Silica and Boron-Containing Ultraphosphate Laser Glass With Low Concentration Quenching and Improved Thermal Shock Resistance

This invention relates to neodymium-doped phosphate laser glasses having a high stimulated emission cross-section together with desirably low self-concentration quenching of the neodymium fluorescence and high thermal shock resistance, in comparison to prior-art and commercially-available phosphate laser glasses.

Glass is an attractive host material for neodymium lasers. It is easy to prepare in high optical quality and large-size relative to crystals, and a broad and continuously adjustable range of properties may be produced.

For high average power, high repetition rate glass lasers useful for materials processing, or other applications such as miniaturized lasers for integrated optics, it is desirable to provide active materials with high induced emission cross-sections (.sigma.) which can tolerate high dopant levels without significant self-concentration quenching, as the gain of the device is proportional to the product of .sigma. and the number of activated ions. Most solid-state laser materials such as YAG or glass exhibit significant self-concentration quenching (i.e., transfer of excited state energy between adjacent activator ions) at relatively low ion concentrations, typically.about.2.times.10.sup.20 cm.sup.-3 for glasses. At higher concentrations the quenching becomes increasingly severe, e.g., in the prior art, commercially-available phosphate glass LG-750.

The first material reported to have extremely low concentration quenching was the crystalline ultraphosphate NdP.sub.5 O.sub.14, (Danielmeyer, H. G.; Weber, H. P.; IEEE J. Quant. Electron. EQ-8, 805 (1972)) and its glassy analog (Damen. et al., U.S. Pat. No. 3,863,177, Jan. 28, 1975). While glassy NdP.sub.5 O.sub.14 has desirable laser properties, it is virtually impossible to produce optical quality glass in commercial quantities and sizes due to its crystallization tendency and the high level of phosphorus volatility, which necessitates production in sealed pressure bombs. Since its introduction, many low quenching rate crystals have been reported.

Low self-concentration quenching rate glasses described in the open literature appear to be confined to two main families, the first in the system R.sub.2 O.Ln.sub.2 O.sub.3.P.sub.2 O.sub.5 and the second in the system R.sub.2 O.Al.sub.2 O.sub.3.Nd.sub.2 O.sub.3.P.sub.2 O.sub.5 where R.sub.2 O is Li.sub.2 O, Na.sub.2 O, or K.sub.2 O and Ln=La+Nd. A summary of compositional effects on concentration quenching rate and laser properties in these systems has been given by Cook, et al., SPIE, Proc. 505, 102 (1984) (ref. 1). The composition with the lowest concentration quenching rate reported in that study was Composition 16. This composition was a simple alkali ultraphosphate. Although its quenching rate was somewhat higher than glassy NdP.sub.5 O.sub.14, its fluorescence lifetime for all neodymium concentrations below approximately 2.times.10.sup.21 ions/cm.sup.3 was substantially above the latter glass. Additionally, its emission cross-section was substantially higher than glassy NdP.sub.5 O.sub.14, making it of greater practical utility for high average power lasers. The glass was also sufficiently stable to allow production by conventional glassmaking techniques.

In the operation of a high average power, high repetition rate glass laser, the exposure of the glass to pump light coupled with absorption of the light by neodymium results in heat build-up within the glass. As laser efficiency decreases rapidly with temperature, it is common for such lasers to be cooled by liquids or gases. This leads to a temperature gradient, and thus a stress gradient, in the glass which can lead to fracture if stresses are great enough. This has been well discussed in the literature; for example, see H. Rawson, PROPERTIES AND APPLICATION OF GLASS, Elsevier 1980, pp. 82-86.

For the case of a sheet of glass of temperature (T.sub.i), which is then cooled to a lower surface temperature, T.sub.o, the surface tensile stress, S.sub.t, produced by the temperature drop is given by,

where E is Young's Modulus, .alpha. is the coefficient of thermal expansion, and .sigma. is Poisson's Ratio. Based on the need for a glass laser to survive the largest possible temperature gradient in operation, a Figure of Merit (FOM) formula derived from the above equation is often used to rank laser glasses for resistance to thermal shock under moderate heat flow conditions,

where K is the thermal conductivity, and S.sub.t is the tensile strength of the glass (S. W. Freiman, "Fracture Mechanics of Glass", pp. 21-78 in GLASS: SCIENCE AND TECHNOLOGY, D. Uhlmann, N. Kreidl, eds. Academic Press 1980).

