OPTICAL FIBER, AND METHOD OF MANUFACTURING AND OF OPERATING
OPTICAL FIBER, AND METHOD OF MANUFACTURING AND OF OPERATING TECHNICAL FIELD OF THE INVENTION The current invention concerns an optical fiber for radiation generation and transfer and a method of manufacturing such an optical fiber and a method of operating such an optical fiber. DESCRIPTION OF THE RELATED ART Optical fibers for radiation generation and transfer are known from thermal photovoltaic (TPV) devices, which comprise an optical fiber that includes luminescence generating material. The optical fiber is heated up to high temperatures and the luminescence generating material emits a radiation at a near-infrared wavelength, which is converted by a photovoltaic cell into electricity. Such a thermal photovoltaic device is known for example from US 5500054 showing a light pipe that comprises emissive material disposed within the light pipe and an optical cladding with a significantly different index of refraction than the core of the light pipe. The cladding reflects the photons generated by the emissive material such that the photons are directed through the light pipe. Further, WO 2016/203012 shows a system that comprises a radiation emitter and a core for energy transfer.
SUMMARY OF THE INVENTION It is the task of the invention to provide an improved optical fiber for radiation generation and transfer and a method of manufacturing such an optical fiber. This task is solved by an optical fiber with the features of claim 1. Further embodiments of the optical fiber as well as a method for manufacturing and operating such an optical fiber are specified by the features of further claims. The invention concerns an optical fiber for radiation generation and transfer. The optical fiber comprises at least one luminescence generating zone of a first material that comprises a luminescence generating material for providing an emitted radiation predominantly at a visible and/or near-infrared wavelength. The optical fiber further comprises at least one wave guiding zone of a second material. The at least one wave guiding zone is configured to concentrate the emitted radiation predominantly within the at least one wave guiding zone. This provides distinct locations with different functionality, one location for generating the emitted radiation and the further location for guiding and concentrating the emitted radiation. Thus, both zones can be specifically designed to achieve optimized performance and efficiency. The invention is particularly advantageous in thermal energy converting applications because the generating of radiation on one hand and the guiding and concentration of
the radiation involve different or even contradicting functional requirements. For example, efficient generation of radiation aims for a high concentration of luminescence generating material, whereas guiding and concentration of the radiation aims for an undisturbed optical path by substantially avoiding radiation attenuation and/or radiation absorption. In the context of the invention the term "zone" is understood as geometrically distinct locations, defined by the border between materials of different characteristics. A zone may consist of multiple distinct parts, including subzones or separated parts. The term "fiber" includes synonyms such as "waveguide" or "light guide". The term "luminescence generating zone", also called "optical active zone" or "active zone", refers to the zone that comprises luminescent material. The term "wave guiding zone", also called "optical passive zone" or "guiding zone" refers to the zone that transfers the emitted radiation towards one or both ends of the optical fiber. The concentration of radiation refers to processes that lead to an increased radiation density, in the sense of more photons per time interval penetrating an area A, in particular where area A is the cross section of the fiber at the fiber end, in particular predominantly at a near-infrared wavelength. The improvement of the optical fiber as a source of radiation in an optical system is reflected in the fiber's ability to capture and guide the radiation originating from luminescence generated in one or more zones, in particular radiation generated by luminescent material embedded
primarily near the large interface area between the luminescence generating zone and the wave guiding zone across the full length of the fiber. The length of the fiber, fiber temperature, spectral distribution and (preferably low) reabsorption determine the spectral power density per area of fiber cross section, typically expressed in [W/mm2]. In a limited spectral region, the power density from such improved optical fiber can exceed the power density from a black body of similar temperature, similar area, within a similar spectral range, resulting from a shift in the spectral distribution due to luminescent materials incorporated in specific zones of the fiber. In an embodiment, the fiber comprises at least one hot zone and at least one cold zone. In particular, the at least one hot zone is located within a hot environment, further in particular inside of a hot cavity, and the cold zone is located outside of the hot environment, further in particular outside and/or remote of the hot cavity. In an embodiment, the fiber is configured to concentrate the emitted radiation by shifting the spectral power distribution of the emitted radiation towards a shorter wavelength. In a further embodiment, the at least one luminescence generating zone forms at least one inner part of the optical fiber, in a particular at least one core, and the wave guiding zone forms an outer part of the optical fiber,
in particular a cladding, that surrounds the at least one luminescence generating zone. In one example, the inner part of the optical fiber and the wave guiding zone may form cylindrical bodies, which may or may not be arranged coaxially to each other. In another embodiment, the at least one wave guiding zone forms an inner part of the optical fiber, in a particular at least one core, and the at least one luminescence generating zone forms an outer part of the optical fiber, in particular a cladding, that surrounds the at least one wave guiding zone. In a further embodiment, the optical fiber comprises a peripheral zone comprising a third material, that surrounds both the wave guiding zone(s) and the luminescence generating zone(s), wherein the refractive index nh of the material of the peripheral zone is lower than the refractive indices of the first material and/or the second material. In a further embodiment, the optical fiber according to any one of the preceding claims, wherein the at least one wave guiding zone is substantially transparent at a near- infrared wavelength and/or the emitted radiation is substantially non-interacting with the material of the wave guiding zone, in particular by substantially avoiding radiation attenuation and/or radiation absorption.
