Local-field engineering in slot waveguide for fabricating on-chip Bragg grating filters with high reflectivity across a flat broadband.

On-chip Bragg gratings with high reflectivities have been found to have widespread applications in filters, resonators, and semiconductor lasers. However, achieving strong Bragg reflections with flat response across a broad bandwidth on the popular 220 nm silicon-on-insulator (SOI) platform still remains a challenge. In this paper, such a high performance device is proposed and fabricated, which is based on a slot waveguide with gratings etched on the inner sidewalls of the slot. By manipulating the local field in the slot region using a chirped and tapered grating-based mode transition, the device achieves a flat response with ultra-high reflection and low transmission for the TE mode across a broad operating bandwidth. Leveraging the ultra-high birefringence of the SOI waveguide, the device functions both as a TE slot waveguide reflector and a TM pass polarizer. Simulation results demonstrate that the device exhibits an ultra-high rejection of more than 50 dB and a reflectivity exceeding 0.99 for the TE mode across a 91 nm wavelength range, while maintaining a high transmittance of larger than 0.98 for the TM mode. Experimental results validate that the device performance is consistent with the simulation results. A fabricated device based on such a gratings exhibits a low insertion loss (<0.8 dB) and high polarization extinction ratio (>30 dB) over 100 nm bandwidth (1484 nm-1584 nm), demonstrating that the performance of the present design is competitive with that of the state-of-the-art SOI Bragg gratings.

Compact and efficient polarization rotator using laterally asymmetric rib waveguides on a lithium-niobate-on-insulator platform.

Photonic devices based on a lithium-niobate-on-insulator (LNOI) are current research hotspots; however, owing to the high refractive index contrast of the LNOI platform and inherent birefringence of lithium niobate itself, such photonic devices are generally polarization sensitive, affecting their further wide application. This paper proposes a simple, compact, and efficient polarization rotator (PR) based on a laterally asymmetric rib waveguide by depositing a layer of semi-infinite silicon nitride dielectric material on one side of the rib waveguide. The results show that a PR with a polarization rotation region length of 15.77 µm is achieved, and the polarization extinction ratio (PER), insertion loss (IL), and polarization conversion efficiency (PCE) are 38.57/68.95 and 0.2/0.22 dB, and 99.99%/almost 100%, respectively, for the fundamental transverse electric mode (T E 0) and transverse magnetic mode (T M 0) at a 1.55 µm wavelength. The operation bandwidth is around 120 nm for the T E 0 mode and T M 0 mode when the PER, IL, and PCE are greater than 20 dB, less than 0.32 dB, and more than 99%, respectively. Fabrication tolerances to the key structural parameters are investigated in detail. In addition, the evolution fields of the T E 0 mode and T M 0 mode along the propagation direction through the proposed device are presented.

All-silicon TM polarizer covering the 1260-1675†nm bandwidth using a band engineered subwavelength grating waveguide.

A TM polarizer working for whole optical communication bands with high performance is proposed on a 220-nm-thick silicon-on-insulator (SOI) platform. The device is based on polarization-dependent band engineering in a subwavelength grating waveguide (SWGW). By utilizing an SWGW with a relatively larger lateral width, an ultra-broad bandgap of ∼476 nm (1238 nm-1714nm) is obtained for the TE mode, while the TM mode is well supported in this range. Then, a novel tapered and chirped grating design is adopted for efficient mode conversion, which results in a polarizer with a compact footprint (3.0 µm × 18 µm), low insertion loss (IL < 1.15 dB) and high polarization extinction ratio (PER > 21 dB) covering O-U bands (1260 nm-1675 nm). Experimental results show that the fabricated device has an IL < 1.0 dB and PER > 22 dB over a 300- nm bandwidth, which is limited by our measurement setup. To the best of our knowledge, no TM polarizer on the 220-nm SOI platform with comparable performance covering O-U bands has ever been reported.

Experimental demonstration of a flexible and high-performance mode-order converter using subwavelength grating metamaterials.

