HOLOGRAPHY
JANUARY
1998
A breadboard
holographic
interferometer
with
photorefractive
crystalsandindustrial
applications
Photorefractive crystals (PRCs), and par-ticularly those belonging to the sillenite family (Bi,2Si02o (BSO), Bi,20e02o
(BOO), Bi'21i020 (BTO», are notoriously
promising recording media for holo-graphic interferometry.' Classical plates are much more sensitive than PRCs, but they need chemical processing requiri~g liquid bridges when in-situ record,ing is required. Photo-thermoplastics have a sensitivity comparable to PRCs and they can be processed in-situ by electrical and thermal processes. However, though they can be erased, the number of exposures is limited. Sillenite PRCs are about I()()()
times less sensitive than the others, but have the advan-tage of being self-developing and indefinitely reusable. One can envisage holographic cameras that, like speckle interferometers, do not require any external operation or manipulation, but here with the higher-quality mea-surement dynamics and lower noise levels that are typi-cal of holographic interferometry.
At a first glance, these advantages are
counterbal-anced by the relatively
weaksensitivity of sillenites.
This difficulty, combined with a weak diffraction effi-ciency and optical dimensions of crystals limited to the cm2, meant that experimental prototypes of PRC inter-ferometers were confined to laboratories where power-fullasers were available. The recent availability of much larger, good-quality, sillenite crystals, powerful com-pact lasers, and sensitive, commercial CCD cameras, has allowed us to overcome these difficulties. Here we discuss an interferometer that was designed to be com-pact (on a breadboard), easily transportable, able to image objects of about 50 x 50 cm" and which made possible the taking of quantitative measurements.The development and optimization of the system are already presented elsewhere.2 Real-time holographic interferometry is performed: reference and object beams are incident onto the crystal for each exposure and the hologram is continuously recorded and read out. Once the object is deformed, an interferogram is observed and disappears slightly within a response time that de-pends on the crystal and illumination conditions. The instrument scheme is shown in figure 1. The ensemble surrounded by the grey line is completely included in a transportable casing (80 x 30 x 20 cm'). The laser is a compact, air-cooled cw DPSS YAO emitting 490 mW @ 532 nm. The crystal is a BOO doped copper grown by J-C Launay (University of Bordeaux) with a 29 x 27 mm2 optical face. Typical working practice is to have a ratio of 200 between recording beams at the level of the crystal, with an object beam of at least 10 IlW/cm2. With a total intensity of2 mW/cm2, the response time is 9 seconds. This limits the use of the system in an only moderately stable environment. The quantitative measurement can be performed by phase-shifting' for sufficiently stable deformations, or by the spatial car-rier technique with Foucar-rier filtering, for the monitoring of dynamic deformations (one interferogram analysis).'
Driviuinlerfau
graphic interferometry with semiconductor PRCs. Camera DPSS YAG laser 490mW CPG LI. L2. L3: lenses MI. M2. M3: mirrors SH I, 5H2: shullers MO: microscope objective VBS: variable beamspliller PZT: piezo translator SF: spatial filter SU: stimulation unit OB: object beam RB: rderence beam PRC:photorefJ1lclive cryslal
between polarizers
Marc P Georges and PhiIippe C
Lemaire
Centre Spatial de Liege
Universite de Liege
4031 Angleur, Liege
Belgium
Tel: +32-4-3676668
Fax: +32-4-3675613
E-mail: mgeorges@ulg.ac.be
Rgure 1.Scheme of the photorefractive holographic camera.
A first application shown in figure 2 (a) and (b) is defects detection in composite panels. The observed area is 55 x.37 cm2. After a hologram is recorded with the object "at rest", the object is heated and the gram is observed after relaxation. The phase interfero-gram (a) is obtained by phase-shifting and is further unwrapped and differentiated, clearly showing defects (b). Also, quantitative measurements of continuous de-formations of large objects have been performed.' A new application of our system is the measurement of vibrations.s The technique used here is to record the hologram of the object at the rest and then excite it with vibration. Directly performed stroboscopic readout is synchronized with the excitation. The observed area is smaller than for other applications because less light is available at the CCD when the stroboscope is working. The object is set closer to the holographic head, so re-ducing the field-of-view (typically 25 x 25 cm2). Fig-ure 2 (c) shows the phasemap of a turbine blade vi-bration mode.
Provided the instru-ment is used in a moder-ately stable environment (merely a good relative stability between the holo-graphic head and the ob-ject), measurement can be performed at high accura-cies on relatively large ob-jects.2.' To our knowledge, this is the first transport-able holographic camera based on photote-fractive materials and used in such varied applications. The present and future work of our group focuses on a compact head design, pulsed illumination ex-periments (for perturbed environments or analysis of transient phenomena), and near-infrared
holo-References
1. M P Petrov. S I Stepanov, and A V Khomenko. Photo refractive Crystals in Coherent Optical Systems, Springer Series in Optical Sciences 59, Springer- Verlag, Berlin, 1991.
2. M P Georges and Ph C Lemaire, Holographic
interferometry using photo refractive crystals for quantitative phase measurement on large objects.
Proc. SPIE 2652, pp 248-257, 1996.
3. M P Georges and Ph C Lemaire, Phase-shifting
real-time holographic interferometry that uses bismuth silicon oxide crystals. Appl. Opt. 34 (32), pp
7497-7506. 1995.
4. M P Georges and Ph C Lemaire, Holographic
interferometry using photo refractive crystals: recent advances and applications. Proc. SPIE 2782, pp
476-485. 1996.
5. M P Georges and Ph C Lemaire, Real-time
stroboscopic holographic interferometry using sillenite crystals for the quantitative analysis of vibrations. Optics Comm., to be published.
(a)
(c)
Figure 2. Industrial applications of the holographic camera. Defect detection in aeronautical composite panels. (a) Phase interterogram. (b) Differentiated phase for easy defect localization. (c) Vibration mode of a turbine blade (phase interterogram).
