JPS6314512B2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a silent discharge pulse laser,
This is intended to eliminate the decrease in laser output that occurs when operating for a long time.

Conventionally, a typical example of this type of laser has been the one shown in FIG. In the figure, 1 is a metal electrode,
A dielectric electrode 2 is made up of a metal part 21 and a dielectric 22 covering it. 4 is a discharge space, 5 is a transformer, 6 is a high frequency power source, 7 is a total reflection mirror, 8 is a partial reflection mirror, 9 is an arrow indicating the flow of laser gas, and 10 is an arrow indicating the laser output. The voltage waveform applied between the electrodes 1 and 2 is as shown in FIG. 2a due to the circuit configuration of the high frequency power source 6. The laser gas is a _CO2 - _N2 -He mixed gas with a molar fraction of 5-60-35 and a pressure of about 200 Torr.

Next, the operation will be explained. An intermittent high frequency high voltage is applied between the metal electrode 1 and the dielectric electrode 2 by a high frequency power source 6 and a transformer 5 as shown in FIG. Electric discharge is occurring. The applied voltage is a sine wave voltage with a period T ₀ (10 μs), and the peak value V ₁ during a period T ₁ (0.5 ms).
(8 kV) and is not applied during period T ₂ (0.5 ms).

Since the excitation molecule lifetime of the laser is about 100 μs,
The laser output is in the form of a pulse as shown in FIG. 2b. In other words, it is a pulse whose period is (T ₁ + T ₂ ), and the pulse width T _p is smaller than T ₁ and is 0.3 ms.
That's about it. The reason why the rise of the pulse is more gradual than the rise of the applied voltage during period T ₁ is because a silent discharge occurs during period T ₁ , which increases the density of excited molecules to the extent that even if the laser gas is excited, it will oscillate. This is because it takes time to do so.

When this pulsed laser is operated continuously, as shown in FIG. 2c, the average value of the laser output significantly decreases after several hours.

However, in the case of continuous oscillation of silent discharge, the decrease in laser output during long-time operation as described above is small.

As a result of research into the causes of this difference,
It was found that there is a difference in the discharge mode between pulsed oscillation and continuous oscillation, and this causes a change in the mole fraction of the laser gas, which causes a decrease in laser output.

FIG. 3 is a schematic diagram of the discharge space 4 observed from the optical axis direction, in which bright line-like discharges 42 are scattered within a uniform glow-like discharge 41. On the other hand, in the case of continuous oscillation, this bright line-like (silk-like) discharge hardly appears.

Analysis of changes in the composition of the laser gas during long-term pulse discharge using gas chromatography revealed that CO ₂ decreased rapidly over time.
It became clear that CO and O ₂ were increasing.
In other words, CO ₂ →
It is presumed that the chemical change of CO+1/2O ₂ progressed at an accelerated pace, which caused the output reduction.

This invention was made based on the above knowledge, and conventionally, the period when no voltage is applied is
The decomposition reaction of CO ₂ is suppressed by applying a voltage with a frequency ₂ lower than the frequency ₀ (1/T ₀ ) of the voltage applied during the main discharge period T ₂ during T ₂ to connect a silent discharge. Therefore, the reduction in laser output is reduced.

FIG. 4 is a diagram for explaining one embodiment of the present invention, and FIG. 4a is a diagram of applied voltage waveforms, in which a voltage of frequency ₂ (5 kHz) is also applied during period _T2 . Same figure b
is an oscillation output waveform diagram, where the solid line shows the output characteristics of this embodiment and the broken line shows the characteristics of the conventional device. Also, the same figure c
is a characteristic diagram showing the change over time in the average oscillation output of this embodiment, where the solid line shows the characteristic of this embodiment and the broken line shows the characteristic of the conventional device.

In this embodiment, as shown in FIG. 4a, the voltage is input for a period _T1 which is sufficiently long for the length of one cycle of the applied high-frequency voltage, _1/0 . Further, during a period T ₂ which is sufficiently long with respect to the length of _1/0 , an alternating current voltage with a frequency of ₂ smaller than ₀ is applied successively to the period T ₁ . In a silent discharge type laser, a discharge power approximately proportional to the frequency of the applied alternating current voltage is input, so the discharge power to be injected can be adjusted by changing the frequency of the applied voltage.

In this way, during the period T ₁ , the discharge power W 1 that is 2 to 5 times the oscillation threshold discharge power W ₀ is input, and during the period T ₂ , the discharge power W ₁ is approximately 1/20 of the discharge power W ₁ . The discharge power _W2 is input.

In this way, it was confirmed by observation that the discharge was a uniform glow-like discharge during both period T ₁ and period T ₂ , but the discharge power during period T ₂
Since W ₂ is much smaller than W ₀ , no oscillation occurs during period T ₂ .

When such driving was performed, the change in laser output over time became extremely small, as shown in FIG. 4c. This is because _the main discharge ( _period
T ₁ ), which is thought to have contributed to the homogenization of the discharge.

Additionally, as an additional effect, both an improvement in the pulse output waveform and an increase in the output were achieved (FIG. 4b). This is because the laser gas is pre-excited in period _T2 prior to period _T1 , and the rise of the output becomes faster and the pulse width also increases.
Furthermore, there is also the effect that a pulse output with a high peak can be obtained, which could not be obtained without increasing V ₁ or ₀ in the conventional example.

In addition, in the above embodiment, the frequency ₂ of the period T ₂ is
I chose 1/20 of ₀ , but I chose a higher frequency,
Even when some oscillation occurs during period _T2 , the same effect as the present invention is achieved in that the output substantially becomes pulse-like.

As described above, the present invention is configured to excite a laser by generating a silent discharge by applying a high frequency voltage in pulses between two electrodes, at least one of which is a dielectric electrode. It is characterized by comprising means for applying an alternating current voltage lower in frequency than the frequency of the high-frequency voltage during the period of applying the high-frequency voltage, so as to maintain silent discharge following the laser excitation period, A pulsed laser having excellent long-term laser output stability, high output, and a pulse waveform with a fast rise speed can be obtained.

[Brief explanation of the drawing]

Figure 1 is a principle diagram showing the structure of a conventional silent discharge pulsed laser, Figure 2a is a diagram of its applied voltage waveform,
Figure b is a diagram of its oscillation output waveform, Figure c is a graph of its oscillation output over time, Figure 3 is a schematic diagram showing the state of discharge in the discharge space of this conventional device, and Figure 4 a.
is an applied voltage waveform diagram of an embodiment of the present invention, and FIG.
is a diagram of its oscillation output waveform, and c of the same figure is a diagram of its oscillation output over time. In the figure, 1 is a metal electrode, 2 is a dielectric electrode,
4 is a discharge space, and 5 is a high frequency power source. In addition, in the figures, the same reference numerals indicate the same or corresponding parts.

Claims (1)

[Claims] 1. In an apparatus configured to apply a high frequency voltage in pulses between two electrodes, at least one of which is a dielectric electrode, to generate a silent discharge and excite a laser, the period during which the high frequency voltage is applied between the two electrodes is A silent discharge type pulsed laser characterized in that the silent discharge type pulse laser is provided with means for applying an alternating current voltage lower in frequency than the frequency of the high frequency voltage in between, so as to maintain silent discharge following the laser excitation period.