JP4211399B2 - Wavelength conversion element and fundamental wave processing method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a wavelength conversion element made of a nonlinear optical crystal such as lithium tetraborate (Li ₂ B ₄ O ₇ ) single crystal, and the converted light when wavelength conversion of a laser beam is performed using this wavelength conversion element. The present invention relates to a fundamental wave processing method that can be safely processed.
[0002]
[Prior art]
Nonlinear optical crystals of this type of wavelength conversion element include BBO (β-BaB ₂ O ₄ ), CLBO (CsLiB ₆ O ₁₀ ), KTP (KTiOPO ₄ ), LN (LiNbO ₃ ), LBO (LiB ₃ O ₅ ). KN (KNbO ₃ ) is known, and in particular, lithium tetraborate (Li ₂ B ₄ O ₇ ) single crystal originally has a wide transparent wavelength region on the short wavelength side, β-barium borate single crystal, etc. In addition to a large laser damage threshold as compared with the nonlinear optical crystal of SHG (second harmonic generation) and SFG (sum frequency generation), it is possible to perform wavelength conversion to a short wavelength region as deep UV region, There is no serious problem in deliquescence and workability, and there is an advantage that it is easy to handle as compared with other nonlinear optical crystals. Therefore, it has been attracting attention as a nonlinear optical crystal for wavelength conversion elements.
[0003]
When the wavelength conversion of the laser beam is performed by these wavelength conversion elements, the laser beam is incident from the incident end face of the wavelength conversion element and is emitted from the emission end face. Two methods have been adopted as to how much to tilt with respect to the direction. The first method is to match the normal line of the emission end face of the wavelength conversion element with the traveling direction of the laser beam, and the second method is, for example, as disclosed in Patent Document 1, the wavelength conversion element. The normal line of the emission end face is slightly inclined with respect to the traveling direction of the laser beam.
[0004]
[Patent Document 1]
Japanese Patent Application Laid-Open No. 2001-296569 (paragraph [0006] column, FIG. 1)
[0005]
[Problems to be solved by the invention]
However, in the first method, since the laser beam is incident at a right angle on the exit end face of the wavelength conversion element, the laser beam cross-sectional area at the exit end face is minimized and the power density is maximized. As a result, the laser output is particularly large. In this case (in the case of a short wavelength laser), the wavelength conversion element may be damaged.
[0006]
On the other hand, in the second method, since a part of the laser beam is reflected obliquely at the emission end face of the wavelength conversion element, the wavelength conversion element is locally heated by the reflected light, and the output becomes optically unstable. There was a bug.
[0007]
In view of such circumstances, a first object of the present invention is to provide a wavelength conversion element that does not cause problems in damage resistance and output stability even when used for wavelength conversion of a high-power laser. Furthermore, it is a second object to provide a fundamental wave processing method with high reliability at the time of wavelength conversion.
[0008]
[Means for Solving the Problems]
First, the present invention described in claim 1 has a block-shaped element body (4) made of a nonlinear optical crystal , and an incident end face (2) is provided at one end of the element body in the longitudinal direction. The other end of the element body in the longitudinal direction is inclined by an angle θ1 that is a polarization angle with respect to the converted light B2 of the laser beam with respect to the longitudinal direction of the element body . emitting end surface (3), the total reflection surfaces inclined by the longitudinal direction relative to the unconverted light B1 of the laser beam reflected by the outgoing end face totally reflected to return to the incident end face angle θ2 and (5) it is characterized in that the formed. Here, the number of the total reflection surfaces is not limited to one and may be plural. By setting the angle (θ1) as the polarization angle, when the converted light of the laser beam is incident on the emission end face of the wavelength conversion element, the ratio of the transmitted light is maximized.
[0009]
The present invention according to claim 2 is a lithium tetraborate (Li ₂ B) in which the longitudinal direction is inclined by a phase matching angle θm with respect to the C-axis direction as a nonlinear optical crystal constituting the element body (4). ₄ O ₇ ) A single crystal is used , the angle θm is set in the range of 65 to 70 °, the θ1 is set in the range of 50 to 70 °, and the θ2 is set in the range of 28 to 40 °. To do .
