Compact double-grating coupler between vertically stacked silicon-on-insulator waveguides

We analyze a compact double-grating coupler that provides coupling through radiation modes between two vertically stacked silicon-on-insulator waveguides. The grating is sufficiently strong to be considered a one-dimensional photonic bandgap structure that facilitates a short coupling length. Simulations suggest that a 29% coupling efficiency is achievable in coupling light from one waveguide to another with 12.9 microm long binary gratings. We found that the coupling efficiency is enhanced by Fabry-Perot resonance between two gratings. The coupling efficiency can be increased by use of a blazed grating. We use the eigenmode expansion method to design and optimize the binary grating coupler, and the results are verified by use of the finite-difference time-domain method.     

Design of a high-efficiency grating coupler based on a silicon nitride overlay for silicon-on-insulator waveguides

A waveguide grating coupler based on a silicon nitride overlay at 1.55 μm for TE polarization is designed with no experimental demonstration. Its coupling efficiency for a fiber is 76%, the 1 dB bandwidth is 75 nm, and the coupling angle is 10°. The effects of different device parameters on the coupling performance for the grating coupler are discussed. The coupling efficiency of our grating coupler is almost equal, yet the 1 dB spectral bandwidth is around 25 nm broader, as compared with the grating coupler design based on a poly-silicon overlay. The coupling performance of our coupling device could still be further improved. The grating coupler presented in this paper is applicable to the optical coupling in nanophotonic integrated circuits.     

Directional coupler based on radiatively coupled waveguides

We investigate a system of two waveguides with leaky modes sharing a common substrate (radiatively coupled waveguides). The main advantage of such a system is the possibility of remote coupling. A perturbation theory is developed for both TE and TM polarization. Numerical calculations of dispersion curves and of the coupling length allow us to determine the limitations of the perturbation theory. We study the influence of multimode interference on the process of beating by considering the propagation of a given initial field. Finally, we propose a new design for an effective, integrated optical TE-TM polarization splitter.     

Controlling the polarization dependence of dual-channel directional couplers formed by silicon-on-insulator slot waveguides

The polarization dependence of directional couplers (DC) formed by silicon-on-insulator (SOI) slot waveguides was studied, and its applications as highly efficient polarization beam splitters (PBSs) and polarization-independent directional couplers (PIDCs) were investigated. The coupling lengths for the quasi-TE and quasi-TM modes may vary with the waveguide geometry due to structural birefringence; thus numerical simulations of the coupling effects in the directional couplers with different aspect ratios and waveguide spacing were conducted to obtain the optimal design parameters for high efficiency as well as compact device size. The lengths of the coupling regions of the designed PBS and PIDC are 47.61 and 23.13 μm, respectively, and they delivered good performance, with an extinction ratio greater than 20 and 1 dB bandwidth larger than 100 nm. The tolerance of fabrication error in the practical device is also discussed.     

Athermal arrayed waveguide gratings in silicon-on-insulator by overlaying a polymer cladding on narrowed arrayed waveguides

Athermal arrayed waveguide gratings (AWGs) in silicon-on-insulator (SOI) are experimentally demonstrated for the first time to our knowledge. By using narrowed arrayed waveguides, and then overlaying a polymer layer, the wavelength temperature dependence of the AWGs is successfully reduced to -1.5 pm/°C, which is more than 1 order of magnitude less than that of normal SOI AWGs. The athermal behavior of the AWGs is obtained with little degradation of their performance. For the central channel, the cross talk is less than -15 dB and the insertion loss is around 2.6 dB. Good characteristics can be maintained with temperatures up to 75 °C. The total size of the device is 350 μm × 250 μm.     

Ultracompact wavelength and polarization directional coupler based on nanowire waveguides

In this paper, we numerically study and propose a wavelength and polarization selective directional coupler based on nanowire waveguides. The plane wave expansion method is used to design the structures and the finite-difference time–domain method is used to evaluate the device performance, which agrees well with each other. The results show that the proposed directional coupler can realize functions of the wavelength and polarization splitters simultaneously without changing the structure parameters. The crosstalks for both wavelength and polarization splitters are lower than ?17 dB at telecommunication wavelengths. The proposed structure has the advantages of small foot print, fabrication without complex nanostructures and complementary metal-oxide-semiconductor compatibility, which is promising for highly dense photonic integration.     

