Bistatic Laser Research Articles

Features of Imaging in a Bistatic Laser Active Optical System

Features of Imaging in a Bistatic Laser Active Optical System

Features of maging formation in a bistatic laser active optics system

Features of maging formation in a bistatic laser active optics system

Two-brightness-amplifier imaging system for energetic-materials-combustion study.

This paper discusses a laser monitor with two channels for acquiring images synchronized in time and its use for visualizing the surface of metal nanopowders during combustion. Each channel is an independent optical system based on a copper bromide vapor brightness amplifier and a high-speed camera that allows visualizing a specified area of the sample. Thus, the laser monitor allows real-time imaging of the same sample area with different spatial resolutions and simultaneously two areas of the sample. In comparison with the "master oscillator-power amplifier" system and a bistatic laser monitor in the proposed system, the radiation pulses of the brightness amplifiers are synchronized in such a way that the inversion in the active media is formed at different time intervals. Such a system makes it possible to register processes in one area of the sample in the mode of double frames with a time shift of 35ns. As an additional option in the proposed system, it is possible to implement a laser illumination mode in which an additional camera is used with a narrow-band filter installed on it for the emission wavelength of brightness amplifiers, and the same radiation from the brightness amplifier is used as an illuminating light source. The proposed technique was used to visualize the combustion of aluminum nanopowders without additives in real time in the area of laser initiation with different spatial resolutions and nanoAl + microAl + microFe powder mixtures in the laser ignition area and the adjacent area of the sample simultaneously as the combustion wave propagates.

The Use of a Bistatic Laser Monitor for High-Speed Imaging of Combustion Processes

We present results of using a laser monitor with an independent illumination source (bistatic laser monitor) for imaging processes blocked from an observer by the background light. The maximal imaging speed was 15 × 103 fps. The possibility to observe the structural transformations in a combustion wave of self-propagating high-temperature synthesis is shown. A bistatic laser monitor is shown to allow significant improvement of the image quality (contrast and brightness) of low-contrast objects.

Brightness amplifier excited by longitudinal capacitive discharge for laser monitors

The paper presents the results of the research and development the brightness amplifier excited by longitudinal capacitive discharge with a pulse repetition frequency (PRF) up to 24 kHz for using in systems with optical signal amplification. The amplification features of the active element were experimentally studied. The results of the use the designed brightness amplifier in the MOPA system and in the laser monitors (bistatic and monostatic) demonstrate the efficiency of the designed active element for an optical signal amplification. The effect of the vapor concentration in the medium on image brightness amplification characteristics was experimentally estimated. The concentration of the CuBr vapors should be lower to provide the optimal features of laser monitors. The difference between concentration provided maximum output power and optimal active optical system parameters reaches 25%–30%. The contrast of the images in the monostatic and bistatic laser monitors higher 90%.

Bistatic laser monitor for imaging objects and processes

The laboratory model of a bistatic laser monitor which images objects and processes with amplified brightness is presented. The bistatic laser monitor is a laser active optical system, which includes two active elements. There are illumination source and optical signal converter (brightness amplifier) with pumping sources and a high-voltage modulator for synchronization of their operating modes. The illumination source and the brightness amplifier are based on active media in copper bromide vapors. The dependence of the image quality on the time lag between amplification and illumination pulses is determined. The parameters of images formed in monostatic and bistatic laser monitors are compared. The results of process imaging under background radiation conditions with the laboratory model developed are presented.

Use of bistatic laser monitor for high-speed imaging of combustion processes

Use of bistatic laser monitor for high-speed imaging of combustion processes

Detection of a Semi-Rough Target in Turbulent Atmosphere by an Electromagnetic Gaussian Schell-Model Beam

The interaction of an electromagnetic Gaussian Schell-model beam with a semi-rough target located in atmospheric turbulence was studied by means of a tensor method, and the corresponding inverse problem was analyzed. The equivalent model was set up on the basis of a bistatic laser radar system and a rough target located in a turbulent atmosphere. Through mathematical deduction, we obtained detailed information about the parameters of the semi-rough target by measuring the beam radius, coherence radius of the incident beam and the polarization properties of the returned beam.

