EP0837835A1

EP0837835A1 - Glasses

Info

Publication number: EP0837835A1
Application number: EP96924085A
Authority: EP
Inventors: Animesh-The School of Materials JHA; Diego Marchese; George Kakarantzas
Original assignee: BTG International Ltd; British Technology Group Ltd
Current assignee: BTG International Ltd
Priority date: 1995-07-13
Filing date: 1996-07-15
Publication date: 1998-04-29
Also published as: KR19990028934A; WO1997003028A1; GB9514345D0; AU6466296A; JPH11508869A; CA2226780A1

Abstract

A method of producing glasses with useful optical transmission characteristics includes the steps of melting a charge comprising group III and group IV chalcogenides. Addition of group I and group II halides enhances the solubility of rare earth dopants and permits the fabrication of stable lasers and amplifiers incorporating these glasses.

Description

Glasses

This invention relates to glasses and, in particular, to glasses suitable for incorporation in planar and fibre optical light guides. It finds application with a range of multicomponent glass compositions without and with rare-earth ions (eg Pr, Nd, Er, Ho, Dy) as dopants. Rare earth dopants are useful in producing energy level transitions of a magnitude suitable for electro-optical applications.

Germanium sulphide is a well known covalently-bonded glass-forming compound and its vibrational spectra suggest that the glass structure with predominant Ge-S bonding has a considerably lower phonon energy (~350cm^"') than either silica glass (1120 cm^"1) or ZBLAN (580cm^"1). The lower phonon-energy structure of this glass is an attractive feature for designing a range of rare-earth ion doped fibre optic and planar waveguide devices for important optoelectronics applications in the telecommunications networks and also for non-telecommunications devices. Germanium sulphide glass fibres are already available commercially for infra-red applications. The solubility of rare-earth ions in pure germanium sulphide glass is extremely low due to structural and thermodynamic factors. The limited solubility of rare-earth ions in this glass renders the pure sulphide host unsuitable for a range of optically active devices which otherwise might have benefited from the lower phonon-energy ofthe host.

The use of binary GeS_x-Ga₂S₃ glass with Pr^3* and Dy³⁺ ions has been proposed for the realisation of a second window optical fibre amplifier. Binary sulphide glass samples were produced by using extremely pure (99.999%) constituent metallic and non-metallic elements, which were mixed in a limited range of compositions inside evacuated silica ampoules. The optical quality of this type of processed glass from elemental constituents suffers due to an incomplete chemical reaction between the constituents. Furthermore the use of elemental constituents in the starting material limits the maximum size ofthe glass samples manufactured because of the kinetic considerations involved in the high- temperature chemical reaction in a restricted volume. A large batch of the starting materials was often found to be extremely inhomogeneous, giving rise to a large number of light scattering centres. This factor greatly affects the ability to fabricate bulk glass samples with minimum crystals for realising optical fibres by drawing core and clad glasses at their softening temperatures. The use of Pr, Nd and Er dopants in Ge-As-S, Ge-P-S and Ge-Sb-S glasses has been reported (D. Simons et al: Proc. Topical Symp. VII on Advanced Materials in Optics, Electro-Optics and Communication Technologies of 8th Cimtech- World Ceramic Congress and Forum On New Materials, Florence, Italy, June 28 to 4th July, 1994. Ed. P. Vincizin and G. C. Righin. Techna, Faenza, 1995.p449). The lifetimes of level in Pr-ions is reported to be 365μs indicating that an efficient fibre device with large gain is potentially possible by using low pump power at 1010 nm. However the emission line for ^*G₄-³H₅ transition, centre at 1300nm in ZBLAN, shifts away from the second window to 1360nm, thereby excluding any possible signal gain within the specified and optimised second window (1290-1330 nm) ofthe installed PON. The peak position ofthe emission line at 1360nm in the Ge-As-S, Ge-P-S and Ge-As-S glasses render these hosts unsuitable for fibre amplifier application within the prescribed second window. These glasses have a very high refractive index (-2.5) which poses additional problem of strong absorption at signal and pump wavelengths as a result ofthe large Rayleigh scattering coefficient. This factor has also hindered the development of rare-earth ion doped arsenic sulphide glass fibres in the past.

