KR20090100393A - Laser pulse generator and method - Google Patents

Abstract

<Task>

A stable pulse high repetition laser oscillation apparatus using a laser medium having a low amplification gain such as a thin plate shape is provided.

<Solution>

In the configuration of the solid state laser oscillator, at least one pair of laser resonators, a thin plate-shaped laser medium to which an active material having a phase life longer than a period of repetitive operation as a laser medium is added, and at least one reflector image in the laser resonator. And a Q switch element provided in the thin plate-shaped laser medium provided in the resonator, an excitation source for optically exciting the laser medium, and a control unit for driving the Q switch element. The control unit causes oscillation in the on state of the Q switch element, and turns off the Q switch by turning off the Q switch during oscillation in which the high-level density of the active substance in the laser medium remains. When the Q switch is in the ON state, the excitation density of the high level is maintained so as to oscillate the Q switch pulse, thereby achieving stable high repetitive pulse oscillation.

Description

LASER PULSE GENERATOR AND GENERATION METHOD}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a solid state laser oscillator, which is very suitable when a laser medium having a low amplification gain such as a thin plate shape doped with rare earth ions including Yb and Nd is used. The present invention provides a method and apparatus for laser oscillation of a Q-switch pulse that realizes high stabilization despite high frequency.

Light shaping to create a three-dimensional three-dimensional shape by irradiating a laser beam along a trajectory of a horizontal cross-sectional shape of a liquid of a photocurable resin, and sequentially superimposing computer-controlled patterns in the horizontal plane Technology is put to practical use. In this technique, the laser beam is scanned in a thin curved line to create a free-form surface. The ultraviolet curable resin is often employ | adopted for the photocuring resin to be used, Therefore, the laser of an ultraviolet range is used for an irradiation laser. Ultraviolet lasers are usually generated by converting a laser such as Nd: YAG having an oscillation wavelength of 1.06 µm into a triplex wave using a nonlinear optical crystal, and are used in accordance with the light absorption characteristics of the ultraviolet curable resin used for molding. Since the wavelength conversion efficiency using the nonlinear optical crystal has a characteristic of conversion efficiency that is proportional to the power of the laser, a pulse laser that obtains high peak power is used. In order to improve the molding speed while maintaining the shape accuracy of the light shaping, the repetition frequency of the pulse laser is increased, the light condensing spots at the irradiation points are continuously overlapped and irradiated so as not to be separated from each other, thereby smoothing the curve. To form.

Typically, laser pulses are generated at a repetitive frequency of tens of kHz using a Q switch control technique from a continuous light excited solid state laser medium. In addition, after converting the laser pulse into an ultraviolet region with a nonlinear optical crystal, a condenser spot is fastened on a hardened resin surface by a beam scanner equipped with a reflection mirror on a galvanometer. Investigate the shape trajectory by using. Generation of high-speed repetitive pulses is conventionally performed using a rod-type solid state laser, but from the viewpoint of miniaturization and high quality, a disk-type laser is studied to solve the defect of the rod laser. It is becoming.

The disc type laser has a structure in which a thin laser medium is attached to a high thermal conductive substrate, which is a heat cooling medium, via a mirror, irradiates excitation light in the thickness direction of the plate of the attached thin laser medium, and amplifies the laser from the surface of the laser medium. The light is emitted. The back substrate absorbs heat generation of the laser medium by irradiation with excitation light and heat dissipates for cooling, thereby preventing the temperature rise of the laser medium. Therefore, since the temperature gradient in the inner direction of the thin plate surface in the laser medium is eliminated, the optical nonuniformity of the oscillation amplification beam in the laser medium is eliminated, and a high quality laser beam having excellent light condensation is obtained.

However, there is a drawback when using a disk type Q switch laser as a high repetition laser light source for such optical shaping. Since it forms and uses a laser oscillation or an amplification optical path in the perpendicular direction with respect to the thin plate of a thin laser medium, the length of the amplification optical path through which a laser beam passes an amplification medium is short, and sufficient amplification gain is not obtained. Therefore, if the on / off repetitive operation of the Q switch is performed at a high frequency of several tens of kHz, the generation of the Q switch pulse cannot follow this repetition frequency, and the output of the pulse is often lost. For this reason, the resin molding by the smooth high precision optical shaping | molding operation was not possible depending on the laser pulse which generate | occur | produced by the Q switch high speed repetition. On the other hand, low-speed repetition in order to prevent missing output of pulses has a problem of insufficient work efficiency and precision.

