Structured silica clad silica optical fiber

Abstract

In structured silica clad silica (SSCS) optical fibers, the cladding provides mode mixing inside the core.SOLUTION: In a structured silica clad silica (SSCS) optical fiber, the cladding is configured to have an average effective refractive index. The structured silica cladding comprises paired layers in which a layer of down-doped silica is followed by a layer of pure silica, or less down-doped silica, or up-doped silica, the number of paired layers typically being from 5 to about 25, and in general, the ratio of the thickness of the high RI layer of silicate down-doped silica within a paired layer is very broad, ranging between about 0.0625 to about 16, depending on the intended application of the SSCS fiber. In some embodiments, the primary core material may be up-doped silica and the primary second component may be pure silica or down-doped silica.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] Over time, photonic sources for optical fibers continue to become smaller, more compact, and have higher power densities. Many medical and industrial applications use fiber lasers and pulsed beams to achieve high power densities for a variety of reasons, including localizing damage and minimizing the temperature of the irradiated surface. In the process, photonic sources have changed from highly scattered beams to highly localized few-mode or single-mode beams.

[0002] Now, most radiation beam sources are highly Gaussian or even confined in shape, with a highly localized, high central peak that decays relatively quickly. For one-dimensional applications such as laser cutting, this is not a problem and can even be an advantage. However, at the atomic / molecular level, such processes are multidimensional and can experience detrimental results. For two-dimensional or higher applications such as laser cleaning, welding, and machining, most cases benefit from a broader, speckle-free output distribution. Smoothing the output to a more speckle-free distribution benefits the application process. The optical fiber disclosed herein makes such output much more likely by incorporating mode mixing into its basic structure in a simple way during the preform manufacturing process. A new class of all-silica optical fiber is fabricated, as described herein. These possess all the advantages of all-silica optical fiber, along with new flexibility in achieving effective numerical apertures and new mode propagation / mixing capabilities.

[0003] Furthermore, fiber lasers that are to be cladding pumped themselves would benefit from incorporating the laser core as the innermost core of a structured silica clad optical fiber, as also described herein. [Background technology]

[0004] Traditionally, all-silica optical fiber has a cladding silica surrounding a core silica, which has a higher refractive index than the cladding silica, and the numerical aperture (NA) is related only to the difference in refractive index (RI) of the two materials at their interface, with the overall optical and physical performance dependent on the specific type of silica used to make the preform used to draw the optical fiber.

[0005] Traditionally, the method for creating mode-mixing fibers has been to break the cross-sectional symmetry of the optical fiber by introducing an off-center core, using a non-circular core, or introducing asymmetric perturbations in the cross-sectional refractive index profile of the fiber. Numerous examples of such efforts over the past approximately 40 years exist in patents and literature, particularly relating to fiber lasers and / or fiber amplifiers.

[0006] The novel all-silica fiber described in this invention provides a symmetrical cross section along the fiber that makes the all-silica optical fiber a good mode-mixing fiber, among other purposes. Summary of the Invention [Problem to be solved by the invention]

[0007] The primary objective is to provide a new (novel) all-silica optical fiber structure with a structured silica cladding, where the numerical aperture (NA) can be varied from preform to preform by varying the structure of the silica layer rather than by varying the doping level within the component.

[0008] Another object is to provide a new approach to mode-mixing optical fibers having a substantially symmetric circular cross-sectional structure.

[0009] Yet another object is to provide improved cladding pumping performance for fiber lasers, fiber amplifiers, etc., which have a substantially symmetric cross-sectional structure.

[0010] Additionally, an objective is to provide optical fibers with a more speckle-free output and a cross section more suitable for optical fiber transmission to remote locations.

[0011] Furthermore, an object is to provide a specialized fiber that can be used to connect a fiber laser source to a standard optical fiber for actual treatment in medical applications.

