Laser medium unit

Abstract

This laser medium unit comprises: a laser medium having a pair of end surfaces and a side surface intersecting the pair of end surfaces; and a holding body that is disposed along the side surface when viewed from a direction intersecting the pair of end surfaces and holds the laser medium. The holding body is formed with a recess that opens to the center side of the laser medium when viewed from the direction intersecting the pair of end surfaces. The holding body holds the laser medium such that a space is formed between the side surface and the inner surface of the recess.

Description

Laser Medium Unit

[0001] The present disclosure relates to a laser medium unit.

[0002] Patent Document 1 describes a laser medium unit including a laser medium and a clad material as a frame that supports the laser medium by surrounding its side surfaces. In this laser medium unit, the entire side surface of the laser medium is fixed to the clad material with an adhesive, and spontaneous emission light generated in the laser medium is absorbed by the clad material. This suppresses parasitic oscillation in the laser medium unit.

[0003] JP 2016-100359 A

[0004] However, in the laser medium unit described above, heat is generated when the cladding material absorbs spontaneously emitted light. The generated heat is transferred from the cladding material to the laser medium, causing the temperature of the laser medium to rise. Although it is possible to cool the laser medium from the side, this can cause the temperature distribution in the pumping region of the laser medium to become non-uniform during cooling. This non-uniform temperature distribution in the pumping region may cause a deterioration in the quality of the laser light emitted from the laser medium unit.

[0005] An object of the present disclosure is to provide a laser medium unit that can improve the quality of laser light while suppressing parasitic oscillation.

[0006] A laser medium unit according to one aspect of the present disclosure is [1] "a laser medium unit comprising: a laser medium having a pair of end faces and side faces intersecting the pair of end faces; and a holder that is arranged along the side faces when viewed from a direction intersecting the pair of end faces and holds the laser medium, wherein a recess that opens toward the center of the laser medium when viewed from the direction intersecting the pair of end faces is formed in the holder, and the holder holds the laser medium such that a space is formed between the side faces and the inner surface of the recess."

[0007] In the laser medium unit, the holder holds the laser medium such that a space is formed between the side surface of the laser medium and the inner surface of the recess formed in the holder. Parasitic oscillation occurs when spontaneous emission light generated in the excitation region of the laser medium repeatedly passes through the excitation region. In the laser medium unit, the spontaneous emission light generated in the excitation region of the laser medium is emitted from the side surface of the laser medium into the space between the side surface and the inner surface of the recess. The emitted spontaneous emission light is scattered by repeated reflections on the inner surface of the recess and is unlikely to return to the excitation region of the laser medium. This suppresses parasitic oscillation. Furthermore, in the laser medium unit, even if heat is generated on the inner surface of the recess by absorbing the spontaneous emission light, the heat is unlikely to be transferred to the laser medium because a space is formed between the inner surface of the recess and the side surface of the laser medium. This suppresses uneven temperature distribution in the excitation region of the laser medium, thereby improving the quality of the laser light. Therefore, the laser medium unit can suppress parasitic oscillation and improve the quality of the laser light.

[0008] A laser medium unit according to one aspect of the present disclosure may be [2] "the laser medium unit according to the above [1], wherein the holder is formed in a frame shape surrounding the laser medium when viewed from the direction intersecting the pair of end faces." In this case, the laser medium can be held in a more stable state by the holder.

[0009] A laser medium unit according to one aspect of the present disclosure may be [3] "the laser medium unit according to the above [1] or [2], wherein the recess has a shape that tapers toward the bottom of the recess." In this case, the number of times that spontaneously emitted light emitted from the side surface of the laser medium is reflected by the inner surface of the recess increases, and the spontaneously emitted light can be efficiently scattered in the recess. This can further suppress parasitic oscillation.

[0010] The laser medium unit according to one aspect of the present disclosure may be [4] "the laser medium unit according to any one of the above [1] to [3], wherein the holder is in contact with the side surface." In this case, the laser medium can be held in a more stable state by the holder while suppressing parasitic oscillation.

[0011] A laser medium unit according to one aspect of the present disclosure may be [5] "the laser medium unit according to any one of the above [1] to [4], wherein the contact area of ​​the side surface with the holder accounts for 90% or less of the entire side surface." In this case, spontaneous emission can be more reliably emitted from the side surface of the laser medium into the space between the side surface and the inner surface of the recess, and parasitic oscillation can be further suppressed.

[0012] A laser medium unit according to one aspect of the present disclosure may be [6] "the laser medium unit according to any one of [1] to [5] above, wherein the contact area of ​​the side surface with the holder accounts for 5% or more of the entire side surface." In this case, it is possible to prevent stress applied from the holder to the side surface of the laser medium from concentrating locally. This allows the laser medium to be held by the holder in an even more stable state while suppressing parasitic oscillation.

[0013] A laser medium unit according to one aspect of the present disclosure may be [7] "the laser medium unit according to any one of the above [1] to [6], wherein the recess is formed to be point-symmetrical with respect to the center of the laser medium when viewed from the direction intersecting the pair of end faces." In this case, even if the temperature of the inner surface of the recess rises due to absorption of spontaneous emission light, the recess is formed to be point-symmetrical with respect to the center of the laser medium, so that non-uniformity in the temperature distribution in the excitation region of the laser medium can be suppressed, thereby improving the quality of the laser light.

[0014] A laser medium unit according to one aspect of the present disclosure may be [8] "the laser medium unit according to any one of [1] to [7] above, wherein the holder has a plurality of recesses, each of which is the recess." In this case, spontaneous emission can be more reliably emitted into a plurality of spaces between the side surface of the laser medium and the inner surfaces of the plurality of recesses, thereby further suppressing parasitic oscillation. In addition, the area of ​​the contact region between the side surface of the laser medium and the holder can be reduced. This further suppresses transfer of heat from the holder, generated by absorption of spontaneous emission, to the laser medium, thereby improving the quality of the laser light.

[0015] A laser medium unit according to one aspect of the present disclosure may be [9] "the laser medium unit according to the above [8], wherein the plurality of recesses are formed to be point-symmetrical with respect to the center of the laser medium when viewed from the direction intersecting the pair of end faces." In this case, even if the temperature of the inner surfaces of the plurality of recesses increases due to absorption of spontaneous emission light, the plurality of recesses are formed to be point-symmetrical with respect to the center of the laser medium, and therefore, it is possible to suppress non-uniformity in the temperature distribution in the excitation region of the laser medium and improve the quality of the laser light.

[0016] A laser medium unit according to one aspect of the present disclosure may be

[10] "the laser medium unit according to the above [8] or [9], wherein a plurality of regions of the surface of the holder located between the openings of the plurality of recesses are in contact with the side surface." In this case, the laser medium can be held in a more stable state by the holder.

[0017] A laser medium unit according to one aspect of the present disclosure may be

[11] "the laser medium unit according to any one of [1] to

[10] above, wherein the holder has a through-hole that communicates the space with an external space of the holder." In this case, spontaneous emission light emitted from a side surface of the laser medium into a space between the side surface and the inner surface of the recess is emitted into the external space of the holder through the through-hole. This can further suppress parasitic oscillation.

[0018] A laser medium unit according to one aspect of the present disclosure may be

[12] "the laser medium unit according to any one of [1] to

[11] above, wherein the holder has a contact portion that contacts the laser medium, and the contact portion is formed of resin." In this case, the contact portion that contacts the laser medium is formed of, for example, a resin that is generally more flexible than metal, thereby reducing stress applied from the holder to the laser medium. This allows the holder to hold the laser medium in a more stable state while suppressing parasitic oscillation. Furthermore, although resin typically has a lower thermal conductivity than metal, the formation of the recesses in the laser medium unit reduces the contact area between the holder and the laser medium. This makes it difficult for heat generated in the holder to be transferred to the laser medium, suppressing uneven temperature distribution in the excitation region of the laser medium and improving the quality of the laser light.

[0019] A laser medium unit according to one aspect of the present disclosure may be

[13] "the laser medium unit according to the above

[12] , wherein the holder has a non-contact portion that does not contact the laser medium, and the non-contact portion is made of metal." In this case, the contact portion that contacts the laser medium provides thermal insulation between the laser medium and the non-contact portion, and the non-contact portion is made of metal that has a higher thermal conductivity than resin, so that heat transferred from the laser medium to the holder via space can be quickly released to the outside.

[0020] A laser medium unit according to one aspect of the present disclosure may be

[14] "the laser medium unit according to any one of the above [1] to

[13] , wherein the holder includes a light absorbing portion formed in the recess and configured to absorb spontaneously emitted light generated in the laser medium." In this case, spontaneously emitted light emitted from a side surface of the laser medium into a space between the side surface and the inner surface of the recess is absorbed by the light absorbing portion. This can further suppress parasitic oscillation.

[0021] A laser medium unit according to one aspect of the present disclosure may be

[15] "the laser medium unit according to the above

[14] , wherein the light absorbing portion is formed of a light absorber, and the light absorber is disposed in the recess away from the laser medium." In this case, spontaneously emitted light emitted from a side surface of the laser medium into a space between the side surface and the inner surface of the recess is absorbed by the light absorber. This can further suppress parasitic oscillation. Furthermore, even if heat is generated in the light absorber due to absorption of the spontaneously emitted light, the generated heat is unlikely to be transmitted to the laser medium because the light absorber is disposed away from the laser medium. This can suppress non-uniform temperature distribution in the excitation region of the laser medium, thereby improving the quality of the laser light.

[0022] The laser medium unit according to one aspect of the present disclosure may be

[16] "the laser medium unit according to the above

[14] , wherein the light absorbing portion is formed of a light absorbing paint." In this case, the light absorbing portion can be easily formed.

