DE19610433A1 - Luminescent glass used e.g. as laser material - Google Patents

Abstract

Luminescent glass doped with rare earth ions has a local structure of rare earth ions (1) inserted in the glass by cluster coordination with tetrahedral anions (2). Production of the glass is also claimed.

Description

The invention relates to luminescent glasses, in particular for applications in optics, integrated optics, optoelectronics and laser technology are applicable, according to the preamble of claim 1.

The invention further relates to a process for the preparation of these glasses according to the preamble of claim 6.

The invention further relates to applications of these glasses according to the The preamble of claim 11.

The possibility of generating laser radiation by means of rare earth ion (SE) -doped glasses has been known since 1961 (E. Snitzer, Optical Maser Action of Nd ³⁺ in a Barium Crown Glass, Phys., Rev. Lett., 7, 444-446 (1961)). Furthermore, it is known that the rare earth ions in glasses - in contrast to SE-doped crystals - have an inhomogeneous space distribution (J. Wong, CA Anglel, Glass-structure by spectroscopy Marcel Dekker (1976)). Naturally, the SE absorption transitions show inhomogeneous broadening (MJ Weber (ed.); CRC Handbook of Laser Science and Technology, Vol. IV, Optical Materials, CRC Press Boca Raton, FL (1986)). This in turn leads to smaller stimulated emission cross sections and higher laser threshold in glasses. However, under special circumstances, this disadvantage can also be exploited: a broad inhomogeneous laser line is used to generate ultrashort laser pulses of high pulse power (W. Koechner, Solid State Laser Engineering, Springer New York (1992).

Glasses have a lower heat conduction than crystals, so that glass lasers can usually only be used in pulse mode. Otherwise there is a risk of thermal-mechanical fracture. Depending on the excitation intensity, on the pump geometry and among other things on the respective cooling conditions, pulse repetition frequencies of not greater than approximately 10 Hz are typically achieved. It must be ensured that the portion of the excitation energy which can not be used as laser energy (loss heat due to radiationless deactivation) can be released to the surrounding medium. For example, special Er ³⁺ glass lasers can also be operated continuously (cw) if the non-radiative deactivation processes are minimized (Ehrt, W. Seeber, E. Heumann, M. Ledig, Sensitized Erbium Glasses of High Efficiency for Laser Technology, Patent DD 2 62 018 A1) and the heat dissipation is optimized. Also, z. For example, the low quantum defect of the Yb ³⁺ laser (absorption band around 970 nm, emission band around 1040 nm) is well known and has recently been used to realize efficient Yb lasers based on fluoroapatite crystals. (ZBLD DeLoach, SA Payne, LK Smith, WL, Kway and WF, Krupke, J. Opt Soc, Am. B. 11 (1994) 269 and SA Payne, LK Smith, LD DeLoach, WL Kway, JB Tassano and others WF Krupke, IEEE J. Quantum Eectron, QE-30 (1994) 170). The production of these crystals in high optical quality and in volumes <1 cm³ causes problems. Experiments published so far mention crystal dimensions <1 cm (e.g., BSA Payne, LK Smith, LD DeLoach, WL Kway, JB Tassano and WF Krupke, IEEE J. Quantum Eectron, QE-30 (1994) 170). In addition, the processing of crystals is usually more difficult than that of glasses. Laser activity at room temperature has been demonstrated, for example, in Yb-doped phosphate glasses (U. Griebner, R. Koch, H. Schonnagel, S. Jiang, MJ Myers, D. Rhonehouse, SJ Hamlin, WA Clarkson, DC Hanna, Laser performance of new ytterbium doped phosphate laser glass, ASSL 1996).

In these materials, the highest differential efficiency so far was 43% reached at a threshold of about 190 mW. Phosphate glasses, however, show naturally an increased tendency to incorporate OH assemblies, the are known to have luminescence quenching properties. Due an advantageous fiber geometry could be achieved by means of Yb-doped silicate glass fibers 77% differential efficiency (D.C. Hanna, R. M. Percival, I. R. Perry, G. S. Smart, P.J Suni and A.C. Tropper; J. Mod. Opt. 37 (1990)), the coupling However, excitation light in the fiber is complicated and achievable Laser output limited.  

In fluoride-phosphate glasses, when excited by Ti-sapphire laser and 3% Output approx. 41% differential efficiency at threshold energies around 145 mW (E.Mix, E. Heumann, G. Huber, D. Ehrt, W Seeber, Adv. Solid State Lasers, Memphis, TN, Tech. Dig., WB 5-1 (1995) 230).

