Bragg Cell Research Articles

Diffraction properties of vector synthetic volume index gratings

We investigate the diffraction properties of vector synthetic gratings, which consist of two volume index gratings with different grating wave vectors, using coupled-wave analysis. These index gratings can be obtained in photorefractive materials or in Bragg cells with two driving acoustic waves. We consider the case of pseudo phase matching in which a Bragg mismatch exists in the scattering from the individual index gratings while a Bragg condition is satisfied for the scattering from the vector sum of the two index gratings. Analytic expressions for the diffraction efficiency as well as the amplitude of the three waves involved are obtained. The results are presented and discussed.

Nonlinear characteristics of magnetooptic Bragg diffraction in bismuth substituted yttrium iron garnet films

In this article, the nonlinear characteristics of magnetostatic forward volume wave (MSFVW)-based guided-wave magnetooptic Bragg cell modulators in bismuth-substituted yttrium iron garnet-gadolinium gallium garnet waveguides using nonuniform bias magnetic field are reported. First, the dispersion characteristics of the MSFVW under nonuniform bias magnetic field are analyzed, and the explicit expression for its bandwidth is determined. The transmission measurements of the MSFVW show that owing to the nonuniform magnetic field, the bandwidth is significantly increased. Next, the results of noncollinear magnetooptic (MO) Bragg diffraction experiments using the MSFVW in the frequency range from 2.0 to 4.0 GHz are presented. Two types of nonlinear process, namely, the four-magnon decay and modulation instabilities, are observed. However, the MO experiments at the carrier frequency of 2.85, 3.10, and 3.25 GHz suggest that the decay instabilities did not play a significant role in the MO interaction because of the larger degree of phase mismatch induced by the satellite waves generated during the nonlinear processes. We find that despite the presence of the decay instabilities, the MO Bragg diffraction characteristics still comply with that predicted by the coupled-mode theory before the nonlinear processes evolve into the modulation instabilities. Once the four-magnon modulation instabilities set in at the threshold powers, the MO Bragg diffraction will incur a drop in diffraction efficiency by as much as 9%. This feature results from perturbation of the satellite waves of smaller wave numbers induced in the modulation instabilities that lead to the MO phase mismatch. A model is established to describe the combined contributions of the initial MSFVW and the excited satellite waves associated with the modulation instabilities to the MO Bragg diffraction characteristics.

Femtosecond-pulse cavity-dumped solid-state oscillator design and application to ultrafast microscopy

The construction, modeling, and performance characteristics of a new resonator design for ultrafast cavity-dumped oscillators are presented. An acousto-optic Bragg cell was incorporated at the end of the longer arm of a Ti:sapphire oscillator rather than in the shorter arm as in several recent studies. The new arrangement improves the pulse intensity stability of the oscillator and significantly reduces the effort required in construction. The experimental findings are supported by comparison of the stability regions of the laser cavities based on the two different designs. To demonstrate the potential of cavity-dumped oscillators for spatially resolved ultrafast spectroscopy studies, the pulse duration is characterized at the focal plane of two achromatic high-N.A. oil-immersion objectives with different amounts of flat-field correction. Transform-limited pulse widths as short as 15 fs are obtained. To our knowledge, this is the shortest pulse duration measured with true high-N.A. (N.A. > 1) focusing conditions.

Increasing the frequency of surface acoustic waves generated by phase velocity scanning of laser interference fringes

To nondestructively detect surface defects that reduce the strength of ceramics and structural materials of micromachines, we previously developed the phase velocity scanning method. By using scanning interference fringes generated by two laser beams whose frequencies are shifted, the method can generate a 110 MHz surface acoustic wave (SAW). However, the frequency of the SAW depends on the frequency of the Bragg cell used, thus making the upper-limit frequency about 110 MHz. In this study, we propose multiple use of Bragg cells in series, and successfully generated a 200 MHz SAW.

