Kevin D. Ridley

Absolute measurement of detector quantum efficiency using parametric downconversion

We show that a parametric downconversion crystal emitting angle resolved coincident photon pairs can be used to measure the absolute quantum efficiency of a photon counting detection system. We have measured the quantum efficiency of a silicon avalanche photodiode, operated in Geiger mode, as a function of operating voltage and compare this to results obtained using a conventional method.

Single-photon counting for the 1300–1600-nm range by use of Peltier-cooled and passively quenched InGaAs avalanche photodiodes

We evaluate the performance of various commercially available InGaAs/InP avalanche photodiodes for photon counting in the infrared at temperatures that can be reached by Peltier cooling. We find that dark count rates are high, and this can partially saturate devices before optimum performance is achieved. At low temperatures the dark count rate rises because of a strong contribution from correlated afterpulses. We discuss ways of suppressing these afterpulses for different photon-counting applications.

Pulsed indirect photoacoustic spectroscopy: application to remote detection of condensed phases.

The technique of pulsed indirect photoacoustic spectroscopy is applied to the examination of free liquid surfaces, and the prospects are assessed for remote detection and identification of chemical species in a field environment. A CO(2) laser (tunable within the 9-11-microm region) provides pulsed excitation for a variety of sample types; the resulting photoacoustic pulses are detected at ranges of the order of a few centimeters. The phenomenon is investigated as a function of parameters such as temperature, sample depth, laser-pulse energy, pulse length, and beam diameter. The results are in good agreement with a theoretical model that assumes the mechanism to be expansion of air resulting from heat conduction from the laser-heated surface of the sample under investigation. Signal and noise processing issues are discussed briefly, and the possible extension of the technique to ranges of the order of 10 m is assessed.

Single-particle laser Doppler anemometry at 1.55 µm

We demonstrate the successful operation of a cw laser Doppler wind sensor at a wavelength of 1.55 mum. At longer ranges (>100 m) the signal conforms closely to complex Gaussian statistics, consistent with the incoherent addition of contributions from a large number of scattering aerosols. As the range is reduced, the probe volume rapidly diminishes and the signal statistics are dramatically modified. At the shortest ranges (<8 m) the signal becomes dominated by short bursts, each originating from a single particle within the measurement volume. These single-particle events can have a very high signal-to-noise ratio (SNR) because (1) the signal becomes concentrated within a small time window and (2) its bandwidth is much reduced compared with multiparticle detection. Examples of wind-signal statistics at different ranges and for a variety of atmospheric backscatter conditions are presented. Results show that single-particle-scattering events play a significant role even to ranges of ~50 m, leading to results inconsistent with complex Gaussian statistics. The potential is assessed for a low-power laser Doppler wind sensor that exploits the SNR enhancement obtained with single-particle detection.

Improved speckle statistics in coherent differential absorption lidar with in-fiber wavelength multiplexing

Remote detection of gaseous pollutants and other atmospheric constituents can be achieved with differential absorption lidar (DIAL) methods. The technique relies on the transmission of two or more laser wavelengths and exploits absorption features in the target gas by measuring the ratio of their detected powers to determine gas concentration. A common mode of operation is when the transmitter and receiver are collocated, and the absorption is measured over a return trip by a randomly scattering topographic target. Hence, in coherent DIAL, speckle fluctuation leads to a large uncertainty in the detected powers unless the signal is averaged over multiple correlation times, i.e., over many independent speckles. We examine a continuous-wave coherent DIAL system in which the laser wavelengths are transmitted and received by the same single-mode optical fibers. This ensures that the two wavelengths share a common spatial mode, which, for certain transmitter and target parameters, enables highly correlated speckle fluctuations to be readily achieved in practice. For a DIAL system, this gives the potential for improved accuracy in a given observation time. A theoretical analysis quantifies this benefit as a function of the degree of correlation between the two time series (which depends on wavelength separation and target depth). The results are compared with both a numerical simulation and a laboratory-based experiment.

Statistics of a filtered telegraph signal

Equations are derived which describe the time evolution of the probability density and corresponding characteristic function of a telegraph signal which has passed through a detuned Lorentzian filter. A closed form expression for the characteristic function is obtained for the tuned case and the predicted joint statistics and correlation properties are reviewed in the context of earlier results. Low-order correlation properties for the more general detuned case are calculated. It is shown that the stationary single interval statistics can be generated by a random phasor moving in a space of fractional dimensions and that a simple transform of variable leads to distributions which are stable in this space.

Signal processing analog of phase screen scattering

The statistical properties of a filtered random phasor are calculated. This problem arises in remote-sensing techniques such as coherent spectroscopy and non-Gaussian scattering and is relevant more generally to the field of nonlinear signal processing. Results are obtained analytically and by numerical simulation for a variety of phase fluctuation models. The problem is found to be analogous to scattering of light from a random-phase-changing screen. This provides a simple interpretation of the results and suggests new ways for characterizing both phase variable and filter.

Intensity-weighted phase-derivative statistics

We have previously shown that amplitude weighting can improve the accuracy of measurements of the frequency offset of a signal contaminated by multiplicative Gaussian noise. We have investigated the more general non-Gaussian case through study of the statistics of a simple phase-screen scattering model and derived formulae for the low-order moments of the intensity-weighted phase-derivative. In this paper we extend numerical simulation of the problem to the case of a phase screen with Kolmogorov spectrum. We also report the results of some preliminary experimental measurements.