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Characteristics of the inductive nitrogen laser generation
Optics Communications 367 (2016) 244–248
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Invited Paper
Characteristics of the inductive nitrogen laser generation A.M. Razhev a,b, D.S. Churkin a,c,n, E.S. Kargapoltsev a a b c
Institute of Laser Physics SB RAS, Ac. Lavrentyev’s, prosp. 15B, 630090 Novosibirsk, Russia NSTU, K. Marx prosp., 20, 630073 Novosibirsk, Russia NSU, Pirogova str., 2, 630090 Novosibirsk, Russia
art ic l e i nf o
a b s t r a c t
Article history: Received 22 September 2015 Received in revised form 19 January 2016 Accepted 22 January 2016 Available online 4 February 2016
The results of the experimental study of energy, temporal, spectral and spatial characteristics of UV inductive laser generation are presented. The study has identified a number of characteristics which demonstrate the differences between electron parameters of inductively coupled plasma and the plasma of longitudinal and transverse electrical discharges. The mechanism of simultaneous occurrence of Le+ wis-Rayleigh afterglow representing transitions between higher vibrational substates of B3Πg and A3 ∑u states; laser generation at C3Πu-B3Πg transition as well as the absence of IR radiation at 1st positive system typical for electrical discharge nitrogen lasers has been thoroughly researched. The major characteristic is ring shaped laser beam which size and width depend on excitation conditions. Inductive UV nitrogen laser is found to operate in ASE regime, but has a low divergence of 0.47 0.1 mrad and high pulse-to-pulse stability (laser pulse deviation amplitude did not exceed 1%). & 2016 Elsevier B.V. All rights reserved.
Keywords: Pulsed inductive discharge Nitrogen laser Lewis–Rayleigh afterglow Ring laser beam shape High pulse-to-pulse stability Low divergence
1. Introduction The various use of nitrogen laser as a source of high-power UV radiation is of great interest [1–5]. The most prevalent nitrogen lasers are the lasers with pumping by longitudinal and transverse pulse electrical discharges. The generation energy of nitrogen lasers with pumping by longitudinal discharge usually does not exceed fractions of mJ and pulse duration at half maximum is not more than several ns. Such lasers may act with pulse repetition frequency up to several hundred Hz [1]. When nitrogen is excited by high-current transverse electrical discharge the generation energy can reach 100 mJ [2] and pulse repetition frequency – 11 kHz [3]. Pulse generation duration with the use of transverse discharge is also limited by several ns at half maximum. To increase pulse duration in that case special measures are required [4]. At the same time the efficiency of nitrogen lasers UV radiation does not exceed 0.2% [5] that limits the use of these lasers, especially after appearance of more efficient excimer lasers. UV nitrogen laser is considered to be well-studied [6]. However, nitrogen as an active medium has quite a number of characteristics which make it a subject of a great scientific interest [7–15]. We have previously suggested the use of pulse inductive discharge as a n Corresponding author at: Institute of Laser Physics SB RAS, Ac. Lavrentyev’s, prosp. 15B, 630090 Novosibirsk, Russia. E-mail addresses: [email protected] (A.M. Razhev), [email protected] (D.S. Churkin), [email protected] (E.S. Kargapoltsev).
http://dx.doi.org/10.1016/j.optcom.2016.01.060 0030-4018/& 2016 Elsevier B.V. All rights reserved.
new alternative method of gas laser active media pumping. Several inductive lasers operating at electronic and vibrational–rotational transitions of gas atoms and molecules at visible and infrared spectrum including UV laser at transitions of nitrogen molecules 2nd positive band system were generated [16–21]. Even during the first experiments we noticed a number of differences in generation characteristics between inductive nitrogen laser and nitrogen lasers pumped with conventional longitudinal and transverse electrical discharges. One of the most marked differences was the absence of IR bands associated with 1st positive band system transitions of molecular nitrogen in the spectrum of spontaneous and laser radiation. At the same time while operating the laser, slow decaying glow in visible spectrum (530–700 nm) also known as Lewis–Rayleigh afterglow was observed. Lewis–Rayleigh afterglow has been known for over 100 years, but is still of scientific and practical interest for investigation of active nitrogen properties along with short-lived pink afterglow [7–15,21–28]. The glow occurs as a result of transitions between + higher vibrational substates of B3Πg and A3 ∑u electron levels of first positive band system of molecular nitrogen. The glow is characterized by long duration which may last over ten seconds (up to few minutes) [24]. There is sufficiently big number of theoretical works on glow formation and existence mechanisms [7,21–28]. In general, these works describe energy redistribution between excited electron levels not paying much attention to the initial excitation of nitrogen molecules from the ground state. Therefore, all the above mentioned investigations do not focus on
A.M. Razhev et al. / Optics Communications 367 (2016) 244–248
population inversion at transitions of nitrogen molecules 2nd positive band system and connection of UV laser generation with Lewis–Rayleigh afterglow. We believe that occurrence of this glow with population inversion and UV laser generation at transition of C3Πu-B3Πg demonstrates the differences in plasma electron parameters of pulsed inductive discharge versus longitudinal and transverse electrical discharges. The purpose of this study is to demonstrate the above mentioned differences based on energy, spatial, timing and spectralresponse characteristics of inductive nitrogen laser radiation. Based on the obtained results and further studies of the processes in the pulsed inductive discharge plasma as an active medium of nitrogen laser, a kinetic model of the inductive nitrogen laser will be created. This model will include the processes of molecular excitation from the ground state, population inversion at transitions of 2nd positive band system and mechanisms of the following energy relaxation resulting in Lewis-Rayleigh afterglow instead of generation on IR transitions of 1st positive system. Ultimate goal is to create a tailored pulsed inductively coupled plasma with specified characteristics (electron energy and their density, excitation specific capacity W ¼UI/V {MW/cm3}, U {kV} and I {kA} – discharge gap voltage and discharge current, V {cm3} – active volume) for excitation of random active laser medium including excimer one.
