JP2013021267A - Pulse laser oscillator and pulse laser oscillation control method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a pulse laser oscillator capable of changing a voltage applied to an electro-optical device over time, extending a pulse width of a laser beam, and reducing a peak energy of a pulse laser outputted.SOLUTION: The pulse laser oscillator comprises: an electro-optical device 6 which polarizes light according to an applied voltage; and a voltage controller 7 which applies a voltage to the electro-optical device 6 and controls the voltage. The voltage controller 7 changes a voltage applied to the electro-optical device 6 over time and controls a pulse width of a laser beam.

Description

  The present invention relates to a pulse laser oscillator and a pulse laser oscillation control method including an electro-optic element that polarizes light according to an applied voltage, and more specifically, changes a voltage applied to the electro-optic element over time, The present invention relates to a pulse laser oscillator and a pulse laser oscillation control method capable of extending the pulse width of laser light and reducing the peak energy of the output pulse laser.

  A conventional pulse laser oscillator has a laser medium, an excitation light source that excites the laser medium, and a resonator that reciprocates and amplifies the light emitted by the laser medium to obtain pulsed laser light. Q-switch oscillation with a Q mirror element and a cavity dump element placed between the resonators with a reflectance mirror and a low reflectance mirror on the other side, and the laser beam is completely confined in the resonator The energy stored in the resonator is instantaneously externalized by causing the cavity dump element to continue to operate and perform the cavity dump near the peak level of the pulsed laser light accumulated in the resonator. There are some which are configured to be taken out (for example, see Patent Document 1).

JP 2003-69118 A

  However, in the conventional pulse laser oscillator, the energy accumulated in the resonator is instantaneously extracted outside, so that the peak energy of the output pulse laser becomes too large, and the object irradiated with the laser is There was a risk of damage.

  Accordingly, the problem to be solved by the present invention that addresses such problems is that a pulse laser oscillator and a pulse laser oscillation that can reduce the peak energy of the output pulse laser by extending the pulse width. It is to provide a control method.

  In order to solve the above problems, a pulse laser oscillator according to the present invention includes an electro-optic element that polarizes light according to an applied voltage, a voltage control device that applies a voltage to the electro-optic element and controls the voltage. And a pulse width of the laser beam is controlled by changing a voltage applied to the electro-optic element over time by the voltage control device.

  Further, the voltage control device changes the rate of change of the voltage applied to the electro-optic element in a stepwise manner.

  Further, the electro-optical element is a Pockels cell and further includes a λ / 4 wavelength plate.

  The pulse laser oscillation control method according to the present invention includes a laser oscillation control method for controlling laser light oscillation by changing a voltage applied to an electro-optic element that polarizes light according to an applied voltage. The voltage applied to the electro-optic element is changed over time to control the pulse width of the laser light.

  The rate of change of the voltage applied to the electro-optic element is changed stepwise.

According to the pulse laser oscillator of the first aspect, the voltage applied by the voltage application circuit to the electro-optic element can be changed over time by the control circuit to control the pulse width of the laser light. Therefore, by changing the voltage applied to the electro-optic element by the control circuit, the pulse width of the output pulse laser can be extended and the peak energy of the pulse laser can be reduced.
Further, since the pulse width can be extended without using a beam splitter that splits the laser light or a mirror for the delay optical system, the pulse laser oscillator can be formed in a compact manner.
Further, when the pulse laser oscillator is used, it is not necessary to adjust the beam splitter and the mirror for the delay optical system, so that the work for using the pulse laser oscillator becomes easy.

  According to the pulse laser oscillator of claim 2, the pulse of the pulse laser output by the voltage application circuit changing the voltage change rate applied to the electro-optic element stepwise by the control circuit. The width can be extended and the peak energy of the pulse laser can be reduced.

  Furthermore, according to the pulse laser oscillator of the third aspect, light can be polarized by the λ / 4 wavelength plate and the Pockels cell. Therefore, a Pockels cell that acts as a λ / 4 wavelength plate by applying a voltage can be used.

According to the pulse laser oscillation control method of the fourth aspect, it is possible to control the pulse width of the laser light by changing the voltage applied to the electro-optic element over time. Therefore, by changing the voltage applied to the electro-optic element, the pulse width of the output pulse laser can be extended and the peak energy of the pulse laser can be controlled.
In addition, a pulse laser oscillator using this pulse laser oscillation control method can be formed compactly because the pulse width can be extended without using a beam splitter that splits the laser beam or a mirror for a delay optical system. can do.
Furthermore, since the pulse laser oscillator does not require adjustment of the beam splitter or the mirror for the delay optical system, the operation at the time of use becomes easy.

