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On the tunable infrared gas lasers
Volume
ON
Institute
October
OPTICS COMMUSICATIONS
4. number
THE
TUNABLE
INFRARED
GAS
LASERS
V. N. BAGRATASHVILI, I. N. KNYAZEV, V. S. LETOKHOV Acndcmgorodok, Podolsky rayon, of Spectroscopy, Acndcnq of Scicxces, Heceivetl
8 September
1971
Moscow,
USSR
1971
The high-pressure (2 10 atm) electrically excited molecular gas lasers with full overlapping of the rotational lines caused by high-pressure broadening are considered as a possible new type of the powerful infrared lasers with tunable frequency. For Cog - laser the overlapping occurs at pressures sho\vn that the transverse-excitation multiple-arc technique allows 10 = 15 ntm. It is experimentally laser action at gas pressures up to 1.S atnl fur CO2 and N 0.9 atm for HF-lasers.
i) The problem of the powerful infrared lasers with tunable frequency is of great importance in quantum electronics. Considerable progress has been made recently in this field by using stimulated Raman scattering of the infrared radiation in the semiconductors at high magnetic field (spin-flip laser) [1,2]. The power output of such lasers is restricted by heating and damage of the semiconductor by the pumping radiation. In the present letter the high pressure gas lasers with continuous and wide gain bands (due to high-pressure broadening) are considered as a possible new type of the tunable lasers. Preliminary experimental results are reported for two laser systems. ii) For gas pressure p 2 10 torr the line width
of the vibrational-rotational laser transition is determined mainly by the molecular collisions and is proportional to the gas pressure. The broadening constant strongly depends on the specific molecular interactions and varies from for collisions of some polar mole1 cm-1 atm-l cules to 0.05 cm-l atm -’ for broadening by the noble gas atoms. The tunability of the gas lasers is restricted by the laser line width and for gas pressures of 10 - 100 torr does not exceed Av: IJ = 10-4. The tunability of the molecular gas lasers can in principle be considerably enlarged up to Au, v = 0.1 if the spectral width of the pressurebroadened laser line is comparable to the vibrational-rotational line separation. The full width
/
Gas
mixture
D/I 1
Pump
4 5
I
B i
\
Fig. 1. Experimental gap, 5 - high-voltage
154
cell, 2 - high-voltage isolator, 3 - resistors, 4 - synchronized spark. set up 1 - high pressure supply, 6 - spark-gap initiation unit, 7 - photodetector, 9 - NaCl-windoxvs, 9 - laser mirrors C - 0.01 PI; capacitor, r - additional resistor.
Volume 4, number 2
October 1971
OPTICS COMMUNICATIONS
A
A (a)
WA
W,Q.
(b)
1.2 P(atm) Fig. 2. The pressure
dependence
of the power output and the relative probability 4 of the laser action for CO2 (a) and HF @) lasers.
of the continuous vibrational band in this case is A v = n( B kTJ1/‘, where n = 6 - 7, B is the vibrational constant in cm-l, kT in cm-l. For the. C02-laser the overlapping of the rotational lines occurs at the He-NZ-CO2 pressure Y 10 - 15 atm. (The pressure broadening constant for CO2 lines due to COZ-He collisions is 0.1 cm-l atm-1 [3], the line separation is 1.0 cm-l for the R-branch and 1.6 cm-l for the P-branch). In the case of full rotational overlapping the tunability is AU” 60 cm-l and 100 cm-l for the 10.6 p and 9.6 JJ C02-laser transitions respectively. iii) The transverse-excitation multiple-arc technique was used in experiments with the atmospheric-pressure CO2 and HF lasers [4] *. The experimental arrangement is shown in fig. 1. The lm system of 200 lka resistors was assembled in the laser cell designed on the maximal pressure up to 10 atm. The 0.01 PF capacitor charged to 30 kV and the synchronized three-electrode high-pressure spark gap were used for laser excitation. The high-voltage pulses up to 150 kV were also available with a pulse transformer. The auration of excitation pulses was 0.5 - 5 IJ-sec. The electrode separation could be changed from 1 to 2 cm. iv) Fig. 2 shows the experimental results. The power output of the CO2-laser averaged over 200 excitation pulses versus the total gas pressure (CO2:NZ:He = 1:l:lO) is shown in fig. 2a. For optimal excitation conditions the pulse to pulse intensity variations of the laser output are about 300/Oat pressures less than 0.4 atm (region I). For * The principal possibility of the increasing of pressure in the pulsed gas-discharge lasers was discussed
in detail earlier
[5].
