LU101456A1

LU101456A1 - A method and a device for heat removal from a flat NIR-MIR laser mirror

Info

Publication number: LU101456A1
Application number: LU101456A
Authority: LU
Inventors: Reza Amani; Jan CVRCEK; Martin Smrz
Original assignee: Fyzikalni Ustav Av Cr V V I
Priority date: 2019-10-29
Filing date: 2019-10-29
Publication date: 2020-10-30
Also published as: WO2021083437A1; LU101456B1

Abstract

The present invention pertains a method and a device for heat removal from optical elements. A method and corresponding device comprises the steps / means for of: attaching the mirror (2) to a heat sink (1) via epoxy layer; and providing a cooling medium flowing through a microchannel provided in the heat sink (1).

Description

| CITT ref.: H19001 | A method and a device for heat removal from a flat NIR-MIR laser mirror 1U107456 ; Technical field ; [1] The present invention pertains to optics. In particular, the present invention is | dedicated to a method and a device for heat removal from a flat NIR-MIR laser mirror, ; 5 more particularly to cooling elements used in a laser system.

; Background of the art ; [2] High-average-power laser systems are suffering from the heat generated in laser ; components. Usually, these laser components such as mirrors, gratings, or Q- : switchers are placed in water-cooled mounts. Between the laser components and the | 10 mounts, a sheet of graphite or indium is usually used. This technique of heat removal | is not very efficient and has resulted in a poor beam pointing in most high-average- power laser systems especially when the average power approaches the kW-class | regime.

; [3] Document US 2005/0019694 discloses a laser amplifying system. The system | 15 comprises heat sink. À laser disc is attached to the heat sink via adhesive layer. In ; between the heat sink and the laser, a reflective layer can be provided. The similar solution is disclosed in document US 2015/0171586. The document relates to a solid- É state laser arrangement capable to withstand high-thermal loads.

| [4] The above mentioned documents disclose a general feature (plate-like solid-state body) and species, an active medium as an embodiment to be cooled.

[6] A thin mirror is known by the skilled person in the art as a mirror with a thickness less than 1 mm with a high thermal conductivity in the substrate.

[6] A flat thin mirror is known by the skilled person in the art as a mirror with a peak to valley (PV) surface deformation less than 60 nm in a clear aperture higher than 80% of the diameter of the optic. Typical radius of curvature is higher than 1 km.

[7] NIR mirror, resp. NIR thin mirror, resp. NIR flat thin mirror are known by skilled person in the art that they are such mirrors capable to reflect radiation having wavelength in near infrared spectrum, it is usually accepted that wavelength of NIR is from 0.8 um | to 1.3 um. Page 1 from 11 Era pr EEE SE EE SE ER

| CITT ref.: H19001 | [8] Mid-IR mirror, resp. mid-IR thin mirror, resp. mid-IR flat thin mirror are known by skilled lu101456 | person in the art that they are such mirrors capable to reflect radiation having | wavelength in mid-wave infrared spectrum, it is usually accepted that wavelength of | mid-IR is from 2 um to 5 um. | 5 [9] Due to overheating, which is generally considered as a heat corresponding to | temperature above 50°C, it also provides several complications such as strong ; fluorescence; coating absorption. ; [10] To avoid heating up of laser mirrors in a high-average-power laser system, it is ; proposed the below defined invention to enhance the performance of a kW-class | 10 laser system. Therefore, it is an object of the present invention to provide better | solution to the currently known in the state-of-the-art. | Description of the Invention | [11] According to a first aspect of the present invention, the above-mentioned problem is ; solved by a method for heat removal from an optical element. The method comprises : 15 steps of: ; attaching the mirror to a polished heat sink via epoxy layer, the epoxy layer having . lower coefficient of thermal expansion comparable to that of material from which the ; heat sink is made of; and | providing a cooling medium flowing through a microchannel provided in the heat sink, . 20 preferably created by laser micro-machining. It shall be understood that the microchannels may be created by other techniques as well. Epoxy layer has various thermal expansion coefficients depending on material. However, it is essential for the present invention to have the thermal expansion coefficient lower than material from which the heat sink is made of. For example, if the material of heat sink is copper, the epoxy layer shall have thermal expansion coefficient lower and preferably being comparable to that of copper.