[0010]
By adopting these configurations, when converting the wavelength of the laser beam, the cross-sectional area of the laser beam by the output end face of the wavelength conversion element is enlarged, and the power density of the converted light is reduced in inverse proportion to it, and at the same time the reflection at the output end face It acts so that local heating of the wavelength conversion element by light is avoided.
[0012]
On the other hand, when the wavelength conversion element (1) according to claim 1 or 2 is used to convert the wavelength of a laser beam, the present invention described in claim 3 has an incident end face (2) in the wavelength conversion element. The unconverted light B1 is reflected at the exit end face (3) of the wavelength conversion element at a polarization angle with respect to the converted light B2, and the reflected light is reflected by the total reflection surface (5) of the wavelength conversion element. The light is totally reflected and returned to the incident end face (2) of the wavelength conversion element.
[0013]
According to a fourth aspect of the present invention, the wavelength conversion element (1) according to the first or second aspect is used to convert the wavelength of the second harmonic of the laser beam oscillated by the solid-state laser, thereby converting the fourth harmonic. When the laser beam is generated, a laser beam is incident on the wavelength conversion element from the incident end face (2), and the unconverted light B1 is reflected by the emission end face (3) of the wavelength conversion element at a polarization angle with respect to the converted light B2. The reflected light is totally reflected by the total reflection surface (5) of the wavelength conversion element and returned to the incident end face (2) of the wavelength conversion element. Here, “solid laser” includes YAG laser, ruby laser, YLF laser, Nd: YVO ₄ laser, and the like.
[0015]
By adopting these configurations, the reflected light at the exit end face of the wavelength conversion element is returned to the entrance end face so that it can be safely processed.
[0016]
Further, when to be reflected at the light output end face of the unconverted light B1 before Symbol laser beam wavelength conversion element (1) (3), by configuring so as to reflect the emission end face at a polarization angle with respect to the converted light, When the converted light of the laser beam is incident on the exit end face of the wavelength conversion element, the ratio of the transmitted light is maximized.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
1A and 1B are diagrams showing a first embodiment of a wavelength conversion element according to the present invention, in which FIG. 1A is a perspective view thereof, and FIG. 1B is a front view thereof.
[0018]
As shown in FIG. 1, the wavelength conversion element 1 has a block-shaped element body 4 made of a lithium tetraborate (Li ₂ B ₄ O ₇ ) single crystal. M and N directions) are inclined with respect to the C-axis direction by a predetermined phase matching angle θm (for example, 65 to 70 °). The element body 4 has a square incident end face 2 formed so as to be orthogonal to the longitudinal direction of the element body 4, and the rectangular exit end face 3 has a predetermined angle θ 1 with respect to the longitudinal direction of the element body 4. (E.g., 50-70 [deg.]). This angle θ1 forms a polarization angle (Brewster angle) with respect to the converted light B2 of the laser beam. Further, a rectangular total reflection surface 5 is formed in the element body 4 in the vicinity of the emission end face 3 so as to be inclined by a predetermined angle θ2 (for example, 28 to 40 °) with respect to the longitudinal direction of the element body 4. Yes. The incident end face 2, the outgoing end face 3 and the total reflection face 5 of the wavelength conversion element 1 are all polished with a surface accuracy of λ / 4 (1/4 of the wavelength of the laser beam) or more. At this time, the converted light B2 and the unconverted light B1 are respectively P-polarized light and S-polarized light with respect to the emission end face 3.
