Polarization-insensitive Filter Research Articles

Power-efficient polarization-insensitive tunable microring filter on a multi-layer Si3N4-on-SOI platform.

We develop and demonstrate a non-duplicate polarization-diversity tunable bandpass optical filter by leveraging the bi-directional transmission of add-drop dual-coupled microring resonators (MRRs) on a multi-layer Si3N4-on-silicon-on-insulator (SOI) platform. By using Euler-bends, we implement compact and low-loss Si3N4 MRRs with an equivalent bending radius of ∼38 µm. Fiber-to-fiber (on-chip) insertion loss of 4.3 dB (1.7 dB) with a low polarization-dependent loss of <0.5 dB and low differential group delay of <2.5 ps is achieved. The extinction ratio is more than 30 dB. The thermo-optic tuning efficiency of the Si3N4 MRRs is improved with a suspended micro-heater design. As a result, a wavelength tuning range of ∼2 nm and a 3-dB bandwidth tuning range of 20 GHz are experimentally demonstrated. The tuning efficiency is 33 pm/mW, which is ∼7.5 times higher than the previous design. This reconfigurable polarization-insensitive filter, with low loss, low cross talk, and high power efficiency, is highly promising for practical applications in optical communication and signal processing.

A Spectrum-Efficient MoF Architecture for Joint Sensing and Communication in B5G Based on Polarization Interleaving and Polarization-Insensitive Filtering

The intelligentization of future society puts forward an urgent demand for high-precision sensing and ultra-high-speed wireless communications in the upcoming beyond fifth-generation (B5G) era. We propose and experimentally demonstrate a novel spectrum-efficient MMW-over-fiber (MoF) architecture for joint sensing and communication in B5G optical-wireless converged networks. The proposed MoF architecture is based on polarization interleaving and polarization-insensitive filtering. In the proposed architecture, the sensing and communication sidebands are generated simultaneously through asymmetrical single-sideband (ASSB) modulation, whereas the two local oscillator (LO) sidebands up-converting the sensing and communication signals to MMW band are obtained by carrier-suppressed double-sideband (CS-DSB) modulation. By interleaving the two sets of sidebands for sensing and communication in two orthogonal polarizations, the demand for higher bandwidth devices and the occupied spectral grid can thus be effectively reduced. The ASSB modulation eliminates the chromatic-dispersion- (CD) induced power fading for long-reach services. The polarization-insensitive filtering removes the need for complicated polarization tracking, resulting in a simple structure at the remote units (RUs) and polarization-free digital signal processing (DSP) at the user ends (UEs). Moreover, the two sets of sidebands originate from a shared laser, so frequency offset estimation (FOE) can be avoided to further reduce the complexity and power consumption of the DSP, thereby facilitating a user-friendly terminal. The experimental results show that a ±15-mm ranging accuracy at B5G MMW band for single-target detection is achieved, and a 30-cm ranging resolution for dual-target detection is also realized. Furthermore, a 23-Gbit/s error-free transmission rate at 28GHz over 5.41-km single-mode fiber (SMF) and 2-m wireless distance is successfully demonstrated without frequency offset compensation.

Polarization-independent guided-mode resonance filtering by all-dielectric gratings in the terahertz region.

We propose an angular-dependent polarization-insensitive filter in the terahertz (THz) region, based on the guided-mode resonance of one-dimensional zero-contrast grating architectural design. Particle swarm optimization combined with the rigorous coupled-wave analysis method is used to design the filter and investigate the influences of the planes of incidence on the characteristics of the proposed all-dielectric THz filter. With the planes of incidence set at 0°, 30°, 45°, and 60°, the polarization-independent resonances occur at 0.458 THz, 0.459 THz, 0.461 THz, and 0.465 THz under oblique incidences of 9.3°, 10.8°, 13.3°, and 19.2°, respectively, which means the oblique incident angle of the polarization-independent THz filter increases with the rotation of the planes of incidence from classic mounting to fully conical mounting. In addition, for the fully conical mounting case, the resonance has high angular stability and is no longer split, compared with classic incidence; meanwhile, there is only a tiny blue shift in resonance of less than 3 GHz when changing the incident angle from 0° to 10°. The physical mechanism of the spectral characteristics is also analyzed in detail. The spectral properties proposed herein enable significant potential application in the fields of spectroscopy, image sensors, communication, etc., in the THz region.

