EP1272816A1 - Capteur a laser a fibre - Google Patents

Capteur a laser a fibre

Info

Publication number
EP1272816A1
EP1272816A1 EP01914952A EP01914952A EP1272816A1 EP 1272816 A1 EP1272816 A1 EP 1272816A1 EP 01914952 A EP01914952 A EP 01914952A EP 01914952 A EP01914952 A EP 01914952A EP 1272816 A1 EP1272816 A1 EP 1272816A1
Authority
EP
European Patent Office
Prior art keywords
bragg grating
temperature
fiber
chirped
sensor according
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP01914952A
Other languages
German (de)
English (en)
Inventor
Etienne Rochat
Karim Haroud
Rene DÄNDLIKER
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Baker Hughes International Treasury Services Ltd
Original Assignee
ABB Research Ltd Switzerland
ABB Research Ltd Sweden
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by ABB Research Ltd Switzerland, ABB Research Ltd Sweden filed Critical ABB Research Ltd Switzerland
Publication of EP1272816A1 publication Critical patent/EP1272816A1/fr
Withdrawn legal-status Critical Current

Links

Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
    • G01L1/00—Measuring force or stress, in general
    • G01L1/24—Measuring force or stress, in general by measuring variations of optical properties of material when it is stressed, e.g. by photoelastic stress analysis using infrared, visible light, ultraviolet
    • G01L1/242—Measuring force or stress, in general by measuring variations of optical properties of material when it is stressed, e.g. by photoelastic stress analysis using infrared, visible light, ultraviolet the material being an optical fibre
    • G01L1/246—Measuring force or stress, in general by measuring variations of optical properties of material when it is stressed, e.g. by photoelastic stress analysis using infrared, visible light, ultraviolet the material being an optical fibre using integrated gratings, e.g. Bragg gratings
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • 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
    • G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
    • G01D5/26—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light
    • G01D5/32—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light
    • G01D5/34—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells
    • G01D5/353—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre
    • G01D5/35306—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre using an interferometer arrangement
    • G01D5/35309—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre using an interferometer arrangement using multiple waves interferometer
    • G01D5/35316—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre using an interferometer arrangement using multiple waves interferometer using a Bragg gratings

