JP4541272B2 - Laser oscillation method and solid-state laser device - Google Patents

Description

  The present invention relates to a laser oscillation method and a solid-state laser device, and more particularly to a laser oscillation method and a solid-state laser device that perform laser oscillation in a single transverse mode.

  Conventionally, it is known that light reciprocating in a laser resonator oscillates in a specific state (mode).

Among these modes, the zero-order state of the transverse mode is referred to as a single transverse mode (TEM ₀₀ mode). On the other hand, when a higher order is included, it is called multimode.

  Here, the laser light obtained by laser oscillation in a single transverse mode has a Gaussian distribution (normal distribution) when viewed from a cross section perpendicular to the optical axis. Compared with laser light obtained by laser oscillation, it is known that it has the best light collecting property.

  For this reason, laser light obtained by laser oscillation in a single transverse mode is used in various technical fields including laser processing. In particular, when laser light obtained by laser oscillation in the single transverse mode is used for laser processing, it is possible to perform so-called fine cutting with a narrow cut width and a smooth cross section.

  By the way, in order to oscillate a solid laser medium formed in a rod shape such as a laser crystal as a laser medium in a single transverse mode, the effective excitation volume diameter and the mode volume diameter in the solid laser medium are made equal, or It is necessary to adjust various settings so that the mode volume diameter is larger than the effective excitation volume diameter.

  Here, the effective excitation volume diameter and the mode volume diameter will be described. First, the effective excitation volume diameter will be described with reference to FIG.

  That is, FIG. 1 shows a rod-shaped solid laser medium 10 having a cylindrical shape extending in the axial direction along a direction perpendicular to the paper surface.

  When excitation light is incident from the outer peripheral surface of the solid laser medium 10, that is, the side surface 10 a by a laser diode (LD) or the like to excite the solid laser medium 10, the solid laser medium 10 is excited. Part occurs.

  In the above-described excited state in the solid-state laser medium 10, a distribution is generated according to the degree of absorption of the excitation light as indicated by the broken line in FIG. 1 (the broken line in FIG. 1 is a region having a different degree of absorption of the excitation light. Conceptually shows the boundaries of.) Of these distributions of excited states, the place that actually contributes to laser oscillation is called the effective excitation volume, and the outermost circle of the concentric circle in the region where the excited states are distributed concentrically in the effective excitation volume. The diameter is referred to as the effective excitation volume diameter. The effective excitation volume diameter is represented by ωp.

  The mode volume diameter is a beam diameter which is a diameter of a laser beam of a single transverse mode determined by the configuration of the laser resonator of the solid-state laser device (see FIG. 2). The effective excitation volume diameter is represented by ωc.

  By the way, the side surface excitation (side) which is an excitation method for exciting the solid laser medium 10 by entering the excitation light from the outer peripheral surface of the solid laser medium 10 described with reference to FIG. In the case of a pump), it has been pointed out that the distribution of excited states is unlikely to be concentric, and it is difficult to obtain a single transverse mode laser oscillation mode.

  For this reason, in the conventional side-pumped solid-state laser device, for example, the resonator length of the laser resonator is changed according to the effective excitation volume diameter, or the curvature of the output mirror and rear mirror in the laser resonator is changed. By changing the mode volume diameter, the mode volume diameter is carefully fine-tuned to obtain a single transverse mode laser oscillation mode.

However, when the intensity of the excitation light (excitation intensity) is changed to change the laser output, the effective excitation volume diameter changes and the size of the thermal lens also changes. The relationship with the diameter changes, and it is difficult to maintain the single transverse mode laser oscillation mode. That is, even if the laser oscillation mode of the single transverse mode can be maintained at a certain excitation intensity, if the excitation intensity is changed to change the laser light output, the effective excitation volume diameter and the mode volume diameter are changed. As a result, the laser oscillation mode becomes a higher order mode (multi-mode), and there is a problem that laser oscillation cannot be stably performed in a single transverse mode.

In the case of pulse oscillation using a Q switch or the like in a conventional side-pumped solid-state laser device, if the repetition frequency of pulse oscillation is changed even if the excitation intensity is kept constant, the solid-state laser medium As the stored energy changes, the size of the thermal lens changes. As a result, the relationship between the effective excitation volume diameter and the mode volume diameter also changes, and there is a problem that it is impossible to maintain the single transverse mode laser oscillation mode.

As a technique for solving the problem that the single transverse mode cannot be maintained by changing the relationship between the effective excitation volume diameter and the mode volume diameter by changing the configuration of the laser resonator as described above, There is known a technique for forcibly limiting a mode in which an aperture is inserted in a laser resonator and reciprocates in the laser resonator.

  However, according to this method, it is possible to maintain stable single transverse mode laser oscillation, but this causes a new problem that efficiency is lowered due to loss in the laser resonator. there were.

In addition, in a mode-locked laser in which a mode locker is arranged in the laser resonator of a conventional side-pumped solid-state laser device, a specific high frequency signal is applied to the mode locker from the outside, and the pulse width is as short as about picoseconds In this case, it is necessary to accurately set the effective length L of the laser resonator so as to satisfy the condition shown in Expression (1) in accordance with the frequency of the applied high-frequency signal.

L = c / 4ν Formula (1)
c: speed of light ν: frequency of the applied high-frequency signal Generally, in order to satisfy the condition shown in the above equation (1), measures such as changing the curvature of the mirror in the laser resonator or inserting a lens are required. It was necessary.

  By the way, in the mode-locked laser as described above, there are two different cases when performing wavelength conversion such as sum frequency generation or difference frequency generation, or when performing spectroscopic measurement by the pump-and-probe method which is one of time-resolved spectroscopy. It is necessary to synchronize the repetition of short pulse laser beams output from two solid-state laser devices accurately.

