Large Air Holes Research Articles

Side-Hole Photonic Crystal Fiber With Ultrahigh Polarimetric Pressure Sensitivity

We propose a side-hole polarization-maintaining photonic crystal fiber (PM-PCF) with ultrahigh polarimetric sensitivity to hydrostatic pressure. The proposed fiber has a very simple structure with a core surrounded by a double row of large air holes and a central row of small air holes. A pair of ultralarge side holes was symmetrically introduced into the silica cladding of the fiber to enhance the polarimetric response to hydrostatic pressure. Modal birefringence B as large as 2.34 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> and polarimetric pressure sensitivity dB/dp as high as -2.30 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> MPa <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> were achieved at 1.55 μm for the proposed fiber. Combining the advantages of both side-hole fibers and PM-PCFs, it is believed to be an excellent candidate for future applications of hydrostatic pressure measurement.

A Novel Birefringent Photonic Crystal Fiber and Its Application to Curvature Measurement

We demonstrate a curvature sensor with a newly designed photonic crystal fiber of high birefringence (Hi-Bi PCF). The proposed PCF has two large air holes in the outer cladding region that give rise to core ellipticity during the fabrication process. The curvature sensor is based on the PCF Sagnac interferometer, the output spectrum shift of which provides information on the PCF curvature. It turns out that the curvature sensitivity depends on the bending directions with respect to the large-air-hole alignment. For the two orthogonal bending directions, the curvature sensitivities near 1480 nm are -1.87 nm/m-1 (parallel to the axis of the two large air holes) and 1.24 nm/m-1 (perpendicular to the axis of the two large air holes), respectively, and are also insensitive to environmental temperature variation. The proposed curvature sensor may be useful in thin structural deflection measurement.

A Novel Birefringent Photonic Crystal Fiber and Its Application to Curvature Measurement

We demonstrate a curvature sensor with a newly designed photonic crystal fiber of high birefringence (Hi-Bi PCF). The proposed PCF has two large air holes in the outer cladding region that give rise to core ellipticity during the fabrication process. The curvature sensor is based on the PCF Sagnac interferometer, the output spectrum shift of which provides information on the PCF curvature. It turns out that the curvature sensitivity depends on the bending directions with respect to the large-air-hole alignment. For the two orthogonal bending directions, the curvature sensitivities near 1480 nm are -1.87 nm/m-1 (parallel to the axis of the two large air holes) and 1.24 nm/m-1 (perpendicular to the axis of the two large air holes), respectively, and are also insensitive to environmental temperature variation. The proposed curvature sensor may be useful in thin structural deflection measurement.

Multi-hole Optical Fiber Surface Plasmon Resonance Sensor

A microstructured-fiber containing six large air holes is proposed to construct the surface plasmon resonance (SPR) sensor. The finite element method is used to analyze characteristics of the surface plasmon resonance sensor. The effects of the thickness of metal films, pitch between air holes, diameter of air hole, and refractive index of liquid on the resonance wavelength are elucidated. The results show that the resonance wavelength is sensitive to the thickness of metal film and refractive index of liquid, while the resonance wavelength doesn't change basically when the pitch between air holes and diameter of air holes vary. The proposed surface plasmon resonance sensor exhibits high sensitivity up to 10−4.

High Performance Large Mode-Area Ytterbium-doped Photonic Crystal Fiber for Fiber Lasers

In this letter, large-mode-area double-cladding ytterbium-doped photonic crystal fiber was designed in theory and fabricated in practice. This fiber we have fabricated successfully has endless single mode operation performance and large inner-cladding numerical aperture of more than 0.75. The struts width between large air-holes in the outer-cladding is about 0.22 μm. The photonic crystal fiber has a mode-area about 1465.7μm2. Due to the material being pure silica and air, such structures have excellent capacity to with-stand high temperature. The laser light can have very good beam quality, even diffraction-limited beam quality because of the single-mode core. This fabrication technical breakthrough of novelty high performance double-cladding ytterbium-doped photonic crystal fibers will give contributions to the high power fiber lasers and promote the progress of technology in the fields of high power lasers.

