Ming-Leung Vincent Tse

High pressure sensor based on photonic crystal fiber for downhole application

We demonstrate a polarization-maintaining (PM) photonic crystal fiber (PCF) based Sagnac interferometer for downhole high pressure sensing application. The PM PCF serves as a direct pressure sensing probe. The sensor is transducer free and thus fundamentally enhances its long-term sensing stability. In addition, the PM PCF can be coiled into a small diameter to fulfill the compact size requirement of downhole application. A theoretical study of its loss and birefringence changes with different coiling diameters has been carried out. This bend-insensitive property of the fiber provides ease for sensor design and benefits practical application. The pressure sensitivities of the proposed sensor are 4.21 and 3.24 nm/MPa at ∼1320  and ∼1550 nm, respectively. High pressure measurement up to 20 MPa was achieved with our experiment. It shows both good linearity in response to applied pressure and good repeatability within the entire measurement range. The proposed pressure sensor exhibits low temperature cross sensitivity and high temperature sustainability. It functions well without any measurable degradation effects on sensitivity or linearity at a temperature as high as 293 °C. These characteristics make it a potentially ideal candidate for downhole pressure sensing.

Fusion Splicing Holey Fibers and Single-Mode Fibers: A Simple Method to Reduce Loss and Increase Strength

We demonstrate a novel method for low-loss splicing Ge-doped holey fibers (HF) with subwavelength core size and high numerical aperture fibers by using a conventional fusion splicer. We found that a large overlap distance of the fibers during fusion would decrease the splice loss and increase the splice strength. The lowest splice loss achieved for a fiber with a core diameter of 1.27 mu m and air-filling fraction of >0.95 was ~ 1 dB at 1550 nm, with a bend failure radius of 0.8 cm. With the same method, we also observed improvement in terms of loss and strength with larger core size HF.

Intermodal coupling of supermodes in a twin-core photonic crystal fiber and its application as a pressure sensor

In this paper, we experimentally demonstrated the fabrication and hydrostatic pressure characteristics of a twin-core photonic crystal fiber (TC-PCF). Mode couplings in the TC-PCF for x- and y-polarizations were analyzed simultaneously using group effective index of guiding modes. The output spectrum of the TC-PCF was modulated due to the combined couplings of the two polarizations. To the best of our knowledge, it is the first time to measure hydrostatic pressure through the dual-polarization mode coupling in a TC-PCF. The measured sensitivity of the pressure sensor was -21 pm/MPa. The length of the TC-PCF used for pressure measurement was 20 cm, which is much shorter than pressure sensor based on PM-PCF, and does not require any external polarizing components, meaning that it is a good candidate for compact pressure sensor.

In-line microfluidic refractometer based on C-shaped fiber assisted photonic crystal fiber Sagnac interferometer.

We propose and demonstrate a highly sensitive in-line photonic crystal fiber (PCF) microfluidic refractometer. Ultrathin C-shaped fibers are spliced in-between the PCF and standard single-mode fibers. The C-shaped fibers provide openings for liquid to flow in and out of the PCF. Based on a Sagnac interferometer, the refractive index (RI) response of the device is investigated theoretically and experimentally. A high sensitivity of 6621 nm/RIU for liquid RI from 1.330 to 1.333 is achieved in the experiment, which agrees well with the theoretical analysis.

Ultrahigh birefringence index-guiding photonic crystal fiber and its application for pressure and temperature discrimination

In this Letter, we reported on an ultrahigh birefringence photonic crystal fiber (PCF) with a germanium-doped elliptical core, which is fabricated in our lab using the stack-and-draw method. An ultrahigh birefringence of 1.1×10(-2) is obtained experimentally, which is close to the theoretical value of 1.4×10(-2) at the wavelength of 1550 nm. To our knowledge, this is the highest birefringence reported to date for fabricated index-guiding PCF. Fiber Bragg gratings (FBG) were written in the fiber to confirm its ultrahigh birefringence, and we demonstrated the capability to simultaneously measure the FBGs pressure and temperature experimentally. Because of the large separation of the two FBG peaks (>12 nm), such fiber is a promising candidate for a single polarization device.

SUPER-LATTICE STRUCTURE PHOTONIC CRYSTAL FIBER

We propose a kind of novel photonic crystal flbers (PCFs) based on a super-lattice structure. Uniform air holes are used to form the basic cell structure. Using the uniform air holes in the PCF has the advantage of minimizing the structural distortion during fabrication while forming a complex-structure cross section. We propose an efiective-circular-hole PCF with the similar properties of the conventional circular-hole PCF to address the concept of the super- lattice structure PCF. An efiective-elliptical-hole PCF based on a super-lattice structure is proposed and investigated, which has similar birefringent and conflnement loss characteristics as the previously reported elliptical-hole PCF. Other PCFs based on super-lattice structures such as the efiective-triangular-hole PCF and efiective- rectangular-hole PCF can also be achieved by using the design method proposed in this paper.

Fabrication, Characterization, and Sensing Applications of a High-Birefringence Suspended-Core Fiber

In this paper, we report on the fabrication, characterization, and sensing applications of a high-birefringence suspended-core fiber (HB-SCF) using the stack-and-draw technique. Capillaries in two sizes were used for stacking, forming an elliptical core, giving rise to geometric modal birefringence of the HB-SCF. Through a finite-element method, the theoretical values of the phase and group modal birefringence of the HB-SCF were found to be 2.59 ×10-4 and 4.75 ×10-4, respectively. Based on a Sagnac interferometer (SI), the group modal birefringence of the HB-SCF was measured to be 4.84 ×10-4 , which agrees well with the theoretical result. The sensing characteristics of the HB-SCF SI for pressure, strain, and torsion measurements were analyzed theoretically and investigated experimentally. It exhibits sensitivities of 2.82 nm/MPa for hydrostatic pressure, 0.43 pm/με for strain, and 0.0157/° for torsion measurements. The HB-SCF is made entirely of pure silica without stress-applying parts, thus its modal birefringence is thermally insensitive, resulting in a low temperature sensitivity of ~1 pm/°C.

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.

Superlattice Microstructured Optical Fiber

A generic three-stage stack-and-draw method is demonstrated for the fabrication of complex-microstructured optical fibers. We report the fabrication and characterization of a silica superlattice microstructured fiber with more than 800 rhomboidally arranged air-holes. A polarization-maintaining fiber with a birefringence of 8.5 × 10−4 is demonstrated. The birefringent property of the fiber is found to be highly insensitive to external environmental effects, such as pressure.

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.