  Here, in order to accurately synchronize the repetitions of the short pulse laser beams output from two different solid-state laser devices, it is necessary to accurately match the effective length of the laser resonator. When the wavelength of the short pulse laser beam to be output is different, the size of the thermal lens generated in the laser resonator is different even when the same solid-state laser medium is used.

For this reason, it is difficult to accurately match the effective lengths of the laser resonators in two different solid-state laser devices, and the laser resonance is performed so that both solid-state laser devices are both in an optimal state, that is, satisfy the condition shown in Equation (1). There is a problem that it is difficult to set the effective length L of the vessel accurately.

The prior art that the applicant of the present application knows at the time of filing a patent is as described above and is not an invention related to a known literature, so there is no prior art information to be described.

  The present invention has been made in view of the above-described various problems of the prior art, and an object of the present invention is to stably maintain a single transverse mode as a laser oscillation mode. An object of the present invention is to provide a laser oscillation method and a solid-state laser device.

  Another object of the present invention is to provide a laser oscillation method and a solid-state laser device that can accurately synchronize the repetition of short pulse laser beams output from two different solid-state laser devices. Is.

  In order to achieve the above object, the present invention provides a rod diameter selected according to the resonator length of a laser resonator and the size of a thermal lens generated in a rod-shaped solid laser medium as a solid laser medium of a solid laser device. The solid-state laser medium is used.

  Therefore, according to the present invention, the rod diameter of the solid-state laser medium of the solid-state laser device is selected according to the resonator length of the laser resonator. Laser oscillation can be maintained stably.

That is, the invention according to claim 1 of the present invention is provided with a cylindrical shape in a laser oscillation method in which laser oscillation is performed in a single transverse mode by side-exciting a solid laser medium disposed in a laser resonator. The rod diameter of the rod-shaped solid laser medium is a curve showing the change of the effective pump volume diameter with the change of the pump intensity of the solid laser medium according to the cavity length of the laser resonator and the pump intensity of the solid laser medium The rod diameter is such that the curve showing the change in the mode volume diameter accompanying the change in the diameter of the rod has a point of contact at only one point, and the effective excitation volume diameter is the distribution of the excited states in the solid-state laser medium. Thus, the concentric circles in the region where the excited states are concentrically distributed in the effective excitation volume, which is actually contributing to the laser oscillation. Outermost circle is the diameter, and is the mode volume diameter, in which as is the diameter of the laser light beam of a single transverse mode determined by the configuration of the laser resonator.

  According to a second aspect of the present invention, in the first aspect of the present invention, the resonator length of the laser resonator is 76 cm, and the rod diameter is 1.7 mm. The laser beam having a wavelength of 1064 nm is oscillated.

  According to a third aspect of the present invention, in the second aspect of the present invention, a Q switch is disposed in the laser resonator to cause pulse oscillation.

According to a fourth aspect of the present invention, in the first aspect of the present invention, a mode locker is disposed in the laser resonator, and the resonator length of the laser resonator is set to the laser. Effective length of resonator Effective length = c / 4ν
c: speed of light
ν: The frequency of the high-frequency signal applied to the mode locker.

According to a fifth aspect of the present invention, the rod-shaped solid laser medium having a cylindrical shape respectively disposed in two laser resonators each having a mode locker is excited on each side. In the laser oscillation method of oscillating synchronized pulsed laser light in a single transverse mode, a first laser resonator including a first mode locker and a first solid state laser medium, a second mode locker, And a second laser resonator including a second solid-state laser medium, the rod diameter of the rod-shaped first solid-state laser medium having a cylindrical shape is set to the resonance of the first laser resonator. Depending on the length of the solid laser medium, the curve indicating the change in the effective excitation volume diameter accompanying the change in the excitation intensity of the solid laser medium and the change in the excitation intensity of the solid laser medium The curve indicating the change in mode volume diameter is a rod diameter that draws a curve having a contact at only one point, and the rod diameter of the rod-shaped second solid laser medium having a cylindrical shape is the second diameter of the second solid laser medium. A curve indicating a change in effective excitation volume diameter associated with a change in excitation intensity of the solid laser medium and a change in mode volume diameter associated with a change in excitation intensity of the solid laser medium according to the cavity length of the laser resonator. The rod diameter is such that the curve draws a curve with a contact at only one point, and the effective excitation volume diameter is a place that actually contributes to laser oscillation in the distribution of excitation states in the solid laser medium. Is the diameter of the outermost circle of the concentric circle in the region where the excited states are distributed concentrically in the effective excitation volume, and the mode volume diameter is A diameter of the laser light beam of a single transverse mode determined by the configuration of the Za resonator, the resonator length of the first laser cavity is 100 cm, the rod diameter of the first solid-state laser medium is 2 mm, and the laser oscillates a laser beam having a wavelength 1064 nm, the resonator length of the second laser resonator is 100 cm, the rod diameter of the second solid-state laser medium is 2.6 mm, the wavelength 1319nm This laser beam is made to oscillate.

According to a sixth aspect of the present invention, a solid-state laser medium is arranged in a laser resonator, and excitation light is incident from a side surface of the solid-state laser medium to laterally excite the solid-state laser medium. In a solid-state laser device that oscillates in a single transverse mode, a rod-shaped solid laser medium with a cylindrical shape is effective as the excitation intensity of the solid-state laser medium changes according to the cavity length of the laser resonator. and the curve showing the change in the mode volume diameter with changes in excitation intensity curve and the solid-state laser medium exhibiting a change in the excitation volume diameter is provided with a rod diameter such as to draw a curve having a contact only one point, the effective excitation The volume diameter is an effective excitation volume that is a place that actually contributes to laser oscillation in the distribution of excitation states in the solid-state laser medium. Is the diameter of the outermost circle of the concentric circles in the region where the excited states are concentrically distributed in the chamber, and the mode volume diameter is a laser of a single transverse mode determined by the configuration of the laser resonator This is the diameter of the light beam .

