Raj Kumar

Publications

• 0.70

  Impact points

  Effects of pressure gradients on laser beam propagation through an optical window for tokamak plasma diagnostics

  Raj Kumar, Ranjeet Singh, Kaushal Pandya, Ajai Kumar

  Fusion Science and Technology. 01/2012; 61(1-1):51.

  In the current study, investigations are reported on the effect of pressure gradient on the optical properties of the window used in optical diagnostics of plasma. Window surface deformations due to pressure gradient (difference between inside and outside pressures of vacuum vessel) at vacuum interf... [more] In the current study, investigations are reported on the effect of pressure gradient on the optical properties of the window used in optical diagnostics of plasma. Window surface deformations due to pressure gradient (difference between inside and outside pressures of vacuum vessel) at vacuum interface are monitored with a digital dial gauge. Effects of these surface deformations on the laser beam propagation through the window are discussed in the context of optical diagnostics. Ultra High Vacuum (UHV) sealing of a large size rectangular glass window with metal assembly using a Helicoflex seal is also discussed.

• Effect of Airy disk on schlieren diffraction interferometer

  Raj Kumar

  Optik. 01/2011; 122:105-109.

  Schlieren is among the oldest and widely used optical techniques for detection of change in refractive index in transparent media. Present work demonstrates the effect of the position of schlieren element in the Airy pattern on the schlieren interferogram. It has been shown that the amplitude of the... [more] Schlieren is among the oldest and widely used optical techniques for detection of change in refractive index in transparent media. Present work demonstrates the effect of the position of schlieren element in the Airy pattern on the schlieren interferogram. It has been shown that the amplitude of the diffracted wave, starting from schlieren element and serving as an inbuilt reference beam for the schlieren diffraction interferometer, becomes maximum when schlieren element diffracts light from the Airy disk. Effect of F-number of the optical system on schlieren pattern is also demonstrated via observations on the amplitude of diffracted light.

• Extraordinary optical transmission by interference of diffracted wavelets

  Kumar Raj

  Optica Applicata. 01/2010;

  The phenomenon of extraordinary optical transmission is drawing much attention of researchers because of its potential applications in diverse emerging areas. In the present work, experimental observations on diffraction-Lloyd-mirror interferometer are reported, where two diffracted wavefronts are s... [more] The phenomenon of extraordinary optical transmission is drawing much attention of researchers because of its potential applications in diverse emerging areas. In the present work, experimental observations on diffraction-Lloyd-mirror interferometer are reported, where two diffracted wavefronts are superimposed using Lloyd’s mirror. These observations provide direct experimental evidence in support of the idea that one of the main reasons of enhanced transmission through subwavelength apertures is the coherent superposition of diffracted wavelets originating from diffractive scattering at the apertures.

• Diffraction Lloyd mirror interferometer

  Raj Kumar

  Journal of Optics. 01/2010; 39:90-101.

  Download

• Interference and diffraction effects in folding mirror schlieren diffraction interferometer

  R. Kumar

  Applied Physics B: Lasers and Optics. 11/2008; 93:415-420.

  Experimental investigations on interference and diffraction phenomenon occurring in a folding mirror schlieren diffraction interferometer have been presented. It is shown that the well-known Lloyd mirror interferometer can easily be converted into a folding mirror schlieren diffraction interferomete... [more] Experimental investigations on interference and diffraction phenomenon occurring in a folding mirror schlieren diffraction interferometer have been presented. It is shown that the well-known Lloyd mirror interferometer can easily be converted into a folding mirror schlieren diffraction interferometer. Various intermediate stages occur as the basic two-beam interference pattern in Lloyd mirror interferometer is transformed into a schlieren diffraction pattern. Study of these intermediate stages gives a good insight in understanding the transition behaviour between the fundamental phenomenon of interference and diffraction taking place as the mirror-edge approaches the Airy disk.

• INVESTIGATIVE STUDIES OF PHASE OBJECTS IN LASER SCHLIEREN DIFFRACTION INTERFEROMETRY AND DEVELOPMENT OF MACHINE-READABLE FEATURES IN SECURITY HOLOGRAMS

  Raj Kumar

  05/2008

  Degree: Ph. D.

  Supervisor: Dr. A. K. Aggarwal and Prof. Devendra Mohan

  Download Paper

• Structure of boundary diffraction wave revisited

  R Kumar

  Applied Physics B -Lasers and Optics. 03/2008; 90:379-382.

