A collimated optical beam is required in several applications such as metrology, optical processing, free space propagation of information, and laser-based instrumentation. Prior to the advent of the laser, the size of the aperture at the focal point of a collimating lens determined the degree of collimation based purely on geometrical optics consideration. Because the laser beam can be focused to a tiny size, effectively making it a point source, the degree of collimation is governed by diffraction at the aperture of a collimating lens. A large number of procedures have been developed to collimate the beam. We examine the development of these techniques from the historical perspective.
Previously, we reported a simple method to obtain lateral shear in both the x - and y -directions using a multiplexing technique. The phase data was extracted using the inherent spatial carrier fringes formed due to the tilt in the two sheared beams. In this article, we report that an error in phase map is introduced when the band-pass-filtered Fourier transform (FT) spectrum is not centered prior to performing the inverse FT to obtain the phase. We also found that intentionally introducing aberrations when capturing dynamic fluctuations in the wave front, resulted in controlling the spread of the Fourier spectrum.
Polarization components can be used as nearly achromatic phase shifters in an interferometers. Here wepresent
an improved achromatic half-wave plate (HWP) phase shifter that can be incorporated at the input end of a white light
interferometer. A nearly achromatic HWP phase shifter that can be used at the input of an interferometer consists of a
rotating HWP followed by a quarter wave plate (QWP) fixed at an azimuth of 45°. An improved achromatic phase
shifter for the input end can be constructed using achromatic circular polarizers. The performance of this achromatic
HWP phase shifter when used at the input end of a white light interferometer is studied using Jones calculus. The
calculated values of the phase shifts between the interfering beams and their amplitudes are very nearly same for all the
wavelengths. This phase shifter thus gives much improved performance.
In applications of surface profilometry with white light interferometry (WLI), the detection of the peak of the fringe contrast function is of prime importance. Several procedures have been proposed for the determination of the fringe contrast function. Fourier transform technique and phase shifting technique are two important methods. In the Fourier transform technique, the interference pattern is scanned to obtain the interference signal which is then subjected to filtering in the frequency domain. This involves two discrete Fourier transform operations. The phase shifting technique makes use of the algorithms introduced in the monochromatic interferometry for the calculation of the fringe contrast function. Both PZT and polarization phase shifters are proposed to be used for WLI. In this paper, it will be shown that polarization phase shifter offers some advantages over PZT phase shifter.
Anomalies in fringes pattern due to weak defect remain practically hidden when the object is subjected to load in between exposures. Increased load results in more number of fringes. In order to bring out the defective region vividly some procedure to reduce the number of undesired fringes should be adopted. Comparative speckle interferometry compares the response of the test object with that of the master object and the fringe pattern corresponds to the defect when the objects are identically loaded. Surface contours can also be compared. Use of phase shifting in comparative speckle interferometry would give the quantitative estimate of the residual difference.
This paper present new optical configurations in speckle shear photography to measure in-plan displacement and the strain in real time using photorefractive (PR) crystals as recording medium. Speckle photography studies are made using a simple two-beam coupling configuration PR beam coupling in barium titanate crystals is characterized experimentally by measuring the signal beam gain and the exponential gain coefficient at 440 nm, 632.8 nm and 780 nm.
In speckle shear photography, a diffused object illuminated with two parallel narrow laser beams is imaged inside the crystal, and a pump beam is added at this plane. The speckle patterns due to each beam and the pump beam produce index gratings. When the object is deformed, the speckle patterns shift consequently. We now have four speckle fields two generated from the interaction of pump beam with the index gratings and two pertaining to deformed states directly transmitted through the crystal. Thus, the fields from respective points on the object interfere after passage through the crystal and produce the Young's fringe patterns. Due to strain, the fringes in each pattern are of different width and orientation, resulting in the generation of a moire pattern. The strain is obtained from the width and orientation of the fringes in the moire pattern. The experiments are conducted on a specimen with a notch, which is subjected to tensile loading. The above studies are carried out at three different laser wavelengths, namely, 441.6 nm, 632.8 nm and 780 nm and the results are compared.
In this paper we present the design and development of volume holographic lenses for white light imaging and solar energy concentration. Details of design analysis and process of manufacturing of these lenses on dichromated gelatin is also given.
The process of two-beam holographic coupling in optically anisotropic media is analyzed. We consider holographic media possessing birefringence, optically, and spatial modulation of these effects. In this paper energy transfer between two beams interfering in an electro-optic is analyzed as a function of the grating period of the recording hologram. The theory for two-beam coupling in Bi12SiO20 (BSO) with an external electric field and a moving grating is developed. We report high coefficients in a wide range of low spatial frequencies. Exponential gain coefficients of (Gamma) equals 7 cm-1 have been reached for high intensity beams ratio and moving gratings. These results have been compared with GaAs crystals.
