WO2014165048A1

WO2014165048A1 - Optical element for mie scattering light from an optical fiber

Info

Publication number: WO2014165048A1
Application number: PCT/US2014/024241
Authority: WO
Inventors: William H. JAMES; Elizabeth Chase; Mark J. DAVIS; Paula Vullo; Sally Pucilowski; Eric H. Urruti; Brianne MORGAN
Original assignee: Schott Corp
Current assignee: Schott Corp
Priority date: 2013-03-12
Filing date: 2014-03-12
Publication date: 2014-10-09
Anticipated expiration: 2015-09-12
Also published as: JP6400072B2; US9585548B2; DE112014001293B4; JP2016519777A; DE112014001293T5; US20140270639A1

Abstract

The invention relates to an optical scattering element suitable for dispersing or scattering light transmitted by optical device by Mie scattering. The optical scattering element comprises a phase-separated or porous borosilicate glass having dispersed phase particles with a particle size of 200 to 500 nanometers or pores with a size of 200 to 500 nanometers, at a number density of 10⁸ to 10¹² mm^-3. The optical scattering element can be prepared by subjecting a borosilicate glass to a controlled heat treatment to induce phase separation, and then optionally leaching out one of the phases with an acid leach. The optical scattering element can be, for example, attached to an end of an optical fiber or bundle of optical fibers. The invention also relates to a method of dispersing or scattering light by transmitting the light through the optical scattering element.

Description

OPTICAL ELEMENT FOR MIE^ATT j NG LIGHT

FROM AN OPTICAL FIBER

S umma rv of the I nye ntlgn

[0001] The invention relates to adjusting Mie scattering of visible light transmitted from an optical fiber. In particular, the invention relates to an optical element that can be affixed to the end of an optical fiber that will scatter light predominantly in the forward direction, a method of manufacturing such an optical element, and the use of such an optica! element in the fields of illumination, sensors and medical devices.

[Q002] Electromagnetic radiation passing through a medium, such as an optical fiber, will be scattered by interactions with defects/inhomogeneities in the medium, that is a volume of variant refractive index, such as a particle, bubble, droplet, or even a density fluctuation. When the defect/inhomogeneity is less than one-tenth of the operating wavelength of light, the scattering is referred to as Rayleigh scattering, named after Lord Rayieigh (John Strutt). For example, the blue color of the sky is caused by Rayleigh scattering as a result of gas molecules in the atmosphere. When the defect/inhomogeneity is comparable in size to the wavelength, i.e., larger than 10% of wavelength, the scattering is known as Mie scattering, named after German physicist Gustav Mie. Thus, for example, the scattering of light induced by aerosols and pollutants in the atmosphere is Mie scattering. The white appearance of clouds is a result of Mie scattering. Rayieigh scattering and Mie scattering are both forms of elastic light scattering, that is the energy (and thus wavelength and frequency) of the light is not substantially changed by the interaction with the scattering particle.

[0003] The amount of scattered radiation and the directions in which the radiation is scattered as a result of Mie scattering are dependent on the size and composition of the particle (feature), as well as the wavelength of the radiation and the refractive index of the medium. [0004] Particles that have a size which is applicable for inducing ie scattering will scatter the radiation in a predictable way. in general the radiation is scattered ail around the particle. While more of the radiation is scattered on the side of the particle facing away from the source of the radiation, some amount of radiation will be scattered back in the direction of the source of the radiation.

[0005] The type of scattering considered herein, Mie scattering, makes use of relatively large features (hundreds of nanometers) to induce scattering efficiently in the forward direction. This is useful to increase the spread from an optic for illumination purposes (i.e., to thereby illuminate a larger area) while maintaining efficiency (i.e., efficient transmission).

[0008] For example, in accordance with the invention, to scatter light emanating from the end of an optical fiber, one can attach an optical element that will induce Mie scattering. In this regard, it is desirable that the optical element be designed to optimize scattering in the forward direction. Therefore, one aspect of the present invention is to provide an optical element (scattering element or scattering optic) for inducing Mie scattering of visible light from an optical fiber, where the element is comprised of nanoscale features that will scatter light predominantly in the forward direction when affixed to the end of an optica! fiber such as a high silica fiber.

[0007] Therefore, in accordance with the invention, there is provided an optical element made of porous or phase-separated glass which is designed to optimize scattering in the forward direction (i.e. increased scattering cross-section in this direction} and minimize scatter in the backward direction, while at the same time attaining the desired solid angle of scattered light in the forward direction (in other words, maintaining good spread out of the optical fiber).

[0008] While the optical element of the invention is generally described for use with an optica! fiber, the element can be used in any optics applications for dispersing or scattering light transmitted by an optica! device. Therefore, in accordance with another aspect of the invention, there is provided a method for dispersing or scattering light transmitted by an optical device by transmitting the light through an optical element made of porous or phase-separated glass which is designed to optimize scattering in the forward direction (i.e. increased scattering cross-section in this direction) and minimize scatter in the backward direction, while at the same time maintaining good spread.

