JPH09307168A - Laser oscillator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a laser oscillator which enables laser beam of high dimensional transverse mode whose phase distribution changes with time to be converted to TEMnm mode which does not change with time and further to laser beam of strength distribution which is close to TEMoo mode as much as possible, that is, regular Gaussian distribution. SOLUTION: Laser beam of high dimensional transverse mode whose phase distribution changes as time passes by interposing an aperture member 10 with a window which is mutually divided of a shape and the number according to mode to be converted between a rear mirror 1a and an output mirror 1b is converted to TEMnm mode which does not changes as time with and laser beam can be stabilized and it can be also converted further to strength distribution which is close to TEMoo mode as much as possible, that is, laser beam of regular Gaussian distribution.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser resonator having a laser medium, a rear mirror, and an output mirror, and particularly, a high-order transverse mode laser beam whose phase distribution changes with time is easily generated. The present invention relates to a structure for stabilizing the transverse mode of a high-power laser resonator.

[0002]

2. Description of the Related Art Generally, as a method of performing laser processing using a laser oscillator, a method of focusing and irradiating laser light is used. It is desirable that the laser beam used for such an application has a TEM ₀₀ mode in which the transverse mode representing the spatial distribution of the light intensity has a clean Gaussian distribution, and this beam is engraved on the surface of a glass substrate, for example. The binary optics, which is focused by the diffraction grating, is used for focusing and irradiation.

Here, the binary optics has a stepwise cross-sectional structure in the order of wavelengths of light,
The traveling wavefront can change its traveling direction due to the difference in the thickness of the stairs. That is, since the transmission length of the light is different from that of the adjacent steps, the phase of the light is shifted, and the light path is bent by diffraction due to the light interference effect. The repetition pitch of the stairs is given as a width that changes the optical path length by one wavelength. For example, the binary optics for converging the YAG laser light has a step structure having a width of several μm and a thickness of about 1 μm. For example, the binary optics 11 having a concentric pattern has a single lens function such as a convex lens. Binary optics with a convex lens function act to focus incident parallel rays at a point, that is, at a focal point (eg, GJ Swanson et al., US Patent 48).
95790, (1990)).

[0004]

By the way, in the above-mentioned laser oscillator, especially when the laser output becomes large, it is easy to oscillate in a higher mode, and the TEM ₁₀ mode or the TEM ₁₁ mode divided into four modes, It becomes a TEM _nm mode having a matrix-like spot pattern, and further a higher-order transverse mode such as a ring mode in which the phase distribution changes with time.

Among the above modes, TEM ₁₀ mode and TE
Since the TEM _nm mode including the M ₁₁ mode is stable to some extent, its control is relatively easy, and it is described in Japanese Patent Application No. 7-305090 filed by the same applicant as the present application. It is possible to focus to some extent using binary optics as described above, but the higher-order transverse mode with time-varying phase distribution has a significantly reduced beam quality and is difficult to focus successfully. .

The present invention has been made in view of such circumstances, and converts a laser beam of a higher-order transverse mode whose phase distribution changes with time into a TEM _nm mode which does not change with time, and further It is an object of the present invention to provide a laser resonator capable of converting into a laser beam having an intensity distribution close to the TEM ₀₀ mode, that is, a Gaussian distribution with a clear distribution.

[0007]

In order to achieve the above object, the present invention provides a laser resonator having a laser medium, a rear mirror, and an output mirror, in which the phase distribution changes with time. In order to convert the laser beam of the next transverse mode into a fixed transverse mode, an aperture member having a window having a shape and a number of partitions defined according to the mode to be transformed is interposed between the rear mirror and the output mirror. A laser resonator characterized by the above.

Here, the beam quality factor M
² is defined as being proportional to the product of the near-field and far-field beamwidths (standard deviation σ ₀ , σ _{s of the} beam profile).

[0009]

[Equation 1]

From the electric field amplitude of the near field E ₀ (x) and the far field distribution P (s),

[0011]

[Equation 2]

(Equation 3)

Here, using the propagation angle θ (angle formed by the optical axis) and the wavelength λ of light,

(Equation 4)

However, since P (s) is considered to be the Fourier transform of E ₀ (x / 2π),

(Equation 5) Becomes

Specifically, for example, in the case of Gaussian beam,
The electric field amplitude is

(Equation 6) Becomes

In the near field,

(Equation 7) Becomes

The spot size parameter ω (z)
Is

(Equation 8)

The far field distribution P (s) is given by

[Equation 9]

[0018] Also, P (s) is

(Equation 10)

It is considered to be the Fourier transform of. Considering the normalization constant,

[Equation 11]

(Equation 12)

Therefore,

(Equation 13) Becomes

On the other hand, since the intensity distribution I of the ring mode TEM ₀₁ ^* is considered to be the superposition of the TEM ₀₁ and TEM ₁₀ modes,

[0022]

[Equation 14] Therefore, it is possible to discuss either one of the electric field strength distributions. That is,

[0023]

(Equation 15) Becomes When this is discussed in the same way as in the Gaussian mode, M ² = 3, that is, the beam quality is 3 times worse than that of the Gaussian beam, and T is stable in time.
It can be seen that it is worse than the EM _nm mode beam.

