- Email: [email protected]
Optically pumped continuous Bi2 and Te2 lasers
Volume 26, number 3
OPTICS COMMUNICATIONS
September 1978
OPTICALLY PUMPED CONTINUOUS Bi 2 AND Te 2 LASERS B. WELLEGEHAUSEN, D. F R I E D E and G. STEGER Institut fur Angewandte Physik, Technische Universitat Hannover, 3000 Hannover, Fed. Rep. Germany Received 22 May 1978
Continuous laser oscillation of optically pumped Bi2 and Te 2 molecules was achieved. By pumping with a single argon laser line more than 80 Te 2 laser lines in the range of 550 nm to 660 nm were observed. For Bi2 conversion efficiences of 10% and multiline output powers up to 350 mW were obtained. Some properties and operating conditions for these new dimer lasers are discussed.
1. Introduction Optically pumped homonuclear diatomic molecules (dimers) show interesting laser properties as extremely low threshold pump intensities, efficient conversion o f pump laser radiation and oscillation on many lines covering broad spectral ranges [ 1 - 4 ] . The dimer molecules are stable and can be easily produced by evaporating suitable elements. Therefore dimer lasers are ideal sources of narrowband line radiation with application possibilities for spectroscopy, kinetics, optical frequency standards and many more [5]. This paper reports cw laser oscillation of two more dimer molecules (Bi 2 , Te2), belonging now to the fifth and sixth group o f the periodic system. Data o f an UV (351 nm) pumped cw S 2 laser with emission in the range o f 420 nm to 500 nm will be reported [6]. Pulsed laser oscillation of these molecular systems was observed before b y several authors [7-9]. The Bi 2 and Te 2 dimer molecules are generated by evaporating the elements in melted off quartz cells or heatpipe systems. The heatpipe has the advantage that operating parameters as the length of the vapor zone or the kind and partial pressure of buffer gases can be changed more easily and that the hot metal vapors are kept away from the optical windows. The Bi 2 system was investigated only by use o f a heatpipe at an optimum temperature around 1300 K, while the Te 2 system was operated either
with a cell or a heatpipe. However, we got a higher output power and a better intensity stability having the Te 2 vapor in a quartz cell with a reservoir temperature of 870 K and a cell temperature o f about 1000 K. The higher cell temperature is necessary to prevent condensation of Te on the optical windows. The worse behaviour o f Te in a heatpipe is mainly caused b y a not uniform circulation of the Te (vapor and liquid). This difficulty may perhaps be circumvented by using other materials and sizes for the metal mesh inside the heatpipe. The set-up for the Bi 2 and Te 2 lasers is very similar to those described recently for the Na and 12 molecules [1,3]. For optical excitation argon laser lines in the range o f 455 nm to 515 nm are well suited, as has been demonstrated in various laser induced fluorescence experiments [ 1 0 - 1 3 ] . As the Doppler width o f the absorption lines is only about 0.75 GHz for Bi 2 at 1300 K and 0.9 GHz for Te 2 at 1000 K, . efficient optical pumping and best laser performance are achieved with single frequency pump lasers tuned to a specific absorption transition.
2. Te 2 laser Fig. 1 shows the energy level diagram of Te 2 as given b y Barrow and Parcq [10]. From the electronic ground state X(0~) transitions to the electronic A(0 +) or B(0 +) states are possible. A specific X ~ B 391
Volume 26, number 3
OPTICS COMMUNICATIONS
E 25
Te2
15 V'
24 23 22
2 i -20 15 10
25
!22074 .......
21
~ --
.... -- --
+ 20
~4508
V'
19 8
J 658.1nm
I
476.5 nm
/
7 t 5575nm
I
6
7' /
/
5 !
