Ravi K. Varshney

Design and Performance Study of a Compact SOI Polarization Rotator at 1.55 μm

We numerically design a compact silicon (Si) based polarization rotator (PR) by exploiting power coupling through phase matching between the TM mode of a Si strip waveguide (WG) and TE mode of a Si-air vertical slot WG. In such structures, the coupling occurs due to horizontal structural asymmetries and extremely high modal hybridness due to high refractive index contrast of Si-on-insulator (SOI) structure. Design parameters of the coupler have been optimized to achieve a compact PR of ~135 μm length at the telecommunication wavelength of 1.55 μm. Maximum power coupling efficiency Cm, which is studied by examining the transmittance of light, is achieved as high as 80% for both polarization conversions. Fabrication tolerances and the band width of operation of the designed PR have also been studied.

An efficient broad-band mid-wave IR fiber optic light source: design and performance simulation

Design of a mid-wave IR (MWIR) broad-band fiber-based light source exploiting degenerate four-wave mixing (D-FWM) in a meter long suitably designed highly nonlinear (NL) chalcogenide microstructured optical fiber (MOF) is reported. This superior FWM bandwidth (BW) was obtained through precise tailoring of the fibers dispersion profile so as to realize positive quartic dispersion at the pump wavelength. We consider an Erbium (Er(3+)) - doped continuous wave (CW) ZBLAN fiber laser emitting at 2.8 μm as the pump source with an average power of 5 W. Amplification factor as high as 25 dB is achievable in the 3 - 3.9 μm spectral range with average power conversion efficiency > 32%.

Design of an efficient mid-IR light source using chalcogenide holey fibers: a numerical study

We report the design of a highly nonlinear holey fiber for making a mid-infrared light source at 4.36 μm. A solid-core chalcogenide-based index-guided holey microstructured optical fiber with circular air holes has been exploited to numerically demonstrate wavelength translation via four-wave mixing. We employ a thulium-doped fiber laser as the pump with a power of 5 W. Our simulations indicate that a maximum parametric gain of 20.5 dB with a bandwidth of 16 nm is achievable in this designed fiber, resulting in a power conversion efficiency of more than 17.6%.

Design and realization of an all-fiber broadband tunable gain equalization filter for DWDM signals

Design and fabrication of a tunable gain equalization filter for dense wavelength division multiplexed (DWDM) signals through erbium doped fiber amplifiers (EDFA) is reported. It is based on a side-polished fiber (SPF) half-coupler block loaded with a displaceable tapered multimode overlay waveguide (MMOW). A simple and accurate normal mode analysis is employed to design this filtering device for its subsequent realization. Equalization of a typical EDFA gain spectrum in the C-band within +/-0.35 dB or even less in the presence of various ITU standard C-band DWDM signal channels is demonstrated under varied operating conditions like add/drop of signals. Tunability of the filter notch is achieved through displacement of the SPF relative to the MMOW.

Ultrafast laser inscribed waveguide lattice in glass for direct observation of transverse localization of light

We present initial results of the direct observation of the signature of localized light in an ultrafast laser-inscribed (ULI) disordered lattice that contains an array of evanescently coupled, one-dimensional optical waveguides in glass in which certain amount of disorder in refractive index was introduced. Numerical simulations were carried out to test the feasibility of the initial experimental design. Such configurable ULI disordered waveguide lattices should open up a platform for investigating the phenomenon of transverse localization of light and its statistical nature.

Design and fabrication of an intrinsically gain flattened Erbium doped fiber amplifier

We report design and subsequent fabrication of an intrinsically gain flattened Erbium doped fiber amplifier (EDFA) based on a highly asymmetrical and concentric dual-core fiber, inner core of which was only partially doped. Phase-resonant optical coupling between the two cores was so tailored through optimization of its refractive index profile parameters that the longer wavelengths within the C-band experience relatively higher amplification compared to the shorter wavelengths thereby reducing the difference in the well-known tilt in the gains between the shorter and longer wavelength regions. The fabricated EDFA exhibited a median gain ≥28 dB (gain excursion below ±2.2 dB within the C-band) when 16 simultaneous standard signal channels were launched by keeping the I/P level for each at ―20 dBm/ channel. Such EDFAs should be attractive for deployment in metro networks, where economics is a premium, because it would cut down the cost on gain flattening filter head.

Design of a Bragg fiber with large mode area for mid-infrared applications

The design of an all-solid, soft glass-based, large mode area Bragg fiber for effective single mode operation with mode effective area exceeding 1100 µm(2) across the wavelength range of 2-4 μm is reported. The design adopts a new strategy to induce large differential loss between the fundamental and higher order modes for effective single-mode operation within few tens of centimetres length of an otherwise multimode fiber. In addition to having the potential for the targeted application in high power laser delivery systems; complemented by a zero dispersion wavelength at 2.04 µm and rapidly developing mid-IR optical sources, the proposed fiber should also be attractive for generation of high power, single mode and less divergent supercontinuum light over this mid-IR window.

Specialty Fibers for Terahertz Generation and Transmission: A Review

Terahertz (THz) frequency range, lying between the optical and microwave frequency ranges covers a significant portion of the electro-magnetic spectrum. Though its initial usage started in the 1960s, active research in the THz field started only in the 1990s by researchers from both optics and microwaves disciplines. The use of optical fibers for THz application has attracted considerable attention in recent years. In this paper, we review the progress and current status of optical fiber-based techniques for THz generation and transmission. The first part of this review focuses on THz sources. After a review on various types of THz sources, we discuss how specialty optical fibers can be used for THz generation. The second part of this review focuses on the guided wave propagation of THz waves for their transmission. After discussing various wave guiding schemes, we consider new fiber designs for THz transmission.

Transverse localization of light and its dependence on the phase front curvature of the input beam in a disordered optical waveguide lattice

We investigate the influence of the phase front curvature of an input light beam on the transverse localization of light, choosing an evanescently coupled disordered one-dimensional semi-infinite waveguide lattice as an example. Our numerical study reveals that a finite phase front curvature of the input beam does indeed play an important role and it could degrade the quality of light localization in a disordered dielectric structure. More specifically, a faster transition from the ballistic mode of beam propagation due to diffraction to a characteristic localized state is observed in the case of a continuous wave (CW) beam whose phase front is plane, as compared to one having a curved phase front.

Calculation of propagation characteristics of surface plasmons in gold/silver nanowires

We obtain the propagation characteristics of the fundamental plasmon mode guided along Au and Ag nanowires using a simple and accurate method proposed recently by our group for solving the complex eigenvalue equations. Our results are found in excellent agreement with the ones obtained by the finite element method. Furthermore, results obtained using Johnson and Christy’s data [Phys. Rev. B6, 4370 (1972)PLRBAQ0556-2805] for Au and Ag are found in excellent agreement with the recently reported experimental results. On the other hand, Palik’s data [Handbook of Optical Constant of Solids (Academic, 1985)] are found to highly overestimate the losses in Ag nanowires. This study establishes a simple and accurate method for theoretical modeling of surface plasmons guided along Au and Ag nanowires for their applications in various plasmonic devices.