*
P(atm)
region II the variations are 2 - 3 times, and for P 2 1.2 atm the laser action is unstable. Fig. 2a also shows the pressure variation of the number ratio of the observed laser pulses to the total number of the excitation pulses. The maximum pressure achieved for lasing is about 1.8 atm, i.e. twice the published figures [6]. The pressure increase causes considerable changes in the form of the discharge. For P (0.5 atm a comparatively bright diffuse l-2mm channel is observed from each pin. For P 2 1 atm and 1 cm gap the diffuse discharge form only takes place for voltages near breakdown. The diffuse luminescence in this case is comparatively weak. The stored electrical energy is realized mainly in the narrow sparks between pins. The SF6:H :He ratio in the HF-laser was varied from 1?I:l:l to 10:1:25. The pressure was increased mainly by using He because of the high electrical strength of SF6. About 6 x 102 successful laser pulses could be achieved owing to weak dissociation of SF6. Fig. 2b shows the experimental results for the HF-laser. Laser action is stable up to 0.55 atm and shows slight instability for higher pressures. The sharp decrease of the power output at 0.95 atm is apparently caused by the fast vibrational relaxation of HF (the HF vibrational relaxation time for collisions with He is 10 iJ.sec/torr [9]). v) At least for the CO2-laser, the main restrictions on the gas pressure are connected with the development of the narrow filament sparks. The spark formation is extremely unfavourable for the laser action. The electron density in the spark is high enough for the effective absorption of the 10 p laser radiation. This effect may cause variable losses in the transverse discharge preven155
Volume ting ing
4, number
laser the
action
giant
2
OPTICS
during
pulse
Besides, the sparks of the medium.
the
discharge
formation, spoil
as the
and
observed
optical
COMMUNICATIONS
favour-
1971
REFERENCES
in [4].
homogeneity
In advanced high-pressure lasers uniformdischarge excitation should be used. Preliminary ionization is the effective method of obtaining the homogeneous pulsed high-pressure plasma. Practically, the high-energy electron beams are the most suitable for pre-ionization. In [lo] the homogeneous pulsed plasma was excited by this method at gas pressures up to z 10 atm **.
** 1n the independent work [ll] the laser action was recently obtained in the high-pressure CO2 laser with the pre-ionization by the high-energy electrons,
156
October
[l] C. K. N. Pate1 and E. D. Shaw, Phys. Rev. Letters 24 (1970) 451. [2] A. Mooradian, S. R. J. Brueck and I‘. A. Blum, Appl. Phys. Letters 17 (1970) 481. [3] T. K. McCubbin, R. Darone and J. Sorrel, Appl. Phys. Letters 7 (1968) 2241. [4] A. J. Beaulieu, Appl. Phys. Letters 16 (1970) 504. [5] I. N. Knyazev, Doctoral thesis, P. N. Lebedev Physical Inst, Moscow (1968). [S] D. C. Smith and A. J. De Maria, J. Appl. Phys. 41 (1970) 5212. [7] C. J. Ultee, IEEE J. Quantum Electron. QE-6 (1970) 647. [8] J. Goldhar, R. M. Osgood and A. Javan, Appl. Phys. Letters 18 (1971) 167. [9] J. R. Airey and S. F. Fried, Chem. Phys. Letters 8 (1971) 23. [lo] B. &I. Kovalchuk, V. V. Kremnev and G. A. Mesyats, Dokl. Akad. Nauk (USSR) 191 (1970) 76. [ll] N. G Basov, V. A. Danilychev et al. , Quantum Electron. (USSR) N3 (1970) 121.
