Abstract
A novel method is presented for determining the group index, intensity enhancement and delay times for waveguide gratings, based on (Rayleigh) scattering observations. This far-field scattering microscopy (FScM) method is compared with the phase shift method and a method that uses the transmission spectrum to quantify the slow wave properties. We find a minimum group velocity of 0.04c and a maximum intensity enhancement of ~14.5 for a 1000-period grating and a maximum group delay of ~80 ps for a 2000-period grating. Furthermore, we show that the FScM method can be used for both displaying the intensity distribution of the Bloch resonances and for investigating out of plane losses. Finally, an application is discussed for the slow-wave grating as index sensor able to detect a minimum cladding index change of 10(-8), assuming a transmission detection limit of 10(-4).
Highlights
Bragg waveguide gratings can be used in many integrated optical architectures to achieve a broad range of functionalities
The presence of the fringes in the transmission spectrum can be explained by longitudinal resonances which occur due to interference of the Bloch harmonics in the structure [6, 11, 12], with each maximum corresponding to a specific intensity distribution along the waveguide grating (WGG)
The group velocity of a propagating mode in the WGG can be determined by calculating dω/dφ, with φ defined as the total phase kL, with L the device length which equals the number of periods N times the period Λ
Summary
Bragg waveguide gratings can be used in many integrated optical architectures to achieve a broad range of functionalities. The limited length causes characteristic fringes in the transmission and reflection spectra of WGGs, occurring near the band edge [12, 13] due to Fabry-Perot resonances of the grating Bloch modes [6]. Several measurement and analyzing methods have been proposed and applied for determining the group velocity or the group delay of a slow light wave in micrometer-sized devices. In this subsection we will present our main experimental results, demonstrating Q factors up to 105, and showing how the “scatterintensity” approach can be applied to slow-wave structures for estimating the corresponding group velocity.
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