Sealed-off CO 2 lasers excited by an all-solid-state 0.6 MHz generator
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1 J. Phys. D: Appl. Phys. 33 (2000) Printed in the UK PII: S (00) Sealed-off CO 2 lasers excited by an all-solid-state 0.6 MHz generator S Wieneke, S Born and W Viöl Department PMF, University of Applied Sciences and Arts, Von-Ossietzky-Str. 99, D Göttingen, Germany wolfgang.vioel@pmf.fh-goettingen.de Received 8 February 2000 Abstract. Sealed-off CO 2 lasers excited by a dielectric barrier discharge are studied. The laser output power is measured at excitation frequencies between 0.3 and 0.6 MHz; discharge gaps are in the range mm. The excitation efficiency of the all-solid-state generator reaches 90%. Due to the scaling laws of radio frequency excitation no laser activity should be possible for the laser under investigation. The use of high-frequency excitation ( MHz) combined with a dielectric barrier discharge gives the possibility of creating a homogeneous stable CO 2 laser gas discharge with high laser activity. The specific laser power follows the 1/d scaling law due to gas cooling by diffusion. Even the optimal gas pressure can be increased by high-frequency excitation compared to the radio frequency excitation. 1. Introduction The CO 2 laser discharge can be excited by dc [1 3], highfrequency (hf) (10 khz 3 MHz) [4, 5], radio frequency (rf) ( MHz) [6] and microwave (2.45 GHz) [7, 8]. The first CO 2 lasers were excited by a dc discharge. Nowadays CO 2 lasers are still excited by a dc discharge because of the low cost of dc high-voltage generators. Microwave generators have also moderate prices, but CO 2 lasers excited by microwaves do not reach the power densities of dc and rf excited lasers. Most of the CO 2 lasers operated today use the rf excitation, which presents many advantages over the dc excitation, for example: (1) the avoidance of anodes and cathodes, which eliminates the associated gas chemistry problem at the cathode; and (2) the occurrence of a stable discharge at higher discharge pressures. One drawback is the high cost of the rf generators which have a relatively low efficiency of 60%. Therefore, low-cost, all-solid-state generators with frequencies of about 0.6 MHz are used in the laser systems under investigation. The excitation efficiency of the all-solid-state generator reaches more than 90%. Up until now, hf high voltage was used only to excite fast axial flow CO 2 lasers [4, 5, 9]. In this paper, a hf excited sealedoff slab CO 2 laser is described which is a new candidate for low-cost, compact CO 2 lasers. 2. Discharge physics 2.1. Gas cooling in a slab CO 2 laser Compared to the CO 2 waveguide laser, the cooling of the slab laser (see figure 1) is only half as efficient because there are only two walls involved. The slab CO 2 laser represents an extension of the CO 2 waveguide laser principle towards higher laser power [10 12]. From the heat diffusion equation, it follows that there exists (for the case of uniform thermal conductivity and energy generation in the laser gas) a parabolic temperature distribution between the discharge walls. Assuming this parabolic temperature distribution, the heat that can be extracted from the laser gas can be calculated [13]. Equation (1) gives the specific laser power P l /A (laser output power per area) assuming a laser efficiency η and neglecting the waveguide losses and the influence of the gap width d on excitation conditions: P l A = η 1 η 8λ g (T max T w ). (1) d For a maximum gas temperature T max 500 K in the middle of the discharge, a mean heat conductivity of the laser gas λ g 0.13 W m 1 K 1 [14, 15], a wall temperature T w = 290 K, and a laser efficiency η 10%, the specific laser power can be estimated as P l A 3 d [mm W cm 2 ]. (2) The walls are slabs made of alumina (Al 2 O 3 ) and act as a dielectric barrier (see figure 1). Due to the high thermal conductivity of alumina (λ Al2O 3 = 19 W m 1 K 1 ) compared to that of the laser gas the heat can be fully transferred through the slabs, so that equation (2) is also valid for a slab CO 2 laser excited by a dielectric barrier discharge Similarity laws for rf discharges It is well known that the extrinsic properties of discharges are related through the similarity laws [16]. In order to preserve discharge or laser excitation conditions during the change of /00/ $ IOP Publishing Ltd
