Tunable Waveguide Research Articles

Disorder effects in tunable waveguide arrays with parity-symmetric tunneling

We investigate the effects of disorder on single particle time-evolution and two-particle correlations in an array of evanescently coupled waveguides with position-dependent tunneling rates. In the clean limit, the energy spectrum of such an array is widely tunable. In the presence of a Hermitian on-site or tunneling disorder, we find that the localization of a wave packet is highly sensitive to this energy spectrum. In particular, for an input confined to a single waveguide, we show that the fraction of light localized to the original waveguide depends on the tunneling profile. We compare the two-particle intensity correlations in the presence of Hermitian, tunneling disorder and non-Hermitian, parity-and-time-reversal ($\mathcal{PT}$) symmetric, on-site potential disorder. We show the two-particle correlation function in both cases is qualitatively similar, since both disorders preserve the particle-hole symmetric nature of the energy spectrum.

Liquid–crystal tunable waveguides for integrated plasmonic components

A broad range of liquid–crystal tunable plasmonic waveguides, based on long-range, dielectric-loaded, and channel surface plasmon polaritons, are theoretically designed and investigated. Liquid–crystal switching is rigorously modeled by solving for the coupled elastic/electrostatic problem, whereas the optical studies are conducted via the finite-element method. Extensive tunability of key optical properties, such as modal index, propagation losses, and modal confinement is demonstrated for waveguides of different optical confinement scale. These highly functional waveguiding structures are proposed as building blocks for the design of functional components, e.g. optical attenuators, directional couplers and switches, in integrated plasmonic chips.

Piezoelectric resonator arrays for tunable acoustic waveguides and metamaterials

One of the outstanding challenges in phononic crystals and acoustic metamaterials development is the ability to tune their performance without requiring structural modifications. We report on the experimental demonstration of a tunable acoustic waveguide implemented within a two-dimensional phononic plate. The waveguide is equipped with a periodic array of piezoelectric transducers which are shunted through passive inductive circuits. The resonance characteristics of the shunts lead to strong attenuation and to negative group velocities at frequencies defined by the circuits' inductance. The proposed waveguide illustrates the concept of a controllable acoustic logic port or of an acoustic metamaterial with tunable dispersion characteristics.

Tunable slow-light multi-mode photonic crystal waveguides based on the coupling of square cavities

The light transmission properties through two-dimensional photonic crystal waveguides based on coupling of square cavities are studied by the finite-difference time-domain technique. Through interlacing the adjacent cavities along the direction vertical to the waveguide, the coupling distance between the adjacent cavities is extended, and the group velocity of the guiding modes can be slowed by five-fold compared with that in vacuum. Because of the different spatial field distributions of various resonant modes, the corresponding group velocities are also different for the same CROW structure.

Combining circuit and packet switching using a large port-count optical cross-connect for data center networks

The exponential increase in the data center network traffic has posed new challenges for achieving high throughput, low latency and energy consumption. To address these challenges, this paper presents a novel scheme that combines optical circuit and packet switching technologies together in a single optical cross-connect for data center networks. By utilizing fast tunable linecards and parallel array waveguide grating routers, the optical cross-connect can offer low latency, large scalability and high throughput in datacenter networks. A scheduling technique is designed to simultaneously accommodate circuit switching and packet switching in the optical cross-connect. The physical performance of this optical cross-connect in circuit and packet switching modes is investigated by experiments. In addition, the network performance of the optical cross-connect is evaluated through simulations under a mixed circuit/packet-switched traffic pattern. Results show that the network performance of the optical cross-connect does not degrade significantly with an increase in the number of racks, but it is affected by the circuit-switched traffic proportion.

Broadly tunable one-way terahertz plasmonic waveguide based on nonreciprocal surface magneto plasmons

One-way-propagating broadly tunable terahertz plasmonic waveguide at a subwavelength scale is proposed based on a metal-dielectric-semiconductor structure. Unlike other one-way plasmonic devices that are based on interference effects of surface plasmons, the proposed one-way device is based on nonreciprocal surface magneto plasmons under an external magnetic field. Theoretical and simulation results demonstrate that the one-way-propagating frequency band can be broadly tuned by the external magnetic fields. The proposed concept can be used to realize various high performance tunable plasmonic devices such as isolators, switches and splitters for ultracompact integrated plasmonic circuits.

Tunable surface plasmon polaritons in metal-strip waveguides with magnetized semiconductor substrates in Voigt configuration

The properties of surface plasmon polaritons (SPPs) in a magnetically tunable strip waveguide geometry comprising of a metal film of finite width deposited on a magnetized semiconductor and covered by an isotropic dielectric material were studied in Voigt configuration. The method of lines was used to compute the dispersion relation of fundamental modes, and the dependence of the propagation constant on metal film dimensions, material parameters and biasing magnetic field was considered. The bounded SPPs are nonreciprocal with respect to the direction of the biasing magnetic field, producing a nonreciprocal phase shift of the order of 2–18 rad mm−1 at a wavelength of excitation 1.55 μm. Moreover, controlled propagation of SPP modes and their effective tuning are possible in this strip geometry, which enables the design and development of tunable optoelectronic devices.

