JP2013254829A - Rare earth-added double clad fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a rare earth-added double clad fiber with a high conversion efficiency and high reliability.SOLUTION: The rare earth-added double clad fiber 1 comprises: an Yb-added core 2; a first clad 3 surrounding the core 2; and a second clad 4 surrounding the first clad 3. The core 2 has a numerical aperture NA between 0.06 and 0.15 inclusive, and a core diameter D0 between 20 μm and 100 μm inclusive. The ytterbium(Yb) is added to only a part of the region of the core 2 ranging from the center of the core 2 to the first clad 3. The Yb-added region expands to 0.11-2.1 times as large as the LP01 mode diameter of the core 2 from the center of the core toward the first clad 3.

Description

  The present invention relates to a rare earth-doped double clad fiber.

  Rare earth doped optical fibers doped (added) with rare earth elements such as Er (erbium) and Yb (ytterbium) are widely used as amplifiers for optical communication signals. In particular, fiber lasers and optical fiber amplifiers using Yb-added optical fibers are widely applied in processing technology 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 technology field, a laser having high output, high quality, and high reliability has been demanded. In order to increase the output, it is conceivable to input a high-intensity laser beam into the optical fiber. As a fiber that can input high-intensity laser light into an optical fiber, an air hole type double clad fiber or a polymer type double clad fiber has been proposed (for example, Non-Patent Document 1).

  On the other hand, when a laser resonator using a Yb-doped fiber is configured, in order to oscillate a high output from the fiber laser, it is necessary to change from a low pump power to a high pump power. However, at a low pumping power, a part of the Yb-doped fiber becomes a saturable absorber, whereby reabsorption by the saturable absorber occurs and self-pulses are generated. This self-pulse is generated in the range of several kW to several hundred kW, causing slight end face damage and breaking the end face of the fiber.

  Non-Patent Document 2 describes a method of configuring an external circuit, offsetting the excitation power, and suppressing the self-pulse, but the self-pulse suppression effect is not perfect and needs to be improved.

  Non-Patent Document 3 describes that a self-pulse can be suppressed by sandwiching a manual fiber between Yb-doped fibers, increasing the resonant reciprocation distance of oscillation light, and adjusting the carrier relaxation time. In addition, it is necessary to prepare a separate fiber or to align the Yb-doped fiber with the alignment, resulting in an extra cost.

  Non-Patent Document 4 describes a center addition method as a method for improving the conversion efficiency of a rare earth-doped fiber. This method relates to a single-mode fiber and an Er-doped fiber, and as a self-pulse suppression method. Is not listed.

  Furthermore, high photodarkening resistance is required in order to increase the reliability of the Yb-doped optical fiber. Here, the photodarkening caused by the mechanism of forming the color center is a main cause for deteriorating the output of the optical fiber, and may occur in proportion to about the seventh power of the excitation inversion distribution or the excitation concentration of Yb ions. It is known (for example, see Non-Patent Document 6). It is generally known that photodarkening occurs due to clustering of added Yb ions. The clustering of Yb ions is a phenomenon caused by oxygen diffusion, because the added Yb ions cannot be sufficiently diffused (separated), and appears significantly when the concentration of Yb addition is increased.

J. Kim et al., Fiber Design For High-Power Low-Cost Yb: Al-Doped Fiber Laser Operating at 980 nm, IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, Vol. 13 (No. 3), 2007 V. Mizrahi et al., J. Lightwave Technol. Vol. 11, p2021 (1993) Elimination of Self-Pulsations in Dual-Clad Ytterbium-Doped Fiber Lasers, LLE Review, Vol. 115 Modeling Erbium-Doped Fiber Amplifiers, C. Randy Giles eral., JOURNAL OF LIGHTWAVE TECHNOLOGY. Vol. 9 (No. 2), 1991 Ajoy Ghatak et al., Introduction to Fiber Optics, Cambridge University Press, p435-437 Joona Koponen. Et al., “Photodarkening Measurements in Large-Mode-Area Fibers”, SPIE Photonics West 2007, 2007, Vol. 6453-50

  By the way, in order to realize high nonlinear tolerance and high beam quality fiber, it is necessary to reduce the core NA (numerical aperture) and widen the effective core area.

