Low-index Core Research Articles

An all-silicon, single-mode Bragg cladding rib waveguide

In this paper, we demonstrate a direct method of fabricating an all-silicon, single-mode Bragg cladding rib waveguide using proton beam irradiation and subsequent electrochemical etching. The Bragg waveguide consists of porous silicon layers with a low index core of 1.4 that is bounded by eight bilayers of alternating high and low refractive index of 1.4 and 2.4. Here, the ion irradiation acts to reduce the thickness of porous silicon formed, creating an optical barrier needed for lateral confinement. Single-mode guiding with losses as low as approximately 1 dB/cm were obtained for both TE and TM polarization over a broad range of wavelengths from 1525 nm to 1625 nm. Such an approach offers a method for monolithic integration of Bragg waveguides in silicon, without the need for multiple processes of depositing alternating materials.

Bragg reflection waveguide diode lasers

What we believe to be the first demonstration of an edge-emitting Bragg reflection waveguide laser is reported. The laser utilized InGaAs quantum wells emitting at 980 nm, with Al(x)Ga(1-x)As core and claddings. The lasing mode is centered in a low-index core with a width of 700 nm, hence providing a large mode volume with strong discrimination against any modes other than the fundamental photonic bandgap mode. Single-transverse mode operation is observed with thresholds as low as 157 A/cm(2). The propagation losses of the mode were measured for the first time and found to be 11.4 cm(-1).

Gain guiding in large-core Bragg fibers

We theoretically analyze gain guiding in large-core Bragg fibers, to be used for large-mode-area laser amplifiers with single-transverse-mode operation. The signal is gain-guided in a low-index core, whereas the pump is guided by the photonic bandgap of the Bragg cladding to achieve good confinement. The high-index layers in the Bragg cladding are half-wave thick at the signal wavelength in order to eliminate Bragg reflection, reducing the Bragg fiber effectively to a step-index fiber for gain guiding.

Efficient fiber Bragg gratings in 2D all-solid photonic bandgap fiber

Fiber Bragg Gratings with reflectivity up to 25 dB have been photo-written in the core of a 2D all-solid Photonic Bandgap Fiber without modification of the guiding properties of the fiber. This result is obtained by combining an appropriate glass composition for the high index inclusions constituting the micro-structured cladding and a photosensitive low index core. Couplings of the fundamental core guided mode with cladding modes are investigated and compared to theoretical predictions.

Color-changing and color-tunable photonic bandgap fiber textiles

We present the fabrication and use of plastic Photonic Band Gap Bragg fibers in photonic textiles for applications in interactive cloths, sensing fabrics, signage and art. In their cross section Bragg fibers feature periodic sequence of layers of two distinct plastics. Under ambient illumination the fibers appear colored due to optical interference in their microstructure. Importantly, no dyes or colorants are used in fabrication of such fibers, thus making the fibers resistant to color fading. Additionally, Bragg fibers guide light in the low refractive index core by photonic bandgap effect, while uniformly emitting a portion of guided color without the need of mechanical perturbations such as surface corrugation or microbending, thus making such fibers mechanically superior to the standard light emitting fibers. Intensity of side emission is controlled by varying the number of layers in a Bragg reflector. Under white light illumination, emitted color is very stable over time as it is defined by the fiber geometry rather than by spectral content of the light source. Moreover, Bragg fibers can be designed to reflect one color when side illuminated, and to emit another color while transmitting the light. By controlling the relative intensities of the ambient and guided light the overall fiber color can be varied, thus enabling passive color changing textiles. Additionally, by stretching a PBG Bragg fiber, its guided and reflected colors change proportionally to the amount of stretching, thus enabling visually interactive and sensing textiles responsive to the mechanical influence. Finally, we argue that plastic Bragg fibers offer economical solution demanded by textile applications.

Large Negative Dispersion in Dual-Core Photonic Crystal Fibers Based on Optional Mode Coupling

Chromatic dispersion of dual-concentric-core photonic crystal fibers (PCFs), which have an inner low-index core and an outer high-index core, is investigated based on a full vector finite-element method. Large negative dispersion can be obtained by introducing avoided crossing between modes of the two asymmetric cores. While increasing the negative dispersion, larger effective mode area is also possible by choosing a higher order mode in the outer core to couple with the fundamental inner core mode. During the analysis, a dual-core PCF with a dispersion peak value of -36 000 ps/(nmldrkm) and effective mode area of 34.8 mum2 is presented.

