The capability of measuring the spectral and temporal phase of an optical signal is of fundamental importance for the advanced characterization of photonic and optoelectronic components, biochemical sensors, structural monitoring sensors and distributed sensor networks. To address this problem, several techniques have been developed (frequency-resolved optical gating (FROG), spectral phase interferometry for direct electric-field reconstruction (SPIDER), stepped-heterodyne technique, laser Doppler vibrometry (LDV) and Doppler optical coherence tomography (OCT)). However, such techniques often lack of versatility for the mentioned applications. Swept-wavelength interferometric techniques and, among these, optical frequency-domain reflectometry (OFDR) are flexible and highly sensitive tools for complete characterization of amplitude and phase of target devices. In this work, we investigate the spectral and temporal phase measurement capabilities of OFDR. Precise characterization of spectral phase information is demonstrated by retrieving the phase response of a commercial optical filter, the Finisar Waveshaper 1000 S/X, programmable in attenuation and phase over C+L band (1530– 1625 nm). The presented results show accurate retrieval of group delay dispersion (GDD) and discrete phase shift as well as filter attenuation profile. Although some intrinsic accuracy limitations of OFDR phase measurements may be encountered (and herein specified), we show that information encoded in OFDR reflectogram data is very rich when adequately exploited. In addition to previously published results, we demonstrate the high sensitivity of the technique to Doppler effects. From practical point of view, such sensitivity can be beneficially exploited for the characterisation of dynamical aspects of samples under test. Unlike LDV, OFDR allows the simultaneous retrieval of the temporal position of several localised reflecting target along the beam path. All these aspects make OFDR a highly promising candidate for the study of both static and dynamic aspects of complex photonic components or to probe a parallel sensor network, as needed for future applications.
In this study, we suggest and validate an all-numerical implementation of a VanderLugt correlator which is optimized for
face recognition applications. The main goal of this implementation is to take advantage of the benefits (detection,
localization, and identification of a target object within a scene) of correlation methods and exploit the reconfigurability
of numerical approaches. This technique requires a numerical implementation of the optical Fourier transform. We pay
special attention to adapt the correlation filter to this numerical implementation. One main goal of this work is to reduce
the size of the filter in order to decrease the memory space required for real time applications. To fulfil this requirement,
we code the reference images with 8 bits and study the effect of this coding on the performances of several composite
filters (phase-only filter, binary phase-only filter). The saturation effect has for effect to decrease the performances of the
correlator for making a decision when filters contain up to nine references. Further, an optimization is proposed based for
an optimized segmented composite filter. Based on this approach, we present tests with different faces demonstrating
that the above mentioned saturation effect is significantly reduced while minimizing the size of the learning data base.
We report for the first time the use of the well-known coherence modulation of light in two-dimensional
arithmetic operations. This method, which allows several signals to be multiplexed on a single light beam,
permits parallel real-time all optical subtractions and additions. In the context of an increased need of security, a
major application of our multiplexed approach could be the optical encryption of images and multi-modalities
techniques in biometric domain. The single-channel optical architecture, which allows carrying out single and
multiple arithmetic operations, is described and more results are presented to show the effectiveness of our
technique. We have evaluated the performance of the processor in terms of noise level as a function of the
continuous optical path-difference ratio.
Interestingly, the past 20 years have provided us many examples of optical correlation methods for pattern recognition,
e.g. VanderLugt correlator (VLC). In recent years, hybrid techniques, i.e. numerical implementation of correlation, have
been also considered an alternative to all-optical methods because they show a good compromise between performance
and simplicity. Moreover, these correlation methods can be implemented using an all-numerical and reprogrammable
target such as the graphics processor unit (GPU), or the field-programmable gate array (FPGA). However, this numerical
procedure requires realizing two Fourier Transforms (FT), a spectral multiplication, and a correlation plane analysis. The
purpose of this study is to compare the performances of a numerical correlator based on the fast Fourier transform (FFT)
with that relying on a simulation of the Fraunhofer diffraction. Different tests using the Pointing Head Pose Image
Database (PHPID) and considering faces with vertical and horizontal rotations were performed with the code MATLAB.
Tests were conducted with a five reference optimized composite filter. The receiving operating characteristics (ROC)
curves show that the optical FT simulating the Fraunhofer diffraction leads to better performances than the FFT. The
implications of our results for correlation are discussed.
The latest developments in optical image processing for security, compression and cryptography require parallel real
time processing and multiplexing. In this paper, we propose the application of the well-know "coherence modulation of
light" technique for real-time encoding and decoding of signals which can be useful for optical image processing. This
method uses the coherence properties of broadband sources for encoding signals onto light beams. One major asset of
this approach, compared to other conventional optical modulation methods, is an original multiplex coding of several
signals through a single light beam. We achieve simultaneous real-time all optical image processing of analog twodimensional
signals and suggest a set of new criteria, based on mean square error, signal to noise ratio and peak to peak
signal to noise, to improve the quality of the decoding image as function of the optical path difference and the coherence
length of the source.
