Thomas C. Hale

Photorefractive optical lock-in vibration spectral measurement.

An optical photorefractive frequency-domain method is described for measuring displacement amplitude and phase of vibrating surfaces. The method is applicable to diffusely scattering surfaces and usable in either a point-detection or imaging configuration. The method utilizes an optical lock-in approach to measure phase modulation of light scattered from continuously vibrating surfaces. Picometer displacement sensitivities have been demonstrated over a frequency range of 100 Hz to greater than 100 kHz. The response of the spectral method is independent of the vibration frequency above the photorefractive cutoff frequency. Two methods are described that produce a readout beam intensity that is a direct function of the vibration amplitude suitable for imaging.

Optical lock‐in vibration detection using photorefractive frequency domain processing

An optical method for vibration detection and spectral analysis based on photorefractive frequency domain processing is presented. The method utilizes the photorefractive effect in selected materials (bismuth silicon oxide) for synchronous detection of the optical phase shift of an object beam scattered from a vibrating specimen surface. Four‐wave mixing and lock‐in detection allow measurement of both the vibration amplitude and phase. Narrow‐bandwidth detection can be achieved at frequencies from the photorefractive response limit to the reciprocal of the photoinduced carrier recombination time.

Ultrasonic vibration modal analysis of ICF targets using a photorefractive optical lock-in

A photorefractive optical lock-in is discussed in relation to ultrasonic vibration modal analysis of inertial confinement fusion (ICF) targets. In this preliminary report, the method is used to analyze specimens with similar response characteristics to ICF targets with emphasis on both the displacement and frequency resolution of the technique. The experimental method, based on photorefractive frequency domain processing, utilizes a synchronous detection approach to measure phase variations in light scattered from optically rough, continuously vibrating surfaces with very high, linear sensitivity. In this photorefractive four-wave mixing technique, a small, point image of the object surface is made to interfere with a uniform, frequency modulated reference beam inside a Bismith Silicon Oxide crystal. Optical interference and the photorefractive effect of electronic charge redistribution leads to the formation of a refractive index grating in the medium that responds to the modulated beams at a frequency equal to the difference between the signal and reference frequencies. By retro-reflecting the reference beam back into the crystal, a diffracted beam, counter-propagating with respect to the original transmitted beam, is generated. Using a beamsplitter, the counter-propagating beam can be picked-off and deflected toward a photodetector. The intensity of this diffracted beam is shown to be a function of the first-order ordinary Bissel function, and therefore linearly dependent on the vibration displacement induced phase modulation depth (delta) , for small (delta) ((delta) < 4 (pi) (xi) /(lambda) < < 1) where (xi) is the vibration displacement and (lambda) is the source wavelength; analytical description and experimental verification of this linear response are given. The technique is applied to determine the modal characteristics of a rigidly clamped disc from 10 kHz to 100 kHz, a frequency range similar to that used to characterize ICF targets. The results demonstrate the unique capabilities of the photorefractive optical lock-in to detect and to measure vibration signals with very narrow bandwidth and high displacement sensitivity. This level of displacement sensitivity is particularly important in detecting changes in vibrational mode shapes and frequencies that might be associated with asymmetries in ICF targets.

Optical resonant ultrasound spectroscopy for fluid properties measurement [fuel capsules]

The properties of fluids are studied using unusually small containment spherical resonators. Proper identification of resonant fluid signatures allows determination of pressure and density of the internal gas with great accuracy using an appropriate equation of state (EOS). Low noise and high sensitivity detection of vibration are critical parameters to characterizing the contained gas when its pressure approaches 1 atm. or less. The benefits of using spherical resonators to determine fluid properties are discussed, and some example calculations of sound speed are presented. In addition to measuring fluids, a comparative experimental approach is taken to explore and, eventually, to optimize vibration detection. In the experiments, two detection methods, a contact piezoelectric transducer (PZT) device and a non-contact optical device, are compared simultaneously and quantitatively. This is done in a unique manner without change in vibration coupling to the sample between tests. A commercially available resonant ultrasound spectroscopy system is used as the contact system, while another commercial device (used as the non-contact vibration detector) combined with the same excitation source (used in the contact system) comprises the other system. The non-contact detector is an optical interferometric receiver that provides adaptation to optically rough surfaces and high sensitivity to acoustic displacements through optical interference in photorefractive GaAs. Both vibration detection systems are compared with particular emphasis on displacement sensitivity, frequency response, and noise level. Furthermore, the results from comparing detection modalities are presented, and their effects on fluid properties measurement are discussed.

