Transmitting volume Bragg grating with longitudinal moiré apodization

Spectral beam combining of fiber lasers based on a transmitting volume Bragg grating

In order to design an angular magnifier based on multiplexed volume holographic grating, the physical model of multiplexed angular magnifier (MAM) is established. The design principle is summarized from two aspects, namely, uniform angular distribution and optimum diffraction efficiency. The effect of production error on MAM is studied. The effect of beam divergence on the performance of MAM is investigated. The results show that the desired angular distribution of MAM is achieved by controlling the spatial frequency and the tilted angle of the grating while the optimum diffraction efficiency is achieved by controlling the thickness and the amplitude of refractive index modulation of the gratings. The 10 VHGs can be multiplexed in the MAM at most. The raising of the grating tilted angle or the ratio between the wavelength of recording beam and working beam can weaken the effect of angular error of reference beam on the output angle distribution of MAM, and the reducing of the thickness is beneficial for reducing the effect of error of thickness and refractive index modulation on diffraction efficiency. When the beam divergence is greater than the angular half width, the optimum diffraction efficiency falls down to 50% or lower and the local minimum values disappear. The raising of the spatial frequency or thickness of the gratings can reduce the desired amplitude of refractive index modulation and multiplex more VHGs, but is not beneficial for achieving uniform diffraction efficiency distribution and weakening the effect of divergent beam.

Get PDF Email Share Share with Facebook Tweet This Post on reddit Share with LinkedIn Add to CiteULike Add to Mendeley Add to BibSonomy Get Citation Copy Citation Text D. Ramecourt, P. Bernage, P. Niay, M. Douay, and I. Riant, "New probe beam modulation scheme for measuring the refractive index modulation along an optical fiber Bragg grating," in Bragg Gratings, Photosensitivity, and Poling in Glass Waveguides, 2001 OSA Technical Digest Series (Optica Publishing Group, 2001), paper BThC8. Export Citation BibTex Endnote (RIS) HTML Plain Text Citation alert Save article

Diffraction contrast profiles have been calculated using the two beam approximation for a thin crystal foil which contains a spherical void. The influence of the foil thickness, foil orientation, void radius, and void depth on the total contrast, a combination of both the strain contrast due to distortion of the foil by the void and the thickness contrast due to the absence of scattering material within the void, was examined, the lengths being expressed in units of ξg* =ξg(1+w2)−1/2 where w=Sgξg with Sg the deviation from the exact Bragg condition and ξg the extinction distance. Maximum strain contrast occurs for the profiles of both beams at depths d*= (N/2)ξg* (N an integer); the black and white sides of these profiles alternate as the depth changes by 1/2ξg*. The amplitude of the strain contrast is maximum at thicknesses t*=Nξg* and (N+1/2)ξg* for the scattered beam and the transmitted beam respectively. In either case, the inbetween thicknesses give a smaller strain contrast for which the white sides of the profiles are suppressed for voids within an extended region of the foil. This region of small amplitude is centered on the midplane of the foil for w=0, but shifts for only the scattered beam toward the entrance (exit) surface as w increases positively (negatively); a much larger amplitude can then occur near the opposite surface. Although the strain contrast for either beam is largest for void radius R0*=1/4ξg*, the thickness contrast at w=0 dominates the image of the void at this and larger radii. For R0*≲1/16ξg*, the strain contrast, when maximum, becomes the dominant image forming mechanism. As the foil is rotated away from the exact Bragg condition, w≠0, the thickness contrast can firstly become quite small compared to the strain contrast even at R0*=1/4ξg*, but upon further rotation it predominates once again as in the case w=0 while showing an image of opposite character to the former case.

ABSTRACTThe incorporation of triple crystal x-ray diffraction (TCD) under conditions of a nearly grazing incident x-ray beam provides a unique capability to characterize surface and subsurface structural damage. The effects of bromine-methanol chemical-mechanical (CM) polishing on the surface quality of GaAs were investigated by inclined Bragg plane triple crystal x-ray diffraction. GaAs samples polished under varying conditions of Br2 concentration, total polish time, polish wheel rotation speed, and force exerted on the samples were analyzed using TCD profiles from inclined (220) planes, with the samples oriented at both the exact Bragg condition, θB, and at deviations from the exact Bragg condition. As the deviation from the 220 reciprocal lattice point was increased, multiple peaks corresponding to both the dynamic and the kinematic components were observed. The peak positions and the relative intensities of the various peaks were used to characterize the effects of the CM polishing variables on the GaAs surfaces.

Investigation of dielectric properties and diffraction efficiency enhancements caused by photothermal effect in DR9 dye-doped nematic liquid crystal

In the last few years, the interest in storing volume holograms in photopolymers has increased enormously due to their applications in industry, the medical field, security, or renewal energy among others. The production of environmentally compatible photopolymers is one of the main focuses of Holography research. In this work, we have studied how to increase the diffraction efficiency of reflection holograms stored in a low-toxicity PVA-based photopolymer called Biophotopol. The holographic material has been doped with different types of nanoparticles (NPs) to achieve an increase in the refractive index modulation during the recording stage. Metallic NPs, obtained by physical and electrochemical methods have been used. The results obtained with all of them have been compared as a function of the concentration used, the size of the NPs, and the stabilization method used for their synthesis. A considerable increase in diffraction efficiency has been achieved by using NPs in the low-toxicity material. By using high refractive index NPs, the average refractive index of the holographic material increases and consequently the diffraction efficiency.

