We report a broadband tunable terahertz source, based on mixing the laser line of CH 2 F 2 . This FIR laser exhibits excellent stability of both power and frequency ( 4, 5) , compared with commonly used linear design radiation from a FIR laser with that of a backward wave oscillator ( BWO ) on an open structure GaAs mixer diode. The tunability of the 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 sideband spectrometer is determined by the frequency coverage of the BWO, which is typically about 100 GHz at 330 GHz center frequency. and Production Co., Fryazino, Russia ). The BWO is phase locked against a harmonic of a frequency synthesizer near 18 GHz ( HP 8673E ) . The Up to now, basically three different types of sideband sources are commonly used together with two different methods of coupling 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 sources to the mixing element. The Berkeley group ( 1 ) uses a FIR laser in conjunction with frequency synthesizers ( up to 26 GHz ) and two additional high-pass filters ( two-dimensional inductive grids ) , which are placed between the monochromator and the absorption cell, the re-multipliers for higher sideband frequencies ( up to 75 GHz ) . Coupling is performed either coaxial ( up to 45 GHz ) or for higher frequencies maining BWO power is reduced to the thermal limit. As a detector we use a liquid helium-cooled InSb hot electron bolometer ( QMC Instruments by waveguide techniques. The Nijmegen group ( 2 ) uses phase-locked klystrons up to 114 GHz which are coupled in the waveguide technique Ltd.) which is magnetically tuned to 1.6 THz with a full width half-power bandwidth of 400 GHz. to the open structure mixer. In all cases diplexers ( Martin Puplett or Fabry Perot type ) have been incorporated to separate the various fre-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 fre-quencies. Evenson et al. ( 3 ) have developed a tunable FIR source by mixing a fixed frequency ( IR ) CO 2 laser with a tunable waveguide ( IR ) quency. The whisker is mounted on a planar low-pass filter to separate the submillimeter power from the DC connector. The Schottky diode (Type CO 2 laser in a metal -insulator -metal ( MIM ) diode. With this technique frequencies up to 5.4 THz have been reached for spectroscopic investi-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. gations.
In our experiment the radiation of both sources, the FIR laser and the 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 BWO, are coupled quasioptically to an open structure mixer (see Fig. 1 ) . To separate the upper sideband from the lower sideband and FIR laser (typically 5 GHz) provided by reflex klystrons, the present BWO-based spectrometer allows 100 GHz continuous tuning range from 1910 to 2010 carrier frequency, we use a grating monochromator ( 6 grooves /mm, blaze angle 34ะ ). No additional diplexer is necessary, because the high-fre-GHz, covering the full atmospheric window at 2 THz. As a demonstration of the capability of this new type of spectrometer, we measured rotational quency separation of the two sidebands enables geometric resolution of the three frequencies on the grating. The optically pumped FIR ring laser transitions of carbon monoxide, CO (J ร
18 R 17 and J ร
17 R 16), and of deuterated chloric acid gas, DCl (J ร
6 R 5). The hyperfine components was built in-house. In the present experiment it is run at the 1626-GHz TABLE 1 Observed and Calculated Frequencies of D 35 Cl for the Rotational Transition J ร
6 R 5 a From Ref. (9).