JPH06310795A - Q switch method for co2 laser - Google Patents

Abstract

PURPOSE:To obtain a laser pulse train having pulse peak outputs variable at the period of pumping discharge pulse by outputting a pulse laser light obtained directly from pulse discharge and a pulse laser light obtained after oscillation thereof. CONSTITUTION:A laser oscillator is constituted of a total reflection mirror 6 and a partial transmission mirror 7. A Q switch device, i.e., a rotary chopper 8, is disposed in a laser resonator along with a cofocal telescopic lens 9 for performing Q switching at a high rate. A slit detector 1 generates a repetitive signal for opening/closing the slit of the rotary chopper 8. A frequency division circuit 2 divides the frequency of the repetitive signal. A delay pulse generating circuit 3 generates a pulse signal using an output signal from the frequency division circuit 2 as a drive trigger signal. A laser pumping power supply 4 can perform pulse modulation at the repetitive frequency and generates a pulse voltage for pumping 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 pulse Q switch CO used for welding or cutting metal, laser photochemistry and the like.
_{The present} invention relates to a CO ₂ laser Q-switch method for controlling the peak output of pulsed laser light at a cycle of several tens of μs in order to control the heat input pattern over time in _two lasers and improve the laser irradiation effect.

[0002]

_2. Description of the Related Art A pulsed CO ₂ laser is capable of irradiating a gas, a solid or the like with a high density of energy in a short time, and it has been difficult to perform laser thermal processing or non-thermal treatment which has been difficult with a continuous wave CO ₂ laser. It can be applied to thermal laser photochemistry. Examples include processing of difficult-to-process metals such as Al and Cu, and laser isotope separation of C, N, and O using infrared multiphoton dissociation reaction.

In order to obtain a pulsed laser beam in a CO ₂ laser, a TEA (transverse excitation atmospheric pressure operation) system is generally used, and a peak output of 50 MW or more can be obtained relatively easily. Since such a large peak output can be obtained, it has been generally used especially in the study of laser photochemistry. However, since high-voltage pulse discharge of 100 kV or more is performed to excite the laser gas, the gas deterioration,
The electrodes are heavily worn, and the window member that transmits the high peak pulse laser light is easily damaged. As a result TEA
CO ₂ lasers have problems in industrial application in terms of running cost and reliability.

Therefore, there is a Q-switch method as another method for obtaining pulsed CO ₂ laser light. This technique is a basic technique of pulsed laser oscillation in which pulsed laser light is obtained using a laser device that performs continuous oscillation. Continuous-wave CO ₂ lasers are widely used in industrial applications at present, and as can be seen from the viewpoint of running cost and reliability, T
It greatly surpasses the EA CO ₂ laser. Therefore, the Q-switched CO ₂ laser is a laser device suitable for industrializing the application technology of the pulsed CO ₂ laser.

A method of Q-switching a CO ₂ laser is described in, for example, the document IEEE J. Quantum Electronics, Vol.QE-4, p762,196.
It is summarized in Section 8, and a typical method is to use an electro-optic crystal, a rotating mirror, and a rotating chopper. Among these methods, the rotary chopper method in which a rotary disk having a slit on the circumference is inserted in the resonator is a Q that mechanically opens and closes an equivalent optical shutter provided on the optical axis.
This is a switching method, and the device is relatively simple, and stable laser oscillation is possible even at high average output operation, and is an optimal Q switching method when industrial application of the Q switch CO ₂ laser is considered.

A general Q-switched CO ₂ laser using a rotary chopper is, for example, Gas Flow and Chemical Lasers (GC
L) Conference Proceedings, Viena, 1988, Vol.1031,
As in SPIE, P48, the laser excitation method by DC discharge is adopted, and a pulse train with a constant peak output is obtained by high pulse repetition of 10 kHz or more.

By the way, when the Q-switch CO ₂ laser is applied to the processing of difficult-to-process materials and laser photochemistry, the energy and peak output of the pulsed laser light are changed at a cycle of several tens of μs, and the heat input with time is changed. It has recently become clear that laser irradiation effects such as processing performance and reaction efficiency can be greatly improved if the pattern can be controlled. For example, there is laser welding of Al, which is difficult to perform laser processing because of its high thermal conductivity and high reflectance for the CO ₂ laser wavelength. Here, by using a periodic laser pulse group whose peak output decreases with time of several tens of μs, a minute melting spot is instantly created on the Al surface with a pulsed laser light with a large peak output, and the laser at that point is generated. It is possible to increase the light absorption rate and to weld Al with high efficiency by the subsequent pulsed laser light having a low peak output. Here, in the conventional example of the Q-switch CO ₂ laser device described above, it is possible to control the peak output of the pulsed laser light by adjusting the average discharge input.