  The physically appealing boundary diffraction wave theory which suggests that diffraction patterns arise due to interference of an undisturbed (geometrical) wave and the boundary diffraction wave generated by edge of the diffracting aperture, simplifies the solution of diffraction problems by reduci... [more] The physically appealing boundary diffraction wave theory which suggests that diffraction patterns arise due to interference of an undisturbed (geometrical) wave and the boundary diffraction wave generated by edge of the diffracting aperture, simplifies the solution of diffraction problems by reducing the Fresnel–Kirchhoff surface integral into a line integral over the illuminated boundary of the diffracting aperture. The present work reports experimental investigations carried out on the structure of the boundary diffraction wave. It has been shown that the boundary diffraction wave is continuous behind the diffracting aperture and apparently there does not exist any discontinuity at the geometrically light to shadow transition boundary, as is required by the theory.

• Comparative study of different Schlieren diffracting elements

  Raj Kumar, Sushil K. Kaura, D. P. Chhachhia, D. Mohan, A. K. Aggarwal

  Pramana. 01/2008; 70:121-129.

  This paper presents an analysis of diffraction effects taking place at different Schlieren diffracting elements. Two types of diffraction effects are prominent in the Schlieren schemes. One is diffraction of direct light (source image) at the Schlieren element, which limits the sensitivity and resol... [more] This paper presents an analysis of diffraction effects taking place at different Schlieren diffracting elements. Two types of diffraction effects are prominent in the Schlieren schemes. One is diffraction of direct light (source image) at the Schlieren element, which limits the sensitivity and resolution of Schlieren systems. The second type is the diffraction of light deflected from the test object at the Schlieren-diffracting element. This second type of diffraction degrades the quality of Schlieren results. Experimental results showing the effect of diffraction of light deflected from the test object at a phase knife-edge, corner of a square phase aperture and an optical fiber tip as Schlieren diffracting elements have been presented and discussed.

• Holographic optics based schlieren diffraction interferometer

  Raj Kumar, SK Kaura, DP Chhachhia, D Mohan, AK Aggarwal

  Current Science. 01/2008; 94:184-188.

  This paper describes a scheme that uses holographic optics for realization of schlieren diffraction interferometer. The interferometer has been realized with single holographic optical element in conjunction with a collimated beam and the focal plane-modifying element. Multiplexing ability of hologr... [more] This paper describes a scheme that uses holographic optics for realization of schlieren diffraction interferometer. The interferometer has been realized with single holographic optical element in conjunction with a collimated beam and the focal plane-modifying element. Multiplexing ability of holographic optics also enables simultaneous operation of more than one technique such as schlieren, shadowgraph and interferometric techniques. Combined schemes where one or more techniques could be applied simultaneously are useful for detailed investigations of phenomena like flow of compressible fluids, because each method has its own characteristic strengths and weaknesses. Procedures for the generation of holographic optical element and realization of various schemes have been presented.

• Interferometric moiré pattern encoded security holograms with concealed phase object

  Raj Kumar, AK Aggarwal

  Optics Communications. 11/2007; 279:120-123.

  This paper describes an interferometry-based scheme for concealing phase objects in a security hologram. The decoding process is based on geometrical moiré technique and requires a key for hologram authenticity verification. Decoding process is straightforward and could be carried out in white light... [more] This paper describes an interferometry-based scheme for concealing phase objects in a security hologram. The decoding process is based on geometrical moiré technique and requires a key for hologram authenticity verification. Decoding process is straightforward and could be carried out in white light, making it suitable for validation of security holograms. The method is described and experimental results are presented.

• Direct visualization of Young’s boundary diffraction wave

  Raj Kumar, Sushil K. Kaura, D.P. Chhachhia, A.K. Aggarwal

  Optics Communications. 08/2007; 279:54-57.

  Experimental investigations on Young’s boundary diffraction wave are presented where a wavefront division interferometric scheme is used on diffracted wavefront to generate two-beam interference fringes in the geometrically shadow region. These fringes have good visibility and are observable in the ... [more] Experimental investigations on Young’s boundary diffraction wave are presented where a wavefront division interferometric scheme is used on diffracted wavefront to generate two-beam interference fringes in the geometrically shadow region. These fringes have good visibility and are observable in the whole space, strongly advocating the physical existence of Young’s boundary diffraction wave as a separate entity. Analysis of these fringes may provide vital information about the structure and nature of boundary diffraction wave e.g. existence in whole space, dependence of amplitude on obliquity factor etc.

