Active Fiber Bragg Grating Flow Sensor Powered By In-Fiber Light

摘要

Tunable fiber Bragg gratings (FBGs) are key components for optical communications and sensing applications. Current tuning mechanisms include on-fiber electric heating, the piezo-electric effect, and mechanical stretching and bending. Unfortunately, all of these tuning mechanisms rely on external electrical power supplied by non-optical means. Additional electrical cabling increases manufacturing cost and the risks of failure associated with additional on-fiber electrical contacts and fragile packaging, which are susceptible to electromagnetic interference. These limitations make current fiber components no longer suitable for use in hostile environments, such as extreme temperature, corrosive, and humid environments.The research herein presents a tunable fiber Bragg grating device without sophisticated packaging and external electrical wiring. Shown for the first time, the resonance wavelength, spectrum width, and chirp can be directly controlled by in-fiber light as well as spectral responses of metal-coated fiber Bragg gratings. In-fiber diode laser light at 910-nm was leaked from the fiber and absorbed by the surrounding metallic coating to raise the grating's temperature and to change the background refractive index distribution of the gratings. Wide tunability of the resonance wavelength and spectral width was demonstrated in both uniform and linear chirped gratings.Applications of in-fiber light-powered active grating sensors are demonstrated for dual function temperature and flow sensors based on self-heated optical hot wire anemometry. A grating flow sensor has been experimentally evaluated for different grating lengths and input laser powers. The grating flow sensor demonstrated a minimum measurable flow velocity for nitrogen gas flow of 0.35-m/s at atmosphere pressure, which is comparable to or better than most MEMS-based flow sensors. Optical fiber is not used only for optical signal delivery, but also as a multi-function cable that can deliver optical power for on-fiber self-heating. This one-fiber solution provides a new dimension to designing multifunctional fiber sensors without compromising their intrinsic advantages, which include immunity to electromagnetic fields, low cost, long lifetime, and the capability to function in harsh environments.