Showing papers on "Radiation mode published in 2011"

Gap-mode waveguide

Abstract: In a gap-mode waveguide embodiment, an interior gap in a tubular waveguide principally condenses a dominant gap mode near the interior gap, and an absorber dissipates electromagnetic energy away from the gap mode. In this manner, the gap mode may dissipate relatively little power in the absorber compared to other modes and propagate with lesser attenuation than all other modes. A gap mode launched into a gap-mode waveguide may provide for low-loss, low-dispersion propagation of signals over a bandwidth including a multimode range of the waveguide. Gap-mode waveguide embodiments of various forms may be used to build guided-wave circuits covering broad bandwidths extending to terahertz frequencies.

High-speed modal decomposition of mode instabilities in high-power fiber lasers.

Abstract: A high-speed mode analysis technique is required to gain fundamental understanding of mode instabilities in high-power fiber laser systems. In this work a technique, purely based on the intensity profile of the beam, is demonstrated to be ideally suited to analyze fiber laser dynamics. This technique, together with a high-speed camera, has been applied to the study of the temporal dynamics of mode instabilities at high average powers with up to 20,000 frames per second. These measurements confirm that energy transfer between the fluctuating transversal modes takes place in millisecond-time-scale.

30GHz Ge electro-absorption modulator integrated with 3 μm silicon-on-insulator waveguide.

Abstract: We demonstrate a compact waveguide-based high-speed Ge electro-absorption (EA) modulator integrated with a single mode 3 µm silicon-on-isolator (SOI) waveguide. The Ge EA modulator is based on a horizontally-oriented p-i-n structure butt-coupled with a deep-etched silicon waveguide, which transitions adiabatically to a shallow-etched single mode large core SOI waveguide. The demonstrated device has a compact active region of 1.0 × 45 µm(2), a total insertion loss of 2.5-5 dB and an extinction ratio of 4-7.5 dB over a wavelength range of 1610-1640 nm with -4V(pp) bias. The estimated Δα/α value is in the range of 2-3.3. The 3 dB bandwidth measurements show that the device is capable of operating at more than 30 GHz. Clear eye-diagram openings at 12.5 Gbps demonstrates large signal modulation at high transmission rate.

Selectively Infiltrated Photonic Crystal Fiber With Ultrahigh Temperature Sensitivity

Abstract: By selective filling of one of the air holes in the photonic crystal fiber, the fundamental core mode can be effectively coupled to the fundamental mode of the adjacent liquid rod waveguide at the resonant wavelength with extremely high temperature sensitivity. The spectral power of the rod mode can be filtered out by fusion splicing the selectively infiltrated photonic crystal fiber with conventional single-mode fiber, resulting in a sharp dip in the transmission spectrum. Such a device is demonstrated in our experiment by filling standard 1.46 refractive index liquid into one of the air holes of the commercially available photonic crystal fiber by use of femtosecond laser-assisted selective infiltration technique. The average temperature sensitivity achieved is ~54.3 nm/°C.

Cross-correlated (C 2 ) imaging of fiber and waveguide modes

Abstract: We demonstrate a method that enables reconstruction of waveguide or fiber modes without assuming any optical properties of the test waveguide. The optical low-coherence interferometric technique accounts for the impact of dispersion on the cross-correlation signal. This approach reveals modal content even at small intermodal delays, thus providing a universally applicable method for determining the modal weights, profiles, relative group-delays and dispersion of all guided or quasi-guided (leaky) modes. Our current implementation allows us to measure delays on a femtosecond time-scale, mode discrimination down to about – 30 dB, and dispersion values as high as 500 ps/nm/km. We expect this technique to be especially useful in testing fundamental mode operation of multi-mode structures, prevalent in high-power fiber lasers.

Coupler for coupling gyrotron whispering gallery mode RF into HE11 waveguide

Abstract: A cylindrical waveguide with a mode converter transforms a whispering gallery mode from a gyrotron cylindrical waveguide with a helical cut launch edge to a quasi-Gaussian beam suitable for conveyance through a corrugated waveguide. This quasi-Gaussian beam is radiated away from the waveguide using a spiral cut launch edge, which is in close proximity to a first mode converting reflector. The first mode converting reflector is coupled to a second mode converting reflector which provides an output free-space HE11 mode wave suitable for direct coupling into a corrugated waveguide. The radiated beam produced at the output of the second mode converting reflector is substantially circular.