[0009]Thus, according to one aspect of the invention there is provided a scattering optical element produced by subjecting a glass composition, formed from a glass system that undergoes phase separation [for example, the classic borosilicate glass system; the systems K₂0-Si0₂, K₂G-Li₂0~8iG2, K₂ONa₂0~ Si0₂, and K₂0-BaOSiQ₂ (Kawamoto and Tomozawa, 1931 , J. Amer. Ceram. Soc, vol. 64 (5), 289-292); the system MgO-AI₂0₃-Si0₂ (Zdaniewski, 1978, J. Amer. Ceram. Soc , vol. 81 (5-6), p. 199-204); and the system CdF₂-LiF-AIF₃- PbF₂ (Randall et at., 1988, J. Amer. Ceram. Soc, vol. 71 (12), p. 1 134-1 141 )] to a phase separation process, such as by means of a controlled thermal treatment, whereby the glass composition separates into two phases, and optionally a further procedure for creating open porosity within the glass by a leach step to obtain a glass having pores on the order of 200 to 700

nanometers, e.g., 200 to 500 nanometers, preferably 300-500 nm, especially 300-450 nm with a number density approaching 10⁸ to 0^'12 mm^"3 preferably 10⁹ to 10¹¹ mm^'3, especially preferably 10¹⁰ to 10¹¹ mm^"3.

[GOU QJAccording to one aspect of the invention there is provided a scattering optical element produced by melting a borosilicate glass (preferably an alkali borosilicate glass) that, after being annealed, is subjected to phase separation using a weil-control!ed thermal treatment whereby the composition separates into a silica-rich phase and a boron-rich phase, and optionally a further procedure for creating open porosity within the glass by an acid leach step and a pore cleaning caustic leach step, to obtain a glass having pores on the order of 200 to 700 nanometers, e.g., 200 to 500 nanometers, preferably 300-500 nm, especially 300-450 nm with a number density approaching 10⁸ to 10¹² mm^"J _, preferably 10⁹ to 10 mm^"3, especially preferably 10¹⁰ to 10 ¹ mm^"3.

[0011]According to one aspect of the invention there is provided a device comprising at least one optical fiber and at least one Mie scattering optical element for dispersing light emitted from the at least one optical fiber, wherein the at least one Mie scattering optical element comprises a phase-separated or porous glass (such as a borosiiicate glass, preferably an alkali borosiiicate glass) having dispersed phase particles with a particle size of 200 to 700 nanometers, e.g., 200 to 500 nanometers or pores having a size of 200 to 700 nanometers, e.g., 200 to 500 nanometers, at a number density of 10⁶ to 10¹² mm^"3,

[0012] According to another aspect of the invention there is provided an endoscope comprising an optical fiber or optical fiber bundle and an optical scattering element attached to the distal end of said optical fiber or optical fiber bundle for dispersing light emitted from the distal end of the optical fiber or optical fiber bundle, the optica! scattering element comprising a phase-separated or porous glass (such as a borosiiicate glass, preferably an alkali borosiiicate glass) having dispersed phase particles with a particle size of 200 to 700 nanometers, e.g., 200 to 500 nanometers, or pores having a size of 200 to 700 nanometers, e.g., 200 to 500 nanometers, at a number density of 10⁸ to 10¹² mm^"3.

[0013]The references to a particle size of, for example, 200 to 700 nanometers and a pore size of, for example, 200 to 700 nanometers is intended to mean that the relevant pores or particles have an effective diameter of 200 to 700 nanometers. So, for example, in an optical element in accordance with the invention the amount, i.e., the number density, of particles or pores having an effective diameter of, e.g., 200 to 700 nanometers will be 10⁸ to 10^!2 mm^"3 _, (preferably 10⁹ to 10¹¹ mm^"3, especially preferably 10¹⁰ to 10¹¹ mm^"3). The optical element may include particles or pores having an effective diameter outside the range of 200 to 700 nanometers, but the number of particles or pores having an effective diameter of 200 to 700 nanometers will be will be 10⁸ to 10¹² mm^"3 (preferably 1 G⁹ to 10¹¹ mm^"3, especially preferably 10¹⁰ to 10¹¹ mm^" ³).

[0014] According to another aspect of the invention there is provided a process for preparing an optical device with a Mie scattering optical element, the process comprising: preparing a Mie scattering optical element by subjecting an annealed glass (such as a borosilicate glass, preferably an alkali borosilicate glass) to a phase separation using a controlled thermal treatment, and optionally subjecting the phase-separated glass to an acid leach to create pores and a caustic leach to clean the resultant pores, and attaching the resultant ie scattering optical element to the end of an optical fiber or to the end of a bundle of optical fibers.