• Schlieren interferometric concealed coded security holograms

  Raj Kumar, AK Aggarwal

  Indian Journal of Pure & Applied Physics. 05/2007; 45:429-433.

  A method has been described to create concealed codes in security holograms. The codes are in the form of pure phase patterns and require an interferometric scheme to convert these invisible phase variations into detectable amplitude/intensity variations. An interferometrically encoded reference bea... [more] A method has been described to create concealed codes in security holograms. The codes are in the form of pure phase patterns and require an interferometric scheme to convert these invisible phase variations into detectable amplitude/intensity variations. An interferometrically encoded reference beam from the key hologram captures a phase modulated object beam to form the security hologram. In the final reading process, a moiré-like fringe pattern gets generated in the observation plane when the same reference beam illuminates security hologram. This moiré pattern is helpful in verifying the genuineness of the hologram and also facilitates in repositioning of security hologram. In the null mode moiré position, true object wavefront gets reconstructed but the information of concealed codes still remains invisible. Use of schlieren technique is proposed as a simple way to reveal the concealed codes for security hologram authentication. These concealed codes can be used as an anti-counterfeiting feature in embossed security holograms.

• Phase knife-edge laser Schlieren diffraction interferometry with boundary diffraction wave theory

  Raj Kumar, D Mohan, Sushil K Kaura, D P Chhachhia, A K Aggarwal

  Pramana. 04/2007; 68:581-589.

  Within the framework of boundary diffraction wave theory it has been shown that the first bright fringe on either side of the central dark fringe of the phase knife-edge Fresnel diffraction pattern could be broadened to cover the whole field of view. Broadening of the first diffraction fringe, inste... [more] Within the framework of boundary diffraction wave theory it has been shown that the first bright fringe on either side of the central dark fringe of the phase knife-edge Fresnel diffraction pattern could be broadened to cover the whole field of view. Broadening of the first diffraction fringe, instead of conventionally modifying the spatial frequency spectrum, enhances the sensitivity of the Schlieren system. The use of phase knife-edge as viewing diaphragm in Schlieren diffraction interferometry not only enhances the fringe contrast but also avoids the loss in phase information as it lets through light from all parts of the test object and its thin interfacing makes the method suitable even for studying weak disturbances.

• Interferometric key readable security holograms with secrete-codes

  Raj Kumar, D Mohan, AK Aggarwal

  Pramana-Journal of Physics. 03/2007; 68:443-450.

  A new method is described to create secrete-codes in the security holograms for enhancing their anti-counterfeiting characteristics. To imitate these codes is difficult as pure phase objects having complex phase distribution function are used to modulate the object beam that is recorded in conjuncti... [more] A new method is described to create secrete-codes in the security holograms for enhancing their anti-counterfeiting characteristics. To imitate these codes is difficult as pure phase objects having complex phase distribution function are used to modulate the object beam that is recorded in conjunction with an encoded interferometric reference beam derived from a key hologram. Lloyd's folding mirror interferometer is used to convert phase variations of the reconstructed wave-front into an intensity pattern for hologram authenticity verification. Creating the secrete-codes through an interferometric reference beam from the key hologram facilitates a multi-stage authenticity verification as well as easy repositioning of the security hologram through a specific Moiré pattern generated during the verification process.

• Knife-edge diffraction pattern as an interference phenomenon: An experimental reality

  Raj Kumar, SK Kaura, AK Sharma, DP Chhachhia, AK Aggarwal

  Optics & Laser Technology. 03/2007; 39:256-261.

  This paper demonstrates that a knife-edge diffraction pattern is, indeed, due to the interference of two superimposing waves: the geometrical wave and the boundary diffraction wave. Within the framework of boundary diffraction wave theory it is shown that this diffraction pattern can easily be broad... [more] This paper demonstrates that a knife-edge diffraction pattern is, indeed, due to the interference of two superimposing waves: the geometrical wave and the boundary diffraction wave. Within the framework of boundary diffraction wave theory it is shown that this diffraction pattern can easily be broadened in such a manner that a single fringe covers the whole field of view. At this point the system converges to a schlieren diffraction interferometer and could be used for the study of phase objects using diffraction-limited optics. Experimental observations show that the method bears a similarity to that of any known two-beam interferometer, e.g. Mach-Zehnder interferometer. The experimental details have been presented and the results are compared with a two-beam holographic interferometer and a point diffraction interferometer.

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

  Raj Kumar, DP Chhachhia, AK Aggarwal

  Journal of Optics A: Pure & Applied Optics. 09/2006; 8:747-751.

  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 re... [more] 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