Birefringent micro-structured optical fiber for sensor application

Patent metadata

Field	Value
Publication number	US-8958676-B2
Application number	US-201013510689-A
Country	US
Kind code	B2
Filing date	Nov 19, 2010
Priority date	Nov 19, 2009
Publication date	Feb 17, 2015
Grant date	Feb 17, 2015

Abstract

Fiber structure including a core and a cladding, a central microstructure having a first plurality of longitudinal holes and which is adapted for guiding optical radiation and for birefringence in the core. Also included is a side microstructure having a second plurality of longitudinal holes is provided, wherein the side microstructure partly surrounds the central microstructure and provides a predetermined mechanical anisotropy, a pressure responsive unit for converting an isotropic pressure force to birefringence changes on the core, a lateral force responsive unit for converting a directional pressure force to birefringence changes on the core, a temperature responsive unit for converting temperature to birefringence changes on the core, and a birefringence responsive unit for converting birefringence in the core to wavelength information.

First claim

The invention claimed is:

1. A fiber structure comprising:
a core and a cladding surrounding the core;
a central microstructure comprising a first plurality of longitudinal holes, wherein the central microstructure is configured for guiding optical radiation and providing an initial level of birefringence in the core via geometrical birefringence;
a side holey structure comprising a second plurality of longitudinal holes, wherein the side holey structure is at least partly surrounding the central microstructure and is configured for providing a predetermined anisotropic response to pressure;
a doped region in the core; and
a birefringence responsive unit comprising at least one fibre grating arranged at least partly in the core, and being configured for converting the birefringence in the core to wavelength information,
wherein the first and/or the second plurality of longitudinal holes comprise(s) a circular cross section, an elliptical cross section, a ring-like cross section or a hexagonal cross section and is/are configured for obtaining at least one of a predetermined birefringence, a predetermined pressure sensitivity, a predetermined force sensitivity, a predetermined temperature sensitivity, a predetermined ratio of pressure sensitivity and temperature sensitivity, and a predetermined ratio of force sensitivity and temperature sensitivity.

2. The fiber structure according to claim 1 , wherein said side holey structure is configured for providing a predetermined anisotropic response to isotropic pressure, or a transverse load that is perpendicular to a fiber axis.

3. The fiber structure according to claim 1 , wherein said birefringence responsive unit is configured for converting the birefringence in the core into Bragg peak separation information.

4. The fiber structure according to claim 1 , wherein said wavelength information comprises a distance in wavelength Δλ between two reflection peaks of said at least one fiber grating written in such a fiber structure.

5. The fiber structure according to claim 1 , further comprising a plurality of gratings arranged at least partially in the core.

6. The fiber structure according to claim 1 , wherein the side holey structure is a side microstructure.

7. A fiber structure according to claim 1 , wherein the side holey structure is a macrostructure.

8. A fiber structure of claim 1 , wherein a central region bordered by at least some of the holes of the central structure defines a transmission pathway for 99% or more of the optical radiation transferred along the optical fiber.

9. The fiber structure according claim 1 , wherein the central micro structure comprises at least one row of air holes, preferably three rows of air holes arranged next to one another, wherein the diameter of the middle row is smaller than the diameter of the two outer rows.

10. The fiber structure according to claim 9 , wherein the central micro structure comprises three rows of air holes and wherein the middle row comprises a plurality of air-holes with a smaller diameter adapted for making the fiber to behave as a single mode fiber by making higher order modes much more lossy than lower order modes, and wherein the two outer rows which are neighboring the middle row comprise larger holes allowing a high confinement of light into the fiber core.

11. The fiber structure according to claim 1 , wherein the birefringence responsive unit is a fiber Bragg grating.

12. The fiber structure according to claim 1 , wherein the first plurality of longitudinal holes is arranged such that their longitudinal axes are parallel relative to the longitudinal axis of the core.

13. The fiber structure according claim 1 , wherein at least a portion out of the first and/or second plurality of longitudinal holes comprises same sizes and/or same shapes.

14. The fiber structure according to claim 7 , wherein the macrostructure comprises at least two side-holes arranged opposite to each other and is configured for determining the phase modal birefringence of the fiber structure.