Optofluidic refractive-index sensor in step-index fiber with parallel hollow micro-channel

Abstract: We present a simple refractive index sensor based on a step-index fiber with a hollow micro-channel running parallel to its core. This channel becomes waveguiding when filled with a liquid of index greater than silica, causing sharp dips to appear in the transmission spectrum at wavelengths where the glass-core mode phase-matches to a mode of the liquid-core. The sensitivity of the dip-wavelengths to changes in liquid refractive index is quantified and the results used to study the dynamic flow characteristics of fluids in narrow channels. Potential applications of this fiber microstructure include measuring the optical properties of liquids, refractive index sensing, biophotonics and studies of fluid dynamics on the nanoscale.

Novel technique for mode selection in a multimode fiber laser

Abstract: A simple technique for transverse mode selection in a large-mode-area (multimode) fiber laser is described. The technique exploits the different spectral responses of feedback elements based on a fiber Bragg grating and a volume Bragg grating to achieve wavelength-dependent mode filtering. This approach has been applied to a cladding-pumped thulium-doped fiber laser with a multimode core to achieve a single-spatial-mode output beam with a beam propagation factor (M2) of 1.05 at 1923 nm. Without mode selection the free-running fiber laser has a multimode output beam with an M2 parameter of 3.3. Selective excitation of higher order modes is also possible via the technique and preliminary results for laser oscillation on the LP11 mode are also discussed along with the prospects for scaling to higher power levels.

Modal Analysis of Dielectric-Filled Rectangular Waveguide With Transverse Slots

Abstract: The dispersion relations and modes for a dielectric-filled rectangular waveguide with transverse slots are investigated. The differences between the air-filled and the dielectric-filled rectangular waveguides with transverse slots are analyzed. The dielectric-filled rectangular waveguide with transverse slots supports a leaky waveguide mode, a surface-wave mode and a proper waveguide mode. The complex propagation wavenumber is calculated by enforcing an aperture magnetic field integral equation using either a space-domain approach or a spectral-domain approach. The physical significance of the solutions is explained. The theoretical results agree very well with the results from HFSS simulation.

Efficient second-harmonic generation in nonlinear plasmonic waveguide.

Abstract: We theoretically studied a nonlinear optical process in a hybrid plasmonic waveguide composed of a nonlinear dielectric waveguide and a metal film with a separation of a thin air gap. Owing to the hybridization effect of guided mode and surface plasmon polariton mode, this particular waveguide is able to confine the optical-field in a deep subwavelength scale together with low propagation loss. Based on this, efficient second-harmonic generations (SHG) were revealed at the fundamental wavelength of λ=1.55 μm with good field confinement. The SHG efficiency, as well as the coupling coefficient and mode area, were analyzed and discussed in detail with respect to the structural parameters.

Hybrid plasmonic waveguide with gain medium for lossless propagation with nanoscale confinement.

Abstract: In this Letter, we propose a hybrid plasmonic nanosystem consisting of a silver cladding layer with a semicylinder bump on top of InGaAsP nanowire. Because of the coupling between the dielectric waveguide mode and surface plasmon polariton mode, the hybrid plasmonic mode can exhibit low loss with strong field localization. The finite element method numerical simulations are employed to evaluate the performances of the hybrid mode. In order to achieve the lossless propagation of the hybrid mode with the mode area of 0.0058(λ²/4) at 1.55 μm, the material gain of 200 nm × 300 nm InGaAsP nanowire should reach 1223 cm⁻¹.

A Waveguide Polarization Toolset Design Based on Mode Beating

Abstract: A toolset of waveguide elements is examined, which can be combined to produce polarization functional devices in a single contiguous waveguide. In particular, waveguide implementations of an optical isolator and a polarization modulator are discussed. The waveguide elements, i.e., quasi-phase-matched nonreciprocal polarization mode converter, reciprocal polarization mode converter (R-PMC), and a differential phase shifter, are all based on mode beating. A universal 3-dB R-PMC specification is identified, which suffices for all the polarization functional devices considered here. A full-vectorial modesolver is used to determine the modes in a number of example III-V waveguide structures, and the polarization state evolution is considered by using an averaged Stokes vector illustrated on the Poincare sphere construct.

