Ultracompact silicon-based polarization splitter and rotator based on asymmetric directional couplers with subwavelength gratings

Jingyuan Chen; Jinbiao Xiao

doi:10.1364/JOSAB.447359

Journal of the Optical Society of America B
Vol. 39,
Issue 1,
pp. 345-354
(2022)
•https://doi.org/10.1364/JOSAB.447359

Ultracompact silicon-based polarization splitter and rotator based on asymmetric directional couplers with subwavelength gratings

Jingyuan Chen and Jinbiao Xiao

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Jingyuan Chen and Jinbiao Xiao, "Ultracompact silicon-based polarization splitter and rotator based on asymmetric directional couplers with subwavelength gratings," J. Opt. Soc. Am. B 39, 345-354 (2022)

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- Wave Guiding and Confinement
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About this Article
History
- Original Manuscript: November 1, 2021
- Revised Manuscript: November 30, 2021
- Manuscript Accepted: December 6, 2021
- Published: December 21, 2021

Abstract

An ultrasmall polarization splitter and rotator (PSR) on a silicon-on-insulator (SOI) platform is proposed based on an asymmetric directional coupler (ADC) consisting of a bridged subwavelength grating (BSWG) and a strip silicon waveguide with another SWG embedded in the middle slot. The inputting TE mode is tightly confined in BSWG and directly passes through a bar port with little coupling to the adjacent waveguide because of the phase mismatch, while the phase-matching condition allows the injected TM mode to be evanescently decoupled from BSWG and simultaneously converted into the TE mode supported by a SWG slot waveguide before propagating out through the cross port. Both polarization-dependent behaviors within the proposed PSR can be readily realized and efficiently manipulated by independently controlling the attributes of the ADC waveguides, which are distinctly engineered by different SWG segments. We have verified what we believe, to the best of our knowledge, is the first combination of BSWG and a SWG slot waveguide that provides an additional degree of freedom in the design and a stronger modal hybridization due to the doubled effects from jointly used SWGs. Further, the increasing number of inserted SWGs in the present device drastically reduces the device footprint, leading to a coupling length of only 5.5 µm. From the results, the conversion loss of $-0.28\;{\rm dB}$ (polarization conversion ratio of 93.7%) has been obtained for a TM-to-TE conversion at 1.55 µm. The TM mode of the resulting PSR shows its cross talk is less than $-14.3\;{\rm dB}$ and the conversion loss is better than $-0.63\;{\rm dB}$ in a 95 nm bandwidth that has covered the entire C and L bands. The cross talk and insertion loss of the TE mode are lower than $-29\;{\rm dB}$ and $-0.42\;{\rm dB}$, respectively, as the operating wavelength varies.

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Data Availability

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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Structures	Lc (µm)	ER (dB)	PCR (%)	IL (dB)	Bandwidth
MMI coupler [1]	1073	20	—	1	35–40 nm for $E R > 12 d B$
SWG-assisted Si waveguide [2]	5.4	$\sim 41$	99.95	1	53 nm for $E R > 25 d B$
Tapered waveguide [3]	$\sim 10$	31.5	—	—	—
Tapered DC [4]	80	37.15	$\sim 98$	0.28	160 nm for $X T < - 30 d B$
Tapered waveguide+MMI [5]	180	20	95	0.6	100 nm for $X T < - 12 d B$
Mode evolution+Y-junction [6]	95	30	99	0.4	400 nm for $E R > 12 d B$
Cascaded MMI couplers [7]	143.8	20.7	99	0.29	35 nm for $X T < 17 d B$
DC + Si3N4 layer [8]	300	29	—	—	300 nm for $X T < - 25 d B$
Parallel strip waveguides [9]	36.8	15.42	97.2	0.6	35 nm for $P C R > 94 %$
Adiabatic taper+ADC [10]	100	16.8	97.94	0.22	70 nm for $E R > 10 %$
Partially etched SWG coupler [11]	8.2	34.7	98.52	0.12	104 nm for $X T < - 20 d B$
Hybrid plasmonic-dielectric DC [12]	7.7	50.9	99.4	1.545	70 nm for $P C R > 95 %$
SWG coupler [39]	6.8	32.1	-	0.36	$\sim 81 n m$ for $E R > 18 d B$
PSR based on SWG [40]	25	10	97.5	0.13	35 nm for $P C R > 91.2 %$
SWG assisted coupler [41]	24.6	13.84	93	0.315	50 nm for $P C R > 80 %$
Tilted SWG coupler on LNOI platform [42]	160	20	—	1	85 nm for $E R > 20 d B$
MMI based on tilted SWG [43]	7.25	20	—	1	200 nm for $E R > 20 d B$
MMI based on angled SWG [44]	92.4	40	—	1	120 nm for $E R > 13 d B$
This work	5.5	30	93.7	0.16	167 nm for $P C R > 80 %$