  However, in a double-clad fiber having a high-order multimode with a core diameter of 20 μm or more and 100 μm or less, since the core area is wide, the high-order mode in the low excitation state is not sufficiently excited and works as a saturable absorber. Therefore, the Q switching state is entered. The peak intensity of the self-pulse varies depending on the Yb-doped fiber length and the total Yb absorption of the fiber, but can reach up to several tens of kW, which is a serious problem that breaks the fiber laser system as well as the end face of the fiber.

  Therefore, there is a problem that the self-pulse occurs in a low excitation state before the fiber laser resonator reaches the quasi-continuous oscillation, and the fiber is broken.

  The present invention has been made in view of this point, and an object of the present invention is to make a rare earth-doped double clad fiber having high conversion efficiency and high reliability.

In order to achieve the above object, in the first invention,
A core to which Yb is added;
A first cladding surrounding the core;
For a rare earth-doped double clad fiber having a second clad surrounding the first clad,
In the rare earth-doped double clad fiber,
The numerical aperture of the core is 0.06 or more and 0.15 or less,
The core has a diameter of 20 μm or more and 100 μm or less,
The Yb is added only in a partial region from the center of the core toward the first cladding,
The partial region is 0.11 to 2.1 times the LP01 mode diameter of the core from the core center toward the first cladding.

  Here, when the numerical aperture NA of the core is smaller than 0.06, light confinement is weak and the light conversion efficiency is lowered, so the core NA needs to be increased to 0.06 or more, and when larger than 0.15, Beam quality is degraded. However, by setting NA to 0.06 or more and 0.15 or less, confinement is strong and high beam quality can be realized.

  If the core diameter is smaller than 20 μm, it is difficult to increase the output. If the core diameter is larger than 100 μm, the pump guide diameter needs to be increased, and the fiber becomes hard and cannot be bent. It is difficult to reduce the weight, and the fiber cooling effect is lowered, so that the laser oscillation characteristics and reliability are lowered. However, in the present invention, since the core diameter is 20 μm or more and 100 μm or less, high output and miniaturization of the apparatus can be realized. If the Yb addition region is smaller than 0.11 times the LP01 mode (fundamental mode) diameter of the core from the core center toward the first cladding, it is necessary to increase the Yb addition concentration in order to increase the Yb absorption coefficient. Photodarkening resistance tends to decrease due to ion clustering, and when it is larger than 2.1, a high-order mode inversion distribution is low in a low excitation state, and a self-pulse occurs because it becomes a saturable absorber. However, in the present invention, since it is 0.11 times or more and 2.1 times or less, a fiber having high photodarkening resistance and no self-pulsing can be obtained. In this way, by adding Yb, which is a rare earth element, only to a partial region of the core center, an inversion distribution state that causes quasi-continuous oscillation even in a low excitation state is created, and self-pulse generation is suppressed.

In the second invention, in the first invention,
The outer diameter of the second cladding is not less than 10 times and not more than 40 times the core diameter.

  For example, when the outer diameter of the second cladding is 10 times the core diameter or smaller, the power of the pumping light input to the fiber is limited, so that a high-power laser becomes difficult. Conversely, if it is larger than 40 times, it is necessary to increase the concentration of Yb added to the core in order to increase the absorption coefficient of the pump guide, and the photodarkening resistance may be deteriorated. However, according to the above configuration, since it is 10 times or more and 40 times or less, it is possible to realize high output of the fiber laser and high photodarkening resistance.

In the third invention, in the first or second invention,
The absorption coefficient of the core in the 915 nm wavelength band is 50 dB / m or more and 300 dB / m or less.

  For example, when the absorption coefficient of the core is smaller than 50 dB / m, it is necessary to lengthen the fiber because the absorption coefficient of the pump guard is low, so that non-linear resistance is reduced and self-pulses are easily generated. On the other hand, if it is higher than 300 dB / m, it is necessary to increase the concentration of Yb added in the core, which may reduce the photodarkening resistance. However, according to the above configuration, since it is 50 dB / m or more and 300 dB / m or less, a reduction in nonlinear characteristics and low photodarkening resistance are not caused, and a high-power fiber laser can be realized.

