WO2010052907A1 - イッテルビウム添加光ファイバ - Google Patents
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- WO2010052907A1 WO2010052907A1 PCT/JP2009/005862 JP2009005862W WO2010052907A1 WO 2010052907 A1 WO2010052907 A1 WO 2010052907A1 JP 2009005862 W JP2009005862 W JP 2009005862W WO 2010052907 A1 WO2010052907 A1 WO 2010052907A1
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- ytterbium
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
- H01S3/06—Construction or shape of active medium
- H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
- H01S3/067—Fibre lasers
- H01S3/06708—Constructional details of the fibre, e.g. compositions, cross-section, shape or tapering
- H01S3/06716—Fibre compositions or doping with active elements
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/14—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range characterised by the material used as the active medium
- H01S3/16—Solid materials
- H01S3/1601—Solid materials characterised by an active (lasing) ion
- H01S3/1603—Solid materials characterised by an active (lasing) ion rare earth
- H01S3/1618—Solid materials characterised by an active (lasing) ion rare earth ytterbium
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/14—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range characterised by the material used as the active medium
- H01S3/16—Solid materials
- H01S3/1691—Solid materials characterised by additives / sensitisers / promoters as further dopants
- H01S3/1693—Solid materials characterised by additives / sensitisers / promoters as further dopants aluminium
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/14—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range characterised by the material used as the active medium
- H01S3/16—Solid materials
- H01S3/17—Solid materials amorphous, e.g. glass
- H01S3/175—Solid materials amorphous, e.g. glass phosphate glass
Definitions
- the present invention relates to an ytterbium-doped optical fiber for optical amplification to which ytterbium is added. More specifically, the present invention relates to an ytterbium-doped optical fiber that suppresses output reduction and nonlinear optical effects called photodarkening.
- This application claims priority based on Japanese Patent Application No. 2008-283165 filed in Japan on November 04, 2008, the contents of which are incorporated herein by reference.
- An optical amplification fiber has a configuration in which a rare earth element is added to the core and / or clad of an optical fiber having an axially symmetric waveguide structure, and is used as a photoactive medium such as a fiber amplifier or a fiber laser.
- a fiber laser using a Yb-doped optical fiber containing ytterbium (Yb) as a rare earth element as an optical fiber for optical amplification has good beam quality and high output power can be obtained.
- this fiber laser has an oscillation wavelength of the output light in the vicinity of 1 ⁇ m, which is almost the same as Nd-YAG, which is one of existing high-power lasers. Therefore, the Yb-doped optical fiber is expected to be put to practical use as a laser medium for a high-power light source for material processing applications such as welding, marking, and cutting.
- photodarkening In fiber-type optical amplifiers and fiber lasers, a phenomenon called photodarkening is known. This is a phenomenon in which transmission loss of an optical fiber caused by pumping light or signal light propagating in the fiber increases. When such transmission loss increases, the gain of the rare earth-doped optical fiber that is an amplification medium decreases. In other words, the output of the fiber-type optical amplifier or the fiber laser decreases with time, which causes a problem in terms of reliability.
- Non-Patent Document 1 Various techniques for suppressing photodarkening have been disclosed so far. For example, a technique for suppressing photodarkening by applying a special manufacturing method called DND (Direct Nanoparticle Deposition) has been disclosed (see Non-Patent Document 1, for example).
- DND Direct Nanoparticle Deposition
- Non-Patent Document 2 a technique for suppressing photodarkening by adding aluminum at a high concentration to an optical fiber is disclosed (for example, see Non-Patent Document 2). Furthermore, a technique for suppressing photodarkening by adding phosphorus at a high concentration to an optical fiber is disclosed (for example, see Non-Patent Document 3).
- the refractive index of silica glass It is disclosed that an increase in the refractive index of the core can be suppressed by co-adding aluminum oxide (Al 2 O 3 ) and diphosphorus pentoxide (P 2 O 5 ) to a base material made of silica glass (SiO 2 ). (For example, see Non-Patent Documents 4 and 5). In particular, it is disclosed that the closer the addition concentration (mol%) of aluminum oxide and diphosphorus pentoxide is to the equivalent, the closer to the refractive index of pure silicon dioxide.
- Patent Document 1 discloses an optical fiber in which a rare earth element, germanium, aluminum, and phosphorus are added to the core of the optical fiber. This Patent Document 1 discloses that by adding these elements to the core, the refractive index difference between the core and the clad is reduced, and recrystallization of the rare earth element is suppressed.
- Non-Patent Document 1 Although photodarkening can surely be suppressed, this technique cannot sufficiently perform dehydration in principle. Therefore, there is a problem that transmission loss due to the hydroxyl group is large. Furthermore, it is difficult to increase the size of the base material, and the yield is low. Therefore, this is a disadvantageous method for reducing the manufacturing cost of the optical fiber.
- Non-Patent Document 2 it is necessary to add a large amount of aluminum in order to sufficiently suppress photodarkening. As a result, there has been a problem that the refractive index of the core of the optical fiber becomes high.
- Rare earth doped optical fibers used for fiber type optical amplifiers and fiber lasers are generally used under conditions of single mode propagation or minority mode propagation. Therefore, when the refractive index of the core is high, the core diameter must be relatively small. When the core diameter is small, the effective core cross-sectional area (A eff ) of the optical fiber is small, so that the power density of the propagating light is high and the nonlinear optical effect is likely to appear. That is, there is a problem that wavelength conversion due to the nonlinear optical effect occurs and desired output light cannot be obtained.
- Non-Patent Document 3 it is necessary to add a large amount of phosphorus in order to suppress photodarkening.
- phosphorus is an additive that increases the refractive index, the refractive index of the core is increased. Therefore, when the optical fiber obtained by this method is used to transmit light by single mode propagation or minority mode propagation, there is a problem that the nonlinear optical effect as described above is easily exhibited.
- Non-Patent Documents 4 and 5 the refractive index of an optical fiber containing aluminum and phosphorus and containing silica glass as a main component is studied in detail. However, the refractive index of an optical fiber containing ytterbium, aluminum and phosphorus and containing silica glass as a main component has not been studied. On the other hand, optical fibers in which ytterbium and other rare earth elements are co-doped in the core are known to be useful for fiber-type optical amplifier applications and fiber laser applications.
- Patent Document 1 there is no description regarding suppression of photodarkening, and there is a possibility that photodarkening cannot be sufficiently suppressed only by adding the above elements to the core in the concentration range described in Patent Document 1.
- the present invention has been made in view of the above circumstances, and an object thereof is to provide an ytterbium-doped optical fiber that can suppress photodarkening and suppress a nonlinear optical effect by suppressing an increase in the refractive index of the core. To do.
- the present invention employs the following means in order to solve the above problems and achieve the object.
- the ytterbium-doped optical fiber of the present invention comprises a core containing at least ytterbium, aluminum, and phosphorus, and a clad surrounding the core, and the molar concentration of phosphorus in the core converted to diphosphorus pentoxide;
- the aluminum oxide equivalent molar concentration of the aluminum in the core is the same, and the ratio of the phosphorus pentoxide equivalent molar concentration of the phosphorus in the core to the ytterbium oxide equivalent molar concentration of the ytterbium in the core is 10
- the relative refractive index difference between the core and the clad is 0.05% or more and 0.30% or less.
- the core and the clad are made of glass based on silica glass.
- the ⁇ preferably satisfies the relationship 0.05 ⁇ ⁇ ⁇ 0.5 ⁇ 0.30.
- the ytterbium-doped optical fiber of the present invention includes a core containing at least ytterbium, aluminum, and phosphorus, and a clad surrounding the core.
- the phosphorous pentoxide conversion molar concentration ratio of the phosphorus is 10 or more and 30 or less, the relative refractive index difference between the core and the cladding is 0.05% or more and 0.30% or less, and the oxidation
- ⁇ is the molar concentration in terms of ytterbium
- ⁇ is the molar concentration in terms of aluminum oxide of the aluminum in the core
- ⁇ is the molar concentration in terms of diphosphorus pentoxide
- the ytterbium-doped optical fiber of the present invention includes a core containing at least ytterbium, aluminum, and phosphorus, and a cladding that surrounds the core.
- the phosphorous pentoxide conversion molar concentration ratio of the phosphorus is 10 or more and 30 or less, the relative refractive index difference between the core and the cladding is 0.05% or more and 0.30% or less, and the oxidation
- ⁇ is the molar concentration in terms of ytterbium
- ⁇ is the molar concentration in terms of aluminum oxide of the aluminum in the core
- ⁇ is the molar concentration in terms of diphosphorus pentoxide
- ⁇ and ⁇ satisfy the relationship of 0.56 ⁇ ( ⁇ / ⁇ ) ⁇ 1.
