Hollow core fiber for transmitting laser light

Abstract

The present invention relates to a microstructured hollow core fiber, comprising a microstructured hollow core extending along the hollow core fiber. The hollow core has at least one microstructure having a first refractive index n, surrounded by an inner fiber cladding having a refractive index n_inner and an outer protective cladding having a protective cladding refractive index n_outer and covering the inner fiber cladding. The hollow core fiber is characterized in that it has at least one further cladding having a further refractive index n_w, disposed between the inner fiber cladding and the outer protective cladding and arranged to cover the inner fiber cladding, the further refractive index n_w being greater than the further refractive index.

Description

Technical Field

[0001] The present invention relates to a microstructure hollow-core fiber configured to transmit a laser beam as set forth in the preamble of claim 1. Such a microstructure hollow-core fiber includes a microstructure hollow core extending along the hollow-core fiber. The hollow core has a microstructure having at least one first refractive index n and is surrounded by an inner fiber cladding having a refractive index n_inner. When the fiber cladding is referred to in the present application, it is intended to be a fiber cladding made of a transparent material having conductivity for the laser beam, and the laser beam is guided by total reflection.

Background Art

[0002] The glass used as the core of a fiber in a well-known optical fiber (solid-core fiber) is replaced by a gas or a vacuum in the case of a hollow-core fiber, giving the fiber a "hollow center". Such hollow-core fibers are known, for example, from the publication "https: / / www.photonics.com / Articles / Hollow-core fibers Outperform Silica glass / a6448?refer=picks#comments."

[0003] It is also known that a microstructure hollow-core fiber can transmit single-mode laser emission with a high pulse peak power. However, a solid-core fiber structure is typically used for transmitting single-mode laser emission with a high average output.

[0004] When transmitting laser output by a hollow-core fiber, higher losses usually occur than when transmitting by a solid-core fiber. These losses are in the range of about 0.5% per meter of fiber length. The laser power lost as power that is not transmitted and lost is emitted into the environment transverse to the longitudinal extension of the hollow-core fiber by the cladding, i.e., the coating of the hollow core that conducts the beam, which is undesirable.

[0005] The covering comprises at least one fiber cladding concentrically surrounding the hollow core, and a protective cladding (jacket or buffer) concentrically surrounding the fiber cladding. At high average laser power (kilowatt range), the jacket material and / or fiber cladding, and therefore the hollow core fiber as a whole, can be damaged by lossy power radiated transversely to the longitudinal extension direction of the hollow core fiber.

[0006] When high-average-power single-mode laser radiation is guided through a solid-core fiber, the intrinsic loss is lower than in the case of transmission through a hollow-core fiber. However, the high electric field intensity of the laser light introduces undesirable nonlinear effects on the fiber material of the solid-core fiber. Therefore, the length of the transmission path is limited, for example, as a function of the laser power. Losses in the transmission characteristics of the fiber material and even fracture of the fiber material in the solid-core fiber can be observed. [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] Against this backdrop, the object of the present invention is to provide a hollow core fiber of the type described above that can transmit higher average laser light power than conventional methods, such as that generated by continuous-wave laser light. The continuous-wave power contained herein is in the kilowatt range. The pulsed peak power reaches the gigawatt range. [Means for solving the problem]

[0008] This objective is achieved by the sum of the features of claim 1. The solution according to the present invention differs from the prior art described at the beginning, in particular, in that the hollow core fibers are arranged to cover the innermost fiber cladding and have at least one further fiber cladding having a further refractive index n_w, wherein the refractive index n_inner of the innermost fiber cladding is greater than the further refractive index n_w.

[0009] Accordingly, the present invention provides at least one further fiber cladding surrounding an inner fiber cladding, the further fiber cladding having a lower refractive index than the inner fiber cladding.

[0010] Therefore, the radially inner first fiber cladding is optically denser than the radially outer second fiber cladding. This facilitates total internal reflection of light propagating within the radially inner first fiber cladding and incident on the interface between the radially inner first fiber cladding and the radially outer second fiber cladding, which is favorable for low-loss wave induction within the radially inner first fiber cladding, and thus reduces the undesirable transmission of lossy light propagating within the radially inner first fiber cladding into the radially outer second fiber cladding.

[0011] In this way, the microstructure of the hollow core fiber enables low-loss guiding for lost light that does not transmit within the hollow core, within the first radially inward fiber cladding. As a result, uncontrolled and undesirable transverse radiation is reduced. Damage to the fiber cladding is prevented as a result of this reduction in lost power radiated transversely to the longitudinal extension of the fiber.

