Design and fabrication of compact 3-dB coupler in silicon-on-insulator

A novel compact 3-dB coupler is proposed and fabricated in silicon-on-insulator. The new coupler provides large output waveguide spacing of 125.0 μm with short device length of 867.0 μm and large cross-section of 6.0 × 4.0 μm. The device length can be remained essentially unchanged even when the output spacing is further enlarged to realize the complex applications in photonic integrated circuits. The fabricated device exhibits excess loss of 6.3 dB and low imbalance of 0.26 dB.     

Novel multimode interference devices for low index contrast materials systems featuring deeply etched air trenches

A novel design of multimode interference (MMI) device is proposed for photonic integrated circuits fabricated in low index contrast materials. In this design, deeply etched air trenches are used to define the boundaries of the multimode section to achieve strong lateral confinement. This results in superior imaging quality and therefore lower loss and imbalance, which can only be achieved in strongly guiding MMIs. However, the access waveguides remain weakly guiding, which is essential for low loss fibre connection. We demonstrate the merit of the design in 1 × 4 MMI power splitters. By introducing air trenches, loss decreases from 0.45 to 0.053 dB and imbalance decreases from 0.15 to 0.058 dB for TE polarization. For TM polarization, loss decreases from 0.46 to 0.003 dB and imbalance decreases from 0.16 to 0.067 dB.     

掩埋式弱限制MMI的性能模拟

采用精确模式分析方法(EMA)对一种掩埋式折射率差为0.47%的弱限制MMI进行了数值模拟,通过和强限制MMI对比表明,这种弱限制的MMI可以实现良好的均衡性(9.7×10-4dB)和附加损耗(0.13dB),比强限制MMI具有更大的带宽,更好的宽度和长度容差性。虽然这种弱限制MMI的最小附加损耗和最佳均衡性不能在同一长度同时获得,但由于其良好的容差性,在最佳均衡性长度处仍能获得低于0.8dB的附加损耗。用紫外写入波导的方法制作掩埋式结构的弱限制MMI器件是可行的。     

Asymmetric multimode interference isolator based on nonreciprocal phase shift

This paper presents an asymmetric multimode interference-based (MMI) optical isolator, by utilizing the magneto-optical nonreciprocal phase shift (NPS). Equivalent beam propagation method (BPM) simulation of symmetric and asymmetric isolators are performed using configuration of Air/Ce:YIG/SiO₂ on silicon substrate for integration. The asymmetric isolator is found to be much more compact in size and efficient in isolation compared with symmetric isolator. Simulation results show that the isolation of asymmetric structure is 23.8 dB higher than that of symmetric one. It is mainly because both symmetric mode and anti-symmetric mode are excited in asymmetric structure and hence can interfere destructively. The proposed device may play an important role in the optical communication systems and photonic integrated devices.     

Loss reduction in silicon nanophotonic waveguide micro-bends through etch profile improvement

Single mode silicon photonic wire waveguides allow low-loss sharp micro-bends, which enables compact photonic devices and circuits. The circuit compactness is achieved at the cost of loss induced by micro-bends, which can seriously affect the device performance. The bend loss strongly depends on the bend radius, polarization, waveguide dimension and profile. In this paper, we present the effect of waveguide profile on the bend loss. We present waveguide profile improvement with optimized etch chemistry and the role of etch chemistry in adapting the etch profile of silicon is investigated. We experimentally demonstrate that by making the waveguide sidewalls vertical, the bend loss can be reduced up to 25% without affecting the propagation loss of the photonic wires. The bend loss of a 2 μm bend has been reduced from 0.039dB/90° bend to 0.028dB/90° bend by changing the sidewall angle from 81° to 90°, respectively. The propagation loss of 2.7 ± 0.1dB/cm and 3 ± 0.09dB/cm was observed for sloped and vertical photonic wires respectively was obtained.     

Compact and efficient coupler to interface hybrid dielectric-loaded plasmonic waveguide with silicon photonic slab waveguide

A coupler is proposed to interface a hybrid dielectric-loaded plasmonic waveguide (HDLPW) with a silicon photonic slab waveguide. The HDLPW is firstly designed and optimized to attain the best tradeoff between the mode confinement and the propagation distance. The designed coupler is inspired from the taper configuration and numerically modeled through finite-difference time-domain (FDTD) simulation. The results demonstrate that a high confinement and low loss of the energy is achieved from a silicon photonic slab waveguide into the dielectric slot of area 50×200 nm² in the HDLPW. The transmission attained through the coupler with a compact size of 400 nm is found to be as high as 80% (1 dB). Further, the planar nature of taper configuration makes the coupler easy to fabricate using the state-of-the-art CMOS facilities. The proposed coupler is useful in enabling the integration between photonic and hybrid plasmonic waveguides and thus realizing on-chip hybrid integrated circuits.     

