Multi-parameter sensing based on few-mode fiber Bragg gratings using femtosecond IR laser
A sensor system includes a femtosecond infrared (fs-IR) laser to generate a laser beam; a reflecting mirror optically receiving the laser beam; a lens optically coupled to the reflecting mirror to focus the laser beam; a phase mask receiving the laser beam from the lens to generate an index modulated pattern; and a few-mode fiber (FMF) receiving the index modulated pattern.
1. A system, comprising
a femtosecond infrared (fs-IR) laser to generate a laser beam;
a reflecting mirror optically receiving the laser beam;
a lens optically coupled to the reflecting mirror to focus the laser beam;
a phase mask receiving the laser beam from the lens to generate an index modulated pattern from a diffraction grating that divides an incident monochromatic beam into two outgoing beams for creating an interference pattern located where outgoing beams overlap and wherein a shift in a grating center wavelength is determined from strain, temperature, a thermal expansion coefficient for the fiber and a thermo-optic coefficient; and
a few-mode fiber (FMF) such that the FMF receives a plurality of index modulated patterns to form a corresponding plurality of sets of Fiber Bragg Gratings, each set being positioned with a grating period corresponding to resonant dips of a mode of a signal within the FMF such that the sets of Fiber Bragg Gratings encode spectral peak shifts for each of the modes.
2. The system of claim 1 , comprising an alignment stage to move the few mode fiber.
3. The system of claim 1 , wherein the each of the plurality of Fiber Bragg Gratings has a reflectivity R (l, λ) with constant modulation amplitude and period of:
R
(
l
,
λ
)
=
Ω
2
sinh
2
(
sl
)
Δ
k
2
sinh
2
(
sl
)
+
s
2
cosh
2
(
sl
)
;
where l is grating length, λ denotes wavelength, Ω represents coupling coefficient, Δk is a detuning wave vector as Δk=k−π/λ; while k=2πn 0 /λ as propagation constant s=√{square root over (Ω 2 −Δk 2 )}.
4. The system of claim 1 , comprising Linearly Wavelength-Dependent Optical Edge Filters and Interrogators to measure reflected Wavelength Shift of Light.
5. The system of claim 1 , wherein a difference in times of arrival of order pairs due to group velocity walk-off results in a two-beam interference pattern by the phase mask.
6. The system of claim 1 , wherein the phase mask forms a non-sinusoidal modulated Fiber Bragg grating (FBG) structure on the FMF to generate a sinusoidal interference field.