We have developed a procedure for making pure multicomponent chalcogenide and chalcohalide glasses using constituent compounds in lieu of constituent elements. We have also developed a method of enhancing the solubility of rare-earth ions in a host glass for use in fibre optic lasers and amplifiers. We have further developed a procedure for making sulphide-oxide glasses with high index of refraction suitable for non-linear fibre application. We have also found that the use of modified germanium sulphide glass is suitable as a host for designing nanocrystalline metal (Cu and Ag) doped glasses for QW structure. These glasses also find application as a host for semiconductor sulphides as dopants in the QW structure. The properties of the modified host glass may be used as laser power transmission cables for mid and far infra-red applications. The bulk glasses also exhibit much wider transmission range in the pure GeS_x and binary GeS_x-Ga₂S₃ glasses thereby making them suitable for a range of bulk infra-red optics applications such as lenses and diffraction gratings. The undoped passive fibre cables may also be used for sensing and detection in food and drinks industry. In particular the dairy and wine industry will tremendously benefit from the monitoring equipment using passive cables for determining the shelf-life ofthe common supermarket dairy products and for the quality of wine and alcoholic drinks. The passive optical fibres may also be used for detecting pollution and the levels of toxic gases in potentially dangerous chemical environment. In polymer recycling plant, these extended IR fibres can be used for screening a wide variety of polymers and therefore assist them in isolating and regrouping by spectroscopic screening procedure prior to recycling.

A particular objective was to device a suitable glass host for Pr-doped single-mode fibre device capable of amplifying a decaying signal within the prescribed second window of the installed PON. Desirable properties for such a glass are that it should have a comparable phonon energy as GeS_x glass and that at least a significant part ofthe emission curve for the 'G₄-³H₅ transition should be well within the prescribed second window ofthe installed PON.

Other desirable properties are the high thermal stability of the glass so that it can be cooled from its homogenisation temperature of 1025°C with minimum volume fraction of crystals formed during glass formation and reduction in the refractive index ofthe glass without either losing its attractive thermal properties or adversely affecting the 'G₄ lifetimes. (It has been reported that the "G₄-³H₅ emission line is strongly dependent on the refractive index ofthe glass host and it red-shifts with the increasing index thereby offering signal gain only in a part ofthe second window.) According to the present invention there is provided a glass comprising a mixture of at least one chalcogenide of a group IV element and a halogen-containing compound.

There is also provided a glass comprising a mixture of at least one chalcogenide of a group IV element and a halogen-containing compound doped with a rare earth element.

The invention will be particularly described with reference to the accompanying drawings, in which:

Figures 1 to 5 show the optical absoφtion at different wavelengths for glasses in accordance with specific embodiments ofthe invention.

The preparation of high-purity chalcogenide and chalcohalide glasses with Pr-ions as dopant was carried out in the following manner during which a particular attention was given to the procedure for impurity removal. The starting raw materials were germanium sulphide (GeS_2+x) where x designates more than the stoichiometric amount of sulphur in

