Hollow-core Waveguides Research Articles

Gas-Sensing Hollow-Core Waveguide Based on a Liquid Film

ABSTRACT In this paper, a strategy for the fabrication of a gas-sensing hollow-core waveguide with liquid film was developed to replace the traditional solid film hollow-core waveguide. The surface morphologies of the solid film and liquid film were characterized by scanning electron microscopy, respectively. Compared with the solid film, liquid film can dissolve gas molecules and enhance the gas sensitivity, especially in the range of low concentration. An improved absorption for detecting 1000 ppm of acetylene has been achieved when liquid film was adopted instead of solid film.

Determination of trace chromium (VI) using a hollow-core metal-cladding optical waveguide sensor

A biosensor capable of highly sensitive detection of trace chromium (VI) with a simple hollow-core metal-cladding waveguide (HCMW) structure is theoretically modeled and experimentally demonstrated. Owing to the high sensitivity of the excited ultrahigh-order modes in the waveguide, a tiny variation of the extinction coefficients in the waveguide guiding layer where the chromate ions reacts with the diphenylcarbazide (DPC) can lead to a significant change of light intensity in the reflection spectrum. The experimental results indicate that using the proposed method, the chromium (VI) sensitivity detection limit can be as low as 1.2 nM, which represents a 16-fold improvement compared to the surface plasmon field-enhanced resonance light scattering (SP-RLS) method, and a 4-fold improvement compared to the flame atomic absorption spectrometry and fluorimetry spectroscopy, respectively.

Determination of glucose concentration in biological liquids using photonic crystal waveguides

The possibility of using hollow-core photonic crystal waveguides as biological sensors is shown. The influence of the glucose concentration in solutions introduced into the waveguide core on the transmission spectrum of photonic crystal waveguides is shown. Two possible methods of determining the glucose concentration in liquids are considered, refractometric and photometric.

Terahertz dielectric waveguides

Several classes of non-planar metallic and dielectric waveguides have been proposed in the literature for guidance of terahertz (THz) or T-ray radiation. In this review, we focus on the development of dielectric waveguides, in the THz regime, with reduced loss and dispersion. First, we examine different THz spectroscopy configurations and fundamental equations employed for characterization of THz waveguides. Then we divide THz dielectric waveguides into three classes: solid-core, hollow-core, and porous-core waveguides. The guiding mechanism, fabrication steps, measured loss, and dispersion are presented for the waveguides in each class in chronological order. The goal of this review is to compare and contrast the current solutions for guiding THz radiation.

Flexible tube lattice fibers for terahertz applications

In this paper a flexible hollow core waveguide for the terahertz spectral range is demonstrated. Its cladding is composed of a circular arrangement of dielectric tubes surrounded by a heat-shrink jacket that allows the fiber to be flexible. Characterization of straight samples shows that the hollow core allows the absorption caused by the polymethylmethacrylate tubes of the cladding to be reduced by 31 times at 0.375 THz and 272 times at 0.828 THz with respect to the bulk material, achieving losses of 0.3 and 0.16 dB/cm respectively. Bending loss is also experimentally measured and compared to numerical results. For large bending radii bending loss scales as Rb-2, whereas for small bending radii additional resonances between core and cladding appear. The transmission window bandwidth is also shown to shrink as the bending radius is reduced. An analytical model is proposed to predict and quantify both of these bending effects.

Analog Signal Transmission in a High-Contrast-Gratings-Based Hollow-Core-Waveguide

In this paper, the performance of an on-chip hollow-core-waveguide (HW) using high-contrast gratings (HCG) for analog signal transmission is analyzed numerically. Simulation results indicate that after propagating 100 m in a HCG-HW with optimally designed parameters, there is very little degradation of either third-order intermodulation distortion spur-free dynamic range (IM3 SFDR) or third-order harmonic distortion (THD) SFDR. Due to the chromatic dispersion of the HCG-HW, the highest second-order harmonic distortion (SHD) SFDR is limited to 107.3 dB · Hz1 / 2. In addition, >; 100 dB · Hz 2/3 IM3 SFDR can be achieved over a radio frequency (RF) range of 80 GHz and an optical wavelength bandwidth of 50 nm after propagation 100 m through a HCG-HW. The parameter dependence of the waveguide performance is also investigated. With a ± 20 nm variation on all parameters, the propagation length in an HCG-HW is limited to ~ 6 m in order to maintain an IM3 SFDR of >; 100 dB · Hz 2 / 3.