[0023] A laser medium unit according to one aspect of the present disclosure may be

[17] "the laser medium unit according to any one of [1] to

[16] above, wherein the laser medium is configured without including a light absorption region having an absorptivity of 90% or more for spontaneously emitted light generated in the laser medium." When the laser medium includes a light absorption region having an absorptivity of 90% or more for spontaneously emitted light, high heat generated when the light absorption region absorbs the spontaneously emitted light is directly transferred to the excitation region of the laser medium. In the laser medium unit described above, the laser medium is configured without including a light absorption region having an absorptivity of 90% or more for spontaneously emitted light, so that direct transfer of heat is suppressed. This makes it possible to suppress non-uniformity in the temperature distribution in the excitation region of the laser medium and improve the quality of the laser light. Furthermore, although stress distortion may occur in the laser medium when forming (bonding) a light absorption region in the laser medium, the occurrence of such stress distortion is suppressed in the laser medium unit described above. This makes it possible to suppress deterioration in the characteristics of the laser medium and improve the quality of the laser light.

[0024] A laser medium unit according to one aspect of the present disclosure may be

[18] "the laser medium unit according to any one of [1] to

[17] above, wherein the laser medium includes a light absorption region having an absorptivity of less than 90% for spontaneously emitted light generated in the laser medium." In this case, the spontaneously emitted light generated in the laser medium is appropriately absorbed by the light absorption region having an absorptivity of less than 90% for spontaneously emitted light, and the spontaneously emitted light transmitted through the light absorption region is emitted, for example, from a side surface of the laser medium into a space between the side surface and the inner surface of the recess, thereby suppressing parasitic oscillation. Furthermore, excessive temperature rise in the light absorption region due to absorption of spontaneously emitted light is suppressed, thereby suppressing non-uniformity in the temperature distribution in the excitation region of the laser medium and improving the quality of the laser light.

[0025] A laser medium unit according to one aspect of the present disclosure may be

[19] "the laser medium unit according to any one of the above [1] to

[18] , wherein the holder is in contact with each of the pair of end faces." In this case, the laser medium can be held in a more stable state by the holder.

[0026] A laser medium unit according to one aspect of the present disclosure may be

[20] "the laser medium unit according to the above

[19] , wherein the holder has a restricting portion that contacts the side surface and restricts movement of the laser medium in a direction perpendicular to the direction intersecting the pair of end faces." In this case, the restricting portion restricts movement of the laser medium, and a predetermined space can be more reliably formed between the side surface of the laser medium and the recess of the holder.

[0027] According to the present disclosure, it is possible to provide a laser medium unit that can improve the quality of laser light while suppressing parasitic oscillation.

[0028] FIG. 1 is a diagram showing the overall configuration of a laser device according to one embodiment. FIG. 2 is a perspective view showing the laser medium unit shown in FIG. 1. FIG. 3 is a perspective view showing the laser medium and holder shown in FIG. 2. FIG. 4 is a plan view showing the laser medium and holder shown in FIG. 2. FIG. 5 is a cross-sectional view of the laser medium and holder taken along line V-V shown in FIG. 4. FIG. 6 is a diagram showing the internal configuration of the laser medium and holder shown in FIG. 2. FIG. 7 is a diagram showing the configuration of the holder shown in FIG. 2. FIG. 8 is a diagram schematically showing the path of spontaneously emitted light in the laser medium of a laser medium unit according to Comparative Example 1. FIG. 9 is a diagram schematically showing the path of spontaneously emitted light in the laser medium of the laser medium unit according to the first embodiment. FIG. 10 is a plan view showing the laser medium and holder according to a first modified example. FIG. 11 is a cross-sectional view of the laser medium and holder taken along line XI-XI shown in FIG. 10. FIG. 12 is a diagram showing the internal configuration of the laser medium and holder shown in FIG. 10. FIG. 13 is a plan view showing a laser medium and a holder according to a second modified example. FIG. 14 is a cross-sectional view of the laser medium and the holder taken along line XIV-XIV shown in FIG. 13. FIG. 15 is a diagram showing the temperature distribution in the laser medium of a laser medium unit according to Comparative Example 2. FIG. 16 is a diagram showing the temperature distribution in the laser medium of a laser medium unit according to Example 1. FIG. 17 is a diagram showing the temperature distribution in the laser medium of a laser medium unit according to Example 2. FIG. 18 is a perspective view showing a laser medium and a holder according to a second embodiment. FIG. 19 is a plan view showing the laser medium and the holder shown in FIG. 18. FIG. 20 is a cross-sectional view of the laser medium and the holder taken along line XX-XX shown in FIG. 19. FIG. 21 is a plan view showing a laser medium and the holder according to a third modified example. FIG. 22 is a cross-sectional view of the laser medium and the holder taken along line XXII-XXII shown in FIG. 21. FIG. 23 is a diagram showing a modified example of the holder. FIG. 24 is a diagram showing a modified example of the laser medium. Fig. 25 is a diagram schematically showing the path of spontaneously emitted light in the laser medium shown in Fig. 24. Fig. 26 is a diagram showing a modified example of the laser medium.

[0029] Hereinafter, an example of the present disclosure will be described in detail with reference to the drawings. In each drawing, the same or corresponding parts are denoted by the same reference numerals, and duplicated explanations will be omitted. [First embodiment] [Laser device]

[0030] As shown in Fig. 1, the laser device 1 of the first embodiment includes multiple laser medium units 10, a chamber 2, a laser light source 3, and multiple excitation light sources 4. The laser medium units 10 are arranged along the Z-axis direction with gaps between them. Hereinafter, a direction intersecting the Z-axis direction will be referred to as the X-axis direction, and a direction intersecting the Z-axis direction and the X-axis direction will be referred to as the Y-axis direction. In this embodiment, the X-axis direction, the Y-axis direction, and the Z-axis direction are perpendicular to one another.

[0031] The chamber 2 has a main body 21, a pair of windows 22, an inlet 25, and an outlet 26. The main body 21 has a space S1 that houses a plurality of laser medium units 10. A pair of openings 21a and a pair of openings 21b are formed in the main body 21. The pair of openings 21a are opposed to each other in the Z-axis direction. The pair of openings 21b are opposed to each other in the Y-axis direction.

[0032] Each window 22 is attached to the main body 21 so as to cover the corresponding opening 21 a. The window 22 includes a frame 23 and a window member 24 held by the frame 23. The window member 24 transmits laser light L1 and excitation light L2, which will be described later.

[0033] The inlet portion 25 has an inlet passage 25a. The inlet portion 25 is attached to the main body 21 so that the inlet passage 25a communicates with the space S1 via the opening 21b. The inlet portion 25 introduces a refrigerant M (e.g., a cooling gas such as low-temperature helium gas) into the space S1. The refrigerant M flows through gaps formed between adjacent laser medium units 10 in the space S1. The outlet portion 26 has an outlet passage 26a. The outlet portion 26 is attached to the main body 21 so that the outlet passage 26a communicates with the space S1 via the opening 21b. The outlet portion 26 leads the refrigerant M out of the space S1.

[0034] The laser light source 3 is disposed on one side in the Z-axis direction relative to the chamber 2 (plurality of laser medium units 10). The laser light source 3 emits laser light L1, which is seed light, to the plurality of laser medium units 10. The laser diameter of the laser light L1 is, for example, approximately 60 mm. The laser light L1 emitted from the laser light source 3 enters the plurality of laser medium units 10 through the window member 24 of the window portion 22 disposed on one side in the Z-axis direction relative to the plurality of laser medium units 10. The laser light L1 is amplified by each of the laser medium units 10.

[0035] The multiple excitation light sources 4 include a pair of first excitation light sources 41 and a pair of second excitation light sources 42. In this example, each of the pair of first excitation light sources 41 and the pair of second excitation light sources 42 has a configuration in which multiple (approximately 10) laser diode (LD) bars, each having multiple light-emitting regions arranged in an array, are stacked. The pair of first excitation light sources 41 is disposed on one side in the Z-axis direction with respect to the chamber 2 (the multiple laser medium units 10). Each first excitation light source 41 emits excitation light L2 to the multiple laser medium units 10. The excitation light L2 emitted from each first excitation light source 41 enters the multiple laser medium units 10 through the window member 24 of the window portion 22 disposed on one side in the Z-axis direction with respect to the multiple laser medium units 10. The pair of second excitation light sources 42 is disposed on the other side in the Z-axis direction with respect to the chamber 2 (the multiple laser medium units 10). Each second excitation light source 42 emits excitation light L2 to the multiple laser medium units 10. The excitation light L2 emitted from each second excitation light source 42 is incident on the plurality of laser medium units 10 through the window member 24 of the window portion 22 arranged on the other side of the plurality of laser medium units 10 in the Z-axis direction. The excitation light L2 excites the laser medium 50, which will be described later. [Laser Medium Unit]

[0036] The laser medium unit 10 will be described in detail with reference to FIGS. 2 to 7 . FIG. 6 is a diagram showing the internal configuration of the laser medium 50 and the holder 60 (described later), and is a half-sectional view in which portions of the laser medium 50 and the holder 60 are cut along the X-axis and Y-axis. The laser medium unit 10 includes the laser medium 50, the holder 60, and a frame 70. The laser medium 50 has a pair of end faces 51 a, 51 b and a side face 51 c. The laser medium 50 has a thickness in the Z-axis direction and is plate-shaped with the pair of end faces 51 a, 51 b as main surfaces. In this example, the laser medium 50 is disk-shaped. The pair of end faces 51 a, 51 b intersect in the Z-axis direction. The end face 51 b is located on the opposite side of the end face 51 a in the Z-axis direction.

