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Iodine laser oscillator in gain switch mode for ns pulses
Volume 13, number 2
OPTICS COMMUNICATIONS
l"ebruary 1975
IODINE LASER OSCILLATOR IN GAIN SWITCH MODE FOR ns PULSES K. HOHLA, W. FUSS, R. VOLK and K.-J. WITTE Max-Ptanck-Institut fiTr Plasmaphysik, 8046 Garching, Germany Received 2 December 1974
An iodine laser oscillator is described, which delivers several mJ in ns-pulses. The oscillator is mode-locked by means of an acoustooptical modulator, the pulse duration varies with the iodine pressure. Apparent self-modelocking is due to saturation effects.
In previous publications it was shown that the iodine laser is suitable for giant pulse operation [1 31 . Basov et al. [2] report l010 W in 5 ns. Shorter pulses with 1011 W in 0.7 ns have been attained by the authors [3]. The oscillator of their iodine laser [3] has been investigated in more detail as described here. A sketch of the oscillator developed by us is shown in fig. 1. The laser quartz tube is terminated with Brewster windows and connected by a line to a filling apparatus. The inner diameter of the tube is 8 mm and the length is 100 cm. Arranged parallel to the tube are four Xe flashlamps. The reflector consists of electropolished aluminum. The flashlamps are parallel to a capacitor. To keep the flash time as short as possible, a capacitor with low inductance (L c = 65 nil) was chosen, the stored energy being 1 kJ at a voltage of 44 kV. The inductances in the conductor circuit were limited to 20 nH by choosing a coaxial spark gap and using stripline connections [6]. The flash in the wavelength range between 2500 ,~ and 3000 A, the absorp-
tion band of C3F71 [7] has a duration of 1.5/~s. The exact tlash profile reproduced in fig. 2 shows that the rise time is approx. 600 ns. Tile short flash times were chosen to operate the oscillator in the gain switch mode. Switchgear such as Pockels cells or Kerr cells is not required in the resonator. A hemiconfocal arrangement with a n f = 2.5 m end mirror acted as resonator. The transmission of the plane coupling mirror is 4054 resulting in a threshold amplification of 1.6. In order to achieve a diffraction limited beam, a mode diaphragm fl~r selecting the TEM00 mode was placed in the beam path. Curve b in fig. 2 shows the time integrated laser signal at a filling pressure of 100 torr C3F71. The ordinate gives the total energy emitted up to a certain time t. Laser emission starts after 500 ns with a dump spike containing 30 mJ and lasts 50 ns. The subsequent enfission curve is ahnost smooth, the total energy being emitted after 5/Is.
a Capacilor /,6 KV, I KJ
C~ F~ I - G a s Loser
~
Modelocker cm
g ' '
r Tot aliy Reflecting Mirror { f :250cm)
~ 1 ~ Partially Reflect,ncj ,, Mirror ( f = ~ , R=LO'/, ) ' Losertube i ", ~ ~ Mode ~ ~! / / Dic~phragm 4 X e - Flashlamps
Fig. 1. Schematic of the iodine laser oscillator. 114
time scale
~ 5 0 0 nsec div
F'ig. 2a. Intensity of the flashlamps in the wavelength range 2500 A to 3000 A. b. Time integrated energy of the oscillator signal.
Volume 13, number 2
OPTICS COMMUNICATIONS
February 1975
The amplification V present at the beginning of the dump spike can be derived from the formula: V = exp (a A N / ) .
(1)
The stimulated emission cross section a at 100 tort C3F71 is 6 × 10 -19 cm 2 [5]. 2tN l is the total inversion per unit area of the amplifier, which is nearly equal to the energy of the dump spike EDS divided by the area of the mode, because the dump spike duration is small compared with the pumping time:
I = EDsG/hva.