  According to the pulse laser oscillation control method according to claim 5, the pulse width of the output pulse laser is extended by changing the change rate of the voltage applied to the electro-optic element stepwise, and the pulse laser The peak energy can be controlled.

It is the schematic which shows the voltage non-application state in embodiment of the pulse laser oscillator by this invention. It is the schematic which shows the voltage application state of the said pulse laser oscillator. It is a graph which shows an example of the relationship between the change of the voltage applied to the Pockels cell with which the said pulse laser oscillator was equipped, and the output energy of the pulse laser output. It is a graph which shows the other example of the relationship as described in FIG. 5 is a graph showing still another example of the relationship described in FIGS. 3 and 4.

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings.
FIG. 1 is a diagram showing an embodiment of a pulse laser oscillator according to the present invention. This pulse laser oscillator generates a giant pulse by a Q-switch method that generates a giant pulse by switching the Q value (energy stored in the optical resonator 3 described later / energy lost outside the optical resonator 3). A YAG laser that includes a YAG rod 1, a flash lamp 2, an optical resonator 3, a polarizer 4, a λ / 4 wavelength plate 5, a Pockels cell 6, and a voltage controller 7.

  The YAG rod 1 emits light when irradiated with light from a flash lamp 2 to be described later, and amplifies the emitted light by stimulated emission. As shown in FIG. It is a solid laser medium that emits light. Instead of the YAG rod 1, other laser media such as an Nd: YAG rod or an Er: YAG rod may be used.

  A flash lamp 2 is provided on a side surface of the YAG rod 1 (upper side of the YAG rod 1 in FIG. 1). The flash lamp 2 irradiates the YAG rod 1 with light and starts emission of light from the YAG rod 1, and a xenon flash lamp or a laser diode is used, for example.

  A front mirror 3a and a rear mirror 3b are provided on the left and right sides of the YAG rod 1 in FIG. The front mirror 3a and the rear mirror 3b reciprocate the light emitted from the YAG rod 1 between the two mirrors. The front mirror 3a and the rear mirror 3b cause stimulated emission in the YAG rod 1. An optical resonator 3 for amplifying coherent light is configured.

  The front mirror 3a is a partial transmission mirror that transmits a part of incident light, and is installed on the side from which the laser light on the optical axis L of the light emitted from the YAG rod 1 is emitted. A part of the laser light instantaneously amplified by the Q switch method is taken out from the optical resonator 3 through the front mirror 3a.

  The rear mirror 3b is a total reflection mirror provided on the optical axis L opposite to the front mirror 3a with the YAG rod 1 in between, and reciprocates light on the optical axis L with the front mirror 3a. Let

On the optical axis L between the rear mirror 3b and the YAG rod 1, a polarizer 4 is provided. The polarizer 4 reflects only s-polarized light, which is a polarized light component perpendicular to the incident surface, in the incident light, and transmits only p-polarized light, which is a polarized light component parallel to the incident surface. The polarizer plays the role of a shutter in the Q-switch method. The material of the polarizer 4 is glass or plastic, and the polarizer 4 is inclined with respect to the optical axis L so that the incident angle θ of incident light becomes a Brewster angle at which the reflectance of p-polarized light becomes zero. A plurality of polarizers 4 may be provided. The polarizer 4 only needs to transmit either s-polarized light or p-polarized light. In addition to the above, a polarizer such as a polarizing prism or a polarizing filter may be used.
The terms s-polarized light and p-polarized light used in the following description refer to s-polarized light and p-polarized light for the polarizer 4.

  A λ / 4 wavelength plate 5 is provided between the polarizer 4 and the rear mirror 3b. The λ / 4 wave plate 5 generates a phase difference of 90 ° (π / 2) in the polarization component of incident light, thereby converting linearly polarized light (s-polarized light or p-polarized light) into circularly polarized light and circularly polarized light into linearly polarized light. The light is converted into polarized light, and is provided on the optical axis L on the left side of the polarizer 4 as shown in FIG. The p-polarized light emitted from the YAG rod 1 and transmitted through the polarizer 4 is converted into circularly polarized light by the λ / 4 wavelength plate 5.

  A Pockels cell 6 is provided between the λ / 4 wavelength plate 5 and the rear mirror 3b. The Pockels cell 6 is an electro-optical element that polarizes light according to an applied voltage, and is provided on the optical axis L on the left side of the λ / 4 wavelength plate 5 as shown in FIG. The Pockels cell 6 does not polarize light when no voltage is applied, but polarizes light when a voltage is applied, and the degree of polarization depends on the applied voltage.

