Birefringent multi-peak optical reference element and birefringent sensor system
Certain aspects of the present disclosure generally relate to an optical reference element having a wavelength spectrum comprising a plurality of wavelength functions having wavelength peaks spaced over a range of wavelengths, wherein adjacent wavelength functions are due to two orthogonal birefringence axes in the optical reference element. Aspects of the present disclosure may eliminate the drift issues associated with residual polarization and polarization dependent loss (PDL) with respect to grating-based sensor and reference element measurements.
1. A reference optical sensor comprising a core with a modulated refractive index, wherein:
in response to light introduced into the reference optical sensor, the modulated refractive index is configured to produce a wavelength spectrum comprising multiple pairs of wavelength functions having wavelength peaks spaced over a range of wavelengths;
adjacent wavelength functions in each pair are due to two orthogonal birefringence axes resulting from anisotropic stress on the core of the reference optical sensor, wherein the anisotropic stress on the core is configured to produce distinct wavelength peaks attributed to each of the two orthogonal birefringence axes;
the wavelength peaks from the multiple pairs of wavelength functions are uniformly spaced over at least a portion of the range of wavelengths at a particular temperature and pressure; and
the refractive index of the core has a periodic modulation corresponding to the uniform spacing of the wavelength peaks from the multiple pairs of wavelength functions.
2. The reference optical sensor of claim 1 , wherein edges of the adjacent wavelength functions do not significantly overlap.
3. The reference optical sensor of claim 1 , wherein the adjacent wavelength functions are distinguishable over a range of environmental operating conditions for the reference optical sensor.
4. The reference optical sensor of claim 1 , further comprising:
a stress feature configured to generate the two orthogonal birefringence axes for light propagated in the core; and
a cladding surrounding the core and the stress feature.
5. The reference optical sensor of claim 4 , wherein an outer diameter of the cladding is at least 0.3 mm.
6. The reference optical sensor of claim 4 , wherein the stress feature has an elliptical shape in cross-section.
7. The reference optical sensor of claim 4 , wherein the stress feature comprises two holes in the cladding and wherein the two holes are parallel to the core.
8. The reference optical sensor of claim 4 , wherein the stress feature comprises two glass rods disposed in the cladding and parallel to the core and wherein the two glass rods have a thermal expansion coefficient different from a thermal expansion coefficient of the cladding.
9. The reference optical sensor of claim 4 , wherein the core comprises a super-structured Bragg grating.
10. The reference optical sensor of claim 4 , wherein edges of the adjacent wavelength functions do not significantly overlap.
11. The reference optical sensor of claim 4 , wherein the adjacent wavelength functions are distinguishable over a range of environmental operating conditions for the reference optical sensor.
12. The reference optical sensor of claim 4 , wherein one or more of the wavelength peaks are characterized for absolute wavelength over a range of environmental operating conditions for the reference optical sensor.
13. The reference optical sensor of claim 1 , wherein one or more of the wavelength peaks are characterized for absolute wavelength over a range of environmental operating conditions for the reference optical sensor.
14. The reference optical sensor of claim 1 , wherein the core comprises a super-structured Bragg grating.
15. The reference optical sensor of claim 1 , further comprising a cladding surrounding the core, wherein an outer diameter of the cladding is at least 0.3 mm.
16. A reference optical sensor, wherein:
in response to light introduced into one core of the reference optical sensor, a modulated refractive index of the core is configured to produce a wavelength spectrum comprising multiple pairs of wavelength functions having wavelength peaks spaced over a range of wavelengths;
adjacent wavelength functions in each pair are due to two orthogonal birefringence axes in the reference optical sensor;
the wavelength peaks from the multiple pairs of wavelength functions are uniformly spaced over at least a portion of the range of wavelengths at a particular temperature and pressure; and
the refractive index of the core has a periodic modulation corresponding to the uniform spacing of the wavelength peaks from the multiple pairs of wavelength functions.
17. The reference optical sensor of claim 16 , wherein the two orthogonal birefringence axes result from anisotropic stress on the core of the reference optical sensor and wherein the anisotropic stress on the core is configured to produce distinct wavelength peaks attributed to each of the two orthogonal birefringence axes.
18. The reference optical sensor of claim 16 , further comprising:
a stress feature configured to generate the two orthogonal birefringence axes for light propagated in the core; and
a cladding surrounding the core and the stress feature.
19. The reference optical sensor of claim 18 , wherein the stress feature has an elliptical shape in cross-section.
20. The reference optical sensor of claim 18 , wherein the stress feature comprises two holes in the cladding and wherein the two holes are parallel to the core.
21. The reference optical sensor of claim 1 , wherein a position of each of the wavelength peaks over the range of wavelengths is independent of a polarization state of the light introduced into the reference optical sensor.
22. The reference optical sensor of claim 1 , wherein at least four wavelength peaks are uniformly spaced over the at least the portion of the range of wavelengths, the at least four wavelength peaks being from at least two different pairs of wavelength functions.