JP5035803B2 - Optical vortex laser beam oscillation method and optical vortex laser beam oscillation apparatus - Google Patents

Description

The present invention relates to a laser beam having a helical equiphase surface and a beam intensity distribution in a ring shape, that is, a so-called optical vortex laser beam oscillation method and apparatus.

Optical vortices represented by Laguerre Gaussian beams are expected to be applied to new optical fields such as optical manipulation and atomic traps. The technology to capture and move a minute object by irradiating laser light and its pressure (a phenomenon in which momentum moves from laser light to the object with absorption, reflection, and refraction) is called optical trap or optical manipulation technology. Recently, it has come to the spotlight rapidly. Such a technique is particularly attracting attention as a means in basic biology research and micromachine engineering. For example, a laser beam is guided into a microscope and is focused with an objective lens to capture and move macromolecules such as DNA, cells, and biological tissues near the focal point (optical tweezers), and to cut and destroy (optical scalpel) ) Is possible. Microscopes used for such optical manipulations are being developed by various research institutes and are entering the practical stage.

On the other hand, various types of laser beam oscillators are actively researched and developed, and their fields of use are expanding. Many types of laser beam oscillators such as semiconductor lasers, gas lasers, and solid-state lasers have been put into practical use so far. Research and development have been actively conducted to use these laser devices as light sources for optical traps and optical manipulation.

However, generally, the cross-sectional light intensity distribution of the output beam of the laser beam oscillation device is a circular normal distribution (Gaussian distribution), and the intensity is highest at the center. For this reason, when an output beam is directly irradiated onto a minute object and an optical trap or optical manipulation is performed, it is easy to move the object, but the object receives force only from one direction, so it is captured. There is a problem that is difficult. Therefore, for the purpose of efficient optical trap and optical manipulation operations, the mainstream method is to generate an optical vortex using a hologram such as a polarizing plate or a liquid crystal spatial modulator for the light intensity distribution of the output beam from the laser beam output device. (Patent Document 1).

However, in this case, it is difficult to generate a high-power optical vortex due to problems such as damage to the diffraction grating element and diffraction loss. Specifically, in the optical vortex generation method using a hologram such as a polarizing plate or a liquid crystal spatial modulator, there is a problem that when a strong optical vortex of 1 W or more is oscillated, the deflecting plate and the liquid crystal are damaged by the strong light. is there. Further, the optical vortex generation efficiency in the optical vortex generation method using a hologram such as a conventional polarizing plate or a liquid crystal spatial modulator is about 10%, and there is a problem that loss is large. Further, the maximum value of the wavelength of the generated optical vortex is about 1 μm, and there is a problem in handling living tissue at such a short wavelength.

In laser processing, a generally used laser beam is a Gaussian beam having a normal distribution (Gaussian distribution) with a circular light intensity distribution. In this case, there is a problem that plasma and sputtering (oxidized product of molten metal) are scattered.

Paragraph 0027, Paragraph 0028 and FIG. 5 of JP 2005-515878 A

The present invention has been made to solve the above problems, and its purpose is to
(1) Optical vortex laser beam oscillation method that can oscillate high output optical vortex laser beam with high generation efficiency in a wide wavelength range and can oscillate directly from the resonator (2) High output optical vortex laser beam in a wide wavelength range It is an object to provide an optical vortex laser beam oscillation device that can oscillate with high generation efficiency and can oscillate directly from a resonator.

