Design of phased-array wavelength division multiplexers using multimode interference couplers

Novel designs for phased-array wavelength-division multiplexers based on self-imaging properties of multimode interference (MMI) couplers are presented. These devices, which operate on N equally spaced wavelength channels, consist of two MMI couplers connected by an array of N monomode waveguides. The MMI couplers function as power splitters/combiners, and the waveguide array is the dispersive element. The excellent characteristics of MMI couplers offer the possibility of designing small-size devices with low loss and with high uniformity among different channels. A general theoretical formulation for an N-channel multiplexer is presented, and a simple procedure for finding an optimum set of lengths for the array guides is given. We show that these multiplexers can function as N x N wavelength-selective interconnecting components. The simulated performance of three variations of a five-channel device, designed in a rib waveguide system, is given. It is demonstrated that sidelobes in the multiplexer spectral response can be suppressed by weighting the power samples in the array waveguides through appropriate design of a nonuniform MMI power splitter.     

Novel approach for design of low index contrast multimode interference devices

Self-imaging theory is widely accepted as a good method in designing 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 the MMI waveguide and the average effective propagation constant of the MMI waveguide are used in the improved self-imaging theory. An approach is given to find the average effective width. 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.     

A 2×2 multimode interference coupler with exponentially tapered waveguide

An exponentially tapered structure is introduced into multimode interference (MMI) devices. Compared with a parabolically tapered structure, which has been successfully used in MMI devices, this structure can further reduce the length of these devices. The performances of the 2×2 MMI coupler with exponentially tapered structure, such as the optical transmission, the splitting ratio, the wavelength response and the fabrication tolerance, are investigated by the 2D finite difference beam propagation method. Results show that the exponentially tapered MMI coupler exhibits a similar property to that with a parabolically tapered structure except for the splitting ratio. The exponentially tapered structure can offer a possible application in MMI couplers with a free choice of the splitting ratio.     

Design of an ultrashort Si-nanowaveguide-based multimode interference coupler of arbitrary shape

A design procedure for an arbitrarily tapered multimode interference (MMI) coupler based on Si nanowaveguides is presented. First a series of the effective indices of the zeroth and first eigenmodes in multimode waveguides are obtained as the core width increases by using a full-vectorial finite-difference method. Two polynomial functions are used to fit the two relations between the effective indices and the core width. The phase difference Delta phi between the zeroth and first eigenmodes can then be easily calculated when the light goes through any given arbitrarily tapered MMI section. By making the phase difference Delta phi equal to a certain value Delta phi 0 required for an N-fold self-imaging, the length of a MMI coupler is determined. With the present design procedure, an ultrashort 2 x 2 parabolic MMI coupler is designed as an example. The size of the designed ultrashort MMI section is only approximately 1.4 microm x 4.7 microm.     

Electro-optic up-conversion mixer amenable to photonic integration

A novel electro-optical up-conversion mixer architecture comprising four electro-optical phase modulators situated in the arms between an interconnected 1 × 4 distribution tree and a complementary 4 × 2 combination tree is proposed. The distribution and combination trees are based on multi-mode interference couplers (MMI). The novelty lies in the use of the intrinsic phase relations between the MMI ports to realize a broadband and free of drift design requiring no static phase shift elements. A transfer-matrix approach is followed to represent the main building blocks in the proposed design, and hence to describe the operation of the entire optical up-conversion mixer. The concept is demonstrated by computer simulations. A single side-band modulation with carrier suppression is obtained at the output of the proposed architecture, which is in agreement with the analytical development. Scenarios considering both ideal and imperfect power balances and phase relations in the MMIs, as well as imperfect phase relations of the electrical drives to the phase modulators are analyzed.     

Passive single-mode fiber depolarizer

We analyze and demonstrate a passive single-mode fiber depolarizer by using cascaded 2 x 2 couplers, recirculating delay lines, and a commercial laser diode. Design criteria and principles are discussed. We reduced the degree of polarization (DOP) to less than 20 dB by using ten cascaded couplers. The DOP can be reduced even further with more couplers. Experiments illustrate that this depolarizer is insensitive to the input polarization state and can eliminate polarization noise in a polarization-sensitive fiber-optic system.     