  According to a seventh aspect of the present invention, in the solid-state laser device according to the sixth aspect, the resonator length of the laser resonator is 76 cm, the rod diameter is 1.7 mm, and the wavelength is 1064 nm. The laser beam is made to oscillate.

  According to an eighth aspect of the present invention, in the solid-state laser device according to the seventh aspect, a Q switch arranged in the laser resonator is provided.

According to a ninth aspect of the present invention, in the solid-state laser device according to the sixth aspect of the present invention, the solid-state laser device includes a mode locker disposed in the laser resonator, and the resonator length of the laser resonator is the laser length. Effective length of resonator Effective length = c / 4ν
c: speed of light
ν: The frequency of the high-frequency signal applied to the mode locker.

In the invention according to claim 10 of the present invention, a synchronized pulsed laser beam can be obtained by exciting the solid laser medium respectively disposed in two laser resonators each having a mode locker. In a solid-state laser device that oscillates in one transverse mode, a first laser resonator including a first mode locker and a first solid-state laser medium, a second mode locker, and a second solid-state laser medium are provided. A rod-shaped first solid-state laser medium having a cylindrical shape, the rod-shaped first solid-state laser medium according to a cavity length of the first laser resonator. A curve showing the change in effective excitation volume diameter with changes in excitation intensity and a curve showing the change in mode volume diameter with changes in the excitation intensity of the solid laser medium. However, the rod-shaped second solid-state laser medium having a rod diameter that draws a curve having a contact point at only one point and having a cylindrical shape depends on the resonator length of the second laser resonator. The curve indicating the change in effective excitation volume diameter associated with the change in the excitation intensity of the solid laser medium and the curve indicating the change in mode volume diameter associated with the change in the excitation intensity of the solid laser medium have a single point of contact. It has a rod diameter that draws a curve, and the effective excitation volume diameter is the excitation volume within the effective excitation volume that is the place that actually contributes to laser oscillation in the distribution of excitation states in the solid laser medium. The diameter of the outermost circle of the concentric circle in the region where the states are distributed concentrically, and the mode volume diameter is a single value determined by the configuration of the laser resonator A diameter of the laser light beam mode, the resonator length of said first laser resonator and 100 cm, the rod diameter of the first solid-state laser medium and 2 mm, to a laser oscillating a laser beam having a wavelength of 1064nm The resonator length of the second laser resonator is 100 cm, the rod diameter of the second solid-state laser medium is 2.6 mm, and laser light having a wavelength of 1319 nm is oscillated.

  Since the present invention is configured as described above, it is possible to provide a laser oscillation method and a solid-state laser device that can stably maintain a single transverse mode as a laser oscillation mode. Has an effect.

  In addition, since the present invention is configured as described above, it has an excellent effect that it is possible to accurately synchronize the repetition of the short pulse laser light output from two different solid-state laser devices. .

  Hereinafter, an example of an embodiment of a laser oscillation method and a solid-state laser device according to the present invention will be described in detail with reference to the accompanying drawings.

  In the following description, the same or corresponding components are denoted by the same reference numerals, and a duplicate description thereof is omitted as appropriate.

FIG. 2 is a conceptual configuration explanatory diagram of a solid-state laser device according to an example of the embodiment of the present invention. In FIG. 2, in order to facilitate understanding, the internal configuration of the excitation unit 110 described later is omitted.

  This solid-state laser device 100 is disposed between a laser resonator having an output mirror 102 and a rear mirror 104, and between the output mirror 102 and the rear mirror 104, and has a rod shape with a cylindrical shape inside. And an excitation unit 110 in which a solid-state laser medium 114 is arranged.

  Here, FIG. 3 shows a schematic configuration end view of the excitation unit 110 taken along line III-III. The detailed configuration of the excitation unit 110 will be described with reference to FIG.

  That is, the excitation unit 110 includes a case 112 in which a window portion transparent to the wavelength of the laser beam output from the solid-state laser device 100 is arranged on the optical path of the laser beam, and a horizontal plane with respect to the paper surface in the case 112. A rod-shaped solid laser medium 114 having a cylindrical shape arranged so that the axial direction extends along the direction and both end faces 114a and 114b are positioned in the left-right direction of the paper surface, and the outer peripheral surface of the solid laser medium 114 In other words, the three laser diodes 116a, 116b, 116c arranged to face the side surface 114c, the flow tube 118 arranged to cover the side surface 114c of the solid-state laser medium 114, and the solid-state laser medium 114 are sandwiched. Are arranged so as to face the laser diodes 116a, 116b, 116c, respectively. Three reflectors 120a which, 120b, is configured to have a 120c.

Hereinafter, each configuration of the above-described excitation unit 110 will be described in more detail. First, the solid laser medium 114 has a cylindrical diameter (hereinafter referred to as “rod diameter”) D set to a predetermined value. Yes. A method for determining the value of the rod diameter D will be described in detail later. As the solid laser medium 114, for example, an Nd: YAG single crystal, Ti: SiO ₄ single crystal, or the like is used.

