rus
Issue #7-8/2023
D.M.Mohovikov, A.A.Guliaeva, I.V.Kulinich, A.A.Talovskaya, A.S.Myrzakhmetov
THE INTEGRAL-OPTICAL POLARIZATION CONVERTER BASED ON SiN
THE INTEGRAL-OPTICAL POLARIZATION CONVERTER BASED ON SiN
DOI: https://doi.org/10.22184/1993-8578.2023.16.7-8.456.461
This paper presents the results of the study of the influence of geometrical design parameters of an integrated-optical (IO) polarization converter, realized based on a comb waveguide structure on silicon nitride on insulator (SNOI), on the polarization rotation efficiency and output optical power of the device. As a result, a mathematical model of an IO polarization converter with polarization rotation efficiency >96 % and output power >98 % was developed. The design of the polarization converter IO with a mirror-reflected polarization rotation section was also proposed, which allows reducing the influence of technological error of the device geometry reproducibility to ±215 nm.
This paper presents the results of the study of the influence of geometrical design parameters of an integrated-optical (IO) polarization converter, realized based on a comb waveguide structure on silicon nitride on insulator (SNOI), on the polarization rotation efficiency and output optical power of the device. As a result, a mathematical model of an IO polarization converter with polarization rotation efficiency >96 % and output power >98 % was developed. The design of the polarization converter IO with a mirror-reflected polarization rotation section was also proposed, which allows reducing the influence of technological error of the device geometry reproducibility to ±215 nm.
Теги: io polarization converter rib optical waveguide silicon nitride гребенчатый оптический волновод ио конвертор поляризации нитрид кремния
INTRODUCTION
The need for faster and more energy-efficient broadband networks is driving global research policy in the field of optical transmission [1]. To create networks capable of handling the growing traffic, photonic integrated circuits (PICs), and power consumption can be reduced by at least 50 % compared to traditional integrated circuits, are widely deployed in modern telecommunication networks [2].
Nowadays, due to increasing degree of integration of PIC elements, the problem of high sensitivity to polarization arises, which is solved by means of polarization diversity or polarization conversion schemes [3]. Hence, there is an active development of passive components allowing to satisfy high requirements to polarization sensitivity of circuits [4, 5].
Polarization conversion is carried out with the help of special passive devices called converters, which are made in the form of asymmetric comb waveguides with different cross-sectional shape [6–8], and length from 20 to 150 μm, providing TE/TM conversion with efficiency >90 %, as well as in the form of cone waveguides [9, 10], length from 200 to 1500 μm, with similar conversion efficiency.
The main problem is to reproduce the geometry of IO polarization converters, since high lithography accuracy, a large number of technological operations, and low roughness of both the waveguide walls and the substrate surface are required [11]. When comparing IO converters obtained on different photonic platforms, polarization converters based on technology presented in [12, 13] stand out for their low insertion loss of less than 1 dB, high polarization conversion efficiency of >95 %, and a wide range of operating wavelengths, and, most importantly, less demanding to the lithographic process, with the possibility of integration with any photonic platforms based on thin-film LiNbO3, silicon-on-insulator (SOI) and InP in comparison with SOI converters with similar characteristics, with insertion losses from ~1 to 5 dB [14, 15].
Thus, NSI converters domestic technology development will expand the possibilities of design and production of Russian DWDM-systems, on any photonic platform.
Therefore, the aim of the work was to develop an integrated-optical polarization converter based on NSI technology.
RESEARCH METHODS
To develop the IO converter, its mathematical model was developed in the specialized software for calculation and design of FIS – ANSYSLumerical.
The modeling was performed using the finite element method (FEM), which is used in the analysis of various photonic devices based on their geometry and properties of the constituent materials. The main mathematical apparatus of this method focuses on mode profiles, cutoff frequencies, and effective refractive indices, due to which it is possible to optimize the waveguide geometry in an integrated photonic system [16].
The model of the IO converter is an NSI-based comb waveguide structure consisting of a rectangular comb waveguide at the input, an asymmetric comb waveguide performing the polarization rotation function and a rectangular comb waveguide at the output (Fig.1).
In this model, plane polarized light (λ = 1.55 μm) propagates in the crest of a SiN-based waveguide (n = 2), then, passing through a turning section (asymmetric waveguide), the light wave experiences a 90-degree polarization rotation and exits the rectangular comb waveguide.