Phase constant peculiarities of cylindrical zero-index anisotropic metamaterial waveguide

Here, we present the phase constant dependencies of propagating eigenmodes of an open cylindrical anisotropic metamaterial waveguide when the metamaterial permittivity and permeability tensor components may accept values close or equal to zero. Dispersion characteristics of rod and hollow-core waveguides with the radii 0.5, 2.5, and 5 mm at the left-handed polarization of the microwave are shown here. There are unusual shapes of eigenmode dispersion characteristics and anomalous sectors of the characteristics at certain frequencies. The first eigenmode of the rod waveguide with the lowest cutoff frequency is a particularly important mode because it is a single one in the frequency range 1.0–1.9 GHz, and some small variations on the frequency produce large changes in the phase constant. We can observe packages of dispersion characteristic branches when their cutoff frequencies closed to the metamaterial electric and magnetic plasma frequencies between 1.9 and 3.5 GHz. There are only three modes in the hollow core anisotropic metamaterial waveguide at the frequency range 1.4–2.8 GHz.

Three-Dimensional Chirped High-Contrast Grating Hollow-Core Waveguide

A 3-D chirped high-contrast grating hollow-core waveguide (HCG-HW) exhibits highly efficient 2-D (transverse and lateral) light confinement. Compared with step-index ones, laterally chirped HCG-HWs (graded index) can pronouncedly reduce the propagation loss (>; 10×) without compromising the lateral index contrast. Simulation results show that a propagation loss as low as 0.04 dB/cm can be achieved. For a chirped HCG-HW with a 6-μm core height, it exhibits 30and 141-nm spectral widths for 0.1and 1-dB/cm loss requirements. The chirped HCG-HW also shows an ultralow nonlinear coefficient, which is on the order of 10-5 /W/m.

Hollow-core waveguides with uniaxial metamaterial cladding: modal equations and guidance conditions

We discuss the conditions in which guided modes exist in a circular waveguide with an anisotropic, uniaxial metamaterial cladding. These hollow-core waveguides can guide modes at deep subwavelength dimensions, with core diameters more than 20 times smaller than the operating wavelength.

Design of a compact mode and polarization converter in three-dimensional photonic crystals

A mode and polarization converter is proposed and optimized for 3D photonic integrated circuits based on photonic crystals (PhCs). The device converts the index-guided TE mode of a W1 solid-core (SC) waveguide to the band-gap-guided TM mode of a W1 hollow-core (HC) waveguide in 3D PhCs, and vice versa. The conversion is achieved based on contra-directional mode coupling. For a 25 μm-long device, simulations show that the power conversion efficiency is over 98% across a wavelength range of 16 nm centered at 1550 nm, whereas the reflection remains below -20 dB. The polarization extinction ratio of the conversion is in principle infinitely high because both W1 waveguides operate in the single-mode regimes in this wavelength range.

Low loss hollow-core waveguide on a silicon substrate

Abstract Optical-fiber-based, hollow-core waveguides (HCWs) have opened up many new applications in laser surgery, gas sensors, and non-linear optics. Chip-scale HCWs are desirable because they are compact, light-weight and can be integrated with other devices into systems-on-a-chip. However, their progress has been hindered by the lack of a low loss waveguide architecture. Here, a completely new waveguiding concept is demonstrated using two planar, parallel, silicon-on-insulator wafers with high-contrast subwavelength gratings to reflect light in-between. We report a record low optical loss of 0.37 dB/cm for a 9-μm waveguide, mode-matched to a single mode fiber. Two-dimensional light confinement is experimentally realized without sidewalls in the HCWs, which is promising for ultrafast sensing response with nearly instantaneous flow of gases or fluids. This unique waveguide geometry establishes an entirely new scheme for low-cost chip-scale sensor arrays and lab-on-a-chip applications.

Biosensor based on hollow-core metal-cladding waveguide

A new hollow-core metal-cladding waveguide (HCMW) has been proposed in order to design and realize optical biosensor for the direct liquid probing. Two peculiar properties are exhibited with respect to the current evanescent wave sensors. First, the effective index of the HCMW can exist in the region of 0<N<1, which is usually prohibited for the conventional guided modes and the surface plasmon resonance modes. Second, the analyte to be detected does not locate in the evanescent field but in the oscillating field. Glucose solution is utilized to characterize the device performance. According to the reflectivity changes and the signal-to-noise ratio, the new biosensor has been shown to be capable of directly detecting concentration of glucose as low as 0.5ppm, which corresponds to a high resolution of 1.4×10−7 RI units. This new biosensor improves the detection limitation and shortens the analysis time significantly.

Coupling external cavity mid-IR quantum cascade lasers with low loss hollow metallic/dielectric waveguides

We report on the optical coupling between hollow core waveguides and external cavity mid-IR quantum cascade lasers (QCLs). Waveguides with 1000 μm bore size and lengths ranging from 2 to 14 cm, with metallic (Ag)/dielectric (AgI or polystyrene) circular cross-section internal coatings, have been employed. Our results show that the QCL mode is perfectly matched to the hybrid HE11 waveguide mode, demonstrating that the internal dielectric coating thickness is effective to suppress the higher losses TE-like modes. Optical losses down to 0.44 dB/m at 5.27 μm were measured in Ag/polystyrene-coated waveguide with an almost unitary coupling efficiency.