Polarization-Independent Filter Based on 2-D Crossed Grating Under Oblique Incidence

The existing polarization-insensitive filters based on two dimensional (2-D) grating are mostly under normal incidence. However, in filtering optical devices, the normal incidence regime is usually avoided because it requires the use of beam separators which yields a loss of efficiency. Herein, we present a compact method to realize polarization-independent filter under oblique incidence, which is implemented on 2-D crossed grating in the telecommunication region. Plane of incidence is set to be in xz plane and three different polarization angles (0°, 45°, and 90°) are used to demonstrate the polarization-insensitivity. Realization of the polarization-insensitivity of the 2-D filter is based on the split feature under oblique incidence, according to the analysis of the physical mechanism. The location of the polarization-independent resonance is mainly determined by the grating period along x-axis and will right shift with increasing the period. Results show that the polarization-insensitive resonances occur at 1356.9 nm, 1372.6 nm and 1398.5 nm when the grating periods along x-axis are 830 nm, 850 nm and 870 nm, respectively. Moreover, the results and methods provided herein can be applied to search for the polarization-insensitive resonances of the 2-D grating with other planes of incidence.

Polarization-Insensitive Filter for Incidence Between Classic and Full Conical Mountings

An angular-dependent polarization-insensitive filter in the telecommunication region is demonstrated in this letter, which is based on a zero-contrast grating for incident plane between classic and full conical mountings. Particle swam optimization combined with the rigorous coupled wave analysis method is applied to design the filter and perform the reflectance spectra. Realization of the polarization-insensitive filter is based on the split feature of a 1D guided mode resonant filter, under oblique incidence in classic mounting. Results show that polarization-independent resonances occur near $1.454~\mu \text{m}$ under the oblique incidences of 4.8°, 5.3°, 6.5°, and 9.3°, with the incident plane angle being set as 15°, 30°, 45°, and 60°, respectively. During the process of varying the incident plane angle from classical mounting to full conical mounting, the mechanism of the increased angle of oblique incidence is also analyzed in detail.

Design of sub-wavelength grating structure for filter regardless of polarization

For isotropic dielectric thin films, polarization effect is an inherent character. As it will make the performance of optical-electric system go to bad, such polarization-dependent properties are often intolerable and should eliminate its effect in many applications. So the design of polarization insensitive filter is an indispensability work. In this paper, based on a four-part period grating, a filter regardless of polarization is gained by combing rigorous wave theory and multiobjective immune optimization algorithm. Its working wavelength is 1315nm which is often used in laser system. The results of our design shows that TE and TM polarized wave have a reflection 0.482 and 0.485, respectively, at designed wavelength 1315nm. And it denotes that two values all approach to the design objective good, and the polarization deviation is very little for their difference is 0.003. Therefore, the effect of polarization deviation at 1315nm wavelength can be eliminated very well by the designed filter.

Angular-dependent polarization-insensitive filter fashioned with zero-contrast grating.

We report here an angular-dependent polarization-insensitive filter fashioned with a free-standing zero-contrast grating (ZCG), which is implemented on an HfO(2)/Silicon platform. The spectral characteristics are investigated by rigorous coupled-wave analysis method and measured on angular-resolved micro-reflectance system. The proposed ZCG structure experimentally shows that the polarization-insensitive resonances occur at 595nm for the incidence angle θ of 12.8° and 500nm for the incidence angle θ of 14.2°. When the incident light is normal to the grating surface, the ZCG device generates yellow and red colors for p- and s-polarization, respectively. The experimental results are in good agreement with the simulations, which indicate that the free-standing ZCG device is promising for polarization-insensitive filter and polarization-controlled tunable color filter.

Polarization circulation effect in microbend-induced long-period fiber gratings for polarization-insensitive filter response

We analyze the reduction mechanism of polarization dependence in loss-peak wavelength for microbend-induced long-period fiber gratings (M-LPFGs) formed in twisted fibers. The analysis model consists of the stress-induced birefringence associated with the microbend as well as the optical activity induced by the twisted fiber. We verify the analysis model by measuring the variation in the state of polarization through the torsion M-LPFG. We reveal that the efficient circulation of the polarization components occurs in the torsion M-LPFG such that the polarization dependence of the loss-peak wavelengths is reduced. In the experiments, the M-LPFG is controlled by an electromagnet in the combination of a coil spring. The experimental results demonstrate a good agreement with the calculated expectation for the polarization-insensitive M-LPFGs.

