Fabrication of novel optical fibers and their sensing applications

Abstract

Photonic crystal fibers (PCFs) or microstructured optical fibers (MOFs) having periodic air holes hexagonally arranged along their entire length can induce unique properties that are difficult to realize in conventional fibers. Specialty optical fibers (PCFs or MOFs) were investigated in this work for sensing applications, e.g. pressure, strain, temperature, torsion, vibration because of the many inherent characteristics of glass which include high temperature operation and elasticity (i.e. very repeatable). The air holes in specialty optical fibers could also be exploited in biomedical applications, such as in optofluidics. Fabrication process of specialty optical fibers using the stack-and-draw method was developed and their practical issues are discussed in this work. A twin-core photonic crystal fiber (TC-PCF) with two ~2.5-μm diameter cores separated by one air-hole was designed and fabricated for pressure sensing applications such as downhole monitoring. The intermodal coupling between the four supermodes in the TC-PCF has been analyzed and verified by experiments. An interferometric pressure sensor using the fabricated fiber was developed. It has a sensitivity of -21 pm/MPa, which is in good agreement with the calculated value of -19 pm/MPa. An ultrahigh birefringence PCF (Hi-Bi PCF) designed for simultaneous measurement of temperature and pressure, by introducing an elliptical Germanium-doped core, was fabricated. The phase and group modal birefringence in the Hi-Bi PCF were measured to be 1.1×10² and 1.3×10², respectively, at the wavelength of ~1550 nm, which are in good agreement with the calculated values. The fiber has the highest birefringence reported for the fabricated index-guiding PCFs. Bragg grating inscribed in the fiber produces two spectral peaks with a large separation of 12 nm due to the exceptionally high birefringence. The two peaks exhibit different coefficients when the Hi-Bi PCF is subjected to different pressure or different temperature, allowing multiple parameters sensing. The measured pressure sensitivities of the Hi-Bi PCF are -1.96 pm/MPa, -5.13 pm/MPa with the x-and y-polarized peaks, respectively, while the temperature sensitivity for both peaks is similar, which is 11 pm/°C. Several types of suspended-core optical fibers (SCFs) having six big air holes surrounding a fiber core (i.e. suspended-core) were designed and fabricated for physical parameter sensing applications. The cores of the SCFs are either elliptical or doped with Germanium. The elliptical core introduces high birefringence (5×10⁴) while doping with Germanium introduces photosensitivity in the fiber, allowing Bragg gratings to be inscribed in the ~2-μm diameter core. The high birefringence SCF was characterized in terms of its transmission, and birefringence. The SCF was constructed into a Sagnac interferometer to measure pressure, strain, and torsion, achieving sensitivities of 2.82 nm/MPa, 0.43 pm/με, 0.0157/°, respectively. The pure silica fiber-based sensor exhibits low temperature dependency (<1 pm/°C). In addition, novel schemes and configurations using specialty optical fibers for biophotonics applications were also studied. Two kinds of optical interferometers using short pieces (~20 μm) of C-shaped fiber were constructed to measure refractive index of liquid. One sensor in the form of a Sagnac interferometer has a short length (11.5 cm) of PM-PCF sandwiched between two short pieces of C-shaped fiber. The C-shaped fiber allows liquid, whose RI is to be measured to get in and out of the air holes in the SOFs. Sensitivity of 6621 nm/RIU in the RI range of 1.330 to 1.333 was attained with the Sagnac interferometer sensor. Another sensor was constructed using one short piece (~20 μm) of C-shaped fiber sandwiched between two singlemode fibers to form a FabryPerot interferometer. The C-shaped fiber served as a spacer and allowed liquid to move into the path of the light beam between the two singlemode fibers. This sensor is smaller and has a simpler configuration but has a lower sensitivity of 1368 nm/RIU than the Sagnac interferometer sensor. A novel microstructure device constructed with a short length (10 mm) of tapered Co²⁺-doped fiber, which exhibits high light-absorption, was developed to measure flow rate in microchannels. The device contains two microchannels and a tapered Co²⁺-doped fiber inscribed with a Bragg grating. The Bragg grating adjacent to the microchannels measures the temperature and thus the cooling rate of the Co²⁺-doped fiber, which is heated by laser light and cooled by liquid flowing inside the microchannels, to determine the liquid flow rate. The flow rate sensor shows excellent detection ability for small amount of liquid with minimum detectable change of flow rate as low as ~16 nL/s.