  As described above, according to the present invention, the numerical aperture of the core is 0.06 or more and 0.15 or less, the core diameter is 20 μm or more and 100 μm or less, and Yb is in a partial region from the core toward the first cladding. By adding this Yb-doped region from the center of the core toward the first cladding, the LP01 mode diameter of the core is 0.11 times or more and 2.1 times or less, thereby making the rare earth-doped double-clad fiber high conversion efficiency and high reliability. It can have a sex.

It is a front view which shows schematic structure of the fiber used by simulation. It is a graph which shows the average inversion distribution with respect to 100% Yb addition. It is a graph which shows the ASE back output with respect to 100% Yb addition. It is a graph which shows the ASE output ratio of each mode with respect to 100% Yb addition. It is a graph which shows the average inversion distribution with respect to 63% of Yb center addition. It is a graph which shows the ASE output ratio of each mode with respect to 63% of Yb center addition. It is a graph which shows the addition profile of 47% of Yb cyclic | annular addition. It is a graph which shows the average inversion distribution with respect to 47% of Yb cyclic | annular addition. It is a graph which shows the ASE output ratio of each mode with respect to the excitation power of 47% Yb cyclic | annular addition. It is a graph which shows the emitted beam pattern depending on a Yb addition profile. It is a graph which shows the outgoing beam pattern depending on a core diameter and a Yb addition profile. It is a table | surface which shows the correlation of a core diameter and the mode filled diameter of LP01. It is a fiber block diagram for simulating the laser output change with respect to Yb center addition ratio change. It is a table | surface which shows the output of the fiber laser with respect to Yb center addition ratio. It is a table | surface which shows the laser evaluation result regarding 63% Yb center addition fiber. It is a table | surface which shows the laser evaluation result of 47% Yb cyclic | annular addition fiber. It is a table | surface which shows the laser evaluation result of 100% Yb addition fiber. It is a figure which shows the pulse waveform and oscillation spectrum of an Example and a comparative example. It is a graph which expands and shows the XIX part of FIG. It is a graph which expands and shows the IIX part of FIG. It is a graph which expands and shows the IIXI part of FIG. It is a graph which expands and shows the IIXII part of FIG. It is a graph which expands and shows the IIXIII part of FIG.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  As shown in FIG. 1, a rare earth-doped double clad fiber 1 of this embodiment has a core 2 doped with Yb. The numerical aperture (NA) of the core 2 is 0.06 to 0.15 (0.06 ≦ NA ≦ 0.15), and the core diameter D0 is 20 μm to 100 μm (20 μm ≦ D0 ≦ 100 μm). is there.

  The core 2 is surrounded by the first cladding 3. Yb is added in a partial region from the core 2 toward the first cladding 3. The Yb-added region is 0.11 to 2.1 times the LP01 mode (fundamental mode) diameter of the core 2 from the center of the core 2 toward the first cladding 3.

  Further, the first cladding 3 is surrounded by the second cladding 4. The outer diameter (fiber diameter) D2 of the second cladding 4 is not less than 10 times and not more than 40 times (10D0 ≦ D2 ≦ 40D0) of the core diameter D0.

  In addition, the absorption coefficient of the core in the 915 nm wavelength band is 50 dB / m or more and 300 dB / m or less.

  In the rare earth-doped double clad fiber 1 of this embodiment, a rare earth element is added only near the center of the core 2 to create an inversion distribution state that causes quasi-continuous oscillation even in a low excitation state, thereby suppressing self-pulse generation. It is configured as follows.

<Simulation 1>
In order to investigate the influence of self-pulse in the low excitation state of the multimode fiber doped with Yb, the ASE rear power ratio of each mode and the inversion distribution ratio of each mode are calculated in the Yb-doped fiber for the entire core (100%). did. Inversion distribution means that when excitation light (Pump Laser Diode = PLD) is input to a fiber, electrons in the ground level (N1) are excited to the excitation level (N2). This is a numerical value indicating how many carriers have been excited up to the level (N2), and appears in the following equation (1).

  Here, N = N1 + N2. Since the inversion distribution of the Yb-doped fiber is affected by the pump wavelength, pump power, core diameter D0, fiber length, fiber structure, Yb-doped concentration, etc., the status of each parameter must be determined.