In a further embodiment, at least two of the materials of the optical fiber, in particular all materials, have substantially equal or similar viscosity at a fiber drawing temperature. In a further embodiment, the fiber drawing temperature is in a range of 1800°C to 2200°C, in particular 1900°C to 2100°C, further in particular 1950°C to 2050°C. In a further embodiment, at least one of the materials of the optical fiber, in particular at least two of the materials, further in particular all materials, comprise a viscosity influencing material, which in particular modifies the viscosity. In a further embodiment, the concentration of the viscosity influencing material is at least 1 at%, in particular at least 10 at%, further in particular at least 20 at%. In a further embodiment, at least one of the materials of the optical fiber, in particular at least two of the materials, further in particular all materials, comprise a refractive index influencing material, which in particular modifies the refractive index. In a further embodiment, the concentration of the refractive index material is at least 1 at%, in particular at least 10 at%, further in particular at least 20 at% not considering oxygen atoms (this omission of oxygen applies to the whole rest of this document). In a further embodiment, the luminescence generating material comprises one or more rare earth materials of a group comprising elements with atomic numbers 21, 39 or 57-
71, in particular optically active element category lanthanide with atomic numbers: 57-71 and transition metals with atomic numbers 21, 39 or 57, in particular at least one element of the group comprising Ytterbium, Neodymium, Erbium and Yttrium. In a further embodiment, the luminescence generating material comprises a concentration of at least 1 at%, in particular at least 4 at%, further in particular at least 10 at%. In a further embodiment, the luminescence generating material provides an emitting radiation predominantly in the near infrared range, in particular 800-1500 nm, in particular 900-1100 nm. In a further embodiment, the first material and/or the second material comprises Si-oxide, in particular in form of silica, and/or Al-oxide, in particular in form of alumina. In a further embodiment, the luminescence generating zone comprises a plurality of mutually separate luminescence generating zones arranged parallel to the fiber axis, in particular in form of strands of luminescent material, further in particular in form of discontinuous strands. Such discontinuities in the luminescence generating zone have little impact on the fiber's wave guiding properties, i.e. the fiber's ability to concentrate and transfer radiation in the wave guiding zone. The fiber's resilience to such discontinuities in the luminescence generating zone
enables the fiber to be composed, structured and drawn in ways that reduce manufacturing cost. In a further embodiment, the wave guiding zone comprises a plurality of mutually separate wave guiding zones arranged parallel to the axis of the optical fiber. In a further embodiment, the optical fiber has a diameter of more than 10 mpi and supports multimode radiation propagation. In a further embodiment, the optical fiber is configured to absorb the part of the emitted radiation that comprises a wavelength longer than a predetermined wavelength lo, in particular by reducing fiber transparency for wavelengths longer than the predetermined wavelength lo, whilst maintaining transmission for wavelengths shorter than said predetermined wavelength lo, further in particular by restricting a dopant concentration within the fiber. The optical fiber of the above-mentioned embodiments of the optical fiber can be used in any combination, unless they contradict each other. Further, the invention concerns an optical system that comprises the optical fiber according to any one of the preceding claims and a photovoltaic cell optically coupled to the optical fiber and/or an energy source operationally
coupled to the optical fiber for providing energy by thermal and/or phononic and/or other excitation. In a further embodiment of the optical fiber or of the optical system the optical fiber, in particular the waveguiding zone thereof, comprises a dopant of rare earth ions. In a further embodiment of the optical fiber or of the optical system the dopant comprises at least one member of a group of Erbium, Thulium, Praseodymium and Holmium and/or a dopant concentration in a range from 0.5%at to 10%at, in particular from 2%at to 5%at. In a further embodiment of the optical fiber or of the optical system the optical fiber comprises at least one of: - Erbium ions (Er3+) for absorbing at 1.5-1.6 pm and/or 2.7 pm; - Thulium ions (Tm3+) for absorbing at 1.7-2.1 pm and/or 1.45-1.53 pm; - Praseodymium ions (Pr3+) for absorbing at 1.3 pm; and - Holmium (Ho3+) for absorbing at 2.1 pm and/or 2.9 pm. The invention further concerns a method for manufacturing the optical fiber according to any one of the preceding optical fiber embodiments.
The invention further concerns a method of manufacturing the optical fiber for radiation generation, concentration and transfer, the method comprising: - providing a first material having a refractive index na and comprising a luminescence generating material for providing an emitting radiation predominantly at a near- infrared wavelength; - providing a second material with a refractive index np, which is higher than the refractive index of the first material; - heating the first material and the second material to a common fiber drawing temperature; and - forming the optical fiber by drawing the first material together with the second material. This method achieves an efficient manufacturing process, particularly with respect to the interaction of the used materials. In a further embodiment the method comprises the step of providing a third material that contains the first and the second material. In a further embodiment the method comprises the step of producing at least one of the materials, in particular at least two of the materials, in particular all of the materials, by fused silica and/or alumina processing. In a further embodiment of the method the fused silica and/or alumina processing comprises sol-gel and/or a granulated powder processing step.
In a further embodiment the method comprises the step of forming a preform by combining at least the first and/or the second material, in particular all the materials, and applying a vitrification process to the preform. In a further embodiment the method comprises the step of placing at least two of the materials, in particular all of the materials, in separate zones, in particular separate zones of the preform. In a further embodiment of the method the drawing of the optical fiber produces a fiber with a diameter of more than 10 (jm. The invention further concerns an optical fiber for radiation generation, concentration and transfer. The optical fiber comprises at least one (highly doped) optical active zone of a first material comprising a luminescent material emitting radiation (e.g. predominantly at a near- infrared wavelength). The optical fiber further comprises at least one optical passive zone of a second material with a refractive index higher than the refractive index of the first material. This provides distinct locations with different functionality, one location for generating the radiation and the further location for guiding and concentrating the radiation. Thus, both zones can be specifically designed to achieve optimized performance and efficiency. The optical active zone of the fiber is designed to generate luminescence but not predominantly to
guide radiation. The optical passive zone of the fiber is designed to guide radiation. The combined impact of spatially separated optically active and passive zones enable a significant amount of luminescence to be generated and guided almost loss-free and without reabsorption along the optical fiber and towards the fiber ends. The radiation density emitted from the fiber end thus scales with fiber length. With this highly actively doped and structured fiber significantly higher radiation densities can be achieved at the fiber end than can be achieved with conventional fibers where the reabsorption-prone luminescent material is dispersed across a single zone used for both generating and guiding radiation. The invention is particularly advantageous in thermal energy converting applications because the generation of radiation on one hand and the guiding and concentration of the radiation involve different or even contradicting functional requirements. For example, in order to generate strong radiation a high concentration of luminescence generating material is required, whereas for guiding (the separated) radiation a low attenuation and no absorption i.e. the absence of luminescent and scattering material is required. In an embodiment, at least one luminescence generating zone forms at least one inner part of the optical fiber, in particular at least one optically active core, and the radiation guiding zone forms an outer part of the optical fiber, in particular an optically passive cladding that surrounds at least one luminescence generating zone. In one