[0012] This invention enables the transmission of high average power laser radiation through a microstructured hollow core fiber by means of targeted guidance of lost light within the fiber cladding during the guidance of light passing through the hollow core fiber.

[0013] In particular, the present invention prevents this lost radiation from escaping laterally from the microstructured fiber in an uncontrolled manner that could damage the jacket, buffer, or environment. The lost light is dissipated in a controlled manner and, in some cases, can be absorbed by the waveguide means achieved in the present invention as it exits the microstructured hollow core fiber.

[0014] Therefore, the present invention provides a hollow core fiber that prevents power loss, which could otherwise cause damage to the hollow core fiber or the surrounding protective cladding. Thus, the present invention enables the transmission of high average power (CW laser light) laser light through the hollow core fiber.

[0015] The present invention enables the intended dissipation and induction of lost light, and therefore allows for the transmission of higher average laser power than the conventional technique consisting of a microstructured hollow core fiber having only one fiber cladding and one protective cladding. Only the present invention makes it possible to use a microstructured hollow core fiber to transmit high CW laser power.

[0016] A preferred embodiment of the present invention is characterized in that the hollow core fiber has at least two further fiber claddings, each having a refractive index, and at least one of the refractive indexes of the at least two further fiber claddings is less than the refractive index of the innermost fiber cladding.

[0017] Furthermore, it is preferable that the refractive index of one of the two further fiber claddings that covers the other of the two further fiber claddings is smaller than the refractive index of the covered further fiber cladding.

[0018] A preferred embodiment is characterized in that there are at least two further fiber claddings such that the innermost (first) fiber cladding is concentrically covered by a second fiber cladding (which may be a protective cladding), and the second fiber cladding is concentrically covered by a third fiber cladding (which may be a protective cladding), each of which fiber claddings has its own refractive index, and the refractive index of the radially outward fiber cladding is always greater than the refractive index of the fiber cladding further extending radially inward.

[0019] Another preferred embodiment of the present invention is characterized in that the material thickness of the fiber cladding and outer protective cladding is such that lost light coupled to further fiber cladding from the internal fiber cladding or microstructured hollow core is fully internally reflected therein. For this purpose, a preferred material thickness is 4 to 6 times, particularly 5 times, the wavelength of the laser light.

[0020] Furthermore, it is preferable that the microstructured hollow core fiber has an input terminal configured to couple laser light to the microstructured hollow core, and an output terminal configured to couple and output laser light from the microstructured hollow core.

[0021] Furthermore, it is preferable that the hollow core fiber is configured to guide the laser light (loss light) coupled to the internal fiber cladding or other fiber cladding from the microstructured hollow core to the output end of the microstructured hollow core fiber using a waveguide means, thereby causing the laser light to be emitted from the fiber cladding.

[0022] Another preferred embodiment is characterized by having a hollow core fiber positioned between the input and output ends, and having at least one mode stripper configured to extract laser light (loss light) coupled to the fiber cladding and / or protective cladding from the microstructured hollow core from the fiber cladding in a direction lateral to the longitudinal extension direction of the fiber cladding.

[0023] Furthermore, it is preferable that the hollow core fiber has multiple mode strippers distributed along the length of the microstructured hollow core fiber.

[0024] This embodiment enables controlled dissipation of lost power. Thus, lost power can be coupled laterally from the hollow core fiber in a controlled manner without causing damage. This prevents the transport of undesirable high-loss power along the longitudinal extension, as the laterally coupled portion no longer needs to be guided to the exit end of the hollow core fiber.

[0025] Another embodiment is the additional or alternative use of a so-called "air cladding" or any further optional cladding between the first fiber cladding and the second fiber cladding.

[0026] By means of these air claddings, advantages of a higher numerical aperture are achieved compared to embodiments without such air claddings.

[0027] Further advantages are described in the dependent claims, the description, and the attached drawings.

[0028] It should be understood that the above-described features and the features further described below can be used not only in the specified combinations but also in other combinations or alone without departing from the scope of the present invention.

[0029] Embodiments of the present invention are shown in the drawings and will be described in more detail in the following description. The same reference numerals in different drawings indicate the same elements respectively. The drawings show the following in a schematic form.

