We developed a pulsed digital shearography system that utilizes the spatial phase-shifting technique. The system employs a commercial micropolarizer camera and a double pulse laser, which allows for instantaneous phase measurements. The system can measure dynamic deformation of objects as large as 1 m at a 2-m distance during the time between two laser pulses that range from 30 μs to 30 ms. The ability of the system to measure dynamic deformation was demonstrated by obtaining phase wrapped and unwrapped shearograms of a vibrating object.
This paper describes an ongoing instrument development project to distinguish genuine manufactured components from counterfeit components; we call the instrument ASSURES (Authentication Sensing System Using Resonance Evaluation Spectroscopy). The system combines Laser Doppler Vibrometry with acoustical resonance spectroscopy, augmented with finite element analysis. Vibrational properties of components, such as resonant modes, damping, and spectral frequency response to various forcing functions depend strongly upon the mechanical properties of the material, including its size, shape, internal hardness, tensile strength, alloy/composite compositions, flaws, defects, and other internal material properties. Although acoustic resonant spectroscopy has seen limited application, the information rich signals in the vibrational spectra of objects provide a pathway to many new applications. Components with the same shape but made of different materials, different fatigue histories, damage, tampering, or heat treatment, will respond differently to high frequency stimulation. Laser Doppler Vibrometry offers high sensitivity and frequency bandwidth to measure the component’s frequency spectrum, and overcomes many issues that limit conventional acoustical resonance spectroscopy, since the sensor laser beam can be aimed anywhere along the part as well as to multiple locations on a part in a non-contact way. ASSURES is especially promising for use in additive manufacturing technology by providing signatures as digital codes that are unique to specific objects and even to specific locations on objects. We believe that such signatures can be employed to address many important issues in the manufacturing industry. These include insuring the part meets the often very rigid specifications of the customer and being able to detect non-visible internal manufacturing defects or non-visible damage that has occurred after manufacturing.
Nanometer-scale patterning of graphite and graphene has been accomplished through local anodic oxidation
using an AFM tip. The underlying mechanism is explained. To date, protrusions, holes, trenches, and even
words have been patterned in HOPG over scales ranging from 1nm2 to 1mm2 and depths ranging from sub nm
to as deep as 200nm with less than 5 nm variation on the feature size and placement. This same method has
also been applied to CVD-grown graphene providing a resist-free process for patterning graphene at the single
nanometer scale. This capability could provide a method to rival e-beam lithography resolution but without any
pre- or post-processing.
A universal, extended dynamic range novel optical inspection system for aspheric optical components and optics that are
not easily inspected with conventional interferometry is presented. Modern optical design and manufacturing procedures
have begun using such components more and more in routine applications to improve optical system capability.
Inspection tools required for these types of optical components have lagged the capability to manufacture them. In this
paper unique measurement procedures employing digital holography combined with a spatial light modulator are
discussed for complex shapes such as aspheres and mandrels.
New generations of infrared transmitting optical domes are currently being developed to improve the drag, range, speed,
and payload capabilities of missiles. Traditionally, these domes have been hemispheres, which can be well characterized
with conventional optical interferometers. These interferometers, however, are not generally well-suited to the new
shapes, such as tangent ogives, because the transmitted and reflected wavefronts can differ by many wavelengths from
the planar or spherical wavefronts that are normally used as a reference. In this paper, we present an innovative
technique to characterize unconventional optical components such as aspheric domes, mirrors, and freeform optics. The
measurements are based on an innovative instrument that combines an instantaneous digital phase-shifting infrared
interferometer with a dynamic spatial light modulator that extends the range of the interferometer. The goal of the
measurement is to determine the wavefront error, within a small fraction of a wavelength, caused by the deviation of the
optical component from a perfect geometrical shape of any type (i.e. not spherical). Experimental results are presented
from several infrared components.
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.
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.
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.
Previous results have shown the potential of acoustic-to-seismic coupling with Laser Doppler Vibrometry for the detection of buried landmines. An important objective of the present technology is to improve the spatial resolution and the speed of the measurement. In this paper, MetroLaser reports on a whole-field digital vibrometer (WDV) that measures an entire one meter area with sub-centimeter spatial resolution in just a few seconds. The WDV is based on a dual-pulsed laser such that each pulse illuminates a one meter area on the ground, and the temporal separation between the two laser pulses can be adjusted to match the ground excitation frequency. By sweeping this excitation frequency, a displacement map of the ground at each frequency can be quickly generated. In addition, an innovative speckle repositioning strategy allows for movement of the measurement platform at reasonable speeds while still obtaining measurements with interferometric precision. This paper describes the WDV instrument and presents preliminary experimental results obtained with this system. This research is being supported by the U.S. Army RDECOM CERDEC NVESD under Contract W909MY04-C-0004.
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-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.
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.
