JPH0730179A - Q switched co2 laser device - Google Patents

Abstract

PURPOSE:To arbitrarily control each pulse energy and crest output within a Q switched pulse train with a simple configuration by providing a mechanism for controlling the discharge time interval for exciting pulse discharge. CONSTITUTION:Q switch timing according to a rotary chopper 6 is detected photoelectrically at a position away from a laser optical axis to generate an output signal (a) of a synchronous signal generator. This signal becomes a delay pulse generator output signal (b) which is delayed by a delay pulse generator 9 and is input as an external trigger of a pulse width modulator 10. The pulse width modulator 10 is synchronized to the external trigger signal (b) and transmits a trigger signal (c) which is subjected to pulse width modulation by a modulation signal which is generated by a computer 11 in advance and then a pulse discharge power supply 12 applies a discharge pulse (d) whose timing and pulse width correspond to those of the signal (c) to a laser medium.

Description

Detailed Description of the Invention

[0001]

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

[0002]

_2. Description of the Related Art With the dramatic increase in the output of continuous wave CO ₂ lasers and the improvement in the quality of laser beam spatial modes in recent years, the fields of application thereof have made great progress centering on various material processing. Furthermore, recently, various developments in new fields have been reported by pulsing the oscillation form of a 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, and laser isotope separation using infrared multiphoton excitation. , Optical excitation of far infrared laser. For application of CO ₂ lasers in such fields, high peak power exceeding 10 kW, 10 k
High average output power is required due to high pulse repetition rate of Hz or higher.

As a CO ₂ laser for such an application, there has been a pulsed TEA laser which operates in a laterally pumped atmospheric pressure. In the TEA laser, high-voltage and high-current pulse discharge can be applied to the laser gas at atmospheric pressure to realize rapid population inversion distribution. Therefore, a high peak output of 10 MW or more with a pulse width of about 100 nsec is achieved by the gain switching phenomenon. Although it can be easily extracted, the pulse repetition frequency is practically about 10 Hz to 100 Hz, and a large average output cannot be extracted. Further, since the high power pulse discharge is performed in a short time, the life of the laser oscillator is short, and the cost required for its operation is increased.

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

In order to deal with such a problem, various methods have been proposed in which the Q switching technique, which has been often used in the conventional solid-state laser, is applied to the 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 cavity of a CW pumped CO ₂ laser. According to the above document, as a Q switching method,
A passive Q-switching method of inserting a saturable absorber into a resonator, a method of using an electro-optical element, a method of incorporating a rotating mirror into the resonator, and a Q-switch device including a combination of a telescope and a rotating chopper. A method of inserting the Fabry-Perot etalon into the resonator and a method of inserting the Fabry-Perot etalon disclosed in JP-A-62-160783 have been proposed. Among them, the method using the rotary chopper and the telescope has a problem that it is necessary to improve the Q switching speed, but it is the mainstream because of other controllability and stability.

Gas Flow & Chemical Lasers (GCL) Confer
ence Proceedings, Viena 1988, Vol.1031, SPIE, p48 has a CWCO ₂ laser with a Q switch by a rotary chopper system, and has achieved pulse energy of 27 mJ, pulse peak output of 150 kW and pulse repetition frequency of 10 kHz. It has been reported, and the inventors of the present application have made various investigations, and applied for a method of using a radio frequency (RF) pulse discharge synchronized with a rotating chopper in order to improve pulse peak output. 4-2
(See Japanese Patent No. 59278). The Q switching method of the CO ₂ laser is a method of extracting a Q switch pulse train of equal energy at a cycle of all fixed intervals.

By the way, when the pulsed Q-switch CO ₂ laser is applied to the processing of a metal material, particularly to the hole processing or the cutting processing, or when it is applied to the infrared multiphoton 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 to be. For example, in material processing, it is possible to change the surface texture with the first pulse to change the surface absorptance with respect to laser light, and to greatly improve the processing efficiency with the second pulse. In photon excitation, it is possible to increase the overall molecular dissociation efficiency by efficiently dissociating the molecular species excited to the intermediate level in the first pulse by the second 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. 4-22182 discloses a method for controlling the direct-current excitation intensity. . Continuous wave excitation Q
If the DC excitation intensity is controlled in the switch laser device,
Since it is possible to change the population inversion distribution accumulation at the time when the resonator Q value becomes high, it is possible to control the energy of the Q switch pulse and the peak output, but to respond to each pulse, 10 kHz It was difficult to control the continuous wave excitation intensity at a high speed. Also, for solid-state lasers, Japanese Utility Model Application Laid-Open No. 63-9170 discloses a device capable of extracting pulses having two kinds of energy characteristics by superposing continuous wave excitation and pulse excitation as excitation methods. ing. However, since this method also modulates the direct current excitation level as in the conventional example, high speed modulation of about 10 kHz is difficult.

