Gain measurements of longitudinally excited CO₂ laser using low-pressure gas without helium

Abstract

The gain characteristics of a CO₂ laser pumped by a longitudinal pulsed discharge were investigated to develop a compact He-free CO₂ laser operating at a high repetition rate. The longitudinal discharge tube had an inner diameter of 0.8 cm and a length of 45 cm. High-voltage pulses with a discharge starting voltage of 13.4–30.2 kV at a rise time of about 321 ns were applied to the discharge tube. CO₂/N₂ mixture gas with mixing ratios of 1:0, 1:1, 1:2 and 1:3 was used. The small signal gain coefficient was measured by inputting a seed CO₂ laser pulse into the longitudinal discharge tube. The small signal gain coefficient depended on the delay time between the start of the discharge in the longitudinal discharge tube and the input of a seed CO₂ laser pulse, the gas mixing ratio, the gas pressure, the repetition rate and the input energy to the longitudinal discharge tube. In this work, in He-free gas, the maximum small signal gain coefficient at a repetition rate of 300 Hz was 2.11%/cm with a delay time of 30.4 μs, a CO₂/N₂ gas mixing ratio of 1:2, a gas pressure of 2.6 kPa and an input energy of 191 mJ.

1 Introduction

CO₂ lasers, quantum cascade lasers (QCLs) and lead-salt lasers are the only lasers that oscillate at wavelengths of 9–11 μm, which is absorbed well by resin, glass and water [1,2,3]. A CO₂ laser can output a high-energy laser pulse and continuous wave (CW) light with a high average power at wavelengths of 9.2–11.4 μm (mainly 9.6 μm and 10.6 μm). A CO₂ laser can produce a long pulse with a pulse width of 1 μs to 6 ms, or a short pulse with a pulse width of 10 ns to 1 μs [4,5,6,7,8,9,10,11,12].

The medium gas in discharge pumped CO₂ lasers is usually a mixture of CO₂, N₂ and He. CO₂ molecules are excited directly from the ground state to the laser upper level (001) by electronic excitation [13, 14]. N₂ molecules are excited by electron excitation. Excited N₂ molecules excite CO₂ molecules by the exchange of vibrational energy because the excited state (v = 1) of N₂ is 2331 cm⁻¹ and the laser upper level (001) of CO₂ is 2349 cm⁻¹ [13, 14]. He gas plays a role in forming a uniform discharge and in enabling CO₂ molecules in lower energy levels to relax to the ground state [13, 14]. The transition from the laser upper level (001) to the laser lower level (100) causes CO₂ laser oscillation in the 10.6 μm band, and the transition from the laser upper level (001) to the laser lower level (020) causes CO₂ laser oscillation in the 9.6 μm band. For example, small signal gain coefficients of CW CO₂ lasers with a direct current (DC) discharge and a longitudinal excitation scheme were reported to be about 1.1%/cm with a discharge volume of 754 cm³, a CO₂/N₂/He gas mixing ratio of 1:3:8, a gas pressure of 10.4 kPa and a discharge current of 90 mA, and 0.64%/cm with a discharge length of 210 cm, a CO₂/N₂/He gas mixing ratio of 1:5:30, a gas pressure of 6.5–6.9 kPa and an input power of 70 kW [15, 16]. Small signal gain coefficients of short-pulse CO₂ lasers with a pulsed discharge and a transverse excitation scheme were reported to be 3.0%/cm with a discharge volume of 360 cm³, a CO₂/N₂/He gas mixing ratio of 1:1:3, a gas pressure of 66.7 kPa and an input energy of 36 J, and 2.9%/cm with an electrode length of 50 cm, a discharge gap of 5 cm, a CO₂/N₂/He gas mixing ratio of 3:1:16, a gas pressure of 97.3 kPa and an input power of 180 W, and about 2.5%/cm with a discharge volume of 1200 cm³, a CO₂/N₂/He gas mixing ratio of 2:1:12 and an input energy of 120 J [17,18,19].

He gas, which is important for CO₂ lasers, is precious, and its availability depends on global supply conditions. Furthermore, the configuration of CO₂ laser devices can be simplified by avoiding the use of He gas. Avoiding the use of He gas is also advantageous for the development of gas-sealed-off systems. The development of a He-free CO₂ laser is thus important. He-free TE CO₂ lasers that operate at repetition rates up to 100 Hz have been reported [20,21,22,23,24]. In the paper reporting 100 Hz operation, a pair of cylindrical electrodes had a length of 80 mm, a width of 15 mm and a discharge gap of 3 mm [20]. The discharge volume was 3.6 cm³ and pre-ionization system was used. The optical cavity was consisted of an ZnSe output mirror with a reflectivity of 95% and a high-reflection mirror. At a repetition rate of 100 Hz, the laser energy was 35.1 mJ and the electro-optic efficiency was 6.2% with a CO₂/N₂ gas mixing ratio of 1:1 and a gas pressure of 101 kPa. The limitation of the repetition rate may be due to the formation of a non-uniform discharge.

