Showing papers on "Radiation mode published in 2009"

Ultrasensitive photonic crystal fiber refractive index sensor

Abstract: We introduce a microfluidic refractive index sensor based on a directional coupler architecture using solid-core photonic crystal fibers. The sensor achieves very high sensitivity by coupling the core mode to a mode in the adjacent fluid-filled waveguide that is beyond modal cutoff, and with strong field overlap. We demonstrate the device through the selective infiltration of a single hole with fluid along a microstructured optical fiber. A detection limit of 4.6x10(-7) refractive index units has been derived from measurements with a sensitivity of 30,100 nm per refractive index unit, which is the highest for a fiber device to date.

Comparison of the lowest-order transverse-electric (TE1) and transverse-magnetic (TEM) modes of the parallel-plate waveguide for terahertz pulse applications.

Abstract: We present a comprehensive experimental study comparing the propagation characteristics of the virtually unknown TE(1) mode to the well-known TEM mode of the parallel-plate waveguide (PPWG), for THz pulse applications. We demonstrate that it is possible to overcome the undesirable effects caused by the TE(1) mode's inherent low-frequency cutoff, making it a viable THz wave-guiding option, and that for certain applications, the TE(1) mode may even be more desirable than the TEM mode. This study presents a whole new dimension to the THz technological capabilities offered by the PPWG, via the possible use of the TE(1) mode.

An investigation of the lowest-order transverse-electric (TE_1) mode of the parallel-plate waveguide for THz pulse propagation

Abstract: We experimentally and theoretically investigate the lowest-order transverse-electric (TE1) mode of the parallel-plate waveguide (PPWG) for the propagation of broadband THz pulses. We demonstrate undistorted THz pulse propagation via the single TE1 mode, solving the group-velocity-dispersion and spectral-filtering problems caused by the mode's low-frequency cutoff. We observe a remarkable counterintuitive property of the TE1 mode: its attenuation decreases with increasing frequency for all frequencies above cutoff. This phenomenon has not been observed with any other THz waveguide to date, and it can enable extremely low-loss propagation. We present a physical interpretation of this frequency-dependent behavior using a simple plane-wave description of the TE1 mode propagation. We also find that it is possible to achieve almost 100% coupling to the TE1 mode from a focused free-space Gaussian beam. In addition, using the above plane-wave analysis, we show how to mitigate the diffraction losses inherent to long path-length PPWGs via the use of transverse-concave plates.

Apparatus for microwave heating of a planar product including a multi-segment waveguide element

Abstract: The invention relates to a microwave waveguide element for matching a standard waveguide input port to an enlarged waveguide output port. In the waveguide element, a plurality of intermediate waveguide segments is cascaded in the propagation direction of the microwave energy to first split the waveguide element into two symmetrical waveguide branches and then combine the branches at the output port. Thus, the width of the waveguide element is gradually enlarged and the input port is matched to the output port. The intermediate waveguide segments are preferably dimensioned such that respective characteristic impedances are approximately matched with each other for the fundamental mode.

Dispersion Relation and Loss of Subwavelength Confined Mode of Metal-Dielectric-Gap Optical Waveguides

Abstract: We have investigated the dispersion relation of a novel metal-dielectric-gap optical waveguide. This structure confines the optical field strongly in the gap region between metals and dielectric materials, and its size can be reduced to less than the wavelength of the transmitted light. In addition, the propagation length of light extends much greater than that of the surface plasmon modes on metal surfaces. We show that this mode of propagation has a cut-off at zero wavenumber, and that it is hollow-waveguide-like for small wavenumbers, while it approaches a surface-plasmon-like mode for large wavenumbers. A typical propagation length at around the communication wavelength is 10-20 mum, and optical fields are confined into an approximately 100 times 200 nm2 cross section.

Investigation of mode coupling in a microdisk resonator for realizing directional emission.

Abstract: Mode coupling between the whispering-gallery modes (WGMs) is numerically investigated for a two-dimensional microdisk resonator with an output waveguide. The equilateral-polygonal shaped mode patterns can be constructed by mode coupling in the microdisk, and the coupled modes can still keep high quality factors (Q factors). For a microdisk with a diameter of 4.5 microm and a refractive index of 3.2 connected to a 0.6-microm-wide output waveguide, the coupled mode at the wavelength of 1490 nm has a Q factor in the order of 10(4), which is ten times larger than those of the uncoupled WGMs, and the output efficiency defined as the ratio of the energy flux confined in the output waveguide to the total radiation energy flux is about 0.65. The mode coupling can be used to realize high efficiency directional-emission microdisk lasers.