Robust and disorder-immune magnetically tunable one-way waveguides in a gyromagnetic photonic crystal

We experimentally realize one-way waveguides that work within the magnetic resonance--induced band gap in a gyromagnetic photonic crystal made from ferrite rods. Our microwave measurements show that the one-way transport band is magnetically tunable and robust against back scattering induced by external disturbance. More importantly, the unidirectional transport is immune to lattice disorders by a much greater extent than the conventional one-way waveguide that works within the Bragg scattering--induced band gap. The experimental observation has been confirmed by numerical simulations, which show that the robust one-way transport property originates from the localized magnetic surface plasmon resonance.

Direct-Coupled Plasma-Assisted Combustion Using a Microwave Waveguide Torch

A tunable microwave waveguide is used to initiate and enhance combustion by coupling an atmospheric plasma discharge to a premixed methane/air flame. The absorbed microwave power ranges from 60 to 150 W, which was generated from a continuous source operating at 2.45 GHz, whereas combustion power ranges from 200 to 1000 W. OH radical number densities were measured using planar laser-induced fluorescence (PLIF), and temperatures were measured using Rayleigh scattering thermometry for various flow rates, equivalence ratios, and power levels. Increases in reaction volume, OH density, and temperature were observed as power increased. In the plasma-coupled premixed flame, OH number densities, which are quantified on the order of 1016 cm-3, increased by up to 50%, and temperature ranging from 2000 to 3000 K increased by up to 40% as the absorbed microwave power was increased from 60 to 130 W. Air-only plasma discharges exhibited a much greater temperature increase, i.e., up to 190%. The power associated with the measured temperature increases varied greatly with flow and input power but are typically three to four times greater in the air-only plasma compared to the flame coupled plasma, demonstrating a greater degree of nonthermal mechanisms present in plasma-enhanced flame discharge.

A tunable waveguide to cavity coupler for high power accelerator cavities

The end iris ridge waveguide couplers are used to couple power to accelerator cavities through a reduced size coupling port. However, higher electric and magnetic fields due to reduced size lead to strict requirements on dimensional tolerances during coupler fabrication process. It is shown by detailed parametric analysis that even small dimensional changes during manufacturing or operation can lead to undesired shift in design frequency and deterioration of return loss. Hence, transmitted power testing of two couplers connected back to back without an intermediate cavity cannot be carried out. Here, we propose cylindrical static tuners on impedance matching section to relax the dimensional tolerance requirements. It is also shown that an iris coupled coupler-cavity system is more tolerant towards coupler dimensional changes than a stand-alone coupler. However, same tuners can find use for tuning the coupling coefficient of coupler-cavity system. The proposed tuning scheme is expected to reduce the coupler manufacturing costs and provide an useful alternative for coupling coefficient tuning over iris machining.

Analog-type millimeter-wave phase shifters based on MEMS tunable high-impedance surface and dielectric rod waveguide

Millimeter-wave phase shifters are important components for a wide scope of applications. An analog-type phase shifter for W-band has been designed, analyzed, fabricated, and measured. The phase shifter consists of a reconfigurable high-impedance surface (HIS) controlled by micro-electromechanical system (MEMS) varactors and placed adjacent to a silicon dielectric rod waveguide. The analog-type phase shift in the range of 0–32° is observed at 75 GHz whereas applying bias voltage from 0 to 40 V to the MEMS varactors. The insertion loss of the MEMS tunable HIS is between 1.7 and 5 dB, depending on the frequency.

A tunable polarization diversity silicon photonics filter

A tunable polarization diversity silicon waveguide based optical filter was demonstrated. With the polarization diversity scheme, less than 0.5dB polarization dependent loss of the silicon optical filter was achieved in the wavelength range from 1525nm to 1600nm. The insertion loss of the whole polarization diversity circuits is 6.3dB. The extinction ratio of the optical filter is more than 27dB.

Tunable waveguide lattices with nonuniform parity-symmetric tunneling

We investigate the single-particle time evolution and two-particle quantum correlations in a one-dimensional $N$-site lattice with a site-dependent nearest neighbor tunneling function $t_\alpha(k)=t_0[k(N-k)]^{\alpha/2}$. Since the bandwidth and the energy levels spacings for such a lattice both depend upon $\alpha$, we show that the observable properties of a wavepacket, such as its spread and the relative phases of its constitutents, vary dramatically as $\alpha$ is varied from positive to negative values. We also find that the quantum correlations are exquisitely sensitive to the form of the tunneling function. Our results suggest that arrays of waveguides with position-dependent evanascent couplings will show rich dynamics with no counterpart in present-day, traditional systems.