Brief Description of the Drawings

[0030] [Figure 1] Shows a cross-section of a known hollow-core fiber. [Figure 2] Shows a longitudinal cross-section of the hollow-core fiber of FIG. 1. [Figure 3] Shows a cross-sectional view of a hollow-core fiber according to the present invention. [Figure 4] Shows a longitudinal cross-section of the hollow-core fiber of FIG. 3.

Embodiments for Carrying Out the Invention

[0031] More specifically, FIG.  shows a cross-section of a microstructured hollow-core fiber 10 assumed to be known.

[0032] This cross-section is perpendicular to the longitudinal extension direction of the hollow core fiber. The cross-section is, for example, the xy-plane of a Cartesian coordinate system. In this case, the longitudinal extension direction is oriented locally parallel to the z-direction of the coordinate system, i.e., within the cross-sectional plane.

[0033] Figure 2 shows a longitudinal section of the microstructured hollow core fiber 10 as shown in Figure 1. The longitudinal section is defined to follow the longitudinal extension direction of the hollow core fiber 10 such that the center of the hollow core 12 of the hollow core fiber 10 is always located in the plane of the drawing.

[0034] The microstructured hollow core fiber 10 has a microstructured hollow core 12 extending along the hollow core fiber 10. The hollow core 12 has a microstructure 14 having at least one first refractive index n and is surrounded by an inner fiber cladding 16 having refractive index n_inner, so that the inner fiber cladding defines the hollow core radially. The inner fiber cladding is covered by an outer protective cladding 18 having protective cladding refractive index n_outer.

[0035] Therefore, Figures 1 and 2 show the overall structure of the hollow core fiber 10, which is assumed to be known.

[0036] In a known hollow core fiber 10, the first refractive index n is typically equal to the refractive index n_inner of the inner fiber cladding 16, while the refractive index n_outer of the protective cladding 18 is typically greater than the refractive index n_inner.

[0037] During the propagation of a single-mode laser beam 20 with a high average power value, losses occur, which are hereafter referred to as lost light 22. In the prior art, this lost light 22 exits laterally from the hollow core fiber 10 uncontrolled through the inner fiber cladding 16 and outer protective cladding 18, potentially damaging the outer protective cladding 18, and in some cases, objects in the environment surrounding the hollow core fiber 10, and / or injuring people in the environment.

[0038] Figure 3 shows a cross-section of an exemplary embodiment of the hollow core fiber 100 according to the present invention for transmitting laser light. Here again, the cross-section is, for example, the xy-plane of a Cartesian coordinate system.

[0039] Figure 4 shows a longitudinal section of the microstructured hollow core fiber 100 as shown in Figure 3. The longitudinal section is defined by following the longitudinal extension direction of the hollow core fiber 100, such that the center of the hollow core of the hollow core fiber is always in the plane of the drawing.

[0040] In this case, the longitudinal extension direction is oriented locally parallel to the z-direction of the coordinate system, i.e., within the cross-sectional plane.

[0041] The microstructured hollow core fiber 100 has a microstructured hollow core 12 extending along the hollow core fiber 100. The hollow core 12 has a microstructure 14 having at least one first refractive index n, surrounded by an innermost fiber cladding having refractive index n_inner, and thus the innermost fiber cladding 16 defines the hollow core 12 radially. The innermost fiber cladding 16 is covered by an outer protective cladding 18 having protective cladding refractive index n_outer.

[0042] The microstructured hollow core fiber 100 has an input end 24 configured to couple laser light into the microstructured hollow core 12 and an output end 26 configured to couple laser light 20 from the microstructured hollow core 12. For this purpose, the input end 24 and the output end 26 have end faces 24.1 and 26.1 oriented laterally with respect to the longitudinal direction of the hollow core fiber 100, respectively. The single-mode laser light 20 propagating within the hollow core 12 strikes the end face 26.1 used for output coupling so that it does not undergo total internal reflection in place but is instead transmitted. Similarly, incoupling occurs, for example, via the end face 24.1 used for incoupling. The end faces used for incoupling and outcoupling may also be located on lateral projections or lateral incisions of the hollow core fiber 100.

[0043] Therefore, Figures 3 and 4 show the overall structure of an exemplary embodiment of the hollow core fiber 100 according to the present invention.

[0044] The hollow core fiber 100 has, in addition to the innermost fiber cladding 16 and the outer protective cladding 18, at least one further cladding 28 positioned between the innermost fiber cladding 16 and the outer protective cladding 18 so as to cover the innermost fiber cladding 16. The cladding referred to in this application is preferably concentric cladding.