[0037] The side surface 51c of the laser medium 50 intersects with the pair of end surfaces 51a, 51b. The side surface 51c is aligned along the Z-axis direction. When viewed from a direction intersecting the pair of end surfaces 51a, 51b (the Z-axis direction), the side surface 51c defines the outer edge of the laser medium 50. That is, when viewed from the Z-axis direction, the side surface 51c has a circular shape. When viewed from the Z-axis direction, the center of the circle defined by the side surface 51c coincides with the center C of the laser medium 50. The side surface 51c has a predetermined width in the Z-axis direction. The thickness of the laser medium 50 (the width of the side surface 51c in the Z-axis direction) is, for example, several millimeters to several tens of millimeters. The diameter of the laser medium 50 (the width in a direction perpendicular to the Z-axis direction) is, for example, about 100 mm.

[0038] The laser medium 50 is a solid-state laser medium. The material of the laser medium 50 is, for example, YAG doped with an additive such as Yb or Nd as an active element. In this example, the entire laser medium 50 is formed from YAG doped with the additive. The laser medium 50 is configured without including a light-absorbing region in which the absorptance for spontaneously emitted light generated in the laser medium 50 is 90% or more. In other words, the absorptance for spontaneously emitted light in the laser medium 50 is less than 90%. The spontaneously emitted light generated in the laser medium 50 may be light of, for example, 1000 nm or more and 1100 nm or less. The absorptance is the absorptance per cm of absorption length.

[0039] The laser medium 50 is excited by the excitation light L2 and outputs emitted light. FIG. 4 shows a region (excitation region 52) irradiated with the excitation light L2. As described above, each of the first excitation light source 41 and the second excitation light source 42 has a configuration in which a plurality of laser diode (LD) bars are stacked. This allows rectangular excitation light L2 to be irradiated onto the laser medium 50. Therefore, the excitation region 52 has a rectangular shape when viewed from the Z-axis direction. In this example, the width of the excitation region 52 in the X-axis direction is wider than the width in the Y-axis direction. The width of the excitation region 52 in the X-axis direction is, for example, approximately 80 mm. The width of the excitation region 52 in the Y-axis direction is, for example, approximately 60 mm. From the viewpoint of energy efficiency, the width of the excitation region 52 is preferably as close as possible to the laser diameter of the laser light L1, which is the seed light, but does not necessarily have to be the same. The emitted light output is, for example, stimulated emission light. The stimulated emission light contributes to the optical amplification of the laser light L1.

[0040] The holder 60 is disposed along the side surface 51 c of the laser medium 50 when viewed from the Z-axis direction, and holds the laser medium 50. In this example, the holder 60 is formed in a frame shape that surrounds the side surface 51 c of the laser medium 50 when viewed from the Z-axis direction. The surface 61 of the holder 60 has an inner surface 61 a, an outer surface 61 b, and a pair of side surfaces 61 c, 61 d.

[0041] The inner side surface 61a extends along the side surface 51c of the laser medium 50 when viewed from the Z-axis direction. In this example, the inner side surface 61a has a circular shape when viewed from the Z-axis direction. The outer side surface 61b has a rectangular shape when viewed from the Z-axis direction. The inner side surface 61a and the outer side surface 61b of the holder 60 each have a predetermined width in the Z-axis direction (thickness direction of the holder 60). The thickness of the holder 60 (i.e., the width of each of the inner side surface 61a and the outer side surface 61b in the Z-axis direction) is, for example, several millimeters to several tens of millimeters. The thickness of the holder 60 is approximately the same as the thickness of the laser medium 50. The pair of side surfaces 61c, 61d intersect in the Z-axis direction. The side surface 61c is located on the opposite side to the side surface 61d in the Z-axis direction.

[0042] As an example, the material of the holder 60 may be resin or metal. The resin may be, for example, a fluororesin such as polytetrafluoroethylene (PTFE). The metal may be, for example, aluminum. As another example, the material of the holder 60 may be, for example, a ceramic (such as black alumina) that absorbs light in the wavelength band (e.g., approximately 1000 nm to 1100 nm) of the spontaneously emitted light generated in the excitation region 52. In this example, the entire holder 60, including the contact portion 63 that contacts the laser medium 50, is formed from the same material. Specifically, the entire holder 60, including the contact portion 63, is formed from resin (PTFE). In this example, the contact portion 63 contacts the side surface 51 c of the laser medium 50.

[0043] The holder 60 has a plurality of recesses 62 formed on an inner surface 61a. As shown in FIG. 4 , the recesses 62 are formed at regular intervals from one another in a direction A (a direction along the side surface 51c of the laser medium 50) surrounding the center C of the laser medium 50 when viewed from the Z-axis direction. The recesses 62 are formed to be point-symmetric with respect to the center C of the laser medium 50 when viewed from the Z-axis direction. In other words, when the recesses 62 are rotated 180° around the center C, the arrangement of the recesses 62 matches the arrangement before rotation. When viewed from the Z-axis direction, the recesses 62 are formed to be line-symmetric with respect to a straight line L passing through the center C and the bottom 62b. When viewed from the Z-axis direction, each recess 62 opens toward the center C of the laser medium 50 (toward the inner region). The recesses 62 have an opening 62a on the inner surface 61a.

[0044] The recess 62 has a shape that tapers from the opening 62a to the bottom 62b of the recess 62. That is, the internal space of the recess 62 narrows from the opening 62a to the bottom 62b. Specifically, the recess 62 has a cone shape. In this example, the recess 62 has an elliptical cone shape. The inner surface 62c of the recess 62 corresponds to the side surface of the cone. As shown in FIG. 7 , the opening 62a of the recess 62 has an elliptical shape with a minor axis along the Z-axis direction. When viewed from the Z-axis direction, the recess 62 is formed to be symmetrical with respect to a line (in this example, line L) passing through the center C (see FIG. 4 ). In this example, when viewed from the Z-axis direction, the shape of the recess 62 has an isosceles triangle shape with the bottom 62b as the apex and the base (the portion corresponding to the inner surface 61a) curving inward.

[0045] The holder 60 further includes a plurality of locking portions (claw portions) 69. The locking portions 69 are shown in FIGS. 2, 3, and 6, and are omitted from the other drawings. A pair of locking portions 69 is provided on the side surface 61c of the holder 60. Similarly, a pair of locking portions 69 is provided on the side surface 61d of the holder 60. On each of the side surfaces 61c and 61d of the holder 60, each locking portion 69 protrudes further inward of the holder 60 than the inner surface 61a when viewed in the Z-axis direction. Each of the pair of locking portions 69 provided on the side surface 61c faces (overlaps with) the corresponding locking portion 69 of the pair of locking portions 69 provided on the side surface 61d in the Z-axis direction.

[0046] The holder 60 surrounds the laser medium 50 when viewed from the Z-axis direction. In other words, the laser medium 50 is disposed inside the holder 60. When viewed from the Z-axis direction, the diameter of the laser medium 50 is approximately the same as the diameter of the inner surface 61 a of the holder 60. Movement of the laser medium 50 in a direction perpendicular to the Z-axis direction is restricted by the holder 60. The laser medium 50 is sandwiched between multiple locking portions 69 in the Z-axis direction. Movement of the laser medium 50 in the Z-axis direction is restricted by the multiple locking portions 69. In this way, the laser medium 50 is held by the holder 60 inside the holder 60.

[0047] The laser medium 50 and the holder 60 are in contact with each other. The laser medium 50 and the holder 60 are in direct contact with each other without any other member interposed therebetween. Specifically, the side surface 51c of the laser medium 50 and the inner surface 61a of the holder 60 are in contact with each other. The holder 60 holds the laser medium 50 such that a space S2 is formed between the side surface 51c of the laser medium 50 and the inner surface 62c of each recess 62. In this example, air is present in the space S2, and no solid or liquid substance is disposed therein. The side surface 51c includes a region R1 (contact region) that contacts the holder 60 (inner surface 61a) and a region R2 (non-contact region) that does not contact the holder 60. In this example, the region R2 is the region facing the space S2.

[0048] A plurality of spaces S2 corresponding to the plurality of recesses 62 are formed in the holder 60. The plurality of spaces S2 are formed at regular intervals from one another in direction A surrounding the center C when viewed from the Z-axis direction. As shown in Fig. 4, a plurality of regions R3 located between the openings 62a of the plurality of recesses 62 on the surface 61 (inner surface 61a) of the holder 60 are in contact with the side surface 51c of the laser medium 50. That is, a plurality of regions R3 aligned in direction A on the inner surface 61a are in contact with the side surface 51c.

[0049] The laser medium 50 and the holder 60 are in continuous contact with each other without any gaps in the region R1. The laser medium 50 and the holder 60 are in close contact with each other in the region R1. On the other hand, the laser medium 50 and the holder 60 are not in contact with each other in the region R2. The laser medium 50 and the holder 60 are spatially separated in the region R2 via a space S2. The proportion of the region R1 to the entire side surface 51c of the laser medium 50 (the sum of the regions R1 and R2) (contact proportion) is, for example, 5% or more and 90% or less. The upper limit of the contact proportion is preferably 90% or less, more preferably 70% or less, even more preferably 50% or less, and even more preferably 30% or less. By reducing the contact proportion, spontaneous emission light can be more reliably emitted from the side surface 51c of the laser medium 50 into the space between the side surface 51c and the inner surface 62c of the recess 62, and thermal insulation between the laser medium 50 and the holder 60 can be more reliably achieved. The lower limit of the contact ratio is preferably 5% or more, and more preferably 10% or more. By increasing the contact ratio, it is possible to prevent the stress applied from the holder 60 to the side surface 51c of the laser medium 50 from concentrating locally. In this example, the ratio of the region R1 to the entire side surface 51c is about 10%.

[0050] The frame 70 is formed in a frame shape that surrounds the outer surface 61b of the holder 60 when viewed from the Z-axis direction. In this example, the frame 70 is formed in a rectangular frame shape when viewed from the Z-axis direction. The frame 70 holds the holder 60. The holder 60 is held by the frame 70 by fitting the holder 60 inside the frame 70. In other words, the inner edge of the frame 70 when viewed from the Z-axis direction coincides with the outer edge (outer surface 61b) of the holder 60.

