WO2012132908A1

WO2012132908A1 - Optical fiber, fiber laser, and method for manufacturing optical fiber

Info

Publication number: WO2012132908A1
Application number: PCT/JP2012/056589
Authority: WO
Inventors: 靖藤本
Original assignee: 国立大学法人大阪大学
Priority date: 2011-03-31
Filing date: 2012-03-14
Publication date: 2012-10-04
Also published as: JPWO2012132908A1; JP5930316B2

Abstract

This optical fiber (10) is provided with a core (12), a first clad (14a) in contact with the core (12), and a second clad (14b) provided around the first clad (12). The core (12) has a higher refractive index than the first clad (14a), and the first clad (14a) has a higher refractive index than the second clad (14b). In a cross-section, the length of the core (12) measured along a first direction is different from the length of the core measured along a second direction.

Description

Optical fiber, fiber laser, and optical fiber manufacturing method

The present invention relates to an optical fiber, a fiber laser, and an optical fiber manufacturing method.

An optical fiber is used as an optical waveguide. The optical fiber includes a core and a clad provided around the core. Typically, the core has a circular shape and the cladding has a cylindrical shape in the cross section of the optical fiber. A fiber laser including an optical fiber has advantages such as high output and miniaturization, and is used for laser processing. For the fiber laser, an optical fiber having a double clad structure in which two clads are provided around a core is used (see, for example, Patent Document 1).

JP 2007-134626 A

As described above, the core is typically circular, and the optical fiber emits a substantially circular light beam. However, when the light beam is used, the light beam may be deformed into a shape other than a circular shape. For example, when performing line scanning of a light beam, the shape of the light beam may be transformed into a line beam using a predetermined optical component. In this case, the light emitted from the optical fiber needs to be deformed using a predetermined optical component.

The present invention has been made in view of the above problems, and an object of the present invention is to provide an optical fiber, a fiber laser, and an optical fiber manufacturing method suitable for emitting or entering a light beam having a desired shape.

An optical fiber according to the present invention is an optical fiber comprising a core, a first clad in contact with the core, and a second clad provided around the first clad, wherein the core is the first clad. The first cladding has a higher refractive index than the second cladding, and the length along the first direction of the core in the cross section is the core. Different from the length along the second direction.

In one embodiment, the first cladding has two regions separated from each other.

In one embodiment, the core is in contact with the second cladding.

In one embodiment, in cross section, the core has a substantially rectangular shape, and the first cladding has a semicircular shape.

In one embodiment, each of the core and the first cladding is doped with a rare earth element.

In one embodiment, the core is doped with neodymium or ytterbium.

In one embodiment, light of a predetermined wavelength propagates in multimode for a length along the first direction of the core and propagates in single mode for a length along the second direction of the core. Is done.

The optical fiber of the present invention is an optical fiber including a core and a clad, and light of a predetermined wavelength is propagated in a multimode with respect to a length along the first direction of the core, and the core Is propagated in a single mode for a length along the second direction.

In one embodiment, in a cross section, the length along the first direction of the core is different from the length along the second direction of the core.

The fiber laser according to the present invention is a fiber laser including a pumping light source and a resonator, and the resonator includes the above-described optical fiber, a rear mirror, and a front mirror.

The optical fiber manufacturing method of the present invention includes a step of preparing a core member having a core material and two first clad members each having a first clad material, and the core member being the two first clad members. Forming a molded body sandwiched between, a second cladding member having a second cladding material, the second cladding member having an opening provided therein, and the molded body as the second cladding member. Including a step of forming a preform by being inserted into the opening, and a step of removing a fiber from the preform.

In one embodiment, the manufacturing method further includes a step of cutting the molded body so as to be aligned with the opening of the second clad member before the step of forming the preform.

In one embodiment, the step of forming the molded body includes a step of polishing at least one of the core member and the two first clad members.

The optical fiber manufacturing method of the present invention includes a step of preparing a rod having a cross section different in length along the first direction from a length along the second direction, a step of forming a core around the rod, Forming a clad around the core.

According to the present invention, it is possible to provide an optical fiber suitable for emission or incidence of a light beam having a desired shape.

(A) is typical sectional drawing which shows embodiment of the optical fiber by this invention, (b) is a graph which shows the change of the refractive index in the optical fiber of this embodiment, (c) is this embodiment. It is a schematic diagram which shows the shape of the light beam radiate | emitted from this optical fiber. (A) is typical sectional drawing of the optical fiber of the comparative example 1, (b) is a schematic diagram which shows the shape of the light beam radiate | emitted from the optical fiber of the comparative example 1. FIG. (A) is a schematic diagram for demonstrating the deformation | transformation to the line beam of the light beam radiate | emitted from the optical fiber of the comparative example 1, (b) is the line of the light beam radiate | emitted from the optical fiber of this embodiment. It is a schematic diagram for demonstrating the deformation | transformation to a beam. It is a schematic diagram of the fiber laser of this embodiment. (A) is a schematic diagram for demonstrating the skew ray generate | occur | produced in the fiber laser provided with the optical fiber of the comparative example 1, (b) is typical sectional drawing of the optical fiber of the comparative example 2. FIG. (A) is a schematic diagram for demonstrating the deformation | transformation to the line beam of the light beam radiate | emitted from the optical fiber of the comparative example 1, (b), (c) and (d) are optical fibers of the comparative example 1. It is a schematic diagram which shows the change of the light beam radiate | emitted from. (A) is a schematic diagram for demonstrating the deformation | transformation to the line beam of the light beam radiate | emitted from the optical fiber of this embodiment, (b), (c) and (d) are the optical fibers of this embodiment. It is a schematic diagram which shows the change of the light beam radiate | emitted from. (A) is a schematic diagram which shows the light beam radiate | emitted toward the optical fiber of the comparative example 3, (b) is a schematic diagram which shows the light beam radiate | emitted toward the optical fiber of this embodiment. (A)-(e) is a schematic diagram for demonstrating the manufacturing method of the optical fiber of this embodiment. It is a schematic diagram which shows the optical fiber of this embodiment. It is a graph which shows the laser oscillation spectrum in the fiber laser of this embodiment. It is a graph which shows the input / output characteristic of the laser oscillation in the fiber laser of this embodiment. It is a graph which shows the input / output characteristic of the laser oscillation in the fiber laser of this embodiment. It is a figure which shows the beam profile in the fiber laser of this embodiment. It is a typical sectional view of the optical fiber of this embodiment. It is a typical sectional view of the optical fiber of this embodiment. It is a schematic diagram of the optical fiber of this embodiment. It is a schematic diagram of the optical fiber of this embodiment. (A) And (b) is the schematic diagram which showed the fiber laser of this embodiment.

