Computer-interferometric holograms used for laser beam scanners

We present a new technique to record holograms and its application to holographic laser beam scanners. “Computer-interferometric holograms” (CIH) are recorded interferometrically, one small facet at a time, and under computer control. Constructing scanner holograms by this technique allows for flexibility in the scan path, previously only achieved by computer-generated holograms (CGH). Experiments, demonstrating the useful application of the CIH recording technique to volume holograms, are described in a subsequent paper.     

Computer-generated volume holograms fabricated by femtosecond laser micromachining

We define computer-generated volume holograms (CGVHs) as arbitrary 3D refractive index modulations designed to perform optical functions based on diffraction, scattering, and interference phenomena. CGVHs can differ dramatically from classical volume holograms in terms of coding possibilities, and from thin computer-generated holograms in terms of efficiency and selectivity. We propose an encoding technique for designing such holograms and demonstrate the concept by scanning focused femtosecond laser pulses to produce localized refractive index modifications in glass. These CGVHs show a significant increase in efficiency with thickness. Consequently, they are attractive for photonic integration with free-space and guided-wave devices, as well as for encoding spatial and temporal information.     

Selective properties of volume phase holograms in photorefractive crystals

The paper deals with the selective properties (both angular and spectral) of volume phase holograms in birefringent and cubic optically active photorefractive crystals. It is shown that in birefringent crystals of LiNbO₃ type, the spectral selectivity of the grating depends remarkably on the type of diffraction (isotropic or anisotropic) and the polarization of the reconstructing plane light wave. In cubic optically active crystals the peculiarities of diffraction reveal themselves in a fine structure of differently polarized peaks in the Bragg maximum that can be easily observed, in particular in Bi₁₂SiO₂₀ samples with a thickness d ≈ 8 mm.     

Efficient and flexible laser beam scanners constructed from volume holograms

We combine the scan flexibility of computer-generated holographic laser beam scanners with the high diffraction efficiency of volume phase holograms to produce a new type of “holo-scanner”. The scanner-hologram consists of numerous, small, volume hologram facets recorded in dichromated gelatin films and produces an arbitrary output pattern with greater than 90% light efficiency. Experimental results from discrete point, continuous line, and 3-D scanners are shown. Multi-wavelength scanners are also considered. This paper is based on the concepts presented in the preceding paper on “computer-interferometric holograms used for laser beam scanners”.     

Generation of optical vortex light beams by volume holograms with embedded phase singularity

Special features of the optical-vortex (OV) beams generated by thick holographic elements (HE) with embedded phase singularity are considered theoretically. The volume HE structure is based on the 3D pattern of interference between an OV beam and a standard reference wave with regular wavefront. The incident beam diffraction is described within the framework of a linear single-scattering model in which the volume HE is represented by a set of parallel thin layers with the “fork” holographic structure. An explicit integral expression is derived for the complex amplitude distribution of the diffracted paraxial beam with OV. The numerical analysis demonstrates that the HE thickness may essentially influence not only selectivity and efficiency of the OV beam generation but also the amplitude and phase profile of the diffracted beam as well as regularities of its propagation. We have studied the generated OV morphology and laws of its evolution; in particular, the possibility of obtaining a circularly symmetric OV beam regardless of the diffraction angle is revealed.     

Localized one-dimensional theory for volume holograms

Coupled partial differential equations are derived, consistent with localized use of one-dimensional theory in a thin volume hologram, from modifications to both one-dimensional and two-dimensional theory. The approximations inherent in the model and the range of validity of the solution are discussed.     

A general solution for two-dimensional volume holograms

We inv estigate the problem in which a volume hologram is produced by two waves of arbitrary amplitude and phase distribution (the only restriction being that they must obey the rules of geometrical optics), and the developed hologram is illuminated by one of the waves having the same phase distribution but not necessarily the same amplitude distribution. Coupled partial differential equations are derived and solved in the form of an infinite series valid for both transmission and reflection holograms. For transmission holograms a simple approximate solution is given for the amplitude of the reconstructed wave in the immediate vicinity of the input surface.     

Anisotropic properties of volume holograms in Bi12SiO20 monocrystals

A theoretical and experimental study is made on light diffraction from holographic diffraction gratings, recorded in cubic photorefractive crystals Bi₁₂SiO₂₀ with (110) cut, for two orientation of the grating wave vector,K [001] andK [110], when an external electric field is applied.     

A power conservation theorem for volume holograms

The two-beam coupled wave analysis of volume holograms given by Solymar, which is a two-dimensional generalisation of Kogelnik's formulation, is extended further to provide an alternative treatment of ‘d.c.’ dephasing effects, and the resulting coupled wave equations are shown to contain an implicit power conservation theorem.     

Diffraction efficiency of volume holograms

In this paper we give an exact solution of the coupled differential equation system as developed by Kogelnik [1] for the case of symmetrical recording without neglecting the second differentials. This generalized solution is applicable to dielectric, absorption and so-called mixed volume holograms. It is shown that under special assumptions and approximations we get the same equations for the diffraction efficiency as derived by Kogelnik. In this first part of the paper transmission holograms are only investigated and in the second part we will give the solution for reflection holograms.     