[0019]
Since the wavelength conversion element 1 has the above-described configuration, the second harmonic (visible light having a wavelength of 532 nm) of the laser beam with a wavelength of 1064 nm oscillated by the YAG laser is converted by the wavelength conversion element 1 to the fourth harmonic. When generating a wave (ultraviolet light having a wavelength of 266 nm), the laser beam is applied to the wavelength conversion element 1. At this time, the longitudinal direction of the element body 4 of the wavelength conversion element 1 is made to coincide with the irradiation direction of the laser beam. Then, as shown in FIG. 1 (b), the laser beam first enters the incident end face 2 of the wavelength conversion element 1, where there are two unconverted light and converted light, that is, the second harmonic (green having a wavelength of 532 nm). Light) B1 and fourth harmonic B2. Then, after the second harmonic B1 advances in the element body 4 in the longitudinal direction (arrow M direction), a part of the second harmonic wave B1 is refracted at the emission end face 3 and is transmitted to the outside of the element body 4, and the remaining part Is reflected by the emission end face 3 and further totally reflected by the total reflection face 5, then retreats in the longitudinal direction of the element body 4 (arrow N direction), and exits from the entry end face 2 to the outside of the element body 4. On the other hand, the fourth harmonic B2 advances in the element body 4 in a direction inclined by a predetermined walk-off angle from the longitudinal direction (direction of arrow M), and then refracts at the emission end face 3 and goes out of the element body 4. Go.
[0020]
As described above, the second harmonic B1 can be safely processed by returning the reflected light at the emission end face 3 of the wavelength conversion element 1 to the incident end face 2, and thus the reliability at the time of wavelength conversion can be improved. Further, since the fourth harmonic B2 is obliquely incident on the emission end face 3 of the wavelength conversion element 1, the cross-sectional area of the emission end face 3 is enlarged, and the power density is reduced in inverse proportion thereto, so that the laser output is large. However, damage to the wavelength conversion element 1 can be prevented. Further, although a part of the second harmonic B1 is reflected obliquely, the reflected light is totally reflected by the total reflection surface 5 and returns to the incident end face 2 side, so that the wavelength conversion element 1 is locally heated. Thus, the output stability of the wavelength conversion element 1 can be improved. Furthermore, since the angle at which the fourth harmonic B2 is incident on the output end face 3 is a polarization angle, the ratio of the transmitted light is the maximum (the ratio of the reflected light is the minimum). The output of the obtained fourth harmonic B2 can be increased to the maximum.
[0021]
In the above-described embodiment, the wavelength conversion element 1 having the element body 4 made of lithium tetraborate (Li ₂ B ₄ O ₇ ) single crystal has been described. However, as the material of the element body 4, lithium tetraborate ( Non-linear optical crystals (for example, β-barium borate single crystal) other than Li ₂ B ₄ O ₇ ) single crystal may be adopted.
[0022]
In the above-described embodiment, the wavelength conversion element 1 having one total reflection surface 5 formed on the element body 4 has been described. However, the number of the total reflection surfaces 5 is not limited to one, and as shown in FIG. Two total reflection surfaces 5 and 6 may be formed, or three or more total reflection surfaces (not shown) may be formed.
[0023]
Moreover, although the above-mentioned embodiment demonstrated the case where 2nd harmonic B1 totally reflected by the total reflection surface 5 was returned in parallel with the longitudinal direction (arrow N direction) of the element main body 4, this 2nd harmonic B1 is It is not always necessary to return the element body 4 in parallel with the longitudinal direction, and as long as the element can be returned to the incident end face 2, it may be returned with an inclination with respect to the longitudinal direction of the element body 4, as shown in FIG. .
[0024]
Furthermore, in the above-described embodiment, a case has been described in which a part of the second harmonic B1 (reflected light at the emission end face 3) is returned to the incident end face 2 side, but the emission is made according to the walk-off angle of the fourth harmonic B2. Similar processing can be performed on the fourth harmonic B2 by appropriately changing the angles θ1 and θ2 of the end surface 3 and the total reflection surface 5.
[0025]
【Example】
Examples of the present invention will be described below.
A wavelength conversion element having an element body (θm = 68.8 °, θ1 = 59.3 °, θ2 = 30.7 °) made of a lithium tetraborate (Li ₂ B ₄ O ₇ ) single crystal according to the present invention ( A conventional wavelength conversion element (conventional product) in which the normal of the emission end face of the element body made of lithium tetraborate (Li ₂ B ₄ O ₇ ) single crystal matches the traveling direction of the laser beam ) Was produced. Using these two types of wavelength conversion elements (the product of the present invention and the conventional product), the laser beam having a wavelength of 1064 nm oscillated by the YAG laser is wavelength converted from the second harmonic (green light having a wavelength of 532 nm) to the fourth. Harmonics (ultraviolet light having a wavelength of 266 nm) were generated, and the damage resistance and output stability at that time were compared. As a result, the conventional product was damaged about 20 hours after the start of wavelength conversion, and the output became unstable, while the product of the present invention was about twice as long, that is, 40 hours passed. However, no damage occurred and the output remained stable.
