CN111326945B

CN111326945B - Optical resonant cavity

Info

Publication number: CN111326945B
Application number: CN201911099407.3A
Authority: CN
Inventors: 田中研太; 河村让一; 万雅史; 冈田康弘
Original assignee: Sumitomo Heavy Industries Ltd
Current assignee: Sumitomo Heavy Industries Ltd
Priority date: 2018-12-17
Filing date: 2019-11-12
Publication date: 2023-05-05
Anticipated expiration: 2039-11-12
Also published as: KR20200074852A; JP2020098814A; JP7262217B2; TWI754854B; TW202025584A; CN111326945A

Abstract

The invention provides an optical resonator capable of suppressing unnecessary oscillation generated by the outward surface of a front mirror even if the outward surface of the front mirror is inclined and a roof mirror is used for a rear mirror. The optical resonator has a front mirror and a rear mirror, and transmits light back and forth through a discharge region exciting the laser gas. The outward facing surface of the front mirror is inclined with respect to an imaginary plane perpendicular to the optical axis of the optical resonator. The rear mirror has two reflection regions in a planar shape in a positional relationship intersecting each other. The intersection line of two virtual planes respectively including the two reflection regions of the rear mirror is not in an orthogonal relationship with the tilt direction of the outward facing surface of the front mirror.

Description

Optical resonant cavity

The present application claims priority based on japanese patent application No. 2018-235278 filed on date 17 of 2018, 12. The entire contents of this japanese application are incorporated by reference into the present specification.

Technical Field

The present invention relates to an optical resonator.

Background

There is known an optical resonator in which an outward-facing surface of a front mirror of the optical resonator is inclined with respect to an optical axis of the optical resonator, thereby suppressing unnecessary resonance caused by reflection on the outward-facing surface (for example, paragraph 0054 of patent document 1 below). In addition, an optical resonator using a roof mirror having two reflection surfaces orthogonal to each other as a back mirror is also known to improve mode stability in order to suppress the generation of a higher order transverse mode.

Patent document 1: international publication No. 2014/046161

When the rear mirror is a flat mirror, if the outward surface of the front mirror is inclined, light that has passed back and forth between the outward surface of the front mirror and the rear mirror is blocked by an aperture disposed in the optical resonator with a small number of passes. Therefore, the light enclosed between the outward-facing surface of the front mirror and the rear mirror does not grow to the laser light.

However, in the case where the roof mirror is used as the rear mirror, the number of trips between the outward facing surface of the front mirror and the rear mirror increases as compared with the case where the flat mirror is used. If the number of round trips increases, light traveling in a direction inclined with respect to the optical axis of the optical resonator grows into laser light in addition to the laser beam that should be oscillated. A laser beam traveling in a direction inclined with respect to the optical axis affects the intensity distribution (transverse mode) of a cross section of the laser beam that should be oscillated.

In order to separate a laser beam traveling in a direction inclined with respect to the optical axis of the optical resonator from a laser beam that should be oscillated outside the optical resonator, a distance of about several meters is required. Therefore, the optical system for separating the two becomes long, which causes the optical device to become expensive.

Disclosure of Invention

The present invention aims to provide an optical resonator capable of suppressing unnecessary oscillation generated by an outward facing surface of a front mirror even if the outward facing surface of the front mirror is inclined and a roof mirror is used for a rear mirror.

According to one aspect of the present invention, there is provided an optical resonator having a front mirror and a rear mirror, and allowing light to pass through a discharge region exciting a laser gas to and from the discharge region, wherein,

the outward facing surface of the front mirror is inclined with respect to an imaginary plane perpendicular to the optical axis of the optical cavity,

the rear mirror has two reflection areas in a planar shape in a positional relationship intersecting each other,

an intersection line of two virtual planes respectively including the two reflection regions of the rear mirror and an inclined direction of an outward-facing surface of the front mirror are not in an orthogonal relationship.

If the intersection line of two virtual planes including the two reflection regions of the rear mirror and the tilt direction of the outward-facing surface of the front mirror are not in an orthogonal relationship, the number of times that light reflected on the outward-facing surface of the front mirror can travel into and out of the optical resonator is reduced. As a result, unnecessary oscillation due to the outward facing surface of the front mirror can be suppressed.