The present invention
“[1] A solid-state laser medium and two cylindrical lenses sandwiching the solid-state laser medium are arranged in an optical resonator including a reflection mirror and an output mirror, and the solid-state laser medium is excited by excitation light from an excitation light source. In the laser beam oscillation method of oscillating a laser beam by optically exciting a laser beam, a thermal lens is formed in the solid-state laser medium by the optical excitation, and the thermal lens, the two cylindrical lenses, and the optical resonator are high. Resonance condition for next mode;
(Where f is the focal length of the thermal lens, L _{M1 is} the resonance distance on the reflecting mirror side, R _{M1 is the} radius of curvature of the reflecting mirror, L _{M2 is} the resonance distance on the output mirror side, R _{M2 is the} radius of curvature of the output mirror, L = L _M1 + L _M2 −L _M1 · L _M2 / f.)
By satisfying the above, and not satisfying the resonance condition for the low-order mode, the high-order mode laser beam is selectively amplified and oscillated while changing the phase of the high-order mode laser beam by 90 degrees. An optical vortex laser beam oscillation method characterized by the above.
[2] A side-excitation method in which the optical excitation is excited from the side surface of the solid-state laser medium, and a thermal lens having a temperature distribution gradient in which the temperature decreases exponentially from the side surface of the solid-state laser medium in the depth direction is formed. The method of oscillating an optical vortex laser beam according to [1].
[3] The optical vortex laser beam oscillation according to [1] or [2], wherein the solid laser medium is Nd: YVO ₄ or Nd: GdVO ₄ doped with Nd at a doping concentration of 1 atm% or more Method.
[4] The optical vortex laser beam oscillation method according to any one of [1], [2], and [3], wherein the excitation light source is a laser diode.
[4-1] An acousto-optic element, electro-optic element or saturable absorber element is arranged between the reflection mirror of the optical resonator and the cylindrical lens to act as a Q switch to oscillate a high-output optical vortex laser beam. An optical vortex laser beam oscillation method according to any one of [1], [2], [3], and [4], wherein
[4-2] A saturable absorber is disposed between the reflecting mirror of the optical resonator and the cylindrical lens to act as a mode-locked laser, or an acousto-optic element is disposed and active mode locking is performed at the nanosecond level. The optical vortex laser beam oscillation according to any one of [1], [2], [3], [4], and [4-1], wherein the optical vortex laser beam is oscillated from picosecond to picosecond level. Method.
[5] A solid laser medium and two cylindrical lenses sandwiching the solid laser medium are arranged in an optical resonator having a reflection mirror and an output mirror, and the solid laser medium is excited by excitation light from an excitation light source. In the laser beam oscillation device that oscillates a laser beam by optical excitation, the laser diode is arranged so that the solid laser medium is optically excited at the confocal position of the two cylindrical lenses, and the resonance condition for the higher-order mode ;
(Where f is the focal length of the thermal lens, L _{M1 is} the resonance distance on the reflecting mirror side, R _{M1 is the} radius of curvature of the reflecting mirror, L _{M2 is} the resonance distance on the output mirror side, R _{M2 is the} radius of curvature of the output mirror, L = L _M1 + L _M2 −L _M1 · L _M2 / f.)
In order to satisfy the resonance condition for the low-order mode, the focal length of the thermal lens, the resonance distance on the reflection mirror, the radius of curvature of the reflection mirror, the resonance distance on the output mirror, and the radius of curvature of the output mirror An optical vortex laser beam oscillation device characterized in that is set.
[6] The optical vortex laser beam oscillator according to [5], wherein the solid-state laser medium is Nd: YVO ₄ or Nd: GdVO ₄ doped with Nd at a doping concentration of 1 atm% or more.
[7] The optical vortex laser beam oscillator according to [5] or [6], wherein the excitation light source is a laser diode.
[7-1] An acoustooptic device, an electrooptic device, or a saturable absorber device is disposed between the reflection mirror of the optical resonator and the cylindrical lens, [5], [6], [6] 7] The optical vortex laser beam oscillation device according to any one of [7].
[7-2] A saturable absorber or an acousto-optic element is disposed between the reflection mirror of the optical resonator and the cylindrical lens, [5], [6], [7], [7] 7-1]. The optical vortex laser beam oscillation device according to any one of [7-1]. "
About.

According to the present invention, a low-order typified by a Gaussian beam is utilized by utilizing the fact that the thermal lens effect formed inside a solid laser medium in a laser amplifier (resonator) varies depending on the beam size (mode size) inside the resonator. It is possible to selectively oscillate higher order Hermitian Gaussian modes than modes, and an optical vortex laser beam can be oscillated directly from the laser resonator by the nature of the resonator and the alignment of the resonator mirror. Therefore, there is no problem of damage to the deflecting plate and the liquid crystal, and it is possible to oscillate an optical vortex laser beam having a low energy loss, a high optical vortex generation efficiency, a wavelength of 1.3 μm class, and an optical power of at least 1 W or more. If this optical vortex laser beam is used for laser processing, it can capture plasma and spatter (oxidized product of molten metal) and control the direction of scattering, thus preventing reversion to the substrate and precise laser processing. Can do.

