Fourier plane image processing using acousto-optic cells: a space-bandwidth analysis

The design, fabrication, and testing of an acousto-optic-based binary data recorder are described. The data to be recorded are applied to an acousto-optic (AO) Bragg cell illuminated by a pulsed laser diode, and the bit sequence present in the cell is imaged onto a linear detector array. Emphasis was placed on incorporating the optical system and the linear detector array with its control board into a rugged module of modest size capable of recording non-return-to-zero data at rates of 200 Mbits/s or greater. The design specifications of the AO cell and the pulsed laser were based on earlier analysis and proof-of-principle experiments. An in-house-built TI3AsS4 AO cell with a bandwidth of 260 MHz centered at 400 MHz gave 11% diffraction efficiency at the 810 nm wavelength used in the system. When used with a laser pulse of 4 ns full-width at half-maximum, this system had a maximum operating rate of 240 Mbits/s. The same optical system used with an AO cell with 500 MHz bandwidth and a 1.7 ns laser pulse had a maximum rate of 500 Mbits/s. The quality of the optical imaging did not appear to limit the performance, suggesting the possibility of either further increasing the data rate or reducing the size. Possible modifications to explore these options are discussed.

Accurate, low-noise, wideband phased array radar antennas depend on the use of true time delays between microwave elements. A photonic means of generating the required element-to-element phase shifts was developed by Toughlian and Zmuda for systems operating in the VHF frequency range, and extended by Monsay and Caccuitto for operation in the radar C band. The photonic beamformer is based on a heterodyne optical system, in which the reference beam acts as a probe of the acoustic wave in the acousto-optic (AO) cell. It is apparent that the benefits of this photonic system rely greatly on the ability of the probe beam to pick out just the right phase shift/time delay from the acoustic waveform in the AO cell. A basic limitation to system performance could come from intrinsic diffraction effects due to the finite beamwidth of the probe in the AO cell. In addition, the finite size of the photodetector in the system will have an effect on the final SNR. These effects have been studied in detail and their implications for design and performance of the photonic beamformer are discussed. © 1997 Society of Photo-Optical Instrumen- tation Engineers. (S0091-3286(97)01312-3)

Dynamic behavior of an analog-to-digital converter (ADC) based on diffractive optical element(s) (DOE)(s) was studied and found to be in agreement with predictions. The analog signal was translated to an angular deflection of a laser beam by means of an acousto-optic (AO) cell. The number of bits in this experimental demonstration was three, using an eight-element DOE array. The maximum sample rate was found to be 2.5 MS/s, the limiting factor being the transit time for the acoustic wave across the width of the laser beam in the AO cell. The study is intended as a first dynamic demonstration of a proposed ADC scheme previously demonstrated in a quasi-static version. The full potential of the ADC scheme will require the use of a fast tunable diode laser to replace the AO deflection scheme used here.

A previously described holographic interconnection system uses an array of self-coherent but mutually incoherent sources to generate a set of overlapping holograms in a photorefractive crystal.1,2 In the current work, we assess the performance of this interconnection system experimentally. A linear array of self-coherent but mutually incoherent sources at visible wavelengths is generated by using an acousto-optic (AO) cell (TeO2) and custom analog electronics. Mutual incoherence is obtained by super-imposing a set of diffraction gratings (one for each source) at different frequencies in the AO cell, so that each diffraction grating yields a frequency shift in the corresponding diffracted beam. We present experimental results in Fe:LiNbO3 at 514.5 nm on the fidelity of the photorefractive interconnection in terms of diffraction efficiency, cross talk, and accuracy, as a function of both the number of multiplexed volume holograms and the degree of spatial overlap of the holograms. Some of these experimental results are compared with results obtained from simulation. The measured fidelity of this incoherent/coherent inter connection system is also compared to that of a fully coherent interconnection system.

A photonic means of generating the element-to-element phase shifts required for the operation of a true time delay radar beamformer was developed previously by Toughlian and Zmuda for systems operating in the VHF frequency range, and extended by Monsay for operation in the radar C band. The photonic beamformer is based on an acousto-optic (AO), heterodyne optical system in which the reference beam acts as a probe of the acoustic wave in the AO cell. The time delay of the acoustic waveform readout by the optical probe from various locations along the length of the AO cell can then be imposed directly on the microwave elements of the radar beamformer, providing the correct phase for each element at whatever frequency drives the AO cell. Fundamental systems-oriented calculations are reported. Results for both optical specifications and radar performance specifications are obtained based on a mathematical model of the photonic delay line previously developed by the author. These results address the use of the photonic delay line in a standard radar beamformer.

We review the role of acousto-optic (AO) modulators in programmable real-time image processing. Using one AO cell, 1D high pass filtering and edge enhancement has recently been demonstrated. Two AO cells in tandem can be used to remove asymmetries for using one AO cell. We also show notch filtering characteristics of an AO cell by developing a technique to determine the spatial transfer functions numerically for various cases of beam incidence on an AO modulator. Normal incidence and incidence at twice the Bragg angle are investigated as examples.