A comparative study of the photorefractive response of some commonly used ferroelectric, nonferroelectric and semiconducting crystals is presented. The figure-of-merit parameters such as steady-state change in refractive index, response time and photorefractive sensitivity tare evaluated for crystals of BaTiO3, LiBnO3, BSO, GaAs and InP on the basis of Kukhtarev's theory, using input parameters form reported data. These parameters are calculated as a function of the grating period with and without an application of 10kV/cm electric field. The results indicate that the choice of the ideal material for a given application can be made only after a careful comparison of the figure-of-merit parameters as trade-offs exist among the various parameters for any material.
We describe a spectrally resolved white light interferometer with polarization phase shifter for use in surface profiling. Phase shifting is introduced by a rotating half- wave plate. The phase shifted intensity values needed for the phase calculation at each pixel are obtained from the same pixel instead of different pixels, thereby avoiding error due to variation in sensitivities of different pixels.
Photorefractive crystals offer several attractive features such as high resolution and in situ processing. As the images are erasable, these crystals are suitable for read-write applications and hence find potential use in speckle photography, image processing and holography. The BaTiO3 crystal as recording medium has been extensively used as a novelty filter for real-time in-plane displacement measurements employing two beam coupling configuration. This paper presents new optical configurations in speckle shear photography to measure in-plan displacement and the strain in real time using BaTiO3 crystal as recording medium. Speckle photography studies are made using a simple two-beam coupling configuration. In speckle shear photography, a diffused object illuminated with two parallel narrow laser beams is imaged inside the crystal, and a pump beam is added at this plane. The speckle patterns due to each beam and the pump beam produce index gratings. When the object is deformed, the speckle patterns shift consequently. We now have four speckle fields: two generated from the interaction of pump beam with the index gratings and two pertaining to deformed states directly transmitted through the crystal. Thus, the fields from respective points on the object interfere after passage through the crystal and produce the Young's fringe patterns. Due to strain, the fringes in each pattern are of different width and orientation, resulting in the generation of a moire pattern. The strain is obtained from the width and orientation of the fringes in the moire pattern. The experiments are conducted on a specimen with a notch, which is subjected to tensile loading. The in-plane displacement is measured separately in another experiment. Time evolution of the growth and the decay of the signal beam also is studied. The above studies are carried out at different laser wavelengths and the results are compared.
Collimation of beam is essential in long path interferometry otherwise significant measurement error may be introduced. Therefore, considerable efforts were spent in developing techniques of collimation testing. In laser interferometers, the source is effectively a point. However, its axial shift from the focus of the collimator introduces a curvature in the wave front and its lateral shift may introduce off-axis aberrations. Therefore, the objective is to place the point source at the focus of the collimator. This task is achieved by collimation testing techniques, which can be grouped under interferometry, shear interferometry, Talbot interferometry, and phase conjugate interferometry. We will discuss these techniques below.
Photorefractive beam coupling in barium titanate crystals is characterized experimentally by measuring the signal beam gain and the exponential gain coefficient at 440 nm, 632.8 nm and 780 nm. The figure of merit parameters such as the change in refractive index, the space charge field and the trap density are estimated using Kukhtarev's theory. A comparative study at multiple wavelengths is presented. The signal beam gain is found to be maximum at 440 nm.
Speckle Phenomenon has been used for the measurement of surface roughness, deformation and shape of the object. Electronic speckle pattern interferometry can deliver the measurement data at the video rate. Some applications of ESPI like defect detection, residual stress measurement etc. are presented.
The paper presents a technique that could be used to measure (1) in-plane displacement, (2) the velocity of a solid body using a principle similar to that used in LDA, and (3) strain at a point. Some initial results for velocity determination are presented and compared with theory.
Shear ESPI has been applied for the NDT of objects under magnification. A method to obtain the thickness of the defect (assumed to be circular diaphragm on a rigid boundary) from the Shear ESPI data is presented. Experiments conducted on programmed defects establish the validity of the procedure; and thickness estimates with less than 10% departure from the nominal values are obtained.
The paper presents a configuration to record out-of-plane displacement component and slope simultaneously using electronic speckle pattern interferometry. The retrieval of information, however, is possible only by Fourier processing.
