JP4873645B2 - Optical fiber Raman laser device - Google Patents

Description

  The present invention relates to an optical fiber Raman laser used for laser processing and laser measurement.

  Conventionally, a laser apparatus using stimulated Raman scattering of an optical fiber is commercially available. Stimulated Raman scattering is a phenomenon in which a light wave incident on a medium is modulated by lattice vibration of a material constituting the medium, and light having a wavelength different from that of the incident light wave is scattered. The frequency of the lattice vibration is a natural frequency determined by the structure of the material, but the frequency is not uniform and distributed.

If the frequency of the incident light wave is ν _p and the frequency of the lattice vibration is ν _m , the frequency of the scattered light wave is given by ν _s given by ν _p −ν _m and ν _p + ν _m . Ν _a . The incident light having the frequency ν _p is called excitation light or pump light, the light wave having the frequency ν _s is called Stokes light, and the light wave having the frequency ν _a is called anti-Stokes light. Since the frequency ν _m of the lattice vibration is distributed, the frequency of the Stokes light ν _s is also distributed. Since there is a relationship of (wavelength) = (speed of light) / (frequency) between the frequency of light and the wavelength, the wavelength is also distributed by the distribution of the frequency.

When the intensity of the pump light ν _p is increased, the pump light ν _p and the Stokes light ν _s cause a non-linear interaction. As a result, a forcing force acts on the lattice vibration, and the lattice vibration becomes intense. When the lattice vibration becomes intense, the Stokes light ν _s becomes stronger, and the lattice vibration becomes more intense through nonlinear interaction.

When strong pump light ν _p is incident on the optical fiber, the Stokes light ν _s becomes stronger as it propagates. This can be easily understood by dividing the optical fiber into minute sections in the light propagation direction. That is, in the middle of a small section with the Stokes light [nu _s is generated in the section, the Stokes light [nu _s were present in the previous section reaches propagated. Since the latter is stronger as the Stokes light intensity, the interaction between the Stokes light ν _s and the pump light ν _p mainly occurs between the propagated Stokes light ν _s and the pump light ν _p . As a result, the intensity of the Stokes light ν _s is greater than the intensity in the previous section.

In order to generate stimulated Raman scattering more efficiently in an optical fiber, a mechanism for reflecting the Stokes light ν _s propagating in the fiber and reversing the traveling direction may be provided near both ends of the optical fiber (see Patent Document 1). . Since the interaction between the Stokes light ν _s and the pump light ν _p occurs regardless of the propagation direction of the light, the Stokes light ν _s is reflected to cause multiple interactions in one optical fiber. Stimulated Raman scattering occurs efficiently. However, in order to extract the Stokes light ν _s , the above-described reflection is not 100% reflection, but a part is extracted as partial reflection. Such an apparatus is called an optical fiber Raman laser or simply a fiber Raman laser.

  FIG. 6 shows a configuration of an optical fiber Raman laser device 100 according to a conventionally known technique. In FIG. 6, 101 is an optical fiber, 102 is an output end, 103 is a pump light source, and 104, 105, and 106 are optical fiber Bragg gratings (hereinafter referred to as FBGs) that serve as reflectors.

  The optical fiber 101 is a normal single mode optical fiber or an optical fiber with enhanced nonlinear effect (see Patent Document 2). The pump light source 103 is a YAG laser having a wavelength of 1064 nm, and the output is 2 W. The FBG 104 is a component that reflects light having a specific wavelength, and reflects light having a wavelength of 1064 nm in the example of FIG. The FBGs 105 and 106 reflect light having a wavelength of 1120 nm, and the reflectivities are approximately 100% for the FBG 106 and 20% for the FBG 105.

  With such a configuration, stimulated Raman scattering occurs at a wavelength of 1120 nm in the optical fiber 101, and a part thereof is extracted as output light from the light output end 102.

Furthermore, in recent years, the output of a fiber laser has been remarkably advanced by the invention of the double clad fiber, and the single mode fiber laser of a commercially available apparatus has reached 2 kW output. A double clad fiber is one in which the clad is doubled, and an inner clad propagates pumping light for optically pumping the core, and an element causing stimulated emission is added to the core. A semiconductor laser is used as an excitation light source, and a rare earth element such as Yb is used as an additive element to the core.
Japanese Patent Laid-Open No. 11-54853 JP-A-56-70683 “Current Status and Trends of High-power Single-mode Yb Fiber Laser”, Laser Research, Vol. 33, No. 4, pp. 254-261

However, when the above-described high-power fiber laser light is used as pump light for an optical fiber Raman laser, the intensity of the pump light is so strong that the intensity of the Stokes light is very strong. Although it was difficult to occur, it is possible that the next Stokes light is further generated using the Stokes light as a new pump light. In this case, the frequency ν _s ′ is ν _s −ν _m . ν _s is called primary Stokes light, and ν _s ′ is called secondary Stokes light. Hereinafter, the second and subsequent orders are referred to as high-order Stokes light.