  A voltage control device 7 is electrically connected to the Pockels cell 6. The voltage control device 7 applies a voltage to the Pockels cell 6 and controls the voltage to be applied, and includes a voltage application circuit 7a and a control circuit 7b.

  The voltage application circuit 7 a applies a voltage to the Pockels cell 6 and is electrically connected to the Pockels cell 6. This voltage application circuit 7a controls the application of voltage to the Pockels cell 6 by the voltage application circuit 7a, thereby changing the degree of polarization of the light incident on the Pockels cell 6 and controlling the oscillation of the laser light. A circuit 7b is connected.

Next, the operation of the thus configured pulse laser oscillator and the pulse laser oscillation control method will be described with reference to FIGS.
When the pulse laser is oscillated by the pulse laser oscillator, first, the control circuit 7b sends a signal to the voltage application circuit 7a to control the applied voltage to the Pockels cell 6 to be 0V. In this state, when the flash lamp 2 emits light and irradiates the YAG rod 1 with light, some atoms in the YAG rod 1 are excited, and light is emitted from the YAG rod 1 along the optical axis L. . As shown in FIG. 1, the light emitted from the YAG rod 1 in the direction of the polarizer 4 (the direction of the arrow A) enters the polarizer 4 at an incident angle θ that is a Brewster angle. Of the incident light, p-polarized light is transmitted through the polarizer 4, and s-polarized light and circular (or elliptical) polarized light are reflected by the polarizer 4 and travel outward of the optical axis L.

  The p-polarized light transmitted through the polarizer 4 is incident on the λ / 4 wavelength plate 5 to generate a phase difference of 90 ° (π / 2), is converted into circularly polarized light, and is incident on the Pockels cell 6. Here, since no voltage is applied to the Pockels cell 6, the incident light is transmitted without being polarized. Therefore, the circularly polarized light incident on the Pockels cell 6 passes through the Pockels cell 6 as circularly polarized light, is reflected by the rear mirror 3b, and is transmitted through the Pockels cell 6 again, as shown in FIG. 5 is incident.

  When the circularly polarized light is incident on the λ / 4 wave plate 5, a phase difference of 90 ° (π / 2) is further generated, and s-polarized light (that is, p-polarized light emitted from the YAG rod 1 and transmitted through the polarizer 4 is the phase Is shifted by 180 ° (π)) and enters the polarizer 4 at an incident angle θ that is a Brewster angle. Since the polarizer 4 has a function of reflecting s-polarized light as described above, the incident s-polarized light is reflected by the polarizer 4 and travels outward of the optical axis L.

  Thus, in a state where no voltage is applied to the Pockels cell 6, the light emitted from the YAG rod 1 is reflected by the polarizer 4 and does not enter the YAG rod 1 again. Does not occur, and the oscillation of the pulse laser is suppressed.

  Next, after maintaining the above voltage non-applied state until the number of excited atoms in the YAG rod 1 becomes an amount necessary for energy to be output as a pulse laser (the inversion distribution becomes sufficiently large), The voltage applied to the Pockels cell 6 by the voltage application circuit 7a is changed by the control circuit 7b. When a predetermined voltage is applied to the Pockels cell 6 by the voltage application circuit 7 a, the Pockels cell 6 functions as the λ / 4 wavelength plate 5.

  In the state where the voltage is applied to the Pockels cell 6, the light emitted from the YAG rod 1 in the direction of the arrow A is converted into p-polarized light by the polarizer 4 as in the above-mentioned voltage non-application state, as shown in FIG. , Converted into circularly polarized light by the λ / 4 wavelength plate 5 and incident on the Pockels cell 6. Since the Pockels cell 6 functions as the λ / 4 wavelength plate 5 as described above, the light incident on the Pockels cell 6 generates a phase difference of 90 ° (π / 2) and is s-polarized (that is, the YAG rod 1 And the p-polarized light that has been transmitted through the polarizer 4 is converted into a phase shifted by 180 ° (π).

  This s-polarized light is reflected by the rear mirror 3b, is incident on the Pockels cell 6 again, and further causes a phase difference of 90 °. Is shifted to 270 ° (3π / 2). This circularly polarized light is incident on the λ / 4 wave plate 5, further generates a phase difference of 90 °, and has a phase difference of 360 ° with p-polarized light (that is, p-polarized light emitted from the YAG rod 1 and transmitted through the polarizer 4). (2π) shifted state).

