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He-Ne laser pumped dimer lasers
Volume 48, number 4
OPTICS COMMUNICATIONS
15 December 1983
He-Ne LASER PUMPED DIMER LASERS
W. LUHS, M. HUBE, U. SCHOTTELIUS and B. WELLEGEHAUSEN Institut ff,r Quantenoptik, UniversitdtHannover, 3000 Hannover, Fed. Rep. Germany Received 10 October 1983
Continuous laser oscillation of the dimer molecules Na2, K2 and 12 on more than 40 lines in the spectral range of 685 nm to 1170 nm has been obtained by optical pumping with a He-Ne laser of up to 25 mW output power. Thresholds as low as 1 mW and output powers up to 1.5 mW could be achieved. Properties of these simple optical conversion systems will be discussed.
1. Introduction
Optically pumped dimer lasers have now been investigated for quite some time and presently dimer laser systems with the molecules Li 2 ,Na 2, K 2, Bi 2, S2, Se2, Te 2 and 12 operate continuously with hundreds of lines in the spectral range of about 380 nm to 1300 nm [ 1 - 3 ] . All these dimer lasers so far have been excited with visible or uv argon- and kryptonion laser lines and, depending on the pump power and the considered system, conversion efficiencies of up to 15%, multiline output powers of 400 mW and single line output powers of 100 mW have been observed. However, high power lasers are not necessary to achieve laser oscillation, as most of these dimer systems typically have thresholds below 50 mW and for some, thresholds of only a few milliwatts have be,,n observed. For a sodium dimer supersonic-beam laser [4] even a threshold of only 17/aW has so far been verified [5]. This means, that also low power and low cost pump lasers might eventually be used for excitation of these dimer molecules, if a good coincidence between the pump laser line and molecular absorption lines exists. A suitable combination of laser and molecules is given by the He-Ne laser and the easy to handle molecules Na2, K 2 and 12. The HeNe laser line at 633 nm lies well within the X 1~+ /5 A 1 ~+ absorption band of Na~ and within the X1Z~g÷~ B1Hu band of K-2 and the X 11~0+ ~ g B 3 li0u band of 12 [6-1 1]. He-Ne laser excited fluorescence investigations of Na 2 molecules have re-
cently been published [6,7] and iodine stabilized HeNe lasers are well known [12,13]. In this communication we report on investigations of He-Ne laser pumped dimer laser systems. By use of commercially available He-Ne lasers with up to 25 mW output power, first continuous laser oscillation of Na 2, K 2 and 12912 molecules on more than 40 lines in the spectral range of 685 nm to 1170 nm could be achieved.
2. Experimental The experimental set-up is shown in fig. 1. A multimode He-Ne laser is focussed into the molecular vapor, which is generated in a heatpipe in case of the alkalies or in a sealed-off quartz cell in case of iodine. The applied optical resonators are simple two element concentric resonators as indicated in fig. 1. The principle of a dimer laser can be taken from fig. 2. The pump radiation excites one or several rotationalvibrational levels in an excited electronic state of the dimer molecule and creates a population inversion R
R
CE
LL/H EATPIPE
Fig. 1. Experimental set-up for dimer laser.
265 0 030-4018/83/0000-0000/$ 03.00 © 1983 North-Holland
Volume 48, number 4
OPTICS COMMUNICATIONS
between these levels and m a n y r o t a t i o n a l - v i b r a t i o n a l levels o f the electronic ground state. Laser oscillation can be e x p e c t e d especially on those transitions which have high Franck- C o n d o n coefficients and terminate in ground state levels that are thermally m u c h less p o p u l a t e d than the initial p u m p level. A detailed discussion o f optically p u m p e d dimer lasers with inclusion o f c o h e r e n t effects (Rarnan gain c o n t r i b u t i o n ) is given in ref. [ 1 ]. Using the therein given formulas and available molecular data, threshold p u m p intensities o f 1 W/cm 2 for m u l t i m o d e He-Ne laser p u m p e d Na 2 molecules can be calculated. E x p e r i m e n t a l l y , lowest
M ~
L
INTERNUCLEAR
15 December 1983
DISTANCE
,91
Fig. 2. Laser cycle within the energy level scheme of a dimer molecule M2.
