Abstract
With diameter close to or below the wavelength of guided light and high index contrast between the fiber core and the surrounding, an optical microfiber shows a variety of interesting waveguiding properties, including widely tailorable optical confinement, evanescent fields and waveguide dispersion. Among various microfiber applications, optical sensing has been attracting increasing research interest due to its possibilities of realizing miniaturized fiber optic sensors with small footprint, high sensitivity, fast response, high flexibility and low optical power consumption. Here we review recent progress in microfiber optical sensors regarding their fabrication, waveguide properties and sensing applications. Typical microfiber-based sensing structures, including biconical tapers, optical gratings, circular cavities, Mach-Zehnder interferometers and functionally coated/doped microfibers, are summarized. Categorized by sensing structures, microfiber optical sensors for refractive index, concentration, temperature, humidity, strain and current measurement in gas or liquid environments are reviewed. Finally, we conclude with an outlook for challenges and opportunities of microfiber optical sensors.
Highlights
In the past 50 years, fiber-optical sensing has been one of the most successful and powerful applications of both fiber optics and sensing technology [1]
By precisely controlling the drawing speed, heating conditions and subsequently the geometric profile of the tapering region, an microfiber drawn from a standard glass fiber, with both ends naturally connected to the original fiber, can exhibit high- efficiency or so-called adiabatic connection to the optical fiber, making it very convenient for in/out-coupling of light via standard optical fiber system
In 2009, Wu et al demonstrated a microelectromechanical system (MEMS) based optical accelerometer combined with a 386 μm diameter microfiber knot resonator fabricated by a 1.1 μm diameter silica microfiber [67]
Summary
In the past 50 years, fiber-optical sensing has been one of the most successful and powerful applications of both fiber optics and sensing technology [1]. It is obvious that, reducing the size of a sensing structure is usually an essential step to bestow the sensor with faster response, higher sensitivity, low power consumption and better spatial resolution, and an optical microfiber is one of the best candidates for this purpose [2,3,4]. With high-index contrast between the microfiber material (e.g., glass or polymer) and the surrounding (e.g., air or water), this kind of micro or nanoscale waveguide guides light with low optical loss, outstanding mechanical flexibilities, tight optical confinement and large fractional evanescent fields [2,10], making it a novel miniaturized platform for optical sensing with special advantages including faster response, higher sensitivity, low power consumption. We review the recent progress in microfiber optical sensors regarding their waveguide properties, fabrication, and sensing applications. We conclude with an outlook for challenges and opportunities of microfiber optical sensors
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