The FOM is directly proportional to the maximum temperature gradient that the glass can withstand before fracture occurs; thus the higher the FOM, the more suitable a glass will be for the above applications. Because the range of tensile strength values for glass is primarily determined by the distribution of surface flaws rather than intrinsic material properties, the term S.sub.t is generally omitted from the FOM when making relative comparisons. FOM values quoted hereafter are so calculated.

Table 1 gives a comparison of thermal FOM and related physical and laser properties for the low quenching rate ultraphosphate no. 16 of ref. 1 and a number of commercially-produced phosphate laser glasses. It is clear that the ultraphosphate is unsuitable for practical applications because of its low FOM.

Again, neglecting tensile strength, the physical properties whose variation produce the largest improvement in FOM are the thermal expansion coefficient, Young's Modulus, and thermal conductivity. Thus, for the highest possible FOM one would desire a glass with the lowest possible .alpha. and E, and the highest possible K. The compositional influence on the aforementioned physical properties in phosphate glasses is generally known, as reviewed in N. H. Ray, "Compositional-Property Relationships in Inorganic Oxide Glasses," J. Non-Cryst. Solids 15 (1974), p. 423-34. In phosphate glasses, improvements in FOM, i.e., reduction in .alpha. and E with increase in K, have been most commonly achieved by additions of Al.sub.2 O.sub.3. Indeed, virtually every prior-art phosphate laser glass contains substantial amounts of aluminum oxide. However, as can be clearly seen in ref. 1, the addition of aluminum to ultraphosphates leads to strong increases in concentration quenching rates and decreases the emission cross-section at those concentrations which are effective in raising the thermal FOM. Thus, in contradiction to prior-art disclosures, the incorporation of Al.sub.2 O.sub.3 is contraindicated for laser glasses suitable for high average power applications.

It is also known that both B.sub.2 O.sub.3 and SiO.sub.2 may be added to phosphate glasses in order to reduce the thermal expansion coefficient and improve the mechanical strength. This is generally discussed in LASER PHOSPHATE GLASSES, M. E. Zhabotinskii, ed., Nauka publ., Moscow, 1980. Phosphate laser glasses incorporating silica, and boron in theory, have also been disclosed very generically in the patent literature (see Japanese Pat. No. 7842334(no B), German Pat. No. DE 3435133 and U.S. Pat. No. 4,075,120). These glasses, particularly those disclosed in German Pat. No. DE 3435133, show thermal expansion coefficients of 80-94.times.10.sup.-7 C.degree. .sup.-1, which generally are significantly lower than the aluminophosphate laser glasses summarized in Table 2 below. Insufficient information was disclosed to allow calculation of thermal F.O.M. However, these silicaphosphate glasses do not have concentration quenching rates significantly different from aluminophosphate glasses (see Table 2, No. DE 3435133). The U.S. patent does not give any data on physical or laser properties of its examples; it incorporates SiO.sub.2 as an antisolarant only.

Accordingly, it is an object of this invention to provide a neodymium-doped silica and/or boron-containing ultraphosphate laser glass, and the corresponding laser devices, optical elements, etc., of relatively low self-concentration quenching rate, high thermal shock resistance and high induced emission cross-section compared to prior-art phosphate laser glasses.

Upon further study of the attached specification and claims, further objects and advantages of the invention will become apparent to those skilled in the art.

The above objects have been attained by the present invention by providing an optical quality glass suitable for use as a laser, containing amounts of Nd effective for lasing, and having an induced emission cross-section, .sigma., of at least 3.5, a first e-folding time of the neodymium fluorescence at a doping level of 10 wt. % Nd.sub.2 O.sub.3 of at least 175 .mu.sec, and a thermal figure of merit of at least 0.70 which consists esentially of, in molar percent:

The glasses of this invention have emission cross-sections (.sigma.), fluorescence linewidths (.DELTA..lambda.) and low Nd.sup.3+ concentration fluorescence lifetimes similar to other commercially-utilized prior-art laser glasses as previously described. They are substantially different in that they combine these properties with desirable thermal shock-related properties; specifically, low thermal expansion coefficients, low Young's Modulus, and high thermal conductivity with extremely low self-concentration quenching rates. They thus provide fluorescence lifetimes at very high Nd.sup.3+ doping levels (.about.10% Nd.sub.2 O.sub.3 by weight) far higher than other prior-art glasses.

