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Experimental investigation of a multichannel N2-laser
Volume 26, number 3
OPTICS COMMUNICATIONS
September 1978
EXPERIMENTAL INVESTIGATION OF A MULTICHANNEL N2-LASER M. HUGENSCHMIDT and K. VOLLRATH Deutsch-Franz6sisch es Forschungsinstitut Saint-Louis, France
Received 1 June 1978 A multichannel UV-laser is described which provides a flexible high repetition rate pulsed light source for the generation of short duration light pulses.
1. Introduction Low inductance transverse discharges of transmission lines are often used to obtain population inversion and high gain factors in nitrogen, hydrogen, carbonmonoxide or rare-gas-halides [ 1 - 7 ] . In N2-1asers at pressures up to several bars, these discharges yield short duration light pulses down to some tens of ps at k = 3371 A [ 8 - 1 3 ] . Our investigations deal with a novel multichannel transverse discharge system delivering a train of high repetition rate pulses at frequencies of 50 to 100 MHz. Preliminary experiments have been performed with nitrogen at atmospheric pressure as a filling gas, so that the half-widths of the individual pulses are of the order of 800 ps to 1 ns. Peak powers are about some hundreds of kW and pulse energies are ranging from 300 to 400/aJ. A new type of preionization ensures stable operating conditions, reproducibility and low jitter so that the multichannel system provides a useful tool for a large number of laser diagnostic applications in the field of high-speed cinematography and spectroscopy.
2. Experimental set up The basic element of the multichannel laser system is a parallel-plate transmission line of the Blumlein type as indicated in fig. 1. In our experiments we use a linear array of five discharge gaps, separated by the parallel transmission line plates, consisting of commercially available copper coated circuit boards. They form the different energy
~mL ¢oltege trigger pulse
trtgffer
~A ¢ Ti
/
~ I
Y I
¢
""
II
Fig. 1. Schematic arrangement of the multichannel laser. storing and transmitting capacitors. According to a capacitance of 5 nF, the application of high voltages up to 20 kV yields electrically stored energies up to 1 J in each section. From the inductivity per unit length of the line of 0.03 nH/cm results a characteristic impedance Z ~ x / ~ of approximately 0.45 ~2. This system is triggered by a low inductance pressurized spark gap which discharges the first transmission-line section, thus building up a high-voltage across the first laser channel. After achieving the breakdown voltage, a discharge is initiated along the 43 cm long gap which provides population inversion of the relevant C311u and B3Ilg states of the N2-molecules and simultaneously gives rise to the formation of a high-voltage drop across the second pair of electrodes. The same procedure holds for all subsequent stages of the multichannel system. Due to the inductivity of the triggering spark 415
Volume 26, number 3
OPTICS COMMUNICATIONS
(~ 25 nil), the high voltage risetimes across the laser channels are of the order of 10 to 20 ns. This confirms some results reported from single channel operation measurements by Schwab and Hollinger [14]. The time delay between two successive pulses in our device is therefore mainly determined by the breakdown voltage, depending upon the gas pressure and the electrode spacing and not primarily by the propagation time of the electromagnetic wave as might be expected at first. Therefore it is of the order of 10 to 20 ns. However, in spite of the slow slope of the electrical pulses, travelling wave excitation of the laser gas can be achieved by properly adjusting the electrode spacings along the gaps in a slightly nonparallel configuration [10]. The gas is introduced through small slits under the electrodes of the laser channels thus forming transverse flows.