Definitions

  • the invention relates to the field of optical sensors. It relates to a fiber laser sensor according to the preamble of claim 1.
  • changes in the refractive index of the fiber due to external influences e.g. Changes in pressure and temperature, in a change in the laser wavelength or in a change in the beat frequency, are implemented between two longitudinal laser modes.
  • Such a fiber laser sensor is known, for example, from GA Ball et al., "Polarimetric heterodyne Bragg-grating fiber-laser sensor", Optic Letters 18 (22), 1993, pp. 1976-1978.
  • the sensor has a fiber laser with two Bragg gratings and a doped birefringent fiber segment arranged between them, which acts as a laser medium.
  • the Bragg gratings are written directly in the fiber core of an optical fiber and form so-called fiber Bragg Grid.
  • Pump light which is guided by means of a feed fiber through one of the Bragg gratings to the doped fiber segment, excites two orthogonally polarized self-polarization modes in the fiber segment.
  • the two natural polarization modes are brought to interference in the emission light of the laser, whereby a common beat frequency is obtained.
  • Any external disturbance that changes the length of the laser cavity or the birefringence leads to a change in this beat frequency.
  • the beat frequency obtained and its changes can be measured using a frequency counter, so that conclusions can be drawn about the magnitude of the external influence, for example an external pressure or a change in temperature.
  • this fiber laser sensor cannot differentiate between individual external influences. In particular, temperature and pressure effects are cumulated in the sensor signal.
  • WO 99/44023 discloses a fiber laser pressure sensor in which, in addition to a fiber segment acting as a laser medium, two sensor fiber segments with a non-rotationally symmetrical structure are present in the laser cavity. If an external pressure is exerted on one of these sensor fiber segments, a pressure-proportional beat frequency is in turn induced between different polarization modes. In order to compensate for temperature effects, it is proposed to expose both sensor fiber segments to the temperature, but only one sensor fiber segment to the external pressure.
  • This fiber laser sensor can also be used for pure temperature measurement by determining a shift in the Bragg wavelength of the fiber Bragg grating and thus the wavelength of the emission light by means of an optical wavelength meter. The use of such an additional device increases the overall cost of the sensor.
  • WO 94/17366 describes a fiber optic sensor with several fiber lasers connected in series. For separate measurement of the temperature, it is proposed to use two fiber lasers with different wavelengths and to subject both to the same temperature and pressure changes. Both fiber lasers experience the same pressure-induced, but different temperature-induced wavelength changes. The temperature change can be determined by subtracting the two signals. It is also proposed in this publication to design the two Bragg gratings of the individual fiber lasers differently, so that each fiber laser has a narrowband and a broadband Bragg grating. By using Bragg gratings of different widths, only the narrow-band Bragg grating determines the Bragg wavelength. This is to prevent disruptions in the cavity and thus so-called mode hopping, caused by different stretching of the two Bragg gratings.
  • the fiber laser sensor according to the invention has a fiber laser with two Bragg gratings, a first Bragg grating being arranged in a temperature-sensitive manner and the second Bragg grating being a chirped-Bragg grating whose reflection zone is temperature-stabilized for its central wavelength.
  • This arrangement ensures that emission light of the laser brought to interference has two beat frequencies. One of them is proportional to a temperature change induced in the first Bragg grating, the other to a pressure change induced in the laser. In particular, the temperature-dependent change is linear. This means that temperature and pressure can be measured simultaneously, but independently of one another.
  • the Chirped-Bragg grating itself is temperature-stabilized, for example by means of a cooling or heating element.
  • the second Bragg grating is preferably a conventional narrow-band Bragg grating with an at least approximately constant grating period.
  • Figure 1 is a schematic representation of a fiber optic sensor according to the invention and Figure 2 shows a reflectivity of a Bragg grating and one
  • FIG. 1 shows a preferred embodiment of a fiber laser sensor according to the invention.
  • the sensor essentially has a pump light source 1, an optical fiber 2 with a fiber laser FL, a detection unit 8 and evaluation electronics 10.
  • the light sources in particular pump lasers, usually used for fiber laser sensors can be used as pump light source 1.
  • the pump light source 1 is in optical connection with the optical fiber 2 and the fiber laser FL via a feed fiber 2a and a fiber coupler 6.
  • a return line fiber 2b, which leads to the detection unit 8, is also optically connected to the fiber coupler 6.
  • the detection unit 8 preferably has a polarization control 80, an analyzer 81 and a detection means, for example a photodiode 82.
  • the detection unit 8, more precisely the photodiode 82, is connected via a signal line 9 to the evaluation electronics 10, which in turn has a frequency filter 11 and a frequency counter 12.
  • An optical isolator 7 is preferably arranged between the fiber coupler 6 and the detection unit 8.
  • Part of the optical fiber 2 is formed by the fiber laser FL.
  • This consists of two end reflectors in the form of a first and a second Bragg grating 4, 5 and a fiber segment 3 arranged between them.
  • the fiber segment 3 is formed in one or more pieces, at least part of the segment acting as a laser medium.
  • the fiber segment 3 are the according the prior art fibers used in fiber laser sensors.
  • the fiber segment should thus carry several, preferably two polarization or spatial modes, between which an external pressure change induces a differential phase shift.
  • Birefringent, polarization-maintaining fibers are suitable, for example an Nd- or erbium-doped fiber.
  • the birefringence can be obtained in various ways, by appropriate refractive indices in the fiber or by appropriate shaping of the fiber.
  • the Bragg gratings 4, 5 are fiber Bragg gratings.
