements. The increased thermal conductivity of the crystal can also significantly expand the temperature range of the laser with high efficiency, up to temperatures significantly higher than room temperature, as well as increase the growth rate of single-crystal boules, due to the absence of cracking of the crystal at high growth rates. High thermal conductivity reduces the temperature gradients along the length and the cross section of the crystal, and consequently, the thermoelastic stresses leading to cracking of the crystal are reduced.

In specific forms of implementation, the technical result:
preventing cracking of the crystal, both during the manufacturing of laser elements and during the operation of the laser, if high-power continuous diodes (1–20 W) or the emission of several diodes through fiber systems are used for pumping,
reduction of passive radiation losses at scattering centers and small-angle boundaries in the range from 0.35 μm to 2 μm
achieved by additional introduction into the substance of at least one of the elements of the group: Li, Be, B, Na, Ca, Mg, Si, K, Ti, Mn, Fe, Co, Ni, Cu, Zn, Ge, Rb, Sr, Zr, Nb, Mo, W, Ta, Hf, Bi.

Small-angle boundaries in orthovanadate crystals arise during crystallization due to the cellular growth of the crystal due to the well-known "concentration supercooling", which is due to the presence of V ⁴⁺ and V ³⁺ ions in the melt. As a result, scattering centers are formed, and the optical quality of the crystals decreases due to the difference in refractive indices in neighboring regions of the crystal separated by small-angle boundaries. In addition, the evaporation of vanadium oxide from the melt surface, as well as the instability of the oxidation state of vanadium +5, increase the passive absorption due to the appearance of vacancies in the crystal in the oxygen sublattice and the formation of undesirable color centers V ⁴⁺ and V ³⁺ in the crystal. Adding to the required amount of a substance ions having a stable oxidation state of +4, +5, +6 (for example, Si, Ti, Mn, Zr, Nb, Mo, Ta, Hf, Bi, W, Ge) use lower melting temperature compared with a nominally pure congruent orthovanadate crystal (3-5 ^° C sufficient) and eliminate the formation of small-angle boundaries during crystal growth. The addition of mono- and divalent impurities in the required laser substance, such as Li, Mg, Ca, Na, B, Fe, Co, Ni, K, Cu, Be, Rb, Zn, Sr, stabilizes the oxidation state of vanadium +5 and eliminates the formation of color centers V ⁴⁺ and V ³⁺ in the crystal, in addition, prevents cracking of the crystal during the cutting and manufacture of laser elements, which (cracking) causes ions having a stable oxidation state of +5, +6.

A new laser substance based on the well-known crystal of orthovanadate (Y, Tm) ₁ VO ₄ , differs in that the crystal has a composition that is expressed by the chemical formula:
A _1-0,4-x B _0,4 Tm _x VO ₄ ,
Where
A at least one of the elements of the group: La, Ce, Gd, Er, Yb, Lu, Nb, Sm, Tb, Du, Eu.

B at least one of the elements of the group Y, Sc, La, Ce, Nb, Sm, Eu, Gd, Tb, Du, Er, Yb, Lu;
x from 0.005 file units up to 0.25 f.ed.

 The limitation in the quantitative composition at positions B is explained by a sharp decrease in the thermal conductivity of crystals when the content of elements such as Y and Sc in the crystal is higher than 0.4 ps. units Such a change in thermal conductivity leads to a decrease in efficiency and an increase in the generation threshold, as well as to a decrease in the temperature range of the laser.

The lower limit for tollium is determined by the fact that when the content of Tm in an amount of less than 0.005 f. units the generation efficiency at the two-micron transition ³ H ₄ → ³ H ₆ of Tm ³⁺ ions becomes insignificant due to weak absorption of the pump energy. As a result, the laser efficiency becomes low (less than 1%).

The upper limit of the content of thallium in the crystal is determined on the basis that when the content of thallium is more than 0.25 f. lasing efficiency decreases due to concentration quenching of the ³ H ₄ → ³ H ₆ laser transition, which is expressed in a sharp decrease in the lifetime of the upper laser level (by more than an order of magnitude) due to nonradiative transitions, and as a result of a sharp decrease in efficiency (less than 1%).

The laser substance including thulium (Tm) in the indicated amount additionally contains at least one of the elements of the group: Li, Be, B, Na, Ca, Mg, Si, K, Ti, Mn, Fe, Co, Ni, Cu, Zn, Ge, Rb, Sr, Nb, Mo, W, Ta, Hf, Bi in an amount of from 1 • 10 ¹⁷ cm ^-3 to 8 • 10 ²⁰ cm ^-3 .

 The lower boundary for these elements is determined by the fact that at concentrations below the specified limit there is no technical result consisting in preventing the formation of small-angle boundaries and distance centers in the growing crystal; reducing passive radiation losses at scattering centers and small-angle boundaries in the range from 0.35 μm to 2 μm in the laser element, as well as preventing cracking of the crystal during the manufacturing and operation of the laser element.