Mode-order converters, transforming a given mode into the desired mode, have an important implication for the multimode division multiplexing technology. Considerable mode-order conversion schemes have been reported on the silicon-on-insulator platform. However, most of them can only convert the fundamental mode to one or two specific higher-order modes with low scalability and flexibility, and the mode conversion between higher-order modes cannot be achieved unless a total redesign or a cascade is carried out. Here, a universal and scalable mode-order converting scheme is proposed by using subwavelength grating metamaterials (SWGMs) sandwiched by tapered-down input and tapered-up output tapers. In this scheme, the SWGMs region can convert, TEp mode guided from a tapered-down taper, into a TE0-like-mode-field (TLMF) and vice versa. Thereupon, a TEp-to-TEq mode conversion can be realized by a two-step process of TEp-to-TLMF and then TLMF-to-TEq, where input tapers, output tapers, and SWGMs are carefully engineered. As examples, the TE0-to-TE1, TE0-to-TE2, TE0-to-TE3, TE1-to-TE2, and TE1-to-TE3 converters, with ultracompact lengths of 3.436-7.71 µm, are reported and experimentally demonstrated. Measurements exhibit low insertion losses of < 1.8 dB and reasonable crosstalks of < -15 dB over 100-nm, 38-nm, 25-nm, 45-nm, and 24-nm working bandwidths. The proposed mode-order converting scheme shows great universality/scalability for on-chip flexible mode-order conversions, which holds great promise for optical multimode based technologies.

Ultra-compact and broadband all-silicon TM-pass power splitter using subwavelength holey-structured metamaterial waveguides.

Power splitters with polarization management features are highly desired to construct high-density silicon photonic integrated circuits. However, few attempts have been made to design a single device that can act as both a power splitter and a TE- or TM-pass polarizer. In this paper, for the first time, we experimentally demonstrate an ultra-compact and broadband all-silicon TM-pass power splitter, where a triple-guide directional coupler (TGDC) composed of three parallel subwavelength holey-structured metamaterial waveguides (SHMWs) is located at central coupling region and three regular strip waveguides are connected at the input/output ports. Such a SHMW can enhance the reflection to realize a wide stop-band for the undesired TE polarized light, while achieving the low loss transmission for the TM polarized light. Besides, the TM dispersion can be significantly flattened by the designed SHMWs, leading to a broadband power splitting for TM polarization. Simulated results show that an ultra-compact device of 1.7 × 4 µm2 in size is obtained with an insertion loss (IL) of 0.34 dB and an extinction ratio (ER) of 36 dB at 1550 nm, and its working bandwidth can be extended to ∼240 nm by keeping IL < 0.9 dB and ER > 16 dB. The measurements of the fabricated devices show low IL (<1 dB) and high ER (>15 dB) over the measured wavelength range of 1460 to 1580 nm, which is consistent with the simulation results.

Highly scalable and flexible on-chip all-silicon mode filter using backward mode conversion gratings.

Mode filters are fundamental elements in a mode-division multiplexing (MDM) system for reducing modal cross-talk or realizing modal routing. However, the previously reported silicon mode filters can only filter one specific mode at a time and multiple modes filtering usually needs a cascade of several filters, which is adverse to highly integrated MDM systems. Here, we propose a unique concept to realize compact, scalable and flexible mode filters based on backward mode conversion gratings elaborately embedded in a multimode waveguide. Our proposed method is highly scalable for realizing a higher-order-mode-pass or band-mode-pass filter of any order and capable of flexibly filtering one or multiple modes simultaneously. We have demonstrated the concept through the design of four filters for different order of mode(s) and one mode demultiplexer based on such a filter, and the measurement of two fabricated 11µm length filters (TE1-pass/TE2-pass) show that an excellent performance of insertion loss <1.0dB/1.5dB and extinction ratio >29dB/28.5dB is achieved over a bandwidth of 51.2nm/48.3nm, which are competitive with the state-of-the-art.

Compact and ultra-broadband all-silicon TM-pass and TE-reflected polarizer using grating based weakly coupled nanowires.

