JP2009010066A - Pulsed laser oscillator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a pulsed laser oscillator which suppresses heat generation in a host crystal, can be miniaturized, has excellent thermal durability, and can obtain an output beam of high quality and high output. <P>SOLUTION: The pulsed laser oscillator concerning this invention includes a conical lens 10 which makes the pumping light generated by a pumping light source 2 into a Bessel beam and makes it incident on a solid laser medium 5. The solid laser medium 5, an aperture 7 and the host crystal 6 are integrally bonded together, and direct bonding is carried out between the solid laser medium 5 and the aperture 7, and between the aperture 7 and the host crystal 6. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a pulse laser oscillator, and more particularly to a pulse laser oscillator having an improved oscillator structure.

  Laser light can be used for material processing such as cutting, drilling, and welding, marking, exposure, and the like, and its application is diverse.

  When laser light is used for machining applications, thermal damage to the workpiece may be a problem. For such applications, a pulse laser that can apply high energy to the workpiece in a short time. Light is used. There are various types of pulse lasers such as a gas laser and a solid-state laser, and laser oscillators using Q switching technology are attracting attention as those capable of outputting high power and short pulses.

  As a laser oscillator using such Q-switching technology, for example, a solid laser medium disposed in a pair of reflecting mirrors constituting a resonator and fluorescence generated from the solid laser medium are absorbed. A host crystal (supersaturated absorber) with reduced transmittance, a semiconductor laser element that emits excitation light for exciting the solid-state laser medium, and suppressing the incidence of the excitation light, and generation of heat from the host crystal due to light emission from the laser medium A passive Q-switched laser including an aperture member that can be suppressed is disclosed (for example, see Patent Document 1).

Further, by providing a recess on at least one of the exit surface of the laser medium and the entrance surface of the host crystal, a space is formed between the laser medium and the host crystal, and an adhesive or A passive Q-switched laser integrated by optical contact, diffusion bonding or the like is also disclosed (for example, see Patent Document 2).
JP 2003-86873 A JP 2006-310604 A

  However, in the laser oscillator having the configuration shown in Patent Documents 1 and 2, etc., since the solid laser medium and the host crystal are arranged side by side, a part of the excitation light emitted from the excitation light source is not absorbed by the solid laser medium. Since the light is incident on the host crystal together with the fluorescence generated in the solid-state laser medium, an unnecessary temperature rise occurs in the host crystal. In order to obtain higher-power pulsed laser light, when pumping light having higher energy from the pumping light source is incident on the solid-state laser medium, it becomes a more significant problem. For example, as disclosed in Patent Document 1, even if an aperture member that can suppress the heat generation of the host crystal is provided, there is a limit to the suppression of the heat generation of the host crystal.

  In addition, a technique for coating the end face of the solid-state laser medium on the host crystal side with a reflection film that reflects only the wavelength of the excitation light, or another reflection mirror other than the pair of reflection mirrors is provided, and is folded by another optical path. A technique for applying a resonator configuration is known.

  However, in the former case, the solid-state laser medium and the host crystal must be separately fixed and adjusted, which increases the number of adjustment points and the number of parts and increases the size of the laser oscillator. In the latter case, since a separate optical path is required, the laser oscillator is similarly increased in size.

  In order to suppress an increase in the size of the apparatus, it is necessary to integrate the solid-state laser medium and the host crystal as described in Patent Document 2.

  However, when the solid-state laser medium and the host crystal are integrated, even if the solid-state laser medium generates heat, it has a thermal durability such that there is no deviation of the bonding surface between the solid-state laser medium and the host crystal. Is required. When such a deviation of the bonding surface occurs, it is natural that a high-quality output beam cannot be obtained.

  The present invention has been made in consideration of the above-described circumstances, and suppresses heat generation of the host crystal, can be downsized, has excellent thermal durability, and provides a high-quality, high-output output beam. An object is to provide a laser oscillator.

  In view of the above problems, the inventors of the present invention are able to solve the above-mentioned problem by using a conical lens to convert the excitation light into a Bessel beam and directly joining and integrating the solid-state laser medium, the aperture, and the host crystal. I found it.

  That is, the pulse laser oscillator according to the present invention absorbs the excitation light source that emits excitation light, the solid-state laser medium that generates fluorescence when the excitation light is incident, and the fluorescence generated from the solid-state laser medium. A host crystal whose transmittance decreases with absorption, and an aperture for controlling the optical path of light incident on the host crystal of fluorescence generated in the solid-state laser medium between the solid-state laser medium and the host crystal. And a first reflecting mirror provided between the excitation light source and the solid-state laser medium, and disposed opposite the first reflecting mirror via the solid-state laser medium, the aperture, and the host crystal. The solid-state laser is provided between the second reflecting mirror, the first reflecting mirror, and the excitation light source, and the excitation light generated by the excitation light source is converted into a Bessel beam. The solid laser medium, the aperture and the host crystal are integrally joined, and the solid laser medium and the aperture, and between the aperture and the host crystal. It is characterized by being directly joined.