*-> germanium disulphide. Other minor constituents were gallium sulphide (Ga₂S₃), alkali and alkaline-earth metal sulphides (M₂S, M'S) and halides (MX, M'X ). Here M and M' designate alkali and alkaline-earth metals. X denotes halide ion eg F\ Cl^", Br and T. These alkali and alkaline-earth metal halide and sulphides were incorporated in the binary GeS_2+x-Ga₂S₃ glasses for modifying the glass network. Gallium sulphide was incorporated in the glass at two different concentrations: small concentrations ( < 3 mole percent) and large concentrations (> 3 mole percent). At small concentrations, it was believed that the ions occupy sites for network forming and continuation whereas at large concentrations, the ions disproportionate between the network-forming sites and network-modifying sites. Chemicals required for making chalcogenide and chalcohalide glasses were accurately weighed inside a dry glove box continuously purged with nitrogen gas. The weighed material was then transferred inside a clean, dry and baked silica ampoule. The mouth of the silica ampoule was closed inside by using a cork and then transferred to outside in order to minimise congestion by atmospheric moisture and CO₂. The silica ampoule was then connected with a vacuum assembly. Followed by this vacuum connection procedure, the ampoule was placed inside a resistance-heated furnace where the charge for making glass was vacuum-dried at less than 0.5 torr in the temperature regime of 200-300°C. The powder mixture was left for drying over 12 to 15 hours. After this period, the ampoule was isolated from the vacuum line and subsequently evacuated using a high vacuum system. The pressure prior to reaching the final phase for evacuation was maintained below IO^"5 torr for several hours. After this stage, the silica ampoule was sealed using a welding torch. After the sealing procedure was completed, the silica ampoule containing melting charge was placed inside a resistance furnace which was slowly heated from the room temperature to 1025°C over a period of approximately three hours. At 1025°C, the charge was left for overnight homogenisation typically for 12-15 hours. Once the melt homogenisation was completed, the silica ampoule was withdrawn from the furnace. The sulphide melt and the ampoule appeared reddish-yellow to deep orange in colour. The ampoule was held outside to air-cool for a minute or so for air-cooling, at which point the temperature dropped leading to a change in the colour appearance ofthe melt from deep orange to deep cherry-red. The cherry-red air-cooled glass sample inside the ampoule was then transferred to an annealing furnace where it was held for over 12 hours after which the furnace was switched-off allowing the ampoule to furnace-cool. The entire annealing process was programmed to reduce quenching stresses in the glass.

For making sulphide glasses with large concentrations of oxide, the vacuum drying was only partially completed at a lower temperature. The glass was melted slowly in a carbon coated ampoule in order to avoid sticking of glass with silica ampoule.

Successful experimental trials were also carried out by allowing the silica ampoule containing molten sulphide to furnace-cool. This experiment was particularly designed to find out whether the glass sample of 25mm by 30-35mm can be produced by furnace cooling and subsequent annealing at T_g. These sample dimensions were consistently tried in order to demonstrate that the larger blocks of this glass than the dimensions specified above can also be made for preform fabrication and fibre drawing. The slow-cooling results confirm our view that the sulphide and chalcohalide glass samples produced are much more stable than the other families of sulphide based glasses belonging to either GeS₂ or binary Ga₂S₃-La₂S₃ (GLS) glass. This is an extremely important result for the realisation of optical devices in which the background loss is expected to be lower than the existing sulphide glass fibres.

In the first instance ternary GeS_2+x-Ga₂S₃-CsI based glasses have been manufactured by slow air~cooling and annealing techniques. The dimensions ofthe glass samples larger than 100mm in length and 25mm in diameter are possible to fabricate by this technique.. Thermal properties of binary sulphide and ternary chalcohalide glasses have been studied. These are summarised in Tables 1 and 2. These glasses exhibit a large T_x -T,, temperature gap, where T_x represents the onset of the crystallisation temperature. The gap typically spans over 115°C reaching a maximum of 228 °C, see table below. The measured characteristic temperatures indicate very high thermal stability of these glasses comparable to oxide glasses.

