A double acousto-optic monochromator for the uv region with improved optical characteristics based on a KDP single crystal

A double monochromator is developed and manufactured on the basis of an acousto-optic filter made of a potassium dihydrogen phosphate (KDP) single crystal. The device has an operating range of 260–540 nm and improved optical characteristics: high aperture ratio (an angular aperture of 2° and a linear aperture of 6.5x15 mm), high polarization contrast (～10⁸), and low level of transmission sidelobes (～10³). The spectral resolution of the monochromator at the edges of the operating range is 0.5 and 3 nm, respectively. The control frequency band is 60–150 MHz. Experimentally, it is shown that, for sound beams with equivalent geometric parameters, the diffraction efficiency provided by the KDP-based acousto-optic filter is an order of magnitude higher than that obtained with a noncollinear acousto-optic filter fabricated on the basis of crystalline quartz.     

Polarizer-Free Acousto-Optic Monochromators

Monochromators based on a noncollinear acousto-optic filter without external polarizers have been studied. As an element to select an operating diffracted light beam, (1) a spatial filter-telescope and (2) an output plane of a filter crystal rotated in a diffraction plane have been used. In the latter case, dispersion of the acousto-optic filter crystal is compensated using a correcting prism made from the same crystal and placed behind an output filter plane along a path of a filtered light beam. As a result, the transmission coefficient of the monochromator is increased upon an effective compensation of the angular drift of the filtered light beam, the monochromator design is simplified, and its sizes are decreased.

     

Controlling the interaction between light and gold nanoparticles: selective suppression of extinction

The interaction of visible light with the particle-plasmon resonance of metallic nanoparticles can be controlled by geometrical arrangement of nanoparticle arrays. These arrays are placed on a substrate that supports guided modes in the wavelength range of the particle plasmon. Coupling of this particle-plasmon resonance to the directly incident light and to the waveguide modes results in almost complete suppression of light extinction within narrow spectral bands due to destructive interference. Variation of the structure parameters allows continuous tuning of these high-transmission bands across the particle-plasmon resonance.     

Conditions for obtaining backward collinear isotropic acousto-optic interaction in germanium and iodic acid crystals

The possibility of experimentally obtaining backward collinear isotropic acousto-optic interaction based on bulk ultrasonic waves and terahertz electromagnetic waves is considered. The magnitudes of the acousto-optic figure of merit and the corresponding acoustic frequencies are calculated for the basic crystallographic axes in germanium and iodic acid crystals. The acousto-optic parameters of both crystals are compared.     

Acousto-Optic Solitons in Fibers

It is shown that the deceleration of light pulses down to the velocity of a sound value can be realized in a case of unidirectional parametric interaction of two electromagnetic waves with an acoustic one in the regime of forming three wave acousto-optic solitons. This nonlinear acousto-optic interaction can be realized in long distance systems like fibers. As the result of such an interaction, two types of acousto-optic envelope solitons can propagate in fibers. Modulation of the amplitude of the electromagnetic pump wave can control the soliton velocity.     

便携式近红外光谱仪器现状及展望

近红外光谱分析技术具有速度快、操作简单等优点,在农业、制药等行业得到了大量应用,其中一些应用需要将仪器携带到分散的分析现场使用,为满足这一需求,很多体积小、便于移动的便携式近红外光谱仪被研制出来。这些仪器的种类较为繁多,采用了很多不同的基本原理,使用了不同的光路结构。文章综述了国内外便携式近红外光谱仪的技术现状,根据光路结构的不同将仪器分成滤光片型、光栅型、傅里叶变换型、声光可调滤波器型以及使用微机电系统(MEMS)的新型光谱仪等类型,重点介绍各类仪器的原理和主要部分的结构,同时简要介绍不同类型仪器的特点,并列举典型产品。也对测量附件、操作、显示等外围部分的设计作简要介绍,这些附件针对便携应用采取了特殊的设计。通过介绍,为新型便携式近红外光谱仪的研制提供借鉴。最后,对便携式近红外光谱仪器的现状作总结,并对未来国内外技术的发展进行展望。     

Diffraction of IR radiation by ultrasound in tellurium single crystals

The optic, acoustic, and acousto-optic properties of tellurium single crystals are considered to analyze the possibilities of applying this material in acousto-optic filters to control mid- and far-IR radiation. The results of measuring the optic and acousto-optic parameters of a tellurium crystal are reported. The isotropic and anisotropic light diffraction by ultrasound in the crystal is investigated. The wide-aperture geometry of interaction in tellurium is implemented and an acousto-optic figure of merit M ₂ of a tellurium crystal exceeding 10 × 10^?15 s³ g^?1 is experimentally obtained.     

