Aluminum phosphate glass composition

Abstract

The present invention relates to phosphate-based glasses doped with Er3 + and sensitized with Yb suitable for use as solid laser media in "eye protection" applications. In particular, the invention relates to the improvement of the physical properties of such phosphate-based laser glass compositions, in particular with respect to the strength of the glass structure and improved thermal shock resistance. The invention also relates to a solid state laser system comprising such a glass and to a method for generating laser beam pulses.

Description

[0001] This application is a divisional application of Chinese patent application No. 201710063521.5. Technical Field

[0002] The present invention relates to a solid laser medium doped with Er suitable for use in "eye protection" applications. 3+ In particular, the present invention relates to improving the physical properties of such phosphate-based laser glass compositions, particularly with respect to the strength of the glass structure and improved thermal shock resistance. Background Art

[0003] "Eye-friendly" lasers are lasers that operate at wavelengths that are less likely to damage the eye, particularly the cornea and retina. The cornea or vitreous humor of the eye absorbs laser beams with wavelengths greater than approximately 1.4 μm (1400 nm). This absorption by the cornea or vitreous humor avoids damage to the sensitive retina. In contrast, wavelengths less than approximately 1.4 μm are not absorbed in the cornea or vitreous humor and can therefore cause damage to the retina. On the other hand, laser beams with wavelengths greater than 1.8 μm (1800 nm) are too strongly absorbed by the cornea and can therefore cause damage to the cornea. Therefore, laser beams with wavelengths in the range of 1400 nm to 1800 nm are generally considered to be "eye-friendly" lasers.

[0004] Eye-safe lasers (Class 1 eye-safe lasers) are classified as lasers that do not emit an amount of available laser radiation exceeding the applicable available laser radiation for any exposure time within the maximum duration inherent in the design or intended use of the laser. See ANSI Z136.1 (Z136.1-2000) of the American National Standards Institute. See also 21 CFR Subchapter J, Section 1040.10.

[0005] Therefore, eye-safe (or retina-safe) operation of lasers outdoors is of great interest for the next generation of lasers used in many industrial, defense, and medical applications. As mentioned above, the wavelength range that is considered optimal for eye-safe operation is 1400nm to 1800nm. Laser emission at these wavelengths can be achieved in many ways. However, solid matrices doped with lanthanides remain the most commonly used method for high-energy laser gain. In particular, trivalent Er ions can produce direct emission at a wavelength of 1540nm. Therefore, Er is the rare earth element emitter of choice for many eye-safe applications. In recent years, the need for high beam quality under passive operation in outdoor applications has rekindled interest in Er-doped bulk glass as a gain material of choice for solid-state eye-safe lasers.

[0006] It is known that phosphate glass has a high sensitivity to Er at 1540 nm.3+ The emission produces high gain, especially when sensitized with Yb. See, for example, Meyer (US 4,962,067), Meyers (US 7,531,473), and Meyers et al. (US 6,911,160). Unfortunately, phosphate glass matrices tend to be weaker than other available amorphous materials. Therefore, there is a need for Er-doped phosphate glasses that can exhibit stronger thermomechanical properties. However, it is well known that the addition of elements that promote glass stability will reduce the laser gain obtained. It is also known that Er 3+ The emission is clearly affected by the phonon energy of the matrix glass. Summary of the Invention

[0007] Therefore, one aspect of the present invention is to provide Er-doped phosphate glasses that can be used as laser gain materials with improved thermomechanical figure of merit (FOM) while maintaining or improving the laser FOM.

[0008] Another aspect of the present invention is to provide a phosphate-based glass composition for use as a solid laser medium, which contains erbium, ytterbium, chromium and cerium like the commercial phosphate-based glass LG940 (Schott), and has a higher thermomechanical FOM and a comparable (if not higher) laser FOM compared to LG940.

[0009] Additional aspects and advantages of the present invention will become apparent to those skilled in the art upon further study of this specification and the appended claims.

[0010] According to the present invention, an Er-doped / Yb-sensitized phosphate glass composition is provided, which exhibits favorable laser and thermomechanical properties and is suitable for use in eye-protecting lasers.

[0011] According to one aspect of the present invention, the phosphate glass composition comprises (based on mol%):

[0012]

[0013]

[0014] in

[0015] R2O = the sum of the amounts of Li2O, Na2O, K2O, Rb2O and Cs2O;

[0016] MO = the sum of the amounts of MgO, CaO, SrO, BaO and ZnO; and

[0017] The total amount of Al2O3, SiO2 and Na2O is 20.00-28.00 mol%.

[0018] According to another aspect of the present invention, the general, preferred and particularly preferred amounts (based on mol%) of the glass components are listed in Table 1 below:

[0019]

[0020] The glass composition according to the present invention uses P2O5 as the main glass network former. The content of P2O5 is preferably maximized. Generally, the P2O5 content is 55.00 - 65.00 mol%, preferably 55.00 - 60.00 mol%, particularly 57.00 - 60.00 mol%. The P2O5 content can also be, for example, 55.0 mol%, 55.5 mol%, 56.0 mol%, 56.5 mol%, 57.0 mol%, 57.5 mol%, 58.0 mol%, 58.5 mol%, 59.0 mol%, 60.0 mol%, 60.5 mol%, 61.0 mol%, 62.0 mol%, 62.5 mol%, 63.9 mol%, 64.0 mol%, 64.5 mol%, 65.0 mol%, etc.

[0021] Al2O3 can also act as a network former and tends to improve the chemical durability of the glass and reduce water solubility. P2O5 acts as the main network former, while Al2O3 acts as an intermediate glass former.

