CN111326945A

CN111326945A - Optical resonant cavity

Info

Publication number: CN111326945A
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: 2020-06-23
Anticipated expiration: 2039-11-12
Also published as: KR20200074852A; JP2020098814A; JP7262217B2; CN111326945B; TWI754854B; TW202025584A

Abstract

The invention provides an optical resonator capable of suppressing unnecessary oscillation generated by the outward surface of a front reflector even if the outward surface of the front reflector is inclined and a roof reflector is used for a rear reflector. The optical resonator has a front mirror and a back mirror and shuttles light through a discharge region that excites the lasing gas. The outwardly 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 reflecting regions in a planar shape in a positional relationship of crossing each other. The intersection line of two imaginary planes each including two reflection regions of the rear mirror and the inclination direction of the outward surface of the front mirror are not in a perpendicular relationship.

Description

Optical resonant cavity

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

Technical Field

The invention relates to an optical resonant cavity.

Background

There is known an optical resonator that suppresses unnecessary resonance due to reflection on an outward-facing surface by inclining an outward-facing surface of a front mirror of the optical resonator with respect to an optical axis of the optical resonator (for example, paragraph 0054 of patent document 1 listed below). Further, an optical resonator using a roof mirror having two reflecting surfaces orthogonal to each other as a back mirror in order to suppress generation of a higher-order transverse mode and improve mode stability is also known.

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 reciprocated between the outward surface of the front mirror and the rear mirror is blocked by the diaphragm disposed in the optical cavity with a small number of times of reciprocation. Therefore, the light confined between the outward 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 round trips between the outward surface of the front mirror and the rear mirror increases as compared to when a flat mirror is used. When the number of round trips increases, light propagating in a direction inclined with respect to the optical axis of the optical resonator in addition to the laser beam that should originally oscillate grows into laser light. A laser beam propagating in a direction inclined with respect to the optical axis affects the intensity distribution (transverse mode) on the cross section of the laser beam that should originally oscillate.

In order to separate a laser beam that propagates in a direction inclined with respect to the optical axis of the optical resonator from a laser beam that should originally oscillate 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 apparatus to become expensive.

Disclosure of Invention

An object of the present invention is to provide an optical resonator capable of suppressing unnecessary oscillation generated by an 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.

According to one aspect of the present invention, there is provided an optical resonator having a front mirror and a back mirror and reciprocating light through a discharge region that excites a lasing gas, wherein,

the outwardly facing face 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 reflecting areas in a planar shape in a positional relationship of crossing each other,

an intersection of two imaginary planes each including the two reflection regions of the rear mirror and an inclination direction of an outward surface of the front mirror are not in a perpendicular relationship.

If the inclination direction of the outward surface of the front mirror is not orthogonal to the intersection line of two virtual planes each including two reflection regions of the rear mirror, the number of times light reflected on the outward surface of the front mirror can travel back and forth in the optical cavity is reduced. As a result, unnecessary oscillation caused by the outward 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 mounted 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 mounted with an optical resonator according to the 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 outward facing surface of the front mirror of the optical resonator according to the embodiment propagates in the xz cross section, and fig. 4 (B) is a diagram showing a state in which light vertically reflected on the outward facing surface of the front mirror of the optical resonator according to the comparative example propagates in the xz cross section.

FIG. 5 is a schematic diagram showing the front aperture, the rear aperture, and light propagating between the front and rear apertures 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 plates, 13A, 13B-opening, 14-bottom plate, 15-partition plate, 16-chamber support member, 21-discharge electrode, 22, 23-discharge electrode support member, 24-discharge region, 25-optical resonator, 25F-front mirror, 25R-rear mirror, 26-common support member, 27-optical resonator support member, 28-light transmission window, 29F-front aperture, 29R-rear aperture, 40, 41, 43, 44, 46, 47-light propagating in 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 to be processed, 251-valley line of rear mirror, 255-inward surface of front mirror, 256-outward surface of front mirror.

Detailed Description

An optical resonator and a gas laser device equipped with the optical resonator according to an embodiment 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 mounted with an optical resonator according to an embodiment. Here, an xyz rectangular coordinate system is defined in which the optical axis direction of the optical resonator is the z-axis direction and the vertical direction upper side is the x-axis direction.

The laser gas is contained within the chamber 10. The internal space of the chamber 10 is divided into an optical chamber 11 located vertically above and a blower chamber 12 located vertically below. The optical chamber 11 and the blower chamber 12 are partitioned by an upper partition plate 13 and a lower partition plate 13. The upper and lower partition plates 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 in the z-axis direction of the optical chamber 11 is longer than the length in the z-axis direction of the blower chamber 12. The chamber 10 is supported on the optical base by a chamber support member 16 at the bottom plate 14 of the optical chamber 11.