  • the first Bragg grating 4 is a conventional, narrow-band Bragg grating with an at least approximately constant grating period. Typical values of the first Bragg grating 4 are 0.1 nm bandwidth and a reflectivity of at least approximately 99%.
  • the second Bragg grating 5 is a chirped Bragg grating, that is to say a Bragg grating with a monotonically changing grating period and / or a changing refractive index of the fiber core. Chirped-Bragg gratings are known in the prior art, and they are used above all in communication technology.
  • Fiber Bragg gratings are given, for example, by A. Othonos, "Fiber Bragg Grätings", Rev. Sei. Instrum .. 68 (12) Dec. 1997, pp. 4309-4340.
  • Typical values of the second Bragg grating 5 are 1-5 nm bandwidth and a reflectivity of at least approximately 95%.
  • Chirped-Bragg gratings are characterized in particular by the fact that they reflect a wider range of wavelengths, ie they are broadband than the conventional Bragg gratings. The individual wavelengths reflect spatially separated reflection zones within the chirped-bragg grating.
  • the chirped-bragg grating exhibits a linear behavior of its group delay over a relatively large spectrum.
  • the reflectivity of the chirped-bragg grating is shown in FIG 5 and the conventional Bragg grating 4 as a function of the wavelength ⁇ , where R CBG denotes the reflectivity of the chirped Bragg grating 5, R BG1 the reflect CTI vity of the conventional Bragg grating 4 at a first temperature, R BG2 at a second temperature and R BG3 at a third temperature.
  • the first Bragg grating 4 is arranged in a temperature-sensitive manner, that is to say it is exposed to external temperature influences without compensation.
  • the position of the central Bragg wavelength is temperature-stabilized.
  • the entire Chirped-Bragg grating 5 is preferably arranged in a temperature-stabilized manner, for example it is operatively connected to a cooling or heating element 50, in particular a Peltier element.
  • the chirped Bragg grating 5 is preferably arranged at the emission end of the fiber laser FL, so that laser light is coupled out via the chirped Bragg grating 5.
  • light from the pump light source 1 is also coupled into the fiber segment 3 via the same chirped Bragg grating 5.
  • the fiber laser sensor functions like the sensors known from the prior art. A detailed description is therefore not given here.
  • pump light emitted by the pump light source 1 is coupled into the fiber segment 3 via a Bragg grating, here via the Chirped-Bragg grating 5.
  • the fiber laser FL represents a laser cavity with longitudinal modes and orthogonal self-polarization modes coupled therewith.
  • the natural frequencies of the longitudinal modes are characterized in that the optical length of the fiber laser FL is filled by integer multiples of half the laser wavelength, that is to say the Bragg wavelength ⁇ B , Since the first Bragg grating 4 is narrow-band, it determines the Bragg wavelength.
  • the fiber segment 3 enables the oscillation of at least, preferably two, longitudinal modes in the Cavity.
  • Emission light which couples out of a grating, here the chirped-Bragg grating 5, has the Bragg wavelength ⁇ B.
  • the emission light is separated from the pump light due to the shifted wavelength and directed into the return fiber 2b.
  • Back reflections in the fiber laser FL are prevented by the optical isolator 7.
  • the preferably two adjacent polarization modes are brought to interference by means of the analyzer 81, so that an interference signal and thus a beat frequency is obtained.
  • the interference signal is converted into an electrical signal proportional to the intensity, from which the desired beat signal is separated in the frequency filter 11 and fed to the frequency counter 12.
  • room modes can also be used for this.
  • Bragg gratings react to changes in temperature by changing their grating constants and thus their Bragg wavelength ⁇ B.
  • the first, temperature-sensitive Bragg grating 4 thus changes its Bragg wavelength ⁇ B. Due to the change in the Bragg wavelength, the changes in the chirped-Bragg grating 5 Reflection zone and thus the optical length of the cavity.
  • the length of the laser cavity is shown schematically in FIG. 1. Let L0 be the optical length at a temperature T 0 and Ll that at a temperature T x ⁇ T 0 .
  • the above-mentioned condition for the oscillation of the longitudinal modes has thus changed, which leads to a relative phase shift of the longitudinal modes.
  • the laser does not break down even when there is a sharp change in temperature.
  • the shift in the reflection zone in the chirped-Bragg grating 5 and thus the change in the cavity length are linear due to the chirped-Bragg grating with respect to the change in wavelength and thus to the change in temperature.
  • the longitudinal modes are emitted analogously to the above pressure measurement method and their beat frequency is analyzed. They do not have to be brought to interference because they already interfere with each other.
  • the change in the beat frequency is proportional, in particular linearly proportional to a temperature change.
  • the sensor comprising an Nd-doped fiber segment, a chirped Bragg grating with a cosine-shaped index profile with a band spectrum of 2 nm FWHM (fill width at half maximum) and a uniform chirp of - 1 nm / cm, a conventional Bragg grating with a Bragg wavelength change of 0.01 nm / K and a cavity length of 5 cm at a temperature T 0 .
  • the Chirped-Bragg grating behaved linearly within a range of the beat frequency of the Longintudinalmoden from 1.15 - 1.35 GHz and showed a sensitivity of 1 MHz / 0.01 nm.
  • the fiber laser sensor By means of the fiber laser sensor according to the invention, temperature and pressure changes can thus be measured simultaneously with the same fiber laser, the beat frequency of the longitudinal modes being used for temperature measurement and the beat frequency of the orthogonal self-polarization modes being used for pressure measurement. It is advantageous that the fiber laser sensor can be multiplexed, that is, several fiber lasers can be arranged in series along a common fiber. This fiber laser sensor, in particular its fiber laser, can be made relatively small, so that it can also be used in confined spaces.
  • Second Bragg grating (Chirped-Bragg grating) 50 Cooling or heating element
  • R BG1 reflectivity of the first Bragg grating at a temperature T :
  • R BG2 reflectivity of the first Bragg grating at a temperature T 2
  • R BG3 reflectivity of the first Bragg grating at a temperature T 3 ⁇ wavelength ⁇ B Bragg wavelength

