Polarization-independent Directional Coupler Research Articles

Ultralow birefringent glass waveguide fabricated by femtosecond laser direct writing.

Optical waveguides prepared by femtosecond laser direct writing have birefringent properties, which can affect polarization encoding and entanglement on chips. Here, we first propose a shape-stress dual compensation fabrication scheme to decrease birefringence. Ultralow birefringent waveguides (1 × 10-9) were obtained by controlling the cross sectional shape of the main waveguide and adjusting the position of the auxiliary lines. In addition, we prepared polarization-independent directional coupler and demonstrated the evolution of polarization-independent waveguide array with different polarized light. In the future, ultralow birefringent waveguides will be widely applied in polarization encoding and entangled quantum photonic integrated circuits.

Polarization-independent, lithium-niobate-on-insulator directional coupler based on a combined coupling-sections design.

We have proposed a polarization-independent directional coupler (DC) by using sections with optimal coupling strength designs on the lithium-niobate-on-insulator platform. With this design, arbitrary polarization-independent coupling ratio ranging from 0% to 100% can be achieved by tuning the length of the identical coupling region. The DC exhibits ultralow excess losses (<0.1dB) and polarization-dependent losses (<0.05dB for the complete coupling) over a bandwidth of 100 nm. Moreover, the proposed DC is compact in size, simple in structure, and easy to fabricate.

A Hybrid-Plasmonic-Waveguide-Based Polarization-Independent Directional Coupler

In optical interconnection field, inconsistent coupling behaviors of TE and TM modes may cause abnormal operation. Therefore, it is very significant to implement a polarization-independent directional coupler. In this paper, a hybrid-plasmonic-waveguide-based polarization-independent directional coupler has been proposed. Since the energy of the hybrid plasmonic waveguides for TE and TM modes is distributed in different layers, we manage to achieve polarization independence by adjusting the material properties and dimensions of the corresponding layer. We optimize the parameters of our proposed directional coupler such as radius of silicon waveguide layer, height of silica waveguide layer, and the distance between the two waveguides, etc. Then performance of the directional coupler has been evaluated. It is worth mentioning that the length of the coupling section is only 4.25μm. Meanwhile, the polarization-dependent loss is only 0.393 dB, and the maximum coupling efficiency of TE and TM modes can reach 86.4% and 94.6%, respectively. Besides, the coupling efficiency of TM mode remains above 90% over the entire C-band, while the coupling efficiency also keeps at least 80% for TE mode. Finally, the manufacture process for the proposed directional coupler has been discussed. In brief, the improved polarization-independent directional coupler features small size, low energy loss, good polarization-independent characteristics, and wavelength insensitivity simultaneously. Compared to the other counterparts ever proposed, our proposed coupler can provide a perfect trade-off between device size and loss, which shows important potential applications in the fields of PIC and optical interconnection.

Broadband Polarization-Independent Directional Coupler Using Asymmetric-Waveguides

A high-performance broadband polarization-independent directional coupler is designed by using asymmetric-waveguides configuration on a silicon-on-insulator platform. Through the design of cascaded directional couplers, the coupling characteristics of the light with the orthogonally polarized transverse electric (TE) and transverse magnetic (TM) can be manipulated, so that the coupling strength could be equivalent between TE and TM polarizations within an ultra-broadband. Couplers with 50% splitting ratio are designed and simulated for operating in the wavelength range from 1450 nm to 1650 nm for both polarizations. Within the 100 nm wavelength range from 1500 nm to 1600 nm, the deviations of splitting ratio for TE polarization and TM polarization are $\pm$ 3.3% and $\pm$ 6.2%, respectively. And within the 200 nm wavelength range from 1450 nm to 1650 nm, the deviations for TE polarization and TM polarization are $\pm$ 9.0% and $\pm$ 7.8%, respectively. The footprint of this polarization-independent directional coupler is only 21 $\mu$ m × 1.6 $\mu$ m.

Ultra-Compact Subwavelength-Grating-Assisted Polarization-Independent Directional Coupler

We propose and experimentally demonstrate an ultra-compact, low-loss polarization-independent directional coupler on the silicon-on-insulator (SOI) platform. By exploiting subwavelength gratings in the directional coupler, the coupling strength could be made equal for both transverse-electric and transverse-magnetic polarizations. The demonstrated polarization-independent directional coupler has a device length of only 4.5 μm and achieves complete cross-coupling with a low excess loss of <;1 dB over a bandwidth of ~65 nm. The reported devices also demonstrate robust fabrication tolerance.

