TW499565B - High stability intensity-based optical fiber sensors - Google Patents

Abstract

This invention provides an optical fiber sensor that can be used to measure physical- or biochemical parameter in embedded cases for multiplexing. This optical fiber sensor is based on the evanescent field of a fiber, and a pair of fiber Bragg gratings across a sensing section is utilized for compensating the variation noises from a light source and environments. The cladding of the sensing section is partially etched away, then it is evaporated or doped with a specific substance that can be reactive with surroundings. As a result, the intensity from a reflecting light could change in response to the measured. The technique of the present invention is based on the ratio of the reflecting intensities from the reference and the sensing fiber Bragg gratings. The ratio has been found to be independent of optical variations noises from a light source or environments. Also, the ratio is immune to the thermal effects.

Description

499565 V. Description of the invention (1) Many backgrounds of fiber optic sensors ^ ^ ^ ^ ^ ^ ^ Fast, small size, light weight, resistant: = sensitivity south, response speed works normally, while using photosensitivity :: ... It can be used in a dangerous environment + π # ^, sp. s Z fiber transmission signals achieve the purpose of remote telemetry. Because the optical fiber sensing state has no shame, its M: Tian Yi to A 4 people, and electromagnetic field interference and multi-point measurement ... etc. characteristics: This is equivalent. 7Suitable for multi-point measurement of physical and biochemical parameters. ^,. ^ ^^ tter and Hocker publish fiber-optic strain sensors m ΐt: For the intelligent structure of optical fiber, it has become a world research and exhibition trend. In recent years, due to the successful development of fiber gratings, not only has the advantages of ordinary fiber sensors, but also has great advantages in multipoint measurement 2 Multiplexed fiber grating sensors, fiber grating multi-parameter measurement sensors Devices and so on are the trends of world research and development. In the development of intensity-type optical fiber sensors, reliability is more questionable by users because it is extremely susceptible to the influence of light sources and the environment. Related intensity type θ fiber sensors developed in recent years have the following forms. U.S. Patent No. 5,11,8,9,31 proposes to use the light intensity change caused by the relative microbending reflected by different fiber gratings as the sensing target, but does not propose a pair of fiber gratings to achieve the compensation function. U.S. Patent No. 4, 7 92, 689 proposes a compensation method between the reference signal and the sensing signal, and uses a special dye (Dye) dye to detect the biochemical reaction, but because the spectroscopic device is designed on the receiving end, so Still can not achieve simple functions like fiber grating multi-point measurement. U.S. Patent 5,646,400 proposes 45. The tapered fiber grating's gradual-instantaneous wave field reacts with the Corrosion Sensitive Compound coated on the fiber grating.

Page 5 499565 V. Description of the invention (2) Changes in light intensity 'but cannot be used for compensation design and are still affected by environmental disturbances. Spillman, WB ·, jr · and Lórd, JR · [SpiUman, WB ·, Jr · and Lord, JR ·, "Self_Referencing Multiplexing Technique for Fiber-Optic Intensity Sensors, IEEE, Journal of Lightwave Technology, LTj, pp · 865 -869, 1 987] The measurement method proposed is a self-compensating technique; a pulsed light wave is cyclically divided into multiple light waves with two couplers, one of which is a reference light wave, and the other is a sensed light wave. Two A sensor is connected between the light waves and the sensor will change the intensity of the sensed light wave due to the change in external pressure. The change of the external environment can be measured by the ratio of the reference light wave and the sensed light wave. The invention is mainly to develop a fiber grating sensor with a simple structure and high reliability, and can avoid the influence of the external environment on the measurement signal, so as to enhance the accuracy of the sensor for the target being measured. Apply this sensing The device can be used for real-time monitoring and multi-point measurement on the mechanical or building structure, which can help improve the safety of the structure and understand its structural health. Do reinforcement or preventive measures; can also be designed as a biochemical sensor, as long as the reaction zone is plated with a reaction medium, so that the light intensity changes due to the interaction between the test object and the reaction medium, the detection purpose can be achieved. The fiber grating sensor is mainly equipped with fast response, small size and weight