Drawings

Fig. 1 is a cross-sectional view including an optical axis of a gas laser device equipped with an optical resonator according to an embodiment.

Fig. 2 is a cross-sectional view perpendicular to the optical axis of a gas laser device equipped with an optical resonator according to an embodiment.

Fig. 3 (a) is a perspective view of an optical resonator according to an embodiment, fig. 3 (b) is a cross-sectional view perpendicular to the y-axis, and fig. 3 (C) is a cross-sectional view perpendicular to the x-axis.

Fig. 4 (a) is a diagram showing a state in which light vertically reflected on the surface facing the outside of the front mirror of the optical resonator according to the embodiment propagates on the xz cross section, and fig. 4 (B) is a diagram showing a state in which light vertically reflected on the surface facing the outside of the front mirror of the optical resonator according to the comparative example propagates on the xz cross section.

FIG. 5 is a schematic diagram showing a front aperture, a rear aperture, and light traveling between the front aperture and the rear aperture obliquely with respect to the optical axis of the optical cavity.

Fig. 6 is a schematic diagram of a laser processing apparatus using a laser oscillator equipped with an optical resonator according to an embodiment.

In the figure: 10-chamber, 11-optical chamber, 12-blower chamber, 13-upper and lower partition, 13A, 13B-opening, 14-bottom plate, 15-partition, 16-chamber support member, 21-discharge electrode, 22, 23-discharge electrode support member, 24-discharge area, 25-optical resonator, 25F-front mirror, 25R-back mirror, 26-common support member, 27-optical resonator support member, 28-light transmission window, 29F-front aperture, 29R-back aperture, 40, 41, 43, 44, 46, 47-light propagating in the optical resonator, 50-blower, 51-1 st gas flow path, 52-2 nd gas flow path, 56-heat exchanger, 58-outflow hole, 59-filter, 70-laser oscillator, 71-beam shaping scanning optical system, 72-stage, 73-control device, 75-object, valley line of 251-back mirror, 255-front mirror facing inner side, face of 256-front mirror facing outer side.

Detailed Description

An optical resonator according to an embodiment and a gas laser device incorporating the same will be described with reference to fig. 1 to 3.

Fig. 1 is a cross-sectional view including an optical axis of a gas laser device equipped with an optical resonator according to an embodiment. Here, an xyz orthogonal coordinate system is defined in which the optical axis direction of the optical resonator is the z-axis direction and the vertical direction is the x-axis direction.

A laser gas is contained within the chamber 10. The internal space of the chamber 10 is divided into an optical chamber 11 located at the upper side in the vertical direction and a blower chamber 12 located at the lower side in the vertical direction. The optical chamber 11 is partitioned from the blower chamber 12 by an upper and lower partition 13. The upper and lower partitions 13 are provided with openings through which laser gas flows between the optical chamber 11 and the blower chamber 12. The bottom plate 14 of the optical chamber 11 protrudes from the side wall of the blower chamber 12 toward both sides in the z-axis direction, and the length of the optical chamber 11 in the z-axis direction is longer than the length of the blower chamber 12 in the z-axis direction. The chamber 10 is supported to the optical base at a floor 14 of the optical chamber 11 by a chamber support member 16.

A pair of discharge electrodes 21 are disposed in the optical chamber 11. The pair of discharge electrodes 21 are supported on the bottom plate 14 via discharge

electrode supporting members

22 and 23, respectively. The pair of discharge electrodes 21 are arranged with a gap therebetween in the x-axis direction, thereby defining a discharge region 24 therebetween. The discharge electrode 21 generates a discharge in the discharge region 24, thereby exciting the laser gas. As will be described later with reference to fig. 2, the laser gas flows through the discharge region 24 in a direction perpendicular to the paper surface of fig. 1.

