Discharge characteristics at high repetition rate in longitudinally excited CO₂ laser pumped by fast pulsed discharge

Abstract

In a previous study, we considered that the laser characteristics of longitudinal discharge pumped CO₂ lasers with fast pulse discharge were affected by changes in the discharge characteristics due to operations with high repetition rates. However, the discharge form has not been investigated in detail. In this study, the effects of repetition rate and gas pressure on the discharge were investigated by observing the discharge using a Pyrex discharge tube with an inner diameter of 12.6 mm and, length of 470 mm, and a CO₂/N₂/He gas mixture ratio of 1:1:2. At repetition rates of 150 Hz or lower, the discharge was uniform throughout the inner diameter of the discharge tube, whereas at repetition rates of 200 Hz or higher, the discharge contracted. The contracted discharge is thought to be caused by the residual charged particles generated by the discharge. At a high repetition rate, the discharge current density changes owing to the contraction of the discharge, which affects the optimal gas pressure and laser output energy. These findings are important for establishing the optimal operating conditions for longitudinally excited CO₂ lasers pumped by fast pulsed discharges.

1 Introduction

CO₂ lasers are mid-infrared gas lasers with wavelengths of 9.2 to 11.4 μm. They have been used in various applications since the development of the first CO₂ laser in 1964 [1]. The CO₂ laser is an important light source for the processing of nonmetallic materials, such as resins, ceramics, and biological tissues, because these materials have high absorption at mid-infrared wavelengths. There are various methods for excitation of CO₂ lasers [2,3,4]. Among these, we focused on the longitudinal discharge excitation scheme. This scheme is known as a compact and simple CO₂ laser with continuous wave (CW) oscillation by DC discharge or long-pulse oscillation with a pulse width of a few milliseconds by pulsed discharge. Recently, we developed a compact and simple CO₂ laser with a short pulse width of approximately 100 ns using a fast pulse discharge [5]. The developed laser is suitable for generating short laser pulses with a high peak intensity for processing resins and other materials with a small heat-affected zone [6]. Laser processing with such short-pulse CO₂ lasers will meet the demand for the micromachining of resins and other materials, which has been increasing in recent years.

The market demand is high-throughput processing, which requires pulsed lasers with high repetition rates. We have previously achieved high repetition rates for longitudinal discharge pumped CO₂ lasers.

As shown in Ref. [7], we investigated the dependence of the laser output and discharge voltage on repetition rates up to 700 Hz using an alumina ceramic discharge tube with an inner diameter of 16 mm and a length of 80 cm. The dependence of the laser power and gas pressure on the CO₂/N₂/He gas mixture ratios of 1:1:4 and 1:1:2 with a coaxial section length of 30 cm showed that the oscillation region shifted to higher gas pressures as the repetition rate increased. The laser beam profile measurement results with a coaxial section length of 30 cm, a CO₂/N₂/He gas mixing ratio of 1:1:4, and a gas pressure of 2.0 kPa indicated that the shape of the laser beam profile was dependent on the repetition rate. Specifically, at a repetition rate of 300 Hz or lower, the laser beam profile was doughnut-shaped. At a repetition rate of 400 Hz or higher, the center of the laser profile had a high energy. At high repetition rates, fast discharges were concentrated at the center of the discharge tube.

As shown in Ref. [8], we have achieved the generation of a short laser pulse of 35.2 mJ laser energy at a repetition rate of 1 kHz without a pre-ionization system in an alumina ceramic discharge tube with an inner diameter of 8 mm and a length of 80 cm at a CO₂/N₂/He gas mixture ratio of 1:1:5 and a gas pressure of 4.6 kPa. However, for CO₂/N₂/He gas mixing ratios of 1:1:2, 1:1:4, and 1:1:6, the laser energy decreased with increasing repetition rate at repetition rates of 400 Hz or higher. Furthermore, a discharge tube with an inner diameter of 16 mm and length of 80 cm could not be operated at a repetition rate of 1 kHz. This is because the charge remained in the center of the discharge tube, causing the discharge to concentrate in the center, increasing the discharge current density, generating heat, and suppressing the relaxation of the lower laser level.