One method of easily forming a uniform discharge at a high repetition rate is to employ longitudinal discharge excitation. Previously, we have reported a longitudinally excited CO₂ laser without pre-ionization, which realized laser pulse waveform control, laser beam shape control and high repetition rate operation [4,5,6,7,8]. In longitudinally excited CO₂ lasers, a discharge tube consists of a dielectric pipe with an inner diameter of 0.8–2.0 cm and a length of 30–80 cm and electrodes attached to both ends of the pipe. The long discharge length gives a high discharge starting voltage (> 20 kV) at a low gas pressure (< 10 kPa). A long, small-diameter discharge at a low gas pressure is formed uniformly by a minute spark discharge and optical ionization, even if residual charges remain after discharge. A longitudinally excited CO₂ laser does not necessarily require pre-ionization or fast gas flow. In fact, in our previous work, our longitudinal excited CO₂ laser without pre-ionization produced a short laser pulse at a repetition rate of 1 kHz [4]. In the discharge tube, an inner diameter was 0.8 cm, a length was 80 cm and a discharge volume was 40.2 cm³. The optical cavity was consisted of a ZnSe output mirror with a reflectivity of 85% and a high-reflection mirror. In the medium gas, a CO₂/N₂/He gas mixing ratio was 1:1:5 and a gas pressure was 4.6 kPa. The input energy was 738 mJ. The laser pulse was a spike pulse width of 200 ns, a tail length of 150 μs and a laser energy of 35.2 mJ at a repetition rate of 1 kHz.

In the present work, gain measurements of a longitudinally excited CO₂ laser using He-free gas were performed to develop a He-free short-pulse CO₂ laser. We investigated the gain characteristics, which depended on the delay time from the start of discharge, the gas mixing ratio, the gas pressure, the repetition rate and the input energy to the longitudinal discharge tube.

2 Experimental setup

The small signal gain was measured with the configuration shown in Fig. 1 [25, 26]. To measure the small signal gain of a longitudinal discharge tube, a short-pulse CO₂ laser serving as a seed pulse source was used as an oscillator. The oscillator produced a tail-free short laser pulse with a pulse width of 112 ns, as shown in Fig. 2, a wavelength of 10.6 μm, a laser energy of 1.48 mJ and a beam diameter of 1.0 cm. The seed laser pulse from the oscillator was input to a longitudinal discharge tube. The longitudinal discharge tube consisted of an alumina ceramic pipe with an inner diameter of 0.8 cm, an outer diameter of 1.2 cm and a length of 45 cm, two metallic electrodes and two anti-reflection coated ZnSe windows. The longitudinal discharge tube did not employ pre-ionization, a fast gas flow or cooling devices. The medium gas was a 1:1, 1:2 or 1:3 mixture gas of CO₂/N₂ or pure CO₂ gas with the purity of over 99.95%. The medium gas flowed through the longitudinal discharge tube at a flow rate of about 0.1 L/min. A fast high-voltage pulse with a rise time of about 321 ns was applied to the longitudinal discharge tube. The discharge starting voltage was 13.4–30.2 kV and depended on the input energy to the longitudinal discharge tube. The repetition rate and input energy were controlled by a pulse power supply (Seidensha Electronics) with a pulse generator (Tektronix, AFG31022).

Fig. 1

Schematic diagram of gain measurement system. a Schematic diagram of gain measurement system with oscillator and longitudinal discharge tube. b Detailed diagram of longitudinal discharge tube

Fig. 2

Seed laser pulse generated by oscillator. Laser intensity was normalized to maximum value of laser pulse

When the longitudinal discharge tube discharged and the seed laser pulse was input to the longitudinal discharge tube, the seed laser pulse was amplified. The small signal gain coefficients were calculated from the amplification factor. The small signal gain coefficients α [%/cm] were obtained by Eq. (1) by the laser energy of seed laser pulse E_in [mJ], the amplified laser energy by the longitudinal discharge E_out [mJ] and the discharge length L [cm].

$$\begin{array}{c}\alpha =\frac{{\text{ln}}\left({{\text{E}}}_{{\text{out}}}/{{\text{E}}}_{{\text{in}}}\right)}{{\text{L}}}\times 100 \left[\mathrm{\%}/{\text{cm}}\right]\end{array}$$

(1)

In the small signal gain measurement, the delay time between the start of the discharge in the longitudinal discharge tube and the input of the seed laser pulse was controlled. The dependencies of the small signal gain of the longitudinal discharge tube on the delay time, medium gas, repetition rate and input energy were investigated.

