Acousto-optic materials and acousto-optic cell applications

A review of physical properties of the materials used in manufacturing real acousto-optic cells for optical and RF signal processing is presented. The overall design of acousto-optic cells requires the knowledge of not only the elastic modulii of the second order, but those of higher orders also, as they determine the acoustic nonlinearity and the dynamic range of any device. The parameters of elastic anisotropy determine the near zone of cell's transducer and directly affect cell's time aperture and number of resolvable spots, and interchannel cross-talks in multichannel devices. The requirements to the properties of crystal materials used in cells with anisotropic and collinear interaction are discussed.

Fourier methods are widely used in optical information processing for time domain waveform synthesis and detection. In such approaches an input pulse is spectrally decomposed and modulated by an appropriately designed spectral filter. In general, various ultrafast applications may need either generation or characterization of waveforms depending whether their frequency response is or is not known a priori. Ultrafast pulses prove useful in driving or probing such systems because their broad, deterministic complex spectral amplitude “sees” a large continuous portion of the system transfer function. Traditionally, space domain spectral decomposition is used to perform optical Fourier processing, where pairs of gratings and lenses are used to decompose the spectral content of the pulsed, optical waveforms. Liquid crystal and acousto-optic (AO) modulators provide a means for introduction of somewhat arbitrary system transfer functions. More recently, compact waveguide-based approaches have used arrayed waveguide gratings (AWGs) to perform an analogous spectral decomposition to those achieved with traditional, free-space coupled gratings. Information processing arrangements using the space domain for transverse spectral decomposition benefit from the maturity of such techniques, with respect to basic understanding, component quality, and the excellent phase control that free-space manipulations provide. Component insensitivity to optical power permits the shaping and detecting of high peak power pulses used to drive nonlinear optical systems. Furthermore, nonlinear optical elements can be introduced directly into these processors to provide unique ultrafast waveform synthesis capabilities with response times in the range of femtoseconds. Drawbacks to free-space approaches include scaling in volume to achieve large time bandwidth products (TBWPs) and limiting the temporal extent of waveforms coupled back into a single-mode fiber (SMF). The origin of this last restraint is the time∕space interrelation inherent in traditional pulse shaping devices. AWG approaches rely on complex waveguide elements to circumvent large processor volumes and time∕space coupling issues, but they too are practically limited by fabrication requirements. The number of resolvable spots in the output waveform for both Fourier synthesis and direct space-to-time AWG approaches is equal to the number of waveguide channels. An alternative approach to Fourier processing relying on a single transverse spatial mode can better integrate with fiber systems and photonic lightwave circuits and will scale in length only for the achievement of large TBWPs. Fiber-based and fiber-integrated processors can manipulate optical waveforms in the time domain exploiting chromatic dispersion for longitudinal spectral decomposition and applying to it Fourier synthesis techniques. Drawing on the identical mathematical treatments of diffraction in space and dispersion in time, an approximate Fourier transform (FT) of an incident optical signal is achieved via chromatic dispersion after reaching the so-called “far-field approximation.” With such an amount of dispersion, the temporal waveform closely resembles the spectrum. Such a waveform is called a longitudinal spectral decomposition wave (SDW). A time-variant element (e.g., modulator) filters this SDW. And it is possible to recompose the pulse waveform through propagation in a conjugate dispersion source matched (i.e., of opposite sign but equal magnitude) to the first dispersive element. The number of resolvable spots that such a processor offers relies on a suitable modulation scheme and the experimental ability to disperse a sub-picosecond pulse using second-order dispersion.

The purpose of this letter is to derive a bound on the number of unique resolvable spots in a photoelastic beam deflection system which is limited by the attenuation of the acoustic wave in the interaction medium. It will also be shown that for a given interaction length, an optimum frequency range exists which yields the maximum number of resolvable spots.