  Next, the laser diodes 116a, 116b, and 116c are arranged at equal intervals around the side surface 114c with the light emitting surfaces 116aL, 116bL, and 116cL facing the side surface 114c so as to face the center O of the solid-state laser medium 114. For example, laser light having a wavelength of 808 nm and an emission width of 1 cm is incident as excitation light into the solid-state laser medium 114 from the light emitting surfaces 116aL, 116bL, and 116cL. Note that the distances X between the light emitting surfaces 116aL, 116bL, 116cL and the center O of the solid-state laser medium 114 are set to 4.5 mm, for example.

  Next, the flow tube 118 is supplied with pure water for cooling in the flow tube 118, and the pure water for cooling flows in the flow tube 118, so that the heat load of the solid laser medium 114 is increased. The influence of is suppressed.

  The reflectors 120a, 120b, and 120c face the light emitting surface 116aL of the laser diode 116a with the solid laser medium 114 sandwiched so that the reflection surface 120aR of the reflector 120a faces the center O of the solid laser medium 114, and the reflector 120b. The reflecting surface 120bR faces the center O of the solid-state laser medium 114 so as to face the light-emitting surface 116bL of the laser diode 116b with the solid-state laser medium 114 interposed therebetween, and the reflecting surface 120cR of the reflector 120c faces the center O of the solid-state laser medium 114. In this manner, the light emitting surfaces 116aL, 116bL, and 116cL of the laser diodes 116a, 116b, and 116c are opposed to the light emitting surface 116cL of the laser diode 116c with the solid laser medium 114 interposed therebetween. They are equally spaced around the side surface 114c so as not to block the excitation light Isa.

  Accordingly, the excitation light emitted from the light emitting surface 116aL of the laser diode 116a and transmitted through the solid laser medium 114 is reflected by the reflecting surface 120aR of the reflector 120a and is incident on the solid laser medium 114 again. Similarly, the excitation light emitted from the light emitting surface 116bL of the laser diode 116b and transmitted through the solid laser medium 114 is reflected by the reflecting surface 120bR of the reflector 120b and is incident on the solid laser medium 114 again. The excitation light emitted from the light emitting surface 116cL and transmitted through the solid laser medium 114 is reflected by the reflecting surface 120cR of the reflector 120c and is incident on the solid laser medium 114 again.

In the above configuration, when the excitation light emitted from the light emitting surfaces 116aL, 116bL, 116cL of the laser diodes 116a, 116b, 116c is incident on the side surface 114c of the solid laser medium 114 and the solid laser medium 114 is side-excited, a solid state laser is obtained. The medium 114 oscillates and emits laser light. The laser light emitted from the solid-state laser medium 114 is amplified by reciprocating in a laser resonator including the output mirror 102 and the rear mirror 104, and output. Output from the mirror 102 to the outside of the laser resonator.

  At this time, if the rod diameter D of the solid-state laser medium 114 is set to a value determined by a method described later, the single transverse mode can be stably maintained as the laser oscillation mode.

Hereinafter, a method for determining the rod diameter D of the solid-state laser medium 114 will be described with reference to the solid-state laser device 100 described above. In the following description, the solid-state laser device 100 is specifically assumed to have the following configuration.

  First, as the solid-state laser medium 114, an Nd: YAG single crystal that oscillates laser light with a wavelength of 1064 nm was used.

  Next, the output mirror 102 and the rear mirror 104 constituting the laser resonator are both configured by a curvature ∞, that is, a flat plane mirror.

  The output mirror 102 is provided with a partial reflection coating (reflectance 90%) so as to reflect 90% of the laser light having a wavelength of 1064 nm, and a part of the laser light having a wavelength of 1064 nm that reciprocates in the laser resonator. Enabled to output to the outside.

  On the other hand, the rear mirror 104 is provided with a reflective coating (reflectance of 99.9% or more) so as to reflect all the laser light having a wavelength of 1064 nm, so that the laser light reciprocating in the laser resonator is not output to the outside. I did it.

  In this laser resonator, the resonator length L1, which is the distance between the output mirror 102 and the rear mirror 104, was set to 76 cm.

  As the laser diodes 116a, 116b, and 116c, those that output laser light having a wavelength of 808 nm and an emission width of 1 cm as excitation light were used.

Therefore, in the solid-state laser device 100 having the above-described configuration, the solid-state laser medium 114 is excited by the laser light having a wavelength of 808 nm of the laser diodes 116a, 116b, and 116c, and the laser having a wavelength of 1064 nm emitted from the excited solid-state laser medium 114. The light reciprocates in the laser resonator, is amplified during the reciprocation, and a part of the light is output from the output mirror 102. At this time, the excited Nd: YAG single crystal as the solid laser medium 114 exhibits the thermal lens effect described above.

By the way, as described above, in order to realize laser oscillation in a single transverse mode, the relationship between the effective excitation volume diameter ωp and the mode volume diameter ωc is
ωp = ωc or ωp <ωc
Must be set to be

That is,
ωp> ωc
In this state, an excitation portion that is not used in the single transverse mode is generated in the solid-state laser medium, so that laser oscillation is performed in multimode.

Also, ωp <ωc
In this state, the modes that can oscillate are limited, and laser oscillation in a single transverse mode is possible. However,
ωp << ωc
In this state, loss occurs and the efficiency of oscillation output decreases.

further,
ωp = ωc
In this state, the energy within the effective excitation volume diameter ωp can be reciprocated most efficiently within the laser resonator, so that the efficiency of the laser output is maximized.

Here, when the resonator length L1 of the laser resonator is 76 cm and an Nd: YAG single crystal having a rod diameter D of 2 mm is used as the solid laser medium 114, according to the experiment of the present inventor, an effective excitation volume is obtained. The diameter ωp and the mode volume diameter ωc change as shown in FIG. 4A according to the excitation intensity (LD excitation intensity) of the laser diodes 116a, 116b, and 116c. Further, the input / output characteristics of the solid-state laser device 100 showing the relationship between the LD excitation intensity and the laser light output at that time are as shown in FIG.

