Amit Lal

Multi-beam laser doppler vibrometer for landmine detection

The method of buried landmine detection based on using elastic waves in the ground and a laser Doppler vibrometer (LDV) as a vibration sensor has shown excellent performance in field tests. To increase the speed of measurements, a multi-beam laser Doppler vibrometer (MB-LDV) was developed. The system is based on a heterodyne interferometer and is capable of simultaneously measuring the vibration of the ground at 16 points over a span of 1 m with a velocity resolution of less than 1 µm/s. Both digital in-phase and quadrature (I&Q) and analog phase-locked loop (PLL) demodulation have been used for signal processing. The MB-LDV can create a velocity image of the ground surface either in stop-and-stare mode or in a continuously scanning mode. The continuously scanning operation results in an increased velocity noise floor due to speckle noise. The speckle noise floor increases with the increase of the speed of the laser beam and can degrade the velocity image of a mine. To overcome the effects of speckle noise, the excitation source must provide a ground vibration velocity higher than the velocity noise floor of the vibrometer. The MB-LDV has been tested at landmine test lanes and shows the ability to detect buried landmine within a one-square-meter area in a time of less than 20 s.

Multi-beam laser Doppler vibrometry for acoustic landmine detection using airborne and mechanically coupled vibration

Acoustic-to-seismic coupling-based technology using a multi-beam laser Doppler vibrometer (LDV) as a vibration sensor has proved itself as a potential confirmatory sensor for buried landmine detection. The multi-beam LDV simultaneously measures the vibration of the ground at 16 points spread over a 1-meter line. The multi-beam LDV was used in two modes of operation: stop-and-stare, and continuously scanning beams. The noise floor of measurements in the continuously scanning mode increased with increasing scanning speed. This increase in the velocity noise floor is caused by dynamic speckles. The influence of amplitude and phase fluctuations of the Doppler signal due to dynamic speckles on the phase locked loop (PLL) demodulated output is discussed in the paper. Either airborne sound or mechanical shakers can be used as a source to excite vibration of the ground. A specially-designed loudspeaker array and mechanical shakers were used in the frequency range from 85-2000 Hz to excite vibrations in the ground and elicit resonances in the mine. The efficiency of these two methods of excitation has been investigated and is discussed in the paper. This research is supported by the U. S. Army Research, Development, and Engineering Command, Night, Vision and Electronic Sensors Directorate under Contract DAAB15-02-C-0024.

Acoustic sensor for landmine detection using a continuously scanning multibeam LDV

Acoustic-to-seismic coupling technology using an LDV as a vibration sensor has proved itself as a potential confirmatory sensor for buried landmine detection. One of the most important objectives of this technology is to increase the speed of measurements over traditional point-by-point scanning LDVs. A moving cart that uses 16 LDVs as well as a continuously-scanning single beam LDV have recently been demonstrated to increase the speed of detection. Recently a multi-beam LDV simultaneously probing 16 positions on the ground has been developed and successfully used for landmine detection. In this work, we report on a continuously-scanning multi-beam LDV as a confirmatory sensor for acoustic landmine detection. The multi-beam LDV simultaneously illuminates the ground in 16 points spread over a 1 meter line. A scanning mirror moves all 16 laser beams across the line. The system enables scanning a 1 meter square area in a much shorter time than previous scanning techniques. This material is based upon work supported by the U. S. Army Communications-Electronics Command Night Vision and Electronic Sensors Directorate under Contract DAAB15-02-C-0024.

Speckle noise in a continuously scanning multibeam laser Doppler vibrometer for acoustic landmine detection

The multi-beam laser Doppler vibrometer (MB-LDV) has been successfully used for acoustic landmine detection in field experiments at an Army test site. Using the MB-LDV in a continuously scanning mode significantly reduces the time of the measurement. However, continuous motion of a laser beam across the ground surface generates noise at the vibrometer output due to dynamic speckles. This speckle noise defines the noise floor and the probability of detection of the system. This paper studies the origins of speckle noise for a continuously scanning LDV. The structure of the speckle field exhibits points of phase singularity that normally coincide with signal dropouts. The signal dropouts and phase singularities can cause spikes in the demodulated velocity signal, which increase the noise in the velocity signal. The response of FM demodulators to input signals causing spikes in the LDV output are investigated in this paper. Methods of spike reduction in the LDV signals have been developed and experimentally investigated.

Silicon ultrasonic horn actuated microprobes based self-calibrating viscosity sensor

We report on a microfabricated silicon ultrasonic horn-driven microprobes with polysilicon strain-gauges and integrated Pt-based immersion depth sensors for fluid viscosity measurement. Non-linear flexural vibration induced in the microprobes by the silicon horn actuated at its longitudinal resonance, is monitored and its damping is used to estimate fluid viscosity. Vibration damping in ethylene glycol solutions of varying viscosities are quantified and modeled, and high sensitivity (∼0.13mV/mPa.s) is demonstrated in the normal blood viscosity range. Whole blood coagulation in a ∼5µl sample is measured by monitoring the frequency of the flexural vibration, demonstrating its applicability as a real-time blood viscosity sensor.