- the core does not contain germanium.
- the ytterbium-doped optical fiber of the present invention comprises a core containing at least ytterbium, aluminum, and phosphorus, and a clad surrounding the core.
- the molar concentration of phosphorus in the core in terms of diphosphorus pentoxide
- the molar concentration of aluminum in terms of aluminum oxide is the same, and the ratio of the molar concentration of phosphorus in the core converted to diphosphorus pentoxide to the molar concentration in terms of ytterbium oxide of the ytterbium in the core is 10 or more and 30
- the relative refractive index difference between the core and the clad is 0.05% or more and 0.30% or less. Therefore, it is possible to realize an optical fiber that can suppress the photodarkening and the background loss, and suppress the nonlinear optical effect by suppressing the increase in the refractive index of the core.
- 4 is a graph showing the relationship between the molar concentration of diphosphorus pentoxide in the core and the relative refractive index difference between the core and the clad in the Yb-doped optical fiber.
- 5 is a graph showing the relationship between the molar concentration of ytterbium oxide and the relative refractive index difference between the core and the clad in the Yb-doped optical fiber. It is a graph which shows the relationship between the ratio of the molar concentration of diphosphorus pentoxide with respect to the molar concentration of ytterbium oxide in the core of a Yb addition optical fiber, and the loss increase amount by photodarkening.
- the concentration of the additive component shown in the unit of “mol%” is an average value unless otherwise specified in an optical fiber having a refractive index distribution.
- Core diameter refers to “a diameter having a relative refractive index difference of 1 / e of the maximum relative refractive index difference of the core”.
- the ytterbium-doped optical fiber of the present invention (hereinafter abbreviated as “Yb-doped optical fiber”) includes a core and a cladding surrounding the core, and the core contains at least ytterbium (Yb), aluminum (Al), and phosphorus (P). is doing. Then, the phosphorus pentoxide (P 2 O 5 ) of phosphorus in the core with respect to the ytterbium oxide (Yb 2 O 3 ) equivalent molar concentration of ytterbium in the core (hereinafter sometimes simply referred to as ytterbium oxide concentration).
- Yb-doped optical fiber includes a core and a cladding surrounding the core, and the core contains at least ytterbium (Yb), aluminum (Al), and phosphorus (P). is doing. Then, the phosphorus pentoxide (P 2 O 5 ) of phosphorus in the core with respect to the ytterbium oxide (Yb
- the ratio of the converted molar concentration (hereinafter sometimes simply referred to as the concentration of diphosphorus pentoxide) is 10 or more and 30 or less.
- the relative refractive index difference between the core and the clad is 0.05% or more and 0.30% or less.
- Ytterbium is a dopant having an optical amplification effect.
- Aluminum is a dopant having an action of increasing the refractive index and an action of suppressing crystallization of silica glass.
- Phosphorus is a dopant having a photodarkening suppressing action and a refractive index raising action.
- ⁇ ⁇ Phosphorus in the core has a photodarkening-inhibiting action.
- the silica glass is crystallized. Therefore, this optical fiber cannot be used as an amplification optical fiber.
- crystallization of silica glass can be suppressed even when the refractive index of the core is set to a desired low value while suppressing photodarkening. It is presumed that aluminum has an action of suppressing crystallization of silica glass because ytterbium and phosphorus are dispersed in silica glass.
- photodarkening can be suppressed by setting the ratio of the molar concentration of phosphorus in the core converted to diphosphorus pentoxide to the molar concentration of ytterbium in the core converted to ytterbium oxide.
- the ratio of the molar concentration of phosphorus in the core converted to diphosphorus pentoxide with respect to the molar concentration of ytterbium oxide in the core is preferably 10 or more and 30 or less, and 15 or more and 20 or less. It is more preferable that If the molar concentration ratio is less than 10, the loss increase due to photodarkening increases rapidly. On the other hand, when the molar concentration ratio exceeds 30, the background loss value increases rapidly. In general, when the background loss value becomes large, when a Yb-doped optical fiber is applied to a fiber laser, the energy conversion efficiency of the fiber laser decreases.
- the molar concentration of ytterbium oxide in the core is ⁇
- the molar concentration of aluminum in the core in terms of aluminum oxide is ⁇
- the phosphorus concentration of diphosphorus pentoxide in the core is ⁇ .
- the relationship of ⁇ ⁇ 0.5 ⁇ 0.15 is satisfied.
- “0.5” is a contribution rate (change rate) that ytterbium oxide per mol% gives to an increase in the refractive index of the core.
- Ytterbium oxide increases the refractive index of the core in proportion to the molar concentration.
- ⁇ ⁇ 0.5 is smaller than 0.05, that is, when the refractive index difference between the core and the clad is smaller than 0.05, bending loss and loss due to external stress to the optical fiber are large, which is not practical.
- ⁇ ⁇ 0.5 is larger than 0.30, that is, when the refractive index difference between the core and the clad is larger than 0.30, wavelength conversion occurs due to the manifestation of nonlinear optical effects typified by stimulated Raman scattering. It is easy to obtain desired output light.
- the molar concentration of ytterbium oxide in the core is ⁇
- the molar concentration of aluminum oxide in the core is ⁇
- the molar concentration of diphosphorus pentoxide in the core is ⁇
- ⁇ > ⁇ may be satisfied.
- the molar concentration of aluminum oxide in the core ( ⁇ ) is higher than the molar concentration of diphosphorus pentoxide ( ⁇ ) in the core ( ⁇ > ⁇ )
- the molar concentration of the core is proportional to the molar concentration of excess aluminum oxide.
- Refractive index increases.
- Ytterbium oxide also increases the refractive index of the core. Therefore, an increase in the refractive index of the core due to excess aluminum oxide and ytterbium oxide is additive. Therefore, ⁇ , ⁇ , and ⁇ can satisfy the relationship 0.05 ⁇ ( ⁇ ) ⁇ 0.19 + ⁇ ⁇ 0.5 ⁇ 0.30 when ⁇ > ⁇ . A high effect of suppressing photodarkening while suppressing crystallization is obtained.
- ⁇ ⁇ may be satisfied, where ⁇ is the molar concentration of ytterbium oxide in the core, ⁇ is the molar concentration of aluminum oxide in the core, and ⁇ is the molar concentration of diphosphorus pentoxide in the core.
- ⁇ is the molar concentration of ytterbium oxide in the core
- ⁇ is the molar concentration of aluminum oxide in the core
- ⁇ is the molar concentration of diphosphorus pentoxide in the core.
- 0.5 is the contribution rate (change rate) of ytterbium oxide per mol% to the increase in the refractive index of the core
- 0.04 is dipentapentoxide per mol%.
- Phosphorus is a contribution rate (rate of change) that contributes to an increase in the refractive index of the core.
- ⁇ , ⁇ , and ⁇ can satisfy the relationship of 0.05 ⁇ ( ⁇ ) ⁇ 0.04 + ⁇ ⁇ 0.5 ⁇ 0.30 when ⁇ ⁇ .
- ⁇ and ⁇ further satisfy the relationship of 0.56 ⁇ ( ⁇ / ⁇ ) ⁇ 1.
- ( ⁇ / ⁇ ) is smaller than 0.56, the relative refractive index difference between the core and the clad may be larger than 0.30, and as described above, desired output light cannot be obtained.
- the core and the clad are preferably made of glass based on silica glass (SiO 2 ).
- Silica glass is not only widely used in general transmission optical fibers, but also can reduce transmission loss and is advantageous for amplifying light with high efficiency.
- the refractive index distribution of the core is not particularly limited, and may be adjusted as appropriate according to the purpose.
- the refractive index distribution of the core may be, for example, a single peak step type, or any known refractive index distribution such as a bell shape, a concave shape, a dual shape, a segment core, a double concave shape, and a W shape.
- the refractive indexes of the core and the clad are preferable to adjust the refractive indexes of the core and the clad in consideration of the structure of the Yb-doped optical fiber, a desired relative refractive index difference, and the like.
- the refractive index of the core is preferably higher than the refractive index of the cladding.
- the relative refractive index difference between the core and the clad is preferably 0.05 to 0.30%, and more preferably 0.08 to 0.20%. If the relative refractive index difference between the core and the clad is less than 0.05%, a sufficient effect of confining light in the optical fiber cannot be obtained. Therefore, if the optical fiber is bent or a lateral pressure is applied to the optical fiber, light cannot be stably propagated. On the other hand, when the relative refractive index difference between the core and the clad exceeds 0.30%, the core diameter is reduced when the optical fiber is substantially used in a single mode condition or when the optical fiber is used with a small number of modes. It becomes smaller and the power density of light becomes higher.