By reducing the phosphate content in these glasses, the Increased efficiency in a glass with 2 mol% of phosphate to about 55%. However, the reduced phosphate content deteriorated the glass formation tendency and the optical quality. There was also an increase in threshold energy 230 mW observed.

The influence of the concrete local structure of built-in rare-earth ions, the their first and second coordination sphere is called the position of Energy levels, transition probabilities and ultimately on Laser properties have been and are the subject of intensive research (eg S.E. Stokowski, R.A. Saroyan and M.J. Weber; Nd doped laser glasses / Spectroscopic and physical properties, LLL University of California, Livermore (1981) and L. D. De Loach, S.A. Payne, L.L. Chase, L.K. Smith, W.L. Kway and W.F. Krupke; IEEE J. Quantum Electron. QE-29 (1993) 1179). That's the electronic situation of terminal Yb laser level in fluoroapatite crystals Electron-phonon interaction processes involving appropriate Brought phonons. About a variation of the local structure of fluoride phosphate glasses with the aim of an effective coupling of phonons to the However, there are no electronic transitions, especially the ytterbium ion Hints.

It is Z. B. according to DE-OS 36 34 676 known to produce special glasses on fluoride-phosphate Ba sis. These glasses are characterized by refractive indices n _e = 1.47-1.50 and Abbe numbers ν _e = 85-80 and were developed with the goal of an extreme optical position in the n _e -ν _e diagram. A targeted variation of the local structure to improve the luminescence or laser properties of these glasses has not occurred.

The object of the invention is the development of luminescent Ytterbium glasses of high efficiency for optics, integrated optics, optoelectronics and laser technology, also known as compact active materials at room temperature, can be used both in pulse and in continuous (cw) operation can and do a wide tuning range of the laser wavelength guarantee. They should be economically cheap and produced in good quality his. Their application is intended in various fields of science and technology bring benefits.

In particular, glasses are to be developed for laser technology, which has the advantage of an adapted electron-phonon interaction of incorporated rare earth ions connect with the benefits of an optimized glass system.

According to the invention, the object with a luminescent glass with the Characteristics of claim 1 solved. The dependent claims 2 to 5 are advantageous Embodiments of the main claim 1.

The invention of the novel luminescent glass is based on the fact that it has been possible to create specific local structures around built-in rare earth ions - hereinafter referred to as SE - in particular of ytterbium (Yb ³⁺ ), these specific structures being largely isolated cluster-like groupings tetrahedral anions are generated around the SE ions.

Host glasses are used which, in addition to cations and fluoride anions, have, in particular, tetrahedral anions which cluster around incorporated rare earth ions, in particular ytterbium (Yb ³⁺ ), forming clusters largely isolated from the remainder of the glass network.

These host glasses are primarily melted from alkaline earth phosphates and fluorides, aluminum fluoride, rare earth compounds and compounds with tetrahedral anions, with the glass melt in the mixture of:
as cations

Mg ²⁺ 0-60 mol% Ca ²⁺ 3-57 mole% Sr ²⁺ 0-39 mol% Ba ²⁺ 0-15 mol% Al ³⁺ 2-39 mol% and 0.001-20 mol% in the sum Ions of lanthanum, cerium, praseodymium, samarium, neodymium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium and / or ytterbium,

continue as anions

1-35 mole% in total
tetrahedral anions as phosphates, sulfates, vanadates, niobates and / or antimonates with:
less than or equal to 15 mol%
in the sum of sulphates, vanadates, niobates and / or antimonates
and
65-99 mol% in the sum of fluoride

available.

It has been found that the incorporation of tetrahedral anions into fluoride glass melts under certain manufacturing conditions allows for the creation of specific local rare earth ion structures. This results mainly from the cluster-like arrangement of tetrahedral anions around rare-earth ions (see schematic representation in Fig. 1). The resulting groupings are largely separated from groups of the same kind.

Electron-phonon couplings of the vibrations of the tetrahedral anions with electronic transitions in the rare earth ion were detected. In Fig. 2, the existence of these couplings is demonstrated using the example of an electronic transition in the rare-earth ion Gd ³⁺ , incorporated in fluoride-phosphate glasses with different content of tetrahedral anions (here: phosphates). These couplings bring about an improvement in the luminescence or laser properties of the material.

