Effects of Bragg periods per grating period on performance of Bragg concave diffraction grating

Abstract

Concave diffraction gratings (CDGs) have the advantages of being compact, time reliability, cost effective, and channel spacing accuracy. These devices can be used in the wavelength division multiplexing (WDM) systems and micro-spectrometer devices. However, comparing with arrayed waveguides gratings (AWGs), the development of traditional CDGs is far from satisfactory. Because the traditional CDGs need deeply etched facets and perfect grating profiles to reduce the insertion losses, which will increase the difficulty in etching process. In order to solve this problem, Bragg reflectors based CDGs (Bragg-CDGs) are proposed. This structure can greatly reduce the insertion loss, and reduce the difficulty in etching process. The performance of the Bragg-CDG is determined by both the reflection condition of the Bragg reflectors and the diffraction condition of the CDG. With the Bragg reflection condition determined, the diffraction condition of Bragg-CDG will have a major influence on the performance of device. For successive strips based Bragg-CDG, the number of Bragg periods per diffraction grating period is an important parameter of Bragg-CDG. The diffraction condition of concave gratings is closely related to this parameter. This parameter has an effect on the performance of Bragg-CDG, specially termed resolution, the free spectrum range, and the diffraction efficiency. The effect of the number of Bragg periods per diffraction grating period on the Bragg diffraction grating is studied by theoretical analysis. In addition, four Bragg-CDGs with different numbers of Bragg periods are studied using the finite-difference time domain method. The results show that with sizes of diffraction gratings fixed, the resolution of Bragg-CDG does not have a significant improvement by changing the number of Bragg periods per diffraction grating period; the total number of diffraction orders is proportional to the number of Bragg periods per diffraction grating period. The Bragg-CDG with a single Bragg period per grating period has a maximum diffraction efficiency, since it has the minimal number of diffraction orders; in addition, with the increase of the number of Bragg periods per diffraction grating period, the free spectrum range of the main diffraction order gradually decreases. This research can contribute to the development of the demultiplexer with the low insertion loss, the high resolution, and the wide operating waveband.