The transition from a TEM-like mode to a plasmonic mode in parallel-plate waveguides

Abstract: We describe subwavelength-resolved measurements of the broadband terahertz field propagating inside a finite-width parallel-plate waveguide We observe a transition in the spatial mode of the waveguide, in which the energy distribution shifts from the waveguide center to the edges with increasing frequency This transition is surprisingly abrupt, and depends sensitively on the gap between the waveguide plates These results may have important implications for a variety of terahertz experiments as well as in the design of optical systems and components in the visible and near-infrared regimes, which rely on plasmonic wave guiding

Optical waveguide, optical waveguide module and method for forming optical waveguide

Abstract: An optical waveguide comprising a core and a clad characterized in that a desired part is heated and transited to machining strain release state, the part transited to the machining strain release state is curved with a specified bending radius and transited to machining strain state. That part of the optical waveguide is heated to a temperature within a range between the bending point and softening point and transited to machining strain state. The optical waveguide is an optical fiber having the outer diameter not shorter than 50 μm. The optical waveguide has the outer diameter not shorter than ten times of the mode field diameter of the optical waveguide. The optical waveguide has a bending radius of 5.0 mm or less and difference equivalent of refractive index Δ1 between the core and clad falls within a range of 0.8-3.5%.

Scaling Fiber Lasers to Large Mode Area: An Investigation of Passive Mode-Locking Using a Multi-Mode Fiber

Abstract: The mode-locking of dissipative soliton fiber lasers using large mode area fiber supporting multiple transverse modes is studied experimentally and theoretically. The averaged mode-locking dynamics in a multi-mode fiber are studied using a distributed model. The co-propagation of multiple transverse modes is governed by a system of coupled Ginzburg-Landau equations. Simulations show that stable and robust mode-locked pulses can be produced. However, the mode-locking can be destabilized by excessive higher-order mode content. Experiments using large core step-index fiber, photonic crystal fiber, and chirally-coupled core fiber show that mode-locking can be significantly disturbed in the presence of higher-order modes, resulting in lower maximum single-pulse energies. In practice, spatial mode content must be carefully controlled to achieve full pulse energy scaling. This paper demonstrates that mode-locking performance is very sensitive to the presence of multiple waveguide modes when compared to systems such as amplifiers and continuous-wave lasers.

Active tapers with reduced nonlinearity

Abstract: Fiber amplifiers and oscillators include tapered waveguides such as optical fibers that permit multimode propagation but produce amplification and oscillation in a fundamental mode. The tapered waveguides generally are provided with an active dopant such as a rare earth element that is pumped with an optical pump source such as one or more semiconductor lasers. The active waveguide taper is selected to taper from a single or few mode section to a multimode section, and seed beam in a fundamental mode is provided to a section of the waveguide taper associated with a smaller optical mode, and an amplified beam exits the waveguide taper at a section associated with a larger optical mode.

Mode coupling and output beam quality of 100-400 μm core silica fibers.

Abstract: Propagation and mode coupling within relatively short (∼1–10 m) large core, nominally multimode, fibers are of interest in a number of applications. In this research, we have studied the output beam quality and mode coupling in various fibers with core diameters of 100–400 μm and lengths of 2 m. Output beam quality (M2) and mode-coupling coefficients (D) have been studied for different clad dimensions, numerical apertures, and wavelengths. The mode-coupling coefficients have been determined based on modal power diffusion considerations. The results show that D scales approximately as the inverse square of the clad dimension and inverse square root of the wavelength. Output from a 2 m length fiber of 100 μm core and 660 μm clad fiber is close to single mode (M2=1.6), while output from a 200 μm core and 745 μm clad fiber also has high beam quality.

Chalcogenide step index and microstructured single mode fibers

Abstract: Chalcogenide glasses are known for their large transparency in the mid infrared, which includes the two atmospheric windows lying from 3–5 μm and 8–12 μm. Chalcogenide single mode fibers present numerous potential applications in the IR field, such as military countermeasures, LIDAR spectroscopy and spatial interferometry. Two routes can be considered for the elaboration of a single mode fiber. The first method consists in preparing a classical step index fiber (SIF) with a core-clad configuration. This procedure is based on two glass compositions (core and clad) with compatible thermal and optical properties and having a refractive index difference allowing the single mode propagation. The second route is based on the design of a microstructured optical fiber (MOF) in which the guiding function is ensured by the refractive index contrast between the core glass and the air contained in the capillaries surrounding the core. Two kinds of fibers exhibiting single mode propagation were fabricated; the first one is a SIF with a 22 μm core diameter and the second one is a three rings of holes MOF. The geometry of the MOF shows a d/Λ around 0.35 and a 40 μm core diameter. In both cases the optical losses in the 2 to 12 μm region were measured and compared.