The optical vortex laser beam in the method for oscillating an optical vortex laser beam (optical vortex) of the first invention, that is, a laser beam having a helical equiphase surface and having a beam intensity distribution in a ring shape, Light with a phase singularity. That is, the Gaussian beam has a large intensity distribution around the optical axis, whereas the optical vortex has a characteristic that the intensity distribution is large at a distance from the optical axis. A typical optical vortex is a Laguerre Gaussian beam. A Laguerre Gaussian beam, like a spiral staircase, has a phase that changes by 2π (or an integer multiple thereof) when it rotates once around the optical axis, and the equiphase surface has a helical structure. The Laguerre Gaussian beam has zero intensity on the optical axis (phase singularity), and the intensity distribution of the optical axis cross section is in a ring shape.

A method for oscillating an optical vortex laser beam according to the first invention, that is, a laser beam having a helical equiphase surface and having a beam intensity distribution in a ring shape will be described with reference to FIG.
FIG. 1 shows an optical vortex laser beam oscillation apparatus A used in the optical vortex laser beam oscillation method from above.
A solid-state laser medium 3 is disposed between the reflection mirror 1 and the output mirror 2 which are optical resonators. A laser diode 5 as an excitation light source is arranged so that the excitation light 4 is irradiated on the side surface 3a of the solid-state laser medium 3. The excitation light source is preferably a laser diode (semiconductor laser), but an excitation light source having equivalent performance may be used instead of the laser diode (semiconductor laser).
A cylindrical lens 6 is disposed between one end surface 3b1 of the solid-state laser medium 3 and the reflection mirror 1, and similarly, a cylindrical lens 7 is disposed between the other end surface 3b2 of the solid-state laser medium 3 and the output mirror 2. Has been. The cylindrical lenses 6 and 7 are arranged so as to have a curved surface in the vertical direction in the cross section of the optical path in the resonator.

When the excitation light 4 is irradiated from the laser diode 5 to the side surface 3a of the solid laser medium 3, the solid laser medium 3 is optically excited. By this optical excitation, the solid-state laser medium 3 is in an inverted distribution state. The solid-state laser medium 3 in the inverted distribution state spontaneously emits, and light of various modes such as a low-order mode and a high-order mode is generated inside the resonator.

Further, the excitation light 4 forms a temperature distribution in the solid laser medium 3 in which the temperature decreases exponentially from the side surface 3a in the depth direction (absorption effect of the solid laser medium). Since the refractive index decreases as the temperature increases, a refractive index distribution 10 in which the refractive index increases exponentially from the side surface in the depth direction is formed in the solid laser medium as shown in FIG. 2 is a schematic diagram of the thermal lens 8 on the side surface 3a of the solid-state laser medium 3 to which the excitation light 4 is irradiated. The thermal lens effect due to the refractive index distribution appears as a quadratic function according to the beam size (mode size) of light inside the optical resonator (8a and 8b in FIG. 2). The higher the temperature, the shorter the focal length. The thermal lens 8 is formed. As described above, since the light traveling back and forth inside the resonator is totally reflected by the side surface 3a (11 in FIG. 2) of the solid-state laser medium 3, the thermal lens 8 is a mirror image 12 that is symmetric at the reflecting surface (mirror surface 11). (Mirror image phenomenon), and becomes the thermal lens 8. Here, the low-order mode thin light such as a Gaussian beam in the light inside the resonator feels the thermal lens 8 as a lens 8a having a short focal length. On the other hand, relatively thick light of higher order modes such as Hermitian Gaussian beam in the light inside the resonator feels that the thermal lens 8 is a lens 8b having a long focal length commensurate with the resonance length capable of resonating.

Inside the optical resonator, light substantially perpendicular to the reflecting mirror 1 and the output mirror 2 of the optical resonator is amplified by stimulated emission. Specifically, a low-order mode thin light such as a Gaussian beam in the light inside the optical resonator diverges by the lens 8a having a short focal length and receives diffraction loss of the mirror of the optical resonator. Only the relatively thick light of the higher order mode is sufficiently reflected by the reflection mirror 1 and the output mirror 2 of the optical resonator. Therefore, the low-order mode thin light diverges and the higher-order mode relatively thick light is selectively amplified and oscillates from the output mirror 2 to the outside. As described above, the thermal lens 8 has mode selectivity of light inside the optical resonator. The light reciprocating in the optical resonator is totally reflected by the side surface 3a of the solid-state laser medium 3, and reciprocates between the reflecting mirror 1 and the output mirror 2 in the resonator.

