Polarization-mode dispersion effect of an acousto-optic tunable filter

The effect of the polarization-mode dispersion (PMD) of an acousto-optic tunable filter (AOTF) is studied both theoretically and experimentally. A coupled-mode method derived from Maxwell's equations is proposed to study the evolution of the polarization of light and hence the deterministic dynamics of the PMD inside an AOTF. It is found that the PMD value of a typical AOTF is around several picoseconds and is adjustable by tuning the strength of the applied sound signal. A set of experimental data acquired by the wavelength scanning method is employed to check the validity of the proposed method. A high degree of similarity is observed between the experimental and the theoretical data.     

Imaging acousto-optic tunable filter with 0.35-micrometer spatial resolution

Image blur in acousto-optic tunable filters (AOTF's) has been a persistent problem. Here we describe the connection between transducer structure and image blur and experimentally measure it by using a 5-cm 12°-cut TeO(2) crystal of our design. With these quantitative results, we develop an image-processing method that minimizes AOTF-related image degradation. The combination of long crystal design and image processing results in substantially improved image contrast and spatial resolution relative to conventional AOTF imaging devices. We present high-magnification images of fluorescent actin fibers in cells in which we obtain a resolution of approximately 0.35 μm, representing the first successful use of an AOTF for ultra-high-resolution microscopy. Further improvements are also predicted.     

基于AOTF的在线红外光谱测量系统

介绍了一种新型的红外光谱测量系统，它由准直发射系统和聚焦接收系统两大部分组成，以声光可调谐滤光器（AOTF）为色散元件，连续改变AOTF的驱动频率就能实现快速波长扫描，达到实时测量目的。该系统具有结构简单、测量快速等特点，可用于在线有害气体检测和红外光谱分析。实验表明，系统光谱测量范围2．5－5μm，光谱分辨力为10nm。     

Multiwavelength thermal lens spectrophotometer based on an acousto-optic tunable filter.

The instrumentation development of a novel, all solid-state, nonmoving parts, fast-scanning and wide-tuning range multiwavelength thermal lens spectrophotometer based on the acousto-optic tunable filter (AOTF) is described. Initially, the essential electronic driver was developed to facilitate the systematic characterization of the paratellurite (TeO2) AOTF and to demonstrate that this filter can be successfully and uniquely used as an all solid-state, nonmoving parts dispersive device to rapidly diffract white incident light into a selected color beam, to amplitude modulate the diffracted monochromatic light, and to keep its intensity constant. The multiwavelength thermal lens instrument was subsequently constructed using this AOTF, and preliminary results on advantages of this spectrophotometer such as its ability to characterize trace chemicals and to analyze multicomponent samples are delineated.     

Compact tunable microwave filter using retroreflective acousto-optic filtering and delay controls

Programmable broadband rf filters are demonstrated using a compact retroreflective optical design with an acousto-optic tunable filter and a chirped fiber Bragg grating. This design enables fast 34 micros domain analog-mode control of rf filter time delays and weights. Two proof-of-concept filters are demonstrated including a two-tap notch filter with >35 dB notch depth and a four-tap bandpass filter. Both filters have 2-8 GHz tunability and a 34 micros reset time.     

White-light imaging by use of a multiple passband acousto-optic tunable filter

White-light imaging was accomplished by operation of a TeO(2) acousto-optic tunable filter (AOTF) with 40 simultaneous overlapping passbands from 400 to 700 nm. The AOTF was chromatically compensated by a wedge applied to the output surface of the AOTF, and the measured spatial resolution correlated well with predictions. Switching off specific rf's applied to the AOTF produced optical rejection corresponding to the inactive passbands. A rejection ratio of 30 dB was demonstrated, and the rejection level was found to be controlled by leakage through the sidelobes of adjacent passbands.     

Investigation of magnesium fluoride crystals for imaging acousto-optic tunable filter applications

Results of an investigation of acousto-optic (AO) cells using single crystals of magnesium fluoride (MgF2) are presented. Two acousto-optic tunable filter (AOTF) cells for imaging application have been designed and tested in the visible and ultraviolet (UV) regions of the spectrum from 190 to 490 nm. The two imaging filters were developed by using the wide-angle AO interaction geometry in the (010) and (11 0) planes of the crystal. These filters were used to obtain spectral images at the shortest wavelengths achieved so far. Advantages and drawbacks of this crystal are discussed and photoelastic, acoustic, and AO properties of MgF2 are examined. The investigation confirmed that MgF2-based AOTF cells can be used in the deep UV region up to 110 nm.     

