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

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.

Broadband and CMOS-compatible polarization splitter and rotator built on a silicon nitride-on-silicon multilayer platform

A broadband and CMOS-compatible polarization beam splitter and rotator (PSR) built on the silicon nitride-on-silicon multilayer platform is presented. The PSR is realized by cascading a polarization beam splitter and a polarization rotator, which are both subtly constructed with an asymmetrical directional coupler waveguide structure. The advantage of this device is that the function of PSR can be directly realized in the SiN layer, providing a promising solution to the polarization diversity schemes in SiN photonic circuits. The chip is expected to have high power handling capability as the light is input from the SiN waveguide. The use of silicon dioxide as the upper cladding of the device ensures its compatibility with the metal back-end-of-line process. By optimizing the structure parameters, a polarization conversion loss lower than 1 dB and cross talk larger than 27.6 dB can be obtained for TM-TE mode conversion over a wavelength range of 1450 to 1600 nm. For TE mode, the insertion loss is lower than 0.26 dB and cross talk is larger than 25.3 dB over the same wavelength range. The proposed device has good potential in diversifying the functionalities of the multilayer photonic chip with high integration density.

Low loss and high extinction ratio all-silicon TM-pass polarizer with reflection removal enabled by contra-mode conversion Bragg-gratings

Bragg-gratings have been frequently used to design compact and high extinction ratio (ER) on-chip polarizers. However, the strong reflection of the unwanted polarization may deteriorate the performance of the light source or cause unwanted interferences. In this paper, we propose a Bragg-grating-based all-silicon TM-pass polarizer with low reflection, low insertion loss (IL) and high ER. Unlike previously reported polarizers based on single mode waveguides, we construct the Bragg grating with a multimode waveguide, which not only acts as a Bragg reflector, but also a mode-order converter to convert the reflected TE light into higher order modes to be eventually filtered out by utilizing a tapered transition. On the other hand, the grating has little adverse influence on the TM input light since it works at sub-wavelength-guided wave propagation regime. Finally, the polarizer obtained has a length of 30µm, an ER of 51.83dB, an IL of 0.08dB, and an operating bandwidth of ∼61nm for ER > 30dB at the wavelength of 1.55µm. More importantly, the reflection of the unwanted polarization is suppressed to -12.6dB, which can be further lowered via additional design optimization. Our work points to a new direction for making better on-chip polarizers.

Compact cross-slot waveguide polarization beam splitter using a sandwich-type coupler

A compact silicon-based cross-slot waveguide (CSW) polarization beam splitter (PBS) utilizing a sandwich-type coupler is proposed. The coupler consists of two identical CSWs separated by a horizontal slot waveguide (HSW), and S-bend sections at each end of the coupling section are introduced to decouple and separate the light. Owing to the strong birefringence of the HSW, the coupling length ratio between two different polarized modes (TE and TM) is adjustable in a suitable range by simply changing the gap width between the CSW and the HSW. The coupling length of the TM mode can be easily made half of that of the TE mode with a proper gap width, which is a favorable condition to realize a short PBS. Then, by making the length of the coupling section equal to the coupling length of the TE mode, the TE and TM modes are split. The PBS has the advantages of a small footprint and a high polarization extinction ratio (PER) and may be useful in the polarization management of a CSW-based on-chip optical system. Numerical results indicate that a PBS of ∼12µm with an insertion loss (IL) of 0.17 (0.53) dB, PER of 24.12 (21.14) dB for the TM (TE) mode is achieved at the wavelength of 1.55 µm, with a bandwidth covering the full C band for keeping PER>15dB and IL<1dB.

Compact polarization beam splitter assisted by subwavelength grating in triple-waveguide directional coupler

A polarization beam splitter (PBS) is one of the key components for manipulating different polarization states in the areas of optical interconnection and communication. In this paper, a PBS with two coupling regions using a subwavelength grating (SWG) waveguide as a bridge is proposed and investigated on a 340-nm silicon-on-insulator platform. The PBS is designed with a triple-waveguide directional coupler consisting of two identical silicon waveguides and an SWG waveguide sandwiched between them. The tailorable dispersion relation of the SWG waveguide offers a flexible design freedom. With the optimized grating period and duty ratio, the input TE mode will experience efficient coupling between the silicon waveguide and the SWG waveguide, while the TM mode can directly pass through without influence. The results show that with a device length of 6.5 μm, the extinction ratios and insertion losses of the two modes are higher than 27 dB and lower than 0.3 dB at a wavelength of 1550 nm, respectively.

Subwavelength-grating-assisted silicon polarization rotator covering all optical communication bands

We propose an ultra-broadband and ultra-compact polarization rotator (PR) structure on the silicon-on-insulator platform. The subwavelength gratings (SWGs) are introduced at the waveguide corner in order to excite the hybridized modes and realize the polarization rotation. The dispersion-engineered SWG can dramatically reduce the polarization conversion length deviation. High polarization extinction ratio > 20 dB and low excess loss < 1 dB can be achieved over 1.26-1.675 μm wavelength range, which covers O-, E-, S-, C-, L-, and U-bands. The total device size is as small as 4.8 × 0.34 μm². To the best of our knowledge, the proposed structure is the first silicon PR that could cover all of the optical communication bands.

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 high-performance adiabatic 3-dB coupler enabled by subwavelength grating slot in the silicon-on-insulator platform

We demonstrate a compact high-performance adiabatic 3-dB coupler for the silicon-on-insulator platform. The refractive index of the gap region between two coupling waveguides is effectively increased using subwavelength grating, which leads to high-performance operation and a compact design footprint, with a mode-evolution length of only 25 µm and an entire device length of 65 µm. The designed adiabatic 3-dB coupler has been fabricated using electron beam lithography and the feature size used in our design is CMOS compatible. The fabricated device is characterized in the wavelength range from 1500 nm to 1600 nm, with a measured power splitting ratio better than 3 ± 0.27 dB and an average insertion loss of 0.20 dB.

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.