J. C. Swartz

Femtosecond demodulation source for high‐resolution submillimeter spectroscopy

A new continuously tunable submillimeter source for spectroscopy and other high‐resolution applications has been developed. In this source the optical spectrum of a mode‐locked femtosecond laser is downconverted into the submillimeter region by the demodulation process of a photoconductive switch. The power generated is subsequently radiated into free space by an antenna which is integrated along with the switch on low‐temperature grown GaAs. The very high resolution is ultimately traceable to the cavity length of the laser and the stable mode‐lock frequency which results. Among the most important attributes of the sources are straightforward absolute frequency calibration, very high spectral purity, and the potential for spectral multiplexing.

Optical transducer for reception of ultrasonic waves

A new optical transducer for the detection of acoustic pressure in the diagnostic ultrasound frequency range is described. This transducer is based on the modulation of an evanescent light field by the incident acoustic energy. Theoretical design considerations are presented for the purpose of developing the most sensitive transducer. Based on these considerations an experimental transducer was constructed. Although less sensitive than predicted this device was capable of transducing ultrasonic pulses with a 1.0-MHz center frequency at diagnostic ultrasound amplitude levels. The techniques developed here are applicable for two-dimensional transduction and may prove a viable alternative to piezoelectric array transducers.

Generation of picosecond pulses at millimeter wavelengths

We report here on an experiment in which picosecond electron pulses, produced by a compact radiation frequency (rf) electron gun, were used to excite a rectangular waveguide, generating 5 ps pulses of radiation with a bandwidth of ∼200 GHz. The interaction of the electron pulses with the waveguide can be modeled quite simply by performing a harmonic expansion of the pulse train produced by the electron gun and employing Poynting’s theorem to compute the power coupled into the modes of the waveguide by each harmonic. The resulting model for the distribution of spectral power yields good agreement with the observed spectrum.

High Resolution Sub-Millimeter Spectroscopy Using Mode-Locked Laser Driven Electro-Optic Antennas

We use a mode-locked laser driven electro-optic antenna as a continuously tunable, high resolution (<1 MHz), high absolute accuracy (1:107) millimeter/sub-millimeter spectroscopic source and to investigate laser phase noise.

Coherent transition radiation produced by a 1.2‐MeV electron beam

We describe a method of generating very high-frequency coherent radiation using an electron beam source with a maximum beam energy of 1.2 MeV. We show that, though the high frequency cutoff for the radiation generated when the beam impacts a target at normal incidence is reduced by transverse beam size effects, it is nevertheless possible to generate much higher frequencies by a judicious choice of the angles of incidence and observation. 5 refs., 4 figs., 1 tab.