Co-precipitation synthesis and characterization of NiO-Ce0.8Sm0.2O1.9 nanocomposite powders: effect of precipitation agents

NiO-Ce0.8Sm0.2O1.9 (NiO-SDC) nanocomposite powders applied as promising anode material for low-temperature solid oxide fuel cells (SOFCs) were synthesized by hydroxide co-precipitation method using NH3 x H2O, NaOH and NH3 x H2O + NaOH as precipitation agents. The crystal phases, morphologies and sintering behavior of the synthesized NiO-SDC nanocomposite powders were investigated by means of X-ray diffraction (XRD), transmission electron microscopy (TEM) and sintering experiments. The effect of precipitation agents on the synthesis of the NiO-SDC nanocomposite powders was discussed. Results show that different precipitation agents influence greatly the synthesis and characteristics of the NiO-SDC nanocomposite powders. The NiO-SDC nanocomposite powders synthesized with NH3 x H2O deviate from the original composition due to the loss of Ni. The loss of Ni is avoided and nano-sized NiO-SDC composite powders are synthesized, when NaOH and NH3 x H2O + NaOH are used as precipitation agents. The NiO-SDC nanocomposite powders can be synthesized at relatively low temperature using NH3 x H2O + NaOH as precipitation agent, and the synthesized NiO-SDC nanocomposite powders show good sintering characteristics.     

新型无色DAR成色剂的合成

释放显影促进剂的成色剂(DAR)对彩色感光材料向高感、微粒方面发展起了重要作用。本文合成了三支无色DAR成色剂,选用N,N’-双-(2-氯-5-正十二烷氧基羰基)苯基丙二酰胺为母体,以2,5-二巯基-1,3,4-噻二唑为吸附基团,分别采用2-甲酰基苯肼、2-乙酰基苯肼和2-三氟乙酰基苯肼为增强基团,研究了它们的合成方法,得到了无色DAR成色剂YKA、YKB、YKC。通过红外光谱、质谱、核磁共振1H谱等测试方法对成色剂的中间体及目标产物进行了表征和确认。     

Coefficient of performance of multistage absorption cycles

A method is presented by which the coefficients of performance of advanced absorption cycles can be calculated very quickly if the efficiencies of single-stage heat pumps and heat transformers are known. The method provides detailed results for the heat input or output of the individual exchange units. These values can be used as a basis for comparison of different cycles, for numerical cycle calculations or for estimates of investment costs. Examples are given for heat pumps, refrigerators, heat transformers and heat pump transformers. Numerical results are presented for the working fluid pairs H₂O/LiBr and NH₃/H₂O.

Résumé

A method is presented by which the coefficients of performance of advanced absorption cycles can be calculated very quickly if the efficiencies of single-stage heat pumps and heat transformers are known. The method provides detailed results for the heat input or output of the individual exchange units. These values can be used as a basis for comparison of different cycles, for numerical cycle calculations or for estimates of investment costs. Examples are given for heat pumps, refrigerators, heat transformers and heat pump transformers. Numerical results are presented for the working fluid pairs H₂O/LiBr and NH₃/H₂O.     

Simultaneous determination of dissolved gases and moisture in mineral insulating oils by static headspace gas chromatography with helium photoionization pulsed discharge detection

This paper presents the development of a static headspace capillary gas chromatographic method (HS-GC) for simultaneously determining dissolved gases (H2, O2, N2, CO, CO2, CH4, C2H6, C2H4, C2H2, C3H8) and moisture from a unique 15-mL mineral oil sample. A headspace sampler device is used to equilibrate the sample species in a two-phase system under controlled temperature and agitation conditions. A portion of the equilibrated species is then automatically split-injected into two chromatographic channels mounted on the same GC for their separation. The hydrocarbons and the lighter gases are separated on the first channel by a GS-Q column coupled with a MolSieve 5-A column via a bypass valve, while the moisture is separated on the second channel using a Stabilwax column. The analytes are detected by using two universal pulsed-discharge helium ionization detectors (PDHID). The performance of the method was established using equilibrated vials containing known amounts of gas mixture, water, and blank oil. The signal is linear over the concentration ranges normally found for samples collected from open-breathing power transformers. Determination sensitivity varies with the nature of the species considered with values as high as 21 500 A x 10(-9) s (microg/ g)(-1) for H2O, 46-216 A x 10(-9) s (microL/L)(-1) for the hydrocarbons and carbon oxides, and as low as 8-21 A x 10(-9) s (microL/L)(-1) for the O2 and N2 permanent gases. The detection limit of the method is between 0.08 and 6 microL/L for the dissolved gases, except for O2, N2, and CO2, where higher values are observed due to air intrusion during sampler operations, and 0.1 microg/g for the dissolved water. Ten consecutive measurements in the low and high levels of the calibration curves have shown a precision better than 12% and 6%, respectively, in all cases. A comparison study between the HS-GC method and the ASTM standard procedures on 31 field samples showed a very good agreement of the results. The advantages of configuring the arrangement with two PDHID over the conventional flame ionization and thermal conductivity detectors were clearly demonstrated.     