 The upper boundary for these elements is determined by their maximum possible content of orthovanadates in crystals. When they are kept above the specified limit, the formation of small centers of the distance of light ("fog") occurs during post-growth annealing of crystals, and as a result, a sharp decrease in the laser efficiency.

In FIG. Figure 1 shows the absorption spectrum of a N 6 crystal for σ (c ⊥ E) and π (c // E) polarizations. Similar to yttrium vanadate crystals, the new active medium has a smoother and more intense absorption spectrum in the 800 nm range than (Y, Tm) ₃ Al ₅ O ₁₂ crystals (analog), which has certain advantages for diode pumping.

In FIG. Figure 2 shows the results of thermal conductivity measurements of the N 2 crystal in the direction <001> (C axis) and the (Y, Tm) ₃ Al ₅ O ₁₂ crystal (analog) in the <001> direction in the temperature range 50–350 K. At 300 K, the thermal conductivity of a new the active medium doped with thulium is y 9.8 W / m • K; this is more than the thermal conductivity of the crystals (Y, Tm) ₃ Al ₅ O ₁₂ , for which y 8.9 W / m • K, and significantly more than the thermal conductivity of the prototype (Y, Tm) ₁ VO ₄ y 5.2 W / m • K) (Handbook of Lasers, edited by A.M. Prokhorov, Moscow, 1978).

In FIG. 3 for the N 3 crystal, the generation spectrum of Tm ³⁺ thulium ions is shown upon pumping by a laser diode with a generation wavelength of 806 nm with a power of 2 W. The maximum radiation amplitude occurs at a wavelength of 1.921 μm, while for the prototype this value is 1.94 μm. At the same time, a smoother and wider spectrum of radiation than that of the prototype indicates the possibility of tuning the radiation on thulium ions in the range of 1915 nm to 1935 nm.

In FIG. 4 shows the dependence of the output energy of a laser on a N 3 crystal and on a (Y, Tm) ₁ VO ₄ crystal (prototype) on the pump energy. It is seen that when the laser is pumped with a sapphire with titanium, the generation threshold of the proposed laser substance is lower, and the efficiency is higher. It also shows the dependence of the output power of a monolithic ("microchip") N 3 crystal laser on the pump power of a laser diode.

 The results of generation tests clearly demonstrate the high efficiency of the proposed laser substance for miniature diode-pumped lasers.

The growth of crystals of orthovanadates was carried out according to the general scheme by drawing from the melt. For example, to obtain a crystal of gadolinium-ytterbium-thallium-europium-yttrium-scandal vanadate, the chemical formula Gd, Yb, TM} _0.6 [Eu, Y, Sc] _0.4 VO ₄ , additionally containing potassium and hafnium, the following was used method: starting reagents: gadolinium oxide, ytterbium oxide, thulium oxide, europium oxide, yttrium oxide, scandium oxide and vanadium oxide were thoroughly mixed, pressed into tablets and synthesized in a platinum crucible for 10 hours at 1200 ^° C. Then, by induction heating tablets were melted in an iridium crucible in a sealed chamber in the atmosphere 98 vol. N ₂ +2 vol. O ₂ . Before growing, K ₂ CO ₃ and hafnium oxide were added to the melt. The crystals were drawn from the melt with a volume of 300 cm ³ at a speed of 4 mm / h, the crystal rotation frequency of 40 rpm. After the growth of the grown crystal from the melt, the crystal was gradually cooled to room temperature for 40 hours. Then, the crystalline boulevard was annealed in air for 48 hours at a temperature of 1200 ^o C.

 Examples of specific compositions of the grown crystals and prototype are shown in table 1.

To measure the absorption spectra, oriented samples were made of N 5 crystal with dimensions 5x5x0.7 mm ³ .

 The absorption spectra were studied using a Glan prism on an SF-20 automated spectrophotometer. As it turned out, the absorption intensity of the proposed substance is higher than that of the prototype, and the intense absorption of crystal N 5 in the region of 806 nm indicates the possibility of using standard laser diodes designed for niode-containing media (808 nm). The latter circumstance indicates a significant advantage of the proposed laser substance over known laser media with thulium ions.

The thermal conductivity of the crystals was measured according to the standard method described in (Orphan N.N. Popov P.A. Sidorov A.A. Ivanov I.A. Izv. AN BSSR, Ser. Phys.-Math. Sciences, 1990, p. 51-55). The experimental study of thermal conductivity was carried out on oriented single-crystal samples of garnet (Y, Tm) ₃ Al ₅ O ₁₂ (analogue) and an N 2 crystal cut in the form of parallelepipeds ³ × ³ × 40 mm ³ in size. The error in determining the thermal conductivity did not exceed 5% in the studied temperature range of 50-350 K.

