Compact polarization splitter based on silicon grating-assisted couplers

Ultra-broadband and high extinction ratio polarization splitter based on triple-tapered directional couplers

The design and numerically investigation of a silicon polarization splitter (PS) is proposed using triple-tapered directional couplers (DCs).The proposed device consists of a triple-tapered DC, a triple-bent DC at through port, and a tapered waveguide at cross port. The coupling length of the device is 20.5 µm. At the 1550 nm wavelength, the realized PS exhibits the extinction ratio (ER) values of 35.7 dB and 37.35 dB as well as the insertion loss values of < 0.58 dB and < 0.4 dB for the TE and TM polarization modes, respectively. Furthermore, the device's bandwidth for ER > 20 dB and ER > 25 dB are 170 nm and 136 nm, respectively.

Compact reconfigurable on-chip polarization beam splitters enabled by phase change material

In this paper, we present the design of a compact reconfigurable polarization beam splitter (PBS) enabled by ultralow-loss phase-changing Sb2Se3. By harnessing the phase-change-mediated mode coupling in a directional coupler (DC), guided light with different polarizations could be routed into different paths and this routing could be dynamically switched upon the phase change of Sb2Se3. With an optimized DC region, the proposed PBS demonstrates efficient polarization splitting with crosstalk less than -21.3 dB and insertion loss less than 0.16 dB at 1550 nm for both phase states of Sb2Se3, and features energy efficient property benefitting from the nonvolatile phase change of Sb2Se3, which holds great potentials for on-chip applications involving polarization control, including polarization-division multiplexing system, quantum photonics, microwave photonics, etc.

Topological polarization selection concentrator

Topological polarization selection devices, which can separate topological photonic states of different polarizations into different positions, play a key role in the field of integrated photonics. However, there has been no effective method to realize such devices to date. Here, we have realized a topological polarization selection concentrator based on synthetic dimensions. The topological edge states of double polarization modes are constructed by introducing lattice translation as a synthetic dimension in a completed photonic bandgap photonic crystal with both TE and TM modes. The proposed device can work on multiple frequencies and is robust against disorders. This work provides a new,to the best of our knowledge, scheme to realize topological polarization selection devices, and it will enable practical applications such as topological polarization routers, optical storage, and optical buffers.

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.

Tilted Nano-Grating Based Ultra-Compact Broadband Polarizing Beam Splitter for Silicon Photonics

An ultra-compact broadband silicon polarizing beam splitter is proposed based on a tilted nano-grating structure. A light cross coupling can be realized for transverse-magnetic mode, while the transverse-electric light can almost completely output from the through port. The length of the coupling region is only 6.8 μm, while an extinction ratio of 23.76 dB can be realized at a wavelength of 1550 nm. As a proof of concept, the device was fabricated by a commercial silicon photonic foundry. It can realize a 19.84 dB extinction ratio and an 80 nm working bandwidth with an extinction ratio of larger than 10 dB. The presented device also shows a good fabrication tolerance to the structure deviations, which is favorable for its practical applications in silicon photonics.

Distributed Bragg deflector coupler for on-chip shaping of optical beams

In integrated optical circuits light typically travels in waveguides which provide both vertical and horizontal confinement, enabling efficient routing between different parts of the chip. However, for a variety of applications, including on-chip wireless communications, steerable phased arrays or free-space inspired integrated optics, optical beams that can freely propagate in the horizontal plane of a 2D slab waveguide are advantageous. Here we present a distributed Bragg deflector that enables well controlled coupling from a waveguide mode to such a 2D on-chip beam. The device consists of a channel waveguide and a slab waveguide region separated by a subwavelength metamaterial spacer to prevent uncontrolled leakage of the guided mode. A blazed grating in the waveguide sidewall is used to gradually diffract light into the slab region. We develop a computationally efficient strategy for designing gratings that generate arbitrarily shaped beams. As a proof-of-concept we design, in the silicon-on-insulator platform, a compact ×75 Gaussian beam expander and a partial beam deflector. For the latter, we also demonstrate a prototype device with experimental results showing good agreement with our theoretical predictions. We also demonstrate via a rigorous simulation that two such couplers in a back-to-back configuration efficiently couple light, suggesting that these devices can be used as highly directive antennas in the chip plane.