example, the inner radiation generating zone of the optical fiber and the outer radiation guiding zone may form cylindrical bodies, which may or may not be arranged coaxially to each other. In another embodiment, at least one radiation guiding zone forms an inner part of the optical fiber, in particular at least one optically passive core, and the at least one luminescence generating zone form an outer part of the optical fiber, in particular an optically active cladding, that surrounds at least one radiation guiding zone. E.g. the inner radiation guiding zone of the optical fiber and the outer radiation generating zone may form cylindrical bodies, which may or may not be arranged coaxially to each other. In a further embodiment, at least one of the materials of the optical fiber, in particular at least two of the materials, further in particular all materials, comprise at least one viscosity influencing material, which in particular changes the viscosity of the material to match the targeted viscosity for all materials at the fiber drawing temperature. In a further embodiment, the concentration of the viscosity influencing material is at least 1 at%, in particular at least 10 at%, further in particular at least 20 at%. In a further embodiment, at least one of the materials of the optical fiber, in particular at least two of the materials, further in particular all materials, comprise at
least one refractive index influencing material. In a further embodiment, the concentration of the refractive index material is at least 1 at%, in particular at least 10 at%, further in particular at least 20 at%. In a further embodiment, the luminescence generating material comprises one or more materials from the rare earth group of elements (atomic numbers: 21, 39, 57-71), optically active element category lanthanide (atomic numbers: 57-71) or transition metals (atomic numbers: 21, 39, 57) in particular Ytterbium, which features a narrow emission band matching closely the Si-Photovoltaic (Si-PV) bandgap, and/or Neodymium and/or Erbium and/or Yttrium. In a further embodiment, the luminescence generating material comprises a concentration of at least 1 at%, in particular at least 5 at%, further in particular at least 10 at%. In a further embodiment, the luminescence generating dopants provide a radiation predominantly in the near-infrared region, in particular 900-1100 nm. In a further embodiment, the host material of the first material and/or the second material is silicon Si and/or aluminum A1 or their respective oxides silicon dioxide (silica) S1O2 and/or transparent aluminum oxide AI2O3 (alumina). In a further embodiment, the radiation guiding zone comprises a plurality of mutually separate luminescence generating zones arranged parallel to the fiber axis, in
particular in form of strands of luminescent material, further in particular in form of continuous and/or discontinuous strands. In a further embodiment, the guiding zone has a diameter and refractive index difference supporting single-mode or multimode radiation. Further, the invention concerns a system that comprises the optical fiber according to any one of the preceding claims and a radiation receiver coupled to the optical fiber and/or an energy source for exciting the luminescent material embedded within the optical fiber for providing excitation through photonic and/or phononic energy transfer mechanisms. The invention further concerns a method of manufacturing the optical fiber for separating radiation generating and radiation guiding zones, the method comprising: - providing a first material having a refractive index and comprising a luminescence generating material for providing an emitting radiation predominantly at a visible and/or near-infrared wavelength; - providing a second optically passive material with a refractive index, which is higher than the refractive index of the first material; - combining in a preform at least the first and/or the second material, in particular all the materials - heating the preform to a fiber drawing temperature; and
- forming the optical fiber by drawing the preform. This method achieves an efficient manufacturing process, particularly with respect to the interaction of the materials used. In a further embodiment the method comprises the step of providing a third material that envelopes the first and the second material. In a further embodiment the method for production of the first and/or second material comprises sol-gel and/or a granulated powder processing step. In a further embodiment the method comprises the step of placing at least two of the materials, in particular all of the materials, in separate zones in the preform. The invention further concerns a method of operating the optical fiber according to any one of the preceding optical fiber embodiments. The invention further concerns a method of operating the optical fiber for radiation generation and concentration, the optical fiber comprising at least one luminescence generating zone and at least one wave guiding zone and the method comprising: - applying heat to the luminescence generating zone for providing an emitted radiation predominantly at a
wavelength in the visible and/or near-infrared radiation spectrum; - providing a second material with a refractive index np, which is higher than the refractive index of the first material; - transferring the emitted radiation to the at least one wave guiding zone; and - concentrating the emitted radiation predominantly within the at least one wave guiding zone. In a further embodiment the fiber comprises at least one hot zone and at least one cold zone. In particular, the hot zone is located within a hot environment, in particular inside of a cavity, and the cold zone is located outside of the hot environment, in particular outside and/or remote of the cavity. In a further embodiment the temperature of the luminescence generating zone and/or the at least one wave guiding zone, in particular the at least one hot zone, is in a range of 800°C to 1700°C, in particular in a range of 1200°C to 1500°C. In a further embodiment the emitted radiation predominantly comprises a wavelength range of 700-1500 nm, in particular 900-1100 nm. In a further embodiment the transferring and/or concentrating of the emitted radiation comprises shifting
the spectral distribution of the emitted radiation towards a shorter wavelength, in particular by at least 20 nm, further in particular by at least 100 nm. In a further embodiment the shifting comprises absorbing the part of the emitted radiation that comprises a wavelength longer than a predetermined wavelength (lo), in particular by reducing fiber transparency for wavelengths longer than the predetermined wavelength (lo), whilst maintaining transmission for wavelengths shorter than said predetermined wavelength (lo), further in particular by restricting a dopant concentration within the fiber. In a further embodiment the method comprises the step of providing the optical fiber, in particular the waveguiding zone thereof, with a dopant that comprises rare earth ions. In a further embodiment the method comprises the step of providing the dopant as at least one member of a group of Erbium, Thulium, Praseodymium and Holmium and/or providing a concentration of the dopant in a range from 0.5%at to 10%at, in particular from 2%at to 5%at. In a further embodiment the method comprises at least one step of: - using Erbium ions (Er3+) and absorbing at 1.5-1.6 pm and/or 2.7 pm; - using Thulium ions (Tm3+) and absorbing at 1.7-2.1 pm and/or 1.45-1.53 pm;
- using Praseodymium ions (Pr3+) absorbing at 1.3 pm; and - using Holmium (Ho3+) and absorbing at 2.1 pm and/or 2.9 pm. The features of the above-mentioned optical fiber embodiments and of the manufacturing method as well as of the operation method can be used in any combination, unless they contradict each other. BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the current invention are described in more detail in the following with reference to the figures. These are for illustrative purposes only and are not to be construed as limiting. It shows Fig. 1 a schematic perspective view of the optical fiber 1 according to the invention and a cross-section "A" thereof, the optical fiber comprising one core 100, a cladding 200 and a hull 300; Fig. 2 an optical fiber according to Fig. 1 with a luminescence generating core 112 and a wave guiding cladding 223 and a hull 321 and a corresponding refractive index profile; Fig. 3 an optical fiber according to Fig. 1 with a wave guiding core 123, a luminescence generating cladding 212 and a hull 321 and a corresponding refractive index profile; Fig. 4 an optical fiber according to Fig. 1, the luminescence generating core 112 comprising
