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Terahertz Laser Sideband Spectroscopy with Backward Wave Oscillators
JOURNAL OF MOLECULAR SPECTROSCOPY ARTICLE NO.
183, 207–209 (1997)
MS967244
LETTER TO THE EDITOR Terahertz Laser Sideband Spectroscopy with Backward Wave Oscillators F. Lewen, E. Michael, R. Gendriesch, J. Stutzki, and G. Winnewisser I. Physikalisches Institut, Universita¨t zu Ko¨ln, D-50937 Ko¨ln, Germany Received October 29, 1996 We report a broadband tunable terahertz source, based on mixing the radiation from a FIR laser with that of a backward wave oscillator ( BWO ) on an open structure GaAs mixer diode. The tunability of the sideband spectrometer is determined by the frequency coverage of the BWO, which is typically about 100 GHz at 330 GHz center frequency. Up to now, basically three different types of sideband sources are commonly used together with two different methods of coupling the sources to the mixing element. The Berkeley group ( 1 ) uses a FIR laser in conjunction with frequency synthesizers ( up to 26 GHz ) and multipliers for higher sideband frequencies ( up to 75 GHz ) . Coupling is performed either coaxial ( up to 45 GHz ) or for higher frequencies by waveguide techniques. The Nijmegen group ( 2 ) uses phase-locked klystrons up to 114 GHz which are coupled in the waveguide technique to the open structure mixer. In all cases diplexers ( Martin Puplett or Fabry Perot type ) have been incorporated to separate the various frequencies. Evenson et al. ( 3 ) have developed a tunable FIR source by mixing a fixed frequency ( IR ) CO2 laser with a tunable waveguide ( IR ) CO2 laser in a metal – insulator – metal ( MIM ) diode. With this technique frequencies up to 5.4 THz have been reached for spectroscopic investigations. In our experiment the radiation of both sources, the FIR laser and the BWO, are coupled quasioptically to an open structure mixer (see Fig. 1 ) . To separate the upper sideband from the lower sideband and FIR laser carrier frequency, we use a grating monochromator ( 6 grooves /mm, blaze angle 347 ). No additional diplexer is necessary, because the high-frequency separation of the two sidebands enables geometric resolution of the three frequencies on the grating. The optically pumped FIR ring laser was built in-house. In the present experiment it is run at the 1626-GHz
laser line of CH2F2 . This FIR laser exhibits excellent stability of both power and frequency ( 4, 5) , compared with commonly used linear design FIR lasers. The fixed FIR laser at 1626.6 GHz is mixed with a tunable 280- to 380-GHz BWO ( Type OB30, supplied by the ISTOK Research and Production Co., Fryazino, Russia ). The BWO is phase locked against a harmonic of a frequency synthesizer near 18 GHz ( HP 8673E ) . The BWO and PLL unit is described in detail in ( 6, 7 ). Only a small fraction of the BWO submillimeter power is directed toward the detector. With two additional high-pass filters ( two-dimensional inductive grids ) , which are placed between the monochromator and the absorption cell, the remaining BWO power is reduced to the thermal limit. As a detector we use a liquid helium-cooled InSb hot electron bolometer ( QMC Instruments Ltd.) which is magnetically tuned to 1.6 THz with a full width half-power bandwidth of 400 GHz. The corner cube mixer is a new design, incorporating a short whisker with 6l length for the laser frequency and about 1.3l for the BWO frequency. The whisker is mounted on a planar low-pass filter to separate the submillimeter power from the DC connector. The Schottky diode (Type 1T6, Semiconductor Device Laboratory of the University of Virginia) is a high cutoff diode with a 0.45-mm diameter and 0.5- f F junction capacitance. The measured beam separation and the position of the two optical axes are in good agreement with theory (8). In contrast to the limited tuning range (typically 5 GHz) provided by reflex klystrons, the present BWO-based spectrometer allows 100 GHz continuous tuning range from 1910 to 2010 GHz, covering the full atmospheric window at 2 THz. As a demonstration of the capability of this new type of spectrometer, we measured rotational transitions of carbon monoxide, CO (J Å 18 R 17 and J Å 17 R 16), and of deuterated chloric acid gas, DCl (J Å 6 R 5). The hyperfine components
TABLE 1 Observed and Calculated Frequencies of D 35Cl for the Rotational Transition J Å 6 R 5
a
From Ref. (9). 207 0022-2852/97 $25.00 Copyright q 1997 by Academic Press All rights of reproduction in any form reserved.
