JPH07142802A - External harmonic generating laser oscillator - Google Patents

Abstract

PURPOSE:To increase the speed of the start of an external harmonic generating laser oscillator and improve the output stability of the oscillator. CONSTITUTION:In an external harmonic generating laser oscillator 10, the second harmonic light (wavelength 532nm) of a CM-Q switch Nd:YAG laser which is emitted from a second harmonic light oscillator 11 is made to enter a beta-BBO crystal 15 to generate the fourth harmonic light (wavelength 266nm) and, at the same time, the switching of the fourth harmonic light is performed by the control of the polarization of the second harmonic light by using a Pockel cell 13 provided between a lambda/2 plate 12 with a rotary mechanism and a condensing lens 14.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an external harmonic generating laser oscillator, and more particularly to an external harmonic generating laser oscillator using an external harmonic generating system using a nonlinear optical crystal.

[0002]

2. Description of the Related Art Conventionally, as an external harmonic generating laser oscillator using an external harmonic generating method using a nonlinear optical crystal, a second harmonic light (SHG) having a wavelength of 532 nm is used for an Nd: YAG laser light having a wavelength of 1.064 μm. That emits light) and the internal cavity type S of the Nd: YAG laser
There is one that generates the third harmonic light (THG light) or the fourth harmonic light (FHG light) of the HG oscillator. Incidentally, such an external harmonic generation laser oscillator performs wavelength conversion for visible light or ultraviolet light outside the resonator, and also stabilizes the generated harmonic light, and performs high-speed switching operation with a short rise time of the optical pulse. Used when needed.

Further, in the case of switching the generated harmonic light, when the laser light which is the fundamental wave is made incident on the nonlinear optical crystal which generates the harmonic light, the laser light itself is turned on / off to generate the harmonic light. I was turning it on and off.

[0004]

However, in the above-described conventional external harmonic generation laser oscillator, the laser light incident on the nonlinear optical crystal is switched or the modulation frequency is changed, so that it is incident on the nonlinear optical crystal. The input average power of laser light also changes. The principle of generating harmonic light by a nonlinear optical crystal is to match the phase velocities of incident light and converted light (harmonic light / mixed wave light) (phase matching). In other words, the refractive index matching of incident light and converted light is taken. That is. Since the refractive index of the crystal changes depending on the temperature of the crystal, phase matching (refractive index matching)
Not only in the case where the temperature matching is realized, but also in the angle matching, if the temperature of the crystal changes, the phase matching condition shifts and the conversion efficiency changes.

As an example, β-B is used as a nonlinear optical crystal.
Using a BO crystal, Nd: YAG second harmonic light (wavelength 5
32 nm) is incident on the β-BBO crystal and converted into the fourth harmonic light (wavelength 266 nm), and one simulation result for obtaining the relationship between the conversion efficiency and the crystal temperature is shown in FIG. In the figure, the vertical axis represents the conversion efficiency η _m , and the horizontal axis represents the change in crystal temperature dT from the crystal temperature in the phase-matched state.
Is shown. From this simulation result, it is understood that the conversion efficiency η _m is reduced to half by only changing the crystal temperature by about 4.6 ° C. from the phase-matched crystal temperature.

Since the non-linear optical crystal has more or less little absorption, if the average power of the laser light incident on the non-linear optical crystal changes, the temperature of the non-linear optical crystal itself changes and the phase matching condition of the non-linear optical crystal changes. Change. From the simulation result of FIG. 3, in order to prevent the temperature of the non-linear optical crystal from rising instantaneously by about 5 ° C., it is conceivable that the non-linear optical crystal is thermostatically controlled. However, since the heat transfer time is usually long, Such constant temperature control is difficult. Particularly, when the power of the laser light incident on the nonlinear optical crystal becomes about 1 W, the influence of the temperature change of the nonlinear optical crystal becomes large, and when the incident laser light is interrupted, the rise of the harmonic light to be converted is increased. There arises a problem that the characteristics (switching characteristics) are extremely deteriorated.

For example, the second harmonic light of an Nd: YAG laser having a power of 5 W is irradiated onto the β-BBO crystal to produce a fourth harmonic light.
When the harmonic light is generated, the second harmonic light is temporarily turned off for a time of about several seconds, and the rising time in seconds is required as the rising time from 0% to 100% of the fourth harmonic light. As a result, when used in a laser beam machine, it is necessary to turn on / off the laser light frequently,
The need for a rise time in seconds has been a major obstacle to practical use.

In order to perform high-speed switching using the Pockels cell, for example, a change in the voltage applied to the Pockels cell after the Q-switch time is changed by a huge pulse depending on the magnitude of the excitation energy of the laser rod. A pulse laser device that stabilizes the laser output against the fluctuations in the excitation energy by providing a Pockels cell control circuit that controls in association with the time of occurrence (Japanese Patent Laid-Open No. Sho 61-61).
However, there is no conventional example that uses a Pockels cell from the viewpoint of speeding up the rise time of the external harmonic generation laser oscillator.