ON
Institute
October
OPTICS COMMUSICATIONS
4. number
THE
TUNABLE
INFRARED
GAS
LASERS
V. N. BAGRATASHVILI, I. N. KNYAZEV, V. S. LETOKHOV Acndcmgorodok, Podolsky rayon, of Spectroscopy, Acndcnq of Scicxces, Heceivetl
8 September
1971
Moscow,
USSR
1971
The high-pressure (2 10 atm) electrically excited molecular gas lasers with full overlapping of the rotational lines caused by high-pressure broadening are considered as a possible new type of the powerful infrared lasers with tunable frequency. For Cog - laser the overlapping occurs at pressures sho\vn that the transverse-excitation multiple-arc technique allows 10 = 15 ntm. It is experimentally laser action at gas pressures up to 1.S atnl fur CO2 and N 0.9 atm for HF-lasers.
i) The problem of the powerful infrared lasers with tunable frequency is of great importance in quantum electronics. Considerable progress has been made recently in this field by using stimulated Raman scattering of the infrared radiation in the semiconductors at high magnetic field (spin-flip laser) [1,2]. The power output of such lasers is restricted by heating and damage of the semiconductor by the pumping radiation. In the present letter the high pressure gas lasers with continuous and wide gain bands (due to high-pressure broadening) are considered as a possible new type of the tunable lasers. Preliminary experimental results are reported for two laser systems. ii) For gas pressure p 2 10 torr the line width
of the vibrational-rotational laser transition is determined mainly by the molecular collisions and is proportional to the gas pressure. The broadening constant strongly depends on the specific molecular interactions and varies from for collisions of some polar mole1 cm-1 atm-l cules to 0.05 cm-l atm -’ for broadening by the noble gas atoms. The tunability of the gas lasers is restricted by the laser line width and for gas pressures of 10 - 100 torr does not exceed Av: IJ = 10-4. The tunability of the molecular gas lasers can in principle be considerably enlarged up to Au, v = 0.1 if the spectral width of the pressurebroadened laser line is comparable to the vibrational-rotational line separation. The full width
/
Gas
mixture
D/I 1
Pump
4 5
I
B i
\
Fig. 1. Experimental gap, 5 - high-voltage
154
cell, 2 - high-voltage isolator, 3 - resistors, 4 - synchronized spark. set up 1 - high pressure supply, 6 - spark-gap initiation unit, 7 - photodetector, 9 - NaCl-windoxvs, 9 - laser mirrors C - 0.01 PI; capacitor, r - additional resistor.
Volume 4, number 2
October 1971
OPTICS COMMUNICATIONS
A
A (a)
WA
W,Q.
(b)
1.2 P(atm) Fig. 2. The pressure
dependence
of the power output and the relative probability 4 of the laser action for CO2 (a) and HF @) lasers.
of the continuous vibrational band in this case is A v = n( B kTJ1/‘, where n = 6 - 7, B is the vibrational constant in cm-l, kT in cm-l. For the. C02-laser the overlapping of the rotational lines occurs at the He-NZ-CO2 pressure Y 10 - 15 atm. (The pressure broadening constant for CO2 lines due to COZ-He collisions is 0.1 cm-l atm-1 [3], the line separation is 1.0 cm-l for the R-branch and 1.6 cm-l for the P-branch). In the case of full rotational overlapping the tunability is AU” 60 cm-l and 100 cm-l for the 10.6 p and 9.6 JJ C02-laser transitions respectively. iii) The transverse-excitation multiple-arc technique was used in experiments with the atmospheric-pressure CO2 and HF lasers [4] *. The experimental arrangement is shown in fig. 1. The lm system of 200 lka resistors was assembled in the laser cell designed on the maximal pressure up to 10 atm. The 0.01 PF capacitor charged to 30 kV and the synchronized three-electrode high-pressure spark gap were used for laser excitation. The high-voltage pulses up to 150 kV were also available with a pulse transformer. The auration of excitation pulses was 0.5 - 5 IJ-sec. The electrode separation could be changed from 1 to 2 cm. iv) Fig. 2 shows the experimental results. The power output of the CO2-laser averaged over 200 excitation pulses versus the total gas pressure (CO2:NZ:He = 1:l:lO) is shown in fig. 2a. For optimal excitation conditions the pulse to pulse intensity variations of the laser output are about 300/Oat pressures less than 0.4 atm (region I). For * The principal possibility of the increasing of pressure in the pulsed gas-discharge lasers was discussed
in detail earlier
[5].
