JP2006041191A - Holey fiber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a holey fiber which can realize a high numerical aperture (NA) in an excitation light for transmitting a solid area and of which manufacturing cost can be reduced. <P>SOLUTION: The holey fiber 1 is provided with a solid area that is extended in the direction of an optical axis L and is added with an optical amplification agent, and a clad area 9 that is provided with a plurality of holes 9A in the extending direction of the optical axis and is provided around the solid area, and it allows an excitation light for exciting the optical amplification agent to be transmitted in the solid area. In the holey fiber 1, the holes are arranged along the outer circumference of the solid area in one layer structure on a cross section 13 almost orthogonally crossing the optical axis, and the shortest distance t between adjoining holes among the holes is 1.0 μm or less. In this case, since the shortest distance between the holes is 1.0 μm or less, the excitation light is hard to leak, so that high NA can be realized. In addition, the number of holes can be reduced because the holes area arranged along the outer circumference of the solid area, and the manufacturing cost can be reduced as a result. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a holey fiber that can be used in an optical fiber laser light source.

  In recent years, for example, a laser light source using a double clad type optical fiber has been developed as an optical fiber laser light source. For example, as described in Patent Document 1, the double-clad optical fiber includes a core region (first core) to which an optical amplifying agent is added, and a refractive index lower than the core region provided around the core region. And a second cladding region provided around the first cladding region and having a lower refractive index than the first cladding region. In this optical fiber, excitation light for exciting the optical amplifying agent is confined in the core region and the first cladding region and propagates to excite the optical amplifying agent in the core region. Since the excited light amplifying agent outputs spontaneous emission light, laser light can be generated using the stimulated emission phenomenon by combining the optical fiber and the optical resonator.

  Accordingly, in order to increase the output of the laser light, the intensity of the stimulated emission light output from the optical amplifying agent by the stimulated emission phenomenon may be increased. The intensity of the stimulated emission light is increased in the core region and the first cladding region. The higher the intensity of the excitation light propagating through the light, the higher. Therefore, in order to increase the intensity of the pumping light propagating in the core region and the first cladding region, an optical fiber having a high numerical aperture (NA) with respect to the pumping light is required.

In order to increase the NA of the optical fiber, it is only necessary to increase the difference in refractive index between the region where the light should propagate and its surroundings. Therefore, it has been proposed to use an optical fiber (holey fiber) having a hole extending in the optical axis direction. For example, Non-Patent Document 1 discloses a solid region around which a pumping light should be confined and propagated. A holey fiber in which a plurality of holes are arranged in a two-layer structure in a cross section orthogonal to the optical axis direction is disclosed.
Japanese Patent No. 3479219 C. Simonneau, et al., “High-power air-clad photonic crystal fiber cladding-pumped EDFA for WDM application in the C-band.”, ECOC-IOOC 2003 Proceedings VOLUME 6-Post-Deadline Papers, September 21-25 , 2003, pp34-35.

  In the holey fiber described in Non-Patent Document 1 described above, a plurality of holes are formed around a solid region where the excitation light should be confined and propagated to improve the NA. However, since the plurality of holes are formed in a layer structure having two layers around the solid region, there is a problem that the number of holes increases and the manufacturing cost increases.

  Accordingly, an object of the present invention is to provide a holey fiber capable of realizing a high numerical aperture (NA) with respect to excitation light propagating in a solid region and reducing the manufacturing cost.

  In a holey fiber, a clad region including a plurality of holes is provided around a solid region, and if the proportion of air in the clad region is about 90%, the numerical aperture ( NA) is expected to be 1, but actually, the NA is smaller than the theoretical value. As a cause of the difference between the theoretical value and the actual side value, the present inventors, even if a plurality of holes are arranged around the solid region in the cross section orthogonal to the optical axis direction, Focused on the remaining glass layer. And it discovered that NA fell because the excitation light which propagates a solid area | region through the glass layer between void | holes leaked, and came to this invention.

  That is, the holey fiber according to the present invention has a solid region extending in the optical axis direction and having a light amplifying agent added to at least a part thereof, and a plurality of holes extending in the optical axis direction and surrounding the solid region. A hollow region that propagates excitation light that excites the optical amplifying agent in the solid region, and in the cross section substantially perpendicular to the optical axis direction, the plurality of holes are solid It is arranged in a one-layer structure along the outer periphery of the region, and the shortest distance between adjacent holes among the plurality of holes is 1.0 μm or less.

