JP2004342681A - Laser oscillator - Google Patents

Abstract

A desired beam mode is obtained by optimizing an optical resonator including a reflecting mirror and an aperture.
A laser oscillator having an electrode for exciting a laser medium having a predetermined gas flow, a reflector for amplifying the excitation light, and an aperture for defining an oscillation mode of the laser is provided. Has a directionality of a gain distribution with respect to the optical axis, the aperture ratio of the aperture in each direction is optimized so that the beam mode orders in the discharge direction and the gas flow direction are the same.
[Selection diagram] Fig. 1

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for improving the roundness of a beam mode of a laser oscillator.
[0002]
[Prior art]
By supplying power between the electrodes, the filled laser gas molecules are excited, the laser light is amplified by an optical resonator composed of a partial reflection mirror and a total reflection mirror, and the laser beam is partially focused on the laser beam optical axis. Output from the reflector.
At this time, by restricting the optical resonance region by the aperture, the desired beam mode shape, its order, and output are obtained.
In other words, the shape of the aperture opening is the desired beam mode, such as making the aperture opening a perfect circular shape if a perfect circular beam mode is desired, and making the aperture opening an elliptical shape if an elliptical beam mode is desired. It had the same shape as.
Further, the size of the aperture opening needs to be determined so as to have a mode order suitable for processing.
The mode order can be estimated by D / w, where D is the aperture diameter of the aperture, and w is the beam diameter on the aperture determined by the reflecting mirror. If D / w is large, the mode order is high. When D / w is small, the mode order is low. For example, when a beam with good condensing properties is required, the aperture diameter of the aperture is reduced to a single mode with a low order, and when a large output is required, the aperture diameter of the aperture is increased. , So that a high-order multi-mode is achieved.
In order to consider the order of the beam mode, for example, as disclosed in Japanese Patent Application Laid-Open No. 1-253977, the laser oscillation mode is changed by changing the aperture cross section.
[0003]
[Patent Document 1] Japanese Patent Application Laid-Open No. 1-253977 (page 2, FIG. 1)
[0004]
[Problems to be solved by the invention]
Conventionally, when a hole is formed in a printed circuit board or the like using the above-described three-axis orthogonal gas laser oscillator, the roundness of the hole to be drilled is important.
Here, to achieve the perfect circularity of the hole, even if the shape of the aperture opening is formed as a perfect circle so that the desired beam mode shape becomes a perfect circle, the beam mode actually output is observed. Then, a case where a perfect circular shape is not always obtained occurs, and there is a problem that a hole having a desired shape is not formed.
[0005]
An object of the present invention is to obtain a desired beam mode by optimizing an optical resonator including a reflecting mirror and an aperture in order to solve such a problem.
[0006]
[Means for Solving the Problems]
A laser oscillator according to the present invention is a laser oscillator including an optical resonator configured by a reflector for amplifying excitation light and an aperture for defining an oscillation mode of the laser, wherein the excitation light has a gain with respect to an optical axis. When the distribution has directivity, the components of the optical resonator in each direction are optimized so that the beam mode order in each direction is the same.
[0007]
BEST MODE FOR CARRYING OUT THE INVENTION
First, the principle of optimization of the optical resonator according to the present invention will be described with reference to FIG.
FIG. 2 shows a three-axis orthogonal gas laser oscillator in which the discharge direction, the gas flow direction, and the optical axis direction are orthogonal to each other, in which the reflecting mirror, the aperture 5, the electrode width, the gap between the electrodes, the gas composition, the gas pressure, and the gas flow rate are the same. When the reflecting mirror has a concentrically curved shape, the opening of the aperture 5 has a perfect circular shape, and the optical resonance region has a maximum diameter that does not hit the electrode 1, the direction of discharge and the direction of gas flow depend on the input power. FIG. 9 is a schematic diagram for explaining how each gain distribution changes.
In the figure, when the input power is changed to W1, W2, W3 (W1>W2> W3), the average gain distribution in the discharge direction is 23, 24, 25, the beam mode is 29a, 30b, 31c, and the gas flow is The average gain distribution in the direction is indicated by 20, 21, 22, the beam mode is indicated by 26e, 27f, 28g, the finally output beam mode is indicated by 32ae, 33bf, 34cg, and the laser oscillation threshold is indicated by 13.