The optical resonator 25 is supported by a common support member 26 disposed in the optical chamber 11. The optical resonator 25 is constituted by a front mirror 25F and a rear mirror 25R. The optical axis of the optical cavity 25 passes through the discharge region 24. The common support member 26 is supported on the base plate 14 via an optical resonator support member 27. A light-transmitting window 28 for transmitting a laser beam is provided at a portion where an extension line extending toward the front mirror 25F (left side in fig. 1) of the optical resonator 25 intersects with the wall surface of the optical chamber 11. The laser beam excited in the optical resonator 25 is emitted to the outside through the light-transmitting window 28.

A blower 50 is disposed in the blower chamber 12. The blower 50 circulates the laser gas between the optical chamber 11 and the blower chamber 12.

Fig. 2 is a cross-sectional view perpendicular to the z-axis of a gas laser device mounted with the optical resonator 25 (fig. 1) according to the present embodiment. The inner space of the chamber 10 is divided into an upper optical chamber 11 and a lower blower chamber 12 by an upper and lower partition 13. A pair of discharge electrodes 21 and a common support member 26 for supporting the optical resonator 25 (fig. 1) are disposed in the optical chamber 11. A discharge region 24 is defined between the discharge electrodes 21.

A partition 15 is disposed in the optical chamber 11. The separator 15 defines a 1 st gas flow path 51 extending from the opening 13A provided in the upper and lower separators 13 to the discharge region 24, and a 2 nd gas flow path 52 extending from the discharge region 24 to the other opening 13B provided in the upper and lower separators 13. The laser gas flows through the discharge region 24 in a direction (y-axis direction) orthogonal to the optical axis. The discharge direction (x-axis direction) is orthogonal to both the direction in which the laser gas flows (y-axis direction) and the optical axis direction (z-axis direction). The blower chamber 12, the 1 st gas passage 51, the discharge region 24, and the 2 nd gas passage 52 constitute a circulation passage through which the laser gas circulates. The blower 50 generates a laser gas flow so that the laser gas circulates in the circulation flow path.

The heat exchanger 56 is accommodated in the circulation flow path in the blower chamber 12. The laser gas heated in the discharge region 24 is cooled by the heat exchanger 56, and the cooled laser gas is supplied again to the discharge region 24.

The upper and lower partitions 13 are provided with outflow holes 58 for allowing the laser gas to flow from the blower chamber 12 to the optical chamber 11. Some of the laser gas flowing through the 1 st gas flow path 51 by the blower 50 flows through the outflow hole 58 to the optical chamber 11. The outflow hole 58 is provided with a filter 59 for removing particulates. For example, the filter 59 closes the outflow hole 58, and the laser gas flowing from the blower chamber 12 to the optical chamber 11 is filtered by the filter 59.

Fig. 3 (a) is a perspective view of the optical resonator 25 according to the present embodiment, fig. 3 (B) is a cross-sectional view perpendicular to the y-axis (vertical cross-sectional view), and fig. 3 (C) is a cross-sectional view perpendicular to the x-axis (horizontal cross-sectional view).

The rear mirror 25R is constituted by a roof mirror having two reflecting surfaces intersecting each other. The two reflecting surfaces form an angle which is approximately a right angle. The rear mirror 25R having two substantially orthogonal reflecting surfaces has a function of suppressing the fluctuation of the light beam in the lateral direction and improving the stability of the light beam intensity distribution. A front aperture 29F is arranged between the discharge region 24 and the front mirror 25F, and a rear aperture 29R is arranged between the discharge region 24 and the rear mirror 25R. In the perspective view of fig. 3 (a), the front diaphragm 29F and the rear diaphragm 29R are omitted. The front diaphragm 29F and the rear diaphragm 29R have a function of shielding unnecessary light propagating in a region separated from the optical axis of the optical resonator 25.

The rear mirror 25R is fixed in a posture in which the valleys 251 of the two reflecting surfaces are parallel to the x axis. The front mirror 25F has a surface 255 facing the inside of the optical resonator 25 and a surface 256 facing the outside. A partially reflective coating is applied to the inward facing surface 255 and a reflective coating is applied to the outward facing surface 256. The inward facing surface 255 is orthogonal to the optical axis (z axis) of the optical resonator 25, and the outward facing surface 256 is inclined with respect to an imaginary plane (a plane parallel to the xy plane) orthogonal to the optical axis. The inward surface 255 may be a concave surface whose focal point is on the optical axis. The angle at which the outward-facing surface 256 of the front mirror 25F is inclined with respect to a virtual plane (a plane parallel to the xy plane) orthogonal to the optical axis is sometimes simply referred to as "the inclination angle of the outward-facing surface".