  As shown in FIG. 1, the core NA is 0.06, the core diameter D0 is 30 μm, the pump guide diameter D1 which is the outer diameter of the first cladding 3 is 260 μm, the fiber length is 6.5 m, and the Yb concentration is 1 wt% (weight). In the rare earth-doped double clad fiber (percentage ratio), the ASE (amplitude stimulated emission) backward output ratio and average inversion distribution of each mode with respect to the excitation power were calculated. Here, in the calculation of the average inversion distribution, the inversion distribution value calculated by (Equation 1) was divided by the length of the fiber. Yb was added to the entire core, and an excitation (PLD) wavelength of 976 nm was used.

  According to FIG. 2, the average inversion distribution of the Yb-doped fiber increases as the pump power is increased, and the inversion distribution is saturated from about 12 W. A 4% average inversion distribution, predicted to be free of self-pulses, was obtained with an excitation power of 5W.

  According to FIG. 3, the ASE output increases with increasing pump power, and the tendency is similar to the change of the inversion distribution with respect to the pump power. Therefore, it is considered that the output of the fiber laser increases in proportion to the increase in inversion distribution.

  According to FIG. 4 showing the ASE output ratio of each mode with respect to the excitation power, there are five superficial modes with respect to the 30 μm core diameter D 0, LP01 being the basic mode, LP11, LP21, LP02, LP31 being the higher order modes. become. The output ratio of LP01 with respect to the 5 W reference of low excitation power near the oscillation threshold is 30.3%, LP11 is 19.9%, LP21 is 9.8%, LP02 is 5.7%, and LP31 is 2.1%. there were. Here, LP11, LP21, and LP31 are propagated toward the front and rear, so that the calculation result is doubled. Note that LP11 / 2, LP21 / 2, and LP31 / 2 in FIG. 4 mean half of the ASE power of LP11, LP21, and LP31. Since the average inversion distribution for 5 W excitation power is 3.9%, the average inversion distribution of each mode is 1.18%, 0.78%, 0.38%, 0.22%, 0.08%. Except for LP01, it is less than 1.0%, so it has an unstable inversion distribution, and there is a high possibility of causing a self-pulse in a low excitation state. Therefore, LP11, LP21, LP02, and LP31 whose average inversion distribution is less than 1.0% are not sufficiently excited, become supersaturated absorbers and reabsorption losses, and are considered to be factors that cause self-pulses.

<Simulation 2>
In the simulation for Yb center addition of 63% core, the rare earth addition double that core NA is 0.06, core diameter D0 is 30 μm, pump guide diameter D1 is 260 μm, fiber length is 6.5 m, and Yb concentration is 2.0 wt%. In the clad fiber, the ASE rear output ratio and the average inversion distribution of each mode with respect to the excitation power were calculated. According to FIG. 5 showing the average inversion distribution with respect to 63% center addition, unlike the average inversion distribution with 100% center addition, the average inversion distribution exceeds 4.3% even at a low excitation power of 5 W. It is considered excited.

  According to FIG. 6, there are five superficial modes for the core 30 μm, which are LP01, LP11, LP21, LP02, and LP31. The output ratio of LP01 with respect to the reference is 96.8%, LP11 is 1.1%, LP21 is 0.049%, LP02 is 0.67%, and LP31 is 0.005 with a low excitation power of 5 W near the oscillation threshold. Only LP01 contributed to the ASE output. Since the average inversion distribution for the excitation power of 5 W is 4.3%, the average inversion distribution in each mode is 4.1%, 0.05%, 0.002%, 0.02%, 0.0002%. LP01 has a considerably high inversion distribution. The inversion distribution of LP11, LP21, LP02, and LP31 is less than 1.0%. However, since Yb is not added to the outer periphery of the core, it does not become a saturable absorber and does not cause a self pulse. Here, LP11, LP21, and LP31 are propagated toward the front and rear, so that the calculation result is doubled.

<Simulation 3>
In the simulation of 47% core Yb addition, rare earth-added double with core NA 0.06, core diameter D0 30 μm, pump guide diameter D1 260 μm, fiber length 6.5 m, and Yb concentration 1.5 wt% In the clad fiber, the output ratio and average inversion distribution of each mode with respect to the excitation power were calculated.

  According to FIG. 7, Yb is not added up to 63% from the center, and is added from 9.4 μm to 15 μm at the interface of the first cladding 3.