  Since Raman scattering is a process through lattice vibration, thermal energy loss is inevitable, and the conversion efficiency from pump light to Stokes light is about 80% at maximum. As higher-order Stokes light is generated, heat dissipation increases and the total light energy decreases.

  The present invention has been made in view of the above problems, and provides an optical fiber Raman laser device that can suppress the generation of high-order Stokes light and improve the conversion efficiency from pump light to primary Stokes light. For the purpose.

  The gist of the present invention for solving the above problems is as follows.

(1) An optical fiber Raman laser device that generates stimulated Raman scattering at a characteristic wavelength in an optical fiber, and is provided on a pump light source, a core region having a first refractive index, and an outer periphery of the core region , comprising a first cladding region having a second refractive index, and a double-clad optical fiber which have a second cladding region having a third refractive index of the outer periphery of the first cladding region, the core region produces a laser radiation by pumping light guided by the first cladding region is emitted from the pump light source, the laser radiation is conveyed to the long side direction, and the Stokes light by stimulated Raman scattering what der those resulting, predetermined size der less so as not to confine the light of longer wavelength than the diameter of the core region primary Stokes light is, the first cladding region, or the pump light source The combined pump light and the light guide is transported in its longitudinal direction, and has an absorption characteristic with respect to wavelength of the higher order Stokes light generated in said core region, wherein the double-clad optical fiber in the longitudinal direction An optical fiber Raman laser device characterized in that at least a part thereof is bent.

( 2 ) The pump light source is a semiconductor laser, the double-clad optical fiber is a silica-based fiber, and the core region is doped with Yb for generating laser radiation and a substance with a large Raman gain. ( 1 ) The optical fiber Raman laser device according to ( 1 ).

  According to the optical fiber Raman laser device of the present invention, it is possible to provide an optical fiber Raman laser device capable of suppressing efficiency reduction due to generation of high-order Stokes light and improving conversion efficiency from pump light to primary Stokes light. Can do.

  Embodiments of the present invention will be described in detail with reference to the drawings. In addition, in each following figure, the same number was attached | subjected to the part which has the same function.

  An example of the optical fiber Raman laser device 40 according to the first embodiment of the present invention will be described with reference to FIG.

  The optical fiber Raman laser device 40 shown in FIG. 1 is configured in substantially the same manner as the configuration of the optical fiber Raman device 100 according to the conventionally known technique shown in FIG. That is, as shown in FIG. 1, an output end 42 and a pump light source 43 are provided at both ends of the optical fiber 1, and an optical fiber Bragg grating (hereinafter referred to as “pump light source”) is provided between the pump light source 43 and the output end 42. Reflectors 46, 45, and 44 (referred to as FBG) are provided in order from the pump light source 43 side.

  The optical fiber 1 is a silica-based optical fiber and is a passive element. In the first embodiment of the present invention, the total length of the optical fiber 1 is, for example, 1 km, and a bent portion 48 is provided by bending the optical fiber 1 for the purpose of introducing a bending loss of laser light propagation. ing. In the case shown in FIG. 1, the bent portion 48 is provided between the FBG 45 and the FBG 46, and the optical fiber 1 is wound 20 times or more in a coil shape having a diameter of 50 mm or less. Note that the bent portion 48 only needs to be provided in at least a part of the optical fiber 1 in the longitudinal direction.

  FIG. 2 is a configuration diagram showing a configuration of the optical fiber 1 provided in the optical fiber Raman laser device 40 according to the first embodiment of the present invention. As shown in FIG. 2, the optical fiber 1 has a configuration in which a cladding region 12 having a second refractive index is provided on the outer periphery of a core region (hereinafter referred to as a core) 11 having a first refractive index. Have. The coated portion of the optical fiber 1 is not shown. The first refractive index is higher than the second refractive index.

  The clad 12 is mixed with an infrared absorbing element such as iron, cobalt, or selenium. As a result, the secondary Stokes light component that has oozed out is absorbed. In addition, the Raman gain can be increased by mixing impurities such as germanium into the core 11.

