JP6022822B2 - Optical fiber - Google Patents

Description

  The present invention relates to an optical fiber in which Yb (ytterbium) is added (doped) to a core, and particularly relates to a fiber added with Al (aluminum) and F (fluorine).

  Optical fibers doped with rare earth elements such as Er (erbium) and Yb are widely used as optical communication signal amplifiers, and the importance of the technology is increasing. In particular, fiber lasers using rare-earth fibers doped with Yb are attracting great interest in the processing industry fields such as cutting, welding, and marking because high light conversion efficiency and excellent beam quality can be obtained.

  In recent years, in such a processing industry field, a high-power and high-quality laser has been demanded. In order to increase the output, it is conceivable to input a high-intensity laser beam into the Yb-doped optical fiber. However, even if a high-intensity laser beam is simply input to the Yb-doped optical fiber, fiber breakdown due to nonlinear characteristics of the fiber such as stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS), In particular, a photo-darkening loss (hereinafter referred to as an increase in PD loss) occurs, and the fiber characteristics deteriorate.

  In order to obtain a high-quality laser from a Yb-doped optical fiber, a wide gain spectrum, low reabsorption, laser reliability, particularly high light conversion efficiency, low transmission loss, and high photodarkening resistance are required. Here, transmission loss is affected by crystal defects, impurities, OH concentration, etc., and photodarkening resistance due to the color center mechanism is the cluster density, crystal defect density, oxygen defect (non-bridging) of rare earth ions. Oxygen Hole Center (hereinafter referred to as NBOHC) energy level and glass band gap.

  Photodarkening due to the mechanism of color center formation is a major factor that deteriorates the output of the optical fiber, and is proportional to the second to seventh power of the excitation inversion distribution (or the excitation concentration of Yb ions). It is known to occur (see, for example, Non-Patent Document 1). It is generally known that photodarkening occurs due to clustering of added rare earth ions (Yb, Er, etc.). The clustering of Yb ions is a phenomenon in which Yb ions in the glass cannot be sufficiently diffused (separated) and NBOHC is generated from the glass network structure. When Yb addition is increased in concentration, the clustering of Yb ions appears remarkably.

  Many methods for suppressing photodarkening are described in the prior art. For example, Non-Patent Document 2 discloses a method of co-adding Al as a method for suppressing clustering of Yb ions and reducing photodarkening. By increasing the molar ratio of Al to Yb element, photo Darkening can be suppressed to some extent.

  As a method for increasing the Al concentration without increasing the core numerical aperture (NA), a method is known in which an additive element for reducing the refractive index of glass such as F or B (boron) is added to the core (for example, Patent Documents). 6).

International Publication No. 2007/049705 Pamphlet International Publication No. 2008/133242 Pamphlet JP 11-314935 A JP-A-3-265537 JP 2009-224405 A JP 2010-1193 A JP 2010-533634 A JP 2007-114335 A International Publication 2010/016245 Pamphlet JP 2009-536785 A US Patent No. 7006752

J. Koponen.et al., "Photodarkening Measurements in Large-Mode-Area Fibers", SPIE Photonics West 2007,2007, Vol.6453-50 T.Kitabayashi.et al., "Population InversionFactor Dependence of Photodarkening of Yb-doped Fibers and its Suppression by Highly AluminumDoping", OFC2006, OThC5,2006 Peter D. Dragic. Etal., "Characterization of defect luminescence in Yb doped silica fibers: part I NBOHC", OPTICS EXPRESS, Vol. 16, 2008, p4688 S. Jetschke.et al., "Efficient Yb laser fibers with low photo-darkening by optimization of the core composition", OPTICS EXPRESS, Vol. 16, 2008, p15540 C. Randy Giles. Et al., "Modeling Erbium-Doped Fiber Amplifiers", Journal of lightwave technology, 1991, Vol. 9 (no. 2), p.271-283 Kazuo Arai.et al., "Evidence of pair generation of an E′center and a nonbridging oxygen-hole center in r-lay-irradiated fluorine-doped low-OH synthetic silica glasses", PhysicalReview B, 1992, Vol45 (no18) , p45 D. J. Di Giovanni et al., "Structure and properties of silica containing aluminum and phosphorus near the AlPO4 join", J. of Non-Crystalline Solids 113, pp. 58-64, 1989

  However, the inventor's research has shown that simply increasing the Al concentration does not provide the targeted low transmission loss and high photodarkening resistance.

  In Non-Patent Document 3 and Patent Documents 7 and 10, by adding P (phosphorus), the NBOHC energy level of the glass network can be blue-shifted and photodarkening can be suppressed to some extent. Have been described. However, the transmission loss of the P-doped fiber is increased by about 300 dB / km, which is not suitable for realizing a high-power and high-reliability laser (for example, see Non-Patent Document 4).

  Further, Patent Documents 1, 2 and 9 describe the photodarkening characteristics of the F-doped rare earth fiber. The photodarkening characteristics of the Yb-doped rare earth fiber are mainly governed by the Al concentration regardless of the F addition. In particular, a decrease in photodarkening resistance was confirmed from the F-doped rare earth fiber (for example, Patent Documents 3 and 5).

  Patent Documents 3 and 4 show a method for producing a rare earth-doped fiber in which rare earth elements, Al and F are added to glass, but are described in detail only for rare earth fibers in which Er is added as a rare earth. Thus, there is no description that can be applied to the rare-earth fiber doped with Yb. In addition, the Yb, Al, and F concentrations do not show appropriate concentrations, and it is difficult to obtain low transmission loss and high photodarkening resistance, which are the problems of the present invention. According to Patent Document 8, it is known to suppress photodarkening by adding hydrogen to a Yb rare earth-doped fiber, but loss of pump light occurs due to light absorption of OH bonds (Hydroxide Bonding). Therefore, the laser conversion efficiency is deteriorated.