The direction in which the outward-facing surface 256 is inclined with respect to a virtual plane (xy-plane) perpendicular to the optical axis is the positive or negative direction of the x-axis. Here, "oblique direction" means: the outward facing surface 256 includes a straight line having the largest inclination angle with respect to the xy plane, and is inclined downward. In other words, "oblique direction" means: the direction in which the distance between the straight line having the largest inclination angle and the inward-facing surface 255 becomes smaller. In other words, the inclined direction of the outward facing surface 256 of the front mirror 25F is parallel to the valley 251 of the rear mirror 25R. As the optical resonator 25, a folded optical resonator including a folded mirror or the like may be used. At this time, the valley line 251 is projected onto a virtual plane perpendicular to the optical axis at a position where the front mirror 25F is disposed via an optical component such as a folding mirror constituting the optical resonator 25, and the line image on the outward facing surface 256 is parallel to the oblique direction. The "parallel relationship" includes a relationship in which a line image in which the valley line 251 is projected is parallel to the oblique direction of the outward facing surface 256.

Next, the excellent effects of the present embodiment will be described with reference to fig. 4 (a) and (B).

Fig. 4 (a) is a diagram showing a state in which light vertically reflected on the outward facing surface 256 of the front mirror 25F of the optical resonator 25 according to the present embodiment propagates on the xz cross section. The intersection of the two reflecting surfaces of the rear mirror 25R and a plane parallel to the xz plane becomes a straight line parallel to the x axis. Thus, in the x z cross section, the rear mirror 25R can be regarded as a plane mirror perpendicular to the optical axis (z axis). In fig. 4 (a), the rear mirror 25R is represented as a plane mirror. The direction of the inclination of the outward facing surface 256 of the front mirror 25F is the negative x-axis direction.

The laser beam to be oscillated is enclosed between the inward facing surface 255 of the front mirror 25F and the rear mirror 25R. The propagation direction of the laser beam is parallel to the optical axis (z-axis) of the optical cavity 25. Although the reflective coating is applied to the outward facing surface 256 of the front mirror 25F, the reflectance is not completely zero, and the outward facing surface 256 of the front mirror 25F has a reflectance of 1% or less. Light naturally emitted in the discharge region 24 is reflected perpendicularly on the outward facing surface 256, and light 40 traveling obliquely in the x z plane with respect to the optical axis of the optical resonator 25 is generated. The x-component of the propagation direction of light 40 is positive.

The obliquely propagating light 40 is reflected obliquely by the rear mirror 25R and then enters the outward facing surface 256 of the front mirror 25F. The x component of the propagation direction of the light 41 obliquely reflected by the rear mirror 25R is positive as is the x component of the propagation direction of the incident light 40. Therefore, the position at which the reflected light 41 is re-incident is shifted on the positive side of the x-axis than the start point of the light 40. The re-incident light 41 is reflected toward an oblique direction having a larger oblique angle with respect to the optical axis of the optical resonator 25. In this way, light reflected perpendicularly on the outward facing surface 256 of the front mirror 25F becomes farther away from the optical axis of the optical cavity 25 as it propagates within the optical cavity 25. Therefore, the light reflected on the outward-facing surface 256 of the front mirror 25F is blocked by the front diaphragm 29F or the rear diaphragm 29R with a small number of round trips. Therefore, the light vertically reflected on the outward surface 256 of the front mirror 25F is less likely to grow to the laser light.