In this study, we take a data-driven approach to study the design efficiency of a variety of optical designs. Efficiency is defined to be the number of resolvable spots across the image per lens element. 3188 designs were selected from a commercially available lens database. Each design was imported into a raytrace code, briefly optimized, and the number of resolvable spots was computed. Examples of efficient designs within this dataset are shown. Four design efficiency groupings are created and discussed separately: 1) all-spherical, monochromatic designs, 2) monochromatic designs with some aspheres, 3) all-spherical, polychromatic designs, and 4) polychromatic designs with some aspheres. Zoom lens systems were excluded from the dataset. The results of the analysis are intended to answer the question of "how many elements does it take, as a minimum, to deliver a certain number of resolved spots?"

Acousto-optic (AO) cells have been used to implement Fourier plane filtering in coherent optical processing systems. AO cells have large time-bandwidth products (>1000) and high frame rates (>10 kHz). The optical input to the AO cell at the Fourier plane consists of several plane waves travelling at different angles. The Bragg matching condition limits the input field of view of the AO cell in the Fourier plane. However, the finite acoustic wave vector spread, optical birefringence, and acoustic beam steering have been used to overcome this limitation. In this paper the relation between the SBW product of an image that can be processed by the AO cell and its time-bandwidth product is derived and experimentally verified. The AO cell in the Fourier plane is suitable for image morphology, and it is shown that the time-bandwidth product requirement for this application is independent of the size of the structuring element, enabling a comparatively large space-bandwidth input to be processed.

A novel method of thermal imaging based on the up-conversion of infrared radiation in nonlinear optical crystals is analyzed. An equation is derived giving the temperature resolution in terms of the pump laser energy E(p). For a proustite-ruby system based on the 8-13-micro band, a temperature resolution of better than 1 degrees C is predicted when the number of resolvable spots is 100 x 100 and E(p) = 10 J. Numerical results are also provided for eighteen possible systems having different combinations of pump laser (neodymium or ruby), infrared band (3-5 micro or 8-13 micro), nonlinear crystal (Ag(3)AsS(3), AgGas(2), ZnGeP(2), or LiNbO(3)), and imaging photocathode [S-1, S-20, GaAs/(Cs,O), or In(As,P)/(Cs,O)].

Summary form only given. The author discusses developments in optical signal processing and highlights the many ways in which the technology of optical and electronic signal processing is converging.

This processor is oriented to studies in the extra-galactic astronomy as well as to searching the extra-solar planets, so that algorithm of the space-and-time integrating is desirable for a wideband spectrum analysis with an improved resolution. It includes 1D -acousto-optic cells as the input devices for a 2D -optical data processing. The importance of this algorithm is based on exploiting the chirp Z − transform technique providing a 2D -Fourier transform of the input signals. The system produces the folded spectrum, accumulating advantages of both space and time integrating. Its frequency bandwidth is practically equal to the bandwidth of transducers inherent in acousto-optical cells. Then, similar processor is able to provide really high frequency resolution, which is practically equal to the reciprocal of the CCD-matrix photo-detector integration time. Here, the current state of designing the triple product acousto-optical processor in frames of the astrophysical instrumentation is presented.

Current research and development in integrated optical signal processing modules such as rf spectrum analyzers using acoustooptic (AO) interaction in planar waveguide1 have been significantly impeded by the poor performance of the waveguide lenses. The waveguide lenses involved suffer from undesirable features that include small light beam aperture, high optical loss, very small tolerance allowable in lens placement and separation, and long focal length. Long focal length will result in a large substrate size and thus high optical throughput loss, severe waveguide scattering, and high cost.