[0051] The frame 70 has a pair of first portions 71 extending along the X-axis direction and a pair of second portions 72 extending along the Y-axis direction. The width of the first portions 71 in the Y-axis direction is wider than the width of the second portions 72 in the X-axis direction. Each of the pair of first portions 71 has a wedge-shaped fin 73 formed thereon. The fins 73 are formed so that their width in the Z-axis direction narrows with increasing distance from the center C of the laser medium 50 along the Y-axis direction. In this example, the fins 73 are formed in a triangular prism shape. The frame 70 having the fins 73 allows the refrigerant M introduced into the space S1 to flow smoothly between the multiple laser medium units 10. The width of the frame 70 in the Y-axis direction (the width of the entire laser medium unit 10 in the Y-axis direction) is, for example, approximately 30 cm to 40 cm. The thickness of the frame 70 in the Z-axis direction is, for example, several millimeters to several tens of millimeters. The thickness of the frame 70 is approximately the same as the thickness of the laser medium 50. The frame 70 may be made of, for example, a metal. The metal may be, for example, aluminum. In this example, the entire frame 70 is made of the same material. Specifically, the entire frame 70 is made of aluminum. [Functions and Effects]

[0052] In the laser medium unit 10, the holder 60 holds the laser medium 50 such that a space S2 is formed between the side surface 51c of the laser medium 50 and the inner surface 62c of the recess 62 formed in the holder 60. Parasitic oscillation occurs when spontaneously emitted light generated in the excitation region of the laser medium repeatedly passes through the excitation region. In the laser medium unit 10, the spontaneously emitted light generated in the excitation region 52 is emitted from the side surface 51c of the laser medium 50 into the space S2. The emitted spontaneously emitted light is scattered by repeated reflections on the inner surface 62c of the recess 62 and is therefore less likely to return to the excitation region 52. This suppresses parasitic oscillation. Furthermore, in the laser medium unit 10, even if heat is generated on the inner surface 62c of the recess 62 by absorbing the spontaneously emitted light, the space S2 is formed between the inner surface 62c of the recess 62 and the side surface 51c of the laser medium 50, so the heat is less likely to be transmitted to the laser medium 50. Therefore, it is possible to suppress non-uniformity in the temperature distribution in the excitation region 52, and improve the quality of the laser light L1. Therefore, according to the laser medium unit 10, it is possible to improve the quality of the laser light L1 while suppressing parasitic oscillation.

[0053] Furthermore, in the laser medium unit 10, parasitic oscillation can be suppressed without forming (or bonding) a member such as a cladding material for absorbing spontaneous emission light on the laser medium 50 itself. Therefore, there is no need to consider the yield when forming such a member on the laser medium 50, and material costs can also be reduced. Therefore, the laser medium unit 10 can improve manufacturing efficiency.

[0054] Here, the operation and effects of the laser medium unit 10 will be described in more detail with reference to Fig. 8 and Fig. 9. Fig. 8 is a diagram schematically showing the path of the spontaneously emitted light L10 in the laser medium 550 of the laser medium unit according to Comparative Example 1. Fig. 9 is a diagram schematically showing the path of the spontaneously emitted light L10 in the laser medium 50 of the laser medium unit 10 according to the first embodiment. In the example shown in Fig. 8, no recess is formed in the holder 560, and the entire side surface 551c of the laser medium 550 is in contact with the holder 560.

[0055] In the example shown in FIG. 8 , no recess is formed in the holder 560 (because the space S2 does not exist), and the spontaneous emission light L10 generated in the excitation region 552 of the laser medium 550 is reflected at the interface between the side surface 551 c of the laser medium 550 and the holder 560 without being emitted to the outside from the laser medium 550. As a result, the spontaneous emission light L10 repeatedly passes through the excitation region 552, making it easy for parasitic oscillation to occur. In contrast, in the example shown in FIG. 9 , the spontaneous emission light L10 generated in the excitation region 52 is emitted from the side surface 51 c of the laser medium 50 into the space S2. The emitted spontaneous emission light L10 is scattered by being repeatedly reflected at the inner surface 62 c of the recess 62 as shown in FIG. 9 , making it difficult for it to return to the excitation region 52. Therefore, the laser medium unit 10 according to this embodiment suppresses parasitic oscillation.

[0056] The holder 60 is formed in a frame shape surrounding the laser medium 50 when viewed from the Z-axis direction. This allows the holder 60 to hold the laser medium 50 in a more stable state.

[0057] The recess 62 has a shape that tapers toward the bottom 62b of the recess 62. This increases the number of times that the spontaneously emitted light emitted from the side surface 51c of the laser medium 50 is reflected by the inner surface 62c of the recess 62, and therefore the spontaneously emitted light can be efficiently scattered in the recess 62. This makes it possible to further suppress parasitic oscillation.

[0058] The contact area (region R1) of the side surface 51c with the holder 60 accounts for 50% or less of the entire side surface 51c of the laser medium 50. This allows spontaneous emission of light from the side surface 51c of the laser medium 50 into the space S2 more reliably, and further suppresses parasitic oscillation.

[0059] The holder 60 is in contact with the side surface 51c of the laser medium 50. This allows the holder 60 to hold the laser medium 50 in a more stable state while suppressing parasitic oscillation.

[0060] The ratio of the region R1 to the entire side surface 51c of the laser medium 50 is 5% or more. This makes it possible to prevent stress applied from the holder 60 to the side surface 51c of the laser medium 50 from concentrating locally. This makes it possible to suppress parasitic oscillation and to hold the laser medium 50 in an even more stable state by the holder 60.

[0061] The holder 60 has a plurality of recesses 62 formed therein. This allows spontaneous emission of light from the side surface 51c of the laser medium 50 to the plurality of spaces S2 more reliably, thereby further suppressing parasitic oscillation. In addition, the area of ​​the contact region (region R1) between the side surface 51c and the holder 60 can be reduced. This further prevents heat from the holder 60, which is generated by absorbing spontaneous emission light, from being transferred to the laser medium 50, thereby improving the quality of the laser light L1.

[0062] The plurality of recesses 62 are formed to be point-symmetrical with respect to the center C of the laser medium 50 when viewed from the Z-axis direction. As a result, even if the temperature of the inner surfaces 62c of the plurality of recesses 62 rises due to the absorption of spontaneous emission light, the plurality of recesses 62 are formed to be point-symmetrical with respect to the center C of the laser medium 50, and therefore, it is possible to suppress non-uniformity in the temperature distribution in the excitation region 52 of the laser medium 50, and it is possible to improve the quality of the laser light L1.

[0063] A plurality of regions R3 located between the openings 62a of the plurality of recesses 62 on the surface of the holder 60 are in contact with the side surface 51c of the laser medium 50. This allows the holder 60 to hold the laser medium 50 in a more stable state.

[0064] The holder 60 has a contact portion 63 that comes into contact with the laser medium 50. The contact portion 63 is made of resin. By forming the contact portion 63 from resin, which is generally more flexible than, for example, metal, the stress applied from the holder 60 to the laser medium 50 is reduced. This allows the holder 60 to hold the laser medium 50 in an even more stable state while suppressing parasitic oscillation. Furthermore, although resin typically has a lower thermal conductivity than metal, the formation of the recess 62 in the laser medium unit 10 reduces the contact area between the holder 60 and the side surface 51c. Therefore, heat generated in the holder 60 is less likely to be transmitted to the laser medium 50, which can suppress non-uniform temperature distribution in the excitation region 52 of the laser medium 50 and improve the quality of the laser light L1.

[0065] The laser medium 50 is configured without including a light-absorbing region having an absorptivity of 90% or more for spontaneously emitted light generated in the laser medium 50. When a laser medium includes a light-absorbing region having an absorptivity of 90% or more for spontaneously emitted light, high heat generated when the light-absorbing region absorbs the spontaneously emitted light is directly transferred to the excitation region of the laser medium. In the laser medium unit 10, the laser medium 50 is configured without including a light-absorbing region having an absorptivity of 90% or more for spontaneously emitted light, so such direct transfer of heat is suppressed. Therefore, it is possible to suppress non-uniformity in the temperature distribution in the excitation region 52 of the laser medium 50, and improve the quality of the laser light L1. Furthermore, when forming (bonding) a light-absorbing region in the laser medium, stress distortion may occur in the laser medium. However, the laser medium unit 10 suppresses the occurrence of such stress distortion. Therefore, it is possible to suppress deterioration in the characteristics of the laser medium 50 and improve the quality of the laser light L1. [Modification of the First Embodiment]

[0066] Next, modified examples of the laser medium unit 10 according to the first embodiment will be described. First, the laser medium unit 10 according to the first modified example will be described with reference to Figs. 10, 11, and 12. Fig. 12 is a diagram showing the internal configuration of the laser medium 50 and the holder 60, and is a half cross-sectional view in which parts of the laser medium 50 and the holder 60 are cut along the X-axis and Y-axis. The locking portion 69 is not shown in Figs. 10 to 12.

[0067] In the first modified example, a plurality of through holes 64 are formed in the holder 60, which connect the plurality of spaces S2 with the external space of the holder 60. Each through hole 64 connects the corresponding space S2 with the external space of the holder 60. Each through hole 64 opens to the side surface 61c and the side surface 61d. The through holes 64 are formed so as to penetrate the recess 62 in the Z-axis direction (so as to pass through the space S2 inside the recess 62). Air is present inside the through holes 64, and no solid or liquid substance is disposed therein.

[0068] An inner surface 64a of the through hole 64 that defines the through hole 64 has a circular shape when viewed from the Z-axis direction. An opening 64b is formed in the inner surface 64a. The space inside the through hole 64 communicates with the space S2 via the opening 64b. As a result, the space S2 is spatially connected to the external space of the laser medium unit 10 via the through hole 64. The refrigerant M (see FIG. 1) introduced into the space S1 enters the space S2 via the through hole 64.