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

electrode supporting members

22 and 23, respectively. The pair of discharge electrodes 21 are disposed with a gap therebetween in the x-axis direction, and define 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.

An optical resonator 25 is supported on 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 resonant cavity 25 passes through the discharge region 24. The common support member 26 is supported by the base plate 14 via an optical resonator support member 27. A light transmission window 28 through which a laser beam passes is attached to a portion where an extension line extending from the optical axis of the optical resonator 25 toward the front mirror 25F (left side in fig. 1) intersects with the wall surface of the optical chamber 11. The laser beam excited in the optical resonator 25 is radiated toward the outside through the light transmitting window 28.

The blower chamber 12 is provided with a blower 50. 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 an optical resonator 25 (fig. 1) according to the present embodiment. The internal space of the chamber 10 is partitioned into an upper optical chamber 11 and a lower blower chamber 12 by an upper partition plate 13 and a lower partition plate 13. A pair of discharge electrodes 21 and a common support member 26 for supporting an 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 separators 15 define a 1 st gas flow path 51 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 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 orthogonal to the optical axis (y-axis direction). 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 to circulate the laser gas in the circulation flow path.

A 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 to the discharge region 24 again.

The upper and lower partition plates 13 are provided with outflow holes 58 through which the laser gas flows from the blower chamber 12 to the optical chamber 11. A part of the laser gas flowing through the blower 50 to the 1 st gas passage 51 flows through the outflow hole 58 to the optical chamber 11. A filter 59 for removing particulates is provided at the outlet port 58. 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 angle formed by the two reflecting surfaces is substantially a right angle. The rear mirror 25R having two substantially orthogonal reflection surfaces has a function of suppressing variation of the light beam in the lateral direction and improving stability of the light beam intensity distribution. A front diaphragm 29F is disposed between the discharge area 24 and the front mirror 25F, and a rear diaphragm 29R is disposed between the discharge area 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 cavity 25.

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

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

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

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 in the xz cross section. The intersection line of the two reflecting surfaces of the rear mirror 25R and the plane parallel to the xz plane is a straight line parallel to the x axis. Therefore, in section x z, the rear mirror 25R can be regarded as a flat mirror perpendicular to the optical axis (z-axis). In fig. 4 (a), the rear mirror 25R is represented as a flat mirror. The direction of inclination of the outward surface 256 of the front mirror 25F is the negative direction of the x-axis.

The laser beam to be oscillated is confined between the inward 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 resonator 25. Although the front mirror 25F has a reflective coating on the outward surface 256, the reflectance thereof is not completely zero, and the outward 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 at the outwardly facing surface 256, producing light 40 traveling obliquely in the x z plane with respect to the optical axis of the optical cavity 25. The x-component of the propagation direction of light 40 is positive.

The obliquely propagating light 40 is obliquely reflected by the rear mirror 25R and then enters the outward surface 256 of the front mirror 25F. The x component in the propagation direction of the light 41 obliquely reflected by the rear mirror 25R is positive as in the propagation direction of the incident light 40. Therefore, the position where the reflected light 41 is incident again is shifted to the positive side of the x-axis from the starting point of the light 40. The re-incident light 41 is reflected toward an inclined direction having a larger inclination angle with respect to the optical axis of the optical cavity 25. In this manner, light reflected perpendicularly on the outward-facing side 256 of the front mirror 25F becomes farther and farther from the optical axis of the optical cavity 25 as it propagates within the optical cavity 25. Therefore, the light reflected by the outward 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 reflected perpendicularly on the outward surface 256 of the front mirror 25F is less likely to grow into the laser light.

Fig. 4 (B) is a diagram showing a state in which light reflected on the outward surface 256 of the front mirror 25F of the optical resonator 25 according to the comparative example propagates in the xz cross section. In the comparative example, the valley lines 251 of the two reflecting surfaces of the rear mirror 25R are arranged parallel to the y-axis. That is, the valley line 251 of the rear mirror 25R is orthogonal to the inclination direction of the outward 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 relationship where one of the straight lines intersects at right angles with the other straight line if the straight line is moved in parallel along the optical axis of the optical cavity 25. When the optical axis of the optical cavity 25 is folded back, the straight line is moved in parallel along 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 have a relationship between an object and an image. The light 43 reflected perpendicularly on the outward surface 256 of the front mirror 25F and propagating obliquely to the optical axis of the optical resonator 25 is reflected twice on the two reflecting surfaces of the rear mirror 25R, and then propagates toward the front mirror 25F.