  Patent Document 11 proposes a method for suppressing the clustering of rare earth ions by adding B, F, Ge (germanium), or the like to the rare earth-added core. However, the target core NA, crystallinity, Yb cluster density, and the like cannot be controlled simply by adding Ge, F, or B to the Yb-doped fiber. For this reason, it is difficult to obtain a desired concentration combination only with the prior art, and it is not possible to easily manufacture a fiber having low transmission loss and high photodarkening resistance within the target core NA range.

  By the way, in order to realize a high beam quality fiber, it is necessary to control the core NA between 0.05 and 0.15, and the concentration of the additive element is limited. In particular, in order to realize a high-power laser with high conversion efficiency in recent years, a core NA is low, and a Yb concentration of about 1 wt% (weight percentage) is required, and Al addition that is influenced by Yb clustering is added. Concentration is limited, and photodarkening resistance may be deteriorated.

  In addition, as a method for obtaining high light conversion efficiency from a single clad fiber, Yb addition at the core center of a rare earth fiber is known (for example, see Non-Patent Document 5), but generally the light conversion efficiency is increased. In addition, the transmission loss of the optical fiber must be reduced. Factors of transmission loss of a single clad fiber include absorption loss (for example, absorption by impurities contained in the core and absorption by crystal defects) and scattering loss (for example, Rayleigh scattering). It is difficult to reduce the concentration of impurities contained in the core because it is largely due to impurities contained in the glass itself and impurities mixed in the rod-in-tube process. In addition, since crystal defects vary depending on the core composition and manufacturing method, it is necessary to control the core composition, control the molar ratio, and optimize the fiber manufacturing conditions.

  As will be described later, for example, Non-Patent Document 6 describes that E` defect formation is suppressed as the OH concentration is higher, and Non-Patent Document 7 describes crystallization when the P addition concentration is too high. Is described to occur. If E` defect formation is suppressed, it is advantageous for the transmission of ultraviolet light, but the effect is small in the 1 μm band which is the wavelength region of the present invention. In addition, when crystallized, it has a structure different from that of glass, which causes a decrease in light conversion efficiency, an increase in transmission loss, and a decrease in photodarkening resistance.

  Thus, a fiber laser using a Yb-doped optical fiber is required to have high light conversion efficiency and high reliability from the viewpoint of application to the communication and processing industries.

  The present invention has been made in view of such a point, and the object of the present invention is to provide a Yb-doped optical fiber with low transmission loss, high photodarkening resistance, low PD loss increase amount, and high nonlinear resistance. To have it.

  In order to achieve the above object, in the present invention, in addition to clarifying the transmission loss and photodarkening resistance of Yb, Al and F doped fibers, combinations of additive elements such as Al, Ge, P and F, and addition The relationship between element molar ratio, transmission loss and photodarkening resistance was established.

Specifically, in the present invention,
A core to which Yb, Al and F are added;
Targeting an optical fiber provided with a cladding provided around the outer periphery of the core,
In the optical fiber,
The Yb concentration added to the core is 0.05 mol% or more and 0.5 mol% or less,
Al concentration added to the core is 0.3 mol% or more and 2 mol% or less,
The F concentration added to the core is 0.1 mol% or more and 1 mol% or less.

According to said structure, since Al is added with the rare earth element Yb to the core, the clustering of Yb can be suppressed. The addition of F to the core softens the glass and has the effect of promoting Yb diffusion and reducing Rayleigh scattering. In addition, the up-conversion of electrons is suppressed because the band gap of the glass is increased, NBOHC defects are difficult to form, and transmission loss in the visible light region due to color center formation is suppressed. If the Yb concentration added to the core is less than 0.05 mol, the Yb absorption coefficient of the core is low, and the fiber length must be increased. For high power and cost reduction due to transfer loss and nonlinear characteristics a disadvantage. When the Yb concentration is higher than 0.5 mol%, crystallization is promoted and transmission loss is increased, and clustering of Yb ions proceeds and photodarkening resistance is reduced. However, in the present invention, since the Yb concentration is 0.05 mol% or more and 0.5 mol% or less, an increase in transmission loss and a decrease in photodarkening resistance do not occur, and a high output laser with high light conversion efficiency is realized. be able to. When the Al concentration added to the core is smaller than 0.3 mol % , Yb clustering proceeds. When the Al concentration is higher than 2 mol%, crystallization is caused and the core NA becomes high. However, in the present invention, since the Al concentration is not less than 0.3 mol% and not more than 2 mol%, Yb clustering can be suppressed while preventing crystallization. If the F concentration added to the core is less than 0.1 mol % , the effect of addition does not appear. If the F concentration is greater than 1 mol%, Al or Ge is used to increase the numerical aperture of the core. It is necessary to increase the concentration of additives such as crystallization, and there is a risk of reducing crystallization and photodarkening resistance. In addition, it is difficult to adjust the concentration of addition and stable production is difficult. However, in the present invention, since the F concentration is 0.1 mol% or more and 1 mol% or less, reduction of Rayleigh scattering and high photodarkening resistance can be obtained while promoting the diffusion of Yb.

The molar ratio of Al to Yb which is added to the core is from 3 to 11,
The molar ratio of Al to F added to the core is 1 or more and 3 or less.