- the "relative refractive index difference between the core and the clad” refers to a refractive index of the core n 1, the refractive index of the cladding in the case of the n 0, the formula: (n 1 -n 0) / n 1 ⁇ 100 Is a value calculated by.
- the core diameter is preferably set as appropriate according to the refractive index of the core, but is usually preferably 10 to 40 ⁇ m, and more preferably 20 to 30 ⁇ m.
- the Yb-doped optical fiber of the present invention can be manufactured by a known method except that a predetermined amount of ytterbium, aluminum and phosphorus is added to the core.
- it can be manufactured by preparing a fiber preform by the MCVD method, VAD method, etc., spinning the fiber preform so as to have a desired outer diameter, and forming a protective coating layer on the outer periphery with a UV curable resin or the like.
- a double clad fiber can also be produced by coating the first UV coating layer with a resin having a refractive index lower than that of silica glass.
- Ytterbium can be added to the soot by a liquid immersion method or a method of spraying liquid droplets in the fiber preform manufacturing process. Further, for example, when the clad shape is non-circular, the fiber preform after the addition of ytterbium is cut off to a desired shape and then spun. Also, for example, when providing a stress applying portion in the clad, in the fiber preform after addition of ytterbium, a hole is provided in the central axis direction, and the inner surface is preferably ground and polished to be mirror-finished here. A stress applying member made of B 2 O 3 —SiO 2 glass produced by MCVD or the like may be inserted and then spun.
- a Yb-doped optical fiber that is excellent in the effect of suppressing photodarkening and obtains desired high-output light by applying a known method such as the MCVD method or the VAD method.
- the size of the fiber preform used at the time of manufacture is not limited. Therefore, Yb-doped optical fibers having excellent characteristics as described above can be provided at low cost and in large quantities. Further, by using such an optical fiber as an optical amplifying medium, it is possible to provide a fiber laser and a fiber amplifier with low optical output and good optical characteristics at low cost.
- the core is basically composed of silica glass containing ytterbium oxide (Yb 2 O 3 ), aluminum oxide (Al 2 O 3 ), and diphosphorus pentoxide (P 2 O 5 ).
- the clad is made of silica glass.
- Example 1 A Yb-doped optical fiber was produced.
- the produced Yb-doped optical fiber is a single clad fiber, in which a clad is provided on the outer periphery of the core, and a protective coating layer is provided on the outer periphery of the clad.
- the fiber preform was produced by the MCVD method. Ytterbium was added by the immersion method. The fiber preform was spun until the glass outer diameter was about 125 ⁇ m, and a protective coating layer was provided on the outer periphery.
- the molar concentration of ytterbium oxide (Yb 2 O 3 ) in the core is constant at 0.20 mol%, and the molar concentration of aluminum oxide (Al 2 O 3 ) in the core is constant at 2.5 mol%.
- the molar concentration of diphosphorus oxide (P 2 O 5 ) was changed, the change in the relative refractive index difference ( ⁇ ) between the core and the clad of the Yb-doped optical fiber was determined. The results are shown in FIG.
- the minimum value of the relative refractive index difference was 0.10%. Also, the absolute value of the slope of the straight line representing the change in the relative refractive index difference in the region where the aluminum oxide is relatively excessive in the core (when the molar concentration of diphosphorus pentoxide is 2.5 mol% or less) is 0.190. there were. Furthermore, the absolute value of the slope of the straight line representing the change in the relative refractive index difference in a region where diphosphorus pentoxide is relatively excessive in the core (when the molar concentration of diphosphorus pentoxide is 2.5 mol% or more) is 0. 040.
- Example 2 Except that the molar concentration of ytterbium oxide in the core was constant at 0.10 mol% and the molar concentration of aluminum oxide in the core was constant at 2.5 mol%, the same procedure as in Experimental Example 1 was carried out. When the molar concentration of phosphorus was changed, a change in the relative refractive index difference between the core and the clad of the Yb-doped optical fiber was obtained, and a graph (not shown) similar to FIG. 1 was created. From this result, it was found that the relative refractive index difference between the core and the clad was minimized when the molar concentration of diphosphorus pentoxide was approximately equal to the molar concentration of aluminum oxide, which was around 2.5 mol%.
- the change in the relative refractive index difference can be approximated by a straight line in a region having a concentration higher than the molar concentration of diphosphorus pentoxide where the relative refractive index difference is minimized.
- the change in the relative refractive index difference can be approximated by a straight line even in a region having a concentration lower than the molar concentration of diphosphorus pentoxide where the relative refractive index difference is minimized.
- the minimum value of the relative refractive index difference was 0.06%.
- the absolute value of the slope of the straight line representing the change in the relative refractive index difference in the region where the aluminum oxide is relatively excessive in the core is 0.186. there were.
- the absolute value of the slope of the straight line representing the change in the relative refractive index difference in a region where diphosphorus pentoxide is relatively excessive in the core is 0. 041.
- Example 3 Except that the molar concentration of ytterbium oxide in the core was constant at 0.40 mol% and the molar concentration of aluminum oxide in the core was constant at 5.0 mol%, the same procedure as in Experimental Example 1 was carried out. When the molar concentration of phosphorus was changed, a change in the relative refractive index difference between the core and the clad of the Yb-doped optical fiber was obtained, and a graph (not shown) similar to FIG. 1 was created. From this result, it was found that the relative refractive index difference between the core and the clad was minimized when the molar concentration of diphosphorus pentoxide was approximately equal to the molar concentration of aluminum oxide, ie, around 5.0 mol%.
- the change in the relative refractive index difference can be approximated by a straight line in a region having a concentration higher than the molar concentration of diphosphorus pentoxide where the relative refractive index difference is minimized.
- the change in the relative refractive index difference can be approximated by a straight line even in a region having a concentration lower than the molar concentration of diphosphorus pentoxide where the relative refractive index difference is minimized.
- the minimum value of the relative refractive index difference was 0.23%.
- the absolute value of the slope of the straight line representing the change in the relative refractive index difference in the region where the aluminum oxide is relatively excessive in the core (when the molar concentration of diphosphorus pentoxide is 5.0 mol% or less) is 0.190. there were. Further, the absolute value of the slope of the straight line representing the change in the relative refractive index difference in the region where the phosphorous pentoxide is relatively excessive in the core (when the molar concentration of phosphorous pentoxide is 5.0 mol% or more) is 0. 042.
- Example 4 In the same manner as in Experimental Example 1, except that the molar concentration of ytterbium oxide in the core was constant at 0.70 mol% and the molar concentration of aluminum oxide in the core was constant at 5.0 mol%, dipentapentoxide in the core. When the molar concentration of phosphorus was changed, a change in the relative refractive index difference between the core and the clad of the Yb-doped optical fiber was obtained, and a graph (not shown) similar to FIG. 1 was created.
- the relative refractive index difference between the core and the clad was minimized when the molar concentration of diphosphorus pentoxide was approximately equal to the molar concentration of aluminum oxide, ie, around 5.0 mol%. It was also found that the change in the relative refractive index difference can be approximated by a straight line in a region having a concentration higher than the molar concentration of diphosphorus pentoxide where the relative refractive index difference is minimized. Similarly, it was found that the change in the relative refractive index difference can be approximated by a straight line even in a region having a concentration lower than the molar concentration of diphosphorus pentoxide where the relative refractive index difference is minimized.
- the minimum value of the relative refractive index difference was 0.35%.
- the absolute value of the slope of the straight line representing the change in the relative refractive index difference in the region where the aluminum oxide is relatively excessive in the core is 0.193. there were.
- the absolute value of the slope of the straight line representing the change in the relative refractive index difference in the region where the phosphorous pentoxide is relatively excessive in the core is 0. 037.
- the ratio of the concentration of diphosphorus pentoxide in the core to the concentration of aluminum oxide in the core is 0.56 or more and less than 1, the relative refractive index difference between the core and the cladding in this optical fiber is 0. It was found to be 1 or more and 0.3 or less. Further, the average absolute value of the slope of the straight line representing the change in the relative refractive index difference in the region where the aluminum oxide was relatively excessive in the core, calculated in Experimental Examples 1 to 4, was 0.19. Further, the average value of the absolute values of the slopes of the straight lines representing the change in the relative refractive index difference in the region where the diphosphorus pentoxide was relatively excessive in the core, calculated in Experimental Examples 1 to 4, was 0.04.