Table 1 Data of He-Ne laser pumped dimer lasers Molecule Pump transition
Na 2 (2, 64) ~ (14, 45) (4, 18)-+ (16, 17)
K2 X 15"~ ~ B 1Flu (0, 82) ~ (7, 81)
12912 X 1 ~0g+ ~ B3170u+ (4, 59) ~ (8, 60)
Laser lines [nm] a)
752.0 766.0 785.8 796.3 800.1 807.0 812.4 814.0
765.0 785.3 792.9 798.5 801.3 808.4 812.8 815.7
685.3 689.1 692.2 692.9 696.0 699.5 700.2 838.8 851.6 892.4 905.9 1031.5 1050.0 1067.5 1104.5 1144.0
a) Accuracy ± 0.2 nm. b) Multiline; pump power 25 mW (633 nm). 266
Threshold [mW]
839.3 852.6 893.0 906.8 1048.5 1066.5 1083.5 1124.0 1163.0
Output power [mW] b)
Remarks
vapor zone 3 cm temperature 930 K 1.5
vapor zone 8 cm temperature 870 K output coupling 5.5%
8
0.1
vapor zone 25 cm temperature 600 K system not optimized
4
0.1
vapor zone 25 cm temperature 315 K system not optimized
Volume 48, number 4
OPTICS COMMUNICATIONS
(b)
Z
5 (a)
z z
r
750
775
WAVELENGTH
825
800
.
[nrn]
Fig. 3. a) Fluorescence spectrum (part) of the He-Ne laser excited Na2. b) Laser oscillation spectrum. Pump power 25 temperature 870 K.
threshold pump power of 1.3 mW has been observed, corresponding to a pump intensity of about 5 W/cm 2 for a focal length of the input lense of 260 mm. Similar threshold pump powers have also been obtained for K 2 and 12 molecules (see table 1). As an example fig. 3a shows a part of the Na 2 A 1~+ ~ X 1 ~.; fluorescence spectrum excited by the He-Ne laser. The total fluorescence spectrum extends from 620 nm to 820 nm. The He-Ne laser mainly excites the (2, 46) --*(14, 45) and the (4, 18) -+(16, 17) transitions in the X ~ A absorption band [6,7] and the fluorescence spectrum therefore consists of two progressions. A corresponding multiline laser oscillation spectrum, obtained from a two element concentric resonator with high mirror reflectivity in the range of 700 nm to 830 nm is shown in fig. 3b. It can be seen, that nearly all of the fluorescence lines around 800 nm oscillate. Additional laser lines at shorter wavelengths can also be obtained (see table 1) if the strongly oscillating lines around 800 nm are suppressed by suitable mirrors or by selection elements. For K 2 the B 1 ii u state is excited and the B 1Hu X 1 Zg+ fluorescence lines ly in the range of 630 nm to
15 December 1983
720 nm [8,9] and laser oscillation is obtained at wavelengths around 690 nm (table 1). For iodine different isotopes may be used for laser investigations. The natural 12712 molecule can be excited with the He-Ne laser on the X 12;0 + B 3II0u+ transition out of the level v " = 5 , J " g 127 [ 10-12]. However, at the given pump power, laser oscillations was not possible, probably due to a not good enough spectral overlap between the molecular absorption line and the He-Ne laser emission profile. Oscillation of 127 i2 molecules upon excitation with shorter wavelengths is well known [ 14]. For the 12912 isotope the spectral overlap is much better, which can be seen from the stronger absorption of the He-Ne laser emission. The main pump level in this case is o" = 4, J " = 59. So far laser oscillation for this isotope has been achieved on lines distributed in the region from 838 nm to 1163 nm (table 1). For optimum operation of a dimer system, temperature (dimer partial pressure), length of vapor zone and output coupling have to be optimized with respect to the used pump intensity (pump power and focussing of the pump radiation). More detailed investigations to this have so far only been performed for the Na 2 system, which gives highest gain and output power, allowing a broader variation of the parameters. For iodine and potassium only a single length of the vapor zone has been used (see table 1). Figs. 4 and 5 show the dependence of threshold and output power for the Na 2 laser of the length of the vapor zone for a fixed concentric resonator (R 1 = R 2 = 260 mm, resonator length 480 mm, output coupling
~L
75O K
760K
r~ 2 =
0
i
i
i
5o
100
150
VAPOR
ZONE
[_mmJ
Fig. 4. Threshold p u m p power versus length of vapor zone.
