Many optical applications requires often totally polarized light. However there are an other applications, such as optical spectrum analyzer, in which incident polarized light is undesirable. Insertion of depolarizer in such devices may stabilize the optical signal of the measured light, in order to reduce offsets in measurements. Liquid crystal are functional materials possessing anisotropies originating from their inner molecular alignment. A vertically aligned nematic liquid crystal with zero pretilts in the off state is isotropic for light impinging at normal incidence. However, the liquid crystal orientation upon electric switching is undefined; therefore the cell usually generates disordered birefringent medium related to undefined switching direction of molecules which produce random polarization of the transmitted light by liquid crystal cell, therefore depolarization effect is produced. In this work, the treatment of problems involving depolarization of incident polarized light beam passing through a depolarizing medium and general physical phenomena associated with it, will be investigated at the speckle scale. A suitable tool for this treatment will be real time Young’s interferometer constructed with a new principle including the possibility to control the fringe pattern in real time with objective to study the dynamics of speckle fluctuation. Modulation of depolarization control with an applied voltage are reported, also.
It is well known that the Young interference experiment is the fundamental setup to combine two beams and to construct the phase modulated light. Moreover, homodyne phase demodulator is based on signal decoding in back Fourier focal plane using bicell photodetector (B-PD). On the above base, we propose a novel experimental approach to the signals demodulation by using the optical interferometer which operates in homodyne mode, combined with liquid crystal spatial light modulators operating both phase as speckle modulator. Dynamic phase changes between the two beams can be controlled by monopixel liquid crystals cell placed in one branch of the interferometer. A phase modulation effect in a signal arm of interferometer is observed as a dynamic shift of the speckle pattern. Simple arithmetic combination of signals from B-PD placed in speckle pattern plane is only one necessary numerical manipulation to obtain exactly phase difference. Concept of signals demodulation in the Fourier focal plane can be only used for exactly defined geometrical (B-PD as well as Young interferometer) and physical parameters (polarization, wavelength). We optimize the setup geometry to obtain extremely high measurement resolution. In this paper we focus on the principles of operation of each part of the system as well as discussion their requirement in order to increase the signal to noise ratio.
We present possibility of demodulation signals with different types of modulation. Our setup gives possibility to obtain
high accuracy and precision of measurements. We explain how to optimize the setup to obtain optimum condition to
phase demodulation for signals with different types of modulation. Real time phase measurements and stable working
conditions are provided experimentally. Thermal stabilization and errors analysis based on geometrical and physical
setup parameters is demonstrated and optimized, too. Our phase demodulation method is effective and simple in opposite
to other phase decoding methods with are overloaded by digital signal processing steps.
Dispersion characteristics are the fundamental properties of microstructured fibres (MSFs) with respect to the nonlinear
applications. The changes of fibre dispersion may strongly influence the whole chain of diverse nonlinear effects
resulting in supercontinuum generation (SG). Transferring the experience from the topics related to tailoring different
properties of MSFs to investigate the potential design freedom of dispersion opens novel possibilities of building the
customized, all-fibre broadband and bright light sources.
The silica nonlinear microstructured fibres, as presented in this paper, become compatible with standard fibre
components and technologies (e.g. splicing, connectorization etc).
Supercontinuum generated in a small-core MSF is a very interesting nonlinear phenomenon from application-oriented
point of view. A development of tailored dispersion of highly non-linear silica MSF offers us the possibility of
constructing a customized broadband light source.
Therefore, in the paper we present a theoretical and experimental investigation of dispersion characteristics of several
different MSFs. Our studies are leading to the development of adapted dispersion properties, allowing construction of
customized supercontinuum sources. All fibre, white light interferometry set-up, resulting in extremely high precision
measurement of chromatic dispersion, is demonstrated, together with fully computer controlled fringe pattern analysis.
Constructed set-up permitted comparison of chromatic dispersion measurements of microstructured fibres with modified
fibre cross-section dimensions during the production process. High correlation between modelling and measured data
gives possibility to control dispersion level in manufacturing process. Additionally, precisely designed and measured
chromatic dispersion, especially around the zero dispersion wavelength, enables superior estimation of MSF nonlinear
effects.