- FIG. 2 is a graph showing the relationship between the molar concentration of ytterbium oxide and the relative refractive index difference between the core and the clad based on the results shown in Table 1.
- the graph shown in FIG. 2 could be fitted as a straight line passing through the origin, and the slope of the straight line was 0.5% / mol%. Therefore, when this value is converted per 1 mol%, ytterbium oxide has a contribution rate (change rate) to the relative refractive index difference between the core and the clad of 0.5% / mol%.
- the molar concentration of ytterbium oxide is involved in the change in the refractive index of the core independent of the molar concentration of aluminum oxide and the molar concentration of diphosphorus pentoxide.
- the contribution ratio (change rate) of the molar concentration of ytterbium oxide to the refractive index of the core was 0.5% / mol%.
- Example 5 A Yb-doped optical fiber was prepared in the same manner as in Experimental Example 1, except that the molar concentration of ytterbium oxide in the core, the molar concentration of aluminum oxide in the core, and the molar concentration of diphosphorus pentoxide in the core were changed. Several types were made. The amount of increase in loss due to photodarkening of the manufactured Yb-doped optical fiber was evaluated by the following method.
- FIG. 3 is a graph showing the relationship between the ratio of the molar concentration of diphosphorus pentoxide in the core to the molar concentration of ytterbium oxide in the core of the Yb-doped optical fiber and the loss increase due to photodarkening. From the results of FIG. 3, it was found that the amount of increase in loss due to photodarkening largely depends on the ratio of the molar concentration of diphosphorus pentoxide in the core to the molar concentration of ytterbium oxide in the core. It was found that when the molar concentration ratio is 10 or more, the loss increase due to photodarkening can be reduced to 0.5 dB or less, and photodarkening can be sufficiently suppressed.
- the increase in loss due to photodarkening is almost zero when the molar concentration ratio is 20 or more.
- the amount of increase in loss due to photodarkening is preferably ideally zero.
- the loss increase of about 0.5 dB is substantially at a level of zero, and there is no practical problem. If the molar concentration ratio is 20 or more, the loss increase due to photodarkening is almost zero, but the molar concentration of diphosphorus pentoxide is high. In this case, the base material is easily broken when the optical fiber base material is manufactured. Accordingly, it becomes difficult to make the concentration distribution of diphosphorus pentoxide in the core uniform, and as a result, it becomes difficult to produce a large base material, which is disadvantageous for cost reduction.
- FIG. 4 is a graph showing the relationship between the ratio of the molar concentration of diphosphorus pentoxide to the molar concentration of ytterbium oxide in the core of the Yb-doped optical fiber and the background loss value at a wavelength of 1200 nm. From the results in FIG. 4, it was found that the background loss value greatly depends on the ratio of the molar concentration of diphosphorus pentoxide in the core to the molar concentration of ytterbium oxide in the core.
- the background loss value increases rapidly.
- the background loss value increases, it is known that when this Yb-doped optical fiber is applied to a fiber laser, the energy conversion efficiency of the fiber laser decreases. Therefore, the smaller the background loss value, the better.
- the background loss value is smaller than a certain level. Therefore, the ratio of the molar concentration of diphosphorus pentoxide in the core to the molar concentration of ytterbium oxide in the core, which is a critical point at which the background loss value changes rapidly, is the background loss value. Is 100 dB / km (wavelength 1200 nm) or less, it is a Yb-doped optical fiber having no practical problem.
- Example 7 In the present invention, for a Yb-doped optical fiber containing ytterbium, aluminum and phosphorus in the core and containing silica glass as a main component, the nonlinear optical effect is suppressed by suppressing photodarkening and suppressing the increase in the refractive index of the core.
- the present inventors have found a configuration of an optical fiber that achieves both suppression. It is ideal to completely suppress photodarkening and nonlinear optical effects, that is, to zero, but in reality it is difficult to completely suppress them. Further, in order to completely suppress the photodarkening and the nonlinear optical effect, there is a problem that the configuration of the manufacturing apparatus becomes complicated and the manufacturing cost increases.
- a realistic numerical value is set as the “target value”.
- the target value for suppression of photodarkening was such that the loss increase due to photodarkening was 0.5 dB or less.
- the target value for suppressing the nonlinear optical effect is such that the relative refractive index difference between the core and the clad is 0.05% or more and 0.30% or less.
- the reason why the target value of loss increase due to photodarkening is set to 0.5 dB or less is that when a fiber laser using an optical fiber that has achieved this target value is continuously operated for 1000 hours, the output optical power after 1000 hours is the initial value. This is because the output light power was 70% or more. Although the required specifications for the output decrease are different for each product or each user, if the output of 70% or more of the initial value is maintained 1000 hours after the start of operation, it is an acceptable level as an industrial product.
- the reason why the relative refractive index difference between the core and the clad is 0.05% or more and 0.30% or less in the suppression of the nonlinear optical effect is that the fiber laser using the optical fiber that has achieved this target value. This is because the output light power and the beam quality of the output light are improved.
- the relative refractive index difference between the core and the clad is too small, for example, less than 0.05%, the light confinement effect is low. For this reason, the propagation state of light becomes unstable with respect to external stress caused by bending of the optical fiber or application of lateral pressure to the optical fiber, or stress caused by volume change due to temperature change, so that it is determined to be impractical.
- the effective core cross-sectional area (A eff ) is decreased, and the influence of the nonlinear optical effect is increased.
- a eff effective core cross-sectional area
- the spectrum width of the laser output light is widened, and the beam quality may be lowered, or the output may be lowered due to the Raman light generation efficiency.
- a wavelength filter is disposed on the exit side of the amplification optical fiber to cut the stimulated Raman scattered light that appears on the longer wavelength side than the laser output light.
- the output light power cannot be sufficiently obtained.
- a phenomenon occurs in which an optical fiber that is an amplification medium is broken due to optical damage.
- the threshold value for optical damage depends greatly on the power density of light. Therefore, in order to avoid destruction, it is necessary to increase the effective core area, that is, to maintain the relative refractive index difference between the core and the cladding low.
- the Yb-doped optical fiber is a core having a step profile with a core diameter of 30 ⁇ m, the cladding diameter is 400 ⁇ m, the outer periphery is coated with a low refractive index resin, and the outer periphery is coated with a high refractive index resin.
- a coated double clad fiber was obtained.
- This double clad fiber was used as an amplification optical fiber, and its entire length was wound around a cylindrical member having a diameter of 300 mm. Pulsed seed light having a wavelength of 1060 nm and an average power of 1 W was incident on the core of the amplification optical fiber wound around a cylindrical member. Further, excitation light having a wavelength of 915 nm and power of 50 W was incident on the clad to amplify light having a wavelength of 1060 nm. And the output optical power of the light of wavelength 1060nm amplified with the optical fiber for amplification was measured. Here, the output light power of the wavelength 1060 nm band (including stimulated Raman scattering light) output from the amplification optical fiber was measured without using the wavelength filter.
- Pulsed seed light having a wavelength of 1060 nm and an average power of 1 W was incident on the core of the amplification optical fiber wound around a cylindrical member. Further, excitation light having a wavelength of 915 nm and power of 50 W was incident on
- FIG. 5 shows the relationship between the relative refractive index difference between the core and the clad of the amplification optical fiber and the laser output light power obtained by this measurement.
- the horizontal axis represents the relative refractive index difference between the core and the cladding of the amplification optical fiber
- the vertical axis represents the laser output light power on the exit side of the amplification optical fiber.
- the target value in which the relative refractive index difference between the core and the clad is 0.05% or more is considered appropriate.
- the Yb-doped optical fiber is a core having a step profile with a core diameter of 30 ⁇ m, the cladding diameter is 400 ⁇ m, the outer periphery is coated with a low refractive index resin, and the outer periphery is coated with a high refractive index resin.
- a coated double clad fiber was obtained.
- This double clad fiber was used as an amplification optical fiber, and pulsed seed light having a wavelength of 1060 nm and an average power of 1 W was incident on the core of the amplification optical fiber. Further, excitation light having a wavelength of 915 nm and power of 50 W was incident on the clad to amplify light having a wavelength of 1060 nm.
- a wavelength filter that cuts light having a wavelength of 1100 nm or more was disposed on the exit side of the amplification optical fiber.
- the second-order stimulated Raman scattered light is generated in the vicinity of a wavelength of 1160 nm.
- the purpose of cutting light having a wavelength of 1100 nm or more is to suppress the generation of stimulated Raman scattering light and to suppress deterioration of beam quality.
- FIG. 6 shows the relationship between the relative refractive index difference between the core and the clad of the amplification optical fiber and the laser output light power obtained by this measurement.