267
Volume 48, number 4
OPTICS COMMUNICATIONS
high temperatures are difficult to operate. The optimum vapor zone for high output power obtained here is valid for the given pump power and pump focussing Due to saturation higher pump powers require hmger vapor zones and/or weaker focussing. For the vapor zone of 80 mm, fig. 6 gives an output coupling curve witt a maximum multiline output power of about 1.5 mW for the lines around 800 mm (see fig. 3b), corresponding to an efficiency of 6%.
870K LO-
~T
9
uJ
3
0
K
~
/ 00.51010K t
5 O
0
50
l
100
VAPOR ZONE
150
[mm]
Fig. 5. Dependence of output power on length of vapor zone.
~0.1%) and a given focussing of the pump laser radiation (focal length 260 mm). The corresponding optimum temperatures for the different vapor zones are indicated in the diagrams. As can be seen, lowest thresholds are obtained for short vapor zones, whereas longer vapor zones yield higher output power. For the threshold a short region of high intensity just around the pump focus is most important to achieve the required gain, while for the output power a certain volume which is sufficiently excited is necessary. A further reduction of the threshold is possible for still shorter vapor zones and stronger focussing of the pump radiation, however, very short vapor zones at
ua
O a. 0.5i--
O 0
I
0
5 OUTPUT
COUPLING [%
J
I0 ]
Fig. 6. Na 2 dimer laser output power in dependence of tile o u t p u t coupling. Length of vapor zone 80 mm, pump power
25 mW, temperature 870 K. 268
l 5 December 1983
3. Conclusion Comparing at present the Na 2, K 2 and 12 system, the He-Ne pumped Na 2 laser gives highest gain and output power and is uncritical to operate. This is mainly due to a better coincidence of the pump frequency with the molecular transition frequency and a more favorable ratio of the laser emission bandwidth to the Doppler halfwidth of the molecular absorption line. However, as the K2and 12 system so far have not been investigated in detail, it is expected that these lasers can be further improved, where especially the iodine laser is attractive, due to its ease of operation with a sealed-off cell at low temperature and a possible broad spectral range. Here, only simple two element resonators have been applied, which give multiline emission, where a single line only carries a small part of the total output power. The single line output power can be appreciably improved if resonators with internal selection elements are used, as all lines start from the same upper laser level and have to share population inversion. The most adequate optical resonator for dimer lasers is thereby a ring resonator, due to the intrinsic forward-backward amplification asymmetry induced by the Raman process [ 1,15]. In some first experiments with an X-shaped ring resonator, simultaneous oscillation of the Na 2 laser in both directions has been observed, with an intensity ratio of about 1000 : 1 in favor of the forward direction (direction of the pump radiation). This is different to the so far investigated single-mode pumped dimer lasers, where practically only the forward direction oscillates and indicates a weaker asymmetry for this multimode pumped system. It should also be mentioned that in case ofmultimode pumping the mode structures of the pump laser and of the dimer laser should be matched be-
Volume 48, number 4
OPTICS COMMUNICATIONS
cause for a Doppler broadened material each pump laser mode only excites a certain velocity subgroup, which is then responsible for the dimer laser emission [1]. The He-Ne laser pumped dimer lasers described here are first examples that simple continuously operating optical conversion systems may be developed, which allow generation of many new laser lines. In combination with the He-Ne laser also Li 2 molecules may eventually be used, however, temperatures of more than 1200 K will be necessary. Looking for similar systems for the visible spectral range, the He-Cd laser seems to be suited. In first experirnents with a He-Cd laser o f only 7 mW output power at 441 nm, we could achieve laser oscillation of 130Te 2 molecules on wavelength at 601.0 nm, 608.6 nm and 616.5 nm. The Te 2 molecules give fluorescence lines in the range of 441 nm to 650 nm, and it is expected that with higher pump power a great portion of this range may be covered with laser lines. Finally, considering applications, these systems may be o f interest in all those cases, where only fixed low power laser lines are required, such as for reference, standard and measuring devices.