That is, when the molar ratio of Al to Yb added to the core is smaller than 3, the clustering of Yb ions tends to proceed. When the molar ratio is larger than 11, crystallization is caused and the core numerical aperture tends to be high. However, in the present invention, since the molar ratio is 3 or more and 11 or less, clustering of Yb can be suppressed while preventing crystallization. Further, the molar ratio of Al to F that is added to the core is less than 1, it tends to glass becomes hard, except that Rayleigh scattering is increased, Al concentration becomes low, clustering of Yb ions As a result, the photodarkening resistance tends to deteriorate. When the molar ratio is larger than 3, it is necessary to increase the additive concentration of Al or Ge in order to match the desired numerical aperture of the core, crystallization progresses, and Rayleigh scattering increases and photodarkening resistance deteriorates. the trend. However, in the present invention, since the molar ratio is 1 or more and 3 or less, low Rayleigh scattering and low Yb cluster density are possible, and lower transmission loss and high photodarkening resistance can be realized.

In particular, Ge is further added to the core,
The concentration of Ge is 0.05 mol% or more and 0.5 mol% or less,
The molar ratio of Al to the Ge may be 4 to 10.

According to said structure, since Ge is further added to the core, there exists an effect which softens glass, promotes Yb spreading | diffusion, and reduces a Rayleigh loss. On the other hand, when the Ge concentration is higher than 0.5 mol%, the NBOHC level is red-shifted, and NBOHC defects tend to be formed. Conversely, when it is less than 0.05 mol%, the effect tends to be hardly exhibited. However, in the present invention, since the Ge concentration is 0.05 mol% or more and 0.5 mol% or less, the effect of addition of Ge is exhibited moderately. The molar ratio of Al to Ge, when less than 4, les over Li scattering is increased, which may lead to an increase in transmission loss. If it is greater than 10, the Al concentration needs to be reduced, clustering of Yb ions and NBOHC progress, and photodarkening resistance may be reduced. However, in the present invention, since the molar ratio is 4 or more and 10 or less, lower transmission loss and higher photodarkening resistance of the Yb-doped fiber can be obtained, and higher output and higher reliability can be realized.

In particular, P is further added to the core,
The concentration of P is from 0.05 to 0.5 mol%,
The molar ratio of Al to the P may be a 3 to 7.

According to the above configuration, since P is further added to the core, the NBOHC level is blue-shifted, and NBOHC defects tend not to be formed easily. On the other hand, if the P concentration is less than 0.05 mol%, the effect tends not to be exhibited. If it is greater than 0.5 mol%, crystallization and the energy level of Yb may change, leading to a decrease in light conversion efficiency. However, in the present invention, since the concentration of P is 0.05 mol% or more and 0.5 mol% or less, the effect of adding P is appropriately exhibited. The molar ratio of Al to P, when less than 3, tends to effect of that can not be achieved, the greater than 7, tends to sintering crystallized by clustering increases and Yb ions Rayleigh scattering, transmission Loss and photodarkening resistance tend to deteriorate. However, in the present invention, since the molar ratio is 3 or more and 7 or less, lower transmission loss and high photodarkening resistance can be obtained, and high output and high reliability can be realized.

The numerical aperture of the core may be 0.15 or less larger than 0.05.

  That is, when the numerical aperture of the core is smaller than 0.05, the light confinement is weak, the light conversion efficiency is lowered, and the bending loss of the fiber tends to increase. If it is larger than 0.15, it is necessary to reduce the core diameter in order to achieve a single mode, and the non-linear resistance tends to decrease. However, according to the above configuration, since the numerical aperture of the core is larger than 0.05 and not larger than 0.15, a high-power laser with high conversion efficiency and high non-linear tolerance can be realized.

  As described above, according to the present invention, in an optical fiber including a core to which Yb, Al and F are added and a cladding around the core, the concentration of the core is 0.05 mol% or more and 0.5% or more. Yb is added so as to be less than mol%, Al is added so that the concentration is 0.3 mol% or more and 2 mol% or less, and F is added so that the concentration becomes 0.1 mol% or more and 1 mol% or less. As a result, it is possible to provide a Yb-doped optical fiber with low transmission loss and high photodarkening resistance or low PD loss increase.

It is a table | surface which shows the influence which an addition element and addition concentration have on glass property. It is a graph which shows the correlation of the transmission loss with respect to Yb addition density | concentration. It is a graph which shows the correlation of the photodarkening loss increase amount with respect to Yb addition density | concentration. It is a table | surface which shows the transmission loss and the photodarkening loss increase amount in the density | concentration of the additive element of Example and a comparative example, and the molar ratio of an additive element.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Note that the description of this embodiment is merely illustrative in nature.

<Configuration of the optical fiber>
An optical fiber according to an embodiment of the present invention is an optical fiber in which Yb is added to a core, and is used for, for example, a fiber laser or an amplifier for optical communication signals. Although not shown in detail, the optical fiber includes a core and a clad provided around the core and having a refractive index lower than that of the core. Here, the clad may be one layer or two or more layers.

  Yb, Al and F are added to the core. Further, Ge or P may be added, and Cl may be added. The concentration of Yb added to the core is 0.05 mol% [mol percentage] or more and 0.5 mol% or less, the Al concentration is 0.3 mol% or more and 2 mol% or less, and the F concentration is The content is preferably 0.1 mol% or more and 1 mol% or less. Further, the Ge concentration is 0.05 mol% to 0.5 mol%, the P concentration is 0.05 mol% to 0.5 mol%, and the Cl concentration is 0.005 mol% to 0.1 mol%. Each may be added as described above.

