JP5612654B2 - Rare earth doped optical fibers for fiber lasers and fiber amplifiers - Google Patents

Description

  This application is priority to US Provisional Application No. 60 / 84,012 (reference number IMRAA.037PR) entitled “Rare Earth Doped Large Effective Area Optical Fibers for Fiber Lasers and Amplifiers” filed on September 20, 2006. And is incorporated herein by reference in its entirety.

  The present application relates to optical fibers that include, for example, rare earth-doped optical fibers and large effective area fibers that can be used, for example, in fiber lasers and fiber amplifiers, and methods of making such fibers.

  Fiber lasers have shown great promise over their semiconductor equivalents over the past decade due to various advantages. Fiber lasers are easy to manufacture, have efficient heat dissipation, are more stable, produce better beam quality, are more reliable and more compact.

  Major limitations in increasing power in fiber lasers include non-linear effects and optical damage, which are a direct result of the severe limitations of optical modes in the laser. There are several nonlinear effects in optical fibers. Self-phase modulation dominates the generation of ultra-short pulses with high peak power. Raman scattering is one of the major limitations for longer pulse and continuous wave operation. Brillouin scattering dominates in narrow spectral linewidth applications.

  Much research has been done on how to counteract these nonlinear effects. A level of self-phase modulation can be balanced by dispersion in self-similaritons. Raman scattering can be reduced by a W-type waveguide design to increase loss at Stoke wavelengths. Brillouin scattering can also be reduced by reducing the acoustic wave guide. All of these non-linear effects are a direct result of the high light intensity within the fiber optic core, so increasing the core size is equivalent to increasing the effective mode area and reducing the light intensity. Therefore, the nonlinear effect can be effectively reduced.

  If there is an appropriate spatial filter and / or selective mode excitation of the fundamental mode, a multimode fiber with a larger core size can be used to serve as a near diffraction limited amplifier. By using multimode fiber, it is possible to increase the size of the core beyond the size of the core provided by single mode fiber. See, for example, Fermann et al., US Pat. No. 5,818,630, which is hereby incorporated by reference in its entirety. As fibers become more multimode with increasing core size, easy emission and reliable propagation of fundamental modes is also increasingly important in these fibers to maintain good beam quality.

  Another effective way to reduce non-linear effects is to use short length fibers. This approach requires a host glass that is heavily doped with rare earths.

US Provisional Patent Application No. 60 / 846,012 US Pat. No. 5,818,630 US patent application Ser. No. 10 / 844,943 US Pat. No. 6,711,918 B1

`` High-power air-clad large-mode-area photonic crystal fiber laser '', Optics Express, vol. 11, pp. 818-823, 2003 "Extended single-mode photonic crystal fiber lasers", Optics Express, vol.14, 2715-2720, 2006

  Therefore, what is needed is a glass that allows a large core and / or high doping.

  Various embodiments described herein include rare earth doped glass compositions that may be used in optical fibers and large core size rods. The refractive index of the glass may be substantially uniform and in some embodiments may be close to the refractive index of silica. Feasible benefits for these features include reducing the formation of additional waveguides within the core, which becomes increasingly problematic as the core size increases.

  For example, the various embodiments described herein include refractive index silica, at least about 10 mole percent phosphorus in the silica, at least about 10 mole percent boron in the silica, and a rare earth in the silica. A doped glass comprising ions is provided. The rare earth ions have a concentration in the silica of at least about 1000 mol ppm. The refractive index of silica containing phosphorus, boron, and rare earth ions is within about ± 0.003 of the refractive index of silica.

Other embodiments described herein include a method of making a glass doped with rare earth ions. The method includes bundling a plurality of rods comprising glass doped with rare earth ions and stretching the bundled rods to form a first rod. In some embodiments, the first rod may be cut into shorter sections that are bundled and stretched to form the second rod. The uniformity of the effective refractive index of this second rod is smaller than the maximum peak-to-peak change of about 5 × 10 ⁻⁴ as measured using a refractive index observation device with a spatial resolution of 0.1 μm. Good.

Other embodiments described herein comprise a rod including a core and a cladding doped with rare earth ions. The uniformity of the effective refractive index of the core is smaller than the maximum peak-to-peak change of about 5 × 10 ⁻⁴ measured using a refractive index observation apparatus with a spatial resolution of 0.1 to 0.5 μm.

Other embodiments described herein comprise a fiber that includes a core and a cladding doped with rare earth ions. The uniformity of the effective refractive index of the core is smaller than the maximum peak-to-peak change of about 5 × 10 ⁻⁴ measured using a refractive index observation apparatus with a spatial resolution of 0.1 to 0.5 μm.

Other embodiments described herein comprise a rod comprising a rare earth ion doped core and a cladding, the core having an average refractive index within about ± 0.003 of the refractive index of silica, at least 200. With a doped region of square microns (μm ² ).

Other embodiments described herein comprise a fiber comprising a core doped with rare earth ions and a cladding, the core having an average refractive index within about ± 0.003 of the refractive index of silica, at least 200. With a doped region of square microns (μm ² ).

Other embodiments described herein include a core having a core radius ρ, a first cladding disposed about the core, and a second cladding disposed about the first cladding. An index optical fiber is provided. The first cladding has an outer radius ρ ₁ . The difference in refractive index between the core and the first clad is Δn, and the difference in refractive index between the first clad and the second clad is Δn ₁ . For this step index fiber, (i) less than 10 modes are supported in the core, and (ii) the first cladding radius ρ1 is greater than about 1.1ρ, less than about 2ρ, and (Iii) The refractive index difference Δn ₁ between the first cladding and the second cladding is greater than about 1.5Δn and less than about 50Δn.

  Other embodiments described herein comprise an optical fiber system that allows optical amplification. The optical fiber system comprises an optical fiber doped with one or more types of rare earth ions. The optical fiber has a tapered input and extends long therefrom. The fiber optic system further comprises an optical pump optically coupled to the optical fiber, and a light source optically coupled to the tapered input of the optical fiber. The tapered input end supports a reduced number of optical modes than the portion that extends longer from the tapered input.

