V. P. Kalosha

Toward the subdiffraction focusing limit of optical superresolution

A superresolving three-zone plate is applied to a Fresnel diffractive lens. It is shown that for radial incident polarization this combination produces a focal spot approaching superresolution allowed subdiffractive limit of 0.36lambda/NA for focusing. For media responsive to longitudinal field component only, our phase engineering scheme results in a focal spot size of 0.368lambda/NA. When used with a solid immersion lens, the scheme can generate the smallest focal spot available for passive optics.

Slow and fast light via SBS in optical fibers for short pulses and broadband pump.

Slow-light effect via stimulated Brillouin scattering (SBS) in single-mode optical fibers was considered for short probe pulses of nanosecond duration relevant to Gb/s data streams. Unlike recent estimations of delay versus pump based on steady-state small-signal approximation we have used numerical solution of three-wave equations describing SBS for a realistic fiber length. Both regimes of small signal and pump depletion (gain saturation) were considered. The physical origin of Stokes pulse distortion is revealed which is related to excitation of long-living acoustic field behind the pulse and prevents effective delay control by pump power increase at cw pumping. We have shown different slope of the gain-dependent delay for different pulse durations. Spectrally broadened pumping by multiple cw components, frequency-modulated pump and pulse train were studied for short pulses which allow to obtain large delay and suppress pulse distortion. In the pump-depletion regime of pumping by pulse train, both pulse delay and distortion decrease with increasing pump, and the pulse achieves advancement.

How to obtain high spectral resolution of SBS-based distributed sensing by using nanosecond pulses

The ultimate spectral and spatial resolutions of distributed sensing based on stimulated Brillouin scattering (SBS) in optical fibers is shown for several-nanosecond Stokes pulses. Precise measurements of the local Brillouin frequency, with a spectral resolution close to the natural linewidth and, simultaneously, the spatial resolution of the pulse length are provided by AC detection of the output pump in the case of a finite cw component (base) of the Stokes pulse. Simulation examples of SBS-based sensing for fibers containing sections with different Brillouin frequencies are presented, demonstrating the high resolution of the sensing.

Frequency-shifted light storage via stimulated Brillouin scattering in optical fibers

A method for storing optical data pulse sequences, frequency shifted with respect to the original data pulse frequency, is theoretically described and experimentally demonstrated. Data pulses are converted into long-living acoustic waves via stimulated Brillouin scattering in optical fiber by counterpropagating write pulses of one frequency, and later they are retrieved by read pulses at a different frequency giving rise to frequency-shifted stored pulses. The shifted frequency is governed by the phase-matching condition between the read pulse and the acoustic wave, which can be satisfied using birefringent fibers. The converted frequency is +/-52 GHz and is tuned by applying strain to the fiber with a slope coefficient of +/-1.8 MHz/micro epsilon, and conversion efficiency can be as high as 13% for the storage time of 8-25 ns.

Signal Processing Technique for Distributed Brillouin Sensing at Centimeter Spatial Resolution

A technique for the accurate processing of experimental data measured with the distributed Brillouin sensor at centimeter spatial resolution is proposed. It uses analytical solutions of the steady-state-coupled intensity equations for stimulated Brillouin scattering. This technique includes experimental parameters such as pulsewidth and extinction ratio, pulse (Stokes) and pump powers, and sensing fiber characteristics. This approach also accounts for the effects induced by the ac and dc parts of the pulse. It is capable of extracting strain components hidden in a distorted single-peak spectrum by the implementation of form factors to analyze the shape of the spectrum. This signal processing technique has been validated by experimental data obtained under controlled laboratory conditions. The good agreement between reconstructed and measured spectra reflects the Brillouin frequency shift and the strain affecting the fiber.

Third-order dispersion impact on mode-locking regimes of Yb-doped fiber laser with photonic bandgap fiber for dispersion compensation

Novel features in stretched-pulse and similariton mode-locked regimes of Yb-doped fiber laser with photonic bandgap fiber used for dispersion compensation are found by means of numerical simulations. We show that the mode-locked pulse may become shorter with increasing third-order dispersion. Analytical estimations explain observed behavior through resonant interaction of the main pulse with dispersive waves involving both resonant sidebands and zero-group-velocity dispersion waves. Switching between the stretched-pulse and the similariton regimes is also studied.

Influence of transient phonon relaxation on the Brillouin loss spectrum of nanosecond pulses

For pump-probe stimulated Brillouin scattering with a probe pulse of a few nanoseconds duration and with a finite DC level, the acoustic wave relaxation time varies with the pump power and the DC level. For a pump power of 1-6 mW, the acoustic wave relaxation changes from approximately 9 to 90 ns for polarization-maintaining fiber at a temperature of -40 degrees C for a 2 ns pulse width. When the pulse DC ratio of the probe varies from 10 to 20 dB, the acoustic relaxation time changes from 24 to 45 ns for single-mode fiber at 25 degrees C. This induces a power-increment spectral feature in the detected AC pump signal in the Brillouin loss spectrum of two temperature or strain sections, where both spectral components appeared at positions far from those related to the natural phonon relaxation time (approximately 10 ns) equivalent length. The theoretical calculations confirm the prolonged phonon relaxation.

Theoretical study of the effect of slow light on BOTDA spatial resolution.

Due to the resonant nature of Brillouin scattering, delay occurs while pulse is propagating in an optical fiber. This effect influences the location accuracy of distributed Brillouin sensors. The maximum delay in sensing fibers depends on length, position, pump and Stokes powers. Considering pump depletion, we have obtained integral solutions for the coupled amplitude equations under steady state conditions and then calculated the group delay. The results show that moderate pump depletion (which is the optimized sensor working range) mitigates significantly the delay, and the maximum delay induced at resonance is only a fraction of Brillouin Optical Time Domain (BOTDA) spatial resolution, which means that the use of pulse width to define the spatial resolution is valid when Brillouin slow light is considered. We have shown that uniform strain and temperature distribution in a fiber gives the maximum delay induced uncertainty.

Enhancement of stimulated Brillouin scattering of higher-order acoustic modes in single-mode optical fiber

Solving the elastic wave equation exactly for a GeO2-doped silica fiber with a steplike distribution of the longitudinal and shear velocities and density, we have obtained the dispersion, attenuation, and fields of the leaky acoustic modes supported by the fiber. We have developed a model for stimulated Brillouin scattering of these modes in a pump-probe configuration and provided their Brillouin gains and frequencies for an extended range of core sizes and GeO2 doping. Parameter ranges close to cutoff of the acoustic modes and pump depletion enhance the ratio of higher-order peaks to the main peak in the Brillouin spectrum and are suitable for simultaneous strain-temperature sensing.

Ultra-short pulse operation of all-optical fiber passively mode-locked ytterbium laser.

The generation regimes of an all-fiber passively mode-locked ytterbium laser with intra-cavity photonic crystal fiber have been studied with the aim to provide recipes for obtaining chirp-free sub-picosecond pulses directly from the cavity. Small-beam area photonic-crystal fiber is used for dispersion compensation of the intra-cavity normal dispersion of b-doped and single-mode fibers as well as for spectrum expanding due to enhanced nonlinearity. Regions of the gain and fiber parameters near the generation threshold were found in both cases of normal and anomalous net intra-cavity dispersion, which provide a stable generation of ultra-short sub-picosecond pulses directly from the cavity. Laser parameters of a transition to the multi-pulsed generation regimes were also found.