discontinuous strands of luminescent material and a corresponding refractive index profile; Fig. 5 an optical fiber according to Fig. 1, with multiple luminescence generating cores 112 and a corresponding refractive index profile; Fig. 6 an optical fiber according to Fig. 3 with multiple wave guiding cores 123 and a corresponding refractive index profile; Fig. 7 a schematic perspective view of a preform 1000 used for manufacturing an optical fiber according to the invention; Fig. 8 a schematic flow diagram showing the steps for manufacturing an optical fiber according to the invention; Fig. 9 an illustration of spectral power density distribution according to the invention; Fig. 10 an illustration of black body radiation at two different temperatures; Fig. 11 an illustration according to Fig. 10 showing an area corresponding to the net radiative power between the two blackbodies at different temperature; Fig. 12 an illustration according to Fig. 11 showing the distribution of the spectral power density between two blackbodies at different temperatures, where the net power corresponds to the area underneath the curve; Fig. 13 an illustration according to Fig. 12 showing the spectral power density distribution and radiative power between two blackbodies at different
temperatures for wavelengths shorter than and longer than a given wavelength l0; Fig. 14 an illustration according to Fig. 13 showing the radiative power emitted from the fiber end in the cold zone, at a temperature below the hot zone temperature in the middle of the fiber, i.e. between the two fiber ends. This is compared with spectral radiation density and radiative power between two blackbodies at different temperatures; Fig. 15 displays only the radiation from the cold fiber end shown in Fig. 14, decomposed into a component of radiation with power density (PFS) at wavelengths shorter than given wavelength loand radiation with a power density (PFID) at wavelengths longer than a given wavelength lo; Fig. 16 an illustration according to Fig. 15 providing a visual interpretation of the Fiber Figure of Merit (FFoM, which is defined as PFa /(PFS + PFID). Fig. 17 a measurement diagram showing the spectral power density at a cold end of the fiber according to the invention; Fig. 18 a schematic perspective view of the optical fiber according to the invention with a hot zone (lOOh, 200h, 300h) and a cold zone (100c,200c,300c with a transition area in between the hot and the cold zone; and Fig. 19 a perspective view according to Fig. 18 but with the fiber ending in two cold zones (100c,200c,300c) with two transition areas in between the hot zone and the two cold zones.
DETAILED DESCRIPTION OF THE INVENTION For the reference numbers of the drawings, the following naming convention of the reference signs applies: X X X ! ! !-Refractive Index: ! ! 0=n undefined, l=n low, 2=n mid, 3=n high ! !- Luminescence: 0=undefined, l=active, 2=passive Zone: l=core, 2=cladding, 3=hull Figure 1 shows a schematic view of the optical fiber 1 according to the invention. The optical fiber comprises a zone referred to as core 100, in this example one core, a zone referred to as cladding 200 and a peripheral zone in form of an outer hull 300. The cladding 200 surrounds the core 100 and the outer hull 300 surrounds the core 100 and the cladding 200. The lower part of figure 1, as indicated by reference "A" shows a cross-section along the dotted line as shown in the upper part of figure 1. The core 100 and the cladding 200 and the outer hull 300 are arranged concentrically. Configurations other than concentrically arranged cylinders and tubes are also possible. Fig. 2 shows an optical fiber according to Fig. 1 with a luminescence generating zone in form of a core 112 and a wave guiding zone in form of a cladding 223 and a hull 321. The upper part of Fig. 2 shows a schematic perspective view of the optical fiber, the middle part the corresponding cross-section "A" and the lower part a corresponding
refractive index profile. The refractive index profile shows the relative refractive index Dh in dependency of the direction x, which extends across the optical fiber. The core 112 comprises a first material with a refractive index na, also referred to as active material, the cladding 223 comprises a second material with a refractive index np, also referred to as passive material, and the outer hull 321 that comprises a third material with a refractive index nh. The following expression is valid: nh < na < np with index "h" denoting the refractive index of the hull, index "a" the active material and index "p" the passive material. Thus, the refractive index nh of the hull 321 is lower than the refractive index na of the core 112, which in turn is lower than the refractive index np of the cladding 223. This situation is illustrated by way of example in the refractive index profile as show in Fig. 2. This configuration provides for an internal reflection, preferably a total internal reflection, of radiation emitted from the core 112 into the cladding 223 so as to thereby confine multiple modes of said emitted radiation within the cladding 223. Fig. 3 shows an optical fiber according to Fig. 1, i.e. a schematic view of the optical fiber 1 according to the invention, but with luminescence generating zone and wave guiding zone in reversed configuration. Thus, the optical fiber 1 comprises the wave guiding zone in form of a core
123, a luminescence generating zone in form of a cladding 212 and a peripheral zone in form of an outer hull 321. The further geometrical configuration is identical to the one of Fig. 2. The core comprises the second material with a refractive index np (also referred to as passive material with refractive index np) and the cladding comprises the first material with a refractive index na (also referred to as active material with refractive index na) and the outer hull that comprises a third material with a refractive index nh. Also in this example the following expression is valid: nh < na < np Thus, the refractive index nh of the hull 321 is lower than the refractive index na of the cladding 212, which in turn is lower than the refractive index np of the core 123. This situation is illustrated by way of example in the refractive index profile show in Fig. 3. In effect, the core is similarly configured as the cladding of Fig. 2, namely configured to confine the emitted radiation predominantly in the wave guiding zone of the optical fiber. Fig. 4 shows an optical fiber according to Fig. 2, but in this example, the luminescence generating core 112 comprises discontinuous strands or discontinuous fiber portions. These strands or portions may reduce or even inhibited optical mutual coupling within the luminescence generating core 112. Since light guiding is accomplished by
the wave guiding cladding 223, these discontinuities do not significantly affect the performance of the fiber. On the other hand, these discontinuities may significantly improve production stability and/or reduce manufacturing costs. Fig. 5 shows an optical fiber according to Fig. 2, but with multiple luminescence generating cores 112. The indicated reference numbers are identical to Fig. 2 and the same expression is valid for the refractive index profile: nh < na < np. An increased number of luminescence generating cores 112 increases the surface area between core and cladding for a given volume of core material per length of fiber, which increases the amount of radiation generated in the luminescence zone that can be coupled into, concentrated and transferred in the waveguide zone. In this example, the number of cores 112 is 5. In another example, the number of cores is more than 5, in particular more than 20. Fig. 6 shows an optical fiber according to Fig. 3 with multiple wave guiding cores 123. The indicated reference numbers are identical to Fig. 3 and the same expression is valid for the refractive index profile: nh < na < np. An increased number of waveguiding cores 123 facilitates the transfer of radiation generated in the luminescent cladding 212. In this example, the number of cores 123 is 5. In another example, the number of cores is more than 5, in particular more than 20.