  Other embodiments described herein include a method of making glass. The method includes introducing boron by vapor deposition and introducing phosphorous acid by vapor deposition, wherein the boron and phosphorous acid are introduced at different times. Boron and phosphorous acid are introduced at different times to prevent reaction of boron and phosphorous acid in the gas phase.

  Illustrative embodiments of the devices, compositions, and methods disclosed herein are shown in the accompanying drawings, which are for illustration purposes only. The drawings include the accompanying figures, wherein like numerals designate like parts.

FIG. 3 schematically illustrates a heavily doped glass in the form of a rod that can be used in the manufacture of an optical fiber in some embodiments described herein. FIG. 2 schematically shows an apparatus for modified chemical vapor deposition (MCVD) on the surface of a tube. It is a figure which shows roughly vapor deposition of the core soot in the tube shown by FIG. 2A. Conventionally having (a) a refractive index across the core region of the glass matrix including ytterbium doping, (b) an average refractive index of the glass, (c) a refractive index of silica, and (d) a similar ytterbium doping level. It is the figure which plotted the example of the average refractive index of the silica base material. FIG. 3 schematically shows a matrix structure that can be manufactured by the apparatus shown in FIGS. 2A and 2B. 3B schematically illustrates a rod manufactured by removing portions of the base material of FIG. 3A that does not include ytterbium. FIG. FIG. 2 schematically shows a stack of rods doped with ytterbium and canning of the stack. It is the figure which plotted the refractive index profile which crosses line AA of FIG. 4A. FIG. 4B schematically illustrates a stack formed by glass cans made by the process shown in FIG. 4A and canning of the stack. FIG. 5B plots the refractive index profile across line BB of FIG. 5A and shows increased uniformity and refractive index. 6 is a photograph of a cross-sectional view of a leaky channel fiber having a ytterbium-doped core manufactured using techniques such as those disclosed above with respect to FIGS. 4A, 4B, 5A, and 5B. FIG. 7 schematically illustrates a cross-sectional view of a fiber leak as shown in FIG. 6. FIG. 7 schematically shows a refractive index profile across the fiber of FIG. 7A formed using a ytterbium-doped rod having an average refractive index that matches that of silica used to form the cladding. It is. FIG. 7 schematically illustrates a refractive index profile across the fiber of FIG. 7A formed using a ytterbium doped rod having an average refractive index higher than that of the silica used to form the cladding. It is. FIG. 7 schematically illustrates a refractive index profile across the fiber of FIG. 7A formed using a ytterbium-doped rod having an average refractive index lower than that of the silica used to form the cladding. It is. FIG. 3 is a photograph of a cross-sectional view of a large diameter core polarization maintaining (PM) fiber including a Yb ³⁺ doped core. FIG. 6 schematically illustrates a fiber end face prepared by slicing with a coreless fiber. FIG. 9B schematically illustrates the coreless fiber of FIG. 9A cleaved to form an end face. It is a figure which shows schematically the fiber end surface prepared by shrinking | reducing the hole in the fiber of a large diameter core. FIG. 10B schematically illustrates the coreless fiber of FIG. 10A cleaved to form an end face. It is a figure which shows roughly the fiber end surface prepared by tapering and reducing the hole in the fiber of a large diameter core. FIG. 10C schematically shows the tapered fiber of FIG. 10C cleaved to form an end face. Step index fiber incorporating a rare earth doped core made by a stack and draw process as described herein having an average refractive index higher than that of silica cladding FIG. A polarization maintaining (PM) step incorporating a doped core manufactured in a stack and draw process as described herein having an average refractive index higher than that of silica cladding FIG. 2 schematically illustrates an index fiber design. FIG. 6 schematically illustrates a design of a bend resistant fiber incorporating a doped core manufactured in a stack and draw process as described herein. FIG. 2 schematically illustrates a double clad fiber formed by the process described herein.

  The details of the invention will be illustrated by examples and figures. One skilled in the art can readily appreciate that many possible variations are also possible and are not limited to the details of the process used in each example.

  As previously mentioned, one limitation to the increase in power in fiber lasers includes non-linear effects and optical damage, which are a direct result of the severe limitations of optical modes in the laser. Since these non-linear effects are a direct result of the high light intensity within the fiber optic core, increasing the core size is equivalent to increasing the effective mode area and reducing the light intensity. Therefore, the nonlinear effect can be effectively reduced.

  By using multimode fiber, it is possible to increase the size of the core beyond the size of the core provided by single mode fiber. Multi-mode fibers with larger core sizes can be used to serve as near-diffraction limited amplifiers if there is an appropriate spatial filter and / or selective mode excitation of the fundamental mode. See, for example, Fermann et al. US Pat. No. 5,818,630, which is incorporated herein by reference in its entirety.

  Conventional fibers reach a core diameter limit of ˜35 μm when reliable fundamental mode operation is required mainly due to emission difficulties and intermode coupling in high multimode fibers. In the past few years, new designs based on photonic crystal fibers (PCF) have been studied. This design allows the demonstration of large core fibers that support single mode or very few modes at the expense of weak waveguide design.

  In “High-power air-clad large-mode-area photonic crystal fiber laser”, Optics Express, vol. 11, pp. 818-823, 2003, which is incorporated herein by reference in its entirety, Lipert et al. A photonic crystal fiber having a core diameter of 28 μm is disclosed. To ensure single mode propagation, this fiber design has a large number of small diameter holes where the hole diameter to hole spacing ratio d / Λ is less than 0.18. The hole diameter d used corresponds to an operating wavelength λ where d / λ≈2. At this small diameter hole size / wavelength ratio, significant optical power penetration into the air holes occurs, which makes the sensitivity to bending more stringent. The design is also limited to ˜35 μm for reliable fundamental mode operation, mainly due to high bend loss.

  In “Extended single-mode photonic crystal fiber lasers”, Optics Express, vol. 14, pp. 2715-2720, 2006, which is incorporated herein by reference in its entirety, Lipert et al. Used the design of photonic crystal fibers. Disclosed the rod. For rods with a diameter of 1.5 mm, core diameters of up to 100 μm have been achieved using photonic crystal designs. The d / Λ ratio for a 100 μm core rod is 0.2. The rod structure keeps the waveguide straight and reduces high bending losses. For practical reasons, the length of the rod is limited to 0.5 meters.

