Ashish Madhukar Vengsarkar

Bio: Ashish Madhukar Vengsarkar is an academic researcher from Alcatel-Lucent. The author has contributed to research in topics: Optical fiber & Fiber Bragg grating. The author has an hindex of 35, co-authored 132 publications receiving 7542 citations. Previous affiliations of Ashish Madhukar Vengsarkar include University of Arizona & AT&T.

Long-period fiber gratings as band-rejection filters

Abstract: We present a new class of long-period fiber gratings that can be used as in-fiber, low-loss, band-rejection filters. Photoinduced periodic structures written in the core of standard communication-grade fibers couple light from the fundamental guided mode to forward propagating cladding modes and act as spectrally selective loss elements with insertion losses act as backreflections <-80 dB, polarization-mode-dispersions <0.01 ps and polarization-dependent-losses <0.02 dB.

Optical fiber long-period grating sensors.

Abstract: We present a novel class of highly sensitive sensors based on long-period fiber gratings that can be implemented with simple and inexpensive demodulation schemes. Temperature, strain, and refractive-index resolutions of 0.65 °C, 65.75 μ∈, and 7.69 × 10−5, respectively, are demonstrated for gratings fabricated in standard telecommunication fibers.

Long-period fiber-grating-based gain equalizers.

Abstract: Long-period fiber gratings are used to flatten the gain spectrum of erbium-doped fiber amplifiers. A broadband amplifier with <0.2-dB gain variation over 30 nm is presented. We also show that a chain of amplifiers can be equalized, leading to a bandwidth enhancement by a factor of 3.

Long-period fibre grating fabrication with focused CO2 laser pulses

Abstract: A new, simple method of fabricating long-period fibre gratings by direct exposure of the fibre to focused 10.6 /spl mu/m wavelength CO/sub 2/ laser pulses is presented. No ultraviolet exposure is used. Hydrogen loading is found to enhance the writing sensitivity.

Article comprising a micro-structured optical fiber, and method of making such fiber

Abstract: Disclosed are non-periodic microstructured optical fibers that guide radiation by index guiding. By appropriate choice of core region and cladding region, the effective refractive index difference Δ between core region and cladding can be made large, typically greater than 5% or even 10 or 20%. Such high Δ results in small mode field diameter of the fundamental guided mode (typically<2.5 μm), and consequently in high radiation intensity in the core region. Exemplarily, a fiber according to the invention has a solid silica core region that is surrounded by an inner cladding region and an outer cladding region. The cladding regions have capillary voids extending in the axial fiber direction, with the voids in the outer cladding region having a larger diameter than those in the inner cladding region, such that the effective refractive index of the outer cladding region is greater than that of the inner cladding region. Non-periodic microstructured fiber potentially has many uses, e.g., as dispersion compensating fiber (with or without dispersion slope compensation), as amplifying fiber, as laser, as saturable absorber, for fiber gratings, and for non-linear elements. A method of making microstructured fiber is also disclosed.

Fiber grating sensors

Abstract: We review the recent developments in the area of optical fiber grating sensors, including quasi-distributed strain sensing using Bragg gratings, systems based on chirped gratings, intragrating sensing concepts, long period-based grating sensors, fiber grating laser-based systems, and interferometric sensor systems based on grating reflectors.

Fiber grating spectra

Abstract: In this paper, we describe the spectral characteristics that can be achieved in fiber reflection (Bragg) and transmission gratings. Both principles for understanding and tools for designing fiber gratings are emphasized. Examples are given to illustrate the wide variety of optical properties that are possible in fiber gratings. The types of gratings considered include uniform, apodized, chirped, discrete phase-shifted, and superstructure gratings; short-period and long-period gratings; symmetric and tilted gratings; and cladding-mode and radiation-mode coupling gratings.

Fiber Bragg grating technology fundamentals and overview

Abstract: The historical beginnings of photosensitivity and fiber Bragg grating (FBG) technology are recounted. The basic techniques for fiber grating fabrication, their characteristics, and the fundamental properties of fiber gratings are described. The many applications of fiber grating technology are tabulated, and some selected applications are briefly described.

Capacity Limits of Optical Fiber Networks

Abstract: We describe a method to estimate the capacity limit of fiber-optic communication systems (or ?fiber channels?) based on information theory. This paper is divided into two parts. Part 1 reviews fundamental concepts of digital communications and information theory. We treat digitization and modulation followed by information theory for channels both without and with memory. We provide explicit relationships between the commonly used signal-to-noise ratio and the optical signal-to-noise ratio. We further evaluate the performance of modulation constellations such as quadrature-amplitude modulation, combinations of amplitude-shift keying and phase-shift keying, exotic constellations, and concentric rings for an additive white Gaussian noise channel using coherent detection. Part 2 is devoted specifically to the "fiber channel.'' We review the physical phenomena present in transmission over optical fiber networks, including sources of noise, the need for optical filtering in optically-routed networks, and, most critically, the presence of fiber Kerr nonlinearity. We describe various transmission scenarios and impairment mitigation techniques, and define a fiber channel deemed to be the most relevant for communication over optically-routed networks. We proceed to evaluate a capacity limit estimate for this fiber channel using ring constellations. Several scenarios are considered, including uniform and optimized ring constellations, different fiber dispersion maps, and varying transmission distances. We further present evidences that point to the physical origin of the fiber capacity limitations and provide a comparison of recent record experiments with our capacity limit estimation.

Structural control: past, present, and future

Abstract: This tutorial/survey paper: (1) provides a concise point of departure for researchers and practitioners alike wishing to assess the current state of the art in the control and monitoring of civil engineering structures; and (2) provides a link between structural control and other fields of control theory, pointing out both differences and similarities, and points out where future research and application efforts are likely to prove fruitful. The paper consists of the following sections: section 1 is an introduction; section 2 deals with passive energy dissipation; section 3 deals with active control; section 4 deals with hybrid and semiactive control systems; section 5 discusses sensors for structural control; section 6 deals with smart material systems; section 7 deals with health monitoring and damage detection; and section 8 deals with research needs. An extensive list of references is provided in the references section.