Reduction of sidewall roughness in silicon-on-insulator rib waveguides

Silicon-on-insulator (SOI) rib waveguides with residual sidewall roughness were achieved through inductive coupled plasma reactive ion etching (ICPRIE) process. Sidewall roughness is the dominant scattering loss source. Conventional ICPRIE could result in the sidewall ripples derived from the etch/deposition cycle steps. Mixed ICPRIE process and hydrogen annealing were used to improve the sidewall roughness of SOI rib waveguides and eliminate the sidewall ripples. Scan electron microscope and atomic force microscope were used to demonstrate the surface profiles of the sidewall. The results indicated that the sidewall roughness could be low down to 0.3 nm level by optimization and combination of these two techniques and the ripples disappeared. According to the scattering theory developed by Payne and Lacey, the scattering loss could be reduced to below 0.01 dB/cm.     

Contradirectional couplers in silicon-on-insulator rib waveguides

We demonstrate contradirectional couplers in silicon-on-insulator rib waveguides using a CMOS compatible technology, in which a periodic dielectric perturbation is introduced in the coupling region between two different-sized rib waveguides. This structure enables high fabrication tolerances for narrow-bandwidth add-drop filters, using commercially available deep-ultraviolet lithography, that do not suffer from having a free spectral range. The simulation using coupled-mode theory and mode-profile calculations shows good agreement with experiment. A narrow bandwidth of 0.35?nm and a low loss of less than 1?dB have been achieved experimentally.     

Low-loss submicrometer silicon-on-insulator rib waveguides and corner mirrors

Rib microwaveguides are demonstrated on silicon-on-insulator substrates with Si film thickness of either 380 or 200 nm and a width of 1 microm. Corner mirrors that allow compact 90 degrees turns between two perpendicular waveguides are characterized. Measured propagation losses are approximately 0.4 dB/cm and approximately 0.5 dB/cm for 380-nm and 200-nm Si film, respectively, and mirror losses are approximately 1 dB. This allows the development of applications such as optical interconnects in integrated circuits over propagation distances larger than several centimeters.     

Direct measurement of sidewall roughness of polymeric optical waveguides

Sidewall roughness (SWR) of fluorinated polyether waveguides fabricated using reactive ion etching in pure oxygen gas was directly measured using atomic force microscopy (AFM). We confirm that SWR is not the replicate of line edge roughness (LER) of the waveguides. Statistical information such as standard deviation of roughness, autocovariance function (ACF), and autocorrelation length (ACL) have been obtained from AFM measurements. The ACL varies in the similar manner as SWR along the depth of the waveguide, and both are dependent on the etch depth. The depth dependence can be explained by the change in the arrival dynamics of etchant ions in a mechanism involving both shadowing and first-order reemission effects.     

SOI纳米线波导导光机理的物理分析

SOI（silicon-on-insulator）纳米线波导及其器件是近年来光电子学领域研究的重点内容之一．文章从基本的导波光学理论出发,引入古斯一汉森位移理论,对SOI纳米线波导导光的物理机制进行了分析并给出了物理解释和模拟结果．     

Side-wall roughness in SOI rib waveguides fabricated by inductively coupled plasma reactive ion etching

SOI rib waveguides were fabricated with vertical side walls using inductively coupled plasma reactive ion etching. The root-mean-square (rms) roughness of the side-wall surface was directly measured by atomic force microscopy. The rms roughness of the side-wall surface obtained by three-mask lithography is 28.73 nm, much higher than that of the one-mask-lithography SOI rib waveguide. The scattering loss induced from side-wall roughness is evaluated using Tiens theory, and is proportional to the square of the side-wall rms roughness. To reduce the rms roughness, hydrogen annealing was used to smooth the side-wall surface obtained by three-mask lithography. After hydrogen annealing, there is a significant drop in the rms roughness of the side-wall surface. PACS 42.82.Et; 42.81.Dp; 52.80.Yr; 68.37.Ps; 81.65.Ps     

SOI纳米线波导导光机理的物理分析

SOI（silicon-on-insulator）纳米线波导及其器件是近年来光电子学领域研究的重点内容之一.文章从基本的导波光学理论出发,引入古斯-汉森位移理论,对SOI纳米线波导导光的物理机制进行了分析并给出了物理解释和模拟结果.     