  In contrast, the design approach disclosed in US patent application Ser. No. 10 / 844,943 (IM-107; Docket No. IMRAA.024A) entitled “Large Core Holey Fibers”, which is incorporated herein by reference in its entirety. Use holes that are much smaller and much larger. This design creates a much larger leaky channel so that higher order modes exit, resulting in higher propagation losses in higher order modes. Using much larger air holes reduces bending sensitivity. The latest implementation of this design uses d / Λ as high as 0.65, resulting in a much lower bending loss. A much larger hole diameter to wavelength ratio d / Λ effectively prevents penetration of optical power into the hole, resulting in a much improved optical waveguiding across the curved fiber. This type of design also reduces inter-mode coupling due to high order mode leakage losses, resulting in much improved single mode propagation.

  To make a practical amplifier fiber using the photonic crystal fiber design and leakage channel design disclosed in US patent application Ser. No. 10 / 844,943, a uniform refractive index on the doped core is used. The properties are preferably better than the uniformity that can be achieved with conventional optical fiber manufacturing.

  When the core becomes larger, the local material with a relatively high refractive index is surrounded by the material with a relatively low refractive index, so the non-uniform refractive index on the core causes the local waveguide to Sometimes formed. The formation of local waveguides depends on the level of refractive index non-uniformity as well as the geometric size of the refractive index non-uniformity. The substantial refractive uniformity on the core can reduce this local waveguiding. Specifically, by keeping the non-uniformity at a small geometric size, local changes in refractive index do not form a waveguide. Thus, in various embodiments, the refractive index non-uniformity is kept below several times the length of the wavelength. Furthermore, the large non-uniformity of the refractive index is made smaller. Such uniformity is easier to achieve for small cores than for large diameter cores.

  Further, a waveguide may be formed in the core if the refractive index in the core doped region is higher than in the core undoped region. The core may include, for example, silica. If each portion of silica doped to obtain optical gain has a higher refractive index than undoped silica, additional local waveguiding may occur. Accurate index matching of the core doped and undoped regions may alleviate this problem. When the doped area is larger, for example when the effective core area is larger, closer index matching may be more useful than when the doped region is small. Thus, in various embodiments described herein, the average refractive index of the doped core is also controlled to be close to the refractive index of the cladding glass also used in the core. It may be difficult to control the refractive index of the doped glass in this way using known matrix materials for rare earth ions with a doping level of 3000 mol ppm or higher.

  In some embodiments described herein, in addition to a higher ytterbium-doped rod that has a higher refractive index in the core, a fluorine-doped silica rod that has a lower refractive index than silica. By bundling and stretching the bundled rod to produce a core rod, the desired refractive index uniformity and average refractive index can be achieved. For example, see “Extended single-mode photonic crystal fiber lasers”, Optics Express, vol. 14, pages 2715-2720, 2006, which is incorporated herein by reference in its entirety. The number of rods doped with fluorine and the number of rods doped with ytterbium are carefully chosen to provide an overall average refractive index close to that of silica. The total number of the two types of rods is large and the rods are evenly distributed in the stack. Each rod is usually bundled into a hexagonal stack and stretched to make a core rod. In this way, a high degree of average refractive index control can be achieved. Because light cannot recognize structures with dimensions much smaller than the wavelength of light, it is very uniform when the initial individual rod is reduced to a size that is about the wavelength of light in the final core. High refractive index can be achieved. However, this technique reduces the average level of ytterbium doping in the final core because it is diluted by fluorine-doped silica rods that do not contain ytterbium. Such fluorine doped rods can be used to achieve an average refractive index, but may not include ytterbium. This approach will lower the average ytterbium level used to provide gain.

  Nevertheless, in theory, precise control of the average refractive index is the ratio of the number of each type of rod in the glass, including two types of rods, one with a high refractive index and one with a low refractive index. It can be achieved by controlling. See, for example, US Pat. No. 6,711,918 B1 issued to Kliner et al., Which is incorporated herein by reference in its entirety. While the clad glass contains a relatively small number of high index rods, the core can be made with a relatively large number of high index rods.

  The traditional matrix for ytterbium in fiber lasers and fiber amplifiers has been a silica glass matrix. A level of aluminum or phosphorus is often added to reduce ytterbium-clustering at high doping levels. Ytterbium clustering is undesirable due to the fact that the interaction between each ytterbium ion results in multiphoton upconversion, and thus additional energy loss in the laser or amplifier.

  Furthermore, photo-darkening is a phenomenon in which the background loss in a fiber is permanently increased by creating a color center as a result of the presence of a large amount of optical power in the fiber. Photodarkening is generally believed to be related to ytterbium clustering at high ytterbium doping levels, and the very high energy at which multiple ions interact to produce photodarkening. Generate photons with levels. The effect may saturate after the period of exposure, but contributes to loss of output power and reduced efficiency in fiber lasers and fiber amplifiers. Light blackening is more severe at high power levels and can contribute to significant power loss in high power fiber laser systems if not handled properly.

  However, adding aluminum and phosphorus to reduce ytterbium clustering has the effect of increasing the refractive index. If needed, a small amount of germanium doping can be added to increase the refractive index. Fluorine can be added to lower the refractive index. The current status of artificial silica fiber manufacturing techniques limits the amount of fluorine that can be incorporated into silica glass, so that the refractive index of a rare earth-doped core is usually higher than the refractive index of silica. In addition to the increase in refractive index due to rare earth doping, this is particularly true for rare earths at higher concentrations, as higher doping levels are required for aluminum or phosphorus to achieve reasonably low levels of clustering. This is true for doped cores. Thus, in certain large core fiber designs where the index of refraction is close to that of silica, making a rare earth heavily doped core glass used for fiber laser power scaling. Is particularly difficult.

  However, various embodiments described herein include glass compositions that are heavily doped with rare earths whose refractive index is close to that of silica. Such glass can be converted to other rods as well as rods that can be used as a base material for manufacturing optical fibers. Furthermore, such glasses can be manufactured with mature technology used in the manufacture of optical fibers in the telecommunications industry.