  According to the pulse laser oscillator of the present invention, there is provided a pulse laser oscillator that suppresses heat generation of the host crystal, can be downsized, has excellent thermal durability, and can obtain a high-quality, high-output output beam. be able to.

  A pulse laser oscillator according to an embodiment of the present invention will be described with reference to the drawings.

  FIG. 1 is a conceptual diagram of a pulse laser oscillator according to an embodiment of the present invention.

  As shown in FIG. 1, a pulsed laser oscillator 1 according to an embodiment of the present invention is an end-pumped passive Q-switched pulsed laser oscillator using a Fabry-Perot resonator, and an excitation light source 2 that emits excitation light. And the solid laser medium 5 that generates fluorescence when the excitation light is incident through the optical fiber 4 and the collimator 3 that guides the excitation light, and the fluorescence generated from the solid laser medium 5 is absorbed and absorbed. Along with the host crystal 6, the transmittance of which decreases, and between the solid-state laser medium 5 and the host crystal 6, an aperture 7 that controls the optical path of light incident on the fluorescent host crystal 6 generated in the solid-state laser medium 5. And the first reflecting mirror 8 provided between the excitation light source 2 and the solid-state laser medium 5, and the first reflecting mirror 8 through the solid-state laser medium 5, the aperture 7 and the host crystal 6. Arrangement The second reflecting mirror 9, the first reflecting mirror 8, and the excitation light source 2 are provided, and the excitation light generated by the excitation light source 2 is converted into a Bessel beam and incident on the solid laser medium 5. A conical lens 10 is provided.

  As the excitation light source 2, for example, a fiber output LD is used, and one having a wavelength of 808 nm and an output of 5 W is used.

  As the solid-state laser medium 5, for example, Nd-added YAG or the like can be used.

  The host crystal 6 is a crystal (supersaturated absorber) that has a predetermined initial transmittance with respect to the fluorescence generated from the solid-state laser medium 5 and becomes transparent when the fluorescence intensity increases to some extent. For example, Cr-added YAG, Cr-added GSGG or the like can be used.

  The aperture 7 can be made of a metal that reflects the excitation light without absorbing it, such as tungsten, molybdenum, or tantalum.

  The first reflecting mirror 8 and the second reflecting mirror 9 are, for example, a radius of curvature (ROC) = ∞ / ∞, no reflection (AR) @ 808 nm / high reflection (HR) @ 1064 nm, a second reflecting mirror (plano-concave lens) ) 9 is ROC = 50 nm / ∞, AR @ 808 nm / T = 70% @ 1064 nm.

  The solid-state laser medium 5 and the aperture 7 and the aperture 7 and the host crystal 6 are directly joined, and the solid-state laser medium 5, the aperture 7 and the host crystal 6 are joined together. The term “direct bonding” as used herein indicates direct bonding without using a coating layer or an adhesive layer. Specifically, it indicates that the bonding is performed by diffusion bonding or integral firing.

  Diffusion bonding is a method in which the solid-state laser medium 5, the aperture 7, and the host crystal 6 are bonded by applying pressure at a high temperature.

  The integral firing is also referred to as simultaneous firing. After the solid laser medium 5, the aperture 7, and the host crystal 6 are stacked at the stage of the molded body before firing, they are integrated using a press or the like and fired at a predetermined temperature. Is the method.

  That is, the solid laser medium 5 and the host crystal 6 are directly bonded via the aperture 7, for example, an Nd: YAG / Pt / Cr: YAG single crystal bonded monolithic product or an integrally bonded synthetic product cofired. Goods are used.

  As described above, since the solid laser medium 5 and the aperture 7 and between the aperture 7 and the host crystal 6 are directly joined without any coating layer or adhesive layer, the thermal durability is excellent. Yes. If the adhesive layer is interposed, these layers are mainly composed of a resin, so that the thermal durability accompanying the heat generation of the host crystal 6 is weak, and a slight deviation of the bonding surface occurs. Such a deviation is not preferable because the quality of the output beam is reduced.

  The aperture 7 formed between the solid-state laser medium 5 and the host crystal 6 cuts off a part of the fluorescence generated in the solid-state laser medium 5 and the excitation light transmitted without being absorbed by the solid-state laser medium 5.