2 CO 2 lasers excited by a 0.6 MHz generator 3. Description of the dielectric barrier discharge slab laser Figure 1. Schematic diagram of a hf excited slab CO 2 laser. Table 1. Relationship between p o, d, and f o for CO 2 laser discharges. p o d f o d p o /f o 240 hpa mm 280 MHz mm 0.86 hpa MHz 1 one parameter, other linked parameters must also be adjusted. Thus, in an rf discharge, the optimal gas pressure p o and the optimal generator frequency f o for a given CO 2 laser gas discharge gap d are related through the similarity laws as outlined in table 1. For smaller excitation frequencies than the optimal generator frequency mentioned in table 1, the specific laser output power decreases. For rf frequencies below f min = 80 d [MHz mm] (3) no laser activity have been measured [12]. The deviation of the specific power from the 1/d scaling law (equation (1)) at low excitation frequencies is considered to be caused by ion losses in the sheath region of the alpha rf discharge [12]. This sheath region is therefore non-laser active. The ion losses are proportional to the sheath thickness d s and increase with decreasing frequency [11, 12] because d s 15 f [mm MHz]. (4) The use of a dielectric barrier discharge gives the possibility of exciting such a laser (with discharge gaps below 6 mm) with frequencies below 1 MHz. These excitation frequencies are more than one magnitude lower than the minimum excitation frequency f min (see equation (3)) of rfexcited CO 2 lasers. From equation (4) it follows that the non-laser active sheath should be d s 25 mm thick at an excitation frequency of 0.6 MHz, so the calculated sheath thickness d s is much higher than the discharge gap d. A schematic diagram showing the construction of the slab waveguide laser excited by a dielectric barrier discharge is shown in figure 1. The alumina plates between the electrodes are used as dielectric barriers. For small discharge gaps, waveguiding is provided also by the dielectric barriers. Ceramic spacers ensure uniform separation of the electrodes. Quartz glass sidewalls are used for spectroscopic observation of the laser gas discharge. These spectroscopic measurements prove that homogeneity of the discharge is excellent. The vacuum enclosure of the CO 2 laser uses O -ring seals and the alumina plates are glued to the quartz glass windows. The electrical power source is capacitively coupled to the discharge load through two electrodes facing each other outside of the discharge tube. The water cooling of the electrodes guarantees the cooling of the discharge through the alumina dielectric barriers. The electrodes have an area of 540 mm 12 mm; discharge gaps of 1.6 mm and 6.0 mm are investigated. The excitation frequency of the all-solid-state generator depends on the capacity of the electrode configuration and varies between 0.3 and 0.6 MHz. About 90% of the source power is transformed into the discharge power. The invar-stabilized cavity is adjustable by means of mechanical mirror adjusters outside of the vacuum chamber. A silicon back mirror with a reflectivity of 99.7% and a curvature of 5 m and a plane ZnSe output resonator mirror with 5% transmission have been employed. The cavity is m in total length including a mirror/waveguide separation of 1.5 mm on each side. 4. Experimental results Several different waveguide dimensions have been investigated. In all cases the output power was measured for the multimode all-waveguide resonator in order to test the maximum power extraction. This output power is converted into a specific output power in order to normalize the power extraction. The laser output power was measured at different gas mixtures, gas pressures, and at different input power densities. The input power density ρ was calculated from timedependent current I(t)and voltage U(t) measurements: T 0 ρ = I(t)U(t)dt (5) TV where T is one time interval of the high-voltage oscillation and V stands for the discharge volume Discharge gap 6 mm The 6 mm discharge gap is excited by the all-solid-state generator with a frequency of 0.6 MHz. Figure 2 shows the variation of output power with gas pressure for a range of different gas compositions (He:N 2 :CO 2 :Xe) for the 6 mm discharge gap at an input power density of 5 W cm 3. From figure 2 it follows that the optimal gas pressure is determined between 90 and 160 hpa. This pressure is higher than the pressure p = 40 hpa calculated with the similarity law for d = 6 mm (see table 1). 1283
3 S Wieneke et al Figure 2. Output power as function of pressure for a6mmdischarge gap and different gas compositions (He:N 2 :CO 2 or He:N 2 :CO 2 :Xe) at an input power density of 5 W cm 3. Figure 3. Laser efficiency as a function of input power density for a6mmdischarge gap and different gas compositions (He:N 2 :CO 2 or He:N 2 :CO 2 :Xe) at a gas pressure between 90 and 120 hpa. The addition of xenon to the active medium increases the optimal pressure value (see figure 2) but decreases the laser efficiency (see figure 3). The maximum laser output power was measured without xenon with a gas mixture of He:N 2 :CO 2 = 70:25:5. The laser power was measured in a sealed-off operation and the power stability was determined to be between 2 and 4% over an operation time of 1 h. The mean laser power did not decrease within 1 h, so that the dissociation of CO 2 seems to be constant over this time interval. Figure 3 demonstrates the variation of the laser efficiency with the electrical input power density ρ. A stable laser gas discharge can be realized at remarkably high input power densities of 20 W cm 3. In a comparable laser system excited by an rf discharge, maximum electrical power densities of 4Wcm 3 at an excitation frequency of 13.6 MHz and 7Wcm 3 at 27.1 MHz have been reached. Taking account of the high efficiency of the low-cost 0.6 MHz generator of about 90%, the sealed-off slab CO 2 laser excited by a dielectric barrier discharge is a new candidate for a low-cost, compact CO 2 laser. A maximum laser output power of 37 W has been measured (see figure 3) corresponding to a specific laser power of 0.57 W cm 2. This value is a little bit higher 1284
