Optical Fiber Sensors And Tunable Whispering-gallery-mode Resonators Based On In-fiber Air Bubble Microcavities

Optical fiber sensors have been widely used in sensing applications of various physical,chemical,and even biological measurements.Meanwhile,fiber optic interferometer sensors also have been widely deployed Interferometer-based fiber optic sensors have been implemented in a wide range of sensing applications,such as temperature,refractive index,strain,pressure measurement,etc,owing to their special characteristics.In this study,the optical fiber devices based on different air bubles in-fiber are discussed in terms of principle of operation,fabrication methods,sensing applications and tunable whispering gallery modes.We demonstrate a sub-micron silica diaphragm based fiber-tip Fabry-Perot interferometer for pressure sensing applications.The silica diaphragm with a thickness of ~320 nm has been achieved by use of an improved electrical-arc-discharge technique.Such a sub-micron silica diaphragm breaks the sensitivity limitation imposed by traditional all-silica Fabry-Perot interferometric pressure sensors and as a result,a high pressure sensitivity of ~1036 pm/MPa at 1550 nm and a low temperature cross-sensitivity of ~960 Pa/oC are achieved when a silica diaphragm of ~500 nm in thickness is used.Moreover,the all-silica spherical structure enhanced the mechanical strength of the micro-cavity sensor,making it suitable for high sensitivity pressure sensing in harsh environment.We demonstrate an ultrahigh-sensitivity gas pressure sensor based on the Fabry-Perot interferometer employing a fiber-tip diaphragm-sealed cavity.The cavity is comprised of a silica capillary and ultrathin silica diaphragm with a thickness of 170 nm,with represents the thinnest silica diaphragm fabricated thus far by an electrical arc discharge technique.The resulting Fabry-Perot interferometer-based gas pressure sensor demonstrates a gas pressure sensitivity of about 12.22 nm/kPa,which is more than two orders of magnitude greater than that of a similarly configured fiber-tip air bubble sensor.Moreover,our gas pressure sensor has a low temperature cross-sensitivity of about 106 Pa/oC,and the sensor functions well up to a temperature of about 1000 oC.As such,the sensor can potentially be employed in high-temperature environments.We demonstrated a high-sensitivity strain sensor based on an in-fiber Fabry-Perot interferometer with an air cavity whose was created by splicing together two sections of standard single mode fibers.The sensitivity of this strain sensor was enhanced to 6.0 pm/?? by improving the cavity length of the Fabry-Perot interferometer by means of repeating arc discharges for reshaping the air cavity.Moreover,such a strain sensor has a very low temperature sensitivity of 1.1 pm/oC,which reduces the cross sensitivity between tensile strain and temperature.We demonstrated a unique rectangular air bubble by means of splicing two sections of standard single mode fibers together and tapering the splicing joint.Such an air bubble can be used to develop a promising high-sensitivity strain sensor based on Fabry-Perot interference.The sensitivity of the strain sensor with a cavity length of about 61 ?m and a wall thickness of about 1 ?m was measured to be up to 43.0 pm/?? and is the highest strain sensitivity among the in-fiber FPI-based strain sensors with air cavities reported so far.Moreover,our strain sensor has a very low temperature sensitivity of about 2.0 pm/oC.Thus,the temperature-induced strain measurement error is less than 0.046 ??/oC.We demonstrated the first optical microresonator based on an in-fiber rectangular air bubble created by means of splicing two sections of standards single mode fibers.Such the optical microresonator supports the whispering-gallery modes(WGMs)with a small mode volume and a high quality factor exceeding ~106,and strain-based fully tuning of WGMs in this resonator is demonstrated.The microresonator,with a 38?m order radius and ~1 ?m order wall thickness,has a superior strain-based tunability,i.e.the sensitivity of tunable resonance peaks is about 14.12 pm/??.Since the tensile strain applied on the in-fiber rectangle air bubble over 1000 ??,the tunable bandwidth of this optical microresonator is more than two times greater than its azimuthal free spectral range.