499565 V. Description of the invention (3) Light, high temperature, high pressure and corrosion resistance of embedded structures. The Bragg grating is used to make the induction mechanism, and the change in the luminous intensity ratio changes the light intensity interference. Optical fiber gratings are designed between optical fiber gratings. The two test signals' the other anti-grating reflective fiber are more than ten years, fast, but the method is not only costly and can be used in quality; it is not only compared with the traditional and parallel internal several pieces of time required for disassembly for construction Controlled steel or light intensity ratio sensing technology is mainly used to measure the amount of corrosion when the measurement is complicated. The keys on the sensing area can be used to obtain a simple sensor to achieve the name of the rotten name. . In addition, the steel in the structure has the characteristics of electromagnetic field interference and other characteristics, so it is quite common or chemical sensing. Although many people have used sensors in the past, the offset of the grating wavelength is used as the sensing mechanism of the reflection of a pair of fiber grating sensors. This novel design is not affected by the enhanced fiber optic sensor. Application value. Based on the asymptotic wave field and paired with a pair of braided fiber sensors, the reflected light of one of the two fiber gratings placed in the sensing area is used as the reference light as the sensing signal, and two fiber lights are used Value 'to achieve the effect of light intensity disturbance compensation. The development of measuring physical changes in measuring objects has been developed. The physical quantities including stress, temperature, pressure, and flow are not sufficient; the traditional method of measuring residual quantities is also inaccurate. In the present invention, a layer of metal film is used as the sensing medium of the corrosion sensing for accurate corrosion prediction, and the most important point in use is that it can be connected in series and can be detected immediately without having to detect the machine. Detach and inspect the parts, not only can avoid the damage, but also save the disassembly and installation of the human-type optical fiber rot # sensor can make the gluten in the prestressed concrete, monitor the situation at any time, In order to make reinforcement in time to avoid corrosion

"々JO :) V. Description of the invention (4) '--- Damage caused by damage. In addition to corrosion sensing, other biochemical sensing can also use the special design of the strength type optical fiber sensor of the present invention to achieve multiple points The purpose of measuring and embedding the main body without being disturbed by the environment. (1) Fabrication of the intensity type fiber grating sensor The intensity type fiber sensor of the present invention is mainly based on the characteristics of a fiber Bragg grating f emitting a specific wavelength, The fiber comprises a core having a core and a cladding 105 surrounding the core. The section of the fiber includes two spaced apart Bragg grating regions 101, 102 in a long axis direction, and a space between the two. The sensing region 1 q 3 between the Bragg grating regions is shown in FIG. 1, wherein: the two Bragg grating regions 101 and 102 are respectively formed with two Bragg gratings with different reflection wavelengths at their cores; and the sensing region The measurement area 1 〇3 has a reduced coating layer with a part of the peripheral coating layer removed and a reaction medium layer attached to the outer surface of the reduced coating layer. In addition, in order to avoid the Bragg grating areas 1 〇, 1 〇2, and the sensing area 丨〇3 Directly received Under the influence of disturbance, a multi-porous sleeve 1 0 4 'is installed on the outside to allow the object to be infiltrated, and at the same time, it can prevent the external external force from causing birefringence changes to the fiber grating, which in turn affects Signal processing. The production system of the fiber Bragg grating sensor of the present invention first uses an excimer laser and a phase mask to produce a Bragg fiber grating (FBG) on a hydrogen-loaded germanium-containing fiber. Periodic phase masks can be used to make a pair of fiber gratings with different center wavelengths. Buffered Oxide Etch (B0E) is used to remove the fiber coating between the two fiber gratings, leaving about 5 # m Thick cover

Page 8 499565 V. Description of the invention (5) layer, and then attach the etched optical fiber by coating or solvent-gel (s〇1-Gel) method to a layer of reaction medium that matches the object to be measured, and it can be completed. Fabrication of Intensity Fiber Grating Sensors. The invention can make a new type fiber grating intensity type sensor with series or parallel connection, which can greatly improve the disadvantage that general intensity type sensors cannot be connected in series or in parallel. The optical receiver suitable for the present invention can use a high-sensitivity (Avalanche Photodetector (APD)) optical receiver to increase the sensitivity of light intensity changes. (1) Theoretical analysis of the intensity type fiber grating sensor. As far as the reflected light intensity of the Bradford fiber grating is concerned, it is very susceptible to external disturbances, changes in the light source supplier and the spectrometer, resulting in changes in the light intensity of the spectrum. Errors and inaccuracies in the measured signals. As the future fiber grating sensor will be used for the detection of machinery, bridges and buildings, the sensor will be buried in the structure. In order to reduce the measurement error caused by the instrument and external disturbances, two reflected lights can be used. Bragg fiber gratings with different center wavelengths are connected in series, and the middle area is separated by a sensing area covered with a reaction medium 103. In Fig. 1, the intensity of the reflected light of the optical fiber grating 101 at the front stage is not affected by the change of the reaction medium layer, and the intensity of the reflected light of the fiber optic grating 100 at the rear stage is affected by the change of the reaction medium layer. After this special design, the fiber grating 1 01 reflected light can be used as compensation for fiber grating 1 〇2 reflected light; that is, after the fiber grating reflected light 1 〇1 compensation, the light source supplier, spectrometer and external disturbance can be eliminated. The impact. For multi-point measurement, multiple chirped fiber grating sensors can be connected in series. In order to simplify the description, two fiber grating sensors 10 and 11 are connected in series as an example, as shown in Figure 2 (a). The sensor 10,