[001 S] According to a further aspect, there is provided a method for dispersing or scattering light transmitted by optical device by transmitting the Sight through an optica! scattering element comprising a phase-separated or porous glass (for example, a borosi!icate glass or an alkali borosilicate glass) having dispersed phase particles with a particle size of 200 to 700 nanometers, e.g., 200 to 500 nanometers or pores having a size of 200 to 700 nanometers, e.g., 200 to 500 nanometers, at a number density of 10⁸ to 10¹² mm^"3. [0016] For example, the optical scattering element can be prepared by melting a borosilicate giass, preferably an alkali borosilicate glass, that, after being annealed, is subjected to a phase separation using a well-controlled thermal treatment, and optionally a further procedure for creating open porosity within the giass by an acid Seach step and a pore cleaning caustic !each step, to obtain a glass having pores on the order of 200 to 700 nanometers, e.g., 200 to 500 nanometers, preferably 300-500 nm, especially 300-450 nm with a number density approaching 10⁸ to 10ⁱ² mm^"3 _, preferably 1 Q⁹ to 10¹¹ mm^"3, especially preferably 10¹⁰ to 10¹¹ mm^'3. [0017] Based on standard Mie scattering calculations, it is believed that the Mie scattering required to optimize scattering in the forward direction while maintaining good spread out of a fiber, and minimal scatter in the backward direction, can be attained with features (scattering centers) on the order of 200 to 700 nm, e.g., 200 - 500 nm, diameter at an approximate number density of 10^e to 10 ^;2 mm^""⁵, and with an approximately equal volume fraction for each phase, e.g., a silica rich phase and a boron rich phase. Although Mie Scattering can occur in principle with larger feature sizes, the preferred upper limit of the features in accordance with the invention is approximately 700 nm. [0018] In accordance with the invention, thermal treatment of the mother borosilicate glass, preferably alkali borosilicate glass (see, for example, the compositions listed in Table 1 ), induces glass~in-glass phase separation. The phases are defined as a silica rich phase and a boron rich phase, After undergoing the heat treatment, the mother glass can then be chemically leached, When the mother glass is chemically leached the host or bulk phase is considered to be the silica rich phase, and the pores resulting from the removal of the boron rich phase is considered the scattering feature. [0019] This combination of feature size and number density can achieve a scattering cross section of 10^'4 to 1 G~² m², preferably 5x10^"3 to 5x10^"z μηι². For this set of conditions, the amount of light scattered in the backward direction is less than 1 % of that scattered in the forward direction. [0020] In order to obtain a glass exhibiting such a combination of nanoscale features on the order of 200 to 700 nm, e.g., 200 - 500 nm, with a number density approaching 10⁸ to 10¹² mm^"3, the glass should be subjected to a well- controlled thermal processing profile. For example, in terms of commercially available products, SCHOTT Cora!Por™ Porous Glass is a glass product which has applications in chromatography media, reference electrode junctions, host material for sensors, and as an additive (filler) in coatings (see James et a!, , US 2013/0017387). During manufacture, Cora!Por™ Porous Glass is subjected to a carefully controlled thermal treatment to induce glass-in-glass phase separation. This heat treatment ultimately dictates the final size of the scattering features present in the material. As a result, the manufacturing process can be

manipulated to produce CoralPor™ Porous Glass in a form which satisfies the desired criteria, i.e., 200 to 700, e.g. , 200 - 500, nm features at a number density of 10^a to 10¹² mm^"3. [0021] Processes are known for the manufacturing of porous glass by phase separation. Hood et al. (US 2, 108,744) disclose a process wherein a borosilicate glass is subjected to a thermal treatment in order to induce separation of two phases, one phase rich in silica (the insoluble phase) and the other rich in alkali and boron oxide (the soluble phase). In the process, the metal oxides are melted in a furnace and then subjected to a heat treatment (e.g., 525°C for three days) which causes the glass to separate into the two phases which is characterized by the glass obtaining bluish opalescence, which indicates Rayieigh scattering rather than ie scattering. Thereafter, the heat treated glass is immersed in an acid bath to leach out the soluble phase,

[0022] Halier et al. (US 3,549,524; US 3,758,284) describe porous separation materials for use in separating components of a liquid mixture by

chromatography. To manufacture the separation materials, a base borosiiicate glass is subjected to a heat treatment to separate two phases, a silica-rich phase and a boron-rich phase. The heat treated glass is then treated with acid to leach out the boron-rich phase. Halier et al. disclose that the one can achieve the desired pore size and narrow distribution thereof through the specific heat treatment and that the dependence of pore size and distribution are expressed by the following equation: r" - kte^{"m r}, wherein r is pore radius, k, m and n are constants, T is heat treatment (in Kelvin), and f is time (in hours). The heat treatment can be at a temperature of 400-950°C for a time period of 2 hours to 4 weeks.

[0023] Hammel et a!. (US 3,850,721) describe a process for forming a microporous borosiiicate glass suitable for filtering tobacco smoke. The glass is prepared by melting the giass composition and then heat treating the glass at a temperature of about 450 °C to about 750 for a time period of 1 hour or more. Hammel et al. discloses that heat treating at higher temperatures for longer time periods causes the borate-rich phase to agglomerate, thereby resulting in the formation of larger pores during the leaching step.

[0024] In accordance with the invention, the scattering optical elements can be produced by, for example, melting a borosiiicate giass, preferably alkali borosiiicate glass that, after being annealed, is subjected to a phase separation using a well-controlled thermal treatment in the temperature range of 500- 800^oC, such as 600-800°C, preferably 650-750°C, more preferably 700- 725^ϋΟ, for a time period of, for example, 1 to 150 hours, such as 24 to 48 hours or 48 to 80 hours. For example, for a given composition, the thermal treatment employed to achieve nanoscale features of approximately 200 nanometers may involve a duration of 20-26 hours at 700 °C.

[0025]The conditions can be adjusted based on the melt parameters used (i.e., temperature and quenching method) and composition. In general, the process employed is dependent on desired phase growth. For example, within the phase separation region, generally, increased temperature for longer durations leads to larger feature size, although the size of the features will also depend on the specific composition.

[0028JThis thermal treatment processing can be considered complete for some applications, in other words, for some applications, no further processing steps are required to achieve the desired results of 200 to 700, e.g., 200 - 500, nm features with a number density approaching 10⁸ to 10¹² mm^"3.

[0027] However, the process can also include a further procedure for creating open porosity within the glass. This open porosity can be obtained by employing conventional chemical leaching protocols comprising the use of a pore forming (i.e., open porosity) acid Ieach and a pore cleaning caustic Ieach. In final, glasses exposed to the entire process (phase separation and chemicai leaching) reveal pore sizes, as measured by mercury intrusion porosimetry, on the order of 200 to 700, e.g., 200 to 500, nanometers with a number density approaching 10^s to 10¹² mm^'3. [0028] Suitable acids and caustic materials for the acid Ieach and pore cleaning caustic Ieach are described in US 2,106,744, US 3,549,524; US 3,758,284, and US 3,650,721 , cited above. Thus, for example, suitable acids include inorganic acids such as hydrochloric acid, sulfuric acid, and nitric acid, and suitable caustic materials include alkali metal hydroxides such as sodium hydroxide or potassium hydroxide.