An object of the present invention is to provide an external harmonic wave generating laser oscillator capable of achieving a faster rise time and improved output stability.

[0010]

An external harmonic generating laser oscillator according to the present invention is a laser oscillator for emitting a fundamental laser beam or a harmonic laser beam having a predetermined polarization direction, and the laser oscillator for emitting an external harmonic wave. A half-wave plate on which the fundamental laser light or the harmonic laser light is incident;
An external harmonic generation laser oscillator including a non-linear optical crystal that converts the fundamental laser light or the harmonic laser light that has passed through a 1/2 wavelength plate into another harmonic laser light,
An electro-optical element provided between the half-wave plate and the non-linear optical crystal and capable of rotating the polarization plane of the fundamental laser light or the harmonic laser light in accordance with an applied voltage. Is characterized by.

Here, an applied voltage control means for controlling the voltage value of the applied voltage may be further included, and the electro-optical element may be a Pockels cell.

Alternatively, a laser oscillator that emits a fundamental wave laser beam or a harmonic wave laser beam having a predetermined polarization direction, and the fundamental wave laser beam or the harmonic wave laser beam emitted from the laser oscillator enters 1 An external harmonic generation laser oscillator including a / 2 wavelength plate and a nonlinear optical crystal that converts the fundamental laser light or the harmonic laser light transmitted through the 1/2 wavelength plate into another harmonic laser light, A magneto-optical element provided between the half-wave plate and the non-linear optical crystal and capable of rotating the polarization plane of the fundamental laser light or the harmonic laser light according to an applied magnetic field. Is characterized by.

Here, an applied magnetic field control means for controlling the value of the applied magnetic field may be further included, and the magneto-optical element may be a Faraday cell.

[0014]

In the external harmonic generating laser oscillator according to the present invention, the fundamental laser light or the harmonic laser light incident on the nonlinear optical crystal is continuously transmitted at a constant output after passing through the electro-optical element or the magneto-optical element. Since the light enters the nonlinear optical crystal in the oscillated state, the temperature at the center of the nonlinear optical crystal does not change. Further, in the phase matching in the nonlinear optical crystal, the conversion efficiency can be adjusted by the polarization direction with respect to the principal axis of the nonlinear optical crystal in both the temperature matching and the angle matching, so that the fundamental laser light or the harmonic laser light incident on the nonlinear optical crystal can be adjusted. By changing the polarization plane of, the intensity of the harmonic generation can be varied while keeping the intensity of the fundamental laser light or the harmonic laser light constant.

[0015]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a schematic configuration diagram showing a first embodiment of an external harmonic generating laser oscillator according to the present invention.

The external harmonic generating laser oscillator 10 is a CW
The Q-switch Nd: YAG laser second harmonic light (wavelength 532 nm) is incident on the β-BBO crystal 15 to generate fourth harmonic light (wavelength 266 nm), and the Pockels cell 13 was used. The fourth harmonic light is switched by controlling the polarization degree of the second harmonic light.

The external harmonic generating laser oscillator 10 is provided with an internal resonator type second harmonic optical oscillator 11 which emits second harmonic light having a wavelength of 532 nm and a second harmonic light which is emitted from the second harmonic optical oscillator 11. Λ / with incident rotation mechanism
2 plate 12 and the 2nd which penetrated λ / 2 plate 12 with a rotation mechanism
The Pockels cell 13 into which the harmonic light is incident, the condenser lens 14 into which the second harmonic light transmitted through the Pockels cell 13 is incident, and the second harmonic light condensed by the condenser lens 14 is converted into the fourth harmonic light. Β-BBO crystal 15 for conversion
And a wavelength of 5 provided on the emission side of the β-BBO crystal 15.
It transmits the second harmonic light of 32 nm and has a wavelength of 266.
The first dichroic mirror 16 coated to totally reflect the fourth harmonic light of nm, and the second harmonic light transmitted through the first dichroic mirror 16 (that is, the β-BBO crystal 15 converts the fourth harmonic light into the fourth harmonic light). A damper 17 for absorbing the unconverted second harmonic light),
The fourth harmonic light totally reflected by the first dichroic mirror 16 is incident, the second harmonic light having a wavelength of 532 nm is transmitted, and the second harmonic light having a wavelength of 266 nm is coated so as to be totally reflected. The dichroic mirror 18 and a cut filter 19 that blocks the second harmonic light transmitted through the second dichroic mirror 18.
And an attenuator 20 provided on the emission side of the cut filter 19 for attenuating the fourth harmonic light, an internal power monitor 21 provided on the emission side of the attenuator 20 using a photodiode for a wavelength of 266 nm, and a Pockels cell. A driver 22 (high-voltage generation circuit) for applying an applied voltage V _S to the driver 13, a photocoupler 23 for turning on / off the output of the applied voltage V _S from the driver 22 to the Pockels cell 13, and an external control device 24. It has and.

