Multimode Channel Research Articles

A G m -C continuous-time analog filter for IEEE 802.11 a/b/g/n wireless LANs

A CMOS transconductor for multi-mode wireless channel selection filter is presented. The linear transconductor is designed based on the flipped-voltage follower (FVF) circuit and an active resistor to achieve the transconductance tuning. The transconductance tuning can be obtained by changing the bias current of the active resistor. A third-order Butterworth low-pass filter implemented with the transconductors was designed by TSMC 0.18-μm CMOS process. The results show that the filter can operate with the cutoff frequency of 10---20 MHz. The tuning range would be suitable for the specifications of IEEE 802.11 a/b/g/n Wireless LANs under the consideration of saving chip areas. In the design, the maximum power consumption is 13 mW with the cutoff frequency of 20 MHz under a 1.8 V supply voltage.

Ion beam irradiated channel waveguides in Er3+-doped tellurite glass

Erbium-doped tellurite glasses are of great interest for the fabrication of active integrated circuits because of their unique properties in terms of bandwidth and rare earth solubility. The fabrication of multimode channel waveguides in a glass of this family, namely, a sodium-tungsten-tellurite glass, is demonstrated using a high-energy ion beam irradiation technique. Nitrogen ions with dose of 1.0×1016ions∕cm2 and 1.5MeV energy were used for this aim. The waveguiding effect was investigated using the end-fire coupling technique.

Impulse response analysis of graded index polymer optical fiber

Polymer optical fibers (POF) are of great interest the last decade in optical communication networks for short and medium distance applications. Based on the theory of generalized impulse response of multimode channels, an analytical method for time response evaluation is proposed for a graded index polymer optical fiber (GIPOF) with polymethylmeta-crylate (PMMA) core material.

High-power multi-mode laser to single-mode pump conversion

A novel pump-sharing module for multi-channel amplifiers based on integration of high-power multimode semiconductor lasers and planar waveguide circuits is proposed. The feedback-free modules are designed for optimum coupling, throughput and power uniformity on silicon using BPM method and fabricated via the sol–gel process of photo patternable precursors. Modal properties of the multimode lasers are considered by studying spectrum and spatial distribution of laser output power at different temperatures. Employing an 1500mW 980nm multimode laser (TE polarized), via lens and Butt coupling up to 1250mW of laser output is coupled into the multimode channel waveguides, which are tapered and branched to four single mode waveguides. The measured pump power from splitter outputs, randomly polarized, exceeds 600mW. At constant temperatures ranging from 22°C to 30°C, the pump-sharing modules exhibit good stability with a power uniformity of <0.5dB over hours.

Ultraviolet direct printing of rare-earth-doped polymer waveguide amplifiers

Polymer channel waveguide amplifier arrays were fabricated using ultraviolet direct printing methodology. The effects of different weight percentage (wt. %) combinations of rare-earth ions erbium (Er3+) and ytterbium (Yb3+) on the absorption spectrum, as well as the internal gain, were investigated. With an input signal power of ∼−18dBm, the highest internal gain obtained is ∼16.5dB at a wavelength of 1533nm for a 20mm long multimode channel waveguide codoped with ∼1wt.% of Er3+ and ∼10wt.% of Yb3+ derivatives. The UV direct printing methodology opens a simple platform for the fabrication of dense and compact polymer waveguide laser arrays.

Elimination of mode coupling in multimode continuous-variable key distribution

A multimode channel can be utilized to substantially increase the capacity of quantum continuous-variable key distribution. Beyond losses in the channel, an uncontrollable coupling between the modes of the channel typically degrades the capacity of multimode channels. For the key distribution protocol with simultaneous measurement of both complementary quadratures we propose a feasible method to eliminate any undesirable mode coupling by only the receiver's appropriate measurement and data manipulation. It can be used to substantially increase the capacity of the channel, which has an important application in practical continuous-variable quantum cryptography.

Coherent electronic transport in a multimode quantum channel with Gaussian-type scatterers

Coherent electron transport through a quantum channel in the presence of a general extended scattering potential is investigated using a T-matrix Lippmann-Schwinger approach. The formalism is applied to a quantum wire with Gaussian type scattering potentials, which can be used to model a single impurity, a quantum dot or more complicated structures in the wire. The well known dips in the conductance in the presence of attractive impurities is reproduced. A resonant transmission peak in the conductance is seen as the energy of the incident electron coincides with an energy level in the quantum dot. The conductance through a quantum wire in the presence of an asymmetric potential are also shown. In the case of a narrow potential parallel to the wire we find that two dips appear in the same subband which we ascribe to two quasi bound states originating from the next evanescent mode.

