JP2012248616A - Single crystal fiber laser device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a laser device capable of applying force having the same magnitude and directions opposite to each other to a pair of upper and lower facets appearing on side faces of a fiber, and including an anchor block for the fiber as a component.SOLUTION: A single crystal fiber laser device includes an anchor block (501) for a fiber having a groove (514) in which a fiber (500) is installed. The groove (514) has a shape obtained by dividing an oval along a long axis of the oval, a depth having a value obtained by subtracting 0-20 μm from a value obtained by dividing a length of one side of a square circumscribing a cross-section of the fiber (500) by 2, and a width having a value obtained by adding 0-40 μm to a length of one side of the square. In the anchor block (501), metal foils (505 and 506) may be interposed in both left and right sides of the fiber (500) and between upper and lower blocks of the anchor block (501). The fiber (500) may be fixed so that a crystal axis vertical to the longitudinal direction of the fiber (500) is in a horizontal direction.

Description

  The present invention relates to a laser device, and more particularly to a laser device used in a measuring device or a processing device.

  In a conventional laser device using a single crystal fiber or a glass optical fiber, as shown in FIG. 1, a fixing base 101 for sandwiching the fiber 100 between a U-shaped groove 110 for fixing and a flat surface 111 is used (for example, non-patent). Reference 1). In this case, the lower part is a curved surface and comes into contact with the fiber 100 over a wide area. However, since the upper part is a flat surface, the fiber 100 is almost in line contact, so the contact area is small. Further, since the fiber 100 is fitted into a groove having the same depth as the diameter of the fiber 100, a strong frictional force acts on the shear stress applying portion 121 on the side surface of the fiber, and a shear stress is generated. The stress distribution generated in the fiber differs greatly from the inversion symmetry as shown by the arrows in FIG. 1 due to the stress applied to the side surfaces of these fibers, and the upper part and the lower part are asymmetric. Here, each arrow represents the direction of an external force.

  In addition, since the fiber cross section of the single crystal fiber used in the present invention has a deviation from the inverted symmetrical shape both vertically and horizontally, the main axis of stress in the center of the fiber is inclined from the horizontal and vertical directions, and the angle Will also have a wide distribution. As a result, the main axis of birefringence due to the photoelastic effect also varies greatly depending on the position in the fiber cross section. When linearly polarized light having a polarization plane parallel to the crystal axis direction is guided through the fiber in such a state, the transmitted light becomes elliptically polarized light, and the direction of the long axis is also inclined from the crystal axis.

  In the laser device, a birefringence filter and a dispersion compensation medium having a Brewster angle incident surface are arranged in the resonator, so that when the polarization plane of the oscillation light is tilted from the polarization plane intended by the designer, a large optical loss occurs. Efficiency will decrease. In addition, there is a temperature gradient generated by the pump light in the oscillating fiber, and the stress caused thereby induces birefringence and tilts the polarization of the oscillating light from the design direction. However, the fixed base shown in FIG. 1 cannot have a function of suppressing this polarization deviation.

  As shown in FIG. 2, when a fixing base 201 sandwiched between a 60 ° V groove 212 and a flat surface 211, which is usually used for fixing a glass optical fiber, is used, a Cr: YAG single crystal having a cross-sectional shape close to a quadrangle. The stress distribution is not symmetrical with respect to the fiber 200, and the principal axis of stress near the center of the fiber is inclined from the horizontal and vertical directions. Here, the arrows in FIG. 2 each represent the direction of the force from the outside.

  Further, as shown in FIG. 3, even when a fixed base 301 that sandwiches the fiber 300 with 90 ° V grooves 313 from above and below is used as an improved version of the fixed base shown in FIG. As shown, the main stress acting on the two opposing surfaces from the stress applying portion 322 and the shear stress caused by friction have asymmetry due to deviation from the four-fold rotational symmetry. As a result, the main axis of birefringence at the center is inclined from the horizontal and vertical directions. Therefore, even when a 90 ° V groove as shown in FIG. 3 is used, there is a problem as a laser device because an optical loss is caused by the intracavity element having the incident surface at the Brewster angle.

Japanese Patent No. 3555098 US Pat. No. 5,851,284

J. L. Nightingale et al. "Monolithic Nd: YAG fiber laser," Optics Letters, Vol. 11, (1986) p. 437. S. P. Tymoshenko et al. Kiyoshi Kanata, “Theory of Elasticity”, Corona, 1973, p. 125 W. Koechner, Springer-Verlag, "Solid-state laser engineering," (1992) p. 387. C. Pfistner, et al., "Thermal beam distortions in end-pumped Nd: YAG, Nd: GSGG, and Nd: YLF rods," IEEE J. Quantum Electron., Vol. 30, (1994) p. 1605.