245
2. Experimental setup and measurements Pulsed inductive discharge in gas was generated by high-voltage excitation system developed on the basis of well-known Blumlein scheme [18,20]. Fig. 1 shows its electrical circuit. In several experiments, С–С scheme of recharging with railgap [21] instead of Blumlein scheme was applied. In the experiments, ceramic discharge tubes DT (Fig. 1) with outside diameter of 40 and 50 mm and inside diameter of 32 and 42 mm, respectively, were used. The length of the tubes was 800 mm. The tubes were sealed by parallel plate windows W1 and W2 made of quartz or MgF2, and placed perpendicularly to its axis. An optical resonator was formed by external flat dielectric mirrors М1 and М2. Rear mirror М1 had a reflection coefficient R1 ¼99% in the spectrum of 300–380 nm. A reflection coefficient of the output mirror М2 was adjusted in the experiment to achieve maximum generation energy. Mirrors with 8–93% reflection coefficient of have been used. Nitrogen or its mixture with other gases under the pressure ranged from 0.1 Torr to 100 Torr was delivered into the tube from the gas system. In the experiment, a longitudinal gas circulation was applied. Spontaneous and laser radiation of pulsed inductive nitrogen discharge as well as laser generation spectrum was registered by SOLAR LS S-150 spectrometer with 0.66 nm resolution in the spectral range of 200–1100 nm. Laser radiation energy was measured by OPHIR Optronics PE50–BB pyroelectric detector. Temporal parameters of electrical pulses were registered by Р6015А high-voltage tester and Tektronix TDS-2024 oscillograph with 200 MHz band. FEK–22 coaxial photocell with resolution of 10 10 s as well as FD-24K photodetector were used to register temporal parameters of optical pulses.
3. Results and discussion
Fig. 1. An electrical drawing of high-voltage system of pulsed inductively coupled plasma.
During the experiments, we were able to generate pulsed inductive discharge in nitrogen only under pressure below 10 Torr. Molecular nitrogen spontaneous emission spectrum was registered in the range of 180–1100 nm. The study has shown that radiation is concentrated within the range of 190–700 nm (Fig. 2). Under relatively high pressure of 5–10 Torr, only bands of the 2nd positive system representing C3Πu-B3Πg transition have been found in the spectrum (0–0) – 337.1 nm and (0–1) – 357.7 nm bands had maximum intensity. When the pressure was reduced to 1.5–2.5 Torr, bands of 1st positive and 1st negative system as well
Fig. 2. Spectrum of spontaneous radiation Isp of pulsed inductive discharge. Peak intensity of 337,1 nm and 357,7 nm were decreased 10 times.
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A.M. Razhev et al. / Optics Communications 367 (2016) 244–248
Fig. 3. Oscillograph record of Lewis–Rayleigh afterglow decay IL–R (registered in the spectrum of 530–700 nm). Diameter of the discharge tube is 50 mm.