For example, one embodiment shown in FIG. 1 has a high level of rare earth ions, for example> 1000 mol ppm, and the refractive index is very close to the refractive index of silica glass, for example a refractive index difference of Δn <1. A glass rod or base material 2 containing a glass composition of × 10 ⁻³ is provided. In this embodiment, most silica glasses have significant levels of phosphorus, such as 10-50 mole percent phosphorus, and boron, such as 8-50 mole percent boron. In various embodiments, phosphorus and boron are present in a compound that includes phosphorus or boron. Most of the silica glass may include, for example, 5 to 25 mol% of P ₂ O _5, and 4 to 25 mol% of B ₂ O _3. For example, elemental analysis using a mass spectrometer or SEM may be used to determine the composition of the glass. It has been found that such glass compositions allow ytterbium doping levels up to 20,000 mol ppm without devitrification, allowing very efficient laser and amplifier operation. For comparable amounts of ytterbium doping levels, it has been found that the optical darkening effect is significantly less in the manufactured fiber. By using this glass, light darkening can be reduced, resulting in a much more stable and efficient high power fiber laser and fiber amplifier.

  Various embodiments described herein also include techniques for fabricating doped fiber cores with very high effective refractive index uniformity. This high uniformity can be achieved by producing a number of rare earth-doped base materials having a core and a cladding. The average refractive index of the doped core glass is made very close to that of silica glass. The base clad glass may be removed in whole or in part by grinding or drilling. As a result, a rod as shown in FIG. 1 can be obtained. The resulting rods are bundled and the stack is stretched into smaller sized rods to ensure substantial fusion of each constituent rod in the stack. In some cases, the stack can be inserted into the tube before stretching. The stretched rod can be cut and rebundled with other similar rods and restretched according to a similar procedure. This process can be repeated many times to finally obtain a uniform rod. In the final fiber, the doped glass may be further mixed by the flow and diffusion of the low viscosity rare earth doped glass at high stretch temperatures. In an alternative configuration where the stack is inserted into the tube, additional steps may be added to fuse the structure. A portion or all of the tube may then be removed by grinding or etching before stretching into a rod.

  In contrast to other doped glasses, this rare earth doped glass described herein allows the refractive index to be very close to that of silica glass. In addition, instead of the two types of glass rods used in other methods, a very uniform rare earth-doped core glass can be obtained by repeating a stack and draw of the same type of glass rod. Can be produced. This approach further improves the uniformity of the glass and allows for a highly effective rare earth doping level within the fiber core.

  Various fibers and processes for manufacturing fibers formed by bundling and preforming the matrix are described in more detail below. A process for manufacturing the matrix is also described. The process for manufacturing the matrix, fiber, and other structures will be described in more detail with examples and figures. However, a wide range of variations in the process and the resulting and intermediate products are possible. Each processing step may be added, removed, or rearranged. Similarly, each component may be added, removed, and configured differently or in different amounts. Specifically, the initial component and the additive component may be changed.

The matrix manufacturing technique used in the following examples is Modified Chemical Vapor Deposition (MCVD), although other known deposition techniques such as external deposition processes can also be used. Silicon and phosphorus are introduced into the deposition zone via a conventional bubbler device using liquid precursors such as SiCl ₄ and POCl ₃ respectively. Boron is introduced into the deposition zone via a gas precursor, for example BCl ₃ . Fluorine is also used and is introduced via CF ₄ . Rare earth ions are introduced through a well-known solution doping process.

  In the early stages of the process, as shown in FIG. 2A, a substrate tube 10 having an outer diameter of 25 mm and an inner diameter of 19 mm is cleaned and placed on a lathe of an MCVD system. In this example, a standard MCVD system for optical fiber manufacturing, manufactured by Nextrom Technologies of Vantaa, Finland, is used, but other systems may be used. The substrate tube 10 is joined at a first end with a similar starting tube 11 and at a second end with a soot tube 12 with a larger diameter. The starter tube 11 is held by a chuck 13 and connected to a gas inlet pipe 16 via a rotary seal 15. The soot tube 12 is also held by the chuck 14. The soot tube 12 is connected to a scrubber that processes the exhaust gas before it is released. A movable burner 17 that moves along the substrate tube 10 can heat a portion of the substrate tube at a time. When the burner 17 reaches the end of the substrate tube 10 near the junction with the soot tube 12, it quickly moves to the starting portion of the substrate 10 near the junction with the starter tube 11. The burner will prepare for the next movement along the substrate tube 10. The chucks 13 and 14 rotate at a speed of 40 rpm.

The substrate tube 10 is first cleaned using an etch path in which 200 sccm of CF ₄ is used to remove a layer of glass from the inner surface of the substrate tube 10. One or more optional cladding passages are used to deposit a cladding that may include silica and whose refractive index is close to the substrate tube. A flow of 500 sccm SiCl ₄ , 300 sccm POCl ₃ , and 10 sccm SF ₆ may be used to form the cladding.

As shown in FIG. 2B, the core layer is deposited by moving the burner 17 upstream from the end of the soot tube. The burner 17 moves upstream prior to soot formation, so the burner does not pass over the core soot 20 to avoid any soot sintering, as shown in FIG. 2B. In some embodiments, the flow used for core formation is 100 sccm SiCl ₄ , 500 sccm POCl ₃ , and 200 sccm CF ₄ . During the passage of the core, the burner 17 advances from the soot tube end to the starting tube end at 20 mm / min to promote soot formation. For the burner, 47 slm (standard liter / min) of hydrogen and an oxygen / hydrogen ratio of 0.45 are used. The core layer is then passed through to solidify the core soot 20 by heating to 1300 ° C.

The substrate tube 10 is then removed from the lathe and the core soot 20 is immersed in the YbCl ₃ solution to incorporate ytterbium into the soot layer. After 1 hour of solution doping, the solution is drained and the substrate tube 10 is returned to the lathe. The substrate tube is dried for several hours by passing through nitrogen and heating to ~ 1000 ° C. The core soot 20 is sintered by heating to 1750 ° C. with a flow of BCl ₃ of 10 to 150 sccm. After sintering, the substrate tube 10 goes through a collapse process with some level of POCl ₃ overdoping, and the burner temperature is significantly increased and the burner speed is decreased. During the collapse process, the surface tension of the tube reduces the outer diameter of the tube. In some embodiments, 3-5 disintegration steps are used. All tubes collapse and eventually become rods. A solid matrix having a core doped with ytterbium is obtained. In this embodiment, boron and phosphorous acid are introduced at different points in time, thereby reducing the possibility of reaction of boron and phosphorous acid in the gas phase.