  As the pump light source 43, for example, a Yb fiber laser with an output of 1 kW and an oscillation wavelength of 1085 nm can be used. In the first embodiment of the present invention, the pump light from the pump light source 43 is configured to be introduced into the core 11 of the optical fiber 1. As a method of coupling the pump light source 43 to the optical fiber 1, splicing between the fibers may be used, or lens condensing from free space may be used. The optical fiber 1 is a silica-based optical fiber and is a passive element. The pump light from the pump light source 43 generates Stokes light having a wavelength of about 1142 nm and secondary Stokes light having a wavelength of about 1206 nm by stimulated Raman scattering.

  The FBG 44 is a component that reflects light having a specific wavelength, and reflects light having a wavelength of 1085 nm in the example of FIG. The FBGs 45 and 46 reflect light having a wavelength of 1142 nm, and the reflectance is approximately 100% for the FBG 46 and 20% for the FBG 45.

  When the optical fiber Raman laser device 40 is operated and laser radiation serving as pump light of stimulated Raman scattering is guided from the pump light source 43 to the core 11 of the optical fiber 1 that generates stimulated Raman scattering, Stokes light is generated. In this way, stimulated Raman scattering occurs at a wavelength of 1142 nm, and a part thereof is extracted from the output end 42 as output light.

  As the pump light propagates, the intensity of the Stokes light gradually increases and secondary Stokes light is generated. Similar to the above-described nonlinear phenomenon, as the second-order Stokes light becomes stronger, the nonlinear interaction with the first-order Stokes light that has become pump light becomes larger and the second-order Stokes light becomes stronger. In the embodiment, energy transfer from the Stokes light to the secondary Stokes light is prevented by weakening the secondary Stokes light before the secondary Stokes light grows as follows.

  According to the first embodiment of the present invention, when the optical fiber 1 is bent, the optical fiber 1 is appropriately used by utilizing the property that the loss of the long wavelength component becomes large in the wavelength dependence of the bending loss. A material having a bent portion 48 wound in a coil shape having a radius to suppress the growth of secondary Stokes light having a wavelength longer than that of Stokes light, and the clad 12 having absorption characteristics with respect to light of the secondary Stokes light wavelength Incorporating, a stronger suppression effect was introduced against the second-order Stokes light component leached into the cladding layer by bending. The provision of the bent portion 48 in the optical fiber 1 is effective only when the intensity of the pump light is, for example, a small output of several W level. However, as described above, the substance having the absorption characteristic is mixed. For example, it can be applied to the case of a high output of, for example, kW level used for laser processing. In the above description, only the second-order Stokes light has been described. Needless to say, the third-order and higher-order Stokes light has a similar suppression effect.

  Furthermore, by using a medium having a large Raman gain for the core 11, the efficiency of Raman conversion can be further improved.

  As described above, according to the high-order Stokes light suppressing optical fiber Raman laser device of the present invention, the light that suppresses the efficiency reduction due to the generation of the high-order Stokes light and maximizes the Raman conversion efficiency from the pump light to the Stokes light. A fiber Raman laser device can be provided.

  In the first embodiment of the present invention, wavelength conversion from pump light to Stokes light in the optical fiber Raman laser device 40 is efficiently performed. The above laser type, wavelength, and fiber length are merely examples, and other values can be selected. The shape of the bent portion 48 provided in the optical fiber 1 and the number of times of bending are not particularly particular. It is further advantageous to add Tm or the like having wavelength dependent absorption to the cladding 12.

  As a second embodiment of the present invention, as shown in FIG. 3, the diameter 2r (that is, the radius r) of the core 11 of the optical fiber 1 provided in the optical fiber Raman laser device 40 will be described later. By setting it to a predetermined size or less, light longer than the Stokes light wavelength may not be confined in the core 11. FIG. 3 is a configuration diagram showing the configuration of the optical fiber 1 provided in the optical fiber Raman laser device 40 according to the second embodiment of the present invention.

  The optical fiber Raman laser apparatus 40 according to the second embodiment of the present invention is configured in substantially the same manner as the optical fiber Raman laser apparatus 40 according to the first embodiment of the present invention shown in FIG. That is, as the pump light source 43, for example, a Yb fiber laser having an output of 1 kW and an oscillation wavelength of 1085 nm is used. Reference numeral 1 denotes a quartz optical fiber, which is a passive element. The total length of the optical fiber 1 is 1 km, and a bent portion 48 wound 20 times or more in a coil shape having a diameter of 50 mm or less is provided in the middle to introduce a bending loss of laser light propagation. The wavelength of Stokes light is 1142 nm, and the wavelength of secondary Stokes light is about 1206 nm.