[0029] The base glass can be, for example, any borosilicate glass that has suitable silica and borate contents for phase separation to occur and a sufficient borate content to achieve the desired number density of nanoscale features, Preferably, the borosilicate glass contains at least some alkali rnetai oxide. For example, in accordance with an embodiment of the invention, the base giass composition comprises (based on wt. %):

B₂0₃ 15,00 - 40.00

Si0₂ 45.00 - 80.00

R₂0 0.0 - 20.0

R'O 0.0 - 20.00

R"G₂ 0.0 - 10.00

AI2O3 0.0 - 10.00

wherein R₂G is the sum of U₂0, Na₂0, K₂0, Rb₂0, and Cs₂0 (and is preferably greater than 0), R'O is the sum of BaO, CaO, gO, SrO, PbO, and ZnO, and R"0₂ is the sum of Ti0_2l Zr0₂, and Hf0₂.

[0030] ith regards to ranges described herein, all ranges include at least the two endpoints of the ranges, as well as ali values between the two endpoints. Thus, for example, a range of 1 to 10 is to be understood as expressly disclosing at least the values of 1.0, 1.5, 2.0, 2.8, 3.0, 3.1 , 4.0, 4,2, 5.0, 5.3, 6.0, 7.0, 7.7 8,0, 9.0, 9.9 and 10.0.

[0031] In the borosilicate base glass, Si0₂ functions as a primary network former. Thus, according to another aspect of the invention, the borosilicate base glass composition contains 45.00 - 80.00 wt.% of SiG₂, for example, 45.00 - 75.00 wt.% of Si0₂, or 45.00 - 70.00 wt.% of Si0₂, or 45.00 - 65.00 wt.% of Si0₂, or 45.00 - 60.00 wt.% of Si0₂, or 50.00 - 60.00 wt.% of Si0₂-

[0032] In the borosiiicate base glass, B2G3 functions as a network former and as

primary former of the nanoscale features of the resultant phase separated/porous glass. Thus, according to another aspect of the invention, the borosllicate base glass composition contains 15.00 - 40.00 wt. % of B₂0₃, for example, 20.00 - 35.00 wt. % of B₂0₃, or 20.00 - 30.00 wt. % of B₂0₃.

[0033]According to another aspect, the borosllicate base glass composition contains 0.00 - 20.00 wt. % of f¾0 (preferably >0.00 - 20.00 wt. % of R₂0), wherein R₂0 is the sum of Li₂0, Na₂0, K₂G, Rb₂0, and Cs₂Q) for example, 1.00 - 15,00 wt. % RaO, or 1.00 - 10.00 wt. % R₂0, or 2,00 - 8.0Q wt. % R₂0, [0034] According to another aspect, the borosiiicate base glass composition contains 0.00 - 20.00 wt.% of R'O (sum of BaO, CaO, MgO, SrO, PbO, and ZnO) for example, 1.00 - 15.00 wt.% R'O, or 1.00 - 10.00 wt.% R'O, or 2,00 - 8,00 wt.% R'O. The R'O metal oxides can be used to adjust the refractive index of each phase. [003S]According to another aspect, the borosiiicate base giass composition contains 0.00 - 10.00 wt. % of R"0₂ (sum of Ti0_2l Zr0₂, and Hf0₂) for example, 0.00 - 8.00 wt. % R"0₂, or 1.00 - 8.00 wt. % R"0_2i or 0.Q0 - 5.00 wt. % R"0₂, or 1.00 - 5.00 wt. % R"G2. These metal oxides can be used to enhance chemical durability, and can be used to adjust the refractive index of each phase. [0036] In the borosiiicate base glass composition, ΑΙ2Ό3 generally acts as a network co-former and can also be used to enhance chemical durability. Thus, according to another aspect, the borosiiicate base glass composition according to the invention contains 0.00 - 10.00 wt.% of Ai₂0₃, for example, 0.00 - 8,00 wt.% of Al₂0₃, or 1 ,00 - 8.00 wt.% of AS₂0₃, or 0.00 - 5.00 wt.% of Al₂0₃, or 1.00 - 5.00 wt.% of Ai₂0₃, or 2.50 - 5,00 wt.% of Al₂0₃.

[0037]According to a further aspect, the base glass can be a glass in

accordance with giass composition described in James et al. (US2013/00 7387), This glass composition comprises (based on wt. %): 40-80% Si0₂, 5-35% Β₂0₃ and 1 -10% a₂0, preferably 45-85% Si0₂₎ 20-30% B₂0₃ and 2-8% Na₂0, and more preferably 50-55% Si0₂, 25-27% B₂0₃ and 5-7% Na₂0. As disclosed in US 2013/0017387 the glass can include other ingredients such as, for example, ZrQz- , Τίθζ, AI2O3, CaO and/or ZnO, and optionally further ingredients, e.g. , oxides of g, Fe, Mn, Ce, Sn, etc.

[0038] The optical element according to the invention can be affixed to an optical fiber or optical fiber bundle via any suitable process such as fusion splicing, optica! contacting via precision polishing, silicone adhesive, organic adhesive, or reactive nanofoil (such as that purchased from indium Corporation of America).

