We propose a way to enhance the temperature sensitivity of single-mode-multimode-single-mode (SMS) fiber structure,
by replacing the cladding of silica multimode fiber with a polymer coating. Based on the mode expansion method, we
analyze transmission characteristics of the polymer-coated SMS fiber structure and optimize design parameters for
sensing. Then we do experiments and verify theoretical analysis. By using the optimized parameters in sample
fabrication, a temperature sensor is obtained with an easily detected and demodulated transmission spectrum; and then
by monitoring valley wavelength shifts, a measuring sensitivity of about 706 pm/°C is achieved.
A simple temperature sensor based on a bent singlemode–multimode–singlemode (SMS) fiber structure fastened on
a polymer base plate is proposed and experimentally investigated. The surrounding refractive index (RI) is higher than
that of the silica fiber and RI changes with temperature will not lead to wavelength shift. This SMS fiber structure
utilizes changing of temperature to control the curvature of SMS fiber which is induced by expanding of polymer base
plate. The shifts of central wavelength are measured at temperature range from 59 to 82 °C. The proposed temperature
sensor offers sensitivity of 3.9 nm/°C, which is significantly higher than that of a normal straight SMS structure or a
grating-based fiber structure.
KEYWORDS: Refractive index, Sensors, Cladding, Liquids, Signal attenuation, Waveguides, Power meters, Lithium, Optical power meters, Single mode fibers
A novel long-period fiber grating (LPG) refractive index sensor is presented for the measurements of ambient refractive
indices which are higher than that of the fiber cladding. As the measurement parameter, the transmission power of the
core mode is interrogated at a single wavelength where the core mode and a leaky mode are phase-matched. Firstly, the
couplings of the core mode to leaky modes in the novel structure are analyzed by using complex coupled-mode theory,
and then classified into three cases analogous to those in the damped oscillations. The power evolutions of the core mode
in the couplings are thus intuitively understood. Based on these, for the first time, we demonstrate, with optimized design
parameters the transmission power of the core mode is rather sensitive to the change of a higher ambient refractive index
at resonant wavelengths. Then we focus on two optimization objectives. One is to enlarge the operational range while
keeping given sensitivity, the other is to enhance the sensitivity within a given operational range. Finally, we demonstrate
the operational range of the LPG refractive index sensor can be from 1.46 to 1.7 with a sensitivity of ~10-4, while for a
given operational range from 1.455 to 1.465, the sensitivity can be ~10-6, if assuming the dynamic range of the power is
less than 30dB and the measurement resolution is 0.01dB for both cases.
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