Vibration modal analysis using all-optical photorefractive processing

A new experimental method for vibration modal analysis based on all-optical photorefractive processing is presented. The method utilizes an optical lock-in approach to measure phase variations in light scattered from optically rough, continuously vibrating surfaces. In this four-wave mixing technique, all-optical processing refers to mixing the object beam containing the frequency modulation due to vibration with a single frequency modulated pump beam in the photorefractive medium that processes the modulated signals. This allows for simple detection of the conjugate wavefront image at a CCD. The conjugate intensity is shown to be a function of the first-order ordinary Bessel function and linearly dependent on the vibration displacement induced phase (delta) , for (delta) equals 4(pi) (xi) /(lambda) << 1 where (xi) is the vibration displacement and (lambda) is the source wavelength. Furthermore, the results demonstrate the unique capabilities of the optical lock-in vibration detection technique to measure vibration signals with very narrow bandwidth (< 1 Hz) and high displacement sensitivity (sub-Angstrom). This narrow bandwidth detection can be achieved over a wide frequency range from the photorefractive response limit to the reciprocal of the photoinduced carrier recombination time. The technique is applied to determine the modal characteristics of a rigidly clamped circular disc from 10 kHz to 100 kHz.

Resonant ultrasonic vibration detection study

Contact and non-contact (optical) vibration detection methods, used in a resonant ultrasound spectroscopy application, are discussed in a comparative manner. The noise-floor for three different methods of vibration detection are quantitatively compared using a unique experimental configuration which employs spherical resonators. Spherical resonators, when resonating at their “breathing mode,” have the special characteristic of vibrating uniformly in all directions with the same displacement. Noise-floor comparisons are made utilizing this unique mode of vibration to simultaneously measure vibration displacements for all three methods without changing the mechanical input coupling to the resonator. Among other things, in this application, we demonstrate that contact vibration detection systems do not necessarily perform better than optical methods in terms of noise-floor and cross-talk considerations.

High Resolution Time-of-Flight Measurements Employing Laser Ultrasonics with Photo-EMF Detection

Laser ultrasonic techniques provide distinct advantages over conventional contact methods for process monitoring and on-line material characterization applications.1 The most important of these is non-contact measurement capability. Non-contact measurement is particularly desirable in industrial environments where high temperatures or hazardous materials are involved. This latter subject is especially significant concerning the types of materials handled, processed, and inspected at Los Alamos National Laboratory where collateral contamination and generation of secondary low-level waste is a major issue. An important ramification of non-contact laser ultrasonic methods is the ability to probe materials that are in motion such as on a conveyor belt or a part being turned on a lathe. Indeed, this all-optical technology can allow corrective action to be taken during a material removal process before the ideal component geometry is irretrievably altered. Also, optical post-process inspection of fabricated parts are not subject to dimensional measurement errors typical associated with conventional contact probes such as probe wear and probe inclination relative to the surface. In contrast, laser generation of ultrasound offers relative insensitivity of the measurement to the incident optical beam inclination, and has the added benefit of averaging surface roughness effects; contact probes usually ride on the outermost surface profile.

Acoustic target characterization for inertial confinement fusion

Numerous hurdles mark the path leading to successful inertial fusion energy and these tasks are being addressed in a multinational effort. Much work at Los Alamos National Laboratory (LANL) has focused on addressing theoretical target design, target fabrication, and target characterization. Favored target designs incorporate millimeter‐size beryllium or plastic shells filled to near‐critical density with hydrogen isotopes. This fuel is then solidified at cryogenic temperatures and allowed to symmetrize through the natural process of beta layering. Implosion physics constraints demand very strict design standards on these targets in terms of layer sphericity, concentricity, and surface smoothness. Design‐size targets for the National Ignition Facility (NIF) have recently been manufactured at LANL. Resonant ultrasound spectrocopy (RUS) is now being implemented as a valuable tool in many aspects of target characterization and it is especially useful for examining the interior of opaque objects. RUS has now b...

Noncontacting resonant ultrasound spectroscopy using a new optical vibration spectral measurement technique

A new noncontacting optical method of vibration detection has been developed that utilizes the photorefractive effect in select materials to form an optical ‘‘lock‐in’’ amplifier. The method synchronously detects the optical phase shift of an object beam scattered from a vibrating specimen surface. Four‐wave mixing and conventional sychronous detection allow measurement of both the vibration amplitude and phase directly as a function of the excitation frequency. Narrow bandwidth detection can be achieved at frequencies from the photorefractive response limit to the reciprocal of the photoinduced carrier recombination time. The method has been implemented using Bismuth silicon oxide providing a resolution bandwidth of about 130 Hz and flat frequency response up to the MHz region. Both synchronous and random vibration excitation methods can be used providing a minimum detectable displacement amplitude of 0.002 nm, at present, with the possibility of further improvement. Application of the method to resonant...