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6t18$$$181
04-15-97 04:28:40
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208
FIG. 1.
Schematic view of the Cologne BWO-based tunable FIR laser spectrometer.
LETTER TO THE EDITOR
Copyright q 1997 by Academic Press
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6t18$$7244
04-15-97 04:28:40
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209
LETTER TO THE EDITOR
FIG. 2. The observed transition of (a) J Å 6 R 5 of D 35Cl and (b) J Å 18 R 17 of 13CO. The DCl spectrum in the central hyperfine component shows saturation.
TABLE 2 Observed and Calculated Frequencies for Carbon Monoxide Isotopomers
a b
From Ref. (10). From Ref. (11).
F Å 9/2 R 9/2 and F Å 13/2 R 13/2 of D 35Cl are well separated from the central four hyperfine components, which are not resolved (Fig. 2a). The dip in the center is caused by saturation effects. The 13CO (Fig. 2b) and C 18O lines are measured in natural abundance. No attempt was made to reduce the atmospheric losses. The ring laser frequency is shifted by É2.9 MHz in comparison to a linear FIR laser, because only one asymmetric velocity class in the Doppler-broadened pump transition profile is excited. From the known spectroscopic data of the species measured (9–11), we determine the ring laser frequency to be 1 626 599.7 MHz. The repeatability of the FIR laser frequency within the gain profile is estimated to be £1 MHz, as is documented by the small scatter quoted in column 5 of Tables 1 and 2. This error is mainly due to the settings of the pump laser and the FIR ring laser resonators, which determine the final accuracy of the measured line position (Tables 1 and 2).
ACKNOWLEDGMENTS
This work was supported by the Deutsche Forschungsgemeinschaft through Grant SFB-301 and through Grant WI 391/7-1. The construction of the laser was partly supported by the Verein der Freunde und Fo¨rderer der Universita¨t Ko¨ln e. V. In addition we also thank the Ministry of Science and Technology of the State Nordrhein–Westfalen for support.
REFERENCES 1. R. C. Cohen, K. L. Busarow, K. B. Laughlin, G. A. Blake, M. Havenith, Y. T. Lee, and R. J. Saykally, J. Chem. Phys. 89, 4494 (1988). 2. P. Verhoeve, E. Zwart, M. Versluis, M. Drabbels, J. J. ter Meulen, W. Leo Meerts, A. Dymanus, and D. B. McLay, Rev. Sci. Instrum. 61, 1612 (1990). 3. K. M. Evenson, D. A. Jennings, and F. R. Petersen, Appl. Phys. Lett. 44, 576 (1984). 4. E. Michael and J. Stutzki, Int. J. Infrared Millimeter Waves 17(8) (1996). 5. E. Michael, F. Lewen, R. Gendriesch, J. Stutzki, and G. Winnewisser, in ‘‘Proceedings, 4th International Workshop on Terahertz Electronics, Erlangen, 1996.’’ 6. G. Winnewisser, A. F. Krupnov, M. Yu. Tretjakov, M. Lietke, F. Lewen, A. H. Saleck, R. Schieder, A. P. Shkaev, and S. A. Volokhov, J. Mol. Spectrosc. 165, 294–300 (1994). 7. G. Winnewisser, Vib. Spectrosc. 8, 241–253 (1995). 8. J. Zmuidzinas, A. L. Betz, and R. T. Boreiko, Infrared Phys. 29, 119– 131 (1989). 9. L. Fusina, P. De Natale, M. Prevedelli, and L. R. Zink, J. Mol. Spectrosc. 152, 55–61 (1994). 10. T. D. Varberg and K. M. Evenson, Astrophys. J. 385, 763 – 765 ( 1992 ) . 11. J. A. Coxon and P. G. Hajigeoriou, Can. J. Phys. 70, 40 – 54 ( 1992 ) .
