Introduction

Many types of CO2 lasers have been widely applied in the laser industry because CO2 lasers can produce pulsed light with a pulse width of 10 ns to 1 ms or continuous wave (CW) light with high power at wavelengths in the range of 9.2  µm to 11.4 µm (mainly 9.6 µm and 10.6 µm). Processing of various materials, such as glass [1,2,3,4,5,6,7,8,9,10,11], polymer resin [12,13,14], fiber-reinforced resin [15,16,17], wood [18, 19], etc., by various CO2 lasers has been reported. In glass processing by a CO2 laser, the CO2 laser light is absorbed at the glass surface because the wavelength of a CO2 laser is 9.2 µm to 11.4 µm. CO2 lasers employ thermal processing because the pulse width of commercial CO2 lasers is 10 ns at the shortest. The heat generated by a laser pulse on a sample may depend on the laser pulse waveform, the pulse width, the fluence, and the repetition rate.

In a transversely excited atmospheric (TEA) CO2 laser, which is a short-pulse CO2 laser, the laser pulse waveform has a spike pulse with a width of about 100 ns and a pulse tail with a length of several microseconds [20,21,22,23,24]. Even in our recently developed longitudinally excited CO2 lasers, the short pulse has a spike pulse with a width of about 100 ns and a pulse tail with a length of several to several hundreds of microseconds, although a short pulse without a pulse tail can be produced [25,26,27,28,29]. Our CO2 laser controls the spike pulse width and the energy ratio of the spike pulse to the pulse tail. The pulse tail of a discharge-pumped CO2 laser is caused by a small current after the main discharge and long-lifetime resonance excitation by N2 contained in the medium gas, and the pulse tail depends on the device configuration and operating conditions. The pulse tail is longer than the spike pulse and causes thermal effects, such as a crack, a heat-affected zone (HAZ), and so on. In our previous work, a short pulse with a spike pulse width of about 250 ns, a pulse tail length of 31.4 µs, and an energy ratio of the spike pulse to the pulse tail of 1:7 to 1:92 produced a crack-less hole in a crown glass with a high thermal expansion coefficient of 100 × 10−7 /K at a repetition rate of 150 Hz to 400 Hz [11]. However, there are various types of glasses, and their physical constants related to heat, such as the thermal expansion coefficient and the softening point, are different. The stress caused by a temperature gradient produces cracks, and glass with a high thermal expansion coefficient tends to crack easily. In addition, glass with a low softening point tends to easily exhibit a HAZ. Therefore, even if the same type of CO2 laser pulse is used to irradiate different glasses, the processing results may differ depending on the glass material.

In this work, the objective is to investigate the influence of the pulse-tail energy of short-pulse CO2 laser drilling of various types of glass with thermal expansion coefficients of 100 × 10−7 /K to 5.5 × 10−7/K and softening points of 724 °C to 1600 °C. Thus, in this work, two types of short CO2 laser pulses, one with a large tail and one with a small tail, were radiated onto four types of glass with different thermal expansion coefficients and softening points, and the processing characteristics were investigated.

Experimental

Figure 1 shows the schematic diagram of the experimental setup consisting of a longitudinally excited CO2 laser, an attenuator, a lens, a stage, and a sample [25,26,27,28,29]. The CO2 laser emitted two types of short laser pulses as shown in Fig. 2. The large-tail short pulse had a spike pulse width of 450 ns, a tail length of 71 µs and an energy ratio of the spike pulse to the pulse tail of 1:31. The small-tail short pulse had a spike pulse width of 570 ns, a tail length of 35 µs and an energy ratio of the spike pulse to the pulse tail of 1:8. In both laser pulses, the wavelength was 10.6 µm, the repetition rate was 200 Hz, and the beam profile was a circular Gaussian-like shape with a correlation coefficient of 0.98. The focal length of the lens was 38.1 mm, the numerical aperture (NA) was 0.16, and the focal spot size was 230 µm. The sample surface was fixed at the focal plane. The fluence per pulse of whole laser pulse waveforms was adjusted by the attenuator to 22 J/cm2 in both laser pulse waveforms with energy ratios of 1:31 and 1:8. The samples were crown glass with a thermal expansion coefficient (CTE) of 100 × 10−7/K, a softening point (SP) of 724 °C and a thickness of 1.1 mm, soda-lime glass with a CTE of 87 × 10−7 /K, a SP of 740 °C and a thickness of 1.2 mm, low expansion borosilicate glass with a CTE of 33 × 10−7/K, a SP of 820 °C and a thickness of 1.1 mm, and synthetic quartz glass with a CTE of 5.5 × 10−7/K, a SP of 1600 °C and a thickness of 2.0 mm. As the thermal expansion coefficient decreased, the softening point tended to increase. In CO2 laser processing of glass with a large thermal expansion coefficient, generally, various treatments before, during, or after irradiation for suppression of crack production have been reported [4,5,6,7]. However, in the present work, such treatments were not used to investigate the HAZ.