Fig. 7 shows a schematic view of a preform 1000 used for manufacturing an optical fiber according to the invention. The preform 1000 provides a hull 1300, which combines the first material and the second material arranged as core 1100 and as cladding 1200. In this example, this is accomplished by applying fused silica and/or alumina production steps, which in turn comprises a sol-gel and/or a granulated powder processing step. The further processing applies a vitrification process to the preform. The preform combines all of the materials in separate zones 1100, 1200 and 1300 respectively. Fig. 8 shows a schematic flow diagram that illustrates the steps for manufacturing an optical fiber according to the invention. In the present example, the method comprises the following steps: SI: Providing a first material having a refractive index and comprising a luminescence generating material for providing an emitting radiation predominantly at a near-infrared wavelength. S2: Providing a second material with a refractive index, which is higher than the refractive index of the first material. S3: Providing a container possibly consisting of a third material, with lower refractive index than the first and the second material. The container holds the first and second material in a suitable arrangement that enables further processing steps (S4, S5)
S4: Heating the materials to a common temperature enabling fiber drawing. S5: Forming the optical fiber by drawing the materials. The choice of materials and methods for the preform production are critical to ensure that the zone serving as a waveguide, for radiation concentration and transfer, has low scattering and higher refractive index than the zone serving as a luminescence generator and, additionally, to ensure that the different regions of the preform, e.g. produced by the techniques mentioned above, have very similar viscosity characteristics, typically at a temperature close to 2000°C, so that the preform can be drawn into a fiber using commercially available equipment. It is important that the preform be composed in such a way that no one region already liquifies while another region still remains solid and also to prevent mixing of materials from different areas during vitrification of the preform and drawing of the fiber. Preferred characteristics of different zones and materials are shown in Table 1 and 2. Elements mentioned and material compositions serve as illustrative examples without limiting the scope of the present invention. Table 1
Table 2
Fig. 9 shows an illustration of a spectral power density distribution by showing the course of spectral power density (arbitrary units) in dependency of radiation wavelength. A predetermined wavelength lo divides the area below the curve in an area PFS, indicating a power density for wavelengths shorter than lo (wave-type shaded), predominantly originating from radiation generated in the hot zone of the fiber, at temperature Ti, and an area PFID, indicating a power density with a wavelength longer than lo (line-type shaded), predominantly originating near the fiber end, at temperature T2. The areas may define a Fiber Figure of Merit (FFoM) in (%) at lo. The FFoM characterizes the temperature dependent and wavelength selective optical properties of the fiber according to the invention according to the expression PFa,Ti /(PFa,Ti + PFID,T2). Further indicated are the spectral power density distributions and respective power densities (RBB,TI) from a black body at temperature Ti (dashed curve at higher spectral density) and (PBB,T2) from a black body at temperature T2respectively (dashed curve at lower spectral density). Fig.9 indicates how the Figure of Merit (%), for a given wavelength lo, can be increased with a fiber based on the invention: 9.1 Raise temperature Ti 9.2 Structure and dope radiation generating zones with thermo-luminescent wavelength shifting materials. 9.3 Structure waveguiding zones for high visible (VIS) and near infrared (NIR) transparency and fiber ends for low VIS/NIR reflection at wavelengths shorter than lo. 9.4 Structure waveguiding zones for high absorption in the infrared (IR) at wavelengths longer than lo.
