Restoration of the intensity profile of a focused laser spot by means of combining optical correlation with a numerical method

A method is presented by which intensity profile of a focused light spot is reconstructed from the correlation pattern between itself and a pinhole, which is obtained by moving the focused spot with the aid of an acousto-optic deflector (AOD). The pattern was formulated into a matrix equation by assuming that it was circular symmetric and was approximated by a number of linear functions. The results were numerically given by solving the matrix equation with a modified least-squares method and were assessed for the degree of focus, influence of signal fluctuations, and degradation of the spot resulting from heat generation within the AOD.     

Excitation with a focused, pulsed optical beam in scattering media: diffraction effects

To gain a better understanding of the spatiotemporal problems that are encountered in two-photon excitation fluorescence imaging through highly scattering media, we investigate how diffraction affects the three-dimensional intensity distribution of a focused, pulsed optical beam propagating inside a scattering medium. In practice, the full potential of the two-photon excitation fluorescence imaging is unrealized at long scattering depths, owing to the unwanted temporal and spatial broadening of the femtosecond excitation light pulse that reduces the energy density at the geometric focus while it increases the excitation energy density in the out-of-focus regions. To analyze the excitation intensity distribution, we modify the Monte Carlo-based photon-transport model to a semi-quantum-mechanical representation that combines the wave properties of light with the particle behavior of the propagating photons. In our model the propagating photon is represented by a plane wave with its propagation direction in the scattering medium determined by the Monte Carlo technique. The intensity distribution in the focal region is given by the square of the linear superposition of the various plane waves that arrive at different incident angles and optical path lengths. In the absence of scattering, the propagation model yields the intensity distribution that is predicted by the Huygens-Fresnel principle. We quantify the decrease of the energy density delivered at the geometric focus as a function of the optical depth to the mean-free-path ratio that yields the average number of scattering events that a photon encounters as it propagates toward the focus. Both isotropic and anisotropic scattering media are considered. Three values for the numerical aperture (NA) of the focusing lens are considered: NA = 0.25, 0.5, 0.75.     

Physical properties of wave scattering by a chiral grating

Wave scattering by a chiral grating is studied in this paper. Numerical results are given, and physical properties are discussed, including the influence of frequency, angle of incidence, and aspect ratio. At high frequencies we find anomalous coupling regions known as Wood's anomalies, which are explained by the excitation and reradiation of leaky waveguide modes in the periodic layer. The chiral grating can possess both frequency-selection and mode-conversion properties.     

Investigation of grating recording in betacarotene-polystyrene film

Abstract

An experimental study of grating recording in polymeric beta-carotene film is reported. Betacarotene was dissolved in a polystyrene host matrix and irradiated with a cw Ar laser. The formation of a holographic grating is demonstrated and the results of the grating stability measurements are presented.     

Nonlinear signal-processing model for scalar diffraction in optical recording

A nonlinear signal-processing model is derived for the optical recording channel based on scalar diffraction theory. In this model, the signal waveform is written in closed form as an explicit function of the channel bits that are stored on an optical disk, thereby comprising both linear and nonlinear terms. Its explicit dependence on the channel bits makes this model well suited for signal-processing purposes. With the model it is also convenient to assess the importance of nonlinear contributions to the signal waveform. The model is applied for one-dimensional optical storage as well as for two-dimensional (2D) optical storage in which bits are arranged on a 2D hexagonal lattice. Signal folding is addressed as a typical nonlinear issue in 2D optical storage and can be eliminated by recording of pit marks of sizes considerably smaller than the size of the hexagonal bit cell. Further simplifications of the model with only a limited number of channel parameters are also derived.     

Material classification of nanoparticles by focused beam scattering

Advanced science and technology frequently encounters the need to detect particles in the micrometer and nanometer range of a given composition. While the scattering process of light by small particles is well documented, most conventional analytic methods employ wide illumination of large ensembles of particles. With such an approach, no information can be obtained about single particles due to their weak interaction. In this paper, we show that single particles can be classified with respect to their material composition by analyzing the scattering pattern of a focused Gaussian beam.     

Improvement in the measurement of index modulation along an optical fiber grating by movement of the probe spot perpendicularly to the fiber axis

We demonstrate a large improvement in the efficiency of the method proposed by Krug et al. [Opt. Lett. 20, 1767 (1995)] to measure the amplitude of the refractive-index modulation along a fiber Bragg grating. The basic idea consists of using what to our knowledge is a new modulation scheme for the probe beam that not only allows the user to get a better discrimination of the probe light incoherently scattered by the fiber from that scattered by the grating but also facilitates alignment of the setup.     

Amplitude checker grating from one-dimensional Ronchi grating and its application to array generation

The Ronchi grating is well known for its many applications in areas such as spectroscopy, grating interferometry, and Talbot interferometry. On the other hand, the checker grating has attracted very little attention. A checker grating also self-images at equidistant planes; the separation between these planes is a quarter of the Talbot distance of a Ronchi grating of the same period. To understand this and several other features, a transition from Ronchi grating (a one-dimensional grating) to checker grating (a two-dimensional grating) has been both theoretically and experimentally studied and results are presented. Because the checker grating self-images closer to the grating and its transmittance is higher than that of a Ronchi grating, the use of its self-image planes for array generation is also emphasized.     