[12] The above mentioned method can efficiently remove heat from the surface, which may further comprises a coating, and provide good pointing stability of a laser which is not affected by vibration of mirrors due to accumulated heat thereon. Lower coefficient of thermal expansion than that of metal from which the heat sink is made of avoids deformation of the optics under high heat load. A polished heat sink provides less deformation on the mirror, especially on the flat mirror, after bonding and ensures a Page 2 from 11 rr re ae re RE

| CITT ref. H19001 | thinner epoxy layer between the thin mirror and the heat sink. Moreover, it assists, | 101456 ; easier flow of the epoxy layer through spin coating technique.

| [13] Mirror, resp. mid-IR mirror, thin mirror, mid-IR flat thin mirror are species in respect to | flat plate-like solid-state body or reflector.

; 5 [14] In a preferred embodiment, the steps are providing for a mid-IR thin mirror, preferably ; flat thin mirror. The definition of mid-IR thin mirror, resp. thin mirror are adopted from | prior art, are generally known by the skilled person in the art and are available in ; textbooks.

[15] In another preferred embodiment which can be combined with any of the mentioned | 10 embodiments, water is flowing through the microchannel. | [16] In another preferred embodiment which can be combined with any of the mentioned . embodiments, the cooling medium is flowing through the microchannel which is from ° metal, preferably Cu, Ni, CuW, or Invar. Preferably the material of heat sink and the 3 material of the microchannel is the same.

[17] According to a second aspect of the present invention, the problem is solved by a ; device for heat removal from a mirror, the device comprises: ; the mirror thermally attached to a polished heat sink via epoxy layer, the epoxy layer , having a lower coefficient of thermal expansion than material from which the heat sink ; is made of, wherein the heat sink comprises a microchannel, wherein cooling medium is ) 20 flowing therein. | [18] In a preferred embodiment, the heat sink is from a single body, including the . embodiment where the material of the heat sink and the microchannels is the same.

| [19] In another preferred embodiment which can be combined with any of the mentioned | embodiments, the heat sink comprises plurality of microchannel.

| 25 [20] In another preferred embodiment which can be combined with any of the mentioned embodiments, the heat sink comprising an inlet for the cooling medium with a large ; diameter so that a high flow rate (>1 I/min) of the cooling water is achieved.

[21] In another preferred embodiment which can be combined with any of the mentioned embodiments, the thickness of the epoxy layer is less than 5 um depending on the viscosity of the epoxy. A higher viscosity results in a thicker epoxy layer due to | Page 3 from 11

| CITT ref: H19001 | difficulty of epoxy flow on the polished heat sink. An epoxy with a low viscosity less lu101456 ; than 3 S (300 cPs), results in a thickness less than 1 um.

j [22] In another preferred embodiment which can be combined with any of the mentioned | embodiments, to improve the mirror flatness after bonding, the mirror thickness can | 5 be up to 1 mm in order to modify the curvature during the bonding process.

| [23] In another preferred embodiment which can be combined with any of the mentioned | embodiments, the mirror has acquired a concave shape using epoxy layer, the | curvature radius of the mirror is more than 1 km.

; [24] In another preferred embodiment which can be combined with any of the mentioned | 10 embodiments, wherein the mirror is a mid-IR thin mirror, preferably mid-IR flat thin ; mirror.

; [25] In another preferred embodiment which can be combined with any of the mentioned ‘ embodiments, the device further comprises a coating on the mirror.

| [26] According to another aspect of the present invention, the above-described device can ] 15 be used especially for heat-removal from the mid-IR mirror, especially in q-switched | laser.

. Brief description of drawings ' [27] Fig. 1 shows a top view and cross section of the device in accordance with the present invention.

| 20 Detailed description of embodiments : [28] To avoid heating up of laser mirror 2 in a high-average-power laser system, it is | proposed a device as described below, the device comprises a water-cooled thin ; mirror 2 to enhance the performance of a kW-class laser system.

