Materials inspection and process control using compensated laser ultrasound evaluation (CLUE): demonstration of a low-cost laser ultrasonic sensor

Abstract

We demonstrate the use of a nonsteady-state photo-inducedemf adaptive photodetector as a robust, low-cost laser ultrasonic sensor. This class of sensor enables high-fractional bandwidth ultrasound detection and, in addition, all-optical compensation of adverse in-factory noise, including vibration, speckle, relative platform motion, and optical fiber modal dispersion. Reference-beam and fiber-based time-delay interferometric configurations were demonstrated, as well as the ue of a diode laser as a compact optical probe. IntroductionThere is a great need in the manufacturing, aerospace (e.g., critical components, aging aircrafi), commercial, energy, automotive, microelectronics and DoD communities for diagnostic systems that can improve the efficiency, yield, and performance of various (laser-based) material processing operations -while reducing cost, labor, scrap, and machine downtime. Such needs

Characterization of the photo-EMF response for laser-based ultrasonic sensing under simulated industrial conditions

“…Laser-based ultrasound [1,2], LBU, has the potential to be a robust, reconfigurable, noncontact diagnostic for many industrial applications. A simple and inexpensive semiconductor sensor based on the nonsteady-state photo-induced-electromotive force (photo-emf) effect [3,4], has been demonstrated [5] to be functional under a variety of manufacturing conditions and in probing various materials, including metals, semiconductors, and organics. This device has the potential to remotely sense ultrasound via speckle motion or coherent detection over a reasonable field-of-view, with good bandwidth and detection sensitivity.…”

Inspection of Flip-Chip Epoxy Underfill in Microelectronic Assemblies Using Compensated Laser-Based Ultrasonic Receivers

“…An A-scan of the compensated LBU system output is shown in Figure 4. For this measurement, we employed the GaAs-based photo-emf detector [7]. In the figure, we show results for two cases: In one case, a sample was tested at a point where it was established by other NDE means that a good epoxy bond existed, whereas, in the second case, a typical void region was probed.…”

“…In addition, a DPCM system using barium titanate (as the nonlinear element) requires one to dwell on the workpiece, for"" 10 to 100 msec, so that the crystal can properly respond. A GaAs-based photo-emf detector, on the other hand, has been shown [7] to provide for substantial compensation of both spatial as well as global whole-body phase shifts, with much greater noise compensation bandwidth ( "" 1 0 kHz) but, at present, has a predicted shot-noise limited sensitivity down by a factor of six relative to that of the DPCM-based system, the latter of which has been demonstrated to be near-shot-noise limited [8].…”

“…In both cases, nonlinear optical interactions in single-crystal ferroelectric oxides or semiconductor crystals provide all-optical compensation of the optical distortions described above. Details of the physics, operational parameters and performance tradeoffs of these adaptive photodetectors are discussed elsewhere [6,7]. Suffice it to say, that the DPCM compensates for spatial wavefront distortions, yet passes temporal information, including the desired ultrasound as well as global piston phase shifts (such as whole-body motion, which represents an undesirable noise source in the present system), imposed onto a laser probe beam by the workpiece.…”

“…These adaptive photodetection systems allow for the sensing of ultrasound over wide bandwidths (in the range of < 1 MHz to > 100 MHz) and wide fields-of-view, while compensating for undesirable industrial optical noise sources, including speckle, relative platform motion, and optical distortions encountered in the transmission of probe beams through multimode optical fibers. One receiver architecture employs a double-pumped phase conjugate mirror (DPCM) with heterodyne detection [5], while the other utilizes a mechanism called nonsteady-state photo-induced electromotive force (photo-emf) with homodyne detection [7]. In both cases, nonlinear optical interactions in single-crystal ferroelectric oxides or semiconductor crystals provide all-optical compensation of the optical distortions described above.…”