Evanescent wave sensor based on permanently bent single mode optical fiber

Abstract: A novel refractive index sensing scheme based on evanescent wave interaction through locally and permanently bent single mode optical fibers is proposed Local and permanent bends in single mode optical fibers enable significant power coupling between core and cladding modes Order and number of excited cladding modes depend on bend features and determine the field profile at the output of the bent region This in turn constitutes a simple mechanism to tailor the field distribution in single mode optical fibers useful for spatial light modulation Moreover, since guided cladding modes are strongly influenced by the surrounding refractive index (SRI), the power transmitted at the output of the bent region as well as its dependence on the optical wavelength are strongly sensitive to the SRI opening new scenarios in sensing applications

Mode converting waveguide for heat assisted magnetic recording

Abstract: Light is excited by a light source in a first waveguide mode. The light is converted to a second waveguide mode via a channel waveguide having a cross-sectional geometry normal to a direction of propagation of the light that rotates a polarity of the first waveguide mode to a second waveguide mode. The light in the second waveguide mode is delivered to a near-field transducer that provides electromagnetic heating for a heat assisted magnetic recording write head.

Large-mode-area leaky optical fiber fabricated by MCVD

Abstract: A large-mode-area single-mode optical fiber based on leaky-mode filtering was prepared by a modified chemical vapor deposition (MCVD) technique. The fiber has a leaky cladding that discriminates the fundamental mode from higher-order modes. A preliminary version has a 25 μm core diameter and 0.11 numerical aperture. A Gaussian-like mode with 22 μm mode field diameter was observed after 3 m propagation, in agreement with modeling.

Very-large-mode-area photonic bandgap Bragg fiber polarizing in a wide spectral range

Abstract: A design of a polarizing all-glass Bragg fiber with a microstructure core has been proposed for the first time. This design provides suppression of high-order modes and of one of the polarization states of the fundamental mode. The polarizing fiber was fabricated by a new, simple method based on a combination of the modified chemical vapor deposition (MCVD) process and the rod-in-tube technique. The mode field area has been found to be about 870 μm2 near λ=1064 nm. The polarization extinction ratio better than 13 dB has been observed over a 33% wavelength range (from 1 to 1.4 μm) after propagation in a 1.7 m fiber piece bent to a radius of 70 cm.

Coupling of a laser diode to single mode circular core graded index fiber via hyperbolic microlens on the fiber tip and identification of the suitable refractive index profile with consideration for possible misalignments

Abstract: With an aim to explore the suitable refractive index profile for maximum coupling, we present a theoretical investigation of coupling optics involving a laser diode and single mode circular core graded index fiber via a hyperbolic microlens on the fiber tip in absence and presence of any possible transverse and angular misalignments. By employing Gaussian field distributions for both the source and the fiber and also ABCD matrix for hyperbolic microlens under paraxial approximation, we formulate analytical expressions for the concerned coupling efficiencies. The investigations are performed for two different light-emitting wavelengths of 1.3 and 1.5 μm for such fibers with different profile exponents in refractive index profile. Further, it is observed that out of the studied refractive index profiles, the triangular index profile having the dispersion-shifted merit comes out to be the most suitable profile to couple laser diode to single mode circular core graded index fiber for two wavelengths of practical interest. The analysis should find use in ongoing investigations for optimum launch optics for the design of hyperbolic microlens either directly on the graded index circular core single mode fiber tip or such fiber attached to single mode fiber to achieve a long working distance.

Semi-guiding high-aspect-ratio core (SHARC) fiber providing single-mode operation and an ultra-large core area in a compact coilable package.

Abstract: A new class of optical fiber is presented that departs from the circular-core symmetry common to conventional fibers. By using a high-aspect-ratio (~30:1) rectangular core, the mode area can be significantly expanded well beyond 10,000 μm2. Moreover, by also specifying a very small refractive-index step at the narrow core edges, the core becomes “semi-guiding,” i.e. it guides in the narrow dimension and is effectively un-guiding in the wide mm-scale dimension. The mode dependence of the resulting Fresnel leakage loss in the wide dimension strongly favors the fundamental mode, promoting single-mode operation. Since the modal loss ratios are independent of mode area, this core structure offers nearly unlimited scalability. The implications of using such a fiber in fiber laser and amplifier systems are also discussed.