time scale
(2)
The area a can be assumed to a good approximation to be equal to an average mode cross section in the active medium, thus a = 7r(1.2 co)2, where co is the beam waist. In the formula G contains the degeneracy of the levels involved; its value is* 2.22 [5]. Thus V is nearly equal to 120. As can be seen from fig. 2 there is almost a linear increasing pump rate P = s t in the first 500 ns. a can be estimated from (2) and the relation z~V l = ½~t 2 to be c~ = 8 × 1031 cm -2 s-2. The amplification as a function of time can therefore be expressed by:
V(t) = exp (½oat 2) ~ exp (2.4 × 1013 t2). This relation allows an approximate calculation o f the build-up time t b o f the dump spike. The threshold amplification o f 1.6 is reached after 140 ns, and therefore t b = 500 ns - 140 ns = 360 ns. This means there are about 21 complete cycles through the oscillator, establishing a mode-controlled beam quality. A more exact time resolution of the dump spike shows distinct modulation (fig. 3a). This self-modelocking can be explained in terms of saturation of the laser medium by a noise pulse. After some runs the pulse enters the saturation regime and the pulse shape is characteristically changed, the start o f the pulse extracting so much energy from the oscillator that the subsequent photons can no longer be amplified quite as much. This typical change can be followed from the first to the second pulse visible in the figure. The third pulse finally contains almost 30% of the total energy in the first 2 - 3 ns. The amplification at the end of the pulse is smaller than the threshold amplification. After * The value 2.22 is based on the assumption that in 50 nsec the upper levels do not, and the lower levels do rapidly relax among each other. Furthermore a factor of 12/7 has to be included as only one of the upper levels is involved in the emission, whereas ~N means the total inversion.
~
20
=
Dsec 20 div
nsec
div
t
:
time scare
Fig. 3a. Self-modulation of the oscillator due to saturation effects, b. Mode-locked pulse train with an acousto-optical modulator. the third pulse the inversion is reduced so much that threshold amplification is no longer ~rchieved, the pulses again become weaker and a change o f the pulse shape is no longer observed. This effect is intensified if the quality o f the resonator is changed by an acousto-optical modulator in step with the photon time o f flight as was done by Ferrar [8] in 1968. We placed a quartz modulator be-
t [nsec] "\x, ~isec/c ~
m 20
60
2nsec/cm 200
100
300 [Torr]
P C3F73
Fig. 4. Dependence of half-width on the pressure of the laser gas. 115
Volume 13, number 2
OPTICS COMMUNICATIONS
tween the mode diaphragm and the coupling mirror and modulated it with 60 MHz. The modulation depth is 50%. Fig. 3b shows a typical pulse train with this arrangement. The pulses are well modulated. The pulse length in this arrangement depends on the pressure chosen in the oscillator. The higher the pressure the higher is the bandwidth due to pressure broadening. In fig. 4 the measured pulse length is plotted as a function of the C 3 F 71 pressure. The half-length decreases almost linearly with the pressure. The shortest measured half-length of 0.7 ns is attained at 180 200 tort. No further shortening at higher pressures was observed owing to the limited resolution time of the diagnostic system (Valvo XA-1003 photocell + Tektronix 519 oscilloscope). The energy in the mode-locked pulses is in the range 1 - 4 mJ. By means of an ordinary pulse-cutting system it is now possible to select one of these pulses from the pulse train and feed it into the amplifier chain.
Acknowledgements The authors are grateful to H. Bauer Jbr construct-
116
February 1975
ing the apparatus and to E. v. Mark for designing tile discharge circuit for the flashlamps. This work was performed under the terms of tlle agreement on association between the Max-Planck-lnstitut fur Plasmaphysik and Euratom.
References [1 ] K. Hohla, K.L. Kompa, Appl. Phys. Lett. 22 (1973) 77. [2] N.G. Basov, L.E. Golubev, V.S. Zuev, V.A. Katulin, V.N. Netemin, V.Yu. Nosach, O.Yu. Nosach, A.L. Petrov, Kvantovaya Elektronika 6 (1973) 116. [3] K. tlohla, G. Brederlow, W. Fuss, K.L. Kompa, J. Raeder. R. Volk, S. Witkowski, K.-J. Witte, submitted to Appl. Phys. Lett. [4] F. Aldridge, Appl. Phys. Lett. 22 (1973) 180. [5] W. Fuss, K. ttohla, IPP Lab. Report IPP IV/67, MaxPlanck-lnstitut fur Plasmaphysik, Garching. Germany, 1974. [6] E.v. Mark, Max-Planck-lnstitut ffir Plasmaphysik, private communication. [7] K. Hohla, in: Laser Interaction and Related Plasma Phenomena, H. Schwarz and H. Hora eds., Vol. llI (Plenum Press, New York). [8] C.M. Ferrar, Appl. Phys. Lett. 12 (1968) 381.