Liquid-core/liquid-cladding integrated silicon ARROW waveguides

The fabrication and characterization of a liquid-core/liquid-cladding integrated antiresonant reflecting optical waveguide (L2-ARROW) is presented. In this waveguide, the light is confined vertically by the ARROW mechanism, whereas the lateral confinement is obtained by using liquid-core/liquid-cladding (L2 waveguides) with different refractive indexes. This approach permits to realize L2 waveguides with very low refractive index core (n≈1.333) and represents a new solution to solve the difficulty to reduce the optical losses in 2D-ARROWs due to the TM polarization in lateral direction. The device has been fabricated with standard silicon technology. The results show that the optical properties can be tuned by changing the type and the flow velocity of the core and the cladding liquids.

Highly dispersive large mode area photonic bandgap fiber

An all-silica photonic bandgap fiber composed of a low-index core surrounded by alternating high- and low-index rings allows us to achieve a large mode area (500 microm(2)) and large chromatic dispersion. Sharp resonances from the even Bragg mode to odd ring modes theoretically lead to 20,000 ps/(nm km) chromatic dispersion when large bends are applied. By nature, sharp resonances are sensitive to inhomogeneities along the fiber length. Under experimental conditions, the resonances are broadened and the dispersion coefficient is decreased to 1000 ps/(nm km). However, to the best of our knowledge, this is the largest dispersion coefficient reported using a large mode area fiber.

Ultra-wide bandpass filter based on long-period fiber gratings and the evanescent field coupling between two fibers

We demonstrate a fiber-based bandpass filter with an ultra-wide spectral bandwidth. The ultra-wide band feature is achieved by inscribing a long-period fiber grating (LPG) in a specially-designed low index core single mode fiber. To get the bandpass function, the evanescent field coupling between two attached fibers is utilized. By applying strain, the spectral shape of the pass-band is adjusted to flat-top and Gaussian shapes. For the flat-top case, the bandwidth is obtained ~ 160 nm with an insertion loss of ~ 2 dB. With strain, the spectral shape is switched into a Gaussian one, which has ~ 120 nm FWHM and ~ 4.18 dB insertion loss at the peak.

Wide-band optical field concentrator for low-index core propagation

We propose a novel chirped structure consisting of a low index polymer core bounded by modulated multilayer claddings, to realize an optical field concentrator with virtually zero propagation losses in a wide spectral range, independent of wave polarization. In spite of the absence of the total internal reflection mechanism, properly designed multilayer claddings ensure the achievement of unitary transmittance in a wide spectral range, including the widely used wavelengths for optical communications. Several cladding geometries obtained by varying the thicknesses of the cladding layers are reported and discussed.

Sharp Bending of On-Chip Silicon Bragg Cladding Waveguide With Light Guiding in Low Index Core Materials

A novel on-chip Bragg cladding waveguide is designed and fabricated using conventional CMOS techniques. This optical waveguide has a low refractive index core surrounded by high index-contrast cladding bilayers. Polysilicon (n=3.5) and silicon nitride (n=2.0) are used for high index-contrast Bragg layers, where index difference is as high as 1.5. Our simulation shows that sharp bending in low index core materials can be achieved, which is not possible using index guiding mechanism. Within our approach, various on-chip applications are expected such as optical integration, high power transmission, biosensor/microelectromechanical system and so on

Guiding light in optically induced ring lattices with a low-refractive-index core

We demonstrate a ring-shaped Bessel-like photonic lattice akin to a photonic bandgap fiber with a low-index core. While the ring lattice is optically induced in a bulk crystal with a self-defocusing nonlinearity, guidance of a probe beam propagating linearly through the core is clearly observed. The possible mechanism for such guidance is also discussed.

Band-Gap Guidance in Optically Induced Photonic Lattices with a Negative Defect

We report the first experimental demonstration of band-gap guidance of light in an optically induced two-dimensional photonic lattice with a single-site negative defect (akin to a low-index core in photonic-crystal fibers). We discuss the difference between spatial guidance at a regular and a defect site, and show that the guided beam through the defect displays fine structures such as vortex cells that arise from defect modes excited at higher band gaps. Defect modes at different wavelengths are also observed.

