Polymer based distributive waveguide sensor for pressure and shear measurement

• US 20050232532A1
• Filed: 02/28/2005
• Published: 10/20/2005
• Est. Priority Date: 03/01/2004
• Status: Active Grant

• Current Assignee: Washington University In St Louis
• Original Assignee: Washington University In St Louis
• Inventors: Wang, Wei-Chih, Reinhall, Per G.
• Granted Patent: US 7,295,724 B2
• Time in Patent Office: Days
• US Class Current: 385/13
• CPC Class Codes: A61B 2562/0266 Optical strain gauges
• CPC Class Codes: A61B 5/1036 Measuring load distribution, e.g. podologic studies
• CPC Class Codes: A61B 5/6892 Mats
• CPC Class Codes: G01D 5/35303 using a reference fibre, e.g. interferometric devices
• CPC Class Codes: G01D 5/35316 using a Bragg gratings
• CPC Class Codes: G01D 5/35354 Sensor working in reflection
• CPC Class Codes: G01L 1/246 using integrated gratings, e.g. Bragg gratings
• CPC Class Codes: G02B 2006/12107 Grating
• CPC Class Codes: G02B 2006/12176 Etching
• CPC Class Codes: G02B 6/12007 forming wavelength selective elements, e.g. multiplexer, demultiplexer
• CPC Class Codes: G02B 6/132 by deposition of thin films

Abstract

According to embodiments of the present invention, a distributed pressure and shear stress sensor includes a flexible substrate, such as PDMS, with a waveguide formed thereon. Along the waveguide path are several Bragg gratings. Each Bragg grating has a characteristic Bragg wavelength that shifts in response to an applied load due to elongation/compression of the grating. The wavelength shifts are monitored using a single input and a single output for the waveguide to determine the amount of applied pressure on the gratings. To measure shear stress, two flexible substrates with the waveguide and Bragg gratings are placed on top of each other such that the waveguides and gratings are perpendicular to each other. To fabricate the distributive pressure and shear sensor, a unique micro-molding technique is used wherein gratings are stamped into PDMS, for example.

Claims

1. An apparatus, comprising:

a pressure sensor having;

a flexible substrate;

at least one waveguide disposed in or on the flexible substrate, the waveguide having an input to receive an optical signal and an output to detect a reflected and/or transmitted optical signal; and

a Bragg grating array having Bragg gratings disposed in series along the length of the waveguide, wherein each Bragg grating comprises a different characteristic grating spacing.

2. The apparatus of claim 1, further comprising at least light source disposed in or on the flexible substrate and coupled to the waveguide input, the light source to transmit the optical signal.

3. The apparatus of claim 2, wherein the light source comprises a broadband light source.

4. The apparatus of claim 2, further comprising a light detector disposed in or on the flexible substrate and coupled to the waveguide output, the light detector to detect light reflected by the Bragg gratings, wherein the wavelength of light reflected by a particular Bragg grating is determined by the grating spacing of the particular Bragg grating.

5. The apparatus of claim 2, further comprising a light detector disposed in or on the flexible substrate and coupled to the waveguide output, the light detector to detect light transmitted by the Bragg gratings, wherein the change in wavelength content of light transmitted by a particular Bragg grating is determined by the grating spacing of the particular Bragg grating.

6. The apparatus of claim 5, wherein at least one Bragg gratings are to change grating spacing in response to an applied pressure.

7. The apparatus of claim 6, wherein the least two Bragg gratings are to reflect a second wavelength different from its characteristic wavelength corresponding to the original grating spacing in response to the change grating spacing.

8. The apparatus of claim 6, wherein the least two Bragg gratings are to transmit a second wavelength content from its characteristic wavelength content corresponding to the original grating spacing in response to the change grating spacing.

9. The apparatus of claim 6, further comprising a wavelength shift detector to light detector disposed in or on the flexible substrate and coupled to the waveguide output, the wavelength shift detector to detect the second wavelength and to determine an amount of shift from the characteristic wavelength to the second wavelength.

10. The apparatus of claim 7, further comprising a wavelength shift detector disposed in or on the flexible substrate and coupled to the waveguide output, the wavelength shift detector to detect the second wavelength content and to determine an amount of shift from the characteristic wavelength content to the second wavelength content.

11. The apparatus of claim 7, further comprising a coupler disposed in or the flexible substrate, the coupler to couple the transmitted optical signal from the light source to the waveguide and the reflected optical signal to the wavelength shift detector from the waveguide.