RESULTS
To perform polarization rotation with maximum efficiency (100 %) and maximum output power (100 %), it is necessary to develop the design of the input comb waveguide in which the TE-mode ΓTE retention factor is 100 % and the TM-mode Γ™ retention factor is 0 %. And it is also necessary to develop the design of polarization rotation section (asymmetric comb waveguide) in which ΓTE and Γ™ will be 50 % for each mode.
Rectangular waveguide
In this work, a mode analysis of a single-mode comb waveguide was performed, for different comb widths w, with h = w (Fig.2).
The results of the performed mode analysis showed that the convergence of the values of the effective refractive indices for TE0 and TM0 is observed at values of the width w and height h of the waveguide crest from 800 nm.
However, since the maximum thickness of the SiN film is limited by the technological capabilities of the research and educational center "Nanotechnology" and is 800 nm, further at h = 800 nm, and influence of the waveguide crest width w on the retention coefficient TE0- and TM0-modes was investigated. The results of the study showed that ΓTE = 100 % and Γ™ = 0 % are observed at the waveguide crest width w = 850 nm.
Turning section
Further, the geometric parameters influence on the asymmetric comb waveguide on efficiency of polarization rotation, which is provided by the flow of optical power from TE0 to TM0 mode, was investigated. As a result of the research, the geometrical parameters of the rotation section with maximum polarization rotation efficiency were determined. Fig.3 shows the geometric parameters of the asymmetric waveguide cross section, and the calculation results are presented in Table 1.
Despite the high polarization rotation efficiency (96.3 %) and output power (98.32 %) of the obtained IO converter design, during reproducibility technological evaluation of its geometry, it was found that technological tolerance of this design is ±60 nm, with increasing of device efficiency drops significantly.
Expansion of the process error range is achieved by increasing the length of the conversion section by a factor of 2, with 25 % of the asymmetric waveguide length to be mirrored (Fig.4).
By using a mirror-reflected polarization converter section, it is possible to increase the allowable process error up to 215 nm, at which polarization conversion efficiency of 96.3 % is observed and the output power of the device reaches 98.32 %. In this case, the polarization converter section length L is 114 μm.
CONCLUSIONS
As a result of this work, a model of an integrated-optical polarization converter based on SixNy/SiO2 was developed, which provides high peak polarization conversion efficiency reaching 96.3 % and output power reaching 98.32 %, with an allowable technological error of ±215 nm at a converter section length of 114 μm. The development of a prototype providing similar characteristics will make it possible to use the promising NSI technology in a wider range of photonics devices.
ACKNOWLEDGMENTS
The work was performed within the framework of the state assignment of the Ministry of Science and Higher Education of the Russian Federation (project № FEWM-2022-0004 "Research and development of manufacturing methods for integrated optical waveguides and elements based on them").
PEER REVIEW INFO
Editorial board thanks the anonymous reviewer(s) for their contribution to the peer review of this work. It is also grateful for their consent to publish papers on the journal’s website and SEL eLibrary eLIBRARY.RU.
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
The need for faster and more energy-efficient broadband networks is driving global research policy in the field of optical transmission [1]. To create networks capable of handling the growing traffic, photonic integrated circuits (PICs), and power consumption can be reduced by at least 50 % compared to traditional integrated circuits, are widely deployed in modern telecommunication networks [2].
Nowadays, due to increasing degree of integration of PIC elements, the problem of high sensitivity to polarization arises, which is solved by means of polarization diversity or polarization conversion schemes [3]. Hence, there is an active development of passive components allowing to satisfy high requirements to polarization sensitivity of circuits [4, 5].
Polarization conversion is carried out with the help of special passive devices called converters, which are made in the form of asymmetric comb waveguides with different cross-sectional shape [6–8], and length from 20 to 150 μm, providing TE/TM conversion with efficiency >90 %, as well as in the form of cone waveguides [9, 10], length from 200 to 1500 μm, with similar conversion efficiency.
The main problem is to reproduce the geometry of IO polarization converters, since high lithography accuracy, a large number of technological operations, and low roughness of both the waveguide walls and the substrate surface are required [11]. When comparing IO converters obtained on different photonic platforms, polarization converters based on technology presented in [12, 13] stand out for their low insertion loss of less than 1 dB, high polarization conversion efficiency of >95 %, and a wide range of operating wavelengths, and, most importantly, less demanding to the lithographic process, with the possibility of integration with any photonic platforms based on thin-film LiNbO3, silicon-on-insulator (SOI) and InP in comparison with SOI converters with similar characteristics, with insertion losses from ~1 to 5 dB [14, 15].