Hereinafter, an optical fiber and an optical fiber manufacturing method according to the present invention will be described with reference to the drawings. In addition, this invention is not limited to the following embodiment.

FIG. 1A is a schematic cross-sectional view of the optical fiber 10 of the present embodiment. The optical fiber 10 includes a core 12, a clad 14a, and a clad 14b. The core 12 is in contact with the clad 14a, and the clad 14b is provided around the clad 14a. Typically, the area of the clad 14 a is larger than that of the core 12 in the cross section of the optical fiber 10.

In the cross section, the clad 14b has a cylindrical shape. For example, the inner diameter of the clad 14b is about 100 to 350 μm, and the outer diameter of the clad 14b is 125 to 400 μm. Here, the clad 14a has regions 14a1 and 14a2 separated from each other, and the regions 14a1 and 14a2 are both semicircular in cross section. The core 12 is in contact with not only the clad 14a but also the clad 14b. The length of the long side of the core 12 is equal to the inner diameter of the clad 14b.

In this specification, the clad 14a may be referred to as a first clad, and the clad 14b may be referred to as a second clad. The clad 14a is also called an intermediate clad, and the clad 14b is also called an outer clad. Although not shown here, it is preferable to protect the optical fiber 10 by coating the periphery of the clad 14b with a resin (for example, polyamide resin). The resin can suppress breakage of the optical fiber 10 when the optical fiber 10 is bent.

FIG. 1B shows a change in refractive index in a cross section taken along the line 1b-1b ′ in FIG. The core 12 exhibits a higher refractive index than the first cladding 14a, and the first cladding 14a exhibits a higher refractive index than the second cladding 14b. For example, the cladding 14b is made of silica, and the core 12 and the cladding 14a are made of silica doped with rare earth elements.

As an example, the core 12 and the cladding 14a are formed by firing a mixture of a rare earth element-fixed zeolite and a medium raw material (for example, a silica raw material). Specifically, a water-soluble rare earth element compound and a zeolite are heated in water and then treated with an ammonium chloride solution to form a rare earth element-fixed zeolite, which is mixed with a silica raw material and calcined. Thus, silica doped with rare earth elements is formed. Thereby, the core 12 and the clad 14a with a high rare earth element concentration can be formed. For example, the cladding 14a may not be doped with a rare earth element.

The light incident on the core 12 is almost totally reflected at the interface between the core 12 and the clad 14 a and travels through the core 12. The light incident on the clad 14a is almost totally reflected at the interface between the clad 14a and the clad 14b, and passes through the interface between the core 12 and the clad 14a and travels into the core 12. Since the optical fiber 10 has the lowest refractive index of the outermost clad 14b, the light incident on the optical fiber 10 can be collected in the core 12, and the intensity of the light output from the core 12 can be increased. . Such an optical fiber 10 is suitably used for an amplifier or a fiber laser.

In the optical fiber 10 of the present embodiment, the length along the direction of the core 12 in the cross section is larger than the length along the other direction. For example, the lengths along two orthogonal directions in the core 12 are different from each other. FIG. 1A is a cross section cut perpendicularly to the direction in which the optical fiber 10 extends. In this cross section, the length of the core 12 along the x direction is larger than the length along the y direction. Here, the cross section of the core 12 is a rectangular shape extending substantially linearly along the longitudinal direction, and the aspect ratio of the core 12 is in the range of 3 (3: 1) to 70 (70: 1). For example, the length of the long side (length along the x direction) of the core 12 is about 100 μm, and the length of the short side (length along the y direction) is about 10 μm.

In the optical fiber 10, the length of the cross section of the core 12 is not constant, and varies depending on the direction. By changing the cross-sectional shape of the core 12 in accordance with the use application of the light beam, the irradiation intensity of the light beam emitted from the optical fiber 10 is made substantially uniform, and the shape of the light beam emitted from the optical fiber 10 is changed. The number of optical components for changing can be reduced. For example, in the case of forming a line beam, if the cross-sectional shape of the core 12 is substantially rectangular as shown in FIG. 1A, the line-shaped light is emitted from the optical fiber 10 as shown in FIG. A beam is emitted. Note that the light beam emitted from the optical fiber 10 may be expanded according to the required length of the line beam. In the optical fiber 10, an optical system for forming a light beam having a desired shape can be simplified.

Hereinafter, advantages of the optical fiber 10 of this embodiment compared to the optical fiber of the comparative example will be described. First, the optical fiber 70 of the comparative example 1 is demonstrated with reference to FIG.