Here, the condition for selectively amplifying the relatively thick light of the higher-order mode by the resonator is that the thermal lens 10, the two cylindrical lenses, and the optical resonator have the following resonance with respect to the higher-order mode. The condition (formula 1) is satisfied, and the low-order mode is not satisfied.
(Where f is the focal length of the thermal lens, L _{M1 is} the resonance distance on the reflecting mirror side, R _{M1 is the} radius of curvature of the reflecting mirror, L _{M2 is} the resonance distance on the output mirror side, R _{M2 is the} radius of curvature of the output mirror, L = L _M1 + L _M2 −L _M1 · L _M2 / f.)
In the above formula, a reflection mirror having a radius of curvature R _M1 that sets a distance of the resonance length L _M1 across a lens having a focal length f, and an output mirror of a radius of curvature R _M2 that sets a distance of the resonance length L _M2 are opposed to each other. This is a general stability condition for an optical resonator. Here, L = L _M1 + L _M2 −L _M1 · L _M2 / f. In the present invention, the focal length f is the focal length of the thermal lens. As described above, since the focal length f of the thermal lens varies depending on the beam size, the values of g ₁ and g ₂ vary depending on the beam size, and the same resonator satisfies the resonance condition of the above formula for the higher-order mode. Therefore, it is possible not to satisfy the low-order mode.

The solid-state laser medium 3, the laser diode 5, and the cylindrical lenses 6 and 7 are arranged so that the thermal lens 8 becomes a confocal position of the cylindrical lenses 6 and 7. The cylindrical lenses 6 and 7 have a curved surface in the longitudinal direction in the optical path section in the resonator, while the thermal lens 8 has an optical action like a cylindrical lens having a curved surface in the lateral direction in the optical path section. Is done. Therefore, the light inside the resonator is given a phase difference of π / 2 in the process of passing through the thermal lens 8. The cylindrical lenses 6 and 7 compensate for vertical light divergence in the thermal lens 8. Although the focal distance of the cylindrical lenses 6 and 7 is not specifically limited, For example, it is 50 mm-100 mm, and is 50 mm in an Example.

In general, inside the resonator, a high-order mode laser beam is selectively amplified and oscillated while being given a phase difference of π / 2 by the thermal lens 8, and has a helical equiphase surface from the output mirror 2. And the optical vortex laser beam 9 whose beam intensity distribution of an optical axis cross section is ring shape is output.

The reflection mirror 1 is preferably a total reflection mirror, but may be a partial transmission mirror as long as it has a mirror reflectivity higher than that of the output mirror 2. Here, the mirror reflectance of the output mirror 2 is not particularly limited, but is preferably 20% to 99%. The reflection mirror 1 and the output mirror 2 are not particularly limited, but are preferably plane mirrors. The reflection mirror 1 and the resonance length are not particularly limited, but are preferably 100 mm to 1000 mm.

Nd: YVO ₄ , Nd: YAG, Nd: GdVO ₄ , Nd: LuVO ₄ , Nd: Gd _× Y _1-x VO _4, Nd doped with Nd at a doping concentration of 1 atm% or more as the solid laser medium 3 : YAB is exemplified. For example, Yb, Tm, or Ho may be doped instead of Nd. The size of the solid laser medium 3 is not particularly limited, but is, for example, 20 mm × 5 mm × 2 mm (side surface 3a: 20 mm × 2 mm, end surfaces 3b1, 3b2: 5 mm × 2 mm). Although not particularly required, the side surface 3a of the solid-state laser medium 3 is preferably provided with an AR coat for the wavelength of the excitation light from the laser diode 5. For example, AR coating for an optical wavelength of 808 nm. Further, the end faces 3b1 and 3b2 of the solid-state laser medium 3 are not particularly required, but it is desirable that AR coating for the wavelength of the output light is applied. For example, AR coating for a light wavelength of 1.3 μm. This is to reduce the loss of the resonator and facilitate oscillation.