The molar ratio of Al (= Al / Yb) to Yb added to the core is adjusted to 3 or more and 11 or less, and the molar ratio of Al (= Al / F) to F added to the core is 1 or more. 3 is adjusted to below. In addition, the molar ratio of Al (= Al / Ge) to Ge added to the core is adjusted to 4 or more and 10 or less, and the molar ratio of Al (= Al / P) to P is 3 or more and 7 or less. It has been adjusted.

  As a result of repeated experiments and analysis by the inventors, it has been found that the physical properties of Yb-added glass are greatly changed by adding and combining various elements in Yb-added glass.

  Specifically, the addition of Al has an effect of suppressing Yb clustering. When the Al concentration is low, Yb clustering proceeds, and when it is too high, crystallization occurs.

  The addition of Ge has the effect of softening the glass and promoting Yb diffusion and reducing Rayleigh loss. However, if the Ge addition concentration is too high, the NBOHC level is red-shifted, and NBOHC defects are easily formed.

  P addition causes a blue shift of the NBOHC level and makes it difficult to form an NBOHC defect. However, if the concentration of P addition is too high, crystallization (for example, Non-Patent Document 7), the energy level of Yb changes, and the quantum level changes. The efficiency was lowered.

  The addition of F has the effect of softening the glass, promoting Yb diffusion and reducing Rayleigh scattering, and increasing the band gap of the glass to suppress electron up-conversion, making it difficult to form NBOHC defects. Transmission loss in the visible light region due to formation was suppressed.

  The addition of B (boron) softens the glass, has the effect of promoting Yb diffusion and preventing Rayleigh scattering, and also tends to cause carrier up-conversion to reduce the band gap of the glass, facilitating the formation of NBOHC defects. It was.

  Since Yb clustering is closely related to NBOHC defect formation, it causes an increase in transmission loss in the visible light band due to the mechanism of photodarkening, and a Yb-doped laser configuration (for example, 915 nm or 976 nm excitation, 1000 to 1100 nm). Oscillation) is an undesirable defect, and for solving the problems of the invention, a low Yb cluster density is desirable, and low concentration Yb and high concentration Al are desirable. However, depending on the application of the Yb doped fiber, it is necessary to increase the Yb absorption coefficient. There is.

  The NBOHC defect is an undesirable defect in a Yb-doped laser configuration (for example, excitation at 915 nm or 976 nm, oscillation at 1000 to 1100 nm) because it causes an increase in loss in the visible light band due to the mechanism of photodarkening. In addition, a low NBOHC defect density and a high energy level are desirable.

  OH has a close relationship with E` defects, and it is known that E` defect formation is suppressed as the OH concentration is higher (for example, Non-Patent Document 6). In order to reduce the loss in the ultraviolet region, a high OH density is desirable, but the Yb laser configuration is less relevant because it does not use the ultraviolet region. Rather, the light absorption by OH is around 940 nm, 1140 nm, 1250 nm, and 1380 nm. A low OH concentration is desirable because of its presence. Since the energy level of glass is closely related to NBOHC defect formation, a higher glass band gap is desirable, and addition of F is desirable (for example, Non-Patent Document 6).

  Furthermore, in Yb-doped glass, crystallization with specific additive elements (eg, Al and P) causes Yb clustering, increased Rayleigh loss, and increased defect density, resulting in low transmission loss and high photodarkening resistance. It is desirable to suppress as much as possible in order to obtain Diffusion of Yb ions is closely related to Yb clustering, and since it is related to crystallization, it is desirable to promote diffusion of not only Yb elements but also added ions, lowering the glass melting point, It is necessary to soften the glass during deposition (or to increase the meltability), and the addition of Ge, B or F is desirable. From this point of view, the manufacturing conditions such as manufacturing method, introduced gas, introduced raw material and deposition temperature have a great influence on parameters such as ion diffusion, crystallization of additive elements, OH concentration, raw material decomposition, adsorption coefficient of additive elements and production yield. Optimization is desirable to give.

  The Rayleigh loss is inversely proportional to the fourth power of the wavelength, and it is desirable to add Ge, B, or F in order to affect the transmission loss in the use wavelength band of the Yb laser such as the excitation wavelength and the oscillation wavelength. Furthermore, in order to suppress Raman scattering and brilliant scattering and increase nonlinear resistance, it is necessary to separate the acoustic wave and the optical wave. The refractive index of the Al acoustic wave is negative, and Ge, P, F, and B are refracted. Since the rate is positive, it is necessary to control the addition profile. For example, it is desirable to combine Al and Ge, P, F, and the like.

<Method of manufacturing an optical fiber>
Next, an example of an optical fiber manufacturing method will be described.