[0045] In the hollow core fiber 100 according to the present invention, the microstructure 14 has a first refractive index n. The innermost fiber cladding 16 has a refractive index n_inner, and the outer protective cladding 18 has a protective cladding refractive index n_outer.

[0046] In a preferred embodiment, at least one further fiber cladding 28, provided to cover the innermost fiber cladding 16 and positioned between the innermost fiber cladding 16 and the outer protective cladding 18, has a further refractive index n_w. ​​The further refractive index n_w is smaller than the refractive index n_inner and larger than the refractive index n_outer of the protective cladding 18.

[0047] Therefore, the inner fiber cladding 16, which is further radially relative to the further fiber cladding 28 and thus closer to the microstructure 14 and the hollow core 12, is optically denser than the further fiber cladding 28. The greater optical density of the innermost fiber cladding 16 is favorable for the occurrence of total internal reflection of lost light 22 propagating within the innermost fiber cladding 16 and incident at the interface to the further fiber cladding 28. Also, the refractive index n_w of the further cladding 18 is greater than the refractive index n_outer of the protective cladding 18.

[0048] The higher optical density of the additional fiber cladding 28 compared to the optical density of the outer protective cladding 18 is advantageous for the occurrence of total internal reflection of lost light 22 that propagates within the additional fiber cladding 28 and is incident at the interface with the outer protective cladding.

[0049] The material thickness of the fiber cladding 16, 28 and the outer protective cladding 18 is sized such that the lost light 22 coupled from the microstructured hollow core 12 to the fiber cladding 16, 28 undergoes total internal reflection therein.

[0050] This results in the advantage of controlled dissipation of lost light 22 by guidance along the innermost fiber cladding 16 and further fiber cladding 28. This desired advantageous effect comes at the expense of the uncontrolled emission load of lost light 22 crossing the hollow core 12 and the innermost fiber cladding 16. In this way, the hollow core fiber 100 is configured to guide the laser light coupled within the fiber cladding 16, 28 from the microstructured hollow core 12 to the exit end 26 of the microstructured hollow core fiber 100, and to allow lost light 22 to exit from the fiber cladding 16, 28.

[0051] As controlled output coupling at the output end 26 of the hollow core fiber 100, or in addition thereto, lost light 22 propagating along the hollow core fiber 100 within the fiber cladding 16, 28 can also be coupled from the fiber cladding 16, 18 in a controlled manner by mode strippers laterally attached to the hollow core fiber 100. This type of mode stripper can be implemented, for example, as a localized projection or incision within the fiber cladding 16, 28 that conducts the lost power 22. This type of projection or incision has an interface oriented such that the lost light 22 impacting it does not undergo total internal reflection, but rather is radially deflected in a controlled manner and thus laterally coupled from the hollow core fiber 100 in a controlled manner.

[0052] One or more mode strippers can be placed between the input end 24 and the output end 26, in which case the lossy light 22, which is coupled out from the microstructured hollow core 12 to the fiber cladding 16, 28 and / or protective cladding 18, can be coupled out from cladding to cladding in a direction laterally with respect to the longitudinal extension direction of the cladding.

[0053] Another possible embodiment is the additional or alternative use of a so-called “air cladding” between the innermost fiber cladding 16 and further fiber cladding 28 or any further fiber cladding.

[0054] The exemplary embodiments of the hollow conductor shown in Figures 3 and 4 have two further fiber claddings 28 and 18 in addition to the radially innermost fiber cladding 16. The radially outermost further fiber cladding 18 is preferably a protective cladding and concentrically surrounds the other further fiber claddings 28. The further fiber claddings 28 concentrically surround the innermost fiber cladding 16.

[0055] At least one of the refractive indices of at least two further fiber claddings 18, 28 is smaller than the refractive index of the innermost fiber cladding 16.

[0056] The refractive index of one of two additional fiber claddings that coat the other of two additional fiber claddings is less than the refractive index of the coated additional fiber cladding, in this case, the additional fiber cladding 28. In this case, the additional coating fiber cladding is fiber cladding 18.

[0057] In one embodiment having only one further fiber cladding, the further fiber cladding can also serve as a protective cladding. Thus, the protective cladding can be formed from silicon and, therefore, can guide the laser beams emitting within the first fiber cladding by means of total internal reflection. Such exemplary embodiments are evident, for example, from the exemplary embodiments in Figures 3 and 4, by omitting the fiber cladding 18 that extends radially furthest away.

