Patent Application
20190097382
The present invention relates to a solid-state laser device including a laser rod, which is made of a solid-state laser crystal, as a laser medium.
For example, a solid-state laser device is composed of a resonator; a rod-shaped solid-state laser medium (laser rod) disposed in the resonator; an excitation light source, such as a flash lamp, for exciting the laser rod; and optical members, such as a Q switch. In many cases, in order to efficiently irradiate the laser rod with excitation light emitted from the excitation light source, the laser rod and the excitation light source are contained in a laser chamber in such a way that at least parts thereof are enclosed in the laser chamber.
JP1999-87806A (JP-H11-87806A) describes that a solid-state laser device has the following problem: return light from an optical component or the like to a laser rod is incident on an O-ring, which is disposed at an exposed root of an end portion of the laser rod exposed from the laser chamber that contains the laser rod; the return light burns the O-ring; and cinders of the O-ring adhere to an end surface of the laser rod and damages an antireflection film on the end surface of the laser rod. In order to protect the end surface of the laser rod from the damage due to the return light of laser light, JP1999-87806A (JP-H11-87806A) proposes a structure in which a holder block, which includes an opening smaller than the diameter of the laser rod, is disposed at a position facing the end surface of the laser rod so that the return light is not incident on the O-ring. JP1999-87806A (JP-H11-87806A) describes that, although the size of the opening of the holder block is smaller than the diameter of the laser rod, this is not a problem because a region in which reflection occurs is limited to a region where laser output power is extremely low, because laser light gradually converges due to a thermal lens effect.
JP2007-96063A discloses a solid-state laser device that includes a mode limiting aperture in order to ensure the beam quality of oscillation laser light. In general, a mode limiting aperture is used to selectively oscillate the lowest-order eigenmode.
In general, optical surfaces of many optical elements are covered with optical films in accordance with the functions of the optical elements. As described in JP11-87806A, a laser rod includes, as an optical film, an antireflection film on an end surface thereof.
An optical film, which is usually disposed on a smooth surface, is not likely to satisfactorily adhere to a rough surface (ground-glass-like) portion. An edge between adjacent surfaces of an optical member is typically chamfered in order to prevent chipping, and the chamfered portion generally has a rough surface. In a general optical member, the chamfered portion is not used as a light beam transmitting region (light path). However, regarding a laser rod, the rod itself is usually the light path, and the chamfered portion is also in the laser light path and is a region through which laser light passes.
Regarding solid-state laser devices, demands for reduction in pulse width and reduction in size are increasing. Use of a small-diameter laser rod is considered as means for realizing shorter pulse width and smaller size. When the same excitation light is absorbed, energy density increases and a shorter-pulse-width laser is obtained as the diameter of the laser rod becomes smaller.
The inventors found that, when a laser rod includes a chamfered portion around an end surface thereof, there is a case where breakage of an optical film on the end surface occurs from the origin on the outer periphery of the end surface, which is the boundary between the optical film and the chamfered portion. To be specific, the inventors found that breakage of the optical film occurs when, as described above, the diameter of the laser rod is small and the energy at the cross section of the light path is very high.
On the background described above, it is an object of the present invention to provide a solid-state laser device that suppresses occurrence of breakage of an optical film on an end surface of a laser rod and that can be stably driven for a long period.
A solid-state laser device according to the present invention includes a resonator composed of a pair of mirrors; a laser rod disposed in the resonator, the laser rod including an antireflection film on an end surface thereof and having a chamfered portion at a peripheral edge of the end surface; and an end-surface protection member that is disposed at a position facing the end surface of the laser rod, that has an opening defining portion that forms an opening whose diameter is smaller than a diameter of an outer periphery of the end surface, and that limits a laser light path region in the end surface of the laser rod to a region inside of the outer periphery of the end surface.
In the solid-state laser device according to the present invention, preferably, a distance between the opening defining portion of the end-surface protection member and the end surface of the laser rod is 0.5 mm or smaller.
In the solid-state laser device according to the present invention, preferably, the opening defining portion of the end-surface protection member and the end surface of the laser rod are in contact with each other.
In the solid-state laser device according to the present invention, preferably, the opening defining portion of the end-surface protection member includes a tapered portion whose opening diameter decreases toward the laser rod.
In the solid-state laser device according to the present invention, preferably, the end-surface protection member has a tubular portion that supports the opening defining portion, and the tubular portion is fitted onto and attached to an end portion of the laser rod including the end surface.