FIG. 2A shows a schematic diagram of the optical fiber 70 of Comparative Example 1. FIG. The optical fiber 70 includes a core 72, a clad 74a, and a clad 74b. The clad 74a is provided around the core 72, and the clad 74b is provided around the clad 74a. The core 72 has a higher refractive index than the clad 74a, and the clad 74a has a higher refractive index than the clad 74b. In the cross section of the optical fiber 70, the core 72 has a circular shape, the

clads

74a and 74b have a cylindrical shape, and the centers of the core 72 and the

clads

74a and 74b are substantially equal. The clad 74a is also called an inner clad, and the clad 74b is also called an outer clad. The optical fiber 70 emits a light beam having a circular cross section, as shown in FIG.

In the case of forming a line beam from the light beam emitted from the optical fiber 70, it is necessary to deform the circular light beam emitted from the optical fiber 70 using an optical member (not shown here). Become. In contrast, the optical fiber 10 of the present embodiment has a core 12 having a substantially rectangular cross section, and the light beam emitted from the optical fiber 10 is suitable for forming a line beam.

As shown in FIG. 3A, the light beam emitted from the optical fiber 70 of Comparative Example 1 is transformed into a line beam. The light beam emitted from the optical fiber 70 is expanded by the magnifying lens F1 and then collected in a specific direction by the cylindrical lens F2, thereby forming a line beam. For example, the diameter of the core 72 of the optical fiber 70 is about 10 μm, the light beam is expanded to a diameter of 5 cm by the magnifying lens F1, and then a line beam having a length of 5 cm is formed by the cylindrical lens F2.

As shown in FIG. 3B, the light beam emitted from the optical fiber 10 of this embodiment is transformed into a line beam. For example, as described above, the core 12 of the optical fiber 10 is substantially rectangular, and the optical fiber 10 emits a substantially rectangular light beam. When forming a line beam having a desired length, the light beam emitted from the optical fiber 10 may be isotropically expanded by the magnifying lens F1 as necessary. For example, when the minor axis and major axis of the core 12 of the optical fiber 10 are about 10 μm and about 100 μm, respectively, the magnifying lens F1 expands the minor axis and major axis of the light beam to about 0.5 cm and about 5 cm, respectively. A line beam having a length of about 5 cm may be formed. Although not shown here, the light beam emitted from the optical fiber 10 may be deformed in a specific direction using a cylindrical lens, if necessary.

The core 12 can propagate light in both multimode and single mode. As shown in FIG. 1A, the length of the cross section of the core 12 varies depending on the direction. Therefore, the length of the core 12 and the length of the core 12 and the cladding are dependent on the wavelength of light propagating through the core 12. By appropriately setting the refractive index of 14a, light of a predetermined wavelength is propagated in multimode with respect to the length along the x direction of the core 12, and with respect to the length along the y direction of the core 12. Propagated in single mode. When light is propagated in a single mode, propagation loss can be suppressed.

Note that the light propagating through the optical fiber 10 may be a pulse wave or a continuous wave. However, when a pulse wave propagates through the optical fiber 10, if the light propagates in a single mode, deformation of the shape of the pulse wave can be prevented and signal delay can be suppressed. In addition, when light propagation is performed in a single mode, the optical fiber 10 is preferably not bent so much.

The optical fiber 10 is preferably used as part of a fiber laser. Hereinafter, the fiber laser 100 of this embodiment provided with the optical fiber 10 will be described with reference to FIGS. 1 and 4.

FIG. 4 shows a schematic diagram of the fiber laser 100 of the present embodiment. The fiber laser 100 includes an excitation light source 110 and a resonator 120. The resonator 120 includes the optical fiber 10, a rear mirror 122, and a front mirror 124. The rear mirror 122 is disposed so as to face one end portion of the optical fiber 10, and the front mirror 124 is disposed so as to face the other end portion of the optical fiber 10. Here, in the optical fiber 10 shown in FIG. 1A, the core 12 is doped with neodymium (Nd) or ytterbium (Yb), and the cladding 14a is doped with lanthanum (La).

In the fiber laser 100, excitation light is emitted from the excitation light source 110 toward the resonator 120. The excitation light that has passed through the rear mirror 122 is incident on the core 12 and the clad 14 a of the optical fiber 10. The excitation light is absorbed by the core 12 and light having an oscillation wavelength of neodymium is generated in the core 12. The light generated in the core 12 travels in a state where it is totally reflected at the interface between the core 12 and the clad 14 a and confined in the core 12. The optical fiber 10 is sandwiched between a rear mirror 122 and a front mirror 124, resonance occurs in the resonator 120, and coherent light having an oscillation wavelength is emitted from the resonator 120. The light emitted from the fiber laser 100 may be a pulse wave or a continuous wave.

The fiber laser 100 is suitably used as a processing laser. For example, the fiber laser 100 is suitably used for surface modification of a solar cell or an annealing process such as a flat panel display.