The basic optical set-up for demodulation of phase signal propagating in singlemode fiber - optics was described in this
article. In this arrangement two fiber - optics (modulated signal in first and reference in second) was used. Both fibers in
the final part of this set-up are parallel to each other at distance between them equals "d". After propagation by fiber
beam goes through the lens. The spatial distribution of light amplitude in the focal plane of the lens is proportional to the
Fourier transform on the output light in fibers and modulated with frequency proportional to "1/d". The power spectral
density of the spatial distribution of light by bicell photodetector was analyzed. The sum and difference of output signals
from channels on the detector is depending on the power spectral density, bicell gap and cosine and sine of phase
respectively. On the ground of this results it is possible to identify demodulate of different types of modulation without
any a priori information.
A shape measurement method combining the fibre-optic interferometric fringe projection method and effective numerical spatial phase unwrapping method is proposed in this paper. Simplicity of the system arrangement determining low set-up cost is the main advantage of the presented approach. The paper presents a detailed theoretical method description. It is based on a specially worked out procedure of an automated analysis of the fringe patterns obtained by the Fourier transform and phase unwrapping based on the Mathematica software procedures. Experimental results of the measurements made for a real object show usefulness and efficiency of the proposed method.
The paper presents review of the selected joined transform correlators (JTC) used for automatic images recognition. Because they are used in non-contact optical measurement systems thus special numerical processing should be used for anlaysis of data collected by them. The neutral network approach, wide development in last decade for such system, seems to be small useful, mainly because it creates computing model. The image is parallel information and such method as the Fourier transform are more useful for its processing. From this reason the wavelet transform as generalization of the Fourier method has been chosen as optimal. On this base the JTC uses wavelet transforms (WT) for analysis because it is a powerful tool for wavelet filtration, image feature extraction and recognition of rationally distorted images. The laboratory system uses the joint wavelet transform correlator (JWTC) and its possibilities were presented on this paper as final result of review.
The construction and experimental results of fiber optic sensor using the ring-wedge photodetector is presented. As sensor head the sample of quasi-monomode optical fiber is used. Changes of intermodal interference condition, due to external perturbation, generated changes of output speckle pattern. This pattern differences are automatically recognized by neural processing. The pattern for neural processing is generated by suitable placed ring-wedge photodetector system.
The system of an acousto-optic processor for phase and frequency measurements is considered in the paper. Interaction between the laser light and acoustic waves result in appearance of the interference pattern in the focal plane a 600 mm lens. All parts of the AO system are discussed in details. The analyze of the disturbing factors influence on the interference pattern is also discussed.
The construction of the fiber-optic sensor for the recognition of perturbations and result of its studies are presented. As a sensor head the sample of two-mode optical fiber is used.Changes of intermodal interference condition, caused by external perturbation, generated changes of output speckle pattern. This output has been concerned as an intensity image and diffraction method, known from an automatic image recognition, has been applied for its recognition. For this reason the ring-wedge photodetector has been placed in Fourier plane of the image studied. The digital signal generated by this detector has been processed by software neural network. As the effect of suitable process of network training, a possibility of the perturbation recognition has been obtained without a necessity of troublesome analysis of intermode interactions. An additional advantage of this solution is the possibility to train the network to eliminate slow environmental perturbations.
Features and parameters of acousto-optic (a-o) frequency shifters and a-o Bragg cell for spectrum analyzer, developed and manufactured, are shown in this work. Three particularly interesting versions of frequency shifter have been made, using Ge single crystal with LiNbO3 transducer for (lambda) equals 10.6 micrometer. They were equipped with specially electric matching circuits in order to widen and shape frequency characteristic.
The parallel processing of the information in real time is very important in the field of computing, pattern recognition, and signal processing. Acousto-optical techniques are particularly interesting from these applications point of view, due to the high degree of parallel processing and broad bandwidth inherent in optical systems. The Bragg cell is the extremely versatile device and may act in combination as a variable pitch diffraction grating, optically taped delay line, single side band mixer, light amplitude modulator and sometimes as an optical polarization switch. This gamut of application possibilities has allowed to develop numerous optical architectures and multifunctional acousto-optic facilities.
Knowledge about optical and thermal properties of tooth tissue is necessary for application and use of new laser techniques in dentistry. Experiments have been carried out with tooth tissue from human and some species of animals at wavelengths reaching from ultraviolet to visible regions, using different spectroscopic methods which give complementary information about the samples. Photothermal spectroscopy, remittance spectroscopy, and fluorescence spectroscopy provide the possibility for differentiation between tooth constituents.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.