Guided wave acousto-optic and electro-optic tunable filter designs with relaxed beam-splitter requirements

New designs for acousto-optic tunable filters and electro-optic tunable filters with the Mach-Zehnder configuration are proposed. The new designs differ from conventional designs in three respects: (1) polarizing beam splitters are not required, (2) the optical path difference for the waveguides between the beam splitters is a half-wavelength, and (3) the relative positions of the polarization coupling regions in the two waveguides are displaced in the propagation direction by half of the spatial period of the perturbation responsible for the coupling. Because the new designs provide an additional degree of freedom in achieving the required beam-splitter performance, they are expected to be much easier to fabricate than conventional filter designs with polarizing beam splitters.     

Spectral interference Mirau microscope with an acousto-optic tunable filter for three-dimensional surface profilometry

A nonmechanical scanning Mirau-type spectral interference microscope has been developed for the measurement of three-dimensional surface profiles of discontinuous objects. An acousto-optic tunable filter (AOTF) is used as a high-resolution spectral filter, which scans the optical frequency of the broadband light emitted from a superluminescent diode. To generate spectral fringes that make full use of the limited coherence length of the filtered light we unbalanced the Mirau interferometric system by positioning the reference mirror nearly halfway between the top and the bottom of the step height. When the frequency of the broadband light source is scanned by an AOTF, the interference fringes move in opposite directions on the top and the bottom of the object. To uniquely determine the sign of the fringe movement over the large area of the object, we developed a three-dimensional Fourier-transform technique, and from the detected sign of the fringe movement and phase information, we determined the three-dimensional step height. Experimental results of the measurement of 100-microm step height are presented. The main advantages of the proposed system are that it provides nonmechanical scanning and a large measurement range without ambiguity in the sign of the phase.     

一种以AOTF为核心的农药荧光检测系统

提出了一种将 AOTF 技术用于农药含量检测的荧光光谱系统。该系统分别采用以α-Quartz,TeO2 晶体作为声光介质的激发和发射色散元件 AOTF,以高压短弧氙灯为激发光源,利用光纤探测并传输荧光,由数据采集卡进行数据接收和 A/D 转换。该系统通过声光调制产生单色光,由超声射频的变化实现光谱扫描,没有机械调谐部件,故波长切换快、结构简单。西维因农药的荧光特性测量结果表明,系统的荧光波长范围为 500-750nm, 光谱分辨率为 10nm。当激发光波长为320nm,荧光波长为 647nm,西维因溶液浓度为 0.0-120.0μg/L 时,系统具有较好的线性关系,线性相关系数 r 为 0.9991。它可用于对粮食作物中的农药残留的检测和荧光光谱分析。     

Hyperspectral imager, from ultraviolet to visible, with a KDP acousto-optic tunable filter

Hyperspectral imaging in the ultraviolet to visible spectral region has applications in astronomy, biology, chemistry, medical sciences, etc. A novel electronically tunable, random-wavelength access, compact, no-moving-parts, vibration-insensitive, computer-controlled hyperspectral imager operating from 220 to 480 nm with a spectral resolution of 160 cm(-1), e.g., 2 nm at 350 nm, has been developed by use of a KDP acousto-optic tunable filter (AOTF) with an enhanced CCD camera and a pair of crossed calcite Glan-Taylor polarizing prisms. The linear and angular apertures of the AOTF are 1.5 x 1.5 cm2 and 1.2 degrees, respectively. Imager setup and spectral imaging results as well as analyses and discussion of various factors affecting image quality are presented.     

Development of an all-sky-scanning spectroradiometer with a visible diode array and a near-infrared acousto-optic tunable filter

A sky scanner was developed that collects spectral radiance data over the wavelength range 390-1732 nm by use of two radiometers, the first being a monochromator with a 512-element silicon diode array and the second being a near-infrared acousto-optic tunable filter (AOTF) coupled to an InGaAs detector. The scanner is capable of completing a set of spectral radiance measurements at 146 points in the sky hemisphere in a period of less than 4 min.     

Ultraviolet-visible imaging acousto-optic tunable filters in KDP

There is a need to develop large-aperture acousto-optic tunable filters (AOTFs) in the UV region for applications in astronomy, environmental sciences, biology, etc. We have developed a high-quality noncollinear AOTF cell that uses a single crystal of KDP that has nearly a four times larger acousto-optic figure of merit, M2, than quartz. The linear and angular apertures of this cell are 1.5 cm x 1.5 cm and 1.2 degrees, respectively. The spectral range is 220-480 nm, with 160-cm(-1) spectral resolution and high transmission in the UV. We present an analysis of the design and describe the characterization results.     