Pre-compensated design formulae for shift-corrected volume reflection holograms

Shifts in the thickness and the refractive-index of holographic recording materials occur naturally, degrading the performance of volume holograms, and reflection-type holograms in particular. In this study, pre-compensated design formulae are derived to correct these effects in fabricating volume reflection holograms. The experimental results are highly consistent with the theory. This method is also applicable to other phase media when the associated film shrinkage and refractive-index shift data are experimentally determined.     

Simulation of diffraction from volume holograms

Coupled wave equations that consider two-dimensional distribution and phases of gratings are derived by using weak grating and slow varying approximation. The result of angular selectivity on rand binary data page shows that each minimum is nonzero and each maximum that is not the middle maximum is a little bit smaller than that on plane wave volume hologram. Angular multiplexing is also modeled using the same method, and the middle page is read out in two different angular spacings. The relationship about PSNR versus angular spacing at different multiplexing pages is analyzed to reflect crosstalk between pages. All simulative results show this method is effective for volume holograms simulation.     

Manipulating the quantum properties of continuous laser beams

The quantum properties of light play an ever increasing role in optics. Thanks to the ability to generate and use special light with modified quantum noise and to the generation of optical entanglement, we can now avoid the conventional limits imposed by the shot noise. In addition, we can create new quantum information protocols. This article concentrates on the case of continuous laser beams, which are used in many precision measurements and applications. The article summarizes some of the concepts, reviews the technical development, provides a simple and reliable model and shows some of the current directions of using optical quantum correlations, such as for the storage of quantum information. PACS 42.50.Dv     

Resisting shrinkage properties of volume holograms recorded in TiO_2 nanoparticle-dispersed acrylamide-based photopolymer

A novel organic–inorganic nanoparticle–photopolymer composite system is developed, and its fundamental holographic recording characteristics are studied. In this hydrophilic TiO2-nanoparticle-dispersed acrylamide photopolymer composite system, the counter-diffusion of monomers and nanoparticles plays a fundamental and key role in hologram grating formation. The experimental results indicate that the volume shrinkage of the nanoparticle–photopolymer film during the holographic recording can be drastically reduced compared with the undoped photopolymer film. It is also found that the diffraction efficiency of the grating recorded in the nanoparticle–photopolymer film depends strongly on the concentration of the TiO2-nanoparticles, and there exists an optimal TiO2-nanoparticle-doping concentration to make the diffraction efficiency and the refractive index modulation reach their maxima. Additionally, the digital data page is stored and reconstructed in the nanoparticle–photopolymer film.     

Recycling of undiffracted laser light for reconstruction of holograms

When a laser source is used to illuminate a transmission hologram, the zero order beam is not utilised and is just wasted. The paper describes a new technique for economical utilisation of laser light by recycling the transmitted light to reconstruct a number of holograms simultaneously for viewing by a large number of observers. There is no danger to viewer who is exposed to the undiffracted laser beams transmitted through the hologram. The paper also discusses the diffraction efficiency requirement of successive hologram and the limit of number of holograms in the scheme.     

Correlation properties of random electromagnetic beams in laser resonators

With the help of the generalized Huygens–Fresnel principle and the tensor approach for electromagnetic stochastic beams we investigate the behavior of their two-point spatial correlation properties on interaction with a laser resonator. In particular, the evolution of the degree of coherence, of the degree of cross-polarization and of the intensity correlations (at one and two points in space) is analyzed for typical beams and cavities. The theory is illustrated by numerical curves pertaining to various correlation properties of typical electromagnetic Gaussian Schell-model beams.     

非傍轴径向阵列高斯激光束的传输特性

推导出相干和非相干非傍轴径向阵列高斯激光束的传输公式,并用以研究了径向阵列光束在自由空间的传输特性,重点分析了阵列光束参量对合成光束光强分布的影响。结果表明：阵列半径与束腰宽度之比是一个关键参量,当它固定时,相干和非相干合成光束的光强分布都保持不变,但合成光束的光强分布和远场轴上光强最大值都与合成方式有关。     

非傍轴径向阵列高斯激光束的传输特性

推导出相干和非相干非傍轴径向阵列高斯激光束的传输公式,并用以研究了径向阵列光束在自由空间的传输特性,重点分析了阵列光束参量对合成光束光强分布的影响。结果表明:阵列半径与束腰宽度之比是一个关键参量,当它固定时,相干和非相干合成光束的光强分布都保持不变,但合成光束的光强分布和远场轴上光强最大值都与合成方式有关。     

Wave-front correction of high-intensity fs laser beams by using closed-loop adaptive optics system

We developed an adaptive optics system to correct the wave-front distortion of an intense fs laser beam from our multi-TW laser system, Jiguang II. In this paper, the instruments of the adaptive optical system are described and the experimental results of the closed-loop wave-front correction are presented. A distorted laser wave-front of 20 wavelengths of P-V values was corrected to 0.15 wavelength of P-V values. The beam quality of the laser system varies from 3.5 diffraction limit to 1.5 diffraction limit.