An ultra‐compact multimode interference coupler with a subwavelength grating slot

Multimode interference couplers (MMIs) are fundamental building blocks in photonic integrated circuits. Here it is experimentally demonstrated, for the first time, a two‐fold length reduction in an MMI coupler without any penalty on device performance. The design is based on a slotted 2 × 2 MMI fabricated on a commercial silicon‐on‐insulator (SOI) substrate. The slot is implemented with a subwavelength grating (SWG) comprising holes fully etched down to the oxide cladding, thereby allowing single etch step fabrication. The device has been designed using an in‐house tool based on the Fourier Eigenmode Expansion Method. It has a footprint of only 3.5 μm x 23 μm and it exhibits a measured extinction ratio better than 15 dB within the full C‐band (1530 nm‒1570 nm). SWG engineered slots thus offer excellent perspectives for the practical realization of MMIs couplers with substantially reduced footprint yet with outstanding performance.     

Uneven splitting-ratio 1×2 multimode interference splitters based on silicon wire waveguides

Two types of 1×2 multi-mode interference(MMI) splitters with splitting ratios of 85:15 and 72:28 are designed.On the basis of a numerical simulation,an optimal length of the MMI section is obtained.Subsequently,the devices are fabricated and tested.The footprints of the rectangular MMI regions are only 3×18.2 and 3×14.3(μm).The minimum excess losses are 1.4 and 1.1 dB.The results of the test on the splitting ratios are consistent with designed values.The devices can be applied in ultra-compact photonic integrated circuits to realize the "tap" function.     

Compact triplexer in two-dimensional hexagonal lattice photonic crystals

We design a compact triplexer based on two-dimensional (2D) hexagonal lattice photonic crystals (PCs).A folded directional coupler (FDC) is introduced in the triplexer beside the point-defect micro-cavities and line-defect waveguides.Because of the reflection feedback of the FDC,high channel drop efficiency can be realized and a compact size with the order of micrometers can be maintained.The proposed device is analyzed using the plane wave expansion method,and its transmission characteristics are calculated using the finite-difference time-domain method.The footprint of the triplexer is about 12× 9 μm,and its extinction ratios are less than –20 dB for 1310 nm,approximately –20 dB for 1490 nm,and under –40 dB for 1550 nm,making it a potentially essential device in future fiber-to-the-home networks.     

新型1×N多模干涉分束器的三维设计与模拟

相比于传统的1×N对称型多模干涉（Mult-Mode Interference,MMI）分束器设计,提出了一种新型埋入式弱限制光波导分束器件.它的干涉区及输入输出波导采用倒锥形式,器件尺寸减小,且不均匀性与附加损耗也减小.以1×4的对称型MMI分束器为例,当只对干涉区采用倒锥形结构后,在TE偏振中心波长为1.55 μm时,器件长度减小了500 μm,均匀性增加了0.131 dB,而附加损耗仅增加了0.02 dB,波长响应较传统设计增加了40 nm.在此基础上,又在输入输出臂上也各增加倒锥形结构后,相比于传统设计附加损耗减小了0.02 dB,均匀性增加了0.139 dB,器件长度减小了500 μm.改进后的器件具有优越的容差性.器件采用掺氟型聚合物材料进行优化设计,通过在合理范围内偏离输出波导位置,使输出光强达到最大值.     

Low-loss, low-cross-talk crossings for silicon-on-insulator nanophotonic waveguides

We present compact crossings for silicon-on-insulator photonic wires. The waveguides are broadened using a 3 microm parabolic taper in each arm. By locally applying a lower index contrast using a double-etch technique, loss of confinement is reduced and 97.5% transmission (-1.7 dB) is achieved with only -40 dB cross talk.     

Low-loss, compact waveguiding with TE mode in metal/dielectric waveguides for planar lightwave circuit

We propose a low-loss metal/dielectric waveguide for compact planar lightwave circuit. The basic waveguide structure is a metal-defined high-index-contrast strip waveguide based on silicon/silica. As the guide is designed for TE single mode waveguiding, extremely low propagation loss (e.g. <0.04 dB/cm), very low bend loss (e.g. 0.0043 dB/90°-turn) and small waveguide pitch of zero-crosstalk are theoretically achievable, and can be further improved by compromising with component size and density. Examples of multi-bends and device integration are demonstrated with numerical simulations. The proposal is compatible with silicon technology and appealing for development of silicon-based planar lightwave circuit.     

A new simulation design of three-mode division (de)multiplexer based on a trident coupler and two cascaded 3 × 3 MMI silicon waveguides

We propose a design of a silicon three-mode (de)multiplexing device based on a trident and two cascaded 3 × 3 multimode interferometers. Input lights at fundamental, first-order, and second-order modes of transverse electric (TE) polarization are simultaneously converted to fundamental TE mode and demultiplexed at different ports at the outputs. The design is carried out through both theoretical analysis and numerical simulation using three dimensional-beam propagation method and effective index method. The results show a successful three-mode multiplexing in 100 nm wavelength range around 1550 nm with low insertion loss (< 0.71 dB) and crosstalk (? 18 dB). The proposed device also exhibit a small footprint (5 µm × 400 µm) that makes it potential for not only wavelength-division multiplexing and multimode-division multiplexing transmission systems, but also high bitrate and compact on-chip silicon photonics integrated circuits.     