The laser energy was measured with an oscilloscope (Tektronix, MSO44) and an energy detector (Gentec, QE50LP-S-MB-D0). The laser pulse waveform was measured with the oscilloscope and a photon drag detector (Hamamatsu Photonics, B749). The discharge voltage was measured with the oscilloscope and a high-voltage probe (Tektronix, P6015A).

3 Results

Figure 3 shows the laser pulse waveform and the discharge voltage waveform of the longitudinal discharge tube with a delay time of 30.6 μs, a CO₂/N₂ gas mixing ratio of 1:2, a gas pressure of 1.4 kPa, a repetition rate of 1 Hz and an input energy to the longitudinal discharge tube of 191 mJ. The discharge voltage reached 14.6 kV at a rise time of 368 ns. The main discharge occurred with a voltage drop of 14.6–4.08 kV. The discharge formation time was 80.4 μs in the main discharge. At a delay time of 30.6 μs, that is 30.6 μs after the start of the discharge in the longitudinal discharge tube, the seed laser pulse was injected into the longitudinal discharge tube. The seed laser pulse was amplified by the discharge in the longitudinal discharge tube. The amplified laser pulse was a tail-free short pulse with a laser energy of 4.62 mJ and a spike pulse width of 112 ns. The amplification factor was 3.11 times and the small signal gain coefficient was 2.52%/cm.

Fig. 3

Laser pulse and discharge voltage waveforms with delay time of 30.6 μs, CO₂/N₂ gas mixing ratio of 1:2, gas pressure of 1.4 kPa, repetition rate of 1 Hz and input energy of 191 mJ. Red and blue lines represent laser pulse waveform and discharge voltage waveform of longitudinal discharge tube, respectively. Laser intensity was normalized to maximum value of seed laser pulse of Fig. 2. a Magnified time scale of laser pulse. b Magnified time scale of rising part of discharge voltage

Figure 4 shows the dependence of the small signal gain coefficient on the delay time with a CO₂/N₂ gas mixing ratio of 1:2, a gas pressure of 1.4 kPa, a repetition rate of 1 Hz and an input energy of 191 mJ. The small signal gain coefficient depended on the delay time. In Fig. 4, the maximum small signal gain coefficient was 2.52%/cm at a delay time of 30.6 μs. Figure 5 shows the spatial distribution of the small signal gain coefficient in the discharge cross section with the same conditions as in Fig. 4 and a delay time of 30.6 μs. The spatial distribution was measured by using a moving slit with a width of 0.1 cm. The distance between the output window of the longitudinal discharge tube and the energy detector was 30 cm. The small signal gain coefficient at the center of the discharge cross section was high and was 2.55%/cm. The small signal gain coefficient was more than 90% of the maximum value at positions of ± 0.2 cm. The inner diameter of the longitudinal discharge tube was 0.8 cm, and the positions of ± 0.4 cm were the wall of the longitudinal discharge tube. The small signal gain coefficient at the positions of ± 0.4 cm was the minimum value of about 1.63%/cm.

Fig. 4

Dependence of small signal gain coefficient on delay time at CO₂/N₂ gas mixing ratio of 1:2, gas pressure of 1.4 kPa, repetition rate of 1 Hz and input energy of 191 mJ. Red circles represent small signal gain coefficient versus delay time, and blue line represents discharge voltage waveform of longitudinal discharge tube

Fig. 5

Spatial distribution of small signal gain coefficient in discharge cross section with delay time of 30.6 μs, CO₂/N₂ gas mixing ratio of 1:2, gas pressure of 1.4 kPa, repetition rate of 1 Hz and input energy of 191 mJ. Position 0 cm represents center of discharge cross section. Positions ± 0.4 cm represent tube wall

Figure 6 shows the dependence of the small signal gain coefficient on the gas pressure with a CO₂/N₂ gas mixing ratio of 1:2 and an input energy of 191 mJ. The discharge starting voltage did not depend on the gas pressure or the repetition rate and was almost constant at 15.1 kV. The small signal gain coefficient depended on the gas pressure. The maximum small signal gain coefficient was 2.52%/cm at a gas pressure of 1.4 kPa and a repetition rate of 1 Hz. The gas pressure at which the maximum small signal gain coefficient was obtained increased with the increase of the repetition rate. The maximum small signal gain coefficient was 2.11%/cm at a gas pressure of 2.6 kPa and a repetition rate of 300 Hz.