4
3
282o
x
Io
(lg)
2 x
1
(o'g I
2 0 0.2
o.a
O.25
o.a5 R[om]
Fig. 1. Energy level diagram of Te 2 (after ref. [10] ) w i t h pump and laser ~xansitions.
pump transition and the range of observed subsequent laser transitions is indicated in the diagram (only the u", u' values are given). Due to the partial overlap of the electronic A and B states and due to the broad ex-
September 1978
citation profile of a pump laser line, many absorption transitions are possible. Furthermore, the structure of the absorption and emission spectra becomes more complicated, as the neutral Te consists of several isotopes. For spectroscopic purposes lasers with different Te isotopes can therefore be of interest. At the optimum temperature of about 870 K found for Te 2 laser oscillation, the Te 2 vapor pressure is about 6 Torr [14]. Compared to the optimum vapor pressure for tile other dimer lasers [ 1-3] this vapor pressure is unusually high and therefore pump transitions from higher rotational vibrational levels are expected. It might also be possible that transitions from another ground state component X(1 g) (see ref. [10,11] )which lies 2820 cm -1 above the X(0g) ground state occur (similar for Bi2). Fig. 2 shows a typical multiline laser oscillation spectrum, which is obtained by pumping with a multimode pump laser line. In this case a resonator with broadband dielectric coatings and without an internal tuning element was used. The spectrum is not corrected for photodetector response and monochromator transmission. Single mode pumping reduces the nun> ber of simultaneously oscillating lines (see table 1) but appreciably increases the stability of the laser emission. In table 1 observed Te 2 laser lines for the 476.5 nm argon laser pump line are summarized. No attempts were made to identify the pump and laser transitions because this would require more accurate wavelength measurements. Each group of lines given in table 1 is obtained for a specific single mode position within the argon laser emission range and should
I (rel,U.)
Te 2 },p: 476.5nm
r 660
--
600
i
,
Wovelen g'lh
c [nm]
550
Fig. 2. Multiline laser oscillation spectrum of Te 2 . Pump line 476.5 nm (Multimode), p u m p power 2W.
392
Volume 26, number 3
OPTICS COMMUNICATIONS
September 1978
Table 1 Cw Te 2 laser lines Pump line 476.5 nm a) Laser lines (nm) b)
c)
564.3 585.9
564.7 592.7
571.5 593.6
572.0 600.4
576.7 600.8
578.4 608.3
579.0 608.7
585.1 616.8
557.5 587,4
557.9
565.0
571.9
572.4
579.4
579.8
587.0
577,4
578.3
584.9
585.7
592.4
593.4
600.2
562.6 584.1 627.8
569.6 585.1 628.7
570.1 600.5 637.1
571.4 600.9 637.9
576.6 608.2 646.5
577.3 608.5 647.3
578.0 616.2 656.1
578,7 616.5 656.9
600.5
600.9
608.5
608.9
616.5
617.0
557.1
563.8
564.2
571.l
571.5
578.5
578.9
586.5
557.5 579.7 648.4
557.8 586.9 657.4
564.6 620.4 658.1
564.9 628.8
570.1 629.5
571.9 638.1
572.1 638.8
579.3 647.7
a) Single mode, typical pump power lW. b) Wavelength accuracy -+0.2 nm. c) Multiline output power 20 roW.
belong to a fluorescence series. Only for this line more detailed investigations o f the possible laser lines have been made. However, laser oscillation occurs for all other blue argon laser lines and similar numbers of transitions and laser lines are possible. The typical output power for a group of lines is several mW. The threshold values are generally below 20 roW, with lowest values below 1 mW for a focal length o f the focussing lens o f 500 m m (beam waist approximately 100/am). For the indicated group of lines a maximum output power of 20 mW (multiline) and up to 3 mW (single fine) was obtained for a pump power o f 1 W. With a length o f the vapor zone of 18 cm a maximum gain of about 15% was observed. However, all these values are preliminary and much higher power values are expected, as not all parameters have been optimized so far. Especially the preparation of the melted o f f quartz cells was insufficient and impurities may quench stronger laser oscillation. To obtain the maximum output power it is also necessary to match a cavity mode o f the dimer laser exactly to the narrow amplification profile which is generated b y the single
mode pump laser (see also discussion below: sub Doppler optical pumping).