Diffraction-Limited Fundamental Mode Operation of Core-Pumped Very-Large-Mode-Area Er Fiber Amplifiers

Abstract: Diffraction-limited fundamental mode amplification is demonstrated in Er-doped fibers with mode areas ranging from 900 to 1800 mum2. The amplifiers are core-pumped with Raman fiber lasers with both signal and pump selectively launched into the fundamental mode. This scheme results in differential gain for the fundamental mode and stabilizes it against mode mixing caused by perturbations in the core. Gains that are greater than 30 dB are demonstrated from a single stage without significant amplified spontaneous emission. The low nonlinearity of the large mode areas enables amplification to high peak powers without resorting to unconventional microstructured or higher order mode fibers.

Discrete mode lasers for communication applications

Abstract: The authors present a novel, low cost laser transmitter for telecommunication systems. This device, called discrete mode (DM) laser, is basically a ridge waveguide Fabry-Perot (FP) laser, whose wavelength spectra has been modified to obtain a single mode operation. This is achieved by perturbing the effective refractive index of the guided mode along very small sections of the laser cavity, by etching features into the ridge waveguide. Suitable positioning of these interfaces allows the mirror loss spectrum of an FP laser to be manipulated in order to achieve single longitudinal mode emission. The waveguide structure requires only a single growth stage and uses optical lithography to realise the ridge. In addition, the fabrication process is re-growth free. Despite this simple and low cost fabrication process, the DM lasers portray many advantages over the distributed feedback and distributed Bragg reflector lasers, such as very high side mode suppression ratio, stable operation over a large temperature range, narrow linewidth and low sensitivity to optical feedback.

Bragg reflection waveguide diode lasers.

Abstract: What we believe to be the first demonstration of an edge-emitting Bragg reflection waveguide laser is reported. The laser utilized InGaAs quantum wells emitting at 980 nm, with AlxGa1−xAs core and claddings. The lasing mode is centered in a low-index core with a width of 700 nm, hence providing a large mode volume with strong discrimination against any modes other than the fundamental photonic bandgap mode. Single-transverse mode operation is observed with thresholds as low as 157 A/cm2. The propagation losses of the mode were measured for the first time and found to be 11.4 cm−1.

Waveguide mode imaging and dispersion analysis with terahertz near-field microscopy

Abstract: Propagation of terahertz waves in hollow metallic waveguides depends on the waveguide mode. Near-field scanning probe terahertz microscopy is applied to identify the mode structure and composition in dielectric-lined hollow metallic waveguides. Spatial profiles, relative amplitudes, and group velocities of three main waveguide modes are experimentally measured and matched to the HE11, HE12, and TE11 modes. The combination of near-field microscopy with terahertz time-resolved spectroscopy opens the possibility of waveguide mode characterization in the terahertz band.

Theory of quantum light emission from a strongly-coupled single quantum dot photonic-crystal cavity system

Abstract: We present a rigorous medium-dependent theory for describing the quantum field emitted and detected from a single quantum dot exciton, strongly coupled to a planar photonic crystal nanocavity, from which the exact spectrum is derived. By using simple mode decomposition techniques, this exact spectrum is subsequently reduced to two separate user-friendly forms, in terms of the leaky cavity mode emission and the radiation mode emission. On application to study exciton-cavity coupling in the strong coupling regime, besides a pronounced modification of the usual vacuum Rabi spectral doublet, we predict several new effects associated with the leaky cavity mode emission, including the appearance of an off-resonance cavity mode and a loss-induced on-resonance spectral triplet. The cavity mode emission is shown to completely dominate the emitted spectrum, even for large cavity-exciton detunings, whereby the usual cavity-QED formulas developed for radiation-mode emission drastically fail. These predictions are in qualitative agreement with several “mystery observations” reported in recent experiments, and apply to a wide range of semiconductor cavities.