Nanoscale material redistribution induced by spatially modulated femtosecond laser pulses for flexible high-efficiency surface patterning

In this paper, we investigated the material redistribution phenomenon controlled by spatially modulated femtosecond laser pulses on a silicon surface. The intensity distribution was shaped by using a spatial light modulator. The material was first selectively melted and then redistributed by the laser-induced plasma. Thus, complex surface patterns were formed conformal to the laser intensity distribution. Sub-diffraction-limit size can be achieved due to the nanoscale material redistribution. Only one pulse was needed in the surface patterning process, thus greatly favoring the efficiency improvement. Combined with multibeam interference, a large-scale nanostructure array can be fabricated with high efficiency of 1600 μm²/pulse. This method offers a simple, flexible and efficient alternative approach for nanoscale surface patterning applications.

Broad bandwidth and large fabrication tolerance polarization beam splitter based on multimode anti-symmetric Bragg sidewall gratings

A novel polarization beam splitter based on an anti-symmetric sidewall Bragg grating in a multimode silicon-on-insulator strip waveguide is demonstrated. Anti-symmetric spatially periodic refractive-index perturbations are designed for strong coupling between the fundamental (TE₀) and the first-order transverse electric modes (TE₁), while not for transfer magnetic modes. An adiabatic coupler is cascaded at the input-port, so as to drop the TE₁ reflection. The Bragg grating has a compact length of ∼20  μm (55 periods). The polarization isolations of the through- and drop-ports at the wavelength of 1557 nm are 34 and 31 dB, respectively. A broad bandwidth of 64 nm and a large fabrication tolerance of 80 nm for polarization isolation over 20 dB are also achieved.

Compact and fabrication-tolerant polarization splitter based on horizontal triple-slot waveguide

A compact and fabrication-tolerant polarization beam splitter (PBS) based on the horizontal triple-slot waveguides is proposed and optimized by using the rigorous H-field-based full-vectorial finite element method and the least-squares boundary residual method. It can be noted from the simulation results that a fabrication-tolerant PBS with a compact length of 33 μm can be yielded based on the horizontal triple-slot waveguides. The polarization extinction ratios are -21.8 and -20.3  dB at 1.55 μm wavelength for the quasi-TE and quasi-TM modes, respectively. The 1 dB bandwidth is 100 nm for both the polarizations. The fabrication tolerances are also thoroughly calculated for the proposed PBS.

Manipulation of beat length and wavelength dependence of a polarization beam splitter using a subwavelength grating

A polarization beam splitter assisted by a subwavelength grating (SWG) is proposed. The SWG enables nearly 20-fold beat length reduction for TE, which makes the high extinction ratio (ER) possible. On the other hand, the embedded SWG preferably affects the refractive index of the even mode in the coupling region and broadens the bandwidth of the splitter. As a result, the ER of 28.7 dB (24.8 dB) for TE (TM) is obtained, while the insertion loss is only 0.10 dB (0.11 dB) at the wavelength of 1550 nm. The ER is more than 10 dB in the wavelength range of 1450-1625 nm for TE and 1495-1610 nm for TM.

High-extinction-ratio silicon polarization beam splitter with tolerance to waveguide width and coupling length variations

We demonstrate a compact silicon polarization beam splitter (PBS) based on grating-assisted contradirectional couplers (GACCs). Over 30-dB extinction ratios and less than 1-dB insertion losses are achieved for both polarizations. The proposed PBS exhibits tolerance in width variation, and the polarization extinction ratios remain higher than 20 dB for both polarizations when the width variation is adjusted from + 10 to -10 nm. Benefiting from the enhanced coupling by the GACCs, the polarization extinction ratio can be kept higher than 15 dB and the insertion loss is lower than 2 dB for both polarizations when the coupling length varies from 30.96 to 13.76 μm.

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.

Cavity-based linear polarizer immune to the polarization direction of an incident plane wave

We herein report a linear polarizer based on a 2D array of substrate integrated waveguide cavities, which can convert an arbitrary linearly polarized (LP) incident wave into an outgoing LP wave in a specified polarization direction with constant transmittance. Two orthogonal slots etched on the front surface of the cavity are utilized to couple a wave of arbitrary polarization into the cavity, while another slot on the back side helps to couple the field out along a desired polarization direction. Microwave experiments are performed as a proof of concept. The proposed polarizer exhibits very good performance with stable transmittance as 50% and a polarization extinction ratio over 45 dB. The new polarizer is potentially useful in novel polarization-selective devices that are immune to the polarization direction of an incident plane wave.