Plasmon excitation by the Gaussian-like core mode of a photonic crystal waveguide

We describe resonant excitation of a plasmon by the Gaussian-like leaky mode of an effectively single mode photonic crystal (PC) waveguide. Plasmon is phase matched by design with a waveguide mode, and travels in a metallic layer on the top of a PC waveguide. We observe that small changes in the ambient refractive index just outside the metal film lead to strong variations in the losses of a waveguide core mode, making such a device a good candidate for compact sensor applications. Using low refractive index core PC waveguides makes phase matching between the plasmon and PC core mode relatively easy to enforce as modal effective index in such waveguides can be readily varied in a wide range.

Efficient antiguiding of TE and TM polarizations in low-index core waveguides without the need for an omnidirectional reflector

While an omnidirectional bandgap is necessary to reflect arbitrary polarized light with a planar periodic reflector at any angle of radiation incidence, we demonstrate that omnidirectionality is not at all necessary for efficient antiguiding of modes of any polarization in low-index core photonic bandgap (PBG) fibers and waveguides. For a given radiation decay rate into the reflector and multilayer bandgap size we characterize the phase space of fiber materials leading to the same effective guiding conditions. We demonstrate that low- and high-index-contrast PBG waveguides can antiguide problematic TM-like modes equally effectively. Finally, the possibility of TE-like guidance of a TM-polarized mode is described.

Design guidelines and characteristics of beam-shaping microstructure optical fibers

Microstructure optical fibers with flat-top fundamental mode are first proposed by introducing a low-index inner core into the core of index-guiding microstructure optical fibers. The design guidelines and characteristics of beam-shaping microstructure optical fibers are demonstrated. The interrelationships of inner-core index with laser wavelength, air hole diameter and size of inner core are investigated. The influence of the relative size of inner core on the spatial profile of the fundamental mode is demonstrated. Moreover, sensitivity of the flat-top fundamental mode profile from the slight change of the optimum inner-core index value is studied. Starting from these results we deduce that it is possible to fabricate beam-shaping microstructure fibers with nowadays technique.

Hollow-core waveguides and 2-D waveguide arrays for integrated optics of gases and liquids

We present integrated antiresonant reflecting optical (ARROW) structures with hollow cores as a new paradigm for integrated optics with gases and liquids. ARROW waveguides with micrometer-sized hollow cores allow for single-mode propagation in low-index nonsolid core materials where conventional index guiding is impossible. Fabrication methods and considerations are described, and measurements of the core-size dependence of the waveguide loss are presented. We analyze the dependence of waveguide loss in hollow-core ARROW waveguides on polarization, shape, and wavelength. We propose two-dimensional hybrid device geometries using solid and nonsolid core ARROWs and show that they can form the basis of integrated fluorescence and Raman sensors. We derive the design principles for simultaneous low-loss propagation in both waveguides and efficient cross-coupling between waveguides.

Mode profiles and dispersion properties of silica/air photonic bandgap fibers

With the advent of microstructured optical fibers, the possibility of guiding light in a low-index core by the photonic bandgap effect has opened up. In this paper, we review the types and properties of photonic bandgap (PBG) fibers realised or suggested in the literature. Particular attention is given to a description of the dispersion properties and their relation to the evolution of the field distribution. Since PBG fibers trap light in a low-index core region over finite frequency intervals, the dispersion properties of these fibers turn out to be quite distinct from those of conventional or microstructured fibers with high-index cores.

All-solid photonic bandgap fiber

We describe the design and fabrication of a photonic bandgap fiber formed with two different glasses. As in a hollow-core fiber, light is guided in a low-index core region because of the antiresonances of the high-index strands in the fiber cladding. The structure described represents an ideal bandgap fiber that exhibits no interface modes and guides over the full width of multiple bandgaps.

Application of an ARROW model for designing tunable photonic devices

Microstructured optical fibers with the low refractive index core surrounded by high refractive index cylindrical inclusions reveal several intriguing properties. Firstly, there is a guiding regime in which the fibers' confinement loss is strongly dependent of wavelength. In this regime, the positions of loss maxima are largely determined by the individual properties of high index inclusions rather than their position and number. Secondly, the spectra of these fibers can be tuned by changing the refractive index of the inclusions. In this paper we review transmission properties of these fibers and discuss their potential applications for designing tunable photonic devices.