-Optical fiber preform manufacturing process-
As a method for manufacturing the optical fiber preform, for example, an MCVD method can be employed. Specifically, SiCl ₄ , Yb (DPM) ₃ (β-diketone metal complex) (or Yb (CP) ₃ ), AlCl ₃ , SiF ₄ are placed in a quartz tube that is rotatably supported by a rotary joint. While feeding, the pipe is heated from the outside by scanning the flame of the oxyhydrogen burner in the axial direction while rotating the quartz tube. At this time, by controlling the supply amount of Yb (DPM) ₃ , AlCl ₃ , SiCl _4, and SiF ₄ using the temperature of the raw material tank or MFC (Mass Flow Controller), Al / Yb molar ratio and Al / F molar ratio can be adjusted. For example, when the temperature of the raw material tank is increased, the addition concentration is increased, and when the MFC flow rate is increased, the addition concentration is increased. Further, since the deposition temperature is affected by the decomposition of the introduced raw material, the adsorption coefficient, and the diffusion of the additive element, it is desirable to perform prior optimization. Then, soot made of SiO ₂ (glass fine particles) to which Yb, Al and F are added is deposited on the inner surface of the glass pipe in the quartz tube. The soot is heated by the heat of the oxyhydrogen flame to become a transparent glass layer. Here, B, Ge, or P may be added by adding BCl ₃ , GeCl _4, or POCl _3. By using temperature control of the raw material tank or MFC, Al / Ge molar ratio, Al / P molar ratio or The Al / B molar ratio can be adjusted.

Next, the supply of Yb (DPM) ₃ , AlCl ₃ and SiF ₄ is stopped, and the quartz tube is heated with an oxyhydrogen flame by reciprocating the oxyhydrogen burner along the quartz tube while rotating the quartz tube. The inner space is reduced and crushed (collapse process). During this collapse process, the additive element on the innermost surface of the glass evaporates, and it is necessary to prevent depression of the molar ratio and refractive index profile of the added element as much as possible, and the inner surface of the deposited pipe is etched immediately before the glass is completely crushed .

  The glass pipe is then completely crushed. Thereby, a cylindrical optical fiber preform having a central core forming portion to which Yb, Al, and F are added and a clad forming portion provided on the outside so as to cover the core forming portion is obtained. In order to reduce the concentration of OH- ions mixed by heating with an oxyhydrogen flame, the diameter may be increased by covering with a quartz tube and collapsing by a rod-in-tube method and then extending.

-Drawing process-
Next, the optical fiber preform is set in a drawing apparatus, heated in a heating furnace, and drawn. As a result, an optical fiber having a core to which Yb, Al, and F are added and a cladding that is provided outside so as to cover the core and has a refractive index lower than that of the core is manufactured.

  In the present embodiment, the manufacturing method of the optical fiber by the MCVD method has been described. However, the present invention is not limited to this, and various known manufacturing methods such as the OVD method and the VAD method may be employed.

  Further, B, Ge, or P may be added according to refractive index adjustment, reduction of transmission loss, improvement of photodarkening resistance, fiber grating, or special fiber application. However, when B, Ge, or P is added to the core, another problem arises. Therefore, the basic concept for obtaining low transmission loss and high photodarkening resistance is the same, although the manufacturing conditions and the optimum values of the additive element concentration are different.

  Fibers with Yb / Al / F added to the core are fabricated by MCVD, and Yb / Al, Yb / Ge, Yb / Al / Ge, Yb / Al / P, Yb / Al are added to the core for comparison. A comparative fiber added with / Ge / P was prepared. In order to examine the Yb concentration, the combination of additive elements (core composition), the transmission loss and the photodarkening resistance in the molar ratio of the additive elements, the core NA was fixed in the vicinity of 0.08. Core NA with adjusting Al, F, Ge or P concentration to investigate Al / Yb molar ratio, Al / F molar ratio, Al / Ge molar ratio, transmission loss to Al / P molar ratio and photodarkening resistance Together. Here, the core NA is fixed and the additive element concentration of Yb, Al, F, Ge or P is adjusted. In order to stably manufacture a single mode optical fiber having a large area core, the core NA is stable. This is because it must be fixed. For example, if the concentration of Yb and F is fixed and the Al concentration is changed, it will be out of the desired NA range, so that the target fiber cannot be produced. In addition, if the core NA is changed, the Al and F concentrations change, There is a problem that fibers exhibiting different behaviors are produced.

- transmission loss evaluation -
To standardize the transmission loss of the drawn fiber, cut 100m, set one end of the fiber to the hundred-color light source side, set the other end to the spectrum analyzer side, and cut back method It was evaluated by.

  According to FIG. 1, the transmission loss of the F-doped fiber is much larger than those of the Yb / Al, Yb / Ge, Yb / Al / Ge, and Yb / Al / Ge / P-doped fibers of Comparative Examples 1 to 11 without F addition. Improvement was confirmed. Specifically, the transmission loss of the Yb / Al / F-doped fiber of Examples 1 and 2 is 3 dB / km, and the transmission loss of the Yb / Al / Ge / F-doped fiber of Examples 3 to 5 is 5 to 6 dB / km. km. The transmission loss of the comparative examples Yb / Al, Yb / Al / Ge, Yb / Al / P-doped fiber, and Yb / Al / Ge / P-doped fiber largely depended on the molar ratio of the additive element. The transmission loss of Al / F-doped fiber and Yb / Al / Ge / F-doped fiber did not depend much on the molar ratio of the additive elements. Moreover, the transmission loss of the Yb / Al / P / F doped fiber of Examples 6-7 is 3-11 dB / km, The transmission loss of the Yb / Al / Ge / P / F doped fiber of Examples 9-12 is The transmission loss of the Yb / Al / P / F doped fibers of Examples 6 to 8 showed a tendency depending on the Yb concentration, but the Yb / Al / Ge of Examples 9 to 12 was 5 to 7 dB / km. The transmission loss of the / P / F-doped fiber did not depend much on the Yb concentration, but showed a tendency to slightly depend on the Al concentration. From this, it was found that the transmission loss of the F-doped fiber basically has a tendency to depend on the Yb concentration, but depending on the combination of the additive elements, it does not depend much on the Yb concentration.