Liquid concentration sensor based on slot waveguide microresonators

A schematic diagram and operating principle are considered for an integrated optical concentration sensor based on slot waveguide microresonators, due to whose use the sensitivity of the sensor to a change in concentration may reach 0.001–0.01%. By means of this device, it is possible the concentration of both still gases and liquids and those flowing with a considerable velocity. The sensor may have a considerable number of sensing elements that makes it possible to monitor concentration simultaneously at different points of the volume of substance.     

Comment on "Low-loss terahertz ribbon waveguides"

We point out an important fundamental limitation associated with the dielectric ribbon waveguide described by Yeh [Appl. Opt.44, 5937 (2005)], which would restrict its practical use in ultrahigh- or terahertz-speed guided-wave applications, contrary to their underlying prediction, and we additionally point out a noteworthy inaccuracy in their introductory discussion.     

基于光纤耦合器的全光纤链路锁相控制

提出了一种基于光纤耦合器的全光纤链路锁相方法。通过光纤交叉互联使得出射光束与本振激光之间以及光纤端面后向回光相互间在光纤耦合器内进行干涉以获得性能指标,利用随机并行梯度下降算法(SPGD)对性能指标进行盲优化控制,实现多路出射光束在光纤端面处的相位锁定。首先建立了这种锁相方法的模型,讨论其锁相控制的稳态条件。搭建了两路全光纤链路锁相实验,并对两路出射光纤激光进行准直发射,通过相机获得的远场干涉条纹图像来判定锁相精度。实验结果表明,在引入正弦相位扰动幅值4个波长和频率2 Hz的情况下,该锁相方法能够将10 s长曝光图像条纹对比度从开环时的0.25提升至闭环时的0.82,单帧短曝光图像条纹对比度从开环时的0.65提升至闭环时的0.98。     

Filtering and sensing properties based on metal-dielectric-metal waveguide with slot cavities

Abstract

A triple-channel plasmonic system, consisting of several slot cavities coupled with the bus and drop waveguides, has been investigated numerically and theoretically. The impacts of adjusting various parameters on transmissivity are researched in detail. The results demonstrate that one can separate arbitrary wavelength and attain high transmissivity by suitably setting parameters. The sensitivity of the structure is about 950 nm shift per refractive index unit and the figure of merit is as high as 25.5. The plasmonic filter system possesses advantage of easy fabrication and compactness, which can find more applications in highly integrated optical devices, optical communication, and high sensitivity nano sensor.     

基于模式耦合器的锁模掺镱光纤激光器

本文提出了一种可以实现光纤高阶模式(HOM)在激光腔内振荡的锁模掺镱光纤激光器。通过使用一对级联的模式选择耦合器(MSC)作为有效的模式转换器,获得光纤锁模激光腔内HOM产生。其中,制备的MSC中心波长为1064 nm,可实现80 nm的模式转换带宽和94%的高阶模式纯度。通过搭建掺镱锁模光纤激光器,实验获得了3 dB谱宽7.4 nm、脉冲重复频率10.9 MHz、射频信噪比55 dB的锁模脉冲激光,输出功率的斜率效率为2.3%。实验证明,这种方法可在激光器内部通过模式级联转换,且能参与腔内锁模过程获得脉冲HOM激光。     

Low-loss waveguides fabricated in BK7 glass by high repetition rate femtosecond fiber laser

For the first time femtosecond-laser writing has inscribed low-loss optical waveguides in Schott BK7 glass, a commercially important type of borosilicate widely used in optical applications. The use of a variable repetition rate laser enabled the identification of a narrow processing window at 1 MHz repetition rate with optimal waveguides exhibiting propagation losses of 0.3 dB/cm and efficient mode matching to standard optical fibers at a 1550 nm wavelength. The waveguides were characterized by complementary phase contrast and optical transmission microscopy, identifying a micrometer-sized guiding region within a larger complex structure of both positive and negative refractive index variations.     

Transmission characteristic of interaction between Airy pulses and solitons in silicon-on-insulator waveguides

In this paper, we numerically simulate the transmission characteristics of Airy pulses and solitons interacting in silicon-on-insulator waveguides by utilizing Split-step Fourier method implemented in MATLAB. During transmission, the energy of Airy pulse is focused, and a new Airy pulse is subsequently separated from frontal edge. The new Airy pulse and the initial pulse exhibit reversal characteristics in time domain. In process of interaction with soliton, partial energy of the main lobe is separated from Airy pulse and transmitted along soliton transmission trajectory. The soliton produce a series of end edge oscillations. When the soliton is affected by Airy pulse, the propagation distance decreases, in constrast to the trailing edge oscillation frequency. In addition, third-order dispersion, Kerr effect and two-photon absorption on Airy pulse and soliton are also mentioned. At the same time, the effect on transmission of Airy pulse and solition will be showed in different truncation coefficients, powers and pulse widths.     