3. Bi 2 laser
Fig. 3 shows a part of the energy level diagram of Bi 2 [13]. Some possible pump and laser transitions for excitation with the 514.5 nm argon laser fine, identified b y G. Gerber [15], are shown in the diagram. Due to this analysis pump transitions start from the higher lying ground state component X(0~-) [12,13]. This is in agreement with the observed higbh optimum operating temperature of 1300 K, which corresponds to a total (Bi and Bi2) vapor pressure o f 28 Tort, with a Bi 2 partial pressure o f about 6 Torr [14[. Optimum heatpipe operation at this temperature would require a buffer gas pressure o f 28 Torr in equilibrium with the Bi, Bi 2 vapor. We found better operating conditions o f the Bi2 laser at a buffer gas pressure around 1 0 - 5 5 Tort, which means that the Bi2 vapor is slightly overheated at a vapor pressure o f 393
Volume 26, number 3
OPTICS COMMUNICATIONS
E llOacn~l] 24 23
....
2,_
/
22 21 2O 19 9 8 35
7 6 5 4 3
V"
2
. Zs_op . . . . . . . . .
11 o~..z
I \
0
\
/
. . . . . . . 0.2
/
:"':
. . . . . . .
0.25
0.3
[.-m] 0.35 R
September 1978
only 2 - 3 Torr. In table 2 some observed dimer laser lines and other laser data are summarized. Again the number of laser lines is incomplete, as the absorption spectrum is very dense and not all single mode pump transitions which are possible with the 514.5 nm argon laser line have been investigated. Much weaker laser oscillation is also possible with some shorter wavelength argon laser lines. A remarkable result is the high output power obtained for multiline and single line emission, which makes the Bi2 laser system an attractive transformation system. A possible reason for the high output power and good conversion efficiency may be an effective relaxation process in the lower laser levels, due to perturbations by other electronic states [13]. Perhaps, quenching by exchange collisions with Bi atoms, similar to the Na/Na 2 system [16], might also be possible. Furthermore, the quenching cross section for a depopulation of the upper laser level seems to be very small, as buffer gas pressures up to 100 Torr were necessary to completely quench laser oscillation at the used pump power. More detailed experiments on high pressure operation with different buffer gases are in preparation, to determine the quenching cross sections and to evaluate the tuning possibilities of heavy molecular dimer lasers.
Fig. 3. Energy level diagram of Bi2 (paxt, after ref. [13] ) with pump and laser transitions. Table 2 Data of the cw Bi2 dimer laser Laser wavelength (nm) a)
Typical threshold (mW)
Output power b) (roW)
592.9 658.2
616.0 665.0
633.9
657.6
1000
20
623.9 660.3
630.0 680.9
641.4 700.6
642.2 701.3
400
100
729.2
730.1
736.6
737.6
300
150
736.4
743.9
733.5 748.2
739.8
740.8
747.1
100
350
746.8
747.5
754.3
755.1
400
150
400
a ) Accuracy +-0.2 nm. b) Multiline; pump power up to 3.5 W, pump line 514.5 nm (single mode). Length of vapor zone 20 cm, maximum gain ~20%. The groups of lines are obtained for different single mode positions (pump transitions) and belong to a fluorescence series (see ref. [15]).
