JP3131079B2 - Q switch CO2 laser device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Q-switched pulse CO having a high peak output and a high pulse repetition rate required for laser photochemistry such as hole drilling and cutting, and laser isotope separation. _Two lasers, especially Q-switched CO ₂ lasers to control the processing shape and heat input pattern corresponding to individual pulses in processing, or high levels of intermediate excitation species in photochemistry using infrared multiphoton excitation The present invention relates to a Q-switched CO ₂ laser device for arbitrarily controlling individual Q-switched pulse energies for efficient excitation.

[0002]

_2. Description of the Related Art With the dramatic increase in the output of a continuous wave CO ₂ laser and the improvement in the quality of a spatial mode of a laser beam in recent years, its application field has been greatly expanded mainly in various material processing. Furthermore, recently, various reports have been made on the development of new fields by pulsing the oscillation mode of the CO ₂ laser. Typical examples are cutting and welding of materials that were difficult to process with conventional CO ₂ lasers such as copper and aluminum, ultra-high-speed and precision processing of steel materials, and laser isotope separation using infrared multiphoton excitation. And light excitation of a far-infrared laser. High peak output for application CO ₂ laser to such field is generally in excess of 10 kW, 10k
High average output performance is required due to a high pulse repetition rate of not less than Hz.

[0003] As a CO ₂ laser corresponding to such an application, there has been a pulse TEA laser operated in a laterally pumped atmospheric pressure mode. In a TEA laser, a high voltage / high current pulse discharge can be applied to a laser gas at atmospheric pressure to rapidly form a population inversion. Therefore, a high peak output of 10 MW or more can be achieved with a pulse width of about 100 nsec by a gain switching phenomenon. Although it can be easily taken out, the pulse repetition frequency is practically about 10 Hz to 100 Hz, and a large output cannot be taken out as an average output. Further, there is a problem that the life of the laser oscillator is short because high power pulse discharge is performed in a short time, and the cost required for the operation increases.

As a method of extracting a pulse laser beam from a CO ₂ laser medium for a TEA laser, there is a method of applying pulse modulation to discharge excitation of a low-pressure continuous wave (hereinafter referred to as CW) laser. The output level was suppressed to about the CW rated output level, and was about several kW at the maximum.

[0005] In order to cope with such a problem, various methods have been proposed for applying a Q-switching technique often used for a solid-state laser to a CO ₂ laser. I
EEEJ. Quantum Electronics Vol. QE-4, p762, 1968 summarizes various Q-switching methods in which a loss controlling element is inserted in the resonator of a CW pumped CO ₂ laser. According to the above document, as a Q switching method,
A passive Q-switching method in which a saturable absorber is inserted into a resonator, a method using an electro-optical element, a method in which a rotating mirror is incorporated in a resonator, and a Q-switch device comprising a combination of a telescope and a rotating chopper And a method of inserting a Fabry-Perot etalon into a resonator disclosed in Japanese Patent Application Laid-Open No. 62-160783. Among them, the method using a rotary chopper and a telescope has a problem that it is necessary to improve the Q switching speed, but it is mainly used because of other controllability and stability.

Gas Flow & Chemical Lasers (GCL) Confer
ence Proceedings, Viena 1988, Vol.1031, SPIE, p48, shows an example of incorporating a rotary chopper type Q switch into a CWCO ₂ laser to achieve a pulse energy of 27 mJ, a pulse peak output of 150 kW, and a pulse repetition frequency of 10 kHz. In addition, the inventors of the present application have been conducting various studies and have applied for a method of using a radio frequency (RF) pulse discharge synchronized with a rotary chopper in order to improve the pulse peak output (Japanese Patent Laid-Open Publication No. 4-2
59278). The Q switching method of the CO ₂ laser is a method of extracting a Q-switch pulse train having the same energy at a constant interval.

By the way, when the pulse Q-switched CO ₂ laser is applied to the processing of metal materials, in particular, drilling and cutting, and when it is applied to the infrared multi-photon excitation process, the individual pulse energy and the peak output are arbitrarily set. It has been found that the laser irradiation effect can be remarkably improved if it can be controlled. For example, in material processing, it is possible to change the surface properties with the first pulse to change the surface absorptivity with respect to the laser light, to greatly improve the processing efficiency with the second pulse, and to improve the infrared efficiency. In photon excitation, it is possible to increase the overall dissociation efficiency of molecules by efficiently dissociating molecular species excited to an intermediate level in energy by the first pulse in the first pulse. .