15. The fiber structure according to claim 14 , wherein the shape of the two side-holes is configured for reducing confinement losses and is chosen from a shape comprising an elliptical, a rectangular and/or a circular shape.

16. The fiber structure according to claim 6 , wherein the cladding comprises an inner cladding and an outer cladding, the inner cladding providing circular features and the outer cladding providing features different from circular features.

17. The fiber structure according to claim 1 having a central microstructure that determines the direction of the slow axis of the optical fiber with respect to the side holey structure with a sensitivity of the Bragg peak wavelengths that correspond to the slow and fast axes for an fiber Bragg grating in that fiber, the fiber structure having a positive or negative sign of the differential Bragg peak sensitivity.

18. A method of use of a fiber structure according to claim 1 with fiber Bragg gratings in at least one of the following sensor applications:
structural health monitoring of composite material structures, such as in aircraft structures; and
monitoring of pressure and strain in the presence of ionizing radiation.

19. A method of producing an optical fiber further comprising the steps of
fabricating a core comprising a doped region and a cladding surrounding the core, a central microstructure comprising a first plurality of longitudinal holes, wherein the central microstructure is adapted for guiding optical radiation and providing an initial level of birefringence in the core via geometrical birefringence, a side holey structure comprising a second plurality of longitudinal holes, wherein the side holey structure is at least partly surrounding the central microstructure and is adapted for providing a predetermined anisotropic response to pressure and a birefringence responsive unit comprising at least one fiber grating arranged at least partly in the core,
wherein fabricating the central microstructure comprises adapting the central microstructure and the core to determine the direction of the slow axis of the optical fiber with respect to the holey side structure, with the result of at least one of tailoring the sensitivity of the Bragg peak wavelengths that correspond to the slow and fast axes for said fiber grating in that fiber and determining the sign of the differential Bragg peak sensitivity to pressure, and wherein fabricating the central microstructure comprises doping the core according to a predetermined doping level in order to control the differential Bragg peak sensitivity to temperature, and
wherein the first and/or the second plurality of longitudinal holes comprise(s) a circular cross section, an elliptical cross section, a ring-like cross section or a hexagonal cross section and is/are configured for obtaining at least one of a predetermined birefringence, a predetermined pressure sensitivity, a predetermined force sensitivity, a predetermined temperature sensitivity, a predetermined ratio of pressure sensitivity and temperature sensitivity, at a predetermined ratio of force sensitivity and temperature sensitivity.

20. The fiber structure according to claim 1 , wherein said second plurality of longitudinal holes in the side holey structure comprises more than two longitudinal holes.

21. The fiber structure according to claim 1 , wherein said doped region in the core has a doping level configured to provide a predetermined differential tempera

Assignees

• Geernaert Thomas
• Berghmans Francis
• Nasilowski Tomasz
• Thienpont Hugo
• Makara Mariusz
• Mergo Pawel
• Poturaj Krzysztof Grzegorz
• Urbanczyk Waclaw
• Martynkien Tadeusz
• Olszewski Jacek Marek
• Wojcik Jan Jozef
• Univ M Curie Sklodowskiej
• Wroclaw University Of Technology
• Univ Bruxelles

Inventors

• Geernaert Thomas
• Berghmans Francis
• Nasilowski Tomasz
• Thienpont Hugo
• Makara Mariusz
• Mergo Pawel
• Poturaj Krzysztof Grzegorz
• Urbanczyk Waclaw
• Martynkien Tadeusz
• Olszewski Jacek Marek
• Wojcik Jan Jozef

Classifications

• G02B6/105

  having optical polarisation effects · CPC title

• G02B6/02338 Primary

  Structured core, e.g. core contains more than one material, non-constant refractive index distribution in core, asymmetric or non-circular elements in core unit, multiple cores, insertions between core and clad · CPC title

• G02B6/02371

  Cross section of longitudinal structures is non-circular · CPC title

• G01D5/3538

  using a particular type of fiber, e.g. fibre with several cores, PANDA fiber, fiber with an elliptic core or the like · CPC title

• G02B6/02109

  having polarization sensitive features, e.g. reduced photo-induced birefringence · CPC title