[0069] In this modification, the holder 60 is formed with a through-hole 64 that connects the space S2 to the space outside the holder 60. As a result, spontaneous emission light emitted from the side surface 51c of the laser medium 50 into the space S2 is emitted into the space outside the holder 60 through the through-hole 64. This makes it possible to further suppress parasitic oscillation. In addition, the refrigerant M enters the space S2 through the through-hole 64 and comes into contact with the side surface 51c. This makes it possible to efficiently cool the laser medium 50.

[0070] Next, a laser medium unit 10 according to a second modification will be described with reference to FIGS. 13 and 14 . In the second modification, the holder 60 includes a light absorbing portion 65a formed by a light absorber 65. The light absorbing portion 65a is formed in the recess 62. The light absorbing portion 65a absorbs spontaneously emitted light generated in the laser medium 50. The absorptance of the light absorbing portion 65a for spontaneously emitted light is 0.1% or more, more preferably 1% or more, even more preferably 5% or more, even more preferably 10% or more, and even more preferably 50% or more. In this example, the absorptance of the light absorbing portion 65a for spontaneously emitted light is 80% or more. Since the absorptance of the light absorbing portion 65a for spontaneously emitted light is 0.1% or more, the spontaneously emitted light is attenuated each time the spontaneously emitted light is incident on (hits) the light absorbing portion 65a, ultimately preventing parasitic oscillation. The material of the light absorbing portion 65a (light absorber 65) may be, for example, YAG doped with Cr.

[0071] The light absorber 65 extends in an annular shape when viewed from the Z-axis direction. In this example, the light absorber 65 extends continuously in an annular shape. The center of the light absorber 65 when viewed from the Z-axis direction coincides with the center C of the laser medium 50. The light absorber 65 is arranged so as to pass through the multiple recesses 62. Specifically, the light absorber 65 is arranged continuously in the portions between the multiple recesses 62 in the holder 60 and throughout the interiors (space S2) of the multiple recesses 62. The light absorber 65 is fitted into annular grooves (spaces opening on the side surfaces 61c and 61d of the holder 60) formed in the holder 60. The light absorber 65 is exposed to the external space of the holder 60 at the side surfaces 61c and 61d. The light absorber 65 is located in the recess 62, away from the side surface 51c of the laser medium 50. That is, a space S2 exists between the light absorber 65 and the side surface 51c. In this example, the light absorber 65 is located inside the bottom 62b of each recess 62 when viewed from the Z-axis direction (there is a space between the light absorber 65 and the bottom 62b).

[0072] In this modification, the holder 60 is provided with a light absorbing portion 65a formed in the recess 62, which absorbs spontaneously emitted light generated in the laser medium 50. As a result, the spontaneously emitted light emitted from the side surface 51c of the laser medium 50 into the space S2 is absorbed by the light absorbing portion 65a. This makes it possible to further suppress parasitic oscillation.

[0073] The light absorbing portion 65a is formed of a light absorber 65, and the light absorber 65 is disposed in the recess 62 away from the laser medium 50. As a result, even if heat is generated in the light absorber 65 due to absorption of spontaneously emitted light, the generated heat is less likely to be transmitted to the laser medium 50 because the light absorber 65 is disposed away from the laser medium 50. This makes it possible to suppress non-uniformity in the temperature distribution in the excitation region 52 of the laser medium 50, and improve the quality of the laser light L1.

[0074] In this modification, the light absorber 65 is located inside the bottom 62b of each recess 62 when viewed from the Z-axis direction, but the light absorber 65 may be located at a position overlapping the bottom 62b when viewed from the Z-axis direction. Also, in this modification, the holder 60 has one continuous light absorber 65, but the holder 60 may have multiple light absorbers 65 (separate light absorbers 65) each located in a corresponding recess 62. [Example]

[0075] The temperature distribution in the laser medium 50 will be described with reference to Fig. 15, Fig. 16, and Fig. 17. Fig. 15 is a diagram showing the temperature distribution in the laser medium 650 of the laser medium unit according to Comparative Example 2. Fig. 16 is a diagram showing the temperature distribution in the laser medium 50 of the laser medium unit 10 according to Example 1. Fig. 17 is a diagram showing the temperature distribution in the laser medium 50 of the laser medium unit 10 according to Example 2. Figs. 15 to 17 show simulation results of the temperature distribution in the laser medium.

[0076] In Comparative Example 2 shown in FIG. 15 , a light absorption region 655 made of Cr-doped YAG is formed on the outer edge of the laser medium 650. Furthermore, in Comparative Example 2, no recesses are formed in the holder 660, and the entire side surface 651 c of the laser medium 650 is in contact with the holder 660. Example 1 shown in FIG. 16 has the configuration according to the first embodiment described above. That is, multiple recesses 62 are formed in the holder 60, and multiple spaces S2 are formed between the side surface 51 c of the laser medium 50 and the inner surfaces 62 c of the multiple recesses 62. Example 2 shown in FIG. 17 has the configuration according to the first modified example described above. That is, in Example 2, in addition to the multiple recesses 62, multiple through-holes 64 are formed in the holder 60. In all of Comparative Example 2 and Examples 1 and 2, the holder material was PTFE resin.

[0077] Other conditions for the simulation are as follows. In Comparative Example 2 shown in Fig. 15, 67 W of heat was generated in the excitation region 652, and 210 W of heat was generated in the light absorption region 655. In Examples 1 and 2 shown in Figs. 16 and 17, 67 W of heat was generated in the excitation region 52, and 210 W of heat was generated on the inner surface 62c of the recess 62. In each of Comparative Example 2 and Examples 1 and 2, the inflowing refrigerant was cooled helium gas, the refrigerant flow rate was 30 m / s, the pressure was 0.5 MPa, and the temperature was 150K.

[0078] As shown in Figures 15 to 17, the temperature distributions in Examples 1 and 2 were simplified and closer to concentric circles around the center of the laser medium compared to the temperature distribution in Comparative Example 2. Specific simulation results for the excitation region (80 mm wide in the X-axis direction × 60 mm wide in the Y-axis direction) are as follows: First, in Comparative Example 2, in the excitation region 652, the central temperature was 159.9 K, the average temperature was 161.3 K, the temperature difference (PV value) was 10.2 K, and the temperature difference (standard deviation) was 1.650 K (rms 1.02%). In Example 1, in the excitation region 52, the central temperature was 158.2 K, the average temperature was 156.7 K, the temperature difference (PV value) was 4.2 K, and the temperature difference (standard deviation) was 0.982 K (rms 0.62%). In Example 2, in the excitation region 52, the central temperature was 158.3 K, the average temperature was 156.8 K, the temperature difference (PV value) was 3.7 K, and the temperature difference (standard deviation) was 0.897 K (rms 0.57%). That is, in Examples 1 and 2, the temperature difference (PV value) was reduced by about 6 K compared to Comparative Example 2. Furthermore, in Examples 1 and 2, the standard deviation of the temperature distribution was reduced by about 0.4% compared to Comparative Example 2. These simulation results confirmed that the laser medium unit 10 suppresses non-uniformity in the temperature distribution in the excitation region 52 of the laser medium 50.

[0079] Furthermore, even in a range narrower than the excitation region (60 mm width in the X-axis direction × 60 mm width in the Y-axis direction), the temperature distributions in Examples 1 and 2 were simplified and closer to concentric circles around the center of the laser medium compared to the temperature distribution in Comparative Example 2. Specific simulation results for this range are as follows: In Comparative Example 2, within this range, the central temperature was 159.9 K, the average temperature was 160.6 K, the temperature difference (PV value) was 5.6 K, and the temperature difference (standard deviation) was 0.867 K. In Example 1, within this range, the central temperature was 158.2 K, the average temperature was 157.0 K, the temperature difference (PV value) was 3.7 K, and the temperature difference (standard deviation) was 0.870 K. In other words, in the above range narrower than the excitation region, it was confirmed that the temperature difference (PV value) in Example 1 was reduced by approximately 2 K compared to Comparative Example 2. [Second Embodiment]

[0080] Next, a laser medium unit 10A according to the second embodiment will be described with reference to Figures 18, 19, and 20. The laser medium unit 10A includes a laser medium 50, a holder 160, and a frame 70; however, in Figures 18 to 20, the frame 70 is not shown, and only the laser medium 50 and the holder 160 are shown. The laser medium unit 10A according to the second embodiment may be used in place of the laser medium unit 10 in the laser device 1. The laser medium unit 10A according to the second embodiment differs from the laser medium unit 10 according to the first embodiment in the configuration of the holder. Therefore, in the following description, the holder 160 will be mainly described, and a description of the commonalities between the second embodiment and the first embodiment may be omitted.

[0081] The holder 160 is disposed along the side surface 51c of the laser medium 50 when viewed from the Z-axis direction, and holds the laser medium 50. In this example, the holder 160 is formed in the shape of a circular frame that surrounds the side surface 51c of the laser medium 50 when viewed from the Z-axis direction. The surface 161 of the holder 160 has an inner surface 161a, an outer surface 161b, and a pair of side surfaces 161c and 161d.

[0082] The inner surface 161a extends along the side surface 51c of the laser medium 50 when viewed from the Z-axis direction. In this example, the inner surface 161a has a circular shape when viewed from the Z-axis direction. When viewed from the Z-axis direction, the inner surface 161a is located more inward (closer to the center C) than the side surface 51c of the laser medium 50. The outer surface 161b has a rectangular shape when viewed from the Z-axis direction. Each of the inner surface 161a and the outer surface 161b of the holder 160 has a predetermined width in the Z-axis direction (thickness direction of the holder 160). The thickness of the holder 160 (i.e., the width of each of the inner surface 161a and the outer surface 161b in the Z-axis direction) is, for example, several millimeters to several tens of millimeters. The thickness of the holder 160 is greater than the thickness of the laser medium 50. The pair of side surfaces 161c and 161d intersect in the Z-axis direction. The side surface 161c is located on the opposite side of the side surface 161d in the Z-axis direction.