On-chip silicon polarizers with broad operating bandwidth and compact footprint have recently attracted increasing attention for their applications in large capacity and high density integrated optical systems. However, strong waveguide dispersion usually limits the bandwidth of the silicon polarizers, especially for the TM-pass polarizers. In this paper, we overcome the bandwidth limit of the TM polarizer by utilizing a novel waveguide structure composed of two weakly coupled nanowires with gratings sandwiched in between. Such a structure can effectively enlarge the bandgap for the undesired TE polarized light, while act as a low loss subwavelength metamaterial for TM polarized light over an extremely large wavelength range. In simulation, we obtain a compact polarizer of 13.6 µm × 1.3 µm in size with an ultra-broad operating bandwidth of ∼362 nm for extinction ratios (ERs) >21 dB and insertion losses (ILs) <1 dB, which covers E-, S-, C-, L-, and U-bands and part of O-band. The measurements of fabricated devices show that the device performed well in the test wavelength range from 1300 to 1600 nm with an ER >15 dB and an average IL ∼1 dB, consistent with the simulation results. This work paves a new way for designing compact and ultra-broadband on-chip polarizers.

Compact and broadband silicon-based polarization beam splitter using asymmetric directional couplers embedded with subwavelength gratings and slots.

A compact and broadband silicon-based polarization beam splitter (PBS) is proposed and investigated in detail, where the two arms of the directional coupler (DC) are, respectively, embedded with subwavelength gratings (SWGs) and vertical slots so that field distributions for the TE mode are significantly changed, effectively weakening coupling strength, whereas those for the TM mode are almost unaffected, nearly analogous to the DC with strip waveguides. By carefully optimizing structural parameters, efficient coupling will emerge between the two waveguides for the TM mode, while TE mode will be confined in the SWG-assisted strip waveguide. Consequently, the two modes can be effectively separated, and thus the realization of a PBS is accomplished. Results show that a compact PBS with a coupling length of 6.45 µm is achieved, together with the extinction ratio (ER) of 27.54/31.88 dB, the insertion loss of 0.12/0.14 dB, and the reflection loss of -43.67/-30.50dB, respectively, for TE/TM mode at the wavelength of 1.55 µm. The bandwidth, for both modes, is up to 230/100 nm when ER is larger than 15/20 dB. In addition, fabrication tolerances to the critical structural parameters and field evolution through the proposed device are analyzed.

Compact silicon-based on-chip wavelength triplexer using directional couplers with double bridged subwavelength gratings.

A compact silicon on-chip wavelength triplexer engineered by double bridged subwavelength gratings (BSWGs) is proposed based on cascaded symmetric directional couplers (SDCs). The SDC consists of a pair of symmetric and identical BSWGs with another SWG waveguide implemented in the middle of the SDC section. It functions well provided that the coupling lengths can match even or odd times of beat lengths of the three wavelengths. Relying on deliberately tailoring the structural dimensions of SWGs at different positions of SDC, the two lowest-mode refractive indices and beat lengths can be efficiently tuned, drastically reducing device footprint. The results show a total device length of 42.2 µm, which is ∼10% of its conventional counterpart based on SDCs. An insertion loss (IL) lower than 0.67 dB, reflection loss (RL) below -21.4dB, and extinction ratio (ER) as high as 34.5 dB are also obtained in the results. The bandwidths around wavelength bands and fabrication tolerances to dimensional variations are investigated and analyzed. The field evolutions for the three injected wavelength bands are presented.

Full-vectorial meshless finite cloud method for an anisotropic optical waveguide analysis.