  Further, the Rayleigh loss near the wavelength of 740 nm largely depends on the additive element. The Rayleigh loss of the Yb / Al / F-doped fibers of Examples 1 and 2 is 190 to 240 dB / km, and the Yb / Al of Examples 3 to 5 is used. / Ge / F doped fiber has 40-50 dB / km, Yb / Al / P / F doped fiber of Examples 6-8 has 120 to 130 dB / km, Yb of Examples 9-12 / Al / Ge / P / F doped fiber has a loss of 30 to 32 dB / km and depends on the Ge addition regardless of the Al, P and F addition concentrations. The addition was found to be more desirable.

Further, the OH loss near the wavelength band of 1300 nm was more influenced by the concentration of H ₂ contained in the organic raw material of Yb (DPM) ₃ and the process than depending on the additive element.

  The transmission loss of the Yb / Al-doped fiber of Comparative Example 1 is 21 to 25 dB / km, the transmission loss of the Yb / Ge-doped fiber of Comparative Example 3 is 22 dB / km, and the Yb / Al / Ge addition of Comparative Examples 4 to 5 The transmission loss of the fiber is 8 to 20 dB / km, the transmission loss of the Yb / Al / P doped fiber of Comparative Examples 6 to 8 is 8 to 15 dB / km, and the Yb / Al / Ge / P addition of the Comparative Examples 9 to 11 is used. The transmission loss of the fiber is 7 to 42 dB / km, which is higher than that of the fiber of the example, and greatly depends on the concentration of the additive element and the core composition.

  In addition, the Rayleigh loss of the Yb / Al-doped fiber of Comparative Examples 1 and 2 is 230 to 300 dB / km, the Rayleigh loss of the Yb / Ge-doped fiber of Comparative Example 3 is 40 dB / km, and the Yb / of Comparative Examples 4 to 5 is Yb /. The Rayleigh loss of the Al / Ge-doped fiber is 30 to 60 dB / km, the Rayleigh loss of the Yb / Al / P-doped fiber of Comparative Examples 6 to 8 is 100 to 130 dB / km, and the Yb / Al / of Comparative Examples 9 to 11 The Rayleigh loss of the Ge / P-doped fiber is 20 to 50 dB / km, and the Ge-doped fiber has reduced Rayleigh scattering, and the fact that Ge addition is advantageous for reducing the Rayleigh loss is the same as the F-doped fiber. there were.

Moreover, since OH loss shows the tendency which is not related to the core composition of Yb / Al, Yb / Ge, Yb / Al / P, and Yb / Al / Ge / P through Comparative Examples 1-11, OH The loss does not depend much on the additive element and is affected by the concentration of H ₂ contained in the Yb (DPM) ₃ organic material and the process.

  According to FIG. 2, the transmission loss of the F-doped fiber is less related to the additive element, and increases with the increase in the Yb concentration. However, the inclination greatly depends on the additive element and the core composition, and Yb / Ge> Yb / Al> Yb / Al / Ge> Yb / Al / P> Yb / Al / Ge / P> Yb / Al / P / The order of the core composition is F> Yb / Al / Ge / P / F> Yb / Al / Ge / F> Al / F, and the F-added fiber shows a lower slope than the fiber not added with F. It became clear that it was more advantageous for increasing the concentration of Yb.

  Since the transmission loss depends on the Yb addition concentration, the following equation was established assuming that the transmission loss is proportional to the sixth power of the Yb addition concentration.

  Here, A is a fitting coefficient depending on the Yb concentration, which means an improvement in transmission loss, and D is the Yb addition concentration [mol%]. According to FIG. 1, the transmission loss largely depends on the core composition, and the improvement in the transmission loss of the Yb / Al / F-doped fibers of Examples 1 and 2 is 27 based on the Yb / Al-doped element of Comparative Example 1. The transmission loss improvement of the Yb / Al / Ge / F doped fibers of Examples 3 to 5 is 40 times, and the transmission loss improvement of the Yb / Al / P / F doped fibers of Examples 6 to 8 is The transmission loss improvement of the Yb / Al / Ge / P / F-doped fibers of Examples 9 to 12 was 57 times. However, the transmission loss improvement of the Yb / Ge-doped fiber of Comparative Example 3 is 1.8 times, and the transmission loss increase improvement of the Yb / Al / Ge-doped fibers of Comparative Examples 4 to 5 is 3.3 times. The transmission loss improvement of the Yb / Al / P-doped fibers of Comparative Examples 6-8 is 1.8 times, and the transmission loss improvement of the Yb / Al / Ge / P-doped fibers of Comparative Examples 9-11 is 1. It was 1 time, and was considerably lower than the Example.

  In addition, when the addition concentration profile of Yb, Al, Ge, P, and F of Example 9 was measured using EPMA (Electron Probe Micro Analyzer), the addition concentration profile of Yb, Al, F, and P was Yb (or Al, F, P) concentration rises steeply at the interface between the core and the cladding, becomes high, becomes substantially constant from the elevated position toward the center, and is a pseudo step index type slightly depressed near the center. The Ge concentration profile is such that the Ge concentration rises steeply near the interface between the core and the clad, becomes high, becomes substantially constant from the elevated position toward the center, and slightly depressed near the center of the core. It was. The refractive index profile rises steeply near the interface between the core and the cladding and becomes high. From the raised position, the refractive index profile becomes almost constant toward the center. It was. In addition, since the addition density | concentration profile and refractive index profile of the optical fiber which concern on Examples 1-11 were the same as that of the optical fiber which concerns on Example 9, detailed description is abbreviate | omitted.