Symbols a to g indicate the mode order, and assume that a>b> c, e>f> g, a 、 e, b> f, and c> g.
[0008]
Next, the relationship between the gain distribution and the beam mode will be described with reference to the drawings.
In the discharge direction, a top hat-shaped gain distribution is obtained as shown from 23 to 25, and a similar system distribution having different magnitudes when the input power is changed.
When this gain distribution is cut out by an aperture and optically resonated, the gain exceeds the laser oscillation threshold value, and the output beam mode changes from the a-order 29a to the c-order 31c.
As described above, since the distribution of the gain equal to or higher than the laser oscillation threshold has a uniform distribution up to the end in the discharge direction, a relatively high-order beam mode can be obtained.
On the other hand, in the gas flow direction, the electric field strength at the electrode end portion is weaker than that near the center of the electrode, so that the input power contributing to the discharge differs depending on the position, and as shown in FIGS. The distribution is affected by the discharge threshold.
When this gain distribution is cut out by an aperture and optically resonated, the beam mode obtained is changed from e-order 26e to g-order 28g.
Thus, in the gas flow direction, when the input power is reduced, the gain above the laser oscillation threshold greatly differs between the center and the end, so that a relatively low-order beam mode can be obtained.
[0009]
As described above, in the gain distribution formed when the input power is W1, a relatively high-order beam mode can be obtained in both the discharge direction and the gas flow direction. There is no higher-order (a-order ≒ e-order) beam mode, that is, a perfect circular and higher-order beam mode 32ae.
On the other hand, in the gain distribution formed when the input power is W2, the discharge direction is a relatively high-order (b-order) beam mode, but the gas flow direction is a relatively low-order (f-order). Since the beam mode is used, the resulting beam mode is a directional beam mode (order a> f), that is, an elliptical beam mode 33bf that is long in the discharge direction.
In the gain distribution formed when the input power is W3, the discharge direction is a relatively low-order (c-order) beam mode, and the gas flow direction is the lowest-order (g-order) beam mode. Is the same as that in the case of W2, and is a lower (c-order> g-order) beam mode 34cg as a whole compared to W2.
[0010]
As described above, FIG. 2 shows the case where the input power is changed. However, when the electrode width, the gap between the electrodes, the gas composition, the gas pressure, the gas flow rate, and the like are changed, the gains in the discharge direction and the gas flow direction are similarly changed. The distribution changes, and consequently the beam mode changes.
Also, when the reflecting mirror and the aperture related to the optical resonance are changed, the gain extraction region changes, and as a result, the beam mode changes.
The present invention focuses on the fact that a beam mode changes due to a difference in gain distribution, and particularly optimizes an optical resonator including a reflecting mirror and an aperture as means for obtaining a desired beam mode.
[0011]
Embodiment 1 FIG.
In the first embodiment, the above-described principle is applied to a case where a three-axis orthogonal gas laser oscillator outputs a pulse laser having a large peak power and a short pulse width.
FIG. 1 is a schematic diagram for explaining the laser oscillator according to the first embodiment.
In the figure, by supplying power between the electrodes 1, the filled laser gas molecules are excited and discharged 2.
Then, stimulated emission is started by the optical resonator constituted by the partial reflection mirror and the total reflection mirror, the laser light is amplified, and the laser beam 8 is outputted from the partial reflection mirror 4 around the laser beam optical axis 6.
At this time, by restricting the optical resonance region by the aperture 5, a desired beam mode shape, its order, and output are obtained.
The laser gas is circulated by a blower 9 to form a predetermined gas flow 10.
Also, 11 is a gain distribution in the gas flow direction, 12 is a gain distribution in the discharge direction, 14 to 16 are beam modes in the gas flow direction, and 17 to 19 are beam modes in the discharge direction.
Symbols D1 to D4 indicate the aperture diameter of the aperture, and it is assumed that D4>D1>D2> D3.
[0012]
Now, in order to output a pulse laser having a large peak power and a short pulse width, it is necessary to apply a large input power in a short time.