  The conical lens 10 is used for condensing the excitation light emitted from the excitation light source 2 on the solid-state laser medium 5. FIG. 2 is a conceptual diagram for explaining the light intensity distribution when the excitation light incident on the conical lens 10 is emitted. As shown in FIG. 2, the intensity distribution of the light collected using the conical lens 10 is such that the intensity up to the condensing portion 10a on the central axis H of the conical lens 10 increases as the distance from the conical lens 10 increases. When further away from the light portion 10a, a donut-shaped intensity distribution having a hollow center portion is shown. Hereinafter, such light is referred to as a Bessel beam (see, for example, JP-A-9-36462). Using this property, the optical axis on which the excitation light is collected, the alignment axis of the resonator, the central axis of the aperture 7 opening 7a, and the like are adjusted, and the light collection region 10a for the excitation light comes into the solid-state laser medium 5. As described above, when the conical lens 10 and the solid-state laser medium 5 are arranged and the aperture 7 having the opening 7a is arranged where the intensity distribution of the Bessel beam is in a donut shape, excitation is performed without blocking the spatial mode of the resonator. Only light can be selectively cut by the aperture 7 (FIG. 3). Therefore, it is possible to prevent the excitation light from being incident on the host crystal 6 without being absorbed by the solid-state laser medium 5, and to suppress an unnecessary temperature rise in the host crystal 6. Excellent thermal durability and high output beam can be obtained. Furthermore, since the solid-state laser medium 5, the aperture 7, and the host crystal 6 can be integrated, the size can be reduced.

  In order to completely cut the excitation light, it is necessary to make the opening diameter of the opening 7 a of the aperture 7 smaller than the inner diameter of the donut-shaped intensity distribution of the Bessel beam at the position of the aperture 7. In order not to prevent laser oscillation, the opening diameter of the opening 7 a of the aperture 7 needs to be larger than the spatial mode diameter of the resonator at the position of the aperture 7. On the other hand, when the diameter of the opening 7a of the aperture 7 is set to a size close to the fundamental mode of the resonator, a function as a mode selector for laser oscillation can be provided.

  When the conventional elliptic lens 10 'is used, as shown in FIG. 4, the excitation light emitted from the elliptic lens 10 does not have a light optical path like the Bessel beam described above. Accordingly, even if the condensing portion of the elliptic lens 10 ′ is disposed so as to be in the solid laser medium 5, the excitation light that is not absorbed by the solid laser medium 5 enters the host crystal 6 from the aperture opening. The generation of an unnecessary temperature rise of the crystal 6 cannot be suppressed.

  The solid-state laser medium 5 and the host crystal 6 are preferably made of a ceramic material. When the solid-state laser medium 5 and the host crystal 6 are composed of a single crystal, the bonding strength between the solid-state laser medium 5 and the host crystal 6 and the aperture 7 made of a metal material may be weak, and the thermal durability May be inferior. However, when the solid-state laser medium 5 and the host crystal 6 are made of a ceramic material, the bonding strength with the aperture 7 becomes stronger than that of a single crystal, and a pulse laser oscillator with more excellent thermal durability can be obtained.

Example 1
a) A solid laser medium of φ10 mm × 10 mm Nd: YAG single crystal is prepared, and one end face of φ10 mm is polished, and a tungsten thin film having an inner diameter of φ0.2 mm and an outer diameter of 8 mm is formed by EB vapor deposition based on the center of the polished surface. An aperture was formed.

  b) A host crystal of Cr: YAG single crystal of φ10 mm × 4 mm was prepared, and one end face was polished.

  c) Laminate in which both the solid laser medium and the host crystal are diffused and integrated by heating and pressing so that the polished surfaces of the solid laser medium and the host crystal become bonded surfaces through an aperture formed of a tungsten thin film. Formed body. Further, laser-grade polishing was performed on the end face that was not previously polished.

  d) A Fabry-Perot resonator as shown in FIG. 1 was prepared, and the laminated body covered with a cooling block was installed therein.

  e) A fiber-coupled semiconductor laser having an output of 5 W and a wavelength of 808 nm was prepared as an excitation light source. A collimator was connected to the output end of the fiber so that it became parallel light, and this output light was condensed on a gain medium by a conical lens. At this time, in the range where the intensity distribution of the excitation light at the aperture position is donut-shaped and the inner diameter is larger than the aperture opening diameter, the resonator length and the stack of the laminated body in the resonator are maximized so that the laser output is maximized. The position, the focal length of the conical lens, the position of the conical lens, etc. were adjusted optimally.

f) As a result, a pulse laser output having a pulse width of 2 nm, an average output of 300 mW, and a beam quality M ² <1.1 was obtained. When this laser was operated continuously for 24 hours, the stability of the average output was within ± 3%.