Polarization-independent grating couplers for silicon-on-insulator nanophotonic waveguides

We propose the use of subwavelength structures in a waveguide grating to achieve polarization-independent coupling of light between an optical fiber and a silicon-on-insulator (SOI) optical waveguide. The subwavelength structure allows the mode effective indices of the TE and TM modes in the grating section to be precisely engineered. We calculate that coupling efficiency of over 64% is possible using the proposed design for polarization-independent coupling between single-mode optical fibers and SOI nanophotonic waveguides.     

Coupled-mode theory of the Raman effect in silicon-on-insulator waveguides

Coupled-mode theory is used to calculate Raman gain and spontaneous efficiency in silicon waveguides with cross-sectional areas ranging from 0.16 to 16 microm2. We find a monotonic increase in the Raman gain as the waveguide cross section decreases for the range of dimensions considered. It is also found that mode coupling between the Stokes modes is insignificant, and thus polarization multiplexing is possible. The results also demonstrate that for submicrometer waveguide dimensions the Einstein relation between spontaneous efficiency and stimulated gain no longer holds.     

基于硅基槽波导的单纤三向滤波器的研究

设计了基于硅基槽波导的微环谐振型单纤三向滤波器。基于电磁场理论得到了二维情形下的硅基槽波导的模场及有效折射率随波长的变化关系,在此基础上,利用硅基槽微环谐振腔谐振波长与微环半径的关系,优化得到了可将1490nm和1550nm两个下载波长分开的谐振环半径,并同时将1310nm波长上传,三个波长信号从不同端口输出,成功地实现了三向器滤波功能。利用传递函数法模拟了所设计的硅基槽微环谐振型三向滤波器的输出光谱,结果显示其串扰可低至-16.9dB。     

Magneto-optical nonreciprocal phase shift in garnet/silicon-on-insulator waveguides

We demonstrate the integration of a single-crystal magneto-optical film onto thin silicon-on-insulator (SOI) waveguides by use of direct wafer bonding. Simulations show that the high confinement and asymmetric structure of SOI allows an enhancement of approximately 3x over the nonreciprocal phase shift achieved in previous designs; this value is confirmed by our measurements. Our structure will allow compact magneto-optical nonreciprocal devices, such as isolators, integrated on a silicon waveguiding platform.     

Scattering at sidewall roughness in photonic crystal slabs

We have simulated the effect of sidewall roughness in photonic-crystallike structures with different vertical refractive-index contrast. We treated the scattering off a sidewall irregularity as a radiating dipole excited by the incident waveguide mode. We show that the loss that is due to this scattering is significantly larger for structures with a low refractive-index contrast (such as GaAs/AlGaAs waveguides) than for structures with a high vertical index contrast (such as silicon-on-insulators and membranes).     

Compact efficient broadband grating coupler for silicon-on-insulator waveguides

We have designed a high-efficiency broadband grating coupler for coupling between silicon-on-insulator (SOI) waveguides and optical fibers. The grating is only 13 microm long and 12 microm wide, and the size of the grooves is optimized numerically. For TE polarization the coupling loss to single-mode fiber is below 1 dB over a 35-nm wavelength range when using SOI with a two-pair bottom reflector. The tolerances to fabrication errors are also calculated.     

ARROW-based silicon-on-insulator photonic crystal waveguides with reduced losses

We employ an antiresonant reflecting layers arrangement with silicon-on-insulator based photonic crystal waveguides. The 3D FDTD numerical modelling reveals improved transmission in such structures with a promising potential for their application in photonic circuits.     