3. Preionization and performance characteristics Experiments revealed that for stable multichannel operation, some kind of preionization is indispensable. Two groups of systems that have been reported in the literature for high-pressure single-channel Blumlein discharges are indicated in figs. 2a and 2b. Thereby stabilization of the discharges is achieved either by UVpreionization similar to that one used in TEA-CO 2laser studies [15,16] or by introducing so-called coronablades where the sharp edges of the blades are leading rt
mare dl~charge i " a ~ ~:~
F--~ ~
September 1978
to high electric field strengths, thus providing the extraction of free carriers. The latter concept, introduced by Bergmann, Hasson [11 13] et al. offers many advantges as a rugged low cost device. By this technique, however, satisfactory performance charactistics could not be obtained for the multichannel laser system. It was necessary to introduce the superposition of deglow discharges as a more efficient new type of preionization. A rather uniform discharge along each laser channel is obtained by a row of 40 resistors (200 Mg2) in parallel as indicated in figs. 1 and 2c. Best results are obtained by combining the two stabilization methods shows in figs. 2b and 2c (the de-glow discharge and the corona-blades). For single channel lasers this is clearly demonstrated in fig. 3. This shows some oscillograms (superpositions of 20 shots in each case) with the coronablades on the left band side and with the new combined method on the right side. The measurements have been done by using a Korad S-20 biplanar photodiode and a Tektronix 519 oscilloscope. As can clearly be seen, the new stabilization technique provides more reproducible laser pulses even at low charging voltages. The lasing threshold decreases considerably. Our preliminary measurements on singJechannel Blumlein discharges also revealed that the new method will considerably improve the laser performance for higher repetition rates (> 100 Hz) and as can be expected also for higher gas pressures. As already mentioned in the multichannel configuration it was only by this type of preionization that reproducibility of the intensities of the five individual
a)
UV emitting sparks
U V- premmzed
tS k V
discharge
U//////////////////////////M _3_ corona-blades
18 k V
b)
main electrodes
,.<.
fast corona-type
F/////
o f discharge
////////~/,////A ._g_
sustomer glowdl3charge "
~ ./¢~\
]-
j
I
K///////////////////////d
20
c)
dc - glow - d/scharge stebil:zed mare discharge
....1_
Fig. 2. Methods for the stabilization of transverse high-pressure discharges of parallel-plate transmission lines. 416
-..
-~,.
5~s/d/v
--~.-
kV
5 ns/d/v
c o r o n a - blade -
corona - blade - and de -
stabtlizatmn
glow
discharge stabihzation
Fig. 3. Single-channel emission for different types of preionization (superposition of 20 shots).
Volume 26, number 3
OPTICS COMMUNICATIONS
September 1978 multlcl~nnel UV-I~er
t'aser"
intensity
~
-
2 0 ns / div
Fig. 4. High repetition rate train of pulses. pulses and small values of temporal jitter could be achieved. Fig. 4 shows a series of pulses as observed with the same diode on the oscilloscope. At an operation voltage of 20.5 kV the jitter is less than -+ 1.5 ns. In fig. 4 ten successive shots are superimposed revealing the good performance characteristics. The energies of the individual pulses have been measured by means of a copper cone calorimeter to be 300 to 400/~J.
Fig. 5. Application of the multichannel laser for plasma diagnostics. phenomena are analyzed that are initiated by the laser beam or by a part of the laser beam which is split off,
target
target
-.w l ornw.-
-~Ic~ ,.-
4. Applications Due to their high repetition rates and short durations, the pulses are well suited for the investigation of rapid transient phenomena such as the formation and expansion of laser produced plasmas by high-speed photographic techniques. Experiments have been performed by using a five channel shadowgraph system to visualize the very early stages of the development of laser absorption waves generated by the absorption of a 20 J CO 2-laser pulse on a plexiglass-target, see fig. 5. The 10.6 tzm-radiation was focussed by means of a 60 cm focal-length KCl-lens thus providing energy-densities of about 100 J/cm 2 corresponding to peak powerdensities of 5 × 108 W/cm 2. The self-luminosity of the phenomenon is suppressed by suitably chosen narrowband interference filters. Optical image separation can also be obtained by using the well-known Cranz-Schardin principle. As compared to the stroboscopic methods with trains of mode-locked laser pulses [17], whereby mostly
very earlf stages of plasma formation
later sta~s of plasma ?ormat/on
A t . 2 O ns
Fig. 6. Temporal development and spatial evolution of CO2laser pulse induced plasmas. 417
Volnme 26, number 3
OPTICS COMMUNICATIONS
the multichannel laser, as described, can be triggered externally with high accuracy thus allowing for precise investigation of processes that are completely independent from the diagnostic laser beam. Furthermore, the different images are separated spatially without any loss of infonnation. For our appfications the pulses are short enough to avoid blurring of the photographs due to the high expansion velocities up to some 107 cm/s. Fig. 6 shows two series of photographs. On the left hand side, breakdown is seen to occur within the first 20 ns. An optically dense plasma is ejected from the target surface. Furthermore, breakdown-centers in the gas, a few centimeters in front of the target, are building up due to the high power densities. The subsequent shadowgrams are revealing the growth of these breakdown-centers in the following 60 ns. On the right, later stages of plasma formation are presented where different centers have already grown until they are intersecting thus forming the well-known nearly cylindrically shaped plasma jet which expands both laterally and even with higher velocities longitudinally towards the incoming laser beam.