[0008]

However, the input adjustment speed of the power source is limited, and it is impossible to control the peak output in a short time as required for improving the irradiation effect.

The object of the present invention is to provide a Q-switched CO ₂ laser device used for processing difficult-to-process materials, laser photochemistry, etc., in order to improve the laser light irradiation effect by pulsing with time. It is an object of the present invention to provide a Q-switch method for obtaining a periodic laser pulse group in which the peak output of laser light is reduced with a simple device configuration.

[0010]

The present invention SUMMARY OF], in CO ₂ laser apparatus for performing Q-switch oscillation using a rotary chopper, the laser medium is a mixed gas containing CO _2, N _2, and CO ₂ and N ₂ The component ratio of each is adjustable, and the laser medium is excited by direct-current or high-frequency discharge pulse-modulated in synchronism with a signal obtained by frequency-dividing the opening / closing repeating signal of the slit of the rotary chopper. By opening and closing the slit of the rotary chopper at least two times from the end to the start of the next pulsed excitation discharge, a laser pulse group that oscillates at the opening and closing repetition cycle of the slit of the rotary chopper and the peak output decreases with time is generated. This is a Q-switch method for a CO ₂ laser obtained with a pulse discharge cycle.

[0011]

The principle of the present invention will be described below. Figure 3 is CO
FIG. 3 is a schematic diagram of vibration energy levels of CO ₂ molecule and N ₂ molecule shown for explaining the oscillation principle of _two lasers. The CO ₂ molecule has three vibration modes, a symmetric stretching mode n1, a bending mode n2, and an asymmetric mode n3, and the vibration energy levels thereof are represented by (n1, n2, n3). Oscillation in the 10.6 μm band, which is a typical oscillation wavelength of a CO ₂ laser, is inverted with the (0 0 ⁰ 1) vibrational level as the laser upper level and the (1 0 ⁰ 0) vibrational level as the laser lower level. A distribution is formed and laser oscillation is obtained. On the other hand, the N ₂ molecule also has a vibrational level, and the first vibrational excitation level (V = 1) is at almost the same energy level as the upper level of the CO ₂ laser, and the energy difference between them is during discharge. It is much smaller than the heat energy distribution of gas, about 18 [1 / c
m]. Therefore, with CO ₂ molecule in the upper level of the laser (V =
1) Between N ₂ molecules in the level, energy transfer occurs extremely efficiently due to collision. Therefore, by mixing N ₂ with the CO ₂ laser medium, C is generated during the discharge excitation period.
More efficient formation of the upper level of the O ₂ laser is performed. Since the (V = 1) level of N ₂ is in a metastable state, the pump life is relatively long at 100 μs or more at the operating pressure of the Q-switch CO ₂ laser, and once the CO ₂ laser oscillation is started, It is possible to compensate the decrease in the number density of the CO ₂ laser upper level by energy transfer, and the N ₂ molecule has a function of an energy tank for forming the CO ₂ laser upper level.

Here, the energy transfer time constant from N ₂ to CO ₂ is almost determined by the collision probability between molecules, that is, the pressure of the laser gas. Reference Rev. Mod. Phys., 41, 2
6, According to 1969, the pressure dependence coefficient of the time constant is 1.9 × 10
⁴ / torr / sec. The operating pressure of a Q-switch CO ₂ laser is generally 100 torr or less, and its time constant is 10 to 50 μs, which is relatively long. On the other hand, since the pulse time width of the Q-switch CO ₂ laser is 100 to 300 ns, the effect of energy transfer from N ₂ to CO ₂ becomes remarkable after the pulse laser oscillation is completed. That is, the effect of energy transfer from N ₂ supplements the number of CO ₂ molecules in the laser upper level consumed by the Q-switch pulse oscillation, and contributes to the reformation of population inversion. Therefore, in the Q-switched CO ₂ laser oscillation process, N ₂ has _two functions of highly efficient formation of the CO ₂ laser population inversion during discharge excitation and reformation of the population inversion consumed by laser oscillation.

