Yonatan Stern

Stable closed-loop fiber-optic delay of arbitrary radio-frequency waveforms.

Thermal drifts in long fiber-optic delay lines are compensated based on chromatic dispersion. An arbitrary input radio-frequency (RF) waveform and a control RF sine wave modulate two different tunable laser sources and are coupled into the fiber delay line. The RF phase of the control tone at the output of the delay line is monitored and used to adjust the wavelengths of both sources, so that the effects of thermal drifts and dispersion cancel out. The input and control waveforms are separated in the optical domain, and no restrictions are imposed on their RF spectra. A figure of merit is proposed, in terms of the fiber delay, range of temperature changes that may be compensated for, and residual delay variations. An upper bound on performance is established in terms of the specifications of the tunable lasers. The principle is used in the stable distribution of sine waves and of broadband linear frequency-modulated (LFM) waveforms, which are commonly employed in radar systems. Lastly, the method is incorporated in stable interrogation of a localized hot-spot within a high-resolution, distributed Brillouin fiber sensing setup. The results demonstrate the applicability of the proposed protocol in the processing of arbitrary waveforms, as part of larger, more complex systems.

Long Microwave-Photonic Variable Delay of Linear Frequency Modulated Waveforms

The long, microwave-photonic variable delay of the impulse response of linear frequency-modulated (LFM) waveforms is proposed and demonstrated experimentally. The delay method is based on the application of a variable frequency offset to one side-band of an LFM-modulated optical carrier. Due to the time-frequency ambiguity properties of the LFM signal, the effect of the frequency offset on the impulse response is nearly equivalent to that of a group delay. Numerical simulations suggest negligible degradations in the resolution, peak-to-side-lobe ratio and integrated side-lobe ratio of the processed LFM waveform. Experiments demonstrate the delay of a 500-MHz-wide LFM signal by up to 250 ns. The method is applicable to LFM waveforms of arbitrary bandwidth and central radio-frequency. The long delays make the method attractive for optical beam-steering in large phased-array radars.

Continuously variable long microwave-photonic delay of arbitrary frequency-chirped signals

A generic method for the continuously variable, long microwave-photonic delay of the impulse response of arbitrarily chirped waveforms is proposed and demonstrated. Nonlinear-frequency-modulated waveforms of 500 MHz bandwidth are delayed by tens of nanoseconds. The principle relies on the specific phase-time relations of the waveforms, and is applicable to chirped pulses of arbitrary durations, central radio frequencies, and bandwidths. The approach is suitable for beam steering in large phased-array antennas.

Frequency-selective filtering and analysis of radio-over-fiber using stimulated Brillouin scattering

A method for tunable microwave-photonic and optical filtering of broadband waveforms is proposed and demonstrated. The principle relies on stimulated Brillouin scattering in standard, birefringent optical fibers, and the associated effect of polarization pulling. The method is employed in the implementation of tunable microwave-photonic band-pass filters with 45 dB selectivity, and in optical spectral analysis of sub-carrier-multiplexed data transmission with tones separation of only 10 MHz.

Brillouin optical spectrum analyzer monitoring of subcarrier-multiplexed fiber-optic signals.

Optical spectral analysis of closely spaced, subcarrier multiplexed fiber-optic transmission is performed, based on stimulated Brillouin scattering (SBS). The Brillouin gain window of a single, continuous-wave pump is scanned across the spectral extent of the signal under test. The polarization pulling effect associated with SBS is employed to improve the rejection ratio of the analysis by an order of magnitude. Ten tones, spaced by only 10 MHz and each carrying random-sequence on-off keying data, are clearly resolved. The measurement identifies the absence of a single subcarrier, directly in the optical domain. The results are applicable to the monitoring of optical orthogonal frequency domain multiplexing and radio over fiber transmission.

Low-Distortion Long Variable Delay of Linear Frequency Modulated Waveforms

The impulse response of 500-MHz-wide, linear frequency modulated (LFM) waveforms is delayed and advanced by 50 ns using microwave-photonic processing. The delays are obtained through the application of a variable frequency offset to a single optical sideband using an acousto-optic modulator. The peak-to-sidelobe ratio (PSLR) and integrated sidelobe ratio (ISLR) of the processed impulse response functions are higher than 18 and 20 dB, respectively, for the entire range of delay and advancement. The ISLR is 14 dB better than that of corresponding previous demonstrations. The results provide, for the first time, 100-ns microwave-photonic delay variations of practical radar signals with a high fidelity that complies with application standards. The method is applicable to LFM waveforms of arbitrary bandwidths and central radio frequencies.

Brillouin Optical Correlation Domain Analysis in Composite Material Beams

Structural health monitoring is a critical requirement in many composites. Numerous monitoring strategies rely on measurements of temperature or strain (or both), however these are often restricted to point-sensing or to the coverage of small areas. Spatially-continuous data can be obtained with optical fiber sensors. In this work, we report high-resolution distributed Brillouin sensing over standard fibers that are embedded in composite structures. A phase-coded, Brillouin optical correlation domain analysis (B-OCDA) protocol was employed, with spatial resolution of 2 cm and sensitivity of 1 °K or 20 micro-strain. A portable measurement setup was designed and assembled on the premises of a composite structures manufacturer. The setup was successfully utilized in several structural health monitoring scenarios: (a) monitoring the production and curing of a composite beam over 60 h; (b) estimating the stiffness and Young’s modulus of a composite beam; and (c) distributed strain measurements across the surfaces of a model wing of an unmanned aerial vehicle. The measurements are supported by the predictions of structural analysis calculations. The results illustrate the potential added values of high-resolution, distributed Brillouin sensing in the structural health monitoring of composites.

Microwave-photonic filters

Microwave-photonic filters (MPF) can overcome the limitations of electrically implemented counterparts. MPF can have a very broad bandwidth, they are tunable and even the filter shape can be changed by programming. Here we review our recent results in the implementation of MPF. The filters are based on a delay-and-sum architecture or on a shaping of the gain bandwidth of stimulated Brillouin scattering. All presented setups are tunable in their filter shape, central wavelength and bandwidth.

High-resolution Brillouin analysis in a carbon-fiber-composite unmanned aerial vehicle model wing

Standard optical fibers are successfully embedded within a model wing of an unmanned aerial vehicle, constructed of carbon fiber and epoxy, during its production. Time-gated Brillouin optical correlation domain analysis along the embedded optical fibers is performed with a spatial resolution of 4 cm. Tests were carried out using a portable measurement setup prototype. The results represent an important step towards applications of high-resolution Brillouin analysis outside the research laboratory.

Stablized fiber-optic delay of arbitrary waveforms with application in distributed sensors

Arbitrary, broadband radio-frequency (RF) waveforms are distributed over long fibers with stabilized group delay. Thermal drifts in delay are compensated based on chromatic dispersion. The input waveform and a control RF sine wave modulate two separate tunable laser diode sources, coupled into a dispersive delay fiber. The output phase of the delayed control tone is used to provide feedback for setting the wavelengths of both lasers in a closed-loop operation. The two waveforms are separated in the optical domain, and no restrictions are imposed on their RF spectra. The method is employed in the distribution of broadband, linear frequency modulated radar waveforms, and in a high-resolution Brillouin analysis sensor experiment. In the latter, the stable interrogation of a 2 cm-wide local hot-spot is demonstrated, in the presence of thermal delay drifts that are several times larger.