Abstract
A numerical model based on a scalar beam propagation method is applied to study light transmission in photonic bandgap (PBG) waveguides. The similarity between a cylindrical waveguide with concentric layers of different indices and an analogous planar waveguide is demonstrated by comparing their transmission spectra that are numerically shown to have coinciding wavelengths for their respective transmission maxima and minima. Furthermore, the numerical model indicates the existence of two regimes of light propagation depending on the wavelength. Bragg scattering off the multiple high-index/low-index layers of the cladding determines the transmission spectrum for long wavelengths. As the wavelength decreases, the spectral features are found to be almost independent of the pitch of the multi-layer Bragg mirror stack. An analytical model based on an antiresonant reflecting guidance mechanism is developed to accurately predict the location of the transmission minima and maxima observed in the simulations when the wavelength of the launched light is short. Mode computations also show that the optical field is concentrated mostly in the core and the surrounding first high-index layers in the short-wavelength regime while the field extends well into the outermost layers of the Bragg structure for longer wavelengths. A simple physical model of the reflectivity at the core/high-index layer interface is used to intuitively understand some aspects of the numerical results as the transmission spectrum transitions from the short- to the long-wavelength regime.
Highlights
Recent advances in microstructured optical fibers (MOFs) indicate their potential for several applications for fiber-based optical devices and telecommunications
The light guidance mechanism in the core allows these fibers to be divided into two categories: (1) air-silica MOFs [1,2,3,4] that rely on total internal reflection from the lower effective index of the cladding for light guidance in the high-index core, and (2) photonic bandgap (PBG) fibers [5,6] in which light propagates in the low-index core due to Bragg scattering off the higher effective index cladding
A cross-section of the PBG fiber is shown below the Bragg fiber in Fig. 1 (a), and it consisted of a solid silica core surrounded by ten hexagonal rings of circular air holes filled with a high-index material
Summary
Recent advances in microstructured optical fibers (MOFs) indicate their potential for several applications for fiber-based optical devices and telecommunications. PBG fibers are attractive as they can serve as an alternative to current transmission fibers for telecommunications with the crucial potential advantages of lower absorption losses and reduced nonlinearities, and potential for dispersion engineering due to light guidance in the air core and manipulation by the periodic cladding region [5,14] These fibers exhibit a spectral response that is approximately periodic with frequency, interpreted as higher order Bragg reflections from the periodic cladding. The ability to predict the spectral transmission of these fibers is of paramount importance as it provides useful information such as the location of bandgaps (for PBG waveguides) and is useful in the design of new fibers and fiber-based devices Both analytical and numerical approaches have previously been used to study some of the properties of MOFs such as dispersion, loss and modal attributes [7,8,9,10,11,12,14]. An analytical model based on an antiresonant reflecting guidance mechanism is used to explain the location of the transmission maxima and minima in the short-wavelength regime
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