Copyright q 1997 by Academic Press
AID
JMS 7244
/
6t18$$$181
04-15-97 04:28:40
mspal
183, 207–209 (1997)
MS967244
LETTER TO THE EDITOR Terahertz Laser Sideband Spectroscopy with Backward Wave Oscillators F. Lewen, E. Michael, R. Gendriesch, J. Stutzki, and G. Winnewisser I. Physikalisches Institut, Universita¨t zu Ko¨ln, D-50937 Ko¨ln, Germany Received October 29, 1996 We report a broadband tunable terahertz source, based on mixing the radiation from a FIR laser with that of a backward wave oscillator ( BWO ) on an open structure GaAs mixer diode. The tunability of the sideband spectrometer is determined by the frequency coverage of the BWO, which is typically about 100 GHz at 330 GHz center frequency. Up to now, basically three different types of sideband sources are commonly used together with two different methods of coupling the sources to the mixing element. The Berkeley group ( 1 ) uses a FIR laser in conjunction with frequency synthesizers ( up to 26 GHz ) and multipliers for higher sideband frequencies ( up to 75 GHz ) . Coupling is performed either coaxial ( up to 45 GHz ) or for higher frequencies by waveguide techniques. The Nijmegen group ( 2 ) uses phase-locked klystrons up to 114 GHz which are coupled in the waveguide technique to the open structure mixer. In all cases diplexers ( Martin Puplett or Fabry Perot type ) have been incorporated to separate the various frequencies. Evenson et al. ( 3 ) have developed a tunable FIR source by mixing a fixed frequency ( IR ) CO2 laser with a tunable waveguide ( IR ) CO2 laser in a metal – insulator – metal ( MIM ) diode. With this technique frequencies up to 5.4 THz have been reached for spectroscopic investigations. In our experiment the radiation of both sources, the FIR laser and the BWO, are coupled quasioptically to an open structure mixer (see Fig. 1 ) . To separate the upper sideband from the lower sideband and FIR laser carrier frequency, we use a grating monochromator ( 6 grooves /mm, blaze angle 347 ). No additional diplexer is necessary, because the high-frequency separation of the two sidebands enables geometric resolution of the three frequencies on the grating. The optically pumped FIR ring laser was built in-house. In the present experiment it is run at the 1626-GHz
laser line of CH2F2 . This FIR laser exhibits excellent stability of both power and frequency ( 4, 5) , compared with commonly used linear design FIR lasers. The fixed FIR laser at 1626.6 GHz is mixed with a tunable 280- to 380-GHz BWO ( Type OB30, supplied by the ISTOK Research and Production Co., Fryazino, Russia ). The BWO is phase locked against a harmonic of a frequency synthesizer near 18 GHz ( HP 8673E ) . The BWO and PLL unit is described in detail in ( 6, 7 ). Only a small fraction of the BWO submillimeter power is directed toward the detector. With two additional high-pass filters ( two-dimensional inductive grids ) , which are placed between the monochromator and the absorption cell, the remaining BWO power is reduced to the thermal limit. As a detector we use a liquid helium-cooled InSb hot electron bolometer ( QMC Instruments Ltd.) which is magnetically tuned to 1.6 THz with a full width half-power bandwidth of 400 GHz. The corner cube mixer is a new design, incorporating a short whisker with 6l length for the laser frequency and about 1.3l for the BWO frequency. The whisker is mounted on a planar low-pass filter to separate the submillimeter power from the DC connector. The Schottky diode (Type 1T6, Semiconductor Device Laboratory of the University of Virginia) is a high cutoff diode with a 0.45-mm diameter and 0.5- f F junction capacitance. The measured beam separation and the position of the two optical axes are in good agreement with theory (8). In contrast to the limited tuning range (typically 5 GHz) provided by reflex klystrons, the present BWO-based spectrometer allows 100 GHz continuous tuning range from 1910 to 2010 GHz, covering the full atmospheric window at 2 THz. As a demonstration of the capability of this new type of spectrometer, we measured rotational transitions of carbon monoxide, CO (J Å 18 R 17 and J Å 17 R 16), and of deuterated chloric acid gas, DCl (J Å 6 R 5). The hyperfine components
TABLE 1 Observed and Calculated Frequencies of D 35Cl for the Rotational Transition J Å 6 R 5
a
From Ref. (9). 207 0022-2852/97 $25.00 Copyright q 1997 by Academic Press All rights of reproduction in any form reserved.