394
Volume 26, number 3
OPTICS COMMUNICATIONS
4. Sub Doppler optical pumping Dimer molecules are most effectively pumped with single frequency pump laser radiation. In this case, however, special effects arise, as dimer lasers are three level laser systems with coupled Doppler broadened transitions (see ref. [ 17] and further refs. given there). The single frequency pump laser only excites a certain velocity group Vp of molecules and thus generates a narrowband homogeneous gain profile. The dimer laser therefore only has gain for frequencies w ± = w32(1 -+ where w32 is the molecular transition frequency. The + signs stand for amplification in the direction o f the pump radiation (+) and opposite to this direction ( - ) . In general the g a i n g + is much larger than g - [18]. Due to the anisotropic amplification, the most adequate laser system (for arbitrary velocities Up) is a ring laser. In such a ring laser system laser oscillation in only one direction and at only a single frequency + co occurs and the dimer laser frequency can be tuned b y tuning the pump laser frequency. We observed unidirectional single frequency laser oscillation for all dimer laser systems investigated so far. In case o f Na 2 output powers up to 200 mW and tuning ranges o f more than 4 GHz have been obtained [19]. In a linear resonator with standing waves best laser performance is achieved for Up = 0, that means, the pump laser must be tuned to the exact absorption frequency and the dimer laser resonator mode must be adjusted to coincide with the molecular transition frequency ~32" The dimer laser then also oscillates (for a selected line) in a single frequency only. Due to fluctuations of the pump laser frequency and the optical resonator length of the dimer laser, the output power and the laser spectrum t o o may fluctuate appreciably. To obtain a more stable laser emission, stabilization of both laser systems is necessary.
Vp/C),
5. Conclusion optically pumped laser oscillation for the heavy dimer molecules Bi 2 and Te 2 has been demonstrated. As the spectroscopic work on these molecules is not yet completed, these dimer lasers may now be very helpful for detailed spectroscopic investigations. In
September 1978
combination with a ring laser set-up, also high resolution saturation spectroscopy can be performed. By proper optimization of laser parameters and the preparation o f cells and heatpipes, higher output powers for Te 2 and efficiencies of more than 10% for Bi 2 should be possible. Furthermore, the Te 2 and Bi 2 systems are interesting candidates for high pressure investigations and discharge excitation experiments.
References [ 1] B. Wellegehausen, S. Shahdin, D. Friede und H. Welling, Appl. Phys. 13 (1977) 97. [2] H. Welling and B. Wellegehausen, Laser Spectroscopy III, Springer Series in Optical Sciences 7, (Springer Verlag New York - Heidelberg - Berlin 1977). [3] B. Wellegehausen, K.H. Stephan, D. Friede and H. Welling, Opt. Commun. 23 (1977) 157. [4] R.L. Byer, R.L. Herbst, H. Kildal and M.D. Levenson, Appl. Phys. Lett. 20 (1972) 463. [5] J.B. Koffend, R.W. Field, D.R. Guyer and S.R. Leone, Laser Spectroscopy III, Springer Series in Optical Sciences 7, (Springer Verlag New York - Heidelberg - Berlin 1977). [6] B. Wellegehausen, G. Steger, to be published. [7] S.R. Leone and K.G. Kosnik, Appl. Phys. Lett. 30 (1977) 346. [8] D.R. Guyer and S.R. Leone, 5th Conf. on Chem. and Mol. Lasers, St. Louis (1977). [9] W.P. West and H.P. Broida, Chem. Phys. Lett. 56 (1978) 283. [10] R.P. Barrow and R.P. Du Parcq, Proc. R. Soc. Lond. A 327 (1972) 279. [11 ] K.K. Yee and R.F. Barrow, J. Chem. Soc. Faraday Trans. II 68 (1972) 1397. [12] G. Gerber, K. Sakurai and H.P. Broida, Journ. Chem. Phys. 64 (1976) 3410. [13] G. Gerber and H.P. Broida, Journ. Chem. Phys. 64 (1976) 3423. [14] A.N. Nesmeyanov, Vapor pressure of the chemical elements (Elsevier Publ. Comp. New York 1963). [15] G. Gerber and J. Janes, priyate communication; to be published J. Mol. Spectrosc. [ 16] W. Demtr6der, K. Bergmann, private communication. [17] I.M. Beterov and V.P. Chebotayev, Progress in quantum electronics, Vol. 3 (Pergamon Press, 1974). [18] N. Skribanowitz, M.S. Feld, R.E. Francke, M.J. Kelly and A. Javan, Appl. Phys. Lett. 19 (1971) 161. [ 19 ] B. Wellegehausen and H.H. Heitmann, to be published; see also: Digest of technical papers, Tenth Internatl. Quant. Electr. Conf., Atlanta 1978.