  The p-polarized light is incident on the polarizer 4 at an incident angle θ that is a Brewster angle, and is transmitted through the polarizer 4. The light transmitted through the polarizer 4 is incident from the left side of the YAG rod 1 in FIG. 2, causes stimulated emission in the YAG rod 1, is emitted from the right side of the YAG rod 1 in FIG. 2, is reflected by the front mirror 3a, The YAG rod 1 passes from the right side to the left side in FIG. Thereafter, the light reciprocates in the optical resonator 3 in the same procedure, and a part of the coherent light amplified by stimulated emission is output as a laser in the direction of arrow B from the front mirror 3a.

  FIG. 3 is a graph showing an example of the relationship between the change in the voltage applied to the Pockels cell 6 and the output energy of the output pulse laser. In the embodiment of the pulse laser oscillator according to the present invention, when the voltage applied to the Pockels cell 6 by the control circuit 7b is changed from about 0 V to about −4000 V in about 100 ns, the peak of the pulse laser is obtained. The energy is about 13.0 mJ and the pulse width is about 10 ns.

  FIG. 4 is a graph showing another example of the relationship between the change in the voltage applied to the Pockels cell 6 and the output energy of the output pulse laser. In this embodiment, the voltage applied to the Pockels cell 6 by the control circuit 7b is gradually changed from about 0V to about −3000V by about 800 ns from the embodiment shown in FIG. 3 by the control circuit 7b. The peak energy of the pulse laser is about 0.6 mJ, and the pulse width is about 70 ns. Thus, by reducing the rate of change of the voltage applied by the voltage application circuit 7a by the control circuit 7b, the pulse width of the pulse laser can be extended and the peak energy can be reduced.

  FIG. 5 is a graph showing still another example of the relationship between the change in the voltage applied to the Pockels cell 6 and the output energy of the output pulse laser. In this embodiment, the control circuit 7b changes the voltage applied to the Pockels cell 6 by the control circuit 7b from about 0V to about -1500V in about 300ns, and then from about -1500V to about -4500V. Further, it is changed at 600 ns.

  As shown in FIG. 5, the voltage change rate changes once between a change in voltage from about 0 V to about -1500 V and a change in voltage from about -1500 V to about -4500 V. That is, the slope of the voltage graph at about −1500 V in FIG. 5 changes. As described above, when the voltage change rate is changed stepwise, a peak can be generated after the point C (hereinafter referred to as “control point”) C where the voltage change rate is changed by the control circuit 7b. In the present embodiment, the first peak energy of the pulse laser is about 0.5 to 0.6 mJ, the second peak energy is also about 0.5 to 0.6 mJ, and the pulse width is about 150 ns. . In this way, the control circuit 7b gradually changes the rate of change of the voltage applied to the Pockels cell 6 by the voltage application circuit 7a, thereby extending the pulse width of the output pulse laser and increasing the peak energy of the pulse laser. Can be reduced.

  The rate of change of the voltage applied by the voltage application circuit 7a and the number of control points C may be determined according to the required pulse width and output energy. Further, instead of using the λ / 4 wavelength plate 5, instead of the Pockels cell 6, an electro-optical element that functions as a λ / 2 wavelength plate according to the application of a voltage may be used.

DESCRIPTION OF SYMBOLS 1 ... YAG rod 2 ... Flash lamp 3 ... Optical resonator 3a ... Front mirror 3b ... Rear mirror 4 ... Polarizer 5 ... λ / 4 wavelength plate 6 ... Pockels cell 7 ... Voltage control device 7a ... Voltage application circuit 7b ... Control circuit B ... Arrow C indicating laser output direction ... Control point L ... Optical axis θ ... Incident angle

Claims (5)

A pulse laser oscillator comprising: an electro-optic element that polarizes light according to an applied voltage; and a voltage control device that applies a voltage to the electro-optic element and controls the voltage.
A pulse laser oscillator, wherein a voltage applied to the electro-optic element is changed with time by the voltage control device to control a pulse width of laser light.   The pulse laser oscillator according to claim 1, wherein the voltage control device changes a change rate of a voltage applied to the electro-optic element in a stepwise manner.   The pulse laser oscillator according to claim 1, wherein the electro-optical element is a Pockels cell and further includes a λ / 4 wavelength plate. In a laser oscillation control method for controlling oscillation of laser light by changing a voltage applied to an electro-optical element that polarizes light according to an applied voltage,
A pulse laser oscillation control method, wherein a voltage applied to the electro-optic element is changed with time to control a pulse width of laser light.   5. The pulse laser oscillation control method according to claim 4, wherein a change rate of a voltage applied to the electro-optic element is changed stepwise.