Thus, we consider that the high-repetition-rate operation changed the laser characteristics due to changes in the discharge characteristics, such as fast discharges concentrated at the center of the discharge tube. However, the discharge form was not examined in detail. In this study, by observing the shape of the discharge, we investigated the form of the discharge required for a high repetition rate of longitudinal discharge pumped CO₂ lasers, which output short pulses in a fast pulse discharge.

2 Experimental setup

Figure 1 shows a schematic of the experimental setup. The discharge tube was a Pyrex tube with an inner diameter of 12.6 mm, an outer diameter of 15 mm, and a length of 470 mm. Two electrodes and two AR-coated ZnSe windows were attached to the ends of the tube. The center section of the tube was covered with a 300 mm-wide grounded metal mesh to avoid the discharge being affected by electric or magnetic fields around the discharge tube. The optical cavity consists of a ZnSe output coupler with a reflectivity of 85% and a highly reflective mirror with a radius of curvature of 5 m. The cavity length was 700 mm. A 1:1:2 mixture of CO₂/N₂/He gas was allowed to flow through the discharge tube at a rate of 0.1 L/min. A mass-flow controller and mass-flow meter were used to adjust and measure the gas flow rate, respectively. The gas pressure was adjusted using a valve on the exhaust side and was estimated from the gas flow rate, pressure of the input, and output of the discharge tube. The power supply produced a high voltage of approximately -35 kV with a rise time of 250 ns. The high voltage charged a capacitance of 700 pF. When the voltage reached the breakdown threshold, a rapid main discharge occurred in the discharge tube. The laser output energy was measured using an energy detector (COHERENT, J-50 MB-LE) and averaged over 100 pulses, calculated using an energy meter (COHERENT LabMax-TOP). The laser pulse, discharge voltage, and discharge current waveforms were measured using a photon drag detector (Hamamatsu Photonics, B749), high-voltage probe (Tektronix, P6015A), and current monitor (PEARSON MODEL110A), respectively. To observe the discharge distribution, a camera (Spiricon SP620U, wavelength band 190–1100 nm) and lens (μTRON HS3514J F:1.4) were used to capture the plasma emission near the center of the discharge tube through a metal mesh.

Fig. 1

Schematic of experimental setup. P, W, OC, M, C, and R represent pressure gauge, AR-coated ZnSe window, output coupler, high-reflectivity mirror, 700 pF capacitor, and 2 MΩ resistor, respectively

3 Results and discussion

Figure 2a shows the dependence of the laser energy on the gas pressure in the discharge tube at repetition rates of 10–400 Hz. The laser energy increased with an increase in gas pressure, peaked when the gas pressure was approximately 4–7 kPa, and then decreased with further increases in gas pressure. At the repetition rates of 150 Hz and less, the gas pressures at which the maximum value of laser energy was obtained at each repetition rate were nearly the same, and the gas pressure range over which the laser output was obtained was also nearly the same. At repetition rates of 200 Hz and more, the gas pressure at which the maximum laser energy was obtained increased with increasing repetition rate, and the gas pressure range over which laser output was obtained shifted to the high-pressure side. Figure 2b shows the repetition-rate dependence of the gas pressure at which the maximum laser output energy (optimal gas pressure) was obtained at each repetition rate. The optimal gas pressure was almost constant at 4.2 kPa at repetition rates of 150 Hz and lower, and increased with increasing repetition rates of 200 Hz or higher. The optimal gas pressure at a repetition rate of 400 Hz was 7.1 kPa. The discharge characteristics differed between repetition rates below 150 Hz and above 200 Hz. The trend of increasing optimal gas pressure with increasing repetition rate was similar to that observed in Ref. [7].

Fig. 2

Dependence of laser energy on gas pressure at gas pressures of 2.6 to 9.0 kPa (a) and gas pressures at maximum laser energy at repetition rates of 10 to 400 Hz (b)

