Fiber laser pressure sensor

• US 6,630,658 B1
• Filed: 11/20/2000
• Issued: 10/07/2003
• Est. Priority Date: 02/25/1998
• Status: Expired due to Term

• Current Assignee: GE Oil & Gas (General Electric Company)
• Original Assignee: ABB Research Ltd. (ABB Ltd.)
• Inventors: Bohnert, Klaus, Brandle, Hubert
• Primary Examiner(s): Bruce, David V.
• Assistant Examiner(s): THOMAS, COURTNEY D
• Application Number:
  US09/623,048
• Time in Patent Office: 1,051 Days
• Field of Search: 250/227.14, 250/227.11, 250/227.16, 250/231.1, 250/227.17, 250/227.18, 250/227.19, 340/555, 340/556, 340/557, 340/626, 356/477
• US Class Current: 250/227.14
• CPC Class Codes: G01D 5/35316 using a Bragg gratings
• CPC Class Codes: G01D 5/3538 using a particular type of fiber, e.g. fibre with several cores, PANDA fiber, fiber with an elliptic core or the like
• CPC Class Codes: G01L 11/025 using a pressure-sensitive optical fibre
• CPC Class Codes: G02B 6/02085 characterised by the grating profile, e.g. chirped, apodised, tilted, helical
• CPC Class Codes: G02B 6/022 using mechanical stress, e.g. tuning by compression or elongation, special geometrical shapes such as "dog-bone" or taper G02B6/02204 takes precedence; optical modulation using photo-elastic effect G02F1/0131
• CPC Class Codes: G02B 6/105 having optical polarisation effects

Abstract

The invention relates to a frequency-coded fiber laser pressure sensor (1) which is especially suitable for measuring isotropic pressures in oil wells. The sensor principle provided for in the invention is based on the fact that in a fiber laser (2) doped with Er³⁺ a monomode or bimodal sensor fiber (5, 5a, 5b) is positioned whose pressure-related birefraction results in a frequency shift and beat frequencies between the orthogonal linear polarisation modes x, y or the spatial modes LP₀₁ and LP₁₁^straight line. The beat frequencies are easily measured using a frequency counter (19). Temperature-related variations in birefraction are compensated in a differential arrangement of two sensor fiber segments (5a, 5b). Fiber-integrated Bragg gratings (4a, 4b) with low bandwidths (0.2 nm) are especially suitable as laser end reflectors. The pressure sensor (1) is characterized by a large measurement range (100 Mpa) and a high triggering capacity , (kPa), is readily multiplexed and can be housed in a very compact, light-weight and robust pressure-resistant housing (25).

Claims

1. A fiber laser pressure sensor comprising in operative communication:

a pumped light source;

feed fibers;

a fiber laser which has at least two end reflectors, a laser-amplifying fiber, and in addition a sensor fiber, wherein the laser-amplifying fiber and the sensor fiber are disposed between the two end reflectors;

a detection unit; and

an electronic evaluation system;

wherein the sensor fiber has a non-rotationally symmetrical structure, in which sensor fiber all-round pressure thereby induces birefringence and a pressure-proportional beat frequency between different polarization modes or spatial modes.

2. The fiber laser pressure sensor as claimed in claim 1, wherein the sensor fiber

(a) has an elliptical core, a bow-tie structure, a panda structure, a side-hole structure, a D-shape structure, an elliptical cladding or an partially buffed cladding; and

(b) is subjected directly as a whole or partially to a pressure to be measured.

3. The fiber laser pressure sensor as claimed in claim 2, wherein

(a) the sensor fiber carries exactly two polarization modes or exactly two spatial modes,

(b) the sensor fiber includes exactly two identical fiber segments which are twisted relative to one another by 90°

(±

30°

) or 0°

(±

30°

), and

(c) in particular, an intermediate fiber is arranged between the fiber segments.

4. The fiber laser pressure sensor as claimed in claim 1, wherein

(a) the end reflectors are fiber Bragg gratings, and

(b) a Bragg wavelength λ

_B is a measure of a temperature.

5. The fiber laser pressure sensor as claimed in claim 1, wherein

(a) the fiber Bragg gratings are written directly into the laser-amplifying fiber and/or into the sensor fiber, and

(b) in particular, one of the fiber Bragg gratings is selected with a larger bandwidth.