Patent Document 1: US Patent No. 6002695

Patent Document 2: US Patent No. 6172996

Patent Document 3: US Patent No. 6922419

Problems to be Solved by the Invention

The problem to be solved by the present invention is to achieve a high repetitive frequency operation of the solid state laser by the Q switch output with stable pulse energy even when using a laser medium having a low amplification gain such as a thin plate shape. Moreover, it is the point which improves the speed and processing precision of the material processing by a laser.

Means for solving the problem

MEANS TO SOLVE THE PROBLEM This invention is a laser pulse generator which generate | occur | produces a pulse at a predetermined repetition frequency, in order to solve the said subject, The laser resonator which has a some reflector, a laser medium, the Q switch provided in the optical path in the said laser resonator, the said laser And an excitation source for optically exciting the medium, and a control unit for driving the Q switch, wherein the control unit has means for turning the Q switch on and off during laser oscillation.

On the other hand, the present invention provides a method of generating a laser pulse, comprising: exciting a laser medium, turning on a Q switch, initiating laser oscillation from the laser medium, and turning off the Q switch during the laser oscillation. Switching and blocking the laser oscillation, and repeating on and off of the Q switch at a predetermined frequency.

Effect of the Invention

In the present invention, since the inverted distribution can always be kept high by forcibly interrupting the generated pulse during oscillation with the Q switch, pulse generation at a high repetition frequency is possible. In particular, pulse generation is possible at a frequency corresponding to a period shorter than the upper level density of the active material added to the laser medium.

1 is a configuration diagram of a first embodiment of a high repetition Q switch pulse according to the present invention.

2 is a configuration diagram of the second embodiment.

3 is an explanatory view of the operation of the present invention.

4 is a characteristic example in a case where the RF switching timing in the configuration of FIG. 1 is changed.

(a) Output and Stability with respect to RF Switching Timing in 100kHz Operation

(b) Pulse train in stable region

(c) Pulse train in unstable region

FIG. 5 is an example of the characteristic in the high repetitive operation of the second harmonic in the configuration of FIG. 2.

(a) Pulse width and stability against energy in 50 kHz and 100 kHz operation

(b) Pulse wave on the low energy side

(c) Pulse waveform on the high energy side

Fig. 1 shows an apparatus used for stable Q switch pulse laser oscillation at a high repetition frequency according to the first embodiment of the present invention. It is very suitable for the laser medium 1 to be a thin laser medium. This is because the thickness of the plate is small, and the optical path length along the laser optical axis perpendicular to the plane of the thin plate is small, so that the amplification gain is small. However, a rod-type medium generally used is particularly effective when the amount of the active substance added is small. In the case of a thin laser medium, the laser medium 1 is fixed to the substrate 2 having high thermal conductivity and excellent cooling ability. The laser medium 1 is used by doping an active material having a high level lifetime which is longer than the period of the target repetitive operation. Alternatively, the device is operated in a cycle of repetitive operation corresponding to a cycle shorter than the phase life of the active substance. The active material of the laser medium 1 is, for example, rare earth ions such as Yb and Nd. Between the substrate 2 and the laser medium 1, a thin layer (not shown) for bonding, such as AuSn (gold tin), used for fusion, and light of a laser wavelength incident from the surface of the laser medium 1 are reflected. A mirror 17 is formed and arranged. The laser medium 1 is irradiated with excitation light 16 from the surface of the excitation light source 12 for optical excitation from the surface side to excite the active material doped in the laser medium. In front of the laser medium 1, a condenser lens 3 is provided for minimizing the diffraction loss of the laser resonator. In addition, the mirror 6 on the output side that forms the laser resonator together with the mirror 17 is provided to face the laser medium 1. In addition, a Q optical switch 5 which operates acousto-optically is provided in the optical path 8 in the laser resonator, and an RF control unit 13 of the Q switch is installed in order to drive a high frequency piezoelectric element which is a diffraction medium in the Q switch 5. do. Moreover, the polarizing plate 4 is provided in the optical path 8 in a resonator as needed.

In the laser medium 1, excitation light 16 from an excitation light source 12 such as a laser diode is focused and irradiated onto a laser resonance optical axis on the laser medium 1. As the wavelength of the excitation light source 12, a wavelength effective for excitation to the high level of laser oscillation of the laser medium 1 is selected. The active material in the laser medium 1 is optically excited to form an inversion distribution of the inner two levels.