[0039] In order to affix the scattering element to an optical fiber or optica! fiber bundle, the output surface of the light guide (single fiber or fiber bundle) and the input surface of the scattering optic or scattering element must be prepared (e.g., cleaned and cleaved). After the surfaces are prepared, the format (e.g. , s ze, geometry) of the optics to be joined is considered, and the method of adhesion is selected. In one example, a scattering element is affixed to a single optica! fiber using a commerciaily available fusion splicing instrument, where arc fusion is employed to create an optica! joint. In a second example an optically clear silicone adhesive is applied to the surface of a fiber bundle and the scattering element is adhered to the surface via the silicone adhesive. A third example employs a flange of nanofoil which when activated (for example, by applying a potential across the foil) produces high temperature in a localized area to join the fiber to the scattering optic. [0040] For example, the optical element according to the invention can be used in in conjunction with a single opticai fiber or a bundle of optical fibers that forms part of an endoscope. Thus, in accordance with a further aspect, the invention relates to an endoscope comprising an opticai fiber or opticai fiber bundle wherein at the distal end of the optica! fiber or opticai fiber bundle (the end away from the user) there is a provided an optical scattering element for dispersing light emitted from the distal end of the optical fiber or optical fiber bundle, the optica! scattering element comprising a phase-separated or porous borosiiicate glass having dispersed phase particles with a particle size of 200 to 700, e.g. , 200 to 500, nanometers or pores having a size of 200 to 700, e.g. , 200 to 500, nanometers, at a number density of 10^s to 10¹² mm^"3. [0041] The optica! elment can also have a spherical shape and thus function as a scattering ball lens. Using such a ball lens, the optical designer can modify the optical output from the end of the fiber.

Brief ¾ c ri tj on of th e D r a wi ngs

[0042] The invention and further details, such as features and attendant advantages, of the invention are explained in more detail below on the basis of the exemplary embodiments which are diagrammatical/ depicted in the drawings, and wherein;

Figure 1 shows a scanning electron microscope image of a porous glass derived from composition A of Table 1 that exhibits pores on the order of 250 nm (see Example 2), and

Figure 2 shows a scanning electron microscope image of a porous optical element prepared from the end of an optica! fiber that exhibits pores on the order of 220-290 nm (see Example 3).

I iPLES

[0043] The base glasses can be made by combining the components in their metal oxide forms and melting the resultant mixture with the help of stirring using a platinum stirrer for better homogeneity. The glasses can then be cast into moulds and appropriately annealed in order to remove the stress as the liquid cools to the amorphous state. The resulting glass slabs can then be shaped into forms required for use with the instruments that measure the various properties of the glasses, drawn to fiber, or machined to numerous geometries for joining to optical fiber or optical fiber bundles,

[0044] The following Tables 1A-1 D present examples of suitable base glasses for use in with invention.

Table 1A. Glass compositions on an oxide-weight percent basis

Table 1 B. Glass compositions on an oxide-weight percent basis

Table 1 C. Glass compositions on an oxide-weight percent basis

Table 1 D, Glass compositions on an oxide-weight percent basis

Example 1

[0045] Glass-in-glass phase separation is employed to create in the glass features on the order of 250 nm to induce Mie scattering primarily in the forward direction. A base glass of composition A from Table 1 in plate form is exposed to a carefully controlled heat treatment wherein the base glass is subjected to a soak temperature of 700 °C for 24 hours. The resultant glass is opaque and white in color.

[0046]To support the proposed ie scattering behavior of the phase separated glass in the visible region of the electromagnetic spectrum (400-800 nm), Mie scattering calculations were performed using the approach outlined in Bohren and Huffman (Absorption and Scattering of Light by Small Particles, Wiley-VCH, 1983, pp. 82-89). Assuming a typical observation wavelength of 550 nm, 300- nm feature sizes and 50% volume proportions of the two phases as actually observed in the phase-separated samples, calculated refractive indices for the two glassy phases (1 ,53 and 1 .46) based on the model of Huggins and Sun (J. Amer. Ceram. Soc , vol. 26, p 4-1 1 , 1943) with compositions estimated from mass-balance consideration of the phase-separated glass, calculations reveal a feature number density of 3.5 x 10¹⁰ mm^"3, a scattering cross-section of 1 .5 x 10 ° pm², and a scattering coefficient of 523 cm^"1. Further, for this particular parameter set, the toward scatter component is over 100 times stronger than the back-scatter component, a desirable feature to increase the efficiency of the out- coupled light and to reduce the thermal load of the fiber. Note that this scenario (550 nm, 300-nm feature sizes, 50% volume proportions of the two phases, and the calculated refractive indices) is meant soieSy as an example of what can be attained using phase-separated glass.

Exaffl t . Z

[0047JA plate of composition A from Table 1 is fabricated and exposed to the same phase separation heat treatment as described in Example 1 . The phase separated sample is then exposed to a chemical leaching process comprised of an acid wash (e.g. , 10% hydrochloric acid, 95 ^CC) to create open porosity and a basic wash (e.g., 0.5 N sodium hydroxide, room temperature) to remove any residual material in the porous matrix. The product of this process is a porous glass that exhibits pores on the order of 250 nm (Figure 1 ). [0048] In this example, the Mie scattering is the result of the size of the nanoscaie features that yield the open porosity, i.e., the pores (average pore size approximately 250 nm), and the difference in refractive index between the silica matrix and the air filled pores. Assuming the same set of conditions as in Example 1 , but now with refractive indices of 1 .50 and 1 .00 (for the voids), the calculated Mie scattering turbidity coefficient is around 2000 mm^"1 at 400 nm wavelength and 850 mm^"1 at 800 nm, much higher than the glass of Example 1 due to the larger refractive index differentia!.

[0049] Following the examples listed above one skilled in the art can produce scattering optical elements in a variety of formats. For example, from melt, the original glass can be cast into plates, discs, other irregular geometries, or drawn to an optical fiber.