There are two methods of exciting the CO ₂ laser medium, continuous discharge and pulse discharge. When continuous discharge excitation is applied to a Q-switched CO ₂ laser, _two discharges of N ₂ occur at the same time because the discharge is performed steadily, and a constant population inversion is always formed at the moment of Q-switching. It As a result, a laser pulse having a constant peak value is output. On the other hand, in pulse discharge excitation, the two effects are clearly distinguished, and there is a difference between the population inversion distribution formed during discharge excitation and the population inversion distribution that is re-formed after performing Q-switch oscillation using the population inversion. Will occur.

In the present invention, based on the principle described above,
The original Q-switched pulse laser light obtained from the population inversion formed directly by the pulse discharge and N ₂ after the oscillation.
A laser pulse train whose pulse peak output changes in the cycle of the excitation discharge pulse can be obtained by using a configuration in which the subsequent Q-switched pulsed laser light obtained by utilizing the population inversion regenerated by energy transfer from obtain.

The Q-switch method of the present invention will be described in detail below with reference to FIGS. 1 and 2. The frequency dividing circuit 2 is driven by the slit opening / closing repeating signal (a) of the rotary chopper 8 output from the slit detector 1 for monitoring the Q switch timing shown in FIG.
Is output. Signals (a), (b), etc. are shown in FIG. Figure 1
In the figure, as an example, the case of 1/4 frequency division is shown. The delayed pulse generation circuit 3 is driven by the frequency-divided signal (b) and outputs a pulse-modulated signal (c) which is synchronized with the frequency-divided signal (b) and has a specific delay time and pulse time width. The laser excitation discharge voltage is pulse-modulated by the pulse modulation signal (c) in the laser excitation discharge power supply 4, and the pulse modulation discharge voltage (d) is applied to the laser medium 5. By the discharge of the laser medium 5 by this voltage (d), the population inversion (e) of the CO ₂ laser is formed. FIG. 1 also shows the population inversion distribution (g) for causing laser oscillation.

Here, the delay pulse generation circuit 3 sets an arbitrary delay time between the frequency-divided signal (b) and the pulse-modulated signal (c), the end timing of the pulse discharge voltage (d) and the rotation chopper 8. Synchronize the slit opening timing,
Q-switching can be performed when a sufficient population inversion is formed. With such a Q-switch method, a Q-switch CO ₂ laser pulse train (f) can be obtained.

Next, the oscillation mechanism of the laser pulse trains f ₁ , f ₂ , f ₃ and f ₄ obtained during one period t _D of the pulsed excitation discharge obtained by the Q switch method described above will be explained. The initial population inversion amount e ₁ is formed by the pulse discharge (d), and the rotary chopper 8 immediately after the end of the pulse discharge (d).
The first laser pulse f ₁ is obtained by the slit opening a ₁ of. Since the Q-switch laser pulse has its pulse peak value at the moment when the population inversion falls below the oscillation threshold, the population inversion occurs within a short time of 300 ns or less in the time width of the Q-switch laser pulse due to this first laser oscillation. It sharply decreases below the oscillation threshold (g). On the other hand, the (V = 1) level of the N ₂ molecule in the metastable state has a lifetime of 100 μs or more. Therefore, immediately after the completion of the first laser oscillation, as shown in FIG. 3 CO on ₂ molecular laser of level number density and N ₂ molecules (V =
1) A large density gradient occurs between the level number densities, and energy transfer from N ₂ to CO ₂ occurs with a time constant of 10 to 50 μs as described above. As a result, the laser population inversion of CO ₂ molecules increases, and the population inversion e ₂ exceeding the oscillation threshold value e ₂
Are reformed. Here, the second pulse laser oscillation f ₂ is obtained by the second slit opening a ₂ after the time t _QS from the first laser oscillation. Population inversion e reformed here
_{Since 2} is smaller than the population inversion amount e ₁ formed during pulse discharge, the peak output of f ₂ is smaller than that of f ₁ . Furthermore, after the second pulse laser oscillation is completed, the third by the same oscillation mechanism and f _2, the fourth pulse laser oscillation f _3, f ₄ is obtained.