  That is, the effective excitation volume diameter ωp changes so as to gradually increase as the LD excitation intensity increases. On the other hand, the mode volume diameter ωc gradually decreases as the LD excitation intensity increases because the size of the thermal lens changes according to the change in the LD excitation intensity, but rapidly increases when the LD excitation intensity exceeds a certain value. It will increase.

Therefore, in FIG. 4A, at the point A where the LD excitation intensity is the lowest, the effective excitation volume diameter ωp <the mode volume diameter ωc.
Therefore, it becomes a single transverse mode, and the effective excitation volume diameter ωp <mode volume diameter ωc until the LD excitation intensity is increased to the point B.
The single transverse mode is maintained and the effective excitation volume diameter ωp = mode volume diameter ωc at point B.
Thus, the single transverse mode is achieved and the efficiency is maximized.

Furthermore, until the LD excitation intensity is increased and exceeds point B to point C,
Effective excitation volume diameter ωp> mode volume diameter ωc
The multi-mode is continued and the effective excitation volume diameter ωp = mode volume diameter ωc at point C.
Thus, the single transverse mode is achieved and the efficiency is maximized.

When the LD excitation intensity is further increased,
Effective excitation volume diameter ωp <mode volume diameter ωc
However, at point D, the value of the mode volume diameter ωc is far away from the value of the effective excitation volume diameter ωp, so that the loss increases and laser oscillation stops.

  Therefore, from FIGS. 4A and 4B, when the resonator length L1 of the laser resonator is 76 cm and the solid laser medium 114 is an Nd: YAG single crystal having a rod diameter D of 2 mm, it is stable. The single transverse mode laser oscillation state can be maintained when the laser light output from point A to point B is low.

Next, when an Nd: YAG single crystal having a rod length D of 1.7 mm is used as the solid-state laser medium 114 with the resonator length L1 of the laser resonator being 76 cm, it is effective according to the experiment of the present inventor. The excitation volume diameter ωp and the mode volume diameter ωc change as shown in FIG. 5A according to the excitation intensity (LD excitation intensity) of the laser diodes 116a, 116b, and 116c. Also, the input / output characteristics of the solid-state laser device 100 showing the relationship between the LD excitation intensity and the laser light output at that time are as shown in FIG.

  That is, a curve indicating a change in effective excitation volume diameter ωp associated with a change in LD excitation intensity and a curve indicating a change in mode volume diameter ωc associated with a change in LD excitation intensity are curves having a contact at only one point A ′. Draw.

That is, the effective excitation volume diameter ωp <mode volume diameter ωc until the LD excitation intensity is increased to the point A ′.
The single transverse mode is maintained and the effective excitation volume diameter ωp = mode volume diameter ωc at the point A ′.
Thus, the single transverse mode is achieved and the efficiency is maximized. At this time, the laser beam output is also maximized.

  Therefore, from FIGS. 5A and 5B, when the cavity length L1 of the laser resonator is 76 cm and the solid laser medium 114 is an Nd: YAG single crystal having a rod diameter D of 1.7 mm, LD The curve indicating the change in the effective excitation volume diameter ωp accompanying the change in the excitation intensity and the curve indicating the change in the mode volume diameter ωc accompanying the change in the LD excitation intensity draw a curve having a contact at only one point A ′. The laser oscillation state of the single transverse mode can be stably maintained from the state where the laser light output is low to the maximum, and the maximum efficiency is obtained at the point where the laser light output is maximum. Can do.

  In addition, at the point A ′ where the maximum laser light output is obtained, the slope of the curve indicating the effective excitation volume diameter ωp and the curve indicating the mode volume diameter ωc is equal. The variation of the mode volume diameter ωc can be minimized with respect to the effective excitation volume diameter ωp.

From the above description with reference to FIGS. 4A and 4B and FIGS. 5A and 5B, a rod as the solid-state laser medium 114 is used for a laser resonator having a resonator length L1 of 76 cm. By selecting one having a diameter of 1.7 mm, it is possible to obtain optimum input / output characteristics in single transverse mode laser oscillation. The beam quality of the output laser light is
M ² <1.1
It becomes.

Here, in FIG. 6, as shown in FIGS. 5 (a) and 5 (b), the laser oscillation state of the single transverse mode can be stably maintained from the state where the laser light output is low to the maximum. In addition, the maximum efficiency can be obtained at the point where the laser light output is maximum, and if the thermal lens varies, the mode volume diameter ωc with respect to the effective excitation volume diameter ωp. The relationship between the resonator length L1 of the laser resonator and the rod diameter D of the solid-state laser medium 114 in the solid-state laser device 100 that can minimize the fluctuation of the solid-state laser is shown.

  That is, when the combination of the resonator length L1 and the rod diameter D on the curve showing the combination of the resonator length L1 and the rod diameter D shown in FIG. Until it becomes possible to maintain the laser oscillation state of the single transverse mode stably, the maximum efficiency can be obtained at the point where the laser light output is maximum, and further, the thermal lens fluctuates. Even in the case where there is, the fluctuation of the mode volume diameter ωc can be minimized with respect to the effective excitation volume diameter ωp. That is, referring to FIG. 6, when the rod diameter of the solid-state laser medium 114 optimum for the resonator length L1 of the resonator is selected, a curve indicating the change in the effective excitation volume diameter ωp accompanying the change in the LD excitation intensity and the LD excitation The curve showing the change in the mode volume diameter ωc accompanying the change in intensity is a curve having a contact point at only one point. Even if the LD excitation intensity is changed during laser light output, it is not easy to change to the multimode. Therefore, it is possible to stably maintain single transverse mode laser oscillation.

