# US 2019/0170609 A1 · Birefringent multi-peak optical reference element and birefringent sensor system

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- Number: US 2019/0170609 A1
- Document ID: US20190170609A1
- Status: Published application
- Kind: A1
- Application number: 16/268,029
- Filed: 2019-02-05
- Published: 2019-06-06
- Earliest claimed: 2015-01-20
- Assignees: Weatherford Technology Holdings, LLC
- Applicants: Weatherford Technology Holdings, LLC
- Inventors: Domino TAVERNER
- Web page: https://patentgrep.com/patent/US20190170609A1

## CPC

- G01M 11/333: {using modulated input signals}
- G01D 5/35303: {using a reference fibre, e.g. interferometric devices}
- G01D 5/35316: {using a Bragg gratings}
- G02B 6/02109: {having polarization sensitive features, e.g. reduced photo-induced birefringence}
- G02B 6/02371: {Cross section of longitudinal structures is non-circular}
- G02B 6/03694: {Multiple layers differing in properties other than the refractive index, e.g. attenuation, diffusion, stress properties}
- G01M: TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
- G01D: MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
- G02B: OPTICAL ELEMENTS, SYSTEMS OR APPARATUS

## Abstract

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.

## Claims

### Claim 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; and
adjacent wavelength functions in each pair are due to two orthogonal birefringence axes in the reference optical sensor.

### Claim 2 (depends on 1)

The reference optical sensor of claim 1, wherein the wavelength peaks are uniformly spaced over at least a portion of the range of wavelengths at a particular temperature and pressure.

### Claim 3 (depends on 1)

The reference optical sensor of claim 1, wherein edges of the adjacent wavelength functions do not significantly overlap.

### Claim 4 (depends on 1)

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.

### Claim 5 (depends on 1)

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.

### Claim 6 (depends on 5)

The reference optical sensor of claim 5, wherein an outer diameter of the cladding is at least 0.3 mm.

### Claim 7 (depends on 5)

The reference optical sensor of claim 5, wherein the stress feature has an elliptical shape in cross-section.

### Claim 8 (depends on 5)

The reference optical sensor of claim 5, wherein the stress feature comprises two holes in the cladding and wherein the two holes are parallel to the core.

### Claim 9 (depends on 5)

The reference optical sensor of claim 5, 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.

### Claim 10 (depends on 5)

The reference optical sensor of claim 5, wherein the core comprises a super-structured Bragg grating.

### Claim 11 (depends on 5)

The reference optical sensor of claim 5, wherein edges of the adjacent wavelength functions do not significantly overlap.

### Claim 12 (depends on 5)

The reference optical sensor of claim 5, wherein the adjacent wavelength functions are distinguishable over a range of environmental operating conditions for the reference optical sensor.

### Claim 13 (depends on 5)

The reference optical sensor of claim 5, 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.

### Claim 14 (depends on 1)

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.

### Claim 15 (depends on 1)

The reference optical sensor of claim 1, wherein the core comprises a super-structured Bragg grating.

### Claim 16 (depends on 1)

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.