Range-finding method using diffraction gratings

A model in geometric optics, along with some preliminary experimental results for a new range-finding method that exploits near-field diffraction phenomena found with plane gratings, is presented. Among the characteristics investigated is a magnification effect applicable to three-dimensional microscopy. A variety of embodiments of the method is disclosed, including an off-axis illumination model and a method of near-field focus compensation that takes advantage of the Scheimpflug condition.     

Polarisation response of mono-order concave diffraction gratings based on an elliptical Bragg reflector

The polarisation dependence of integrated concave diffraction gratings with extended metallic or dielectric elliptical Bragg reflectors was investigated. Equal grating efficiency of 99% for both polarisations was achieved for the dielectric configuration using finite-difference time-domain (FDTD) modelling, with a polarisation-dependent loss (PDL) of 0.07 dB.     

Simulation of mechanically ruled concave diffraction gratings by use of an original geometric theory

A geometric theory is developed for a spherical concave diffraction grating that has variable line spacing and curved grooves and can be produced with existing technology. The aberration coefficients of this grating are determined. A comparison of these coefficients with the corresponding coefficients of holographic gratings and with CODE V polynomial coefficients gives a clearer understanding of the similarity and difference between mechanically ruled and holographically recorded concave gratings and allows the optimization of these gratings with standard computer programs.     

Optical demultiplexer using a silicon concave diffraction grating

A demultiplexer composed of a concave diffraction grating and a multimode slab waveguide is attractive since it has many advantageous features. However, this type of demultiplexer has had high demultiplexing losses until now, because the concave diffraction gratings used had poor diffraction efficiency. A silicon concave diffraction grating has been developed to overcome this problem, manufactured by cylindrically bending a thin silicon plane diffraction grating. The diffraction efficiency of this grating was 82% at a blaze wavelength. The five-channel demultiplexer was assembled with this grating as well as with a multimode slab waveguide and a fiber array. Its branching loss was in the 1.4-1.8-dB range.     

Angular momentum flux of counterpropagating paraxial beams

We study the angular momentum (AM) of the arbitrary superposition of counterpropagating paraxial beams that have the same magnitude of the wavenumber. We derive compact analytical expressions for the total AM in a transverse cross section (linear AM density) and the total AM flux through the cross section. We demonstrate that whereas for the time-averaged linear AM density its separation into the spin and orbital parts is not, generally, observed, the total time-averaged AM flux is separated into well-identifiable spin and orbital constituents. Moreover, we show that such a flux is also naturally separated into the fluxes of forward- and backward-propagating beams.     

Recording performance of holographic diffraction gratings in dry films containing hyperbranched polyisophthalesters as polymeric binders

By use of a photopolymerization-diffusion model, the diffraction efficiency of photopolymerizable recording dry films prepared from hyperbranched polyisophthalesters as polymeric binders was investigated. The recording characteristics of these films, i.e., spatial frequency, polymeric binder structure, exposure intensity, and modulation depth, are discussed in detail. For a given total exposure dose the diffraction efficiency first increases and then decreases with increasing exposure intensity, and this effect becomes more remarkable as the unsaturated concentration of polymeric binder increases. An optimum total exposure dose of 36 mJ cm(-2) and an exposure intensity of 0.4 mW cm(-2) were determined. A modulation depth of 1 was found to produce the highest diffraction efficiency. Longer-lasting gratings could be obtained by use of polymeric binders with higher cross-linking densities.     

Stationary anastigmatic mounts of concave gratings

The general equations for parameters of concave grating mounts that provide stationary and superstationary astigmatism at the wavelength of correction are derived for the first time, to the best of our knowledge. These can be used to design grating multi/demultiplexers for wavelength-division multiplexed optical communication systems and high-resolution, narrow-band spectrographs. Important special cases of stationary anastigmatic mounts and their performance are presented.     

Recording of holographic diffraction gratings in photopolymers: Theoretical modelling and real-time monitoring of grating growth

Abstract

Recording of holographic volume diffraction gratings in Du Pont's photopolymer HRS-150 is studied theoretically and experimentally. Particular attention is paid to the dynamics of the recording process. The temporal evolution of holographic gratings is monitored for a range of illumination intensities in two ways: (i) by means of a multiple-exposure approach when intensity profiles of recording beams are uniform, many holographic exposures are necessary and each exposure is carried out for a particular value of the total illumination intensity; (ii) by means of a single-exposure approach when intensity profiles of recording beams are strongly Gaussian and a volume grating with spatially distributed diffraction efficiency arises, depending on a local (average) value of the total illumination intensity. The second approach proves to be a useful tool providing us very quickly with qualitative information about the dynamics of the recording process while the first one, which is much more time-consuming, is more reliable for quantitative evaluations. The grating evolution is analysed theoretically by means of the well known photopolymerization-diffusion model. Discrepancies are found between the model and experimental results for low illumination intensities while qualitative agreement is found for higher intensities. For a given grating period, the discrepancies are interpreted in terms of insufficient change of the refractive index due to photopolymerization for low illumination intensities.     

