Recent Advances in Adaptive Receivers For Laser-Generated Ultrasound
G. David Bacher
6059 Bristol Parkway
Culver City, CA 90230,USA
- International Symposium on NDT Contribution to|
the Infrastructure Safety Systems, 1999 NOV 22-26 Torres,
published by UFSM, Santa Maria, RS, Brazil
Laser ultrasonics is a promising new technique for remote, noncontact ultrasonic inspection. In this technique, a pulsed laser is used to generate an ultrasonic pulse in the sample under inspection and a cw laser interferometer is used to detect the small displacements generated where the ultrasonic pulse reaches the surface.
Lasers are quite effective in generating ultrasound, but until recently, laser ultrasonic receivers were not as sensitive as required. Most laser ultrasonic receivers use an interferometer for detection. Conventional interferometers suffer from loss of performance because they can not process speckled beams from rough surfaces. They also require exact path length matching for linear signal detection. In this paper, we will describe a new family of compensated interferometers using photorefractive crystals as real-time holograms or as adaptive photodetectors. These receivers require no path-length stabilization and can process speckled beams with no loss in signal-to-noise. They can also compensate for dynamic wavefront distortions due to mechanical vibrations or turbulence in the propagation path.
Keywords: laser-based ultrasound, homodyne, photo-emf, two-wave mixing
Laser-based ultrasound (LBU) is a noncontact inspection technique with the potential for in-process sensing, manufacturing quality control and in-service inspection (see Figure 1). Although the methods of generation and detection are very different from contact ultrasonics, the ultrasonic wave interactions inside the part are very similar. The normal complement of waves (compressional, shear, surface and plate) can be generated, and familiar inspection geometries (pulse-echo, through-transmission and pitch-catch) can be used. Particularly large wave amplitudes are observed when the generation laser is allowed to ablate a thin layer of material from the incident surface.
LBU has a number of unique features:
- Its noncontact nature avoids mechanical loading of the workpiece and allows inspection of parts moving at high speeds (up to at least 20 m/s)
- Its remote standoff capability allows inspection of parts in adverse manufacturing conditions, including high temperatures, vacuum or plasmas
- The use of scanning mirrors and fiber optics allows reconfigurable probing
of complex-shaped parts without conformal surface tracking and (4) the high bandwidth of laser generated waves enhances spatial resolution and provides more reliable defect detection, compared with transducer-based measurements.
Fig 1: Schematic diagram of laser ultrasonic inspection system in pulse echo mode.
In general, ultrasonic signals produced by ablation have larger amplitudes and higher bandwidths than those produced by conventional contact transducers. However, interferometric receivers, while having higher bandwidth, are generally less sensitive than contact transducers for detection of ultrasonic signals. In recent years, there has been considerable interest in improving the performance of laser ultrasonic receivers. In response, a new class of adaptive interferometers has been developed to detect the small ultrasonic surface displacements encountered in typical measurement conditions. Unlike conventional homodyne or heterodyne interferometers, these adaptive laser ultrasonic receivers allow the efficient processing of speckled beams that are received from rough surfaces and/or from multimode fibers. In addition, they can compensate for dynamic wavefront changes resulting from beam scanning, workpiece motion or atmospheric turbulence.
Laser interferometers have been used for many years (1) to detect small-amplitude surface displacements that are produced when an ultrasonic wave reaches the detected surface. As originally developed, passive homodyne or heterodyne interferometers with coherent detection could not operate effectively with the speckled input beams that result from interrogating a rough surface with a laser probe beam. In addition, accurate path length stabilization (homodyne) or postprocessing electronics (heterodyne) were required for effective operation. The later development of time-delay interferometers, such as the confocal Fabry-Perot, has allowed the processing of light scattered from a rough surface with a large field of view. The confocal Fabry-Perot can respond very rapidly to changing input wavefronts and has high sensitivity, with measured values approaching the shot noise limit. However, the confocal Fabry-Perot still requires stabilization of the interferometer length to a fraction of an optical wavelength, thereby adding complexity and cost to the receiver.