9.5 Reduce the temperature T2, at the fiber end(s), below the temperature Ti of the hot zone of the fiber, between the fiber ends. The radiation indication PBBI and PBB2 also apply to figures 10-11. In Fig. 12 the shaded area indicates the radiation between two black bodies at temperatures Ti and T2. In Fig. 13 PBBa indicates the radiation at wavelengths shorter than lo and PBBb the radiation at wavelengths longer than lo. In Fig. 14 PF indicates the spectral power density at the fiber end shows two peaks, reflecting radiation originating from different zones within the fiber and the temperature of the fiber varying between Ti in the hot zone and T2 in the cold zone(s) at the fiber end(s). Also shown, for comparison, is the spectral power density distribution and power density PBB between two black bodies at different temperatures Ti and T2. In Fig. 15 and 16 PFS indicates radiation with wavelengths shorter than lo and PFb denotes radiation with wavelengths longer than lo. Together, PFS + PFb compose the overall radiation PF emitted from the fiber end. Fig. 17 shows a measurement of radiation emitted from one end of a fiber of 380 pm diameter with a cold zone temperature of 20°C for two hot zone temperatures of 1005°C (diagram 17.1) and 1332°C (diagram 17.2) respectively. Within the wavelength range between 900nm and 1080nm the measured power output from the fiber end was 4.15pW and
25.4pW and the peak spectral power densities were 0.5pW/nm/mm2 and 1.9pW/nm/mm2 at 1005°C (diagram 17.1) and 1332°C (diagram 17.2) respectively. This increase in power output with rising temperature is stronger with a fiber, according to the invention, than the increase for a blackbody at these temperatures (diagram 17.3 and 17.4). The measured results from a fiber, according to the invention, is in line with theoretical predictions. For any given wavelength lo, the power of radiation with shorter wavelength than lo is a fraction of the total radiation. According to Planck's law - The overall radiation power increases fast with rising temperature . - The fraction of radiation with a wavelength shorter than lo increases even faster than overall radiation when temperature increases. According to Wien's law - The spectral power density peak wavelength APk shifts towards shorter wavelengths when temperature increases. As a result - The fraction of radiation with a wavelength shorter than lo increases very fast, for any wavelength lo, in particular for wavelengths lo below the spectral power density peak wavelength APk for a given temperature. For many technical applications using radiative energy transfer, it would be desirable to be able to separate radiation with wavelengths below wavelength lo from
radiation with wavelengths above wavelength lo. Hence the interest in materials or devices that display such properties : - wavelength selective high (low) transmission over a range of wavelengths shorter than wavelength lo, with a sharp transition to low (high) transmission at wavelength lo, and a corresponding change in absorption. - stability of material and optical properties during operation at very high temperatures Fig. 14 to 16 show the spectral power density distribution and power density PF for a fiber, as described in this invention. Given two temperatures Ti and Tå, where Ti is the temperature of the hot zone of the fiber and T2 is the temperature of the fiber end. In particular, temperature Ti could be well above 1000°C and temperature T2 could be well below 400°C, e.g. close to ambient temperature 20°C. The radiation between the two black bodies at two distinct temperatures Ti and T2 is reflected in the difference between the two blackbody spectral power density distributions. The spectral power density peaks at a wavelength determined by the temperatures of the two blackbodies, but the curve showing the difference between two blackbodies now deviates from the spectral power density distribution of single a blackbody . According to the invention, the use of a fiber results in a spectral power density distribution at the fiber end that
displays two pronounced peaks in the spectral power density distribution, at two well separated wavelengths, instead of only one broad peak for radiation between two blackbodies. For a given wavelength lo the share of radiation with a wavelength shorter than lo to overall radiation is expressed by the FFoM [%]. The Figure of Merit for radiation measured at the end of a fiber that is designed and operated according to the invention can be larger than the Figure of Merit for other thermal radiation emitters at comparable temperatures. According to the invention, radiation is captured, generated and guided inside a micro-structured optical fiber, consisting of several zones doped with rare earth atoms to achieve the desired optical properties. The spectral power density distribution for radiation originating in the hot zone of the fiber, by virtue of the rare earth dopants, will be modified in the sense of being shifted towards a higher share of radiation with shorter wavelengths which due to the high transmission of the fiber for wavelengths shorter than lo, is effectively guided towards and emitted from the cold fiber end. The use of specific rare earth dopants in different zones of the fiber allow FFoM to be maximized, given a wavelength lo and temperatures Ti and Tå, to match the needs of specific applications, such as photovoltaic conversion devices. Doping the waveguide zone of the fiber with rare earth atoms to raise infrared absorption without lowering transmission for radiation with wavelengths shorter than lo. This results in visible and near infrared radiation being transmitted from the hot fiber zone towards the cold
fiber ends where it exits the fiber, whereas infrared radiation is strongly absorbed and thus constrained to the hot zone of the fiber, effectively lowering the amount of infrared radiation being emitted from the fiber end. In a publication by G. Torsello et al., 22 August 2004, doi:10.1038/nmat1197, an explanation is provided for why rare earth materials display the quantum thermodynamical properties as described in this invention. Due to their spectroscopic and atomic properties, rare earth atoms embedded in a lattice material can collect radiation from a broad band of wavelengths and re-emit radiation via much narrower emission lines. This is explained by the fact that thermal energy is absorbed by the whole rare earth atom. This includes the rare earth atomic nucleus which captures phononic energy from the surrounding lattice, in this case the fiber material in which the rare earth atom is embedded, whilst the relaxation of excited atomic states and radiation emission involves transitions between energy levels within the rare earth atom that are shielded from interaction with other atoms in the lattice material. Hence respective emission lines are narrow, with strong spectral power densities, even at very high lattice temperatures. The impact of the fiber on the spectral power density of radiation emitted from the fiber end is best described by the Fiber Figure of Merit (FFoM), in %, for a given lo. Fig. 16 shows the spectral power density distribution for radiation at the cold fiber end (T2) and a hot zone (Tl), with a high FFoM for a given wavelength lo. A high FFoM is
desirable for applications that require radiation with short wavelengths i.e. high photon energy. According to the invention, radiation is generated and radiative power is concentrated at short wavelengths due rare earth wavelength shifting, temperature dependent wavelength selective transmission and absorption along the fiber with one or more zones of the fiber serving specific purposes. The invention is of significant interest to industrial applications that rely upon wavelength dependent radiative energy transfer and conversion. Fig. 17 shows a measurement of the spectral power density at cold end of a fiber, at a temperature T2 of 20°C, for two different hot zone fiber temperatures T1 of 1005°C (diagram 17.1) and of 1332°C. The radiation from the fiber end within the wavelength interval from 900nm to 1080nm increases significantly with increasing temperature: from 5.15pW at 1005°C (diagram 17.1) to 25.4pW at 1332°C (diagram 17.2). This increase in power output with temperature for a fiber according to the invention is stronger than the increase in power output with temperature from a black body (diagrams 17.3 for blackbody at 1005°C and 17.4 for blackbody at 1332°C). Fig. 18 shows a schematic perspective view of the optical fiber according to the invention with a hot zone (lOOh, 200h, 300h) and a cold zone (100c,200c,300c with a transition area in between the hot and the cold zone
characterized by a temperature gradient along the fiber axis. Fig. 19 shows a perspective view according to Fig. 18 but with the fiber ending in two cold zones (100c,200c,300c) with two transition areas in between the hot zone and the two cold zones characterized by temperature gradients along the fiber axis, wereby the two cold zones and transition areas may, in particular, have similar characteristics.