Preferably, the solid-state laser device according to the present invention includes a laser chamber that contains at least a part of the laser rod and that has a hole having a columnar shape that is shorter than a longitudinal axial length of the laser rod, the laser rod is inserted through the hole of the laser chamber and is supported by the laser chamber in a state in which both end portions of the laser rod are exposed from the hole, and the tubular portion of the end-surface protection member has a shape that covers an entire region of a side surface of the end portion exposed from the hole.
In the solid-state laser device according to the present invention, preferably, a laser chamber that contains at least a part of the laser rod and that has a hole having a columnar shape that is shorter than a longitudinal axial length of the laser rod; the laser rod is inserted through the hole of the laser chamber and is supported by the laser chamber in a state in which both end portions of the laser rod are exposed from the hole; an O-ring is disposed at an exposed root of an end portion of the laser rod including the end surface where the end-surface protection member is disposed, the exposed root being exposed from the hole of the laser chamber; and the solid-state laser device includes a cover member that is disposed on a side surface of the laser rod between the O-ring and the end surface and that blocks incidence of stray light on the O-ring.
In the solid-state laser device according to the present invention, preferably, the end-surface protection member is made of at least one of a ceramic, glass, or a fluororesin.
In the solid-state laser device according to the present invention, preferably, the laser rod is made of an alexandrite crystal.
In the solid-state laser device according to the present invention, preferably, a rod diameter of the laser rod is 3 mm or smaller.
The solid-state laser device according to the present invention includes a resonator composed of a pair of mirrors; a laser rod disposed in the resonator, the laser rod including an antireflection film on an end surface thereof and having a chamfered portion at a peripheral edge of the end surface; and an end-surface protection member that is disposed at a position facing the end surface of the laser rod, that has an opening defining portion that forms an opening whose diameter is smaller than a diameter of an outer periphery of the end surface, and that limits a laser light path region in the end surface of the laser rod to a region inside of the outer periphery of the end surface. Therefore, occurrence of breakage of the antireflection film on the end surface can be effectively suppressed. Moreover, by suppressing breakage of the antireflection film on the laser rod end surface, the solid-state laser device can be driven stably for a long period.
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
A solid-state laser device 1 includes a pair of mirrors 11 and 12 that constitute a resonator, a laser rod 13 that is disposed in the resonator, and a laser chamber 30 that contains at least a part of the laser rod 13. The solid-state laser device 1 further includes optical members between the mirror 12 and the laser rod 13. The optical members are an aperture member 15, a polarizer 16, a shutter 17, a Q switch 18, and a wedge prism pair 19. The mirrors 11 and 12, the laser rod 13, and the optical members 15 to 19 are disposed in a housing 50. A part of the laser chamber 30 is exposed from the housing 50 to the outside, and a flash lamp 20 is contained in the part of the laser chamber 30 exposed from the housing 50. The housing 50 is composed of a base 51, a side wall portion 53, and a lid portion 55, and includes an emission opening 56, for outputting laser light, in a part of the side wall portion 53.
The pair of mirrors 11 and 12 are disposed so as to face each other with the laser rod 13 therebetween and constitute a linear resonator. The mirror 11 is a partially transmissive mirror and functions as a so-called output mirror that outputs laser light. The mirror 12 is a high reflection mirror and functions as a so-called rear mirror. In the present embodiment, the mirror 11 is a planar mirror, and the mirror 12 is a concave mirror. Hereinafter, the mirrors 11 and 12 may be respectively referred to as an output mirror 11 and a rear mirror 12. In the present embodiment, the output mirror 11 and the rear mirror 12 face each other and are attached to side surfaces, in the transversal direction, of the side wall portion 53 of the housing 50.
In the solid-state laser device 1 according to the present embodiment, a linear optical resonator is constituted by the mirrors 11 and 12. However, instead of a linear resonator, a solid-state laser device according to the present invention may include, for example, an L-shaped, Z-shaped, or an X-shaped resonator structure that includes prisms and mirrors, for changing the direction of light, in the light path. On the other hand, in view of reduction in size and cost, the number of optical members is preferably smaller, and a linear resonator is most preferable.