Moire contours are seen on a curved surface viewed through a grating when the surface is illuminated with a matched grating. We have been able to replace both the projection and viewing physical gratings with gratings of variable spacing generated in acousto-optic (AO) cells and have been able to observe and record moire contours. The gratings are generated by amplitude modulating 70 MHz AO cells with a 1 to 10 MHz square wave. The gratings become visible on the curved object when the illuminating laser beam is strobed with 50 ns pulses by means of an AO modulator. We give data showing the variation in grating spacing with AO modulation frequency. Figures show the variation of the moire contour spacing with the AO modulation frequency and with target contour. The results for an angled flat plate target are compared with those in the literature.

The structure of the acoustic field defines the key parameters of acousto-optical (AO) devices. To confirm their compliance with the expected values in the presence of multiple real factors, AO crystalline cells require accurate experimental investigation of the acoustic field after being totally assembled. For this purpose, we propose to detect and quantify all the acoustic waves propagating in AO cells using an impulse acoustic microscopy technique. To validate this approach, we have analyzed both theoretically and experimentally the modes, amplitudes, propagation trajectories, and other features of the ultrasonic waves generated inside an AO modulator made of fused quartz. Good correspondence between theoretical and experimental data confirms the effectiveness of the proposed technique.

Photonic-based dual-chirp microwave waveforms generation with multi-carrier frequency and large time-bandwidth product

We present a novel photonic approach based on heterodyne between a multiple-dispersed optical pulse and a continuous optical signal to generate tunable linearly chirped microwave waveform (LCMW) with large time-bandwidth product (TBWP). The carrier frequency of the generated waveform can be changed by tuning the frequency of continuous optical wave outputted from the tunable laser source (TLS),while the TBWP of the LCMW can be changed by optically pumping the linearly chirped fiber Bragg grating (LCFBG) in the dispersive loop with different pumping powers. An extremely large equivalent dispersion will be obtained because of the dispersive loop used in the system, which leads to the generation of LCMW with long temporal duration. The capability of the presented method is verified via numeric simulation.

Photonic approach to generating linearly chirped waveforms with large time–bandwidth product (TBWP) is proposed and demonstrated. In the approach, a dual-output dual-parallel Mach–Zehnder modulator (DPMZM) is employed to generate dual-tone wavelengths in the two output ports. The polarization modulator (PolM) is employed to conduct complementary phase modulation of the dual-tone wavelengths. After driving PolM with parabolic signal, frequency-doubled chirped microwave waveform will be obtained from photodetector (PD). To enlarge the TBWP, parabolic waveform is split into time domain and convolved with binary sequences to increase the bandwidth and temporal duration of the generated signal simultaneously. Simulation results show that linearly chirped microwave waveforms with carrier frequency of 40 GHz, bandwidth of 12.5 GHz, time duration of 819.2 ns are generated. After splitting and code-convolving with binary sequence, TBWP of the generated signal is enlarged to 10,240. Pulse compression performances of the generated signal are analyzed, and structure stability is also investigated.

The effect of Bragg diffraction occurring in the tangential geometry of the acousto-optic (AO) interaction on the speckle structure of the zero and first diffraction orders is investigated. It is established that in the first order, the fine-grained background of the speckle structure is significantly suppressed in comparison with the zero-order structure. Experiments have been performed on filtering speckle-containing optical radiation with a wavelength of 0.63 microns using an AO cell from TeO operating in tangential diffraction geometry at a sound frequency of 100 MHz. A decrease in the fine-grained background in the first order was revealed by ~7 times in comparison with the speckle structure of the zeros order of diffraction.

A multichannel acousto‐optic joint transform correlator is presented. On‐plane acousto‐optic (AO) cells and a square‐law converter are employed. The AO cells are driven by frequency multiplexed temporal signals to produce spatially modulated signals. The joint transform power spectrum is generated with a square‐law converter. By inverse Fourier transformation with a coherent readout, the cross correlation of input signals can be obtained. Since the architecture can process wideband signals, it may be applied to detect multiple frequency shift keying (MFSK) signals.

In order to improve both the density and particularly the temporal resolution beyond previous dispersion interferometers (DIs), a heterodyne technique based on an acousto-optic (AO) cell has been added to the DI. A 40 MHz drive frequency for the AO cell allows density fluctuation measurements into the MHz range. A CO2 laser-based heterodyne DI (HDI) installed on DIII-D has demonstrated that the HDI is capable of tracking the density evolution throughout DIII-D discharges, including disruption events and other rapid transient phenomena. The data also show good agreement with independent density measurements obtained with the existing DIII-D two-color interferometer. The HDI line-integrated density resolution sampled over a 1 s interval is ∼9 × 1017 m-2. Density fluctuations induced by MHD instabilities are also successfully measured by the HDI.

A coherent optical processing system is usually nonredundant because any attempt to secure redundancy by band-limiting the input leads to a corresponding underuse of the Fourier plane. Methods are suggested for putting additional copies of the signal into the unused region. This results in a certain departure from strict coherence but is in no case as extreme as using noncoherent methods.