The propagation direction of the light 43 incident toward the rear mirror 25R and the propagation direction of the reflected light 44 are in an antiparallel relationship. Since the propagation direction of the light 43 is perpendicular to the outward surface 256, the light 44 traveling from the rear mirror 25R toward the front mirror 25F is incident perpendicularly on the outward surface 256. A part of the light 44 perpendicularly incident on the outward surface 256 is reflected by the outward surface 256, and the reflected light propagates in a direction opposite to the path of the light 43 and 44 and is incident again on the outward surface 256. As a result, light directed obliquely to the optical axis may be confined in the optical cavity 25 and grow into laser light. The laser beam propagating 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 originally, and therefore the stability of the intensity distribution of the beam in the cross section is degraded.

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

Next, variations of the above embodiments will be described.

In the above-described embodiment, as shown in (a) to (C) of fig. 3, two sets of the front mirror 25F and the rear mirror 25R are used, but a folding mirror or the like may be disposed between the two to configure a folding optical resonator.

In the above embodiment, the valley line 251 of the rear mirror 25R and the inclination direction of the outward surface 256 of the front mirror 25F are in parallel with each other, but they do not necessarily have to be in parallel with each other. If the two are not in an orthogonal relationship, the number of times light reflected by the outward surface 256 of the front mirror 25F can make a round trip at the optical resonator 25 is reduced as compared with the case of the orthogonal relationship. As a result, a 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, a roof mirror is used as the rear mirror 25R, but a mirror having two planar reflection regions in a positional relationship of intersecting with each other may be used as the rear mirror 25R. At this time, the direction of the intersection line of the two virtual planes each including the respective reflection regions corresponds to the direction of the valley line 251 of the roof mirror.

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

Fig. 5 is a schematic diagram showing the front diaphragm 29F, the rear diaphragm 29R, and light propagating between the front diaphragm 29F and the rear diaphragm 29R obliquely with respect to the optical axis of the optical resonator 25. The outward surface 256 of the front mirror 25F is inclined in the x-axis direction by an inclination angle θ with respect to a virtual plane perpendicular to the z-axis. The light 46 reflected perpendicularly on the outward surface 256 of the front mirror 25F propagates in a direction inclined by the inclination angle θ with respect to the z-axis. 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 shifted in the x-axis direction from the first light 46 passing position at the position of the front diaphragm 29F. The offset Δ d 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 amount Δ D, the light reflected perpendicularly on the outward surface 256 of the front mirror 25F is blocked by the front diaphragm 29F while the light makes one round trip in the optical resonator 25. If light that makes a round trip within the optical cavity 25 is blocked before making a round trip twice, it is considered that the light does not grow to a laser. In order to shield light that has reciprocated inside the optical resonator 25 before twice reciprocating, the inclination angle θ, the interval L of the diaphragm, and the aperture diameter D of the diaphragm preferably satisfy the following relationship.

θ≥tan^-1(D/4L)……(2)

Actually, the rear mirror 25R is disposed outside the rear diaphragm 29R. Therefore, the offset amount Δ d becomes larger than the value expressed by equation (1). As described with reference to fig. 4 a, the inclination angle of the propagation direction of the light 47 reflected by 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 going back and forth within the optical cavity 25 is shielded before going back and forth twice is more relaxed than the above conditional expression (2). When the conditional expression (2) is satisfied, an effect of suppressing the generation of unnecessary laser oscillation due to the outward surface 256 of the front mirror 25F can be obtained in the actual gas laser device.

Next, a laser processing apparatus mounted with the optical cavity 25 according to the above-described 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 pulsed laser beam in accordance with an instruction from the control device 73. The pulsed 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 circuit 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 in response to a command from the control device 73. This laser processing apparatus is used for drilling of an object 75 to be processed by a 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 pulsed 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 processing quality of the drilling process can be improved.

The above embodiments are merely examples, and the present invention is not limited to the above embodiments. For example, it will be apparent to those skilled in the art that various modifications, improvements, combinations, and the like can be made to the present invention.

Claims

1. An optical resonator having a front mirror and a back mirror and reciprocating light through a discharge region that excites a lasing gas,

2. The resonant optical cavity of claim 1,

an intersection line of two imaginary planes respectively including the two reflection regions of the rear mirror and an inclination direction of an outward surface of the front mirror are in parallel relation.

3. The resonant optical cavity of claim 1 or 2,

apertures having openings are also provided on both sides of the discharge region in the optical axis direction of the optical resonator,

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