Next, FIG. 7 shows a conceptual configuration explanatory diagram of a solid-state laser device provided with a Q switch.

  Here, when the solid-state laser device 200 provided with the Q switch is compared with the solid-state laser device 100 described above, they are different only in that the solid-state laser device 200 includes the Q switch 202.

  FIG. 8 shows the experimental results of using the solid-state laser device 200 provided with the Q switch 202 to output the pulsed laser light by fixing the LD excitation intensity to 75 W and changing the oscillation repetition frequency of the Q switch 202. Has been. In addition, about the solid laser medium 114, the experimental result about each with the rod diameter D of 2 mm and 1.7 mm was shown.

  In this solid-state laser device 200, since the thermal lens changes due to the change in the repetition frequency of the Q switch 202, the mode volume diameter ωc determined by the configuration of the laser resonator with the change in the repetition frequency of the Q switch 202 is shown in FIG. To change.

  At this time, since the LD excitation intensity is fixed, even if the repetition frequency of the Q switch 202 is changed, the value of the effective excitation volume diameter ωp hardly changes.

From the experimental results shown in FIG. 8, when the solid laser medium 114 having a rod diameter of 2 mm is used, when the repetition frequency of the Q switch 202 is 1 kHz or less,
Effective excitation volume diameter ωp <mode volume diameter ωc
In this state, single transverse mode laser oscillation is obtained. However, when the repetition frequency of the Q switch 202 exceeds 1 kHz,
Effective excitation volume diameter ωp> mode volume diameter ωc
In this state, laser oscillation occurs in multimode.

On the other hand, when a solid laser medium 114 having a rod diameter of 1.7 mm is used, the effective excitation volume diameter ωp <mode volume diameter ωc at all repetition frequencies of the Q switch 202.
Therefore, it is possible to always achieve single transverse mode laser oscillation.

  That is, when a solid laser medium 114 having a rod diameter of 1.7 mm is used, even if the repetition frequency of the Q switch 202 is increased during pulse laser oscillation in the single transverse mode by the Q switch 202, a stable single transverse medium can be obtained. Pulse laser oscillation can be maintained in the mode. Thus, even when pulse laser oscillation is performed using the Q switch 202, the single transverse mode can be maintained.

Next, FIG. 9 shows an explanatory diagram of a conceptual configuration of a solid-state laser device provided with a mode locker.

  Comparing the solid-state laser device 300 having the mode locker shown in FIG. 9 with the solid-state laser device 100 described above, the solid-state laser device 300 is different only in that the mode-locker 302 is provided.

Here, the mode locker generates a short pulse laser beam by applying a high-frequency signal having a specific frequency from the outside. At this time, the effective length L of the laser resonator corresponds to the applied high-frequency signal,
L = c / 4ν Formula (1)
c: speed of light ν: it is necessary to accurately satisfy the condition of the frequency of the applied high-frequency signal.

  In the effective length L of the laser resonator determined by this mode locker, by selecting the solid laser medium 114 having the optimum rod diameter, the laser beam output can be stably changed from a low state to a maximum in a single transverse mode. The laser oscillation state can be maintained, and the maximum efficiency can be obtained at the point where the laser light output is maximum. Further, when the thermal lens varies, the effective excitation volume diameter The variation of the mode volume diameter ωc can be minimized with respect to ωp.

  That is, by selecting a solid laser medium having a rod diameter most suitable for the effective length of the laser resonator calculated from the frequency of the mode locker, a laser resonator having the above characteristics can be realized and a single transverse mode can be realized. Can keep the light.

  For example, when a mode locker having a frequency ν of 75 MHz is used, the value of the effective length L needs to be 100 cm. Referring to FIG. 6, the rod diameter corresponding to the effective length L of 100 cm is 2 mm. Therefore, if a solid laser medium 114 having a rod diameter of 2 mm is selected, a stable single transverse mode can be obtained as described above. It becomes possible.

By the way, when the pulses are output in synchronization with two different mode-locked lasers, it is necessary to accurately match the repetition of outputs from the two different mode-locked lasers. And in order to match this repetition correctly, it is necessary to match | combine the effective length of the laser resonator of two different mode lock lasers correctly.

  At this time, when the oscillation wavelengths of two different mode-locked lasers are different, the size of the thermal lens will be different. Therefore, if a solid laser medium having the same rod diameter is used, the optimum effective length will be different. It will be.

  Here, FIG. 10 shows the relationship between the resonator length L1 of the laser resonator and the rod diameter D of the Nd: YAG single crystal as the solid-state laser medium 114 when the solid-state laser device 100 oscillates at a wavelength of 1319 nm. It is a graph. As the output mirror 102, a partial reflection mirror having a reflectance of 95% with respect to a laser beam having a wavelength of 1319 nm is used. As the rear mirror 104, 99.9% or more of a laser beam having a wavelength of 1319 nm is used. A total reflection mirror with reflectivity was used.

  When laser oscillation was performed under these conditions, a laser beam output of 4 W or more was obtained if the rod diameter of the fixed laser medium 114 was 2 to 3 mm.

Here, when the Nd: YAG single crystal serving as the solid-state laser medium 114 is laser-oscillated with an oscillation line having a wavelength of 1319 nm, the thermal lens becomes stronger than that at the time of laser oscillation at a wavelength of 1064 nm, and the effective length of the optimum laser resonator is short. Become.

  That is, generally, in order to weaken the effect of the thermal lens, it is necessary to shorten the effective length of the laser resonator.