Low index, large mode field diameter optical coupler

Abstract: An optical coupler is formed of a low index material and exhibits a mode field diameter suitable to provide efficient coupling between a free space optical signal (of large mode field diameter) and a single mode high index waveguide formed on an optical substrate. One embodiment comprises an antiresonant reflecting optical waveguide (ARROW) structure in conjunction with an embedded (high index) nanotaper coupling waveguide. Another embodiment utilizes a low index waveguide structure disposed in an overlapped arrangement with a high index nanotaper coupling waveguide. The low index waveguide itself includes a tapered region that overlies the nanotaper coupling waveguide to facilitate the transfer of the optical energy from the low index waveguide into an associated single mode high index waveguide. Methods of forming these devices using CMOS processes are also disclosed.

Refractive-index sensor based on long-range surface plasmon mode excitation with long-period waveguide grating

Abstract: We propose a refractive-index sensor that operates on the principle of exciting the long-range surface plasmon mode of a metal-coated waveguide with a long-period grating formed in the waveguide, where the wavelength at which the mode excitation occurs serves as a measure of the refractive index of the external medium. We analyze the sensor with a coupled-mode theory and highlight the effects of the waveguide parameters on the loss of the surface plasmon mode and the performance of the sensor. Our results show that the sensor can provide a sharp resonance for high precision measurements and a high sensitivity comparable to that of an optimized bulk prism-based surface plasmon sensor. Our sensor also offers much flexibility in the choice of waveguide parameters for different applications.

Multiband antenna using +1, −1, and 0 resonant mode of DGS dual composite right/left handed transmission line

Abstract: The multiband antenna using defected ground structure (DGS) dual composite right/left handed transmission line (D-CRLH TL) is proposed. The D-CRLH TL resonator can provide a multiband using positive, zero, and negative modes without the different length resonators because the D-CRLH TL has inherent three bands in half periodicity. To design and analyze the proposed antenna, the circuit simulation of the two stage DGS D-CRLH resonator is executed by the equivalent circuit and the results are compared with those of full wave simulation and experiment. The results show that the presented antenna has such a reasonable radiation characteristics of efficiency, bandwidth, and size that it is suitable for compact multiband antenna. In addition, a theoretically unexpected radiation mode, which is generated by the DGS, is observed in the stop band. This additional mode could be utilized for the multi-band antenna. © 2009 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 2485–2488, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24649

Grating induced transparency (GIT) and the dark mode in optical waveguides

Abstract: We propose and describe a new class of optical modes consisting of superposition of three waveguide modes which can be supported by a few-mode waveguide spatially modulated by two co-spatial gratings. These supermodes bear a close, but not exact, formal analogy to the three-level quantum states involved in EIT and its attendant slow light propagation characteristics. Of particular interest is the supermode which we call the dark mode in which, in analogy with the dark state of EIT, one of the three uncoupled waveguide modes is not excited. This mode has unique dispersion characteristics that translate into a slow light propagation which possesses high bandwidth-delay product and can form the basis for a new generation of optical resonators and lasers.

Optical mode coupler

Abstract: An optical coupler includes a first waveguide configured to supply a first optical signal having a wavelength and a second waveguide. The first optical signal having a first mode. The first waveguide has a tapered portion being spaced from the second waveguide by a distance sufficient to facilitate evanescent coupling of the first optical signal from the first waveguide to the second waveguide. A first effective refractive index of the first waveguide at a location in the tapered portion being equal to a second effective refractive index at a location in the second waveguide. The first effective refractive index being associated with the first mode and the second effective refractive index being associated with a second mode of a second optical signal having the wavelength. The second mode having a different order than the first mode, and the second waveguide being configured to supply the second optical signal.

Guided Wave Analysis of Hollow Optical Fiber for Mode-Coupling Device Applications

Abstract: In this paper, optical mode characteristics of hollow optical fibers are thoroughly analyzed using finite element method. The guided modes along the ring core and cladding are identified and their optical properties are investigated. For the core modes, we investigated intensity distribution, higher order mode cutoff, propagation constant, and chromatic dispersion. The mode coupling between the fundamental mode and the excited modes in both core modes and cladding modes are discussed for applications in mode-coupling devices.

External cavity tunable laser module

Abstract: The present invention relates to an external cavity tunable laser module by the thermo-optic effect of a semiconductor optical waveguide. The tunable laser module according to the present invention comprises: a light source which generates the broadband light; a semiconductor optical waveguide of which one end is optically coupled with the light source; a Bragg grating which is formed on the optical waveguide; a thin film heater which is positioned in the upper part of the Bragg grating, and adjusts the reflection band of the Bragg grating by the thermo-optic effect; a first temperature sensor which is positioned in the upper part of the optical waveguide; a thermo-electric cooler (TEC) which is disposed in the lower part of the optical waveguide; a thermal insulating layer which is equipped between the optical waveguide and the TEC; and an optical fiber which is optically coupled with the other end of the optical waveguide.