5. Discussions and conclusions The multichannel UV-laser as described provides a very flexible high repetition rate pulsed light source for the generation of trains of short duration light pulses. By applying higher pressures o f the filling gas the pulse duration can be shortened down to the psrange [10]. As our experiments have shown, even larger numbers of discharges carl be operated. We have been limited to five channels due to the linear arrangement that has been chosen because the electrode gaps were easily accessible for optimization o f the electrode spacings and incorporation of additional electrodes for the preionization. However, more compact sys}ems can be built for example by folding the transmission lines. Furthermore, other gases can be applied as well
418
September 1978
such as tor example the raregas-halides. Of still more importance m a y be the use of the multichannel-system as a pumping source for dye-lasers. We have investigated a large number of dyes which in a simple form can be excited to emit in the superradiant mode thus requiring only one focusing lens and a dye-cell for each channel. By this way the wavelength range is extended over the large spectral range from the UV to the near IR. The authors wish to thank Mr. Wey and Mr. Baca for their generous cooperation and help in performing the experiments.
References [1] D.A. Leonard, Appl. Plays. Letters 7 (1965) 4. [2] C.P. Wnag, Rev. Sci. Instrum. 47 (1976) 92. [3] W.A. Fitzsimons, L.W. Anderson, C.E. Riedhauser and J.M. Vrtilek, IEEE J. of Quantum Electr. QE-12 (1976) 624. [4] J.D. Shipman Jr., Appl. Phys. Letters 10 (1967) 3. [5 ] R.W. Waynant, J.D. Shipman Jr., R.C. Elton and A.W. Ali, Proc. IEEE 59 (1971) 679. [6] J.P. Girardeau-Montaut, L'onde ~l~ctrique 54 (1974) 456. [7] D.G. Sutton, S.N. Suchard, O.L. Gibb and C.P. Wang, AppL Phys. Letters 28 (1976) 522. [8] tl. Salzmann and tt. Strohwald, Optics Commun. 12 (1974) 37O. [9] W. Herden, Phys. Letters 54A (1975) 96. [10] H. Strohwald and H. Salzmann, Appl. Phys. Letters 28 (1976) 272. [11 ] H.M. yon Bergmann, V. Hasson and D. Preussler, Appl. Phys. Letters 27 (1975) 553. [12] V. Hasson, H.M. von Bergmann and D. Preussler, Appl. Phys. Letters 28 (1976) 17. [13] E.E. Bergmann, Appl. Phys. Letters 28 (1976) 84. [14] A.J. Schwab and F.W. Hollinger, IEEE J. Quantum Electr. QE-12 (1976) 183. [15] V. Hasson, D. Preussler, J. Klimek and H.M. yon Bergmann, Appl. Phys. Letters 25 (1974) 1. [16] N.A. Kurnit, S.J. Tubbs, K. Bidhichand, L.W. Ryan Jr. and A. Javan, IEEE J. Quantum Electr. QE-I 1 (1975) 174. [17] K. Vollrath and M. Hugenschmidt, Proc. 12th Int. Congress on high-speed photography, ISL-Report CO 214/76.