  As described above, when the conventional fiber fixing base is applied to a Cr: YAG single crystal fiber laser, the main axis of birefringence is in the horizontal and vertical directions at the center of the fiber where most of the guided light energy passes. There is a risk of tilting. In that case, the plane of polarization of the laser oscillation light also has an angle with respect to the horizontal direction (crystal axis direction). The light source designer sets the crystal axis direction in which the maximum gain of the single crystal fiber is obtained within the Brewster angle incident surface. However, when the main axis of birefringence described above does not coincide with this direction, the oscillation polarization is inclined from the design direction, resulting in a decrease in oscillation efficiency. This problem has been unavoidable with conventional fiber mounts.

  An object of the present invention is to provide a fiber fixing base capable of applying an opposite force with an equal size from the upper surface and the lower surface of a fiber in view of the above-described problems of the prior art. .

  The present invention is a laser device composed of a single crystal fiber and a fixed base composed of a set of two copper blocks that sandwich and fix the single crystal fiber from above and below, and includes two copper blocks of the fixed base. Each has a groove for fixing the single crystal fiber on the single crystal fiber installation surface. The groove has a semi-oval shape in a cross section perpendicular to the longitudinal direction. Of the facets appearing on the side face of the crystal fiber, the value obtained by dividing the distance between the opposing facet pairs by 2 is a value obtained by subtracting one of the values in the range of 0 μm to 20 μm, and the width of the groove is opposed. The distance between the facet pairs is a value obtained by adding one value in the range of 0 μm to 40 μm.

  In one embodiment of the present invention, the single crystal fiber is grown by bringing a seed crystal into contact with a melt of a crystal having a garnet-type structure and pulling up the seed crystal. One direction within the range of 10 degrees or more from the <100> direction of the crystal, 20 degrees or more from the <110> direction, and 20 degrees or more from the <211> direction, or a crystallographic equivalent direction in one direction Are matched with each other.

  In one embodiment of the present invention, a quartz film having a thickness of 1 μm is formed on a side surface of a single crystal fiber, and is coated with a non-reflective coating for the oscillation wavelength of laser light.

  In one embodiment of the present invention, an indium foil is sandwiched between each of the two copper blocks of the fixed base and the single crystal fiber, and the thickness of the indium foil ranges from 5 μm to 50 μm. It is any one of these.

  In one embodiment of the present invention, one or a plurality of metal foils are sandwiched between two copper blocks on each of the left and right sides of the single crystal fiber and on the upper and lower sides of the fixed base. The material is copper or gold, and the total thickness of one or a plurality of metal foils is equal on the left and right of the single crystal fiber.

  In one embodiment of the present invention, the single crystal fiber is characterized in that the crystal axis <001> perpendicular to the longitudinal direction of the single crystal fiber is fixed so as to be horizontal with the single crystal fiber installation surface.

  In one embodiment of the present invention, a laser device includes a pump light source, a condensing lens that condenses excitation light from the pump light source, and a resonator including two concave mirrors. .

  In one embodiment of the present invention, the laser device further includes a wavelength filter, a saturable absorber, and a dispersion compensation medium.

  The fixing base according to the present invention applies upside down symmetrical stress to the fiber perpendicular to the crystal axis from the upper surface and the lower surface of the fiber. Also in the fixing base according to the present invention, as in the prior art shown in FIG. 1, shear stress acts from the side, but the magnitude is weaker than that in the prior art. As a result, the main axis of stress coincides with the horizontal and vertical directions in most of the fiber cross-sectional central region. Therefore, the polarization rotation of the oscillation light due to the stress from the fixed base does not occur, and the influence of birefringence caused by the temperature distribution by the pump light can be suppressed.

It is sectional drawing for demonstrating the structure using the conventional fiber fixing stand for U-grooves for single crystal fibers or glass optical fibers, and a plane. It is sectional drawing for demonstrating the structure using the conventional fiber fixing stand pinched | interposed by a 60 degreeV-shaped groove and a plane. It is sectional drawing for demonstrating the structure using the conventional fiber fixing stand pinched | interposed with a 90 degreeV-shaped groove. It is explanatory drawing for demonstrating embodiment of this invention. It is sectional drawing for demonstrating embodiment of this invention. It is a perspective schematic diagram for explaining an embodiment of the present invention. It is explanatory drawing for demonstrating embodiment of this invention. It is explanatory drawing for demonstrating the Example of this invention.