  The concentration of the additive element was determined again by EPMA quantitative analysis of the flat corrugated portion between the core center and the clad from the EPMA line analysis. The EPMA quantitative analysis region is generally near the middle between the core and the clad.

- Photodarkening loss evaluation -
In order to standardize the evaluation of photodarkening resistance of optical fibers having different Yb concentrations, by changing the fiber length of the optical fiber, the total absorption coefficient of the optical fiber is substantially matched, and the pump wavelength is 976 nm and the pump power is 500 mW. While the core was excited, the transmittance (transmission power) of 670 nm HeNe laser light was continuously measured and recorded for 48 hours to evaluate the increase in loss of photodarkening (for example, see Non-Patent Document 1). . Here, the core diameter of the fiber is 7.5 μm, the total Yb absorption of the core is 15 dB / fiber in the wavelength 915 nm band, and the optical fiber fused to the Yb-doped optical fiber has the same core diameter as the Yb-doped optical fiber 1 A 3 μm zero-dispersion optical fiber was adopted. Since the fusion loss between fibers is affected by an increase in photodarkening loss, the transmittance after fusion was measured, and it was confirmed that there was no problem.

  The PD loss increase amount of the Yb / Al / F-doped fibers of Examples 1 and 2 is 5 to 7 dB / 2 days, and the PD loss increase amount of the Yb / Al / Ge / F-doped fibers of Examples 3 to 5 is 5 to 5 dB. It was 11 dB / 2 days. The PD loss increase amount of the Yb / Al / F-doped fiber and the Yb / Al / Ge / F-doped fiber tended to depend mainly on the Al / Yb molar ratio. Further, the PD loss increase amount of the Yb / Al / P / F-doped fiber of Examples 6 to 7 is 4 to 8 dB / 2 days, and the PD loss of the Yb / Al / Ge / P / F-doped fiber of Examples 9 to 12 The increase amount is 5 to 7 dB / 2 days, and the PD loss increase amount of the Yb / Al / P / F doped fiber tended to depend on the Yb concentration, but the Yb / Al / Ge / P / F doped fiber The amount of increase in PD loss was less dependent on the Yb concentration. From this, it was found that the PD loss increase amount of the F-doped fiber tends to depend on the Yb concentration, but depending on the core composition, it became clear that it does not depend much on the Yb concentration. In particular, the PD loss increase amount of Comparative Examples 1 to 11 greatly depends on the Al, Ge, and P concentrations, and showed a value exceeding 15 dB / 2 days. However, the PD loss increase amount of the F-doped fiber is 10 dB / 2 days or less. Yes, it was basically governed by the addition of F rather than a combination of addition concentrations of Al, Ge and P and additive elements.

  The PD loss increase amount of the Yb / Al-doped fiber of Comparative Examples 1 and 2 is 15 to 45 dB / 2 days, the PD loss increase amount of the Yb / Ge-doped fiber of Comparative Example 3 is 55 dB / 2 days, and Comparative Examples 4 to 5 The PD loss increase amount of the Yb / Al / Ge doped fiber is 16 to 30 dB / 2 days, and the PD loss increase amount of the Yb / Al / P doped fiber of Comparative Examples 6 to 8 is 10 to 19 dB / 2 days, Comparative Example 9 to 11 Yb / Al / Ge / P-doped fiber has a PD loss increase of 45 to 47 dB / 2 days, which is considerably higher than that of the fiber of the example, and greatly depends on the combination of Yb concentration and additive elements. did.

  According to FIG. 3, the amount of increase in PD loss showed a tendency to increase with increasing Yb concentration regardless of the additive element. However, the inclination greatly depends on the additive element, and Yb / Ge> Yb / Al> Yb / Al / Ge> Yb / Al / Ge / P> Yb / Al / P> Yb / Al / Ge / F> Yb / The order of the core composition is Al / F> Yb / Al / P / F> Yb / Al / Ge / P / F, and the PD loss increase amount of the F-doped fiber shows a considerably lower slope than that of the fiber not containing F, It became clear that addition of F is more advantageous for increasing the concentration of Yb.

  Since the transmission loss depends on the Yb addition concentration, assuming that the PD loss increase amount is proportional to the sixth power of the Yb addition concentration, the following equation was established.

  Here, B is a fitting coefficient depending on the Yb concentration, which means improvement in photodarkening resistance, and D is the Yb addition concentration [mol%]. According to FIG. 1, the PD resistance improvement greatly depends on the core composition. Based on the Yb / Al-added elements of Comparative Examples 1 and 2, the PD resistance improvement of the Yb / Al / F-doped fibers of Examples 1 and 2 is used. Of the Yb / Al / Ge / F-doped fiber of Examples 3-5 is 40 times, and the PD resistance of the Yb / Al / P / F-doped fiber of Examples 6-8 is 40 times. The resistance improvement was 40 times, and the PD resistance improvement of the Yb / Al / Ge / P / F-doped fibers of Examples 9 to 12 was 57 times. However, the PD resistance improvement of the Yb / Ge doped fiber of Comparative Example 3, which is a comparative example, is 1 time, and the transmission loss increase improvement of the Yb / Al / Ge doped fibers of Comparative Examples 4 to 5 is 4 times. Yes, the PD resistance improvement of the Yb / Al / P-doped fibers of Comparative Examples 6 to 8 is 20 times, and the PD resistance improvement of the Yb / Al / Ge / P-doped fibers of Comparative Examples 9 to 11 is 15 times. Yes, lower than Comparative Examples 9-11.