[0022] In the glass composition of the present invention, SiO2 acts as a modifier. The amount of SiO2 can increase the thermal conductivity. However, a large amount of SiO2 can increase the crystallization tendency and / or cause phase separation, and can reduce the emission cross-section. The amount of SiO2 used to prepare the glass is generally 0.0 - 12.0 mol%, preferably 0.0 - 11.0 mol%, particularly 4.0 - 11.0 mol%, such as 9.0 - 10.0 mol%. The SiO2 content can also be, for example, 0.5 mol%, 1.0 mol%, 2.0 mol%, 3.0 mol%, 4.0 mol%, 5.0 mol%, 6.0 mol%, 7.0 mol%, 8.0 mol%, 9.0 mol%, 9.5 mol%, 10.0 mol%, 10.5 mol%, 11.0 mol% or 11.5 mol%.

[0023] As described above, Al2O3 acts as an intermediate glass former. As a result, Al2O3 exhibits the characteristics of both a glass former and a glass modifier. Al2O3 can not only provide better chemical durability but also better thermomechanical properties. However, a large amount of Al2O3 can induce crystallization and reduce the emission cross-section and the coefficient of thermal expansion. The content of Al2O3 is generally 4.0 to 20.00 mol%, preferably 4.0 to 18.0 mol%, particularly 5.0–7.0 mol% or 10.0-12.0 mol% or 15.0–18.0 mol%. Other suitable contents of Al2O3 are, for example, 5.0 mol%, 5.5 mol%, 6.0 mol%, 7.0 mol%, 7.5 mol%, 7.8 mol%, 8.0 mol%, 8.5 mol%, 9.0 mol%, 9.5 mol%, 10.0 mol%, 10.5 mol%, 11.0 mol%, 11.5 mol%, 12.0 mol%, 12.5 mol%, 13.0 mol%, 13.5 mol%, 14.0 mol%, 14.5.mol%, 15.0 mol%, 15.5 mol%, 16.0 mol%, 16.5 mol%, 17.0 mol% and 17.5 mol%.

[0024] The alkaline earth metal MO can improve the chemical durability of the glass. Generally, the amount of MO is 0.0–7.0 mol%, preferably 0.0–6.0 mol%, particularly 0.0–5.0 mol%. If present, the preferred alkaline earth metal is MgO because the amount of MgO tends to provide a higher thermomechanical FOM.

[0025] The alkali metal inclusion R2O will affect certain properties of the glass, such as the linear coefficient of thermal expansion and the emission cross-section. Generally, the amount of R2O is 12.00–25.00 mol%, preferably 14.00–22.00 mol%, particularly 15.00–21.00 mol%. Other contents of R2O are, for example, 14.5 mol%, 15.0 mol%, 15.5 mol%, 16.0 mol%, 16.5 mol%, 17.0 mol%, 17.5 mol%, 18.0 mol%, 18.5.mol%, 19.0 mol%, 19.5 mol%, 20.0 mol%, 20.5 mol%, 21.0 mol% and 21.5 mol%.

[0026] Preferably, the alkali metals present include both K2O and Na2O. Generally, the amount of K2O in the glass is 4.00–8.00 mol%, preferably 5.0 mol% to 7.0 mol%, especially 5.50-6.50 mol%. Other suitable contents of K2O are, for example, 4.5 mol%, 4.8 mol%, 5.1 mol%, 5.2 mol%, 5.4 mol%, 5.6 mol%, 5.7 mol%, 5.8 mol%, 5.9 mol%, 6.0 mol%, 6.1 mol%, 6.2 mol%, 6.3 mol%, 6.4 mol%, 6.8 mol%, 7.1 mol%, 7.5 mol% and 7.8 mol%. Generally, the amount of Na2O in the glass is 8.00–18.50 mol%, preferably 9.0 mol% to 16.0 mol%, especially 10.0-15.0 mol%. Other suitable contents of Li2O are, for example, 9.5 mol%, 10.5 mol%, 11.0 mol%, 12.0 mol%, 13.0 mol%, 14.0 mol% and 17.0 mol%. The amounts of Li2O, Rb2O and Cs2O are generally 0.0-2.0 mol% respectively, preferably 0.0–1.0 mol%.

[0027] The total amount of Al2O3, SiO2 and Na2O is generally 20.00–28.00 mol%, preferably 22.00–28.00 mol%, especially 25.00–28.00 mol%.

[0028] The laser-active ion of the glass composition is Er 3+ , while Yb 3+ , Cr 3+ and / or Ce 3+ acts as a sensitizer. Generally, the amount of Er2O3 is 0.03–1.00 mol%, preferably 0.03–0.50 mol%, especially 0.03–0.10 mol%, for example, 0.03–0.08 mol%, especially 0.05–0.08 mol%. Other suitable contents of Er2O3 are 0.04 mol%, 0.06 mol%, 0.07 mol%, 0.09 mol%, 0.11 mol%, 0.12 mol%, 0.15 mol%, 0.18 mol%, 0.20 mol%, 0.22 mol%, 0.25 mol%, 0.28 mol%, 0.30 mol%, 0.33 mol%, 0.35 mol%, 0.4 mol%, 0.45 mol%, 0.55 mol%, 0.6 mol%, 0.65 mol%, 0.7 mol%, 0.75 mol%, 0.8 mol%, 0.85 mol%, 0.9 mol% and 0.95 mol%.