- the horizontal axis represents the relative refractive index difference between the core and the clad of the amplification optical fiber
- the vertical axis represents the laser output light power that has passed through the wavelength filter.
- the output optical power can be increased by setting the relative refractive index difference between the core and the clad of the amplification optical fiber to be 0.30% or less.
- the target value in which the relative refractive index difference between the core and the clad is 0.30% or less is considered to be appropriate.
- Example 10 A Yb-doped optical fiber containing ytterbium, aluminum, phosphorus, and germanium in the core was fabricated. It was produced in the same manner as in Experimental Example 1 except that 1 mol% of germanium dioxide (GeO 2 ) was added. Similarly to Experimental Example 1, the molar concentration of ytterbium oxide in the core was constant at 0.20 mol%, the molar concentration of aluminum oxide in the core was constant at 2.5 mol%, and the diphosphorus pentoxide in the core The change in the relative refractive index difference between the core and the clad of the Yb-doped optical fiber was determined by changing the molar concentration. The obtained results are shown in Table 1 and FIG. As shown in FIG.
- the refractive index of the core is increased by containing germanium dioxide in the core.
- silica glass containing germanium has a refractive index increase of about 0.1% per mol% of germanium dioxide.
- the value of the relative refractive index difference was increased by about 0.1% under any condition of the concentration of diphosphorus pentoxide. This is considered to be caused by an increase in refractive index that is generally known as a result of containing germanium.
- the relative refractive index difference between the core and the clad is 0.30% or less.
- the range of the molar concentration of diphosphorus pentoxide in which the relative refractive index difference is 0.30% or less was a wide range of 1.5 to 7.5 mol%.
- the result shown in FIG. 7 is narrowed to 1.8 to 5.0 mol%. Therefore, it can be said that it is preferable not to contain germanium in the core from the viewpoint of the relative refractive index difference.
- germanium dioxide should not be added as much as possible unless there is a special purpose of adding germanium dioxide, such as providing a grating. This is because, as described above, the addition of germanium dioxide does not have a special effect on photodarkening, but the addition of germanium dioxide increases the refractive index. This is because the beam quality is deteriorated. Similarly, when the refractive index increasing agent (Ti or the like) other than Al and P is not particularly effective in adding the element, it is desirable not to add the refractive index increasing element as much as possible.
- the refractive index increasing agent Ti or the like
- the loss increase due to photodarkening is 0.5 dB or less, and the relative refractive index difference between the core and the cladding is 0.05% or more and 0.30% or less. If it is possible to achieve both, it is considered to be a realistic optical fiber that can suppress photodarkening and nonlinear optical effects. At this time, it is preferable that the core does not contain a dopant that increases the refractive index like germanium.
- the present invention can be used as a laser medium for a high-power light source for material processing applications such as welding, marking, and cutting.
Abstract
Description
本願は、2008年11月04日に、日本国に出願された特願2008-283165号に基づき優先権を主張し、その内容をここに援用する。
例えば、DND(Direct Nanoparticle Deposition)と呼ばれる特殊な製造方法を適用することにより、フォトダークニングを抑制する手法が開示されている(例えば、非特許文献1参照)。
さらに、光ファイバにリンを高濃度に添加することによって、フォトダークニングを抑制する手法が開示されている(例えば、非特許文献3参照)。
シリカガラス(SiO2)からなる母材に、酸化アルミニウム(Al2O3)と五酸化二リン(P2O5)を共添加することにより、コアの屈折率上昇を抑制できることが開示されている(例えば、非特許文献4、5参照)。特に、酸化アルミニウムと五酸化二リンの添加濃度(mol%)が等量に近付くほど、純粋な二酸化ケイ素の屈折率に近付くことが開示されている。
一方、コアにイッテルビウムと他の希土類元素を共添加した光ファイバは、ファイバ型光増幅器用途やファイバレーザ用途に有用であることが知られている。
(1)本発明のイッテルビウム添加光ファイバは、イッテルビウム、アルミニウムおよびリンを少なくとも含有するコアと、このコアを囲むクラッドと、を備え、前記コア中の前記リンの五酸化二リン換算モル濃度と、前記コア中の前記アルミニウムの酸化アルミニウム換算モル濃度と、が同じであり、前記コア中の前記イッテルビウムの酸化イッテルビウム換算モル濃度に対する前記コア中の前記リンの五酸化二リン換算モル濃度の比が10以上かつ30以下であり、前記コアと前記クラッドとの比屈折率差が0.05%以上かつ0.30%以下である。
この形態は、発明の趣旨をより良く理解させるために具体的に説明するものであり、特に指定のない限り、本発明を限定するものではない。
