Compact 3-dB tapered multimode interference coupler in silicon-on-insulator

We have fabricated a compact 3-dB multimode interference coupler with a large silicon-on-insulator cross section. To reduce the length of the usual symmetric interference multimode interference coupler, we propose using a parabolically tapered structure. The length of the device is 398microm . The device has a uniformity of 0.28 dB.     

Ultra-low-loss inverted taper coupler for silicon-on-insulator ridge waveguide

An ultra-low-loss coupler for interfacing a silicon-on-insulator ridge waveguide and a single-mode fiber in both polarizations is presented. The inverted taper coupler, embedded in a polymer waveguide, is optimized for both the transverse-magnetic and transverse-electric modes through tapering the width of the silicon-on-insulator waveguide from 450 nm down to less than 15 nm applying a thermal oxidation process. Two inverted taper couplers are integrated with a 3-mm long silicon-on-insulator ridge waveguide in the fabricated sample. The measured coupling losses of the inverted taper coupler for transverse-magnetic and transverse-electric modes are ∼ 0.36 dB and ∼ 0.66 dB per connection, respectively.     

Properties, applications and fabrication of photonic crystals with ring-shaped holes in silicon-on-insulator

We show that photonic crystals with ring-shaped holes (RPhCs) exhibit superior properties compared to conventional photonic crystals (PhCs). At low air-fill factors RPhCs can have a larger bandgap than conventional PhCs. Moreover, RPhC waveguides with both high group index and small group velocity dispersion can be designed. RPhC waveguides are also more sensitive to external refractive index changes, which is attractive for sensor applications. Finally we set up a procedure to pattern RPhCs in silicon-on-insulator.     

Optical wave breaking of high-intensity femtosecond pulses in silicon optical waveguides

We have investigated numerically the propagation of high-intensity femtosecond optical pulses with pulsewidth of 100 fs (half width at 1/e maximum) on the silicon-on-insulator (SOI) optical waveguide when the central wavelength of the pulse locates in the normal dispersion region. Results show that the combined effects of group-velocity dispersion (GVD), third-order dispersion (TOD), self-phase modulation (SPM), and free-carrier dispersion (FCD) can lead to the phenomenon of optical wave breaking that manifests as an asymmetric profile and oscillation near the trailing edge of the pulse. Moreover, the optical wave breaking will be experienced from generation to disappearance during propagation.     

Simultaneously All-Optical Inverted and Noninverted Wavelength Conversion Employing Ultra-Small Silicon-on-Insulator Waveguide

We propose single-to-dual all-optical wavelength conversion based on stimulated Raman effect on silicon waveguides. Simulation results of non-return-to-zero (NRZ) pseudorandom bit sequence (2⁷–1 code) at 10, 50, and 200-Gbit/s rates of conversion in an ultra-small silicon-on-insulator waveguide are presented. The main goal in the proposed scheme is that the converted signal can be obtained simultaneously inverted, at the 1550 nm channel (down-conversion) and non-inverted, at the 1846 nm channel (up-conversion), for original signal fixed at 1686 nm.     

Design, analysis and optimization of silicon-on-insulator photonic crystal dual band wavelength demultiplexer

A highly efficient photonic crystal dual band wavelength demultiplexer (DBWD) using silicon-on-insulator (SOI) substrates is proposed for demultiplexing two optical communication wavelengths, 1.31 μm and 1.55 μm. Demultiplexing of two wavelength channels is obtained by modifying the propagation properties of guided modes in two arms of Y type photonic crystal structure. Propagation characteristics of proposed DBWD are analyzed utilizing 3D finite difference time domain (FDTD) method. Enhancement in spectral response is further obtained by optimizing the Y junction of demultiplexer giving rise to high transmission and extinction ratio for the wavelengths, 1.31 μm and 1.55 μm. Hence it validates the efficiency of proposed optimized DBWD design for separating two optical communication wavelengths, 1.31 μm and 1.55 μm. Tolerance analysis was also performed to check the effect of variation of air hole radius, etch depth and refractive index on the transmission characteristics of the proposed design of SOI based photonic crystal DBWD.     