Figures 1 and 2 and 3 and 4 summarise the effect ofthe minimisation ofthe OH impurities dissolved in this type of glass. When the glass contains large quantities of OH, as can be seen in Figure 1. the corresponding Pr-absorption spectrum in Figure 2 appears insignificant. By removing the OH ions from glass (see Figure 3), the absorption bands of Pr-ions become more resolved from the background (see Figure 4). The addition of Csl also enhances the solubility of Pr-ions by modifying the closed packed sulphide network. Csl modifies the network by creating more non-bridging Ge-S and S-S bonds. The structural modification creates excess space thereby enhances the solubility of large ions such as Pr in the glass. As indicated above, the network modification can be brought about by dissolving either alkali or alkaline-earth ions or both in compound forms (sulphide or halide or both). The solubility of Pr-ions in the network in the presence of Csl also has a pronounced effect on the UV and multiphonon absoφtion bands. These results are shown in Figures 5, 3 and 4 respectively. The presence of Csl in amounts larger than 0.75% begins to destroy the S-S ring structure in the glass which is dominating the IR edge in the binary composition. The reduction of S-S vibration in the ternary glasses has a favourable effect on the metastable lifetimes of "G₄ level of Pr-ions. The lifetimes of this level has been measured using 101 Onm Ti-sapphire tunable pump wavelength of 750mW power for binary and ternary glasses with 2000ppmw concentrations of Pr-ions. These are summarised together with the measured refractive indices of these glasses at 633nm wavelength in Table 3. The longest lifetime measured in a 2000ppmw glass is 200μs which is expected to be longer in a 700ppmw-doped samples due to a reduced ion-ion cross relaxation process. The ion-ion cross-relaxation adversely affects the lifetime of the metastable level. We anticipate that in a 700ppmw doped sample, the measured lifetime to be at least as long as 300μs, which will compare favourably with other types of sulphide glasses if we combine the spectroscopic properties of this type glass with the thermal properties and the lower refractive index. Currently the measurements are ongoing using a 700ppmw sample together with the emission spectroseopy which will indicate the peak ofthe position ofthe emission line.

Table 1: Chalcohalide and chalcogenide glass compositions.

Glass Composition (mole %) Dopant (ppmw)

1 GeS₂ (92) -Ga₂S₃ (08) - Csl (00) Pr₂S₃(1000)

2 GeS₂(85) -Ga₂S₃(15) - Csl (00) Pr₂S₃ (1000)

3 GeS₂ (75) -Ga₂S₃ (25) - Csl (00) Pr₂S₃(1000)

4 GeS₂ (85) -Ga₂S₃ (13) - Csl (02) Pr₂S₃ (2000)

5 GeS₂ (85) -Ga₂S₃ (09) - Csl (06) Pr₂S₃ (2000)

6 GeS₂(85)-Ga₂S₃(ll)-CsI(04) Pr₂S₃ (2000)

7 GeS₂ (85) -Ga₂S₃ (07) - Csl (08) Pr₂S₃ (2000)

8 GeS₂ (79) -Ga₂S₃(15) - Csl (06) Pr₂S₃ (2000)

9 GeS₂ (74) -Ga₂S₃ (20) - Csl (06) Pr,S₃ (2000)

10* GeS₂ (69) -Ga₂S₃ (25) - Csl (06) Pr₂S₃ (2000)

11 GeS₂(85) -Ga₂S₃ (10) - Csl (05) Pr₂S₃ (2000)

12 GeS₂(80) -Ga₂S₃(10)-CsI(10) Pr₂S₃ (2000)

13 GeS₂ (75) -Ga₂S₃ (10) - Csl (15) Pr₂S₃ (2000)

14* GeS₂ (70) -Ga₂S₃ (10) - Csl (20) Pr₂S₃ (2000)

15 GeS₂ (95) -Ga₂S₃ (00) - Csl (05) Pr₂S₃ (2000)

*Crvstallised batch

Table 2: Chalcohalide and chalcogenide glass characteristic temperatures.