Curve 100 in FIG. 2C shows the refractive index profile across the core portion of the ytterbium-doped matrix that is produced. The average index of refraction of this matrix across the core, as well as the target index of silica shown by line 102, is shown by line 101. The average refractive index of the base material over the entire core indicated by 101 can be within ± 1 × 10 ⁻⁴ of the target refractive index indicated by 102. An example of the average refractive index made using a conventional silica host is illustrated by line 103, which is much higher. The much higher level of B ₂ O ₃ incorporated in the glass significantly increased the matching, thereby reducing the refractive index of the doped glass.

The core is doped with 3500-17500 mol ppm of Yb ³⁺ ions resulting in an absorption of 300-1500 dB / m at 976 nm. In this example, the core 15 to 25 mol% of P ₂ O _5, are further doped with 0.1 to 0.5 mole% of F, and 10 to 25 mol% of B ₂ O _3. For a 50 μm core fiber, in some embodiments, the difference between the average refractive index and the refractive index of silica Δn = n−n _silica is greater than −5 × 10 ^{−3 at} 1 μm wavelength, 5 × 10 5. ^Less than ^-4 . In some cases, at least one contains phosphorus to prevent ytterbium clustering, thus significantly reducing the photodarkening effect in the manufactured fiber for comparable ytterbium doping levels Be made. Variations in the parameters used to manufacture the fiber may be used and the results may vary.

  3A is a matrix comprising a core 201, an optional deposited cladding layer 202, and a silica glass layer 203 from a substrate tube 10 that may be manufactured in the process described above with reference to FIGS. 200 structures are shown. FIG. 3B further shows a core rod 210 made from the preform 200 by removing the silica layer 203, the deposited cladding layer 202, and possibly a small portion of the core 201. The sectional view 211 includes all or part of the core 201. In one alternative embodiment, the rod 211 includes a portion of the cladding layer 202.

  The iterative stack and draw process begins with the production of a plurality of preforms having a rare earth doped core. The matrix is then ground to remove the silica layer 203, and the deposited cladding layer 202 is entirely removed to produce a plurality of core rods 210 comprising mostly rare earth doped core glass. Or partially removed. As an alternative, etching away the glass layer free of ytterbium and drilling away the doped core can be used for this purpose.

  The doped core rods 210 can then be bundled as shown in FIG. 4A. Due to its high packing density, the hexagonal stack 300 shown in FIG. 4A may be used, but other stack configurations may be used. While the stack is held in place, both ends of the stack 300 are fused. The fused ends can then retain the shape of the stack 300 during canning. Fusing the entire stack 300 is optional. In the subsequent canning process shown in FIG. 4A, the doped core rod 210 can also be joined with an undoped rod at one or both ends to reduce wasted core rod waste. . The stack 300 can then be canned into a single rod 310 by choosing appropriate stretching conditions to fuse all the rods in the stack. The stack 300 can also be inserted into a tube (not shown in FIG. 4A) before stretching. While stretching, the tube does not have to hold the stack 300 in place. The canned rod 310 has a cross-sectional view that is substantially fused. FIG. 4B shows a refractive index profile 320 across the cross-sectional view along AA shown in FIG. 4A. The refractive index profile 320 mainly includes the refractive index of the first core rod 210 with reduced dimensions. At higher canning temperatures, flow and diffusion may occur. The peaks and troughs of the refractive profile of the first rod 210 can be greatly smoothed.

  The glass rod 310 can then be cut into multiple sections and rebundled to form the stack 400 for the iterative process shown in FIG. 5A. This process can be repeated many times. FIG. 5B shows the refractive index profile 420 along line BB of the resulting glass rod 410 in FIG. 5A. The refractive index profile 420 mainly includes the refractive index profile of the first glass rod 310 with reduced dimensions. Again, if the canning temperature is high, the refractive index profile 420 (displayed along BB) can be significantly smoothed by flow and diffusion. Thereby, a more uniform refraction profile can be created. In some embodiments, at least two stages are used because the second stage can eliminate inconsistencies in the manufactured matrix. In the example stack in FIGS. 4A and 5A, a stack of 37 rods is used. Other stack sizes can be used. The number of rods used in each stage need not be the same. In various embodiments, the configuration may vary.

  In another embodiment of this process, at each stage of the two-stage process, the stack is inserted into the tube before stretching. Further, additional steps can be used to fuse the stack and tube prior to stretching. The tube may then be totally or partially removed by grinding or etching. Furthermore, the doped rod is incorporated into the core region of the large core fiber. Such a fiber may be manufactured by bundling and stretching a doped core with an undoped rod as well as a hollow (undoped) rod. A cross-sectional view of the fiber 500 produced thereby is shown in FIG. The fiber 500 includes six holes 502 that define a core 501. The six holes may be formed from six hollow rods used in the drawing process. Other processes and configurations may be used. The doped portion of core 503 comprises a doped rod manufactured by an iterative stack and draw process as described above with respect to FIGS. 4A and 5A showing the first and second stacks, respectively. Each of the rods used in the second stack is visible because the optional silica tube used in the first stack has a slightly lower refractive index than that of the glass doped with ytterbium. .

  In the embodiment shown in FIG. 6, silica glass is used as the matrix for the doped rods in holey fiber 500 as well as holes 502. In various other embodiments, silica may be used as a matrix for doped rods and / or holes in holey fibers. Thus, silica may be used as a base material for the doped regions in the core 503 and may be used as a base material for making the cladding. In other embodiments, other materials may be used as the base material. In addition, various matrix materials may be used for either the core or cladding region, for example to surround a doped region (eg, rod) or hole in a holey fiber.