6. The fiber laser pressure sensor as claimed in claim 5, wherein one bandwidth Δ

ν

_B⁽¹⁾=0.6 nanometers and another bandwidth Δ

ν

_B⁽²⁾=0.2 nanometers.

7. The fiber laser pressure sensor as claimed in claim 1, wherein

(a) a fiber polarizer is arranged in or near the fiber laser, and

(b) in particular, the Bragg wavelength λ

_B is selected in a spectral region of a vanishing group refractive index birefringence of the sensor fiber.

8. The fiber laser pressure sensor as claimed in claim 1, wherein

(a) the pumped light source, the fiber laser, and the detection unit are optically connected by means of a fiber coupler,

(b) the detection unit comprises an analyzer, a detector, and in particular, a polarization controller, and

(c) the electronic evaluation system has a frequency filter and a frequency counter.

9. The fiber laser pressure sensor as claimed in claim 8, wherein the fiber coupler is frequency selective.

10. The fiber laser pressure sensor as claimed in claim 1, wherein

(a) the pumped light source, the fiber laser, and the detection unit are optically connected by means of a fiber coupler,

(b) the detection unit has a polarization maintaining fiber coupler with two analyzers oriented at 0°

and 45° and

two detectors, and

(c) the electronic evaluation system has a frequency filter, a frequency counter, and an adder.

11. The fiber laser pressure sensor as claimed in claim 1, wherein

(a) a plurality of fiber lasers of different emission wavelengths are optically connected to a pumped light source and a detection unit,

(b) the detection unit has a wavelength division multiplier and a multichannel detector, and

(c) the electronic evaluation system is a multichannel electronic evaluation system.

12. The fiber laser pressure sensor as claimed in claim 1, wherein

(a) the fiber laser is mounted in a pressure-resistant housing with a plurality of pressure chambers and pressure-tight fiber feedthroughs, and

(b) a first pressure chamber which contains a measuring fiber segment of the sensor fiber is in direct pressure exchange with a surrounding medium.

13. The fiber laser pressure sensor as claimed in claim 12, wherein

(a) a fiber Bragg grating and the laser amplifying fiber are mounted in a second pressure chamber, and

(b) a fiber Bragg grating and a fiber segment of the sensor are mounted in a third pressure chamber, and in particular, the second and third pressure chambers are inner capillaries with pressure-tight fiber feedthroughs.

14. A fiber laser pressure sensor as in claim 1, wherein the sensor fiber is between the laser-amplifying fiber and one of the reflectors.

15. A fiber laser sensor for measuring a variable comprising in operative communication:

a pumped light source;

feed fibers;

a fiber laser which has at least two end reflectors, a laser-amplifying fiber, and in addition a sensor fiber, wherein the laser-amplifying fiber and the sensor fiber are disposed between the two end reflectors;

a detection unit;

an electronic evaluation system;

wherein the sensor fiber carries exactly two spatial modes, and

wherein the measured variable acts on the sensor fiber or on a segment of the sensor fiber.

16. A fiber laser sensor comprising in operative communication:

a pumped light source;

feed fibers;

a fiber laser which has at least two end reflectors, a laser-amplifying fiber, and a sensor fiber, wherein the laser-amplifying fiber and the sensor fiber are disposed between the two end reflectors;

a detection unit;

an electronic evaluation system;

wherein the sensor fiber carries exactly two spatial modes, and in which sensor fiber all-round pressure thereby induces birefringence and a pressure-proportional beat frequency between the two spatial modes.

17. A fiber laser pressure sensor comprising in operative communication:

a pumped light source;

feed fibers;

a fiber laser which has at least two end reflectors, a laser-amplifying fiber, and in addition a sensor fiber, wherein the laser-amplifying fiber and the sensor fiber are disposed between the two end reflectors;

a detection unit;

an electronic evaluation system;

wherein the end reflectors are fiber Bragg gratings,

the fiber sensor has a non-rotationally symmetrical structure, in which fiber sensor all-round pressure thereby induces birefringence and a pressure-proportional beat frequency between different polarization modes or spatial modes,

the fiber laser is mounted in a pressure-resistant housing with a plurality of pressure chambers and pressure-tight fiber feedthroughs,

a first pressure chamber, which contains a measuring fiber segment of the sensor fiber, is in direct pressure exchange with a surrounding medium, and

one of the fiber Bragg gratings and the laser amplifying fiber are mounted in a second pressure chamber.