as Vegard–Kaplan system A3 ∑u – X1 ∑g occurred in the spectrum. Instead of well-known IR bands (800–1200 nm) of 1st positive system in electric discharge nitrogen lasers, we registered the radiation of 1st positive band in green-red region. This radiation is called “Lewis–Rayleigh afterglow”. It reflects the transitions between higher vibrational substates (ν′, ν″ ¼ 6–15) of B3Πg and + A3 ∑u states. Its intensity and duration highly depended on pumping conditions and nitrogen pressure in the discharge tube. Fig. 3 shows the process of radiation decay for various values of charging voltage under nitrogen total pressure of 1–2 Torr (diameter of discharge tube is 50 mm). In these experiments, decay time was up to one second. When the discharge tube of 50 mm diameter was replaced by the tube with less diameter (40 mm) decay time decreased in 1.5– 2 times. Moreover, in one of the experiments, high-voltage switch – TPI1-10k/50 thyratron was substituted by self-made triggered discharger. In this case, inductor oscillation of voltage decreased from 300 to 150 ns due to reduced integral inductor. This resulted in increase of inductive nitrogen laser pumping intensity and, as a consequence, energy generation (λ ¼337.1 nm) and total laser efficiency growth. However, these effects were accompanied by the abrupt decrease of the intensity and decay time of Lewis–Rayleigh afterglow (2–5 times depending on the tube diameter). At the same time, as it is shown in Fig. 4, correlation pattern of recombination radiation intensity with pressure was similar to laser generation energy. Therefore, the conclusion can be made that highest efficiency of Lewis-Rayleigh afterglow generation is reached under the same conditions as population inversion at the C3Πu-B3Πg transition. The mechanism of Lewis-Rayleigh afterglow generation during population inversion at transition C3Πu-B3Πg of the second positive band system of nitrogen molecules with pulse inductive discharge pumping is not fully understood and requires further investigations. Based on the available experimental data, we can make the following assumption. Unlike longitudinal and transverse electrical discharges, inductive discharge is formed in the absence of electrodes; therefore, it does not have electron emission from cathode. The initial concentration of electrons occurs as a result of capacitive discharge appearing immediately before inductive discharge out of voltage between inductor turns [29]. However, this concentration of electrons may be insufficient for population +
+
Fig. 4. Dependence of Lewis–Rayleigh afterglow intensity IL–R (registered in the spectrum of 530–700 nm) and energy generation value E on nitrogen pressure. Diameter of the discharge tube is 50 mm. Laser pulse duration is 15 7 1 ns (FWHM).
inversion based on direct excitement of nitrogen molecule from the ground state by an electron impact and for reaching the generation regimen (ne ¼1014–1015 cm 3). Other electrons appear under the influence of inductive discharge when some of nitrogen molecules are ionized. In this case, nitrogen molecules ionization may be stepwise through the excitement of intermediate levels. It is also possible that besides electron excitation of operating laser level other higher levels get excited. Further, when nitrogen molecular ions recombinate, N2 molecules get to higher vibrational substates of B3Πg electron state and then according to schemes described in [7,21–28] create recombination radiation. The suggested consequence of the processes explains simultaneous occurrence of Lewis-Rayleigh afterglow and UV laser generation at the transition of C3Πu-B3Πg as well as presence of significant amount of bands of nitrogen molecular ion and absence of 1 þ system IR bands in the spectrum of both laser and spontaneous radiation. Study of laser generation temporal parameters found that pulse duration (FWHM) is relatively longer for nitrogen lasers and reaches 13–20 ns (Fig. 5). Pulse duration was 40 ns close to the base (level 0.1 from intensity maximum). It is important to note that such changes in pulse duration can be done in the same laser emitter. For this purpose, only reflection coefficient of the output mirror R should be changed (Fig. 6). In the experiments the optimal reflection coefficient R was 40%. The study of UV inductive N2 laser generation spatial characteristics demonstrated that the laser beam in cross-section has a ring shape. This special characteristic of radiation is explained by a hollow cylinder form of active medium. Ring diameter and thickness depended on the laser tube size and shape, excitation power and Q factor of resonator. For detailed studies we used diode array which was placed along ring diameter. Because the diode array length did not exceed 29 mm, the tube with 20 mm inner diameter and 800 mm length was used in these studies. As it was shown in the experiments (Fig. 7(a)), laser radiation intensity of was the lowest at the ring outer border, close to the inner surface of the discharge tube. The intensity of generation sharply increased towards the center of the ring, and reached its maximum at 1–1.5 mm from the outer ring border. Then it was decreasing to almost zero at 3–4 mm. Therefore, the width of the ring close to the base was approximately 3 mm. The width of the ring depended on Q factor of the resonator. It was minimal (usually, 2–3 mm) in the resonator with low Q factor in the
A.M. Razhev et al. / Optics Communications 367 (2016) 244–248
247
Fig. 5. Oscillograph record of inductive nitrogen laser pulse. Diameter of the discharge tube is 50 mm. Output mirror reflection coefficient is 80%.
Fig. 7. Transverse profile of laser beam Ilas (from (a) to (c)) at 0.5 m, 3 m and 6 m distance from the output mirror. Discharge tube inner diameter is 20 mm, tube length is 800 mm.