By using the meshless finite cloud method, an efficient full-vectorial mode solver based on the transverse-magnetic-field components is developed to analyze the optical waveguides made of anisotropic materials, in which the waveguide cross-section enclosed by the perfectly matched layers is divided into an appropriate number of homogeneous clouds. The point collocation technique is utilized to create a scattered set of nodes over the cloud, and then the continuity conditions of the longitudinal field components are imposed to appropriately deal with the discrete nodes at the interfaces shared by the adjacent clouds. In comparison with conventional mesh-based numerical techniques, the distributions of solution nodes of the present method can be applied to the area of complexity in a completely free manner. Moreover, an interior nodal distribution adaptively updating along the propagation direction is adopted to accomplish higher computational efficiency while improving numerical accuracy. To validate the proposed method, an anisotropic square waveguide, a magneto-optical raised strip waveguide, and a nematic liquid-crystal channel waveguide are considered as numerical examples, and their modal field distribution and corresponding effective refractive indexes are presented. Results are in good agreement with those published earlier, showing the effectiveness of the present method.

Ultracompact polarization-insensitive power splitter using subwavelength-grating-based MMI couplers on an SOI platform.

An ultracompact and polarization-insensitive power splitter using a subwavelength-grating-based multimode interference (MMI) coupler on an SOI platform is proposed and analyzed in detail. By properly tailoring the structural parameters of the subwavelength gratings embedded in the center of the MMI coupler, the effective reflective indices for TE and TM modes supported by this MMI coupler can be engineered, leading to equal coupling lengths for the two polarizations and an efficient reduction in length for the used MMI coupler. As a result, an ultracompact polarization-insensitive power splitter can be realized. Moreover, to effectively minimize the loss, tapered waveguides are used, and two right angles are cut at both corners of the used MMI coupler. Results show that a footprint of ${2}.{2}\;\unicode{x00B5} {\rm m} \times {3}.{8}\;\unicode{x00B5} {\rm m}$2.2µm×3.8µm for the MMI region is achieved with an insertion loss of 0.07 dB for both TE and TM modes (polarization dependent loss ${\sim\;{0}\;{\rm dB} }$∼0dB) and a reflection loss of $ - {28}.{29}\;{\rm dB}$-28.29dB ($ - {31}.{25}\;{\rm dB}$-31.25dB) for TE mode at the wavelength of 1.55 µm. Insertion loss below 0.3 dB is obtained over the bandwidth of 200 nm, covering the C-band. In addition, fabrication tolerances to the structural parameters are analyzed, and the injected light propagating through the power splitter is also presented.

Compact silicon-based polarization-independent directional coupler using subwavelength gratings.

A compact and high extinction ratio polarization-independent directional coupler (PIDC) using silicon-based subwavelength gratings (SWGs) is proposed and analyzed in detail, where the SWG structures are located between the two input/output strip waveguides together with input/output SWG-based transitions. The introduced SWG-based structures can effectively enhance the coupling strength for the TE mode, while that for the TM mode is hardly affected. According to the coupled-mode theory, by carefully choosing the structural parameters, the coupling length for both polarizations can be identical; thus a PIDC is realized. Results show that a PIDC with a coupling length of 9.2 µm is achieved, and the extinction ratio, insertion loss, and reflection loss are 33.25/31.41 dB, 0.224/0.121 dB, and -23.11/-26.31 dB, respectively, for the TE/TM mode. In addition, fabrication tolerances to the key structural parameters are analyzed in detail, and field evolution through the present device is also presented.

Broadband TE-pass slot waveguide polarizer using an asymmetrical directional coupler.

A compact TE-pass polarizer for silicon-based slot waveguides is numerically proposed based on a directional coupler where an asymmetrical slot waveguide (ASW) and hybrid plasmonic waveguide (HPW) are involved. The input section is linearly tapered to an ASW to markedly enlarge the modal birefringence. Beneficially, only the TM mode can be coupled into the adjacent HPW and attenuated while the injected TE mode just passes through the slot waveguide without coupling by properly choosing the dimensions. As a consequence, an efficient TE-pass polarizer can be implemented. From the simulation results, the proposed polarizer has an extinction ratio (ER) of 45 dB and insertion loss (IL) of 0.44 dB at 1.55 µm, and its bandwidth exceeds 130 nm with the ER>30 dB and IL<1 dB. In addition, fabrication tolerances to the key structural parameters are analyzed and field evolution along the propagation distance is also demonstrated.

