- Email: [email protected]
Excitation characteristics of a wire-preionized, ultraviolet nitrogen laser
Volume 66, number
2.3
OPTICS
COMMUNICATIONS
I5 April 1988
EXCITATION CHARACTERISTICS NITROGEN LASER*
OF A WIRE-PREIONIZED,
A. DIAS TAVARES
‘, J.L. FABRIS ‘, and C.A. MASSONE
Jr., M. MULLER
ULTRAVOILET
Laser and Spectroscopy Laboratory, Physics Insfuure, Gnwerstdade Federal Flumirwnse. C‘P 100296. ,YuerPol, CEP 24210 RJ, Bra:11 Received
I2 November
1987
Experimental optimum conditions for maximum ionization efficiency are determined in an NZ UV laser (337.1 nm) by the analysis of ionization and excitation voltage pulse modifications. Temporal width of excitation voltage pulse and the delay between this pulse and the stimulated emission are compared with previous results, suggesting a possible explanation for the short laser pulse-width obtained
1. Introduction Generation of ultra-short dye laser pulse-widths ( = 1O- ” s) is a very interesting topic of research because of the wide field of applications it offers. In order to excite a dye by a distributed feedback technique, a nitrogen laser (337.1 nm) with high-power and short pulse-width as well as high pulse-to-pulse reproducibility is a convenient tool. In the development of NZ lasers for this purpose the ionization process is very important in order to apply the excitation pulse in optimized conditions. Generally, ionization processes are by pin arrangement [ 11, corona blade electrodes [ 2,3 1, DC [ 4 ] or pulsed [ 51 wire systems, and UV light generation [ 61 among several others. The above mentioned references generally analyse modifications in laser output characteristics due to preionization effects. It would be interesting to have also a corresponding view of electrical excitation condition behaviour. A study about the role of a SIC paper preionizer has recently been published [ 71 where voltage and current pulse behaviours were presented, with a correlation between excitation and laser output characteristics. * Work partially supported by FINEP, CNPq and TW.4S. ’ Fellow ofthe National Research Council of Brazil (CNPq)
140
The present work analyses excitation voltage and current pulse modifcations in an N2 laser under different conditions together with the time evolution behaviour of the electric field and discharge energy, in preionization and excitation processes. A careful choice of all these factors resulted in a very short laser pulse-width.
2. Experimental
apparatus and procedure
The excitation circuit used (without a preionization system) was similar to that of ref. [ 8 ] and is shown in fig. 1 (a), while in figs. 1 (b) and 1 (c) the same circuit is presented with a preionization system similar to that of ref. [ 5 1. These last two circuits differ in polarities between preionization wires and one of the main discharge electrodes. Preionization wires were made of stainless steel of 0.3 mm in diameter. The discharge channel was 30 cm in length and the distance between the main electrodes (d) was 3 mm. Charging capacitance (C) and transmission line (C’ ) were both 10 nF, while preionization capacitance (C”) was 0.5 nF or 2.5 nF. The distance between ionization wires and one of the main electrodes (d’ ) - on a plane perpendicular to that determined by d - was 6, 4 or 2 mm. The charging voltage was kept constant at 10 kV DC. A 6.4 pH, 3.8 uH or 3.1 uH 0 030-401 S/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
Volume 66, number
2,3
OPTICS
COMMUNICATIONS
a
15 April 1988
3. Results Fig. 2 shows current (upper trace), voltage (middle trace) and laser (lower trace) pulses for the laser model A, fig. lc circuit, d’ = 2 mm, C” = 2.5 nF and 150 mb Nz pressure. From its current and voltage behaviours it is clearly seen that almost all energy is applied during the second voltage pulse. The laser radiation appears in this second voltage pulse (the time interval between corresponding peaks varies from 1 ns to 10 ns for all experimental tested conditions). Therefore this second pulse is considered to be responsible for excitation (the first one being the ionization process generator). For that reason an analysis has been carried out about how the relative voltage pulse amplitude is modified with the gas pressure in the laser discharge tube. The analysis has been made for the three laser models mentioned before (A, B and C), and for the two circuits shown respectively in figs. 1 (b) and 1 (c) with a complete set of possible combination values of d’ and C”. A typical result is shown in fig. 3 (laser model A, fig. 1 (b) circuit, d’ = 6 mm, C” = 2.5 nF) where at low pressures (below 150 mb) the first voltage pulse amplitude is bigger than the second pulse, while the situation is inverted as the pressure increases. On account of the typical results shown in fig. 2, the analysis has been concentrated on the electric field distribution (voltage pulse amplitudes ) since almost
Fig. 1. (a) Excitation circuit without preionization. (b), (c) Excitation circuit with preionization arrangement, for two different polarities between stainless steel wires and one of the main electrodes. C= C’ = 10 nF, L= 6.4 pH, 3.8 pH or 3. I FH, SG = sparkgap, LT = discharge tube, d= 3 mm, d’ = 6, 4 or 2 mm, C” = 0.5 nF or 2.5 nF.