Optical characteristics of one-dimensional photonic crystals composed of high-aspect-ratio Si walls fabricated on V-grooved wafer

We demonstrate optical properties of one-dimensional photonic crystals (PC), which are fabricated using high-aspect-ratio etching on a V-grooved silicon wafer. The measured transmission spectrum has an obvious band gap; the suppression is over 30 dB. The quite small insertion loss of 1.9 dB is achieved by induced coupled plasma (ICP) cryogenic etching and direct coupling to the optical fiber aligned in the V-groove. We also successfully observed peaks originating from a localized cavity mode. Such a microcavity enables control of the light, which qualifies photonic crystal as a fundamental structure of optical functional devices. These results lead to achievement of integrated Si-based photonic circuits.     

Integrated thin film InGaAsP laser and 1 x 4 polymer multimode interference splitter on silicon

We report on a thin film InGaAsP laser integrated with a 1 x 4 polymer multimode interference (MMI) splitter on a silicon substrate for planar optical signal distribution. The thin film laser had a threshold current of 40 mA and was endfire coupled to the integrated passive polymer MMI splitter, and the optical signal from the laser was distributed to the four output waveguides of the MMI coupler. The measured loss of the MMI splitter was 0.79 dB. The normalized powers of the four MMI output ports in the integrated system were measured to be 0.823, 0.978, 0.852, and 1.     

Extraneous self-imaging phenomenon with weak-guiding condition

We discuss a self-imaging phenomenon in a multimode interference (MMI) coupler. From experiment, different self-images, which are undefined in MMI theory, are observed. These undefined self-images are named "extraneous self-images" (Ex_SIs) for convenience. To estimate the applicability of the Ex_SIs, the characteristics of both the single self-image (0 dB self-image, 0 dB SI), which is defined in MMI theory, and the Ex_SI are compared and analyzed through simulation and experiment. The results show that the Ex_SI has an imaging period that is the same as the 0 dB SI and that the excess loss and the extinction ratio of the Ex_SI improve more than that of the 0 dB SI as the imaging period increases.     

Compact four-channel reconfigurable optical add-drop multiplexer using silicon photonic wire

We designed and fabricated a four-channel reconfigurable optical add-drop multiplexer based on silicon photonic wire waveguide, which is controlled through the thermo-optic effect. The effective footprint of the device is about 1000 × 500 μm². The minimum insertion loss including the transmission loss and coupling loss is about 10.7 dB. The tuning bandwidth is about 17 nm, the average tuning efficiency about 6.11 mW/nm and the tuning speed about 24.5 kHz.     

基于自准直效应的光子晶体异质结偏振分束器

基于光子晶体的自准直效应和禁带特性,提出了一种具有非正交异质结结构的光子晶体偏振分束器.无需引入缺陷或波导,可使光波在该结构中准直无发散地传输并实现分束功能,对制造工艺的要求大大降低.利用Rsoft软件,结合平面波展开法和二维时域有限差分法,对提出的偏振分束器进行了仿真研究.结果表明,该偏振分束器在一个较大的频率范围f=0.275—0.285(a/λ)内可实现横电(TE)和横磁(TM)模的大角度偏振分离,TE和TM模的透过率均在88%以上,偏振消光比分别大于26.57 dB和17.50 dB.该结构可应用到太赫兹波段的传输系统中,a=26μm,尺寸大小为572μm×546μm,在91—95μm波长范围内可实现TE和TM模的分离.利用该结构可设计用于光通信系统(n=3.48)的偏振分束器,a=426.25 nm,结构仅为9.38μm×8.95μm.本方案结构简单,易于集成,有望在集成光路的发展中发挥重要作用.     

High-speed compact silicon digital optical switch with slot structure

A high-speed compact silicon digital optical switch (DOS) is proposed in this paper. The direct electro-optic effect is applied by filling electro-optic polymer in the void slot of the branches, which compensates the limitation of silicon itself. The crosstalk of about 35 dB and the insertion loss of 0.7 dB is obtained, the switching speed is less than 1 ps, and the whole device length can be shortened to 616 μm even using the basic mode-evolving principle and a simple Y-type structure. Analysis also shows that the device has good fabrication tolerance and wavelength independence over the C-band.     

Coupling loss minimization of slow light slotted photonic crystal waveguides using mode matching with continuous group index perturbation

We experimentally demonstrate highly efficient coupling into a slow light slotted photonic crystal waveguide. With optical mode converters and group index tapers that provide good optical mode matching and impedance matching, a nearly flat transmission over the entire guided mode spectrum of 68.8 nm range with 2.4 dB minimum insertion loss is demonstrated. Measurements also show up to 20 dB baseline enhancement and 30 dB enhancement in the slow light region, indicating that it is possible to design highly efficient and compact devices that benefit from the slow light enhancement without increasing the coupling loss.