Flexible, Low-loss Waveguide Designs for Efficient Coupling to Quantum Cascade Lasers in the Far-infrared

We coupled linearly polarized and azimuthally polarized Terahertz quantum cascade lasers (QCLs) to the low-loss optical modes of hollow core waveguides having a sequence of different metallic or dielectric inner coatings. The latter waveguides have been specifically designed to force the propagation of a dominant optical mode once the thickness (d) of the inner dielectric coating is properly chosen. Our results demonstrate that both the TE01 and the TE11 modes can be easily converted to a hybrid one when d > 6 μm allowing the propagation of THz QCL beams with transmission losses as low as 1.5 dB/m, bending losses < 1.1 dB and reasonably high coupling efficiencies (87%).

Characterization of Plasmonic Modes in a Low-Loss Dielectric-Coated Hollow Core Rectangular Waveguide at Terahertz Frequency

In this paper, a low-loss hollow-core rectangular plasmonic waveguide with a dielectric coating of Teflon is analyzed for terahertz (2.5 THz) propagation using a full-vectorial finite-element method (FEM). The modal properties of the waveguide, their effective indices, and power confinements have been calculated with a particular emphasis on the loss characteristics of the different modes. It has been observed that the loss characteristics of the guide are greatly affected by the thickness of the dielectric coating. It has been identified that, in contrast to the fundamental H10x mode, the H12x mode shows interesting modal properties and offers the lowest possible loss for the structure. This mode also tends to yield a near-Gaussian field profile when the dielectric coating thickness is optimized. The optimization of the loss values has been evaluated by comparing the loss characteristics for different dielectric materials, as well as by using different metal claddings.

Design and fabrication of gas sensing hollow core waveguides in near infrared

A category of gas sensing hollow core waveguide that was operated in near infrared was designed to solve their major obstacles in application including high scattering and high losses. An attenuated total reflection structure was developed by using MS theory and dynamic liquid phase processing. An explanation on how the loss was reduced by total reflectance was provided and a suitable thickness of Ag film by controlling the deposition time was achieved. The guide we designed had a simple structure, low loss and hence increased the sensitivity.

Near-Infrared Hollow Waveguide Gas Sensors

The development of a hollow core waveguide (HWG) gas sensor in combination with a fast and compact near-infrared (NIR) spectrometer is presented. The spectrometer operates in the spectral range of 1200-1400 nm and may thus be applied for the detection of gas-phase analytes providing NIR absorptions in that spectral window such as, e.g., methane. Since mid-infrared spectroscopy in combination with HWGs has already been successfully demonstrated for probing hydrocarbons in the gas phase, the present study investigates the achievable sensitivity in the NIR spectral regime. Methane has been selected as an exemplary analyte due to the fact that it shows strong absorption features in the mid-infrared (mid-IR) fingerprint area, but also overtone bands in the NIR. Since the HWG simultaneously serves as a miniaturized absorption gas cell and as an optical waveguide for NIR radiation, a compact yet optical and cost-efficient sensor device was established providing an interesting alternative in target sensing for mid-IR devices. The achieved limit of detection (LOD) was 5.7% (vol./vol.) methane for a 9.5 cm long HWG, 1.6% (vol./vol.) methane for a 39.1 cm long HWG, and 1.3% (vol./vol.) methane for a setup using a 77.4 cm long HWG, which provides the most practical HWG dimensions among the three investigated setups. Limit of quantitation (LOQ) values were calculated at 20.1% (vol./vol.) methane, 8.7% (vol./vol.) methane, and 5.6% (vol./vol.) methane, respectively.

Few cycle pulse compressor based on hollow-core fiber waveguide results in pulse tails

The hollow-core fiber structure influence of few-cycle pulse compressor on pulse contrast is investigated experimentally. It is found that a periodic structure of pulses at the tail of the main pulse extended hundreds of picoseconds away. It can be due to the fiber structure and could be eliminated by modifying the structure.

Selective Thin-Film Deposition for Optofluidic Platforms With Optimized Transmission

Selective deposition of dielectric thin films on an optofluidic biosensor provides the means for low loss in hollow-core waveguides and increased solid- to hollow-core coupling. Polymethylglutarimide (PMGI) is used over nonplanar topographies to provide liftoff patterning of dielectric films deposited at temperatures up to 250°C . This fabrication method provides throughput improvement in devices with hollow-cores longer than 2 mm.

Ultrasmall-V high-Q photonic crystal nanobeam microcavities based on slot and hollow-core waveguides

Photonic crystal nanobeam microcavities based on slot and hollow-core waveguides were proposed and analyzed. Three-dimensional finite-difference time-domain simulations show that both an ultrasmall modal volume (V) and a high quality (Q) factor can be obtained simultaneously in hollow-core-nanobeam mirocavities (HCNMs) due to the strong confinement of fields in the two lateral directions. For a 6.5-μm-long HCNM, the Q factor and V are on the order of 10⁵ and 10⁻²(λ/2n)³, respectively. With compactness, lower fabrication requirements as well as ultrahigh Q/V, the proposed microcavities would be very promising in a variety of applications.