inductance was used characterizing the laser model A, B and C, respectively. Laser emission was detected with a 1850 ITL vacuum photodiode and a 7 104 Tektronix oscilloscope. Voltage and current pulse measurements were made with an RFT, HTR-2 model (voltage) and HP 1110 AC (current) probes plus a 7904 Tektronix oscilloscope. Fig. 2. Current (upper trace), voltage (middle trace) and laser (lower trace) pulses for the laser model A, fig. 1(c) circuit, d’ =2 mm and C” ~2.5 nF, at 150 mb gas pressure.
141
Volume 66. number
SC
OPTICS
2.3
RELATIVE
PEAK
VOLTAGE
100
;50
COMMUNICATIONS
15 April 1988
BC
70
60
50
40
30
2c
IO
50
2 00
2 50
Ftg. 3. Relative peak voltage pulse behaviour for ionization model A, fig. lb circutt. d’ = 6 mm, C” = 2.5 nF.
30
--zo
400
450
N
2
PRESSURE(mbj
( I’<.)pulses as a function of gas pressure, for the laser
and for the laser model B, fig. 1(b) circuit, d’ = 4 mm, C” ~0.5 nF. From a complete set of fig. 4 type results, it appears that: (a) C” is not critical (differences in V,- V, are not very significant when changing only C” ). (b) For any d’ and/or C”, the maximum efficiency for the ionization process is always obtained
_---
290
3 50
( V,) and excitatron
all stored energy is always discharged with the second pulse (excitation pulse). It is also clear that an increase in ionization efficiency would correspond to a reduction of first pulse amplitude when compared with the second one. Fig. 4 shows excitation ( Vc) minus ionization ( V,) peak voltages for the laser model C, fig. 1 (c) circuit, d’ =2 mm, C” =2.5 nF
I50
0
300
350
400
450 N2P~ESSURE
Imb)
Fig. 4. Variation of excitation peak voltage minus tomration peak voltage ( I;- I’,) as a function of N: pressure for the laser model C, and thelaser model B. fig. l(b) circuit, d’=4 mm and C” =0.5 nF (-----). fig. I (c) circuit. d’ =2 mm and C” ~2.5 nF (---)
142
Volume 66, number
2.3
OPTICS
COMMUNICATIONS
15 April 1988
4. Discussion 1 nd
H
a)
50mb
I n5 n
_Jo 9L b) 100
mb
-IL 07
c)200mb
Fig. 5. Laser pulse-width versus N: gas pressure for the highest value of the obtained V,- V, (laser model C, fig. 1(c) circuit, d’=2 mm, c”‘=2.5 nF).
for laser model C. This model corresponds to the lowest inductance in the circuit which means the highest discharge frequency. Clearly in this case the electrical pulse has the maximum breakdown capacity when applied to the gas volume. (c) Combining d’, C” and excitation circuit parameters, V,- V, can be strongly modified. In the present case it was possible to go from V,- Vi= - 15 (laser model A, fig. 1 (b ) circuit, d’ = 6 mm, C” = 2.5 nF) up to I’c-Vi =55 (laser model C, d’=2 mm, C” =2.5 nF). Fig. 5 presents the laser pulse-width as a function of gas pressure for the highest obtained value V, - Vi mentioned before. The laser pulse-width reduces as the Nz pressure increases, being 0.7 ns in the best possible conditions.
In gas laser systems with a preionization device the first point to be usually considered is if ionization is generated or not by UV light emisssion. In the present case, and because of fig. 2 type results, UV light emission can be in a first approximation disregarded as major responsible for the ionization process. That comes our from the fact that a very reduced current is detected during the first voltage pulse, resulting in almost no light emission between the first and second voltage pulses. Hence, the preionization process is basically attributed to electric-field effect. Figs. 3 and 4 allow us (a) to obtain the distribution function between excitation and ionization voltage pulses in connection with gas pressure, and (b) to establish the set of values for d’, C” and the excitation circuit to give highest efficiency in preionization operation. Figs. 3 and 4 together with fig. 5 make it possible to detect the influence of (a) a high impedance at low pressure for NZ laser operation, resulting in a distorted electrical pulse when gas breakdown is obtained, giving a broader laser pulse, (b) a collisional process which reduces the available time for N2 inversion population as pressure increases, in accordance with the life-time of the C ‘II” state [ 9 1, and (c) a combination of (a) and (b). Further work is necessary at this point for a definitive answer. The temporal delay between the excitation pulse and peak laser power is in the 1 to 10 ns range. Serafetinides et al. [ 71 reported a value of 20 ns for the interval. They also detected a mean value of 50 ns for the excitation voltage pulse-width. It was 17 ns in our case. Both differences probably influence and justify the very short laser pulse-width obtained (0.7 ns) which shows the importance of excitation pulse characteristics on the NZ laser emission processes
[lOI.