In the optical fiber 10 of the fiber laser 100, since the clad 14a is provided between the core 12 and the clad 14b, the excitation light is efficiently incident. In general, the intensity of light output from the optical fiber 10 also increases until the intensity of light from the excitation light source 110 increases to some extent. However, the power density of light emitted from the optical fiber 10 (power density: unit W / cm ² ) is substantially constant even if the intensity of light from the excitation light source 110 increases to some extent due to the influence of the nonlinear phenomenon. . In the optical fiber 10 of the present embodiment, since the cross-sectional area of the core 12 can be made relatively large, the power of the light beam output from the optical fiber 10 can be increased. In the optical fiber 70 having the circular core 72, when a single mode light beam is emitted at a relatively high power, the refractive index difference between the core 72 and the clad 74a is reduced by simply increasing the cross-sectional area of the core 72. However, since the size of the area is limited due to manufacturing limitations, high power cannot be realized. On the other hand, in the optical fiber 10 of this embodiment, since the cross-sectional area of the core 12 can be made relatively large, the power of the light beam output from the optical fiber 10 can be increased. For example, when the area of the core 12 in the cross section of the optical fiber 10 is 10 times or more than the area of the core 72 in the cross section of the optical fiber 70, the maximum output of the fiber laser 100 including the optical fiber 10 includes the optical fiber 70. 10 times or more than a fiber laser. Thus, since the output of the fiber laser 100 can be improved, the fiber laser 100 is suitably used for processing a large area.

Here, the optical fiber 10 of this embodiment is compared with a fiber laser including the optical fiber 70 of Comparative Example 1. For example, the diameter of the core 74 of the optical fiber 70 of Comparative Example 1 is 25 μm. When the absorption coefficient of the excitation light wavelength 915 nm in this fiber laser is 0.8 dB / m, the fiber length that absorbs 99% (−20 dB) of the excitation light is 25 m (= (20 dB) / (0.8 dB / m). In this case, the stimulated Raman amplification threshold is 3.14 kW and the stimulated Brillouin amplification threshold is 41 W. Typically, a fiber laser including the optical fiber 70 of Comparative Example 1 has a light beam of about 1 kW. Is output.

On the other hand, for example, in the fiber 10 of this embodiment, the length of the long side of the core 12 is 100 μm, and the length of the short side is 10 μm. When the core 12 is formed using zeolite, the absorption coefficient of the excitation light wavelength 915 nm in the fiber laser 100 is about 100 dB / m, and the fiber length for absorbing 99% (−20 dB) of the excitation light is 0.2 m ( = (20 dB) / (100 dB / m) In this case, the stimulated Raman amplification threshold is 628 kW, and the stimulated Brillouin amplification threshold is 825 W. According to such a fiber laser 100, a general margin is taken into consideration. Even in this case, it is possible to obtain an output of about 200 kW As described above, according to the fiber laser 100 of the present embodiment, it is possible to emit a light beam having a higher output than a general fiber laser.

Although not shown here, as described above, the optical fiber 10 is preferably protected with a resin. In the fiber laser 100, since the optical fiber 10 has the clad 14b provided around the clad 14a, the seizure of the resin can be suppressed.

Thus, the clad 14b is used for light confinement. Moreover, since the leakage of light to the outside can be suppressed by the clad 14b, contact cooling with metal or the like can be suitably performed. The clad 14b does not only confine light. As will be described later, since the clad 14b supports the core 12 when the optical fiber 10 is manufactured by drawing, the optical fiber 10 can be easily manufactured.

As described above, in the optical fiber 10, the length of the cross section of the core 12 varies depending on the direction. For this reason, light of a predetermined wavelength propagates in multimode with respect to the length along the x direction of the core 12 and propagates in single mode with respect to the length along the y direction of the core 12. In the case of the fiber laser 100, light having an oscillation wavelength propagates in both multimode and single mode. In FIG. 1A, the length dx along the x direction of the core 12 is larger than 2.405λ / (π√ (n1 ² −n2 ² )), and the length dy along the y direction of the core 12 is When less than 2.405λ / (π√ (n1 ² −n2 ² )), the core 12 can propagate in multimode in the x direction and single mode in the y direction. Here, λ is the wavelength of light propagating through the core 12 in the air, n1 is the refractive index of the core 12, and n2 is the refractive index of the clad 14a.

For example, when the core 12 is doped with neodymium, the oscillation wavelength is 1062 nm. When the refractive index of the core 12 is 1.452 and the refractive index of the clad 14a is 1.450, the calculation result of the above formula is 2.405λ / (π√ (n1 ² −n2 ² )) = 10.7 μm. is there. If this calculation result is between the lengths along different directions of the core 12, the light is propagated in multimode with respect to the length along one direction of the core 12, and along the other direction of the core 12. Propagated in single mode for a given length. For example, when the length of the core 12 along the x direction is 100 μm and the length of the core 12 along the y direction is 10 μm, the light having a wavelength of 1062 nm has a length along the x direction of the core 12. On the other hand, it propagates in multimode and propagates in single mode for the length of core 12 along the y direction.

In addition, although one excitation light source 110 is shown in FIG. 4, the excitation light incident on the optical fiber 10 may be emitted from a plurality of excitation light sources 110 (for example, laser diodes).

When producing a fiber laser using the optical fiber 70 of Comparative Example 1, skew rays may occur. On the other hand, in the fiber laser 100 including the optical fiber 10 of the present embodiment, the occurrence of skew ray can be suppressed.

FIG. 5A shows skew rays generated in the optical fiber 70 of Comparative Example 1. FIG. The excitation light incident on the clad 74a of the optical fiber 70 is almost totally reflected at the interface between the clad 74a and the clad 74b. A certain component of the pumping light passes through the interface between the core 72 and the cladding 74a inside the cladding 74a and reaches the core 72, while another component of the pumping light is generated by the cladding 74a and the cladding 74b inside the cladding 74a. Even if total reflection is repeated at the interface, the light may be emitted from the optical fiber 70 without reaching the core 72 located at the center.

In recent years, it has also been known to form a clad having a cross-sectional shape in which a part of a circle is cut out in order to suppress skew rays. Hereinafter, the optical fiber 80 of Comparative Example 2 will be described with reference to FIG.