Page 9 499565 V. Description of the invention (6) is a phase mask (QPS Tech., Λριη = 106), Grating Size = 10mm x 5mm, Zero order = 0.71%, +1 order = 39 · 63%, -1 order = 38 · 26%), after excimer laser processing, 'made two identical fiber Bragg gratings, one of which was prestressed' to shift the center wavelength, thus producing two different Grating fiber 101 with a center wavelength. The sensor uses a phase mask (QPS Tech ·, Apm = 1 070, Grating Size = 10mm x .5mm, Zero order = 0.51%, +1 order = 39 · 48%, -1 order = 38 · 88%), made by the same step (2) as the sensor j 〇 (including the grating fiber 1 1 1, 1 丨 2 and the sensing area 丨 丨 3). As shown in the spectrum of the sensor in Figure 2 (b), the reflected light spectrum produced by the sensor can be expressed in the spectrum analyzer using formula (i). The first pair can be expressed as: B1 λ Β1

RbI (into Β1) S

Ai (1)

The second pair can be expressed as: B2 (ti, AB2) = RB2 (λB2) · S · vs · A〗 Pm2 (tl, t2, λM2) = KM2 V into M2) · AM2 (t2) A2mi (S · Vc ) · V • A, 2 · A2, (2)

Where PBi and PMi represent Bragg fibers that are not affected by the reaction medium layer, respectively, and fiber grating reflection spectra that are not affected by the reaction medium layer; and 1 represents two different Bragg wavelengths; and κ)

Page 10 499565 V. Description of the invention (7) Do not indicate the reflectance of two Bragg fiber gratings; t is the thickness of the dielectric layer; i — 1 ', the sensing area anti-grid sensor; In addition, the # Continuing eight squeezes, 矣 矣-the first degree i first fiber silver. V surface dry first 5 Jin knife analysis of the light obtained by the S surface light source = as the coefficient of variation of the source density; Detector attenuation coefficient of the layer; \ represents the decrement coefficient caused by the external force disturbance on the path. According to equations (1) and (2), the ratio of the reference signals of the two sensors can be obtained.

Rl (tj) = ---- (ti, λΜ1) RmI (into Ml) AM1 ()

Pbi (λΒ1)