395
OPTICS COMMUNICATIONS
September 1978
OPTICALLY PUMPED CONTINUOUS Bi 2 AND Te 2 LASERS B. WELLEGEHAUSEN, D. F R I E D E and G. STEGER Institut fur Angewandte Physik, Technische Universitat Hannover, 3000 Hannover, Fed. Rep. Germany Received 22 May 1978
Continuous laser oscillation of optically pumped Bi2 and Te 2 molecules was achieved. By pumping with a single argon laser line more than 80 Te 2 laser lines in the range of 550 nm to 660 nm were observed. For Bi2 conversion efficiences of 10% and multiline output powers up to 350 mW were obtained. Some properties and operating conditions for these new dimer lasers are discussed.
1. Introduction Optically pumped homonuclear diatomic molecules (dimers) show interesting laser properties as extremely low threshold pump intensities, efficient conversion o f pump laser radiation and oscillation on many lines covering broad spectral ranges [ 1 - 4 ] . The dimer molecules are stable and can be easily produced by evaporating suitable elements. Therefore dimer lasers are ideal sources of narrowband line radiation with application possibilities for spectroscopy, kinetics, optical frequency standards and many more [5]. This paper reports cw laser oscillation of two more dimer molecules (Bi 2 , Te2), belonging now to the fifth and sixth group o f the periodic system. Data o f an UV (351 nm) pumped cw S 2 laser with emission in the range o f 420 nm to 500 nm will be reported [6]. Pulsed laser oscillation of these molecular systems was observed before b y several authors [7-9]. The Bi 2 and Te 2 dimer molecules are generated by evaporating the elements in melted off quartz cells or heatpipe systems. The heatpipe has the advantage that operating parameters as the length of the vapor zone or the kind and partial pressure of buffer gases can be changed more easily and that the hot metal vapors are kept away from the optical windows. The Bi 2 system was investigated only by use o f a heatpipe at an optimum temperature around 1300 K, while the Te 2 system was operated either
with a cell or a heatpipe. However, we got a higher output power and a better intensity stability having the Te 2 vapor in a quartz cell with a reservoir temperature of 870 K and a cell temperature o f about 1000 K. The higher cell temperature is necessary to prevent condensation of Te on the optical windows. The worse behaviour o f Te in a heatpipe is mainly caused b y a not uniform circulation of the Te (vapor and liquid). This difficulty may perhaps be circumvented by using other materials and sizes for the metal mesh inside the heatpipe. The set-up for the Bi 2 and Te 2 lasers is very similar to those described recently for the Na and 12 molecules [1,3]. For optical excitation argon laser lines in the range o f 455 nm to 515 nm are well suited, as has been demonstrated in various laser induced fluorescence experiments [ 1 0 - 1 3 ] . As the Doppler width o f the absorption lines is only about 0.75 GHz for Bi 2 at 1300 K and 0.9 GHz for Te 2 at 1000 K, . efficient optical pumping and best laser performance are achieved with single frequency pump lasers tuned to a specific absorption transition.
2. Te 2 laser Fig. 1 shows the energy level diagram of Te 2 as given b y Barrow and Parcq [10]. From the electronic ground state X(0~) transitions to the electronic A(0 +) or B(0 +) states are possible. A specific X ~ B 391
Volume 26, number 3
OPTICS COMMUNICATIONS
E 25
Te2
15 V'
24 23 22
2 i -20 15 10
25
!22074 .......
21
~ --
.... -- --
+ 20
~4508
V'
19 8
J 658.1nm
I
476.5 nm
/
7 t 5575nm
I
6
7' /
/
5 !