  As shown in FIG. 8, the average inversion distribution for 47% cyclic addition is different from the average inversion distribution of 63% center addition, and the average inversion distribution at a low excitation power of 5 W is 1.5%. Shows a considerably low value. Therefore, it is predicted that 63% center addition will give higher light-light conversion efficiency than 47% cyclic addition.

  According to FIG. 9 showing the ASE rear output ratio of each mode with respect to the excitation power, there are five modes for the core 30 μm, which are LP01, LP11, LP21, LP02, and LP31. The output ratio of LP01 was 33.1%, LP11 was 5.0%, LP21 was 29.5%, LP02 was 7.08%, and LP31 was 15% at a low excitation power of 5 W near the oscillation threshold. Since the average inversion distribution for the excitation power of 5 W is 1.5%, the average inversion distribution in each mode is 0.48%, 0.67%, 0.42%, 0.10%, 0.20%. Yes, the inversion distribution of all modes was 1.0% or less. Here, LP11, LP21, and LP31 are propagated toward the front and rear, so that the calculation result is doubled. In particular, the inversion distribution of LP01 and LP11 is relatively high, but Yb is not added to the LP01 and LP11 regions, so it is not related to the self pulse. LP21, LP02, and LP31 below 1.0% become a saturable absorber and reabsorption loss, which causes self-pulse. In addition, due to the characteristics of the cyclic addition profile, even if the ASE ratio of LP01 and LP11 is high, the Yb addition profile and the propagation mode do not combine, and thus do not contribute to stimulated emission. Therefore, higher order modes such as LP21, LP02, and LP31 having a low inversion distribution contribute to laser oscillation, become a saturable absorber unstable to oscillation, cause self-pulses, and reduce light-light conversion efficiency. .

  As can be seen from the simulation result for the addition profile in FIG. 10 showing the output beam pattern with respect to the Yb addition profile, the output beam pattern greatly depends on the addition profile. When added to the entire core, the exit beam related to the fundamental mode of LP01 and the higher-order mode can be seen around the core. In the case of 47% annular addition, the fundamental mode of LP01 is not observed, and only the outgoing beam related to the higher order mode is seen. However, with respect to 63% center addition, the fundamental mode of LP01 appears remarkably at the core center, and no higher-order mode emission beam is seen at all. From this result, it can be seen that self-pulsing is suppressed by eliminating the supersaturated absorption region in the high-order mode region in the low excitation state by adding the center.

  The core diameter D0 and the Yb addition diameter were changed, and the shape change of the emitted beam was simulated. According to FIG. 11, when the core NA is 0.06 or more and 0.15 or less and the core diameter D0 is 20 μm or more and 100 μm or less, the intensity of the low-mode emission beam such as LP01 is low when the Yb-added region is 70% or less of the core. It was found to be higher than the outgoing beam intensity of the surrounding higher-order modes. From this result, it is determined that the self-pulse can be suppressed up to 70% of the core diameter D0 in the Yb addition region.

  According to FIG. 12, the LP01 (basic mode) mode filled diameter (Non-Patent Document 5) greatly depends on the core NA and the core diameter D0, and the larger the core diameter D0, the larger the mode filled diameter MFD of LP01. It was. Further, the mode field diameter (MFD) of LP01 increases as the core diameter D0 decreases. For example, when the core NA is 0.08 and the core diameter D0 is 20 μm, the mode field diameter of LP01 is 23. When the core NA is 0.15 and the core diameter D0 is 40 μm, the mode filled diameter of LP01 is 19.7 μm. Furthermore, when the core NA is 0.08 and the core diameter D0 is 40 μm, the ratio of the LP01 mode filled diameter to 40% of the center addition is 0.27, and the ratio of the LP01 mode filled diameter to 80% of the center addition is 1.4. The Yb addition diameter can be expressed as a function of the LP01 mode diameter, and a preferable Yb center addition range is a range of 0.1 to 2 times the LP01 mode filled diameter.

<Simulation 4>
FIG. 13 is a fiber configuration diagram for simulating the laser output change with respect to the Yb center addition ratio change. Pump power in rare earth-doped double clad fiber with core NA of 0.08, core diameter D0 of 40 μm, pump guide diameter D1 of 260 μm, fiber length of 6.5 m, and Yb concentration of 0.5 wt% to 1.5 wt% The output ratio of each mode and the average inversion distribution were calculated. The excitation wavelength was 976 nm.