AID
JMS 7244
/
6t18$$$181
04-15-97 04:28:40
mspal
208
FIG. 1.
Schematic view of the Cologne BWO-based tunable FIR laser spectrometer.
LETTER TO THE EDITOR
Copyright q 1997 by Academic Press
AID
JMS 7244
/
6t18$$7244
04-15-97 04:28:40
mspal
209
LETTER TO THE EDITOR
FIG. 2. The observed transition of (a) J Å 6 R 5 of D 35Cl and (b) J Å 18 R 17 of 13CO. The DCl spectrum in the central hyperfine component shows saturation.
TABLE 2 Observed and Calculated Frequencies for Carbon Monoxide Isotopomers
a b
From Ref. (10). From Ref. (11).
F Å 9/2 R 9/2 and F Å 13/2 R 13/2 of D 35Cl are well separated from the central four hyperfine components, which are not resolved (Fig. 2a). The dip in the center is caused by saturation effects. The 13CO (Fig. 2b) and C 18O lines are measured in natural abundance. No attempt was made to reduce the atmospheric losses. The ring laser frequency is shifted by É2.9 MHz in comparison to a linear FIR laser, because only one asymmetric velocity class in the Doppler-broadened pump transition profile is excited. From the known spectroscopic data of the species measured (9–11), we determine the ring laser frequency to be 1 626 599.7 MHz. The repeatability of the FIR laser frequency within the gain profile is estimated to be £1 MHz, as is documented by the small scatter quoted in column 5 of Tables 1 and 2. This error is mainly due to the settings of the pump laser and the FIR ring laser resonators, which determine the final accuracy of the measured line position (Tables 1 and 2).
ACKNOWLEDGMENTS
This work was supported by the Deutsche Forschungsgemeinschaft through Grant SFB-301 and through Grant WI 391/7-1. The construction of the laser was partly supported by the Verein der Freunde und Fo¨rderer der Universita¨t Ko¨ln e. V. In addition we also thank the Ministry of Science and Technology of the State Nordrhein–Westfalen for support.
REFERENCES 1. R. C. Cohen, K. L. Busarow, K. B. Laughlin, G. A. Blake, M. Havenith, Y. T. Lee, and R. J. Saykally, J. Chem. Phys. 89, 4494 (1988). 2. P. Verhoeve, E. Zwart, M. Versluis, M. Drabbels, J. J. ter Meulen, W. Leo Meerts, A. Dymanus, and D. B. McLay, Rev. Sci. Instrum. 61, 1612 (1990). 3. K. M. Evenson, D. A. Jennings, and F. R. Petersen, Appl. Phys. Lett. 44, 576 (1984). 4. E. Michael and J. Stutzki, Int. J. Infrared Millimeter Waves 17(8) (1996). 5. E. Michael, F. Lewen, R. Gendriesch, J. Stutzki, and G. Winnewisser, in ‘‘Proceedings, 4th International Workshop on Terahertz Electronics, Erlangen, 1996.’’ 6. G. Winnewisser, A. F. Krupnov, M. Yu. Tretjakov, M. Lietke, F. Lewen, A. H. Saleck, R. Schieder, A. P. Shkaev, and S. A. Volokhov, J. Mol. Spectrosc. 165, 294–300 (1994). 7. G. Winnewisser, Vib. Spectrosc. 8, 241–253 (1995). 8. J. Zmuidzinas, A. L. Betz, and R. T. Boreiko, Infrared Phys. 29, 119– 131 (1989). 9. L. Fusina, P. De Natale, M. Prevedelli, and L. R. Zink, J. Mol. Spectrosc. 152, 55–61 (1994). 10. T. D. Varberg and K. M. Evenson, Astrophys. J. 385, 763 – 765 ( 1992 ) . 11. J. A. Coxon and P. G. Hajigeoriou, Can. J. Phys. 70, 40 – 54 ( 1992 ) .
Copyright q 1997 by Academic Press
AID
JMS 7244
/
6t18$$$181
04-15-97 04:28:40
mspal