Here, as a method for controlling the energy of the Q switch pulse, which is intended for a solid-state laser, Japanese Patent Application Laid-Open No. H4-222182 discloses a method for controlling the DC excitation intensity. . Continuous wave excitation Q
By controlling the DC excitation intensity in a switch laser device,
Since the amount of population inversion distribution at the time when the resonator Q value becomes high can be changed, it is possible to control the energy and the peak output of the Q switch pulse, but in order to correspond to each pulse, 10 kHz It has been difficult to control the continuous wave excitation intensity at such a high speed. Japanese Utility Model Application Laid-Open No. 63-9170 discloses a solid-state laser capable of extracting a pulse having two types of energy characteristics by superimposing continuous wave excitation and pulse excitation. ing. However, since this method also modulates the DC excitation level in the same manner as in the conventional example, high-speed modulation of about 10 kHz was difficult.

In order to cope with such a problem, the inventors of the present invention disclosed in Japanese Patent Application No. 5-93429 a Q-switched CO ₂ laser excited by a pulse discharge, and a pulse discharge of two or more times for one pulse discharge. By applying a Q-switch method, a pulse group in which pulse energy and peak output gradually decrease by opening the switch is obtained. In this method, the degree of the above-described decrease can be controlled by changing the laser gas composition, but it has been difficult to arbitrarily control the individual energy and the peak output of the Q switch pulse train.

[0010]

SUMMARY OF THE INVENTION An object of the present invention is to address the above-mentioned problems of the prior art, and to provide an irradiation effect of a laser pulse of a Q-switched CO ₂ laser device used in metal material processing or laser photochemistry. The object of the present invention is to make it possible to arbitrarily control individual pulse energies and peak outputs of a Q-switch pulse train with a simple configuration as a Q-switch laser output that can improve the above.

The above object is to provide a pulse discharge power supply, a casing in which a CO ₂ laser medium is sealed to excite the pulse discharge, a total reflection mirror and a partial transmission mirror which constitute a resonator with the casing interposed therebetween. A Q-switched CO ₂ laser device having a Q-switch device disposed in the resonator operating in synchronization with pulse-discharge excitation, wherein the pulse-discharge excitation has a mechanism for controlling the discharge time, A synchronization signal generator for generating a pulse signal that is time-synchronized with the Q switch, and a delay pulse generator for generating a pulse having a fixed delay time with respect to the synchronization signal. And a pulse width modulator for transmitting a width signal that is time-synchronized with the delay pulse and is pulse-modulated according to a preset pattern. A Q-switched CO ₂ laser device, characterized in that a discharge excitation pulse is started in synchronization with the imaging, and the discharge excitation pulse is ended in synchronization with the falling timing of the pulse width modulated signal. Achieved by providing.

[0012]

FIG. 5 schematically shows the temporal behavior of various parameters related to laser oscillation for explaining the principle of laser Q switching. In FIG. 5, a Q-switched laser pulse, which is a laser output, has a sharp peak with a short time width, as a result of a sudden rise in the Q value of the resonator and the inversion distributed energy accumulated up to that time being released at once. It is extracted as a pulse with a head output. Here, when the Q switch speed is sufficiently fast, the output pulse energy has a strong positive correlation with the amount of population inversion distribution. Therefore, when the laser pulse energy is controlled, that is, the amount of population inversion distribution is controlled, which is determined by a balance between the amount of energy injected from outside for laser excitation and the lifetime of the laser upper level. Parameters.