  7A-7D show various embodiments where the doped core rods are chosen to have different average refractive indices. FIG. 7A shows a leaky channel fiber 500 having six holes 502 that form a core 501. A portion 503 of the core 501 is made from a doped rod. 7B, 7C, and 7D are the refractive index profiles illustrated along line 504 in FIG. 7A, where the average refractive index of the doped core is that of the silica glass used, respectively. Same as refractive index, higher or lower.

FIG. 7B shows the refractive index 611 of the doped portion 503 of the core 501 where the average refractive index 612 matches the refractive index of the silica glass 610. The refractive index 613 of air is also shown in FIG. 7B. FIG. 7C shows the refractive index 621 of the doped portion 503 of the core 501 where the average refractive index 622 is higher than the refractive index of the silica glass 610. In this case, this high average refractive index results in additional waveguides. In some embodiments, the difference between the average refractive index and the refractive index of silica, Δn = n−n _silica, is small enough so that V = 2πρNA / λ is kept below about 6. However, X is an optical wavelength, ρ is a core radius, and NA = (n ² −n _silica ² ) ^1/2 ≈n _silica (2Δn / n _silica ) ^1/2 . In various embodiments, V is less than about 2.4, and therefore higher order modes are not supported in this additional waveguide. If this additional waveguide is made with a locally increasing refractive index, the holes 502 can be reduced to reduce the overall waveguide effect to reduce higher order mode propagation. The gap between each hole provides a leakage channel. To increase leakage, either the size or / and the number of holes can be reduced. Thereby, higher order modes can be leaked and a few mode or single mode fiber is provided.

FIG. 7D shows the refractive index 631 of the doped portion 503 of the core 501 where the average refractive index 632 is lower than the refractive index of the silica glass 610. In this case, this low average refractive index will result in an additional negative waveguide. In various embodiments, the difference between the average refractive index and the refractive index of silica, Δn = n−n _silica, is greater than −0.005, and therefore does not cancel the waveguide effect from the hole 502 and is less than −0.001. May be larger. If this additional negative waveguide is present, the hole 502 may be increased to increase the overall waveguide effect. Either the size or number of holes may be increased.

  FIG. 8 shows a cross-sectional view of a polarization maintaining (PM) fiber 700 that incorporates two stress elements 704 in addition to four holes 702. The core 701 comprises a doped portion 703 that is formed using a doped rod, such as a rod manufactured using the process described with reference to FIGS. 4A and 5A.

  In the embodiment shown in FIG. 8, silica glass is used as the base material for the doped rod, two stress elements 704, and the hole 702 in the holey fiber 500. Thus, in various other embodiments, silica may be used as a matrix for doped rods, stress elements, and / or holes in holey fibers. Silica may be used as a base material for the doped regions in the core 703 or may be used as a base material for making the cladding. In other embodiments, other materials may be used as the base material. In addition, various materials are used in either the core or cladding region, for example, as a matrix for doped regions (eg, rods), holes in holey fibers, stress elements, or other components. It's okay.

  Thus, silica glass and air holes are used in the above example, but many different transparent optical media with different refractive indices can be used to implement the design. As an example, soft glass can replace silica, and another soft glass with a lower refractive index can be used to replace air holes. Rare earth ions other than ytterbium can also be used. Still, other variations in configuration, materials, dimensions, or other design parameters are possible, for example.

  Due to the presence of large holes, undesirable cracks may occur during cleavage. Cracks can create cleaved end faces that are not suitable for use. Furthermore, for various applications, it is necessary to seal the hole at the fiber end, for example, to prevent contaminants from entering the hole. Several techniques can be used to solve these problems. In one embodiment shown in FIG. 9A, coreless fiber 802 is joined to large diameter core holey fiber 801. The coreless fiber 802 is then cleaved to form the end face 810. A cross-sectional view of the cleaved end surface 810 is shown in FIG. 9B. Junction point 804 and hole 803 are clearly shown in FIG. 9A. The coreless fiber may serve as an end cap for beam expansion when placed at the output of the amplifier. The end cap provides a uniform medium that allows the beam to expand before exiting the glass. In order to reduce or minimize end face damage, this beam expansion can advantageously reduce light intensity at the glass / air interface.

  In another approach, the hole 901 in the holey fiber 900 collapses entirely into a solid fiber 903 by heating the holey fiber section to a temperature at least as high as the glass softening temperature. The fiber 900 can then be cleaved at the collapsed portion to form an end face 910 as shown in FIG. 10A. FIG. 10B shows a cross-sectional view of the collapsed and cleaved end face 910 of the PM fiber. The stress element 911 is visible on the cross-sectional view of the fiber shown in FIG. 10B. A doped rod as described above is also visible in the doped section 912 of the core.

  FIG. 10C shows a further technique in which a holey fiber 920 having a hole 921 is tapered and then collapsed. This process should not be so limited, but stretching and / or heating can be used to create a taper and collapse hole 921. The holey fiber 920 has a taper 923 and a cleaved and tapered end 922. The end face 922 in this case is shown in FIG. 10D. This taper 923 supports a smaller number of modes and may further help increase transmission loss for higher order modes, especially when placed at the input of the amplifier. Such an input taper can help to efficiently emit optical power into the core.

  The iterative stack and draw process described herein for making doped cores with very high effective index uniformity is used in conventional step index fibers as well as photonic crystal fibers You can also. In various embodiments in which stack and draw is repeated to create a doped core region in a conventional step index fiber, the average refractive index of the doped matrix core is the cladding glass The refractive index can be made slightly higher than the refractive index. The doped matrix may include, for example, a doped rod that forms part of the fiber core. In some embodiments, the final rod made from a plurality of doped rods is inserted into a tube containing clad glass.

  The fiber 1000 is shown in FIG. In FIG. 11, doped core 1001 is made from an iterative stack and draw process. As described above, the doped core 1001 may be formed from a plurality of doped rods. The clad layer 1003 includes clad glass. The fiber 1000 is further coated with a coating 1002.