[0050] Furthermore, glass compositions such as those found in Table I can be drawn to form an optical fiber using well-known, accepted industrial processes. Here, a single-fiber can be employed, where a porous fiber can be partially exposed to a pore closing heat treatment as taught by Hood and Nordberg (US 2, 108,744) to produce a high silica fiber with a porous end that act as the scattering optical element as described in Example 2. Thus, according to a further aspect of the invention there is provided a process of preparing an optica! device comprising an optical fiber and a Mie scattering optical element, the process comprising preparing a porous glass, formed from a glass system that undergoes phase separation (for example, a borosilicate glass, preferably an alkali borosilicate glass), drawing the glass into a porous fiber and subjecting the drawn porous glass to a pore closing heat treatment to produce an optical fiber with a porous end, the porous end being a fvlie scattering optical element having pores having a size of 200 to 700 nanometers (e.g., 200 to 500 nanometers), at a number density of 10⁸ to 10¹² mm^"3.

[0051] Additionally, a single fiber produced from glass compositions such as those found in Table 1 can be exposed to a thermal treatment to induce glass-in- glass phase separation. The phase separated fiber can then be partially chemically leached such that the end of the fiber remains phase separated (e.g., non-porous). The pores created by the chemical leaching procedure can then be closed with a pore closing thermal treatment such as that taught by Hood and Nordberg (US 2,106,744). Here, the entire fiber is non-porous, and comprised of two regions - a high silica, non-scattering region and a phase separated ie scattering region. Thus, according to another aspect of the invention there is provided a process of preparing an optical device comprising an optical fiber and a Mie scattering optical element, the process comprising preparing a glass, formed from a glass system that undergoes phase separation (for example, a borosilicate glass, preferably an a!ka!i borosilicate glass), drawing the glass into a fiber, subjecting the fiber to controlled thermal treatment to induce phase separation, partially subjecting the phase-separated fiber to an acid leach to create pores to form a fiber having a porous region and a non-porous end, and subject the porous region of the fiber to a pore closing heat treatment to produce an optical fiber with a phase-separated non-porous end, the non-porous end being a Mie scattering optical element having dispersed phase particles with a particle size of 200 to 700 nanometers (e.g., 200 to 500 nanometers), at a number density of 10^s to 10¹² mm^"3. [0052] Furthermore, glass compositions such as those found in Table 1 can be drawn to an optical fiber. The resultant optical fiber can be partially inserted into a furnace to be heat treated only at one end. Here, a single optica! fiber is produced that is capable of Mie scattering Sight. Thus, according to a further aspect of the invention there is provided a process of preparing an optica! device comprising an optica! fiber and a Mie scattering optical element, the process comprising preparing a glass, formed from a glass system that undergoes phase separation (for examp!e, a borosilicate glass, preferabl an alkali borosilicate glass), drawing the g!ass into a fiber, subjecting one end of the fiber to controlled thermal treatment to induce phase separation in that end of the fiber, and optionally subjecting the phase-separated fiber end to an acid leach to create pores and a caustic leach to clean the resultant pores, to convert the treated end of the fiber to a Mie scattering optica! element having dispersed phase particles with a particle size of 200 to 700 nanometers (e.g., 200 to 500 nanometers) or pores having a size of 200 to 700 nanometers (e.g., 200 to 500 nanometers), at a number density of 10^s to 10¹² mm^"3.

[0053] Alternatively, rather than heating the fiber end in a furnace, the end of the single optica! fiber can be irradiate with a high power laser to induce g ass in glass phase separation.

[0054] In order to c!ose the pores of a porous glass, the glass is exposed to a mu!tistep thermal treatment. Initially, the porous glass is held at a low

temperature (-"200 °C) to vaporize water contained within the glass. Next, the temperature is slowly increased to consolidate the open porosity (i.e., sinter) to create a non-porous glass. Here, temperatures exceeding 1000 ^aC are employed.

[005S]The scattering optica! elements comprised of phase separated glass or porous glass, in accordance with the invention, have commercial value in the general field of illumination, optical devices and sensors. For example, by affixing the scattering optical element of the present invention to an optical fiber or optica! fiber bundle, the resultant fiber + scattering optical element can be employed in medical devices, such as optical fibers used in ocular surgery. Two specific advantages of the inventive scattering optical elements in the fields of optical devices and sensors are the tunability of the scattering features over a range of sizes (e.g., 200 - 700 nm) by varying the heat treatment/leaching conditions, and the ability to fabricate the glass products in a variety of formats (e.g., piate, fiber, etc.).

Example 3

[0056JA fiber of composition A from Table 1 is redrawn to a nominal diameter of 40 microns. One end of the fiber is then exposed to a thermal treatment for phase separation of 700 °C for 2 hours. Upon phase separation, the end of the fiber is opaque and opalescent when illuminated with white light and observed using an opticai microscope. [0057] The phase separated portion of the sample is then exposed to a chemical leaching process comprised of an acid wash (e.g., 10% hydrochloric acid, 95 °C) to create open porosity and a basic wash (e.g., 0,5 N sodium hydroxide, room temperature) to remove any residual materia! in the porous matrix. The product of this process is a porous giass that exhibits pores on the order of 220-290 nm (see Figure 2).

[0058] in this example, the ie scattering is the result of the size of the nanoscaie features that yield the open porosity, i.e., the pores (average pore size approximately 255 nm), and the difference in refractive index between the silica matrix and the air filled pores. Assuming the same set of conditions as in

Example 1 (typical observation wavelength of 550 nm. 300-nm feature sizes and 50% volume proportions of the two phases), but now with refractive indices of 1.50 (for the glass) and 1.00 (for the voids), the calculated Mie scattering turbidity coefficient is around 2000 mm^"1 at 400 nm wavelength and 850 mm^{' 1} at 800 nm, much higher than the glass of Example 1 due to the larger refractive index differential.