[0012] A further object is to provide a specialty optical fiber that is compatible with standard optical fibers and that can be used in industrial or military applications. [Means for solving the problem]

[0013] A new type of all-silica optical fiber is described: a structured silica clad silica (SSCS) optical fiber, whose cladding is configured to achieve mode mixing in the core. Its cross section is substantially symmetric. This optical fiber can be used to achieve flatter, more speckle-free output from fiber lasers or other limited-mode photonic sources. By constructing a new fiber structure around the laser core, a better fiber laser / amplifier for cladding pumping is realized. The structured silica cladding includes pairs of down-doped silica layers followed by pure or less down-doped silica layers within the pair. The number of pairs is preferably 5 to about 25. Typically, the thickness ratio of pure silica to down-doped silica within the pair ranges quite widely, from about 0.0625 to 16, depending on the intended application of the SSCS fiber. In some configurations, the primary core material may be up-doped silica, and the secondary component may be pure silica or down-doped silica. [Brief explanation of the drawings]

[0014] [Figure 1] 1A and 1B show basic cross sections of a preform and a drawn fiber. [Figure 2] FIG. 2 shows the refractive index profile of the fiber of FIG. 1. [Figure 3] FIG. 1 is a cross-sectional view of a fiber with pure, lightly doped silica around a basic SSCS structure. [Figure 4] 1 shows a cross section of a novel fiber laser or fiber amplifier according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0015] This specification describes a new type of all-silica optical fiber structure drawn from a preform with a similar internal structure. In the preform and the resulting optical fiber, the cladding is composed of alternating layers of pure low-index (cladding-type) and pure high-index (core-type) materials, called paired layers, and in some instances, the cladding-type layers are typically the same thickness as or thicker than the core-type layers. This new cladding, called a structured silica cladding (SSC), most commonly surrounds / covers a core of pure silica or high-index material, resulting in the basic SSCS structure of the optical fiber. In many instances, to increase the all-silica optical fiber's resistance to static or dynamic fatigue, an additional low-index (RI) material is added over the SSC before the resulting optical fiber is coated with a protective coating / jacket.

[0016] Layered silica structures can also have a thinner low-index layer followed by a thicker high-index layer in a pair of layers, which would result in a structured section with an average refractive index closer to the core than described above. A lower refractive index difference is more beneficial in fibers / preforms designed primarily as enhanced mode-mixing fibers with a structured silica section inside the core section of the optical fiber / preform. The latter is described and claimed in a related patent application by two of the inventors of the present invention, U.S. Patent Application No. 62 / 981151. In the related patent application, the optical fiber is an asymmetric or non-circular core optical fiber with a structured silica section inside the core, which differs significantly from the symmetric circular cross-section of the inventive optical preforms and fibers described in this invention.

[0017] The new SSCS optical fiber provides a new all-silica optical fiber in much the same way that the hard-clad silica fiber discovered by one of the inventors of the present invention in the 1980s provided improved plastic-clad fiber (see U.S. Pat. No. 4,511,208, BJ Skutnik). In this application, the new fibers and the new preforms from which they are drawn provide, among other different capabilities, better mode-mixing output and the ability to make better cladding-pumped fiber lasers, fiber amplifiers, and the like.

[0018] Within SSCS fibers, the effective numerical aperture can be adjusted without changing materials other than the traditional method of varying the chemical composition of the low-index layer. Rather, in the fabrication of the optical preform, the relative thickness of the lightly doped silica layer to the thickness of the high-index silica layer can change the effective refractive index of the structured silica cladding. The use of structured silica cladding thus effectively provides an additional degree of freedom in the design of optical fiber, in addition to other novel performance capabilities.

[0019] The number of pairs, as well as the actual thickness of the individual layers required, depends on the evanescent field structure in a particular application and the core size of the optical fiber used in that application. In some cases where the core size is small, i.e., close to or smaller than 100 μm, it is beneficial for the preform and optical fiber construction to have additional cladding material on the SSC before adding the protective coating to the drawn optical fiber. The effectiveness of mode mixing performance and cladding efficiency also depend somewhat on the wavelength of light used in a given application and the number of pairs, as well as the thickness of the individual layers within the SSC.

[0020] For ease of discussion, this specification generally uses pure silica as the core material (high refractive index material) and fluorine-doped silicon as the down-doped material (low refractive index material). The present invention works equally well with cores constructed of up-doped silica, such as germanium-doped silicon, as well as graded-index core materials. In combination with highly RI-doped silicon as the core material, the cladding, or low RI-doped material, may be pure silicon.