5. Conclusion In this work an efficient ionization is obtained in an Nz laser by a careful adjustment of ionization and excitation voltage pulses. Results have been compared with some other previous works concerning the excitation pulse-width and the temporal delay 143
Volume 66. number
2.3
OPTICS
between excitation pulse and laser emission very short laser pulse-width obtained.
COMMUNICATIONS
with the
Acknowledgments The authors wish to thank CEPEL (EletrobrBs) for making available the 7 104 Tektronix oscilloscope and R. Francke (UFRGS) AND J. Mendes F” (UFCe) for their collaboration.
References [ I ] A. Rothem 227.
144
and S. Rosenwacks.
Optics Comm.
30
( 1979)
I 5 April 1988
[2] V. Hasson and H.M. van Bergmann, Rev. Sci. Instru. 50 (1979) 59. [3] H.M. van Bergmann and V. Hasson. J. Phys. E. I1 ( 1978) 2341. [4] U. Rebhan. J. Hildebrandt and G. Skopp, Appl. Phys. 23 (1980) 341. [ 51C.H. Brito Cruz. V. Loureiro, A. Dias Tavares Jr. and 4. Scalabrin. Appl. Phys. B 35 ( 1984) 13 1. [6] T. Efthimiopoulos and Ch. Bacharides. Opt. Engineer. 25 (1986) 1055. [7] A.A. Serafetinides, A.D. Papadoupoulos and K.R. Rickwood, Optics Comm. 53 ( 1987) 264. [8] R. Polloni, Opt. Quant. Elect. Lett. 8 ( 1976) 565. [ 91 B.E. Cherrington, Gaseous electtronics and gas lasers (Pergamon Press, 1979). [ IO] C.S. Willet, Introduction to gas lasers: population inversion mechanisms (Pergamon Press. 1974)
2.3
OPTICS
COMMUNICATIONS
I5 April 1988
EXCITATION CHARACTERISTICS NITROGEN LASER*
OF A WIRE-PREIONIZED,
A. DIAS TAVARES
‘, J.L. FABRIS ‘, and C.A. MASSONE
Jr., M. MULLER
ULTRAVOILET
Laser and Spectroscopy Laboratory, Physics Insfuure, Gnwerstdade Federal Flumirwnse. C‘P 100296. ,YuerPol, CEP 24210 RJ, Bra:11 Received
I2 November
1987
Experimental optimum conditions for maximum ionization efficiency are determined in an NZ UV laser (337.1 nm) by the analysis of ionization and excitation voltage pulse modifications. Temporal width of excitation voltage pulse and the delay between this pulse and the stimulated emission are compared with previous results, suggesting a possible explanation for the short laser pulse-width obtained
1. Introduction Generation of ultra-short dye laser pulse-widths ( = 1O- ” s) is a very interesting topic of research because of the wide field of applications it offers. In order to excite a dye by a distributed feedback technique, a nitrogen laser (337.1 nm) with high-power and short pulse-width as well as high pulse-to-pulse reproducibility is a convenient tool. In the development of NZ lasers for this purpose the ionization process is very important in order to apply the excitation pulse in optimized conditions. Generally, ionization processes are by pin arrangement [ 11, corona blade electrodes [ 2,3 1, DC [ 4 ] or pulsed [ 51 wire systems, and UV light generation [ 61 among several others. The above mentioned references generally analyse modifications in laser output characteristics due to preionization effects. It would be interesting to have also a corresponding view of electrical excitation condition behaviour. A study about the role of a SIC paper preionizer has recently been published [ 71 where voltage and current pulse behaviours were presented, with a correlation between excitation and laser output characteristics. * Work partially supported by FINEP, CNPq and TW.4S. ’ Fellow ofthe National Research Council of Brazil (CNPq)
140
The present work analyses excitation voltage and current pulse modifcations in an N2 laser under different conditions together with the time evolution behaviour of the electric field and discharge energy, in preionization and excitation processes. A careful choice of all these factors resulted in a very short laser pulse-width.