Simultaneous Measurement of Temperature and Hydrostatic Pressure Using Bragg Gratings in Standard and Grapefruit Microstructured Fibers

A novel fiber-optic sensor based on Bragg gratings in standard and grapefruit microstructured fibers (GMF) for simultaneous measurement of temperature and hydrostatic pressure is proposed and experimentally demonstrated. The temperature responses of these two gratings are almost the same due to the similar material composition in the core. The existence of large air holes in the cladding of GMF makes it experience larger axial strain than standard single-mode fiber (SMF), and therefore the pressure sensitivity of Bragg grating in GMF is much larger than that for SMF. Hence, simultaneous measurement of temperature and hydrostatic pressure can be achieved.

HIGHLY BIREFRINGENT FOUR-HOLE FIBER FOR PRESSURE SENSING

A highly birefringent four-hole flber (FHF) with a pair of large air holes and a pair of small air holes are proposed for air/hydrostatic pressure sensing. The birefringence of the FHF can be up to 0.01 due to the rectangle-like flber core surrounded by four air holes. Therefore, a FHF with a length of only several centimeters is required for high-sensitivity pressure sensing based on a Sagnac interferometer. Optical properties of the FHF such as efiective index and birefringence are investigated. Pressure sensor based on the FHF depends on the pressure-induced refractive index change or pressure- induced birefringence. The stress distribution of the FHF subjected to an air/hydrostatic pressure is represented. Simulations show that the principal stress component parallel to the slow axis of the of the FHF under the air/hydrostatic pressure is greatly enhanced due to the existence of two large air holes, which consequently results in a high sensitivity of the FHF-based pressure sensor. Relationships between the pressure-induced bireflrngence and the radius of the large air hole, the external diameter of the FHF, or the ellipticity of the elliptical FHF are investigated. The polarimetric pressure sensitivity of the FHF can be up to 607rad/MPa/m.

Dual-Core Side-Hole Fiber for Pressure Sensing Based on Intensity Detection

We propose a novel dual-core side-hole fiber (DCSHF) for air/hydrostatic pressure sensing. Two large air holes are employed in the cross-section of the DCSHF, which result in a difference of the pressure-induced index changes for two fiber cores. The mode coupling between two fiber cores of the DCSHF is sensitive to the pressure-induced index change which ensures the intensity detection of the DCSHF-based pressure sensor. A segment of DCSHF with a length equal to the coupling length of the mode coupling between two fiber cores is used as a pressure sensor. When the light is injected into the central fiber core of the DCSHF, there is a one-to-one correspondence between the pressure and the output power on the output side of the central fiber core. A pressure sensor based on a typical DCSHF can achieve a pressure measurement range of 27 MPa. DCSHFs with different structure parameters for pressure sensing are investigated.

Investigation of V-type photonic crystal fiber with high birefringence

According to the full-vector finite element method, a new kind of V-shape photonic crystal fiber with high birefringence is proposed. Simulation results show that the V-shape photonic crystal fiber has a birefringence of 1.239×10-2 at a wavelength of 1.55 μm, when its cladding air hole pitch, large air hole diameter, small air hole diameter are 1.0 μm, 0.95 μm and 0.6 μm, respectively. There are two zero dispersion wavelengths in the visible and the near-infrared region respectively, and the abnormal dispersion zone includes the operation wavelength of Ti:sapphire laser, which is advantageous for nonlinear generation of photonic crystal fibers. The difference in air filling fraction between fast and slow axis can be used as a sensor for the weak strain.