The output of the laser diode 5 is large enough to form a thermal lens in the solid-state laser medium 3, and is preferably greater than 10W, and in the embodiments is 35W, 45W, 50W, 55W. Further, the wavelength of the excitation light 4 is not particularly limited, but is 780 to 970 nm, for example, and 808 nm in the embodiment. A cylindrical lens having a curved surface in the longitudinal direction in the optical path cross section may be disposed between the laser diode 5 and the side surface 3a of the solid-state laser medium 3.

In the laser beam oscillation method having the helical equiphase surface of the present invention and the beam intensity distribution being a ring shape, the output laser beam 9 is not particularly limited, but in reality, the output is 1 W to 50 W. In the embodiment, it is 5.2 W, 5.8 W, and 6 W. The wavelength range is wide, preferably 1 μm to 2 μm, and 1.3 μm in the examples. The method of generating an optical vortex according to the present invention can be applied to all solid-state lasers.

Further, an acousto-optic device can be arranged between the reflection mirror 1 and the cylindrical lens 6 to act as a Q switch, so that the output of the optical vortex laser beam can be 10 to 100 times. Further, instead of the acousto-optic element, a Q-switch oscillation is possible as a saturable absorbing element such as an electro-optic element or Cr: YAG.

It is also possible to arrange a saturable absorber between the reflecting mirror 1 and the cylindrical lens 6 and oscillate an optical vortex pulse laser of nanosecond level to picosecond level as a mode-locked laser. Further, instead of the saturable absorber, an acoustooptic device can oscillate an optical vortex pulse laser of nanosecond level to picosecond level by active mode lock.

An optical vortex laser beam oscillation apparatus A according to the second invention will be described with reference to FIG.

The optical vortex laser beam oscillation apparatus A according to the second invention comprises a solid-state laser medium 3 and two cylindrical lenses 6 and 7 sandwiching the solid-state laser medium 3 in an optical resonator. In the laser beam oscillation device that oscillates a laser beam by optically exciting with a laser diode 5 as an excitation light source, the solid-state laser medium 3 is optically excited at the confocal position of the two cylindrical lenses 6 and 7. The focal length of the two cylindrical lenses 6 and 7 and the resonance length of the optical resonator are resonant conditions for higher-order modes;
(Where f is the focal length of the thermal lens, L _{M1 is} the resonance distance on the reflecting mirror side, R _{M1 is the} radius of curvature of the reflecting mirror, L _{M2 is} the resonance distance on the output mirror side, R _{M2 is the} radius of curvature of the output mirror, L = L _M1 + L _M2 −L _M1 · L _M2 / f.)
And the resonance condition is set so as not to satisfy the low-order mode. Each component is as described in the optical vortex laser beam oscillation method of the first invention.

The optical vortex laser beam oscillation method, optical vortex laser beam oscillation apparatus A, and generated optical vortex laser beam of the present invention will be specifically described below. In Example 1, a continuous wave of an optical vortex laser beam was oscillated. In Example 2, a pulse wave of an optical vortex laser beam was oscillated.
[Example 1]
[Conditions for optical vortex laser beam generation]
In the optical vortex laser beam generating method of the first invention and the optical vortex laser beam oscillation apparatus A of the second invention, the resonance condition (Equation 1) is satisfied for the higher order mode, and The following parameters were set so as not to satisfy the resonance condition (Equation 1), and a continuous wave of the optical vortex laser beam was generated. In addition, the output of the excitation light 4 was made into four steps, 35W, 45W, 50W, and 55W. The following description is based on FIG.
(1) Parameters related to the following resonance condition (Formula 1)
Reflection mirror resonance length L _M1 : 24cm
Reflective mirror radius of curvature R _M1 : ∞
Output mirror resonance length L _M2 : 16cm
Output mirror radius of curvature R _M2 : ∞
Approximate value of thermal lens focal length f for low-order modes: 14 to 26 cm (see Table 1)
Approximate value of thermal lens focal length f for higher-order modes: 22 to 43 cm (see Table 1)

L = 40cm for low-order modes (L is the total resonator length)
L = 40cm for higher-order modes (L is the total resonator length)
Approximate value of g ₁ g ₂ for low-order modes: -0.04 to -0.007 (see Table 2)
Approximate value of g ₁ g ₂ for higher order modes: 0.007 to 0.22 (see Table 2)