OPTICS COMMUNICATIONS
September 1978
EXPERIMENTAL INVESTIGATION OF A MULTICHANNEL N2-LASER M. HUGENSCHMIDT and K. VOLLRATH Deutsch-Franz6sisch es Forschungsinstitut Saint-Louis, France
Received 1 June 1978 A multichannel UV-laser is described which provides a flexible high repetition rate pulsed light source for the generation of short duration light pulses.
1. Introduction Low inductance transverse discharges of transmission lines are often used to obtain population inversion and high gain factors in nitrogen, hydrogen, carbonmonoxide or rare-gas-halides [ 1 - 7 ] . In N2-1asers at pressures up to several bars, these discharges yield short duration light pulses down to some tens of ps at k = 3371 A [ 8 - 1 3 ] . Our investigations deal with a novel multichannel transverse discharge system delivering a train of high repetition rate pulses at frequencies of 50 to 100 MHz. Preliminary experiments have been performed with nitrogen at atmospheric pressure as a filling gas, so that the half-widths of the individual pulses are of the order of 800 ps to 1 ns. Peak powers are about some hundreds of kW and pulse energies are ranging from 300 to 400/aJ. A new type of preionization ensures stable operating conditions, reproducibility and low jitter so that the multichannel system provides a useful tool for a large number of laser diagnostic applications in the field of high-speed cinematography and spectroscopy.
2. Experimental set up The basic element of the multichannel laser system is a parallel-plate transmission line of the Blumlein type as indicated in fig. 1. In our experiments we use a linear array of five discharge gaps, separated by the parallel transmission line plates, consisting of commercially available copper coated circuit boards. They form the different energy
~mL ¢oltege trigger pulse
trtgffer
~A ¢ Ti
/
~ I
Y I
¢
""
II
Fig. 1. Schematic arrangement of the multichannel laser. storing and transmitting capacitors. According to a capacitance of 5 nF, the application of high voltages up to 20 kV yields electrically stored energies up to 1 J in each section. From the inductivity per unit length of the line of 0.03 nH/cm results a characteristic impedance Z ~ x / ~ of approximately 0.45 ~2. This system is triggered by a low inductance pressurized spark gap which discharges the first transmission-line section, thus building up a high-voltage across the first laser channel. After achieving the breakdown voltage, a discharge is initiated along the 43 cm long gap which provides population inversion of the relevant C311u and B3Ilg states of the N2-molecules and simultaneously gives rise to the formation of a high-voltage drop across the second pair of electrodes. The same procedure holds for all subsequent stages of the multichannel system. Due to the inductivity of the triggering spark 415
Volume 26, number 3
OPTICS COMMUNICATIONS
(~ 25 nil), the high voltage risetimes across the laser channels are of the order of 10 to 20 ns. This confirms some results reported from single channel operation measurements by Schwab and Hollinger [14]. The time delay between two successive pulses in our device is therefore mainly determined by the breakdown voltage, depending upon the gas pressure and the electrode spacing and not primarily by the propagation time of the electromagnetic wave as might be expected at first. Therefore it is of the order of 10 to 20 ns. However, in spite of the slow slope of the electrical pulses, travelling wave excitation of the laser gas can be achieved by properly adjusting the electrode spacings along the gaps in a slightly nonparallel configuration [10]. The gas is introduced through small slits under the electrodes of the laser channels thus forming transverse flows.
