Vladimir B. Markov

Matrix laser vibrometer for transient modal imaging and rapid nondestructive testing

This article describes the development and application of a 16x16 array (matrix) laser vibrometer based on a parallel architecture which supports fast 2D measurement of arbitrary (steady state, non-steady state, transient) solid body vibrations without beam scanning. The small size and low weight of the measurement probe, which is linked to a remote detector/processor unit via a flexible armoured fibre-optic umbilical, enables deployment in areas with restricted access. Incorporating aspects of high-speed electronic speckle pattern interferometry (ESPI) and laser Doppler vibrometry (LDV), the design is based on a hybrid fiber-optic/bulk optic interferometer which operates at a wavelength of 1550 nm. Test data illustrate high-speed capture of transient vibrations, showing the full 2D temporal evolution of surface deformation, including multiple resonant modes, of a center-pinned metal plate excited by a 1-50 kHz frequency chirp of 109 ms duration. We discuss preliminary data showing detection of sub-surface defects in composite materials, based on non-contact (frequency chirped) acoustic resonance of the locally damaged structure. For large area NDT the probe can be mounted on a lightweight XY gantry for automated multi-frame measurements.

System concept and some characteristics of the coupled-cavity laser system for active target tracking

We performed architecture and design analyses of coupled-cavity laser systems to arrive at a concept for tracking distant objects. We also conducted an experimental study of such a system using a pulsed ruby laser as a prototype for the laboratory tests. Both laser cavities were coupled through the dynamic holographic grating. Special attention was paid to characterization of the coupled-cavity laser system and its operation. In particular, we studied the formation of holographic grating that serves to couple the cavity; the slope efficiency of the master and slave arms and their dependence upon cavity parameters. The resulted experimental data and analyses verified the tracking principle and proved the feasibility of the proposed phase-conjugate double cavity laser system for tracking moving object at distance with high accuracy.

Applications, challenges, and approaches for electronic, digital holography

Digital holography has been around for over thirty years; however only in recent years has necessary and affordable recording and computing hardware become widely available. Some of the first works in digital holography are described as far back as 1966, with others following shortly after that. Today many researchers are working in this area for both application and further development of the technique. Indeed, digital holography may represent the best hope for holography as a thriving area of optics, and science. In this paper we discuss of the many applications, advantages, challenges, and opportunities that exist in the field of digital holography.

3D speckle-shift hologram and its storage capacity

Here a method for data storage by means of multiplexing 3D holograms is considered. The method consists in shifting the recording material with respect to a reference speckle wave. The experimental data on the dependence of the diffraction efficiency of the hologram with respect to the shift is presented. The general scheme of data storage in 3D-speckle shift hologram is considered and presented. Its shown that this kind of hologram substantially outperforms all other types for the number of cells (pixels) in which information can be stored. At the same time, the noises that are inherent in a 3D-speckle shift hologram restrict the density of the information storage.

Coherent-optical localization and assessment of importance of damage and defects of cultural heritage

Abstract Some experimental results are presented, related to practical applications of a holographic nondestructive technique for inspection of museum item conditions, especially in the detection of deformation, stress concentration and defect localization. These results are obtained using the basic principles of traditional double-exposure holographic interferometry, as well as its electronic variant with computer image processing. In the latter case the accuracy in displacement measurements was studied on a test object. Two museum items have been selected for their condition inspection: a XVIIIth century icon on wooden panel and a precolumbian terracotta anthropomorphic. The technique described can give quite accurate metrological information, and is also rather promising for rapid qualitative analysis of the object condition before and after restoration, as well as during its storage.

Holographic investigation of brittle crack propagation in plastic pipes

Abstract Plastic pipes dominated the low pressure gas distribution piping market during the last decade. Research on steel pipes has long since indicated that the susceptibility to rapid crack propagation (RCP) increases with increasing diameter. Although most plastic pipes used in gas distribution systems are of small diameter (

Selective characteristics of single-layer color holograms

We report some results of theoretical analysis and experimental study of selective properties of 3D holographic grating. In particular, the case of strong deviation of the propagation vector of the reconstructing wave from Bragg conditions by tuning the wavelength and/or the angle of the incident illuminating beam is considered. In this case, it is more convenient, to operate with joint function of angular-spectral selectivity. The character of this joint function should be taken into consideration, if true color holographic images are of interest. Key words: angular and spectral selectivity, 3D reflection grating, diffraction efficiency.

Injection seeded single frequency Nd:YAG MOPA Q-switched laser

This paper discusses the realization of an injection-seeded single frequency Q-switched Nd:YAG ring laser with pulse width adjustability from several nanoseconds to 20 ns and an output energy in excess of 100 mJ from a ring cavity slave oscillator. In the cavity configuration under study, the slave ring oscillator was injected by a pulse formed from a CW single frequency non-planar ring oscillator (NPRO) Nd:YAG laser with a Pockels cell modulator. The injected pulse duration is slightly shorter than the emission round trip time inside the slave laser cavity. The use of this injection seeding results in unidirectional Q-switched laser oscillation with suppression of bidirectional Q-switched oscillation that otherwise may be initiated from spontaneous emission. The advantage of these regime is in stable laser operation with no need in adjustment of the seeded light wavelength and the mode of the cavity because there are never two waves of the same polarization existing inside the cavity at the same location and the same time. The cavity contains two Pockels-cells, in which the first cell serves to open the oscillator cavity and the second one performs cavity dumping, generating a pulse with optimized characteristics and enables variation of the duration of the Q-switched output pulse. Proper selection of the pump parameters and Pockels-cell gating enables further operation of the system in a mode when the Q-switched pulse can be formed only under the seeding conditions. It is found that the wavelength of the Q-switched laser radiation matches well to the injected seed NPRO laser longitudinal mode. By using two-stage amplifiers, an output energy better than 300 mJ has been achieved in a MOPA configuration without active control of the cavity.

Laser scanning technique for fatigue damage evolution detection

Metal components subjected to cyclic stress develop surface-evident defects (microcracks, slip bands, etc). Monitoring the formation and evolution of these fatigue damage precursors (FDPs) with increasing numbers of cycles can be an effective tool for determining the fatigue state of the component, which can be used in remaining fatigue life prognostics. In this paper a laser scanning technique (LST) for FDP detection is described and experimental results from examination of specimens made of nickel-based superalloy and aluminum are presented. The proposed detection technique is based on scanning a focused laser beam over the specimen surface and detecting variations in spatial characteristics of the scattered light signal. These variations indicate the presence of surface abnormalities and therefore can be associated with incremental fatigue damage formation. The studies performed show that the proposed LST can serve as a basis for design of a portable non-contact instrument for in situ structural health monitoring.

The nonlinear OPC technique for laser beam control in turbulent atmosphere

This paper discusses a novel type of beam director for effective laser beacon formation in deep turbulence conditions. The concept of the proposed beam director is based on an innovative approach employing a Brillouin enhanced four-wave mixing (BEFWM) mechanism for generating a tight (small spot size) laser beacon on a remote image-resolved target. The BEFWM technique enables both amplification and total (phase and amplitude) conjugation of the beacon-forming beam without the need for wavefront sensors, deformable mirrors or predictive feedback algorithms. Total conjugation is critical for beam control in the presence of strong turbulence, whereas conventional adaptive optics methods do not have this capability. The phase information retrieved from the beacon beam can be used in conjunction with an AO system to propagate laser beams in deep turbulence.