B. D. Guenther

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.

Spectral purity and sources of noise in femtosecond-demodulation terahertz sources driven by Ti:sapphire mode-locked lasers

Direct measurements of the spectral purity in terahertz femtosecond-demodulation sources are reported and compared to theory. Because these sources operate at very high harmonics (/spl sim/102-10/sup 4/) of the mode-lock frequency, a high spectral purity source is very dependent on a low-jitter femtosecond laser. Conversely, the spectral content of the terahertz sources provides detailed information about timing jitter and stringent tests of models used to describe the jitter. We find that both the behavior of the central core, and the noise skirts of the power spectrum of our sources, can be quantitatively related to measured ripple and continuum amplitude noise on the Ar/sup +/ pump laser by use of modulation theory.

Microwave generation from picosecond demodulation sources

A mode‐locked picosecond laser has been used to prebunch electron beams at a photocathode. These electrons are subsequently accelerated through coupling structures and microwaves radiated. The relation between the microwave output and the properties of the picosecond pulse train and the electron beam output coupling process are reported and related to theory. Among the important attributes of these devices are their ability to generate almost arbitrary microwave waveforms, to operate in the 100–200 kV region with very simple power supplies and excellent spectral purity, and to generate substantially more microwave power than contained in the optical drive.

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.