In general, in CO₂ laser discharge pumping, the discharge energy is used not only for excitations that directly affect the laser output, such as the excitation of CO₂ to the upper laser level (001) and the vibrational excitation of N₂, but also for excitations that do not directly contribute to the laser output, such as the excitation of CO₂ to other levels, the rotational excitation of N₂, and the electronic excitation or ionization of CO₂ and N₂. At high discharge current densities, a higher percentage of the energy of the discharge is lost owing to the loss from the excitation of the electronic levels of CO₂ and N₂ and the loss from the ionization of CO₂, N₂, and He. At higher laser medium gas densities, the rate of discharge energy loss increases because of elastic collisions with CO₂, N₂, and He, the rotational excitation of N₂, and the excitation of CO₂ to levels other than the upper laser level (001) [2]. As the discharge current density increases at low gas pressures and the laser medium gas density increases at high gas pressures, the gas pressure dependence of the laser output has a mountain-like shape with a peak at a certain gas pressure. The measured gas pressure dependence of the laser energy, shown in Fig. 2a, was also mountain-shaped, with a peak at the optimum gas pressure. The optimal gas pressure, which is the gas pressure at the top of the mountain shape, increased with an increase in the repetition rate at repetition rates of 200 Hz or higher, as shown in Fig. 2b. In practical applications, we aim to operate the system near the optimum gas pressure; hence, it would be beneficial to determine the reason for the increase in the optimum gas pressure. This behavior of the optimal gas pressure may be related to the change in the discharge current density, which is discussed in detail later.

Figure 3 shows the distribution of the plasma emissions. The emission intensity tended to be higher at lower gas pressures and lower at higher gas pressures. At a repetition rate of 150 Hz or lower, either the discharge spread throughout the discharge tube or along the inner wall of the tube was observed. At repetition rates of 200, 300, and 400 Hz, the discharge contracted at gas pressures below 5.0, 7.8, and 8.6 kPa, respectively. At repetition rates of 200 Hz or higher, the discharge contracted, and the higher the repetition rate, the more the discharge contracted at higher gas pressures.

Fig. 3

Plasma emission distribution at repetition rates of 10 to 400 Hz and gas pressure of 2.6 to 8.6 kPa. The white dashed lines represent the inner diameter of the discharge tube

Figure 4 shows the cross-sectional intensity distribution obtained from the plasma emission intensity distribution at gas pressures of 3.8 and 7.8 kPa (Fig. 3). At both gas pressures, there was little change in the plasma emission intensity at repetition rates of 150 Hz or lower. At a gas pressure of 3.8 kPa, the discharge contracted at repetition rates of 200 Hz or higher, and at a gas pressure of 7.8 kPa, the discharge contracted at repetition rates of 300 Hz or higher. When the discharge did not contract, no significant difference in the emission intensity was observed; however, when the discharge did contract, the emission intensity became stronger as the repetition rate increased.

Fig. 4

Plasma emission intensity distribution at gas pressures of 3.8 kPa (a) and 7.8 kPa (b). Position indicates the distance in the radial direction of the discharge tube. Position = 0 represents the center of the discharge tube, and the minus side represents the top of the distribution images. The intensity is normalized by the peak value at a gas pressure of 3.8 kPa and a repetition rate of 400 Hz

Figure 5 shows the discharge diameters under contracted discharge conditions. At the repetition rates of 300 and 400 Hz, the discharge diameters were almost the same from 2.6 to 5.0 kPa, but above 5.0 kPa, the discharge diameter at 300 Hz was larger. When the discharge contracted, the discharge diameter decreased with an increasing repetition rate. Observation of the discharge revealed that the repetition rate dependence of the emission intensity changed with and without the contraction of the discharge. When the discharge did not contract, the emission intensity was almost constant for the same gas pressure. However, when the discharge did contract, the emission intensity increased with increasing repetition rate, and the diameter of the discharge decreased, even when the gas pressure was the same. The higher the repetition rate, the more the discharge contracted at higher gas pressures.

Fig. 5

Dependence of discharge diameter on gas pressure at repetition rates of 200 to 400 Hz. The discharge diameter is the full width at half-maximum obtained by Gaussian fitting of the plasma emission distribution as shown in Fig. 4. Only those discharges with contracted discharges are shown

Figure 6 shows the laser pulse, discharge voltage, and current waveforms at a gas pressure of 3.8 kPa and a repetition rate of 50 Hz, which are typical waveforms obtained in this experiment. The breakdown voltage of the discharge was −33.1 kV. The discharge current rose with breakdown, and the maximum current was 14.5 A, the pulse width was 1.0 µs at half maximum width. Further, 1.5 µs after the breakdown, a high-spike laser pulse was generated. This laser pulse had a high spike laser pulse with a pulse width of 180 ns and a tail length of 65 μs. The pulse tail length is defined as the length from the end of the spike pulse to the end of the pulse tail. The overall laser pulse was 21.4 mJ.