[0013] Next 6 CO ₂ laser of a typical vibration energy levels of CO ₂ molecules involved in oscillation transition, as well as generally schematically vibration energy level of the N ₂ molecule is added as a laser medium of a CO ₂ laser FIG. 3 is an energy diagram schematically shown.
The CO ₂ molecule has a symmetric stretching mode n ₁ , a bending mode n _{2 and an} asymmetric stretching mode n ₃ , and its vibration level is (n ₁ , n
₂ ^l , n ₃ ). Here, the subscript l of n ₂ represents a sub-level, and l = 0, 1,... N ₂ -1, n ₂ . CO
Oscillation in the 10.6μm band, which is a typical oscillation wavelength of ₂ laser, the laser on the level of the (0 0 ⁰ 1) level, (1
(0 ⁰ 0) is obtained by a transition in which the level is a lower level. On the other hand, the N ₂ molecule also has a vibration level, and the first vibrationally excited level (v = 1) is at almost the same energy level as the (0 ⁰ 1) level of the CO ₂ molecule, and the difference is About 18c
m ^-1 and the thermal energy of the gas at the time of discharge (about 20 at 300 K ゜).
0 cm ^-1 ). As a result, energy is transferred between the CO ₂ molecule at the upper laser level and the N ₂ molecule at the (v = 1) level extremely efficiently due to collision. Therefore, by adding N ₂ gas to the CO ₂ laser medium, efficient excitation to the upper level of the CO ₂ laser is performed. Here, the life of the excited N ₂ molecule (v = 1) is relatively long at 100 μsec or more in the operating pressure range (100 torr or less) of a general CO ₂ laser. Furthermore, N ₂
The energy transfer time constant from a molecule to a CO ₂ molecule is determined by the collision probability between molecules, that is, the pressure of the laser medium. The coefficient for determining the time constant is described in Rev. Mod. Phys., 41, 2
According to 6,1969, k _l = 1.9 × 10 ⁴ / torr / se
c, which is 50 μsec to 100 when converted to a time constant.
μsec, which is about the same as the lifetime of the N ₂ molecule. Therefore, the presence of the N ₂ molecule has a function of replenishing the population of population inversion that decreases by spontaneous transition due to the lifetime of the upper level of the CO ₂ molecule.

From the above discussion, it is difficult to unambiguously determine the equivalent upper level lifetime of a CO ₂ laser, but it is at least equivalent to the lifetime of N ₂ molecules and the energy transfer time constant of about 100 μsec. Above the pulse repetition frequency of about 10 kHz, the effect of the reduction of the population of the population inversion caused by the lifetime of the upper level of the laser is small, and the magnitude of the population inversion is substantially determined by the amount of externally injected energy. You can think. Considering that the amount of injected energy is changed, there are two cases, that is, a case where the amplitude of the discharge power is changed in a fixed time width and a case where the discharge time width is changed while the discharge power is kept constant. However, any of the above time scales may be adopted. The above is the description of the principle of the present invention.

Generally, in the case of a pulse-pumped Q-switched solid-state laser, when controlling the laser pulse energy, the former of the above two cases, that is, the pump pulse width is fixed and the pulse amplitude is changed to control the pump energy. The method is adopted. This is because it is possible to achieve a simpler control structure by changing the pulse power while keeping the arc discharge time constant of the flash lamp power supply constant and the pulse width fixed, but the pulse repetition frequency is several tens of times.
0 Hz is the limit.

On the other hand, in the RF pulse modulation discharge system capable of modulating at a high pulse repetition frequency in glow discharge excitation of a CO ₂ laser, the former pulse amplitude is changed due to impedance matching conditions between a load and a power supply. It is difficult to perform this operation at high speed, but conversely, it is easy to change the pulse width while keeping the latter pulse amplitude constant. Therefore, the point of the present invention for performing the energy modulation of the output pulse in the Q switching of the CO ₂ laser is to control the amount of the injected energy by the pulse width modulation and to control the amount of the population inversion and, consequently, the pulse energy of the Q switch laser. It is in.

FIG. 7 shows the results of an experiment conducted to confirm the validity of the above principle description. In the figure, the horizontal axis represents the discharge pulse width. The peak power of the discharge pulse is 1
The amount of energy injected into the laser medium was controlled by changing the pulse width while fixing the pulse width to 5 kW. The Q switch device used a combination of a telescope and a rotary chopper, and the Q switch opening timing was synchronized so that it was immediately after the end of the discharge pulse. The pulse repetition frequency is 10 kHz
Because of z, the maximum width of the discharge pulse is 100 μsec. Referring to the figure, a substantially linear relationship is obtained between the discharge pulse width and the Q switch pulse energy at least up to a pulse width of about 60 μsec, and it has been confirmed that the Q switch pulse energy can be controlled by discharge pulse width modulation. Was done. When the pulse width is 60 μsec or more, the laser output tends to be saturated. This is because the above-mentioned laser upper level lifetime has a slight effect, and the amount of population inversion distribution becomes saturated.