Numerical investigation of a depressed-index core photonic crystal fiber for gas sensing

Abstract: A depressed-index core photonic crystal fiber (PCF) was numerically analyzed for gas sensing application. The dependence of relative sensitivity, effective mode area, confinement loss, and bend loss on the fiber parameters as well as wavelength are investigated. Lowering the refractive index of core reduces the index contrast between the core and the cladding weakens the index-guidance of the fiber, and enhances the relative sensitivity and the mode area simultaneously. Relative sensitivity and effective mode area increase with the cladding background index, especially in the short wavelength region, which are opposite to that in a conventional PCF. The enhancements of the relative sensitivity and the normalized effective mode area are more than 3700% (from 0.0065 to 0.25) and 680% (from 3.85 to 30), respectively, as compared with that of the non-depressed-index core PCF.

Influence of Mode Loss on the Feasibility of Grating-Assisted Optical Fiber Surface Plasmon Resonance Refractive Index Sensors

Abstract: In this paper, the influence of mode loss on the feasibility of grating-assisted optical fiber surface plasmon resonance (SPR) refractive index (RI) sensors is investigated. The loss of surface plasmon polarition (SPP) mode plays a key role in the design and implementation of such sensors. It is demonstrated through simulation that the grating length should be smaller than or comparable with the propagation length of SPP mode in order to achieve effective coupling. The loss of SPP mode is the severe limiting factor for the implementation of the grating-assisted SPR-RI sensors. More generally, in order to achieve effective mode coupling with the help of waveguide grating, the grating length is bounded by the shortest propagation length of the modes in lossy waveguides.

Quality factor (q-factor) for a waveguide micro-ring resonator

Abstract: The waveguide in the ring and the bus waveguide in the immediate vicinity of the ring are made wider than the optimal single mode size. The bus waveguide has adiabatic tapers which serve to connect single mode portions in the bus waveguide to the wider portion of the bus waveguide to expand the mode from the narrower waveguide to the wider waveguide. Since the light is now spread out over a larger area in the wider waveguides, the scattering loss from the sidewalls is reduced and the loss is lower. This lower loss gives rise to a higher Q in the ring since the Q of the ring is directly proportional to the round trip loss.

Characterization of a photonic crystal fiber mode converter using low coherence interferometry.

Abstract: The relative group delay of the different modes present in an all-fiber LP11 mode converter at a central wavelength of 750 nm is observed using low coherence interferometric imaging. We have simultaneously measured the relative group delay and computed the intensity and the phase distribution of the modes emitted from the mode converter end face using a Fourier technique, providing unequivocal identification of the modes involved.

Extremely large mode area optical fibers formed by thermal stress.

Abstract: We report a strictly single-mode optical fiber with a record core diameter of 84µm and an effective mode area of ~3600µm2 at 1µm. We also demonstrate fundamental mode operation in an optical fiber with a record core diameter of 252µm and a measured mode field diameter (MFD) of 149µm at 1.03µm, i.e. an effective mode area (Aeff) of ~17,400µm2 at 1.03µm, an Aeff of 31,600µm2 at 1.5µm. All these fibers have near parabolic index profiles with a peak refractive index difference ΔN≈~6×10-5, i.e. a record low numerical aperture (NA) of ~0.013 in an optical fiber. This low refractive index difference was achieved by frozen-in thermal stress as a result of two different types of glass in the fibers. When the fundamental mode was excited in the 252µm core fiber using a 1.03µm ASE source, the output beam was measured to have M2x=1.04 and M2y=1.18.

Comparison of amplification in large area fibers using cladding-pump and fundamental-mode core-pump schemes.

Abstract: We compare the amplification of various modes in large mode area fibers when the pump is coupled into the fundamental core mode versus the standard cladding-pump scheme. Our simulations show that pumping in the LP01 core mode results in differential gain for the fundamental signal mode which suppresses the higher order modes and amplified spontaneous emission compared to the cladding pump scheme. This differential gain effect is predicted to increase with core size and may provide a path to scale fiber mode area to several thousand square micron.