Fig. 6. Dependence of generation energy E and pulse duration τ (FWHM) on output mirror reflection coefficient R of laser resonator.
absence of front mirror, in high-density resonator it was approximately 6–7 mm. The same results were observed for the tubes with 34 mm and 42 mm inner diameters, respectively. Experiments showed that laser radiation inside the beam had variable intensity and certain granular structure (it is clearly shown on the lower profile of the ring in Fig. 7(c)). This structure is random both in width and length of the ring and is not reproduced from pulse to pulse. This phenomenon is well known and described in the literature [30]. It was observed in lasers with high amplification coefficient (i.e. amplified spontaneous emission – ASE). This condition is observed in inductive nitrogen laser as well. The most contrast and clear granular generation structure was observed in laser in the absence of resonator. After mirrors have been installed the contrast decreased, and the ring profile became relatively smooth and changed from pulse to pulse insignificantly. We also studied the divergence of UV inductive N2 laser. Fig. 7 (from (a) to (c)) shows transverse profiles of laser beam at 0.5 m, 3 m and 6 m distance from output mirror. As the Fig. demonstrates, a beam profile is represented by two symmetrical to
optical axis peaks with sharp outlines near the output mirror. The farther from the output mirror, the peaks intensity decreased and the width increased. It was visible as an increase in ring width and reduction in picture contrast. We defined divergence as θ ¼ arctg((sL s0)/L) (L-distance from the output mirror, sL – a peak width at half maximum for L distance, s0 – peak width at half maximum near the output mirror). The results showed that divergence was 0.4 70.1 mrad. In our case it can be explained by the fact that an active medium of inductive nitrogen laser was long (approx. 700 mm), had relatively small transverse size of 4– 6 mm and existed in the open Fabry–Perot interferometer type resonator formed by two flat mirrors (with semi-transparent output mirror). Maximum generation energy in the experiments was 4.5 mJ and pulse duration was 15 71 ns (FWHM) that corresponded with pulse power of 300 kW. These parameters were observed only in nitrogen without addition of other gases and under lower pressure (1–2 Torr). On the contrary, when additions common for electric discharge nitrogen lasers (He, NF3 and SF6) were used, their increased concentration resulted in continuous reduction and following UV generation failure. In pulse-periodic regimen of laser (capacity of power system did not allow for the frequency higher than 10 Hz), light pulse deviation amplitude was no higher than 1% that indicates high consistency of inductive discharge. Such combination of active medium and N2 laser generation parameters of at 337.1 nm wavelength has been discovered for the first time and also indicates the differences in plasma electron parameters between pulse inductive and electrical discharge.
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4. Conclusion The study of UV inductive laser has identified a number of characteristics which suggest the differences between electronic parameters of inductively coupled plasma and the plasma of longitudinal and transverse electrical discharges. We suggested the explanation of the mechanism of simultaneous occurrence of Lewis-Rayleigh afterglow representing transitions between B3Πg + higher vibrational substates and A3 ∑u states; laser generation at C3Πu-B3Πg transition, as well as the absence of IR radiation at 1st positive system typical for electrical discharge nitrogen lasers. The major characteristic is ring shaped laser beam which size and width depend on excitation conditions. Inductive UV nitrogen laser was found to operate in ASE regime, but has a low divergence of 0.4 70.1 mrad and high pulse-to-pulse stability (laser pulse deviation amplitude did not exceed 1%).
References [1] A. Gorlov, V. Kyun, V. Skoz, Yu Tokunov, Sov. J. Quantum Electron. 19 (9) (1992) 1144. [2] I. Konovalov, A. Panchenko, V. Tarasenko, A. Tel’minov, Quantum Electron. 37 (7) (2007) 623. [3] V. Atezhev, S. Vartapetov, A. Zhigalkin, K. Lapshin, A. Obidin, Quantum Electron. 34 (9) (2004) 790. [4] E. Baksht, A. Panchenko, V. Tarasenko, Quantum Electron. 28 (12) (1998) 1058. [5] V. Apollonov, V. Yamshchikov, Quantum Electron. 32 (2) (2002) 183.