15 December 1983
References [1] B. WeUegehausen, IEEE J. Quantum Electron. QE-15 (1979) 1108. [2] B. Wellegehausen, A. Topouzkhanian, C. Effantin and J. d'Incan, Optics Comm. 41 (1982) 437. [ 3] A. Topouzkhanian, B. Wellegehausen, C. Effantin and J. d'Incan, Laser Chemistry 1 (1983) 195. [4] P.L. Jones, U. Gaubatz, U. Hefter, K. Bergmarm and B. WeUegehausen, Appl. Phys. Lett. 42 (1983) 222. [ 5 ] K. Berman, personan communication. [6] K.K. Verma, T.H. Vu and W.C, Stwalley, J. Molecular Spectr. 85 (1981) 131. [7] K.K. Verma, W.C. StwaUey and W.T. Zemke, J. Appl. Phys. 52 (1981) 5419. [8] W.J. Tango, J.K. Link and R.N. Zare, J. Chem. Phys. 49 (1968) 4264. [9] M. Allegrini, P. Bicchi, M. Civilini and L. Moi, Chem. Phys. Lett. 91 (1982) 63. [10] J.I. Steinfeld, R.N. Zare, L. Jones, M. Lesk and W. Klemperer, J. Chem. Phys. 42 (1965) 25. [11] P. Luc, J. Molecular Spectr. 80 (1980) 41. [12] G.R. Hanes and C.E. Dahistrohm, Appl. Phys. Lett. 14 (1969) 362. [131 A.J. Wallard, J. Phys. E5 (1972) 926. [14] B. Wellegehausen, K.H. Stephan, D. Friede and H. Welling, Optics Comm. 23 (1977) 157. [15] B. Wellegehausen and H.H. Heitmann, Appl. Phys. Lett. 34 (1979) 44.
269
OPTICS COMMUNICATIONS
15 December 1983
He-Ne LASER PUMPED DIMER LASERS
W. LUHS, M. HUBE, U. SCHOTTELIUS and B. WELLEGEHAUSEN Institut ff,r Quantenoptik, UniversitdtHannover, 3000 Hannover, Fed. Rep. Germany Received 10 October 1983
Continuous laser oscillation of the dimer molecules Na2, K2 and 12 on more than 40 lines in the spectral range of 685 nm to 1170 nm has been obtained by optical pumping with a He-Ne laser of up to 25 mW output power. Thresholds as low as 1 mW and output powers up to 1.5 mW could be achieved. Properties of these simple optical conversion systems will be discussed.
1. Introduction
Optically pumped dimer lasers have now been investigated for quite some time and presently dimer laser systems with the molecules Li 2 ,Na 2, K 2, Bi 2, S2, Se2, Te 2 and 12 operate continuously with hundreds of lines in the spectral range of about 380 nm to 1300 nm [ 1 - 3 ] . All these dimer lasers so far have been excited with visible or uv argon- and kryptonion laser lines and, depending on the pump power and the considered system, conversion efficiencies of up to 15%, multiline output powers of 400 mW and single line output powers of 100 mW have been observed. However, high power lasers are not necessary to achieve laser oscillation, as most of these dimer systems typically have thresholds below 50 mW and for some, thresholds of only a few milliwatts have be,,n observed. For a sodium dimer supersonic-beam laser [4] even a threshold of only 17/aW has so far been verified [5]. This means, that also low power and low cost pump lasers might eventually be used for excitation of these dimer molecules, if a good coincidence between the pump laser line and molecular absorption lines exists. A suitable combination of laser and molecules is given by the He-Ne laser and the easy to handle molecules Na2, K 2 and 12. The HeNe laser line at 633 nm lies well within the X 1~+ /5 A 1 ~+ absorption band of Na~ and within the X1Z~g÷~ B1Hu band of K-2 and the X 11~0+ ~ g B 3 li0u band of 12 [6-1 1]. He-Ne laser excited fluorescence investigations of Na 2 molecules have re-
cently been published [6,7] and iodine stabilized HeNe lasers are well known [12,13]. In this communication we report on investigations of He-Ne laser pumped dimer laser systems. By use of commercially available He-Ne lasers with up to 25 mW output power, first continuous laser oscillation of Na 2, K 2 and 12912 molecules on more than 40 lines in the spectral range of 685 nm to 1170 nm could be achieved.