Since the ring mode (TEM ₀₁ ^* ) is a linear combination of TEM ₀₁ and TEM ₁₀ modes, it is considered that the phase distribution changes with time. First,
The circular aperture set in the resonator is replaced with an elliptical aperture so that only one of TEM _{01 and} TEM ₁₀ oscillates in mode.

[0025]

DETAILED DESCRIPTION OF THE INVENTION The present invention will now be described in detail with reference to the accompanying drawings. FIG. 1 shows a main configuration of a laser processing apparatus to which the present invention is applied. For example, the light output from the resonator 1 of the Nd: YAG laser for laser processing having an average output of 100 W is expanded through the beam expander 2 composed of a combination of two lenses having different focal lengths, and becomes parallel light. After the conversion, the light is incident on the binary optics 3 of the present invention. It is assumed that the laser light has a circular beam shape and is incident so that its optical axis coincides with the center O of the binary optics 3. And
The beam-deformed light is used for known laser processing by removing low-quality components that are difficult to be converted into parallel light by the beam reforming device 5 through the binary optics 4 for converting the light into parallel light.

Here, an aperture member 10 for fixing the transverse mode of the beam is provided in the resonator 1. This is because the transverse mode of the beam output from the resonator 1 is not a time-changing mode such as a ring mode but a fixed TEM _nm mode. It is a shielding member provided. In this embodiment, as shown in FIG. 2, an aperture member 10 having an elliptic opening is provided between the rear mirror 1a and the output mirror 1b of the resonator 1 so as to fix the TEM ₀₁ mode. In addition to this, for example, the aperture member 20 having four openings centered on the optical axis as shown in FIG. 3 fixes the TEM ₁₁ mode, or 12 openings centered on the optical axis as shown in FIG. Aperture member 3 having
Any TEM, such as fixing to TEM ₃₂ mode by 0
Can be fixed to _nm mode.

The binary optics 3 and 4 are formed by using a semiconductor microfabrication technique, exposing a quartz substrate (diameter 50 mm) with a mask and exposing it, and then etching it to obtain V in FIG. 5 (a).
The cross-sectional shape as shown in FIG. Here, in the binary optics 3, the left and right thicknesses with respect to the optical axis O differ by the amount by which one phase is delayed (or advanced) by π.

FIG. 6 is a schematic diagram of optical path conversion and phase conversion by the binary optics 3 and conversion into parallel light by the binary optics 4 in this embodiment. The laser beam fixed in the TEM ₀₁ mode in the resonator 1 by the aperture member 10 has two peaks on the left and right at the time of entering the binary optics 3, and the center thereof has a dark bright-dark pattern. (Part A). The electric field amplitude is left and right, that is, with O as the origin, x
The sign is different between> 0 and x <0, that is, the phase is shifted by π (section B).

When such a laser beam is incident on the binary optics 3, not only is it diffracted so that it is condensed near the center, but also the phase of either the right half or the left half is shifted by π, and the electric field amplitude is changed. Are made to have the same sign and are returned to parallel light by the binary optics 4. Then, the intensity distribution (part C) and the electric field amplitude (D
(Part) is close to the Gaussian distribution except that there is a gap in the central part.

Here, the intensity distribution as shown in the C portion of FIG. 6 results in the problem that the Gaussian beam passes through the linear shield having the width D corresponding to the width of the gap in the central portion (FIG. 7).
This is complementary to Fraunhofer diffraction due to a slit of width D, so binary optics 4
Linear distance R ₀ from the center of, at the field amplitude E (x ₀₎ to a point of the distance x ₀ from the optical axis,

[0031]

(Equation 16)

## EQU1 ## That is, the component diffracted from the slit having the width D may be subtracted from the Gaussian distribution. It is calculated that a Gaussian beam of ω = 15 mm passes through a linear block having a width D, and for any width D of the gap, a beam brofield (| E (X ₀ ) | ² -30 mm
≦ x ₀ ≦ 30 mm) is shown in FIGS. This allows
It can be seen that the diffracted component due to blocking the center of the emitted light is scattered left and right as the distance from the binary optics increases (R becomes larger). In particular, it can be seen that the smaller D is, the more scattered the particles are. Actually D is 30-50
Below μm, the beam profile is almost the same as the Gaussian distribution.

Since the above-mentioned diffractive component cannot be converted into parallel light by the binary optics 4,
In the far field, a component having bad beam quality occurs at a position separated from the center as shown in the E portion of FIG. The beam reforming device 5 is used to remove this diffraction component. First, the light incident from the binary optics 4 is focused by the lens 7 and passed through the small-diameter opening 8a provided in the aperture member 8 in the vicinity of the focal position (FIG. 13). In this way, the diffracted components scattered left and right cannot pass through the opening 8a. Then, by returning the light to parallel light again with the lens 9, a beam close to the Gaussian mode can be extracted. In this way, since the electric field amplitude is 0 at x = 0 in the near field, even if the beam quality is inferior to the original Gaussian mode due to the definition of M ² , the component with poor beam quality is separated into space, and only the good quality part is You can leave.