  Therefore, when the effective length of the laser resonator that oscillates at a wavelength of 1064 nm and the laser resonator that oscillates at a wavelength of 1319 nm is equal to 100 cm, the rod of the solid laser medium 114 in the laser resonator that oscillates at a wavelength of 1064 nm. When the diameter is 2.0 mm and the rod diameter of the solid laser medium 114 in the laser resonator that oscillates at a wavelength of 1319 nm is 2.6 mm, it becomes possible to output a pulse laser beam that is completely synchronized between the two. The laser oscillation state of the single transverse mode can be stably maintained from the state where the laser light output is low to the maximum, and the maximum efficiency can be obtained at the point where the laser light output is maximum. If there is a change in the thermal lens It will be able to minimize the variation of the mode volume diameter ωc the effective pumping volume diameter .omega.p.

The above-described embodiment described above may be modified as described in the following (1) to (3).

  (1) In the above-described embodiment, the case where an Nd: YAG single crystal is used as the solid laser medium has been described. However, the solid laser medium is not limited to the Nd: YAG single crystal. Various solid-state laser media can be used as long as they have a rod-like shape with a cylindrical shape.

  (2) In the above-described embodiment, three laser diodes are used as the excitation light source. However, the number of excitation light sources is not limited to three. One, two, or four or more light sources may be provided.

  (3) You may make it combine the above-mentioned embodiment and the modification shown in above-mentioned (1) thru | or (2) suitably.

  The present invention can be used for laser processing in which the cutting width is narrow and the section is smooth, that is, so-called fine cutting.

FIG. 1 is an explanatory diagram of a conceptual configuration of a rod-shaped solid laser medium having a cylindrical shape whose axial direction extends along a direction perpendicular to the paper surface. FIG. 2 is a conceptual configuration explanatory diagram of a solid-state laser device according to an example of an embodiment of the present invention. FIG. 3 is a schematic end view of the excitation unit taken along line III-III. FIG. 4 (a) shows an effective excitation volume diameter ωp associated with a change in LD excitation intensity when the cavity length of the laser resonator is 76 cm and an Nd: YAG single crystal having a rod diameter of 2 mm is used as the solid laser medium. 4B is a graph showing the input / output characteristics of the solid-state laser device showing the relationship between the LD excitation intensity and the laser beam output at that time. FIG. 5A shows an effective excitation volume associated with a change in LD excitation intensity when a laser resonator has a resonator length of 76 cm and an Nd: YAG single crystal having a rod diameter of 1.7 mm is used as a solid laser medium. FIG. 5B is a graph showing the input / output characteristics of the solid-state laser device showing the relationship between the LD excitation intensity and the laser beam output at that time. FIG. 6 shows that the laser oscillation state of the single transverse mode can be stably maintained from the low state to the maximum when the laser light output at the wavelength of 1064 nm is maximized, and the maximum laser light output is at the maximum. In the solid-state laser device that can obtain the efficiency, and further, when the thermal lens varies, the variation of the mode volume diameter ωc with respect to the effective excitation volume diameter ωp can be minimized. It is a graph which shows the relationship between the resonator length of a laser resonator, and the rod diameter of a solid laser medium. FIG. 7 is an explanatory diagram of a conceptual configuration of a solid-state laser device including a Q switch. FIG. 8 shows an effective excitation volume diameter ωp when a solid-state laser device provided with a Q switch is used and the LD excitation intensity is fixed at 75 W and the pulse repetition is output by changing the oscillation repetition frequency of the Q switch. 5 is a graph showing the relationship between the change in the mode volume and the change in the mode volume diameter ωc. FIG. 9 is an explanatory diagram of a conceptual configuration of a solid-state laser device including a mode locker. FIG. 10 shows that the laser oscillation state of the single transverse mode can be stably maintained from the low state to the maximum when the laser light output at the wavelength of 1319 nm is maximized, and the maximum laser light output is at the maximum. In the solid-state laser device that can obtain the efficiency, and further, when the thermal lens varies, the variation of the mode volume diameter ωc with respect to the effective excitation volume diameter ωp can be minimized. It is a graph which shows the relationship between the resonator length of a laser resonator, and the rod diameter of a solid laser medium.

Explanation of symbols

10, 114 Solid laser medium 10a, 114c Solid laser medium side surface 100, 200, 300 Solid laser device 102 Output mirror 104 Rear mirror 110 Excitation unit 112 Case 114a, 114b Solid laser medium end face 116a, 116b, 116c Laser diode 116aL, 116bL, 116cL Light emitting surface of laser diode 118 Flow tube 120a, 120b, 120c Reflector 120aR, 120bR, 120cR Reflector surface of reflector 202 Q switch 302 Mode locker

Claims (10)