Fig. 4 (B) is a diagram showing a state in which light reflected on the surface 256 of the front mirror 25F facing outward of the optical resonator 25 according to the comparative example propagates in an xz cross section. In the comparative example, the valleys 251 of the two reflecting surfaces of the rear mirror 25R are arranged parallel to the y-axis. That is, the valley 251 of the rear mirror 25R is in an orthogonal relationship with the tilting direction of the outward facing surface of the front mirror 25F. Here, the "orthogonal relationship" includes not only a case where two straight lines intersect at right angles in three-dimensional space, but also a case where one straight line intersects at right angles with another straight line if the other straight line is moved in parallel along the optical axis of the optical resonator 25. When the optical axis of the optical resonator 25 is folded back, the straight line is moved in parallel with the optical axis so that the straight line moving along the optical axis before folding back and the straight line moving along the optical axis after folding back are in relation to the object and the image. The light 43 that is reflected perpendicularly on the outward-facing surface 256 of the front mirror 25F and propagates obliquely with respect to the optical axis of the optical resonator 25 propagates toward the front mirror 25F after being reflected twice on the two reflection surfaces of the rear mirror 25R.

The propagation direction of the light 43 incident toward the rear mirror 25R is in an antiparallel relationship with the propagation direction of the reflected light 44. Since the propagation direction of the light 43 is perpendicular to the outward-facing surface 256, the light 44 from the rear mirror 25R toward the front mirror 25F is perpendicularly incident on the outward-facing surface 256. A part of the light 44 perpendicularly incident on the outward-facing surface 256 is reflected by the outward-facing surface 256, and the reflected light propagates in a direction opposite to the path of the light 43, 44 and is re-incident on the outward-facing surface 256. As a result, light directed in an oblique direction with respect to the optical axis may be confined in the optical resonator 25 and may grow into laser light. The laser beam traveling obliquely with respect to the optical axis of the optical cavity 25 affects the intensity distribution in the transverse direction of the laser beam that should be oscillated, and thus the stability of the intensity distribution of the beam in the cross section decreases.

In the present embodiment, since the laser oscillation caused by the reflection on the outward-facing surface 256 of the front mirror 25F is suppressed, the decrease in the stability of the intensity distribution of the laser beam to be oscillated in the cross section can be suppressed.

Next, modified examples of the above embodiment will be described.

In the above-described embodiment, as shown in (a) to (C) of fig. 3, two sets of mirrors, that is, the front mirror 25F and the rear mirror 25R, are used, but a folded optical resonator may be configured by disposing a folded mirror or the like between the two mirrors.

In the above embodiment, the valley 251 of the rear mirror 25R is parallel to the inclined direction of the outward surface 256 of the front mirror 25F, but the two are not necessarily parallel. If the two are not in an orthogonal relationship, the number of times that light reflected on the outward facing surface 256 of the front mirror 25F can travel back and forth through the optical resonator 25 is reduced as compared to when the two are in an orthogonal relationship. As a result, the decrease in the stability of the intensity distribution of the laser beam can be suppressed as compared with the comparative example shown in fig. 4 (B).

In the above embodiment, the roof mirror is used as the back mirror 25R, but the back mirror 25R may be a mirror having two planar reflection regions in a positional relationship intersecting each other. At this time, the intersecting line direction of the two virtual planes respectively including the respective reflection regions corresponds to the direction of the valley line 251 of the roof mirror.

Next, a preferred relationship among the interval L between the front diaphragm 29F and the rear diaphragm 29R, the opening diameters D of the front diaphragm 29F and the rear diaphragm 29R, and the tilt angle θ of the outward facing surface 256 of the front mirror 25F will be described with reference to fig. 5.

Fig. 5 is a schematic diagram showing the front aperture 29F, the rear aperture 29R, and light traveling between the front aperture 29F and the rear aperture 29R obliquely with respect to the optical axis of the optical resonator 25. The outward facing surface 256 of the front mirror 25F is inclined by an inclination angle θ in the x-axis direction with respect to an imaginary plane perpendicular to the z-axis. Light 46 reflected perpendicularly at the outward facing surface 256 of the front mirror 25F propagates in a direction inclined by an inclination angle θ with respect to the z-axis. It is assumed that a planar rear mirror is disposed at the position of the rear diaphragm 29R. The light 47 reflected by the rear mirror passes through a position offset in the x-axis direction from the first passing position of the light 46 at the position of the front aperture 29F. The offset Δd is expressed by the following equation.