As for the diameter of the core 11, a predetermined size that does not confine the secondary Stokes light can be determined as follows, for example. The characteristic values of the optical fiber 1 shown in FIG. 3 are, for example, the Stokes light wavelength λ, the secondary Stokes light wavelength λ ₂ , the core 11 refractive index n ₁ , the core 11 radius r, and the cladding refractive index n _2. When,
2.405λ / {2π (n ₁ ² −n ₂ ² ) ^1/2 } <r <2.405λ ₂ / {2π (n ₁ ² −n ₂ ² ) ^1/2 }
By designing the radius r of the core 11 so that the secondary Stokes light out of the Stokes light and the secondary Stokes light leaks out of the core. As a result, only the secondary Stokes light receives a lot of loss, and the intensity growth can be suppressed. For example, when n ₁ = 1.5 and n ₂ = 1.499, 8.0 μm <r <8.4 μm.

  In the above case, as the light confinement effect, the longer the wavelength, the larger the diameter of the fundamental mode of the spatial mode. Therefore, in the case of the same core diameter, the longer the wavelength, the greater the penetration of the lightwave electric field into the cladding region. A phenomenon that is easily affected by absorption in the cladding region was used.

  According to the second embodiment of the present invention, the diameter 2r of the core 11 of the optical fiber 1 exceeds a predetermined size (for example, (8.0 × 2 =) 16.0 μm (8.4 × 2). =) Less than 16.8 μm), the Stokes light can be confined in the core 11 and the secondary Stokes light having a longer wavelength than the Stokes light can be oozed out of the core 11. Thereby, the efficiency fall by generation | occurrence | production of high-order Stokes light can be suppressed more, wavelength conversion from the pump light by the optical fiber Raman laser apparatus 40 to primary Stokes light can be performed efficiently, and conversion efficiency can further be improved. The second embodiment of the present invention also has the same effect as the first embodiment of the present invention shown in FIG.

  The above laser type, wavelength, and fiber length are merely examples, and other values can be selected. The shape of the bent portion 48 provided in the optical fiber 1 and the number of times of bending are not particularly particular. It is further advantageous to add Tm or the like having wavelength dependent absorption to the cladding 12.

  As a third embodiment of the present invention, pump laser light is not introduced into the core 11 from the outside as in the case of the first embodiment of the present invention and the second embodiment of the present invention. A semiconductor laser beam having a poor beam quality as a heavy cladding structure may be guided to the first cladding 13 on the inner side and used as pump light to induce stimulated emission in the core 11 itself to be a fiber laser. FIG. 5 is a diagram for exemplarily explaining the structure of the pump light source portion in the third embodiment of the present invention. As shown in FIG. 5, the optical fiber 1 is provided with a first clad (hereinafter referred to as a first clad) 13 having a second refractive index on the outer periphery of a core region 11 having a first refractive index. It has the structure which was made. Further, a second cladding region (hereinafter referred to as a second cladding) 14 having a third refractive index is provided on the outer periphery of the first cladding. The coated portion of the optical fiber 1 is not shown. The first refractive index is higher than the second refractive index, and the second refractive index is higher than the third refractive index.

  The third embodiment of the present invention is configured in substantially the same manner as the optical fiber Raman laser device 40 according to the first embodiment of the present invention shown in FIG. That is, as shown in FIG. 4, an output end 42 and a pump light source 43 are provided at both ends of the optical fiber 1, and a reflector 47 as an FBG is provided between the pump light source 43 and the output end 42. 46, 45, and 44 are provided in order from the pump light source 43 side. As the pump light source 43, for example, a semiconductor laser having an output of 1 kW and an oscillation wavelength of 980 nm is used. The optical fiber 1 is a silica-based double clad optical fiber and is an active element having a core doped with Yb.

  Laser light with an oscillation wavelength of 980 nm from the semiconductor laser of the pump light source 43 is coupled to the first cladding 13 and is excited by the Yb-doped core 11 while propagating through the first cladding 13 and stimulated emission at a wavelength of 1085 nm. It is designed to cause laser operation. This laser beam becomes pump light for stimulated Raman scattering. The wavelength of Stokes light is about 1142 nm, and the wavelength of secondary Stokes light is about 1206 nm.