4
3
282o
x
Io
(lg)
2 x
1
(o'g I
2 0 0.2
o.a
O.25
o.a5 R[om]
Fig. 1. Energy level diagram of Te 2 (after ref. [10] ) w i t h pump and laser ~xansitions.
pump transition and the range of observed subsequent laser transitions is indicated in the diagram (only the u", u' values are given). Due to the partial overlap of the electronic A and B states and due to the broad ex-
September 1978
citation profile of a pump laser line, many absorption transitions are possible. Furthermore, the structure of the absorption and emission spectra becomes more complicated, as the neutral Te consists of several isotopes. For spectroscopic purposes lasers with different Te isotopes can therefore be of interest. At the optimum temperature of about 870 K found for Te 2 laser oscillation, the Te 2 vapor pressure is about 6 Torr [14]. Compared to the optimum vapor pressure for tile other dimer lasers [ 1-3] this vapor pressure is unusually high and therefore pump transitions from higher rotational vibrational levels are expected. It might also be possible that transitions from another ground state component X(1 g) (see ref. [10,11] )which lies 2820 cm -1 above the X(0g) ground state occur (similar for Bi2). Fig. 2 shows a typical multiline laser oscillation spectrum, which is obtained by pumping with a multimode pump laser line. In this case a resonator with broadband dielectric coatings and without an internal tuning element was used. The spectrum is not corrected for photodetector response and monochromator transmission. Single mode pumping reduces the nun> ber of simultaneously oscillating lines (see table 1) but appreciably increases the stability of the laser emission. In table 1 observed Te 2 laser lines for the 476.5 nm argon laser pump line are summarized. No attempts were made to identify the pump and laser transitions because this would require more accurate wavelength measurements. Each group of lines given in table 1 is obtained for a specific single mode position within the argon laser emission range and should
I (rel,U.)
Te 2 },p: 476.5nm
r 660
--
600
i
,
Wovelen g'lh
c [nm]
550
Fig. 2. Multiline laser oscillation spectrum of Te 2 . Pump line 476.5 nm (Multimode), p u m p power 2W.
392
Volume 26, number 3
OPTICS COMMUNICATIONS
September 1978
Table 1 Cw Te 2 laser lines Pump line 476.5 nm a) Laser lines (nm) b)
c)
564.3 585.9
564.7 592.7
571.5 593.6
572.0 600.4
576.7 600.8
578.4 608.3
579.0 608.7
585.1 616.8
557.5 587,4
557.9
565.0
571.9
572.4
579.4
579.8
587.0
577,4
578.3
584.9
585.7
592.4
593.4
600.2
562.6 584.1 627.8
569.6 585.1 628.7
570.1 600.5 637.1
571.4 600.9 637.9
576.6 608.2 646.5
577.3 608.5 647.3
578.0 616.2 656.1
578,7 616.5 656.9
600.5
600.9
608.5
608.9
616.5
617.0
557.1
563.8
564.2
571.l
571.5
578.5
578.9
586.5
557.5 579.7 648.4
557.8 586.9 657.4
564.6 620.4 658.1
564.9 628.8
570.1 629.5
571.9 638.1
572.1 638.8
579.3 647.7
a) Single mode, typical pump power lW. b) Wavelength accuracy -+0.2 nm. c) Multiline output power 20 roW.
belong to a fluorescence series. Only for this line more detailed investigations o f the possible laser lines have been made. However, laser oscillation occurs for all other blue argon laser lines and similar numbers of transitions and laser lines are possible. The typical output power for a group of lines is several mW. The threshold values are generally below 20 roW, with lowest values below 1 mW for a focal length o f the focussing lens o f 500 m m (beam waist approximately 100/am). For the indicated group of lines a maximum output power of 20 mW (multiline) and up to 3 mW (single fine) was obtained for a pump power o f 1 W. With a length o f the vapor zone of 18 cm a maximum gain of about 15% was observed. However, all these values are preliminary and much higher power values are expected, as not all parameters have been optimized so far. Especially the preparation of the melted o f f quartz cells was insufficient and impurities may quench stronger laser oscillation. To obtain the maximum output power it is also necessary to match a cavity mode o f the dimer laser exactly to the narrow amplification profile which is generated b y the single
mode pump laser (see also discussion below: sub Doppler optical pumping).