Refractive Index Sensor Using a Two-Hole Fiber

We propose to use a twin-hole fiber to measure refractive index of liquids The key idea is to have a single mode fiber (SMF) having two large air-holes running along the fiber length, the holes do not interact with the core. However, using wet chemical etching we can have access to the hole around the fiber, and further etching increases the holes diameter. The diameter is increased until the fiber exhibits a specific birefringence. Since the holes are open, by immersing the fiber in different liquids (n=1.33 to n=1.42) the value of the birefringence is modified and the refractive index of the liquid can be estimated from the change on the beat length. This process provides a very simple and highly sensitive mechanism for sensing refractive index in liquids, and can also be used for other applications.

Dispersion-modified, highly nonlinear holey fibre with a high index, slot-structurecore

The group velocity dispersion (GVD) characteristics of a holey fibre design with ahigh index, slot structure in the core region are modified by introducing a simplecladding structure with a few large air holes. Our simulation results show thatultra-small mode effective areas and a high nonlinearity of the TE mode canbe achieved, which is confined in the narrow slot by two silicon slabs in a glassfibre. This slot structure can be optimized to simultaneously achieve a very smallAeff and a single-TE-mode propagation. On the other hand, the proposed claddingcan significantly reduce and flatten the large negative GVD of the slotstructure, which could improve the nonlinear efficiency and interactionlength for nonlinear applications. Our results indicate that an ultra-smallAeff of0.16 µm2 and a flattened and reduced GVD with less than10 ps nm − 1 km − 1 dispersion ripple within a 40 nm range at the 1.55 mm wavelength are achievable.

Enhanced transverse load sensitivity by using a highly birefringent photonic crystal fiber with larger air holes on one axis

We report on a transverse load sensor with enhanced sensitivity through the use of a birefringent interferometer based on a highly birefringent photonic crystal fiber (HB-PCF). The transverse load sensitivity can be enhanced by using a fabricated HB-PCF having larger air holes on its fast axis. The transverse load sensitivity was measured to be high: approximately 2.17 nm/(N/cm). The temperature-induced undesirable effects can be ignored because transmission outputs of our HB-PCF were stable with the change of the temperature. The sensing probe can be compact because of its high birefringence with the order of 10(-3) and no bending loss.

An asymmetric dual-core single-polarization single-mode photonic crystal fiber coupler

A vectorial finite element method (VFEM) is adopted to investigate the novel single-polarization single-mode (SPSM) photonic crystal fiber (PCF) coupler which has asymmetric dual-core and two lines of enlarged air holes. It is demonstrated that the SPSM region of the designed fiber can be more than 250 nm wide with a set of optimized air-hole parameters and the width of the SPSM region could change slightly by fine adjustment of the inner large air holes. The coupling length between the odd and even x-polarization modes is also investigated through fine-tuning the large and small air-hole diameters.

Characterization of Pressure Response of Bragg Gratings in Grapefruit Microstructured Fibers

In this paper, the pressure response of fiber Bragg gratings in grapefruit microstructured fibers was experimentally investigated and theoretically analyzed with full-vector finite element method. The theoretical calculation agrees well with the experimental results. The pressure changes the refractive index and induces axial strain of the fiber. The radial pressure induces positive strain, but axial pressure induces negative strain. The negative strain dominates. The existence of large air holes make grapefruit microstructured fibers experience larger negative strain than standard single-mode fiber, therefore fiber Bragg gratings in grapefruit microstructured fibers have larger pressure sensitivity than that in SMF.

Equal channel spacing Sagnac filter using high-birefringent photonic crystal fibers

An equal channel spacing Sagnac filter is proposed using a high-birefringent photonic crystal fiber (PCF) with birefringence square to the operation wavelength. A PCF with four smaller central air holes surrounded by larger air holes is applied in a Sagnac filter with optimized parameters to achieve equal channel spacing of 0.8 nm. Given that the birefringence of this PCF is square to the operation wavelength, the channel spacing of the proposed filter changes by only 0.03 nm within a wavelength range from 1400 to 1650 nm; this is about one order of magnitude less than that constructed with conventional high-birefringent fibers.