The laser rod 13, which is a solid-state laser medium, is made by forming a solid-state laser crystal, such as alexandrite (Cr:BeAl2O3), neodymium YAG (Nd:YAG (yttrium aluminum garnet)), titanium-sapphire (Ti:Al2O3), into a rod shape. The term “rod shape” refers to an elongated cylindrical shape such that the distance between two discs, which are end surfaces, is larger than the diameter of the discs. A solid-state laser medium is not limited to the one described above, and another known laser medium may be used, as appropriate. Preferably, the laser rod 13 has a small diameter in order to reduce the size of the entire device. In the present embodiment, particularly preferably, the laser rod 13 is made of alexandrite; and, preferably, the diameter of a cross section (circular cross section) perpendicular to the longitudinal direction of the laser rod 13 (hereinafter, referred to as “the rod diameter”) is 4 mm or smaller. More preferably, the rod diameter is 3 mm or smaller, and further preferably, 2.5 mm or smaller. The length of the laser rod is preferably 75 mm or smaller, and more preferably, 60 mm or smaller.
The flash lamp 20 is an excitation light source that emits excitation light for exciting the laser rod 13. The entirety of the flash lamp 20, including terminals 21 at both ends thereof, is substantially bar-shaped. The length of the flash lamp 20 is, for example, about 10 cm. The length of the flash lamp 20 is defined as the length of the flash lamp 20 including the terminals 21 in the longitudinal direction. Lead wires (not shown) are connected to the two terminals 21, and the flash lamp 20 is connected to a lighting power source via the lead wires. To be more specific, for example, a xenon flash lamp or the like can be used as the flash lamp 20. An excitation light source of a solid-state laser device according to the present invention is not limited to the flash lamp 20. For example, a light source in which a plurality of LEDs (light-emitting diodes) are arranged in a transparent linear tube and that has a bar-like shape as a whole may be used.
The laser chamber 30 is made of, for example, a metal, and is configured to contain the laser rod 13 and the flash lamp 20. The laser chamber 30 has an inner space for containing the laser rod 13 and the flash lamp 20, and transmits light that is emitted from the flash lamp 20 in the inner space to the laser rod 13. For example, a reflection surface is formed inside the laser chamber 30, and the laser rod 13 is directly irradiated with light emitted from the flash lamp 20 or is irradiated with light emitted from the flash lamp 20 and reflected by the reflection surface.
Pipes 42 and 44 are connected to a side wall of the laser chamber 30. As schematically illustrated in
The hole 34 of the laser chamber 30 has a cylindrical shape that is shorter than the longitudinal axial length of the laser rod 13. The laser rod 13 is inserted into the hole 34 and supported in a state in which both end portions thereof are exposed. The flash lamp 20 is supported by being inserted through the hole 33 (see
As illustrated in
As illustrated in
The width δ of the chamfered portion 13d, which corresponds to the difference between the radius of the laser rod 13 and the radius of the outer periphery of the end surface, is about 1 to 5% of the rod diameter ϕ0, and preferably, about 2% of the rod diameter ϕ0. For example, when the rod diameter ϕ0 is 2.5 mm, the width δ of the chamfered portion 13d is 0.05 mm.
As illustrated in
Because the solid-state laser device 1 includes the end-surface protection member 60 as described above, the laser light path region is limited to the region inside of the outer periphery of the rod end surface 13b, that is, a region inside of the inner periphery of the chamfered portion 13d. Accordingly, during laser oscillation, the boundary between the rod end surface 13b and the chamfered portion 13d is not irradiated with laser light. As described above, the boundary between the rod end surface 13b and the chamfered portion 13d is a region in which coating of an optical film is not satisfactory and that tends to become the origin of coating breakage. However, as long as the boundary is not irradiated with high-energy laser light, occurrence of coating breakage can be suppressed. That is, because the solid-state laser device 1 includes the end-surface protection member 60, occurrence of coating breakage at the rod end surface 13b can be suppressed.