Efficient fiber coupler for vertical silicon slot waveguides.

Abstract: A mode size converter for efficient fiber coupling to silicon slot waveguides was proposed and demonstrated. It consists of two inverted lateral tapers that extend from the two strips of the silicon slot waveguide, and an overlaid low index waveguide with expanded mode size. Parameters including taper length and taper tip width were optimized with computer simulations. Samples were fabricated with a combined electron beam and photolithography process on a silicon-on-insulator wafer. The measured coupling loss to a standard single mode optical fiber was reduced by 8 dB for TE mode and 5.2 dB for TM mode with the converter.

Nanoscale Mode Selector in Silicon Waveguide for on Chip Nanofocusing

Abstract: We demonstrate a nanoscale mode selector supporting the propagation of the first antisymmetric mode of a silicon waveguide. The mode selector is based on embedding a short section of PhC into the waveguide. On the basis of the difference in k-vector distribution between orthogonal waveguide modes, the PhC can be designed to have a band gap for the fundamental mode, while allowing the transmission of the first antisymmetric mode. The device was tested by directly measuring the modal content before and after the PhC section using a near field scanning optical microscope. Extinction ratio was estimated to be ∼23 dB. Finally, we provide numerical simulations demonstrating strong coupling of the antisymmetric mode to metallic nanotips. On the basis of the results, we believe that the mode selector may become an important building block in the realization of on chip nanofocusing devices.

Nanoscale mode selector in silicon waveguide for on chip nanofocusing applications.

Abstract: We demonstrate a nanoscale mode selector supporting the propagation of the first antisymmetric mode of a silicon waveguide. The mode selector is based on embedding a short section of PhC into the waveguide. On the basis of the difference in k-vector distribution between orthogonal waveguide modes, the PhC can be designed to have a band gap for the fundamental mode, while allowing the transmission of the first antisymmetric mode. The device was tested by directly measuring the modal content before and after the PhC section using a near field scanning optical microscope. Extinction ratio was estimated to be ∼23 dB. Finally, we provide numerical simulations demonstrating strong coupling of the antisymmetric mode to metallic nanotips. On the basis of the results, we believe that the mode selector may become an important building block in the realization of on chip nanofocusing devices.

Unified Approach for Coupling to Cladding and Radiation Modes in Fiber Bragg and Long-Period Gratings

Abstract: Couplings of core mode to guided cladding and unguided radiation modes in fiber Bragg and long-period gratings are investigated by a unified approach based on complex mode matching method (CMMM). With the combination of the perfectly matched layer (PML) and the perfectly reflecting boundary (PRB), the continuous radiation modes can be well represented by a set of discrete complex modes so that simulation of coupling to radiation modes is greatly simplified and may be treated in the same fashion as guided modes. Numerical results of fiber Bragg and long-period gratings with refractive index of the outer cladding lower, equal, and higher than that of the inner cladding indicate that the unified approach is highly effective in the simulation of couplings to cladding and radiation modes.

Single mode mid-infrared silver halide asymmetric flat waveguide obtained from crystal extrusion

Abstract: A flat waveguide for the middle infrared was made by co-extrusion of two silver halide crystals of different chemical compositions. The transmission of the waveguide and its modal behavior was studied using a Fourier Transform Spectrometer and a dedicated optical bench. Analyzing this spectrum, we were able to obtain the cut-off wavelength of the waveguide. We observed a single mode behavior for wavelengths longer than 8.83μm, in good agreement with the theoretically expected values. This novel procedure is ideal for tailoring the properties of the waveguide for specific applications, in particular the spectral range where it exhibits a single-mode behavior. It can thus be applied to achieve modal filtering for mid-IR astronomical interferometers (e.g. beam combiners, nullers, etc.).

An investigation of the mode characteristics of SOI submicron rib waveguides using the film mode matching method

Abstract: The mode characteristics of SOI (silicon-on-insulator) submicron rib waveguides are very different from those of micrometer-sized ones. Using the full-vector film mode matching method, we propose a simple criterion to determine whether a waveguide mode is guided or not. The single-mode condition for deep-etched waveguides is obtained using this criterion. We also obtain the inherent TM mode leakage and sharp cancelation effects due to TE–TM mode coupling in shallow-etched rib waveguides from numerical simulations, which agree well with the analytical results based on total internal reflection and interference theories.