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Ultra-broadband, compact, and high-reflectivity circular Bragg grating mirror based on 220 nm silicon-on-insulator platform

Yang Wang, Shitao Gao, Ke Wang, Hongtao Li, and Efstratios Skafidas

Open Access Open Access

Abstract

A compact (4.49 μm × 4.54 μm) and ultra-broadband circular Bragg grating mirror with relaxed fabrication requirements is proposed and demonstrated based on the 220 nm silicon-on-insulator (SOI) platform. Based on FDTD-simulations, the proposed grating mirror can achieve a reflectivity of >90% over a ultrabroad bandwidth of 500 nm (1263 - 1763 nm), and a high reflectivity of >95% over a broad bandwidth of 397 nm (1340 - 1737 nm), which covers the entire E- to U-bands. The circular grating is fabricated, and the experimental measurement results exhibit a high reflectivity of 93% - 98% within the measured band of 1530 to 1610 nm, which agrees well with simulations. Based on the proposed broadband and high-efficiency circular Bragg mirror, a compact notch filter with high rejection ratio (>10 dB) and low transmission loss (<0.5 dB) is also fabricated and presented, and the proposed filter could find various potential applications in optical communications and sensing applications. With its ultrabroad bandwidth, high reflectivity and compact size, the proposed circular Bragg mirror is expected to be a promising element for large-scale photonic integrated circuits and applications which require ultra-broadband and high-efficiency on-chip reflections.

© 2017 Optical Society of America

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Figures (5)
Fig. 1
Fig. 1 (a): The lateral schematic of the circular Bragg grating proposed in [7]. (b): The lateral schematic of the new circular Bragg grating design proposed in this paper (size scaled).

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Fig. 2
Fig. 2 The field distribution (1550 nm) within the 2 µm long taper simulated with angles (θ) of (a) 270°, (b) 180°, (c) 60°, and (d) 20°. Note that the above field distributions were captured to illustrate the wave diffraction from strip waveguide to taper, and no grating blades were set in the simulation. The reflection observed was due to the refractive index contrast between taper and cladding, rather than Bragg reflection. The red circles in (a) and (b) indicate the branches formed in the 270° taper and the index-contrast interface in the 180° taper, respectively. The dashed line in (b) shows the 60° boundary. The tapers were all covered by 2 µm thick SiO2.

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Fig. 3
Fig. 3 (a): The grating reflection spectra simulated with different sets of W1, Wt, Wb, and the reflection spectrum of the optimized grating when TM-polarized light is launched. (b - e): The vertical field intensity distributions of the proposed grating captured at y = 0 plane, based on different input wavelengths (1.3 µm and 1.75 µm). The width of the first grating trench (W1) is adjusted while the widths of all other grating trenches and blades are fixed at 181 nm.

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Fig. 4
Fig. 4 (a): The grating spectra simulated with different W1 values. (b): The change of the grating reflectivity (1550 nm) and ∆λ with dc. (c): The grating spectra simulated with different angles (θ). (d): The change of the reflectivity (1550 nm) and ∆λ with the taper length (L) for gratings with a 20° and 60° angle. (e): The change of the grating reflectivity (1550 nm) and ∆λ with the number of blade (N). (f): The grating spectra simulated with 6, 8, and 10 periods.

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Fig. 5
Fig. 5 (a): The schematic of the measurement setup and the SEM images of the fabricated grating mirror (right), and a notch filter (left) with the following parameters: Lw = 4 μm, Lc = 0 μm, rb = 5 μm. (b): The measured spectra of the circular grating mirror, reference waveguide, and chip-facet reflections. (c): The measured IL of the circular grating mirror. (d): The transmission spectra of the fabricated notch filters with different cavity and coupling lengths.

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Equations (2)

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n t W t + n b W b = λ 0 2
filte r transmission (dB)= filte r output power -W G output power
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