Design and analysis of a highly efficient coupler between a micro/nano optical fiber and an SOI waveguide

A compact coupling structure is proposed for highly efficient coupling between a micro/nano fiber and a silicon-on-insulator waveguide. The proposed structure is characterized by high coupling efficiency, wavelength insensitivity, large misalignment tolerance, and easy fabrication. Theoretical analysis and numerical simulation results show that coupling efficiency of >90% can be achieved with a taper length of ～4.5 μm.     

利用WDM光纤耦合器的光纤光栅传感解调技术

根据 WDM 光纤耦合器波长解调方案的工作原理、偏振特性以及影响系统波长分辨力的因素,提出一种改进的利用 WDM 光纤耦合器的光纤光栅传感解调技术。该技术在原技术的基础上,采用偏振控制器控制入射光偏振状态,提高了解调的精度和稳定性。对 WDM 光纤耦合器的多次波长扫描结果表明,采用偏振控制器后,其波长误差可减小到 5pm 左右。实验采用 1540/1560nm的 WDM 光纤耦合器对单点光纤光栅应变传感器进行静态解调,结果表明:按此技术开发的解调系统具有 0.01nm 波长分辨力和 10nm 的波长线性解调范围。     

Impact of two-photon absorption on soliton trapping in silicon-on-insulator waveguides

In this paper, we present a numerical simulation study of the effects of two-photon absorption on soliton trapping occurring between two components decomposed from a soliton pulse. The two pulse components polarize along the polarization directions of the quasi-TE mode and quasi-TM mode in a silicon-on-insulator waveguide. The effects of free carriers generated by two-photon absorption are also investigated, leading to free-carrier absorption and free-carrier dispersion. The simulation image of the propagation of solitons is provided by solving the coupled nonlinear Schrödinger equation satisfied by the quasi-TE mode component and the quasi-TM mode component utilizing the split-step Fourier method with MATLAB. The simulation shows that two-photon absorption and free carriers will hinder the formation of soliton trapping. As the incident pulse power increases, the soliton trapping will occur, inspite of the presence of two-photon absorption and free carriers. The simulation also indicates that if the value of the free-carrier’s lifetime is comparable to the value of the pulse width, the formation of soliton trapping is susceptible to the lifetime of free carriers. With the incident pulse width increasing, soliton trapping forms between the mean peak of two polarization components, whereas it originally formed between the mean peak of TE mode and secondary peak of TM mode at small pulse width.     

Manipulation of polarization-dependent effects by magnetostrictive stress on silicon-on-insulator rib waveguides

Polarization dependent loss (PDL) can cause deterioration in optical network performance owing to the resultant fluctuation in received power because of random changes in fiber polarization. We describe the use of a magnetostrictive layer integrated on a planar light-wave circuit that can offer the functionality of modifying the variable PDL or differential group delay produced within the typical tolerances of volume manufacturing. This approach provides an alternative to the sizable delay lines and splitters that were previously employed for polarization compensation. We demonstrate adjustment of polarization-dependent parameters by the application of an external magnetic field to a ferromagnetic layer adjacent to the waveguide.     

Direct integration of nanoscale conventional and slot waveguides

Slot waveguides can provide high optical confinement in a nanoscale low-index layer. While a conventional waveguide has a Gaussian-like Eigenmode profile, the Eigenmode profile of a slot waveguide is quite non-Gaussian type, due to the large discontinuity of refractive indices and thus the transverse electric field component between the high and low index layers of a slot waveguide. Although the field profiles of the two types of waveguides seem different, here we show that direct integration of conventional and slot waveguides yields efficient coupling of light into and out of slot waveguides using the rigorous finite-difference time domain method. The proposed direct coupling method has comparable performance to recently proposed taper based coupling methods, while having advantages in easier integration with conventional waveguide optics and higher integration density. We also show that coupling efficiency is not sensitive to the symmetricity of the slot waveguide, resulting in good manufacturing tolerance. The proposed direct coupler may have a number of applications in lightwave interconnects, sensing and data storage.