In order to cope with such a problem, the inventors of the present invention have disclosed in Japanese Patent Application No. 5-93429 that a Q-switched CO ₂ laser excited by a pulsed discharge causes a Q discharge twice or more for one pulse discharge. We have applied for a Q-switch method to obtain a pulse group in which the pulse energy and the peak power are gradually reduced by opening the switch. In this method, it is possible to control the degree of the above reduction 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 The object of the present invention is to solve the above-mentioned problems of the prior art, and to irradiate the laser pulse of the Q-switch CO ₂ laser device used in metal material processing, laser photochemistry and the like. As a Q-switched laser output capable of improving the above, it is possible to arbitrarily control individual pulse energy and peak output of the Q-switched pulse train with a simple configuration.

[0011]

The above-mentioned object is to provide a pulse discharge power source, a housing in which a CO ₂ laser medium is sealed for pulse discharge excitation, and a total reflection mirror which forms a resonator with the housing sandwiched therebetween. Also, in a Q-switch CO ₂ laser device having a partial transmission output mirror and a Q-switch device arranged in the resonator that operates in synchronization with pulse discharge excitation, the discharge time width of pulse discharge excitation is controlled. This is accomplished by providing a Q-switched CO ₂ laser device characterized by having a mechanism.

[0012]

FIG. 5 schematically shows the temporal behavior of various parameters related to laser oscillation in order to explain the principle of laser Q switching. In FIG. 5, the Q-switched laser pulse, which is the laser output, has a high sharpness in a short time width as a result of the population inversion energy accumulated until then being released at once due to the Q value of the resonator rising rapidly. It is taken out 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 population inversion distribution. Therefore, when the laser pulse energy is controlled, that is, the population inversion distribution is controlled, which is determined by the balance between the amount of energy injected for laser excitation from the outside and the lifetime of the laser upper level. Parameter.

[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 It is the energy diagram which expressed typically.
The CO ₂ molecule has a symmetric stretching mode n ₁ , a bending mode n _{2 and an} asymmetric stretching mode n ₃ , and their vibration levels are (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
_For oscillation in the 10.6 μm band, which is a typical oscillation wavelength of _two lasers, the (0 0 ⁰ 1) level is set as the upper laser level, and (1
This is obtained by the transition with the 0 ⁰ 0) level as the lower level. On the other hand, the N ₂ molecule also has a vibrational level, and its first vibrational excitation level (v = 1) is almost at the same energy level as the (0 0 ⁰ 1) level of the CO ₂ molecule, and the difference is About 18c
m ^-1 and heat energy of gas at the time of discharge (about 20 at 300K °
Much smaller than 0 cm ^-1 ). As a result, energy transfer occurs extremely efficiently due to collision between CO ₂ molecules in the laser upper level and N ₂ molecules in the (v = 1) level. Therefore, by adding N ₂ gas to the CO ₂ laser medium, the CO ₂ laser upper level is efficiently excited. Here, the lifetime of the excited N ₂ molecule (v = 1) is 100 μsec or more, which is relatively long in the operating pressure range (100 torr or less) of a general CO ₂ laser. Furthermore, N ₂
The energy transfer time constant from the molecule to the CO ₂ molecule is determined by the collision probability between molecules, that is, the pressure of the laser medium. The coefficient that determines the time constant is 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 life of N ₂ molecule. Therefore, the presence of N ₂ molecules has a function of replenishing the population inversion distribution accumulated which decreases by spontaneous transition due to the lifetime of the upper level of CO ₂ molecules.