Fig. 1
figure 1

Schematic diagram of experimental setup

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Fig. 2
figure 2

Laser pulse waveforms. Red and blue lines represent laser pulse waveforms with energy ratios of 1:31 and 1:8, respectively. a Magnified time scale of spike pulse. b Overall waveform

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Results and discussion

In this work, none of the samples exhibited cracks. Figure 3 shows an image of crown glass irradiated with 200 large-tail short pulses with an energy ratio of 1:31. The irradiation diameter was 230 µm, and the total irradiation fluence was 4400 J/cm2. The surface hole diameter as shown in Fig. 3a was 144 µm, and the ratio of the surface hole diameter to the irradiation diameter was 61.1%. The HAZ diameter as shown in Fig. 3b was 425 µm, and the ratio of the HAZ diameter to the irradiation diameter was 181%. The drilling depth as shown in C of Fig. 3c was 854 µm.

Fig. 3
figure 3

Image of crown glass irradiated with 200 short pulses with a large tail a Surface image. a and b show surface hole diameter and HAZ diameter, respectively. b Side image. c shows the drilling depth

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Figure 4 shows the dependence of the ratio of the surface hole diameter to the irradiation diameter on the total irradiation fluence, that is the fluence per single pulse multiplied by the number of irradiation pulses in crown glass with a CTE of 100 × 10−7/K and a SP of 724 °C, soda-lime glass with a CTE of 87 × 10−7/K and SP of 740 °C, low expansion borosilicate glass with a CTE of 33 × 10−7/K and a SP of 820 °C, and synthetic quartz glass with a CTE of 5.5 × 10−7/K and a SP of 1600 °C. In this work, the results were presented using the ratio of the surface hole diameter to the irradiation diameter to illustrate the relationship between the irradiation diameter and the surface hole diameter. Figure 4a presents the results obtained with the large-tail short laser pulse with the energy ratio of the spike pulse to the pulse tail of 1:31, and Fig. 4b presents the results obtained with the small-tail short laser pulse the energy ratio of 1:8. In the laser pulse waveform with the energy ratio of 1:31, the ratio of the surface hole diameter increased slightly with the increase of the total irradiation fluence but did not depend on the glass material. The average ratio was about 59.8%. In the laser pulse waveform with the energy ratio of 1:8, the ratio of the surface hole diameter did not depend on the total irradiation fluence or the glass material and was constant at about 59.8%.

Fig. 4
figure 4

Dependence of ratio of surface hole diameter to irradiation diameter on total irradiation fluence. Circles, squares, diamonds, and triangles represent crown glass, soda-lime glass, borosilicate glass, and synthetic quartz glass, respectively. a Laser pulse waveform with an energy ratio of 1:31. b Laser pulse waveform with an energy ratio of 1:8

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Figure 5 shows the dependence of the ratio of the HAZ diameter to the irradiation diameter on the total irradiation diameter in crown glass with a CTE of 100 × 10−7 /K and a SP of 724 °C, soda-lime glass with a CTE of 87 × 10−7/K and SP of 740 °C, low expansion borosilicate glass with a CTE of 33 × 10−7/K and a SP of 820 °C, and synthetic quartz glass with a CTE of 5.5 × 10−7 /K and a SP of 1600 °C. In this work, the results were presented using the ratio of the HAZ diameter to the irradiation diameter to illustrate the relationship between the irradiation diameter and the HAZ diameter. Figure 5a presents the results obtained with the large-tail short laser pulse with the energy ratio of the spike pulse to the pulse tail of 1:31, and Fig. 5b presents the results obtained with the small-tail short laser pulse with the energy ratio of 1:8. In both laser pulse waveforms with the energy ratios of 1:31 and 1:8, the ratio of the HAZ diameter increased with the increase of the total irradiation fluence, and the decrease of the softening point of the glass. The ratio of the HAZ diameter produced by the laser pulse waveform with the energy ratio of 1:31 was larger than that produced by the laser pulse waveform with the energy ratio of 1:8. In particular, in a glass material with a low softening point, the difference became significant when the total irradiation fluence was over 2000 J/cm2.