  In this case, the cladding region provided around the solid region to which at least a part of the optical amplifying agent is added has a plurality of voids arranged along the outer periphery of the solid region in a cross section orthogonal to the optical axis direction. Since it has a hole, the excitation light is confined in the solid region and propagates due to a difference in refractive index between the solid region and the cladding region. In the cross section, since the shortest distance between the holes is 1.0 μm or less, the excitation light hardly leaks between the holes and the NA is high with respect to the excitation light. Can be realized. In addition, since the plurality of holes are arranged in a single layer structure in the cross section, for example, the number of holes can be reduced compared to the case where the plurality of holes are arranged in a two layer structure, and the manufacturing cost is reduced. Can be planned.

  Furthermore, in the holey fiber according to the present invention, the shortest distance is preferably 0.5 μm or less. In this case, since the excitation light is less likely to leak from between the holes, it is effective for realizing a high NA with respect to the excitation light.

  Furthermore, in the holey fiber according to the present invention, the shortest distance is preferably 0.1 μm or more. According to this configuration, the strength of the holey fiber can be maintained, and the holey fiber can be prevented from breaking.

  The solid region in the holey fiber according to the present invention includes a first region to which an optical amplifying agent is added and a second region having a refractive index lower than the refractive index of the first region. It is preferable to have a region.

  In this case, since the optical amplifying agent is added to the first region of the solid region and the refractive index of the second region around the first region is smaller than the refractive index of the first region, it is excited by the excitation light. The light output from the optical amplifying agent can be confined and propagated in the first region.

  Moreover, as an optical amplifying agent in the holey fiber which concerns on this invention, a metal ion is mentioned, for example.

  According to the holey fiber of the present invention, it is possible to realize a high NA while reducing the manufacturing cost.

  Hereinafter, preferred embodiments of a holey fiber according to the present invention will be described in detail with reference to the drawings. In the following description, the same reference numerals are used for the same elements, and duplicate descriptions are omitted.

A holey fiber (hereinafter, simply referred to as a fiber) 1 of the present embodiment shown in FIG. 1 is made of linear quartz glass. The fiber 1 is, for example, an Er ³⁺ as an optical amplifying agent and an optical amplifying auxiliary agent. , Yb ³⁺ , Bi ³⁺ , Al ³⁺ and other core ions (first region) 3 doped with metal ions. The core region 3 is a region for propagating light output from the light amplifying agent excited by the excitation light. Around the core region 3, in order to confine and propagate the light output from the optical amplifying agent in the core region 3, a first cladding region (second cladding region) having a refractive index smaller than the refractive index of the core region 3. Region) 5 is provided.

  The solid region 7 composed of the core region 3 and the first cladding region 5 functions as a pumping guide for propagating excitation light for exciting the optical amplifying agent in multimode. The second cladding region (cladding region) 9 is made of quartz glass for confining and propagating the excitation light in the solid region 7.

  The second cladding region 9 is configured to include a plurality of holes 9A provided around the first cladding region 5 and extending along the direction of the optical axis L of the fiber 1. Since the second cladding region 9 has a plurality of holes 9A, the average refractive index is smaller than the refractive index of the first cladding region 5, so that the excitation light is confined in the solid region 7. A third cladding region 11 made of quartz glass is provided around the second cladding region 9 to support the second cladding region 9.

  FIG. 2 is a cross-sectional view orthogonal to the optical axis L of the fiber 1. As shown in FIG. 2, the shape of the fiber cross section 13 which is a cross section orthogonal to the optical axis L of the fiber 1 is a circular shape having a diameter D1. In the fiber cross section 13, the shape of the solid region 7 is a circular shape having a diameter D 2, and the plurality of holes 9 A are annularly arranged in a single layer structure along the outer periphery of the first cladding region 5. Yes.

FIG. 3 is an enlarged cross-sectional view of the second cladding region 9. The shape of each of the holes 9A is a long oval shape thickness d ₁ of the holes 9A is than the circumferential thickness d ₂ in the radial direction of the fiber 1, the holes 9A adjacent to each other among the plurality of holes 9A, The shortest distance t between 9A is 1.0 μm or less. The shortest distance t is more preferably 0.5 μm or less from the viewpoint of increasing the numerical aperture (NA) with respect to the excitation light propagating through the solid region 7, while maintaining the strength of the fiber 1 so that the fiber 1 does not break. It is preferable that it is 0.1 μm or more. The plurality of holes 9A satisfy the above-described arrangement relationship, and in the fiber 1, the hole volume ratio of all the holes 9A in the second cladding region 9 is 90% or more. The size and number are adjusted. In the present specification, the NA of the fiber 1 means the NA with respect to the excitation light propagating through the solid region 7 as described above.

  As understood from the above configuration, the fiber 1 is a fiber called a double clad type having two cladding regions, that is, a first cladding region 5 and a second cladding region 9.