[0029] Preferably, at least Yb2O3 is used as a sensitizer. The amount of Yb2O3 preferably reaches the solubility limit of Yb2O3 in the glass. Generally, the amount of Yb2O3 is 5.00–10.00 mol%, preferably 6.00–10.00 mol%, especially 7.00–10.00 mol%. Other suitable contents of Yb2O3 are, for example, 5.5 mol%, 6.5 mol%, 7.5 mol%, 7.8 mol%, 8.0 mol%, 8.3 mol%, 8.5 mol%, 8.7 mol%, 9.0 mol% and 9.5 mol%. The amount of Cr2O3 is usually 0.00–0.30 mol%, preferably 0.01–0.10 mol%, especially 0.02–0.05 mol%. Other suitable contents of Cr2O3 are, for example, 0.02 mol%, 0.03 mol%, 0.04 mol%, 0.06 mol%, 0.07 mol%, 0.08 mol% and 0.09 mol%. The amount of CeO2 is usually 0.00–0.30 mol%, preferably 0.00–0.20 mol%, especially 0.10–0.20 mol%. Other suitable contents of CeO3 are, for example, 0.02 mol%, 0.03 mol%, 0.04 mol%, 0.06 mol%, 0.07 mol%, 0.08 mol%, 0.09 mol%, 0.10 mol%, 0.13 mol%, 0.15 mol%, 0.18 mol%, 0.21 mol%, 0.25 mol% and 0.28 mol%.

[0030] The total amount of Er2O3, Yb2O3 and CeO2 is preferably 5.03–11.30 mol%, preferably 6.0–9.5 mol%, especially 7.0–9.5 mol%, such as 8.0–9.5 mol%, 6.0-9.0 mol%, 7.0–9.0 mol% and 8.0–9.0 mol%. In addition, except for Er2O3, Yb2O3, CeO2 and Cr2O3, the glass does not contain perceptible amounts of other laser-active ions and sensitizers. For example, the glass preferably contains 0.0 mol% Nb2O3.

[0031] Regarding other components, the glass optionally contains fining agents (e.g., As2O3 and Sb2O3) and / or antisolarants (e.g., Nb2O3). Generally, the amount of Sb2O3 (or As2O3) is 0.00–0.50 mol%, preferably 0.05–0.30 mol%, especially 0.05–0.20 mol%. The amount of Nb2O5 is usually 0.00–2.00 mol%, preferably 0.50–2.00 mol%, especially 0.50–1.50 mol%.

[0032] In addition, the glass composition may contain additional modifiers such as Bi2O3, TeO2, and GeO2. The amount of each of Bi2O3 and TeO2 is generally from 0.00 - 3.00 mol%, preferably from 0.00 - 2.50 mol%, particularly from 0.00 - 2.00 mol%. However, even small amounts of some modifiers such as GeO2 can cause instability. Therefore, the amount of GeO2 is preferably minimized, generally 2 mol% or less, preferably 0.05 mol% or less, particularly 0.10 mol% or less.

[0033] The glass according to the present invention can be characterized as a high lanthanide glass system, in which all of the rare earth elements and sensitizer inclusions (Ce + Cr + Er + Yb) can be replaced by La to produce an optical glass, such as for the cladding of an optical fiber. Therefore, another aspect of the present invention is an optical glass composition, such as for the cladding of a fiber, which comprises the following components (based on mol%):

[0034]

[0035] where

[0036] R2O = the sum of the amounts of Li2O, Na2O, K2O, Rb2O, and Cs2O;

[0037] MO = the sum of the amounts of MgO, CaO, SrO, BaO, and ZnO; and

[0038] where the sum of Al2O3, SiO2, and Na2O is from 20.00 - 28.00 mol%.

[0039] To contribute to generating high average power, phosphate-based laser glasses should have favorable thermo-mechanical properties. During operation, the cooling of the outer surface of the solid laser material will result in the formation of a thermal gradient, where the internal temperature of the material is higher than the temperature of the outer surface. This thermal gradient can then cause a stress gradient within the solid laser material, which can ultimately lead to the fracture of the active solid laser material.

[0040] Generally, the thermo-mechanical properties of a laser are evaluated by a parameter called the thermo-mechanical figure of merit TM-FOM. The thermo-mechanical figure of merit is proportional to the maximum thermal gradient that the material can withstand without fracture and also reflects the magnitude of the thermal gradient for a given situation.

[0041] According to the present invention, the thermo-mechanical figure of merit TM-FOM is calculated by the following formula:

[0042] TM-FOM = K 90C K IC (1 - ν) / (αE)

[0043] where

[0044] K 90C is the thermal conductivity [W / mK] measured at 90 °C,

[0045] K IC is the indentation fracture toughness [MPa·m 0.5 ,

[0046] ν is the Poisson's ratio,

[0047] E is the Young's modulus [GPa]; and

[0048] α is the linear thermal expansion coefficient in the range of 20 - 300 °C [10 -7 / K].

[0049] Therefore, as can be seen from the above equations, to increase the TM-FOM, it is desirable to have a high thermal conductivity and low thermal expansion coefficient, Poisson's ratio, and Young's modulus. For a given thermal gradient, when the product of the thermal expansion and Young's modulus is low, the amount of stress in the glass component is reduced. A higher thermal conductivity value helps to reduce the magnitude of the thermal gradient due to a given amount of heat deposited in the glass.

[0050] In addition, according to the present invention, the laser figure of merit L-FOM is calculated by the following formula:

[0051] L-FOM = σ em* (τ meas / τ rad )

[0052] where

[0053] σ em is the maximum emission cross-section, [×10 -20 cm 2 ,

[0054] τ meas is the measured radiative lifetime, (μsec);

[0055] τ rad is the calculated radiative lifetime, (μsec).

