PHASE-LOCKED BACKWARD-WAVE OSCILLATOR PULSED-BEAM SPECTROMETER IN THESUBMILLIMETER-WAVE RANGE

We have developed a new submillimeter wave pulsed molecular beam spect rometer with phase stabilized backward wave oscillators (BWOs), In the frequency ranges of 260-380 and 440-630 GHz, the BWOs output power va ries between 3 and 60 mW. Part of the radiation was coupled to a novel designed harmonic mixer for submillimeter wavelength operation, which consists of an advanced whiskerless Schottky diode driven by a harmon ic of the reference synthesizer and the BWO radiation. The resulting i ntermediate frequency of 350 MHz passed a low noise high electron mobi lity transistor amplifier, feeding the phase lock loop (PLL) circuit. The loop parameters of the PLL have been carefully adjusted for low ph ase noise. The half power bandwidth of the BWO radiation at 330 GHz wa s determined to be as small as 80 MHz, impressively demonstrating the low phase noise operation of a phase locked BWO. A double modulation t echnique was employed by combining an 80 Hz pulsed jet modulation and a 10-20 kHz source modulation of the BWO and reaching a minimum detect able fractional absorption of 2 x 10(-7). For the first time, a number of pure rotational (K-a = 3 <-- 2, K-a = 4 <-- 3) and rovibrational t ransitions in the van der Waals bending and stretching bands of the Ar -CO complex were recorded. (C) 1998 American Institute of Physics.