Gerald David Bacher

In-process detection of weld defects using laser-based ultrasound

Laser-based ultrasonic (LBU) measurement shows great promise for on-line monitoring of weld quality in tailor-welded blanks. Tailor-welded blanks are steel blanks made from plates of differing thicknesses and/or properties butt- welded together, they are used in automobile manufacturing to produce body, frame, and closure panels. LBU uses a pulsed laser to generate the ultrasound and a continuous wave laser interferometer to detect the ultrasound at the point of interrogation to perform ultrasonic inspection. LBU enables in-process measurements since there is no sensor contact or near-contact with the workpiece.

Ultralong dark decay measurements in BaTiO 3

We have observed dark decay rates in BaTiO(3) crystals at elevated temperatures that correspond to dark decay times as long as 2200 years at room temperature. Our investigations have shown that cerium is a desirable dopant in BaTiO(3) for enhancing the dark storage time.

On-machine characterization of moving paper using a photo-emf laser ultrasonics method

Stiffness properties of paper materials can readily be characterized in the laboratory using conventional ultrasonic techniques. For on-line inspection on a paper machine, due to the high translation velocity and the somewhat fragile nature of the moving paper web, contact ultrasonic techniques using piezoelectric transducers are of limited use. To overcome this limitation, non-contact laser- based ultrasonic techniques can be used. Due to the rough surface of the paper, the reflected light is composed of many speckles. For efficient detection, the receiver must be able to process as many speckles as possible. Adaptive receivers using the photorefractive or photo-emf effects are characterized by a large etendue, and thus, are well suited for detection on paper and paperboard. Moreover, the translation velocity of the moving web implies that the detection system must adapt extremely quickly to the changing speckle pattern. In this work, a photo-emf receiver was used to detect Lamb waves excited using a pulsed Nd:YAG laser in moving paper. Experiments were performed using a variable-speed web simulator at speeds much higher than 1 m.s-1. Results corresponding to various translation speeds are shown, demonstrating the feasibility of laser- based ultrasound for on-machine inspection of paper and paperboard during production.

Robust laser-based detection of Lamb waves using photo-EMF sensors

Lamb waves are easily generated and detected using laser techniques. It has been shown that both symmetric and antisymmetric modes can be produced, using single-spot and phased array generation. Detection has been demonstrated with Michelson interferometers, but these instruments can not function effectively on rough surfaces. By contrast, the confocal Fabry-Perot interferometer can interrogate rough surfaces, but generally is not practical for operation below 300 kHz. In this paper we will present Lamb wave data on a number of parts using a robust, adaptive receiver based on photo-emf detection. This receiver has useful sensitivity down to at least 100 kHz, can process speckled beams and can be easily configured to measure both out-of-plane and in- plane motion with a single probe beam.

Enhanced responsivity of photo-induced-EMF adaptive photodetectors using asymmetric interdigitated contacts: scaling limits and application to laser ultrasound sensing

We demonstrate enhanced responsivity in GaAs photo-EMF adaptive photodetectors using a novel asymmetric interdigitated electrode configuration with fine spacing. We observe enhancements even when only a fraction of an optical fringe is collected between the contacts.

Design and engineering of photorefractive materials for use in adaptive laser ultrasonic receivers

We describe the use of photorefractive material in adaptive interferometers for the detection of small surface displacements generated by ultrasonic waves. Recent results on devices based on two-wave mixing and photo-emf detection will be presented. We will discuss techniques for tailoring materials properties for improved device performance.

Homodyne detection of ultrasonic surface displacements using two-wave mixing in photorefractive polymers

Summary form only given. Laser-based ultrasound is a promising technique for remote ultrasonic inspection. One approach for the measurement of small ultrasonic surface displacements uses dynamic holography in a photorefractive material to combine a distorted signal beam with a plane-wave reference beam and match their wavefronts for homodyne detection. Slow variations in the wavefront of the signal beam are compensated by the real-time hologram and thus do not produce noise or interfere with the quality of the wavefront overlap in the output beams.

Novel adaptive single-beam laser vibrometer for vector surface-velocity measurement

In this work, we describe a homodyne laser vibrometer that can measure all three components of surface velocity using a single probe beam and a single return beam along the line- of-sight. Our device is based on a photodetector using the photo-emf effect in GaAs. When used in a homodyne reference- beam interferometer, this detector provides an analog output signal that is proportional to the out-of-plane surface velocity. In this configuration the instrument is insensitive to slow drifts in the path length difference and can operate on speckled beams with no loss in performance. In the absence of a reference beam the same detector provides a signal proportional to the in-plane surface velocity. Both configurations can process highly speckled beams, require no setting to quadrature and are insensitive to slow phase drifts, thereby eliminating the need for path- length stabilization. We will describe the principles of operation of this instrument and report measurements to characterize its performance.