Design of a wavelength demultiplexer based on a parabolically tapered multimode interference coupler

A compact wavelength demultiplexer is designed for the operation at 1.30 and 1.55 μm wavelengths using the three-dimensional semi-vectorial beam propagation method. The parabolically tapered multimode interference (MMI) coupler based on the deep-etched SiO₂/SiON rib waveguide is introduced into the present demultiplexer for reducing the length. The numerical results show that a MMI section of 330.0 μm in length, which is only 76% length of a straight MMI coupler, is achieved with the contrasts of 42.3 and 39.2 dB in quasi-TE mode, and 38.4 and 37.8 dB in quasi-TM mode at wavelengths 1.30 and 1.55 μm, respectively and the insertion losses below 0.2 dB. The modified finite difference scheme is applied to approximate the resulting equations along the transverse directions, in which the discontinuities of the derivatives of magnetic field components H_y and H_x along the vertical and horizontal interfaces, respectively, are involved.     

The cascaded exponential-tapered multimode interference couplers based triplexer design for FTTH system

A novel compact triplexer, which consists of series cascaded exponential-tapered multimode interference couplers, is introduced in this paper. General self-image properties of the exponential-tapered multimode interference couplers are investigated thoroughly to fulfill the compact integrated triplexer. By combining four exponential-tapered MMI couplers, a novel compact 0.6 cm-long triplexer that is with ITU standard wavelength of 1310 nm, 1490 nm and 1550 nm for FTTH is achieved without requiring complicated design. And it can be fabricated by common planar lightwave circuit (PLC) technology. From the simulation results for wavelength of 1310 nm, 1490 nm and 1550 nm, the insertion losses are 0.34 dB, 0.9 dB and 1.17 dB, with isolations for each output waveguide ranging from 12.74 dB to 37.15 dB. The 3 dB passband widths for 1310 nm, 1490 nm and 1550 nm are 112.39 nm, 29.44 nm and 26.7 nm.     

An ultra-short double-wavelength optical power splitter for two waveguides operation based on photonic crystal multimode interference

In this paper, we design an ultra-short 1 × 2 1310/1550 nm double-waveguide optical power splitter based on photonic crystal multimode interference. The device can be used to divide the input beam equally for both 1310 nm and 1550 nm at the same time. The total multimode waveguide length of this device is only about 13 μm, which is one 210th of the conventional dielectric counterparts reported. On the basis of the guided-mode propagation analysis method, the self-imaging effect is discussed for the case of symmetric incidence. The finite-difference time-domain method is used to simulate the propagation of the beam in the multimode interference. The results show that the repetitive appearances of single image and twofold image of the input field occur alternatively in this device.     

Optimal design of multimode interference couplers using an improved self-imaging theory

Self-imaging theory is widely accepted as a good method in designing 1 × N multimode interference (MMI) couplers, but it is also true that self-imaging theory is not suitable for low-contrast structures. An improved self-imaging theory is proposed in this paper for the optimal design of low-contrast 1 × N MMI couplers. The average effective width of MMI waveguide and the average effective propagation constant of MMI waveguide are used as the basis to modify the conventional self-imaging theory. A direct calculation of the average effective width of low-contrast MMIs is presented. We use this approach in the optimal design of a 1 × 4 silica MMI coupler, and the results show that the improved self-imaging theory is more accurate than conventional self-imaging theory for low-contrast structures, the results also show that if the material parameters and the width of an MMI waveguide are fixed, the average effective width of the MMI waveguide will increase with the decrease of the height of the core layer.     

Design and simulation of a novel 32 × 32 photonic bandgap power switch based on SOI waveguide

In this paper, we propose a novel 32 × 32 photonic bandgap (PBG) power switch based on silicon-on-insulator (SOI) substrate. The 32 × 32 PBG power switch merges the design concept of PBG structure and multimode interference (MMI) structure. With controlling the voltages of 256 blocks for changing the refractive index, we can demonstrate that the light propagation direction can be switched to the assigned output port as a 32 × 32 power switch. According to the simulations, the power switching control tables are searched and selected for successful switching. The size of our designed 32 × 32 PBG switch can be reduced to 252 × 6040 μm which is much smaller than the conventional opto-electronic switches.     