FIG. 5B shows a schematic diagram of the optical fiber 80 of Comparative Example 2. The optical fiber 80 includes a core 82, a clad 84a, and a clad 84b. The clad 84a is provided around the core 82, and the clad 84b is provided around the clad 84a. The clad 84a and the clad 84b are also referred to as an inner clad and an outer clad, respectively.

The core 82 has a higher refractive index than the clad 84a, and the clad 84a has a higher refractive index than the clad 84b. In the cross section of the optical fiber 80, the core 82 has a circular shape and the clad 84b has a cylindrical shape. The clad 84a has a partially cut shape of a cylindrical shape. Even in the optical fiber 80, the centers of the core 82 and the

clads

84a and 84b are substantially equal.

In the fiber laser including the optical fiber 80 of Comparative Example 2, the excitation light incident on the clad 84a of the optical fiber 80 is almost totally reflected at the interface between the clad 84a and the clad 84b. A certain component of the excitation light reaches the core 82 through the interface between the core 82 and the clad 84a inside the clad 84a. Another component of the excitation light is totally reflected at the interface between the notched portion of the clad 84a and the clad 84b inside the clad 84a and finally reaches the core 82 located at the center. For this reason, in the optical fiber 80, the occurrence of skew ray can be suppressed.

In the optical fiber 80, the light incident on the clad 84a is often incident on the core 72 when totally reflected at the interface between the clad 84a and the clad 84b a plurality of times, but the light incident on the clad 84a is often incident on the clad 84a. If the number of total reflections at the interface between the clad and the clad 84b is relatively small, the core 82 may not be incident. On the other hand, as shown in FIG. 1A, in the optical fiber 10, since the core 12 extends so as to be in contact with the clad 14b, the occurrence of skew ray can be effectively suppressed. .

In the above description, the silica 12 is doped with a rare earth element to form the core 12 and the clad 14a having a higher refractive index than the clad 14b. However, the present invention is not limited to this. Similarly, elements such as germanium (Ge), aluminum (Al), yttrium (Y), phosphorus (P), magnesium (Mg), calcium (Ca), strontium (Sr) or barium (Ba) are added to silica glass. By doping, the core 12 and the clad 14a having a higher refractive index than the clad 14b may be formed. Alternatively, the

claddings

14a and 14b having a lower refractive index than the core 12 may be formed by doping with fluorine or boron. For example, the cladding 14a may not be doped with an impurity element.

Interference may occur when a single mode light beam emitted from a fiber laser including the optical fiber 70 of Comparative Example 1 is transformed into a line beam. Below, with reference to FIG. 6 and FIG. 7, the advantage of the optical fiber 10 of this embodiment compared with the optical fiber 70 of the comparative example 1 is demonstrated. Here, a line beam having a length of about 5 cm is formed.

FIG. 6A is a schematic diagram for explaining the deformation of the light beam emitted from the optical fiber 70 of Comparative Example 1 into a line beam. 6 (b), 6 (c) and 6 (d), the light beam L1 emitted from the optical fiber 70, the light beam L2 expanded by the magnifying lens F1, and the specific direction by the cylindrical lens F2 are shown. Each of the collected light beams L3 is shown in FIG.

The light beam emitted from the optical fiber 70 is expanded by the magnifying lens F1, and then collected in a specific direction by the cylindrical lens F2, thereby forming a line beam. For example, the diameter of the core 72 of the optical fiber 70 is about 10 μm, the light beam is expanded to a diameter of 5 cm by the magnifying lens F1, and then a line beam having a length of 5 cm is formed by the cylindrical lens F2.

FIG. 7A is a schematic diagram for explaining the deformation of the light beam emitted from the optical fiber 10 of the present embodiment into a line beam. 7B, 7C, and 7D, the light beam L1 emitted from the optical fiber 10, the light beam L2 expanded by the magnifying lens F1, and the specific direction by the cylindrical lens F2 are shown. Each of the collected light beams L3 is shown in FIG.

The light beam emitted from the optical fiber 10 is expanded by the magnifying lens F1, and then collected in a specific direction by the cylindrical lens F2, thereby forming a line beam. For example, as described above, the core 12 of the optical fiber 10 has a substantially rectangular shape, and a substantially rectangular light beam is emitted from the optical fiber 10. Thereafter, the light beam isotropically expanded to the length required for the line beam by the magnifying lens F1.

For example, the short diameter and long diameter of the core 12 of the optical fiber 10 are about 10 μm and about 100 μm, respectively, and when the desired length of the line beam is 5 cm, the short diameter and long diameter of the light beam are respectively set by the magnifying lens F1. It is enlarged to about 0.5 cm and about 5 cm. Thereafter, the light beam is collected by the cylindrical lens F2 until the minor axis becomes about 10 μm.

When the light beam emitted from the optical fiber 70 is deformed to obtain a line-shaped light beam as shown in FIG. 6D, the circular light beam shown in FIG. Interference may occur due to gathering. On the other hand, when the line-shaped light beam as shown in FIG. 7D is obtained by deforming the light beam emitted from the optical fiber 10, only the x direction is expanded. , Light overlap hardly occurs and interference hardly occurs.

Here, the light beam emitted from the optical fiber 10 is deformed by the cylindrical lens F2 to form a line beam. However, the light beam emitted from the optical fiber 10 is used as a line beam without passing through the cylindrical lens. Also good. Further, by moving the light beam emitted from the optical fiber 10 relatively along the x direction (longitudinal direction of the core 12), the light beam can be continuously irradiated over a wide region in a single mode.