Example 4 [0059] A fiber of composition A from Table 1 is redrawn to a diameter of 40 +/- 10 microns and cut to a length of 200 microns. The cut fiber (in the form of a short cyciinder) is affixed to an optica! fiber (e.g., by fusion sp!icing), and the cut fiber is exposed to a thermal treatment for phase separation of 875 °C for 2 hours. The cut fiber forms a sphere at the end of the optical fiber upon phase separation and is opalescent when illuminated with white light and observed using an optica! microscope.

[0060] The sphere formation is the result of the length of the cut fiber, the time/temperature of the phase separation and the surface tension/viscosity of the glass.

[0081] To one skilled in the art the spherical element can function as a scattering bail lens. For example, after heat-treating a fiber of composition A that is approximately 1 .3 mm in length and affixed to the end of an optical fiber, a baSi Sens of about 80 micron radius will form on the end of the optica! fiber. Such a bail lens can thus be used to additionally modify the optica! output from the end of the fiber according to the wishes of the optica! designer.

[0062] The entire disclosure[s] of ail applications, patents and publications, cited herein, are incorporated by reference herein.

[0063] The preceding examples can be repeated with similar success by substituting the generical!y or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.

[0064] 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.

Claims

1. A device comprising at least one optical fiber and at least one Mie scattering optical element for dispersing Sight emitted from said at least one optical fiber, when said least one optical fiber is in operation, wherein said at least one Mie scattering optica! eiement comprises a phase-separated or porous giass having dispersed phase particles with a particle size of 200 to 700 nanometers or pores having a size of 200 to 700 nanometers, at a number density of 10⁸ to 10¹² mm^"3.

2. A device according to claim 1 , wherein said phase-separated or porous glass has dispersed phase particles with a particle size of 200 to 500 nanometers or pores having a size of 200 to 500 nanometers, at a number density of 10^B to 10¹² mm^"3

3. A device according to claim 2, wherein said phase-separated or porous glass is a phase-separated or porous borosiiicate glass.

4. A device according to claim 2, wherein said phase-separated or porous giass is a phase-separated or porous alkali borosiiicate glass.

5. The device according to claim 2, wherein said at least one Mie scattering optica! element comprises a phase-separated borosiiicate glass wherein the particles of the dispersed phase have particle size of 200 to 500 nanometers at a number density of 10⁸ to 10¹² mm^" .

8. The device according to claim 5, wherein said at least one Mie scattering optical eiement comprises an alkali phase-separated borosiiicate giass wherein the particles of the dispersed phase have particle size of 200 to 500 nanometers at a number density of 10^s to 10¹² rnrrf³ .

7. The device according to claim 2, wherein said at least one Mie scattering optical element comprises a porous borosiiicate glass wherein glass have pores with a pore size of 200 to 500 nanometers at a number density of 10^e to 10¹² mm^"3.

8. The device according to claim 7, wherein said at least one M\e scattering optical element comprises an alkali porous borosilicate glass wherein glass have pores with a pore size of 200 to 500 nanometers at a number density of 10⁸ to 10¹² mm^"3.

9. The device according to claim 5 or claim 6, wherein said particles of the dispersed phase have particle size of 300 to 500 nanometers.

10. The device according to claim 5 or claim 8, wherein said particles of the dispersed phase have particle size of 300 to 450 nanometers. 1 1 . The device according to claim 7 or ciairn 8, wherein said pores have a pore size of 300 to 500 nanometers.

12, The device according to claim 7 or ciairn 8, wherein said pores have a pore size of 300 to 450 nanometers.

13, The device according to any of claims 1 to 12, wherein said dispersed phase particles or said pores have a number density of 10³ to 10ⁱ mm^"3.

14. The device according to claim 13, wherein said dispersed phase particles or said pores have a number density of 10¹⁰ to 1 G ^' mm^"3,

15. The device according to any of claims 1 to 14, wherein said device is capable of providing a scattering cross section of 10^'4 to 1 Q^~2 μπι² for light emitted though said at least one Svlie scattering optica! element.

18. The device according to claim 1 5, wherein said device is capable of providing a scattering cross section of 5x10^"3 to 5x10^"2 pm².

17, The device according to any of claims 1 to 18, wherein said at least one Mie scattering optica! element is prepared by subjecting an annealed borosiiicate glass to a phase separation using a controlled thermal treatment, and optionally subjecting the phase-separated borosiiicate giass to an acid leach to create pores and a caustic leach to dean the resultant pores,

18. The device according to claim 17, wherein said controiied thermal treatment comprising heating the annealed borosiiicate glass at a temperature of 500~800°C for a time period of 1 to 150 hours. 19. The device according to claim 17, wherein said controlled thermal treatment comprising heating the annealed borosiiicate glass at a temperature of 800-800°C for a time period of 1 to 150 hours.

20. The device according to claim 17, wherein said controlled thermal treatment comprising heating the annealed borosiiicate glass at a temperature of 850-75GX for a time period of 1 to 150 hours.

21 . The device according to claim 17, wherein said controiied thermal treatment comprising heating the annealed borosiiicate glass at a temperature of 700-725°C for a time period of 1 to 150 hours.

22. The device according to any one of claims 18 to 21 , wherein said time period is 24 to 48 hours. 23. The device according to any one of claims 18 to 21 , wherein said time period is 48 to 80 hours.