  According to FIG. 14, the output of the fiber laser with respect to the Yb center addition ratio is slightly different between the low excitation state and the high excitation state, but is basically similar. The oscillation output of the fiber with respect to the Yb center addition ratio seems to decrease with a decrease in the Yb center addition ratio, but a similar difference is shown when the addition concentration is optimized. However, Yb ion clustering occurs by increasing the Yb addition concentration, and the Yb addition concentration has a limit in order to cause a problem of reduction in photodarkening resistance and increase in loss. Actually, about 2 wt% or less is preferable, and 1.5 wt% or less is more preferable.

  The fiber length is preferably 5.0 m or more and 20.0 m or less.

  In addition, HR coating and AR coating may be applied to the end face of the optical fiber as needed. The HR coating is an optical thin film having a reflectance of 99% or more in the HR spectrum range of 1010 to 1100 nm. It is preferable to apply. As the AR coat, it is preferable to apply an optical thin film having an AR spectrum having a transmittance of 97% or more from 800 to 1000 nm.

(Example)
In the example, a Yb-doped optical fiber employing Yb as a rare earth element was produced using an MCVD apparatus. Yb (DPM) ₃ was used as the rare earth element supply source. Anhydrous quartz was adopted as the quartz tube.

  The rare earth-doped double clad fiber 1 obtained through the drawing process has a core diameter D0 of 40 ± 3 μm, a pump guide diameter D1 of 600 ± 10 μm, a fiber diameter D2 of 1000 ± 20 μm, and a core numerical aperture NA of 0.07 to 0.075.

  Fiber laser evaluation was performed using the drawn rare earth doped double clad fiber 1. The fiber laser evaluation configuration was the same as that shown in FIG.

  The fiber length was 15 m, HR / AR coating was applied to one end face of the fiber, and flat end face processing was performed on the other end face. The HR spectroscopy was made to have a transmittance of 97% in the range of 1010 to 1100 nm, and the AR spectroscopy was made to have a transmittance of 95% or more from 800 to 1010 nm. The excitation wavelength was 975 nm.

  According to FIG. 15, when the PLD application current is 90 A, the maximum light-light conversion efficiency is 70%, and the maximum internal conversion efficiency is oscillation power / (pumping power-leakage power) × 100%, which is 74.3%. It was. In particular, no pulse was observed at the excitation power in the evaluation region, and it was found that the addition of the center was effective in suppressing the self-pulse as in the simulation results.

(Comparative Example 1)
In order to confirm the center addition effectiveness, fiber fabrication and laser evaluation were performed on a 47% annularly added fiber. Core production and fiber formation were similar to Example 1, but Yb was added near the outer periphery of the core in the production of the core, and Yb was not added to the core center.

  The optical fiber obtained through the drawing process has a core diameter D0 of 40 ± 3 μm, a pump guide diameter D1 of 600 ± 10 μm, a fiber diameter D2 of 1000 ± 20 μm, and a core numerical aperture NA of 0.07 to 0.075. there were. The absorption coefficient in the 915 nm band of the core forming part was 80 to 120 dB / m.

  Fiber laser evaluation was performed using the drawn fiber. The fiber laser evaluation configuration was the same as that shown in FIG. The fiber length was 15 m, HR / AR coating was applied to one end face of the fiber, and flat end face processing was performed on the other end face. The HR spectroscopy was made to have a transmittance of 97% in the range of 1010 to 1100 nm, and the AR spectroscopy was made to have a transmittance of 95% or more from 800 to 1010 nm. The excitation wavelength was 975 nm.

  According to FIG. 16, the maximum light-light conversion efficiency is 64.8% and the internal conversion efficiency is 72.3% at the PLD application current of 90 A, and the maximum light-light conversion efficiency of 74% in Example 1 is 74. It was lower than the maximum internal conversion efficiency of 3%. In particular, self-pulses were observed over the entire range of pump powers evaluated. Thus, it can be seen from the simulation results that the origin of the self-pulse is due to the added region on the outer periphery of the multi-mode core.

(Comparative Example 2)
In order to confirm the effectiveness of center addition, laser evaluation was performed on a fiber added to the entire core (100 center addition). The core fabrication and fiberization are similar to the examples, but Yb was added to the entire core in the core fabrication.