  Hereinafter, the present invention will be described in detail with reference to the drawings.

  The laser apparatus according to the present invention has a configuration in which a single crystal fiber is fixed to a fixing base having a semi-oval groove. Cr, Ca: YAG is used as the fiber single crystal. As other fiber single crystals, Cr, Mg: YAG, Nd: YAG, Yb: YAG, or the like can also be used.

  In the present invention, a single crystal is grown by bringing a seed crystal into contact with a melt of a crystal having a garnet structure and pulling up the seed crystal. Specifically, the tip of the base material made of a crystal having a garnet structure is melted, and the seed crystal is brought into contact with the melted tip. Next, a fiber-like single crystal is grown by pulling up the seed crystal. Alternatively, a rod-shaped single crystal may be grown by bringing a seed crystal into contact with a crystal melt having a garnet structure housed in a crucible and then pulling up the seed crystal while rotating. One direction or one direction in the direction in which the seed crystal is pulled up, in the range of 10 degrees or more from the <100> direction of the seed crystal, 20 degrees or more from the <110> direction, and 20 degrees or more from the <211> direction The crystallographic equivalent directions are matched (see Patent Document 1 and Patent Document 2).

  In the present invention, using the method described above, the Cr, Ca: YAG single crystal is pulled up to an orientation rotated by 15 ° from the <100> orientation toward the <010> orientation. As a result, as shown in FIG. 4, facets (crystal habits) appear on the side surfaces of the fiber, and the cross-sectional shape of the fiber becomes a shape close to a quadrangle with rounded corners. If the two facet surfaces facing each other are regarded as one set, the cross section is surrounded by two facet pairs, and the direction perpendicular to one of the pairs is the crystal axis <001>. The cross section of the fiber used in the present invention does not have inversion symmetry, and the square shown inside the cross section is not strictly inscribed in the fiber as shown in FIG.

  As shown in FIG. 5, the groove shape of the fixing base for fixing the single crystal fiber has a semi-oval shape in a cross section perpendicular to the longitudinal direction in order to dissipate and fix the light source single crystal. FIG. 5 is a cross-sectional view when the single crystal fiber 500 is installed on the fixed base 501 according to the present embodiment, and details will be described later.

  As shown in FIG. 6, a fixing base 601 composed of a pair of copper blocks each having a vertically symmetrical shape is fixed on a copper base 603 serving as a base by screws 602 at four corners.

  Reference is again made to FIG. In the cross-sectional area of the single crystal fiber 500, when the interval between the facet pairs facing each other was 120 μm, the width of the semi-ellipse groove was 150 μm and the depth was 55 μm. However, this is merely an example, and the depth of the semi-ellipse groove may be set to be smaller than ½ of the interval between the opposing facet pairs. More specifically, the depth of the semi-oval groove may be a depth obtained by subtracting one value in the range of 0 μm to 20 μm from the value obtained by dividing the interval between the opposing facet pairs by 2. The width of the oval groove may be a value obtained by adding a value in the range of 0 μm to 40 μm to the interval between the opposing facet pairs. Indium foil 504 (having a thickness of 30 μm) is sandwiched between each of the upper and lower blocks of the fixing base 501 and the fiber 500. When the cross-sectional size of the fiber is different from that of the above embodiment, the size of the semi-ellipse groove and the thickness of the indium foil are also changed accordingly. The thickness of the indium foil can take a value in the range of 5 μm to 50 μm. The left and right positions of the upper and lower semi-long circular grooves are aligned and fixed. At this time, the orientation of the fiber 500 is set so that the facet pair perpendicular to the crystal axis <001> is the side surface. That is, the crystal axis <001> is installed in a direction perpendicular to the longitudinal direction of the fiber in the horizontal plane. A metal foil is sandwiched between the both sides of the fiber 500 and the upper and lower blocks of the fixing base 501 and screwed. As the metal foil to be sandwiched, one metal foil or a combination of a plurality of metal foils is used. By increasing or decreasing the thickness of the entire foil, the magnitude of the force applied to the fiber can be adjusted. By using a combination of a plurality of metal foils, delicate adjustment of the force becomes possible. The thickness of the entire foil is about 30 μm to 60 μm, and the thickness of the sandwiched metal foil is made equal on both the left and right sides. In this embodiment, a copper foil 505 having a thickness of 40 μm and a copper foil 506 having a thickness of 5 μm were used. In addition, gold can be used as the material of the foil.