3. Preionization and performance characteristics Experiments revealed that for stable multichannel operation, some kind of preionization is indispensable. Two groups of systems that have been reported in the literature for high-pressure single-channel Blumlein discharges are indicated in figs. 2a and 2b. Thereby stabilization of the discharges is achieved either by UVpreionization similar to that one used in TEA-CO 2laser studies [15,16] or by introducing so-called coronablades where the sharp edges of the blades are leading rt
mare dl~charge i " a ~ ~:~
F--~ ~
September 1978
to high electric field strengths, thus providing the extraction of free carriers. The latter concept, introduced by Bergmann, Hasson [11 13] et al. offers many advantges as a rugged low cost device. By this technique, however, satisfactory performance charactistics could not be obtained for the multichannel laser system. It was necessary to introduce the superposition of deglow discharges as a more efficient new type of preionization. A rather uniform discharge along each laser channel is obtained by a row of 40 resistors (200 Mg2) in parallel as indicated in figs. 1 and 2c. Best results are obtained by combining the two stabilization methods shows in figs. 2b and 2c (the de-glow discharge and the corona-blades). For single channel lasers this is clearly demonstrated in fig. 3. This shows some oscillograms (superpositions of 20 shots in each case) with the coronablades on the left band side and with the new combined method on the right side. The measurements have been done by using a Korad S-20 biplanar photodiode and a Tektronix 519 oscilloscope. As can clearly be seen, the new stabilization technique provides more reproducible laser pulses even at low charging voltages. The lasing threshold decreases considerably. Our preliminary measurements on singJechannel Blumlein discharges also revealed that the new method will considerably improve the laser performance for higher repetition rates (> 100 Hz) and as can be expected also for higher gas pressures. As already mentioned in the multichannel configuration it was only by this type of preionization that reproducibility of the intensities of the five individual
a)
UV emitting sparks
U V- premmzed
tS k V
discharge
U//////////////////////////M _3_ corona-blades
18 k V
b)
main electrodes
,.<.
fast corona-type
F/////
o f discharge
////////~/,////A ._g_
sustomer glowdl3charge "
~ ./¢~\
]-
j
I
K///////////////////////d
20
c)
dc - glow - d/scharge stebil:zed mare discharge
....1_
Fig. 2. Methods for the stabilization of transverse high-pressure discharges of parallel-plate transmission lines. 416
-..
-~,.
5~s/d/v
--~.-
kV
5 ns/d/v
c o r o n a - blade -
corona - blade - and de -
stabtlizatmn
glow
discharge stabihzation
Fig. 3. Single-channel emission for different types of preionization (superposition of 20 shots).
Volume 26, number 3
OPTICS COMMUNICATIONS
September 1978 multlcl~nnel UV-I~er
t'aser"
intensity
~
-
2 0 ns / div
Fig. 4. High repetition rate train of pulses. pulses and small values of temporal jitter could be achieved. Fig. 4 shows a series of pulses as observed with the same diode on the oscilloscope. At an operation voltage of 20.5 kV the jitter is less than -+ 1.5 ns. In fig. 4 ten successive shots are superimposed revealing the good performance characteristics. The energies of the individual pulses have been measured by means of a copper cone calorimeter to be 300 to 400/~J.
Fig. 5. Application of the multichannel laser for plasma diagnostics. phenomena are analyzed that are initiated by the laser beam or by a part of the laser beam which is split off,
target
target
-.w l ornw.-
-~Ic~ ,.-
4. Applications Due to their high repetition rates and short durations, the pulses are well suited for the investigation of rapid transient phenomena such as the formation and expansion of laser produced plasmas by high-speed photographic techniques. Experiments have been performed by using a five channel shadowgraph system to visualize the very early stages of the development of laser absorption waves generated by the absorption of a 20 J CO 2-laser pulse on a plexiglass-target, see fig. 5. The 10.6 tzm-radiation was focussed by means of a 60 cm focal-length KCl-lens thus providing energy-densities of about 100 J/cm 2 corresponding to peak powerdensities of 5 × 108 W/cm 2. The self-luminosity of the phenomenon is suppressed by suitably chosen narrowband interference filters. Optical image separation can also be obtained by using the well-known Cranz-Schardin principle. As compared to the stroboscopic methods with trains of mode-locked laser pulses [17], whereby mostly
very earlf stages of plasma formation
later sta~s of plasma ?ormat/on
A t . 2 O ns
Fig. 6. Temporal development and spatial evolution of CO2laser pulse induced plasmas. 417
Volnme 26, number 3
OPTICS COMMUNICATIONS
the multichannel laser, as described, can be triggered externally with high accuracy thus allowing for precise investigation of processes that are completely independent from the diagnostic laser beam. Furthermore, the different images are separated spatially without any loss of infonnation. For our appfications the pulses are short enough to avoid blurring of the photographs due to the high expansion velocities up to some 107 cm/s. Fig. 6 shows two series of photographs. On the left hand side, breakdown is seen to occur within the first 20 ns. An optically dense plasma is ejected from the target surface. Furthermore, breakdown-centers in the gas, a few centimeters in front of the target, are building up due to the high power densities. The subsequent shadowgrams are revealing the growth of these breakdown-centers in the following 60 ns. On the right, later stages of plasma formation are presented where different centers have already grown until they are intersecting thus forming the well-known nearly cylindrically shaped plasma jet which expands both laterally and even with higher velocities longitudinally towards the incoming laser beam.