2. Experimental
apparatus and procedure
The excitation circuit used (without a preionization system) was similar to that of ref. [ 8 ] and is shown in fig. 1 (a), while in figs. 1 (b) and 1 (c) the same circuit is presented with a preionization system similar to that of ref. [ 5 1. These last two circuits differ in polarities between preionization wires and one of the main discharge electrodes. Preionization wires were made of stainless steel of 0.3 mm in diameter. The discharge channel was 30 cm in length and the distance between the main electrodes (d) was 3 mm. Charging capacitance (C) and transmission line (C’ ) were both 10 nF, while preionization capacitance (C”) was 0.5 nF or 2.5 nF. The distance between ionization wires and one of the main electrodes (d’ ) - on a plane perpendicular to that determined by d - was 6, 4 or 2 mm. The charging voltage was kept constant at 10 kV DC. A 6.4 pH, 3.8 uH or 3.1 uH 0 030-401 S/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
Volume 66, number
2,3
OPTICS
COMMUNICATIONS
a
15 April 1988
3. Results Fig. 2 shows current (upper trace), voltage (middle trace) and laser (lower trace) pulses for the laser model A, fig. lc circuit, d’ = 2 mm, C” = 2.5 nF and 150 mb Nz pressure. From its current and voltage behaviours it is clearly seen that almost all energy is applied during the second voltage pulse. The laser radiation appears in this second voltage pulse (the time interval between corresponding peaks varies from 1 ns to 10 ns for all experimental tested conditions). Therefore this second pulse is considered to be responsible for excitation (the first one being the ionization process generator). For that reason an analysis has been carried out about how the relative voltage pulse amplitude is modified with the gas pressure in the laser discharge tube. The analysis has been made for the three laser models mentioned before (A, B and C), and for the two circuits shown respectively in figs. 1 (b) and 1 (c) with a complete set of possible combination values of d’ and C”. A typical result is shown in fig. 3 (laser model A, fig. 1 (b) circuit, d’ = 6 mm, C” = 2.5 nF) where at low pressures (below 150 mb) the first voltage pulse amplitude is bigger than the second pulse, while the situation is inverted as the pressure increases. On account of the typical results shown in fig. 2, the analysis has been concentrated on the electric field distribution (voltage pulse amplitudes ) since almost
Fig. 1. (a) Excitation circuit without preionization. (b), (c) Excitation circuit with preionization arrangement, for two different polarities between stainless steel wires and one of the main electrodes. C= C’ = 10 nF, L= 6.4 pH, 3.8 pH or 3. I FH, SG = sparkgap, LT = discharge tube, d= 3 mm, d’ = 6, 4 or 2 mm, C” = 0.5 nF or 2.5 nF.
inductance was used characterizing the laser model A, B and C, respectively. Laser emission was detected with a 1850 ITL vacuum photodiode and a 7 104 Tektronix oscilloscope. Voltage and current pulse measurements were made with an RFT, HTR-2 model (voltage) and HP 1110 AC (current) probes plus a 7904 Tektronix oscilloscope. Fig. 2. Current (upper trace), voltage (middle trace) and laser (lower trace) pulses for the laser model A, fig. 1(c) circuit, d’ =2 mm and C” ~2.5 nF, at 150 mb gas pressure.
141
Volume 66. number
SC
OPTICS
2.3
RELATIVE
PEAK
VOLTAGE
100
;50
COMMUNICATIONS
15 April 1988
BC
70
60
50
40
30
2c
IO
50
2 00
2 50
Ftg. 3. Relative peak voltage pulse behaviour for ionization model A, fig. lb circutt. d’ = 6 mm, C” = 2.5 nF.
30
--zo
400
450
N
2
PRESSURE(mbj
( I’<.)pulses as a function of gas pressure, for the laser
and for the laser model B, fig. 1(b) circuit, d’ = 4 mm, C” ~0.5 nF. From a complete set of fig. 4 type results, it appears that: (a) C” is not critical (differences in V,- V, are not very significant when changing only C” ). (b) For any d’ and/or C”, the maximum efficiency for the ionization process is always obtained
_---
290
3 50
( V,) and excitatron
all stored energy is always discharged with the second pulse (excitation pulse). It is also clear that an increase in ionization efficiency would correspond to a reduction of first pulse amplitude when compared with the second one. Fig. 4 shows excitation ( Vc) minus ionization ( V,) peak voltages for the laser model C, fig. 1 (c) circuit, d’ =2 mm, C” =2.5 nF
I50
0
300
350
400
450 N2P~ESSURE
Imb)
Fig. 4. Variation of excitation peak voltage minus tomration peak voltage ( I;- I’,) as a function of N: pressure for the laser model C, and thelaser model B. fig. l(b) circuit, d’=4 mm and C” =0.5 nF (-----). fig. I (c) circuit. d’ =2 mm and C” ~2.5 nF (---)
142
Volume 66, number
2.3
OPTICS
COMMUNICATIONS
15 April 1988
4. Discussion 1 nd
H
a)
50mb
I n5 n
_Jo 9L b) 100
mb
-IL 07
c)200mb
Fig. 5. Laser pulse-width versus N: gas pressure for the highest value of the obtained V,- V, (laser model C, fig. 1(c) circuit, d’=2 mm, c”‘=2.5 nF).