Dephasing in Rashba spin precession along multichannel quantum wires and nanotubes

Coherent Rashba spin precession along interacting multi-mode quantum channels is investigated, revisiting the theory of coupled Tomonaga-Luttinger liquids. We identify susceptibilities as the key-parameters to govern exponents and Rashba precession lengths. In semiconducting quantum wires spins of different transport channels are found to {\em dephase} in their respective precession angles with respect to one another, as a result of the interaction. This could explain the experimental difficulty to realize the Datta Das transistor. In single walled carbon nanotubes, on the other hand, interactions are predicted to suppress dephasing between the two flavor modes at small doping.

Selective mode launching in a multimode channel waveguide by planar coupler

Selective mode couplers representing a planar variant of input/output prism couplers have been fabricated. The planar couplers developed demonstrate selective excitation of solitary modes of different orders in channel waveguides. Mode division/multiplexing units based on these couplers can provide practical applications of modal data compression in multimode fibre links, resulting in multiplication of the transmission capacity.

A dry electromigration process for fabricating deep optical channel waveguides on glass and their characterization

A dry electromigration technique was developed to fabricate deep multimode channel and planar waveguides on BK7 optical glass. In contrast to earlier ion exchange processes based on thermal diffusion, a relatively high electric field (∼440 V/mm) was applied on the glass to accelerate the field-driven ion exchange process by expeditiously replacing host sodium ions in the glass with silver ions. As a result, the process temperature is significantly reduced, leading to lower optical attenuation on the waveguides. Lowest optical loss was achieved on waveguides fabricated at 320 °C. The increase on the index of refraction for the planar waveguide was determined to be 0.076 using the prism coupling method. The optical loss for channel waveguides was measured using the edge coupling technique with a 0.6328 μm He–Ne laser. Loss of 2 dB/cm was obtained for channel waveguides of 25 μm in depth, relatively low for waveguides of such depth at red wavelength. The scanning electron microscope (SEM) with energy-dispersive X-ray (EDX) spectroscope was utilized to obtain the concentration profile of silver ions in the waveguide region. A modified Huggins–Sun model with Gladstone–Dale relation was employed to deduce the refractive index profile from the silver ion concentration profile. A nearly step-like profile was observed from every deep multimode waveguide fabricated.

Low-loss multimode waveguides using organic-inorganic hybrid materials

Multimode channel waveguides were fabricated using a direct UV patterning technology from thick films deposited by the one-step dip-coating of an organic/inorganic hybrid material (ORMOCER®). The core size of the covered ridge waveguide was 43 × 51 μm2; the waveguides can be readily prepared for multimode applications by direct UV patterning. The waveguides exhibited smooth surface profiles and a low optical loss of 0.07 dB/cm at the most important wavelength (850 nm) used for optical interconnects.

A planar prism for detection and selective excitation of modes in a multimode channel waveguide

A fully planar element for selective excitation and detection of various modes in a multimode channel waveguide has been manufactured and tested. A mode multiplexer/demultiplexer based on this element will provide for the mode data compression in fiber-optic communication lines employing multimode fibers.

Theory of a one-atom laser in a photonic band-gap microchip

We present a quantum theory of a coherently pumped two-level atom in a photonic band gap (PBG), coupled to both a multimode waveguide channel and a high-quality microcavity embedded within a photonic crystal. One mode is engineered to exhibit a sharp cutoff within the PBG, leading to a large discontinuity in the local photon density of states near the atom, and the cavity field mode is resonant with the central component of the Mollow spectrum of atomic resonance fluorescence. Another mode of the waveguide channel is used to propagate the pump beam. We derive analytical expressions for the optical amplitude, intensity, second-order correlation functions, and conjugate quadrature variances for the light emitted by the atom into the microcavity. The quantum degree of second-order coherence in the cavity field reveals enhanced, inversionless, nearly coherent light generation when the photon density of states jump between the Mollow spectral components is large. The cavity field characteristics are highly distinct from that of a corresponding high-$Q$ cavity in ordinary vacuum. In the case of a vanishing photon density of states on the lower Mollow sideband and no dipolar dephasing, the emitted photon statistics is Poissonian, and the cavity field exhibits quadrature coherence.

Er 3+ – Yb 3+ codoped polymeric optical waveguide amplifiers

Codoping of erbium (Er3+) and ytterbium (Yb3+) rare-earth ions in polymer was studied for optical amplifier applications. The absorption spectrum confirms that the presence of Yb3+ ions enhances the absorption efficiency of Er3+ ions. Typical Er3+ luminescence at ∼1540 nm wavelength was observed, and the full width at half maximum bandwidth is ∼47 nm wide. Multimode Er3+–Yb3+ codoped polymeric channel waveguides were fabricated using electron beam direct writing. With an input signal power of &amp;lt;−18 dB m, an optical gain of 13 dB at a wavelength of 1533 nm was measured in an 18 mm long multimode channel waveguide.