In this paper, we present the developmental details of an all fiber optic phase shifting Electronic Speckle Pattern Interferometer (ESPI) and its applications in Non-Destructive Evaluation (NDE) of Glass Fiber Reinforced Plastics (GFRP). Laminates made of GFRP are studied using the developed ESPI system in its shear configuration. This provides a full-field, non-contact method for NDE, and also observation of surface strains which are proportional to displacement derivatives. Since defects in objects usually induce strain concentrations, shear ESPI reveals defects by identifying anomalies in strain concentration. Experiments have been carried out for NDE of unidirectional GFRP specimens at different fiber orientations and having programmed defects. The experimental results are compared with the theoretical predictions.
Two simple Electronic Speckle Pattern Interferometer (ESPI) configurations have been devised based on diffused reference beam which provides out-of-plane displacement data over the whole field. Both configurations use a tiny diffuser to generate the reference beam. This makes the system insensitive to reference beam misalignment, simplifies the construction of ESPI setup and allows larger area of observation unlike the conventional ESPI system. Thermal deflection studies on a cantilever plate have been carried out. Experimental results and features over the conventional ESPI system are discussed.
A double shear speckle interferometer is presented that can provide information about the
curvature, i.e., the second order derivative of the out of plane displacement of an object under study.
Here, two shear interferometers are kept in sequence or tandem. The sheared images formed by the
first interferometer are sheared once again by the second interferometer. The shears at the image
plane can be adjusted for the required magnitude and orientation. A double exposure record is made
on a photographic plate before and after the object deformation. When the processed plate is
subjected to Fourier filtering, the curvature information is seen as a Moire of the two sheared slope
fringe patterns. The results for a centrally loaded diaphragm are presented. The advantages of the
present technique over the existing methods are discussed.
High birefringence optical fiber embedded in composite laminates can be used for real time strain measurement. The use of such an embedded optical fiber for detection of damage in composites through strain monitoring has been investigated.
Photorefractive crystals are gaining importance as a recording medium due to the possibility of real-time operations. A BaTiO3 crystal can be used as a recording medium in two configurations, i.e., when the phase shift of (Phi) between the dynamic grating and the interference pattern is (Phi) = +/- (pi) /2. The optical characteristics are different for the two configurations. We discuss the optical characteristics of BaTiO3 crystal under the two configurations, and use the BaTiO3 for speckle photography and image plane holography.
The plane parallel plate (PPP) and wedge plate (WP) interferometers are the simplest configurations for collimation testing. These elements are usually used in reflection due to high fringe visibility. The reflection coated wedge plate acting as a shearing interferometer both in reflection and transmission is recently reported. This multiple beam wedge plate lateral shear interferometer (MBWPLSI) used for collimation testing gives superior performance compared to an uncoated wedge plate interferometer. In this paper, some aspects of the multiple beam interferometer and its use in the double pass configuration are presented. Double pass results in a system of fringes in which adjacent fringes rotate and split in opposite directions for a non-collimated beam resulting in further improvement in the setting sensitivity.
A two-beam coupled photorefractive novelty filter is presented for real-time measurement of displacement components. In the optical configuration, the BaTiO3 crystal is oriented such that the spatial phase shift between the dynamic grating and the interference pattern is + π/2. Time evolution of fringe patterns is analyzed. The technique is applied to speckle photography for real-time measurements of displacement components. Experimental results are given.
An imaging system with its aperture fully open or with a multiaperture mask in front has been used for carrying out speckle shear interferometry. The shearing is achieved by installing shear elements in front of the imaging system. The multiaperture configuration has built-in in-plane displacement sensitivity, and hence shear fringes obtained from multiaperture shear interferometer always carry the in-plane displacement contribution. This problem does not arise when the full aperture of the lens is used. A three-aperture speckle shear interferometric technique is introduced here to separate the in-plane displacement and slope contributions from the shear interferogram. Detailed theoretical and experimental results are presented.
This paper presents the use of holographic optical elements for the measurement of slope and curvature by video techniques. the curvature fringes occur as moire between two speckle slope patterns. A good agreement between theory and experimental results is demonstrated.
The integrated optics waveguide devices should have low coupling losses with optical fibers. We present a procedure for determining the ion-exchange process parameters for fabrication of such low loss waveguides in glass and discuss the single mode waveguide fabrication by the inverse ion-exchange.
Self-referencing collimation testing techniques are briefly reviewed. New self-referencing configurations using Talbot and doublewedge plate shear interferometric techniques are described. Setting sensitivities of various arrangements are compared.
The inverse ion-exchange process in glass is
studied with the aim of fabricating single mode
waveguide devices. Single mode channel
waveguides arc fabricated by ion-exchange. The
process is modeled using finite difference technique
and the effects of various experimental
parameters on the refractive index profile of the
waveguide are studied.
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