Examination of beam propagation in misaligned holographic gratings and comparison with the acousto-optic transfer function model for profiled beams

A transfer function formalism developed earlier for the propagation of profiled optical beams through acousto-optic Bragg cells is revisited and applied to a thick holographic grating. The results based on the holographic coupled wave model and the acousto-optic multiple scattering model are shown to be compatible, and equivalent parameters such as the Q and grating strength are defined for the two systems. Results for a Gaussian spatial profile are numerically computed and compared. For the holographic grating, a profiled beam may be interpreted as an angular misalignment or Bragg-angle mismatch problem. The case of Bragg-wavelength mismatch is also investigated for the case of a polychromatic READ beam with a uniform and a Gaussian amplitude spectrum. The resulting spatial amplitude distribution of the scattered order at the grating output is plotted as a function of the departure from the correct Bragg direction.

Integrated acousto-optic and magneto-optic Bragg cell modulators and their applications

Significant advances have been made in both integrated acousto-optic (AO) and magneto-optic (MO) Bragg cell modulators in recent years. Presented are similarities in the basic interaction geometry and the characteristics associated with AO and MO Bragg diffraction and a comparison of the resulting AO and MO Bragg cell modulators in terms of the substrate material, center carrier frequency, bandwidth and their electronic tunability, modulation and switching speed, diffraction effi- ciency, fabrication and integration technologies, and applications. While the technology toward realization of monolithic integrated AO device modules in indium phosphide (InP)-based waveguide substrate is con- siderably ahead of its MO counterpart in yittrium iron garnet/gadolinium garnet (YIG/GGG)-based waveguide substrate, the basic MO Bragg cell modulators have demonstrated a number of unique and important ad- vantages over their AO counterparts. Further research and development toward ultimate realization of integrated AO and MO device modules is also discussed. © 1999 Society of Photo-Optical Instrumentation Engineers. (S0091-3286(99)00907-1) Subject terms: integrated acousto-optic Bragg cell modulators; integrated magneto-optic Bragg cell modulators; acousto-optic Bragg diffraction; magneto- optic Bragg diffraction; integrated optics; surface acoustic waves; magnetostatic waves; light beam scanning and switching; rf signal processing.

Multichannel acousto-optic Bragg cell for real-time electroholography

A multichannel acousto-optic Bragg cell was made by the joining of acousto-optic cells that have the shape of symmetrical trapezoidal prisms. This acousto-optic Bragg cell has an aperture length equal to the sum of its component acousto-optic cells but has an aperture time equal to that of each acousto-optic cell. We tested the performance of the multichannel acousto-optic Bragg cell by joining six TeO(2) acousto-optic cells together with 300-microm gaps between one another. The holographic fringe data prepared by computer were divided into six equal segments. Each data segment was again divided into six equal parts and then fed into the corresponding acousto-optic cell. The holographic images were reconstructed by the periodical illumination of a green (lambda = 0.5145 microm) pulse laser beam onto the multichannel acousto-optic Bragg cell. The reconstructed images are sharp and clear. This indicates that the multichannel acousto-optic Bragg cell is working properly.

A magnetostatic forward volume wave oscillator-based magneto-optic Bragg cell modulator

A guided-wave magneto-optic (MO) Bragg cell modulator that utilizes a magnetostatic forward volume wave (MSFVW) oscillator through a feedback loop is reported. The carrier frequency of the MSFVW signals involved in the MO interaction is tuned linearly from 2.112 to 3.274 GHz by increasing an external bias magnetic field from 2350 to 2900 Oe. Compared with a conventional delay line-based MO Bragg cell modulator, the oscillator-based MO modulator can provide higher diffraction efficiency by twofold to fourfold owing to reduction of insertion losses in the MSFVW oscillator. Furthermore, such oscillator-type MO modulator can be more easily integrated with other monolithic microwave integrated circuit components.

A High Power, High Resolution LDA/PDA System Applied to Gasoline Direct Injection Sprays

A new generation LDA/PDA transmitter system has been designed and constructed. The heart of the optical system is a new type of Bragg cell. Advances in laser power handling and symmetrical splitting at high Bragg angles of the shifted and unshifted beams have made it possible to construct a simple yet elegant LDA/PDA transmitter. The optical system integrates the Bragg cell with a laser beam expander to offer variable beam separation and high beam expansion ratios to produce a measurement volume with a high spatial resolution and at high power levels. The transmitter is proving to be a significant contribution to LDA/PDA optical system design and, not only applicable as a research tool for use in flows of a demanding nature but, due to its simplicity, flexibility and cost, an asset to teaching. The LDA/PDA system is being applied to characterise the atomization of fuel by high pressure automotive injectors as found in Gasoline Direct Injection (GDI) engines. Although the PDA system was configured to measure two orthogonal velocity components (2D) and size, its operation was not restricted to this configuration. An analysis of the spray with the PDA system in 1D and size and the LDA system in 2D and 1D configurations indicated the complexity of the atomization and break-up processes occurring in the spray. Single-shot imaging was used to study the spatial structure of the spray as a function of time. Use of the light sheet imaging technique alone could lead to false impressions of the atomization process.

