Multi-parameter Sensing based on Few-mode Fiber Bragg Gratings using Femtosecond IR Laser

Abstract

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.

Claims

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; and a few-mode fiber (FMF) receiving the index modulated pattern.

2. The system of claim 1, comprising an alignment stage to move the few mode fiber. The system of claim 1, comprising

3. The system of claim 1, wherein the phase mask comprises a diffraction grating that divides an incident monochromatic beam into two outgoing beams creating an interference pattern located where the outgoing beams overlap.

4. The system of claim 3, wherein a reflectivity R(l, ) with constant modulation amplitude and period comprises: $R\left(l,\right)=\frac{{}^2.Math.{\sinh }^2\left(\mathrm{sl}\right)}{.Math..Math.k^2.Math.{\sinh }^2\left(\mathrm{sl}\right)+s^2.Math.{\cosh }^2\left(\mathrm{sl}\right)};$ where l is grating length, denotes wavelength, represents coupling coefficient, k is a detuning wave vector as k=k/; while k=2n.sub.o/ as propagation constant s={square root over (.sup.2k.sup.2)}.

5. The system of claim 1, wherein the laser comprises a pulse duration fs-IR laser.

6. The system of claim 1, wherein the fs-IR laser multi-photon absorption forms defects.

7. 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.

8. 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.

9. The system of claim 1, comprising a wavelength-change interrogator extracts the measurand information from the light signals after passing a filter based on intensity measurement.

10. The system of claim 1, wherein the fiber provides multi-point sensing along the sensing fiber.

11. The system of claim 1, wherein the FMF comprises a Bragg grating resonant wavelength that depends on the effective index of refraction of the core and the periodicity of the grating.

12. The system of claim 11, wherein a shift in the Bragg grating center wavelength .sub.B due to strain and temperature changes and T is comprises:
.sub.B=.sub.B[(1p.sub.e)+(.sub.+.sub.n)T+C]; where p.sub.e is effective strain-optic constant, .sub. denotes thermal expansion coefficient for the fiber, .sub.n represents thermo-optic coefficient.

13. The system of claim 1, comprising a circulator coupled to the IR laser and to the FMF.

14. The system of claim 13, comprising an edge filter coupled to the circulator.

15. The system of claim 14, comprising an interrogator coupled to the edge filter.

16. The system of claim 15, wherein the interrogator comprises a detector and a reflection spectrum coupled to the edge filter.

17. The system of claim 13, wherein the FMF is coupled to an optical spectrum analyzer (OSA).

18. The system of claim 1, comprising an FBG sensor coupled to the FMF.

19. The system of claim 18, wherein the FBG sensor generates a transmission spectrum chart.

20. The system of claim 1, wherein the FMF senses multi-parameter measurements.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] FIG. 1 shows an exemplary schematic of FM-FBG Fabrication Method using Holographic/UV Laser Irradiation Source.

[0008] FIG. 2 shows an exemplary schematic of FM-FBG Fabrication Method using Phase Mask/Femtosecond IR Laser Source

[0009] FIG. 3A shows an exemplary wavelength Shift of FM-FBG as a Function of Temperature for Different Modes

[0010] FIG. 3B shows an exemplary wavelength Shift of FM-FBG as a Function of Strain for Different Modes

[0011] FIG. 4 shows an exemplary Flow Chart of FM-FBG sensor using Femtosecond IR Laser Source.

[0012] FIG. 5 shows an exemplary FM-FBG sensor using Femtosecond IR Laser Source.

[0013] FIG. 6 shows an exemplary configuration of FM-FBG sensor using Femtosecond IR Laser Source.

DESCRIPTION

[0014] FIG. 1 shows an exemplary schematic of FM-FBG Fabrication Method using Holographic/UV Laser Irradiation Source. An embodiment uses a harsh-environmental multi-parameter fiber Bragg grating sensors using femtosecond IR laser irradiation, which have exhibited enhanced thermal stability up to about 1000 C. Although the SM-FBG only has one resonant dip in the transmission spectrum, FBGs based on few-mode fibers (FMF) may have more than one resonant dips. By analyzing the changing spectra of the dips, one can distinguish the changes induced by bending, stressing or temperature fluctuations. Different dips have different sensitivities in bending, due to the difference in structural deformation when temperature & strain was applied to the fiber. Temperature fluctuations tended to influence the dips uniformly across different dips for similar reasons. In other words, bending and ambient temperature fluctuations affect the shapes of multiple transmission dips in different ways. Besides, one of major advantages of grating based fiber optic sensors is that they can be multiplexed. As each grating is inscribed at different locations on the sensing fiber with different grating periods, the signals coming from each mode are encoded at different positions in the wavelength domain. Therefore, the physical quantities can be easily and simultaneously measured by the spectral peak shift in the wavelength range, through multi-point sensing along the sensing fiber. The Bragg grating resonant wavelength depends on the effective index of refraction of the core and the periodicity of the grating. The shift in the Bragg grating center wavelength _B due to strain and temperature changes and T is give by:

_B=_B[(1p_e)+(_+_n)T+C];

[0015] Where p_e is the effective strain-optic constant, _ denotes the thermal expansion coefficient for the fiber, while _n represents the thermo-optic coefficient. Last but not least, the proportional constant C stands for the FBG wavelength shift caused by other parameters such as pressure, chemical concentration or PH values, etc.