[0021] The new fiber can be manufactured with high precision because its structural features are carefully engineered into the preform. The fiber drawing process typically uses a large drawdown ratio, which allows for very well-defined layers and structures. Key to this invention is that the preform is fabricated by plasma vapor deposition (PVD), either outside-air vapor deposition (POVD) or closed-air vapor deposition (PCVD). Precise control of the vapor composition is critical to creating well-defined layers within the structured silica cladding section of the preform, especially when transitioning between materials with different refractive indices. Careful drawing of the preform using standard fiber draw towers and techniques results in the drawn optical fiber having a symmetry proportionally equivalent to that of the preform. These structured silica-clad silica optical fibers excel at achieving speckle-free output and distal output for low-mode, high-power sources such as fiber lasers. They are also useful in fiber laser and amplifier design, with the appropriate selection of the innermost active core.

[0022] Some examples of the present invention are optical fibers with high to medium NAs that support multimode transmission. In these fibers, the basic structure is a thicker low-index layer followed by a high-index layer within each pair. The thickness ratio of the low-RI to high-RI layers ranges from about 2 to 15. The actual thickness will depend on the conditions and capabilities of the plasma vapor deposition equipment / process at hand. For most mode-mixing type applications, the range of the number of pairs depends somewhat on the application area of ​​the fiber, including the light source used. Generally, a useful number of pairs ranges from about 5 to 30. A more preferred range for these parameters would be a thickness ratio of 7 to 15 and a number of pairs of about 10 to 25.

[0023] If the application benefits from a low to medium NA, thicker high-index layers and thinner low-index layers are beneficial for the structured silica cladding. To benefit from a larger relative thickness of the high-RI layers compared to the low-RI layers within each pair, the thickness ratio can be varied using the same starting material. When the effective refractive index of the structured silica cladding approaches that of the core material, optical fiber with a very low NA can be drawn from a properly designed preform.

[0024] Alternatively, a high RI structured silica section can be achieved by using updoped silica as the high RI layer and fluorosilicate for the low RI layer, adjusting the relative thicknesses to achieve the desired very low effective NA of the fiber.

[0025] The ratio of high RI layers to low RI layers may be useful in the range of about 3 to 20. A generally useful range for the number of pairs is in the range of 5 to 30. More preferred ranges for these parameters would be in the range of 7 to 15 for the thickness ratio and in the range of about 10 to 25 for the number of pairs.

[0026] Particularly in mode-mixing applications, a slightly different cross-section may be advantageous: adding a thin layer on top of the core material, the added layer having a higher refractive index than the refractive index of the core material, followed by a selected low-index layer or similar, followed by a conventional structured silica cladding such as that described above. This configuration may increase the evanescent field effect and improve the mode-mixing efficiency of the structured silica cladding.

[0027] 1-4 illustrate several examples of having a thin low RI layer compared to a thick high RI layer within each pair of structured silica cladding, as described below.

[0028] A pure silica core rod 101 is placed in a POVD chamber, and a series of alternating layers of down-doped and pure silica layers 121 are added to achieve the structured section 103 shown in FIG. 1 . The difference between the diameter 102 of the pure silica core and the diameter 104 of the structured silica cladding defines the overall thickness of the mode-mixing structured silica cladding 103. Within the cladding 103, there are a number of layered pairs 120, which can vary but are typically in the range of 8 to 30 pairs. Within each layered pair 120, the pure silica layer 121 is often much thicker than the down-doped silica layer 123. The ratio of the two thicknesses typically ranges from about 3 to 20. This is summarized in FIGS. 1 and 1(A). Particularly useful ranges for these two parameters are 7 to 13 for the thickness ratio within a pair and 12 to 20 for the number of pairs.

[0029] Of course, to start with a silica core of appropriate size, the inner core 101, 201, ... may in some cases be made from a thinner silica rod onto which pure silica is deposited by plasma deposition of additional pure silica to achieve the desired core diameter.

[0030] FIG. 2 shows the refractive index (RI) profile of the preform 100 in cross section. FIGS. 2(A) and 2(B) show how the RI varies across the cross section. The lines represent the drop in the refractive index of the downdoped silica layer relative to the refractive index of the core material. The abruptness of the RI change indicates the abrupt change in material over the course of deposition, and the speckle-free bottom demonstrates the speckle-free dopant levels in each downdoped layer. In one set of examples, Δn=5×10 -3 is.

[0031] In Figure 3, a cladding type layer 307 has been added that has a constant RI that is lower than the RI of the core 301 and is generally lower than the average RI of the structured silica cladding 303. In this case, this is either a fluorosilicate deposit deposited during the manufacturing of the preform, or a plastic cladding added during the fiber drawing process to provide an additional barrier to contain light transmitted through the SSCS optical fiber.