以下、「モル%」の単位で示す添加成分の濃度は、屈折率分布を有する光ファイバにおいては、特に断りのない限り平均値である。
「コア径」とは、「コアの最大比屈折率差の1/eの比屈折率差を有する径」のことを指す。
本発明のイッテルビウム添加光ファイバ(以下、「Yb添加光ファイバ」と略記する)は、コアおよびそれを囲むクラッドを備え、コアに少なくともイッテルビウム(Yb)、アルミニウム(Al)およびリン(P)を含有している。そして、コア中のイッテルビウムの酸化イッテルビウム(Yb2O3)換算モル濃度(以下、単に酸化イッテルビウムの濃度と略記する場合がある)に対する、コア中のリンの五酸化二リン(P2O5)換算モル濃度(以下、単に五酸化二リンの濃度と略記する場合がある)の比が10以上かつ30以下である。コアとクラッドとの比屈折率差が0.05%以上かつ0.30%以下である。
アルミニウムは、屈折率上昇作用およびシリカガラスの結晶化抑制作用を有するドーパントである。
リンは、フォトダークニング抑制作用および屈折率上昇作用を有するドーパントである。
このモル濃度比が10未満では、急激にフォトダークニングによる損失増加量が増える。
一方、このモル濃度比が30を超えると、急激にバックグラウンド損失値が大きくなる。
一般に、バックグラウンド損失値が大きくなると、Yb添加光ファイバをファイバレーザに適用した場合、そのファイバレーザはエネルギー変換効率が低下する。
上記の関係式において、「0.5」は1mol%当たりの酸化イッテルビウムが、コアの屈折率の上昇に与える寄与率(変化率)である。
したがって、α、βおよびγは、β=γの場合、0.05≦α×0.5≦0.30なる関係を満たすようにすれば、ガラスの結晶化を抑制しつつフォトダークニングを抑制する高い効果が得られる。
α×0.5が0.05より小さくなると、すなわち、コアとクラッドとの屈折率差が0.05より小さくなると、曲げ損失や光ファイバへの外部応力による損失が大きく、実用的ではない。一方、α×0.5が0.30より大きくなると、すなわち、コアとクラッドとの屈折率差が0.30より大きくなると、誘導ラマン散乱に代表される非線形光学効果の発現により波長変換が生じやすく、所望の出力光が得られない。
上記の関係式において、「0.5」は1mol%当たりの酸化イッテルビウムが、コアの屈折率の上昇に与える寄与率(変化率)であり、「0.19」は1mol%当たりの酸化アルミニウムが、コアの屈折率の上昇に与える寄与率(変化率)である。
したがって、α、βおよびγは、β>γの場合、0.05≦(β-γ)×0.19+α×0.5≦0.30なる関係を満たすようにすれば、ガラスの結晶化を抑制しつつフォトダークニングを抑制する高い効果が得られる。
(β-γ)×0.19+α×0.5が0.05より小さくなると、すなわち、コアとクラッドとの屈折率差が0.05より小さくなると、曲げ損失や光ファイバへの外部応力による損失が大きく、実用的ではない。一方、(β-γ)×0.19+α×0.5が0.30より大きくなると、すなわち、コアとクラッドとの屈折率差が0.30より大きくなると、誘導ラマン散乱に代表される非線形光学効果の発現により波長変換が生じやすく、所望の出力光が得られない。
この際、β及びγが、1<(β/γ)≦3なる関係を更に満たすのが好ましい。(β/γ)が3より大きくなると、コアとクラッドとの比屈折率差が0.30より大きくなる場合があり、上述したように所望の出力光が得られなくなる。
上記の関係式において、「0.5」は1mol%当たりの酸化イッテルビウムが、コアの屈折率の上昇に与える寄与率(変化率)であり、「0.04」は1mol%当たりの五酸化二リンが、コアの屈折率の上昇に与える寄与率(変化率)である。
したがって、α、βおよびγは、β<γの場合、0.05≦(γ-β)×0.04+α×0.5≦0.30なる関係を満たすようにすれば、ガラスの結晶化を抑制しつつフォトダークニングを抑制する高い効果が得られる。
(γ-β)×0.04+α×0.5が0.05より小さくなると、すなわち、コアとクラッドとの屈折率差が0.05より小さくなると、曲げ損失や光ファイバへの外部応力による損失が大きく、実用的ではない。一方、(γ-β)×0.04+α×0.5が0.30より大きくなると、すなわち、コアとクラッドとの屈折率差が0.30より大きくなると、誘導ラマン散乱に代表される非線形光学効果の発現により波長変換が生じやすく、所望の出力光が得られない。
この際、β及びγが、0.56≦(β/γ)<1なる関係を更に満たすのが好ましい。(β/γ)が0.56より小さくなると、コアとクラッドとの比屈折率差が0.30より大きくなる場合があり、上述したように所望の出力光が得られなくなる。
例えば、導波する光を閉じ込めるためには、コアの屈折率がクラッドの屈折率よりも高いことが好ましい。
コアとクラッドとの比屈折率差が0.05%未満では、光ファイバにおいて光を閉じ込める十分な効果が得られない。そのため、光ファイバを曲げたり、光ファイバに側圧を加えたりすると、光を安定に伝搬できなくなる。一方、コアとクラッドとの比屈折率差が0.30%を超えると、光ファイバを実質的にシングルモード条件で使用する場合、または、光ファイバを少ないモード数で使用する場合、コア径が小さくなり、光のパワー密度が高くなる。そのため、光によるコアガラスの損傷や光学的非線形現象を抑制する効果が得られ難くなる。これにより、高出力の光が得られ難くなる。
ここで「コアとクラッドとの比屈折率差」とは、コアの屈折率をn1、クラッドの屈折率をn0とした場合に、式:(n1-n0)/n1×100で算出される値である。
例えば、MCVD法、VAD法などでファイバプリフォームを作製し、これを所望の外径となるように紡糸して、その外周上にUV硬化樹脂などで保護被覆層を形成することで製造できる。1層目のUV被覆層にシリカガラスよりも屈折率の低い樹脂をコーティングすることにより、ダブルクラッドファイバを製造することもできる。
イッテルビウムは、ファイバプリフォーム作製過程において、スートに液浸法で添加する手法や、液滴を噴霧する手法で添加できる。
また、例えば、クラッドの形状を非円形状とする場合、イッテルビウム添加後のファイバプリフォームを所望の形状に外削し、これを紡糸すればよい。
また、例えば、クラッド中に応力付与部を設ける場合、イッテルビウム添加後のファイバプリフォームにおいて、その中心軸方向に孔を設け、好ましくはその内表面を研削および研磨して鏡面化した後、ここにMCVD法などで作製したB2O3-SiO2ガラス製の応力付与部材を挿入し、次いで紡糸すればよい。
また、このような光ファイバを光増幅媒体として使用することで、経時に伴う出力低下が抑制され、光学特性が良好なファイバレーザおよびファイバアンプを安価に提供できる。
Yb添加光ファイバを作製した。作製したYb添加光ファイバはシングルクラッドファイバであり、コアの外周上にクラッドが設けられ、クラッドの外周上に保護被覆層が設けられたものである。
ファイバプリフォームは、MCVD法で作製した。また、イッテルビウムは液浸法で添加した。そして、ファイバプリフォームをガラス外径が約125μmになるまで紡糸し、外周上に保護被覆層を設けた。
コア中の酸化イッテルビウム(Yb2O3)のモル濃度を0.20mol%で一定とし、コア中の酸化アルミニウム(Al2O3)のモル濃度を2.5mol%で一定として、コア中の五酸化二リン(P2O5)のモル濃度を変化させた場合、Yb添加光ファイバのコアとクラッドとの比屈折率差(Δ)の変化を求めた。
結果を図1に示す。
これらの結果を表1に示す。
コア中の酸化イッテルビウムのモル濃度を0.10mol%で一定とし、コア中の酸化アルミニウムのモル濃度を2.5mol%で一定とした以外は実験例1と同様にして、コア中の五酸化二リンのモル濃度を変化させた場合、Yb添加光ファイバのコアとクラッドとの比屈折率差の変化を求め、図1と同様のグラフ(図示略)を作成した。
この結果から、五酸化二リンのモル濃度が、酸化アルミニウムのモル濃度とほぼ等しい、2.5mol%近傍である場合、コアとクラッドとの比屈折率差が最小になることが分かった。また、比屈折率差が最小になる五酸化二リンのモル濃度よりも高濃度の領域において、比屈折率差の変化をほぼ直線で近似できることが判明した。同様に、比屈折率差が最小になる五酸化二リンのモル濃度よりも低濃度の領域においても、比屈折率差の変化をほぼ直線で近似できることが判明した。また、比屈折率差の最小値は、0.06%であった。また、コアにおいて相対的に酸化アルミニウムが過剰な領域(五酸化二リンのモル濃度が2.5mol%以下の場合)の比屈折率差の変化を表す直線の傾きの絶対値は0.186であった。さらに、コアにおいて相対的に五酸化二リンが過剰な領域(五酸化二リンのモル濃度が2.5mol%以上の場合)の比屈折率差の変化を表す直線の傾きの絶対値は0.041であった。
これらの結果を表1に示す。
コア中の酸化イッテルビウムのモル濃度を0.40mol%で一定とし、コア中の酸化アルミニウムのモル濃度を5.0mol%で一定とした以外は実験例1と同様にして、コア中の五酸化二リンのモル濃度を変化させた場合、Yb添加光ファイバのコアとクラッドとの比屈折率差の変化を求め、図1と同様のグラフ(図示略)を作成した。
この結果から、五酸化二リンのモル濃度が、酸化アルミニウムのモル濃度とほぼ等しい、5.0mol%近傍である場合、コアとクラッドとの比屈折率差が最小になることが分かった。また、比屈折率差が最小になる五酸化二リンのモル濃度よりも高濃度の領域において、比屈折率差の変化をほぼ直線で近似できることが判明した。同様に、比屈折率差が最小になる五酸化二リンのモル濃度よりも低濃度の領域においても、比屈折率差の変化をほぼ直線で近似できることが判明した。また、比屈折率差の最小値は、0.23%であった。また、コアにおいて相対的に酸化アルミニウムが過剰な領域(五酸化二リンのモル濃度が5.0mol%以下の場合)の比屈折率差の変化を表す直線の傾きの絶対値は0.190であった。さらに、コアにおいて相対的に五酸化二リンが過剰な領域(五酸化二リンのモル濃度が5.0mol%以上の場合)の比屈折率差の変化を表す直線の傾きの絶対値は0.042であった。