Nonlinearity: magneto-optic—microwave interactions. Towards new devices

The properties of ferrimagnetic yttrium—iron-garnet films are investigated with respect of mutual interactions between optical wave guided modes and spin waves excited by means of microwaves. Various physical effects are reviewed and their practical applications are discussed. Traditional methods of microwave—optic waves interaction are extended to the case where the microwave power becomes high and the nonlinearity of the magnetic film plays a crucial role. Along with traditional devices such as magneto-optic Bragg cells, frequency shifters, modulators a new class of nonlinear devices is considered and practical recommendations are given.

Influence of nonparallelism of the surfaces of a piezoelectric layer on the frequency band of an electroacoustic piezoelectric transducer

The influence of nonparallelism of the surfaces of a piezoelectric transducer on its impedance and frequency band is analyzed theoretically. The model of a wedge-shaped piezoelectric layer taken for the analysis consists of a set of n plane-parallel elements arranged in succession along the length of the transducer with a gradual increase in the thickness of the piezoelectric layer. The analysis performed demonstrates the possibility of significantly expanding the band of working frequencies of a piezoelectric transducer when its electroacoustic conversion efficiency is reduced. In some cases, however, a decrease in conversion efficiency can be employed to optimize an acoustic device, for example, in creating a filter-type piezoelectric transducer, where electromagnetic energy is converted into acoustic energy successively from one cell of the filter to another. The proposed method for expanding the frequency band can be useful, for example, in creating high-frequency acousto-optic Bragg cells.

A 4×1 GHz Array Acousto-Optical Spectrometer

In this paper an array acousto-optical spectrometer (AOS) with four 1 GHz bands and a frequency channel spacing of 1 MHz is presented. A discussion about the requirements which have to be fulfilled by the presented instrument for applications in radio astronomy is given as well as e detailed description of the optical design. Remarkable test results have been achieved such as an Allan-Variance minimum time of about 1000 s, identical noise performance, and identical frequency calibration of all bands. The reproducibility of measurements has been demonstrated by tests with CH3CN, using an absorption cell. The question of the maximum number of bands per array AOS is discussed. Although it may seem promising to increase the number of bands per optical unit to e.g. eight or more instead of four, a strong limitation has been found due to the increasing r.f. power level on the Bragg cell which affects the baselines of the spectra.

A two-color interferometer using a frequency doubled diode pumped laser for electron density measurements

A new tangentially viewing interferometer has been operated on the Alcator C-Mod tokamak. This system uses both the near infrared and visible beams from a continuous wave, diode pumped, frequency doubled Nd:YAG laser to produce the two wavelengths needed for vibration subtraction. A single Bragg cell driven at two harmonically related frequencies provides the basis for a novel heterodyne detection technique. Comparison of the data from the tangentially viewing interferometer and a vertically viewing CO2 interferometer will be discussed.

Experimental and numerical results for intensity modulated laser ultrasonics

This paper describes the experimental confirmation of a numerical model of laser-generated ultrasound, where a 100-μs-long laser pulse was intensity modulated using a Bragg cell. Temporal modulation was used to produce narrow-band ultrasound signals, which can substantially improve signal-to-noise ratios over ultrasound generated by Q-switched laser pulses. A curve fit function that represents Bragg cell modulation is given. Experimental results were obtained using a piezoelectric transducer (PZT) on an aluminum test block, and good agreement between theory and experiment was obtained while the modulated pulse train was on. Though the simple PZT model used here results in some discrepancies between theory and experiment late in the signals, it is concluded that the numerical model reliably predicts Bragg cell modulated laser-generated displacements.