The nonreciprocal effect under low- and high-frequency collinear acousto-optic interactions

The nonreciprocal effect under collinear acousto-optic interaction in the low- and high-frequency regimes is studied theoretically. The magnitudes of nonreciprocity determined from the ultrasonic frequency and from the wavelength of light are shown to be quantitatively identical. An expression that governs the magnitude of the nonreciprocity and that is valid for both low- and high-frequency regimes of the collinear acousto-optic interaction is obtained. The shape and width of the frequency characteristic of the collinear acousto-optic interaction calculated in the low diffraction efficiency approximation are shown to be the same in the low- and high-frequency regimes. The dependence of the frequency bandwidth of the collinear acousto-optic interaction on the ultrasonic-wave attenuation and diffraction efficiency is obtained. The magnitude of the nonreciprocal effect in some of the crystals used in acousto-optics is estimated numerically. The nonreciprocity of the collinear interaction is shown to be substantially stronger in the high-frequency regime relative to the low-frequency regime. Sapphire is proved to be an optimal material for experimental realization of the nonreciprocal effect in the high-frequency regime.     

Multiple parameter tuning of the bandwidth, wavelength and attenuation of a fiber-based acousto-optic tunable filter

In this paper we presented a method, which will allow multiple parameter control of the 3-dB bandwidth, center notch wavelength and attenuation level in a fiber-based acousto-optic tunable filter (AOTF). The method basically involves the variation of the interaction length, RF frequency and RF power on a single-fiber device to achieve multiple parameters tuning of the optical filter. The acousto-optic (AO) interaction inside a single-mode fiber (SMF) was studied theoretically and experimentally.     

Determination of the Klein-Cook parameter in ultrasound light diffraction

For weak and strong acousto-optic interaction analytical solutions are available which describe the diffraction problem. In the intermediate range between these limiting cases, however, the light-sound interaction is far more difficult to analyze for two reasons: (i) Only numerical approaches are available and (ii) the diffraction problem depends on two parameters: the Raman-Nath parameter and the Klein-Cook parameter. This paper describes a technique which makes possible the determination of the Klein-Cook parameter with an adequate accuracy for many fields of application. Once the Klein-Cook parameter is known, the Raman-Nath parameter can easily be obtained from numerical approaches.     

Design of the bandpass width, the spectral resolution and the diffraction efficiency of a non-collinear acousto-optic tunable filter of TeO2

The spectral dependencies of the bandpass width, the spectral resolution and the diffraction efficiency of a non-collinear acousto-optic tunable filter of TeO₂ are designed by the method proposed by Chang. The effect of four parameters (the directions of propagation of the acoustic and optic waves, the length of acousto-optic interaction and the power density of the controlling RF-signal) on the filter spectral characteristics is discussed and illustrated from the point of view of practical application. An example of filter design using these dependencies is presented.     

Particular features of detecting the acousto-optic interaction signal in visualization of scattering media

Experimental dependences of the detected acousto-optic interaction signal and of the signal-to-noise ratio on parameters of a scattering medium have been obtained using single- and three-slit diaphragms in front of the photocathode of the receiver. Under the aforesaid conditions, the visualization parameters of the absorbing object are compared. The conditions of preferable use of a multislit diaphragm are determined.     

A new acousto-electro-optic improved resolution deflector for high frequency operation

The two main constrains on the characteristics of the acousto-optic deflector are the acoustic sound attenuation and velocity. These two parameters will determine the resolution and the radio frequency bandwidth.Due to a double degenerated phase matching condition on a specific acousto-optic deflector configuration, the possibility of an improvement on the resolution of the acousto optic deflector becomes evident. This degenerescence is the result of the action of a D.C. electric field by the electro-optic effect. This configuration presents several advantages upon the early related works based on optical activity of selected materials and enables a wideband interaction in such a way that the number of resolvable channels is kept high. This has implications on the acousto-optic spectrometers for wideband radio astronomy research.     

