Compact and broadband design of an 850&#x00A0;nm 2 &#x00D7; 2 3-dB directional coupler with a shallowly etched SWG gap

Yue Dong; Hao Shen; Yin Xu; Bo Zhang

doi:10.1364/AO.471114

Applied Optics
Vol. 61,
Issue 31,
pp. 9154-9162
(2022)
•https://doi.org/10.1364/AO.471114

Compact and broadband design of an 850 nm 2 × 2 3-dB directional coupler with a shallowly etched SWG gap

Yue Dong, Hao Shen, Yin Xu, and Bo Zhang

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Yue Dong, Hao Shen, Yin Xu, and Bo Zhang, "Compact and broadband design of an 850 nm 2 × 2 3-dB directional coupler with a shallowly etched SWG gap," Appl. Opt. 61, 9154-9162 (2022)

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- Integrated Optics
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About this Article
History
- Original Manuscript: July 20, 2022
- Revised Manuscript: September 20, 2022
- Manuscript Accepted: October 4, 2022
- Published: October 21, 2022

Abstract

A compact and broadband ${{2}} \times {{2}}$ 3 dB directional coupler (DC) is designed at the 850 nm wavelength region based on the silicon nitride platform. The proposed DC is equipped with a shallowly etched subwavelength gratings (SWG) gap so that the length of the coupling region is effectively reduced to 7.8 µm for equally splitting the fundamental TE polarization state. Such a DC coupling region is much more compact than that using empty and pure SWG gaps. Meanwhile, the DC working bandwidth is determined to be broader than 106 nm, and the insertion loss is always kept under an acceptable level of 0.6 dB over the entire wavelength region. Considering that the shallowly etched device requires two photoetching processes in manufacturing, we did a numerical analysis to characterize the fabrication tolerance under a minimum overlay accuracy of 20 nm in both $x$ and $y$ directions. Such overlay mismatch only increases the maximal imbalance from 0.55 to 0.6 dB at the center wavelength. In addition, we discussed the impact of the proposed DC in its potential application, i.e., optical coherence tomography. The results show that the proposed DC is able to support a high axial resolution of 3.77 µm, and the 20 nm overlay mismatch leads to neglected axial resolution degradation.

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Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.

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Duty Cycle ( $a / Λ$ )	0.33	0.36	0.39	0.42	0.45	0.48	0.51
Smallest BLV^a	0.1887	0.1168	0.0978	0.0686	0.0599	0.0593	0.0575
Period (nm)	221	218	218	215	215	215	212

Principles	Platform	Center Wavelength (nm)	Bandwidth (nm)	IL (dB)	Coupling Region (µm)
SWG-assisted tapered coupler [9]	SOI	1550 nm	130 nm	0.5 dB	50 µm
Tapered adiabatic coupler [10]	SOI	1550 nm	$> 100 n m$	0.3 dB	200 µm
Tapered adiabatic coupler [11]	SOI	1550 nm	$> 100 n m$	0.3 dB	60 µm
Tapered adiabatic coupler [18]	SOI	1550 nm	35 nm	0.5 dB	30 µm
Angled MMI [12]	SOI	1550 nm	250 nm	1.2 dB	75 µm
MMI [13]	SOI	1550 nm	50 nm^a	0.6 dB	$< 300 µ m$
SWG-assisted DC [28]	SOI	1550 nm	100 nm	∼	19.2 µm
SWG-assisted DC [30]	SOI	1550 nm	100 nm	0.8 dB	14 µm
SWG-assisted DC [29]	$S i_{3} N_{4}$ -on-insulator	850 nm	110 nm	0.6 dB	12.6 µm
This work	$S i_{3} N_{4}$ -on insulator	850 nm	106 nm	0.6 dB	7.8 µm

Duty Cycle ( $a / Λ$ )	0.33	0.36	0.39	0.42	0.45	0.48	0.51
Smallest BLV^a	0.1887	0.1168	0.0978	0.0686	0.0599	0.0593	0.0575
Period (nm)	221	218	218	215	215	215	212

Principles	Platform	Center Wavelength (nm)	Bandwidth (nm)	IL (dB)	Coupling Region (µm)
SWG-assisted tapered coupler [9]	SOI	1550 nm	130 nm	0.5 dB	50 µm
Tapered adiabatic coupler [10]	SOI	1550 nm	$> 100 n m$	0.3 dB	200 µm
Tapered adiabatic coupler [11]	SOI	1550 nm	$> 100 n m$	0.3 dB	60 µm
Tapered adiabatic coupler [18]	SOI	1550 nm	35 nm	0.5 dB	30 µm
Angled MMI [12]	SOI	1550 nm	250 nm	1.2 dB	75 µm
MMI [13]	SOI	1550 nm	50 nm^a	0.6 dB	$< 300 µ m$
SWG-assisted DC [28]	SOI	1550 nm	100 nm	∼	19.2 µm
SWG-assisted DC [30]	SOI	1550 nm	100 nm	0.8 dB	14 µm
SWG-assisted DC [29]	$S i_{3} N_{4}$ -on-insulator	850 nm	110 nm	0.6 dB	12.6 µm
This work	$S i_{3} N_{4}$ -on insulator	850 nm	106 nm	0.6 dB	7.8 µm

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Applied Optics