Features of Operation of a Brightness Amplifier on Copper Bromide Vapors in the Bistatic Scheme of a Laser Monitor

The influence of the brightness amplifier operation mode on images formed with a bistatic laser monitor is studied. The bistatic laser monitor is an active optical system with two active elements. A possibility of imaging remote (to more than 5 m) objects with this instrument is evaluated. It is shown that a change in the concentration of active substance (copper bromide) of the amplifier significantly affects the amplification of the input signal. The active substance temperature rise from 480 to 550°C increases the gain throughout the input signal range. A further increase in the temperature (up to 570°C) increases the gain only at a relatively weak input signal (less than 100 mW). The resulting amplification characteristics of the active optical system are described and compared with the parameters of images formed (distortion and brightness).

Optical System With Brightness Amplification for Monitoring the Combustion of Aluminum-Based Nanopowders

This work discusses the optical systems with brightness amplification based on two copper bromide brightness amplifiers for studying the combustion of aluminum-based nanopowders. It is proposed to use a system with brightness amplification and independent illumination–bistatic laser monitor to increase the brightness and sharpness of images. Independent illumination is realized using a second copper bromide laser and makes it possible to increase the exposure of the object under investigation in comparison with the conventional laser monitor with one brightness amplifier. In addition to independent illumination, a two-channel laser monitor is proposed to examine the surface of burning materials with different magnifications in one area of the sample, as well as simultaneously in two areas of the sample. The delay between the lasing pulses of the brightness amplifiers can be adjusted with high accuracy so that the radiation from one amplifier does not affect the images of the other. The proposed experimental technique makes it possible to investigate the surface of powder materials during high-temperature combustion, accompanied by intense glowing and scattering of combustion products. This article presents the results of real-time visualization of the surface of burning metal nanopowders and mixtures using bistatic and two-channel laser monitors.

Bistatic laser polarimeter calibrated to 1% at visible-SWIR wavelengths.

This paper documents the accuracy and precision of the U. S. Air Force Research Laboratory APCL laser polarimeter in arbitrary bistatic geometries at the three laser wavelengths 633nm, 1064nm, and 1550nm. The difference between measured and theoretical-truth Mueller matrices of calibration components is used as the calibration metric and justified relative to block ellipsometer calibration methods. Calibration of the polarimeter ellipsometry mode is demonstrated first, at quasi-monostatic and large bistatic angles, employing a metallic mirror and a dielectric window as the calibration component, respectively, the latter in order to avoid uncertainty in the retardance of typical metallic mirrors at large incident angles. This uncertainty is demonstrated in measurements of COTS protected-silver mirrors from two vendors, revealing an approximately λ/8 retardance difference, for reflection through 90°, between nominally-identical mirrors from the two vendors. Polarimeter calibration is finally extended beyond ellipsometry by calibrating depolarization measurements using a new technique employing ensembles of polarized states as calibration components.

A bistatic laser monitor

A prototype of a bistatic laser projection system employing copper bromide vapor based active media is presented that allows remote objects and high-speed processes to be monitored online. The proposed scheme employs two active elements: one in the source for lighting the object and another in the image brightness amplifier.

Space debris orbit prediction errors using bi-static laser observations. Case study: ENVISAT

Large and massive space debris objects – such as abandoned satellites and upper stages – in the Low Earth Orbit (LEO) segment pose an increasing threat to all space faring nations. For collision avoidance measures or the removal of these objects, the quality of orbit predictions is one of the most relevant issues. Laser ranging has the potential to significantly contribute to the reliability and accuracy of orbit predictions. The benefit of “conventional” two-way laser ranges for this purpose has recently been demonstrated. For the first time, in this contribution we focus on bi-static laser observations – a new observation type for orbit determination and prediction. Our investigations deal with orbit predictions of the defunct ENVISAT satellite. In order to compensate for the sparseness of “conventional” tracking data, we found that the concept of bi-static laser observations improves the prediction accuracy by one order of magnitude compared to the results based on two-way laser ranges only.

Speckle-metric-optimization-based adaptive optics for laser beam projection and coherent beam combining

Maximization of a projected laser beam's power density at a remotely located extended object (speckle target) can be achieved by using an adaptive optics (AO) technique based on sensing and optimization of the target-return speckle field's statistical characteristics, referred to here as speckle metrics (SM). SM AO was demonstrated in a target-in-the-loop coherent beam combining experiment using a bistatic laser beam projection system composed of a coherent fiber-array transmitter and a power-in-the-bucket receiver. SM sensing utilized a 50 MHz rate dithering of the projected beam that provided a stair-mode approximation of the outgoing combined beam's wavefront tip and tilt with subaperture piston phases. Fiber-integrated phase shifters were used for both the dithering and SM optimization with stochastic parallel gradient descent control.