The refractive index profile 1004 shows the refractive index difference Δn between the doped core and the cladding glass. Doped cores with very high effective index uniformity can improve mode quality and ease of use, and may allow smaller Δn to be used. In many current designs, Δn is greater than 1 × 10 ⁻³ and is chosen to some extent by refractive index uniformity, manufacturing process repeatability, and fundamental mode bending losses. A smaller Δn makes it possible to implement a larger core diameter. For example, if Δn is reduced to (50/30) ² ≈2.8, mode quality with a 30 μm core fiber can be achieved with a 50 μm core fiber. Since the bending loss is dependent on Δn, for the same core size, a higher bending loss is expected for a lower Δn. In some embodiments, if the bend performance can be compromised and Δn can be reduced to less than 8 × 10 ⁻⁵ , eg, NA ≦ 0.015, for a core diameter of 50 μm at a wavelength of ˜1 μm. Single mode fiber is possible. However, values outside these ranges are possible.

  As shown in FIG. 12, a stress rod 1104 can be included in the cladding 1103 of the fiber 1100 to create a large core polarization maintaining (PM) fiber having a very uniform doped core 1101. it can. An additional cladding layer having a refractive index lower than that of 1103 may be added between the cladding 1103 and the coating 1102 to form a pump guide within the double-clad fiber structure. The coating 1102 can also be chosen to have a low refractive index to form a double clad structure. It is also possible to dope only a portion of the core 1101 and use the same stack and draw technique to make the rest of the core with a very uniform refractive index. Such selective doping of a portion of the core can be used to further improve mode selection within the fiber.

  As previously mentioned, the smaller Δn in conventional fibers allows the use of larger core diameters. However, higher bending losses are expected for lower Δn. The various embodiments described herein also provide a design to reduce this bending loss.

For example, the design in FIG. 13 can be used to improve (eg, reduce) the bending loss of a large core fiber 1200 with very small NA, eg, NA <0.05, and weak waveguide. . The first cladding layer 1202 is placed next to the core 1201 to provide a small Δn and supports 1 to 10 modes in a partially or fully rare earth doped core. The refractive index difference Δn ₁ between the first layer 1202 and the second layer 1203 can be chosen to be much larger than Δn, for example Δn ₁ > 1.5Δn to reduce bending loss. The higher the Δn ₁ , the more effective. The fiber further comprises a coating 1204 surrounding the second cladding layer 1203. The diameter ρ ₁ of the first cladding layer 1202 only needs to be slightly larger than the diameter ρ of the core 1201, for example, ρ ₁ > 1.1ρ. If ρ ₁ and Δn are too large, multiple modes may be supported in the combined waveguide formed by the core 1201 and the first cladding layer 1202. If the core 1201 and the first cladding 1202 support too many modes, emission of the fundamental mode may be difficult and inter-mode coupling may be increased. Since the modes supported in the core 1201 overlap with the rare earth doped core and therefore should have a greater gain, the light of each mode with a significant amount of power in the first cladding 1202 is It will be distinguished.

  Similar to the fiber shown in FIG. 13, a double clad fiber 1300 is shown in FIG. 14 and includes a core 1301, a first clad 1302 and a second clad 1303, and a coating 1304. The fiber shown in FIG. 14 further comprises a third cladding layer 1305 between the second cladding layer 1303 and the coating 1304.

A small Δn may be provided between the first cladding layer 1302 and the core 1301 to support 1 to 10 modes in a partially or fully rare earth doped core. The refractive index difference Δn ₁ between the first layer 1302 and the second layer 1303 can be chosen to be much larger than Δn, for example Δn ₁ > 1.5Δn to reduce bending loss. The higher the Δn ₁ , the more effective. The diameter ρ ₁ of the first cladding layer 1302 only needs to be slightly larger than the diameter ρ of the core 1301, for example, ρ ₁ > 1.1ρ. If ρ ₁ and Δn are too large, multiple modes may be supported in the combined waveguide formed by the core 1301 and the first cladding layer 1302. If the core 1201 and the first cladding 1202 support too many modes, emission of the fundamental mode may be difficult and inter-mode coupling may be increased.

In some embodiments, the difference in refractive index between the second layer 1303 and the third layer 1305 can be chosen to be much greater than Δn and even greater than Δn ₁ . The third cladding 1305 can provide a propagation region comprising a core 1301, a first cladding 1302, and a second cladding 1303 for propagating pump radiation, for example in a fiber amplifier or laser.

  The fibers 1200 and 1300 may have a large effective area and may be doped with rare earth elements. Glass compositions whose refractive index is close to that of silica may be used in the aforementioned core. In addition, a stack and draw process as described above may be used to increase refractive index uniformity across the core. Other features and methods described herein may also be used with the fibers 1200, 1300 shown in FIGS.

  A wide variety of variations are possible. Each component may be added, removed, or rearranged. It may be replaced with various components. The arrangement and configuration may be different. Similarly, each processing step may be added, removed, or rearranged.

  While certain embodiments of the invention have been described, these embodiments have been presented by way of example only and are not intended to limit the scope of the invention. Indeed, the novel methods and systems described herein may be implemented in a variety of other forms, and further, without departing from the spirit of the present invention. Various omissions, substitutions, and changes in form may be made. The appended claims and their equivalents are intended to include such forms or modifications as fall within the scope and spirit of the invention.

Claims (38)