Two dimensional silicon waveguide chirped grating couplers for vertical optical fibers

Shallow etched two dimensional gratings for coupling light between silicon-on-insulator nanophotonic waveguides and vertical optical fibers were designed and experimentally characterized. We show that the large second order back reflection could be suppressed effectively by applying a linear chirp in the grating period for two dimensional grating couplers. The total coupling efficiency from an optical fiber to two orthogonal silicon waveguides is independent of the input polarization. An almost linear relationship between the average effective index of the grating region and the fill factor for 1D grating is obtained and verified. We also show that the average effective index for the two dimensional grating is similar to the one dimensional grating with the same fill factor in the light propagation direction, when the fill factor of the etched holes was larger than 0.5.     

Micro-optical 3-dB mode converters

We present a detailed design of a 3-dB TE₀-TE₁ micro-optical mode converter using a chirped grating to phase-match the two propagation modes in an optical waveguide. Devices were fabricated in titanium in-diffused lithium niobate by ion-beam etching the tapered grating pattern into the waveguide surface. The power transfer between the two coupled modes was observed experimentally by varying the angle between the light beam and the normal grating wave-vector. Insertion loss of the TE₀-TE₁ mode converter due to spurious scattering was estimated at 1.5 dB.     

Design and fabrication of a three-dimensional polymer optical waveguide polarization splitter

We present a design and fabrication of a three-dimensional polymer optical waveguide polarization splitter by taking into consideration of the induced birefringence effect of the polymer. We show that it is not possible to couple TM light from one waveguide to the other but evanescent coupling for TE light is possible. Hence the polarization splitter can be designed by considering TE mode coupling alone. This has an advantage of short interaction length of the device. Based on this consideration, we fabricated a polarization splitter with a TE extinction ratio of 15 dB and TM extinction ratio of 21 dB.     

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.     

A simple integrated ratiometric wavelength monitor based on a directional coupler

A simple integrated ratiometric wavelength-monitoring device based on a single directional coupler (DC) is proposed and designed. To meet the desired spectral response, a computationally fast method is proposed to optimize the separation distance between two waveguides and the interaction length of the DC based on the local supermodes solution. The wavelength discrimination of the designed structure is demonstrated numerically.     

Ultrafast all-optical XOR logic gate based on a symmetrical Mach-Zehnder interferometer employing SOI waveguides

We present an integrated Silicon-on-Insulator (SOI) based Mach-Zehnder interferometer (MZI) in order to perform ultrafast all-optical XOR logic gate operation with a bit rate of ∼ 0.33 Tb/s. A numerical simulation is carried out in order to study various parameters such as extinction ratio and eye-opening parameters, characterizing the performance of the XOR logic gate. The output XOR logic gate signal can have improved extinction and eye margin if the initial powers of primary signals and the probe continuous-wave (CW), and SOI waveguide length are judiciously adjusted.     

Characterization of GaN layers grown on silicon-on-insulator substrates

In this study, we report growth and characterization of GaN layers on (1 0 0)- and (1 1 1)-oriented silicon-on-insulator (SOI) substrates. Using metalorganic chemical vapor deposition (MOCVD) technique, GaN layers are grown on KOH treated Si (1 0 0) overlayers of thin SIMOX SOI substrates. Growth of GaN on such surface with an AlN buffer leads to c-axis orientated textured GaN. This is evident from high-resolution X-ray diffraction (HRXRD) measurement, which shows a much broader rocking curve linewidth. Significantly enhanced photoluminescence (PL) intensity and partial stress relaxation is observed in GaN layers grown on these SOI substrates. Furthermore, GaN grown on (1 1 1)-oriented bonded SOI substrates shows good surface morphology and improved optical quality. Micro-Raman, micro-PL, and HRXRD measurements reveal single crystalline hexagonal GaN oriented along (0 0 0 1) direction. We also report growth and characterization of InGaN/GaN multi-quantum well structures on (1 1 1)-oriented bonded SOI. Such an approach to realize nitride epilayers would be useful to fabricate GaN-based micro-opto-electromechanical systems (MOEMS) and sensors.     