5. Discussions and conclusions The multichannel UV-laser as described provides a very flexible high repetition rate pulsed light source for the generation of trains of short duration light pulses. By applying higher pressures o f the filling gas the pulse duration can be shortened down to the psrange [10]. As our experiments have shown, even larger numbers of discharges carl be operated. We have been limited to five channels due to the linear arrangement that has been chosen because the electrode gaps were easily accessible for optimization o f the electrode spacings and incorporation of additional electrodes for the preionization. However, more compact sys}ems can be built for example by folding the transmission lines. Furthermore, other gases can be applied as well
418
September 1978
such as tor example the raregas-halides. Of still more importance m a y be the use of the multichannel-system as a pumping source for dye-lasers. We have investigated a large number of dyes which in a simple form can be excited to emit in the superradiant mode thus requiring only one focusing lens and a dye-cell for each channel. By this way the wavelength range is extended over the large spectral range from the UV to the near IR. The authors wish to thank Mr. Wey and Mr. Baca for their generous cooperation and help in performing the experiments.
References [1] D.A. Leonard, Appl. Plays. Letters 7 (1965) 4. [2] C.P. Wnag, Rev. Sci. Instrum. 47 (1976) 92. [3] W.A. Fitzsimons, L.W. Anderson, C.E. Riedhauser and J.M. Vrtilek, IEEE J. of Quantum Electr. QE-12 (1976) 624. [4] J.D. Shipman Jr., Appl. Phys. Letters 10 (1967) 3. [5 ] R.W. Waynant, J.D. Shipman Jr., R.C. Elton and A.W. Ali, Proc. IEEE 59 (1971) 679. [6] J.P. Girardeau-Montaut, L'onde ~l~ctrique 54 (1974) 456. [7] D.G. Sutton, S.N. Suchard, O.L. Gibb and C.P. Wang, AppL Phys. Letters 28 (1976) 522. [8] tl. Salzmann and tt. Strohwald, Optics Commun. 12 (1974) 37O. [9] W. Herden, Phys. Letters 54A (1975) 96. [10] H. Strohwald and H. Salzmann, Appl. Phys. Letters 28 (1976) 272. [11 ] H.M. yon Bergmann, V. Hasson and D. Preussler, Appl. Phys. Letters 27 (1975) 553. [12] V. Hasson, H.M. von Bergmann and D. Preussler, Appl. Phys. Letters 28 (1976) 17. [13] E.E. Bergmann, Appl. Phys. Letters 28 (1976) 84. [14] A.J. Schwab and F.W. Hollinger, IEEE J. Quantum Electr. QE-12 (1976) 183. [15] V. Hasson, D. Preussler, J. Klimek and H.M. yon Bergmann, Appl. Phys. Letters 25 (1974) 1. [16] N.A. Kurnit, S.J. Tubbs, K. Bidhichand, L.W. Ryan Jr. and A. Javan, IEEE J. Quantum Electr. QE-I 1 (1975) 174. [17] K. Vollrath and M. Hugenschmidt, Proc. 12th Int. Congress on high-speed photography, ISL-Report CO 214/76.