RbI (long B1

Pm2 (t, t2, λ M2) Rm2 (into M2) · Am2 (t 2) () ---------------- Pβ2 (^ B2) Rb2 (into B2) ( 3) From equation (3), it can be easily found that these two ratios have eliminated the attenuation coefficient of the light source and external force disturbance. In other words, the magnitude of these two ratios is not affected by the light source and external force. Therefore, in equation (3), the sensing attenuation coefficient of only the remaining reaction medium layer related to the measurement parameter. In other words, the ratio of the reflected light generated by the Bragg fiber grating 1 01, Π 1 not affected by the reaction medium layer and the Bragg fiber grating 1 0 2, 1 1 2 affected by the reaction medium layer, can establish the measurement The relationship between the state of matter and the reaction medium layer. Figure 3 shows a basic architecture diagram illustrating that the present invention can be applied to tandem sensing without 499565. 5. Description of the invention (8) Affected by external disturbances. The architecture is composed of a white light source 201 and optical fibers 20 2 to 205, a 2x2 fiber coupler 2 06, 5 gold-degree fiber grating sensors 10 and 11 and a spectrum analyzer 30. The spectrum analyzer is only used to prove the feasibility of this design, but to avoid the problem of signal distortion, you can use a modulation spectrum filter (such as Optical Fiber Tunable Fabry-Perot Filters) instead of the spectrum analyzer to improve signal acquisition. speed. Four different external forces are applied to the optical fibers 202 to 205 to measure the effect of the external force on the sensing signal. The spectral signals at the four positions are made into Figures 4 (a) ~ (d). Figure 4 (d) is the optical spectrum of the optical fiber at the position of the optical fiber at position 05. Because the external force is generated at the optical fiber at 05, it only interferes with the sensor 11 and causes the spectrum of the sensor 11 to be attenuated. The Bragg fiber grating 1 in 11 which is not affected by the reaction medium belongs to 11. The reflected light intensity is from 1.825 nw to 1.049 nw at the highest. The reflection of the Bragg fiber grating 112 in the sensor 11 which is affected by the reaction medium belongs to 112. The intensity ranges from the highest 1.378 nw to 0 · 80 3 nw, but the range of the ratio of the two reflected light intensities ranges from only 1 · 3 2 4 to 1 · 2 9 9, indicating that although the reflected light intensity will be affected by external force disturbances However, it has little effect on the ratio of the reflected light intensity. Similarly, an external force disturbance is generated in the optical fibers 20 2 ~ 204. The external force disturbance will cause disturbance to the sensor 10 and the sensing ^ § 11 at the same time, as shown in Figure 4 (a). ~ (C). Therefore, the Bragg fiber gratings 101 and 111 that are not affected by the reaction medium layer and the Bragg fiber gratings 102 and Π2 that are affected by the reaction medium layer in the sensors 10 and 11 will have attenuation, but the ratio changes are relatively small, and the error is About 2%. The reflected light ratio of the Bragg fiber grating generated at the four positions 2 0 2 to 2 0 5 is made into Fig. 4 (e). From Fig. 4 (e), the reflections at the 4 positions 2 0 2 to 2 05 can be found.

Page 12 499565 V. Description of the invention (9) The light intensity ratios are all maintained at a certain value, the reflected light intensity ratio of the sensor 10 is about 1 · 0 6 and the sensor 11 is maintained at about 1 · 30 The change is quite small. If the design is not compensated, it will cause great changes under the same external force interference. Take the external force disturbance at the position of fiber 205 as an example. The reflected fiber energy of the Bragg fiber grating 1 12 affected by the reaction medium layer is from the highest 〖· 3 ^ 8 nw Up to 0 · 803 nw, the variation of the reflected light intensity is as high as 40%, and the ratio of the reflected light intensity is only about 2%. Therefore, the compensation design of the intensity fiber grating sensor is caused by external force disturbance. Interference is minimized, and the purpose of the present invention is achieved. When not measured, the ratio of the two signals of each pair of fiber gratings of the sensors 丨 〇 and 丨 丨 has been quantified first as the measurement reference for each pair of sensing areas. When the measurement factor starts to affect, the ratio of the two signals of a pair of fiber gratings will change with the change of the measurement factor, and the situation is detected. Therefore, the effect of the measurement factor can be judged. For each pair of optical fiber bridge sensing, the frequency spectrum of each reflected light is used as a separation to prevent crosstalk between the sensing of each pair of optical fiber gratings. Figure 5 shows the effect of temperature effects on the sensor of the present invention. After the money-cut optical fiber with a diameter of 19 / zm was electroless copper treated, the thickness of the reaction medium layer was increased to a sensing area with a diameter of 2 2 // m 1 0 3, and the effect of temperature on the ratio of reflected light intensity was recorded. It can be found when the temperature is from 2 to 5. (: The change in 'reflected light intensity ratio when changing to 90 ° C is about 4%. The possible reason is that the diameter of the etched fiber is quite small, which has a microbend effect on the etched fiber during the electroless copper plating process. The temperature rise causes micro-tension to the etched optical fiber, so that the reflected light intensity ratio increases by 4%, but the reflected light intensity ratio changes due to the metal layer in the sensing area.