From the above discussion, it is difficult to uniquely determine the equivalent upper level lifetime of the CO ₂ laser, but at least it corresponds to the lifetime of the N ₂ molecule and the energy transfer time constant of about 100 μsec. Above a pulse repetition frequency of about 10 kHz, the effect of reducing the population inversion distribution accumulation due to the upper level lifetime of the laser is small, and the size of the population inversion is almost determined by the amount of energy injected from the outside. You can think. Considering changing the amount of injected energy, there are two cases: changing the amplitude of the discharge power in a constant time width and changing the discharge time width while keeping the discharge power constant. However, any case may be adopted in the above time scale. The above is the description of the principle of the present invention.

Generally, in the case of a Q-switched solid-state laser of pulse excitation, when controlling the laser pulse energy, the former of the above two cases, that is, the excitation pulse width is kept constant and the pulse amplitude is changed to control the excitation energy. The method is adopted. This is because keeping the arc discharge time constant of the flash lamp power supply constant and changing the pulse power by fixing the pulse width can be realized with a simpler configuration for control, but on the other hand, the pulse repetition frequency is several tens.
The limit is 0 Hz.

On the other hand, in the RF pulse modulation discharge method capable of modulating at a high pulse repetition frequency in the glow discharge excitation of the CO ₂ laser, the former pulse amplitude is changed due to the impedance matching condition between the load and the power supply. It is difficult to do so at a high speed, but on the contrary, it is easy to realize the latter in which the pulse amplitude is kept constant and the pulse width is changed. Therefore, the point of the present invention for performing energy modulation of the output pulse in the Q switching of the CO ₂ laser is to control the injection energy amount by pulse width modulation and to control the population inversion amount and thus the Q switch laser pulse energy. It is in.

FIG. 7 shows the result of an experiment conducted to confirm the validity of the above-described principle description, and the horizontal axis in the figure represents the discharge pulse width. Discharge pulse peak power is 1
The amount of energy injected into the laser medium was controlled by fixing the pulse width at 5 kW and changing the pulse width. The Q switch device used a combination of a telescope and a rotary chopper, and the Q switch opening timing was synchronized so as to be immediately after the end of the discharge pulse. Pulse repetition frequency is 10 kHz
Since z is set, 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 up to a pulse width of at least about 60 μsec, and it is confirmed that the Q switch pulse energy can be controlled by the discharge pulse width modulation. Was done. It should be noted that when the pulse width is 60 μsec or more, the laser output tends to be saturated, because the above-mentioned upper level life of the laser exerts a slight influence and the population inversion distribution is saturated.

Next, assuming that the Q switch laser pulse is output at a constant interval, the discharge pulse width changes, so that the Q switch opening timing should always be synchronized with the end point 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, 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. The result is shown in FIG. Since the pulse repetition frequency is 10 kHz, the maximum delay time is 80 μs.
ec, it was found that the influence on the output pulse energy can be suppressed to about 15% even if there is a delay of 4 times the excitation discharge pulse width. This is as described in the above principle,
This is because the equivalent upper level lifetime is relatively long.
From the above results, it was found that the laser output pulse energy can be controlled substantially linearly with the discharge pulse width even if the Q switch opening timing is aligned with the expected end timing of the longest discharge pulse.

Next, the construction and operation of an actual output pulse energy modulation Q-switch CO ₂ laser will be described with reference to FIGS. 1 and 2.
Will be explained. FIG. 1 shows a Q switch CO according to the present invention.
₂ shows the configuration of a laser device. The laser resonator is composed of a total reflection mirror 3 and a partial transmission output mirror 4, and telescope lenses 5, 5'and a rotary chopper 6 are provided in the resonator.
A Q switch device constituted by and is installed. The laser medium is excited by the 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 away from the laser optical axis to generate an output signal a of the synchronizing 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 uses this external trigger signal b
A trigger signal c that is pulse-width modulated by a modulation signal generated in advance by the computer 11 is sent to the pulse discharge power supply 12, and the pulse discharge power supply 12 has 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 of these various signals, and only three pulses are schematically shown in order to clarify the time relationship.