  The rare earth-doped double clad fiber obtained through the drawing process has a core diameter D0 of 40 ± 3 μm, a pump guide diameter D1 of 600 ± 10 μm, a fiber diameter D2 of 1000 ± 20 μm, and a core numerical aperture NA of 0.07-0. 0.075. The absorption coefficient in the 915 nm band of the core forming part was 100 to 120 dB / m.

  Fiber laser evaluation was performed using drawn rare-earth doped double clad fiber. The fiber laser evaluation configuration was the same as that shown in FIG. The fiber length was 9.6 m, HR / AR coating was applied to one end face of the fiber, and flat end face processing was performed on the other end face. The HR spectroscopy was made to have a transmittance of 97% in the range of 1010 to 1100 nm, and the AR spectroscopy was made to have a transmittance of 95% or more from 800 to 1010 nm. The excitation wavelength was 975 nm.

  According to FIG. 17, at a PLD application current of 90 A, the maximum light-light conversion efficiency is 62.8% and the internal conversion efficiency is 71.2%, which is 70% of the maximum light-light conversion efficiency of Example 1, 74 It was lower than the maximum internal conversion efficiency of 3%. In particular, self-pulses were observed up to an excitation power of 57 W, but were not observed thereafter. This is because the inversion distribution of the higher-order core mode (the outer periphery of the core) is low at low pumping power and becomes a supersaturated absorber. However, at high pumping power, the inversion distribution increases to the core and the core outer periphery. This is because the area is completely gone. Thus, it can be seen from the simulation results that the origin of the self-pulse is due to the addition of the outer periphery of the multi-mode core.

  According to FIG. 18, the total fiber absorption of the center-added fiber of the example is 12.6 dB, the fiber total absorption of Comparative Example 1 is 9.4 dB, and the fiber total absorption of Comparative Example 2 is 9.4 dB. Total absorption is higher than the comparative example. Since the self-pulse of the fiber appears more conspicuously as the total fiber absorption is higher, the self-pulse should not be observed although the example should be more likely to generate the self-pulse. However, in Comparative Examples 1 and 2 where the total absorption coefficient of the fiber is low, a pulse was already observed from a low excitation state, and self-pulse generation was particularly remarkable from Comparative Example 2.

  Since the oscillation spectrum at the excitation power of 62 W is wider in Comparative Examples 1 and 2 than in the Example, it was considered that the self-pulse was due to higher-order mode oscillation.

  From the above viewpoint, the core has a multimode core 2 with a Yb absorption coefficient of 50 to 300 dB / m and a core diameter D0 of 20 to 100 μm in a 915 nm wavelength band, and a core NA of 0.06 to 0.15. In a rare earth-doped double clad fiber, it is preferable to add Yb in a range of 40% to 70% of the core diameter. If the Yb center addition is 40% or less, the Yb concentration must be increased, so that Yb clustering increases, and photodarkening resistance and conversion efficiency may deteriorate. On the other hand, if the ratio is 70% or more, the higher-order mode effect appears remarkably, a self-pulse is generated, and the fiber is broken.

  In addition, the above embodiment is an essentially preferable illustration, Comprising: It does not intend restrict | limiting the range of this invention, its application thing, or a use.

  As described above, the present invention is useful for a rare earth-doped double clad fiber having a first cladding surrounding a core and a second cladding surrounding the first cladding.

1 Rare earth doped double clad fiber
2 core
3 First cladding
4 Second clad

Claims (3)

A core to which Yb is added;
A first cladding surrounding the core;
A rare earth-doped double clad fiber having a second clad surrounding the first clad,
The numerical aperture of the core is 0.06 or more and 0.15 or less,
The core has a diameter of 20 μm or more and 100 μm or less,
The Yb is added only in a partial region from the center of the core toward the first cladding,
The rare earth-doped double clad fiber, wherein the partial region is 0.11 to 2.1 times the LP01 mode diameter of the core from the center of the core toward the first clad. In the rare earth-doped double clad fiber according to claim 1,
The rare earth-doped double clad fiber, wherein an outer diameter of the second clad is 10 to 40 times the core diameter. In the rare earth-doped double clad fiber according to claim 1 or 2,
A rare earth-doped double clad fiber, wherein the core has an absorption coefficient of 50 dB / m to 300 dB / m in the 915 nm wavelength band.