[0056] The laser performance can be measured according to the Judd-Ofelt theory, the Fuchtbauer-Ladenburg theory, or the McCumber method. Discussions of the Judd-Ofelt theory and the Fuchtbauer-Ladenburg theory can be found in E. Desurvire, Erbium Doped Fiber Amplifiers, John Wiley and Sons (1994). The McCumber method is described, for example, in Miniscalco and Quimby, Optics Letters 16(4) pp 258-266 (1991). See also Kassab, Journal of Non-Crystalline Solids 348(2004) 103–107. The Judd-Ofelt theory and the Fuchtbauer-Ladenburg theory evaluate the laser performance based on the emission curve, while the McCumber method uses the absorption curve of the glass.

[0057] Regarding the emission bandwidth, if there is a measured emission curve (collected in Judd-Ofelt or Fuchtbauer-Ladenburg analysis) or a calculated emission curve (from McCumber analysis), the emission bandwidth can be obtained in two ways. The first method is simply to measure the width at half of the maximum value (referred to as the full width at half maximum of the emission bandwidth or Δλ FWHM ).

[0058] The following examples shown are part of a study to strengthen the commercially available laser glass LG940 (SCHOTT) by adding modifiers in an attempt to improve the strength of the starting phosphate glass structure while having no adverse effect on the laser performance.

[0059] LG940 is an erbium-ytterbium-chromium-cerium doped phosphate-based laser glass that is used in flashlamp-pumped and diode-pumped solid-state laser systems. LG940 exhibits a high cross-section and high solubility for rare earth elements and is relatively easy to fabricate.

[0060] Large compositional studies have previously been carried out using systematic variations of alkali and alkaline earth metals in commercial phosphate laser glasses. See Hayden et al., “Effect of composition on the thermal, mechanical, and optical properties of phosphate laser glasses,” Proc. SPIE 1277, High-Power Solid State Lasers and Applications, 121 (August 1, 1990). However, that study investigated phosphate glasses doped with Nd 3+ rather than Er 3+ . Examples

[0061] In this example, 21 variations of LG940 glass were prepared. First, the glass was prepared by mixing various powdered raw materials in specified proportions such that the total amount of each batch would yield approximately 200 g of cast glass. These batches were placed in fused quartz crucibles and put into a resistance furnace with a temperature exceeding 1,000 °C. Once melted and clarified, the molten glass was cast and annealed for a period of time. Those glasses that were determined to well conform to the thermodynamic conditions involved in the standard manufacturing process (e.g., easily formed into glass and showed a decrease in the coefficient of thermal expansion) were repeated on a larger scale (e.g., 0.5 L). The larger-scale melting was accomplished using induction heating, and the molten liquid was stirred and clarified at a temperature exceeding 1,000 °C. Once the casting and forming processes were completed, samples for measurement were made from these glasses. Analyses of all required properties and measurements were completed for each fabricated composition.

[0062] The density was measured using the Archimedes method with a standard accuracy of ±0.003 g / cm 3 . The coefficient of thermal expansion CTE (α 20-300℃ , ±0.03 ppm / °C) and the glass transition point T g (±5 °C) were determined using dilatometry. The temperatures corresponding to specific viscosities, including the annealing point (3.16×10 14 poise), the strain point (1×l0 13 poise), and the softening point (3.98×10 7 poise), were determined using dilatometry, beam bending (3-point), and softening point methods. The melt viscosity around the working point of the glass (1×10 4 poise) was determined using high-temperature rheometry. Subsequently, these individual points were fitted using the well-known Volger-Fulcher-Tammann (VFT) model.

[0063] Differential thermal analysis (DTA) is used to detect the relative devitrification stability of each composition. The Vickers indentation method with a 3N load is used to measure hardness and fracture toughness. The pulse excitation technique is used to determine the Young's modulus. The refractive index is measured using the standard V-block method, and the dispersion value, the V"' refractive index at the emitted laser wavelength, and the nonlinear refractive index n2 are then calculated using these measurements. Using a Perkin Elmer Lambda or Lambda spectrophotometer, the transmission curve is obtained using the specified scanning conditions within the 200 nm to 2500 nm window. The dn / dT measurement is performed using the solid etalon method. The measurements are made at the respective wavelengths of the ions of interest and in the temperature range of 25 °C - 30 °C. As the temperature cycles up and down over the entire temperature range, the instrument measures the temperature-dependent displacement (Δλ) of the wavelength of the interference fringes. The dn / dT of the sample within the temperature interval is then calculated using the collected data. See S. George et al., SPIE Photonics West, Paper No. 9342-46, PW15L-LA101-71, 2015 presentation, which is incorporated herein by reference.

[0064] The presence of hydroxyl impurities in the glass can non-radiatively quench the laser excited state. See G. C. Righini et al., “Photoluminescence of Rare-Earth-Doped Glasses,” Rivista del Nuovo Cimento, 28(12), 1-53(2005). The conventional melt quenching process used to fabricate the glass can relatively easily introduce residual OH" species, which will subsequently affect the fluorescence decay of Er 3+ ions at 1.5 μm, resulting in a reduced quantum efficiency. This effect is usually most significant in lifetime measurements. Therefore, the residual hydroxyl content of all the glasses produced is monitored by taking advantage of the absorption features present near 3333 cm -1 (3.0 μm) and 3000 cm -1 (3.333 μm). The method employed assumes proportionality between the concentration of the OR species and the measured absorption. The magnitude of the hydroxyl absorption at the two previously mentioned wavelengths allows for the concentration estimation through the Beer-Lambert law. The ppm content concentration in the glass is not explicitly calculated but is set to the value of the maximum admissible absorption. For laser-grade glass gain elements, regardless of the active ions present in the glass, it is desired that the absorption at a wavelength of 3000 nm be less than 2.0 cm -1 , and most preferably less than 1.8 cm -1。