(Example 2)
a) Granulated powder of Nd: YAG polycrystalline ceramics was prepared and press-molded to a size of 13 mm in thickness using a φ13 mm mold. The Nd: YAG molded body was not taken out from the mold, and a tungsten foil having an outer diameter of φ10 mm and an inner diameter of φ0.2 mm was set on the Nd: YAG molded body so that the central axes were aligned.

  b) A granulated powder of Cr: YAG polycrystalline ceramics is prepared, and an amount of 10 mm after molding is put on the molded body in the above mold, and pressed together to form tungsten (aperture) ) Was embedded to obtain an Nd: YAG, Cr: YAG integral molded body.

  c) After the above-mentioned molded body was vacuum fired, an annealing treatment for controlling the valence of the added ions was performed. The sintered body was ground so that the outer diameter was 10 mm, the Nd: YAG portion thickness was 10 mm, and the Cr: YAG portion thickness was 4 mm, and laser grade polishing was performed on both end faces to obtain a laminate.

  d) A Fabry-Perot resonator as shown in FIG. 1 was prepared, and the laminated body covered with a cooling block was installed therein.

  e) A fiber-coupled semiconductor laser having an output of 5 W and a wavelength of 808 nm was prepared as an excitation light source.

  A collimator was connected to the output end of the fiber so that it became parallel light, and this output light was condensed on a gain medium by a conical lens. At this time, the resonator length and the position of the laminate in the resonator are set so that the laser output is maximized in the range where the intensity distribution of the excitation light is donut-shaped and the inner diameter is larger than the aperture opening diameter at the aperture position. The focal length of the conical lens, the position of the conical lens, etc. were adjusted optimally.

f) As a result, a pulse laser output having a pulse width of 2 ns, an average output of 300 mW, and a beam quality M ² <1.3 was obtained. When this laser was operated continuously for 24 hours, the stability of the average output was within ± 3%.

(Comparative Example 1)
a) Same as Example 1 except that the polished surfaces of the YAG single crystal laser gain medium and the YAG single crystal passive Q switch element are bonded and integrated with an organic optical adhesive. The test was conducted.

b) As a result, a pulse laser output having a pulse width of 2 ns, an average output of 250 mW, and a beam quality M ² <2.1 was obtained. When this laser was operated continuously for 24 hours, the stability of the average output was within ± 10% (the average output was also reduced by 40 mW). The decrease in output and beam quality are presumed to be caused by the occurrence of loss and alignment change due to thermal damage to the adhesive layer.

1 is a conceptual diagram of a pulse laser oscillator according to an embodiment of the present invention. The conceptual diagram explaining the intensity distribution of the light when the excitation light which injected into the conical lens inject | emitted. The conceptual diagram explaining a mode that the excitation light made into the Bessel beam is shielded with an aperture. The conceptual diagram explaining the intensity distribution of the light at the time of the excitation light which injects into the conventional elliptical lens radiate | emitted.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Pulse laser oscillator 2 Excitation light source 3 Collimator 4 Optical fiber 5 Solid-state laser medium 6 Host crystal 7 Aperture 8 1st reflective mirror 9 2nd reflective mirror 9
10 Conical lens

Claims (5)

An excitation light source that emits excitation light;
A solid-state laser medium that generates fluorescence when the excitation light is incident;
A host crystal that absorbs fluorescence generated from the solid-state laser medium and has a transmittance that decreases with absorption;
An aperture that is provided between the solid-state laser medium and the host crystal and controls an optical path of light incident on the host crystal of fluorescence generated in the solid-state laser medium;
A first reflecting mirror provided between the excitation light source and the solid-state laser medium;
A second reflecting mirror disposed opposite to the first reflecting mirror via the solid-state laser medium, the aperture and the host crystal;
A conical lens that is provided between the first reflecting mirror and the excitation light source, converts the excitation light generated by the excitation light source into a Bessel beam and enters the solid laser medium;
The solid-state laser medium, the aperture, and the host crystal are integrally bonded, and the solid-state laser medium and the aperture are directly bonded, and the aperture and the host crystal are directly bonded. Pulse laser oscillator. The conical lens and the solid-state laser medium are arranged so that a condensing region for condensing by the Bessel beam is in the solid-state laser medium, and an opening is formed at a place where the intensity distribution of the Bessel beam is in a donut shape. The pulse laser oscillator according to claim 1, wherein the aperture having the following is arranged. The pulse laser oscillator according to claim 1, wherein the aperture is made of a metal material that reflects the excitation light. 4. The pulse laser oscillator according to claim 1, wherein the solid-state laser medium and the host crystal are made of a ceramic material. 5. 5. The pulse laser oscillator according to claim 1, wherein the solid laser medium, the aperture, and the host crystal are directly bonded by diffusion bonding or integral firing. 6.

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