  Here, when Examples 1 and 2 are compared with Comparative Example 1, although the Al concentration of Comparative Example 2 is higher, the transmission loss and PD loss increase amount of Example 1 of the F-doped fiber are low. In addition, a desirable result was not obtained from the comparative example even in a wide concentration range. In addition, when Examples 3 to 5 and Comparative Examples 4 and 5 are compared, although the Ge concentration is higher in the comparative example, the increase in transmission loss and PD loss is superior in the example, and a wide concentration range. However, a desirable result was not obtained from the comparative example. Further, when Examples 6 to 8 and Comparative Examples 6 to 8 are compared, even if the P concentration is not similar, the transmission loss and the PD loss increase amount are superior to those in the example, and a wide concentration range. However, a desirable result was not obtained from the comparative example. Further, when Examples 9 to 10 and Comparative Examples 9 to 11 were compared, even though the addition concentrations of Ge and P were the same, the transmission loss and the PD loss increase amount showed a low value in Examples, Even in a wide concentration range, desirable results were not obtained from the comparative example.

  FIG. 4 shows a table summarizing the effects of the present invention on the glass properties with respect to the type of additive element and the concentration of the additive element. According to FIG. 4, compared with the characteristics of the comparative example, the photo-darkening and transmission loss of the F-doped fiber are greatly improved because Yb diffusion is promoted by improving the melting property of the glass by adding F, and crystallization is suppressed. This is because the defect density is reduced, the band gap of the glass is widened, NBOHC formation is suppressed, the Al addition concentration is increased, and the Al / Yb molar ratio is increased. Further, the further improvement by adding Ge or P was confirmed that Yb clustering suppression and Rayleigh loss were reduced by improving the meltability of Ge addition, the NBOHC level by P addition was blue-shifted, This is due to improved photodarkening resistance. Moreover, improvement in transmission loss and photodarkening was recognized by adding Ge and P. However, if Ge is added too much, the NBOHC level is red-shifted and photodarkening is likely to occur. If the P addition concentration is too high, the Yb energy level is changed, and the Yb absorption coefficient and conversion efficiency are lowered. In addition, care must be taken to cause an increase in transmission loss due to crystallization.

  From this, in order to sufficiently obtain the invention effect from a fiber having a specific core NA, it is necessary to suppress transmission loss, photodarkening and nonlinear characteristics, and to improve conversion efficiency. It is very difficult to simply add Ge, P, and F. Not only optimization of manufacturing conditions such as introduction gas, introduction raw material, and glass deposition temperature, but also precise addition concentration control and combination of additive elements It became clear that adjustment was necessary.

<Evaluation of laser characteristics>
In order to evaluate the laser characteristics, an F-doped fiber was prepared as in the example, and Yb, Al, Ge, or P was added to the core as in Comparative Examples 3 to 11, and the core diameter was 11 ± 1 μm. A so-called polymer clad double clad optical fiber having a guide diameter of 400 μm and a fiber length of 15 m was produced. Then, a dielectric high reflection film for a band of 950 to 1100 nm was formed on one end face of the optical fiber, and the laser oscillation characteristics were evaluated with an excitation wavelength of 976 nm and an incident NA of 0.45. The doped fiber had a light-to-light conversion efficiency at 50 W output of 75 to 80%, which was higher than the light-to-light conversion efficiency (60 to 70%) of the comparative example.

  As described above, the present invention is useful for an optical fiber in which Yb is added to the core.

Claims (5)

A core to which Yb, Al, and F are added;
An optical fiber comprising a clad provided on the outer periphery of the core,
The Yb concentration added to the core is 0.05 mol% or more and 0.5 mol% or less,
Al concentration added to the core is 0.3 mol% or more and 2 mol% or less,
F concentrations are added to the core state, and are less than 1 mol% 0.1 mol% or more,
The optical fiber , wherein the molar ratio of Al to F is 1 or more and 3 or less . A core to which Yb, Al, P and F are added;
An optical fiber comprising a clad provided on the outer periphery of the core,
The Yb concentration added to the core is 0.05 mol% or more and 0.5 mol% or less,
Al concentration added to the core is 0.3 mol% or more and 2 mol% or less,
F concentration added to the core is 0.1 mol% or more and 1 mol% or less,
The concentration of P added to the core is 0.05 or more and 0.5 mol% or less,
The molar ratio of Al to P is 3 or more and 7 or less. A core to which Yb, Al, P and F are added;
An optical fiber comprising a clad provided on the outer periphery of the core,
The Yb concentration added to the core is 0.05 mol% or more and 0.5 mol% or less,
Al concentration added to the core is 0.3 mol% or more and 2 mol% or less,
F concentration added to the core is 0.1 mol% or more and 1 mol% or less,
  The concentration of P added to the core is 0.05 or more and 0.5 mol% or less,
The molar ratio of Al to Yb is 3 or more and 11 or less, the molar ratio of Al to F is 1 or more and 3 or less, and the molar ratio of Al to P is 3 or more and 7 or less.
An optical fiber characterized by that. The optical fiber according to any one of claims 1 to 3 ,
Ge is further added to the core,
The concentration of Ge is 0.05 mol% or more and 0.5 mol% or less,
The molar ratio of Al to Ge is 4 or more and 10 or less. The optical fiber according to any one of claims 1 to 4,
An optical fiber characterized in that the numerical aperture of the core is larger than 0.05 and not larger than 0.15.

Images