Fig. 6

Laser pulse and discharge voltage waveforms at gas pressure of 3.8 kPa and repetition rate of 50 Hz. Blue, green, and orange lines represent laser pulse, discharge current, and discharge voltage waveforms, respectively. Spikes near time zero in the laser pulse waveform and discharge current waveform were due to switching noise. a Magnified time-scale view of spike pulse. b Overall waveform

Figure 7 shows the relationship between the repetition rate and the peak current (maximum value of the discharge current). The peak current was significantly affected by the gas pressure. The peak current increased with decreasing gas pressure. At gas pressures below 5.0 kPa and repetition rates of 200 Hz or higher, the peak current increased with the repetition rate. At higher gas pressures, the effect of the repetition rate on the peak current was less significant. The peak current at the optimal gas pressure was nearly constant for all repetition rates.

Fig. 7

Dependence of maximum discharge current on repetition rate at gas pressures of 2.6 to 7.8 kPa. Red circles indicate peak current values at the optimal gas pressure shown in Fig. 2b

Figures 3 and 7 were compared to examine the relationship between the contraction of the discharge and the peak current. At a gas pressure of 2.6 kPa (Fig. 7), the peak current did not change significantly at a repetition rate of 150 Hz or lower; however, at a repetition rate of 200 Hz or higher, the peak current increased as the repetition rate increased. At a gas pressure of 2.6 kPa (Fig. 3), no contraction of the discharge occurred at a repetition rate of 150 Hz or lower. However, at repetition rates of 200 Hz or higher, a contraction of the discharge occurred. Similarly, at gas pressures of 3.8 kPa and 5.0 kPa, a contraction of discharge and an increase in peak current were observed at repetition rates above 200 Hz. Increasing the repetition rate at the same gas pressure indicated that the peak current changed slightly when the discharge was not contracting but the peak current increased when the discharge contracted. In contrast, at a gas pressure of 3.8 kPa and a repetition rate of 200 Hz, where the discharge contracted, the peak current was approximately 15 A. At a gas pressure of 2.6 kPa and a repetition rate of 150 Hz, where the discharge did not contract, the peak current was approximately 45 A. It can also be seen that a large current does not necessarily cause contraction of the discharge.

Next, we investigate the reason for the discharge contraction. As previously mentioned, when the discharge contracts, the current increases; however, this does not imply that the discharge contracts when the current is high. If the discharge contraction is due to the pinch effect, the same amount of current would be expected to produce the same degree of contraction. Therefore, it is thought that the cause of discharge contraction is not the pinch effect but is related to the speed of movement of the charged particles produced by the discharge and the internal structure of the discharge tube, as shown below. The charged particles produced by the discharge are relaxed by collisions with the inner wall of the discharge tube. Relaxation occurs quickly near the inner wall; however, in the central region of the discharge tube, farther from the wall, relaxation time is required. Therefore, at high repetition rates, some of the charged particles remain near the center of the discharge tube until the next pulse. While the duration of the discharge is on the order of microseconds, considering that the speed of ion migration is approximately 10^{^4} cm/s, the time it takes to move through an inner diameter of 12.6 mm is on the order of milliseconds, which is consistent with a contraction of the discharge occurring at a repetition rate of 200 Hz or higher. The discharge tends to concentrate on the charged particles remaining near the center of the discharge tube, resulting in the contraction of the discharge in the next pulse. This discharge contraction causes the current to concentrate at the center of the discharge tube, increasing the density of the charged particles produced. In the following pulse, as the number of charged particles remaining in the center increases, the discharge current increases further, and the discharge diameter decreases. By repeating these processes, the discharge diameter gradually decreases and stabilizes when a balance between the pulse interval time and relaxation rate is achieved. Whether the discharge contracts also depends on the interval time between pulses and the relaxation rate. The conditions for the contraction of the discharge also depend on the diameter of the discharge tube and the gas pressure, which affects the relaxation rate. This may be the reason for the drastic increase in emission intensity at the center, decrease in discharge diameter, and increase in current observed in Figs. 3, 4, and 6, when the discharge contracted. It also explains why the higher the repetition rate, the more the discharge contracts at higher gas pressures, as shown in Fig. 3.