Next, the operation of the fiber 1 will be described. In the following description, it is assumed that excitation light having a wavelength capable of exciting the optical amplifying agent in the core region 3 is output from a light source (not shown) and is incident from the end face of the fiber 1. For example, when Yb ³⁺ is used as the light amplifying agent, the excitation light is light having a wavelength of 0.98 μm.

  The pumping light incident from the end face of the fiber 1 is confined and propagated in the solid region 7 due to the refractive index difference between the first cladding region 5 and the second cladding region 9, so that the optical amplifying agent is excited by this pumping light. Is done. As a result, spontaneous emission light is output from the optical amplifying agent, and the spontaneous emission light is confined in the core region 3 by the refractive index difference between the core region 3 and the first cladding region 5 and propagates through the core region 3.

  Then, by combining the fiber 1 and the optical resonator, the spontaneous emission light can be used to generate stimulated emission light propagating in the core region 3 from the optical amplifying agent excited by the excitation light. As a result, laser light is generated. Therefore, the fiber 1 shown in FIGS. 1 to 3 can be suitably used for an optical fiber laser light source. Note that spontaneous emission light or stimulated emission light propagating in the core region 3 is normally propagated in a single mode, but may be propagated in a multimode.

  In the fiber 1, a high NA of at least 0.3 or more is realized by setting the shortest distance t between adjacent air holes 9A and 9A to 1.0 μm or less. Thereby, since the incident efficiency when the light from the light source is incident on the solid region 3 is improved, the intensity of the excitation light in the solid region 3 is higher than that when the NA is low. As a result, the optical amplifying agent is efficiently excited and the output of the stimulated emission light is improved. As a result, a high output optical fiber laser light source can be realized.

  Here, the fact that the high NA can be realized since the shortest distance t between the air holes 9A and 9A is 1.0 μm or less will be specifically described with reference to FIG.

表１に示したファイバ径Ｄ１は、ファイバ１の直径であり、直径Ｄ２は中実領域７の直径であり（図２参照）、厚さｄ_１はファイバの半径方向の空孔９Ａの厚さである（図３参照）。また、空孔体積比率は、第２クラッド領域９内で全空孔９Ａが占める割合であり、第２クラッド領域９内に空気が含まれる割合に対応している。 FIG. 4 is a graph showing the relationship between the shortest distance t between adjacent air holes 9A and 9A and NA. The measured values of NA of the fibers A and B as examples shown in Table 1 and the fiber C as a comparative example are plotted on the vertical axis, and the shortest distance t between holes of the fibers A to C is plotted on the horizontal axis. A tungsten lamp was used as the light source for NA measurement.

Fiber diameter D1 shown in Table 1, the diameter of the fiber 1, the diameter D2 is the diameter of the solid region 7 in (see FIG. 2), the thickness d ₁ is the thickness of the holes 9A in the radial direction of the fiber (See FIG. 3). The hole volume ratio is a ratio occupied by all the holes 9 </ b> A in the second cladding region 9, and corresponds to a ratio in which air is contained in the second cladding region 9.

尚、（１）式中のφは、中実領域７を伝播できるように励起光をファイバ１に入射する際の最大入射角（図１参照）である。ＮＡは入射角φのｓｉｎを取った値であり、（１）式から求めたＮＡの計算値が１．０を越える場合には、ＮＡは、１．０であることを意味している。 The point where the shortest distance between holes t shown in FIG. 4 is 0 is a calculated value. When t is 0, a simple glass rod exists in the air, the refractive index n ₀ of the first cladding region 5 (glass) is 1.45, and the refraction of the second cladding region 9 (air). It is possible to calculate from the formula (1) by setting the rate n ₁ to 1.0.

In the equation (1), φ is the maximum incident angle (see FIG. 1) when the excitation light is incident on the fiber 1 so that it can propagate through the solid region 7. NA is a value obtained by taking the sin of the incident angle φ, and when the calculated value of NA obtained from the equation (1) exceeds 1.0, it means that NA is 1.0.

The hole volume ratios of the fibers A and B shown in Table 1 are both 90% or more, and the average refractive index n ₁ of the second cladding region 9 is 1.045 or less. Accordingly, the NA calculated from the average refractive index n ₁ of the second cladding region 9 by the equation (1) is 1.0 (1.0 or more in calculation). The hole volume ratio of the fiber C is 87%, and the average refractive index n ₁ of the second cladding region 9 is 1.0585. Therefore, NA obtained from the equation (1) is 0.99. However, as shown in FIG. 4, the measured NA is greatly different from the value calculated from the equation (1) and shows a small value. From this, it can be seen that factors other than the average refractive index n ₁ of the second cladding region 9 obtained based on the hole volume ratio affect the NA.