An SOI based polarization insensitive filter for all-optical clock recovery

We fabricate and demonstrate a compact polarization insensitive filter for all-optical clock recovery (CR) based on silicon-on-insulator (SOI), which consists of a microring resonator (MRR) and two modified two-dimensional (2D) grating couplers. The distributed Bragg reflectors (DBRs) are introduced to improve the coupling efficiency of the 2D grating coupler. The MRR works as a comb filter for CR, while the 2D grating couplers serve as the polarization diversity unit to achieve a polarization insensitive operation. A subsequent semiconductor optical amplifier (SOA) performs the amplitude equalization. Based on this scheme, a good clock signal with 970 fs timing jitter can be achieved at 44 Gb/s from input signals with arbitrary polarization states.

Design of polarization insensitive filters with micro- and nano-grating structures

For isotropic dielectric thin films, polarization effect is an inherent characteristic. As it will make the performance of optical-electric system go to bad, such polarization-dependent properties are often intolerable and should be eliminated in many applications. In this paper, based on a micro- and nano-optical structure whose period consists of four parts, a polarization insensitive filter is obtained by combining rigorous wave theory and multi-objective immune optimization algorithm. Its working wavelength is 1315 nm which is often used in laser systems. The results of our design show that TE and TM polarized waves have reflectivities of 0.482 and 0.485, respectively at designed wavelength of 1315 nm. And it denotes that two values are both close to the design values, their difference is only 0.003, and polarization deviation is also very little. Therefore, the designed filter can eliminate the effect of polarization deviation very well at 1315 nm wavelength.

Silicon based polarization insensitive filter for WDM-PDM signal processing

We propose and fabricate a novel circuit that combines two two-dimensional (2D) grating couplers and a microring resonator (MRR). According to the polarization states, one 2D grating coupler first splits the input signals into two orthogonal paths, which co-propagate in the loop and share a common MRR, and then the two paths are combined together by the other 2D grating coupler. The proposed circuit is polarization insensitive and can be used as a polarization insensitive filter. For demonstration, the wavelength division and polarization division multiplexing (WDM-PDM) non return-to-zero differential-phase-shift-keying (NRZ-DPSK) signals can be demodulated successfully. The bit error ratio measurements show an error free operation, reflecting the good performance and the practicability.

SOI based ultracompact polarization insensitive filter for PDM signal processing

We propose and fabricate a scheme that combines a two-dimensional (2D) grating coupler and a delay interferometer (DI) in a loop configuration. According to the polarization states, the 2D grating coupler first splits the input signals into two paths, which copropagate in the loop and share a common DI, and then combines the two paths together to the same input port. The proposed device is polarization insensitive and can be used for polarization division multiplexed (PDM) signal processing. For demonstration, the PDM non return-to-zero differential-phase-shift-keying signals can be demodulated successfully. The bit error ratio measurements show an error free operation, reflecting the good performance and the practicability of the proposed device.

Bragg 격자구조가 집적된 편광 무의존성 방향성 결합기와 다중모드 간섭 결합기의 설계

This paper presents a rigorous comparison of the design characteristics of polarization-insensitive directional coupler (DC) and multimode interference (MMI) coupler based on rib type waveguides, by using longitudinal modal transmission-line theory (L-MTLT). It shows that the multimode mixing and interference property of MMI can be structurally designed through the continuous evolution of the two-mode coupling property of DC. It also compares and analyzes the coupling efficiency along with the coupling length and the wavelength between polarization-insensitive DC and MMI. From the design properties obtained, it demonstrates for the first time the integration of polarization-insensitive DC or MMI with a Bragg grating and evaluates precisely the filtering characteristics. The numerical results reveal that the DC, as long as it is designed to have the same coupling length for TE and TM modes, has better performance than the MMI in polarization-insensitive filtering behaviour. However, it shows that the MMI with much less coupling length than DC is preferred in the miniaturization of integrated devices.

Design of a fast tunable wavelength filter based on an arrayed waveguide grating

We present the design of a tunable wavelength filter based on an active arrayed waveguide grating (AWG). We show a novel layout and simulated performance of a tandem filter configuration that covers 43 nm in C band with 0.2-nm (25-GHz) channel spacing at –35-dB crosstalk level. Design of the device is based on Fourier-Fresnel formalism with special emphasis on tuning mechanisms. The Gerchberg-Saxton phase retrieval method is used to estimate phase errors and generate phase patterns required for device tuning. A polarization insensitive filter is presented with its main parameters and simulation results. The technological considerations of achieving nanoseconds-scale tunability by exploiting the electro-optical effect in LiNbO3 crystals are discussed in detail. Such a filter may find various applications in packet-switched coarse and dense multiwavelength dynamic networks.