Experimental studies of the generation characteristics were carried out on active elements made of crystal N 3. The size of the active elements: diameter 5 mm and length 1 mm. To compare the parameters of laser radiation, active elements of a similar size were made of a yttrium orthovanadate crystal Y, Tm} ₁ VO ₄ prototype. One of the ends of the active element (AE) was flat, the other had a concave spherical surface with a radius of curvature of 500 cm. Complex - dichroic coatings with the following parameters were applied to these ends: reflection of R at a wavelength l 800 nm (± 10 nm) R ₈₀₀ < 2% and transmittance T at l 1950 nm (± 50 nm) T ₁₉₅₀ <0.2% for a flat surface; T ₈₀₀ <0.2% and T ₁₉₅₀ = 2.4% for a concave surface.

Studies of the lasing characteristics of Y, Tm} ₁ VO ₄ crystal lasers and a N 3 crystal were carried out using the following scheme. The laser elements were pumped into the absorption band l 806 nm (see Fig. 1), corresponding to the ³ H ₄ → ³ H ₆ transition. Both Ti-sapphire laser and semiconductor laser were used as pump sources. The output radiation of both lasers was modulated at a frequency of 20 Hz with a duty cycle of 2. The radiation of these lasers was focused in the active element into a spot with a diameter of 25 μm for a Ti laser and a spot with a size of 20 × 75 μm for a diode laser. The pump radiation power reached 1.2 watts. A monochromator was used to control the radiation wavelength. The radiation after the monochromator was recorded by a PbS photodetector, the signal from which was automatically processed and recorded by a computer. The output characteristics of a N 3 crystal laser depended weakly on the pump radiation wavelength, which is in good agreement with the relatively wide absorption spectrum of thulium ions (Fig. 1). In our experiments, the optimal wavelength for pumping was 806 ± 2 nm. Note that this wavelength practically coincides with the absorption peak for pumping neodymium lasers; therefore, effective pumping of a thulium laser is possible with widely used and cheap laser diodes used to pump Nd ³⁺ neodymium ions at a temperature of 20 ^o C. In our experiments, we used the standard SDL diode at a temperature of 2 ^o C.

 The emission spectrum of a N 3 crystal laser was measured using a monochromator. The experimentally measured emission spectrum of a N 3 crystal laser is shown in FIG. 3, the measurement accuracy was 1 nm. The generation line has a relatively flat top with a maximum at a wavelength of 1923nm. The relatively large width of the radiation spectrum makes it possible to realize tunable generation by thulium ions.

When radiation was pumped by a Ti-spherical laser, the generation threshold for a N 3 crystal laser was approximately 174 mW. The average output power of the laser radiation reached 125 mW with a differential efficiency of 36%. However, for short intervals of time it was possible to fix the maximum output power of 180 mW, which corresponds to an absolute efficiency of 36% and a differential efficiency of 55%. Instability of the laser at high output powers, such as It turned out to be due to poor-quality coating of the output mirror, which was destroyed when the output radiation power was more than 125 mW. For comparison, in FIG. Figure 4 shows the dependence of the output power of a laser on a (Y, Tm) ₁ VO ₄ crystal on the absorbed pump power Ti of a sapphire laser. In the case of a (Y, Tm) ₁ VO ₄ crystal, the generation threshold was 370 mW, and the differential efficiency was 25%
When pumped by a laser diode, the differential efficiency of the N 3 crystal laser was 10% (Fig. 4). The highest output radiation power reached 43 mW with a pumping rate of 1.18 watts. A decrease in the lasing efficiency when pumped by a laser diode as compared to pumping with a Ti-sapphire laser is in good agreement with an increase in the size of the focus area of the pump radiation in the AE.

It should be noted that AEs in laser experiments worked without water cooling, and this led to a local conversation in the pump region up to 120 ^o C.

 For comparison, the main properties of the grown crystals and the prototype, as well as the results of laser tests are shown in table 2.3

Claims (1)

A laser substance based on a crystal of yttrium orthovanadate containing thulium T _m , characterized in that it has a composition described by the chemical formula
A _{1 - 0 , 4 - x} B _{0 , 4} Tm _x VO ₄ ,
where A is at least one of the elements of the group La, Cl, Gd, Er, Yb, Lu, Nd, Sm, Tb, Du, Eu;
In at least one of the elements of the group Y, Sc, La, Ce, Nd, Sm, Eu, Gd, Tb, Du, Er, Yb, Lu;
x 0.005 0.25 f. and further comprises at least one of the elements of the group Li, Be, B, Na, Ca, Mg, Si, K, Ti, Mn, Fe, Co, Ni, Cu, Zn, Ge, Rb, Zr, Nb, Mo, W, Ta, Hf, Bi in an amount from 1 • 10 ^{1 7} to 5 • 10 ^{2 0} cm ^{- 3} .

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