Pair production in counterpropagating electron and laser beams

An analysis is made of the collision between a high-energy electron beam and laser radiation. It is shown that when the electron energy is 800 GeV and the laser pulse has an intensity of 10²⁰ W/cm², a wavelength of 248 nm, and a duration of 300 fs, a single seed electron produces 60 electrons and positrons. Pis’ma Zh. Tekh. Fiz. 23, 72–75 (July 26, 1997)     

Rigorous theory of the diffraction of Gaussian beams by finite gratings: TE polarization

A rigorous modal theory for the diffraction of Gaussian beams from N equally spaced slits (finite grating) in a planar perfectly conducting thin screen is presented. The case of normal incidence and TE polarization state is considered; i.e., the electric field is parallel to the slits. The characteristics of the far-field diffraction patterns, the transmission coefficient, and the normally diffracted energy as a function of several optogeometrical parameters are analyzed within the so-called vectorial region, where the polarization effects are important. The diffraction pattern of an aperiodic grating is also considered. In addition, one diffraction property known to be valid in the scalar region is generalized to the vectorial region: the existence of constant-intensity angles in the far field when the incident beam wave is scanned along the N slits. The classical grating equation is tested for incident Gaussian beams under several conditions.     

Rigorous theory of the diffraction of Gaussian beams by finite gratings: TM polarization

Diffraction of TM-polarized Gaussian beams by N equally spaced slits (finite grating) in a planar perfectly conducting thick screen is treated. We extend to the TM polarization case the results of a previous paper where the TE polarization was considered. The far-field diffraction patterns, the transmission coefficient tau, and the normally diffracted energy E as a function of several optogeometrical parameters are analyzed within the so-called vectorial region. The existence of constant-intensity angles in the far field when the incident beam wave is scanned along the N slits is shown. In addition, the property E=Ntau/lambda, valid in the scalar region, is extended to the TM polarization case in the vectorial region, lambda being the wavelength. The coupling between slits is analyzed, giving an oscillating amplitude-decreasing function as the separation between slits increases, where the period for these oscillations is the wavelength lambda. Finally, the extraordinary optical transmission phenomena that appear when the wavelength is larger than the slit width (subwavelength regime) are analyzed.     

Optimized superstationary anastigmatic mounts of concave gratings

Two subfamilies of concave grating superstationary anastigmatic mounts that provide minimum chromatic aberrations are described. The obtained approximate formulas can be used to design flat-field spectrographs and multi/demultiplexers for optical communication networks. Two specific mounts and their performance are presented.     

Analysis of diffraction gratings by using an edge element method

Typically the grating problem is formulated for TE and TM polarizations by using, respectively, the electric and magnetic fields aligned with the grating wall and perpendicular to the plane of incidence, and this leads to a one-field-component problem. For some grating profiles such as metallic gratings with a triangular profile, the prediction of TM polarization by using a standard finite-element method experiences a slower convergence rate, and this reduces the accuracy of the computed results and also introduces a numerical polarization effect. This discrepancy cannot be seen as a simple numerical issue, since it has been observed for different types of numerical methods based on the classical formulation. Hence an alternative formulation is proposed, where the grating problem is modeled by taking the electric field as unknown for TM polarization. The application of this idea to both TE and TM polarizations leads to a two-field-component problem. The purpose of the paper is to propose an edge finite-element method to solve this wave problem. A comparison of the results of the proposed formulation and the classical formulation shows improvement and robustness in the new approach.     

Role of evanescent waves in power calculations for counterpropagating beams

A general expression is obtained for the time-average power passing through a plane transverse to the direction of propagation for two counterpropagating electromagnetic beams. Each beam is represented by its plane-wave spectrum, which contains both propagating and evanescent plane waves. The expression clearly shows that, under certain conditions, the evanescent plane waves contribute to the time-average power passing through the plane. This is in contrast to the case of a single electromagnetic beam, in which only the propagating plane waves contribute to the time-average power passing through the plane. The utility of the expression is demonstrated with a practical example: a line current placed over a dielectric slab. Here the counterpropagating beams are the incident and reflected fields in the region between the current and the slab. The expression is applied to a plane in this region, and it is used to determine the time-average power associated with the evanescent waves passing through this plane. This power is then shown to be equal to the time-average power carried by the guided modes of the slab.     

Scattered-light properties of diffraction gratings

One of the many calibrations performed for a scientific-quality spectrometer is the characterization of its scattered-light properties. The scattered light can arise from any optical surface, and light leaks or scattering from baffles can also contribute to the instrumental stray-light level. For a diffraction-grating spectrometer the primary contribution to instrumental scattered light has been found to be the scattered light from the grating. The results from measuring the scattered-light properties of 10 diffraction gratings are discussed along with the application of these results in analyzing the total scattered light measured for three spectrometers. It has been found from these measurements that there are two components of the grating scattered light: a Lorentzian-type component and a constant background component. The Lorentzian component is predicted from the diffraction theory for a grating, and the constant background component is consistent with Rayleigh scattering from the microscopic surface im erfections. It was also discovered that multiple replicas of gratings from the same master grating exhibit significantly more scattered light than the preceding replica by factors of 1.1-2.     

Ultralow-voltage generator of chaotic microwave oscillations on counterpropagating electron beams

Technical Physics Letters - The new scheme of an ultralow-voltage generator of broadband chaotic microwave oscillations is considered, which operates on counterpropagating turbulent electron beams....