A.K. Aggarwal

Highly Sensitive Pulsed Digital Holography for Built-in Defect Analysis with a Laser Excitation.

A highly sensitive method is presented for noninvasive defect analysis on thin structures with a Q-switched double-pulsed ruby laser with frequency doubling (347 nm). In our research we feature an all-optical arrangement, where a focused laser pulse derived from the same ruby laser (694 nm) acts as a built-in synchronous excitation source for digital holographic interferometry. The recordings are made with a CCD camera for capturing two holograms (two states of the specimen) corresponding to the two UV laser pulses with a short time separation (10-50 mus). Subtraction of the phase distribution in two digital holograms gives a fringe phase map that shows the change in deformation of the specimen between the recordings. The advantage of the proposed method is two fold. First, the use of a shorter wavelength results in a higher sensitivity. Second, owing to the induced synchronous built-in optical excitation, the specimen is not subjected to any external physical excitation devices. Experimental results are presented on identification and evaluation of defects in thin metal sheets.

Measurement of the focal length of a collimating lens using the Talbot effect and the moiré technique

A method using the magnification of the Talbot image and the moire technique to measure the focal length of a collimating lens is described.

Comparative study of various endoscopes for pulsed digital holographic interferometry

A comparison of several endoscopes as object image carriers in pulsed digital holography is presented. Three multicore flexible fiber endoscopes of different spatial resolution and one rigid endoscope are investigated. The four endoscopes are integrated in a setup for the recording of digital holograms on a CCD camera. A double-pulsed ruby laser is used as the light source. A spatial carrier is introduced by an off-axis reference beam, which permits quantitative evaluation of the phase difference between two holograms recorded with a short time separation (5-600 micros). From reported studies it may be inferred that the quality of the phase maps so derived from digital holographic interferometry has a strong correlation to the spatial resolution of the multicore fiber used in these endoscopes. With the endoscopic technique combined with pulsed digital holography a number of useful applications (in areas such as medical endoscopy, micromechanics, and microelectronics) are envisaged for which access to the objects of interest is otherwise difficult.

Interferometric moiré pattern encoded security holograms

This paper describes a simple method for making interferometric moire pattern encoded security holograms. These security holograms contain multi-fold concealed and encoded anti-counterfeit security features which can only be decoded by using an encoded key hologram in the final reading process. The concealed codes in these holograms are recorded with an encoded feature, so that they remain invisible to the counterfeiters, thereby enhancing the anti-counterfeiting ability of security holograms. In the final reading process, a specific moire-like fringe pattern is formed on the security hologram only when a reconstructing beam generated from the encoded key hologram illuminates this hologram. Further, a careful spatial filtering results in the generation of a specific moire pattern in the observation plane and it disappears when perfectly repositioned. These can also be used as security codes for better protection against counterfeiting in embossed holograms. Recording schemes for the formation of such security holograms and typical experimental results have been presented.

Encoded reference wave security holograms with enhanced features

A simple and cost effective two-step method for making encoded reference wave security holograms with enhanced features, which are readable with an encoded key hologram, has been presented. In the final reading process, two spatially separated sharp focus spots (bright spots) emerge only when the security hologram is illuminated by the decoding, reconstructing beam, generated from the encoded key hologram. In addition these focused spots, upon divergence in the longitudinal direction, further generate variable interferometeric features, i.e. circular and linear interference fringe patterns contained respectively in them. Since the verification/identification pattern in these security holograms are variable interferometric features in addition to the spatially separated sharp focus spots, this type of security hologram is suitable for both visual as well as machine inspection. They can also be used as security codes for better protection against counterfeiting embossed holograms. Recording schemes for the formation of such security holograms and typical experimental results have been presented.

Simple fiber-optic technique for in-situ corrosion sensing in structures

Corrosion of structures is a serious problem involving man and material safety. Over the years, though several methods of monitoring corrosion have been devised with some success, but there is a persistent need for devising non-destructive and in-situ techniques for monitoring corrosion in structures. Fiber optic techniques are capable of meeting these requirements, besides offering several other important advantages. Fiber optic corrosion sensors have thus become quite attractive and are currently being investigated to address the high costs associated with the existing structural maintenance procedures. Fiber optics based direct absorption spectroscopic techniques investigated by some groups for estimating corrosion have used single fiber elements for recording the signal reflected from specimen at different wavelengths. As the light coupling efficiency of the single fiber elements is relatively poor in comparison with that of fiber bundles and the signal available for processing is weak, the paper presents a simple and alternate technique based on the color matching principle of fiber optic colorimetry to detect corrosion induced color changes. It employs a thin Y- shaped fiber optic bundle which increases the quantity of light energy coupled from a whitelight source. The light reflected off the sample is made incident on a PIN photo- detector through a complementary filter. A series of such probes can be safety embedded and or bonded to structures at pre-determined locations. The experimental set up for this sensor was implemented and feasibility of in-situ corrosion detection in structures demonstrated. Measurement data was acquired for steel samples corroded both in concrete embedded and open ambience conditions and results analyzed.

Retrieval of infinite-fringe mode information from a beam folding interferometer for direct phase visualization

A recently reported one-beam interferometer using beam folding is quite suitable for carrying out studies on phase objects in finite-fringe mode, but in its present form it is difficult to work with this interferometer in infinite-fringe mode for direct and quick phase visualization, which may be required for certain problems of physical interest. The present paper describes a modification to the beam folding interferometer, where interferometric fringes are superimposed onto a grating to get both finite- as well as infinite-fringe mode information of the test object.

Full-view rainbow-reflection holograms

Abstract A simple method of recording full-view rainbow-reflection holograms on a single flat plate is described: only one white-light illuminating source is required in the reconstruction process. The hologram can be viewed from opposite sides, where each side of a three-dimensional object, with its original spatial relationship, is displayed in bright colours.

Moiré pattern encoded extended fractional Fourier transform security hologram

This paper describes a simple method for making extended fractional Fourier transform (EFRT) based Moiré pattern encoded security holograms. These security holograms contain multifold concealed and encoded anticounterfeit security features that can only be read through a key hologram and periodic patterns in the final reading process. The encoded features in these holograms are concealed and unknown to the counterfeiter. These features are encoded separately for each individual recording in angularly multiplexed extended fractional Fourier transform hologram (EFRTH). The principle of recording and reconstruction of the proposed security hologram along with experimental results are presented.

Realization of an optical interferometer based on holographic optics for real-time testing of phase objects

The paper describes a simple and cost effective method for the realization of an optical interferometer based on holographic optics, which use minimal bulk optical components. The optical arrangement in the proposed method involves a very simple alignment procedure and inexpensive holographic recording material is used in the formation of holographic optical elements. The proposed interferometer set-up is quite suitable for performing optical test studies on phase (transparent) objects in real-time. Recording schemes for the formation of holographic optical elements and the related technique for the realization of the interferometer set-up along with the experimental results have been presented.