Ultra-low power fiber-coupled gallium arsenide photonic crystal cavity electro-optic modulator

We demonstrate a gallium arsenide photonic crystal cavity injection-based electro-optic modulator coupled to a fiber taper waveguide. The fiber taper serves as a convenient and tunable waveguide for cavity coupling with minimal loss. Localized electrical injection of carriers into the cavity region via a laterally doped p-i-n diode combined with the small mode volume of the cavity enable ultra-low energy modulation at sub-fJ/bit levels. Speeds of up to 1 GHz are demonstrated with photoluminescence lifetime measurements revealing that the ultimate limit goes well into the tens of GHz.

Bottom-Heating Approach for the Realization of Thermooptic Polymer Waveguide Devices

We propose a bottom-heating approach to realizing a thermo-optically tunable polymer waveguide device, where the electrode heater is deposited at the bottom of the waveguide through a hole-etched substrate. Compared with the conventional top-heating approach, the bottom-heating approach can provide more effective optical isolation from the electrode, reduce the risk of damaging the polymer material in the fabrication process, and simplify electric wiring. We demonstrate the idea experimentally with an ultraviolet written long-period grating on a polymer waveguide fabricated by an imprinting technique, which incorporates a bottom electrode heater for phase-shift tuning. The grating requires an electric power of 1.74 mW to produce a -phase shift.

Tunable waveguides based on liquid crystal‐infiltrated silicon photonic crystals

AbstractA methodology for the study of the practical implementation of tunable waveguides based on Silicon Photonic Crystals with liquid crystal‐infiltrated pores is presented. First, by using the FDTD method, the transmission properties of the waveguide depending on the liquid crystal optical axis orientation are studied. Then by means of the plane wave expansion method and taking into account the anisotropy of the photonic crystal components and considering adequate supercells, the transmission or rejection of the optical beam are explained. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Tunable and Robust Reflection-free Waveguides Based on a Gyromagnetic Photonic Crystal

We propose and numerically investigate a kind of tunable and robust reflection-free waveguides based on a gyromagnetic photonic crystal (PC). The time reversal symmetry breaking of the gyromagnetic PC under out-of-plane dc magnetic fields gives rise to one-way edge modes in both the second and third band gaps. It is found that the propagating properties of one-way modes in the waveguides can be easily controlled by adjusting the directions of external magnetic fields. Moreover, the one-way modes are robust against the disorder in the waveguides.

Tunable coplanar waveguide band-stop and band-pass filters based on open split ring resonators and open complementary split ring resonators

In this study, the authors show that open split ring resonators (OSRRs) and open complementary split ring resonators (OCSRRs) can be loaded with varactor diodes in order to implement tunable filters in coplanar waveguide (CPW) technology. By loading a CPW with shunt-connected varactor-loaded OSRRs, a band-stop filter with tunable central frequency results. The electronically tunable band stop can be switched to a pass band by replacing the shunt resonators with varactor-loaded OCSRRs. To illustrate the possibilities of the approach, a tunable band-stop filter and a tunable band-pass filter have been designed and fabricated. The measured characteristics are in good agreement with the co-simulations (electric and electromagnetic) and with the circuit simulations of the electric model. The measured tuning range is roughly 120% for the stop-band structure (for varying voltages between 0 and 28 V) and over 30% for the band-pass filter (for varying voltages between 5 and 25 V).

Tunable Compact Asymmetric Coplanar Waveguide Zeroth-Order Resonant Antenna

A tunable asymmetric coplanar waveguide (CPW)-type antenna using the 0th order resonance is presented in this paper. The reduction in antenna size is achieved by exploiting the unique zeroth-order resonant (ZOR) property. In order to electronically tune the operating frequency, the varactor diodes are inserted between the shunt chip inductor and ground in the CPW configuration. In addition, a bias network is realised to isolate the antenna from the bias line. By adjusting the reverse bias voltage, the shunt reactance is continuously varied, so that the resonant frequency is tuned. This proposed antenna is electronically tunable from 2.19 GHz to 2.51 GHz. Moreover, good impedance matching is achieved over the desired frequency range.

A tunable 3D optofluidic waveguide dye laser via two centrifugal Dean flow streams

This paper presents a tunable optofluidic waveguide dye laser utilizing two centrifugal Dean flows. The centrifugal Dean flow increases the light confinement of the dye laser by shaping a three-dimensional (3D) liquid waveguide from curved microchannels. The active medium with the laser dye is dissolved in the liquid core and pumped with an external pump laser to produce stimulated emission. The laser's Fabry-Pérot microcavity is formed with a pair of aligned gold-coated fiber facets to amplify the fluorescent emission. The advantage of the 3D optofluidic waveguide dye laser is its higher efficiency, thus to obtain lasing at a reduced threshold (60%) with higher output energy. The demonstrated slope efficiency is at least 3-fold higher than its traditional two-dimensional equivalent. In addition, the laser output energy can be varied on demand by tuning the flow rates of the two flows. This technique provides a versatile platform for high potential applications microfluidic biosensor and bioanalysis.