Higher moments of scattered light fields by heterodyne analysis

Lidar frequency modulation vibrometry in the presence of speckle

We report laboratory target vibration measurements that use an easily aligned and adjusted fiber-based 1.5-microm heterodyne lidar. The targets are simple spherically curved retroreflectors with well-controlled vibration frequencies and amplitudes. A rotating ground-glass screen creates Gaussian speckle. We wish to understand the modulated and fast-fading lidar returns seen from real target. We frequency demodulated the recorded laboratory data by phase differencing to provide estimates of dphi/dt, where phi is the phase of the received carrier-plus-noise phasor. Experimental results for signal strength and signal-to-noise ratio, for specific target modulation parameters, agree well with our recently developed dphi/dt correlation-function theory.

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.

Continuous-wave bistatic laser Doppler wind sensor

A coherent laser radar has been built by use of a master-oscillator power-amplifier arrangement in which the master oscillator is an external-cavity semiconductor laser and the power amplifier is an erbium-doped fiber amplifier with approximately 1-W output at a wavelength of 1.55 microm. The beams are routed within single-mode optical fiber, allowing modular construction of the optical layout with standard components. The system employs separate transmit and receive optics (a bistatic configuration) and has sufficient sensitivity for reliable Doppler wind-speed detection in moderate scattering conditions at short range (to as much as approximately 200 m). The bistatic arrangement leads to a well-defined probe volume formed by the intersection of the transmitted laser beam with the virtual backpropagated local-oscillator beam. This could be advantageous for applications in which the precise localization of wind speed is required (e.g., wind tunnel studies) or in which smoke, low cloud, or solid objects can lead to spurious signals. The confinement of the probe volume also leads to a reduction in the signal power. A theoretical study has been carried out on the reduction in wind signal strength compared with the monostatic arrangement, and the results are compared with experimental observation.

Single-Particle Laser Doppler Anemometry at 1.55 mum

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.

Multistage optical system for broadcasting and switching information

Conventional switching systems connect each input channel to one output channel. Broadcasting systems permit the connection of each input channel to more than a single output. A broadcast 2 x 2 switch is presented. This switch is an extension of the standard bypass-exchange switch. It allows for the broadcasting of the inputs in addition to the conventional modes. Multistage interconnection networks can be constructed with this switch as the basic building block. Such networks will extend their capabilities, allowing for broadcasting features. Three implementations of this type are described, and experimental results for the 2 x 2 switch are also presented.

Intensity correlations in filtered phase-diffusing light

Light undergoing phase diffusion displays a Lorentzian line shape: Here, electrical filtering techniques are used to examine the correlations among different spectral regions under this line shape. The experiment involves delayed self-heterodyne measurements of the output from a single-mode semiconductor laser. Two filters are tuned to isolate the signal contributions from opposite wings of the spectrum, and the transmitted intensities are shown to be strongly correlated in some regimes and strongly anticorrelated in others.

Heterodyne detection of enhanced backscatter

Heterodyne detection has been used to measure the enhanced backscattering from a standard target consisting of a plane mirror positioned behind a moving ground-glass disk (assumed to act as a phase screen). A tilt of the plane mirror in combination with spectral filtering of the detector output allows isolation of the double-scattered component of the light signal. When the illuminating laser and the local oscillator beams are correctly mode matched, the intensity of this signal displays the factor-of-2 increase that denotes full coherent enhancement. This full enhancement is preserved for a wide range of mirror to phase-screen spacing, in contrast with earlier observations in which direct-detection techniques were used.