It is important that the opening defining portion 62 be disposed at a position that is extremely close to the rod end surface 13b so as to cover the boundary between the rod end surface 13b and the chamfered portion 13d. The term “extremely close to” means that the distance d between the rod end surface 13b and the opening defining portion 62 (see
As illustrated in
The distance between the opening defining portion 62 and the rod end surface 13b is defined as the distance between a part of the opening defining portion 62 that has the diameter ϕ2 and that is most adjacent to the rod end surface 13b and the rod end surface 13b. For example, as shown in
The end-surface protection member 60 may be disposed at each of two end surfaces of the laser rod 13, or may be disposed at one of the two end surfaces. Even when the end-surface protection member 60 is disposed at only one of the end surfaces, the end-surface protection member 60 provides an advantageous effect of limiting the laser oscillation region. When a small-diameter laser rod is used to meet the demands for reduction in size of the device and reduction in pulse width, opening limitation greatly affects laser output power. That is, when disposing the end-surface protection members at both end portions of a small-diameter laser rod, the laser output power becomes highly sensitive to the production accuracy and the positional accuracy of the end-surface protection members, and may lead to decrease in stability and increase in manufacturing cost. Accordingly, preferably, the protection member is disposed at only one end surface.
Because the opening defining portion 62 of the end-surface protection member 60 contacts the laser light path, it is necessary that the opening defining portion 62 be made of a material that is not damaged or deformed by laser light and that generates only small amounts of dust and outgas. Accordingly, a ceramic or a fluororesin is suitable as the material of the opening defining portion 62. Preferably, the entirety of the end-surface protection member 60, including the opening defining portion 62, is made of at least one of a ceramic or a fluororesin.
An end-surface protection member 60 illustrated in
Next, other elements of the solid-state laser device will be described concisely.
As described above, the solid-state laser device 1 includes optical members on the rear mirror 12 side of the laser rod 13. The optical members are the aperture member 15, the polarizer 16, the shutter 17, the Q switch 18, and the wedge prism pair 19. The aperture member 15 is disposed most adjacent to the laser rod 13. Due to such disposition, the aperture member 15 can suppress propagation of stray light, which is generated by the polarizer 16, the shutter 17, the Q switch 18, the wedge prism pair 19, the rear mirror 12, or the like, toward the laser rod 13. The aperture member 15, which has an opening in the light path, can block stray light that propagates from the optical members 16 to 19 toward the laser rod 13 with a comparatively large angle and that considerably deviates from the light path, and can prevent the stray light from being incident on the laser chamber 30.
It is necessary that the aperture member 15 generate only small amounts of dust and outgas, absorb only a small amount of laser light, and have heat resistance. Preferably, the aperture member 15 is made of a material that can diffuse laser light. Accordingly, a ceramic, ground glass, or a fluororesin such as polytetrafluoroethylene (polytetrafluoroethylene; PTFE) is suitable as the material of the aperture member 15.
In order to prevent stray light from being incident on the laser chamber 30, preferably, the aperture member 15 is disposed between the laser chamber 30 and the other optical members 16 to 19 as in the present embodiment. The opening diameter of the aperture member 15 is preferably larger than or equal to the rod diameter ϕ0, and more preferably, larger than the rod diameter ϕ0. In particular, when a small-diameter laser rod having a rod diameter ϕ0 of 4 mm or smaller is used as the laser rod 13 for the purpose of reduction in size of the device and reduction in pulse width of laser light, opening limitation by the aperture member 15 considerably affects the laser output power. That is, because the laser output power is highly sensitive to the positional accuracy of the aperture member 15 for a small-diameter laser rod, if the positional accuracy of the aperture member 15 is low, decrease in stability occurs. On the other hand, increasing the positional accuracy leads to increase in manufacturing cost. Accordingly, when using a small-diameter laser rod, preferably, the opening diameter of the aperture member is larger than the rod diameter. However, because the effect of blocking stray light may not be sufficiently obtained if the opening diameter of the aperture member is too large, preferably, the opening diameter is 120% of the rod diameter or smaller. Preferably, the opening shape of the aperture member 15 is similar to the end surface shape of the laser rod 13.
In the present embodiment, the aperture member 15 is disposed only on the rear mirror 12 side of the laser rod 13. Preferably, the aperture member 15 is disposed on each of two sides on which the end surfaces of the laser rod 13 are located, in view of protection by blocking of stray light. However, when disposing the aperture member 15 on each of two sides on which the end surfaces of the laser rod 13 are located, requirement for positional accuracy becomes stricter, and leads to increase in manufacturing cost. This predominantly occurs particularly when the rod has a small diameter. In the present embodiment, by disposing the optical members 16 to 19 only on the rear mirror 12 side of the laser rod 13, main points where stray light is generated are shifted toward one side. Therefore, a sufficiently high protection effect can be obtained even when the aperture member 15 is disposed only on one side.