Abbreviations used in Figures 1-20 B One Black body (radiative emitter acc. to Planck's law) BB Two black bodies (at different temperatures) F Fiber T Temperature [°C] P Power or Power density[W/mm2] or Spectral Power Density [W/mm2/nm) PF (Spectral) Power (Density) of radiation from fiber end PB (Spectral) Power (Density) of radiation from one black body RB,TI (Spectral) Power (Density) of radiation from one black body at temperature Ti PBB (Spectral) Power (Density) of radiation between two black bodies PBB,Tl,T2 (Spectral) Power (Density) of radiation between two black bodies at temperatures Ti and T2 lo a particular wavelength PFa (Spectral) Power (Density) of radiation, from one fiber end, at wavelengths shorter than wavelength lo
PFb (Spectral) Power (Density) of radiation, from one fiber end, at wavelengths longer than wavelength lo PBBa (Spectral) Power (Density) of radiation, between two black bodies, at wavelengths shorter than wavelength lo PBBb (Spectral) Power (Density) of radiation, between two black bodies, at wavelengths longer than wavelength lo
The invention concerns an optical fiber (1) for radiation generation, concentration and transfer. The optical fiber comprises at least one luminescence generating zone of a first material comprising a luminescence generating material for providing an emitting radiation predominantly at a visible and/or near-infrared wavelength. The optical fiber further comprises at least one wave guiding zone. The at least one wave guiding zone is configured to concentrate the emitted radiation predominantly within the wave guiding zone. This provides distinct locations with different functionality, one location for generating the emitted radiation and the further location for guiding and concentrating of the emitted radiation. Thus, both zones can be specifically designed to provide optimized performance and efficiency. CLAIMS 1. An optical fiber (1) for radiation generation and transfer, comprising two zones: - at least one luminescence generating zone (112; 212) of a first material comprising a luminescence generating material for providing an emitted radiation predominantly at visible or near-infrared wavelength, and - at least one wave guiding zone (123; 223) of a second material, wherein the at least one wave guiding zone is configured to concentrate the emitted radiation predominantly within the at least one wave guiding zone. 2. The optical fiber (1) according to claim 1, wherein the fiber, in particular the at least one luminescence generating zone (112; 212) and/or the at least one wave guiding zone (123, 223), comprises at least one hot zone (100c, 200c, 300c) and at least one cold zone (100c, 200c, 300c). 3. The optical fiber (1) according to claim 1 or 2, wherein the at least one hot zone (100c, 200c, 300c) is located within a hot environment, in particular inside of a cavity, and/or the cold zone (100c, 200c, 300c) is located outside of the hot environment, in particular outside and/or remote of the cavity. 4. The optical fiber (1) according to any one of the preceding claims, wherein at least one zone is
configured to concentrate the emitted radiation by shifting the spectral distribution of the emitted radiation towards a shorter wavelength. 5. The optical fiber (1) according to any one of the preceding claims, wherein the first material has a refractive index (na) and the second material has a refractive index (np) higher than the refractive index (na) of the first material. 6. The optical fiber according to any one of the preceding claims, wherein the at least one luminescence generating zone forms at least one inner part of the optical fiber, in a particular at least one core, and the wave guiding zone forms an outer part of the optical fiber, in particular a cladding, that surrounds the at least one luminescence generating zone. 7. The optical fiber according to any one of claims 1 to 5, wherein the at least one wave guiding zone forms an inner part of the optical fiber, in particular at least one core, and the luminescence generating zone forms an outer part of the optical fiber, in particular a cladding, that surrounds the at least one core forming the wave guiding zone. 8. The optical fiber according to any one of the preceding claims, wherein the optical fiber comprises a peripheral zone (321) of a third material, that surrounds the wave guiding zone and the luminescence generating zone, wherein the refractive index (nh) of
the material of the peripheral zone is lower than the first material and the second material. 9. The optical fiber according to any one of the preceding claims, wherein the at least one wave guiding zone is substantially transparent at a near-infrared wavelength and/or the emitted radiation is substantially non interacting with the material of the wave guiding zone, in particular by substantially avoiding radiation attenuation and/or radiation absorption. 10. The optical fiber according to any one of the preceding claims, wherein at least two of the materials of the optical fiber, in particular all materials, have substantially equal or similar viscosity at a fiber drawing temperature. 11. The optical fiber according to any one of the preceding claims, wherein the fiber drawing temperature is in a range of 1800°C to 2200°C, in particular 1900°C to 2100°C, further in particular 1950°C to 2050°C. 12. The optical fiber according to any one of the preceding claims, wherein at least one of the materials of the optical fiber, in particular at least two of the materials, further in particular all materials, comprise a viscosity influencing material, which in particular modifies the viscosity. 13. The optical fiber according to the preceding claim, wherein the concentration of the viscosity influencing
material is at least 1 at%, in particular at least 10 at%, further in particular at least 20 at%. 14. The optical fiber according to any one of the preceding claims, wherein at least one of the materials of the optical fiber, in particular at least two of the materials, further in particular all materials, comprise a refractive index influencing material, which in particular modifies the refractive index. 15. The optical fiber according to the preceding claim, wherein the concentration of the refractive index material is at least 1 at%, in particular at least 10 at%, further in particular at least 20 at%. 16. The optical fiber according to any one of the preceding claims, wherein the luminescence generating material comprises one or more rare earth materials of a group comprising elements with atomic numbers 21, 39 or 57- 71, in particular optically active element category lanthanide with atomic numbers: 57-71 and transition metals with atomic numbers 21, 39 or 57, in particular at least one element of the group comprising Ytterbium, Neodymium, Erbium and Yttrium. 17. The optical fiber according to any one of the preceding claims, wherein the luminescence generating material comprises a concentration of at least 1 at%, in particular at least 4 at%, further in particular at least 10 at%.