Therefore, the gas pressure must be high, the gas composition must include a large amount of laser medium components, and the gas flow rate must be high, and each is theoretically limited to a certain range. Further, the input power, the gap between the electrodes, and the electrode width have the following restrictions.
Generally, to increase the laser output, it is necessary to increase the discharge area.
The discharge area is determined by gas pressure, gas composition, gas flow rate, input power, gap between electrodes, and electrode width.
In particular, the inter-electrode gap and the electrode width are parameters that spatially limit.
Accordingly, a physically large discharge region cannot be obtained unless the gap between the electrodes and the electrode width are increased. However, in order to increase them, it is necessary to increase the input power.
Regarding the input power, there are restrictions on the production of the power supply due to reasons such as cost and installation area. Therefore, there is a limit in increasing the electrode gap and the electrode width. As a result, in order to obtain a large laser output, it is necessary to maximize the ratio of the optical resonance region to the discharge region.
When configuring a laser oscillator that maximizes the laser output from various restrictions as described above, it may be difficult to form a concentric gain distribution.
[0013]
FIG. 1 shows this state. Since the gain distributions in the discharge direction and the gas flow direction are different from 11 and 12, respectively, the aperture of the aperture has a perfect circular shape and the maximum diameter D1 at which the optical resonance region does not hit the electrode. , The output beam mode is an elliptical mode long in the discharge direction.
In such a case, there is a method of obtaining a perfect circular beam mode by reducing the aperture diameter of the perfect circular aperture to D3 and equalizing the beam modes in the discharge direction and the gas flow direction as 15 and 18. In this method, since the mode volume is greatly reduced, it is difficult to obtain a large peak power.
[0014]
Therefore, in the present embodiment, this problem is solved by making the opening of the aperture an elliptical shape having a short discharge direction.
That is, as described above, in the case of a perfect circular aperture, the discharge direction has a high mode order, so that the optical path capable of optical resonance is reduced by reducing the aperture of the aperture to D2 so as to reduce the mode order, and conversely, the gas flow direction. Since the mode order is relatively low, the aperture of the aperture is widened to D4 so as to increase the mode order.
As a result, the mode orders in the discharge direction and the gas flow direction match, and a concentric beam mode, that is, a perfect circular beam mode is obtained.
In this case, a large peak power is obtained because the mode volume does not decrease so much.
[0015]
Specifically, the relationship between the opening D2 in the discharge direction and the opening D4 in the gas flow direction is set as follows.
First, a gain distribution determined by the gas pressure, the gas composition, the gas flow rate, the input power, the gap between the electrodes, and the electrode width is calculated.
Next, the laser beam mode order obtained by the optical resonator and the aperture is calculated, and the aperture is optimized so that the mode order in the gas flow direction and the discharge direction becomes the same.
At this time, the opening D4 in the gas flow direction is set to be approximately the same as the discharge width, and the opening D2 in the discharge direction is optimized so as to be the same as the mode order in the gas flow direction. And a large laser output can be obtained.
In the laser oscillator constructed in the experiment, the width of D2 was about 0.7 to 0.9 times the width of D4, so that the mode order in the gas flow direction was the same, and a concentric beam mode was obtained.
[0016]
As described above, according to the present invention, in a laser oscillator in which the gain distribution is not concentric, by using an aperture having an elliptical opening with a short discharge direction, the circularity of the beam mode can be reduced without lowering the output. It has the effect of improving.
In the above description, the aperture of the aperture is elliptical. However, the present invention can be applied to an elliptical shape including a straight line in the direction of the major axis. It has the effect of improving.
In the above description, the case where a pulse laser having a large peak power and a short pulse width is output in a three-axis orthogonal gas laser oscillator is applied.However, when the gain distribution is not concentric in other laser oscillators, the gain distribution is considered. By optimizing the optical resonator in this way, there is an effect of improving the roundness of the beam mode without lowering the output.
[0017]
The technique disclosed in the conventional Japanese Patent Application Laid-Open No. 1-253977 discloses that the projection as viewed from the optical axis cross section is made elliptical by rotating an aperture whose opening is a perfect circle and orthogonal to the resonance optical axis. Is the way.