Finally, as shown in Fig. 2b, the reasons for the shift of the optimal gas pressure to the high-pressure side at repetition rates of 200 Hz or higher are considered.

Figure 8 shows the discharge current density calculated from the discharge diameter and peak current. The values shown in Fig. 5 were used to determine the discharge diameter when the discharge contracted. The discharge diameter when the discharge did not contract was set to 12.6 mm, which was the inner diameter of the discharge tube. At each repetition rate, the lower the gas pressure, the higher the current density. As the gas pressure increased, the current density decreased rapidly, followed by a gradual slope. The optimal gas pressure was observed at the point where the slope changed slowly. At repetition rates of 150 Hz or lower, the dependence of the current density on the pressure was almost the same regardless of the repetition rate. At repetition rates of 200 Hz or higher, the higher the repetition rate, the higher the current density, and the higher the repetition rate, the higher the current density up to higher gas pressures (the gas pressure-current density curve shifts to the upper right).

Fig. 8

Dependence of discharge current density on pressure at repetition rates of 10 to 400 Hz. Red circles indicate optimal gas pressure at each repetition rate

However, the laser output energy showed a mountain shape with a peak at the optimal gas pressure, as shown in Fig. 2a. At pressures lower than the optimal gas pressure, the pulse energy increased with increasing gas pressure, whereas the current density decreased, as shown in Fig. 8, indicating a negative correlation between pulse energy and current density. This indicates that the discharge energy was used for electronic excitation and ionization of the laser medium gas.

This indicates that the optimal gas pressure moved to the higher-pressure side with a higher repetition rate because the discharge current density increased to a higher gas pressure at a repetition rate of 200 Hz or higher, and the pulse energy decreased with a higher current density.

At repetition rates of 200 Hz or higher, the optimal gas pressure is considered to have moved to the high-pressure side because the current density increases owing to discharge contraction, and the energy loss due to electronic excitation and ionization of the laser medium gas increases.

4 Conclusions

In a longitudinally excited CO₂ laser pumped by a fast pulsed discharge with a discharge tube having an inner diameter of 12.6 mm and length of 470 mm, we investigated the effect of the repetition rate on the laser energy and discharge characteristics.

At a repetition rate of 150 Hz or lower, discharge spread throughout the discharge tube or along the inner wall of the tube was observed. At repetition rates of 200 Hz or higher, the discharge contracted, and the higher the repetition rate, the more the discharge contracted at higher gas pressures. Discharge contraction is thought to be caused by the residual charged particles generated by the discharge.

At a gas pressure of 5 kPa or lower, the peak current value was almost constant at the same gas pressure in the range of repetition rates where the discharge did not contract. In the repetition rate range where the discharge contracted, the peak current value increased with increasing repetition rate, even at the same gas pressure. However, at gas pressures above 5 kPa, the peak current value was almost constant and independent of the repetition rate. When the discharge contracts, the increase in the peak current with increasing repetition rate may be due to the residual charged particles generated by the discharge.

The optimal gas pressure at which the maximum laser energy could be obtained at each repetition rate was constant at approximately 4.5 kPa under conditions where the discharge did not contract but increased with increasing repetition frequency under conditions where the discharge contracted. This may be due to an increase in the current density caused by discharge contraction, which increases the energy loss due to electronic excitation and ionization of the laser medium gas.

At a high repetition rate, the discharge current density changes because of the contraction of the discharge, which affects the optimum gas pressure and laser output energy.

The discharge contraction can be controlled by optimizing the high-voltage power supply voltage to match the repetition rate, increasing the relaxation rate of the charged particles by reducing the diameter of the discharge tube, and reducing the effect of the residual charged particles by performing sufficient pre-ionization over the entire discharge tube diameter before the main discharge.

These findings serve as a reference for establishing the best operational parameters for a longitudinally excited CO₂ laser pumped by a fast pulsed discharge. Based on this knowledge, in future research we aim to realize 10 kHz He-free operation, which acts as a uniformizer of the discharge.