*
P(atm)
region II the variations are 2 - 3 times, and for P 2 1.2 atm the laser action is unstable. Fig. 2a also shows the pressure variation of the number ratio of the observed laser pulses to the total number of the excitation pulses. The maximum pressure achieved for lasing is about 1.8 atm, i.e. twice the published figures [6]. The pressure increase causes considerable changes in the form of the discharge. For P (0.5 atm a comparatively bright diffuse l-2mm channel is observed from each pin. For P 2 1 atm and 1 cm gap the diffuse discharge form only takes place for voltages near breakdown. The diffuse luminescence in this case is comparatively weak. The stored electrical energy is realized mainly in the narrow sparks between pins. The SF6:H :He ratio in the HF-laser was varied from 1?I:l:l to 10:1:25. The pressure was increased mainly by using He because of the high electrical strength of SF6. About 6 x 102 successful laser pulses could be achieved owing to weak dissociation of SF6. Fig. 2b shows the experimental results for the HF-laser. Laser action is stable up to 0.55 atm and shows slight instability for higher pressures. The sharp decrease of the power output at 0.95 atm is apparently caused by the fast vibrational relaxation of HF (the HF vibrational relaxation time for collisions with He is 10 iJ.sec/torr [9]). v) At least for the CO2-laser, the main restrictions on the gas pressure are connected with the development of the narrow filament sparks. The spark formation is extremely unfavourable for the laser action. The electron density in the spark is high enough for the effective absorption of the 10 p laser radiation. This effect may cause variable losses in the transverse discharge preven155
Volume ting ing
4, number
laser the
action
giant
2
OPTICS
during
pulse
Besides, the sparks of the medium.
the
discharge
formation, spoil
as the
and
observed
optical
COMMUNICATIONS
favour-
1971
REFERENCES
in [4].
homogeneity
In advanced high-pressure lasers uniformdischarge excitation should be used. Preliminary ionization is the effective method of obtaining the homogeneous pulsed high-pressure plasma. Practically, the high-energy electron beams are the most suitable for pre-ionization. In [lo] the homogeneous pulsed plasma was excited by this method at gas pressures up to z 10 atm **.
** 1n the independent work [ll] the laser action was recently obtained in the high-pressure CO2 laser with the pre-ionization by the high-energy electrons,
156
October
[l] C. K. N. Pate1 and E. D. Shaw, Phys. Rev. Letters 24 (1970) 451. [2] A. Mooradian, S. R. J. Brueck and I‘. A. Blum, Appl. Phys. Letters 17 (1970) 481. [3] T. K. McCubbin, R. Darone and J. Sorrel, Appl. Phys. Letters 7 (1968) 2241. [4] A. J. Beaulieu, Appl. Phys. Letters 16 (1970) 504. [5] I. N. Knyazev, Doctoral thesis, P. N. Lebedev Physical Inst, Moscow (1968). [S] D. C. Smith and A. J. De Maria, J. Appl. Phys. 41 (1970) 5212. [7] C. J. Ultee, IEEE J. Quantum Electron. QE-6 (1970) 647. [8] J. Goldhar, R. M. Osgood and A. Javan, Appl. Phys. Letters 18 (1971) 167. [9] J. R. Airey and S. F. Fried, Chem. Phys. Letters 8 (1971) 23. [lo] B. &I. Kovalchuk, V. V. Kremnev and G. A. Mesyats, Dokl. Akad. Nauk (USSR) 191 (1970) 76. [ll] N. G Basov, V. A. Danilychev et al. , Quantum Electron. (USSR) N3 (1970) 121.