This paper explores the effects of humidity on gratings recorded in a polyvinylalcohol–acrylamide photopolymer medium. Investigation of the behaviour of transmission gratings exposed to high humidity is of significant interest for two reasons, firstly because the grating’s sensitivity to humidity can be exploited for the development of irreversible humidity indicators, secondly because too much sensitivity to humidity can limit the use of these materials in applications where an environmentally stable hologram is needed. In this paper we focus on the effect of high humidity on the properties of volume phase transmission gratings recorded in PVA/AA photopolymer layers in the temperature range 8–24 ° C. It has been found that although exposure to humidity changes the diffraction efficiency and Bragg angle of gratings, the effects are fully reversible if the temperatures are kept low. For example, when gratings were subjected to relative humidities of 80% and 90% at a temperature of 8 ° C the observed changes were fully reversible. However, irreversible changes in diffraction efficiency, thickness, refractive index modulation and Bragg angle were observed when the temperature during the humidity exposure was higher than 16 ° C. The magnitude of the irreversible changes depends strongly on the ambient temperature during the humidity exposure, the humidity level and also on the duration of the humidity exposure.

Dynamics of plasma formation and permanent structural transformation in ZBLAN excited by tightly focused femtosecond laser pulses

In the last three decades several kinds of organic mixtures for holographic recording were developed in order to achieve a new class of DVD-like optical memories for high-density optical data storage. The holographic materials should satisfy the following requirements: high sensitivity to blue light, low losses, high spatial resolution and long term stability. To this aim we developed new organic photosensitive mixtures based on only three components. We recorded high spatial frequency reflection gratings up to 7400 lines/mm with blue laser light (405 nm) by using a conventional holographic setup. We obtained a macro grating diffraction efficiency up to 67%, refractive index modulation over 0.01, optical shrinkage < 2 % and overall losses ~5%. In order to characterize data-storage materials independently on the experimental conditions, the sensitivity has been evaluated through the S parameter which takes into account the diffraction efficiency, recording light intensity, exposure time and sample thickness. The amazing obtained values of <i>S</i> >10⁵ cm/J evidences a very fast recording process with a very low writing intensity (less than 20 mW/cm²) corresponding to a recording energy density of few mJ/cm². The performance of these materials have been also tested in the microholographic geometry.

We investigated the photorefractivity enhancement of polymeric composites by introducing gold nanoparticles (NPs). The gold NPs enhance the photocharge generation rate of sensitizers through plasmon resonance coupling achieved between NPs and sensitizers. Systematic studies show that the presence of gold NPs has increased photocharge generation efficiency, photoconductivity, diffraction efficiency, refractive index modulation, and photorefractive (PR) grating formation rate. The gold-NP-doped PR sample exhibits 2 times larger photocharge generation efficiency and photoconductivity, and 2.5 times faster PR grating formation rate compared to the control sample without the NPs.

A new material based on 1,2-naphthoquinone in polymethylmethacrylate is proposed for phase holograms showing diffusion enhancement. The spectral sensitivity of this material allows forming holograms using 488–530 nm wavelengths, while the hologram reconstruction can be implemented at 633 nm. The amplitude of the refractive index modulation achieved makes it possible to reach diffraction efficiencies close to 100% at a thickness of several hundreds of µm. It was found by means of UV–visible and NMR spectroscopy that the diffusion of 1,2-naphthoquinone caused by photoproduct attachment to macromolecules of a polymer matrix is the main reason for the hologram enhancement. A relatively high diffusion coefficient (4.8 × 10−17 m2 s−1 at 50 °C) was measured, leading to fast enhancement at moderate temperature.

Features of recording holograms in polymethylmethacrylate doped by boron difluoride anthraceneacetonates

The dynamic manipulation of light can be achieved b y the interaction of a signal pulse propagating thr ough or reflected from a refractive index front. Both the frequency a nd the wave vector of the signal are changed in thi s case, which is generally referred to as an indirect transition. We have developed a theory to describe such transitio ns in integrated photonic crystal waveguides. Through indirect trans itions, the following effects can be envisaged: lar ge frequency shifts and light stopping and order of magnitude pulse compression and broadening without center frequency sh ift. All effects can be potentially realized with a refractive index modulation as small as 0.001. For the experimental realization, we have used slow light photonic crystal waveguides in silicon. The refractive index front was obtained by free carriers generation with a switching pulse co-propagating with the signal i n the same slow light waveguide. The group velocities of the signal and the front could be varied arbitrarily by choos ing the right frequencies of the signal and switching pulses. The indirect transition was unambiguously demonstrated by considering two situations: a) the front overtaking the signal and b) the signal overtaking the front. In both cas es, a blue shift of the signal frequency was observed. This blue shift can only be explained by the occurrence of the expected indirect transition and not by a direct transition without w ave vector variation.

The dynamical theory for perfect crystals in the Laue case was reformulated using the Riemann surface, as used in complex analysis. In the two-beam approximation, each branch of the dispersion surface is specified by one sheet of the Riemann surface. The characteristic features of the dispersion surface are analytically revealed using four parameters, which are the real and imaginary parts of two quantities specifying the degree of departure from the exact Bragg condition and the reflection strength. By representing these parameters on complex planes, these characteristics can be graphically depicted on the Riemann surface. In the conventional case, the absorption is small and the real part of the reflection strength is large, so the formulation is the same as the traditional analysis. However, when the real part of the reflection strength is small or zero, the two branches of the dispersion surface cross, and the dispersion relationship becomes similar to that of the Bragg case. This is because the geometrical relationships among the parameters are similar in both cases. The present analytical method is generally applicable, irrespective of the magnitudes of the parameters. Furthermore, the present method analytically revealed many characteristic features of the dispersion surface and will be quite instructive for further numerical calculations of rocking curves.