[6] A.M. Razhev, G.G. Telegin, Zarubezh. Élektron 3 (1978) 76, In Russian. [7] A. Kirillov, Kinetics of Electron-Excited and Vibration-Excited Molecules in Disturbed Atmosphere (Ph.D. Phys.-Math. Sc. dissertation), Apatity, 2013. [8] J. Amorim, IEEE Trans. Plasma sci. 33 (2) (2005). [9] Y. Akishev, M. Grushin, V. Karalnik, A. Petryakov, N. Trushkin, Plasma Phys. 33 (9) (2007) 828. [10] J. Levaton, J. Amorim, A. Souza, D. Franco, A. Ricard, J. Phys. D: Appl. Phys. 35 (2002) 689. [11] Et. Essebbar, Y. Benilan, A. Jolly, M.-C. Gazeau, J. Phys. D: Appl. Phys. 42 (2009) 135206 11pp. [12] D. Burnette, A. Montello, I.V. Adamovich, W.R. Lempert, Plasma Sources Sci. Technol. 23 (4) (2014) 045007. [13] Chenrui Jing, et al., Laser Phys. Lett. 12 (1) (2015) 015301. [14] D. Kartashov, et al., J. Phys B: At. Mol. Opt. Phys. 48 (9) (2015) 094016. [15] S. Sarikhani, A. Hariri, J. Opt. 15 (5) (2013) 055705. [16] A. Razhev, V. Mkhitaryan, D. Churkin, JETP Lett. 82 (5) (2005) 259. [17] A. Razhev, D. Churkin, JETP Lett. 86 (6) (2007) 420. [18] A. Razhev, D. Churkin, Opt. Commun. 282 (2009) 1354. [19] A. Razhev, D. Churkin, A. Zavyalov, Vestnik NSU Ser. Fiz. 4 (3) (2009) 12, in Russian. [20] A. Razhev, D. Churkin, E. Kargapol’tsev, Laser Phys. Lett. 7 (10) (2013) 075002 (4pp). [21] A. Razhev, D. Churkin, A. Zhupikov, Quantum Electron. 39 (10) (2009) 901. [22] M. Pillow, A. Rogers, Proc. Phys. Soc. 81 (1963) 1034. [23] J. Berkowitz, W. Chupka, G. Kistiakowsky, J. Chem. Phys. 25 (3) (1956) 457. [24] K. Bayes, G. Kistiakowsky, J. Chem. Phys. 32 (4) (1960) 992. [25] I. Campbell, B. Thrush, Proc. R. Soc. A 296 (1967) 201. [26] S. Benson, J. Chem. Phys. 48 (4) (1968) 1765. [27] R. Brown, J. Chem. Phys. 52 (9) (1970) 4604. [28] G. Hays, H. Oskam, J. Chem. Phys. 59 (3) (1973) 1507. [29] R.B. Piejak, V.G. Godyak, B.M. Alexandrovich, Plasma Sources Sci. Technol. 1 (1992) 179. [30] V. Ischenko, V. Lisitsyn, A. Razhev, S. Rautian, A. Shalagin, JETP Lett. 19 (11) (1974) 669.
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Invited Paper
Characteristics of the inductive nitrogen laser generation A.M. Razhev a,b, D.S. Churkin a,c,n, E.S. Kargapoltsev a a b c
Institute of Laser Physics SB RAS, Ac. Lavrentyev’s, prosp. 15B, 630090 Novosibirsk, Russia NSTU, K. Marx prosp., 20, 630073 Novosibirsk, Russia NSU, Pirogova str., 2, 630090 Novosibirsk, Russia
art ic l e i nf o
a b s t r a c t
Article history: Received 22 September 2015 Received in revised form 19 January 2016 Accepted 22 January 2016 Available online 4 February 2016
The results of the experimental study of energy, temporal, spectral and spatial characteristics of UV inductive laser generation are presented. The study has identified a number of characteristics which demonstrate the differences between electron parameters of inductively coupled plasma and the plasma of longitudinal and transverse electrical discharges. The mechanism of simultaneous occurrence of Le+ wis-Rayleigh afterglow representing transitions between higher vibrational substates of B3Πg and A3 ∑u states; laser generation at C3Πu-B3Πg transition as well as the absence of IR radiation at 1st positive system typical for electrical discharge nitrogen lasers has been thoroughly researched. The major characteristic is ring shaped laser beam which size and width depend on excitation conditions. Inductive UV nitrogen laser is found to operate in ASE regime, but has a low divergence of 0.47 0.1 mrad and high pulse-to-pulse stability (laser pulse deviation amplitude did not exceed 1%). & 2016 Elsevier B.V. All rights reserved.
Keywords: Pulsed inductive discharge Nitrogen laser Lewis–Rayleigh afterglow Ring laser beam shape High pulse-to-pulse stability Low divergence
1. Introduction The various use of nitrogen laser as a source of high-power UV radiation is of great interest [1–5]. The most prevalent nitrogen lasers are the lasers with pumping by longitudinal and transverse pulse electrical discharges. The generation energy of nitrogen lasers with pumping by longitudinal discharge usually does not exceed fractions of mJ and pulse duration at half maximum is not more than several ns. Such lasers may act with pulse repetition frequency up to several hundred Hz [1]. When nitrogen is excited by high-current transverse electrical discharge the generation energy can reach 100 mJ [2] and pulse repetition frequency – 11 kHz [3]. Pulse generation duration with the use of transverse discharge is also limited by several ns at half maximum. To increase pulse duration in that case special measures are required [4]. At the same time the efficiency of nitrogen lasers UV radiation does not exceed 0.2% [5] that limits the use of these lasers, especially after appearance of more efficient excimer lasers. UV nitrogen laser is considered to be well-studied [6]. However, nitrogen as an active medium has quite a number of characteristics which make it a subject of a great scientific interest [7–15]. We have previously suggested the use of pulse inductive discharge as a n Corresponding author at: Institute of Laser Physics SB RAS, Ac. Lavrentyev’s, prosp. 15B, 630090 Novosibirsk, Russia. E-mail addresses: [email protected] (A.M. Razhev), [email protected] (D.S. Churkin), [email protected] (E.S. Kargapoltsev).