On the other hand, as described above, the aperture member 20 having the four openings centered on the optical axis as shown in FIG.
The laser beam output from the resonator 1 by the TEM ₁₁
When the mode is fixed, the upper left and lower right thicknesses and the upper right and lower left thicknesses with respect to the optical axis O instead of the binary optics 3 are delayed by (or advanced) one phase by π / 2. Use different binary optics 13. Similarly, with respect to the higher-order mode TEM _nm that can also obtain the maximum output, it can be converted into a beam close to a Gaussian beam by using binary optics according to the mode, and then, similarly to the above, the portion with poor beam quality can be spatially separated and You can leave only the part.

In the above embodiment, each binary optics is of the transmissive type, but the reflective type can improve the space efficiency in the axial direction. Further, in the above embodiment, for example, a laser beam of a higher-order transverse mode whose phase distribution changes with time is converted into a beam of TEM _nm mode, and then converted into a beam close to a Gaussian beam with binary optics. A laser beam having a desired shape and a desired intensity distribution can be obtained by changing the pitch and pattern of the binary optics 3, 13.

[0036]

According to the laser resonator having the above structure, the aperture member having the shape and the number of windows which are partitioned from each other is interposed between the rear mirror and the output mirror, so that the phase is changed. A high-order transverse mode laser beam whose time distribution changes with time can be converted into a TEM _nm mode that does not change with time, and the laser beam can be stabilized. Furthermore, the intensity close to that of the TEM ₀₀ mode is possible. It is also possible to convert the laser beam to a distribution, that is, a Gaussian distribution having a clear distribution.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a main part of a laser processing apparatus to which the present invention is applied.

FIG. 2 is a perspective view showing a simplified configuration of the resonator shown in FIG.

FIG. 3 is a perspective view showing a simplified structure of another embodiment of the resonator shown in FIG.

4 is a perspective view showing a simplified configuration of another embodiment of the resonator shown in FIG.

5A is a plan view of binary optics according to the present invention, and FIG. 5B is a sectional view taken along line VV of FIG.

FIG. 6 is a schematic diagram showing optical path conversion and phase conversion by binary optics according to the present invention, and conversion of a beam after the conversion into parallel light.

FIG. 7 is an explanatory view schematically showing the intensity distribution of a TEM ₀₁ mode laser beam that has passed through binary optics according to the present invention.

FIG. 8: Center gap D of TEM ₀₁ mode laser beam passed through binary optics according to the present invention
Is 200 μm, the distance from the binary optics is R = 500 (a), 300 (b), 200
Beam profile at positions (c), 100 (d) and 50 mm (e).

FIG. 9 Gap in the center of a TEM ₀₁ mode laser beam passed through binary optics according to the present invention.
Is 100 μm, the distance from the binary optics is R = 500 (a), 300 (b), 200
Beam profile at positions (c), 100 (d) and 50 mm (e).

FIG. 10 shows the distance R from the binary optics R = 500 (a), 300 (b), 200 when the gap D at the center of the TEM ₀₁ mode laser beam passing through the binary optics according to the present invention is 50 μm.
Beam profile at positions (c), 100 (d) and 50 mm (e).

FIG. 11 shows a distance R from the binary optics R = 500 (a), 300 (b), 200 when the center gap D of the TEM ₀₁ mode laser beam passing through the binary optics according to the present invention is 30 μm.
Beam profile at positions (c), 100 (d) and 50 mm (e).

FIG. 12 is a distance R = 500 (a), 300 (b), 200 from the binary optics when the gap D at the center of the laser beam of the TEM ₀₁ mode passing through the binary optics according to the present invention is 20 μm.
Beam profile at positions (c), 100 (d) and 50 mm (e).

FIG. 13 is a diagram showing a configuration of a beam reforming apparatus of a laser processing apparatus to which the present invention is applied.

14A is a plan view of binary optics according to another embodiment of the present invention, FIG. 14B is a sectional view taken along line BB of FIG. 14A, and FIG. C-C
Sectional drawing which looked at the line.

[Explanation of symbols]

 1 Resonator 1a Rear mirror 1b Output mirror 2 Beam expander 3,4 Binary optics 5 Beam reforming device 7 Lens 8 Aperture member 8a Aperture 9 Lens 10 Aperture member 13 Binary optics 20, 30 Aperture member

Claims (2)

[Claims] 1. A laser resonator having a laser medium, a rear mirror, and an output mirror, which converts a laser beam of a higher-order transverse mode whose phase distribution changes with time into a fixed transverse mode. In order to achieve this, a laser resonator comprising an aperture member having a window having a shape and a number of windows defined according to a mode to be converted, interposed between the rear mirror and the output mirror. 2. In the case where the aperture member converts a TEM ₀₁ ^* mode into a TEM ₀₁ mode or a TEM ₁₀ mode, the aperture member has one window having an elliptical shape, and in the case where the aperture member converts into a TEM _nm mode. Has a window of nxm, The laser resonator according to claim 1 characterized by things.