Simulation of light propagation in anisotropic,optically active and slightly inhomogeneous medium,concerning the acousto-optic interaction

A novel modeling technique is introduced for modeling light propagation in slightly inhomogeneous, anisotropic and optically active materials. With the aid of the model the phenomenon of acousto-optic interaction can be efficiently and accurately simulated in a completely general approach. The applied inhomogeneity caused by the sound waves can be arbitrary, similarly the incident and propagating light beams can be also arbitrary, nonparaxial. The basis of the model is described in our previous paper, in addition present work introduces further improvements. The calculation speed is significantly enhanced by increasing the speed of convergence in the case of large spatial frequency ranges and arbitrary angles between the acoustic and optical waves. We also extend our model to handle optical activity, which considerably influences the acousto-optic effect in widely used materials, like TeO2.Mathematical confirmation of the calculated field convergence to the exact solution of the Maxwell's equations is included. The simulation precisely describes the acousto-optic interaction, as a physical verification, simulation results satisfy accurately principal theoretical expectations i.e. diffracted light intensity, phase and polarization distribution.     

Influence of acoustic anisotropy of paratellurite crystal on the double acousto-optic Bragg light scattering

Influence of acoustic anisotropy on acousto-optic interaction in optically and acoustically anisotropic media is theoretically and experimentally studied. A specific type of acousto-optic diffraction is analyzed with allowance for the phase-matching conditions for two diffraction maxima. Analytical expressions for the phase-mismatch parameters versus the angle between the phase and group velocities of acoustic wave are derived. Light intensity in the diffraction peaks is numerically calculated, and experimental data on the diffraction in the paratellurite crystal at an acoustic walk-off angle of 54° are presented.     

Acousto-optic interaction in a non-homogeneous acoustic field excited by a wedge-shaped transducer

The article is devoted to theoretical analysis of light diffraction in a non-homogeneous acoustic field created by a wedge-shaped piezoelectric transducer. Relationships describing electrical, acoustic and acousto-optic properties of the acousto-optic cell are derived in the approximation of a small thickness of the piezoelectric plate. Principal characteristics of acousto-optic interaction are examined such as dependences of the light diffraction efficiency on the phase mismatch and the acoustic wave amplitude. It is shown that the acoustic field has a complicated amplitude-phase structure changing with the acoustic frequency. It is demonstrated that the diffraction efficiency in the Bragg regime can approach 100% in spite of a noticeable phase mismatch. The appropriate optimal values of ultrasound power and incidence angles of light are found.     

Collinear acousto-optic interaction in a single tellurium crystal

The problem of designing acousto-optic imaging filters based on tellurium crystal is discussed. The measurement and calculation results of acousto-optic properties of tellurium are presented. The collinear acousto-optic interaction in tellurium crystal is experimentally implemented for the first time at the light wavelength of 10.6 μm.     

Acousto-optic diffraction with allowance for the influence of the acousto-optic cell boundaries

The influence of the dimension of an acousto-optic cell in the plane orthogonal to the plane of acousto-optic diffraction on the acoustic field structure and, consequently, on acousto-optic diffraction is studied theoretically and experimentally. A method is suggested for approximate calculation of the acoustic field with regard to the acoustic column reflection from the crystal faces. Based on this method, the influence of the acousto-optic cell dimensions is analyzed in a wide range of ratios between the height of the piezoelectric transducer and the height of the crystal. A region of acoustic power concentration is predicted to arise both for parallel faces and for slight wedging. A number of statements are demonstrated with paratellurite used as an interaction medium. The results may be useful in designing acousto-optic devices.     

Spectral characteristics of non-collinear acousto-optic tunable filters

The bandpass width and the diffraction efficiency of a non-collinear acousto-optic tunable filter are considered as functions of the optical wavelength for fixed values of the direction of propagation of the optic wave, the length of the acousto-optic interaction and the power density of the controlling RF-signal. A comparison between the calculated and the experimentally measured spectral dependencies is discussed and illustrated from the viewpoint of practical application for a TeO₂ filter. It is shown that in the approximation used, satisfactory agreement can be achieved between the experimental and calculated dependencies.     

Optical phase shifting with acousto-optic devices

A novel optical phase-shifting method based on a well-known acousto-optic interaction is proposed. By using a pair of acousto-optic modulators (AOMs) and properly aligning them, we construct an optical phase shifter that can directly control the phase of a collimated beam. The proposed phase shifter is insensitive to the polarization of the incident beam when polarization-insensitive AOMs are used, and no calibration is necessary. The proposed approach is confirmed with experimental results.