An interferometric spectrum analyzer with high dynamic range

The dynamic range improvement and realization of the acousto-optic spectrum analyzer are reported. When two RF signals of equal strengths and frequencies f 1 and f 2 are applied simultaneously to the signal Bragg cell, the analyzer demonstrates a dynamic range of 50 dB and frequency resolution 10 kHz.

Paratellurite: Propagation of the slow shear wave in the (001) plane. New formulations for the acoustic velocity and divergence

The propagation of the slow shear wave in paratellurite with an angular tilt θ from the [110] axis in the (001) plane is considered. Original formulations are given for the acoustic velocity, the power flow angle, and the acoustic divergence. For a small angular deviation, the acoustic spreading is greatly reduced. This configuration can be used for multichannel Bragg cells. In order to demonstrate the feasibility of such a device, a three-channel Bragg cell (θ=5°) has been designed and successfully tested.

Enlarged shift of light frequency in a modified Bragg cell

Ultrasonic light interaction in a slightly modified Bragg cell in the form of a truncated pyramid is considered theoretically. It is shown that the duration of the interaction can be increased by a factor of . The ultrasonic wave not only modulates the light wave in space but can also significantly increase its frequency and energy. Properties of moving optical resonators formed by the ultrasonic wave in the cell in relation to its taper and the intensity of the ultrasonic wave, optimal regimes of the interaction, maximally possible frequency shift, physical mechanisms responsible for the frequency shift, and possible generalization of the cell are considered.

Visualization of a phase structure of sound field in a Bragg cell

A phase structure of a sound field in a Bragg cell was investigated experimentally using a dual-beam, dual-frequency laser system. The technique used allows the recording of a spatial phase structure of diffracted light field to visualize phase nonuniformities of a sound field. The underlying physical principle of this technique is to probe an analyzed phase object using two slightly separated in space light beams with different optical frequencies. A difference frequency signal, which is obtained as a photodetector output in an optical heterodyne system, contains information about a phase difference of the adjacent areas of the sound field. When the probing beams are diffracted by a Bragg cell, their optical frequencies are shifted by the same acoustic frequency, so that the difference is the same at any Bragg cell operating frequency. A Bragg cell on paratellurite operating around 100 MHz was investigated. Focused laser beams with a difference of optical frequencies equal to 1 MHz were used as probing beams. Pictures were obtained which demonstrated dependencies of a phase structure of diffracted light on frequency and sound power. Phase nonuniformities across the sound field up to three wavelengths have been estimated. [Work supported by EOARD.]

Application of nondestructive testing techniques to materials testing : Kino, G.S. Stanford University, California (United States), AD-A172-005, 77 pp. (Feb. 1986)

Abstract : During the last year, we finished our electronic imaging. We have developed an new type of acoustic microscope which is capable of measuring both the phase and amplitude of an acoustic wave. With this system, we have been able to measure the thickness of films of a few thousand Angstroms thick using a 50 MHz microscope with an acoustic wavelength in water of 30 microns. An interesting feature has been that we can now measure the amplitude and phase of the V (z) curves (the variation in the amplitude of the signal received at the microscope determined as a function of the distance z of the surface of the solid object from the focus). The two papers on this subject to be published in the IEEE Ultrasonic Transactions in April 1985 form part of the appendix of this report. During the year we have begun to work on a new type of scanning optical microscope which used a Bragg cell to scan an otptical beam. Early results indicate that we can measure the thickness of transparent and opaque films to high accuracies.

Recent Advances in Magnetostatic Waves-Based Integrated Magnetooptic Bragg Cell Modulators in Yig-Ggg Waveguides

AbstractThe most recent advances in the integration architecture and diffraction efficiency of magnetostatic wave (MSW)-based guided-wave magnetooptic (MO) Bragg cell modulators in yttrium iron garnet-gadolinium gallium garnet (YIG-GGG) waveguides are reported. A curved ion-milled hybrid waveguide lens pair has been integrated with a MO Bragg cell modulator in a taper waveguide with dimensions of 6.0x16.0mm2 to facilitate the collimation and focusing functions. An enhancement in the Bragg diffraction efficiency by two-to six-fold has been accomplished using a non-uniform bias magnetic field. An oscillator-based MO Bragg cell modulator has also been constructed and has provided a Bragg diffraction efficiency which is higher by a factor of two to four than that of a conventional delayline-based modulator.