2. Experimental The experimental set-up is shown in fig. 1. A multimode He-Ne laser is focussed into the molecular vapor, which is generated in a heatpipe in case of the alkalies or in a sealed-off quartz cell in case of iodine. The applied optical resonators are simple two element concentric resonators as indicated in fig. 1. The principle of a dimer laser can be taken from fig. 2. The pump radiation excites one or several rotationalvibrational levels in an excited electronic state of the dimer molecule and creates a population inversion R
R
CE
LL/H EATPIPE
Fig. 1. Experimental set-up for dimer laser.
265 0 030-4018/83/0000-0000/$ 03.00 © 1983 North-Holland
Volume 48, number 4
OPTICS COMMUNICATIONS
between these levels and m a n y r o t a t i o n a l - v i b r a t i o n a l levels o f the electronic ground state. Laser oscillation can be e x p e c t e d especially on those transitions which have high Franck- C o n d o n coefficients and terminate in ground state levels that are thermally m u c h less p o p u l a t e d than the initial p u m p level. A detailed discussion o f optically p u m p e d dimer lasers with inclusion o f c o h e r e n t effects (Rarnan gain c o n t r i b u t i o n ) is given in ref. [ 1 ]. Using the therein given formulas and available molecular data, threshold p u m p intensities o f 1 W/cm 2 for m u l t i m o d e He-Ne laser p u m p e d Na 2 molecules can be calculated. E x p e r i m e n t a l l y , lowest
M ~
L
INTERNUCLEAR
15 December 1983
DISTANCE
,91
Fig. 2. Laser cycle within the energy level scheme of a dimer molecule M2.
Table 1 Data of He-Ne laser pumped dimer lasers Molecule Pump transition
Na 2 (2, 64) ~ (14, 45) (4, 18)-+ (16, 17)
K2 X 15"~ ~ B 1Flu (0, 82) ~ (7, 81)
12912 X 1 ~0g+ ~ B3170u+ (4, 59) ~ (8, 60)
Laser lines [nm] a)
752.0 766.0 785.8 796.3 800.1 807.0 812.4 814.0
765.0 785.3 792.9 798.5 801.3 808.4 812.8 815.7
685.3 689.1 692.2 692.9 696.0 699.5 700.2 838.8 851.6 892.4 905.9 1031.5 1050.0 1067.5 1104.5 1144.0
a) Accuracy ± 0.2 nm. b) Multiline; pump power 25 mW (633 nm). 266
Threshold [mW]
839.3 852.6 893.0 906.8 1048.5 1066.5 1083.5 1124.0 1163.0
Output power [mW] b)
Remarks
vapor zone 3 cm temperature 930 K 1.5
vapor zone 8 cm temperature 870 K output coupling 5.5%
8
0.1
vapor zone 25 cm temperature 600 K system not optimized
4
0.1
vapor zone 25 cm temperature 315 K system not optimized
Volume 48, number 4
OPTICS COMMUNICATIONS
(b)
Z
5 (a)
z z
r
750
775
WAVELENGTH
825
800
.