Glass No. T °C τ_x°c τ_p °c

1 487 639 657 152

2 459 619 642 160

3 432 613 654 181

4 412 640 663 228

5 422 624 661 202

6 464 603 622 139

7 440 593 647 153

8 415 605 627 190

9 415 530 568 115

11 433 556 701 123

12 421 607 658 186

13 421 615 703 194

Table 3: Summary of the measured 'G₄ lifetimes in Pr-doped chalcogenide and chalcohalide glasses and their refractive indices

Glass No. Composition (mole%) Lifetime Refractive GeS -Ga₂S₃ - Csl (μs)±3μs Index (λ=633nm)

2 85%-15%-0% — 1.994

6 85%-l l%-4% 190 2.160

7 85%-7%-8% 110 2.133

8 79%-15%-6% 140 2.156

9 74%-20%-6% 158 ~

1 1 85%-10%-5% 160 2.150

12 80%- 10%- 10% 190 2.118

13 75%- 15%- 10% 200 The measured UV edges, shown in Figure 5, of these glasses indicate a strong dependence on the Csl concentration ofthe glass. This is extremely useful in tailoring the core/clad composition for designing a large numerical aperture fibre. The large value of index in this glass is also important for designing large third-order non-linear devices for optical switching. In particular, the device in fibre and planar geometry will be extremely attractive since it will offer a long interaction length while the signal propagates in the core of the fibre waveguides. This is a distinct advantage over crystalline semiconductor devices which have a major design drawback due to geometrical restrictions on waveguide length. Additionally these glasses, being in sulphide, can be ideal candidates for dissolving CdS, CdSe and CdTe and for forming QW structures for nonlinear devices.

As well as Pr-ions other rare-earth ions can also be dissolved. Dy-doped chalcogenide and chalcohalide glass fibres, utilising two different transitions, ^'4F_9/2-⁶F_5/2 at around 1000 nm and F_9/2 - ⁶F_3/2 at around 1294 nm ( both transitions reported in a fluoride glass ZBLANP) yields two very useful devices. Because ofthe high-index ofthe sulphide glasses, the characteristic transitions from the metastable ⁴F_9/2 level to ⁶F_5/2 and °F_3/2 levels is at longer wavelengths than ZBLANP, thereby making the former transition ideally suited for a fibre laser pump at around lOlOnm for Pr-ion doped devices which currently utilise an expensive Nd-YLF pump at 1040nm for power amplifier. The latter transition at 1294nm in ZBLANP will also be shifted to longer wavelength than 1294nm in sulphide glass due to its high refractive index. In a thermally-less stable germanium sulphide based glass, there is a ⁴F_9/2- ⁶F_3/2 transition whose peak is centred at 1340nm. A significant part of this peak is well within the prescribed second window. The absoφtion and emission cross-section for this transition are also large, which offers a clear advantage for restricting the fibre length to few tens of centimetres up to a maximum of metre. In such applications thermally stable chalcogenide and chalcohalide glasses in accordance with the invention have a distinct advantage over thermally less stable glasses such as GLS for drawing long lengths of single-mode optical fibres.

Chalcogenide and chalcohalide glasses can be doped with Nd ions for realising amplifiers outside the second telecom window in new networks. The new installed silica networks are virtually OH free and hence amplifiers between 1 lOOnm and 1400nm will be required in future. The F_3/2 transition in Nd-doped in the above host will prove quite useful.

The dopant ions Er and Ho can also used to realise devices for surgical applications. The optical transitions in ⁴Iπ_/2- I_!3/2 in Er and- - 1₇ in Ho are at 2719 mn and 2848 nm respectively and match with the strong bands of OH at around 2900 nm. The rare-earth- doped glasses utilising this transition in the form of a pulsed laser capable of delivering several mW of power will be useful for laser ablation of malign tissues which will strongly respond to absoφtion of 2.9μm light corresponding to OH peak. This device can easily complement the viewing cable, microscopic surgical tools attached to the endoscopic cable. The incoφoration of either Er or Ho ions in the chalcogenide glass fibre will therefore make a compact endoscopic device.

Other types of applications with semiconductor and metal dopants can be incoφorated for making nonlinear devices.

Large refractive index glasses with long wavelength UV cut-off edges have been fabricated for non-finer fibre optic and planar devices.