Multichannel silicon WDM ring filters fabricated with DUV lithography

We have fabricated 9-channel silicon wavelength-division-multiplexing (WDM) ring filters using 193 nm deep-ultraviolet (DUV) lithography and investigated the spectral properties of the ring filters by comparing the transmission spectra with and without an upper cladding. The average channel-spacing of the 9-channel WDM ring filter with a polymeric upper cladding is measured about 1.86 nm with the standard deviation of the channel-spacing about 0.34 nm. The channel crosstalk is about −30 dB, and the minimal drop loss is about 2 dB.     

A 1 × 2 optical switch using one multimode interference region

A 1 × 2 optical switch using only one multimode interference (MMI) region is designed and demonstrated in GaAs/AlGaAs. This design makes a single MMI region works as MMI coupler using paired interference at “off” state and symmetric interference at “on” state. By injecting a current of 110 mA, the measured on/off ratio and crosstalk are 23 dB and 33 dB, respectively in the demonstrated device with GaAs/GaAlAs.     

Fabrication of low OH loss holey fibers with varying air hole sizes and their optical properties

We experimentally investigate a flexible fabrication technique for low OH and transmission losses holey fibers with a Ge-doped core and air holes in a silica cladding region. Versatile holey fibers of different size, pitch, and shape of air holes were achieved by controlling the temperature and heating time of the holey fiber preform. In addition, we suppress the OH loss of less than ∼0.323 dB/km at 1383 nm. After fabricating holey fibers, we measure their optical properties including cut-off wavelength, mode field diameter, splicing loss, dispersion, bending loss, and polarization dependent loss based on the size of air holes. The total transmission loss was measured to be ∼0.226 dB/km at 1550 nm by improving the fabrication process. After fabricating optical patch cord based on holey fibers, we measured the long-term stability of the fabricated holey fiber by using the temperature cycling technique for 24 and obtained low power fluctuation of 0.2 dB. We achieve the high quality holey fiber with a low bending loss of ∼0.04 dB/turn under a bending radius of 2.5 mm at 1550 nm. We also obtain a tunable band rejection filter with a number of bending turns.     

Low-loss amorphous silicon-on-insulator technology for photonic integrated circuitry

We report the fabrication of low-loss amorphous silicon photonic wires deposited by plasma enhanced chemical vapor deposition. Single mode photonic wires were fabricated by 193 nm optical lithography and dry etching. Propagation loss measurements show a loss of 3.46 dB/cm for photonic wires and 1.34 dB/cm for ridge waveguides.     

Fabrication of Triplexers Based on Flattop SOI AWG

A triplexer is fabricated based on SOI arrayed waveguide gratings （AWGs）. Three wavelengths of the triplexer operate at different diffraction orders of an arrayed waveguide grating. The signals of 1490nm and 1550nm, which are input from central input waveguide of an AWG, are demultiplexed and the signal of 131Onto, which is input from central output waveguide of an AWG, is uploaded. The tested results show that the downloaded and uploaded signals have fiat-top response. The insertion loss is 9 dB on chip, the nonadjacent crosstalk is less than -30 dB for 1490nm and 1301 nm, and is less than -25 dB for 1550nm, the 3dB bandwidth equates that of the input light source.     