for laser model C. This model corresponds to the lowest inductance in the circuit which means the highest discharge frequency. Clearly in this case the electrical pulse has the maximum breakdown capacity when applied to the gas volume. (c) Combining d’, C” and excitation circuit parameters, V,- V, can be strongly modified. In the present case it was possible to go from V,- Vi= - 15 (laser model A, fig. 1 (b ) circuit, d’ = 6 mm, C” = 2.5 nF) up to I’c-Vi =55 (laser model C, d’=2 mm, C” =2.5 nF). Fig. 5 presents the laser pulse-width as a function of gas pressure for the highest obtained value V, - Vi mentioned before. The laser pulse-width reduces as the Nz pressure increases, being 0.7 ns in the best possible conditions.
In gas laser systems with a preionization device the first point to be usually considered is if ionization is generated or not by UV light emisssion. In the present case, and because of fig. 2 type results, UV light emission can be in a first approximation disregarded as major responsible for the ionization process. That comes our from the fact that a very reduced current is detected during the first voltage pulse, resulting in almost no light emission between the first and second voltage pulses. Hence, the preionization process is basically attributed to electric-field effect. Figs. 3 and 4 allow us (a) to obtain the distribution function between excitation and ionization voltage pulses in connection with gas pressure, and (b) to establish the set of values for d’, C” and the excitation circuit to give highest efficiency in preionization operation. Figs. 3 and 4 together with fig. 5 make it possible to detect the influence of (a) a high impedance at low pressure for NZ laser operation, resulting in a distorted electrical pulse when gas breakdown is obtained, giving a broader laser pulse, (b) a collisional process which reduces the available time for N2 inversion population as pressure increases, in accordance with the life-time of the C ‘II” state [ 9 1, and (c) a combination of (a) and (b). Further work is necessary at this point for a definitive answer. The temporal delay between the excitation pulse and peak laser power is in the 1 to 10 ns range. Serafetinides et al. [ 71 reported a value of 20 ns for the interval. They also detected a mean value of 50 ns for the excitation voltage pulse-width. It was 17 ns in our case. Both differences probably influence and justify the very short laser pulse-width obtained (0.7 ns) which shows the importance of excitation pulse characteristics on the NZ laser emission processes
[lOI.
5. Conclusion In this work an efficient ionization is obtained in an Nz laser by a careful adjustment of ionization and excitation voltage pulses. Results have been compared with some other previous works concerning the excitation pulse-width and the temporal delay 143
Volume 66. number
2.3
OPTICS
between excitation pulse and laser emission very short laser pulse-width obtained.
COMMUNICATIONS
with the
Acknowledgments The authors wish to thank CEPEL (EletrobrBs) for making available the 7 104 Tektronix oscilloscope and R. Francke (UFRGS) AND J. Mendes F” (UFCe) for their collaboration.
References [ I ] A. Rothem 227.
144
and S. Rosenwacks.
Optics Comm.
30
( 1979)
I 5 April 1988
[2] V. Hasson and H.M. van Bergmann, Rev. Sci. Instru. 50 (1979) 59. [3] H.M. van Bergmann and V. Hasson. J. Phys. E. I1 ( 1978) 2341. [4] U. Rebhan. J. Hildebrandt and G. Skopp, Appl. Phys. 23 (1980) 341. [ 51C.H. Brito Cruz. V. Loureiro, A. Dias Tavares Jr. and 4. Scalabrin. Appl. Phys. B 35 ( 1984) 13 1. [6] T. Efthimiopoulos and Ch. Bacharides. Opt. Engineer. 25 (1986) 1055. [7] A.A. Serafetinides, A.D. Papadoupoulos and K.R. Rickwood, Optics Comm. 53 ( 1987) 264. [8] R. Polloni, Opt. Quant. Elect. Lett. 8 ( 1976) 565. [ 91 B.E. Cherrington, Gaseous electtronics and gas lasers (Pergamon Press, 1979). [ IO] C.S. Willet, Introduction to gas lasers: population inversion mechanisms (Pergamon Press. 1974)