http://dx.doi.org/10.1016/j.optcom.2016.01.060 0030-4018/& 2016 Elsevier B.V. All rights reserved.
new alternative method of gas laser active media pumping. Several inductive lasers operating at electronic and vibrational–rotational transitions of gas atoms and molecules at visible and infrared spectrum including UV laser at transitions of nitrogen molecules 2nd positive band system were generated [16–21]. Even during the first experiments we noticed a number of differences in generation characteristics between inductive nitrogen laser and nitrogen lasers pumped with conventional longitudinal and transverse electrical discharges. One of the most marked differences was the absence of IR bands associated with 1st positive band system transitions of molecular nitrogen in the spectrum of spontaneous and laser radiation. At the same time while operating the laser, slow decaying glow in visible spectrum (530–700 nm) also known as Lewis–Rayleigh afterglow was observed. Lewis–Rayleigh afterglow has been known for over 100 years, but is still of scientific and practical interest for investigation of active nitrogen properties along with short-lived pink afterglow [7–15,21–28]. The glow occurs as a result of transitions between + higher vibrational substates of B3Πg and A3 ∑u electron levels of first positive band system of molecular nitrogen. The glow is characterized by long duration which may last over ten seconds (up to few minutes) [24]. There is sufficiently big number of theoretical works on glow formation and existence mechanisms [7,21–28]. In general, these works describe energy redistribution between excited electron levels not paying much attention to the initial excitation of nitrogen molecules from the ground state. Therefore, all the above mentioned investigations do not focus on
A.M. Razhev et al. / Optics Communications 367 (2016) 244–248
population inversion at transitions of nitrogen molecules 2nd positive band system and connection of UV laser generation with Lewis–Rayleigh afterglow. We believe that occurrence of this glow with population inversion and UV laser generation at transition of C3Πu-B3Πg demonstrates the differences in plasma electron parameters of pulsed inductive discharge versus longitudinal and transverse electrical discharges. The purpose of this study is to demonstrate the above mentioned differences based on energy, spatial, timing and spectralresponse characteristics of inductive nitrogen laser radiation. Based on the obtained results and further studies of the processes in the pulsed inductive discharge plasma as an active medium of nitrogen laser, a kinetic model of the inductive nitrogen laser will be created. This model will include the processes of molecular excitation from the ground state, population inversion at transitions of 2nd positive band system and mechanisms of the following energy relaxation resulting in Lewis-Rayleigh afterglow instead of generation on IR transitions of 1st positive system. Ultimate goal is to create a tailored pulsed inductively coupled plasma with specified characteristics (electron energy and their density, excitation specific capacity W ¼UI/V {MW/cm3}, U {kV} and I {kA} – discharge gap voltage and discharge current, V {cm3} – active volume) for excitation of random active laser medium including excimer one.
245
2. Experimental setup and measurements Pulsed inductive discharge in gas was generated by high-voltage excitation system developed on the basis of well-known Blumlein scheme [18,20]. Fig. 1 shows its electrical circuit. In several experiments, С–С scheme of recharging with railgap [21] instead of Blumlein scheme was applied. In the experiments, ceramic discharge tubes DT (Fig. 1) with outside diameter of 40 and 50 mm and inside diameter of 32 and 42 mm, respectively, were used. The length of the tubes was 800 mm. The tubes were sealed by parallel plate windows W1 and W2 made of quartz or MgF2, and placed perpendicularly to its axis. An optical resonator was formed by external flat dielectric mirrors М1 and М2. Rear mirror М1 had a reflection coefficient R1 ¼99% in the spectrum of 300–380 nm. A reflection coefficient of the output mirror М2 was adjusted in the experiment to achieve maximum generation energy. Mirrors with 8–93% reflection coefficient of have been used. Nitrogen or its mixture with other gases under the pressure ranged from 0.1 Torr to 100 Torr was delivered into the tube from the gas system. In the experiment, a longitudinal gas circulation was applied. Spontaneous and laser radiation of pulsed inductive nitrogen discharge as well as laser generation spectrum was registered by SOLAR LS S-150 spectrometer with 0.66 nm resolution in the spectral range of 200–1100 nm. Laser radiation energy was measured by OPHIR Optronics PE50–BB pyroelectric detector. Temporal parameters of electrical pulses were registered by Р6015А high-voltage tester and Tektronix TDS-2024 oscillograph with 200 MHz band. FEK–22 coaxial photocell with resolution of 10 10 s as well as FD-24K photodetector were used to register temporal parameters of optical pulses.