Generally, when a linearly polarized laser beam is incident on a nonlinear optical crystal for frequency multiplication, a λ / 2 plate (1/2 wavelength plate) is provided on the incident side of the nonlinear optical crystal.
By inputting linearly polarized light that is rotated by θ ° with respect to the crystal axis of the λ / 2 plate, it is possible to rotate the polarized light of the laser light that has passed through the λ / 2 plate by 2θ °. The intensity of the higher harmonics can be adjusted by rotating the target.
However, the method of manually rotating the λ / 2 plate in this way cannot turn on / off the laser light at high speed or modulate the laser light.

Therefore, the external harmonic generating laser oscillator 10
Then, the Pockels cell 13, which is an electro-optical element capable of rotating the plane of polarization at high speed, is arranged between the λ / 2 plate 12 with the rotation mechanism and the β-BBO crystal 15 (nonlinear optical crystal). Now, the phase difference α in the Pockels cell 13
And the voltage V applied from the driver 22 to the Pockels cell 13
The relationship with _S (modulation signal) is α = K · V _S (1) Here, K is represented by a constant, and therefore the phase difference α is continuously changed by continuously changing the applied voltage V _S. Can be made.

Therefore, by setting the voltage value of the applied voltage V _{S to} the half-wave voltage value v _{a at} which the phase difference α is exactly one half wavelength, the light enters the β-BBO crystal 15 via the condenser lens 14. Since the polarization direction of the second harmonic light can be rotated by 90 °, the conversion efficiency from the second harmonic light to the fourth harmonic light in the β-BBO crystal 15 is about 0.
% Or about 100%. When the voltage value of the applied voltage V _S is made smaller than the half-wave voltage value v _a , the second harmonic light transmitted through the Pockels cell 13 is converted into elliptically polarized light. This elliptically polarized light is converted into β-BBO crystal 1
If is incident to 5, since one size conversion light intensity commensurate with the component that contributes to harmonic generation of the polarization (electric field) component decomposed elliptically polarized light is obtained, the applied voltage V _S
By continuously changing the phase of the electric field components of the elliptically polarized light which are orthogonal to each other, the intensity of the fourth harmonic light converted in the β-BBO crystal 15 is freely controlled between 0% and 100%. You can also Since the response time of the Pockels cell 13 is on the order of nanoseconds (10 ⁻⁹ seconds) or less, high-speed switching can be performed without any problem as a light source of a laser processing machine.

The β-BBO crystal 15 is generally called "Type".
It is called "1" and is cut so that phase matching can be achieved by angle matching or temperature matching. To complete this phase matching, β-B
The polarization direction of the second harmonic light incident on the BO crystal 15 is β
-It is necessary to uniquely adjust to a predetermined angle with respect to the optical axis of the BBO crystal 15. However, since the polarization direction of the second harmonic light incident on the β-BBO crystal 15 is defined by the polarizing plate and the Brewster plate (both not shown) that constitute the second harmonic light oscillator 11, It is difficult to freely rotate the polarization direction of harmonic light. Therefore, it is necessary to rotate the β-BBO crystal 15 itself to make fine adjustment. However, since the β-BBO crystal 15 has an angle adjusting mechanism and a crystal thermostat unit attached, a fine adjustment rotating mechanism is attached to this portion. It is difficult to provide. Therefore, the external harmonic generation laser oscillator 1
At 0, the output of the fourth harmonic light is maximum or in a state where the applied voltage V _S is not applied to the Pockels cell 13 from the driver 22 or the voltage value of the applied voltage V _S is a half wavelength voltage value v _a. The λ / 2 plate 12 with a rotating mechanism is rotated and adjusted while looking at the internal power monitor 21 so as to minimize the adjustment.

FIG. 2 is a schematic configuration diagram showing a second embodiment of the external harmonic generating laser oscillator according to the present invention.

The external harmonic generating laser oscillator 50 is a CW
The Faraday cell 53 was used for making the second harmonic light (wavelength 532 nm) of the -Q switch Nd: YAG laser incident on the β-BBO crystal 55 to generate the fourth harmonic light (wavelength 266 nm). The fourth harmonic light is switched by controlling the polarization degree of the second harmonic light.

The external harmonic wave generating laser oscillator 50 differs from the external harmonic wave generating laser oscillator 10 of the first embodiment shown in FIG. 1 in the following points. (1) Instead of the Pockels cell 13 shown in FIG. 1 between the λ / 2 plate 52 with a rotating mechanism and the condenser lens 54, a magneto-optical element capable of rotating the polarization plane according to the applied magnetic field H. Faraday cell 53 is included. (2) The driver 62 causes the Faraday cell 53 to receive the current I.
To apply the applied magnetic field H to the Faraday cell 53.