[0065] Fluorescence emission lifetime measurements were performed on 10 mm cubic samples and on powdered glass layers (to avoid Er ion self-pumping, which results in longer decay times from the cubic samples). Samples were prepared from each melt, with two adjacent sides polished and the remaining four sides finely ground. The samples were excited through one polished face with a laser diode at a nominal 980 nm and the emission was collected through the orthogonal polished face. The fluorescence lifetimes of erbium and ytterbium were measured separately by selecting the light emitted at 1550 nm and 1000 nm with a 10 nm FWHM interference filter. Careful analysis of the temporal emission from ytterbium also allowed determination of the energy transfer efficiency for each sample doped with erbium and ytterbium. The fluorescence lifetime, designated τ, was then calculated by fitting the data from t = 0 to the point where the intensity dropped below 1 / e of its initial value. Additional details are described in S. George et al., SPIE Photonics West, Paper No. 9342-46, PW15L-LA101-7, presented in 2015, which is incorporated herein by reference. See also http: / / www.pti-nj.com / brochures / QuantaMaster.pdf , E. Desurvire, Erbium-doped Fiber Amplifiers Principles and Applications, John Wiley and Sons, pg. 244-245 (1994) and S. George et al., Tougher Glasses for Eye-safe Lasers, Proc. SPIE 9466, Laser Technology for Defense and Security XI, 94660E (May 20, 2015) [http: / / spie.org / Publications / Proceedings / Paper / 10.1117 / 12.2176235?origin_id=x4318], which are incorporated herein by reference.

[0066] QuantaMaster from Photon Technology International TM A 50 NIR steady-state spectrofluorometer [see D. E. McCumber, Phys. Rev. 134, A299 (1964)] was used for all emission measurements. The instrument employed a TE-cooled InGaAs detector, where the sensitivity was enhanced by modulating the excitation light with an optical chopper and a lock-in amplifier at the detector end.

[0067] For Er- and Yb-doped glasses, the laser performance of the radiative lifetimes and cross-sections of stimulated absorption and emission as a function of wavelength is calculated by a simplification of the Judd-Ofelt (JO) theory, which is commonly referred to in the literature as the Fuchtbauer-Ladenburg (FL) relation. A brief description is provided by S. George et al., Tougher Glasses for Eye-safe Lasers, Proc. SPIE 9466, Laser Technology for Defense and Security XI, 94660E (May 20, 2015), which is incorporated herein by reference. S. George et al. reported in SPIE Photonics West, Paper No. 9342-46, PW15L-LA101-71, 201(citation continued on next page)5 presentation, http: / / www.pti-nj.com / brochures / QuantaMaster.pdf , and additional details are provided by E. Desurvire, Erbium-doped Fiber Amplifiers Principles and Applications, John Wiley and Sons, pg. 244-245 (1994), which is incorporated herein by reference.

[0068] The radiative lifetime is determined by Equation (1) below:

[0069] (1) 1 / τ rad = 8πcn 2 [(2J′ + 1) / λ 4 abs max (2J + 1)] ∫α(λ)dλ

[0070] where J′ and J are the total momenta of the lower and higher levels, respectively, in the case of erbium 15 / 2 and 13 / 2, and the integration is performed from 1400 nm to 1700 nm.

[0071] Subsequently, the emission cross-section is determined by Equation (2) below:

[0072] (2) σ emm (λ) = λ 4 g(λ) / [8πcn 2 τ rad

[0073] where g(λ) is the line shape function obtained from the emission data I(λ) collected using a PTIQM50 fluorescence spectrometer,

[0074] g(λ) = I(λ) ∫I(λ)dλ (Equation (3)).​

[0075] The compositions of 22 glasses prepared in the initial small-scale fabrication (200 g) are listed in Tables 1A and 1B below. During the initial small-scale fabrication of the 22 glasses (LG940 and 21 variants), one composition (Example 14) was found to be completely unsuitable for the melt quenching process employed. Another two compositions (Example 13 and Example 22) showed a tendency towards devitrification. The remaining nineteen glasses were stable for all processes. A set of properties of these 19 glasses were collected, including refractive index (measured at the Fraunhofer "D" line, the center of the yellow sodium doublet emission at 589 nm), dispersion, density, coefficient of thermal expansion (CTE), glass transition temperature (Tg), and fluorescence lifetimes of Er and Yb. Based on these properties, seven compositions were selected for larger-scale fabrication and detailed characterization. Table 2 lists the standard material properties of the 7 glasses for comparison. The nominal ion concentrations in all these glasses are 0.2×10 20 ions / cm 3 of Er and 23.5×l0 20 ions / cm 3 of Yb.

[0076] From the perspective of glass strength, low values of CTE, high thermal conductivity, and high fracture toughness are key properties. Fracture of the laser assembly occurs when the stress induced during pumping exceeds the tensile strength. The theoretical tensile strength of a defect-free material can be approximately estimated using the following equation (4):

[0077] (4) σ max ≈ E / 10.

[0078] where E is the Young's modulus (GPa). See, for example, R. Feldman et al., "Thermochemical strengthening of Nd:YAG laser rods", Proc. SPIE 6190, Solid State Lasers and Amplifiers II, 619019 (April 17, 2006).

[0079] In the real world, there is a huge difference between the theoretical fracture limit of materials in the Giga-Pa range and the component fracture limit achievable in cavities within the Mega-Pa limit. This difference is particularly large in the case of laser rods actively cooled under repeated thermal loads. The descriptive formalism starts with the heat dissipated per unit volume of the rod as a function of the absorbed pump power. In the case of Er, there is also energy transfer upconversion that affects the heat dissipated in the gain material, but this is ignored in a simple treatment. A part of the absorbed laser pump energy can be converted into heat through quantum defect heating. Thus, the total heat dissipated P h is a function of the optical pump power and the partial thermal load, as shown in Equation (5):

[0080] (5)P h =(1 - λP / λL)P p .