3. Bi 2 laser
Fig. 3 shows a part of the energy level diagram of Bi 2 [13]. Some possible pump and laser transitions for excitation with the 514.5 nm argon laser fine, identified b y G. Gerber [15], are shown in the diagram. Due to this analysis pump transitions start from the higher lying ground state component X(0~-) [12,13]. This is in agreement with the observed higbh optimum operating temperature of 1300 K, which corresponds to a total (Bi and Bi2) vapor pressure o f 28 Tort, with a Bi 2 partial pressure o f about 6 Torr [14[. Optimum heatpipe operation at this temperature would require a buffer gas pressure o f 28 Torr in equilibrium with the Bi, Bi 2 vapor. We found better operating conditions o f the Bi2 laser at a buffer gas pressure around 1 0 - 5 5 Tort, which means that the Bi2 vapor is slightly overheated at a vapor pressure o f 393
Volume 26, number 3
OPTICS COMMUNICATIONS
E llOacn~l] 24 23
....
2,_
/
22 21 2O 19 9 8 35
7 6 5 4 3
V"
2
. Zs_op . . . . . . . . .
11 o~..z
I \
0
\
/
. . . . . . . 0.2
/
:"':
. . . . . . .
0.25
0.3
[.-m] 0.35 R
September 1978
only 2 - 3 Torr. In table 2 some observed dimer laser lines and other laser data are summarized. Again the number of laser lines is incomplete, as the absorption spectrum is very dense and not all single mode pump transitions which are possible with the 514.5 nm argon laser line have been investigated. Much weaker laser oscillation is also possible with some shorter wavelength argon laser lines. A remarkable result is the high output power obtained for multiline and single line emission, which makes the Bi2 laser system an attractive transformation system. A possible reason for the high output power and good conversion efficiency may be an effective relaxation process in the lower laser levels, due to perturbations by other electronic states [13]. Perhaps, quenching by exchange collisions with Bi atoms, similar to the Na/Na 2 system [16], might also be possible. Furthermore, the quenching cross section for a depopulation of the upper laser level seems to be very small, as buffer gas pressures up to 100 Torr were necessary to completely quench laser oscillation at the used pump power. More detailed experiments on high pressure operation with different buffer gases are in preparation, to determine the quenching cross sections and to evaluate the tuning possibilities of heavy molecular dimer lasers.
Fig. 3. Energy level diagram of Bi2 (paxt, after ref. [13] ) with pump and laser transitions. Table 2 Data of the cw Bi2 dimer laser Laser wavelength (nm) a)
Typical threshold (mW)
Output power b) (roW)
592.9 658.2
616.0 665.0
633.9
657.6
1000
20
623.9 660.3
630.0 680.9
641.4 700.6
642.2 701.3
400
100
729.2
730.1
736.6
737.6
300
150
736.4
743.9
733.5 748.2
739.8
740.8
747.1
100
350
746.8
747.5
754.3
755.1
400
150
400
a ) Accuracy +-0.2 nm. b) Multiline; pump power up to 3.5 W, pump line 514.5 nm (single mode). Length of vapor zone 20 cm, maximum gain ~20%. The groups of lines are obtained for different single mode positions (pump transitions) and belong to a fluorescence series (see ref. [15]).