In this method, when D / w is large, the optical resonance length is short, and the rotation angle needs to be large, a mode bias in the rotation direction occurs.
As a result, it becomes a new cause of deteriorating the axial symmetry of the mode, and it becomes difficult to obtain the concentric beam mode which is the subject of the present invention.
[0018]
Embodiment 2 FIG.
In the first embodiment, the aperture has a fixed shape. However, the aperture may have a variable opening diameter in the discharge direction.
3 and 4 are diagrams according to the second embodiment. FIG. 3 is a schematic diagram for explaining a laser oscillator according to the second embodiment. FIG. 4 is a schematic diagram for explaining a mechanism of an aperture mounted on the laser oscillator. FIG.
The laser oscillator shown in FIG. 3 is a laser oscillator that outputs a pulse laser having a large peak power and a short pulse width in a three-axis orthogonal gas laser oscillator, as in the first embodiment.
In the figure, 35 is a beam mode in the gas flow direction, 36 to 38 are beam modes in the discharge direction, and 39 to 41 are regions where optical resonance can be performed.
Further, since the difference between FIG. 1 and FIG. 3 is only the shape of the aperture, the gain distribution formed in the laser oscillator of FIG. 3 is the gain distribution 11 formed by the laser oscillator of FIG. Same as 12.
In FIG. 4, reference numeral 42 denotes a shielding object having a perfect circular opening, 43 and 44 denote shielding plates, 45 denotes a driving unit that fixes the shielding plate and drives the Y direction, 46 denotes a base, and 47 denotes a driving unit. The axis of the portion 45, 48 is the center of the opening of the aperture, and 49 and 50 are the central axes centered on 48, respectively.
Symbols Dy and D5 to D7 denote inter-shield plate distances, and h to k denote mode orders, where D5>D6> D7, h>i> j, and i ≒ k.
[0019]
Next, the operation will be described.
As shown in FIG. 4, the aperture according to the present invention restricts a beam mode by a beam shielding structure in which an opening 42 having a perfect circular shape and two shielding plates 43 and 44 are combined. Each of the two sheets has a structure for driving around a center axis 51.
Therefore, by changing the distance Dy between the shielding plates, it is possible to change the region where optical resonance in the Y direction can be performed.
Further, by circulating the cooling water through the base 46, the shield 42 in the aperture passes through the base 46, and the shielding plates 43 and 44 indirectly pass through the drive unit 45 and the base 46, respectively. It has a cooling structure.
[0020]
In FIG. 3, the driving direction of the shield plate of the aperture is used as the discharge direction, and the opening of the shield 42 is formed into a perfect circle of φD5.
ΦD5 is the maximum diameter at which the optical resonance region does not hit the electrode.
First, considering the case where the distance Dy between the shielding plates is D5, the region where optical resonance can occur is a perfect circle 39 of φD5.
Accordingly, the beam mode obtained at this time is the same as the case of D1 shown in the first embodiment, and the discharge direction is a high-order (h-order) mode and the gas flow direction is a low-order (k-order) mode. As a result, an elliptical beam mode having a poor roundness (hth order> kth order) and long in the discharge direction is obtained.
Next, considering the case where the distance Dy between the shielding plates is D6, the area where optical resonance can occur is 40.
Therefore, the mode order in the discharge direction is reduced while keeping the gas flow direction unchanged (i-order <h-order).
Therefore, in the beam mode obtained at this time, the order in the discharge direction and the gas flow direction is substantially equal (i-th order ≒ k-th order), and as a result, the beam mode has good circularity.
Next, assuming that the distance Dy between the shielding plates is D7, the region where optical resonance can occur is 41.
Accordingly, the mode order in the discharge direction is further lower than that in the case of D6 while keeping the gas flow direction as it is (j order << h order).
Therefore, the beam mode obtained at this time has a lower order in the gas flow direction than in the discharge direction (j-order <k-order). As a result, an elliptical beam mode with poor circularity and long in the gas flow direction is obtained. Become.