This desirable combination of properties is achieved by substituting SiO.sub.2 and B.sub.2 O.sub.3 for P.sub.2 O.sub.5 and K.sub.2 O in an aluminum-free, ultraphosphate base glass of composition essentially similar to those disclosed in reference 1. The effectiveness of this substitution was not suggested in prior-art laser glass disclosures, which utilize almost entirely Al.sub.2 O.sub.3 to achieve requisite physical properties and neglect compositional effects on self-concentration quenching of the activator ion. The closest prior-art patent (Germany No. DE 3435133) utilizes a combination of SiO.sub.2 and, in theory, B.sub.2 O.sub.3 in combination with P.sub.2 O.sub.5 and, optionally, with Al.sub.2 O.sub.3, to provide glasses with low expansion coefficient. However, as mentioned above, the range of composition used leads to glasses with relatively high self-concentration quenching. The glasses of this invention exclude amounts of Al.sub.2 O.sub.3, e.g., all substantial amounts having an effect on the glass properties. Additionally, the theoretical maximum range of B.sub.2 O.sub.3 (0-5 mol. %) is below the range for the present invention (5-20% and preferably >5 to 20%, including 5.01, 5.1, 5.2, 5.3, 5.4, 5.5, etc.) Also, in that patent, B.sub.2 O.sub.3 is disclosed as interchangeable with La.sub.2 O.sub.3 and Y.sub.2 O.sub.3. This is not the case for the present invention, as the three compounds do not play the same structural role in the phosphate glasses of the invention. The criticality of B.sub.2 O.sub.3 per this invention is not suggested.

The preferred compositional ranges of this invention are generally 50-60 mol. % P.sub.2 O.sub.5, 10-20 mol. % SiO.sub.2, 10-15 mol. % B.sub.2 O.sub.3, 5-10 mol. % K.sub.2 O, 0-10 mol. % ZnO, 5 mol. % or 4.5-5.5 mol % sum La.sub.2 O.sub.3 +Nd.sub.2 O.sub.3, where the sum SiO.sub.2 +B.sub.2 O.sub.3 is 15-35 mol. %. As regards the criticality of the preferred ranges, the most critical are P.sub.2 O.sub.5, SiO.sub.2, B.sub.2 O.sub.3 and K.sub.2 O.

For P.sub.2 O.sub.5, above 60 mol. %, no advantage with respect to concentration quenching is obtained while thermal expansion coefficient increases and thermal conductivity decreases, making thermal shock resistance poorer. Below 50 mol. %, glasses become more difficult to melt due to increased viscosity. Below 40 mol. %, concentration quenching of Nd.sup.3+ increases sharply.

For SiO.sub.2, below 10 mol. % incorporation gives little improvement in thermal shock-related properties, specifically thermal expansion coefficient and thermal conductivity. Above 20 mol. %, concentration quenching of Nd.sup.3+ rapidly increases and emission cross-section becomes undesirably low. Additionally, glasses become increasingly unstable, giving devitrification and phase separation. Also, the melt viscosity becomes undesirably high.

For B.sub.2 O.sub.3, concentrations below 10 mol. % give only slight improvements in thermal FOM, and below 5 mol. % there is no noticeable improvement at all. Above 15 mol. %, while thermal conductivity continues to increase, Young's Modulus also increases sharply, which is undesirable. Above 20 mol. %, volatile losses of B.sub.2 O.sub.3 and P.sub.2 O.sub.5 become excessive and melt viscosities become undesirably high. Additionally, concentration quenching begins to increase strongly, emission cross-section becomes undesirably low, and the fluorescence linewidth increases, indicating that there is a distinct change in the structural role of B.sub.2 O.sub.3 at those levels.

Alkali content is particularly critical. Preferred glasses utilize potassium only as concentration quenching rates increase in the order K.sub.2 O<Na.sub.2 O<Li.sub.2 O. Below 5 mol. %, P.sub.2 O.sub.5 volatility becomes undesirably high; and, in the presence of B.sub.2 O.sub.3 and SiO.sub.2, melt viscosity becomes undesirably high. Above 10 mol. %, the thermal expansion coefficient becomes undesirably high if good thermal shock resistance is desired. Above 20 mol. % or below 5 mol. %, concentration quenching rates increase strongly.