The RF switch 13 is turned off and on to control the transducer (not shown) of the Q switch 5 to turn the Q switch 5 on and off. . When the power of the RF is on, the Q switch 5 is turned off since the straight path of the optical path 8 inside the laser resonator is turned off, and the Q value is lowered and the oscillation is blocked by increasing the optical loss in the laser resonator. When the RF power is turned off or the power is lowered, the straight line of the optical path inside the laser resonator cannot be prevented, so that the Q switch 5 is turned on. When the Q switch 5 is in the ON operation, if the laser medium 1 has a laser amplification gain, oscillation is reached, the Q switch pulse is raised between the mirrors 6 and 17 constituting the resonator, and the mirror 6 on the output side is ), The output beam 7 is obtained externally.

According to the present invention, when the Q switch pulse is oscillated, a stable Q switch pulse can be obtained until the high repeat operation. It is by controlling the timing of the Q switch on and off, and also by using the active material with a long life of high level to efficiently generate residual energy to obtain a substantially high density inversion distribution. The state will be described with reference to FIG. 3.

Fig. 3A shows the concept of inversion distribution number Ni between laser oscillation levels in the laser medium. The case where there is no inversion distribution is set to 0 level, and the number of inversion distributions necessary for stable operation is N1, and the state where the number of inversion distributions immediately before oscillation is N2.

It is assumed that the laser medium 1 is excited by the excitation light source 12 from before time t0. After t0, the inversion distribution number Ni gradually increases (20). At a time point t1 where the inversion distribution is larger than the oscillation threshold, when the RF power is cut off by the RF control unit 13 to reduce the loss of the resonator, the laser oscillation starts to rise in the laser resonator, and at time t2, as shown in FIG. As indicated by the slope, the rise 27 of the pulse occurs. If the RF power is cut off as described above, the inversion distribution is lowered to near zero level by the induced emission of light, and the falling portion 28 is generated at that time to generate the Q switch pulse of the tail portion.

As shown in Fig. 3C, it is assumed that the switching of the RF power to on (Q switch operation is off) is performed at the times t15, t16 and t17. In this case, since the energy is released by the pulses of the falling portion 28 between the times t3 and t15, the inversion distribution number Ni sees a significant decrease 21. The RF power is turned on at time t15, and an increase 24 of the inversion distribution number Ni is achieved by gradually exciting the RF power to the high level due to optical excitation until time t6 at which the next pulse is generated. When the amplification gain of the laser medium is low, such as making the laser medium thin, the inversion distribution number necessary for stable rise of oscillation is insufficient at time t6, and the rise of the Q switch oscillation is insufficient or impossible. Therefore, the Q switch pulse starting from this t6 becomes a pulse 29 of lower energy than the immediately preceding pulse or becomes missing.

Subsequently, when the Q switch is turned on again with the RF power off again at time t9, induction emission is not performed due to insufficient oscillation at time t6 before that t16 to t10. The optical excitation energy in between is given and the inversion distribution is further increased. Therefore, since the inversion distribution number exceeds N2, a sudden pulse rise 30 occurs at t10. In addition, the induced emission amount is increased by a sudden pulse, and the number of remaining inverted distributions is reduced again, so that a stable output cannot be taken out.

Normally, the Q-switched pulse laser device is designed so that inversion distribution energy sufficient to cause oscillation is accumulated and further emitted while the RF power is turned off. When a medium having a small amplification gain is used, such as a thin laser medium, the time required for giving sufficient inversion distribution energy becomes longer, so that the upper limit of the frequency of the high repetitive pulse oscillation is lower than that of the rod-type solid state laser.

In the present invention, the RF switch is turned off and the Q switch is turned on, and then, at the time when the laser pulse starts oscillation, the Q switch is forcibly turned off to cut off the laser oscillation, whereby the upper limit of the frequency of the high repetitive pulse oscillation is increased. It did not become low. This will be described below with reference to FIGS. 3B and 3D. When the laser pulse oscillation is started at time t2, the laser switch is interrupted by forcibly turning off the Q switch during the oscillation at t3.

When a medium having a small amplification gain is used, such as a thin laser medium, pulse oscillation can be stopped immediately by turning off the Q switch. It is assumed that the Q switch-off and oscillation stop are performed at the times t3, t7 and t11 shown in Fig. 3D. By blocking oscillation during oscillation in this manner, the inversion distribution number Ni is always higher than zero level and can be maintained between N1 and N2. From the time t3 at which the RF power is turned on to the time t5 at the next off time, the inversion distribution number continues to increase (23) from the time t6 at the next pulse to the value at the start of the preceding Q switch pulse oscillation. It oscillates after the number of distribution increases. After starting oscillation at time t6, the pulses are stopped by turning off the Q switch at time t7 during oscillation. The transition of energy of the inversion distribution to the Q switch pulse is interrupted, and the inversion distribution remains. After the pulse oscillation stops at time t7, the inverted distribution number is increased to the next oscillation operation start time t10. In this way, the Q switch was turned off during the oscillation operation of the Q switch pulse to maintain the inversion distribution in the laser medium, and the repeat pulse operation was continued while always maintaining the high inversion distribution state. Branches can continue to oscillate with a pulse train.