(2) Cylindrical lenses 6 and 7 The focal lengths of the cylindrical lenses 6 and 7 are both 50 mm.
(3) About the reflection mirror 1 and the output mirror 2 The reflection mirror 1 and the output mirror 2 are both plane mirrors. The reflection mirror 1 is a total reflection mirror, and the mirror reflectivity of the output mirror 2 is 85%.
(4) Solid Laser Medium 3 The solid laser medium 3 is Nd: YVO ₄ doped with Nd at a doping concentration of 1 atm%, and its size is 20 mm × 5 mm × 2 mm (side surface 3a: 20 mm × 2 mm, End faces 3b1, 3b2: 5 mm × 2 mm). The side surface 3a of the solid-state laser medium 3 is provided with an AR coating for the light wavelength of 808 nm. The end faces 3b1 and 3b2 of the solid-state laser medium 3 are subjected to an AR coating for a light wavelength of 1.3 μm.
(5) About the laser diode 5 The laser diode 5 output four-stage excitation light 4 of 35 W, 45 W, 50 W, and 55 W. The wavelength of the excitation light 4 is 808 nm. Although not shown, a cylindrical lens having a curved surface in the vertical direction in the cross section of the optical path is disposed between the laser diode 5 and the side surface 3 a of the solid-state laser medium 3.

[Generated optical vortex]
(1) Optical vortex output The optical vortex laser beam 9 generated by the above method will be described. The output laser beam 9 was measured with a power meter. The results are shown in Table 3. The wavelength of these optical vortices was 1.3 μm.
(2) Confirmation of optical vortex The interferometer used for confirming the optical vortex is a Mach-Zehnder interferometer, and its optical system is shown in FIG. This optical system divides the optical vortex laser beam 9 into two paths by a half mirror 13 to form optical vortex laser beams 9a and 9b. A mirror 14, a pinhole 15 and a lens 16 are provided in the path of the optical vortex laser beam 9a. Then, a part of the optical vortex laser beam 9 a is cut out by the pinhole 15 and converted into the spherical wave 17 by the lens 16. The spherical wave 17 and the optical vortex laser beam 9b are superposed by the half mirror 18, the mirror 19 and the prism mirror 20, and the interference fringes 21 are observed. The interference fringe 21 drew a vortex from the middle without light intensity (right side of FIG. 3).
FIG. 4A shows the intensity distribution of the generated optical vortex. FIG. 4B shows vortex-like interference fringes observed when an optical vortex is superimposed on a spherical wave.
FIG. 4C shows interference fringes observed when the optical vortices are caused to interfere with each other without converting the optical vortex into a spherical wave without using a pinhole in the optical system of FIG. Since the interference fringes are cut, it can be confirmed that there is a phase singularity in the middle. FIG. 4D is a numerical simulation image of interference fringes observed when the optical vortices interfere with each other. (C) and (d) in FIG. 4 correspond to experiments and theories. From the above, it can be seen that the output laser beam is an optical vortex.

[Example 2]
[Conditions for optical vortex laser beam generation]
In the optical vortex laser beam generating method of the first invention and the optical vortex laser beam oscillation apparatus B of the second invention, the resonance condition (Equation 1) is satisfied for the higher order mode, and The following parameters were set so as not to satisfy the resonance condition (Equation 1), and a Q-switch mode-lock pulse of the optical vortex laser beam was generated. The output of the excitation light 4 was set to 34W and 42W. The following description is based on FIG.
(1) Parameters related to the following resonance condition (Formula 1)
Resonant mirror resonance length L _M1 : 425mm
Reflective mirror radius of curvature R _M1 : ∞
Output mirror resonance length L _M2 : 440mm
Output mirror radius of curvature R _M2 : ∞
Approximate value of thermal lens focal length f for low-order modes (see Table 4)
Approximate value of focal length f of thermal lens for higher order mode (see Table 4)

L = 865mm for low-order modes (L is the total resonator length)
L = 865mm for higher-order modes (L is the total resonator length)
Approximate value of g ₁ g ₂ for low-order mode: see Table 5 Approximate value of g ₁ g ₂ for high-order mode: see Table 5

