An in-line shearography setup based on circular polarization gratings Pascal Blain *, Pierre Piron, Yvon Renotte, Serge Habraken University of Liège, Department of Astrophysics, HOLOLAB, Liège, Belgium article info Article history: Received 17 January 2013 Received in revised form 25 February 2013 Accepted 5 March 2013 Available online 12 April 2013 Keywords: Speckle interferometry Shearography Circular polarization grating Polarization holography abstract The shearing amount defines the resolution of a speckle shearing interferometer and the shearing direction defines the sensitivity direction of the setup. The properties of circular (cycloid) holographic polarization gratings recorded in liquid crystal polymers can be used to build a new multi-shearing direction and amount shearography setup. The polarization states of the diffracted beams offer an easy way to produce phase shifts and thus to compute the phase contained in the shearograms. The theoretical bases of such a device are highlighted and an original compact and full in-line setup is proposed. First experimental results of delamination detection and flaw detection by shearography are presented. A discussion on the importance of the grating recording and the polarization orientation points out the best working requirements. & 2013 Elsevier Ltd. All rights reserved. 1. Introduction Shearography is widely used in the field of non-destructive Testing [1]. Many shearing elements for shearographic devices have been proposed [2]. Among these, the most suited with phase shifting techniques (which are the only way to obtain quantitative, i.e. numerical, evaluation of a shearogram [3]) and the most widely used interferometer, is the modified Michelson [3]. The main drawback of this interferometer comes from the crossed reference and object beams can be altered differently by external vibrations and environmental disturbances. To overcome this drawback, self- referenced almost common path interferometers based on a diffractive element [4] and on a birefringent element [5] (i.e., a Savart Plate [6]) have been developed for shearography. Those devices take advantage of polarization states separation [7]. Unfortunately their constant shearing direction [4] and amount [5] limit their use. Indeed the shearing amount defines the resolution of the interferometer and the shearing direction defines the direction in which the first derivative of the displacement is evaluated. It has also been established that quantitative topologi- cal shearography is restricted along the shearing direction [8,9]. Other devices based on polarization states separation have been proposed, such as Wollaston and Babinet compensator [1,7] based interferometers. Valera et al. [10,11] used a birefringent fiber as a way to produce a two orthogonal polarization illumination and a Wollaston as shearing device. The shearing amount can be changed by translating the Wollaston. Phase shifts were induced by applying a sinusoidal time-dependent constraint to the fiber with a piezoelectric transducer. The shearing element from Zhao et al. [12] is a liquid crystal reflective spatial light modulator. Although not in line, this setup benefits from numerically drivable phase shifting and shearing amount. Holographic gratings [13,14] and sinusoidal diffraction gratings [15] have successfully been used in several shearography setups. Although compact, those devices do not permit a full in line phase shifting techniques. A robust interferometer, which allows at the same time phase shifting and variation of the shearing amount and direction, would better fit with for shearography needs. The aim of this study consists in associate birefringence together with diffraction. Circular polariza- tion gratings (CPG) recorded in liquid crystal polymers (LCP) [16–21] are proposed. For the first time a shearography setup is built full in line and enables straightforward phase shifting, changing the shear- ing direction and changing the shearing amount. After a short reminder of CPGs properties, a new shearography setup and some experimental results are depicted. 2. Theory The following results and statements are inspired from pre- vious related works [16–25] on CPGs. This section uses those results to demonstrate that CPGs are well suited for shearography. 2.1. Circular polarization grating (CPG) The construction of a CPG consists in recording an interference pattern of two orthogonal circular polarizations beams. When the Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/optlaseng Optics and Lasers in Engineering 0143-8166/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.optlaseng.2013.03.003 * Correspondence to: Hololab, Allée du 6 Août 17, Bat. B5a, 4000, Liège, Belgium. Tel.: þ32 43669646; fax: þ32 43664516. E-mail addresses: pascal.blain@ulg.ac.be (P. Blain), pierre.piron@ulg.ac.be (P. Piron), y.renotte@ulg.ac.be (Y. Renotte), shabraken@ulg.ac.be (S. Habraken). URL: http://www.hololab.ulg.ac.be (P. Blain). Optics and Lasers in Engineering 51 (2013) 1053–1059