Experimental investigation on submicron rib waveguide microring/racetrack resonators in silicon-on-insulator

In experiment, characteristics of silicon microring/racetrack resonators in submicron rib waveguides have been systematically investigated. It is demonstrated that only a transverse-electric mode is guided for a ratio of slab height to rib height h/H = 0.5. Thus, these microring/racetrack resonators can only function for quasi-transverse-electric mode, while they get rid of transverse-magnetic polarization. Electron beam lithography and inductively coupled plasma etching were employed and improved to reduce sidewall roughness for low propagation loss and high performance resonators. Then, the effects of waveguide dimensions, coupling region design, waveguide roughness, and oxide cladding for the resonators have been considered and analyzed.     

Effect of patterning on thermal agglomeration of ultrathin silicon-on-insulator layer

Effect of patterning on thermal agglomeration of ultrathin silicon-on-insulator (SOI) layer has been studied. A square-shaped 12 nm thick SOI layer was patterned by lithography and by selective etching with a KOH solution. The structural change by ultrahigh vacuum annealing in a temperature range of 900–1100 °C was observed by atomic force microscopy. The agglomeration takes place preferentially from the pattern edges at a lower annealing temperature than that for the unpatterned layer, indicating enhanced diffusion of Si atoms at the edges. Additionally, the patterning causes formation of smaller islands than those for the unpatterned layer, reflecting that the patterning limits the amount of Si atoms supplied for the island formation.     

Efficient and compact TE-TM polarization converter built on silicon-on-insulator platform with a simple fabrication process

An efficient TE-TM polarization converter built on a silicon-on-insulator nanophotonic platform is demonstrated. The strong cross-polarization coupling effect in air-cladded photonic-wire waveguides is employed to realize the conversion. A peak TE-TM coupling efficiency of 87% (-0.6 dB insertion loss) is measured experimentally. A polarization conversion efficiency of >92% with an overall insertion loss of <-1.6 dB is obtained in a wavelength range of 40 nm. The proposed device is compact, with a total length of 44 μm and can be fabricated with one lithography and etching step.     

Lightwave splitting in two dimensional photonic crystal analogue of coupler

Two bent channels are created in an otherwise periodically arrayed photonic crystal structure by removing some dielectric pillars. If lightwave is introduced into one channel, a part of it would couple into the second channel and propagate down the guide. Factors deciding the amount of light couple from the first channel into the second channel are discussed thoroughly. This coupling scheme could be an important part for photonic integrated devices.     

Design, fabrication and characterization of a high-performance microring resonator in silicon-on-insulator

A high-performance microring resonator in a silicon-on-insulator rib waveguide is realized by using the electron beam lithography followed by inductively coupled plasma etching. The design and the experimental realization of this device are presented in detail. In addition to improving relevant processes to minimize propagation loss, the coupling efficiency between the ring and the bus is carefully chosen to approach a critical coupling for high performance operating. We have measured a quality factor of 21,200 and an extinction ratio of 12.SdB at a resonant wavelength of 1549.32nm. Meanwhile, a low propagation loss of 0.89dB/mm in a curved waveguide with a bending radius of 40μm is demonstrated as well.     

硅基光波导中啁啾高斯脉冲在反常色散区的传输特性

本文利用激光脉冲在硅基光波导(SOI)中传播时满足的非线性薛定谔方程,采用分步傅里叶变换,数值模拟了硅基光波导中啁啾高斯脉冲在反常色散区的传播特性.通过模拟发现,三阶色散正负主要决定了脉冲的振荡波形性质,克尔效应与双光子吸收对脉冲波形起调制作用,自由载流子效应则可以忽略.初始啁啾参数的正负存在影响了脉冲的振荡强弱变化和脉冲中心的漂移的趋势,并且无论是正啁啾还是负啁啾,脉冲振荡与漂移都随着啁啾的增大而更加剧烈,同时啁啾正向增大与负向增大,都将导致脉冲展宽越严重致主峰峰值越低.     

Design and fabrication of the star coupler based on SOI material

A 1×25 star coupler is designed through calculation and beam propagation method (BPM) simulation. Improvement methods are focused on the design of the tapered waveguides in the device, improving the uniformity of the output light power of the star coupler. Utilizing the conventional Si process technology, the device is fabricated based on silicon-on-insulator (SOI) material. The test result shows that the star coupler has a perfect function of power splitting.