Ultracompact and broadband silicon-based TE-pass 1 × 2 power splitter using subwavelength grating couplers and hybrid plasmonic gratings.

An ultracompact TE-pass 1 × 2 power splitter, using subwavelength grating (SWG) couplers and hybrid plasmonic gratings (HPGs), is proposed and analyzed in detail. As the input strip waveguide is tapered to a narrow wire, where TE mode is cutoff, the launched TE-mode can be evenly divided with high efficiency with the help of the SWG couplers, which are placed between the central silicon wire and two nearby output branches. The injected TM-mode will be perfectly reflected by the carefully designed HPG, whose periodically varied metal layer is located above the bottom strip and SWG waveguides. Consequently, a single device combining both the functions of polarization selection and power division can be realized. This is valuable for highly dense integrated circuits. Results show that, with a period number of 4 in HPG, the present device is only ~6.2-µm-long with an extinction ratio (ER) and an insertion loss (IL) of 25.4 and 0.53 dB at 1.55 µm, respectively, and its bandwidth of ER > 20 dB is ~180 nm with the IL < 0.8 dB, showing a broadband property. Besides, fabrication tolerances to the key dimensions are analyzed and modal field evolution through the device is also presented.

Compact polarization rotator for silicon-based cross-slot waveguides using subwavelength gratings.

A compact and broadband polarization rotator (PR) for silicon-based cross-slot waveguides using subwavelength gratings (SWGs) is proposed and analyzed. To significantly break the symmetry of the waveguide structure, the diagonal regular Si wires of the cross-slot waveguides are replaced with the full etching SWGs. Moreover, the special properties of the SWGs-whose effective index is adjustable-can effectively enhance the modal birefringence between the two lowest-order hybrid modes, resulting in a more compact device. By utilizing interference effect of the hybrid modes, both transverse electric to transverse magnetic (TE-to-TM) and TM-to-TE conversion can be efficiently realized. Numerical results show that a PR of 12.6 µm in length at a wavelength of 1.55 µm is achieved, where the polarization conversion efficiency (PCE) and insertion loss (IL) are, respectively, 97.2% and 0.71 dB, and the reflection loss is below -20.5 dB for both cases. Moreover, a wide bandwidth of ∼260 nm for both polarizations is obtained for keeping the PCE over 90% and IL below 1 dB. In addition, fabrication tolerances to the structural parameters are analyzed in detail, and field evolution along the propagation distance is also presented.

Design and numerical study of a compact, broadband and low-loss TE-pass polarizer using transparent conducting oxides.

A compact, broadband, and low-loss TE-pass polarizer using transparent conducting oxides (TCOs) embedded in the center of the strip waveguide and deposited on its top is proposed and analyzed in detail. With the tunable permittivity of TCO, epsilon-near-zero (ENZ) of its real part and significant increase of its imaginary part can be achieved around the wavelength of 1.55 µm under a certain electron concentration. By introducing this ENZ material into the strip waveguide, huge polarization dependence can be realized, that is, the TE mode is almost not affected due to its quite weak interaction with TCOs, while the TM mode is extremely confined in the accumulation layers of TCO with high absorption loss, leading to a great reduction in length for the present polarizer. Moreover, the top TCO layer is applied to further enhance the polarizer performance. Results show that a polarizer of only 4.5 µm in length with an extinction ratio (ER) of 25.26 dB and an insertion loss of 0.21 dB is achieved at 1.55 µm, and its bandwidth can be extended to ~140 nm for an ER>20 dB. In addition, the ER can also be increased only by enlarging the length of the TCO-based polarizer.

Ultracompact and high efficient silicon-based polarization splitter-rotator using a partially-etched subwavelength grating coupler.