Er-Yb co-doped glass waveguide amplifiers using ion exchange and field-assisted annealing

Er³⁺-Yb³⁺ co-doped waveguide amplifiers fabricated using thermal two-step ion-exchange are demonstrated. K⁺-Na⁺ ion-exchange process was first carried out in pure KNO₃ molten bath, and then field-assisted annealing (FAA) was used to make the buried waveguides. The effective buried depth is estimated to be ∼3.4 μm for the buried FAA waveguides. With the use of cut-back method, the fiber-to-guide coupling loss of ∼4.38 dB, the waveguide loss of ∼2.27 dB/cm, and Er³⁺ absorption loss ∼5.7 dB were measured for a ∼1.24-cm-long waveguide. Peak relative gain of ∼7.0 dB is obtained for a ∼1.24-cm-long waveguide. The potential for the fabrication of compact optical amplifiers operating in the range of 1520-1580 nm is also demonstrated.     

Analysis and optimization of an InGaAsP/InP waveguide variable optical attenuator

An InGaAsP/InP waveguide variable optical attenuator (VOA) is proposed in this paper. The device consists of straight input and output waveguides and an S-bend waveguide. An electrode is deposited on a portion of the waveguide to form an active region so that its refractive index can be modified by a current injection, resulting in the variation of the transmitted optical power. The beam propagation method is employed in the numerical simulation and the device structure is optimized using a genetic algorithm. The optimized VOA has a low excess loss (<1 dB) and a large dynamic range of about 40 dB.     

Investigation on push-pull polymer Mach-Zehnder interferometer electro-optic switches using improved 3-D mode propagation analysis method

By introducing normalized mode excitation coefficient and total mode excitation coefficient, we improve the 3-D mode propagation analysis (MPA) method for convenient design and analysis of multimode interference (MMI) coupler. With the improved 3-D MPA method and point-matching method, we present a novel formulation technique to analyze the low- and high-frequency characteristics for the impedance-matched polymer Mach-Zehnder interferometer electro-optic (EO) switch based on MMI couplers. As an application, under 1550 nm, optimization and simulation performed for the designed device reveal low driving voltage of 1.375 V with short EO region length of 5 mm. The insertion loss and extinction ratio are less than 3.75 dB and more than 42 dB, respectively. The microwave characteristic impedance is about 49.6 Ω, and due to the less mismatch between lightwave velocity and microwave velocity, the estimated cutoff switching frequency is up to 263 GHz with the 10–90% rise time and fall time about 1.90 ps under the operation of step-style square-wave switching signal. This theoretical cutoff switching frequency is almost 1.53 times of that of our previous reported shielded EO switch with similar design technique.     

1x2 Multimode interference couplers based on semiconductor hollow waveguides formed from omnidirectional reflectors

A 1x2 hollow multimode interference (MMI) coupler based on semiconductor hollow waveguides formed from omnidirectional reflectors (SHOW-ODR) is demonstrated. The device has a shorter coupling length than a conventional silicon-on-insulator MMI coupler. A 2 dB uniformity was achieved at operating wavelengths between 1520 and 1562 nm. The device exhibited a weak polarization dependence in the TE and TM modes.     

Resonant coupling into hybrid 3D micro-resonator devices on organic/biomolecular film/glass photonic structures

We have designed and realized an integrated photonic family of micro-resonators (MR) on multilayer SU8/lipidic film/glass materials. Such a family involves hybrid 3D-MR structures composed of spherical glass-MR arranged upon organic pair-SU8-waveguides, an efficient coupling being ensured with a Langmuir-Blodgett Dipalmitoylphosphatidylcholine (DPPC-lipid from Avanti Polar^®) film whose thickness is ranging from 12 to 48 nm. We have characterized such add/drop filters, respectively, in intensity and spectral measurements, and experimentally achieved an evanescent resonant-photonic-coupling between the 3D-MR and the 4-ports structure through the DPPC-gap. Spectral resonances have been measured for 4-whispering gallery-modes (WGM) into such 3D-structures, respectively, characterized with a 0.97 nm free spectral range (FSR) and a high quality Q-factor up to 4.10⁴.     

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.     