Claims

1. A glass comprising a mixture of at least one chalcogenide ofa group IV element and a halogen-containing compound.

2. A glass comprising a mixture of at least one chalcogenide of a group IV element and a halogen-containing compound as claimed in claim 1 characterised in that it further includes a chalcogenide of a Group III element.

3. A glass comprising a mixture of at least one chalcogenide of a group IV element and a halogen-containing compound as claimed in claim 1 characterised in that the chalcogenide ofthe group IV element is present at a quantity in the range 70-95 mole%, the chalcogenide ofthe Group III element is present at a quantity in the range 0-25 mole% and the halide is of an alkali or alkaline earth element present at a quantity in the range 0-20 mole%.

4. A glass comprising a mixture of at least one chalcogenide of a group IV element and a halogen-containing compound as claimed in claim 3 characterised in that the glass is doped with a rare earth element.

5. A glass comprising a mixture of at least one chalcogenide ofa group IV element and a halogen-containing compound as claimed in claim 4 characterised in that the rare-earth dopant is selected from the group comprising Pr, Nd, Er, Ho and Dy.

6. A glass comprising a mixture of at least one chalcogenide of a group IV element and a halogen-containing compound as claimed in claim 4 characterised in that the group IV element is germanium and the chalcogenide is sulphur.

7. A glass comprising a mixture of at least one chalcogenide of a group IV element and a halogen-containing compound as claimed in claim 4 characterised in that the group III element is gallium and the chalcogenide is sulphur.

8. A glass comprising a mixture of at least one chalcogenide of a group IV element and a halogen-containing compound as claimed in claim 4 characterised in that the halide is cesium iodide.

9. A method of enhancing the solubility of dopants in glasses containing chalcogenides comprising adding a halide of an alkali or alkaline earth metal to said chalcogenide containing glass.

10. A method of making a glass comprising the steps of melting a mixture of the chalcogenide ofa group IV element with a chalcogenide ofa group III element and adding a halide ofa group I or a group II element thereto.

1 1. A method of making a glass comprising the steps of placing a charge of constituent compounds including a chalcogenide of a group IV compound, a chalcogenide of a group III compound and a halide of an element selected from the group comprising the alkali metals and the alkaline earth metals in a container, heating said container to remove water from said constituent compounds, further heating said container to allow said charge to melt, maintaining said container at an elevated temperature to allow said charge to homogenise and subsequently allowing said charge to cool.

12. A method of making a glass as claimed in claim 11 further including the step of annealing the charge to reduce quenching stresses in said glass.

13. An infra-red radiation transmission guide incoφorating a glass as claimed in any one ofthe previous claims 1 to 9.

14. An optical component incoφorating a glass as claimed in any one of the previous claims 1 to 9.

15. An optical component as claimed in claim 14 incoφorating a nanocrystalline metal dopant.

16. An optical component as claimed in claim 14 characterised in that the metal is selected from the group consisting of copper and silver.

17. A measuring instrument incoφorating an optical component as claimed in claim 14.

18. A recycling plant incoφorating a measuring instrument according to claim 17.

19. A fibre laser or amplifier incoφorating a glass according to any one of claims 1 to 9.

20. A fibre laser or amplifier including a glass as claimed in any one of claims 1 to 9 characterised in that it utilises the transition F_9/2-⁶F₅,₂.

21 A fibre laser or amplifier including a glass as claimed in any one of claims 1 to 9 characterised in that it utilises the transition ⁴F_9/2 - ⁶F₃,₂.

22. A fibre laser or amplifier including a glass according to either claim 19 or claim 20 characterised in that it incoφorates a dopant selected from the group consisting of Pr, Nd,

Er and Dy.

23. A fibre laser or amplifier including a glass as claimed in any one of claims 1 to 9 characterised in that it utilises the transition ⁴In_/2- I-₃π.

24. A fibre laser or amplifier including a glass as claimed in any one of claims 1 to 9 characterised in that it utilises the transition ⁵I₆ -⁵I₇ .

25. A devise for use in therapy incoφorating a laser according to claim 19.