P. V. Mitchell

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

We demonstrate the use of a nonsteady-state photo-induced-emf 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 use of a diode laser as a compact optical probe.

High-power x-ray point source for next-generation lithography

An x-ray power of 2.8 Watts at the 1 nm x-ray lithography wavelength was generated by a copper plasma formed by a single laser beam focused to an intensity of greater than 1014 W/cm2 on a copper tape target. The all solid state BritelightTM YAG laser has 700 ps pulse duration, 300 Hz pulse repetition rate, average power of 75 Watts, and less than 2 times diffraction limited beam quality at the fundamental 1.064 micrometer wavelength. The single beam laser system has a master oscillator, a preamplifier and one power amplifier, all diode pumped. Measurements confirmed negligible copper vapor debris at 8 cm from the laser-plasma source with atmospheric pressure He gas and modest gas flow. The point source x-ray radiation was collimated with either a polycapillary or grazing mirror collimator. The near-parallel beam of x-rays has good divergence both globally (0.5 mrad) and locally (less than 3 mrad), good uniformity (2% achievable goal) and large uniform field size (20 mm X 20 mm full field and 25 mm X 36 mm scanning system). High-resolution lithography was performed for the first time with collimated 1 nm point source x-rays. A power scaling system is being built with eight amplified beams in parallel on the x-ray target, and is expected to achieve 24 - 30 Watts of x-rays. A 16 beam laser plasma x-ray lithography system could achieve a throughput of 24 wafer levels per hour using 300 mm diameter wafers.

Compensated High-Bandwidth Laser Ultrasonic Detector Based on Photo-Induced Emf in GaAs

The trend toward intelligent manufacturing has produced an increase in the need for sensors which can nondestructively evaluate components and processes in real-time. One commonly used nondestructive approach is ultrasonic inspection. The most common method for generating and sensing ultrasound in materials makes use of contacting piezoelectric transducers. A gel or water interface is often used to match the acoustic impedance between the sensor and part. This constraint can impose limitations on their applicability for some types of in-process industrial control or inspection, specifically, inspection of moving parts at elevated temperatures or in vacuum. While noncontact receivers have been made using capacitance or magnetic induction, often their spacing to the workpiece must be maintained within a close tolerance. The lack of a substantial standoff distance for these sensors also reduces their usefulness in some industrial inspection and process control applications. Specifically, contacting schemes and close proximity sensors are not well suited for conditions such as extreme vibrations and fast moving parts with irregular surfaces. Furthermore, it may be more cost effective, from the users perspective, to use a long standoff, remote sensing system which could be applied to a wide range of materials including metals, semiconductors and composites.

Remote Laser-Based Ultrasonic Inspection of Weld Joints for High Volume Industrial Applications

The ability for manufacturers to implement economic techniques for process control and quality assurance can significantly contribute to their competitiveness in the world market. Consequently, many manufacturers would like to determine the quality of their products by conducting the appropriate quantitative tests at suitable stages during production. In addition, an in-line capability for real-time intelligent process-control is desirable. Many of the weld joints produced today are inspected off-line using destructive sectioning and visual examination. Due to the limited number of parts available for this type of testing, the process control is statistical in nature and 100% certification is impossible. Furthermore, visual inspection cannot directly or accurately measure many important material parameters of the weld and the parent material. A better solution to the problem would be a real-time, nondestructive, noncontact technique for weld joint inspection and feedback control of the welder.

Optical real-time defect-enhancement diagnostic system.

We have demonstrated an all-optical diagnostic system that enhances the observation of defects in periodic structures. This real-time technique employs a spatial light modulator as a smart-pixel array for information processing in the Fourier transform plane of a lens. The system also includes a phase-conjugate mirror for autoalignment and for correction of optical wave-front aberrations that are imparted on the object light by the smart-pixel processor and its associated optical train.

Single-pixel demonstration of innovative adaptive optics by use of a charge-transfer membrane light modulator

We have constructed and demonstrated a single-pixel implementation of an all-optical membrane-based spatial light modulator as a compact optical wave-front error correction device. High rates of response of as much as 20 kHz in an open-loop configuration were obtained. The device was then used in an adaptive-optics servo to compensate successfully for a 1-kHz sinusoidal phase error with a peak-to-peak excursion of approximately pi/7 rad. A small-signal servo gain of the order of 10 was inferred from the closed-loop measurements.

Optimizing the photo-induced-emf response for broadband remote sensing

A simple and inexpensive semiconductor based sensor based on the photo-induced electromotive effect has been demonstrated under a variety of remote sensing conditions. We have performed parametric investigations in an effort to optimize the performance of this adaptive photodetection system. These measurements range from fundamental device physics and materials studies to optical architectures and electronic packaging. We studied a monolithic, hybridized detector module, which achieved high-performance sensing at low cost, in a lightweight, compact, and rugged package.

Advanced laser ultrasonic receivers for industrial applications

Laser-based ultrasound (LEU) is a promising method for noncontact-testing of materials, components, and structures. One critical element of an LEU system is the laser ultrasonic receiver. We describe recent progress on developing rugged, adaptive receivers that meet all the above requirements. Laser interferometers have been used for many years to detect small amplitude surface displacements that are produced when an ultrasonic wave reaches the detected surface.