Thus, NSI converters domestic technology development will expand the possibilities of design and production of Russian DWDM-systems, on any photonic platform.
Therefore, the aim of the work was to develop an integrated-optical polarization converter based on NSI technology.
RESEARCH METHODS
To develop the IO converter, its mathematical model was developed in the specialized software for calculation and design of FIS – ANSYSLumerical.
The modeling was performed using the finite element method (FEM), which is used in the analysis of various photonic devices based on their geometry and properties of the constituent materials. The main mathematical apparatus of this method focuses on mode profiles, cutoff frequencies, and effective refractive indices, due to which it is possible to optimize the waveguide geometry in an integrated photonic system [16].
The model of the IO converter is an NSI-based comb waveguide structure consisting of a rectangular comb waveguide at the input, an asymmetric comb waveguide performing the polarization rotation function and a rectangular comb waveguide at the output (Fig.1).
In this model, plane polarized light (λ = 1.55 μm) propagates in the crest of a SiN-based waveguide (n = 2), then, passing through a turning section (asymmetric waveguide), the light wave experiences a 90-degree polarization rotation and exits the rectangular comb waveguide.
RESULTS
To perform polarization rotation with maximum efficiency (100 %) and maximum output power (100 %), it is necessary to develop the design of the input comb waveguide in which the TE-mode ΓTE retention factor is 100 % and the TM-mode Γ™ retention factor is 0 %. And it is also necessary to develop the design of polarization rotation section (asymmetric comb waveguide) in which ΓTE and Γ™ will be 50 % for each mode.
Rectangular waveguide
In this work, a mode analysis of a single-mode comb waveguide was performed, for different comb widths w, with h = w (Fig.2).
The results of the performed mode analysis showed that the convergence of the values of the effective refractive indices for TE0 and TM0 is observed at values of the width w and height h of the waveguide crest from 800 nm.
However, since the maximum thickness of the SiN film is limited by the technological capabilities of the research and educational center "Nanotechnology" and is 800 nm, further at h = 800 nm, and influence of the waveguide crest width w on the retention coefficient TE0- and TM0-modes was investigated. The results of the study showed that ΓTE = 100 % and Γ™ = 0 % are observed at the waveguide crest width w = 850 nm.
Turning section
Further, the geometric parameters influence on the asymmetric comb waveguide on efficiency of polarization rotation, which is provided by the flow of optical power from TE0 to TM0 mode, was investigated. As a result of the research, the geometrical parameters of the rotation section with maximum polarization rotation efficiency were determined. Fig.3 shows the geometric parameters of the asymmetric waveguide cross section, and the calculation results are presented in Table 1.
Despite the high polarization rotation efficiency (96.3 %) and output power (98.32 %) of the obtained IO converter design, during reproducibility technological evaluation of its geometry, it was found that technological tolerance of this design is ±60 nm, with increasing of device efficiency drops significantly.
Expansion of the process error range is achieved by increasing the length of the conversion section by a factor of 2, with 25 % of the asymmetric waveguide length to be mirrored (Fig.4).
By using a mirror-reflected polarization converter section, it is possible to increase the allowable process error up to 215 nm, at which polarization conversion efficiency of 96.3 % is observed and the output power of the device reaches 98.32 %. In this case, the polarization converter section length L is 114 μm.
CONCLUSIONS
As a result of this work, a model of an integrated-optical polarization converter based on SixNy/SiO2 was developed, which provides high peak polarization conversion efficiency reaching 96.3 % and output power reaching 98.32 %, with an allowable technological error of ±215 nm at a converter section length of 114 μm. The development of a prototype providing similar characteristics will make it possible to use the promising NSI technology in a wider range of photonics devices.
ACKNOWLEDGMENTS
The work was performed within the framework of the state assignment of the Ministry of Science and Higher Education of the Russian Federation (project № FEWM-2022-0004 "Research and development of manufacturing methods for integrated optical waveguides and elements based on them").
PEER REVIEW INFO
Editorial board thanks the anonymous reviewer(s) for their contribution to the peer review of this work. It is also grateful for their consent to publish papers on the journal’s website and SEL eLibrary eLIBRARY.RU.
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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