$W_{A}$ (nm)	$W_{B}$ (nm)	$n_{A} / η_{1}$	$n_{B} / η_{2}$	PCL (dB)	$L_{C}$ (µm)
350	390	2.8/0.61	2.3/0.38	−0.38	4.5
350	400	2.966/0.7	2.3/0.38	−0.74	4.5
370	400	2.8/0.61	2.3/0.38	−0.26	5.5
380	400	2.8/0.61	2.4/0.43	−0.51	5.5
300	400	2.8/0.61	2.1/0.3	−0.63	5.5

Structures	Lc (µm)	ER (dB)	PCR (%)	IL (dB)	Bandwidth
MMI coupler [1]	1073	20	—	1	35–40 nm for $E R > 12 d B$
SWG-assisted Si waveguide [2]	5.4	$\sim 41$	99.95	1	53 nm for $E R > 25 d B$
Tapered waveguide [3]	$\sim 10$	31.5	—	—	—
Tapered DC [4]	80	37.15	$\sim 98$	0.28	160 nm for $X T < - 30 d B$
Tapered waveguide+MMI [5]	180	20	95	0.6	100 nm for $X T < - 12 d B$
Mode evolution+Y-junction [6]	95	30	99	0.4	400 nm for $E R > 12 d B$
Cascaded MMI couplers [7]	143.8	20.7	99	0.29	35 nm for $X T < 17 d B$
DC + Si3N4 layer [8]	300	29	—	—	300 nm for $X T < - 25 d B$
Parallel strip waveguides [9]	36.8	15.42	97.2	0.6	35 nm for $P C R > 94 %$
Adiabatic taper+ADC [10]	100	16.8	97.94	0.22	70 nm for $E R > 10 %$
Partially etched SWG coupler [11]	8.2	34.7	98.52	0.12	104 nm for $X T < - 20 d B$
Hybrid plasmonic-dielectric DC [12]	7.7	50.9	99.4	1.545	70 nm for $P C R > 95 %$
SWG coupler [39]	6.8	32.1	-	0.36	$\sim 81 n m$ for $E R > 18 d B$
PSR based on SWG [40]	25	10	97.5	0.13	35 nm for $P C R > 91.2 %$
SWG assisted coupler [41]	24.6	13.84	93	0.315	50 nm for $P C R > 80 %$
Tilted SWG coupler on LNOI platform [42]	160	20	—	1	85 nm for $E R > 20 d B$
MMI based on tilted SWG [43]	7.25	20	—	1	200 nm for $E R > 20 d B$
MMI based on angled SWG [44]	92.4	40	—	1	120 nm for $E R > 13 d B$
This work	5.5	30	93.7	0.16	167 nm for $P C R > 80 %$

$W_{A}$ (nm)	$W_{B}$ (nm)	$n_{A} / η_{1}$	$n_{B} / η_{2}$	PCL (dB)	$L_{C}$ (µm)
350	390	2.8/0.61	2.3/0.38	−0.38	4.5
350	400	2.966/0.7	2.3/0.38	−0.74	4.5
370	400	2.8/0.61	2.3/0.38	−0.26	5.5
380	400	2.8/0.61	2.4/0.43	−0.51	5.5
300	400	2.8/0.61	2.1/0.3	−0.63	5.5

Abstract

Data Availability

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Figures (10)

Tables (2)

Equations (9)

Journal of the Optical Society of America B