Moreover, in the optical fiber 10 of this embodiment, the line beam is efficiently incident. Hereinafter, with reference to FIG. 8, the advantage of the optical fiber 10 compared with the optical fiber 90 of the comparative example 3 is demonstrated.

As shown in FIG. 8A, the line beam is incident on the optical fiber 90 of Comparative Example 3. The light source R1 emits a line beam having a length of about 100 μm toward the optical fiber 90. The optical fiber 90 includes a core 92 having a diameter of 100 μm and a clad 94. Typically, the light beam emitted from the light source R1 is deformed by the lens R2. However, the core 92 of the optical fiber 90 has a circular shape, and the shape of the core 92 of the optical fiber 90 is significantly different from the shape of the light beam emitted from the light source R1, and the beam area becomes large. The brightness will be low.

As shown in FIG. 8B, the line beam is incident on the optical fiber 10 of the present embodiment. The light source R1 emits a line beam having a length of about 100 μm toward the optical fiber 10. For example, the light source R1 is a semiconductor laser. Since the core 12 of the optical fiber 10 has a substantially rectangular shape, it is possible to suppress a reduction in light intensity when the light from the light source R1 enters the optical fiber 10. In FIG. 8B, the light beam from the light source R1 is incident on the optical fiber 10 through the lens R2, but the light beam from the light source R1 enters the optical fiber 10 without passing through an optical component such as the lens R2. It may be directly incident. In this case, the long side of the core 12 is preferably larger than the emitter width of the light source R1. Since the optical fiber 10 has the clad 14a and the clad 14b, the optical fiber 10 is easily coupled with the light source R1 with high efficiency and high luminance.

Hereinafter, a method of manufacturing the optical fiber 10 will be described with reference to FIG. First, as shown in FIG. 9A, a core member A including a material for forming the core 12 and first cladding members B1 and B2 including a material for forming the first cladding 14a are prepared. In the following description, the first cladding members B1 and B2 may be simply referred to as cladding members B1 and B2. Here, the materials and sizes of the clad members B1 and B2 are equal, but the materials and sizes of the clad members B1 and B2 may be different. Here, the width and length of the core member A are equal to the width and length of the cladding members B1 and B2, respectively, but they may be different.

For example, the core member A is silica glass doped with rare earth elements. The core member A is doped with neodymium (Nd). The clad members B1 and B2 are silica glass doped with rare earth elements. For example, the cladding members B1 and B2 are doped with lanthanum (La). For example, the concentration of the rare earth element in the core member A is higher than the concentration of the rare earth element in the cladding members B1 and B2, and the refractive index of the core member A is higher than the refractive index of the cladding members B1 and B2. In addition, the core 12 of the optical fiber 10 to be manufactured can be obtained by setting the refractive index of the core member A and the clad members B1 and B2 and the aspect ratio (width / thickness ratio) of the core member A in advance. It becomes possible to propagate light in both the mode and the single mode.

For example, the core member A is doped with neodymium (Nd). The weight percentage of neodymium is 1.25%. The core member A is further doped with alumina (Al ₂ O ₃ ). The thickness of the core member A is 3 mm, and the width and length of the core member A are 7 mm and 40 mm, respectively.

The clad members B1 and B2 are doped with lanthanum (La) and alumina (Al ₂ O ₃ ). The weight% of La ₂ O ₃ and Al ₂ O ₃ is 0.8 wt% and 1.74 wt%, respectively. The thickness of the clad members B1 and B2 is 3 mm, and the width and length of the clad members B1 and B2 are 7 mm and 40 mm, respectively. Here, the concentration of neodymium in the core member A is higher than the concentration of lanthanum in the clad members B1 and B2, and the ratio of Nd and Al in the core member A is substantially equal to the ratio of La and Al in the clad members B1 and B2. The refractive index of the core member A is higher than that of the clad members B1 and B2.

Next, as shown in FIG. 9B, the clad members B1 and B2 are bonded to both surfaces of the core member A, and the core member A and the clad members B1 and B2 are joined. Thereby, the molded body S in which the clad member B1, the core member A, and the clad member B2 are laminated is formed.

Before joining the core member A and the clad member B1, at least one of the surface of the core member A that joins the clad member B1 and the surface of the clad member B1 that joins the core member A is optically polished. It is preferable to do. By such an optical polishing process, the core members A and B1 are joined by optical contact. The core member A and the clad member B2 are joined in the same manner. The core member A may be relatively thick until just before the clad member B2 is bonded to the core member A. The core member A is bonded to the clad member B1 and then bonded to the clad member B2. It may be thinned by polishing. For example, after the core member A is joined to the clad member B1, the core member A is thinned from about 3 mm to about 0.5 mm and joined to the clad member B2. The molded body S may be formed as described above.

As shown in FIG. 9C, the molded body S is cut to form the molded body S into a rod shape. For example, the diameter of the molded body S is about 5 mm, and the thickness of the core member A is about 0.5 mm.

As shown in FIG. 9D, the rod-shaped molded body S is inserted into the opening of the second clad member C containing the material for forming the second clad 14b. The molded body S ′ thus obtained is also called a preform C ′. For example, the inner diameter and the outer diameter of the second cladding member C are 5 mm and 9 mm, respectively.

Then, as shown in FIG. 9 (e), the optical fiber 10 is taken out from the preform C '. For example, the preform C 'is inserted for drawing an optical fiber, and necked down by heating. Due to the neck-down, the preform C 'whose viscosity has been reduced by heating is pulled downward by its own weight and falls with a small diameter. Thereafter, the optical fiber is drawn in a heated state.