By suitable synthesis compositions, in particular by the choice a suitable ratio of tetrahedral anions to the sum of Fluorides and the fluorides with each other, succeed the construction of such clusters, at the same time a glass stabilizing effect by reducing the Crystallization tendency of fluorides occurs. At a salary of in total 1-25 mol% of tetrahedral anions becomes a good amorphous solidification reached. This is due to the concentration of the fluorides according to the invention to each other, the appropriate use of alkaline earth, rare earth and / or Aluminum phosphate and the use of alkaline earth sulfate and / or vanadate and / or niobate and / or antimonate. Become as fluorides predominantly MgF₂, CaF₂, SrF₂, BaF₂ and AlF₃ and as compounds with tetrahedral anions strontium, sulfate, vanadate, niobate, antimonate and / or metaphosphate. However, according to the invention, the Cations of phosphates, sulfates, vanadates, niobates, antimonates and Fluorides exchanged while maintaining the mol%, as well as oxides of Vanadium, niobium and / or antimony are used.

According to the invention the object is further achieved in that after the steps of claim 6 luminescent glasses are produced. The Steps are that

• 1) Mixtures of components of the respective synthesis composition are melted at temperatures in the range of 400 K to 700 K above the corresponding transformation temperature T _g, preferably in the crucible (eg of platinum or carbon). The temporary use of reactive gases according to internationally known RAP technology (reactive atmospheric processing) may be advantageous.
• 2) refining and homogenization of the glass melt at Temperatures 50 K to 150 K above at step 1) used maximum temperature.
• 3) Cooling of the melt to temperatures of 200 K to 300 K. above the respective transformation point.
• 4) Cast the glass in preheated to about T _g forms, which preferably consist of graphite.
• 5) Cool the glass filled molds to room temperature.
• 6) Depending on the type of glass may optionally also a renewed Melting of the glass (remelting) done.
• 7) Optionally, a fine cooling of the glasses by Warming to temperatures about 20 K above the respective Transformation point, holding this temperature for about 30 minutes and controlled cooling with cooling rates <15 K / h.

The invention further relates to applications of the luminescent glasses Claim 11 for

• a) active tunable laser materials in a compact form (bulk geometry) for generating laser radiation in the wavelength range of about 1020 nm to 1085 nm at room temperature,
• b) active tunable laser materials in fiber form for the production of Laser radiation in the wavelength range of about 1020 nm to 1085 nm at Room temperature
• c) gain media for laser radiation in the wavelength range of about 1020 nm to 1085 nm, in particular for the construction of amplifier chains for giant pulses,
• d) media for generating ultra-short pulses, in particular for applications of communication technology, metrology and Spectroscopy and continue
• e) media for the optical cooling of solids.

The invention will be described below with reference to figures. It demonstrate:

FIG. 1 Schematic structural model for forming a cluster-like grouping of a rare earth ion (here Yb ³⁺ ) and in each case 3 tetrahedral anions in a fluoridic glass matrix.

Figure 2 Detection of electron-phonon couplings in fluoride-phosphate glasses by taking up Gd ³⁺ -phone side band spectra at room temperature.

Fig. 3 threshold energy and differential efficiency of various glasses according to the invention.

Fig. 4 compositions of the mixture of glasses according to the invention in mole percent and characteristic optical properties.

Fig. 1 shows a schematic structural model to form a cluster-like array of a rare-earth ion 1, in the example Yb ^3+, and three tetrahedral anions 2, embedded in a fluoridic glass matrix. Depending on the ionic radius of the respective rare earth ion 1 , a displacement of the tetrahedra along the given spatial directions a, b, c occurs due to steric reasons.

This local structure of the rare-earth ion 1 forms the basis for effective couplings of vibrations of the tetrahedral anion 2 with electronic transitions on the rare-earth ion 1 , which for the light generation based on the laser effect and for the optical cooling based on the anti-Stokes luminescence are usable.

Fig. 2 illustrates the detection of electron-phonon couplings in fluoride-phosphate glasses by recording Gd ³⁺ phonon sideband spectra at room temperature. Emission spectra at excitation wavelength λ _exc = 273 nm are shown. In addition to purely electronic transitions, in this example ⁶P _7/2 → ⁸S _7/2 and ⁶P _5/2 → ⁸S _7/2 , phonon sides bound at about 323 nm, in the example ⁶P _7/2 → ⁸S _7/2 + ν₃ (PO₄ ^3- ). They are due to vibrations of the PO₄ tetrahedron known from IR / Raman spectroscopy (Blasse, Blase, Vibrational Structure in the Luminescence Spectra of Ions in Solids, in Topics in Current Chemistry, 171, Springer Verlag Berlin (1994). ).

These phonon sidebands are based on the steric arrangement of the tetrahedral anions 2 around the rare-earth ion 1 and on the efficient coupling of the tetrahedral anions 2 to the electronic transitions of the rare-earth ion 1 . They are decisive for the luminescence properties of these glasses.