More recently, a number of laser ultrasonic receivers based on adaptive reference-beam interferometers have been developed to process speckled beams with time-varying wavefronts resulting from mechanical disturbances or workpiece motion. These adaptive reference-beam interferometers have several advantages over passive reference-beam interferometers such as the Michelson or Mach-Zehnder:
- No path-length stabilization is required
- The intrinsic device field-of-view is higher
- Mechanical stability tolerances are greatly reduced
- Low frequency wavefront disturbances resulting from turbulence and mechanical disturbances can be compensated.
Current receiver development efforts are focused on approaches based on two-wave mixing or on photo-emf detection. In the two-wave mixing approach (see Figure 2), the photorefractive medium acts as an adaptive beamsplitter, combining a distorted signal beam with a plane-wave reference beam and matching their wavefronts for homodyne detection. To provide linear detection of a temporally phase-modulated signal beam, the phases of the two combined beams must be biased in quadrature for optimum sensitivity. In contrast with the confocal Fabry-Perot interferometer, no path-length stabilization is required to maintain this condition.
One feature of the two-wave mixing approach is that there is no material-related upper limit on the ultrasonic signal bandwidth. The upper limit is determined only by the bandwidth of the conventional photodetector. By contrast, the wavefront distortion compensation bandwidth is determined by the response time of the photorefractive grating. The grating response time also determines the maximum scan rate for scanning applications. For many industrial applications, a compensation bandwidth of at least 1 kHz is required. The requirement for a short response time favors the photorefractive semiconductors. In recent experiments using bulk InP and CdTe as well as photorefractive multiple quantum wells, such bandwidths have been observed. For inspection applications in a static environment, slower, more efficient materials such as BaTiO3 may be used.
Fig 2: Schematic diagram of a laser ultrasonic receiver based on two-wave mixing.
The other approach under development at this time is based on a reference-beam interferometer with photo-emf detection, as shown in Figure 3. In this case, the photo-emf element performs the dual function of laser-based ultrasonic detection as well as optical distortion compensation in a single semiconductor crystal. As before, the speckled signal beam interferes with the reference beam in a photorefractive material, producing a spatially modulated conductivity pattern, which leads to the production of a spatially periodic space charge field via the normal carrier migration and trapping process. The small phase modulation on the signal beam imparted by the surface motion causes a lateral vibration of the periodic free carrier grating, which induces an ac current that is proportional to the modulation amplitude and to the total power. This current is only produced when the frequency of the ultrasonic phase modulation is faster than the grating relaxation rate. When the modulation frequency is lower than the grating relaxation rate, the space charge field grating can follow the motion of the fringes and no current is produced. Thus, the grating relaxation rate is equivalent to the compensation bandwidth defined above and the photo-emf receiver has the desirable property of reduced sensitivity for noise-related frequencies below this bandwidth.
As mentioned above, the major advantage of the photo-emf approach is that it combines the separate optical compensation and detection stages in the two-wave mixing approach into a single semiconductor element, without the need for an optical readout beam or an electro-optical response. Since no transmitted beams are required, a laser wavelength with a photon energy larger than the bandgap can be used. At these wavelengths the large value of absorption coefficient provides a very fast grating relaxation rate for modest levels of probe laser power. In the case of GaAs, this rapid relaxation rate allows compensation of wavefront distortions at bandwidths exceeding 100 kHz, as well as measurements on samples moving at speeds greater than 20 m/s. The upper limit on the ultrasonic signal processing bandwidth is determined by the recombination rate, which is ~80 MHz in conventional semi-insulating GaAs. In recent experiments, we have also shown that photo-emf detectors with interdigitated electrodes can produce an improvement of at least 10x in the responsivity of these devices.
Fig 3: Schematic diagram of a laser ultrasonic receiver based on photo-emf detection.
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