4 CO 2 lasers excited by a 0.6 MHz generator Figure 4. Output power as function of input power density for a 1.6 mm discharge gap (He:N 2 :CO 2 = 60:35:5). Figure 5. Laser efficiency as function of input power density for a 1.6 mm discharge gap (He:N 2 :CO 2 = 60:35:5). than the value 0.5 W cm 2 calculated by equation (2). Therefore, the specific power of the laser is only limited by thermal conductivity and not by the onset of discharge instabilities. The laser efficiency is about 13% at a low input power density of 1 W cm 3. An efficiency of 6% at the relatively high input power density of 15 W cm 3 has been realized. The resonator efficiency of this small laser system is very low due to the high resonator losses. Therefore, there is a potential to improve the laser efficiency by using a longer active length and by optimizing the transmission of the output mirror and the curvatures of the resonator mirrors Discharge gap 1.6 mm Due to the higher capacity of the 1.6 mm discharge gap the all-solid-state generator delivers an excitation frequency of between 0.3 and 0.4 MHz. The dependence of the laser output power and the laser efficiency on the input power density for a 1.6 mm discharge gap are shown in figures 4 and 5, respectively. The highest output power was achieved using a gas pressure of about 260 hpa. This pressure is also higher than the pressure p = 150 hpa calculated with the similarity law for d = 1.6 mm (see table 1). The electrical power density reaches the same values of about 100 W cm 3 as obtained in rf discharges [10, 11, 16]. The maximum specific 1285
5 S Wieneke et al laser power of 0.46 W cm 2 (see figure 4) achieved is not the value 1.88 W cm 2 calculated by formula (2). So far the laser efficiency of approximately 4% is relatively low. A higher laser output power and a laser efficiency can be expected by using a better resonator efficiency, a better alignment of the optical axis relative to the waveguide, and additional mirror cooling. 5. Conclusion The use of hf excitation ( MHz) combined with a dielectric barrier discharge gives the possibility of creating a homogeneous stable CO 2 laser gas discharge. The optimal gas pressure and the input power density can be increased compared to rf excitation. The laser is only thermally limited and not by the onset of discharge instabilities. The efficiency of the all-solid-state generator used to excite the dielectric barrier discharge reaches 90%. Experimental results show that the sealed-off CO 2 laser excited by a hf dielectric barrier discharge is a promising candidate for a low-cost, commercial, high-power CO 2 laser for material processing. Acknowledgment The authors wish to thank the Lower Saxony Ministry for Science and Culture for their support of this work. References [1] Cheo P K 1971 Lasers vol 3 (New York: Dekker) ch 2 [2] Witteman W J 1987 CO 2 Laser (Berlin: Springer) [3] Hügel H 1992 Strahlwerkzeug Laser (Stuttgart: Teubner) [4] Kuzumoto M, Ogawa S, Tanaka M and Yagi S 1990 IEEE J. Quantum Electron [5] Klein J, Otto G, Habich U and Loosen P 1994 EuroLaser 2 48 [6] Witteman W J and Ochkin V N 1996 Gas Lasers Recent Developments and Future Prospects (Dordrecht: Kluwer) [7] Bielesch U, Budde M, Fischbach M, Freisinger B, Schäfer J H, Uhlenbusch J and Viöl W 1992 Laser Optoelektronik [8] Viöl W and Uhlenbusch J 1996 J. Phys. D: Appl. Phys [9] Nagai H, Hishii M, Tanaka M, Myoi Y, Wakata H, Yagi T and Tabata N 1993 IEEE J. Quantum Electron [10] Nowack R, Opower H, Schaefer U, Wessel K and Hall Th 1990 SPIE CO 2 Lasers Application II [11] Nowack R, Opower H, Wessel K, Krüger H, Haas W and Wenzel N 1991 Laser Optoelektronik 3 68 [12] Kobayyashi S, Murata T, Terai K and Tamagawa T 1997 SPIE Gas Flow Chemical Lasers High-Power Laser [13] Thomas L C 1992 Heat Transfer (Englewood Cliffs, NJ: Prentice-Hall) p 73 [14] Ladermann A and Byron S 1971 J. Appl. Phys [15] Bird R B, Stewart W E and Lightfoot E N 1960 Transport Phenomena (New York: Wiley) [16] Colley A D, Villarreal F, Cameron A A, Vitruk P P, Baker H J and Hall D R 1996 Gas Lasers Recent Developments and Future Prospects (Dordrecht: Kluwer) p
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