In a laser oscillation method in which laser oscillation is performed in a single transverse mode by side-exciting a solid-state laser medium disposed in a laser resonator,
The rod diameter of a rod-shaped solid laser medium having a cylindrical shape is a curve showing the change in effective pumping volume diameter accompanying the change in pumping intensity of the solid laser medium according to the cavity length of the laser resonator and the solid The rod diameter is such that the curve showing the change in the mode volume diameter accompanying the change in the excitation intensity of the laser medium draws a curve with a contact at only one point ,
The effective excitation volume diameter is a region in which excitation states are concentrically distributed in an effective excitation volume that is a place that actually contributes to laser oscillation in the distribution of excitation states in the solid-state laser medium. The diameter of the outermost circle of the concentric circles of
The mode volume diameter is a laser beam diameter of a single transverse mode determined by the configuration of the laser resonator . The laser oscillation method according to claim 1,
The laser resonator has a resonator length of 76 cm,
The rod diameter is 1.7 mm,
A laser oscillation method characterized by lasing laser light having a wavelength of 1064 nm. The laser oscillation method according to claim 2,
A laser oscillation method comprising: oscillating a pulse by arranging a Q switch in the laser resonator. The laser oscillation method according to claim 1,
A mode locker is disposed in the laser resonator,
The resonator length of the laser resonator is the effective length of the laser resonator. Effective length = c / 4ν
c: speed of light
ν: A laser oscillation method characterized in that the frequency of the high frequency signal applied to the mode locker is used. In a laser oscillation method of oscillating a synchronized pulsed laser beam in a single transverse mode by laterally exciting solid laser media respectively disposed in two laser resonators each having a mode locker,
A first laser resonator comprising a first mode locker and a first solid state laser medium;
A second laser resonator comprising a second mode locker and a second solid state laser medium;
Have
The rod diameter of the rod-shaped first solid-state laser medium having a cylindrical shape is determined based on the effective pumping volume diameter associated with the change in the excitation intensity of the solid-state laser medium according to the resonator length of the first laser resonator. A rod diameter that draws a curve having a contact point at only one point, a curve indicating a change in mode and a curve indicating a change in mode volume diameter accompanying a change in excitation intensity of the solid-state laser medium,
The rod diameter of the rod-shaped second solid-state laser medium having a cylindrical shape is determined based on the effective pumping volume diameter accompanying the change in the excitation intensity of the solid-state laser medium according to the resonator length of the second laser resonator. A rod diameter that draws a curve having a contact point at only one point, a curve indicating a change in mode and a curve indicating a change in mode volume diameter accompanying a change in excitation intensity of the solid-state laser medium,
The effective excitation volume diameter is a region in which excitation states are concentrically distributed in an effective excitation volume that is a place that actually contributes to laser oscillation in the distribution of excitation states in the solid-state laser medium. The diameter of the outermost circle of the concentric circles of
The mode volume diameter is a diameter of a laser beam of a single transverse mode determined by the configuration of the laser resonator,
The resonator length of the first laser cavity is 100 cm, the rod diameter of the first solid-state laser medium is 2 mm, and the laser oscillates a laser beam having a wavelength of 1064 nm,
The resonator length of the second laser resonator is 100 cm, the rod diameter of the second solid-state laser medium is 2.6 mm, laser oscillation method characterized by lasing the laser beam having a wavelength of 1319nm . In a solid-state laser device that oscillates in a single transverse mode by arranging a solid-state laser medium in a laser resonator and injecting excitation light from a side surface of the solid-state laser medium to excite the solid-state laser medium side-by-side,
A rod-shaped solid laser medium having a cylindrical shape includes a curve indicating a change in effective pumping volume diameter according to a change in pumping intensity of the solid laser medium and a solid laser medium according to a cavity length of the laser resonator. With a rod diameter that draws a curve with a contact point at only one point, a curve showing a change in mode volume diameter with a change in excitation intensity ,
The effective excitation volume diameter is a region in which excitation states are concentrically distributed in an effective excitation volume that is a place that actually contributes to laser oscillation in the distribution of excitation states in the solid-state laser medium. The diameter of the outermost circle of the concentric circles of
The mode volume diameter is a diameter of a single transverse mode laser beam determined by the configuration of the laser resonator . The solid-state laser device according to claim 6,
The laser resonator has a resonator length of 76 cm,
The rod diameter is 1.7 mm,
A solid-state laser device that oscillates laser light having a wavelength of 1064 nm. The solid-state laser device according to claim 7,
A solid-state laser device comprising a Q switch disposed in the laser resonator. The solid-state laser device according to claim 6,
Comprising a mode locker disposed in the laser resonator;
The resonator length of the laser resonator is the effective length of the laser resonator. Effective length = c / 4ν
c: speed of light
ν: A solid-state laser device characterized by having a frequency of a high-frequency signal applied to a mode locker. In a solid-state laser device that laser-oscillates a synchronized pulsed laser beam in a single transverse mode by laterally exciting solid laser media respectively disposed in two laser resonators each having a mode locker,
A first laser resonator comprising a first mode locker and a first solid state laser medium;
A second laser resonator comprising a second mode locker and a second solid state laser medium;
The rod-shaped first solid-state laser medium having a cylindrical shape has a change in effective pumping volume diameter associated with a change in pumping intensity of the solid-state laser medium according to the resonator length of the first laser resonator. The curve shown and the curve showing the change in the mode volume diameter accompanying the change in the excitation intensity of the solid-state laser medium have a rod diameter that draws a curve with a contact at only one point,
The rod-shaped second solid-state laser medium having a cylindrical shape has a change in effective pumping volume diameter accompanying a change in pumping intensity of the solid-state laser medium according to the resonator length of the second laser resonator. The curve shown and the curve showing the change in the mode volume diameter accompanying the change in the excitation intensity of the solid-state laser medium have a rod diameter that draws a curve with a contact at only one point,
The effective excitation volume diameter is a region in which excitation states are concentrically distributed in an effective excitation volume that is a place that actually contributes to laser oscillation in the distribution of excitation states in the solid-state laser medium. The diameter of the outermost circle of the concentric circles of
The mode volume diameter is a diameter of a laser beam of a single transverse mode determined by the configuration of the laser resonator,
The resonator length of the first laser resonator is 100 cm, the rod diameter of the first solid-state laser medium is 2 mm, and laser light with a wavelength of 1064 nm is laser-oscillated.
A solid-state laser device, wherein a resonator length of the second laser resonator is 100 cm, a rod diameter of the second solid-state laser medium is 2.6 mm, and laser light having a wavelength of 1319 nm is laser-oscillated.

Images