Fig. 5
figure 5

Dependence of ratio of HAZ diameter to irradiation diameter on total irradiation fluence. Circles, squares, diamonds, and triangles represent crown glass, soda-lime glass, borosilicate glass, and synthetic quartz glass, respectively. a Laser pulse waveform with an energy ratio of 1:31. b Laser pulse waveform with an energy ratio of 1:8

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Figure 6 shows the dependence of the drilling depth on the total irradiation diameter in crown glass with a CTE of 100 × 10−7/K and a SP of 724 °C, soda-lime glass with a CTE of 87 × 10−7/K and SP of 740 °C, low expansion borosilicate glass with a CTE of 33 × 10−7/K and a SP of 820 °C, and synthetic quartz glass with a CTE of 5.5 × 10−7/K and a SP of 1600 °C. Figure 6a presents the results obtained with the large-tail short laser pulse with the energy ratio of the spike pulse to the pulse tail of 1:31, and Fig. 6b presents the results obtained with the small-tail short laser pulse with the energy ratio of 1:8. In both laser pulse waveforms with the energy ratios of 1:31 and 1:8, the drilling depth increased with the increase of the total irradiation fluence but did not depend on the glass material. At a total irradiation fluence of about 220 J/cm2, the drilling depth produced by the laser pulse waveform with the energy ratio of 1:31 was almost the same as that produced by the laser pulse waveform with the energy ratio of 1:8 and was about 323 µm. However, at a total irradiation fluence of 500 J/cm2 or more, the drilling depth produced by the laser pulse waveform with the energy ratio of 1:31 was smaller than that produced by the laser pulse waveform with the energy ratio of 1:8. This may be attributed to the fluence of the spike pulse part of the laser pulse waveform. In this work, the fluence per single pulse of whole laser pulse waveforms was 22 J/cm2 in both laser pulse waveforms with the energy ratios of 1:31 and 1:8. Therefore, the fluence per pulse of the spike pulse parts was 0.69 J/cm2 and 2.44 J/cm2 in the laser pulse waveforms with the energy ratios of 1:31 and 1:8, respectively. In drilling with a large number of irradiation pulses, the fluence per pulse of the spike pulse part affected the drilling depth.

Fig. 6
figure 6

Dependence of drilling depth on total irradiation fluence. Circles, squares, diamonds, and triangles represent crown glass, soda-lime glass, borosilicate glass, and synthetic quartz glass, respectively. a Laser pulse waveform with an energy ratio of 1:31. b Laser pulse waveform with an energy ratio of 1:8

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Conclusions

Two types of short CO2 laser pulses, with a large or small tail, were radiated onto four types of glass with thermal expansion coefficients of 100 × 10−7/K to 5.5 × 10−7/K and softening points of 724 °C to 1600 °C at a repetition rate of 200 Hz and a fluence per pulse of 22 J/cm2, and the processing characteristics, in terms of the ratios of the surface hole diameter and the HAZ diameter to the irradiation diameter, as well as the drilling depth, were investigated. The surface hole diameter did not depend on the pulse-tail energy of the short CO2 laser pulse and the glass material. The ratio was about 59.8%. In glass with a large softening point of 820 °C or more, the HAZ was not affected by the pulse-tail energy of a short CO2 laser pulse. In glass with a small softening point of 740 °C or less, the HAZ produced by the short CO2 laser pulse with a small tail at a total irradiation fluence of 2000 J/cm2 or more was smaller. The drilling depth increased with the increase of the total irradiation fluence but did not depend on the glass material. In drilling with a large number of irradiation pulses, the fluence per pulse of the spike pulse part affected the drilling depth. Therefore, in glass processing by a short-pulse CO2 laser, the pulse-tail energy of the laser pulse waveform is an important parameter to be considered. However, with a small total irradiation fluence produced by a small number of irradiation pulses, the pulse-tail energy of the laser pulse waveform does not affect the results of glass drilling.