Spectral characterization of near-infrared acousto-optic tunable filter (AOTF) hyperspectral imaging systems using standard calibration materials

In this study, we propose and evaluate a method for spectral characterization of acousto-optic tunable filter (AOTF) hyperspectral imaging systems in the near-infrared (NIR) spectral region from 900 nm to 1700 nm. The proposed spectral characterization method is based on the SRM-2035 standard reference material, exhibiting distinct spectral features, which enables robust non-rigid matching of the acquired and reference spectra. The matching is performed by simultaneously optimizing the parameters of the AOTF tuning curve, spectral resolution, baseline, and multiplicative effects. In this way, the tuning curve (frequency-wavelength characteristics) and the corresponding spectral resolution of the AOTF hyperspectral imaging system can be characterized simultaneously. Also, the method enables simple spectral characterization of the entire imaging plane of hyperspectral imaging systems. The results indicate that the method is accurate and efficient and can easily be integrated with systems operating in diffuse reflection or transmission modes. Therefore, the proposed method is suitable for characterization, calibration, or validation of AOTF hyperspectral imaging systems.     

High-performance tunable filter

The tuning performance of a liquid-crystal, electro-optically tunable, Lyot filter is described. This filter is widely tunable across the visible region up to a wavelength of approximately 630 nm and has very high performance characteristics that are preserved over the entire tuning range. These include a narrow passband of 0.1 nm, a large-aperture performance of at least 30 mm, a wide field of view of ?8.5 degrees , and the ability to be tuned to an accuracy that is one tenth of the linewidth of the passband. Possible applications for such a filter include remote sensing, multispectral sensing, and laser environmental and underwater monitoring.     

Proposal for acousto-optic tunable filters with near-ideal bandpass characteristics

A scheme for achieving near-ideal-filter characteristics in the acousto-optic-tunable-filter design is proposed. When applied to bandpass filter applications, the proposed scheme should produce a filter thatis far superior to those from conventional designs in terms of such ideal-filter characteristics as a flat top and no sidelobes, which are of immense importance in the optical-frequency-division-multiplexing scheme of optical-communication network systems.     

Monolithic integration of continuously tunable plasmonic nanostructures

We demonstrate precise three-dimensional integration of smooth bumps, grooves, and apertures in optically thick metal films using template stripping. Patterned silicon wafers are used as high-quality, reusable templates. The heights or depths of the metallic features are controlled to within 2 nm, giving continuously tunable optical properties with sharp and intense plasmonic resonances. Furthermore, we demonstrate a pick-and-place template stripping method in situ, enabling versatile three-dimensional micromanipulation, imaging, and characterization of nanoscale devices.     

Polarization-based tunable interferometric filter

A polarization-based tunable interferometric filter essentially consisting of a two-beam interferometer with birefringence elements is described. The analysis of the filter is done through the concept of a geometric phase in optics-namely, the Pancharatnam phase. The transmission characteristics of the filter can be controlled through three parameters: the thickness of the birefringent elements, the optical path difference, and the orientation angle of an analyzer placed at the interferometer output. It is demonstrated theoretically that, with a particular choice of these parameters, the chromatic dispersion of the filter is compensated in a given spectral range. Some properties of the device are confirmed by an experimental demonstration.     

Optically tunable optical filter

We experimentally demonstrate an optically tunable optical filter that uses photorefractive barium titanate. With our filter we implement a spectrum analyzer at 632.8 nm with a resolution of 1.2 nm. We simulate a wavelength-division multiplexing system by separating two semiconductor laser diodes, at 1560 nm and 1578 nm, with the same filter. The filter has a bandwidth of 6.9 nm. We also use the same filter to take 2.5-nm-wide slices out of a 20-nm-wide superluminescent diode centered at 840 nm. As a result, we experimentally demonstrate a phenomenal tuning range from 632.8 to 1578 nm with a single filtering device.     

Telecentric confocal optics for aberration correction of acousto-optic tunable filters

A telecentric confocal optical arrangement is presented that greatly reduces the diffraction aberrations of the acousto-optic tunable filter (AOTF). Analytical expressions for the aberrations were identified based on the fundamental properties of Bragg diffraction, and additional aberrations due to focusing through the AOTF were also included. The analysis was verified by use of a geometrical ray trace optical code, and an experimental AOTF system was analyzed. Considerable improvement in the potential spatial resolution is predicted with confocal optics, which could accommodate large pixel-limited image fields of greater than 10(6) pixels. When the image quality of the experimental system was assessed, the resolution was found to be improved by the confocal optics and was diffraction limited. Higher resolution could have been obtained with the use of larger optics to increase the throughput before being limited by the aberrations.