Michael R. Harris

All-fiber multifunction continuous-wave coherent laser radar at 1.55 µm for range, speed, vibration, and wind measurements

The design and performance of a simple, multifunction 1.55-mum continuous-wave (cw) and frequency-modulated cw coherent laser radar system with an output power of 1 W is presented. The system is based on a semiconductor laser source plus an erbium-doped fiber amplifier, a polarization-independent fiber-optic circulator used as the transmit-receive switch, and digital signal processing. The system is shown to be able to perform wind-speed measurements even in clear atmospheric conditions when the visibility exceeds 40 km. The aerosol measurements indicate the potential to use single-particle detection for wind measurements with enhanced sensitivity. The system can perform range and line-of-sight velocity measurements of hard targets at ranges of the order of several kilometers with a range accuracy of a few meters and a velocity accuracy of 0.1 m/s by use of triangular-wave frequency modulation with compensation of the frequency-modulation response of the semiconductor laser. The system also demonstrates a capability for vibration sensing.

The role of laser coherence length in continuous-wave coherent laser radar

Abstract The effect of laser coherence length on the performance of continuous-wave coherent laser radar (lidar) is examined. A laboratory lidar investigation using semiconductor lasers with linewidths of the order of 1 MHz has been carried out for ranges much shorter than, comparable with and much longer than the laser coherence length (about 50m). The signal strength, spectrum and fluctuation statistics are all shown to be sensitive to the various effects resulting from limited laser coherence. Different laser line-broadening mechanisms are considered, including phase diffusion and jitter, and the fundamental difference between single- and multiple-mode sources is examined. Limited laser coherence is also shown to give rise to an excess noise contribution, and this can be severely detrimental to the lidar system performance.

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.

Aircraft wake vortices: a comparison of wind-tunnel data with field trial measurements by laser radar

Abstract A method is presented for comparison of measurements on aircraft wake vortices obtained using two very different techniques: 1) five-hole probe measurements on a 1/13.6 (7.35%) scale half-model of an Airbus A321 were made in a wind tunnel, and 2) coherent laser radar (lidar) measurements were made in full-scale field trials at Toulouse Blagnac Airport. The lidar measurements provide line-of-sight tangent velocities across the vortex pair to over half the peak core velocity, whereas the wind tunnel measurements cover the full core region, extending nearly to the mid point. Measurements from the two systems may thus be compared over the overlap region of the vortex profile. Over this range the measured tangent velocity profiles show good agreement for vortices of comparable age. Vortex disturbance is brought about by atmospheric turbulence, and the effects are evident in the lidar data, leading to significant distortion of older vortices. Some implications of the results are discussed together with points of comparison for the two measurement systems. The complementary character of the information that may be derived from the two techniques is emphasised.

Output characteristics of a compact 1 J carbon dioxide laser with a Gaussian reflectivity resonator

The authors describe the performance of a compact (41-cm-long) e-beam sustained atmospheric pressure CO/sub 2/ laser using a Gaussian reflectivity mirror in an unstable resonator configuration. The device operated on a single longitudinal and transverse mode (W/sub 1L/=12.9 mm) at an output energy of around 1 J. The frequency characteristics were studied and acoustic effects were observed as a result of the near-threshold operation. >

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.

Lidar measurement of B747 wakes : observation of a vortex within a vortex

Abstract Measurements of aircraft wake vortices obtained by laser radar techniques at London Heathrow Airport have revealed many interesting features of vortex structure. Notably, the vortices from Boeing 747 aircraft are shown to contain an intense, narrow-core vortex embedded within a larger co-rotating diffuse vortex. We propose an empirical ‘double-dispersion’ model consisting of the linear superposition of two concentric dispersion functions. This new model provides a good description of the B747 vortex, and should therefore be applicable in simulations of aircraft encounters with B747 vortices.

Polarimetry of scattered light using heterodyne detection

Abstract Heterodyne detection has been used to measure the polarization state of light back-scattered from various targets (including flame-sprayed aluminium, sandpaper and painted surfaces). The samples are illuminated with a linearly polarized single-frequency continuous-wave CO2 laser operating at a wavelength of 10.6 μm. The back-scattered co-polarized and cross-polarized components are both coherently detected by beating with an optical local oscillator. This process allows the relative amplitudes and phases of the two components to be measured and hence the lights polarization state can be evaluated. When the target undergoes movement, the scattered light demonstrates the usual properties of dynamic speckle, and the technique allows observation of the time evolution of the polarization ellipse.