[0016] FIG. 2 shows the schematic diagram of the fabrication method for FM-FBG using phase mask and femtosecond IR laser source. Phase mask essentially serves as a precision diffraction grating that divides an incident monochromatic beam into two outgoing beams, thus creating an interference pattern located where the outgoing beams overlap. The reflectivity of FM-FBG R(l, ) with constant modulation amplitude and period is given by:

[00001]$R\left(l,\right)=\frac{{}^2.Math.{\sinh }^2\left(\mathrm{sl}\right)}{.Math..Math.k^2.Math.{\sinh }^2\left(\mathrm{sl}\right)+s^2.Math.{\cosh }^2\left(\mathrm{sl}\right)};$

[0017] Where l is the grating length, denotes the wavelength, represents the coupling coefficient, k stands for the detuning wave vector as k=k/; while k=2n.sub.o/ as propagation constant; s={square root over (.sup.2k.sup.2)}.

[0018] FIG. 3A shows measured spectra of FBG in FMF inscribed by IR femtosecond laser at a spectral range from room temperature to 700 C., which depicts the spectra of dip locations as a function of temperature for different modes. Four spectral curves corresponding to four modes are parallel to each other. Similarly, FIG. 3B shows that there exists a linear relationship between mechanical strain and wavelength variation.

[0019] FIG. 4 shows an exemplary Flow Chart of FM-FBG sensor using Femtosecond IR Laser Source. During transmission (TX) femtosecond pulse duration infrared (fs-IR) laser systems to generate ultrahigh peak power radiation (20). Multi-photon absorption/ionization process by the fs-IR laser leading to defect formation (22). The difference in times of arrival of the order pairs due to group velocity walk-off results in a pure two-beam interference pattern by a phase mask (24). Non-sinusoidal modulated FBG structure written on FMF by phase-mask generates sinusoidal interference field (26). During receiving phase (RX), the wavelength-change interrogator extracts the measurand information from the light signals after passing the filter at the detector, based on intensity measurement (28). Multi-parameter sensing with signals from each mode is encoded and multiplexed at different positions in the wavelength domain (30).

[0020] FIG. 5 shows an exemplary FM-FBG sensor using Femtosecond IR Laser Source. The Multi-parameter Fiber Bragg Grating (FBG) Sensors using Femtosecond IR Laser Irradiation system can be used as an intelligent multi-parameter optical fiber sensor system for real-time temperature, pressure, vibration or chemical concentration monitoring in high temperature (above 1000 C.) harsh environments. The system provides Multi-parameter Measurement Schemes for Temperature, Strain, Acceleration, Vibration, Displacement, etc. The system has advantages in: [0021] 1) Electrically Passive Operation and Multiplexibility [0022] 2) Better Sensitivity, Improved Reliability, High Spatial Resolution; [0023] 3) Quasi-distributed sensing capability [0024] 4) Stable Operation Temperature Up to 1000 C. [0025] 5) Immunity to Electromagnetic Interference.

[0026] In one Passive Detection Scheme the system can measure the Wavelength Shift of Light Reflected from an FBG using Linearly Wavelength-Dependent Optical Edge Filters/Interrogators. The Measuring Multiple Parameters can be used with smart Buildings, Cars, Machines, Process Control Systems and Gas Safety Installation, with Inherent Self-Referencing Capability.

[0027] FIG. 6 shows an exemplary configuration of FM-FBG sensor using Femtosecond IR Laser Source. From FIG. 6 above, a wavelength-change interrogator at receiver side extracts the measurand information from the light signals after passing the filter at the detector, based on intensity measurement. There is a tradeoff between the measurable range and the sensitivity. The relatively high cost of the optical signal interrogators for SM-FBG sensor systems present a major limitation in many applications, in spite of the advantages over their electronic counterparts, whilst FM-FBG in a large capacity multiplexing system provide simultaneous measurements of various physical or chemical changes with a large number of modes, thus making its per-sensor-cost even more competitive in harsh environment applications.

[0028] Of course, any number of fibers may be used and the plates may assume any size and shape to obtain acceleration measurements. Still further, other housing shapes, fiber shapes, sizes, and arrangements may be used, as desirable.

[0029] It should also be noted that any suitable digital or analog signal processing technique(s) may be utilized by the processor in any of the embodiments herein to process the outputs of the sensor(s), including a filter function (such as a low pass filter). The signal processing techniques may be undertaken by a digital or analog signal processing circuit. The circuit may be programmable, hard-wired, a microcontroller, an ASIC, an analog filter, etc.

[0030] One or more features of an embodiment disclosed herein may be combined with one or more features of one or more other embodiments. Modifications may be made to any embodiment as should be evident to one of ordinary skill in the art.

[0031] It is to be noted that a sensor according to at least one example of the present presently disclosed subject matter can comprise any desired number of mounting structures (singly, or in faced pairs, or in staggered pairs, for example), with any desired combination or permutation of different configurations of mounting structures being provided for clamping the respective support ring to the respective housing members, for example any combination of the examples of the mounting structures illustrated herein or alternative variations thereof.

[0032] Finally, it should be noted that the word comprising as used throughout the appended claims is to be interpreted to mean including but not limited to.

[0033] While there has been shown and disclosed example examples in accordance with the presently disclosed subject matter, it will be appreciated that many changes can be made therein without departing from the spirit of the presently disclosed subject matter.