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

[0022]

1 is a block diagram showing an embodiment of a Q-switch CO ₂ laser according to the present invention. Laser cavity is GaAs
Made of a total reflection mirror 3 having a reflectance of 99.5% and a concave curvature of 30 m and a flat partial transmission output mirror 4 made of ZnSe having a reflectance of 50%, and the resonator length is about It is 6m. The Q switch device is a ZnSe lens 5 with a focal length of 127 mm, which is coated with an antireflection coating,
5'is configured by a combination of a telescope installed under the confocal condition and a rotary chopper 6 installed at the confocal position. The rotary chopper is a rotary disc having 10 slits at equal intervals on the circumference, and the pulse repetition frequency is 10 by rotating at a rotation speed of 1000 rps.
Q switching of kHz is realized. Laser medium is C
Gas mixed at a ratio of O ₂ / N ₂ /He=1.2/1.7/8, the total pressure is about 75 torr, and it circulates at high speed in the discharge unit housing 2 composed of a discharge glass tube. To do. Q
The signal a corresponding to the opening timing of the switch is photoelectrically detected as the output of the synchronization signal generator formed by the combination of the laser diode 7 and the photodiode 8 mounted on the upper part of the chopper. The synchronizing signal generator output signal a is given a constant delay by the delay pulse generator 9. In the configuration of the present embodiment, the photoelectrically detected position is a position shifted by exactly one-half circumference from the laser optical axis, and this signal and the Q switch opening timing e are synchronized in phase. Therefore, as the delay amount in the delay pulse generator 9, when exciting with a pulse having a width of 80 μsec, 15 μse is set so that the Q switch is opened at the end of the excitation.
c. The remaining 5 μsec is the total response delay of the other signal processing equipment 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 the pulse width modulator 10 constituted by the synthesizer, and the signal c pulse-width modulated according to the modulation signal preset by the computer 11 is output. It is supplied to the pulse discharge power supply 12. The pulse discharge power supply 12 has a radio frequency (R) of carrier frequency 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 according to 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. 20 μsec discharge pulse width
As a result of modulating from 20 to 80 μsec with a pitch of 20 μsec, laser pulse energy is 22, 47, 69, 83 m
J, pulse peak power 80,170,250,300 kW pulse energy and peak power modulation were realized. Next, FIG. 4 shows the result when the discharge pulse width is modulated with 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, by using the configuration of the Q-switch CO ₂ laser device according to the present invention, it is possible to perform arbitrary modulation control of pulse energy and peak output, which has been difficult in the past. The irradiation effect can be greatly improved in material processing and photochemistry applications.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an embodiment of a Q-switch 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 the 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 operating principle of the Q-switch laser oscillation process.

[6] CO ₂ according to the laser oscillation CO ₂ molecule and N ₂
It is an energy level diagram which shows the excited vibrational level of a molecule.

FIG. 7 is a graph showing the experimental results of measuring the characteristics of the Q switch pulse energy when the peak energy of the discharge pulse is fixed and the width thereof is changed to change the injection energy amount in the Q switch CO ₂ laser. Is.

FIG. 8 is an experiment in which the peak power of a discharge pulse and its width are fixed in a Q-switch CO ₂ laser, and the characteristics of the Q-switch pulse energy are measured 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]

 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 Delayed pulse generator output signal c Pulse width modulator output signal d Discharge pulse e Q switch opening timing f Q switch laser pulse

Claims (4)

[Claims] 1. A pulse discharge power source, a casing in which a CO ₂ laser medium is sealed for pulse discharge excitation, a total reflection mirror and a partial transmission output mirror which form a resonator with the casing sandwiched, and pulse discharge A Q-switch CO ₂ having a Q-switch arrangement arranged in said resonator operating in synchronization with the excitation
A Q-switched CO ₂ laser device having a mechanism for controlling a discharge time width of pulse discharge excitation in the laser device. 2. The discharge time width control mechanism, a synchronization signal generator that generates a pulse signal that is time-synchronized with the Q switch, and a delay that generates a pulse having a constant delay time with respect to the synchronization signal. 2. The Q switch according to claim 1, comprising a pulse generator and a pulse width modulator which is time-synchronized with the delayed pulse and which outputs a pulse width modulated signal according to a preset pattern. CO ₂ laser device. 3. The Q-switch CO _{2 according} to claim 1 or 2, wherein the Q-switch device is a combination of a telescope and a rotary chopper which are composed of a pair of condensing optical systems. Laser device. 4. The Q-switch CO ₂ laser device according to claim 1, wherein the Q-switch device comprises a combination of an electro-optical element and a polarizer.