[0032] A cross-sectional sketch of a cladding-pumped fiber laser / amplifier according to the present invention is shown in Figure 4. A rare-earth doped core 410 is surrounded by a pure silica core / cladding 401, and a "second" cladding, SSC 403, surrounds the pure silica first cladding, resulting in a more efficient cladding-pumped device.

[0033] In summary, structured silica cladding includes twin layers, e.g., in some types of SSCS fiber, a low index layer followed by a thinner layer of pure silica. The number of twin layers is typically from 3 to about 30, and generally, within a twin layer, the ratio of the thickness of the low index material to the thickness of the high index material is between 2 and 20.

[0034] In another variation of the SSCS fiber, where a low to very low effective NA is desired in the SSCS, the thicknesses within a pair may be the same or reversed: a thin (down-doped) low-index layer followed by a thicker high-index (e.g., pure silica) material. Here, the thickness ratio within a pair is relative to the thicker high-RI layer, and the ratio of high-index layers to low-index layers is typically between 2 and 20. The number of pairs is generally about 3 to 30.

[0035] In either form, the optical fiber may have a low refractive index polymeric material applied over the structured silica cladding during the drawing process, with an outer jacket applied to the optical fiber for mechanical protection.

[0036] In some versions of the invention, the structured silica cladding region of the preform may be further surrounded by a layer of down-doped silica or other reflective coating to add a low refractive index material to the optical fiber as it is drawn from the preform. All fibers generally have an outermost coating for mechanical protection in application / use.

[0037] Although pure silica (Si) is primarily envisioned as the core material, and the down-doped silica is fluorosilica, other materials such as up-doped silica or graded-index silica can also be used as the core material paired with pure silica as a "down-doped" material. Depending on the intended application, other down-doped silicas can also be used in place of fluorosilica (F-Si).

[0038] (Addendum) (Appendix 1) An optical fiber comprising: a structured silica cladding having an average refractive index; and a silica core having a refractive index higher than the average refractive index of the structured silica cladding.

[0039] (Appendix 2) 2. The optical fiber of claim 1, wherein the structured silica cladding comprises a plurality of pairs of layers of downdoped silica followed by less downdoped, updoped, or pure silica, the less downdoped, updoped, or pure silica layers typically being thinner than the downdoped layers.

[0040] (Appendix 3) 3. The optical fiber of claim 2, wherein a ratio of the thickness of the downdoped layer to the thickness of the less downdoped silica layer is between about 2 and 15.

[0041] (Appendix 4) 4. The optical fiber of claim 2 or 3, wherein the number of pairs is between about 2 and 30.

[0042] (Appendix 5) 5. The optical fiber of any one of claims 1 to 4, further comprising an outer coating selected from the group consisting of pure silica, high refractive index plastic, and down-doped silica, low refractive index plastic cladding material.

[0043] (Appendix 6) 6. The optical fiber of any one of claims 1 to 5, wherein the silica core contains an innermost core doped with a rare earth material and having a higher refractive index than the core silica, the doped innermost core enabling the optical fiber to function as a cladding-pumped fiber laser or fiber amplifier.

[0044] (Appendix 7) 10. A method for manufacturing the optical fiber having a structured silica cladding and a high refractive index silica core of claim 1, comprising: making a preform having a circular silica core surrounded by a section of a circular layered pair consisting of a downdoped layer followed by an overall thinner high refractive index silica layer; drawing the preform under standard drawing parameters to produce an optical fiber having a selected fiber core dimension; Including, Optical fiber manufacturing method.

[0045] (Appendix 8) 7. A method for manufacturing an optical fiber laser / amplifier having a structured silica second cladding, a silica first cladding, and a rare-earth-doped innermost core according to claim 6, comprising the steps of: making a preform having a circular rare earth doped innermost core surrounded by a silica first cladding, which in turn is surrounded by a circular layered pair of sections consisting of a downdoped layer followed by a thicker pure silica layer; drawing the preform under standard drawing parameters to produce an optical fiber having a selected fiber core dimension; Including, Fabrication method for optical fiber lasers / amplifiers.