[nrn]
Fig. 3. a) Fluorescence spectrum (part) of the He-Ne laser excited Na2. b) Laser oscillation spectrum. Pump power 25 temperature 870 K.
threshold pump power of 1.3 mW has been observed, corresponding to a pump intensity of about 5 W/cm 2 for a focal length of the input lense of 260 mm. Similar threshold pump powers have also been obtained for K 2 and 12 molecules (see table 1). As an example fig. 3a shows a part of the Na 2 A 1~+ ~ X 1 ~.; fluorescence spectrum excited by the He-Ne laser. The total fluorescence spectrum extends from 620 nm to 820 nm. The He-Ne laser mainly excites the (2, 46) --*(14, 45) and the (4, 18) -+(16, 17) transitions in the X ~ A absorption band [6,7] and the fluorescence spectrum therefore consists of two progressions. A corresponding multiline laser oscillation spectrum, obtained from a two element concentric resonator with high mirror reflectivity in the range of 700 nm to 830 nm is shown in fig. 3b. It can be seen, that nearly all of the fluorescence lines around 800 nm oscillate. Additional laser lines at shorter wavelengths can also be obtained (see table 1) if the strongly oscillating lines around 800 nm are suppressed by suitable mirrors or by selection elements. For K 2 the B 1 ii u state is excited and the B 1Hu X 1 Zg+ fluorescence lines ly in the range of 630 nm to
15 December 1983
720 nm [8,9] and laser oscillation is obtained at wavelengths around 690 nm (table 1). For iodine different isotopes may be used for laser investigations. The natural 12712 molecule can be excited with the He-Ne laser on the X 12;0 + B 3II0u+ transition out of the level v " = 5 , J " g 127 [ 10-12]. However, at the given pump power, laser oscillations was not possible, probably due to a not good enough spectral overlap between the molecular absorption line and the He-Ne laser emission profile. Oscillation of 127 i2 molecules upon excitation with shorter wavelengths is well known [ 14]. For the 12912 isotope the spectral overlap is much better, which can be seen from the stronger absorption of the He-Ne laser emission. The main pump level in this case is o" = 4, J " = 59. So far laser oscillation for this isotope has been achieved on lines distributed in the region from 838 nm to 1163 nm (table 1). For optimum operation of a dimer system, temperature (dimer partial pressure), length of vapor zone and output coupling have to be optimized with respect to the used pump intensity (pump power and focussing of the pump radiation). More detailed investigations to this have so far only been performed for the Na 2 system, which gives highest gain and output power, allowing a broader variation of the parameters. For iodine and potassium only a single length of the vapor zone has been used (see table 1). Figs. 4 and 5 show the dependence of threshold and output power for the Na 2 laser of the length of the vapor zone for a fixed concentric resonator (R 1 = R 2 = 260 mm, resonator length 480 mm, output coupling
~L
75O K
760K
r~ 2 =
0
i
i
i
5o
100
150
VAPOR
ZONE
[_mmJ
Fig. 4. Threshold p u m p power versus length of vapor zone.
267
Volume 48, number 4
OPTICS COMMUNICATIONS
high temperatures are difficult to operate. The optimum vapor zone for high output power obtained here is valid for the given pump power and pump focussing Due to saturation higher pump powers require hmger vapor zones and/or weaker focussing. For the vapor zone of 80 mm, fig. 6 gives an output coupling curve witt a maximum multiline output power of about 1.5 mW for the lines around 800 mm (see fig. 3b), corresponding to an efficiency of 6%.
870K LO-
~T
9
uJ
3
0
K
~
/ 00.51010K t
5 O
0
50
l
100
VAPOR ZONE
150
[mm]
Fig. 5. Dependence of output power on length of vapor zone.
