Vipul Rastogi

Long-period gratings in planar optical waveguides

We present a theoretical analysis of light propagation in a four-layer planar waveguide that consists of a long-period grating (LPG) having a period of the order of 100 microm. By means of the coupled-mode theory, we show that such a structure is capable of coupling light from the fundamental guided mode to the cladding modes at specific wavelengths (resonance wavelengths) and thus results in sharp rejection bands in the transmission spectrum of the waveguide. Our numerical results show that the resonance wavelengths as well as the transmission spectrum can be significantly changed with the waveguide and grating parameters. A waveguide-based LPG should provide a useful approach to the design of a wide range of integrated-optic devices, including wavelength-tunable filters, switches, and environmental sensors.

Widely tunable long-period gratings fabricated in polymer-clad ion-exchanged glass waveguides

Long-period-grating filters were fabricated in polymer-clad ion-exchanged BK7 glass waveguides. The transmission spectra of the filters exhibited strong polarization dependence. A contrast as high as 25 dB at the resonance wavelength was obtained. The temperature sensitivity of the filters was measured to be /spl sim/9.0 nm//spl deg/C, which allows potential wavelength tuning over the entire S+C+L band of /spl sim/180 nm with a temperature control over a range of /spl sim/20/spl deg/C.

Analysis of corrugated long-period gratings in slab waveguides and their polarization dependence

We analyze theoretically the light transmission characteristics of corrugated long-period gratings formed in slab waveguides. The transmission spectra of the gratings show distinct rejection bands at specific wavelengths, known as the resonance wavelengths. We investigate in detail the phase-matching curves of the gratings, which govern the relationship between the resonance wavelength and the grating period. Thanks to the flexibility in the choice of the waveguide parameters, the phase-matching curves of a long-period waveguide grating can be different characteristically from those of a long-period fiber grating (LPFG), which implies that the former can exhibit much richer characteristics than the latter. Unlike an LPFG, the transmission spectrum of a long-period waveguide grating is in general sensitive to the polarization of light. Nevertheless, a proper choice of the waveguide and grating parameters can result in a polarization-independent rejection band. Long-period waveguide gratings should find potential applications in a wide range of integrated-optic waveguide devices and sensors.

Propagation characteristics of a segmented cladding fiber

We propose a novel optical fiber design that consists of a uniform core and a segmented cladding formed by alternate regions of high and low refractive indices in the azimuthal direction. The structure is analyzed by use of the radial effective-index method, and the propagation characteristics of the structure are studied. The fiber has a highly dispersive cladding and shows characteristics similar to those of photonic-crystal fibers and holey fibers. The novel fiber offers the possibility of single-mode operation over a wide range of wavelengths with a large core diameter.

Temperature-compensated fiber-Bragg-grating-based magnetostrictive sensor for dc and ac currents

A compact and inexpensive optical fiber sensor for the measurement of dc and ac currents is demonstrated. The sensor consists of a fiber Bragg grating bonded on two joined pieces of metal alloys, one being Terfenol-D and the other MONEL 400. Because Terfenol-D has much stronger magnetostriction, a magnetic field applied along the two alloy bars stretches the two portions of the grating in different proportions and, thus, results in a split in the reflection spectrum of the grating, which can be measured with a simple technique using a single photodetector. On the other hand, because Terfenol-D and MONEL 400 have almost identical thermal expansion coefficients, a change in temperature does not cause the reflected spectrum of the grating to split and, therefore, does not affect the magnetic field measurement.

Analysis of segmented-cladding fiber by the radial-effective-index method

A segmented-cladding fiber consists of a uniform core of high refractive index and a cladding with regions of high and low refractive index alternating angularly. This type of fiber provides an effective approach for achieving widely extended single-mode operation with a large core size. We analyze the fiber in detail by the radial-effective-index method, which replaces the fiber with an effective circular fiber. The accuracy of the method is confirmed by comparison with results obtained from the finite-element method. By applying the transverse-matrix method to the effective fiber, the leakage losses of the first two modes of the fiber are calculated. These then form the basis for discussion of the single-mode operation of the fiber. The analysis elucidates not only the physics of the fiber, but also the dependence of the performance of the fiber on various fiber parameters. With illustrations, we demonstrate the possibility of designing an ultralarge-core, segmented-cladding fiber that is single moded over the entire S+C+L band. The fiber should be able to suppress nonlinear optical effects and therefore prove useful for broadband optical communication employing dense-wavelength-division multiplexing.

Design and analysis of a multilayer cladding large-mode-area optical fibre

We present a large-mode-area optical fibre design consisting of a multilayer cladding. The cladding is formed by alternate high and low index regions specially designed to strip-off higher-order modes, while causing a nominal loss to the fundamental mode. The fibre is analysed by the transfer matrix method and the leakage losses of the modes are calculated. A high differential leakage loss between the first two modes ensures single-mode operation. With the proposed design a fibre with 30 µm core diameter and 0.16 numerical aperture can exhibit single-mode operation at 1550 nm wavelength. Such a large-core fibre can suppress the nonlinear effects and should be useful for high power applications, including fibre lasers and optical communication systems employing DWDM.

Large-mode-area leaky optical fiber fabricated by MCVD

A large-mode-area single-mode optical fiber based on leaky-mode filtering was prepared by a modified chemical vapor deposition (MCVD) technique. The fiber has a leaky cladding that discriminates the fundamental mode from higher-order modes. A preliminary version has a 25 μm core diameter and 0.11 numerical aperture. A Gaussian-like mode with 22 μm mode field diameter was observed after 3 m propagation, in agreement with modeling.

Holey optical fiber with circularly distributed holes analyzed by the radial effective-index method.

We analyze a holey fiber that consists of a circular distribution of air holes by the radial effective-index method. By this method, we show that the holey fiber is a leaky structure and its extended single-mode operation is governed by the differential leakage loss between the first two modes of the fiber. The effects of the hole size and the hole separation on the leakage losses of the first two modes are calculated. The leakage loss of the fundamental mode of the fiber is found to be comparable to that of a conventional holey fiber that has a hexagonal distribution of air holes.

Co-axial dual-core resonant leaky fibre for optical amplifiers

We present a co-axial dual-core resonant leaky optical fibre design, in which the outer core is made highly leaky. A suitable choice of parameters can enable us to resonantly couple power from the inner core to the outer core. In a large-core fibre, such a resonant coupling can considerably increase the differential leakage loss between the fundamental and the higher order modes and can result in effective single-mode operation. In a small-core single-mode fibre, such a coupling can lead to sharp increase in the wavelength dependent leakage loss near the resonant wavelength and can be utilized for the suppression of amplified spontaneous emission and thereby gain equalization of an optical amplifier. We study the propagation characteristics of the fibre using the transfer matrix method and present an example of each, the large-mode-area design for high power amplifiers and the wavelength tunable leakage loss design for inherent gain equalization of optical amplifiers.