OPTICS COMMUNICATIONS
l"ebruary 1975
IODINE LASER OSCILLATOR IN GAIN SWITCH MODE FOR ns PULSES K. HOHLA, W. FUSS, R. VOLK and K.-J. WITTE Max-Ptanck-Institut fiTr Plasmaphysik, 8046 Garching, Germany Received 2 December 1974
An iodine laser oscillator is described, which delivers several mJ in ns-pulses. The oscillator is mode-locked by means of an acoustooptical modulator, the pulse duration varies with the iodine pressure. Apparent self-modelocking is due to saturation effects.
In previous publications it was shown that the iodine laser is suitable for giant pulse operation [1 31 . Basov et al. [2] report l010 W in 5 ns. Shorter pulses with 1011 W in 0.7 ns have been attained by the authors [3]. The oscillator of their iodine laser [3] has been investigated in more detail as described here. A sketch of the oscillator developed by us is shown in fig. 1. The laser quartz tube is terminated with Brewster windows and connected by a line to a filling apparatus. The inner diameter of the tube is 8 mm and the length is 100 cm. Arranged parallel to the tube are four Xe flashlamps. The reflector consists of electropolished aluminum. The flashlamps are parallel to a capacitor. To keep the flash time as short as possible, a capacitor with low inductance (L c = 65 nil) was chosen, the stored energy being 1 kJ at a voltage of 44 kV. The inductances in the conductor circuit were limited to 20 nH by choosing a coaxial spark gap and using stripline connections [6]. The flash in the wavelength range between 2500 ,~ and 3000 A, the absorp-
tion band of C3F71 [7] has a duration of 1.5/~s. The exact tlash profile reproduced in fig. 2 shows that the rise time is approx. 600 ns. Tile short flash times were chosen to operate the oscillator in the gain switch mode. Switchgear such as Pockels cells or Kerr cells is not required in the resonator. A hemiconfocal arrangement with a n f = 2.5 m end mirror acted as resonator. The transmission of the plane coupling mirror is 4054 resulting in a threshold amplification of 1.6. In order to achieve a diffraction limited beam, a mode diaphragm fl~r selecting the TEM00 mode was placed in the beam path. Curve b in fig. 2 shows the time integrated laser signal at a filling pressure of 100 torr C3F71. The ordinate gives the total energy emitted up to a certain time t. Laser emission starts after 500 ns with a dump spike containing 30 mJ and lasts 50 ns. The subsequent enfission curve is ahnost smooth, the total energy being emitted after 5/Is.
a Capacilor /,6 KV, I KJ
C~ F~ I - G a s Loser
~
Modelocker cm
g ' '
r Tot aliy Reflecting Mirror { f :250cm)
~ 1 ~ Partially Reflect,ncj ,, Mirror ( f = ~ , R=LO'/, ) ' Losertube i ", ~ ~ Mode ~ ~! / / Dic~phragm 4 X e - Flashlamps
Fig. 1. Schematic of the iodine laser oscillator. 114
time scale
~ 5 0 0 nsec div
F'ig. 2a. Intensity of the flashlamps in the wavelength range 2500 A to 3000 A. b. Time integrated energy of the oscillator signal.
Volume 13, number 2
OPTICS COMMUNICATIONS
February 1975
The amplification V present at the beginning of the dump spike can be derived from the formula: V = exp (a A N / ) .
(1)
The stimulated emission cross section a at 100 tort C3F71 is 6 × 10 -19 cm 2 [5]. 2tN l is the total inversion per unit area of the amplifier, which is nearly equal to the energy of the dump spike EDS divided by the area of the mode, because the dump spike duration is small compared with the pumping time:
I = EDsG/hva.