[0026]
【The invention's effect】
As described above, according to the first and second aspects of the present invention, when the wavelength of the laser beam is converted, the cross-sectional area of the laser beam by the emission end face of the wavelength conversion element is enlarged, and the converted light is inversely proportional to it. At the same time as the power density decreases, local heating of the wavelength conversion element due to the reflected light at the output end face is avoided, causing problems in damage resistance and output stability even when used for wavelength conversion of high-power lasers It is possible to provide a wavelength conversion element that does not occur.
[0027]
Further, when the converted light Le Zabimu is incident on the emission end face of the wavelength conversion element, since the ratio of the transmitted light is maximized, to increase the maximum output of the resulting harmonic wavelength conversion by the wavelength conversion element Can do.
[0028]
On the other hand, according to the third or fourth aspect of the present invention, since the reflected light at the emission end face of the wavelength conversion element can be returned to the incident end face and processed safely, the fundamental wave processing with high reliability at the time of wavelength conversion is achieved. A method can be provided.
[0029]
Further, when the converted light Le Zabimu is incident on the emission end face of the wavelength conversion element, since the ratio of the transmitted light is maximized, to increase the maximum output of the resulting harmonic wavelength conversion by the wavelength conversion element Can do.
[Brief description of the drawings]
1A and 1B are views showing a first embodiment of a wavelength conversion element according to the present invention, wherein FIG. 1A is a perspective view thereof, and FIG. 1B is a front view thereof.
FIG. 2 is a front view showing a second embodiment of a wavelength conversion element according to the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Wavelength conversion element 2 ... Incidence end face 3 ... Output end face 4 ... Element main body 5 ... Total reflection surface B1 ... Second harmonic (unconverted light)
B2 ... Fourth harmonic (converted light)
θ1 …… Angle

Claims (4)

It has a block-shaped element body (4) made of a nonlinear optical crystal,
At one end of the element body in the longitudinal direction, an incident end face (2) is formed orthogonal to the longitudinal direction of the element body,
An emission end face (3) inclined at an angle θ1 that is a polarization angle with respect to the converted light B2 of the laser beam with respect to the longitudinal direction of the element body at the other end portion in the longitudinal direction of the element body, and the longitudinal direction wavelength conversion element characterized by forming a total reflection surface (5) for tilting the unconverted light B1 of the laser beam reflected by the outgoing end face by an angle θ2 that total reflection to return to the incident end face against . With the longitudinal direction as the nonlinear optical crystal to adopt the phase matching angle θm only inclined by lithium tetraborate (Li ₂ B ₄ O ₇₎ single crystal with respect to the C-axis direction constituting the element body (4), wherein The wavelength conversion element according to claim 1 , wherein the angle θm is set in a range of 65 to 70 °, the θ1 is set in a range of 50 to 70 °, and the θ2 is set in a range of 28 to 40 ° . When converting the wavelength of a laser beam using the wavelength conversion element (1) according to claim 1 or 2, the laser beam is incident from the incident end face (2) into the wavelength conversion element, and the unconverted light B1 Is reflected at the emission end face (3) of the wavelength conversion element at a polarization angle with respect to the converted light B2, and the reflected light is totally reflected by the total reflection face (5) of the wavelength conversion element, so that the incident end face ( 2. A fundamental wave processing method characterized by returning to 2). When the wavelength conversion element (1) according to claim 1 or 2 is used to generate a fourth harmonic by converting the wavelength of the second harmonic of the laser beam oscillated by the solid-state laser,
A laser beam is incident on the wavelength conversion element from its incident end face (2), and the unconverted light B1 is reflected at the exit end face (3) of the wavelength conversion element at a polarization angle with respect to the converted light B2, and this reflected light is reflected. A fundamental wave processing method, comprising: totally reflecting the total reflection surface (5) of the wavelength conversion element and returning the light to the incident end face (2) of the wavelength conversion element.

Images