For example, neck-down is performed at 2040 ° C. and drawing is performed at 1910 ° C. For example, the delivery speed of the preform C ′ is 2.0 mm / min, and the take-up speed is 4.0 mm / min. The optical fiber 10 can be manufactured as described above.

FIG. 10 shows the optical fiber 10 formed in this way. The thickness of the core 12 is about 10 μm, the diameter of the clad 14 a is about 100 μm, and the diameter of the clad 14 b is about 180 μm. For example, the refractive index of the core 12 is about 1.452 and the refractive index of the clad 14a is about 1.447 at 1062 nm, which is the oscillation wavelength of neodymium. The refractive index of the cladding 14b is about 1.443. The aperture ratio between the core 12 and the clad 14a is about 0.12, and the aperture ratio between the clad 14a and the clad 14b is about 0.11. In general, the aperture ratio of an optical fiber that is commercially available is almost equal, and the optical fiber 10 can be used in a wide range.

FIG. 11 shows the spectrum of the fiber laser 100 of this embodiment including the optical fiber 10 shown in FIG. In FIG. 11, the peak at a wavelength of 800 nm is due to the leakage light of excitation. Narrowing of the spectrum is confirmed at a wavelength of 1062 nm. It is inferred that laser oscillation was obtained by such narrowing of the spectrum.

FIG. 12 shows input / output characteristics of the fiber laser 100 of the present embodiment. The oscillation threshold of the fiber laser 100 is 1.6W. When the output of the excitation light source 110 is 1.84 W, the fiber laser 100 exhibits a maximum output of 5.25 mW. There was a threshold in the narrowing of the oscillation spectrum and the input / output characteristics, confirming laser oscillation.

FIG. 13 shows input / output characteristics of another fiber laser 100 according to this embodiment. The oscillation threshold of the fiber laser 100 is 480 mW. When the output of the excitation light source 110 is 1657 mW, the fiber laser 100 exhibits a maximum output of 75.1 mW. There was a threshold in the narrowing of the oscillation spectrum and the input / output characteristics, confirming laser oscillation.

FIG. 14 shows a beam profile of the fiber laser 100 of the present embodiment. FIG. 14 shows a profile of amplified spontaneous emission (ASE) immediately before oscillation of the fiber laser 100. As understood from FIG. 14, the region having a high beam intensity is formed corresponding to the core 12 and is long in the x direction and short in the y direction. In this way, a substantially rectangular light beam is emitted from the fiber laser 100.

In addition, although an example of the manufacturing method of the optical fiber 10 was demonstrated with reference to FIG. 9, the manufacturing method of the optical fiber 10 is not limited to this. The optical fiber 10 may be manufactured by an OVD (Outside Vapor Deposition) method. For example, the optical fiber 10 having the substantially rectangular core 12 may be manufactured by preparing a target rod in the OVD method.

First, prepare different length rods according to the direction in the cross section. For example, a rod having a rectangular cross section is prepared. Next, a core is formed around the rod, a clad is formed around the core, and finally a part of the clad is cut. In this way, the optical fiber 10 having a circular cross section can be manufactured.

In the above description, the center of the core 12 in the cross section of the optical fiber 10 is substantially equal to the centers of the

clads

14a and 14b, and the cross section of the optical fiber 10 is substantially point symmetric, but the present invention is not limited to this. Not. The core 12 is disposed at a position different from the center of the clad 14b, and the cross section of the optical fiber 10 may not be point symmetric. For example, the center of the clad 14 b in the cross section of the optical fiber 10 may be located outside the core 12.

In the above description, in the optical fiber 10, the core 12 is in contact with both the clad 14a and the clad 14b, but the present invention is not limited to this. As shown in FIG. 15, the core 12 is surrounded by the clad 14a, and the core 12 may not be in direct contact with the clad 14b.

In the above description, the clad 14b is provided around the clad 14a. However, the present invention is not limited to this. The clad 14b may not be provided around the clad 14a. Such an optical fiber 10 is suitably used as a waveguide.

Hereinafter, the optical fiber 10 will be described with reference to FIG. The optical fiber 10 includes a core 12 and a clad 14. The clad 14 has a higher refractive index than the core 12. For example, the minor axis and major axis of the core 12 are about 10 μm and about 100 μm, respectively.

In the optical fiber 10, the core 12 propagates light in both multimode and single mode. The length dx along the x direction of the core 12 is larger than 2.405λ / (π√ (n1 ² −n2 ² )), and the length dy along the y direction of the core 12 is 2.405λ / (π√ Smaller than (n1 ² -n2 ² )), the core 12 can propagate in both multimode and single mode. Here, λ is the wavelength of light propagating through the core 12 in the air, n1 is the refractive index of the core 12, and n2 is the refractive index of the cladding 14.

In the optical fiber 10, for example, when the core 12 is doped with neodymium, the oscillation wavelength is 1062 nm. When the refractive index of the core 12 is 1.452 and the refractive index of the cladding 14 is 1.450, the calculation result of the above formula is 2.405λ / (π√ (n1 ² −n2 ² )) = 10.7 μm. is there. If this calculation result is between the lengths along different directions of the core 12, the light is propagated in multimode with respect to the length along one direction of the core 12, and along the other direction of the core 12. Propagated in single mode for a given length. For example, when the length of the core 12 along the x direction is 100 μm and the length of the core 12 along the y direction is 10 μm, the light having a wavelength of 1062 nm has a length along the x direction of the core 12. On the other hand, it propagates in multimode and propagates in single mode for the length of core 12 along the y direction.

The optical fiber 10 shown in FIG. 16 is manufactured in the same manner as described above with reference to FIG. 9 except that the optical fiber 10 is formed without using the second cladding member. Or as above-mentioned, the optical fiber 10 may be produced by OVD method.