3. Results and discussion
Fig. 1. An electrical drawing of high-voltage system of pulsed inductively coupled plasma.
During the experiments, we were able to generate pulsed inductive discharge in nitrogen only under pressure below 10 Torr. Molecular nitrogen spontaneous emission spectrum was registered in the range of 180–1100 nm. The study has shown that radiation is concentrated within the range of 190–700 nm (Fig. 2). Under relatively high pressure of 5–10 Torr, only bands of the 2nd positive system representing C3Πu-B3Πg transition have been found in the spectrum (0–0) – 337.1 nm and (0–1) – 357.7 nm bands had maximum intensity. When the pressure was reduced to 1.5–2.5 Torr, bands of 1st positive and 1st negative system as well
Fig. 2. Spectrum of spontaneous radiation Isp of pulsed inductive discharge. Peak intensity of 337,1 nm and 357,7 nm were decreased 10 times.
246
A.M. Razhev et al. / Optics Communications 367 (2016) 244–248
Fig. 3. Oscillograph record of Lewis–Rayleigh afterglow decay IL–R (registered in the spectrum of 530–700 nm). Diameter of the discharge tube is 50 mm.
as Vegard–Kaplan system A3 ∑u – X1 ∑g occurred in the spectrum. Instead of well-known IR bands (800–1200 nm) of 1st positive system in electric discharge nitrogen lasers, we registered the radiation of 1st positive band in green-red region. This radiation is called “Lewis–Rayleigh afterglow”. It reflects the transitions between higher vibrational substates (ν′, ν″ ¼ 6–15) of B3Πg and + A3 ∑u states. Its intensity and duration highly depended on pumping conditions and nitrogen pressure in the discharge tube. Fig. 3 shows the process of radiation decay for various values of charging voltage under nitrogen total pressure of 1–2 Torr (diameter of discharge tube is 50 mm). In these experiments, decay time was up to one second. When the discharge tube of 50 mm diameter was replaced by the tube with less diameter (40 mm) decay time decreased in 1.5– 2 times. Moreover, in one of the experiments, high-voltage switch – TPI1-10k/50 thyratron was substituted by self-made triggered discharger. In this case, inductor oscillation of voltage decreased from 300 to 150 ns due to reduced integral inductor. This resulted in increase of inductive nitrogen laser pumping intensity and, as a consequence, energy generation (λ ¼337.1 nm) and total laser efficiency growth. However, these effects were accompanied by the abrupt decrease of the intensity and decay time of Lewis–Rayleigh afterglow (2–5 times depending on the tube diameter). At the same time, as it is shown in Fig. 4, correlation pattern of recombination radiation intensity with pressure was similar to laser generation energy. Therefore, the conclusion can be made that highest efficiency of Lewis-Rayleigh afterglow generation is reached under the same conditions as population inversion at the C3Πu-B3Πg transition. The mechanism of Lewis-Rayleigh afterglow generation during population inversion at transition C3Πu-B3Πg of the second positive band system of nitrogen molecules with pulse inductive discharge pumping is not fully understood and requires further investigations. Based on the available experimental data, we can make the following assumption. Unlike longitudinal and transverse electrical discharges, inductive discharge is formed in the absence of electrodes; therefore, it does not have electron emission from cathode. The initial concentration of electrons occurs as a result of capacitive discharge appearing immediately before inductive discharge out of voltage between inductor turns [29]. However, this concentration of electrons may be insufficient for population +
+
Fig. 4. Dependence of Lewis–Rayleigh afterglow intensity IL–R (registered in the spectrum of 530–700 nm) and energy generation value E on nitrogen pressure. Diameter of the discharge tube is 50 mm. Laser pulse duration is 15 7 1 ns (FWHM).
