Daniel B. Lang

An improved laser-Rayleigh scattering photodetection system

An improved photodetection system for high resolution laser-Rayleigh scattering measurements has been developed that utilizes a solid state detector coupled to a custom-designed, low-noise, transimpedance amplifier. The resulting system, based on a PIN photodiode is less expensive, inherently safer, less delicate and, depending on the detected light level, may exhibit higher signal-to-noise ratios than photodetection systems based on photomultiplier tubes. The frequency response of the system was designed to be uniform (3% peak variation) from DC to nearly 100 kHz. Concentration fluctuation spectra of a high scattering cross-section label (jet fluid) gas discharging into a density-matched, low scattering cross-section quiescent reservoir gas were measured using this system. Spectral signal-to-noise ratios as high as 7 decades were achieved under some conditions in parts of the spectrum.

Imaging through turbulence with a quadrature-phase optical interferometer

We present an improved technique for imaging through turbulence at visible wavelengths using a rotation shearing pupil-plane interferometer, intended for astronomical and terrestrial imaging applications. While previous astronomical rotation shearing interferometers have made only visibility modulus measurements, this interferometer makes four simultaneous measurements on each interferometric baseline, with phase differences of pi/2 between each measurement, allowing complex visibility measurements (modulus and phase) across the entire input pupil in a single exposure. This technique offers excellent wavefront resolution, allowing operation at visible wavelengths on large apertures, is potentially immune to amplitude fluctuations (scintillation), and may offer superior calibration capabilities to other imaging techniques. The interferometer has been tested in the laboratory under weakly aberrating conditions and at Palomar Observatory under ordinary astronomical observing conditions. This research is based partly on observations obtained at the Hale Telescope.