(2) Cylindrical lenses 6 and 7 The focal lengths of the cylindrical lenses 6 and 7 are both 50 mm.
(3) About the reflection mirror 1 and the output mirror 2 The reflection mirror 1 and the output mirror 2 are both plane mirrors. The reflection mirror 1 is a total reflection mirror, and the mirror reflectivity of the output mirror 2 is 40%.
(4) Solid-state laser medium 3 The solid-state laser medium 3 is Nd: GdVO ₄ doped with Nd ³⁺ at a doping concentration of 1 atm%, and its size is 20 mm × 5 mm × 2 mm (side surface 3a: 20 mm × 2 mm, end faces 3b1, 3b2: 5 mm × 2 mm). An AR coating for the light wavelength 809 nm is applied to the side surface 3 a of the solid-state laser medium 3. Further, the end surfaces 3b1 and 3b2 of the solid-state laser medium 3 are subjected to an AR coating for a light wavelength of 1064 nm. The c-axis direction is shown in the solid-state laser medium 3 in FIG. The angle formed by the light traveling back and forth between the side surface 3a of the solid-state laser medium 3 and the inside of the resonator is 28 °.
(5) Laser diode 5 34 W and 42 W excitation light 4 was output from the laser diode 5. The wavelength of the excitation light 4 is 809 nm. Between the laser diode 5 and the side surface 3a of the solid-state laser medium 3, a cylindrical lens 22 having a curved surface in the vertical direction in the optical path cross section is disposed.
(6) Acoustooptic Element (AOM) 23 An acoustooptic element (AOM) 23 is disposed between the reflection mirror 1 and the cylindrical lens 6, and the acoustooptic element (AOM) 23 and the solid-state laser medium 3 A telescope 24 having a focal length of 50 mm and a telescope 25 having a focal length of 100 mm are disposed between the two (FIG. 5). The telescope 24 is disposed at a position 50 mm from the reflection mirror 1, and the telescope 25 is disposed at a position 200 mm from the reflection mirror 1.
The acousto-optic device (AOM) 23 was operated as a Q-switched mode locker, and a Q-switched mode-lock pulse 26 of an optical vortex laser beam was output. The repetition frequency of the acousto-optic element (AOM) 23 was set to 100 kHz, and the mode lock frequency was set to ~ 170 MHz (approximately 170 MHz).

[Q-switched mode-locked pulse of generated vortex laser beam]
FIG. 6 shows a Q-switch mode lock pulse of an optical vortex laser beam generated in the case of an excitation light output of 42 W. The wavelength of the Q switch mode lock pulse was 1063 nm. In addition, the vertical axis | shaft of FIG. 6 is light intensity (a unit is au (arbitrary unit)), and a horizontal axis is time. The pulse time width of the Q switch mode lock pulse was about 100 ns. It can also be seen that there are a plurality of spikes having a time interval of about 5 ns on one pulse. Therefore, the optical vortex laser beam generating method of the first invention and the optical vortex laser beam oscillation apparatus B of the second invention were able to obtain an optical vortex laser beam with a high peak power of maximum output> 10 kW.

The optical vortex laser beam oscillation method and optical vortex laser beam oscillation apparatus of the present invention are useful for optical manipulation and atomic trapping. It is also useful for laser processing while capturing plasma and spatter (oxidized product of molten metal).

FIG. 1 is a plan view of the optical vortex laser beam oscillation apparatus A. FIG. FIG. 2 is a schematic diagram of the thermal lens 8 on the side surface 3 a of the solid-state laser medium 3 irradiated with the excitation light 4. FIG. 3 is a schematic diagram of an interferometer for confirmation of the optical vortex laser beam 9. FIG. 4A shows the intensity distribution of the generated optical vortex laser beam 9. FIG. 4B shows vortex interference fringes observed when the optical vortex laser beam is superimposed on a spherical wave. FIG. 4C shows interference fringes observed when the optical vortex laser beams interfere with each other without converting the optical vortex laser beams into spherical waves without using pinholes in the optical system of FIG. FIG. 4D shows a numerical simulation image of interference fringes observed when optical vortex laser beams interfere with each other. FIG. 5 is a plan view of the optical vortex laser beam oscillator B. FIG. 6 is a graph showing the intensity of the Q switch mode lock pulse of the optical vortex laser beam.