inversion based on direct excitement of nitrogen molecule from the ground state by an electron impact and for reaching the generation regimen (ne ¼1014–1015 cm 3). Other electrons appear under the influence of inductive discharge when some of nitrogen molecules are ionized. In this case, nitrogen molecules ionization may be stepwise through the excitement of intermediate levels. It is also possible that besides electron excitation of operating laser level other higher levels get excited. Further, when nitrogen molecular ions recombinate, N2 molecules get to higher vibrational substates of B3Πg electron state and then according to schemes described in [7,21–28] create recombination radiation. The suggested consequence of the processes explains simultaneous occurrence of Lewis-Rayleigh afterglow and UV laser generation at the transition of C3Πu-B3Πg as well as presence of significant amount of bands of nitrogen molecular ion and absence of 1 þ system IR bands in the spectrum of both laser and spontaneous radiation. Study of laser generation temporal parameters found that pulse duration (FWHM) is relatively longer for nitrogen lasers and reaches 13–20 ns (Fig. 5). Pulse duration was 40 ns close to the base (level 0.1 from intensity maximum). It is important to note that such changes in pulse duration can be done in the same laser emitter. For this purpose, only reflection coefficient of the output mirror R should be changed (Fig. 6). In the experiments the optimal reflection coefficient R was 40%. The study of UV inductive N2 laser generation spatial characteristics demonstrated that the laser beam in cross-section has a ring shape. This special characteristic of radiation is explained by a hollow cylinder form of active medium. Ring diameter and thickness depended on the laser tube size and shape, excitation power and Q factor of resonator. For detailed studies we used diode array which was placed along ring diameter. Because the diode array length did not exceed 29 mm, the tube with 20 mm inner diameter and 800 mm length was used in these studies. As it was shown in the experiments (Fig. 7(a)), laser radiation intensity of was the lowest at the ring outer border, close to the inner surface of the discharge tube. The intensity of generation sharply increased towards the center of the ring, and reached its maximum at 1–1.5 mm from the outer ring border. Then it was decreasing to almost zero at 3–4 mm. Therefore, the width of the ring close to the base was approximately 3 mm. The width of the ring depended on Q factor of the resonator. It was minimal (usually, 2–3 mm) in the resonator with low Q factor in the
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Fig. 5. Oscillograph record of inductive nitrogen laser pulse. Diameter of the discharge tube is 50 mm. Output mirror reflection coefficient is 80%.
Fig. 7. Transverse profile of laser beam Ilas (from (a) to (c)) at 0.5 m, 3 m and 6 m distance from the output mirror. Discharge tube inner diameter is 20 mm, tube length is 800 mm.
Fig. 6. Dependence of generation energy E and pulse duration τ (FWHM) on output mirror reflection coefficient R of laser resonator.
absence of front mirror, in high-density resonator it was approximately 6–7 mm. The same results were observed for the tubes with 34 mm and 42 mm inner diameters, respectively. Experiments showed that laser radiation inside the beam had variable intensity and certain granular structure (it is clearly shown on the lower profile of the ring in Fig. 7(c)). This structure is random both in width and length of the ring and is not reproduced from pulse to pulse. This phenomenon is well known and described in the literature [30]. It was observed in lasers with high amplification coefficient (i.e. amplified spontaneous emission – ASE). This condition is observed in inductive nitrogen laser as well. The most contrast and clear granular generation structure was observed in laser in the absence of resonator. After mirrors have been installed the contrast decreased, and the ring profile became relatively smooth and changed from pulse to pulse insignificantly. We also studied the divergence of UV inductive N2 laser. Fig. 7 (from (a) to (c)) shows transverse profiles of laser beam at 0.5 m, 3 m and 6 m distance from output mirror. As the Fig. demonstrates, a beam profile is represented by two symmetrical to
optical axis peaks with sharp outlines near the output mirror. The farther from the output mirror, the peaks intensity decreased and the width increased. It was visible as an increase in ring width and reduction in picture contrast. We defined divergence as θ ¼ arctg((sL s0)/L) (L-distance from the output mirror, sL – a peak width at half maximum for L distance, s0 – peak width at half maximum near the output mirror). The results showed that divergence was 0.4 70.1 mrad. In our case it can be explained by the fact that an active medium of inductive nitrogen laser was long (approx. 700 mm), had relatively small transverse size of 4– 6 mm and existed in the open Fabry–Perot interferometer type resonator formed by two flat mirrors (with semi-transparent output mirror). Maximum generation energy in the experiments was 4.5 mJ and pulse duration was 15 71 ns (FWHM) that corresponded with pulse power of 300 kW. These parameters were observed only in nitrogen without addition of other gases and under lower pressure (1–2 Torr). On the contrary, when additions common for electric discharge nitrogen lasers (He, NF3 and SF6) were used, their increased concentration resulted in continuous reduction and following UV generation failure. In pulse-periodic regimen of laser (capacity of power system did not allow for the frequency higher than 10 Hz), light pulse deviation amplitude was no higher than 1% that indicates high consistency of inductive discharge. Such combination of active medium and N2 laser generation parameters of at 337.1 nm wavelength has been discovered for the first time and also indicates the differences in plasma electron parameters between pulse inductive and electrical discharge.
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4. Conclusion The study of UV inductive laser has identified a number of characteristics which suggest the differences between electronic parameters of inductively coupled plasma and the plasma of longitudinal and transverse electrical discharges. We suggested the explanation of the mechanism of simultaneous occurrence of Lewis-Rayleigh afterglow representing transitions between B3Πg + higher vibrational substates and A3 ∑u states; laser generation at C3Πu-B3Πg transition, as well as the absence of IR radiation at 1st positive system typical for electrical discharge nitrogen lasers. The major characteristic is ring shaped laser beam which size and width depend on excitation conditions. Inductive UV nitrogen laser was found to operate in ASE regime, but has a low divergence of 0.4 70.1 mrad and high pulse-to-pulse stability (laser pulse deviation amplitude did not exceed 1%).
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