[0081] In a cooled laser rod (or in other geometries such as a flat plate), there is a thermal gradient where the center of the rod is hotter than the surface of the rod in contact with the cooling medium. In this case, the power of the dissipated heat can be related to the temperature difference by Equation (6):

[0082] (6)P h =T(0) - T(r0)·4πKL

[0083] where T(0) and T(r0) are the temperatures at the center and the surface of the rod, K is the thermal conductivity of the material, and L is the total length of the rod. Thus, the thermal difference from the edge to the center is proportional to the absorbed power and the thermal conductivity, and these temperature gradients induce mechanical stresses tangentially, radially, and axially in the gain component. When these stresses exceed the tensile strength of the rod, it results in fracture. See, for example, W. Koechner, Solid State Laser Engineering, 6th Edition, Springer, Berlin, (2006) pp. 439 - 481.

[0084] In the case of the rod, the total surface stress (up to rupture) is the vector sum of the tangential and axial components, and taking into account the basic material properties, as shown in Equation (7):

[0085] (7)σ T =[αE / 8πK(1 - v)]·P h L = √2σ φ .

[0086] where K is the thermal conductivity measured at 90 °C (K 90C ) [W / mK], v is the Poisson's ratio, E is the Young's modulus (GPa), and α is the linear coefficient of thermal expansion (K-1 ), and σ φ is the circumferential (tangential) stress. See, e.g., R. Feldman et al., "Thermochemical strengthening of Nd:YAG laser rods", Proc. SPIE 6190, Solid State Lasers and Amplifiers II, 619019 (April 17, 2006) and W. Koechner, Solid State Laser Engineering, 6th Edition, Springer, Berlin, (2006) pp. 439 - 481.

[0087] In addition, the actual fracture stress is a function of the surface finish of the component and is related by Equation (8):

[0088] (8) σ T = [αE / 8πK(1 - v)]·P h L = YK IC / √a

[0089] where K IC is the indentation fracture toughness (MPa·m 1 / 2 ), Y is the fracture orientation / geometry factor and is of the order of one, and a is the average depth of surface flaws introduced during the grinding and polishing steps of the fabrication process.

[0090] In terms of the heat dissipated per unit length of the rod, it becomes Equation (9), where R s is the thermal shock resistance:

[0091] (9) P h = [8πK(1 - ν) / αE]·σT·L = 8πR s L

[0092] For the purpose of materials research and development, the intrinsic material property from Equation 9 is used as the TM - FOM in order to evaluate the applicability by ranking through comparison. Thus, the TM - FOM is described by the thermal shock resistance parameter, as described in Equation (10):

[0093] (10) TM - FOM = R s = K(1 - v)K IC / αE [W / m 1 / 2

[0094] ​See, for example, W. Koechner, Solid State Laser Engineering, 6th Edition, Springer, Berlin, (2006) pp. 439 - 481; J. H. Campbell, J. S. Hayden, and A. Marker, High-Power Solid-State Lasers: a Laser Glass Perspective. International Journal of Applied Glass Science, 2:3–29 (2011); and W. F. Krupke, M. D. Shinn, J. E. Marion, J. A. Caird, and S. E. Stokowski, "Spectroscopic, optical, and thermomechanical properties of neodymium- and chromium-doped gadolinium scandium gallium garnet," J. Opt. Soc. Am. B 3, 102 - 114 (1986).

[0095] Equation (10) directly provides the maximum thermal load that a surface-cooled glass component can withstand before complete failure, especially when considering higher repetition rate operation. Thus, the best materials will have the maximum value of R s (TM-FOM).

[0096] In Table 3, the TM-FOMs of seven compositions selected for larger scale manufacturing are compared.

[0097] A brief treatment of thermally induced wavefront distortion due to gain material properties is described in Davis et al., "Thermal lensing of laser materials", in Laser-Induced Damage in Optical Materials: 2014, Gregory J. Exarhos; Vitaly E. Gruzdev; Joseph A. Menapace; Detlev Ristau; MJ Soileau, Editors, Proceedings of SPIE Vol. 9237 (SPIE, Bellingham, WA 2014), 92371. In that article, the classical case of a uniformly heated cylindrical rod with its outer surface at a constant temperature is presented, such as would be encountered in the case of a strongly cooled rod pumped CW under steady-state conditions. As presented in that article, for the relative ranking of materials during the R & D process, the thermo-optical response as a function of the exponent change is considered to be a function of temperature, as shown in Equation (11):

[0098] (11) \(n(T)=n(T_0)+(\frac{dn}{dT})(T - T_0)\).

[0099] Regarding the medium where the exponent change occurs, the following relationship is known from W. Koechner, Solid State Laser Engineering, 6th Edition, Springer, Berlin, (2006) pages 439 - 481:

[0100] (12) \((\frac{dn}{dT}) = n_{(absolute)}(\frac{dn}{dT}) + n_{(relative)}(\frac{dn}{dT})\) abs / dT) = n med (dn rel / dT) + n rel (dn med / dT)

[0101] where \(n_{(absolute)}\) is the refractive index with respect to vacuum and \(n_{(relative)}\) is the exponent with respect to the medium of interest (e.g., air). The \(dn / dT\) of air at -0.93 ppm / K is a non-negligible quantity, and it can be in the measured \(dn / dT\) and \(dn\) abs (absolute) refers to the refractive index with respect to vacuum and \(n_{(relative)}\) rel (relative) refers to the exponent with respect to the medium of interest (e.g., air). The \(dn / dT\) of air at -0.93 ppm / K is a non-negligible quantity, and it can be in the measured \(dn / dT\) and \(dn\) med / dT is a non-negligible quantity, and it can be in the measured \(dn / dT\) and \(dn\) abs / dT and \(dn\) relA significant difference in the value of / dT was produced [Davis et al., "Thermal lensing of laser materials", in Laser-Induced Damage in Optical Materials: 2014, Exarhos et al. (eds.); Proceedings of SPIE Vol. 9237 (SPIE, Bellingham, WA 2014), 92371].