これらの結果を表1に示す。
コア中の酸化イッテルビウムのモル濃度を0.70mol%で一定とし、コア中の酸化アルミニウムのモル濃度を5.0mol%で一定とした以外は実験例1と同様にして、コア中の五酸化二リンのモル濃度を変化させた場合、Yb添加光ファイバのコアとクラッドとの比屈折率差の変化を求め、図1と同様のグラフ(図示略)を作成した。
この結果から、五酸化二リンのモル濃度が、酸化アルミニウムのモル濃度とほぼ等しい、5.0mol%近傍である場合、コアとクラッドとの比屈折率差が最小になることが分かった。また、比屈折率差が最小になる五酸化二リンのモル濃度よりも高濃度の領域において、比屈折率差の変化をほぼ直線で近似できることが判明した。同様に、比屈折率差が最小になる五酸化二リンのモル濃度よりも低濃度の領域においても、比屈折率差の変化をほぼ直線で近似できることが判明した。また、比屈折率差の最小値は、0.35%であった。また、コアにおいて相対的に酸化アルミニウムが過剰な領域(五酸化二リンのモル濃度が5.0mol%以下の場合)の比屈折率差の変化を表す直線の傾きの絶対値は0.193であった。さらに、コアにおいて相対的に五酸化二リンが過剰な領域(五酸化二リンのモル濃度が5.0mol%以上の場合)の比屈折率差の変化を表す直線の傾きの絶対値は0.037であった。
これらの結果を表1に示す。
また、実験例1~4において算出した、コアにおいて相対的に酸化アルミニウムが過剰な領域の比屈折率差の変化を表す直線の傾きの絶対値の平均値は0.19であった。また、実験例1~4において算出した、コアにおいて相対的に五酸化二リンが過剰な領域の比屈折率差の変化を表す直線の傾きの絶対値の平均値は0.04であった。以上の結果から、これら直線の傾きの値を1mol%当たりに換算すると、コアにおいて相対的に過剰な酸化アルミニウムが、コアとクラッドとの比屈折率差に対する寄与率(変化率)は0.19%/mol%であり、コアにおいて相対的に過剰な五酸化二リンが、コアとクラッドとの比屈折率差に対する寄与率(変化率)は0.04%/mol%であることが分かった。
図2に示したグラフは、原点を通る直線としてフィッティングでき、その直線の傾きが0.5%/mol%であった。したがって、この値を1mol%当たりに換算すると、酸化イッテルビウムが、コアとクラッドとの比屈折率差に対する寄与率(変化率)は0.5%/mol%である。
また、図2に示したグラフは、酸化アルミニウムのモル濃度を2.5mol%(実験例1、2)または5.0mol%(実験例3、4)とした場合に、酸化イッテルビウムのモル濃度と比屈折率差との関係を示すものである。図2に示したように、異なる2つの酸化アルミニウムのモル濃度における屈折率差をほぼ同じ直線上にプロットできることから、酸化イッテルビウムのモル濃度の比屈折率差に対する寄与は、酸化アルミニウムのモル濃度や五酸化二リンのモル濃度から独立していることが分かった。
コアにイッテルビウム、アルミニウムおよびリンを含有し、シリカガラスを主成分とするYb添加光ファイバでは、五酸化二リンのモル濃度と、酸化アルミニウムのモル濃度とがほぼ等量である場合、コアの屈折率がシリカガラスの屈折率に近付く。
また、コアにおいて相対的に過剰な酸化アルミニウムが、コアとクラッドとの比屈折率差に対する寄与率(変化率)は0.19%/mol%であった。コアにおいて相対的に過剰な五酸化二リンが、コアとクラッドとの比屈折率差に対する寄与率(変化率)は0.04%/mol%であった。
さらに、酸化イッテルビウムのモル濃度は、酸化アルミニウムのモル濃度や五酸化二リンのモル濃度から独立してコアの屈折率の変化に関与する。その酸化イッテルビウムのモル濃度のコアの屈折率に対する寄与率(変化率)は0.5%/mol%であった。
コア中の酸化イッテルビウムのモル濃度、コア中の酸化アルミニウムのモル濃度、および、コア中の五酸化二リンのモル濃度を変化させたこと以外は実験例1と同様にして、Yb添加光ファイバを数種類作製した。
以下の方法により、作製したYb添加光ファイバのフォトダークニングによる損失増加量を評価した。
コアのYb吸収量が340dBとなるような中心軸方向における長さのYb添加光ファイバを使用し、そのコアに、波長976nmの励起光を入射光量が400mWとなるように100分間照射した。そして、波長800nmにおける照射前後の損失の差分を「フォトダークニングによる損失増加量」とした。
図3の結果から、フォトダークニングによる損失増加量は、コア中の酸化イッテルビウムのモル濃度に対する、コア中の五酸化二リンのモル濃度の比に大きく依存することが分かった。このモル濃度の比が10以上であれば、フォトダークニングによる損失増加量を0.5dB以下にでき、フォトダークニングを十分に抑制できることが分かった。一方、このモル濃度の比が10未満では、フォトダークニングによる損失増加量が0.5dBを超える。そのため、このようなYb添加光ファイバを用いたファイバレーザは、長期間レーザ発振させた場合に出力低下が生じ、信頼性の点で問題となる。
実験例5にて作製したYb添加光ファイバについて、波長1200nmにおける損失波長特性を測定した。
図4は、Yb添加光ファイバのコア中の酸化イッテルビウムのモル濃度に対する五酸化二リンのモル濃度の比と、波長1200nmにおけるバックグラウンド損失値との関係を示すグラフである。
図4の結果から、バックグラウンド損失値は、コア中の酸化イッテルビウムのモル濃度に対する、コア中の五酸化二リンのモル濃度の比に大きく依存していることが分かった。また、このモル濃度の比が30以下であれば、バックグラウンド損失値は100dB/km以下であり、損失値の低減効果が十分に得られることが分かった。一方、このモル濃度の比が30を超えると、バックグラウンド損失値が100dB/kmを超えてしまう。そのため、このYb添加光ファイバをファイバレーザに適用した場合、そのファイバレーザのエネルギー変換効率が著しく低下する。
そこで、バックグラウンド損失値が急激に変化する臨界点である、コア中の酸化イッテルビウムのモル濃度に対する、コア中の五酸化二リンのモル濃度の比が30以下の領域、すなわち、バックグラウンド損失値が100dB/km(波長1200nm)以下であれば、実用上問題ないYb添加光ファイバである。
本発明では、コアにイッテルビウム、アルミニウムおよびリンを含有し、シリカガラスを主成分とするYb添加光ファイバについて、フォトダークニングを抑制すること、および、コアの屈折率上昇を抑えることにより非線形光学効果を抑制することを両立する光ファイバの構成を見出した。
フォトダークニングおよび非線形光学効果を完全に抑制する、すなわちゼロにすることが理想であるが、現実にはこれらを完全に抑制することは困難である。また、フォトダークニングおよび非線形光学効果を完全に抑制するには、製造装置の構成が複雑になり、製造コストが増加するという問題がある。したがって、工業製品としてある程度許容できるレベルまで、フォトダークニングおよび非線形光学効果を抑制するのが現実的な対応である。そこで、本発明では、現実的な数値を「目標値」として設定した。
フォトダークニングの抑制における目標値は、フォトダークニングによる損失増加量が0.5dB以下とした。
非線形光学効果の抑制における目標値は、コアとクラッドとの比屈折率差が0.05%以上かつ0.30%以下とした。
一般に、コアとクラッドとの比屈折率差があまりにも小さい場合、例えば、0.05%未満の場合、光の閉じ込め効果が低いことが知られている。そのため、光ファイバの曲げや光ファイバへの側圧の印加による外部応力や、温度変化による体積変化に伴う応力に対して、光の伝搬状態が不安定になるので実用的でないと判断される。
コア中の酸化イッテルビウムのモル濃度、コア中の五酸化二リンのモル濃度およびコア中の酸化アルミニウムのモル濃度が大きく変わらない範囲にて、これらのドーパントのモル濃度を微調整し、コアとクラッドとの比屈折率差を変化させたこと以外は実験例1と同様にして、Yb添加光ファイバを数種類作製した。
比屈折率差の小さい光ファイバを作製する場合、屈折率を下げる効果を有するフッ素を適量添加し、比屈折率差を調整した。
この実験例8では、Yb添加光ファイバを、コア径が30μmのステッププロファイルを有したコアとし、クラッド径を400μmとし、その外周を低屈折率樹脂でコーティングし、さらにその外周を高屈折率樹脂コーティングしたダブルクラッドファイバとした。
円柱状の部材に巻き付けた状態の増幅用光ファイバのコアに、波長1060nm、平均パワー1Wのパルス状の種光を入射した。また、クラッドに波長915nm、パワー50Wの励起光を入射して、波長1060nmの光を増幅した。
そして、増幅用光ファイバにより増幅した波長1060nmの光の出力光パワーを測定した。
ここでは、波長フィルタを用いずに、増幅用光ファイバから出力される波長1060nm帯(誘導ラマン散乱光を含む)の出力光パワーを測定した。増幅用光ファイバの長さを、出力光パワーが最大になるように、適宜調整した。
この測定によって得られた、増幅用光ファイバのコアとクラッドとの比屈折率差と、レーザ出力光パワーとの関係を図5に示す。図5に示すグラフにおいて、横軸は増幅用光ファイバのコアとクラッドとの比屈折率差、縦軸は増幅用光ファイバの出口側のレーザ出力光パワーを示す。
以上、本発明において、コアとクラッドとの比屈折率差を0.05%以上とした目標値は、妥当であると考えられる。
コア中の酸化イッテルビウムのモル濃度、コア中の五酸化二リンのモル濃度およびコア中の酸化アルミニウムのモル濃度が大きく変わらない範囲にて、五酸化二リンのモル濃度と酸化アルミニウムのモル濃度を微調整し、コアとクラッドとの比屈折率差を変化させたこと以外は実験例1と同様にして、Yb添加光ファイバを数種類作製した。
この実験例9では、Yb添加光ファイバを、コア径が30μmのステッププロファイルを有したコアとし、クラッド径を400μmとし、その外周を低屈折率樹脂でコーティングし、さらにその外周を高屈折率樹脂コーティングしたダブルクラッドファイバとした。