With the laser oscillation mechanism described above, in the Q-switch method of the present invention, a CO ₂ laser pulse group that oscillates at the opening / closing repetition cycle t _QS of the rotary chopper 8 and whose peak output decreases with time is generated. , Laser excitation pulse discharge period t _D can be obtained. Further, in the Q switch method of the present invention, one Q discharge is generated by one pulse discharge excitation.
The energy accumulated in the (V = 1) level of the N ₂ molecule, which was hardly used in the method of obtaining the switch laser pulse, can be effectively used, so that the laser output obtained for the same discharge input is There is also an advantage that large and highly efficient Q-switched CO ₂ laser oscillation can be performed.

Further, in the Q switch method of the present invention, similar to the conventional Q switch method using the rotary chopper, the pulse repetition period t is changed by changing the rotation speed of the rotary chopper 8.
_{Since QS} can be controlled and the frequency dividing ratio by the frequency dividing circuit 2 can be selected freely, energy that can reform the population inversion enough to exceed the laser oscillation threshold in N ₂ molecules. There during sustained can be selected arbitrarily t _QS and t _D, respectively, it is possible to thereby freely control the number of laser pulses contained in one cycle of the pulse excitation discharge.

Further, the change in the peak output of the pulse within one cycle of the pulse excitation discharge is the energy stored in the N ₂ molecule in view of the principle of the present invention, that is, the number of N ₂ molecules in the (V = 1) level. , Depends on. Therefore, as shown in FIG.
By changing the relative ratio of N ₂ in the component composition of the laser medium, it is possible to adjust the reduction rate of the pulse peak output and output a laser pulse train having various peak output changes.

[0021]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an example of the time relationship of control signals when the Q switching method of the present invention is carried out in one mode. FIG. 2 shows the Q switching method of a CO ₂ laser according to the present invention. The apparatus structure used for implementation is shown. A continuous wave CO ₂ laser, which is a basic device of the Q-switched CO ₂ laser, uses a high frequency discharge as an excitation method. The laser resonator is composed of a total reflection mirror 6 and a partial transmission mirror 7, and the resonator length is about 6 m. The laser medium is CO ₂ , N ₂ and He.
The mixed gas is a total pressure of about 90 torr and is enclosed in a discharge glass tube. The gas composition of the laser medium is (C
It can be arbitrarily adjusted within the ratio limit of O ₂ + N ₂ ): He = 27: 73. A rotary chopper 8 is provided as a Q switch device in the laser resonator. Also, in order to perform Q switch at high speed, a confocal telescope lens 9
Is provided, and the Q-switch is performed by chopping the laser light by the rotary chopper 8 at the focal position.
The slit detector 1 generates a slit opening / closing repeating signal (a) of the rotary chopper 8. The frequency dividing circuit 2 is a device that outputs a signal (b) obtained by dividing the repetitive signal (a).
FIG. 1 shows the signal waveform of each part when the frequency dividing circuit 2 performs 1/4 frequency division. The frequency dividing circuit 2 used here is a simple circuit in which flip-flop IC circuits are combined, and the frequency dividing rate is a power of 2. However, it goes without saying that it is possible to manufacture a frequency dividing circuit that obtains an arbitrary frequency dividing rate by using a pulse counter IC or the like.

The delay pulse generating circuit 3 uses the output signal (b) of the frequency dividing circuit 2 as a drive trigger signal and generates a pulse signal (c) having an arbitrary time width t _W. Further, an arbitrary time delay can be set between the trigger signal (b) and the output pulse signal (c), and the end of the laser excitation pulse discharge driven by the pulse signal (c) and the opening of the slit of the rotary chopper 8. Timing is synchronized.

The laser excitation power source 4 has a carrier frequency of 1
It is a high frequency power supply of 3.56MHz and can perform pulse modulation up to a repetition frequency of 100kHz. The power supply 4 generates a pulse voltage (d) that is pulse-modulated by the pulse signal (c) and discharge-excites the laser medium.

Three examples according to the present invention in which the slit opening / closing repetition period t _QS of the rotary chopper 8 and the discharge pulse modulation period t _D and the laser gas composition are set as variable parameters in the above apparatus settings will be described below.