[0102] For the purposes of this article, when an internal change in refractive index is encountered, dn abs / dT is the relevant property. The temperature-induced diopter of the gain material can be related to the heat dissipated in the rod (P h ) and the thermal conductivity by Equation (13):

[0103] (13) D thermo = (P h / πr o 2 K)(dn abs / dT).

[0104] The experimental glass data collected are compared in Table 4.

[0105] The absorption caused by water molecules present in the glass structure is evaluated, and this absorption is shown in Table 5 for the glasses evaluated. As previously mentioned, for laser-grade glass gain elements, regardless of the active ions present in the glass, the absorption at a wavelength of 3000 nm must be less than 2.0 cm -1 , and most preferably less than 1.8 cm -1 .

[0106] Table 6 shows for Er 3+J-O calculations of laser-active ions and measured lifetimes, as well as measured lifetimes for Yb-sensitized ions. An estimate of the quantum yield can be obtained from the ratio of the calculated and measured lifetimes [J.S. Hayden, Y.T. Hayden, J.H. Campbell; Effect of composition on the thermal, mechanical, and optical properties of phosphate laser glasses. Proc. SPIE 1277, High-Power Solid-State Lasers and Applications, 121 (August 1, 1990)]. This is also given in Table 6. Since the fact that various non-radiative loss mechanisms will affect the emission lifetime, the quantum efficiency will never be unity. In the presence of hydroxyl species and transition metal ion impurities such as Cu 2+ 、Ni 2+ 、Fe 2+ 、Co 2+ etc., a significant shortening of the lifetime is observed. In the absence of hydroxyl and ionic impurities, as observed from the data presented in Table 6, the measured lifetime tends to be longer than the calculated radiative lifetime due to Er self-pumping. The only exception is Example 17, which shows that the measured lifetime is shorter than the calculated lifetime. Considering the high OH absorption for this particular glass found in Table 5, this is in line with the expected result.

[0107] Calculated laser performance for Er3+ laser-active ions is given in Table 7. As described above, the L-FOM is calculated by the equation L-FOM = σ em *(τ meas / τ rad ).

[0108] Table 1A. Examples of phosphate-based glass compositions (mol%) doped with Er 3+ and sensitized with Yb 3+

[0109]

[0110] Table 1B. Examples of phosphate-based glass compositions (mol%) doped with Er 3+ and sensitized with Yb 3+

[0111]

[0112] Table 2. Material properties of glasses selected for larger-scale manufacturing

[0113]

[0114] Table 3. TM-FOM Comparison

[0115]

[0116] Table 4. Thermo-Optical Response

[0117]

[0118] Table 5. Hydroxyl Content

[0119]

[0120] Table 6. Calculated and Measured Lifetimes

[0121]

[0122] Table 7. Er Laser Performance (J-O Method)

[0123]

[0124] The entire disclosures of all applications, patents, and publications cited herein are hereby incorporated by reference.

[0125] The foregoing examples can be reproduced in a similar and successful manner by substituting the reactants and / or operating conditions of the general or specific description of the present invention for those used in the foregoing examples.

[0126] From the foregoing description, those skilled in the art can readily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.

Claims

1. A phosphate glass composition, which comprises (based on mol%): wherein R2O = the sum of the amounts of Li2O, Na2O, K2O, Rb2O and Cs2O; MO = the sum of the amounts of MgO, CaO, SrO, BaO and ZnO; and wherein the sum of Al2O3, SiO2 and Na2O is 20.00 - 28.00 mol%.