[0083] The holder 160 has a pair of plate members 170. The holder 160 is composed of a pair of stacked plate members 170. Each plate member 170 includes a main body portion 171 and a wall portion 172. The main body portion 171 has a plate shape with an opening 173 formed therein. In this example, the main body portion 171 has a rectangular plate shape. The opening 173 is formed in the center of the main body portion 171 when viewed from the Z-axis direction. The main body portion 171 has an inner surface 171a that defines the opening 173. The opening 173 (inner surface 171a) has a circular shape when viewed from the Z-axis direction. When the pair of plate members 170 are stacked, the inner surface 171a of one plate member 170 and the inner surface 171a of the other plate member 170 form the inner surface 161a of the holder 160. When viewed from the Z-axis direction, the shape of the inner surface 171a is the same as the shape of the inner surface 161a.

[0084] The wall portion 172 extends along the outer edge of the main body portion 171 when viewed from the Z-axis direction. The wall portion 172 has an inner surface 172a, an outer surface 172b, and a top surface 172c. The inner surface 172a extends along the side surface 51c of the laser medium 50 when viewed from the Z-axis direction. In this example, the inner surface 172a has a circular shape when viewed from the Z-axis direction. When viewed from the Z-axis direction, the inner surface 172a is located outward from the side surface 51c of the laser medium 50. The outer surface 172b has a rectangular shape when viewed from the Z-axis direction. When a pair of plate members 170 are stacked, the outer surface 172b of one plate member 170 and the outer surface 172b of the other plate member 170 form the outer surface 161b of the holder 160.

[0085] The pair of plate members 170 are stacked on top of each other so that the top surface 172c of one plate member 170 contacts the top surface 172c of the other plate member 170. A gap corresponding to the height of the two wall portions 172 in the Z-axis direction is formed between one main body portion 171 and the other main body portion 171. The material of the holder 160 (each plate member 170) may be the same as the material of the holder 60 according to the first embodiment.

[0086] A recess 162 is formed in the holder 160. The recess 162 is formed on an inner surface 161a. As shown in FIG. 19 , the recess 162 is continuously formed in a circular ring shape along a direction A surrounding the center C of the laser medium 50 when viewed from the Z-axis direction. The recess 162 is formed to be point-symmetric with respect to the center C of the laser medium 50 when viewed from the Z-axis direction. In other words, the arrangement (shape) of the recess 162 when rotated 180° about the center C matches the arrangement (shape) before rotation. The recess 162 is formed to be line-symmetric with respect to a line LA passing through the center C when viewed from the Z-axis direction. The recess 162 opens toward the center C of the laser medium 50 (toward the inner region) when viewed from the Z-axis direction. The recess 162 has an opening 162a on the inner surface 161a. A bottom 162b of the recess 162 is formed by the inner surfaces 172a of two wall portions 172. The width of the recess 162 in the Z-axis direction is constant, that is, the width of the recess 162 in the Z-axis direction does not change from the opening 162a toward the bottom 162b.

[0087] The holder 160 surrounds the laser medium 50 when viewed from the Z-axis direction. When viewed from the Z-axis direction, the diameter of the laser medium 50 is larger than the diameter of the inner surface 161a of the holder 160 and smaller than the diameter of the bottom 162b of the recess 162 (the inner surface 172a of the wall portion 172). When viewed from the Z-axis direction, the outer edge of the laser medium 50 is located between a pair of plate members 170 (main body portions 171). The holder 160 holds the laser medium 50 by sandwiching the laser medium 50 between the pair of plate members 170 in the Z-axis direction. The holder 160 is in contact with each of a pair of end faces 51a, 51b of the laser medium 50. Specifically, one plate member 170 is in direct contact with the end face 51a, and the other plate member 170 is in direct contact with the end face 51b. The laser medium 50 is sandwiched between the holder 160 (a pair of plate members 170), thereby restricting movement of the laser medium 50 in the Z-axis direction and in a direction perpendicular to the Z-axis direction.

[0088] As shown in FIG. 20 , the holder 160 has a pair of restricting portions 163. Each restricting portion 163 is integrally formed with the main body portion 171 of the corresponding plate member 170. The restricting portion 163 extends in an annular shape when viewed from the Z-axis direction. In this example, the restricting portion 163 extends along the side surface 51 c of the laser medium 50 when viewed from the Z-axis direction. The restricting portion 163 is a convex portion that protrudes from the inner surface 162 c of the recess 162 toward the space S2 (inside the recess 62). The restricting portion 163 is in contact with the side surface 51 c of the laser medium 50. The restricting portion 163 restricts movement of the laser medium 50 in a direction perpendicular to the Z-axis direction.

[0089] The holder 160 holds the laser medium 50 so that a space S2 is formed between the side surface 51c of the laser medium 50 and the inner surface 162c of the recess 162. The side surface 51c does not include a region (contact region) that contacts the holder 160 (inner surface 161a). In other words, the side surface 51c only includes a region (non-contact region) that does not contact the holder 160.

[0090] In this embodiment, the holder 160 includes a light absorbing portion 165a formed by a light absorber 165. The light absorbing portion 165a is formed in the recess 162. The light absorbing portion 165a absorbs spontaneously emitted light generated in the laser medium 50. The light absorbing portion 165a has an absorptivity for spontaneously emitted light of 0.1% or more, more preferably 1% or more, even more preferably 5% or more, even more preferably 10% or more, and even more preferably 50% or more. The material of the light absorbing portion 165a (light absorber 165) may be, for example, Cr-doped YAG. The light absorber 165 extends in an annular shape when viewed from the Z-axis direction. In this example, the light absorber 165 extends in an annular shape. The center of the light absorber 165 when viewed from the Z-axis direction coincides with the center C of the laser medium 50. The light absorber 165 is continuously disposed on the bottom 162b of the recess 162. The light absorber 165 is located in the recess 162 away from the side surface 51 c of the laser medium 50. A space S2 exists between the light absorber 165 and the side surface 51 c. Air exists in the space S2 between the light absorber 165 and the side surface 51 c, and no solid or liquid substance is disposed therein.

[0091] In the laser medium unit 10A according to the second embodiment, the holder 160 holds the laser medium 50 such that a space S2 is formed between the side surface 51c of the laser medium 50 and the inner surface 162c of the recess 162 formed in the holder 160. As a result, according to the laser medium unit 10A, for the same reasons as in the laser medium unit 10 according to the first embodiment, it is possible to improve the quality of the laser light L1 while suppressing parasitic oscillation.

[0092] When viewed from the Z-axis direction, the recess 162 is formed to be point-symmetrical with respect to the center C of the laser medium 50. As a result, even if the temperature of the inner surface 162c of the recess 162 rises due to absorption of spontaneous emission light, the recess 162 is formed to be point-symmetrical with respect to the center C of the laser medium 50, and therefore, it is possible to suppress non-uniformity in the temperature distribution in the excitation region 52 of the laser medium 50, and it is possible to improve the quality of the laser light L1.

[0093] The holder 160 is provided with a light absorbing portion 165a formed in the recess 162, which absorbs spontaneously emitted light generated in the laser medium 50. As a result, the spontaneously emitted light emitted from the side surface 51c of the laser medium 50 into the space S2 is absorbed by the light absorbing portion 165a. This makes it possible to further suppress parasitic oscillation.

[0094] The light absorbing portion 165a is formed of a light absorber 165, and the light absorber 165 is disposed in the recess 162 away from the laser medium 50. As a result, even if heat is generated in the light absorber 165 due to absorption of spontaneously emitted light, the generated heat is less likely to be transmitted to the laser medium 50 because the light absorber 165 is disposed away from the laser medium 50. As a result, it is possible to suppress non-uniformity in the temperature distribution in the excitation region 52 of the laser medium 50, and it is possible to improve the quality of the laser light L1.

[0095] The holder 160 is in contact with each of the pair of end faces 51a, 51b of the laser medium 50. This allows the holder 160 to hold the laser medium 50 in a more stable state.

[0096] The holder 160 has a restricting portion 163 that comes into contact with the side surface 51c of the laser medium 50 and restricts movement of the laser medium 50 in a direction perpendicular to the Z-axis direction. The restricting portion 163 restricts movement of the laser medium 50, making it possible to more reliably form a predetermined space S2 between the side surface 51c of the laser medium 50 and the recess 162 of the holder 160. [Modification of the Second Embodiment]

[0097] Next, modifications of the laser medium unit 10A according to the second embodiment will be described. First, a laser medium unit 10A according to a third modification will be described with reference to Figures 21 and 22. In the third modification, the shapes of the laser medium and holder are different from the shapes of the laser medium 50 and holder 160 according to the second embodiment. Below, a laser medium 250 and holder 260 according to the third modification will be described, but a description of points in common with the laser medium 50 and holder 160 according to the second embodiment will sometimes be omitted.

[0098] The laser medium 250 has a pair of end faces 251a, 251b and a side face 251c. The laser medium 250 has a thickness in the Z-axis direction and is plate-shaped with the pair of end faces 251a, 251b as its main faces. In this example, the laser medium 250 is rectangular plate-shaped. The pair of end faces 251a, 251b intersect in the Z-axis direction. The end face 251b is located on the opposite side of the end face 251a in the Z-axis direction.

[0099] The side surface 251c of the laser medium 250 intersects with the pair of end surfaces 251a and 251b. The side surface 251c is aligned along the Z-axis direction. When viewed from a direction intersecting the pair of end surfaces 251a and 251b (the Z-axis direction), the side surface 251c defines the outer edge of the laser medium 250. That is, the side surface 251c has a rectangular shape when viewed from the Z-axis direction. When viewed from the Z-axis direction, the center of the rectangle defined by the side surface 251c coincides with the center C of the laser medium 250. When viewed from the Z-axis direction, the length of one side of the laser medium 250 is, for example, approximately 100 mm. The material of the laser medium 250 may be the same as the material of the laser medium 50.