Fig. 6

Dependence of small signal gain coefficient on gas pressure with CO₂/N₂ gas mixing ratio of 1:2 and input energy of 191 mJ. Filled circle and unfilled circle symbols represent repetition rates of 1 Hz and 300 Hz, respectively. Delay times were optimum

Figure 7 shows the dependence of the small signal gain coefficient on the repetition rate and the gas mixing ratio at an input energy of 385 mJ. The discharge starting voltage did not depend on the repetition rate and was almost constant at 22.2 kV. In He-free gases with CO₂/N₂ gas mixing ratios of 1:1, 1:2 and 1:3, the small signal gain coefficient was about 1.90%/cm or more at a repetition rate of 200 Hz or less. At repetition rates from 200 to 500 Hz, the small signal gain coefficient decreased with the increase in a repetition rate. The small signal gain coefficient was 0.52%/cm or less at a repetition rate of 400 Hz and 0.29%/cm or less at a repetition rate of 500 Hz. On the other hand, in a CO₂/N₂/He mixture with a mixing ratio of 1:1:2, the small signal gain coefficient was 1.74%/cm at a repetition rate of 500 Hz. Thus, in the He-free gas, nonuniformity of the discharge or heat generation that caused by the absence of helium gas may have resulted in lower small gain coefficients at repetition rates of 400 Hz and 500 Hz. Although one of the features of a longitudinally excited CO₂ laser is its simplicity, it may be possible to obtain a sufficiently high small signal gain coefficient even at a high repetition rate of 400 Hz or more by the use of pre-ionization, a fast gas flow or a cooling system in the longitudinal discharge tube.

Fig. 7

Dependence of small signal gain coefficient on repetition rate and gas mixing ratio at input energy of 385 mJ. Black, orange, red, and blue symbols represent CO₂/N₂ gas mixing ratios of 1:0, 1:1, 1:2 and 1:3, respectively

Figure 8 shows the dependence of the small signal gain coefficient on the repetition rate and the input energy with a CO₂/N₂ gas mixing ratio of 1:2. The discharge starting voltage depended on the input energy and did not depend on the repetition rate. The discharge starting voltage was 15.1 kV, 22.2 kV, 24.5 kV and 28.1 kV at input energies of 191 mJ, 385 mJ, 478 mJ and 738 mJ, respectively. Especially at repetition rates of 200 Hz and 300 Hz, a low input energy gave a high small signal gain coefficient. For example, at a repetition rate of 300 Hz, input energies of 191 mJ and 738 mJ produced small signal gain coefficients of 2.11%/cm and 0.36%/cm, respectively. A high input energy gives a high discharge starting voltage, but the energy loss increases, which generates heat and prevents the relaxation of the lower levels, possibly resulting in lower small signal gain coefficients. Conversely, at a low input energy, a high small signal gain is produced, but the laser energy may have been reduced when an optical cavity is added and an oscillator is constructed. On the other hand, in an amplifier, it is desirable to obtain a high gain at a low input energy.

Fig. 8

Dependence of small signal gain coefficient on repetition rate and input energy with CO₂/N₂ gas mixing ratio of 1:2. Circle, square, diamond and triangle symbols represent input energies of 191 mJ, 385 mJ, 478 mJ and 738 mJ, respectively

4 Conclusion

The gain characteristics of a longitudinally excited CO₂ laser using a low-pressure He-free gas were investigated. The longitudinal discharge tube had an inner diameter of 0.8 cm and a length of 45 cm and did not employ pre-ionization, fast gas flow or a cooling system. High-voltage pulses were applied to the discharge tube. The small signal gain coefficient depended on the delay time, the gas mixing ratio, the gas pressure, the repetition rate and the input energy. In this work, the small signal gain coefficient reached a maximum at 10–50 μs after the discharge started, not immediately after the discharge started. In He-free gases with CO₂/N₂ gas mixing ratios of 1:1, 1:2 and 1:3, the small signal gain coefficient was 1.90%/cm or more at a repetition rate of 200 Hz or less and 0.52%/cm or less at a repetition rate of 400 Hz or more. A longitudinally excited CO₂ laser will oscillate at repetition rates up to 300 Hz even in a He-free gas. In addition, at repetition rates of 200 Hz and 300 Hz, a low input energy gave a high small signal gain coefficient. At a repetition rate of 300 Hz and a CO₂/N₂ gas mixing ratio of 1:2, input energies of 191 mJ and 738 mJ produced small signal gain coefficients of 2.11%/cm and 0.36%/cm, respectively.

Our next work is to measure the saturated intensity, to compare gains and saturated intensities with and without He gas and to develop a short-pulse CO₂ laser pumped by a longitudinal pulsed discharge without He gas, operating at a high repetition rate.