Rapid optical coherence tomography and recording functional scattering changes from activated frog retina

Optical coherence tomography (OCT) has important potential advantages for fast functional neuroimaging. However, dynamic neuroimaging poses demanding requirements for fast and stable acquisition of optical scans. Optical phase modulators based on the electro-optic effect allow rapid phase modulation; however, applications to low-coherence tomography are limited by the optical dispersion of a broadband light source by the electro-optic crystal. We show that the optical dispersion can be theoretically estimated and experimentally compensated. With an electro-optic phase modulator-based, no-moving-parts OCT system, near-infrared scattering changes associated with neural activation were recorded from isolated frog retinas activated by visible light.     

Piecewise polynomial dielectric function model and its application for the retrieval of optical functions

A new expression of dielectric function model based on piecewise polynomials is introduced. Its association with spline and more recent shape preserving interpolation algorithms allows easy reproduction of every kind of experimental spectra and thus retrieval of all the linear optical functions of a material. Based on a pure mathematical framework, the expression of the model is always applicable and does not necessitate any knowledge of the microscopic mechanisms of absorption responsible for the optical response. The potential of piecewise polynomial dielectric functions is shown through synthetic examples and the analysis of experimental spectra.     

Four-flux model for a multilayer, plane absorbing and scattering medium: application to the optical degradation of white paint in a space environment

We generalize the four-flux radiative transfer model to the case of a multilayer medium. A concrete application, that of the study of the optical degradation of white paint in a simulated space environment, is presented. The degraded material is decomposed in a damaged layer and in an unaffected layer, and we assume that the degradation is due to a variation Dkappa of the imaginary part of the refractive index in the damaged layer. Then we find an empirical law for variation Dkappa with dose, taking into account possible saturation.     

Studying the temperature field in the recording and reproduction of information by means of focused radiation

The functions of an instantaneous spot source of heat acting on the boundary of layer separation have been constructed for a two-layer plate. The temperature field generated by a moving normally distributed source of radiation is studied in the recording and reproduction of information.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 57, No. 6, pp. 983–990, December, 1989     

Pitch-modulated phase grating and its application to displacement encoder

Abstract

A binary phase grating with modulated pitch is investigated for a simple displacement encoder. The grating consists of a binary phase grating to eliminate the zeroth-order diffraction, and the pitch of the grating is modulated to compensate the higher harmonics of the encoder displacement signal. Therefore, an undistorted sinusoidal signal as a function of displacement is obtained by simply superimposing a conventional binary grating on the pitch-modulated phase gratings for any air gap between the gratings. The characteristics of the proposed gratings and the encoder signal are investigated by the Fresnel diffraction theory. The proposed grating has been fabricated lithographically, and the signal was examined experimentally. Considering these results, the proposed technique can suppress interpolation error and will be useful for an encoder in precision machining.     

Comment on "Excitation with a focused,pulsed optical beam in scattering media: diffraction effects"

To incorporate the wave properties of light, it was recently proposed to modify the Monte Carlo-based photon transport model to a semi-quantum-mechanical representation by considering each ray as a photon wave packet [Appl. Opt. 39, 5244 (2000)]. It is not clear from the paper whether each photon wave packet is considered representative of an entire plane wave or if each is spatially localized. However, for each interpretation we identify problems with the approach suggested for combining the wave-packet contributions. These include violation of the principle of conservation of energy and the use of a scattering phase function that is incompatible with the suggested way of calculating the intensity values. These issues render the approach impractical.     

Tunable dual-wavelength optical short-pulse generation by use of a fiber Bragg grating and a tunable optical filter in a self-seeding scheme

A simple self-seeding scheme is developed to generate tunable dual-wavelength optical short pulses in a flexible manner and with an increased wavelength-tuning range. The wavelength selection and tuning are achieved by simultaneous use of a fiber Bragg grating and a tunable optical filter. The side-mode suppression ratio of the output pulses is better than 30 dB over a wavelength-tuning range of 33.8 nm. The system is compact and convenient for dual-wavelength tuning.     

Debye series analysis of scattering of a plane wave by a spherical Bragg grating

The Debye series decomposition of the partial-wave scattering amplitudes of a multilayer sphere is derived. The partial-wave transmission and reflection terms appearing in the Debye series are multiple-scattering amplitudes written in terms of four basic quantities and combined together layer by layer in an identical way. The resulting expressions are then used to calculate the scattered intensity of a spherical Bragg grating covering a dielectric core particle and to analyze a number of new structures appearing in the scattered intensity.     

Levelling the focal spot intensity of the focused gaussian beam

Abstract

It is experimentally and numerically shown that a simple binaryphase optical element can be used for levelling the light energy in the focal plane of the focused Gaussian beam, generating a square-shaped focal beam, and transforming the Gaussian beam into a uniform beam which preserves its radius at a definite length of its path.