2. The glass composition according to claim 1, wherein the amount of P2O5 is 55.00–60.00 mol%.

3. The glass composition according to claim 1, wherein the amount of P2O5 is 57.00–60.00 mol%.

4. The glass composition according to any one of claims 1 to 3, wherein the amount of Al2O3 is 5.00–7.00 mol%.

5. The glass composition according to any one of claims 1 to 3, wherein the amount of Al2O3 is 10.00–12.00 mol%.

6. The glass composition according to any one of claims 1 to 3, wherein the amount of Al2O3 is 15.00–18.00 mol%.

7. The glass composition according to any one of claims 1 to 6, wherein the amount of K2O is 5.00–7.00 mol%.

8. The glass composition according to any one of claims 1 to ⑥, wherein the amount of K2O is 5.50–6.5 mol%.

9. The glass composition according to any one of claims 1 to 8, wherein the amount of Na2O is 9.00–16.00 mol%.

10. The glass composition according to any one of claims 1 to 8, wherein the amount of Na2O is 10.00–15.00 mol%.

11. The glass composition according to any one of claims 1 to 10, wherein the amount of Li2O is 0.00–1.00 mol%.

12. The glass composition according to any one of claims 1 to 11, wherein the amount of Rb2O is 0.00–1.00 mol%.

13. The glass composition according to any one of claims 1 to 12, wherein the amount of Cs2O is 0.00–1.00 mol%.

14. The glass composition according to any one of claims 1 to 13, wherein the amount of SiO2 is 0.00–11.00 mol%.

15. The glass composition according to any one of claims 1 to 13, wherein the amount of SiO2 is 4.00–11.00 mol%.

16. The glass composition according to any one of claims 1 to 15, wherein the amount of MO is 0.00–6.00 mol%.

17. The glass composition according to any one of claims 1 to 15, wherein the amount of MO is 0.00–5.00 mol%.

18. The glass composition according to any one of claims 1 to 17, wherein the amount of TeO2 is 0.00–2.50 mol%.

19. The glass composition according to any one of claims 1 to 17, wherein the amount of TeO2 is 0.00–2.00 mol%.

20. The glass composition according to any one of claims 1 to 19, wherein the amount of GeO2 is 0.00–0.50 mol%.

21. The glass composition according to any one of claims 1 to 19, wherein the amount of GeO2 is 0.00–0.10 mol%.

22. The glass composition according to any one of claims 1 to 21, wherein the amount of Nb2O3 is 0.50–2.00 mol%.

23. The glass composition according to any one of claims 1 to 21, wherein the amount of Nb2O3 is 0.50–1.50 mol%.

24. The glass composition according to any one of claims 1 to 23, wherein the amount of Sb2O3 is 0.05–0.30 mol%.

25. The glass composition according to any one of claims 1 to 23, wherein the amount of Sb2O3 is 0.05–0.20 mol%.

26. The glass composition according to any one of claims 1 to 25, wherein the amount of Cr2O3 is 0.01–0.10 mol%.

27. The glass composition according to any one of claims 1 to 25, wherein the amount of Cr2O3 is 0.02–0.05 mol%.

28. The glass composition according to any one of claims 1 to 27, wherein the amount of CeO2 is 0.00–0.20 mol%.

29. The glass composition according to any one of claims 1 to 27, wherein the amount of CeO2 is 0.10–0.20 mol%.

30. The glass composition according to any one of claims 1 to 29, wherein the amount of Er2O3 is 0.03–0.10 mol%.

31. The glass composition according to any one of claims 1 to 29, wherein the amount of Er2O3 is 0.03–0.08 mol%.

32. The glass composition according to any one of claims 1 to 29, wherein the amount of Er2O3 is 0.05–0.08 mol%.

33. The glass composition according to any one of claims 1 to 32, wherein the amount of Yb2O3 is 6.00–10.00 mol%.

34. The glass composition according to any one of claims 1 to 32, wherein the amount of Yb2O3 is 7.00– ten.00 mol%.

35. The glass composition according to any one of claims 1 to 34, wherein the amount of R2O is 14.00–22.00 mol%.

36. The glass composition according to any one of claims 1 to 34, wherein the amount of R2O is 15.00–21.00 mol%.

37. The glass composition according to any one of claims 1 to 36, wherein the total amount of Al2O3, SiO2 and Na2O is 22.00–28.00 mol%.

38. The glass composition according to any one of claims 1 to 36, wherein the total amount of Al2O3, SiO2 and Na2O is 25.00–28.00 mol%. It should be noted that in the above translation, there is an error in the English expression of "ten.00" in the translation of claim 14. It should be "10.00". The corrected translation is as follows:

20. The glass composition according to any one of claims 1 to 19, wherein the amount of GeO2 is 0.00–0.50 mol%.

21. The glass composition according to any one of claims 1 to 19, wherein the amount of GeO2 is 0.00–0.10 mol%.

22. The glass composition according to any one of claims 1 to 21, wherein the amount of Nb2O3 is 0.50–2.00 mol%.

23. The glass composition according to any one of claims 1 to 21, wherein the amount of Nb2O3 is 0.50–1.50 mol%.

24. The glass composition according to any one of claims 1 to 23, wherein the amount of Sb2O3 is 0.05–0.30 mol%.

25. The glass composition according to any one of claims 1 to 23, wherein the amount of Sb2O3 is 0.05–0.20 mol%.

26. The glass composition according to any one of claims 1 to 25, wherein the amount of Cr2O3 is 0.01–0.10 mol%.

27. The glass composition according to any one of claims 1 to 25, wherein the amount of Cr2O3 is 0.02–0.05 mol%.

28. The glass composition according to any one of claims 1 to 27, wherein the amount of CeO2 is 0.00–0.20 mol%.

29. The glass composition according to any one of claims 1 to 27, wherein the amount of CeO2 is 0.10–0.20 mol%.

30. The glass composition according to any one of claims 1 to 29, wherein the amount of Er2O3 is 0.03–0.10 mol%.

31. The glass composition according to any one of claims 1 to 29, wherein the amount of Er2O3 is 0.03–0.08 mol%.

32. The glass composition according to any one of claims 1 to 29, wherein the amount of Er2O3 is 0.05–0.08 mol%.

33. The glass composition according to any one of claims 1 to 32, wherein the amount of Yb2O3 is 6.00–10.00 mol%.

34. The glass composition according to any one of claims 1 to 32, wherein the amount of Yb2O3 is 7.00–10.00 mol%.

35. The glass composition according to any one of claims 1 to 34, wherein the amount of R2O is 14.00–22.00 mol%.

36. The glass composition according to any one of claims 1 to 34, wherein the amount of R2O is 15.00–21.00 mol%.

37. The glass composition according to any one of claims 1 to 36, wherein the total amount of Al2O3, SiO2 and Na2O is 22.00–28.00 mol%.

38. The glass composition according to any one of claims 1 to 36, wherein the total amount of Al2O3, SiO2 and Na2O is 25.00–28.00 mol%.

39. A solid-state laser system comprising a solid gain medium and a pump source, wherein the solid gain medium is a glass having the composition according to any one of claims 1 to 38.

40. A method for generating laser beam pulses, comprising flashlamp pumping or diode pumping of the glass composition according to any one of claims 1 to 38.

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