[0058] If the three concentrically arranged fiber claddings 16, 28, and 18, which extend radially between the innermost and outermost fiber claddings, have a lower refractive index than the innermost fiber cladding, then the outermost fiber cladding does not necessarily need to have a lower refractive index, since the laser beam is already guided through the central fiber cladding within the innermost fiber cladding. Furthermore, it is sufficient if only one of the two further fiber claddings has a lower refractive index than the radially innermost fiber cladding in order to guide the laser beam into the array by means of total internal reflection.

Claims

1. A hollow core fiber (100) configured to transmit laser light (20), It comprises a microstructured hollow core (12) extending in the fiber direction, The microstructured hollow core (12) has at least one microstructure (14) having a first refractive index n, and is surrounded by an inner fiber cladding (16) having a refractive index n_inner. The hollow core fiber (100) is arranged to cover the inner fiber cladding (16) and has at least one further fiber cladding (28) having a further refractive index n_w. The refractive index n_inner of the inner fiber cladding (16) is greater than the further refractive index n_w. A hollow core fiber (100) characterized by, The hollow core fiber (100) has at least one mode stripper positioned between the input end (24) and the output end (26) and configured to extract lost light (22) coupled from the microstructured hollow core (12) to the inner fiber cladding (16) or the at least one further fiber cladding (28), The mode stripper is configured to extract lost light from the hollow core fiber (100) in a direction lateral to its longitudinal extension direction, It has at least two further fiber claddings, each having a refractive index, At least one of the refractive indices of the two further fiber claddings is smaller than the refractive index of the innermost fiber cladding (16). Hollow core fiber (100).

2. The hollow core fiber according to claim 1, characterized in that, of the two further fiber claddings, the refractive index of the other of the two further fiber claddings covering one of the aforementioned further fiber claddings is smaller than the refractive index of the covering further fiber cladding.

3. Having at least two further fiber cladding, The innermost first fiber cladding is concentrically covered by the second fiber cladding. The second fiber cladding is concentrically covered by the third fiber cladding. Each of the fiber claddings has an intrinsic refractive index, The refractive index of the fiber cladding extending radially outward is always lower than that of the fiber cladding extending radially inward. Therefore, the refractive index in the fiber cladding arrangement decreases from the inside outwards. The hollow core fiber (100) according to claim 2, characterized in that...

4. The hollow core fiber (100) according to any one of claims 1 to 3, characterized in that the thickness of the inner fiber cladding (16) and the further fiber cladding (28) is such that the lost light (22) coupled from the microstructured hollow core (12) to the inner fiber cladding (16) and / or the further fiber cladding (28) undergoes total internal reflection within them.

5. The hollow core fiber (100) is The microstructured hollow core (12) has an input end (24) configured to couple laser light (20), The output terminal (26) is configured to combine and output laser light (20) from the aforementioned microstructured hollow core (12), A hollow core fiber (100) according to any one of claims 1 to 4, characterized in that...

6. The lost light (22) coupled to the inner fiber cladding (16) is guided by means of guiding it from the microstructured hollow core (12) to the exit end (26) of the microstructured hollow core (12), This enables the lost light (22) to be emitted from the inner fiber cladding (16). A hollow core fiber (100) according to any one of claims 1 to 5, characterized in that it is configured as described above.

7. The mode stripper is configured to extract the lost light (22) coupled to the protective cladding (18) covering the inner fiber cladding from the microstructured hollow core (12) in a direction laterally to its longitudinal extension direction from the hollow core fiber (100). A hollow core fiber (100) according to any one of claims 1 to 6, characterized in that...

8. The hollow core fiber (100) according to claim 7, characterized in that it has multiple mode strippers distributed over the length of the hollow core fiber (100).

9. A hollow core fiber (100) according to any one of claims 1 to 8, characterized in that an air cladding layer is disposed between the inner fiber cladding (16) and the further fiber cladding (28).

10. A hollow core fiber (100) according to any one of claims 1 to 8, characterized in that an air cladding layer is disposed between the radially outermost fiber cladding and a protective cladding (18) covering the inner fiber cladding.

11. The hollow core fiber (100) according to any one of claims 1 to 10, characterized in that the refractive index of the microstructure (14) is equal to the refractive index n_inner of the inner fiber cladding (16).

12. The hollow core fiber (100) according to any one of claims 1 to 11, characterized in that the further fiber cladding (28) concentrically surrounds the inner fiber cladding (16).

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