Sensitivity analysis of refractive index measurement based on intermodal interference in photonic crystal fiber

The sensitivity of an interferometer based on intermodal interference in photonic crystal fiber (PCF) has been investigated theoretically and numerically. Using the intermodal interference theory and the normalized frequency of PCF, we have analyzed the principle of the method for measuring refractive index by PCF interferomter,and obtained a variation, δns, to show the sensitivity of the PCF interferometer. From the analysis of the underlying physics of δns, we find that air holes of the first layer play an important role on the sensitivity.Compared with the conventional PCF structure,the sensitivity can be improved remarkably via employing large air holes with the shape of a combination of half circle and half oval.

Investigation of an Arc-induced Long Period Fiber Grating Inscribed in a Photonic Crystal Fiber with Two Large Air Holes

A photonic crystal fiber with two large air holes outside the holey cladding region is fabricated to induce an effective long periodic grating (LPG) in the core by an electric arc discharge. We believe that the two large air holes lead to the asymmetric perturbation in the core under the electric arc discharge, thereby introducing the coupling to the first higher-order mode. The transmission characteristics of the PCF with the LPG for the external perturbation such as strain, curvature, and temperature are also investigated. It was found that the shift of resonance peak in the transmission spectrum depends on the bending direction. The curvature of 8.55 <TEX>$m^{-1}$</TEX> results in the center wavelength shifts of 1.8, 4.3, and 11 nm for a vertical, diagonal, and horizontal direction of the curvature to the large air-hole alignment, respectively.

Design of photosensitive microstructured polymer optical fibers

We propose a new hole-assisted polymer optical fiber design to eliminate the influence of dopant diffusion and to increase the ultra violet (UV) writing efficiency in fiber Bragg grating inscription. The optical waveguide is formed inside a solid core polymethyl methacrylate (PMMA) doped with photosensitive trans-4-stilbenemethanol, surrounded by a ring of three large air holes with double cladding. We determined a map of the single-mode and multi-mode phase transitions using a finite-element-based vectorial optical mode solver. A wide range of geometrical configurations for the single-transverse-mode (HE11) propagation in the visible was obtained. The design is optimized to operate at the low optical loss wavelengths of 580 and 770 nm.

Ultrahigh Birefringent Photonic Crystal Fiber With Ultralow Confinement Loss Using Four Airholes in the Core

A simple photonic crystal fiber (PCF) design with a rectangular array of four airholes in the core region and a traditional circular-airhole cladding is proposed in this work. The modal birefringence is induced by the asymmetry of the rectangular distribution of four airholes and/or the elliptical shape of the holes. The traditional symmetric cladding structure results in good confinement loss performance by limiting the light in the core region. Therefore, the proposed design enables simultaneous realization of high birefringence and low confinement loss. Simulations based on the full-vector finite element method (FEM) with anisotropic (PML) show that ultrahigh single-mode birefringence (~ 10-2) and ultralow confinement losses ( < 0.002 dB/km) can be achieved at 1.55 mum wavelength. Dependence study of the birefringence and losses on several key parameters is also provided. Compared to previously studied PCF with asymmetric core or cladding structures, by having just four relatively large airholes in the core, this design could be much easier to be implemented with even better performance.

Chalcogenide As$_{2}$S$_{3}$ Suspended Core Fiber for Mid-IR Wavelength Conversion Based on Degenerate Four-Wave Mixing

A chalcogenide optical fiber of special design is proposed to convert a short-wavelength infrared radiation (around 2 mum) up to a second transparency window of atmospheric air (around 4.5 mu m) by degenerate four-wave mixing. The fiber supports a small core surrounded by three large air holes. The zero-dispersion wavelength is shifted down to 2 mum in this fiber by properly tailoring the fiber core. We demonstrate by solving the nonlinear Schrodinger equation that efficient wavelength-conversion can be obtained by pumping the fiber with a Tm:SiO2 pulsed fiber laser.