In this paper we have studied effect of depth etching on the Bragg gratings (BGs) realized by Focused Ions Beam. This
technique has the advantage to induce a direct waveguide structuring without intermediate media, comparing to
traditional methods. A reflectivity of 96% within a window centred at 1550 nm is obtained. The effect of the depth
etching on the transmittance and the bandwidth at half maximum is demonstrated.
We present an extension of an early work on external optical feedback in semiconductor lasers. A more general
formalism has been developed, which takes into consideration the anisotropy properties of an external cavity. The
expressions are derived for description of the feedback phenomena in a system composed of a laser diode and a Fabry-Perot cavity which is optically birefringent. We show that the emission behavior of such a system can be strongly
affected by the polarization states of the feedback waves, and that therefore multiple solutions become possible for
stabilization of a composite mode. Particular attention is paid to the angle-dependent phase condition. Examples are
given for a laser emitting at the wavelength of 1.54 μm and for an external cavity made of a quartz crystal.
Proton exchange is becoming widely accepted as a complementary technique to titanium indiffusion for the fabrication
of integrated optical waveguides in LiNbO3. In this paper, we propose, for the first time to our knowledge, a novel
approach to analyze the optical distributed waveguides formed by local index variation, combining two processes,
titanium indiffusion (Ti), and a localized patterned proton exchange (PE), yielding to the Ti-PE: LiNbO3 distributed
parameter waveguides. We present a straightforward extension of the Wentzel Kramers and Brillouin (WKB) technique
covering the computation of a single diffusion's effective index, to multiple successive diffusions having a specific
graded-index profile. The efficiency of the method is shown by varying some of its parameters, like the index
modulation or the proton exchanged depth, for example. The simulation's result proves that the evolution of the
reflectivity's spectrum has been found to be in a good agreement with prediction.
An electro-optically tunable erbium-doped fiber ring laser with a side mode suppression ratio of ~ 51.2 dB and a 0.062 nm linewidth is demonstrated. Wavelength tuning is achieved with a hybrid liquid crystal Solc structure used as an intracavity tunable filter. The laser wavelength is tuned over 28.6 nm with a tuning rate of 2.38 nm/V.
We show that the spontaneous-emission behavior of an emitter-embedded periodic photonic band-gap (PBG) stack, described in general on the basis of the approximation of dipole emission, can be considerably modified if the macroscopic features of the emitter are considered. We have developed a model which takes into account the optical thickness of the emitter in the usual formalism. The extended model is presented in comparison with the classical dipole-emission model. Some numerical results are given and discussed, by using GaAs-embedded SiO2 / TiO2-coated quarter-wave stacks as a specific configuration. Our model provides quantitative arguments for optimization of spontaneous-emission power in terms of radiation frequency and emitter localization. It can be directly applied to optimum design of more complex systems, such as multi-emitter-embedded periodic stacks and any other passive structure.
In this paper, we present the feasibility of a wavelength Bragg reflector based on a set of liquid crystal plates. The tunability of the structure, using the fast variations of birefringence of the plates, is demonstrated and simulated. The tailoring of the transmittance by changing the orientation of the plates between them is also discussed and simulated.
We show that cascading two finite one-dimensional periodic photonic band-gap structures may result, due to the multiple confinement of the electromagnetic field, in a significant modification of the modal distribution of the electromagnetic density of modes (DOM) around a band edge. As a result of the combination of the band-gap effects and the band-edge effects, the enhancement and the optimum localization of the DOM can be simultaneously obtained. The general approach is presented. The basic rules and the main steps for the design of such a structure are described. Some numerical results are given and discussed, by using alternating Ga0.7Al0.3As/AlAs layer quarter-wave and mixed quarter-wave/half-wave stacks. The principle of the present work, although described within the limits of linear and dispersionless materials, may be directly applied to nonlinear parametric processes.
We propose, based on our recent investigations of a novel type of optical birefringent filter (called intracavity birefringent filter), a wavelength selective configuration, which may be used for Dense WDM applications. We show that this structure, formed with a classical Lyot-Ohman filter inside a ring cavity, takes the advantages of two typical filters: Fabry-Perot filters and birefringent filters, and in consequence gives a quite good compromise among usual filters parameters such as the working range, the bandwidth and the crosstalk. The theoretical considerations have been summarized. The first experimental results by use of a tunable nematic liquid-crystal light modulator have been shown.
In this paper, we present the feasibility of wavelength filtering by a hybrid liquid crystal Solc/Fabry-Perot filter device. The influence of the structure opto-geometric defects, such as the variation of the thickness, on the spectral response of the filter have been reported. Frequency and temporal characteristics are presented and discussed.
In this paper, we report for the first time the feasibility of coding by coherence modulation in 2D optical correlator using a spatially and temporally incoherent light source. This technique allows to carry out simultaneously several correlation products. The reported results have been obtained with a broadband source, such as a white-light source.
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