  On the other hand, as shown in FIG. 4, when the shortest distance t between adjacent air holes 9A and 9A becomes shorter, the NA increases and approaches the value obtained from the equation (1). In the NA of the fiber 1, the shortest distance t between adjacent air holes 9A and 9A is an important parameter, and it can be seen that the NA can be improved by reducing the shortest distance t. This is because, as the shortest distance t becomes shorter, the thickness of the glass layer between the holes 9A and 9A becomes thinner than the wavelength of the excitation light, and as a result, the excitation leaks from the solid region 7 through the glass layer. This is because the light is reduced, so that the excitation light is easily confined by the solid region 7.

  Since the shortest distance t of the fibers A and B as examples is 1.0 μm or less, the NA is practically about 0.3 or more, which is known as high NA. It is higher than fiber C. In addition, FIG. 4 shows that the NA increases sharply when the shortest distance t is 0.5 μm or less, so that the shortest distance t is more preferably 0.5 μm or less from the viewpoint of increasing the NA.

  The result shown in FIG. 4 is one example for explaining the relationship between the shortest distance t between the holes 9A and 9A and NA, and the fiber 1 of the present embodiment is shown in Table 1, for example. It is not limited to the fibers A and B shown.

  By the way, the fiber 1 having a plurality of holes 9A extending in the direction of the optical axis L is manufactured as follows, for example. That is, first, a solid optical fiber preform is prepared, and a plurality of holes extending in the longitudinal direction are formed in the optical fiber preform. Subsequently, the optical fiber preform formed with the plurality of holes is drawn while pressurizing the holes. At the time of this drawing, the fiber is controlled while controlling the pressure applied to the optical fiber preform so that the hole 9A has a desired size and the shortest distance t between the adjacent holes 9A, 9A is 1.0 μm or less. 1 is manufactured. In the production of the fiber 1, the smaller the number of holes, the easier the fiber 1 can be produced, and the production cost can be reduced.

  In the fiber 1 of the present embodiment, the plurality of holes 9A in the fiber cross section 13 are formed in a single layer structure along the outer periphery of the first cladding region 5, so that, for example, the plurality of holes are formed in two or more layers. As a result of the reduction in the number of vacancies, the manufacturing cost can be reduced as compared with the case of forming with a layer structure. That is, in the fiber 1 shown in FIGS. 1 to 3, a high NA can be realized while reducing the manufacturing cost.

As mentioned above, although preferred embodiment of this invention was described, it cannot be overemphasized that this invention is not limited to the said embodiment. For example, the solid region 7 includes two regions of the core region 3 and the first cladding region 5 having a refractive index lower than the refractive index of the core region 3, but is not limited to this case. The solid region may be a structure that can confine and propagate the excitation light that excites the optical amplifying agent and can propagate the light output from the optical amplifying agent. In Equation (1), n ₀ is the refractive index of the first cladding region 5. However, except when the solid region 7 is composed of the core region 3 and the first cladding region 5, for example, The refractive index of the outermost region that is in contact with the second cladding region (cladding region) 9 may be used.

  In the above-described embodiment, the holey fiber 1 has been described as being used as an optical fiber laser light source. However, for example, the holey fiber 1 can also be used as an optical amplifier by inputting signal light together with excitation light.

It is a longitudinal cross-sectional view of one embodiment of the holey fiber of the present invention. It is sectional drawing orthogonal to the optical axis of the holey fiber shown in FIG. FIG. 3 is an enlarged cross-sectional view of a second cladding region of the holey fiber shown in FIG. 2. It is a graph which shows the relationship between the shortest distance between adjacent air holes, and a numerical aperture (NA).

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Holey fiber, 3 ... Core area | region (1st area | region), 5 ... 1st clad area | region (2nd area | region), 7 ... Solid area | region, 9 ... 2nd clad area | region (cladding area | region), 9A ... Hole, L …optical axis.

Claims (5)

A solid region extending in the optical axis direction to which an optical amplifying agent is added at least in part, a clad region having a plurality of holes extending in the optical axis direction and provided around the solid region; A holey fiber that propagates excitation light for exciting the light amplification agent in the solid region,
In the cross-section substantially orthogonal to the optical axis direction, the plurality of holes are arranged in a single layer structure along the outer periphery of the solid region, and among the plurality of holes, between the adjacent holes. The shortest distance is 1.0 μm or less.   The holey fiber according to claim 1, wherein the shortest distance is 0.5 μm or less.   The holey fiber according to claim 1, wherein the shortest distance is 0.1 μm or more. The solid region is
A first region to which the light amplification agent is added;
The first region according to any one of claims 1 to 3, further comprising a second region provided around the first region and having a refractive index lower than that of the first region. Holy fiber.   The holey fiber according to any one of claims 1 to 4, wherein the light amplification agent is a metal ion.

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