The polarizer 16 selectively outputs a component of oscillated laser light that is linearly polarized in a predetermined direction. The shutter 17, which controls emission of laser light, mechanically blocks emission of laser light by being controlled to be opened or closed. The Q switch 18 performs a so-called Q switch operation so as to generate high-power pulsed laser light. However, a solid-state laser device according to the present invention is not limited to a device that generates pulsed laser light in this way, and may be configured to perform a CW (continuous wave) operation. The wedge prism pair 19 is used to perform optical-system adjustments, such as correction of the optical axis, by adjusting the positions and the angles of the prism pair. With the wedge prism pair 19, it is possible to perform highly-accurate optical axis adjustment.
The optical members 15 to 19 are respectively attached to holders 25 to 29, and the holders 25 to 29 are disposed on the base 51 of the housing 50. Note that the optical members 15 to 19 may be provided as necessary. For example, a solid-state laser device according to the present invention may include, among the optical members, only the Q switch. As necessary, the solid-state laser device may include another optical member.
With the solid-state laser device 1, when the Q switch 18 is switched to a light blocking state and the flash lamp 20 is turned on, the laser rod 13 is excited by excitation light emitted from the flash lamp 20, and a strong population inversion state is formed. Then, when the Q switch 18 is switched to a light transmitting state, light that is emitted from the laser rod 13 by stimulated emission resonates between the mirrors 11 and 12, the light becomes high-power giant pulse and passes through the output mirror 11, and the light is emitted to the outside of the resonator. The flash lamp 20 and the laser rod 13, which generate heat, are cooled by a coolant that flows through the laser chamber 30.
The solid-state laser device 1, which includes the end-surface protection member 60 at the end surface 13b of the laser rod 13, can suppress coating breakage of the antireflection film 14 on the rod end surface 13b and can obtain laser output power that is stable for a long period.
In the present design modification, the solid-state laser device 1 includes a cover member 38 that suppresses incidence of stray light on the O-ring 36 at the exposed root of the laser rod 13 exposed from the laser chamber 30.
As illustrated in
With the solid-state laser device according to the present embodiment, which includes the cover member 38, incidence of stray light on the O-ring 36 can be prevented, as in the case where the solid-state laser device includes the end-surface protection member 60A shown in
The cover member 38 is preferably disposed at each of two end portions of the laser rod, but may be disposed at only one end portion. When disposing the cover member 38 only at one end portion of the laser rod, preferably, the cover member 38 is disposed on a side on which a larger number of optical members such as the Q switch and the polarizer, which may cause generation of stray light, are disposed.
It is necessary that the cover member 38 generate only small amounts of dust and outgas, absorb only a small amount of laser light, and have heat resistance. Preferably, the cover member 38 can diffuse laser light. Accordingly, the cover member 38 is preferably made of at least one of a ceramic, ground glass, or a fluororesin such as polytetrafluoroethylene (polytetrafluoroethylene; PTFE). In order to prevent incidence of stray light on the O-ring 36, a soft material that can closely contact the laser rod 13 is preferable. Accordingly, a fibrous ceramic or glass, an unbaked fluororesin, or the like is particularly suitable.
In
As the O-ring 36, a general O-ring made of a rubber, which is not a fluorocarbon rubber, may be used. However, preferably, the O-ring 36 is made of a material that generates only small amounts of dust and outgas, such as a fluorocarbon rubber.
As illustrated in
As illustrated in
In each of the embodiments, a solid-state laser device in which a resonator, a laser rod, and optical members are disposed in a housing has been described. However, a solid-state laser device according to the present invention is not limited to a device in which these elements are disposed in a housing, and need not include a housing.
A solid-state laser device according to the present invention can be used for various purposes, which are not particularly limited. For example, a solid-state laser device according to the present invention can be used as a measurement light source that generates laser light (in particular, pulsed laser light), with which a subject is irradiated for detection of photoacoustic waves, in a photoacoustic measurement device described in JP2012-196430A, JP2014-207971A, or the like.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2016-105871 | May 2016 | JP | national |
This application is a continuation application of International Application No. PCT/JP2017/019805, filed May 26, 2017, the disclosure of which is incorporated herein by reference in its entirety. Further, this application claims priority from Japanese Patent Application No. 2016-105871, filed May 27, 2016, the disclosure of which is incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP2017/019805 | May 2017 | US |
| Child | 16200117 | US |