The rotation angle β (Faraday rotation angle) of the plane of polarization in the Faraday cell 53 is β = K _V · L · H (2) where K is the relationship with the length L of the Faraday cell 53 and the applied magnetic field H. _V is given by the Verdet constant. Therefore, since the applied magnetic field H can be freely controlled by the current I, the rotation angle β can be changed between 0 ° and 90 ° by changing the current I. As a result, high-speed switching is possible in the same manner as when the Pockels cell 13 is used.

In the above description, the second harmonic light is converted into the fourth harmonic light, but the external harmonic generating laser oscillator of the present invention is not limited to this, and for example, the fundamental laser light is converted into arbitrary harmonic light. It is also effective in cases.

[0028]

Since the present invention is configured as described above, it has the following effects.

By including an electro-optical element capable of rotating the plane of polarization in accordance with an applied voltage or a magneto-optical element capable of rotating the plane of polarization in response to an applied magnetic field, high-speed switching is possible, and thus high It is possible to improve the on / off characteristics (rise time response) of a laser in an output external harmonic generation laser oscillator from the order of seconds or minutes to the order of nanoseconds. as a result,
For example, using the fourth harmonic light (wavelength 266 nm) of the CW-Q switch pulse laser of the Nd: YAG laser,
The external harmonic generating laser oscillator of the present invention can also be applied to a light source of a laser marking device that controls dots in units of one shot of a Q switch pulse. Further, the external harmonic generating laser oscillator of the present invention can also be used for a laser trimming device, a laser scriber device and the like which require high-speed switching of a laser beam. Further, not only in applications requiring high-speed switching, the stability can be greatly improved even when the frequency of ON / OFF of harmonic light becomes random or changes in the middle.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing a first embodiment of an external harmonic generation laser oscillator of the present invention.

FIG. 2 is a schematic configuration diagram showing a second embodiment of the external harmonic generation laser oscillator of the present invention.

FIG. 3 shows a conventional external harmonic generation laser oscillator,
A β-BBO crystal is used as the nonlinear optical crystal, and Nd: Y
In the graph which shows one simulation result which calculated | required the relationship between the conversion efficiency at the time of making the 2nd harmonic light (wavelength 532nm) of AG enter into (beta) -BBO crystal, and converting into 4th harmonic light (wavelength 266nm). is there.

[Explanation of symbols]

 10,50 External harmonic generation laser oscillator 11,51 Second harmonic optical oscillator 12,52 λ / 2 plate 13 Pockels cell 14,54 Condensing lens 15,55 β-BBO crystal 16,56 First dichroic mirror 17, 57 Damper 18,58 Second dichroic mirror 19,59 Cut filter 20,60 Attenuator 21,61 Internal power monitor 22,62 Driver 23,63 Photocoupler 24,64 External control device 63 Faraday cell

Claims (6)

[Claims] 1. A laser oscillator which emits a fundamental wave laser beam or a harmonic wave laser beam having a predetermined polarization direction, and the fundamental wave laser beam or the harmonic wave laser beam emitted from the laser oscillator is incident 1 1/2 wave plate and the 1 /
An external harmonic generating laser oscillator including a nonlinear optical crystal that converts the fundamental laser light or the harmonic laser light that has passed through a two-wave plate into another harmonic laser light, wherein the half-wave plate and the nonlinear An external harmonic generating laser including an electro-optical element that is provided between the optical crystal and can rotate a polarization plane of the fundamental laser light or the harmonic laser light according to an applied voltage. Oscillator. 2. The external harmonic wave generating laser oscillator according to claim 1, further comprising applied voltage control means for controlling a voltage value of the applied voltage. 3. The external harmonic generating laser oscillator according to claim 1, wherein the electro-optical element is a Pockels cell. 4. A laser oscillator which emits a fundamental wave laser beam or a harmonic wave laser beam having a predetermined polarization direction, and the fundamental wave laser beam or the harmonic wave laser beam emitted from the laser oscillator is incident 1 1/2 wave plate and the 1 /
An external harmonic generating laser oscillator including a nonlinear optical crystal that converts the fundamental laser light or the harmonic laser light that has passed through a two-wave plate into another harmonic laser light, wherein the half-wave plate and the nonlinear An external harmonic generation laser including a magneto-optical element that is provided between the optical crystal and can rotate the polarization plane of the fundamental laser light or the harmonic laser light according to an applied magnetic field. Oscillator. 5. The external harmonic wave generating laser oscillator according to claim 4, further comprising an applied magnetic field control means for controlling the value of the applied magnetic field. 6. The external harmonic generation laser oscillator according to claim 4, wherein the magneto-optical element is a Faraday cell.