Landscapes

  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Measuring Fluid Pressure (AREA)
  • Measuring Temperature Or Quantity Of Heat (AREA)
  • Optical Transform (AREA)

Abstract

Capteur qui possède un laser à fibre (FL) doté de deux réflecteurs sous forme de premier et second réseaux de Bragg (4, 5) et d'un segment de fibre (3) placé entre ces deux réseaux. Le premier réseau de Bragg (4) est placé de manière sensible à la température. Le second réseau de Bragg est un réseau de Bragg à glissement de la période (5) dont la longueur d'onde centrale est stabilisée par rapport aux changements de température. Ledit capteur permet la mesure séparée et simultanée de la température et de la pression, les fréquences de battement de modes d'auto-polarisation orthogonaux du laser à fibre (FL) constituant une mesure de la pression et les fréquences de battement de modes longitudinaux constituant une mesure de la température.
EP01914952A 2000-04-11 2001-04-05 Capteur a laser a fibre Withdrawn EP1272816A1 (fr)

Applications Claiming Priority (3)

Application Number Priority Date Filing Date Title
DE10017946A DE10017946A1 (de) 2000-04-11 2000-04-11 Faserlaser-Sensor
DE10017946 2000-04-11
PCT/CH2001/000220 WO2001077623A1 (fr) 2000-04-11 2001-04-05 Capteur a laser a fibre

Publications (1)

Publication Number Publication Date
EP1272816A1 true EP1272816A1 (fr) 2003-01-08

Family

ID=7638338

Family Applications (1)

Application Number Title Priority Date Filing Date
EP01914952A Withdrawn EP1272816A1 (fr) 2000-04-11 2001-04-05 Capteur a laser a fibre

Country Status (5)

Country Link
US (1) US6901187B2 (fr)
EP (1) EP1272816A1 (fr)
AU (1) AU2001242218A1 (fr)
DE (1) DE10017946A1 (fr)
WO (1) WO2001077623A1 (fr)

Cited By (1)

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Publication number Priority date Publication date Assignee Title
CN109932113A (zh) * 2019-02-20 2019-06-25 天津大学 力触觉超高空间分辨率的啁啾光纤测量系统

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US6845194B2 (en) * 2001-06-27 2005-01-18 Furukawa Electric North America Inc. Optical bandpass filter using long period gratings
AU2003203087A1 (en) * 2002-02-07 2003-09-02 Teraxion Inc. Power efficient assemblies for applying a temperature gradient to a refractive index grating
WO2006032116A1 (fr) * 2004-09-22 2006-03-30 Faculte Polytechnique De Mons Procede pour evaluer l'effet de la temperature et de la contrainte sur le spectre reflechi par un reseau de bragg en fibre
EP1640692A1 (fr) * 2004-09-22 2006-03-29 Faculté Polytechnique de Mons Méthode d'évaluation de l'influence de la température et de la contrainte sur le spectre réfléchi par un réseau de Bragg sur fibre
WO2012075474A2 (fr) * 2010-12-02 2012-06-07 Ofs Fitel, Llc Capteur courbe de laser à fibre dfb et microphone hétérodyne optique
WO2015076969A1 (fr) * 2013-11-22 2015-05-28 Baker Hughes Incorporated Utilisation de réseaux de bragg avec otdr cohérente
US9341532B2 (en) 2014-03-24 2016-05-17 General Electric Company Systems and methods for distributed pressure sensing
US9240262B1 (en) 2014-07-21 2016-01-19 General Electric Company Systems and methods for distributed pressure sensing
DE102017201524A1 (de) 2017-01-31 2018-08-02 Hochschule für angewandte Wissenschaften München Faseroptische Erfassungseinrichtung sowie Verfahren zum Betreiben einer solchen faseroptischen Erfassungseinrichtung
CN107046219B (zh) * 2017-04-18 2023-10-24 中国工程物理研究院激光聚变研究中心 一种啁啾体布拉格光栅冷却系统及冷却方法
CN109282913A (zh) * 2018-07-26 2019-01-29 孝感锐创机械科技有限公司 一种基于布拉格光纤光栅的非接触式测温装置
CN114675053B (zh) * 2022-02-23 2023-08-01 广东工业大学 基于啁啾光纤光栅的强度解调型风速传感器
CN115693359A (zh) * 2022-11-08 2023-02-03 上海频准激光科技有限公司 一种光纤激光器及其偏振态切换方法

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CN109932113A (zh) * 2019-02-20 2019-06-25 天津大学 力触觉超高空间分辨率的啁啾光纤测量系统
CN109932113B (zh) * 2019-02-20 2021-07-06 天津大学 力触觉超高空间分辨率的啁啾光纤光栅测量系统

Also Published As

Publication number Publication date
US20040071400A1 (en) 2004-04-15
WO2001077623A1 (fr) 2001-10-18
AU2001242218A1 (en) 2001-10-23
US6901187B2 (en) 2005-05-31
DE10017946A1 (de) 2002-01-17

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