Planar Bragg Waveguide Research Articles

Free-electron masers based on planar Bragg waveguides

A new scheme of the free-electron maser (FEM) with a ribbon electron beam is proposed in which the radiation is channeled in an open planar Bragg waveguide structure of two parallel plates with corrugated inner surfaces. The direction of corrugation is parallel to the radiation group velocity vector. The electron beam propagates in the waveguide at a large angle to its axis. The electron translation velocity is parallel to the direction of propagation of one of the partial waves that form the waveguide mode, which makes the maximum Doppler up-shift of the radiation frequency possible. A nonlinear analysis shows that the proposed FEM scheme provides a high gain (up to 30 dB). Using the Bragg waveguide, it is possible to ensure effective mode filtration, thus controlling the spatial structure of the output radiation.

Surface waves on the boundary of photonic crystals and tunnelling coupling between two photonic crystals via these waves

Based on coupled Nb2O5—SiO2 waveguides, one-dimensional photonic crystals are fabricated and surface waves at 0.63 μm are generated at their boundary with air. A planar Bragg waveguide with an air core is fabricated due a contact of two photonic crystals through an air gap. The propagation constants of the symmetric and antisymmetric modes of the Bragg waveguide are measured and the value of the air gap of the waveguide is estimated. It is shown that, to obtain the fundamental modes of the Bragg waveguide based on the used waveguides, it is necessary to shift the working wavelength to the red and to increase the thickness of the air gap of the Bragg waveguide.

Structure of the core—cladding interface and radiation losses in hollow planar Bragg waveguides

A planar Bragg waveguide with a hollow core is considered. Analytic dependences of the mode properties of the waveguide on its geometrical parameters are presented. By using these dependences, optimal parameters of the Bragg waveguide are determined. The dependence of the properties of the waveguide modes on the structure of the internal boundary of its cladding is considered for the first time. In addition, the dependence of the waveguide radiation losses on the number of its layers is considered for wavelengths 1.55, 10.6 and 0.245 μm of practical interest.

Fundamental losses in planar Bragg waveguides

This paper considers a planar Bragg waveguide. The guided modes and their dissipation due to the fundamental absorption are described. In the interacting-wave approximation, an analytical relation between the characteristics of the modes and parameters of the Bragg-waveguide geometry was established. Absorption losses in hollow Bragg waveguides were shown to be significantly lower than in a regular waveguide fabricated from the same material. Quantitative estimates of the fundamental and radiation losses in Bragg waveguides are given for materials used at wavelengths of 1.55, 10.6, and 0.245 µm.

Disappearance of modes in planar Bragg waveguides

We show how the forbidden bands of a Bragg reflector may shrink to points and how some classes of the guided modes of a planar Bragg waveguide may disappear altogether by shrinking with the forbidden bands. We derive the general conditions to determine the missing modes and explain these conditions with examples. It is possible, for example, to design a Bragg waveguide that rejects all antisymmetric modes and supports only symmetric modes for the TE polarization. We also highlight the effect of Brewster incidence on the interpretation of the missing modes for the TM polarization.

Open planar Bragg waveguides for mode selection in quantum and classical amplifiers

Open Bragg structures for radiation waveguiding with simultaneous effective mode selection (filtration) are studied. The basic theory of both free electron and quantum laser amplifiers is developed and the effective amplification is shown.

Propagation properties of planar Bragg waveguides studied by an analytical Bloch-mode method

We use an analytic Bloch-mode approach to investigate one-dimensional planar Bragg waveguides (PBWGs) with an air- or a glass-core layer sandwiched within two symmetric Bragg gratings formed by alternate arrays of air and glass layers. The calculation results show that there exist simultaneously two kinds of guided modes as the gap-guided modes resulting from the photonic band-gap effect and the index-guided modes formed by the total internal reflection effect. We calculate the electromagnetic (EM)-field distributions of these guided modes with different parallel wave vectors and find remarkably different EM-field profiles between the index-guided modes and the gap-guided modes. In order to explore the propagation properties of the PBWGs, we analyze the group velocity and the group-velocity dispersion of different types of guided modes. The results show that the guided mode properties are significantly influenced by the core material as well as the cladding structure. The analytic Bloch-mode approach can help to investigate and design the PBWGs in a simple, efficient, and accurate manner.

Design of weakly guiding Bragg fibres for chromatic dispersion shifting towards short wavelengths

The waveguiding in Bragg optical fibres is based on a radial photonic bandgap effect due to their periodic cladding. Even with a weak index contrast, these fibres can be used to strongly bind the electromagnetic field in the core. With an adequate index profile, the resulting field localization allows us to shift the zero-chromatic-dispersion wavelength of the unique mode below 1.3 µm, i.e. into a spectral range where nonlinear phenomena such as solitons or continuum generation become possible, in contrast to standard monomode fibres. A preliminary analysis, performed with the planar Bragg waveguide for simplicity, gives conditions to obtain a fast decrease of the field distribution in the periodic area. These conditions are then used to develop a simple method for determining an adequate geometry of weak-guiding fibres to achieve dispersion shifting. An approximate formula is given for index transition positions in the case of step index profiles. Examples of such profile determinations are presented. The possibility of reducing the zero-dispersion wavelength to 0.8 µm by means of a realistic index profile is proven. The validity of the method is confirmed by first measurements on an experimental Bragg fibre.