As a first embodiment, t _QS = 71 μs (1 / t
_QS = 14kHz), 1/4 frequency division is performed, and t _D = 286μs (1 /
t _D = 3.5 kHz), discharge pulse time width t _W = 57 μs, and laser gas composition having a relatively high N ₂ ratio CO ₂ : N ₂ :
He = 0.6: 2.3: 8.0 was selected, and Q-switched CO ₂ laser oscillation according to the present invention was performed. The slit opening / closing repeating signal, the discharge pulse voltage and the output laser pulse waveform of the rotary chopper 8 are shown in FIGS. 5 (a), (d) and (f), respectively.
Shown in. Under this condition, f during one cycle of pulse discharge
A pulse group whose peak output decreased with time from _{1 to} f ₄ was generated in the cycle of pulse discharge. The pulse peak powers of f _{1 to} f ₄ were 495, 153, 55 and 18 kW, respectively.

Next, as a second embodiment, the N ₂ ratio of the laser gas component is reduced and CO ₂ : N ₂ : He = 1.2: 1.7:
Was subjected to oscillation at 8.0, the same laser pulse group in the first embodiment are obtained to f ₁ ~f ₃ as shown in FIG. The pulse peak outputs in this case are 541 and 148, respectively.
And 32 kW. Here, the energy transfer effect from N ₂ is smaller than in the case of the first embodiment,
The fourth pulse oscillation was not obtained.

As a third embodiment, the frequency division ratio is 1/2
, T _QS = 125μs (1 / t _QS = 8kHz), t _D = 250
μs (1 / t _D = 4kHz) and discharge pulse time width t _W = 50μ
Adjusted to s and oscillated. Here, the laser gas composition was the same as in the second embodiment. Under this oscillation condition, as shown in FIG. 7, two laser pulses f are generated during one cycle of the pulse discharge.
₁ and f ₂ occur and pulse peak output is 480 each
And 105 kW were obtained.

[0028]

As described above, in the Q-switch CO ₂ laser device using the rotary chopper, the Q-switch method of the present invention oscillates at the slit opening / closing cycle of the rotary chopper, and the pulse peak output decreases with time. Since the laser pulse group can be obtained in the excitation pulse discharge cycle, the laser light irradiation effect in laser processing of difficult-to-process materials and laser photochemistry can be improved.

[Brief description of drawings]

1 is a time chart showing output signals of the circuit elements and the like shown in FIG. 2, (a) is a slit opening / closing repeating signal of the rotary chopper 8 shown in FIG. 1, and (b) is a frequency dividing circuit 2;
Output signal of the pulse generator 3, (d) the laser excitation discharge voltage waveform, and (e) C
The time variation of the inversion number density of O ₂ molecules, the (f) is Q-switched CO ₂ laser pulse time waveform shows a population inversion density threshold (g) is a CO ₂ laser.

FIG. 2 is a block diagram showing an example of a device configuration for implementing the present invention.

FIG. 3 is a graph showing vibrational energy levels of CO ₂ and N ₂ molecules.

FIG. 4 is a graph showing a change tendency of the laser pulse peak output when the N ₂ ratio of the laser medium is changed in the present invention.

FIG. 5 is a graph showing a laser pulse peak output obtained in the first embodiment of the present invention.

FIG. 6 is a graph showing a laser pulse peak output obtained in the second embodiment of the present invention.

FIG. 7 is a graph showing a laser pulse peak output obtained in the third embodiment of the present invention.

[Explanation of symbols]

1: Slit detector for rotating chopper slit open / close signal monitoring 2: Dividing circuit 3: Delay pulse generator 4: High frequency power source for laser excitation 5: Laser medium 6: Total reflection mirror 7: Partial transmission mirror 8: Rotation Chopper 9: Telescope lens t _QS : Slit opening / closing repetition cycle of rotating chopper t _D : Laser excitation pulse discharge period t _W : Laser excitation pulse discharge time width

Claims (2)

[Claims] 1. A CO ₂ laser device for performing Q-switch oscillation using a rotary chopper, wherein a laser medium is excited by a DC or high-frequency discharge, and the discharge is a signal obtained by dividing a signal for repeatedly opening and closing a slit of the rotary chopper. The pulse modulation is performed synchronously, and the slit of the rotary chopper is opened and closed at least twice from the end of one pulse discharge to the start of the next pulse discharge, and the laser pulse oscillates in the opening and closing cycle of the slit of the rotary chopper. A method of Q-switching a CO ₂ laser, characterized in that a laser pulse group whose peak output decreases with time is obtained in a pulse discharge cycle. Wherein the laser medium is a mixed gas containing CO ₂ and N _2, CO ₂ and CO ₂ laser Q switching method of claim 1, wherein changing the composition ratio of N _2.