12. The apparatus of claim 10, further comprising two couplers disposed in or the flexible substrate, wherein a first coupler is to couple the transmitted optical signal from the light source to the waveguide and a second coupler is to couple the reflected optical signal to the wavelength content shift detector from the waveguide.

13. The apparatus of claim 11, further comprising a time domain multiplexer (TDM) coupled to the wavelength shift detector, wherein the TDM is to separate the reflected optical signal of one Bragg grating from the reflected optical signal of another Bragg grating by a time delay.

14. The apparatus of claim 1, wherein the flexible substrate comprises a polydimethylsiloxane (PDMS) elastomer and/or any elastic polymer.

15. The apparatus of claim 1, wherein an individual characteristic grating spacing corresponds to a range of operating wavelengths for a Bragg grating, wherein the range of operating wavelengths for one Bragg grating does not overlap the range of operating wavelengths for a second Bragg grating.

16. An apparatus, comprising:

a shear stress sensor having;

a first sensor having;

a first flexible substrate; and

a first detector array disposed in or on the first flexible substrate along a first series path, wherein the first series path includes a first input and a first output;

a second sensor having;

a second flexible substrate; and

a second detector array disposed in or on the second flexible substrate along a second series path, wherein the second series path includes a second input and a second output,

wherein the first sensor is disposed on the second sensor, and

wherein the first series path is disposed perpendicular to the second series path.

17. The apparatus of claim 16, wherein the first detector array comprises piezoelectric sensors.

18. The apparatus of claim 16, wherein the first detector array comprises capacitive sensors.

19. The apparatus of claim 16, wherein the first detector array comprises Bragg gratings.

20. The apparatus of claim 19, wherein the first and second series paths comprise first and second waveguides, respectively.

21. A method, comprising:

passing a light beam through a waveguide disposed in or on a flexible substrate;

deforming a first Bragg grating and a second Bragg grating disposed in or on the waveguide in response to a load being applied orthogonal to the surface of the flexible substrate, the first Bragg grating having a first wavelength Bragg wavelength, the second Bragg grating having a second Bragg wavelength;

monitoring an output of the waveguide to detect a first shift in the first Bragg wavelength and a second shift in the second Bragg wavelength, the first and second wavelength shifts being in response to deforming the first and second Bragg gratings; and

determining an amount of deformation of the first and second Bragg gratings based on the first and second shifts in the first and second Bragg wavelengths, respectively.

22. The method of claim 21, further comprising:

passing a second light beam through a second waveguide disposed in or on a second flexible substrate, the second flexible substrate disposed on the first flexible substrate, the second waveguide being perpendicular to the first waveguide;

deforming at least two Bragg gratings disposed in or on the second waveguide in response to a shear force being applied along the surface of the first and second flexible substrates, a third Bragg grating having a third Bragg wavelength, a fourth Bragg grating having a fourth Bragg wavelength;

monitoring an output of the waveguide to detect a third shift in the third Bragg wavelength and a fourth shift in the fourth Bragg wavelengths, the third and fourth wavelength shifts being in response to deforming the third and fourth Bragg gratings; and

determining an amount of shear stress based on the first, second, third, and fourth wavelength shifts.

23. A method, comprising:

forming a first waveguide on a first flexible optical medium;

forming a first Bragg grating array in the first waveguide;

forming a second waveguide on a second flexible optical medium;

forming a second Bragg grating array in the second waveguide; and

disposing the first flexible optical medium on the second optical medium.

24. The method of claim 23, further comprising:

constructing two molds of an interference pattern on two silicon substrates;

spinning a first photoresist on the silicon substrates;

writing a Bragg grating pattern on the first photoresist on the two silicon substrates;

developing the first photoresist;

transferring the Bragg grating pattern from the molds to two pieces of flexible optical material, and the Bragg grating pattern having perturbations with spaces therebetween, the two pieces of flexible optical material having a first index of refraction;

spin-coating a second optical material onto the two pieces of flexible optical material to substantially fill the grating spaces, the second optical material having a second index of refraction different from the first index of refraction;

etching away a portion of the second optical material; and

patterning the first and waveguides in the second optical material.

25. The method of claim 24, further comprising:

forming a top support layer and bottom support layer for each of the two pieces of flexible optical material; and

laminating together the top support layer and bottom support layer for each of the two pieces of flexible optical material and the two pieces of flexible optical material.