[0021]
As described above, according to the present invention, in the laser oscillator whose gain distribution is not concentric as described above, by adjusting the interval between the shield plates 43 and 44 driven in the discharge direction, the beam can be reduced without lowering the output. This has the effect of improving the roundness of the mode.
[0022]
Further, the present invention is particularly effective against assembly errors which are problematic in manufacturing.
In the laser oscillator shown in FIG. 3, the electrode width, the gap between the electrodes, the positional relationship between the electrodes, etc., have some assembling errors, each of which affects the gain distribution. different.
Therefore, the present invention has the effect of continuously adjusting the mode order to cancel out the assembly error and to improve the roundness of the beam mode without lowering the output.
[0023]
Further, in the above description, the opening of the shield 42 is a perfect circle, but may be an ellipse.
As an effect, in a laser oscillator whose gain distribution is not concentric as described above, if it is known in advance that the ratio of the minor axis to the major axis is within a certain range of variation, fine adjustment of a difference in gain distribution due to an assembly error is performed. Therefore, there is an effect that the aperture increased by the driving unit can be reduced to the minimum necessary.
[0024]
Embodiment 3 FIG.
In the first and second embodiments, the optimization of the aperture shape has been described. However, the inner surface curvature of the partial reflecting mirror, the total reflecting mirror, or both may be optimized.
5 and 6 are diagrams according to the third embodiment, FIG. 5 is a schematic diagram for explaining a laser oscillator of the present invention, and FIG. 6 is a schematic diagram for explaining a structure of a reflecting mirror mounted on the laser oscillator. FIG.
The laser oscillator shown in FIG. 5 is a three-axis orthogonal gas laser oscillator that outputs a pulse laser having a large peak power and a short pulse width, similarly to the first and second embodiments.
In the drawing, reference numeral 51 denotes a beam mode in a gas flow direction, 52 to 54 denote beam modes in a discharge direction, and 55 to 57 denote beam optical paths for optical resonance.
Also, the difference between FIG. 1 and FIG. 5 is only the shape of the reflecting mirror, so that the gain distribution formed in the laser oscillator of FIG. 5 is the gain distribution 11 formed by the laser oscillator of FIG. , 12.
Symbols Ry, R0 to R3 indicate the inner surface curvature of the reflecting mirror, w1 to w3 indicate the beam diameter in the discharge direction on the aperture determined by the reflecting mirror, and l to o indicate the mode orders, respectively, R1 <R2 <R3. , R0 = R1, l>m> n, m ≒ o.
[0025]
The reflecting mirror in the present invention has a shape whose inner surface curvature differs in the X and Y directions as shown in FIG.
In FIG. 5, the gas flow direction is the X direction, the discharge direction is the Y direction, the reflecting mirror 3 has a concentric curvature R0, the inner surface curvature of the reflecting mirror 4 in the X direction is R0, and the inner surface curvature Ry in the Y direction is R1. The operation in the case where R2 and R3 are changed will be described.
However, it is assumed that the aperture of the aperture has a perfect circular shape and has a maximum diameter such that the optical resonance region does not hit the electrode.
[0026]
First, when the inner surface curvature Ry of the reflecting mirror 4 is R1 (same as the inner surface curvature R0 in the X direction), that is, when the reflecting mirror is a concentric reflecting mirror, an optically resonating beam in the discharge direction determined by the reflecting mirrors 3 and 4 is used. The optical path is as shown in FIG.
Accordingly, the beam diameter in the discharge direction on the aperture is w1, and the beam mode obtained at this time is the same as that of D1 shown in the first embodiment, the order of the discharge direction is high (l order), and the gas flow direction is Since the mode is a low-order (o-order) mode, as a result, an elliptical beam mode that is inferior in roundness (1st> o-order) and is long in the discharge direction is obtained.
Next, assuming that the inner surface curvature Ry of the reflecting mirror 4 is R2, the beam optical path for optical resonance in the discharge direction determined by the reflecting mirrors 3 and 4 is as shown in FIG.
Accordingly, the beam diameter in the discharge direction on the aperture is w2 (w2> w1), and D / w is smaller than that in the case of R1, so that the mode order is reduced (m-order <l-order). As a result, the order of the discharge direction and the order of the gas flow become substantially equal (m-order ≒ o-order), so that a beam mode with good circularity is obtained.