In Fig. 2, emission spectra of glass No. 6 (from the table in Fig. 4) with 20% of phosphate, glass No. 5 with 3% of phosphate as a tetra-erated anion and a fluoride comparative glass without phosphate are shown. The glass with no phosphate content shows no phonon side band in the range around 323 nm. The glasses with phosphate content show an increase in the phonon side band with increasing phosphate content. In the example of FIG. 2, the energy gap between the electronic transition ⁶P _7/2 → ⁸S _7/2 and the phonon side band as a function of the phosphate content is about 1050 cm ^-1 for glass No. 5 and about 1200 cm ^-1 at glass no. 6.

The glasses according to the invention have improved laser properties compared to the known glasses. For example, in experiments using diode excitation of Yb-doped glasses, threshold energies of less than 80 mW and differential efficiencies of up to 69% were achieved (see also FIG. 3).

The scope of the invention is demonstrated by the examples tabulated in FIG. 4. However, the invention is not limited to the examples given. It gives some compositions of the mixture of glasses according to the invention in mole percent and characteristic optical properties.

LIST OF REFERENCE NUMBERS

1 rare earth ion
2 tetrahedral anion
a, b, c spatial directions corresponding to D₃ symmetry

Claims (11)

1. Luminescent rare-earth ion-doped glasses of high efficiency for optics, integrated optics, optoelectronics and laser technology, characterized in that the local structure of the stored in these glasses rare earth ions ( 1 ) is characterized by cluster-like coordination with tetrahedral anions ( 2 ) , 2. Luminescent glasses according to claim 1, characterized in that ytterbium is incorporated as a rare earth ion in host glasses of fluoride-phosphate type, these glasses continue to

• - 99.0 to 65.0 mol% of fluorides, mainly MgF₂, CaF₂, SrF₂, BaF₂ and AlF₃ and from
• - 1.0 to 35.0 mol% of phosphates are melted and
• the concentration of all incorporated rare earth ions La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and / or Yb is less than or equal to 20 mol%.

3. Luminescent glasses according to claim 2, characterized in that Up to 15 mol% sulfates are used at the expense of the phosphate content. 4. Luminescent glasses according to claim 2, characterized in that in the sum up to 15 mol% vanadates and / or niobates and / or Antimonates are charged at the expense of the phosphate content. 5. Luminescent glasses according to claim 2, claim 3 and claim 4, characterized in that as cations Mg ²⁺ 0-60 mol% Ca ²⁺ 3-57 mole% Sr ²⁺ 0-39 mol% Ba ²⁺ 0-15 mol% Al ³⁺ 2-39 mol% and 0.001-20 mol% in the sum Ions of lanthanum, cerium, praseodymium, samarium, neodymium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium and / or ytterbium, furthermore as anions1-35 mol% in the sum of tetrahedral anions as phosphates, sulfates, vanadates, niobates and / or antimonates with:
less than or equal to 15 mol% in the sum of sulfates, vanadates, niobates and / or antimonates and
65-99 mole% in total fluoride. 6. A process for the preparation of luminescent rare-earth ion-doped glasses, characterized by the following production steps:

• a) mixtures of components of the respective synthesis composition are melted at temperatures in the range from 400 K to 700 K above the corresponding transformation temperature T _g ,
• b) refining and homogenization of the glass melt at temperatures of 50 K to 150 K above the maximum temperature used in step a),
• c) cooling of the melt to temperatures 200 K to 300 K above the respective transformation point,
• d) casting the glass in preheated to about T _g forms and
• e) cooling the filled molds to room temperature.

7. The method according to claim 6, characterized in that a temporary use of reactive gases according to the RAP technology takes place. 8. The method according to claim 6, characterized in that depending on the type of glass a re-melting of the glass (Remelting) takes place.   9. The method according to claim 6, characterized in that
a fine cooling of the glasses by heating to temperatures about 20 K above the respective transformation point, holding this temperature for about 30 minutes and controlled cooling with cooling rates <15 K / h. 10. The method according to claim 6, characterized in that the melting of the glass in a crucible made of graphite (carbon) or made of platinum. 11. Applications of the luminescent SE doped glasses with special local structure as

• a) active tunable laser materials in a compact form (bulk geometry) for generating laser radiation in the wavelength range of about 1020 nm to 1085 nm at room temperature,
• b) active tunable laser materials in fiber form for generating laser radiation in the wavelength range of about 1020 nm to 1085 nm at room temperature,
• c) amplification media for laser radiation in the wavelength range of about 1020 nm to 1085 nm, in particular for the construction of amplifier chains for giant pulses,
• d) media for generating ultrashort pulses, in particular for applications of communication technology, measurement technology and spectroscopy, and continue
• e) media for the optical cooling of solid-state lasers.