  A resonator is configured by combining a single crystal fiber, an end mirror, a wavelength selection element, and the like installed on the fixed base according to the present embodiment. Excitation light from a semiconductor laser, solid laser, or the like is collected on the end face of the single crystal fiber and absorbed while being guided in the longitudinal direction. The resonator oscillates with the light emitted from the single crystal fiber in the longitudinal direction as a gain.

The effect of the semi-ellipse-shaped groove fixture according to this embodiment was modeled, and the birefringence in the fiber was calculated by numerical calculation. As shown in FIG. 7, it is assumed that a vertically symmetrical force is applied to a cylindrical YAG single crystal fiber having a diameter of 120 μm from a straight line in the longitudinal direction at a vertically symmetrical position toward the center of the fiber. In order to simplify the model, an infinite length fiber is assumed. It is assumed that translational symmetry in the longitudinal direction of the fiber (z-axis direction in FIG. 7) is established, and inversion symmetry of the fiber is also established for all planes perpendicular to the z-axis. The x and y coordinates are set as shown in FIG. This model is almost the same as the case of a disk provided with symmetrical forces from two upper and lower points (see Non-Patent Document 2). However, the condition that there is no stress component in the z-axis direction in the model of Non-Patent Document 2 changes to the condition that there is no displacement in the z-axis direction as a result of the symmetry in the current model. When the strain in the coordinate axis direction is ε _x , ε _y , ε _z and the shear strain is γ _xy , γ _yz , γ _zx , the following equation (1) is obtained.

The calculation results of the normal stress components σ _x and σ _y and the shear stress component τ _{xy in} the fiber vertical cross section are as follows.

Here, f represents the magnitude of force per unit length.

  From Hook's law and equation (1), the following equation (5) holds.

  This

It becomes. Here, E represents Young's modulus and ν represents Poisson's ratio.

Since YAG is a cubic crystal (the crystal point group is m3m), the strain optical coefficient is represented by the following three values. That is, p ₁₁ = −0.029, p ₁₂ = + 0.0091, and p ₄₄ = −0.0615 (see Non-Patent Document 3). The Young's modulus is 310 GPa and the Poisson's ratio is 0.30 (see Non-Patent Document 4). If the distortion components of the equations (6) to (8) are transformed into the crystal axis coordinates of the single crystal fiber in the present invention shown in FIG. 4 and these distortion optical coefficients are used, the refractive index ellipse of this fiber is obtained. The body can be calculated. From there, the birefringence in the light traveling direction, that is, the fiber longitudinal direction is calculated. When f is 40000 N / m, the birefringence is 4.3 × 10 ⁻⁴ at the fiber center, 5.7 × 10 ⁻⁴ at 30 μm from the center in the y-axis direction, and 30 μm from the center in the x-axis direction. 2.1 × 10 ⁻⁴ . These values are about the same as the birefringence in the polarization maintaining fiber used for optical communication, and are sufficiently large. Therefore, it is considered that the influence of birefringence on the polarization rotation caused by the temperature distribution by the pump light can be sufficiently suppressed. Note that 40000 N / m is 800 N for a normally used fiber having a length of 20 mm, which is the magnitude of force that can be applied by four M2 screws.

  Hereinafter, an embodiment of the present invention will be described.

  As shown in FIG. 8, a Cr, Ca: YAG single crystal fiber 800 is placed in a resonator composed of two concave mirrors (807, 808) having a radius of curvature of 100 mm to produce a laser device. The length of the single crystal fiber 800 is 20 mm, and the length of one side of the square circumscribing the cross-sectional shape is 120 μm. The concave mirror 807 transmits pump light having a wavelength of 1.064 μm, and has a reflectance of 99.9% for oscillation light having a wavelength of 1.40 μm to 1.60 μm. The concave mirror 808 is an output coupling mirror and has a transmittance of 1% with respect to oscillation light with a wavelength of 1.40 μm to 1.60 μm. A quartz film having a thickness of 1 μm is formed on the side surface of the single crystal fiber, and an antireflective film for oscillation light having a wavelength of 1.40 μm to 1.60 μm is provided on the end surface.