Next, assuming that the inner surface curvature Ry of the reflecting mirror 4 is R3, the beam optical path for optical resonance in the discharge direction determined by the reflecting mirrors 3 and 4 is as shown in FIG.
Accordingly, the beam diameter in the discharge direction on the aperture becomes w3 (w3 >> w1), and D / w becomes smaller as compared with the case of R2, so that the mode order further decreases (n-order << l-order). . As a result, an elliptical beam mode that is inferior in circularity (nth order <oth order) and is long in the gas flow direction is obtained.
[0027]
From the above description, the inner surface curvature of the reflecting mirror is set as follows.
First, a gain distribution determined by the gas pressure, the gas composition, the gas flow rate, the input power, the gap between the electrodes, and the electrode width is calculated.
Next, the laser beam mode order obtained by the optical resonator and the aperture is calculated, and the inner surface curvature of the reflecting mirror is optimized so that the mode order in the gas flow direction and the discharge direction becomes the same.
At this time, the inner surface curvature in the gas flow direction is selected so that a desired beam mode order can be obtained in the gas flow direction, and then the inner surface curvature in the discharge direction is optimized so that the mode order is the same. Optimization can be performed efficiently.
[0028]
As described above, according to the present invention, in a laser oscillator having a gain distribution that is not concentric as described above, by mounting a reflecting mirror having different inner surface curvatures in the discharge direction and the gas flow direction, without lowering the output, This has the effect of improving the roundness of the beam mode.
Further, a mechanism having a mechanism for continuously changing the inner surface curvature of the reflecting mirror may be employed.
As an effect, similarly to the second embodiment, the mode order can be continuously adjusted, thereby canceling an assembly error and improving the circularity of the beam mode without lowering the output.
[0029]
【The invention's effect】
As described above, by using the laser oscillator of the present invention, it is possible to improve the roundness of the beam mode, which has not been achieved by the conventional technology.
[Brief description of the drawings]
FIG. 1 is a schematic diagram illustrating a laser oscillator according to a first embodiment.
FIG. 2 is a schematic diagram for explaining the principle of the present invention.
FIG. 3 is a schematic diagram for explaining a laser oscillator according to a second embodiment.
FIG. 4 is a schematic diagram for explaining a structure of an aperture according to a second embodiment.
FIG. 5 is a schematic diagram illustrating a laser oscillator according to a third embodiment.
FIG. 6 is a schematic diagram for explaining a structure of a reflecting mirror according to a third embodiment.
[Brief description of reference numerals]
1 electrode, 2 excitation discharge, 3 partial reflection mirror, 4 total reflection mirror, 5 aperture, 6 laser beam optical axis, 10 gas flow.

Claims (6)

In a laser oscillator including an optical resonator configured by a reflector for amplifying the pump light and an aperture that defines an oscillation mode of the laser,
A laser oscillator characterized in that, when the pump light has a directionality of a gain distribution with respect to an optical axis, components of the optical resonator in the respective directions are optimized so that the beam mode order in the respective directions is the same. 2. The laser oscillator according to claim 1, wherein the aperture of the aperture is made elliptical or the inner surface curvature of the reflecting mirror is made different in each direction so that the beam mode order in each direction is the same. . An electrode for exciting a laser medium having a predetermined gas flow, a reflecting mirror for amplifying the excitation light, and a laser oscillator having an aperture that defines an oscillation mode of the laser,
When the excitation light has the directionality of the gain distribution with respect to the optical axis, the aperture ratio of the aperture in each direction is optimized so that the beam mode orders in the discharge direction and the gas flow direction are the same. Laser oscillator. 4. The laser oscillator according to claim 3, wherein the aperture of the aperture has an elliptical shape with a short diameter in the discharge direction. 5. The laser oscillator according to claim 4, wherein the elliptical shape having a shorter diameter in the discharge direction has a width of about 0.7 to 0.9 times the diameter in the gas flow direction. 6. 4. The laser oscillator according to claim 1, wherein the opening of the aperture is made variable by moving the shielding plate around the optical axis in the discharge direction.

Images