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.     

Supermode dispersion and waveguide-to-slot mode transition in arrays of silicon-on-insulator waveguides

In this Letter, we report group index measurements of the supermodes of an array of two strongly coupled silicon-on-insulator waveguides. We observe coupling-induced dispersion that is greater than the material and waveguide dispersion of the individual waveguides. We demonstrate that the system transforms from supporting the two supermodes associated with two coupled waveguides to the single mode of a slot waveguide within the investigated spectral range. During the cutoff of the antisymmetric supermode, an anti-crossing between the symmetric TM and antisymmetric TE supermodes has been observed.     

Reduced lateral leakage losses of TM-like modes in silicon-on-insulator ridge waveguides

We numerically investigate the lateral leakage loss behavior for TM-like modes in silicon-on-insulator ridge waveguides. In order to improve the leakage loss properties, we propose a novel ridge waveguide structure where a dimple is introduced at the ridge center. It is shown that the ridge waveguide with a dimple is both low loss and fabrication tolerant. This behavior is predicted by not only an accurate finite-element-based analysis but also a simple, phenomenological effective-index-based analysis.     

Dispersion characteristics of nanometer-scaled silicon rib waveguides

<正>We investigate the dispersion properties of nanometer-scaled silicon waveguides with channel and rib cross section around the optical fiber communication wavelength and systematically study their relationship with the key structural parameters of the waveguide.The simulation results show that the introduction of an extra degree of freedom in the rib depth enables the rib waveguide more flexible in engineering the group velocity dispersion(GVD) compared with the channel waveguide.Besides,we get the structural parameters of the waveguides that can realize zero-GVD at 1550 nm.     

Ultrafast femtosecond all-optical modulation through nondegenerate two-photon absorption in silicon-on-insulator waveguides

We present numerical investigations of ultrafast femtosecond (with time duration of 100 fs at 1/e intensity point) all-optical modulation of a pump-probe wave arrangement by using nondegenerate two-photon absorption (TPA), namely cross absorption, inside silicon-on-insulator (SOI) optical waveguides. Our results show that when a pump pulse with femtosecond duration and a continuous probe wave are co-propagating along the SOI, the probe wave can be modulated inversely by the ultrafast pump pulse, whose modulation depth depends strongly on the system parameters such as the waveguide length, the peak power and initial chirp of the pump wave, the group velocity dispersion (GVD), etc.; this means that the modulation depth can be improved by an appropriate increase of the waveguide length, the pump peak, and the initial chirp, in addition, which has a larger value for the probe wavelength in the normal dispersion regime compared with the case of abnormal dispersion when the center wavelength of the pump wave is located at the zero-dispersion wavelength.     

Design for beam splitting components employing silicon-on-insulator rib waveguide structures

We present a new design for beam splitting components employing a silicon-on-insulator rib waveguide structures. In the new design, a high-index thin-film layer is deposited in the rib section to reduce the wave field dispersive tails in the slab section and accordingly render the mode field a confined spot. This in turn improves the beam splitting performance of some conventional waveguide components such as y branches and multimode interference couplers (MMICs), in terms of the excess loss, fiber coupling loss, and compactness of these components. For a 1 x 2 y-branch beam splitter, the excess loss can be as small as 0.43 dB in the new design, which is much lower than that for a conventional rib waveguide structure (which is 1.28 dB). For a 1 x 2 MMIC in our example, the new rib waveguide structure presents an excess loss of 0.064 dB for the TE mode and 0.046 dB for the TM mode, with negligible nonuniformity in dimensions of 30 microm x 1040 microm, whereas its counterpart (i.e., the one with the same dimensions but without a thin-film layer) presents an excess loss of approximately 0.86 dB for both modes. A conventional MMIC must have dimensions larger than 70 microm x 5650 microm to maintain almost the same low excess loss.