Avishay Eyal

Vector analysis of stimulated Brillouin scattering amplification in standard single-mode fibers.

The polarization properties of stimulated Brillouin scattering (SBS) amplification or attenuation in standard single-mode fibers are examined through vectorial analysis, simulation and experiment. Vector propagation equations for the signal wave, incorporating SBS and birefringence, are derived and analyzed in both the Jones and Stokes spaces. The analysis shows that in the undepleted pump regime, the fiber may be regarded as a polarization-dependent gain (or loss) medium, having two orthogonal input SOPs, and corresponding two orthogonal output SOPs, for the signal, which, respectively, provide the signal with maximum and minimum SBS amplification (or attenuation). Under high Brillouin gain conditions and excluding zero-probability cases, the output SOP of arbitrarily polarized input signals, would tend to converge towards that of maximum SBS gain. In the case of high SBS attenuation the output SOP of an arbitrarily polarized signal would approach the output SOP corresponding to minimum attenuation. It is found that for a wide range of practical pump powers ( < or = 100mW) and for sufficiently long fibers with typical SBS and birefringence parameters, the signal aligned for maximum SBS interaction will enter/emerge from the fiber with its electric field closely tracing the same ellipse in space as that of the pump at the corresponding side of the fiber, albeit with the opposite sense of rotation. The analytic predictions are experimentally demonstrated for both Stokes (amplification) and anti-Stokes (attenuation) signals.

Extended delay of broadband signals in stimulated Brillouin scattering slow light using synthesized pump chirp

Judicious chirping of a directly modulated pump laser is used to broaden the intrinsic linewidth of stimulated Brillouin scattering in an optical fiber. The modulation waveform is designed to obtain a spectrum with sharp edges, resulting in phase gradients stronger that those obtained for random pump modulation. The gain and phase frequency response of the slow light process are measured by a vector network analyzer, and the delays obtained for our tailored modulation are compared with the case of random direct modulation. For equal pump powers and gain bandwidths (FWHM), the tailored modulation waveform introduces 30-40% longer delays. Using this technique, pseudo random bit sequences of 5 Gb/s were successfully delayed by up to 120 ps (BER<10(-5)) and 80 ps (BER<10(-9)).

Stimulated Brillouin scattering slow light in optical fibers (Invited)

Stimulated Brillouin scattering (SBS) has become a favorable underlying mechanism in many demonstrations of all-optical variable delay in standard fibers, often referred to as slow and fast light. Over 100 journal papers and numerous conference sessions have been dedicated to SBS slow light since 2005. In this paper, recent research in this area is reviewed. Following a short introduction to the topic, several specific trends in contemporary work are highlighted: the optimization of the SBS pump spectrum for extended slow light delay and reduced pulse distortion; SBS slow light demonstrations in nonstandard, highly nonlinear fibers; applications of SBS slow light to the delay of analog waveforms; and the role of polarization. Finally, a brief concluding perspective is provided.

Gigahertz-Wide Optically Reconfigurable Filters Using Stimulated Brillouin Scattering

Flat-top sharp optical filters of gigahertz bandwidth are realized using stimulated Brillouin scattering (SBS). Pump chirp control of the SBS process enables versatile programming of the filter shape and bandwidth. The operating wavelength of the filters is tunable, and their frequency response is inherently aperiodic. Full widths at half maximum of 1.3-2.5 GHz are demonstrated, with a filtering selectivity up to 30 dB and an rms ripple of 0.5-1 dB. The filters are used to convert double sideband to single sideband (SSB) modulations for 1-GHz-wide linear-frequency-modulated (LFM) signals of arbitrary radio-frequency carrier. Such SSB modulation is highly instrumental for photonic implementations of true time delay, for example, in antenna beam-forming. The peak-side-lobe ratio of the processed LFM signal was -32 dB; its main lobe was broadened by only 4%. The integrated side-lobe ratio, which is limited by noise from spontaneous Brillouin scattering, was better than 21 dB, which is a sufficient value for most systems. The technique results in a large modulation index and does not lead to harmonic distortions.

Design of broad-band PMD compensation filters

We describe a new design approach for broad-band polarization-mode dispersion (PMD) compensation filters. An efficient algorithm for minimization of the maximum differential group delay within a given frequency band is described.

All-Optical Polarization Control Through Brillouin Amplification

Amplification through stimulated Brillouin scattering in optical fibers can totally modify the polarization properties of the light, enabling an all-optical polarization control and a direct estimate on the fiber birefringence.

Dynamic optical frequency domain reflectometry

We describe a dynamic Optical Frequency Domain Reflectometry (OFDR) system which enables real time, long range, acoustic sensing at high sampling rate. The system is based on a fast scanning laser and coherent detection scheme. Distributed sensing is obtained by probing the Rayleigh backscattered light. The system was tested by interrogation of a 10 km communication type single mode fiber and successfully detected localized impulse and sinusoidal excitations.

Photoacoustic Doppler measurement of flow using tone burst excitation.

In this paper a novel technique for flow measurement which is based on the photoacoustic (PA) Doppler effect is described. A significant feature of the proposed approach is that it can be implemented using tone burst optical excitation thus enabling simultaneous measurement of both velocity and position. The technique, which is based on external modulation and heterodyne detection, was experimentally demonstrated by measurement of the flow of a suspension of carbon particles in a silicon tube and successfully determined the particles mean velocity up to values of 130 mm/sec, which is about 10 times higher than previously reported PA Doppler set-ups. In the theoretical part a rigorous derivation of the PA response of a flowing medium is described and some important simplifying approximations are highlighted.

Dual-pump push-pull polarization control using stimulated Brillouin scattering

Stimulated Brillouin scattering (SBS) amplification of probe signals is highly polarization dependent. Maximum and minimum gain values are associated with a pair of orthogonal states of polarization (SOP), which are related to the pump SOP. Since the maximum gain is much higher than the minimum, the SOP of the output probe is pulled towards that of the maximum amplification. Polarization pulling is restricted, however, by pump depletion. In this work, a new method is proposed, analyzed and demonstrated for enhanced SBS polarization pulling, using two orthogonal pumps. Here, one pump amplifies one polarization component of the probe wave, and at the same time the other pump attenuates the corresponding orthogonal component, resulting in a push-pull effect. In the undepleted regime and for equal total power, the same degree of pulling is achieved as in the single pump case, but at a significantly less signal gain. Thus, the dual pump technique can provide high pulling efficiency for stronger input signals, deferring the onset of depletion.

Measurement of high-order polarization mode dispersion

We demonstrate a new method to measure high-order polarization mode dispersion (PMD) using the Jones matrix exponential expansion. High-order PMD is characterized by measuring a series of characteristic matrices, which are convenient quantities for analyzing PMD effects in the time-domain. An experimental method is developed to estimate the validity range of the exponential expansion.