In this paper, we consider a class of N-th order lattice filter with gains as an extension to the traditional lattice filter structures and develop analysis and synthesis of such filters. In the analysis-synthesis parts, we present recursive methods to derive the input-output transfer function of the filter in terms of parameters of the lattice structure such as the time delay, gains, transmission and reflection coefficients and vice versa. Stability of the filter is analyzed and an algorithm to test the stability is proposed. This class of MIMO lattice filters with adjustable gains models integrated photonic devices under development in our labs for optical communication and high speed signal processing applications. Our signal processing approach to characterizing this type of lattice structures can also be used in filter realization by VLSI, FPGA, or programmable processors for acoustic or speech applications

Magneto-optical devices based on Bragg diffraction of light by magnetostatic waves (MSWs) offer the potential of large time-bandwidth optical signal processing at microwave frequencies of 1 to 20 GHz and higher. A thin-film integrated-optical configuration, with the interacting MSW and guided-optical wave both propagating in a common ferrite layer, is necessary to avoid shape-factor demagnetization effects. The underlying theory of the MSW-optical interaction is outlined, including the development of expressions for optical diffraction efficiency as a function of MSW power and frequency, device geometry, materials properties, and other relevant parameters. Recent experimental observations of anisotropic Bragg diffraction and collinear TE↔TM mode conversion induced by MSWs in yttrium iron garnet (YIG) thin films suggest that high-performance MSW integrated-optical devices are feasible. Diffraction levels as large as 4% (7-mm interaction length) and a modulation dynamic range of ∼30 db have been demonstrated. Potential signal processing applications are mentioned, including: spectrum analyzers, convolvers/correlators, deflectors, non-reciprocal optical isolators, and tunable narrowband optical filters. Advantages of these MSW-based devices over the analogous acousto-optical devices include: much greater operating frequencies, tuning through the MSW dispersion relation by varying either the rf frequency or the applied bias magnetic field, simple MSW transducer structures (e.g., a single stripline), and the potential for very high diffraction efficiencies.

: High efficient nonlinear optical materials (NOMS) are highly required for optical computing and optical signal processing. However, currently no such practical NOMs are available. In this project, new organic NOMs have been researched and developed. New organic NOM design principle and approach have been reviewed and investigated. Four new organic NOMs, namely, CMVI, DVDA, MOHNS and HMONS have been synthesized and purified. Nonlinear optical responses of these four samples have been demonstrated and preliminary been characterized. Device fabrication and application for optical computing and optical signal processing have been investigated.

Silicon – based devices for optical signal processing has established itself as a mainstay in the field of photonics. Their special significance lies in their ability to be manufactured alongside CMOS electronics, thus enabling commercial applications in transceivers and interconnects. In this paper, we present recent research progress pertaining to ultrafast optical signal processing on silicon – based platforms. Advances in on – chip wavelength and mode division multiplexing strategies with the potential for meeting 200GHz dense wavelength division multiplexing standards across the C – and L – bands will be discussed. In addition, the development of a silicon – based nonlinear optics platform with high nonlinear figures of merit will be presented. Nonlinear optical devices fabricated on the aforementioned platform possess nonlinear parameters 500 times larger than that in silicon nitride waveguides, while possessing negligible nonlinear losses at $1.55\mu \mathrm{m}$

Acoustooptic devices have been used in a variety of applications, such as laser beam deflectors, modulators, tunable optical filters, and signal processors. This paper discusses an architecture for an acoustooptic correlator. Two parallel on-plane acoustooptic cells are employed to convert electrical signals into acoustic strain fields which modulate incident laser light. The Fourier transforms of the acoustic fields are produced with a lens on the input plane of a liquid crystal light valve. The inverse Fourier transform of the readout beam of the liquid crystal light valve provides the cross-correlation of two input signals. Since two acoustooptic cells are in the same plane, the proposed architecture can be minimized and implemented in a compact system. This architecture can be easily extended to the system which yields more than two cross-correlations by adding an adequate number of acoustooptic cells in the input plane. This feature is useful for digital communications in which different waveforms are used to represent different messages. Due to the wide bandwidth of acoustooptic cells, this correlator can process broadband communication signals such as frequency-hopping signals.

This paper summarizes recent advances on optical processing using InAs/InP quantum dot semiconductor mode-locked lasers. In particular, all-optical clock recovery at 40 and 160 Gbit/s with excellent phase noise characteristics will be demonstrated using these lasers.