増幅用光ファイバの出口側に、波長1100nm以上の光をカットする波長フィルタを配置した。
波長1060nm付近の出力光を増幅する場合、1次の誘導ラマン散乱光は、波長1110nm付近に発生する。
さらに、2次の誘導ラマン散乱光は、波長1160nm付近に発生する。波長1100nm以上の光をカットする目的は、誘導ラマン散乱光の発生を抑制し、ビーム品質の低下を抑制するためである。
この測定によって得られた、増幅用光ファイバのコアとクラッドとの比屈折率差と、レーザ出力光パワーとの関係を図6に示す。図6に示すグラフにおいて、横軸は増幅用光ファイバのコアとクラッドとの比屈折率差、縦軸は波長フィルタを通過したレーザ出力光パワーを示す。
一方、コアとクラッドとの比屈折率差が0.30%を超えると、誘導ラマン散乱光の発生が大きくなり、得られる出力光パワーが著しく低下することが分かった。この実験例9で得られた出力光パワーは、実験例8で得られた出力光パワーよりも僅かに低下している。これは、増幅用光ファイバの出口側に波長フィルタを配置したことによる損失の影響を受けたものと考えられる。そこで、増幅用光ファイバのコアとクラッドとの比屈折率差を0.30%以下とすることにより、出力光パワーを高くできるのは明らかである。
以上、本発明において、コアとクラッドとの比屈折率差を0.30%以下とした目標値は、妥当であると考えられる。
コアにイッテルビウム、アルミニウム、リンおよびゲルマニウムを含有させたYb添加光ファイバを作製した。
二酸化ゲルマニウム(GeO2)を1mol%添加したこと以外は実験例1と同様にして作製した。また実験例1と同様に、コア中の酸化イッテルビウムのモル濃度を0.20mol%で一定とし、コア中の酸化アルミニウムのモル濃度を2.5mol%で一定とし、コア中の五酸化二リンのモル濃度を変化させて、Yb添加光ファイバのコアとクラッドとの比屈折率差の変化を求めた。得られた結果を表1及び図7に示す。
図7に示すように、五酸化二リンのモル濃度が、酸化アルミニウムのモル濃度とほぼ等しい、2.5mol%近傍である場合、コアとクラッドの比屈折率差が最小になることが分かった。また、比屈折率差が最小になる五酸化二リンのモル濃度よりも低濃度の領域、及び高濃度の領域において比屈折率差の変化をほぼ直線で近似できることが判明し、その傾きの絶対値はそれぞれ0.192、0.040であった。これらの傾きは実験例1~4で得られた結果とほぼ同じであった。実験例1と異なるのは、比屈折率差の最小値の値が0.20%となっていたことであって、実験例1の比屈折率差の最小値の値よりも大きくなった。これは、コアに二酸化ゲルマニウムを含有させたことでコアの屈折率が上昇したことに起因する。
一般的に、ゲルマニウムを含有させたシリカガラスは、二酸化ゲルマニウム1mol%あたり約0.1%の屈折率上昇が起きることが知られている。図7に示す結果を図1に示す結果と比較したところ、いずれの五酸化二リンの濃度の条件においても、比屈折率差の値が0.1%程度大きくなっていた。これは、ゲルマニウムを含有させたことで、一般的に知られている程度の屈折率上昇が生じたものであると考えられる。
すでに述べたように、コアとクラッドの比屈折率差は0.30%以下にすることが好ましい。図1に示す結果では比屈折率差が0.30%以下になる五酸化二リンのモル濃度の範囲は1.5~7.5mol%と広い範囲であった。一方、図7に示す結果では1.8~5.0mol%と狭くなってしまっている。したがって、コアにゲルマニウムを含有しないほうが、比屈折率差の観点では好ましいといえる。
Claims (8)
- イッテルビウム、アルミニウムおよびリンを少なくとも含有するコアと、このコアを囲むクラッドと、を備え、
前記コア中の前記リンの五酸化二リン換算モル濃度と、前記コア中の前記アルミニウムの酸化アルミニウム換算モル濃度と、が同じであり、
前記コア中の前記イッテルビウムの酸化イッテルビウム換算モル濃度に対する前記コア中の前記リンの五酸化二リン換算モル濃度の比が10以上かつ30以下であり、
前記コアと前記クラッドとの比屈折率差が0.05%以上かつ0.30%以下である
ことを特徴とするイッテルビウム添加光ファイバ。 - 前記コアおよび前記クラッドがシリカガラスをベースとするガラスから構成されている
ことを特徴とする請求項1に記載のイッテルビウム添加光ファイバ。 - 前記酸化イッテルビウム換算モル濃度をαとすると、前記αは、0.05≦α×0.5≦0.30なる関係を満たす
ことを特徴とする請求項1に記載のイッテルビウム添加光ファイバ。 - イッテルビウム、アルミニウムおよびリンを少なくとも含有するコアと、このコアを囲むクラッドと、を備え、
前記コア中の前記イッテルビウムの酸化イッテルビウム換算モル濃度に対する前記コア中の前記リンの五酸化二リン換算モル濃度の比が10以上かつ30以下であり、
前記コアと前記クラッドとの比屈折率差が0.05%以上かつ0.30%以下であり、
前記酸化イッテルビウム換算モル濃度をα、前記コア中の前記アルミニウムの酸化アルミニウム換算モル濃度をβ、前記五酸化二リン換算モル濃度をγとすると、前記α、前記βおよび前記γは、β>γの場合、0.05≦(β-γ)×0.19+α×0.5≦0.30なる関係を満たす
ことを特徴とするイッテルビウム添加光ファイバ。 - 前記β及び前記γが、1<(β/γ)≦3なる関係を満たすことを特徴とする請求項4に記載のイッテルビウム添加光ファイバ。
- イッテルビウム、アルミニウムおよびリンを少なくとも含有するコアと、このコアを囲むクラッドと、を備え、
前記コア中の前記イッテルビウムの酸化イッテルビウム換算モル濃度に対する前記コア中の前記リンの五酸化二リン換算モル濃度の比が10以上かつ30以下であり、
前記コアと前記クラッドとの比屈折率差が0.05%以上かつ0.30%以下であり、
前記酸化イッテルビウム換算モル濃度をα、前記コア中の前記アルミニウムの酸化アルミニウム換算モル濃度をβ、前記五酸化二リン換算モル濃度をγとすると、前記α、前記βおよび前記γは、β<γの場合、0.05≦(γ-β)×0.04+α×0.5≦0.30なる関係を満たす
ことを特徴とするイッテルビウム添加光ファイバ。 - 前記β及び前記γが、0.56≦(β/γ)<1なる関係を満たすことを特徴とする請求項6に記載のイッテルビウム添加光ファイバ。
- 前記コアにゲルマニウムを含有しないことを特徴とする
請求項1、4または6のいずれか1項に記載のイッテルビウム添加光ファイバ。
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EP09824607.7A EP2348587B1 (en) | 2008-11-04 | 2009-11-04 | Ytterbium-doped optical fiber |
DK09824607.7T DK2348587T3 (da) | 2008-11-04 | 2009-11-04 | Ytterbium-doteret optisk fiber |
CA2742138A CA2742138C (en) | 2008-11-04 | 2009-11-04 | Ytterbium-doped optical fiber |
CN200980143297.4A CN102197550B (zh) | 2008-11-04 | 2009-11-04 | 掺镱光纤 |
JP2010536689A JP5470266B2 (ja) | 2008-11-04 | 2009-11-04 | イッテルビウム添加光ファイバ |
US13/097,563 US8774590B2 (en) | 2008-11-04 | 2011-04-29 | Ytterbium-doped optical fiber |
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JP2008283165 | 2008-11-04 | ||
JP2008-283165 | 2008-11-04 | ||
WOPCT/JP2009/052064 | 2009-02-06 | ||
PCT/JP2009/052064 WO2010052940A1 (ja) | 2008-11-04 | 2009-02-06 | イッテルビウム添加光ファイバ |
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US13/097,563 Continuation US8774590B2 (en) | 2008-11-04 | 2011-04-29 | Ytterbium-doped optical fiber |
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Publication number | Priority date | Publication date | Assignee | Title |
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JP2013232591A (ja) * | 2012-05-01 | 2013-11-14 | Mitsubishi Cable Ind Ltd | Yb添加光ファイバ |
CN113300195A (zh) * | 2020-02-21 | 2021-08-24 | 丰田自动车株式会社 | 放大光纤及激光射出装置 |
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JP2013232591A (ja) * | 2012-05-01 | 2013-11-14 | Mitsubishi Cable Ind Ltd | Yb添加光ファイバ |
CN113300195A (zh) * | 2020-02-21 | 2021-08-24 | 丰田自动车株式会社 | 放大光纤及激光射出装置 |
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