Next, assuming that the Q-switch laser pulse is output at a constant interval, the discharge pulse width changes, so that the opening timing of the Q-switch is always synchronized with the end of the discharge pulse. Will be difficult. Therefore, the effect on the Q switch pulse energy output when the discharge peak output is fixed at about 55 kW and the discharge pulse width is fixed at 20 μsec and the delay time from the end of the discharge pulse to the opening of the Q switch is sequentially changed Was evaluated. FIG. 8 shows the result. Since the pulse repetition frequency is 10 kHz, the maximum delay time is 80 μs
Although it is ec, it has been found that the influence on the output pulse energy can be suppressed to about 15% even when there is a delay as much as four times the excitation discharge pulse width. This is similar to the principle explained above,
This is due to the relatively long equivalent upper level lifetime.
From the above results, it was found that the laser output pulse energy can be controlled in a linear relationship with the discharge pulse width even when the Q switch release timing is set to the expected end timing of the longest discharge pulse.

Next, the configuration and operation of an actual output pulse energy modulation Q-switched CO ₂ laser will be described with reference to FIGS.
This will be described with reference to FIG. FIG. 1 shows a Q switch CO according to the present invention.
₂ shows a configuration of a _two- laser device. The laser resonator is constituted by a total reflection mirror 3 and a partial transmission output mirror 4, and includes telescope lenses 5, 5 'and a rotary chopper 6 in the resonator.
Is provided. The laser medium is excited by a pulse-modulated RF glow discharge, which is the output of the pulse discharge power supply 12. The Q switch timing by the rotary chopper 6 is photoelectrically detected at a position distant from the laser optical axis to generate an output signal a of the synchronization signal generator. This signal becomes a delayed pulse generator output signal b to which a fixed delay is given by the delayed pulse generator 9, and is input as an external trigger of the pulse width modulator 10. The pulse width modulator 10 receives the external trigger signal b
And sends a trigger signal c pulse-width-modulated by a modulation signal generated in advance by the computer 11 to the pulse discharge power supply 12, and the pulse discharge power supply 12 generates a discharge pulse whose timing and pulse width correspond to this signal c. d is applied to the laser medium.

FIG. 2 is a timing chart showing the time relationship between these various signals. In order to clarify the time relationship, only three pulses are schematically shown.

In the above description, a combination of a telescope and a rotary chopper, which is excellent in the scaling property of the average output, has been described as an example of the Q-switch device, but CdTe or the like often used in a low average output area is described. A Q-switch device using a combination of the electro-optical element and the polarizer described above may be used. Further, although the RF discharge method, which is the easiest for pulse width modulation, has been described as the discharge excitation method, pulse modulation of DC discharge or microwave discharge may be used.

[0022]

FIG. 1 is a block diagram showing an embodiment of a Q-switched CO ₂ laser according to the present invention. Laser resonator is GaAs
The reflector comprises a total reflection mirror 3 having a reflectivity of 99.5% and a curvature of a concave shape of 30 m and a partially transmitting output mirror 4 made of ZnSe and having a reflectivity of 50%. 6m. The Q switch device is a ZnSe lens 5 having a non-reflective coating and a focal length of 127 mm,
5 'is constituted by a combination of a telescope installed under confocal conditions and a rotary chopper 6 installed at the confocal position. The rotating chopper is a rotating disk having 10 slits at equal intervals on the circumference, and rotating at a rotation speed of 1000 rps, thereby obtaining a pulse repetition frequency of 10 slits.
A Q-switching of kHz is realized. Laser medium is C
O ₂ / N ₂ /He=1.2/1.7/8 mixed gas, the total pressure is about 75 torr, and the gas circulates at high speed in the discharge unit housing 2 composed of a discharge glass tube. I do. Q
The signal a corresponding to the opening timing of the switch is photoelectrically detected as an output of a synchronizing signal generator constituted by a combination of a laser diode 7 and a photodiode 8 mounted on the upper part of the chopper. This synchronization signal generator output signal a is given a fixed delay by the delay pulse generator 9. In the configuration of the present embodiment, the photoelectrically detected position is a position that is just half a circle deviated from the laser optical axis, and the phase of this signal and the Q switch opening timing e are synchronized. Therefore, the amount of delay in the delay pulse generator 9 is set to 15 μsec so that when a pulse having a width of 80 μsec is excited, the Q switch is opened at the end of the excitation.
c. The remaining 5 μsec is the sum of the response delays of the other signal processing devices and the pulse discharge power supply 12. The output signal b of the delay pulse generator 9 output with a constant pulse width is supplied to a pulse width modulator 10 composed of a synthesizer, and a signal c that has been pulse width modulated according to a modulation signal set in advance by a computer 11 is obtained. It is supplied to a pulse discharge power supply 12. The pulse discharge power supply 12 has a radio frequency (R) of a carrier frequency of 13.56 MHz.
F) A power supply, which is a device capable of pulse modulation up to a repetition frequency of 100 kHz, and supplies a discharge pulse d to the laser excitation unit in accordance with the timing and pulse width of a signal supplied from the outside during an external trigger operation. FIG. 3 shows an operation example when the computer 11 generates a stepwise modulated signal with the above configuration. Discharge pulse width 20μsec
To 80 μsec at a pitch of 20 μsec, resulting in a laser pulse energy of 22, 47, 69, 83 m
J, pulse power of 80, 170, 250, 300 kW pulse energy and modulation of the peak power were realized. Next, FIG. 4 shows the result when the discharge pulse width is modulated in a completely random pattern. As a result, a Q-switched CO ₂ laser pulse train of 12 to 80 mJ was obtained in a time-series random pattern.