Entanglement assisted capacity of the broadband Lossy channel.

We calculate the entanglement assisted capacity of a multimode bosonic channel with loss. As long as the efficiency of the channel is above 50%, the superdense coding effect can be used to transmit more bits than those that can be stored in the message sent down the channel. Bounds for the other capacities of the multimode channel are also provided.

근접장 주사 광학현미경을 이용한 광 도파로 특성 연구

광 도파로를 따라 전파하는 빛의 특성을 측정하기 위해 근접장 주사 광학현미경(Near-field scanning optical microscope, NSOM)으로 광 도파로의 표면에 형성된 에바네슨트 파 evanescent wave)의 분포를 측정하였다. 사용된 NSOM은 photon scanning tunneling microscope방식으로 본 연구의 목적에 적합하도록 직접 제작한 것이다. 광원 파장 1550㎚에서 단일 모드 다중 모드 채널형 광 도파로에 대해 도파로 표면에 형성된 에바네슨트 파의 분포를 측정하였으며, 3차원 빔전파방법(Beam Propagation Method)으로 계산된 수치 해석 결과와 두 모드 간의 간섭 형상을 직접적으로 확인할 수 있었다. The propagation characteristic of an optical waveguide was investigated by measuring with a near-field scanning optical microscope (NSOM) the evanescent field formed at the neighbor of its core-cladding interface. For this purpose, the NSOM system was developed specially as a form of Photon scanning tunneling microscope. The evanescent field distributions of several channel waveguides were measured at the wavelength of 1550 ㎚, and the usefulness of the system was verified by comparing experimental results with simulation results. In particular, the interference phenomena of the guided modes during their propagation along a multimode channel waveguide could be observed directly from the measured evanescent field distribution.

Models and measurements for the transmission of submicron-width waveguide bends defined in two-dimensional photonic crystals

One of the essential building-blocks of miniature photonic crystal (PC)-based photonic integrated circuits (PICs) is the sharp bend. Our group has focused on the 2-D photonic crystal based on a triangular lattice of holes perforating a standard heterostructure. The latter, GaAlAs-based or InP-based, is vertically a monomode waveguide. We consider essentially one or two 60/spl deg/ bends defined by one to five missing rows, spanning both cases of monomode and multimode channel waveguides. From intensive modeling and various experimental measurements (both on GaAs and InP), we point out the origin of the present level of bend insertion losses and discuss the merits of the many roads open for improved design.

Orthogonal frequency-division multiplexing in wireless communication systems with multimode fiber feeds

The feasibility of using multimode fiber as an inexpensive cell feed in broad-band indoor picocellular systems is investigated in this paper. The performance of coded orthogonal frequency-division multiplexing (OFDM) for a variety of multimode fiber profiles, including stepped index and /spl alpha/-profile graded index fibers, is assessed. In addition to its ability to perform well in a frequency-selective multipath environment, OFDM is shown to offer good protection against the frequency selectivity of a dispersive multimode fiber. Data rates in excess of 100 Mb/s (without equalization) over a multimode fiber channel are possible, whereas they may be limited to some 20-30 Mb/s using conventional ASK modulation.

Demonstration of optical gain at 1.06 μm in a neodymium-doped polyimide waveguide

Neodymium-doped polyimide waveguides were studied for optical amplifier applications. Photoluminescence at 890 nm, 1.06 μm, and 1.33 μm from neodymium ions in the polymer matrix was observed. Optical gain of about 8 dB was measured at 1.06 μm in a 5-cm-long multimode channel waveguide. Phase separation was observed in the doped polyimide. Large scattering losses in the waveguides due to the phase separation were reduced by planarizing the layer of doped polyimide with another layer of undoped polyimide. Such a waveguide amplifier has potential applications in lossless splitters, lossless fiber, and waveguide systems with large fanout.

Real-time implementation of a reconfigurable IMT-2000 base station channel modem

Research suggests that joint methods combining smart antennas, RAKE reception, multi-user detection or other adaptive methods may be practically implemented for IMT-2000 channel modems using computationally simplified algorithms. Using software-defined radio methods, these modems can be employed in a new generation of adaptive multimode base stations which permit software reconfiguration from second- to third-generation air interfaces. Practical implementation is made possible by corresponding advances in hardware technology, including new processors and high-bandwidth I/O fabrics which replace traditional computer buses with their inherent limitations in bandwidth and scalability. In this article adaptive processing research is reviewed, implementation requirements for second- and third-generation base stations are considered, and the capabilities of selected new monolithic silicon devices are examined. A possible implementation approach for a reconfigurable multimode base station channel modem using software defined radio (SDR) design methods is proposed.