~0.1%) and a given focussing of the pump laser radiation (focal length 260 mm). The corresponding optimum temperatures for the different vapor zones are indicated in the diagrams. As can be seen, lowest thresholds are obtained for short vapor zones, whereas longer vapor zones yield higher output power. For the threshold a short region of high intensity just around the pump focus is most important to achieve the required gain, while for the output power a certain volume which is sufficiently excited is necessary. A further reduction of the threshold is possible for still shorter vapor zones and stronger focussing of the pump radiation, however, very short vapor zones at
ua
O a. 0.5i--
O 0
I
0
5 OUTPUT
COUPLING [%
J
I0 ]
Fig. 6. Na 2 dimer laser output power in dependence of tile o u t p u t coupling. Length of vapor zone 80 mm, pump power
25 mW, temperature 870 K. 268
l 5 December 1983
3. Conclusion Comparing at present the Na 2, K 2 and 12 system, the He-Ne pumped Na 2 laser gives highest gain and output power and is uncritical to operate. This is mainly due to a better coincidence of the pump frequency with the molecular transition frequency and a more favorable ratio of the laser emission bandwidth to the Doppler halfwidth of the molecular absorption line. However, as the K2and 12 system so far have not been investigated in detail, it is expected that these lasers can be further improved, where especially the iodine laser is attractive, due to its ease of operation with a sealed-off cell at low temperature and a possible broad spectral range. Here, only simple two element resonators have been applied, which give multiline emission, where a single line only carries a small part of the total output power. The single line output power can be appreciably improved if resonators with internal selection elements are used, as all lines start from the same upper laser level and have to share population inversion. The most adequate optical resonator for dimer lasers is thereby a ring resonator, due to the intrinsic forward-backward amplification asymmetry induced by the Raman process [ 1,15]. In some first experiments with an X-shaped ring resonator, simultaneous oscillation of the Na 2 laser in both directions has been observed, with an intensity ratio of about 1000 : 1 in favor of the forward direction (direction of the pump radiation). This is different to the so far investigated single-mode pumped dimer lasers, where practically only the forward direction oscillates and indicates a weaker asymmetry for this multimode pumped system. It should also be mentioned that in case ofmultimode pumping the mode structures of the pump laser and of the dimer laser should be matched be-
Volume 48, number 4
OPTICS COMMUNICATIONS
cause for a Doppler broadened material each pump laser mode only excites a certain velocity subgroup, which is then responsible for the dimer laser emission [1]. The He-Ne laser pumped dimer lasers described here are first examples that simple continuously operating optical conversion systems may be developed, which allow generation of many new laser lines. In combination with the He-Ne laser also Li 2 molecules may eventually be used, however, temperatures of more than 1200 K will be necessary. Looking for similar systems for the visible spectral range, the He-Cd laser seems to be suited. In first experirnents with a He-Cd laser o f only 7 mW output power at 441 nm, we could achieve laser oscillation of 130Te 2 molecules on wavelength at 601.0 nm, 608.6 nm and 616.5 nm. The Te 2 molecules give fluorescence lines in the range of 441 nm to 650 nm, and it is expected that with higher pump power a great portion of this range may be covered with laser lines. Finally, considering applications, these systems may be o f interest in all those cases, where only fixed low power laser lines are required, such as for reference, standard and measuring devices.
15 December 1983
References [1] B. WeUegehausen, IEEE J. Quantum Electron. QE-15 (1979) 1108. [2] B. Wellegehausen, A. Topouzkhanian, C. Effantin and J. d'Incan, Optics Comm. 41 (1982) 437. [ 3] A. Topouzkhanian, B. Wellegehausen, C. Effantin and J. d'Incan, Laser Chemistry 1 (1983) 195. [4] P.L. Jones, U. Gaubatz, U. Hefter, K. Bergmarm and B. WeUegehausen, Appl. Phys. Lett. 42 (1983) 222. [ 5 ] K. Berman, personan communication. [6] K.K. Verma, T.H. Vu and W.C, Stwalley, J. Molecular Spectr. 85 (1981) 131. [7] K.K. Verma, W.C. StwaUey and W.T. Zemke, J. Appl. Phys. 52 (1981) 5419. [8] W.J. Tango, J.K. Link and R.N. Zare, J. Chem. Phys. 49 (1968) 4264. [9] M. Allegrini, P. Bicchi, M. Civilini and L. Moi, Chem. Phys. Lett. 91 (1982) 63. [10] J.I. Steinfeld, R.N. Zare, L. Jones, M. Lesk and W. Klemperer, J. Chem. Phys. 42 (1965) 25. [11] P. Luc, J. Molecular Spectr. 80 (1980) 41. [12] G.R. Hanes and C.E. Dahistrohm, Appl. Phys. Lett. 14 (1969) 362. [131 A.J. Wallard, J. Phys. E5 (1972) 926. [14] B. Wellegehausen, K.H. Stephan, D. Friede and H. Welling, Optics Comm. 23 (1977) 157. [15] B. Wellegehausen and H.H. Heitmann, Appl. Phys. Lett. 34 (1979) 44.
269