[0100] The holder 260 is disposed along the side surface 251c of the laser medium 250 when viewed from the Z-axis direction, and holds the laser medium 250. In this example, the holder 260 is formed in a rectangular frame shape that surrounds the side surface 251c of the laser medium 250 when viewed from the Z-axis direction. The surface 261 of the holder 260 has an inner surface 261a, an outer surface 261b, and a pair of side surfaces 261c and 261d. The inner surface 261a extends along the side surface 251c of the laser medium 250 when viewed from the Z-axis direction. In this example, the inner surface 261a has a rectangular shape when viewed from the Z-axis direction. When viewed from the Z-axis direction, the inner surface 261a is located inside (closer to the center C) than the side surface 251c of the laser medium 250. The outer surface 261b has a rectangular shape when viewed from the Z-axis direction.

[0101] The holder 260 has a pair of plate members 270. The holder 260 is formed by a pair of stacked plate members 270. Each plate member 270 includes a main body portion 271 and a wall portion 272. The main body portion 271 has a plate shape with an opening 273 formed therein. In this example, the main body portion 271 has a rectangular plate shape. The main body portion 271 has an inner surface 271a that defines the opening 273. The opening 273 (inner surface 271a) has a rectangular shape when viewed from the Z-axis direction. When the pair of plate members 270 are stacked, the inner surface 271a of one plate member 270 and the inner surface 271a of the other plate member 270 form the inner surface 261a of the holder 260.

[0102] The wall portion 272 extends along the outer edge of the main body portion 271 when viewed from the Z-axis direction. The wall portion 272 has an inner surface 272a, an outer surface 272b, and a top surface 272c. The inner surface 272a extends along the side surface 251c of the laser medium 250 when viewed from the Z-axis direction. In this example, the inner surface 272a has a rectangular shape when viewed from the Z-axis direction. When viewed from the Z-axis direction, the inner surface 272a is located outward from the side surface 251c of the laser medium 250. The outer surface 272b has a rectangular shape when viewed from the Z-axis direction. When a pair of plate members 270 are stacked, the outer surface 272b of one plate member 270 and the outer surface 272b of the other plate member 270 form the outer surface 261b of the holder 260. The material of the retainer 260 (each plate member 270) may be similar to the material of the retainer 160 (i.e., the retainer 60).

[0103] A recess 262 is formed in the holder 260. The recess 262 is formed on an inner surface 261a. As shown in FIG. 21 , the recess 262 is formed continuously in a rectangular ring shape along a direction A surrounding the center C of the laser medium 250 when viewed from the Z-axis direction. The recess 262 has an opening 262a on the inner surface 261a. A bottom 262b of the recess 262 is formed by the inner surfaces 272a of the two wall portions 272.

[0104] The holder 260 surrounds the laser medium 250 when viewed from the Z-axis direction. When viewed from the Z-axis direction, the size of the laser medium 250 is larger than the rectangle defined by the inner surface 261a of the holder 260 and smaller than the rectangle defined by the bottom 262b of the recess 262 (the inner surface 272a of the wall portion 272). The holder 260 holds the laser medium 250 by sandwiching the laser medium 250 in the Z-axis direction between a pair of plate members 270. The holder 260 is in contact with each of a pair of end faces 251a, 251b of the laser medium 250. Specifically, one plate member 270 is in direct contact with the end face 251a, and the other plate member 270 is in direct contact with the end face 251b.

[0105] As shown in FIG. 22 , the holder 260 has a pair of restricting portions 263. Each restricting portion 263 is integrally formed with the main body portion 271 of the corresponding plate member 270. When viewed from the Z-axis direction, the restricting portion 263 extends in a rectangular ring shape. In this example, when viewed from the Z-axis direction, the restricting portion 263 extends along the side surface 251 c of the laser medium 250. The restricting portion 263 is a convex portion that protrudes from the inner surface 262 c of the recess 262 toward the space S2 (inside the recess 262). The restricting portion 263 is in contact with the side surface 251 c of the laser medium 250. The restricting portion 263 restricts movement of the laser medium 250 in a direction perpendicular to the Z-axis direction. The holder 260 holds the laser medium 250 such that a space S2 is formed between the side surface 251 c of the laser medium 250 and the inner surface 262 c of the recess 262.

[0106] In this embodiment, the holder 260 includes a light absorbing portion 265a formed by a light absorber 265. The light absorbing portion 265a is formed in the recess 262. The light absorbing portion 265a absorbs spontaneously emitted light generated in the laser medium 250. The absorptance of the light absorbing portion 265a for spontaneously emitted light is 0.1% or more, more preferably 1% or more, even more preferably 5% or more, even more preferably 10% or more, and even more preferably 50% or more. The light absorber 265 extends in a rectangular ring shape when viewed from the Z-axis direction. The center of the light absorber 265 when viewed from the Z-axis direction coincides with the center C of the laser medium 250. The light absorber 265 is continuously disposed on the bottom 262b of the recess 262. The light absorber 265 is located in the recess 262 away from the side surface 251c of the laser medium 250. A space S2 exists between the light absorber 265 and the side surface 251c.

[0107] In the laser medium unit 10A according to the third modification, the holder 260 holds the laser medium 250 such that a space S2 is formed between the side surface 251c of the laser medium 250 and the inner surface 262c of the recess 262 formed in the holder 260. This makes it possible to improve the quality of the laser light L1 while suppressing parasitic oscillation for the same reasons as in the laser medium unit 10A according to the second embodiment. [Modification]

[0108] The present disclosure is not limited to the above-described embodiments and modifications. For example, as shown in FIG. 23 , the holder may have a light-absorbing paint 365 applied to the inner surface of the recess as a light-absorbing portion 365a. That is, the light-absorbing portion 365a is formed of the light-absorbing paint 365. Part (a) of FIG. 23 illustrates a configuration in which the holder 60 according to the first embodiment includes the light-absorbing paint 365. The light-absorbing paint 365 absorbs spontaneously emitted light generated in the laser medium 50. The absorptivity of the light-absorbing portion 365a (light-absorbing paint 365) with respect to spontaneously emitted light is 0.1% or more, more preferably 1% or more, even more preferably 5% or more, even more preferably 10% or more, and even more preferably 50% or more. The light-absorbing paint 365 is, for example, a black paint containing metal oxide, carbon black, or the like. The light-absorbing paint 365 is applied to the inner surface 62c of the recess 62. In this example, the light-absorbing paint 365 is applied to the entire inner surface 62c, but it may be applied to only a portion of the inner surface 62c. A space S2 is formed between the light-absorbing paint 365 and the side surface 51c of the laser medium 50. By applying the light-absorbing paint 365 to the inner surface 62c, spontaneously emitted light emitted from the side surface 51c of the laser medium 50 into the space S2 is absorbed (attenuated) by the light-absorbing paint 365. This further suppresses parasitic oscillation. Furthermore, by forming the space S2 between the light-absorbing paint 365 and the side surface 51c of the laser medium 50, even if heat is generated in the light-absorbing paint 365 due to the absorption of spontaneously emitted light, the heat is less likely to be transmitted from the light-absorbing paint 365 to the excitation region 52. This suppresses uneven temperature distribution in the excitation region 52 and improves the quality of the laser light L1. The light-absorbing portion 365a is formed by the light-absorbing paint 365, making it easy to form the light-absorbing portion 365a.

[0109] FIG. 23B shows a configuration in which the holder 160 according to the second embodiment has a light-absorbing paint 365 instead of the light absorber 165. Similar to the example shown in FIG. 23A, the light-absorbing paint 365 is applied to the inner surface 162c of the recess 162 as a light-absorbing portion 365a. In this example, the light-absorbing paint 365 is applied to the entire inner surface 162c, but it may also be applied to only a portion of the inner surface 162c. A space S2 is formed between the light-absorbing paint 365 and the side surface 51c of the laser medium 50. In the example shown in FIG. 23B, similar to the example shown in FIG. 23A, parasitic oscillation can be further suppressed and the quality of the laser light L1 can be improved. Because the light-absorbing portion 365a is formed of the light-absorbing paint 365, the light-absorbing portion 365a can be easily formed.

[0110] 24 , the laser medium 50 (including the laser medium 250) may have a light-transmitting region 55 having an absorptance of 10% or less for spontaneously emitted light generated in the laser medium 50. The light-transmitting region 55 is made of YAG that is not doped with an additive such as Yb or Nd (non-doped YAG). The light-transmitting region 55 is provided in an annular shape along the side surface 51 c of the laser medium 50 when viewed from the Z-axis direction. In the laser medium 50, the light-transmitting region 55 is located outside (away from the center C) the region 56 made of YAG doped with an additive.

[0111] FIG. 25 is a diagram schematically illustrating the path of spontaneously emitted light L10 in a laser medium 50 having a configuration similar to that of the example illustrated in FIG. 24 . In FIG. 25 , the spontaneously emitted light L10 is indicated by a dashed line, but for ease of explanation, a portion of the spontaneously emitted light L10 is illustrated by a thick solid line. In the example illustrated in FIG. 25 , the spontaneously emitted light L10 generated in the excitation region 52 is incident on the side surface 51c. When the spontaneously emitted light L10 is incident on the side surface 51c at an angle greater than the total reflection angle with respect to the normal to the side surface 51c when viewed from the Z-axis direction, the spontaneously emitted light L10 is reflected by the side surface 51c. Because the side surface 51c has a circular shape when viewed from the Z-axis direction, the spontaneously emitted light L10 reflected by the side surface 51c continues to be reflected by the side surface 51c at the same angle thereafter (looping in the laser medium 50).

