Andrey Kobyakov

Radio Over Fiber for Picocellular Network Architectures

Picocellular network designs at lower frequencies up to 6 GHz are compared to implementations at 60 GHz. System design aspects are discussed, including the implementation of MIMO. Very high bit rates of >10 Gb/s are achieved in mm-wave OFDM RoF systems.

Stimulated Brillouin scattering in optical fibers

We present a detailed overview of stimulated Brillouin scattering (SBS) in single-mode optical fibers. The review is divided into two parts. In the first part, we discuss the fundamentals of SBS. A particular emphasis is given to analytical calculation of the backreflected power and SBS threshold (SBST) in optical fibers with various index profiles. For this, we consider acousto-optic interaction in the guiding geometry and derive the modal overlap integral, which describes the dependence of the Brillouin gain on the refractive index profile of the optical fiber. We analyze Stokes backreflected power initiated by thermal phonons, compare values of the SBST calculated from different approximations, and discuss the SBST dependence on the fiber length. We also review an analytical approach to calculate the gain of Brillouin fiber amplifiers (BFAs) in the regime of pump depletion. In the high-gain regime, fiber loss is a nonnegligible effect and needs to be accounted for along with the pump depletion. We provide an accurate analytic expression for the BFA gain and show results of experimental validation. Finally, we review methods to suppress SBS including index-controlled acoustic guiding or segmented fiber links. The second part of the review deals with recent advances in fiber-optic applications where SBS is a relevant effect. In particular, we discuss the impact of SBS on the radio-over-fiber technology, enhancement of the SBS efficiency in Raman-pumped fibers, slow light due to SBS and SBS-based optical delay lines, Brillouin fiber-optic sensors, and SBS mitigation in high-power fiber lasers, as well as SBS in multimode and microstructured fibers. A detailed derivation of evolutional equations in the guided wave geometry as well as key physical relations are given in appendices.

Design concept for optical fibers with enhanced SBS threshold

We propose a criterion to predict the relative value of the stimulated Brillouin scattering (SBS) threshold in single-mode optical fibers with different refractive index profiles. We confirm our results by several representative measurements. We show that with the proper profile design one can achieve more than 3 dB increase in the SBS threshold compared to the standard single-mode optical fiber.

Plasmonic nano-structures for optical data storage

We propose a method of optical data storage that exploits the small dimensions of metallic nano-particles and/or nano-structures to achieve high storage densities. The resonant behavior of these particles (both individual and in small clusters) in the presence of ultraviolet, visible, and near-infrared light may be used to retrieve pre-recorded information by far-field spectroscopic optical detection. In plasmonic data storage, a femtosecond laser pulse is focused to a diffraction-limited spot over a small region of an optical disk containing metallic nano-structures. The digital information stored in each bit-cell modifies the spectrum of the femtosecond light pulse, which is subsequently detected in transmission (or reflection) using an optical spectrum analyzer. We present theoretical as well as preliminary experimental results that confirm the potential of plasmonic nano-structures for high-density optical storage applications.

Brillouin gain analysis for fibers with different refractive indices

We studied an anomalous result in the measured stimulated Brillouin scattering threshold for single-mode optical fibers with different refractive index profiles and effective areas. It was found that, in addition to the optical effective area, the refractive-index-dependent interaction between the optical and the acoustic fields must be considered in order to account for the differences in the stimulated Brillouin scattering threshold between fiber types. A detailed analytical formalism describing the acousto-optic interaction was derived to accommodate the influence of the index profile design on Brillouin scattering.

Stimulated Brillouin scattering in Raman-pumped fibers: a theoretical approach

We develop a theoretical approach to describe stimulated Brillouin scattering (SBS) in Raman-pumped optical fibers. We derive the condition for the SBS threshold as a function of fiber parameters and the input power of forward and/or backward pump. A particular emphasis is given to the effect of the fiber length and unequal absorption coefficients for pump and signal wavelengths on the SBS threshold. Simple, yet accurate, analytical expressions for the SBS threshold in pumped fibers are also obtained. We show that in pumped fibers, the SBS threshold power is inversely proportional to the path-average integral of Raman gain. Validity ranges of derived formulas are considered in detail. The theoretical predictions are verified experimentally. The calculated values of the SBS threshold powers are in a good agreement with the measured threshold power in Raman-pumped dispersion-compensated fibers.

Two-photon spectroscopy and analysis with a white-light continuum probe

We present a powerful experimental tool and analysis for characterization of two-photon absorption (2PA) spectra. We demonstrate this method with ZnS and then apply it to organic dyes in solution. We also compare the results with those from other methods such as two-photon fluorescence spectroscopy. This femtosecond pump-probe method uses a white-light continuum (WLC) as the probe to produce a nondegenerate 2PA spectrum. The extreme chirp of the WLC requires that transmittance data be collected over a range of temporal delays between pump and probe pulses. These data then need to be corrected for the effects of this chirp as well as for the temporal walk-off of the pulses in the sample that result from the frequency nondegenerate nature of the experiment. We present a simple analytic solution for the transmitted fluence through the sample, which is applicable for most practical cases.

Fiber design considerations for 40 Gb/s systems

In this paper, we review the fundamental advantages and drawbacks of 40-Gb/s systems from a fiber manufacturers perspective. Based on modeling, experimental results, and fundamental understanding, we correlate the fiber design parameters with the expected performance of long-haul systems operating at 40 Gb/s. Nonlinear penalties, dispersion tolerances, modulation formats, polarization-mode dispersion, and Raman amplification are covered. We also present the fiber features required for both metro and submarine networks at this specific data rate.

40 × 112 Gb/s transmission over an unrepeatered 365 km effective area-managed span comprised of ultra-low loss optical fibre

We experimentally demonstrate transmission of 40 × 112 Gb/s PM-QPSK channels over a 365 km unrepeatered span enabled by ultra-low loss fibres in an effective area-managed configuration using only backward-pumped Raman with 25 dB gain and EDFA amplification.

MIMO radio signals over fiber in picocells for increased WLAN coverage

We show that separating base station antennas in an optically-fed 2×3 MIMO WLAN system increases the overall coverage area in picocells. A peak throughput of 130 Mb/s was measured for antenna spacings of both 16 cm and 7 m.