Criteria and a method for estimating the information content of the attributes of objects in tasks of laser detection and ranging

This paper presents an analysis of the criteria and develops a method for estimating the information content of the attributes of objects in problems of laser-radar detection, measurement, guidance, and recognition. A description is given of the generalized structure and the basic component of this method - an algorithm for the mathematical simulation of the detection-and-ranging conditions and the characteristics of the recorded signals. Examples are given of calculations of the amplitude, temporal, spatial, and spatiotemporal reflective characteristics (normalized signals) of objects of complex shape (aircraft) under conditions of monostatic and bistatic laser detection and ranging.

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.

Enhancement of laser-based ultrasonic wave detection through signal processing techniques

Laser generation and detection of ultrasonic signals is a technology that expands the applications for ultrasonic nondestructive evaluation. It is a remote, noncontacting method of generating and detecting acoustic energy in materials. Laser-based ultrasonics has been shown to have less sensitivity than traditional ultrasonic testing because the amount of optical energy that can be introduced to a material to generate ultrasonic waves nondestructively is limited by the ablation threshold of the material. In addition, detection sensitivity is limited by the surface conditions of the material as well as the feasible power of the laser used in the interferometer. To increase the signal-to-noise ratio of laser-based acoustic detection, beamforming techniques were applied to modeled and measured data. A thermoacoustic wave propagation model of laser generation and detection was used to simulate data from a bistatic array. The corresponding measured bistatic data set was beamformed in the same manner as the simulated data. Filters derived by matching measured and modeled beamformed data were applied to other measured data sets with good results. These filters can be used to enhance images generated from bistatic laser- based ultrasonic data. [Work performed under auspices of the U.S. Department of Energy by the Lawrence Livermore National Laboratory under contract No. W-7405-ENG-48.]

Coherence properties of a reflected optical wave in atmospheric turbulence

The transverse spatial coherence radius is studied for both the monostatic and bistatic laser radar problems involving an optical wave propagating through atmospheric turbulence in the weak-fluctuation regime over a path of length L and then reflected in the reverse direction from a finite mirror with finite focal length FR. Formal expressions are developed for the wave structure function and the modulus of the complex degree of coherence in the general Gaussian-beam wave case, and tractable analytic results are derived for the special case of a diverging (or spherical) wave at the transmitter and observation points in the beam symmetrically located with respect to the beam centerline. By varying the focal length of the mirror, one minimizes the spatial coherence radius of a reflected spherical wave when the receiver is located near the plane defined by the radius of curvature of the mirror (i.e., L/FR ~ 2) and maximizes it when L/FR is approximately 6–7. Effects of inner scale, outer scale, and the high wave-number deviation from pure power-law behavior are taken into account in the assumed spectral models for refractive-index fluctuations. Analogous to line-of-sight propagation, the spatial coherence radius based on a modified spectrum is generally less than that based on the von Karman spectrum, particularly when the coherence radius is of the order of the inner scale of turbulence.

Pulsed heterodyne CO_2 laser rangefinder and velocimeter with chirp correction

A pulsed hybrid CO(2) transversely excited atmosphere (TEA) laser has been used in a bistatic laser rangefinder-velocimeter system with heterodyne detection. Several techniques have been applied to improve the performance of the system. These include the stabilization of the hybrid CO(2) TEA-laser and the stabilization of the frequency offset of the local oscillator (better than ±74 kHz peak to peak), phase-front matching at the detector surface resulting in a heterodyne beat efficiency of 0.4-0.6, and chirp correction. With this system, targets at distances of up to 25 km can be detected with an accuracy of 15 m. The velocity of the targets can be estimated with an accuracy of approximately ±0.5 m/s.

Remote measurements of ambient air pollutants with a bistatic laser system

The ambient air pollutants ozone, nitric oxide, and ethylene have been monitored in the Pasadena area with a bistatic ir laser apparatus. These pollutants were measured with a differential absorption technique using selected wavelengths in the 9.5-microm, 5.2-microm, and 10.5-mum regions, respectively. The transmitted laser radiation was detected using both direct and heterodyne detection techniques. In the direct detection case cube corner retroreflectors provided the return, and the heterodyne detection responded to scattered radiation from various rough surfaces, ranging from 400 m to 1.9 km in distance from the apparatus. Significant departures from ambient background concentration levels were noticed in the region near a local freeway during periods of moderate and heavy traffic.