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 9.2μm is achieved, and the extinction ratio, insertion loss, and reflection loss are 33.25/31.41dB, 0.224/0.121dB, 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.

Silicon On-Chip PDM and WDM Technologies Via Plasmonics and Subwavelength Grating

Silicon photonics, which is compatible with standard CMOS technology, is the most promising platform for large-scale photonic integration. The rapid growth of data volume demands larger link capacity and higher integration density of photonic integrated circuits. Multiplexing technology is an effective approach to enhance the capacity of on-chip optical interconnects. This review provides an introduction to the silicon on-chip multiplexing devices using plasmonics and subwavelength grating (SWG). To expand the data transmission capacity, one solution is introducing wavelength-division-multiplexing structures where diplexer and triplexer are fundamental elements. Another is adopting polarization-division-multiplexing (PDM) technology. Polarizer, polarization beam splitter, polarization rotator, polarization beam splitter-rotator, and polarization-independent directional coupler are all essential components for integrated PDM scheme. By using plasmonics and SWG, the mode profile distribution, refractive index, birefringence, and dispersion can all be manipulated, which enables compact devices with high performance.

Polarization-independent directional coupler and polarization beam splitter based on asymmetric cross-slot waveguides.

The polarization dependence of a directional coupler (DC) based on asymmetric cross-slot waveguides is investigated. Due to structural birefringence, the coupling behaviors of the quasi-TE and quasi-TM modes in the DC vary with the waveguide geometry. A polarization-independent directional coupler (PIDC) and polarization beam splitter (PBS) are proposed by tailoring the ratio of the coupling length for quasi-TE and quasi-TM modes. The simulated results show that the coupling lengths of the designed PIDC and PBS are 8 and 28.25μm, respectively. Both the PIDC and PBS show an insertion loss (IL) <0.7 dB on a bandwidth over 100nm. The extinction ratios are ∼20 dB for PIDC and ∼14 dB for PBS. The fabrication-error tolerance of the practical devices is also discussed. In this study, we employ a commercial software tool for finite difference eigenmode and three-dimensional finite difference time domain methods to perform the numerical simulations.

Subwavelength-grating-assisted broadband polarization-independent directional coupler.

This Letter presents both numerical and experimental results of a polarization-independent directional coupler based on slot waveguides with a subwavelength grating. The measured coupling efficiency is 97.4% for TE and 96.7% for TM polarization at a wavelength of 1550 nm. Further analysis shows that the proposed subwavelength grating directional coupler has a fabrication tolerance of ±20 nm for the grating structure and that the coupling efficiencies for the two polarizations are both higher than -0.5 dB (∼89%), exceeding the entire C-band (1525-1570 nm) experimentally.

Polarization-independent hybrid plasmonic coupler for a silicon on insulator platform

Hybrid plasmonic waveguides consisting of a high-index slab separated from a metal plane by a low-index spacer provide an optimal compromise between the loss and confinement for surface plasmon waves in passive medium. In such hybrid structures, because power for the TE and TM modes are concentrated in two different regions of the guide, the characteristics of the two modes can be controlled independently by changing the waveguide dimensions. We propose to use this property to implement a hybrid plasmonic polarization-independent directional coupler for a silicon on insulator platform. We also investigate the effects of variations of wavelength and device dimensions on the performance of the proposed device.

基于slot波导的硅基长波红外偏振无关定向耦合器分析

The research of long-wave infrared silicon waveguided components is triggered by the development of integrated optic devices.The validity of Scale-up principle for designing the size of slot waveguides in this waveband is demonstrated,the characteristics such as contain factor and polarization were explained.The polarization-independent optical directional coupler based on slot waveguides was attained at the wavelength of 10.6μm.

Birefringence compensation for polarization-independent directional coupler wavelength filters

We show how polarization-independent optical wavelength filters can be achieved using asymmetric directional couplers with birefringence-compensation. Simulations demonstrate that excellent polarization behavior can be achieved with practical tolerances and without sacrificing filter bandwidth or insertion loss. >

Four-sectioned polarization-independent directional coupler with extremely relaxed fabrication tolerances

A new type of polarization-independent directional coupler is proposed. The principle, which utilizes an electrode pattern in four sections, gives extremely large fabrication tolerances in terms of allowed values on the L/l/sub c/ ratios (physical length/coupling length) for the two polarizations. The concept is analyzed by coupled wave theory, and the result is checked for one case with the beam propagation method.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>