Multiple-beam LDV system for buried landmine detection

This paper discusses the performance and experimental results of a multiple beam laser Doppler vibrometer designed to locate buried landmines with the laser-acoustic technique. The device increases the speed of landmine detection by simultaneously probing 16 positions on the ground over a span of 1 meter, and measuring the ground velocity at each of these positions. Experimental results are presented from controlled laboratory experiments as well as from landmine test lanes at the University of Mississippi. In the mine lanes, the multiple beam system is raised to a height of 2.5 meters with a forklift, with the 16 beams spread over a 1 meter line along the mine lane. A motor system then allows the 16 beams to be translated across the mine lane, enabling the system to scan a 1 x 1 meter area in a much shorter time than with previous scanning techniques. The effects of experimental parameters such as platform motion, angle of incidence, speckle dropout, and system depth-of-field will be presented and discussed.

Advanced LDV instruments for buried landmine detection

Several experiments have demonstrated the potential of Laser Doppler Vibrometry, in conjunction with acoustic-toseismic coupling or mechanical shakers, for the detection of buried landmines. For example, experiments conducted by The University Of Mississippi and MetroLaser, Inc. have shown the ability to scan a one square meter area in less than 20 seconds with a 16-beam multi-beam LDV (MB-LDV), and find the landmines under a variety of soil conditions. Some critical requirements for this technology are to reduce the measurement time, increase the spatial resolution, and reduce the size of the systems. In this paper, MetroLaser presents data from three optical systems that help achieve these requirements: 1) A Compact MB-LDV, 2) A two dimensional, or Matrix Laser Doppler Vibrometer (MX-LDV), and 3) A Whole-field Digital Vibrometer (WDV). The compact MB-LDV produces a 1-D array of beams, which may be scanned over the target surface with a scanning mirror. The size of the new, compact MB-LDV system has been reduced to approximately 17 x 11 x 9, thus enhancing its capability for field applications. The MX-LDV, to be developed in 2006, produces a 16x16 array of beams over a one meter area, allowing the ground velocity of the entire area to be measured in a single measurement. The WDV uses a camera-based interferometry system to take a snapshot of the ground vibration over a one meter square area with very high spatial resolution. Field tests for this system are scheduled for mid-2006.

Wireless transmission of cardiac action potentials with ultrasonically guided insertion of silicon probes

This paper reports on the coupling of ultrasonically guided cardiac probes with wireless transmission of cardiac action potentials for applications in monitoring the 3D electrical onset of ventricular fibrillation. An application specific integrated circuit has been designed with a 40 dB amplifying stage and a frequency modulating oscillator to wirelessly transmit the recorded action potentials. Combined with the ultrasonically inserted cardiac probe that reduces penetration force, this system demonstrates the initial results in wireless monitoring of 3D action potential propagation.

Whole-field laser vibrometer for buried land mine detection

This paper discusses the development and performance of a multi-beam laser Doppler vibrometer specifically designed to locate buried landmines with a laser-acoustic technique. The device aims at increasing the speed of landmine detection with this technique by at least one order of magnitude. The present system is capable of simultaneously probing sixteen positions on the ground over a span of one meter, and of measuring the ground velocity at each of these positions with a velocity resolution of about 1 micrometers /s. This architecture could also be scaled to a larger number of beams or into two dimensions. The present system uses a low (100 kHz) carrier frequency, which enables digital signal processing in a simple architecture. This paper also discusses a numerical model to simulate and predict the performance of the multi-beam vibrometer. In particular, the model attempts to address issues associated with speckle dropout, signal/noise, and maximum scanning velocity.

Ultrasonically actuated silicon-microprobe-based testicular tubule metrology

We report on a microfabricated silicon microprobe integrated with an ultrasonic actuator and polysilicon strain gauges for Microdissection TEsticular Sperm Extraction (TESE) surgery. Multiple microprobe insertion experiments were performed on rat testis tissue and, by monitoring the tubule puncture artifacts in the force signal sensed by the microprobe, we were able to estimate the average diameter of the sperm-carrying tubules in the sample. We have demonstrated the ability to sense the existence of larger tubules embedded in a mass of thinner tubules, by means of an Area-Ratio based metric using an analytically calculated expression for the distribution of sizes measured by the microprobe. This information is important in microdissection TESE to distinguish tubules with and without fertile sperm, potentially eliminating the large incision currently required for optical spermatazoa localization.