Hadamard transform microchip electrophoresis combined with diode laser fluorometry

The application of a Hadamard transform technique to microchip electrophoresis is described. The sample is electrokinetically injected into a separation channel and is then detected by diode laser-induced fluorometry. The sample and buffer solutions are introduced into the channel by controlling the high voltages applied to the solutions, according to a code determined by a Hadamard matrix. The S/N ratio of the signal in the electropherogram can be improved by a factor of 5 in comparison with that obtained by a conventional single-injection method, although an 8-fold improvement is theoretically predicted when a 255-order matrix is used.     

Design and modeling of ultrashort MMI-based couplers

In this work, based on the Gaussian-modes theory, modeling and analysis of ultrashort (<10 µm) MMI couplers is performed. Using the introduced model, temporal behaviors and also other features such as insensitivity to input polarization, insensitivity to operating wavelength and manufacturing tolerances of these devices are studied. In addition, the optimal refractive-index structures with no sensitivity to the polarization are obtained. A comparison is made between this model and the semi-sinusoidal model and it is shown that the Gaussian model is necessary to handle these ultrashort devices. In addition, accuracy of the model is justified using a comparison with results of direct simulations obtained using the FEMLAB environment and FDTD method.     

Tunable power splitter based on an electro-optic multimode interference device

Power splitters based on multimode interference (MMI) devices that offer the possibility of dynamically tuning the power-splitting ratio using electro-optic (EO) polymers are presented. The so-called 1 x 2 electro-optic MMI (EO-MMI) is demonstrated to provide a tuning range of 6 dB at approximately 54 V as theoretically predicted. Also a method is discussed to reduce the driving voltage by generating multiple beats, which provide 15 V for a tunable range of 10 dB for r(33)=15 pm/V at wavelength 1.55 microm.     

Compact silicon-on-insulator-based multimode interference coupler with bilevel taper structure

A novel compact silicon-on-insulator- (SOI-)based multimode interference (MMI) coupler with bilevel taper structures was designed. The MMI section and the S-bend sections of the input-output waveguides are deeply etched. The input-output waveguides connecting to single-mode fibers or other photonic light circuits are etched shallowly to yield single-mode operation. A bilevel taper is introduced in the transition region between the shallowly and deeply etched regions. It is predicted theoretically that this design will not only improve the quality of the self-imaging in the MMI section but will also make the structure compact. Both the excess loss and the nonuniformity of the MMI coupler are reduced. By use of a three-dimensional beam propagation method, the performance of a 1 x 4 MMI coupler based on a SOI is simulated as a numerical example of the novel design. The simulated nonuniformity and the excess loss are approximately 0.0285 and 0.2 dB, respectively.     

Compact tuneable single and dual mode ferrite left-handed coplanar waveguide coupled line couplers

For the first time, single and dual mode novel left-handed coupled line couplers implemented on ferrite substrates are reported. Both of the proposed couplers demonstrate novel characteristics of coupler operation with properties of tunability and non-reciprocity, which have not been demonstrated before. The proposed couplers are realised in a left-handed coplanar waveguide configuration, constructed using interdigital capacitors and various types of planar shunt inductors. The performance of the couplers has been studied theoretically and verified numerically. The single mode tuneable coupler demonstrates a coupling level as high as 4 dB with a maximum isolation level better than 25 dB. The dual mode tuneable coupler exhibits nonreciprocal operations with a near-zero dB coupling level and a maximum non-reciprocal isolation level of more than 30 dB. Moreover, both the couplers have very compact sizes: 10.32 mm x 19.8 mm for the single mode coupler and 10.2 mm x 19.8 mm for the dual mode tuneable coupler, respectively.     