The function of additions of alkaline earth elements, MgO, CaO, SrO, BaO, and particularly ZnO, is to improve chemical durability without changing the emission cross-section. Above 10 mol. % incorporation, concentration quenching rates increase sharply. Below 5 mol. %, there is little effect.

La.sub.2 O.sub.3 is included as a structural replacement of Nd.sub.2 O.sub.3 where necessary, thereby enabling attainment of the desirable thermal properties of the invention at all useful doping levels. This allows continuously variable adjustment of Nd.sup.3+ from very low to very high doping levels (.about.20 wt. %) without significant change in physical, optical, or laser properties as long as the molar sum Nd.sub.2 O.sub.3 +La.sub.2 O.sub.3 is kept in the range of the invention. The amount of Nd ions is not critical. The amount used in a given case will be selected routinely to be effective for lasing under the chosen conditions. Often, the amount will be 0.1 to about 3 or 4 mol. %, but significant lower or higher amounts can be employed where suitable.

It is possible to add conventional refining agents such as As.sub.2 O.sub.3 and Sb.sub.2 O.sub.3 ; amounts of 0.1-0.3 wt. % being generally sufficient.

It is additionally possible to add conventional antisolarants such as TiO.sub.2, CeO.sub.2, and Nb.sub.2 O.sub.5 in amounts as needed, e.g., 0.1-0.5 wt. %. In strong contrast to U.S. Pat. No. 4,075,120, SiO.sub.2 is not added as an antisolarant in this invention but is an integral structural component added in amounts well in excess of what would be needed to suppress solarization.

The glasses of this invention have the following properties:

(a) refractive index, nd, .gtoreq.1.520, generally 1.525-1.537;

(b) Abbe number, Vd, >60, generally 63-67;

(c) densities <3.0 g/cm.sup.3, generally 2.6-2.8;

(d) thermal expansion coefficients .ltoreq.110.times.10.sup.- 7.degree.C.sup.-1, generally 80-110;

(e) Young's Modulus, E, <70.times.10.sup.3 N/mm.sup.2, generally 45-55;

(f) Poisson's Ratio, .nu., <28, generally 0.20-0.25;

(g) thermal conductivity, K, >0.5 W/m.K, generally 0.6-0.8;

(h) thermal FOM's (1-.nu.).multidot.K/.alpha.E>0.7 generally 0.9-1.2;

(i) induced emission cross-section, .sigma., >3.5.times.10.sup.-20 cm.sup.2, typically 3.8-4.5 or 3.87-4.5;

(j) fluorescence linewidth (.DELTA..lambda..sub.f1)<23.5 nm, typically 21-23 nm; and

(k) first e-folding time of the Nd.sup.3+ fluorescence >375 .mu.sec at 0.5 wt % and >175 at 10 wt. %, typically 385-400 .mu.sec and 180-185 .mu.sec, respectively.

These glasses can be prepared by conventional methods used to prepare phosphate glasses, usually utilizing platinum crucibles. Similarly, optical elements or laser devices can be prepared from the glasses using conventional techniques.

Without further elaboration, it is believed that one skilled in the art can, using the preceeding description, utilize the present invention to its fullest extent. The following preferred embodiments are, therefore, merely illustrative and not limitative of the remainder of the disclosure in any way. In the following examples, all temperatures are in degrees Celsius; and, unless otherwise indicated, all parts and percentages are molar.

A number of typical glass compositions within the scope of this invention together with their properties are given in Table 2 in mole %. Also included in this table for comparative purposes is the simple ultraphosphate of reference 1 previously mentioned. Of the compositions given, compositions D, F and G are preferred. Weight % compositions are given in Table 3.

All of the glasses of the examples were fabricated by batch melting. The requisite weights of each batch ingredient corresponding to the desired wt.% were mixed and added to a fused silica crucible and melted by induction heating at .about.1350.degree. C. for 3 hours. The glass was then refined at 1450.degree. C. for 3 hours with stirring and cast at .about.1200.degree. C. The cast glass was annealed at Tg plus 20.degree. C. for 3 hours and cooled at 30.degree. C./hour.

Typically, batch ingredients for each final glass component were P.sub.2 O.sub.5 -P.sub.2 O.sub.5, SiO.sub.2 -SiO.sub.2, B.sub.2 O.sub.3 -H.sub.3 BO.sub.3,K.sub.2 O - K.sub.2 CO.sub.3,

The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.

From the foregoing description, one skilled in the art can easily 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.