time scale
(2)
The area a can be assumed to a good approximation to be equal to an average mode cross section in the active medium, thus a = 7r(1.2 co)2, where co is the beam waist. In the formula G contains the degeneracy of the levels involved; its value is* 2.22 [5]. Thus V is nearly equal to 120. As can be seen from fig. 2 there is almost a linear increasing pump rate P = s t in the first 500 ns. a can be estimated from (2) and the relation z~V l = ½~t 2 to be c~ = 8 × 1031 cm -2 s-2. The amplification as a function of time can therefore be expressed by:
V(t) = exp (½oat 2) ~ exp (2.4 × 1013 t2). This relation allows an approximate calculation o f the build-up time t b o f the dump spike. The threshold amplification o f 1.6 is reached after 140 ns, and therefore t b = 500 ns - 140 ns = 360 ns. This means there are about 21 complete cycles through the oscillator, establishing a mode-controlled beam quality. A more exact time resolution of the dump spike shows distinct modulation (fig. 3a). This self-modelocking can be explained in terms of saturation of the laser medium by a noise pulse. After some runs the pulse enters the saturation regime and the pulse shape is characteristically changed, the start o f the pulse extracting so much energy from the oscillator that the subsequent photons can no longer be amplified quite as much. This typical change can be followed from the first to the second pulse visible in the figure. The third pulse finally contains almost 30% of the total energy in the first 2 - 3 ns. The amplification at the end of the pulse is smaller than the threshold amplification. After * The value 2.22 is based on the assumption that in 50 nsec the upper levels do not, and the lower levels do rapidly relax among each other. Furthermore a factor of 12/7 has to be included as only one of the upper levels is involved in the emission, whereas ~N means the total inversion.
~
20
=
Dsec 20 div
nsec
div
t
:
time scare
Fig. 3a. Self-modulation of the oscillator due to saturation effects, b. Mode-locked pulse train with an acousto-optical modulator. the third pulse the inversion is reduced so much that threshold amplification is no longer ~rchieved, the pulses again become weaker and a change o f the pulse shape is no longer observed. This effect is intensified if the quality o f the resonator is changed by an acousto-optical modulator in step with the photon time o f flight as was done by Ferrar [8] in 1968. We placed a quartz modulator be-
t [nsec] "\x, ~isec/c ~
m 20
60
2nsec/cm 200
100
300 [Torr]
P C3F73
Fig. 4. Dependence of half-width on the pressure of the laser gas. 115
Volume 13, number 2
OPTICS COMMUNICATIONS
tween the mode diaphragm and the coupling mirror and modulated it with 60 MHz. The modulation depth is 50%. Fig. 3b shows a typical pulse train with this arrangement. The pulses are well modulated. The pulse length in this arrangement depends on the pressure chosen in the oscillator. The higher the pressure the higher is the bandwidth due to pressure broadening. In fig. 4 the measured pulse length is plotted as a function of the C 3 F 71 pressure. The half-length decreases almost linearly with the pressure. The shortest measured half-length of 0.7 ns is attained at 180 200 tort. No further shortening at higher pressures was observed owing to the limited resolution time of the diagnostic system (Valvo XA-1003 photocell + Tektronix 519 oscilloscope). The energy in the mode-locked pulses is in the range 1 - 4 mJ. By means of an ordinary pulse-cutting system it is now possible to select one of these pulses from the pulse train and feed it into the amplifier chain.
Acknowledgements The authors are grateful to H. Bauer Jbr construct-
116
February 1975
ing the apparatus and to E. v. Mark for designing tile discharge circuit for the flashlamps. This work was performed under the terms of tlle agreement on association between the Max-Planck-lnstitut fur Plasmaphysik and Euratom.
References [1 ] K. Hohla, K.L. Kompa, Appl. Phys. Lett. 22 (1973) 77. [2] N.G. Basov, L.E. Golubev, V.S. Zuev, V.A. Katulin, V.N. Netemin, V.Yu. Nosach, O.Yu. Nosach, A.L. Petrov, Kvantovaya Elektronika 6 (1973) 116. [3] K. tlohla, G. Brederlow, W. Fuss, K.L. Kompa, J. Raeder. R. Volk, S. Witkowski, K.-J. Witte, submitted to Appl. Phys. Lett. [4] F. Aldridge, Appl. Phys. Lett. 22 (1973) 180. [5] W. Fuss, K. ttohla, IPP Lab. Report IPP IV/67, MaxPlanck-lnstitut fur Plasmaphysik, Garching. Germany, 1974. [6] E.v. Mark, Max-Planck-lnstitut ffir Plasmaphysik, private communication. [7] K. Hohla, in: Laser Interaction and Related Plasma Phenomena, H. Schwarz and H. Hora eds., Vol. llI (Plenum Press, New York). [8] C.M. Ferrar, Appl. Phys. Lett. 12 (1968) 381.