The use of a laser Doppler vibrometer (LDV) to sense the acoustic-to-seismic coupling ratio for buried landmine detection has previously been demonstrated. During these experiments, the LDV is mounted on a fixed platform and the beam moves continuously across the ground. Experiments show that fixed mounted LDV can achieve scanning speeds up to 3.6 km/h for successful detection of buried landmines in outdoor ground. The problems associated with taking a fixed-mount, scanning LDV and transitioning to a mobile system involve such issues as vehicle vibration, additional Doppler bandwidth due to vehicle speed, speckle noise, and sample time vs. spatial averaging. This paper presents the results of field tests with the moving platform on U.S. Army mine lanes showing that many of these issues can be overcome with an appropriately designed moving platform. The testing involved scanning different types of mines at varying depths and different speeds. Different aspects of the experiment are also discussed.
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.
Carbon nanotube tips (CNT) offer many advantages over the standard SFM probes, namely high aspect ratio, high resolution, durability, minimal tip or sample damage and, perhaps most important, tailoring. We demonstrate here the value of CNT as probes for surface metrology. Their high- aspect ratio enables profiling morphologies that are inaccessible to conventional probes. We report method for controlling the end-form of a nanotube bundle (mounted on a Si tip) so that a single nanotube protrudes from it. We did not observe any tip or sample wear over time with CNT probes, contrary to results with conventional probes. We also demonstrate that a combination of tuning forks and nanotubes can be used as probes for SPM.
The use of ultrasonic pulses incident on surface micromachines has been shown to reduce dormancy-related failure. We applied ultrasonic pulses from the backside of a silicon substrate carrying SUMMiT processed surface micromachined rotors, used earlier as ultrasonic motors. The amplitude of the pulses was less than what is required to actuate the rotor (sub-threshold actuation). By controlling the ultrasonic pulse exposure time it was found that pulsed samples had smaller actuation voltages as compared to non-pulsed samples after twelve-hour dormancy. This result indicates that the micromachine stiction to surfaces during dormant period can be effectively eliminated, resulting in long-term stability of surface micromachines in critical applications.
This paper presents experimental results using a ZnSe Risley prism scanner in which diffractive gratings were etched into the prism faces to correct for chromatic aberrations. Risley prism scanners, which consist of independently rotating prisms, offer distinct advantages over mirrored systems. The faces of the two scanner elements are parallel and adjacent to one another, resulting in a simple, lightweight, and compact system with extremely high pointing stability and accuracy. Laboratory results for the scanner, when used in a midwave infrared imaging system, demonstrated a total field of view +/- 22.5 degrees with almost no aberrations. The optical performance of the scanner demonstrated a factor of two improvement in resolution when compared to an equivalent scanner using no diffractive correction. We conclude that the use of diffractively corrected prisms offer a new potential for using Risley prisms as a alternative lightweight scanner in missile seekers.
Anisotropically etched silicon structures were impacted against stainless steel to measure relative impact strength of different silicon surface treatments. In order to study the effects of surface treatment, the beams were coated with thermal silicon dioxide, LPCVD silicon nitride, and silicon dioxide/silicon nitride sandwich. A jig was made to controllably impact the sample and also measure the stresses during impact. It was found that silicon-dioxide treatment resulted in the highest strength structures and the nitride coated samples being weaker than even the uncoated sample. An explanation based on surface stress effect on crack initiation is presented.
In this paper we demonstrate the use of diffractive gratings to optically measure strain in miniature ultrasonic transducers. Aluminum diffraction gratings were fabricated on silicon-microfabricated ultrasonic horns and beams which were actuated by bonded piezoelectric PZT (Lead-Zirconate Titanate) plates. A He-Ne laser beam was diffracted from the grating and a knife-edge was used to measure small changes in the diffraction angle as a result of time varying grating space and width. The measured strain and displacement profiles agreed with the expected mode patterns for the silicon resonators.
Bacteriorhodopsin (BR) has been proven to be an effective non-linear media for a variety of applications, such as optically addressable spatial light modulators, volumetric memories, optical image processing systems, optical sensors, and optical correlators. However, practical realization of such systems with BR depends upon the specific characteristics of this material. In this report we present experimental results of the time evolution and intensity dependent characteristics of a BR gelatin film. In particular we studied the spectral dependence of the optical density/refraction index modulation. A holographic technique was used to investigate the exposure characteristics of photorefraction, recording versus storage time, as well as the connection between the diffraction efficiency of the recorded grating and light induced scattering (noise)--the parameters that are of primary importance for such applications as high density memory systems and optical correlators.
A two-frequency CO2 laser beam was used to beat-excite a large amplitude electron plasma wave in a resonant density plasma. The accelerating fields of the relativistic plasma wave were probed with collinear injected 2.1 MeV electrons from an electron linac. Some electrons gained at least 7 MeV in traversing the approximately 1 cm length of the beat wave accelerator, with the measurement limited by the 9.1 MeV high energy cut-off of the detection system. The corresponding average acceleration gradient is > 0.7 GeV/m and the average wave amplitude n1/n0 is > 8%. Estimates based on collective Thomson scattering indicate that peak wave amplitudes of 15 - 30% may have been achieved.