基于Si3N4/SiNx/Si3N4三明治结构的偏振无关1×2多模干涉型解复用器的设计

提出一种基于Si3N4/SiNx/Si3N4三明治结构多模干涉波导的偏振无关1×2解复用器,用于分离1310和1550 nm两个波长.通过合理选择三明治结构中间层SiNx的折射率,可以调节同一波长两个正交偏振态的拍长相等,实现偏振无关;根据多模干涉原理,通过合理选择多模干涉波导的长度与宽度,可以使两个波长的输出像点分别成正像和反像,实现解复用功能.运用三维有限时域差分法进行建模仿真,对结构参数进行优化,并对器件关键结构参数的制作容差进行了分析.结果表明:该器件多模干涉波导的尺寸为4.6μm×227.7μm,插入损耗低至0.18dB,输出波导间的串扰低至–25.7dB, 3dB带宽可达60 nm.另外,本文提出的器件采用Si3N4/SiO2平台,可有效减小波导尺寸,提高集成度,不仅实现了偏振无关,而且结构紧凑、损耗低,在未来的集成光路中具有潜在的应用价值.     

Ring resonator with multimode waveguide turning-mirror couplers in InGaAsP-InP

Compact In_0.67Ga_0.33As_0.6P_0.4/In_0.71Ga_0.29As_0.74P_0.26 on InP single ring resonators incorporating 2 × 2 multimode interference (MMI) turning-mirror couplers with cross coupling factor of 0.15 have been demonstrated. The form of race tracks is a 15-degree arc of 260 μm radius joined with a 60-degree arc of 110 μm radius, and finished with another 15-degree arc of 260 μm radius. The MMI turning-mirror coupler of 128 μm in length is used in the single ring resonators, which correspond to free spectral ranges of 82 GHz. A contrast of 4 dB, a finesse of nearly 3 and full-width at half-maximum (FWHM) of 0.24 nm for the drop port have been achieved in this single ring resonator. From the experimental value T_max/T_min of 21 dB, the experiment coupling factor coincides with the simulation.     

Compact multimode interference coupler with tapered waveguide geometry

In this paper, a novel MMI coupler, based on general interference, with tapered waveguide geometry has been proposed for reduction of coupling length. The coupling characteristics and power imbalance of the proposed structure are compared with conventional MMI structures by using a mathematical model based on sinusoidal modes. It is seen that the beat length for tapered MMI coupler with angle of taper ∼1.05° is reduced by ∼24% of that of conventional MMI coupler and the coupling characteristics obtained with the mathematical model, match well with those obtained by more sophisticated BPM computer aided design software. The power imbalance for tapered 3 dB MMI coupler is more sensitive to the wavelength than that for conventional 3 dB MMI coupler and variation of power imbalance with fabrication tolerance for both the MMI coupler is almost same.     

Spectroscopic investigation of Tm:TeO2-WO3 glass

We studied the spectroscopic characteristics of telluride glass with the host composition (0.85)TeO₂-(0.15)WO₃, containing 0.25 and 1.0 mol% thulium oxide (Tm₂O₃). By analyzing the absorption spectra with the Judd-Ofelt theory, the average radiative lifetimes of 305±7.5 μs and 1.95±0.02 ms were determined for the ³F₄ and ³H₄ levels, respectively. Measured fluorescence lifetime of the ³F₄ level decreased from 218 to 51 μs for the 0.25 and 1.0 mol% Tm₂O₃ doped samples, respectively, indicating the effect of boosted non-radiative decay at higher doping concentrations. A similar trend was observed for the ³H₄ level, where the fluorescence lifetime decreased from 1.86 ms to 350 μs at these concentrations. The quenching of the 1460 nm (³F₄→³H₄) emission in favor of the 1800 nm (³H₄→³H₆) emission due to cross relaxation was further evident in the fluorescence spectra of the samples. The calculated stimulated emission cross sections (3.73±0.1×10⁻²¹ cm² at 1460 nm and 6.57±0.07×10⁻²¹ cm² at 1808 nm) reveal the potential importance of the Tm³⁺:(0.85)TeO₂-(0.15)WO₃ glass for applications in fiber-optic amplifiers and fiber lasers.