On-chip polarization manipulation is pivotal for silicon-on-insulator material platform to realize polarization-transparent circuits and polarization-division-multiplexing transmissions, where polarization splitters and rotators are fundamental components. In this work, we propose an ultracompact and high efficient silicon-based polarization splitter-rotator (PSR) using a partially-etched subwavelength grating (SWG) coupler. The proposed PSR consists of a taper-integrated SWG coupler combined with a partially-etched waveguide between the input and output strip waveguides to make the input transverse-electric (TE) mode couple and convert to the output transverse-magnetic (TM) mode at the cross port while the input TM mode confine well in the strip waveguide during propagation and directly output from the bar port with nearly neglected coupling. Moreover, to better separate input polarizations, an additional tapered waveguide extended from the partially-etched waveguide is also added. From results, an ultracompact PSR of only 8.2 µm in length is achieved, which is so far the reported shortest one. The polarization conversion loss and efficiency are 0.12 dB and 98.52%, respectively, together with the crosstalk and reflection loss of -31.41/-22.43 dB and -34.74/-33.13 dB for input TE/TM mode at wavelength of 1.55 µm. These attributes make the present device suitable for constructing on-chip compact photonic integrated circuits with polarization-independence.

Design of a compact and integrated TM-rotated/TE-through polarization beam splitter for silicon-based slot waveguides.

A compact and integrated TM-rotated/TE-through polarization beam splitter for silicon-based slot waveguides is proposed and characterized. For the input TM mode, it is first transferred into the cross strip waveguide using a tapered directional coupler (DC), and then efficiently rotated to the corresponding TE mode using an L-shaped bending polarization rotator (PR). Finally, the TE mode for slot waveguide at the output end is obtained with the help of a strip-to-slot mode converter. By contrast, for the input TE mode, it almost passes through the slot waveguide directly and outputs at the bar end with nearly neglected coupling due to a large mode mismatch. Moreover, an additional S-bend connecting the tapered DC and bending PR is used to enhance the performance. Results show that a total device length of 19.6 µm is achieved, where the crosstalk (CT) and polarization conversion loss are, respectively -26.09 and 0.54 dB, for the TM mode, and the CT and insertion loss are, respectively, -22.21 and 0.41 dB, for the TE mode, both at 1.55 µm. The optical bandwidth is approximately 50 nm with a CT<-20 dB. In addition, fabrication tolerances and field evolution are also presented.

Compact and high extinction ratio polarization beam splitter using subwavelength grating couplers.

A compact and high extinction ratio polarization beam splitter using subwavelength grating (SWG) couplers is proposed and characterized, where the SWG couplers are located between the two input/output strip waveguides, including SWG-based transitions combined at both ends. The TM mode can be confined well in the strip waveguide and transmits along it with nearly neglected coupling, while the TE mode undergoes a strong coupling and is transferred to the adjacent waveguide with the help of SWG couplers due to dissimilar modal characteristics and cutoff conditions between these two polarizations. To further enhance the performance, an additional tapered waveguide is added in the lateral end of the input SWG-based transition. Results show that a total length of 6.8 µm with an insertion loss of 0.08 (0.36) dB, extinction ratio (ER) of 32.19 (20.93) dB, and reflection loss of -34.76 (-32.59) dB for TE (TM) mode is obtained at 1.55 µm; its bandwidth can be enlarged to ∼81 nm for an ER>18 dB. In addition, fabrication tolerances and mode-field evolution are also presented.

Compact polarization rotator for silicon-based slot waveguide structures.

A compact polarization rotator (PR) for silicon-based slot waveguides is proposed, where the slot region including the upper claddings is filled with liquid crystals (LCs). With the anisotropic features of the LCs, the transverse field components of eigenmodes have almost identical amplitudes, leading to a high modal hybridness. As a result, the TE (TM) polarization can be rotated efficiently to the TM (TE) polarization within a short length. The numerical results show that a PR 11.3 µm in length at an operating wavelength of 1.55 µm is achieved with an extinction ratio (ER) (insertion loss) of 12.6 (0.22) dB for TE-to-TM and 11.5 (0.30) dB for TM-to-TE. Moreover, the optical bandwidth for TE-to-TM (TM-to-TE) mode must be ∼64(∼29) nm to keep the ER over 12 (10) dB. In addition, fabrication tolerances to the structural parameters are investigated, and field evolution along the propagation distance through the PR is also demonstrated.