In order to effectively maintain the inversion distribution, it is preferable to use an active substance whose phase life is long enough for the cycle of the repeated operation. For example, this can be realized by using Yb, which has a high-level life of almost 1 ms as the active material, for a period of less than 50 mu sec corresponding to a repetitive operation of more than 20 kHz repetition frequency.

In addition, it is important to perform the timing of turning on the RF power and forcibly turning off the Q switch during oscillation of the Q switch pulse. This method simply controls the time from turning off the RF power to the RF control unit 13 of the Q switch element (t1, t5, t9 in Fig. 3) and then on (t3, t7, t11) to a predetermined value. Can be realized. The predetermined value can be determined experimentally. On the other hand, a configuration in which a sensor or the like for detecting the laser oscillation is additionally added, and a means for turning off the RF power through the appropriate time delay means by receiving the oscillation detection signal from the sensor is included in the RF control unit 13. It is also possible.

2 shows a second embodiment of this invention. The component parts of the same function as 1st Embodiment are shown with the same number. In this embodiment, a function of harmonic conversion of the generated laser pulse is added to the first embodiment. In this embodiment, the wavelength selective mirror 14 inclined at 45 ° has a total reflection characteristic with respect to the fundamental wave 11 of the laser oscillation, and the harmonic converting means 9 and 10 provided in the resonator are separated from the fundamental wave. The wavelength of the converted harmonics has a transmissive characteristic. As the harmonic conversion means 9 and 10, nonlinear optical elements, such as a nonlinear optical crystal, can be used. In order to give a nonlinear effect to a nonlinear optical element, input light needs to be polarized. In the case where there is an element with polarization in the resonator, polarization oscillation is possible even without a polarizing means in particular, and thus the polarizing plate 4 may not be inserted. However, the polarizing plate 4 is essential when the laser medium is free of elements that accompany polarization.

The wavelength selective mirror 14 has a property of reflecting the fundamental wave and transmitting the harmonic wave. For the fundamental wave, the resonator is constituted by the mirrors 6 and 17 and the wavelength selective mirror 14. The fundamental wave 11 on the optical path is reflected by the wavelength selective mirror 14 and directed to the harmonic converting means 9 and 10, and converted into the second harmonic or the third harmonic while passing through the harmonic converting means 9 and 10. . Harmonics of short wavelength are transmitted through the wavelength selective mirror 14 as high repetitive Q switch pulses and are emitted as a harmonic output beam 15. Therefore, when Yb: YAG having an oscillation wavelength of 1.03 μm of the fundamental wave is used as the laser medium, when the second harmonic is outputted to the harmonic converting means 9 and 10, the third harmonic generates a laser pulse having a wavelength of 515 nm as a visible region. When outputting the laser beam, a laser pulse having a wavelength of 343 nm, which is an ultraviolet region, can be obtained.

Experimental Example 1

Fig. 4 shows an example of the characteristic change in output power and stability when the timing of RF power in pulse frequency 100 kHz operation is changed by using a thin laser medium of Yb: YAG in the first embodiment. The thickness of this laser medium is 300 μm. As the excitation light source, a laser diode having a wavelength of 940 µm was used. And the oscillation wavelength of the laser was 1.03μm, which is to the energy level corresponding to the transition of 10327cm ^-1 and 612cm ^-1, the above sangjun life of 950μ sec. In FIG. 4A, the horizontal axis represents an off period of RF, that is, t1 to t3, t5 to t7, and t9 to t11 in FIG. 3. The output power increases when the RF off period is increased, but when the optimum point X is exceeded, the stability of the power deteriorates. This is because the energy does not come out in the middle, or does not come out so that a normal inversion distribution is not obtained. In the stable region in which the off period of the RF is less than or equal to the optimum point X, a sufficient inversion distribution is obtained from the remaining energy, so that the operation is stable as in (b). Likewise, it becomes unstable operation to repeat the high and low.

