Spectral characterization of integrated acousto-optic tunable filters by means of laser frequency modulation spectroscopy

The spectral characteristics of an integrated acousto-optic tunable filter (AOTF) as well as its responsivity to the rf driving signal and sensitivity to temperature changes are experimentally investigated and quantified using a diode-laser-based interrogation system. A spectroscopic technique, exploiting the rf frequency modulation of the laser beam and the phase-sensitive detection of the AOTF transmission, has been used for this purpose. That allows for the generation of a dispersivelike signal, which serves as a reference for tracking any wavelength change of the filter's peak with high resolution. The possibility of using the integrated AOTF as a spectrum analyzer with this interrogation scheme for fiber Bragg grating (FBG) strain sensing is also discussed.     

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.     

Development and characterization of two-transducer imaging acousto-optic tunable filters with extended tuning range

We developed two high-quality large-aperture acousto-optic tunable filter cells in TeO2 with more than two octaves spectral coverage for hyperspectral imaging applications from the visible to the midwave infrared: the first cell covers from 0.43 to 2.1 microm and the second from 0.69 to 4.0 microm. The key feature of these cells is a special design of two transducers in tandem with a special bonding technique that results in such a wide spectral coverage with virtually no acoustic and electrical loss due to careful matching of both acoustic and electrical impedances. Each of these cells has high spectral transmission, as well as low power requirement. We discuss the design, characterization, and performance results for these cells.     

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.     

Spectral LADAR: active range-resolved three-dimensional imaging spectroscopy

We present the concept and experimental results for Spectral LADAR, an augmented LADAR imager combining three-dimensional (3D) time-of-flight ranging with active multispectral sensing in the shortwave infrared (1080-1620 nm). The demonstrated technique is based on a nanosecond regime pulsed supercontinuum transmitter and spectrally multiplexed receiver that computes a high-resolution range value for each of 25 spectral bands. A low frame-rate prototype unit is described. Results demonstrating 3D imaging and material type classification of objects, especially those obscured by camouflage, are shown at effective stand-off ranges exceeding 40 m. These capabilities and the highly eye safe wavelengths at which the system operates make it suitable for applications in military imaging and robotic perception.     

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.     

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.     

Rapid two-dimensional optical spectroscopy through acousto-optic pulse shaping

Abstract

Phase-matching techniques are widely used to retrieve nonlinear optical signals of electronic and vibrational transitions. Here, collinear, phase cycled pulses are used to collect the same nonlinear signals in direct analogy to nuclear magnetic resonance studies. An acousto-optic pulse shaper is used to create suitable sequences of ultrashort pulses with arbitrary relative delays and phases. The rapid update rate of the acousto-optic modulator allows for impressive data rates.     

Suppression of sidelobe levels for guided-wave acousto-optic tunable filters using weighted coupling

An analysis of guided-wave acousto-optic tunable filters (AOTFs) that employ simple acousto-optic (AO) weighted coupling techniques for sidelobe reduction and the calculated and experimental results from a specific example that involves only variation of the width of a surface acoustic wave (SAW) slot waveguide are presented. The calculations on single- and multi-stage AOTFs consisting of an optical channel waveguide and a SAW slot waveguide in LiNbO(3) substrate show that waveguide width weighting using generalized Hamming functions would provide significant improvement in sidelobe suppression. Calculated results together with the design, fabrication, and measured performance characteristics of a single-stage AOTF that utilizes a weighted-aperture SAW slot waveguide in YX-LiNbO(3) substrate at the optical wavelength of 1.55 mum and the acoustic center frequency of 175 MHz are reported. The measured sidelobe level is -13.6 dB and the measured FWHM bandwidth is 26 A, as compared to the theoretical values of -15.0 dB and 15 A, respectively. The RF drive power was measured to be 1.0 W at a mode-conversion efficiency of 100%     

Combined optical-coherence and spectral microscopy based on tunable acousto-optic filters of images

A new optical scheme for the microscopic investigation of biological objects is proposed, which combines principles of the optical-coherence and spectral microscopy. The proposed system is based on the acousto-optic filtration of images in the reception channel of a microinterferometer. A prototype of the instrument has been manufactured and it has been demonstrated that the acousto-optic filtration does not violate the interference of two beams carrying images, which makes it possible to extract information both about three-dimensional spatial features and about spectral properties of the object studied.     

Spectral characterization and calibration of AOTF spectrometers and hyper-spectral imaging systems

The goal of this article is to present a novel method for spectral characterization and calibration of spectrometers and hyper-spectral imaging systems based on non-collinear acousto-optical tunable filters. The method characterizes the spectral tuning curve (frequency-wavelength characteristic) of the AOTF (Acousto-Optic Tunable Filter) filter by matching the acquired and modeled spectra of the HgAr calibration lamp, which emits line spectrum that can be well modeled via AOTF transfer function. In this way, not only tuning curve characterization and corresponding spectral calibration but also spectral resolution assessment is performed simultaneously over the whole imaging plane. The obtained results indicated that the proposed method is efficient, accurate and feasible for routine calibration of AOTF spectrometers and hyper-spectral imaging systems and thereby a highly competitive alternative to the existing calibration methods.     

Three-dimensional acousto-optic imaging in biological tissues with parallel signal processing

An original acousto-optic method is described that allows one to reveal optical contrasts through biological tissues that are several centimeters thick with a millimeter-sized resolution. This technique is based on the interaction of scattered laser light with a focused ultrasonic field. The modulation depth of the optical speckle is related to local optical properties of the sample. Our parallel-processing approach to the demodulation of the speckle improves the observed degree of modulation by 2 orders of magnitude and quickly yields a good statistical value. Optically absorbing objects were imaged inside 35-mm-thick biological tissues.     

Position of the prism in a dispersion-compensated acousto-optic deflector for multiphoton imaging

The performance of a dispersion-compensated acousto-optic deflector (AOD) for steering femtosecond laser pulses was examined with the prism located before or after the AOD, which is regarded as prism-AOD and AOD-prism, respectively. Comparisons are made over parameters including the spot spatial pattern, output pulse width, scanning linearity, the field of view, and the transmission rate. Fluorescence images of 170 nm diameter beads and cells were measured to provide an overall evaluation for these femtosecond laser beam scanning configurations. On the basis of these experiments, the prism-AOD configuration is concluded to be more advantageous for the purpose of simultaneous compensation for the spatial and temporal dispersion.     

Hybrid acousto-optic Fourier processor for imaging spatially inhomogeneous acoustic fields

A new method is proposed for imaging spatially inhomogeneous acoustic fields. The approach is based on the Fourier transform of a coherent light field formed as a result of the Bragg diffraction of a probing laser beam on an ensemble of quasi-plane acoustic waves in a lithium niobate crystal. These waves appear as a result of the transformation of an imaged acoustic field by a spherical acoustic lens. The proposed method has been experimentally verified using model multielement acoustic sources.     

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.     

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.     

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.     

Selection of optimal filters for multispectral imaging

Preece and Claridge [IEEE Trans. Pattern Anal. Mach. Intell. 26, 913 (2004)] have proposed a technique for selecting filters for the maximally accurate recovery of object parameters such as chromophore concentrations from a multispectral image of an object. Their selection criteria are derived from an analysis of a model of light propagation in the object and take into account both errors in the modeling process and errors in the image acquisition process, as well as the inherent behavior and structure of the model. We investigate their method on simulated image data and show that filters selected according to their criteria are demonstrably superior to other choices.