18. The optical fiber according to any one of the preceding claims, wherein the luminescence generating material provides an emitting radiation predominantly at a wavelength range of 800-1500 nm, in particular 900-1100 nm. 19. The optical fiber according to any one of the preceding claims, wherein the first material and/or the second material comprises Si-oxide, in particular in form of silica, and/or Al-oxide, in particular in form of alumina. 20. The optical fiber according to any one of the preceding claims, wherein the luminescence generating zone comprises a plurality of mutually separate luminescence generating zones (112) arranged parallel to the axis of the optical fiber, in particular in form of strands of luminescent material, further in particular in form of discontinuous strands. 21. The optical fiber according to any one of the preceding claims, wherein the wave guiding zone comprises a plurality of mutually separate wave guiding zones (123) arranged parallel to the axis of the optical fiber. 22. The optical fiber according to any one of the preceding claims, wherein the optical fiber has a diameter of more than 10 mpi and supports multimode radiation propagation. 23. An optical system that comprises the optical fiber according to any one of the preceding claims and a
photovoltaic cell optically coupled to the optical fiber and/or an energy source coupled to the optical fiber for providing energy by photonic and/or phononic excitation. 24. A method for manufacturing the optical fiber according to any one of the claims 1 to 23. 25. A method of manufacturing the optical fiber (1) for radiation generation and concentration, the method comprising: - providing a first material having a refractive index (na) and comprising a luminescence generating material for providing an emitting radiation predominantly at a near-infrared wavelength; - providing a second material with a refractive index (np) , which is higher than the refractive index (na) of the first material; - heating the first material and the second material to a common fiber drawing temperature; and - forming the optical fiber by drawing the first material together with the second material. 26. The method according to claim 24 or 25, wherein the method comprises the step of providing a third material that contains the first and the second material. 27. The method according to any one of claims 24 to 26, wherein the method comprises the step of producing at least one of the materials, in particular at least two of the materials, in particular all of the materials, by fused silica and/or alumina processing.
28. The method according to the preceding claim, wherein the fused silica and/or alumina production comprises a sol-gel and/or a granulated powder processing. 29. The method according to any one of claims 24 to 28, wherein the method comprises the step of forming a preform (1000) by combining at least the first and the second material in particular all the materials, and applying a vitrification process to the preform. 30. The method according to any one of claims 24 to 29, wherein the method comprises the step of placing at least two of the materials, in particular all of the materials, in separate zones (1100, 1200, 1300), in particular separate zones of the preform in particular drawing the preform. 31. The method according to any one of claims 24 to 30, wherein the drawing of the optical fiber produces a fiber with a diameter of more than 10 jjm and/or supports multimode radiation propagation. 32. A method of operating the optical fiber according to any one of the claims 1 to 23. 33. A method of operating an optical fiber (1) for radiation generation and concentration, the optical fiber (1) comprising at least one luminescence generating zone (112; 212) and at least one wave guiding zone (123; 223) and the method comprising:
- applying heat to the luminescence generating zone (112; 212) for providing an emitted radiation predominantly at a visible or near-infrared wavelength; - transferring the emitted radiation to the at least one wave guiding zone (123; 223); and - concentrating the emitted radiation predominantly within the at least one wave guiding zone (123; 223). 34. The method according to the proceeding claim, wherein the fiber comprises at least one hot zone (lOOh, 200h, 300h) and at least one cold zone (100c, 200c, 300c), wherein the concentrating of the emitted radiation is predominantly accomplished within the at least one cold zone (100c, 200c, 300c). 35. The method according to the proceeding claim, wherein the at least one hot zone (lOOh, 200h, 300h) is located within a hot environment, in particular inside of a cavity, and the cold zone (100c, 200c, 300c) is located outside of the hot environment, in particular outside and/or remote of the cavity. 36. The method according to any one of the claims 32 to 35, wherein the temperature of the luminescence generating zone (112; 212) and/or the fiber, in particular the at least one hot zone (lOOh, 200h, 300h), is in a range of 800°C to 1700°C, in particular in a range of 1200°C to 1500°C. 37. The method according to any one of the claims 32 to 36, wherein the emitted radiation predominantly comprises a
wavelength range of 700-1600 nm, in particular 900-1100 nm. 38. The method according to any one of the claims 32 to 37, wherein the transferring and/or concentrating of the emitted radiation comprises shifting the spectral distribution of the emitted radiation towards a shorter wavelength. 39. The method according to claims 32 to 38, wherein the shifting comprises absorbing the part of the emitted radiation that comprises a wavelength longer than a predetermined wavelength (lo), in particular by reducing fiber transparency for wavelengths longer than the predetermined wavelength (lo), whilst maintaining transmission for wavelengths shorter than said predetermined wavelength (lo), further in particular by restricting a dopant concentration within the fiber. 40. The method according to any one of the claims 32 to 39, wherein method comprises the step of providing the optical fiber (1), in particular the waveguiding zone thereof, with a dopant that comprises rare earth ions. 41. The method according to claims 40, wherein method comprises the step of providing the dopant as at least one member of a group of Erbium, Thulium, Praseodymium and Holmium and/or providing a concentration of the dopant in a range from 0.5%at to 10%at, in particular from 2%at to 5%at.
42. The method according to claim 41 comprising at least one step of: - using Erbium ions (Er3+) and absorbing at 1.5-1.6 pm and/or 2.7 pm; - using Thulium ions (Tm3+) and absorbing at 1.7-2.1 pm and/or 1.45-1.53 pm; - using Praseodymium ions (Pr3+) absorbing at 1.3 pm; and - using Holmium (Ho3+) and absorbing at 2.1 pm and/or 2.9pm. 43. The optical fiber (1) according to claim 4 configured to absorb the part of the emitted radiation that comprises a wavelength longer than a predetermined wavelength (lo), in particular by reducing fiber transparency for a wavelength longer than the predetermined wavelength (lo), whilst maintaining transmission for wavelengths shorter than said predetermined wavelength (lo), further in particular by restricting a dopant concentration within the fiber. 44. The optical fiber (1) according to any one of the claims 1 to 22 or the optical system according to claim 23, wherein the optical fiber (1), in particular the waveguiding zone thereof, comprises a dopant of rare earth ions. 45. The optical fiber (1) according to claim 44 or the optical system according to claim 44, wherein the dopant comprises at least one member of a group of Erbium, Thulium, Praseodymium and Holmium and/or a
dopant concentration in a range from 0.5%at to 10%at, in particular from 2%at to 5%at. 46. The optical fiber (1) according to claim 45 or the optical system according to claim 45, wherein the optical fiber (1) comprises at least one of: - Erbium ions (Er3+) for absorbing at 1.5-1.6 pm and/or 2.7 pm; - Thulium ions (Tm3+) for absorbing at 1.7-2.1 pm and/or 1.45-1.53 pm; - Praseodymium ions (Pr3+) for absorbing at 1.3 pm; and - Holmium (Ho3+) for absorbing at 2.1 pm and/or 2.9 pm.