24. The device according to any one of claims 17 to 23, wherein said annealed borosiiicate giass comprises (based on wt. %):

B₂0₃ 15.00 - 40.00

Si0₂ 45.00 - 80.00

R₂0 0.0 - 20.0

R'O 0.0 - 20,00

R"0₂ 0.0 - 10.00

A!₂0₃ 0.0 - 10.00 wherein R₂0 is the sum of Li₂0, Na₂0, K₂Q, and Cs₂0, R'O is the sum of BaO, CaG, MgO, SrO, and ZnO, and R'O₂ is the sum of Ti0₂, Zr0_2l and Hf0₂. 25. The device according to any one of ciaims 17 to 24, wherein said annealed borosilicate g!ass is an annealed aika!i borosi!icate glass.

26, The device according to any one of claims 17 to 23, wherein said annealed borosilicate glass comprises {based on wt. %): 40-80% Si0_2i 5-35% B₂0₃ and 1-10% Na₂0.

27. The device according to claim 26, wherein said annealed borosilicate glass comprises (based on wt. %): 45-65% Si0₂, 20-30% B₂0₃ and 2-8% Na₂0 28. The device according to claim 27, wherein said annealed borosilicate glass comprises (based on wt. %): 50-55% Si0_2l 25-27% B₂0₃ and 5-7% Na₂0.

29. The device according to any one of claims 1 to 28, wherein each optical fiber of said device has one of said Mie scattering optical elements attached to an end thereof.

30. The device according to any one of claims 1 to 28, wherein said device comprises a piuraiity of said optical fibers in the form of an optica! fiber bundle, and one of said Mie scattering optical elements is attached to an end of said

optical fiber bundle.

31. The device according to any of claims 1 to 30, wherein said optical element has a spherical shape.

32. A process for preparing a device according to any one of claims 1 to 31 , said process comprising: preparing said Mie scattering optical element by subjecting an annealed glass (such as a borosilicaie glass, preferably an alkali borosiiicate glass) to a phase separation using a controlled thermal treatment, and optionally subjecting the phase-separated glass to an acid leach to create pores and a caustic leach to clean the resultant pores, and attaching the resultant Mie scattering optical element to the end of said optical fiber or to an end of a bundle of optical fibers.

33. The process according to claim 32, wherein said resultant Mie scattering optica! element is attached to the end of said optical fiber or to an end of a bundle of optical fibers by fusion splicing.

34. The process according to claim 32, wherein said resultant Mie scattering optical element is attached to the end of said optical fiber or to an end of a bundle of optical fibers by an optically clear silicone adhesive. 35. The process according to claim 32, wherein said resultant Mie scattering optical element is attached to the end of said optical fiber or to an end of a bundle of optical fibers by a nanofoii.

38. A process for preparing a device according to any one of claims 1 to 31 , said process comprising: subjecting at least a portion of an anneaied glass opticai fiber (such as a borosilicaie glass, preferably an alkali borosilicaie glass) to a phase separation using a controlled thermal treatment, and optionally subjecting at least a part of the resultant phase-separated region of said optical fiber to an acid leach to create pores and a caustic leach to clean the resultant pores.

37. The process according to claim 38, wherein the entire optica! fiber is subjected to phase separation, then, except for an end region thereof, the phase- separated optical fiber is subjected to an acid leach to create pores and thereby form a fiber having a porous region and a non-porous end, and then the porous region of the optical fiber is subjected to a pore closing heat treatment to produce an optical fiber having a phase-separated, non-porous end, and said end is a Mie scattering optica! element having dispersed phase particles with a particle size of 20Ό to 700 nanometers (e.g., 200 to 500 nanometers), at a number density of 10^s to 1 Q ² mm^"3.

38. The process according to claim 36, wherein only an end of optical fiber is subjected to phase separation, then only the phase-separated end of the optical fiber is subjected to an acid leach to create pores and thereby form a fiber having a non-poous, non-phase separated region and a porous end, and said porous end is a Mie scattering optical element having pores with a size of 200 to 700 nanometers (e.g., 200 to 500 nanometers} at a number density of 10^e to 10¹² mm^"3.

39. An endoscope comprising an optical fiber or optical fiber bundle and an optical scattering element attached to the distal end of said optical fiber or optica! fiber bundle for dispersing light emitted from the distal end of said optical fiber or optical fiber bundle, the optical scattering element comprising a phase-separated or porous glass (such as a borosilicate glass, preferably an alkali borosilicate glass) having dispersed phase particles with a particle size of 200 to 700 nanometers or pores having a size of 200 to 700 nanometers, at a number density of 10^s to 1 Q¹² mm^"3.

40. The endoscope according to claim 39, wherein said a phase-separated or porous glass has dispersed phase particles with a particle size of 200 to 500 nanometers or pores having a size of 200 to 500 nanometers, at a number density of 10⁸ to 10¹² mm^"3.

41. A method for dispersing or scattering light transmitted by optical device comprising transmitting the light through an optical scattering element comprising a phase-separated or porous glass (such as a borosilicate glass, preferably an a!kaii borosiiicate glass) having dispersed phase particles with a particle size of 200 to 700 nanometers or pores having a size of 200 to 700 nanometers, at a number density of 10^s to 10¹² mm^"3. 42. The method according to claim 41 wherein said a phase-separated or porous glass has dispersed phase particles with a particle size of 200 to 500 nanometers or pores having a size of 200 to 500 nanometers, at a number density of 10⁸ to 10¹² mm^"3.

43. The method according to claim 41 , wherein said optical device is an optical fiber.

414, The method according to claim 3941 , wherein said optical device is an optical fiber bundle.