In the third embodiment of the present invention, the FBGs 44 and 47 shown in FIG. 4 are components that reflect light of a specific wavelength. In the example of FIG. 4, the stimulated emission laser light having a wavelength of 1085 nm is reflected. The FBGs 45 and 46 reflect Stokes light having a wavelength of 1142 nm, and the reflectivities are approximately 100% for the FBG 46 and 20% for the FBG 45. As described above, the FBG 47 reflects light having a wavelength of 1085 nm, and confines the stimulated emission laser light in the core 11 together with the FBG 44. The total length of the optical fiber 1 is 30 m, and a bending portion 48 wound 20 times or more in a coil shape with a diameter of 50 mm is provided in the middle to introduce bending loss. In addition, an infrared absorbing element such as iron, cobalt, or selenium is mixed into the first cladding 13 to absorb the secondary Stokes light component that has oozed out. With such a configuration, stimulated Raman scattering occurs at a wavelength of 1142 nm in the optical fiber 1, a part of which is transmitted through the FBG 44, and is extracted from the output end 42 as output light.

  According to the third embodiment of the present invention, stimulated emission is caused in the core 11 itself so that a fiber laser can be obtained, so that pump light of Raman scattering is introduced into the optical fiber 1 from the outside. Therefore, the processing and the optical system can be omitted, and the total efficiency is improved. Note that the third embodiment of the present invention also has the same effect as the first embodiment of the present invention shown in FIG.

  In the third embodiment of the present invention, wavelength conversion from pump light to Stokes light in the optical fiber Raman laser device 40 is efficiently performed. It is further advantageous to add Tm or the like having wavelength dependent absorption to the first cladding 13. The above laser type, wavelength, and fiber length are merely examples, and other values can be selected. Further, the shape of the bent portion 48 provided in the optical fiber 1 and the number of times of bending are not particularly limited. Further, the cross-sectional shape of the first cladding 13 may be a shape other than the substantially circular shape shown in FIG. Further, the optical fiber 1 may be cut once after the FBG 44 and connected to a normal optical fiber that is not double-clad by splicing or the like.

  The present invention can be used for an optical fiber Raman laser used for laser processing and measurement.

It is a block diagram which shows the structure of the optical fiber Raman laser apparatus 40 which concerns on the 1st Embodiment of this invention. It is a block diagram which shows the structure of the optical fiber 1 with which the optical fiber Raman laser apparatus 40 which concerns on the 1st Embodiment of this invention is provided. It is a block diagram which shows the structure of the optical fiber 1 with which the optical fiber Raman laser apparatus 40 which concerns on the 2nd Embodiment of this invention is provided. It is a block diagram which shows the structure of the optical fiber Raman laser apparatus 40 which concerns on the 3rd Embodiment of this invention. It is a figure explaining illustratively the structure of the pump light source part in the 3rd Embodiment of this invention. It is a figure explaining the conventional Raman optical fiber laser exemplarily.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Optical fiber by this invention 11 Core 12 Clad 13 1st clad 14 2nd clad 40 Optical fiber Raman laser apparatus 42, 102 output end 43, 103 Pump light source 44-47, 104-106 FBG by this invention
48 Bending portion of optical fiber 100 Optical fiber Raman laser device 101 according to conventionally known technology 101 Optical fiber according to conventionally known technology

Claims (2)

An optical fiber Raman laser device that generates stimulated Raman scattering at a characteristic wavelength in an optical fiber,
A pump light source;
A core region having a first refractive index; a first cladding region having a second refractive index provided on the outer periphery of the core region; and a third refractive index on the outer periphery of the first cladding region. comprising a double clad optical fiber which have a second cladding region having,
Said core region produces a laser radiation by pumping light guided by the first cladding region is emitted from the pump light source, the laser radiation is conveyed to the long side direction, and the Stokes light by stimulated Raman scattering the I der those resulting, Ri predetermined size der less so as not to confine the light of longer wavelength than the diameter of the core region primary Stokes light,
The first cladding region transports and guides the pump light coupled from the pump light source in the longitudinal direction, and has an absorption characteristic with respect to the wavelength of higher-order Stokes light generated in the core region. Have
An optical fiber Raman laser device, wherein the double clad optical fiber is bent at least partially in the longitudinal direction. The pump light source is a semiconductor laser;
The double clad optical fiber is a silica-based fiber,
The optical fiber Raman laser apparatus according to claim 1, wherein the core region is doped with Yb for generating laser radiation and a substance having a large Raman gain .

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