  As described above, the single crystal fiber 800 of FIG. 8 is fixed to a fixing base 801 having a semi-long circular groove with an indium foil having a thickness of 30 μm interposed therebetween (see FIG. 5). The fixed base is placed horizontally, and the single crystal fiber is installed so that the crystal axis is horizontal. The upper and lower blocks of the fixed base are fixed by aligning the positions of the grooves.

  Again, this embodiment will be described with reference to FIG. The oval groove 514 has a depth of 55 μm and a width of 150 μm. One set of each of a copper foil 505 having a thickness of 40 μm and a copper foil 506 having a thickness of 5 μm (total thickness of 45 μm) is sandwiched between the upper and lower blocks and on both sides of the single crystal fiber 500.

  In the configuration of the laser light source as shown in FIGS. 5 and 8, pump light having a wavelength of 1.064 μm was condensed on a single crystal fiber by a condensing lens 809 having a focal length of 120 mm and oscillated. When the pump light incident power was 2.3 W, the wavelength of the oscillating light was 1.43 μm, the output power of the oscillating light was 110 mW, and the oscillating light was linearly polarized light whose polarization plane coincided with the horizontal plane.

  For comparison, an experiment was performed with the same configuration as described above, except that the conventional fiber fixing base 301 of the 90 ° V groove upper and lower set shown in FIG. 3 was used. In this case, when the pump light incident power is 2.3 W, the wavelength of the oscillation light is 1.43 μm, the output power of the oscillation light is 110 mW, and the polarization plane of the oscillation light is inclined by 15 ° from the horizontal. Therefore, the effectiveness of the laser apparatus according to the present example could be confirmed.

  The laser device of the present invention can further include a wavelength filter, a saturable absorber, and a dispersion compensation medium as necessary.

100, 200, 300, 400, 500, 700, 800 Single crystal fiber 101, 201, 301, 501, 601, 801 Fixing base 110 U-shaped groove 1111, 211 Plane 120 Main stress applying part 121 Shear stress applying part

212 60 ° V-shaped groove 222,322 Stress applying part

313 90 ° V-shaped groove

504 Indium foil 505, 506 Copper foil 514 Half-long circular groove

602 screw 603 base 615 oval groove

807, 808 Concave mirror 809 Condensing lens

Claims (8)

Single crystal fiber,
A laser device composed of a fixed base composed of a set of two copper blocks that sandwich and fix the single crystal fiber from above and below,
Each of the two copper blocks of the fixing base includes a groove for fixing the single crystal fiber on a single crystal fiber installation surface,
The shape of the groove is a semi-elliptical cross section perpendicular to the longitudinal direction,
The depth of the groove is a value obtained by subtracting one value in the range of 0 μm to 20 μm from the value obtained by dividing the interval between the facet pairs facing each other among the facets appearing on the side surface of the single crystal fiber by 2. Yes,
The width of the groove is a value obtained by adding one value in the range of 0 μm to 40 μm to the interval between the facing facet pairs.   The single crystal fiber is grown by bringing the seed crystal into contact with a melt of a crystal having a garnet structure and pulling up the seed crystal. In the direction of pulling up the seed crystal, the <100> direction of the seed crystal 10 directions or more from the <110> direction and 20 degrees or more from the <110> direction, and a direction that is 20 degrees or more away from the <211> direction, or a crystallographic equivalent direction to one direction. The laser device according to claim 1, characterized in that:   2. The laser device according to claim 1, wherein a quartz film having a thickness of 1 μm is formed on a side surface of the single crystal fiber and is anti-reflective coated with respect to an oscillation wavelength of the laser beam.   An indium foil is sandwiched between each of the two copper blocks of the fixing base and the single crystal fiber, and the thickness of the indium foil is any one of a range of 5 μm to 50 μm. The laser apparatus according to claim 1.   One or more metal foils are sandwiched between the left and right sides of the single crystal fiber and between the upper and lower copper blocks of the fixed base, and the material of the metal foil is copper or gold The laser apparatus according to claim 1, wherein the entire thickness of the one or more metal foils is equal on the left and right of the single crystal fiber.   2. The laser device according to claim 1, wherein the single crystal fiber is fixed so that a crystal axis <001> perpendicular to a longitudinal direction of the single crystal fiber is parallel to the single crystal fiber installation surface. .   The laser apparatus according to claim 1, further comprising: a pump light source; a condenser lens that condenses the excitation light from the pump light source; and a resonator including two concave mirrors.   The laser device according to claim 7, further comprising a wavelength filter, a saturable absorber, and a dispersion compensation medium.

Images