A core having a core radius ρ and a core refractive index n _core ;
A first cladding disposed around the core;
A second cladding disposed around the first cladding;
The first cladding has an outer radius ρ ₁ and a refractive index n _c1 , and the core and the first cladding have a refractive index difference Δn = n _core −n _c1 and a numerical aperture (NA) less than about 0.05. And the NA is determined by n _core and n _c1 ,
The first cladding and the second cladding have a refractive index difference Δn ₁ ,
A number of modes less than 10 and greater than 1 are supported in the core,
The outer radius ρ1 of the first cladding is greater than about 1.1ρ but less than about 2ρ,
The difference [Delta] n ₁ of the refractive index between the first cladding and the second cladding, rather smaller than about 1.5Δn greater than about 50Δn
A large diameter core optical fiber , wherein the core includes a doped region of at least 200 square microns . The large-core optical fiber is bent, and has a reduced bending loss compared to a large-core optical fiber having substantially the same core radius ρ and a refractive index difference Δn between the cladding and the core. configured associated optical fiber of a large diameter core of claim 1. The large diameter core optical fiber of claim 1 , wherein Δn is less than about 10 ⁻³ . 2. The large core optical fiber of claim 1 , wherein the core is about 50 [mu] m in size and [Delta] n is less than about 8 * 10 < ^-5 >. 2. The large core optical fiber of claim 1 , wherein the core is as large as about 50 [mu] m and the NA is less than about 0.015. The large core optical fiber of claim 1 , wherein the core includes a doped region having an average refractive index that is within about ± 0.003 of the refractive index of silica. 2. The large-diameter core according to claim 1 , wherein the first cladding is slightly larger than the core, and an outer radius ρ ₁ of the _first cladding is in a range of 1.1ρ <ρ ₁ <1.5ρ. Optical fiber. 1.5Derutaenu <a Δn ₁ <10Δn, optical fiber with a large diameter core of claim 1. The large-core optical fiber according to claim 1 , wherein the core is partially doped with rare earth. The large-core optical fiber according to claim 1 , wherein the core is entirely doped with rare earth. The large-core optical fiber according to claim 1 , wherein the core has a radius of about 25 μm. Wherein the third cladding disposed about the second cladding, said core, said first further comprising a pump propagation region comprising cladding and the second cladding, a large diameter core of claim 1 Optical fiber. Coating and, for forming a pump guide further comprises an additional cladding disposed between the second cladding and the coating, the optical fiber with a large diameter core of claim 1. The large core optical fiber of claim 1 , wherein the core has an effective refractive index uniformity with a change in maximum peak-to-peak less than about 5 × 10 ⁻⁴ . The large-diameter core optical fiber according to claim 1 , wherein a part of the large-diameter core optical fiber includes a holey fiber. Silica having a refractive index;
At least about 10 mole percent phosphorus in the silica;
At least about 10 mole percent boron in the silica;
Rare earth ions in the silica having a concentration of at least about 1000 mol ppm in the silica;
The phosphorus, the silica having the boron and the rare earth ions therein has about ± 0.003 Within the a refractive index of the refractive index of the sheet Rica optical fiber with a large diameter core of claim 2 . An optical fiber system that provides optical amplification,
The fiber optic system is:
An optical fiber having a large core according to claim 1 ;
An optical pump optically coupled to the large core optical fiber;
An optical source optically coupled to the input of the large core optical fiber;
The core of the large core optical fiber is doped with one or more types of rare earth ions, and the large core optical fiber is a combination formed by the core and the first cladding. Fiber optic system including an optical waveguide. 18. The light of claim 17 , wherein the combined waveguide is configured such that a mode supported in the core has increased gain compared to a mode having substantial power in the first cladding. Fiber system. The core is configured to receive an input beam emitted within the core;
The optical fiber system of claim 17 , wherein the input beam is in a fundamental mode of the large core optical fiber. 18. The fiber optic system of claim 17 , wherein the large core optical fiber is configured to support from 1 to 10 modes in the core. The number of modes supported in the combined waveguide and the inter-mode coupling between the number of supported modes, wherein the modes supported in the core have a substantial power in the first cladding; 18. The fiber optic system of claim 17 , wherein the first cladding outer radius [rho] ₁ and the [Delta] n values are taken to be limited to increased gains. The optical fiber system of claim 17 , wherein the core has substantial refractive index uniformity. The optical fiber system of claim 17 , wherein the core radius is as large as about 25 μm. The optical fiber system of claim 17 , wherein a portion of the large core optical fiber comprises a holey fiber. The large-core optical fiber is bent, and has a reduced bending loss compared to a large-core optical fiber having substantially the same core radius ρ and a refractive index difference Δn between the cladding and the core. The fiber optic system of claim 17 , configured to accompany. The optical fiber of the large-diameter core further includes a third cladding disposed around the second cladding, and a pump propagation region including the core, the first cladding, and the second cladding. The optical fiber system of claim 17 . The fiber optic system of claim 17 , further comprising a coating and an additional cladding disposed between the coating and the second cladding to form a pump guide. 18. The optical fiber system of claim 17 , wherein the first cladding is slightly larger than the core, and the first cladding has an outer radius ρ ₁ in the range 1.1ρ <ρ ₁ <1.5ρ. . The input of the large core optical fiber is tapered, the optical fiber of the large core has a length extending from the tapered input, and the tapered input is tapered. The optical fiber system of claim 17 , wherein the optical fiber system supports a reduced number of optical modes than the length extending from the input. The core of the large diameter core is
Silica having a refractive index;
At least about 10 mole percent phosphorus in the silica;
At least about 10 mole percent boron in the silica;
Rare earth ions in the silica having a concentration of at least about 1000 mol ppm in the silica;
The phosphorus, the silica has approximately ± refractive index is 0.003 within the refractive index of the sheet Rica optical fiber system of claim 17 having the boron and the rare earth ions therein. 17. The large core optical fiber of claim 16, wherein the rare earth ions comprise ytterbium ions. 32. The large core optical fiber of claim 31 , wherein the rare earth ions comprise at least about 3000 mole ppm Yb3 ⁺ . 32. The large core optical fiber of claim 31 , wherein the rare earth ions comprise at least about 3000 mol ppm Yb ³⁺ and at least about 100 mol ppm Er ³⁺ . 34. A large core optical fiber according to claim 32 or 33 , wherein the light darkening effect of the doped glass is lower than that of a fiber having an equivalent amount of ytterbium doping level. 34. A large core optical fiber according to claim 32 or 33 , wherein the doped glass comprises ytterbium doping levels up to 20,000 mol ppm without exhibiting devitrification. 32. A large core optical fiber according to claim 31 , wherein the doped glass exhibits absorption in the range of 300-1500 db / m at the pump wavelength. The doped glass is disposed as part of an optical gain medium;
The doped glass includes rare earth ions having at least about 3000 mole ppm Yb;
34. A large core optical fiber according to claim 32 or 33 , wherein the optical gain medium provides a lower optical darkening effect than a gain medium having an equivalent amount of ytterbium doping level. The large light blackening effect of claim 37 , wherein the low light darkening effect occurs at least in part to include the phosphorus in the doped glass, thereby preventing ytterbium clustering. An optical fiber with a large core

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