Explanation of symbols

A Optical vortex laser beam oscillation device 1 Reflecting mirror 2 Output mirror 3 Solid laser medium 3a Side surface 4 of solid laser medium Excitation light 5 Laser diode 3b1 One end face 6 of solid laser medium 3 Cylindrical lens 3b2 The other end face of solid laser medium 3 7 Cylindrical lens 8 Thermal lens 8a Thermal lens 8b in which low-order mode light is sensed Thermal lens 9 in which higher-order mode light is sensed relatively light 9 Optical vortex laser beam 9a Optical vortex laser beam 9b Optical vortex laser beam 10 Refractive index distribution 11 Mirror surface 12 Mirror image 13 Half mirror 14 Mirror 15 Pinhole 16 Lens 17 Spherical wave 18 Half mirror 19 Mirror 20 Prism mirror 21 Interference fringe 22 Cylindrical lens 23 Acoustooptic device (AOM)
24 Telescope (focal length 50mm)
25 Telescope (focal length 100mm)
26 Q-switched mode-locked pulse of optical vortex laser beam

Claims (7)

A solid laser medium and two cylindrical lenses sandwiching the solid laser medium are arranged in an optical resonator including a reflection mirror and an output mirror, and the solid laser medium is optically excited by excitation light from an excitation light source. In the laser beam oscillation method for oscillating a laser beam by the optical excitation, a thermal lens is formed in the solid laser medium by the optical excitation, and a high-order mode is formed by the thermal lens, the two cylindrical lenses, and the optical resonator. Resonance conditions;
(Where f is the focal length of the thermal lens, L _{M1 is} the resonance distance on the reflecting mirror side, R _{M1 is the} radius of curvature of the reflecting mirror, L _{M2 is} the resonance distance on the output mirror side, R _{M2 is the} radius of curvature of the output mirror, L = L _M1 + L _M2 −L _M1 · L _M2 / f.)
By satisfying the above, and not satisfying the resonance condition for the low-order mode, the high-order mode laser beam is selectively amplified and oscillated while changing the phase of the high-order mode laser beam by 90 degrees. An optical vortex laser beam oscillation method characterized by the above. The optical excitation is a side excitation method in which excitation is performed from a side surface of the solid-state laser medium, and a thermal lens having a temperature distribution gradient in which the temperature decreases exponentially from the side surface of the solid-state laser medium in the depth direction is formed. The optical vortex laser beam oscillation method according to claim 1. 3. The optical vortex laser beam oscillation method according to claim 1, wherein the solid-state laser medium is Nd: YVO ₄ or Nd: GdVO ₄ doped with Nd at a doping concentration of 1 atm% or more. The optical vortex laser beam oscillation method according to any one of claims 1 to 3, wherein the excitation light source is a laser diode. A solid laser medium and two cylindrical lenses sandwiching the solid laser medium are arranged in an optical resonator including a reflection mirror and an output mirror, and the solid laser medium is optically excited by excitation light from an excitation light source. In the laser beam oscillation device that oscillates the laser beam, the laser diode is arranged so that the solid-state laser medium is optically excited at the confocal position of the two cylindrical lenses, and the resonance condition for the higher-order mode;
(Where f is the focal length of the thermal lens, L _{M1 is} the resonance distance on the reflecting mirror side, R _{M1 is the} radius of curvature of the reflecting mirror, L _{M2 is} the resonance distance on the output mirror side, R _{M2 is the} radius of curvature of the output mirror, L = L _M1 + L _M2 −L _M1 · L _M2 / f.)
In order to satisfy the resonance condition for the low-order mode, the focal length of the thermal lens, the resonance distance on the reflection mirror, the radius of curvature of the reflection mirror, the resonance distance on the output mirror, and the radius of curvature of the output mirror An optical vortex laser beam oscillation device characterized in that is set. 6. The optical vortex laser beam oscillator according to claim 5, wherein the solid-state laser medium is Nd: YVO ₄ or Nd: GdVO ₄ doped with Nd at a doping concentration of 1 atm% or more. The optical vortex laser beam oscillation apparatus according to claim 5 or 6, wherein the excitation light source is a laser diode.