Multimode optical coupler interfaces

Abstract: Optical interfaces that may be employed between large-core optical fibers and chip-scale optoelectronic devices. Described herein are couplers that improve the tolerance of misalignment when a single mode (SM) fiber is used as waveguide input. This enables the possibility of passive/automatic alignment and therefore reduces the production cost. The coupler also serves as a spot-size converter that reduces the spot size and is suitable for applications where a waveguide mode with small cross-section area is of particular importance. One such example can be a waveguide-based SiGe or III-V semiconductor photodetector in which the vertical size of its waveguide mode should be as small as few microns.

Equilibrium mode distribution and steady-state distribution in 100-400 μm core step-index silica optical fibers

Abstract: Using the power flow equation, the state of mode coupling in 100-400 μm core step-index silica optical fibers is investigated in this article. Results show the coupling length L(c) at which the equilibrium mode distribution is achieved and the length z(s) of the fiber required for achieving the steady-state mode distribution. Functional dependences of these lengths on the core radius and wavelength are also given. Results agree well with those obtained using a long-established calculation method. Since large core silica optical fibers are used at short distances (usually at lengths of up to 10 m), the light they transmit is at the stage of coupling that is far from the equilibrium and steady-state mode distributions.

Effects of twisting and bending on LP21 mode propagation in optical fiber.

Abstract: Twisting and bending characteristics of low-multimode LP21 mode propagation in optical fibers is presented for the first time. Theoretical fiber mode modeling, combining geometrical rotation with opto-elastic effects, demonstrates that the propagation of the LP21 mode is bending-effect-immune. Experimental testing verifies that the LP21 mode specklegram rotates 0.9112 of the fiber twist angle in a fused silica fiber, independent of any fiber bending. This characteristic allows for the LP21 mode to be highly applicable in fiber specklegram sensors.

High Gain Antipodal Fermi Antenna with Low Cross Polarization

Abstract: Antipodal Fermi antenna (APFA) that uses an antipodal feeding section is proposed and its fundamental characteristics are presented. It is shown that the cross polarization level is decreased by 5-10dB by the presence of the corrugation. It is also found that high gain, low VSWR and low side lobes and low back lobes are obtained. The mechanism of operation principles is discussed by using FDTD analysis. It is found that the corrugation transforms the current of parallel line mode to the current of traveling wave radiation mode and the effective aperture is enlarged which yields high gain characteristics.

Single Transverse Whispering-Gallery Mode AlGaInAs/InP Hexagonal Resonator Microlasers

Abstract: AlGaInAs/InP hexagonal resonator microlasers with an output waveguide connected to one vertex of the hexagon are fabricated using standard photolithography and inductively coupled plasma (ICP) etching process. Room-temperature continuous-wave electrically injected operation with a threshold current of 18 mA is realized for a hexagon laser with an edge length of 16 μm and an output waveguide width of 2 μm. Single-mode operation is achieved with a side mode suppression ratio of 21 and 33 dB at the injection current of 30 and 60 mA, respectively. The peak wavelength intervals of the laser spectrum agree very well with the longitudinal mode intervals of the whispering-gallery modes, which indicates single transverse mode operation. The mode Q factor of 6.53 × 103 is measured for the lasing mode at 1547 nm at the threshold current, which is in the same magnitude as the Q factor obtained by finite-difference time-domain (FDTD) simulation. The numerical simulations also indicate that the hexagonal resonator with an output waveguide is suitable to realize single transverse mode operation.

Low-loss plasmonic hybrid optical ridge waveguide on silicon-on-insulator substrate

Abstract: To achieve low loss and complete confinement of light, we propose and study a plasmonic hybrid optical waveguide fabricated on an silicon-on-insulator substrate. Using controlled oxidation and etching processes, a deep submicrometer Si core is fabricated based on UV photolithography patterning. The as-fabricated waveguide demonstrates a very low propagation loss of 1.6 dB/mm at 1550 nm. The numerical study discloses the hybrid characteristics of the mode. It is shown that the optical power of the waveguiding mode distributes more in the high-index Si-core region, and the metal cover further confines the light beyond the diffraction limit. The hybrid waveguide may be applied in compact, high-density Si photonic integrated circuits.