[29] In accordance with the present invention, a high-power laser diode (2-3 kW) provides a | 25 laser beam directed to a flat thin mirror 2. The laser beam at mid-IR wavelength heated up the flat thin mirror 2. Thickness of the mirror 2 was 1 mm. The mirror 2 was | thermally attached to a heat sink having a micro-channel structure therein. Through the micro-channel structure, a cooling medium was flowing. In a preferred embodiment, the cooling medium is water. There is an epoxy layer 4 in-between the heat sink 1 and flat thin mirror 2 providing thermal conduction between those two Page 4 from 11

| CITT ref.: H19001 , elements. The epoxy layer 4 has coefficient of thermal expansion less than material 101456 ; from which the heat sink 1 is made of. For example, if the heat sink 1 is made of , copper, the epoxy layer has a thermal expansion of 16x10° m/(m K).

[30] The heat sink 1 must be made of thermally conductive material, preferably metal, more preferably Cu, Ni, CuW, or Invar. However, the thermal conductivity is implicit feature | of the heat sink 1. | [31] In a preferred embodiment, the heat sink 1 is made of single body. The microchannel is | preferably made of the same material as the heat sink, i.e. Cu, Ni, CuW, or Invar. . [32] More preferably, the heat sink 1 further comprises plurality of microchannel through ; 10 which the cooling medium, preferably water, flows. ; [33] The thickness of the epoxy layer 4 is ranging from 1 um to 5 um depending on the ; viscosity of the epoxy layer 4.

[34] To the heat sink 1, in particular to the plurality of microchannel, an inlet 3 is mounted. ) [35] The above-described device can be also used in a g-switched laser system for removal of heat from a saturable absorber with a coating 2.

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Claims

| CITT ref.: H19001 Claims lu101456 [Claim 1] A method for heat removal from a mirror (2), the method comprises the steps of: attaching the mirror (2) to a polished heat sink (1) via epoxy layer (4), the epoxy layer (4) having a coefficient of thermal expansion less than material from which the heat sink (1) is made of; and providing a cooling medium flowing through a microchannel provided in the heat sink (1). [Claim 2] The method according to claim 1, wherein the steps are providing for a mid-IR thin mirror, preferably flat thin mirror. [Claim 3] The method according to anyone of the preceding claim, wherein water is flowing through microchannel. [Claim 4] The method according to anyone of the preceding claim, wherein the cooling | medium is flowing through the microchannel made of metal, preferably Cu, Ni, | CuW, or Invar. [Claim 5] A device for heat removal from a mirror (2), the device comprises the mirror (2) thermally attached to a polished heat sink (1) via epoxy layer (4), the epoxy layer (4) having a lower coefficient of thermal expansion than material | from which the heat sink (1) is made of, wherein | the heat sink (1) comprises a microchannel, wherein cooling medium is flowing | therein. | [Claim 8] The device according to claim 5 wherein the heat sink (1) is from a single body. [Claim 7] The device according to claim 5 or 6 wherein the heat sink (1) comprises plurality | of microchannel. | [Claim 8] The device according to anyone of the claims 5 to 7 wherein the heat sink (1) is | from metal, preferably Cu, Ni, CUW, or Invar. [Claim 9] The device according to anyone of the claims 5 to 8 wherein the heat sink (1) | comprising an inlet (3) for the cooling medium. | [Claim 10] The device according to anyone of the claims 5 to 9, wherein the cooling medium | is water. | Page 6 from 11 | ee eee

| CITT ref.: H19001 | [Claim 11] The device according to anyone of the claims 5 — 10, wherein the thickness of 1101456 the epoxy layer (4) is less than 5 um. [Claim 12] The device according to anyone of the claim 5 — 11, wherein the mirror (2) is a mid-IR thin mirror, preferably mid-IR flat thin mirror. [Claim 13] The device according to anyone of the claim 5 — 12, wherein thickness of the mirror (2) is up to 1 mm. [Claim 14] The device according to anyone of the claim 5 — 13, wherein curvature radius of the mirror (2) is more than 1 km. [Claim 15] The device according to anyone of the claim 5 — 13 further comprises a coating on the mirror. [Claim 16] Use of the device according to anyone of the claims 5 — 14 for cooling a mirror or a saturable absorber with a coating (2) in a g-switched laser. | Hanes —~ Page 7 from 11