Experimental Example 2

FIG. 5 is a second embodiment and shows the characteristic example of the second harmonic (wavelength 515 nm) in pulse frequency 50 kHz and 100 kHz operation using a Yb: YAG thin-film laser medium similarly to the first embodiment. LiB ₃ O ₅ , which is a nonlinear optical crystal, was used for the harmonic converting means 9, 10. In (a), the horizontal axis is the average energy of one second harmonic pulse. As shown in Fig. 5 (b) and (c), the pulse waveform is cut off at the trailing tail, forcing is controlled forcibly even at the time of harmonic oscillation, thereby achieving stable operation.

In the present invention, in the repetitive operation of the Q switch using the laser medium to which the active substance is added, the optical path of the Q switch is transmitted and the laser pulse is forcibly cut off by turning off the Q switch optical path during the oscillation. This stops the energy release from within the laser medium halfway, further increasing the residual excitation high level energy density. When the Q switch is in the ON state, an oscillation method in which the high-level excitation density is maintained to oscillate the Q switch pulse is obtained, and thus stable high repetition pulse oscillation can be realized. It is possible to generate a laser pulse at a repetition frequency corresponding to a period shorter than the phase life of the active material. This high repetition energy beam is used to improve the speed and processing accuracy of laser material processing.

Conventionally, in a laser using a laser medium having a low amplification gain such as a thin plate shape, since the oscillation gain is small, it is difficult to increase the oscillation in a short time. The operation was limited. In the laser pulse generating apparatus according to the present invention, during generation of the Q switch pulse, the oscillation is forcibly higher than a constant level by forcibly blocking oscillation in a state where the state density of the high level is higher than the oscillation threshold to maintain a high laser amplification gain state. Since the Q switch operation was controlled to perform a high repetitive operation under the condition in which the distribution was formed, generation of a stable pulse at a high repetition frequency could be realized. The present invention has a high effect on a disk type laser. This is because, in the disk laser, a thin plate medium is used, the laser oscillator gain optical path length corresponding to the thickness of the thin plate is short, and the amplification gain is low.

When the laser generating device according to the present invention is used as a light source in a three-dimensional light molding resin model production process using a photocurable resin, fine high-precision image shapes can be drawn by thin lines on the liquid surface of the photocurable resin. Therefore, high precision three-dimensional molding is attained. Since the repetition frequency of the Q switch pulse is high and stable pulse output energy is obtained, a high speed scanning can be achieved by overlapping the condensing spot of the pulse laser, so that a smooth curve can be drawn and the shape accuracy is high. Three-dimensional resin model formation is possible at high speed. In addition, when applied to semiconductor-related micromachining which can apply high repetition frequency Q pulses, a laser system with high processing capability can be realized.

The embodiment of the present invention has been described above. It is clear that changes can be made to these without departing from the technical idea of the invention described in the claims.

The present invention obtains a small laser oscillator with high repeatability and high stability such as infrared, visible and ultraviolet light, and thus is used in a three-dimensional optical molding process using a photocurable resin. In addition, the electrode forming process of a solar cell, the processing of the transparent electrode film used for display devices, such as a liquid crystal, the correction process of various electronic devices, the silicon wafer redundancy circuit of a semiconductor memory, etc. It can be utilized in laser processing areas requiring high-speed scanning, such as processing of circuit elements of conductive links, other removal processing, marking, and heat treatment.

Claims (9)

A laser pulse generator for generating pulses at a predetermined repetition frequency, A laser resonator having a plurality of reflectors, a laser medium, a Q switch provided on an optical path in the laser resonator, an excitation source for optically exciting the laser medium, and a control unit for driving the Q switch, And the control unit has means for turning the Q switch on and off during laser oscillation. The method of claim 1, An active material is added to the laser medium, and the laser pulse generator is longer in life than the period corresponding to the repetition frequency. The method according to claim 1 or 2, And the laser medium is a thin laser medium provided on at least one reflector in the laser resonator. The method according to any one of claims 1 to 3, And a harmonic converting means. The method according to any one of claims 2 to 4, And the active material contains at least one of rare earth ions including Yb and Nd. The method of claim 5, And the repetition frequency is 20 kHz or more. Exciting the laser medium, Turning on the Q switch, Initiating laser oscillation from the laser medium, Turning off the Q switch and blocking the laser oscillation during the laser oscillation; and repeating on and off of the Q switch at a predetermined frequency. The method of claim 7, wherein An active material is added to the laser medium, and the predetermined repetition frequency is a frequency corresponding to a period shorter than the phase life of the active material. The method according to claim 7 or 8, And a step of converting the generated laser pulses into harmonics.

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