Shlomo Zach

Photonic Beamformer Receiver With Multiple Beam Capabilities

A multiple-beam photonic beamforming receiver is proposed and demonstrated. The architecture is based on a large port-count demultiplexer and fast tunable lasers to achieve a passive design, with independent beam steering for multiple beam operation. A single true time delay module with four independent beams is experimentally demonstrated, showing extremely smooth RF response in the -band, fast switching capabilities, and negligible crosstalk.

Optical Under-Sampling and Reconstruction of Several Bandwidth-Limited Signals

We demonstrate experimentally an optical system for under-sampling several bandwidth-limited signals with carrier frequencies that are not known apriori and can be located anywhere within a very broad frequency region between 0-18 GHz. The system is based on under-sampling asynchronously at three different sampling rates. The optical pulses required for the under-sampling are generated by a combination of an electrical comb generator and an electro-absorption optical modulator. To reduce loss and improve performance the implementation of the optical system is based on a wavelength division multiplexing technique. An accurate reconstruction of both the phase and the amplitude was obtained when two chirped signals each with a bandwidth of about 150 MHz were sampled.

Radio frequency photonic filter for highly resolved and ultrafast information extraction.

We present a method and devices for highly resolved carrier and information extraction of optically modulated radar signals. The extraction is done by passing the optical beam through a monitoring path that constitutes a finite impulse response filter. Replications of the monitoring signal realize the required spectral scan of the filter. Despite the fact that the filter configuration is fixed, each replication experiences different spectral filtering. The radar carrier is detected by observing the energy fluctuations in a low-rate output detector. The RF information is extracted by positioning a low-rate tunable filter at the detected carrier frequency.

Optical under-sampling by using a broadband optical comb with a high average power

We demonstrate a new method to improve the performance of photonic assisted analog to digital converters (ADCs) that are based on frequency down-conversion obtained by optical under-sampling. The under-sampling is performed by multiplying the radio frequency signal by ultra-low jitter broadband phase-locked optical comb. The comb wave intensity has a smooth periodic function in the time domain rather than a train of short pulses that is currently used in most photonic assisted ADCs. Hence, the signal energy at the photo-detector output can be increased and the signal to noise ratio of the system might be improved without decreasing its bandwidth. We have experimentally demonstrated a system for electro-optical under-sampling with a 6-dB bandwidth of 38.5 GHz and a spur free dynamic range of 99 dB/Hz(2/3) for a signal with a carrier frequency of 35.8 GHz, compared with 94 dB/Hz(2/3) for a signal at 6.2 GHz that was obtained in the same system when a pulsed optical source was used. The optical comb was generated by mixing signals from two dielectric resonator oscillators in a Mach-Zehnder modulator. The comb spacing is equal to 4 GHz and its bandwidth was greater than 48 GHz. The temporal jitter of the comb measured by integrating the phase noise in a frequency region of 10 kHz to 10 MHz around comb frequencies of 16 and 20 GHz was only about 15 and 11 fs, respectively.

A novel photonic Rotman-lens design for radar phased array antennas

Future high performance large radar phased array antennas will require true time delay (TTD), rather than phase shifters, in order to support high bandwidth pulses over a range of scanning angles. Currently, RF Rotman lenses are used as TTD elements, offering a somewhat limited performance with respect to their RF characteristics. Recently, photonic architectures, comprising optical transmitters and receivers, together with some optical circuitry to optically process RF signals, have been proposed as candidates for inclusion within wideband RF systems, which need true time delay to cover an RF bandwidth as large as 18 GHz. In this paper a novel microwave photonic implementation of a Rotman-lens is proposed, offering improved functionality and performance. The proposed scanning unit is an optical module, where photo-detectors connected to the transmitting/receiving antennas are the interfaces, converting between the RF signals and their respective optical waves. The optical module is basically a photonic Rotman lens, quite similar in design to its RF counterpart. However, in addition to the improvement that is obtained by realizing the solution in a photonic module, the proposed photonic Rotman lens improved design is capable of realizing a linear phase profile with a varied slope, that is obtained at the output of the lens for any possible position at the input to the lens. This is in contrast to what is currently available with the conventional RF Rotman lens, where output phase front linearity is obtained for a small and discrete number of input positions. The improved performance is obtained by numerically optimizing the curves of the photonic input and output surfaces of the lens, having an off-centered elliptical profiles, rather than the classically used spherical curvatures. Performance characteristics will be reviewed.

Contour superresolved imaging of static ground targets using satellite platform

We propose a method for increasing the contour resolution of static ground targets and to overcome the diffraction limit of an optical system installed on top of a satellite. The resolution improvement is obtained by using a sequence of low-resolution images taken from different angles realized by the movement of the satellite platform. The superresolving process is obtained by the generation of relative movement between the inspected object and the a priori known high-resolution background. The relative movement is caused because the images are taken from different angles. The captured set of low-resolution images are decoded by the a priori known high-resolution background obtained from a set of reference images taken only once by a high-resolution camera. The proposed concept is demonstrated via Matlab simulation and laboratory experiments.

Passive and periodically ultra fast RF-photonic spectral scanner.

In this paper we present passive photonic device performing periodic and ultra fast spectral analysis of RF signals modulated on optical carrier. The spectral scanning is demonstrated in two approaches. First by passing the light through a couple of special bulk periscopes that split the beam into a set of parallel channels or combine a set of channels into one beam. One surface of each periscope is coated with high reflectivity coating such that the set of parallel beams travel several times through the structure due to their partial back reflection in each passage through the periscope. In each passage in the system the channel experience different delay in comparison with the original signal. This relative delay is accumulative and it is generated by placing glass bars with different length for each one of the channels. This structure realizes Finite Impulse Response (FIR) filter that performs the spectral scanning. The second approach involves similar configuration but it is realized with fibers and Y couplers rather than bulk optics. In this case the filter that performs the spectral scanning is an Infinite Impulse Response (IIR) filter having much sharper spectral sampling capability.

Optical synthetic aperture radar

A method is proposed for increasing the resolution of an object and overcoming the diffraction limit of an optical system installed on top of a moving imaging system, such as an airborne platform or satellite. The resolution improvement is obtained via a two-step process. First, three low resolution differently defocused images are captured and the optical phase is retrieved using an improved iterative Gershberg–Saxton based algorithm. The phase retrieval allows numerical back propagation of the field to the aperture plane. Second, the imaging system is shifted and the first step is repeated. The obtained optical fields at the aperture plane are combined and a synthetically increased lens aperture is generated along the direction of movement, yielding higher imaging resolution. The method resembles a well-known approach from the microwave regime called the synthetic aperture radar in which the antenna size is synthetically increased along the platform propagation direction. The proposed method is demonstrated via Matlab simulation as well as through laboratory experiment.

Spurious-free dynamic range characterization of a χ 2 -based PPLN waveguide

We experimentally characterize the spurious-free dynamic range performance of a χ2-Based PPLN waveguide by measuring the distortion on a microwave photonic link of two RF tones around 9.9GHz and characterizing the SFDR of a generated idler in a PPLN waveguide.

Analog performance of multiple, discretely tunable time delays based on a frequency comb and a chromatic dispersion element

We demonstrate the analog performance of multiple, coherent, and discretely tunable time delays based on a frequency comb and a chromatic dispersion element. The amount of 0.6ns delay is measured between adjacent selected comb lines. The linear phase frequency response of the system is verified and the SFDR is measured.