394
Volume 26, number 3
OPTICS COMMUNICATIONS
4. Sub Doppler optical pumping Dimer molecules are most effectively pumped with single frequency pump laser radiation. In this case, however, special effects arise, as dimer lasers are three level laser systems with coupled Doppler broadened transitions (see ref. [ 17] and further refs. given there). The single frequency pump laser only excites a certain velocity group Vp of molecules and thus generates a narrowband homogeneous gain profile. The dimer laser therefore only has gain for frequencies w ± = w32(1 -+ where w32 is the molecular transition frequency. The + signs stand for amplification in the direction o f the pump radiation (+) and opposite to this direction ( - ) . In general the g a i n g + is much larger than g - [18]. Due to the anisotropic amplification, the most adequate laser system (for arbitrary velocities Up) is a ring laser. In such a ring laser system laser oscillation in only one direction and at only a single frequency + co occurs and the dimer laser frequency can be tuned b y tuning the pump laser frequency. We observed unidirectional single frequency laser oscillation for all dimer laser systems investigated so far. In case o f Na 2 output powers up to 200 mW and tuning ranges o f more than 4 GHz have been obtained [19]. In a linear resonator with standing waves best laser performance is achieved for Up = 0, that means, the pump laser must be tuned to the exact absorption frequency and the dimer laser resonator mode must be adjusted to coincide with the molecular transition frequency ~32" The dimer laser then also oscillates (for a selected line) in a single frequency only. Due to fluctuations of the pump laser frequency and the optical resonator length of the dimer laser, the output power and the laser spectrum t o o may fluctuate appreciably. To obtain a more stable laser emission, stabilization of both laser systems is necessary.
Vp/C),
5. Conclusion optically pumped laser oscillation for the heavy dimer molecules Bi 2 and Te 2 has been demonstrated. As the spectroscopic work on these molecules is not yet completed, these dimer lasers may now be very helpful for detailed spectroscopic investigations. In
September 1978
combination with a ring laser set-up, also high resolution saturation spectroscopy can be performed. By proper optimization of laser parameters and the preparation o f cells and heatpipes, higher output powers for Te 2 and efficiencies of more than 10% for Bi 2 should be possible. Furthermore, the Te 2 and Bi 2 systems are interesting candidates for high pressure investigations and discharge excitation experiments.
References [ 1] B. Wellegehausen, S. Shahdin, D. Friede und H. Welling, Appl. Phys. 13 (1977) 97. [2] H. Welling and B. Wellegehausen, Laser Spectroscopy III, Springer Series in Optical Sciences 7, (Springer Verlag New York - Heidelberg - Berlin 1977). [3] B. Wellegehausen, K.H. Stephan, D. Friede and H. Welling, Opt. Commun. 23 (1977) 157. [4] R.L. Byer, R.L. Herbst, H. Kildal and M.D. Levenson, Appl. Phys. Lett. 20 (1972) 463. [5] J.B. Koffend, R.W. Field, D.R. Guyer and S.R. Leone, Laser Spectroscopy III, Springer Series in Optical Sciences 7, (Springer Verlag New York - Heidelberg - Berlin 1977). [6] B. Wellegehausen, G. Steger, to be published. [7] S.R. Leone and K.G. Kosnik, Appl. Phys. Lett. 30 (1977) 346. [8] D.R. Guyer and S.R. Leone, 5th Conf. on Chem. and Mol. Lasers, St. Louis (1977). [9] W.P. West and H.P. Broida, Chem. Phys. Lett. 56 (1978) 283. [10] R.P. Barrow and R.P. Du Parcq, Proc. R. Soc. Lond. A 327 (1972) 279. [11 ] K.K. Yee and R.F. Barrow, J. Chem. Soc. Faraday Trans. II 68 (1972) 1397. [12] G. Gerber, K. Sakurai and H.P. Broida, Journ. Chem. Phys. 64 (1976) 3410. [13] G. Gerber and H.P. Broida, Journ. Chem. Phys. 64 (1976) 3423. [14] A.N. Nesmeyanov, Vapor pressure of the chemical elements (Elsevier Publ. Comp. New York 1963). [15] G. Gerber and J. Janes, priyate communication; to be published J. Mol. Spectrosc. [ 16] W. Demtr6der, K. Bergmann, private communication. [17] I.M. Beterov and V.P. Chebotayev, Progress in quantum electronics, Vol. 3 (Pergamon Press, 1974). [18] N. Skribanowitz, M.S. Feld, R.E. Francke, M.J. Kelly and A. Javan, Appl. Phys. Lett. 19 (1971) 161. [ 19 ] B. Wellegehausen and H.H. Heitmann, to be published; see also: Digest of technical papers, Tenth Internatl. Quant. Electr. Conf., Atlanta 1978.
395