[0112] As shown in the modified examples in FIGS. 24 and 25 , the laser medium 50 includes the light-transmitting region 55. This increases the distance from the excitation region 52 to the side surface 51 c compared to a configuration without the light-transmitting region 55, thereby reducing the distance the looping spontaneous emission light L10 travels through the excitation region 52. This reduces parasitic oscillation. Furthermore, the light-transmitting region 55 has a low absorptivity for spontaneous emission light, thereby reducing heat generation in the light-transmitting region 55 due to absorption of spontaneous emission light. Even if the holder 60 absorbs the spontaneous emission light L10 that has passed through the light-transmitting region 55 and generates heat in the holder 60, the light-transmitting region 55 functions as a buffer, making it difficult for the heat to be transferred from the holder 60 to the excitation region 52. This reduces the unevenness of the temperature distribution in the excitation region 52, thereby improving the quality of the laser light L1. When viewed from the Z-axis direction, the outer edge of the light-transmitting region 55 is not limited to a circular shape, but may also be polygonal. 21 may have a light-transmitting region 55 with a rectangular outer edge. When the outer edge of the light-transmitting region 55 is polygonal, it is preferable that the outer edge of the light-transmitting region 55 has a polygonal shape other than a square (a polygon with many vertices that are close to a circle) from the viewpoint of suppressing the spontaneous emission light L10 from passing through the excitation region 52.

[0113] As shown in FIG. 26 , the laser medium 50 (including the laser medium 250) may have a light-absorbing region 57 having an absorptance of 0.1% or more and less than 90% for spontaneously emitted light generated in the laser medium 50. In this example, the absorptance is approximately 30%. The light-absorbing region 57 is, for example, YAG doped with Cr. The absorptance of the light-absorbing region 57 is adjusted by the additive rate (the amount of Cr doped into the YAG). The light-absorbing region 57 is provided in an annular shape along the side surface 51 c of the laser medium 50 when viewed from the Z-axis direction. The absorptance of the spontaneously emitted light in the light-absorbing region 57 is higher than the absorptance of the spontaneously emitted light in a region 58 located inside the light-absorbing region 57 when viewed from the Z-axis direction.

[0114] When the laser medium 50 has a light absorption region 57 as in the modified example shown in FIG. 26 , the spontaneous emission light generated in the excitation region 52 is appropriately absorbed by the light absorption region 57, and the spontaneous emission light transmitted through the light absorption region 57 is emitted, for example, into the space S2, thereby suppressing parasitic oscillation. Furthermore, the absorptance of the spontaneous emission light in the light absorption region 57 is less than 90%. In this case, excessive temperature rise in the light absorption region 57 due to absorption of the spontaneous emission light is suppressed, and uneven temperature distribution in the excitation region 52 can be suppressed, thereby improving the quality of the laser light L1. The upper limit of the absorptance of the spontaneous emission light in the light absorption region 57 is less than 90%, more preferably 70% or less, and even more preferably 50% or less. The lower limit of the absorptance of the spontaneous emission light in the light absorption region 57 is 0.1% or more, more preferably 1% or more, even more preferably 5% or more, even more preferably 10% or more, and even more preferably 20% or more. By setting the absorptance to 0.1% or more, the spontaneously emitted light is gradually absorbed by the light absorbing region 57, and as a result, parasitic oscillation can be suppressed.

[0115] The inner surface 62c (including the inner surfaces 162c and 262c) of the recess 62 may be a rough surface (sandy ground) having irregularities. This makes it easier for spontaneous emission light emitted from the side surface 51c of the laser medium 50 into the space S2 to be scattered by the inner surface 62c, thereby further suppressing parasitic oscillation. The side surface 51c (including the side surface 251c) of the laser medium 50 may be a mirror surface, a mirror surface coated with an AR coating (an anti-reflection film formed on the mirror surface), or a rough surface.

[0116] The holder 60 (including the holders 160 and 260) does not have to surround the entire laser medium 50 when viewed from the Z-axis direction, and may be arranged along only a portion of the side surface 51 c (including the side surface 251 c of the laser medium 250) of the laser medium 50. For example, in a third modified example shown in Fig. 21 , the holder 260 may have only a pair of portions 268 along a pair of regions of the side surface 251 c that extend in the X-axis direction, and the pair of portions 268 may be arranged to sandwich the laser medium 250 in the Y-axis direction.

[0117] The proportion of the region R1 to the entire side surface 51c is not limited to the above example. The proportion of the region R1 to the entire side surface 51c may be 30% or more and 40% or less. The proportion of the region R1 to the entire side surface 51c may be greater than 90% or less than 5%.

[0118] The shape of the recess 62 is not limited to an elliptical cone shape. The recess 62 may have, for example, a cone shape or a pyramid shape. Examples of pyramid shapes include a triangular pyramid shape, a square pyramid shape, and a pentagonal pyramid shape.

[0119] 6 , the holder 60 has a non-contact portion 66 that does not contact the laser medium 50, and the non-contact portion 66 may be made of a different material from the contact portion 63. For example, the contact portion 63 may be made of resin, and the non-contact portion 66 may be made of metal. In this case, the contact portion 63 that contacts the laser medium 50 provides thermal insulation between the laser medium 50 and the non-contact portion 66, and the non-contact portion 66 is made of metal that has a higher thermal conductivity than resin, so that heat transferred from the laser medium 50 to the holder 60 via the space S2 can be quickly released to the outside.

[0120] The holder 160 may not have one or both of the pair of restricting portions 163. Each restricting portion 163 may be formed discontinuously. That is, a plurality of restricting portions 163 (protrusions) may be formed on each plate member 170. The holder 260 may not have one or both of the pair of restricting portions 263. Each restricting portion 263 may be formed discontinuously. That is, a plurality of restricting portions 263 (protrusions) may be formed on each plate member 270.

[0121] The light absorber 165 may be arranged discontinuously on the bottom 162b of the recess 162. That is, a plurality of light absorbers 165 may be arranged at intervals on the bottom 162b of the recess 162. The light absorber 265 may be arranged discontinuously on the bottom 262b of the recess 262. That is, a plurality of light absorbers 265 may be arranged at intervals on the bottom 262b of the recess 262.

[0122] 10, 10A...laser medium unit, 50, 250...laser medium, 51a, 51b, 251a, 251b...end surface, 51c, 251c...side surface, 60, 160, 260...holding body, 61, 161, 261...surface, 62, 162, 262...recess, 62a...opening, 62b...bottom, 62c, 162c, 262c...inner surface, 63...contact portion, 64...through hole, 65, 165, 265...light absorber, 65a, 165a, 265a...light absorbing portion, 163, 263...regulating portion, 365...light absorbing paint, R1...region (contact region), R3...region, S2...space.

Claims

1. A laser medium unit comprising: a laser medium having a pair of end faces and side faces intersecting the pair of end faces; and a holder that is arranged along the side faces when viewed from a direction intersecting the pair of end faces and holds the laser medium, wherein the holder has a recess that opens towards the center of the laser medium when viewed from the direction intersecting the pair of end faces, and the holder holds the laser medium so that a space is formed between the side faces and the inner surface of the recess.

2. The laser medium unit according to claim 1, wherein the holder is formed in a frame shape surrounding the laser medium when viewed from the direction intersecting the pair of end faces.

3. The laser medium unit according to claim 1 or 2, wherein the recess has a shape that tapers toward the bottom of the recess.

4. The laser medium unit according to any one of claims 1 to 3, wherein the holder is in contact with the side surface.

5. The laser medium unit according to any one of claims 1 to 4, wherein the contact area of ​​the side surface with the holder accounts for 90% or less of the entire side surface.

6. The laser medium unit according to any one of claims 1 to 5, wherein the contact area of ​​the side surface with the holder accounts for 5% or more of the entire side surface.

7. A laser medium unit according to any one of claims 1 to 6, wherein the recess is formed so as to be point-symmetric with respect to the center of the laser medium when viewed from the direction intersecting the pair of end faces.

8. The laser medium unit according to any one of claims 1 to 7, wherein a plurality of recesses, each of which is the recess, are formed in the holder.

9. The laser medium unit according to claim 8, wherein the plurality of recesses are formed so as to be point-symmetric with respect to the center of the laser medium when viewed from the direction intersecting the pair of end faces.

10. The laser medium unit according to claim 8 or 9, wherein a plurality of areas on the surface of the holder that are located between the openings of the plurality of recesses are in contact with the side surface.

11. The laser medium unit according to any one of claims 1 to 10, wherein the holder has a through-hole formed therein that connects the space to an external space of the holder.

12. The laser medium unit according to any one of claims 1 to 11, wherein the holder has a contact portion that comes into contact with the laser medium, and the contact portion is made of resin.

13. The laser medium unit according to claim 12, wherein the holder has a non-contact portion that does not contact the laser medium, and the non-contact portion is made of metal.

14. The laser medium unit according to any one of claims 1 to 13, wherein the holder has a light absorbing portion formed in the recess for absorbing spontaneously emitted light generated in the laser medium.

15. The laser medium unit according to claim 14, wherein the light absorbing portion is formed of a light absorber, and the light absorber is disposed in the recess away from the laser medium.

16. The laser medium unit according to claim 14, wherein the light absorbing portion is formed of a light absorbing paint.

17. A laser medium unit according to any one of claims 1 to 16, wherein the laser medium is configured so as not to include a light absorbing region having an absorption rate of 90% or more for spontaneously emitted light generated in the laser medium.

18. A laser medium unit according to any one of claims 1 to 17, wherein the laser medium includes a light absorbing region having an absorption rate of less than 90% for spontaneously emitted light generated in the laser medium.

19. The laser medium unit according to any one of claims 1 to 18, wherein the holder is in contact with each of the pair of end faces.

20. The laser medium unit according to claim 19, wherein the holder has a restricting portion that contacts the side surface and restricts movement of the laser medium in a direction perpendicular to the direction intersecting the pair of end faces.

Images