Δd＝2L×tanθ……(1)

When the aperture diameter D of the front diaphragm 29F is equal to or smaller than the offset Δd, the front diaphragm 29F shields the light vertically reflected on the outward surface 256 of the front mirror 25F while the light travels once in the optical resonator 25. If the light traveling back and forth in the optical resonator 25 is shielded before traveling back and forth twice, it is considered that the light does not grow to the laser light. In order to shield the light traveling in the optical resonator 25 before traveling twice, it is preferable that the following relationship be satisfied among the inclination angle θ, the aperture interval L, and the aperture diameter D of the aperture.

θ≥tan ^-1 (D/4L)……(2)

In practice, the rear mirror 25R is disposed further outside than the rear aperture 29R. Therefore, the offset Δd becomes larger than the value represented by the formula (1). As described with reference to fig. 4 a, the inclination angle of the propagation direction of the light 47 reflected on the outward surface 256 of the front mirror 25F with respect to the optical axis (z axis) is larger than the inclination angle θ. Therefore, the condition that light traveling back and forth in the optical resonator 25 is shielded before traveling back and forth twice is more relaxed than the above-mentioned condition (2). When the above conditional expression (2) is satisfied, in an actual gas laser device, an effect of suppressing unnecessary laser oscillation due to the outward facing surface 256 of the front mirror 25F can be obtained.

Next, a laser processing apparatus mounted with the optical resonator 25 according to the above embodiment will be described with reference to fig. 6.

Fig. 6 is a schematic view of a laser processing apparatus. The laser oscillator 70 outputs a pulse laser beam in accordance with an instruction from the control device 73. The pulse laser beam output from the laser oscillator 70 is incident on the object 75 through the beam shaping scanning optical system 71. The beam shaping scanning optical system 71 shapes the beam cross-sectional shape of the laser beam and scans the laser beam in two dimensions.

The object 75 is, for example, a printed board, and is held on the table 72. The table 72 can move the object 75 in two directions parallel to the surface to be processed according to a command from the control device 73. The laser processing apparatus is used for drilling the object 75 based on the pulse laser beam.

The laser oscillator 70 uses the optical resonator 25 based on the above-described embodiment. Therefore, the stability of the intensity distribution of the pulse laser beam output from the laser oscillator 70 can be improved. As a result, the roundness of the beam cross section of the pulse laser beam is improved, and the machining quality of the drilling machining can be improved.

The above-described embodiments are merely examples, and the present invention is not limited to the above-described embodiments. For example, various alterations, modifications, combinations, etc. of the invention will be apparent to those skilled in the art.

Claims

1. An optical resonator having a front mirror and a rear mirror and allowing light to travel back and forth through a discharge region exciting a laser gas, the optical resonator characterized in that,

the inwardly facing surface of the front mirror is orthogonal to the optical axis of the optical cavity,

the optical axis of the optical resonant cavity is a straight line, or the optical resonant cavity comprises a foldback mirror,

in the case where the optical axis of the optical resonator is a straight line, an intersection line of two virtual planes respectively including the two reflection regions of the rear mirror and an inclined direction of the outward-facing surface of the front mirror are not in an orthogonal relationship,

when the optical resonator includes a fold mirror, an intersection line of two virtual planes including the two reflection regions of the rear mirror is projected via the fold mirror onto a virtual plane perpendicular to an optical axis at a position where the front mirror is disposed, and a line image of an outward-facing surface of the front mirror is parallel to an oblique direction of the outward-facing surface.

2. The optical resonator according to claim 1, characterized in that,

an intersection line of two virtual planes respectively including the two reflection regions of the rear mirror is in parallel relation to an inclination direction of an outward-facing surface of the front mirror.

3. An optical resonator according to claim 1 or 2, characterized in that,

on both sides of the discharge region in the optical axis direction of the optical resonator, diaphragms having openings are also respectively arranged,

when the distance between a pair of diaphragms is denoted by L and the aperture diameter of the diaphragms is denoted by D, the outward facing surface of the front mirror is inclined at an angle tan with respect to an imaginary plane orthogonal to the optical axis of the optical resonator ^-1 (D/4L) or more.