[0023]

As described above, when the configuration of the Q-switched CO ₂ laser device according to the present invention is used, arbitrary modulation control of pulse energy and peak output, which has been difficult in the past, can be performed. The irradiation effect can be greatly improved in application to material processing and photochemistry.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an embodiment of a Q-switched CO ₂ laser device of the present invention.

FIG. 2 is a timing chart showing the time behavior of various control signals.

FIG. 3 is a graph showing an example of measuring the time behavior of an energy-modulated Q-switched laser pulse obtained by the configuration of the embodiment of the present invention and various signals related thereto.

FIG. 4 is a graph showing another example of measuring the time behavior of the energy-modulated Q-switched laser pulse obtained by the configuration of the embodiment of the present invention and various signals related thereto.

FIG. 5 is a schematic diagram showing the time behavior of various oscillation parameters for explaining the operation principle of the Q-switch laser oscillation process.

[6] CO ₂ according to the laser oscillation CO ₂ molecule and N ₂
5 is an energy level diagram showing excited vibration levels of molecules.

FIG. 7 is a graph showing experimental results obtained by measuring the characteristics of the Q-switch pulse energy when the peak power of the discharge pulse is fixed, the width is changed, and the amount of injected energy is changed in the Q-switched CO ₂ laser. It is.

FIG. 8 shows an experiment in which the peak power and the width of a discharge pulse are fixed in a Q-switched CO ₂ laser and the characteristics of the Q-switch pulse energy when the discharge pulse and the opening timing of the Q-switch are changed. It is a graph which shows a result.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Laser beam 2 Discharge part housing 3 Total reflection mirror 4 Partial transmission output mirror 5 Telescope lens 5 'Telescope lens 6 Rotating chopper 7 Laser diode 8 Photodiode 9 Delay pulse generator 10 Pulse width modulator 11 Computer 12 Pulse discharge Power supply a Synchronous signal generator output signal b Delay pulse generator output signal c Pulse width modulator output signal d Discharge pulse e Q switch opening timing f Q switch laser pulse

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-4-259278 (JP, A) JP-A-2-202081 (JP, A) JP-A-4-255283 (JP, A) 52945 (JP, Y2) Special Table 62-500135 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) H01S 3/10-3/139

Claims (3)

(57) [Claims] 1. A pulse discharge power supply, a case in which a CO ₂ laser medium is sealed to excite pulse discharge, a total reflection mirror and a partial transmission mirror forming a resonator with the case interposed therebetween,
A Q-switched CO ₂ laser device having a Q-switch device disposed in the resonator that operates in synchronization with pulsed-discharge excitation; and a mechanism for controlling a discharge time of the pulse-discharge excitation. A synchronization signal generator for generating a pulse signal that is time-synchronized with the Q switch, and a delay pulse generator for generating a pulse having a fixed delay time with respect to the synchronization signal. And a pulse width modulator that is time-synchronized with the delay pulse and sends out a pulse width-modulated signal according to a preset pattern, and the discharge excitation pulse is synchronized with the rising timing of the delay pulse. Start and end the discharge excitation pulse in synchronization with the fall timing of the pulse width modulated signal. Q-switched CO ₂ laser device. 2. The Q-switched CO ₂ laser device according to claim 1, wherein the Q-switched device is formed by a combination of a telescopic device constituted by a pair of condensing optical systems and a rotary chopper. 3. The Q-switched CO ₂ laser device according to claim 1, wherein the Q-switched device comprises a combination of an electro-optical element and a polarizer.