[0046] (Appendix 9) A new subclass of all-silica optical fibers particularly useful for speckle-free output from low-mode power sources, comprising: a core having a refractive index or refractive index profile; a structured silica cladding surrounding the core and having an average refractive index lower than that of the core, the structured silica cladding being composed of multiple alternating layers of different refractive indices and thicknesses; Equipped with the structured silica cladding is composed of alternating pairs of layers, each pair consisting of a low refractive index silica layer and a high refractive index silica layer, each having a different layer thickness; The ratio of the thickness of the low refractive index layer to the thickness of the high refractive index layer in any pair of layers is in the range of 3 to 20, and the number of paired layers comprising the structured silica cladding is in the range of 5 to 30. Optical fiber.

[0047] (Appendix 10) 10. The optical fiber of claim 9, wherein the ratio of the thickness of the low refractive index layer to the thickness of the high refractive index layer is between about 7 and 15.

[0048] (Appendix 11) 10. The optical fiber of claim 9, wherein the number of pairs is between about 10 and 25.

[0049] (Appendix 12) An optical fiber comprising: a structured silica cladding having an average refractive index; and a silica core having a refractive index higher than the average refractive index of the structured silica cladding.

[0050] (Appendix 13) 13. The optical fiber of claim 12, wherein the structured silica cladding comprises multiple pairs of layers of downdoped silica followed by less downdoped or pure silica, the less downdoped or pure silica layers typically being thicker than the downdoped layers.

[0051] (Appendix 14) 14. The optical fiber of claim 13, wherein a ratio of the thickness of the high-index silica layer to the thickness of the down-doped layer is between about 2 and 15.

[0052] (Appendix 15) 15. The optical fiber of claim 13 or 14, wherein the number of pairs is between about 2 and 30.

[0053] (Appendix 16) 16. The optical fiber of any of claims 12 to 15, further comprising an outer coating selected from the group consisting of added pure silica, high refractive index plastic, and down-doped silica, low refractive index plastic cladding material.

[0054] (Appendix 17) 17. The optical fiber of any of claims 12 to 16, wherein the silica core has an innermost core doped with a rare earth material and having a higher refractive index than the silica core, the doped innermost core enabling the optical fiber to function as a cladding-pumped fiber laser or fiber amplifier.

[0055] (Appendix 18) 13. A method for manufacturing the optical fiber of claim 12 having a structured silica cladding and a silica core, comprising: making a preform having a circular silica core surrounded by a section of a circular layered pair consisting of a downdoped layer followed by an overall thicker high refractive index silica layer; drawing the preform under standard drawing parameters to produce an optical fiber having a selected fiber core dimension; Including, Optical fiber manufacturing method.

[0056] (Appendix 19) 18. A method for manufacturing an optical fiber laser / amplifier having a structured silica second cladding, a silica first cladding, and a rare-earth doped innermost core according to claim 17, comprising the steps of: making a preform having a circular rare earth doped innermost core surrounded by a silica first cladding, which in turn is surrounded by a circular layered pair of sections consisting of a downdoped layer followed by a thicker pure silica layer; drawing the preform under standard drawing parameters to produce an optical fiber having a selected fiber core dimension; Including, Fabrication method for optical fiber lasers / amplifiers.

[0057] (Appendix 20) A new subclass of all-silica optical fibers particularly useful for speckle-free output from low-mode power sources, comprising: a core having a refractive index or refractive index profile; a structured silica cladding surrounding the core and having an average refractive index lower than that of the core, the structured silica cladding being composed of multiple alternating layers of different refractive indices and thicknesses; Equipped with the structured silica cladding is composed of alternating pairs of layers, each pair consisting of a low refractive index silica layer and a high refractive index silica layer, each having a different layer thickness; The ratio of the thickness of the high refractive index layer to the thickness of the low refractive index layer in any pair of layers is in the range of 3 to 20, and the number of paired layers constituting the structured cladding is in the range of 5 to 30. Optical fiber.

[0058] (Appendix 21) 21. The optical fiber of claim 20, wherein a ratio of the thickness of the high refractive index layer to the thickness of the low refractive index layer is between about 7 and 15.

[0059] (Appendix 22) 21. The optical fiber of claim 20, wherein the number of pairs is between about 10 and 25.

Claims

[Claim 1] An optical fiber comprising: a structured silica cladding having an average refractive index; and a silica core having a refractive index higher than the average refractive index of the structured silica cladding.

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