Characteristics of polymeric optical passive single-mode waveguiding devices fabricated by an argon-ion laser

The fabrication of polymeric single-mode Gaussian profile optical waveguides is described. We used poly(methyl methacrylate) and a mixture of two intermiscible monomers for the cladding and the core, respectively, of the waveguides. The cores of the waveguides were fabricated by with an argon-ion laser beam. The waveguides had single-mode Gaussian refractive-index profiles. By using such waveguides, we fabricated directional couplers for power coupling to the adjacent waveguides and also parallel waveguide arrays for preventing power coupling to adjacent waveguides for use in interconnect chips. We analyzed the characteristics of these couplers by using the coupled-mode theory and compared the results with those obtained with the beam propagation method. Experimental results showed good correlation with the theoretical values. We designed optical bus arrays for interconnect chips, considering the variation of normalized frequency V, the power penalty, and the dimensions of the waveguides and the separation between them. The number of waveguides in the bus array increased with increasing V. For a known value of V, a waveguide's density increases with a decrease of its dimensions. The theoretical maximum number of waveguides is ~1490/cm and ~846/cm for 2 mum x 2 mum and 4 mum x 4 mum single-mode waveguides, respectively, to satisfy a 1-dB power penalty criterion at bit-error rate of 10(-9). We fabricated interconnect bus arrays with fifteen 4 mum x 4 mum waveguides for a 3-cm coupling length, and the experimental results were verified to be in good agreement with the theoretical values.     

Multimode interference devices with input-output ports on the sides

Multimode interference (MMI) devices are useful for power splitting and for the separation or combination of wavelengths or polarizations, usually in integrated optics. Input-output guides connect to the MMI region by ports. In all previously reported MMI devices, the input and output guides connect only to the ends of the MMI region; i.e., they are end ported. What is believed to be a novel arrangement of the input-output ports on MMI devices is described. By placing input-output ports either partially or entirely on the sides of the MMI region (i.e., side porting), a variety of benefits are achieved and a variety of new devices can be made.     

硅酸盐氧基磷灰石的溶胶凝胶法制备与表征

为了提高电导率,以溶胶-凝胶法制备了硅酸盐氧基磷灰石.通过TG-DTA、XRD和IR分析,研究了从凝胶转变为硅酸盐氧基磷灰石过程中凝胶焙烧温度的影响.结果表明,制备Sr2La8(SiO4)6O2、La9.33(SiO4)6O2和La10(SiO4)6O3时凝胶的焙烧温度分别为850、950℃和950℃;间隙氧可能诱发晶体结构扭曲,晶胞体积变大.通过IR和XRD分析,证实了在La9.33(SiO4)6O2和La10(SiO4)6O3磷灰石中间隙氧的存在.     

Phase Engineering of Transition Metal Dichalcogenides with Unprecedentedly High Phase Purity,Stability, and Scalability via Molten-Metal-Assisted Intercalation

The crystalline phase of layered transition metal dichalcogenides (TMDs) directly determines their material property. The most thermodynamically stable phase structures in TMDs are the semiconducting 2H and metastable metallic 1T phases. To overcome the low phase purity and instability of 1T-TMDs, which limits the utilization of their intrinsic properties, various synthesis strategies for 1T-TMDs have been proposed in phase-engineering studies. Herein, a facile and scalable synthesis of 1T-phase molybdenum disulfide (MoS₂) via the molten-metal-assisted intercalation (MMI) approach is introduced, which exploits the capillary action of molten potassium and the difference between the electron affinity of MoS₂ and the ionization potential of potassium. Highly reactive molten potassium metal can readily intercalate into the MoS₂ interlayers, inducing an efficient phase transition from the 2H to 1T crystal structure. The ionic bonding between the intercalated potassium and sulfur lowers the energy barrier of the 1T-phase transition, enhancing the phase stability of the 1T crystals. Owing to the high purity and stability of the 1T phase, the electrocatalytic performance for the hydrogen evolution reaction is significantly higher in 1T-MoS₂ (MMI) than in 2H-MoS₂ and even in 1T-MoS₂ synthesized using n-butyllithium.