In addition, in the optical fiber 10 shown in FIG. 16, although the edge part along the major axis direction of the core 12 is not enclosed by the clad | crud 14, this invention is not limited to this. The core 12 may be surrounded by a clad at the end along the minor axis direction of the core 12 and may be surrounded by another clad at the end along the major axis direction of the core 12.

FIG. 17 shows a schematic diagram of the optical fiber 10 of the present embodiment. The typical sectional view of optical fiber 10 of this embodiment is shown. The optical fiber 10 includes a core 12, a clad 14a, and a clad 14b. The core 12 is in contact with the clad 14a, and the clad 14b is provided around the clad 14a. The core 12 is surrounded by the clad 14 a at the end along the minor axis direction of the core 12, and is surrounded by the clad 14 b at the end along the major axis direction of the core 12. In the optical fiber 10 shown in FIG. 17, it propagates in multimode with respect to the length along the major axis direction of the core 12, and propagates in single mode with respect to the length along the minor axis direction of the core 12.

The core 12 has a higher refractive index than the

clads

14a and 14b, and the refractive index of the clad 14a is equal to that of the clad 14b. For example, the

clads

14a and 14b are made of silica, and the core 12 is made of silica doped with a rare earth element. The optical fiber 10 also has a three-layer structure, and when the optical fiber 10 is manufactured by drawing, the cladding 14b supports the core 12, so that the optical fiber 10 can be easily manufactured.

In the above description, the shape of the core 12 in the cross section is substantially rectangular, but the present invention is not limited to this. Depending on the required shape of the light beam, the shape of the core 12 may be triangular, square, L-shaped or elliptical. Further, by effectively connecting optical components between the light beam emitted from the optical fiber 10 and the light beam actually used, a line beam having an arbitrary shape can be formed.

In the above description, one core is provided for one optical fiber, but the present invention is not limited to this. As illustrated in FIG. 18, the optical fiber 10 may be provided with a plurality of cores 12.

Further, the above-described optical fibers 10 that perform incidence or emission of a light beam may be coupled to each other. FIG. 19 shows a schematic diagram of a fiber laser 100A of this embodiment including

optical fibers

10a, 10b, 10c, and 10d. In FIG. 19, the excitation light source is omitted. As shown in FIG. 19A, the optical fiber 10b is provided with a plurality of cores 12, and light beams are incident on the optical fiber 10b from the plurality of optical fibers 10a.

Further, as shown in FIG. 19B, the light beam from the optical fiber 10b provided with the plurality of cores 12 enters the optical fiber 10c. Specifically, the light beam from the optical fiber 10b is incident on the clad 14a of the optical fiber 10c, and the optical fiber 10c functions as a power combiner. The core 12 of the optical fiber 10d is doped with rare earth elements such as neodymium (Nd) or ytterbium (Yb), and the optical fiber 10d performs amplification and laser oscillation based on the light from the optical fiber 10c. As described above, a plurality of optical fibers 10 may be used for the fiber laser 100A.

According to the present invention, it is possible to provide an optical fiber suitable for emission or incidence of a light beam having a desired shape. The optical fiber of the present invention is preferably used for a high-power single mode fiber laser.

10 optical fiber 12 core 14 clad 14a first clad 14b second clad

Claims

The core,
A first cladding in contact with the core;
An optical fiber comprising a second cladding provided around the first cladding,
The core has a higher refractive index than the first cladding;
The first cladding has a higher refractive index than the second cladding;
In cross-section, the length of the core along the first direction is different from the length of the core along the second direction.
The optical fiber according to claim 1, wherein the first cladding has two regions separated from each other.
The optical fiber according to claim 1 or 2, wherein the core is in contact with the second cladding.
The optical fiber according to any one of claims 1 to 3, wherein the core has a substantially rectangular shape in cross section, and the first cladding has a semicircular shape.
The optical fiber according to any one of claims 1 to 4, wherein each of the core and the first cladding is doped with a rare earth element.
The optical fiber according to any one of claims 1 to 5, wherein the core is doped with neodymium or ytterbium.
The light of a predetermined wavelength is propagated in a multimode for a length along the first direction of the core, and propagated in a single mode for a length along the second direction of the core. The optical fiber according to any one of 1 to 6.
The core,
An optical fiber comprising a cladding,
An optical fiber in which light of a predetermined wavelength is propagated in a multimode with respect to a length along the first direction of the core and is propagated in a single mode with respect to a length along the second direction of the core. .
9. The optical fiber according to claim 8, wherein, in a cross section, a length along the first direction of the core is different from a length along the second direction of the core.
A fiber laser comprising an excitation light source and a resonator,
The resonator includes a fiber laser including the optical fiber according to any one of claims 1 to 9, a rear mirror, and a front mirror.
Providing a core member having a core material and two first cladding members each having a first cladding material;
Forming a molded body in which the core member is sandwiched between the two first clad members;
Preparing a second clad member having a second clad material, the second clad member having an opening;
Forming a preform by inserting the molded body into the opening of the second cladding member;
And a step of taking out the fiber from the preform.
The method for producing an optical fiber according to claim 11, further comprising a step of cutting the molded body so as to be aligned with the opening of the second clad member before the step of forming the preform.
The method for producing an optical fiber according to claim 11 or 12, wherein the step of forming the molded body includes a step of polishing at least one of the core member and the two first clad members.
Preparing a rod having a cross section in which the length along the first direction is different from the length along the second direction;
Forming a core around the rod;
Forming a clad around the core.