Page 13 499565 V. Description of the invention (ίο) can reach more than 50%, so the temperature effect on the intensity optical fiber grating sensor will cause relatively small interference. It has been experimentally proven that the reflected light intensity is not affected by temperature. In addition, temperature and axial stress will only cause the center wavelength of the optical fiber to drift. Because this sensor uses the reflected light energy ratio as a measurement mechanism, temperature and axial stress will not affect the signal processing. Figure 6 shows the variation of the ratio of the reflected light intensity of the Bragg fiber grating and the reaction medium of the fiber sensor of the present invention. Take a metal corrosion sensor as an example to measure the range of the reflected light intensity variation. In the procedure of removing the optical fiber coating, three optical fibers with diameters of 19, 21, and 2 8 // m _ were produced, and the etched optical fibers were electrolessly plated on the reaction medium layer ore. Copper film, in order to increase the effect of electroless plating, first put the etched optical fiber into a ratio of (1 g-4 m 1) / 1 0 0 m 1 made of gasified stannous (SnCl2 · 2Η20) and hydrochloric acid (HC1) The sensitizing solution was immersed for 20 minutes, and then the activator solution soaked with palladium (PdCl2) and hydrochloric acid at a ratio of (0.025g-0.25ml) / 100ml was immersed for 10 minutes. To provide the electroless copper plating at the catalytic site where the reaction started, followed by copper sulfate (CuS04 · 5Η20) at lg / 100ml, formaldehyde (HCH0) at 0.5ml / 100ml, potassium sodium tartrate (NaKC4H4 06 · 4Η20) An electroless copper solution was prepared at 2.5 g / 100 ml and sodium hydroxide (NaOH) at 0.9 g. The remaining fiber is immersed in an electroless copper solution, and the spectrometer is used to measure the change of the reflected light every 30 seconds. The Bragg fiber grating 1 0 2 and 1 1 2 as the reaction medium and the Bragg fiber grating not affected by the reaction medium are measured.丨 0 1,1 丨 丨 Calculate the ratio of the intensity of the hetero-reflected light according to formula (3) and make it into Figure 6. The main trend It can be found from this experiment that the range of the ratio of the reflected light energy can be changed.

Page 14 499565 V. Description of the invention (11) In order to reach 50 to 70% of the left grating 1 0 2, 1 1 2 and the ratio of the reflected light intensity, the value of the received corrupted money is reduced or changed, or it does not appear quite small. The change of the film coating layer is the strongest of the present invention, such as the interference factors of light source changes, or the value achieved in a parallel manner. In other words, when the reaction medium μ, ........ a Bragg fiber grating 1 0 1, 111 not affected by the reaction medium reaches a change of more than 50%, it can be concluded that. At the same time, the reflected light intensity ratio in Figure 6 begins to show a decaying change. The main reason may be the effect of microbending caused by the electroless plating process of the etched optical fiber. The main reason is that the external loop, the bend of the outgoing / incoming optical fiber, and the underweight effect can be designed as an embedded optical fiber. The measurement of the crying temple to the multi-point measurement Function; because: Asia uses tandem diary to check and grant patent for team 11 'very commercial price 499565

FIG. 1 is a schematic diagram of an intensity type optical fiber sensor according to the present invention. (Take sensor 10 as an example): /, τ 1 01 fiber grating that is not affected by the reaction medium layer; 1 0 2 ^: reaction medium layer Affected fiber gratings; 103 fiber sensing area; 104 porous sleeve. Fig. 1 (a) is a perspective view of a series-strength fiber-optic sensor according to the present invention, where: 10 and 11 are two series-connected sensors. FIG. 1 (b) is a reflection spectrum diagram of the sensors 10 and 11 of FIG. 2 (a). FIG. 3 is a basic structural diagram of a series intensity optical fiber sensor for testing the external disturbance to the present invention, wherein: photo-sensing sensing is a 021 white light source; outgoing / incoming 2 0 ~ 2 0 5 optical fiber; 20 6 2x2 fiber coupler and 30 spectrum analyzer. Figure 4 shows the spectrum of the optical fiber sensors 10 and 11 when the optical fiber is subjected to lateral stress. (A) 20 2 The outgoing / incoming fiber is disturbed by external force. (B) 20 3 The outgoing / incoming fiber is disturbed by external force. Spectrum (c) 20 4 Spectrum of outgoing / incoming fiber disturbed by external force

(d) Spectrum of 205 outgoing / incoming fiber disturbed by external force, (e) is the ratio of reflected light intensity of fiber sensor 10 and u in (a) to (d). Figure 5 shows the temperature effect on the sensing of the present invention. Effect of the device. The effect of the change in the reaction medium i of the optical fiber sensor of the present invention on the reflection intensity ratio is shown.

Claims (1)

499565 VI. Application for patent scope 1 · An intensity type optical fiber sensor, which includes a core fiber and a coaxial fiber with a coating layer surrounding the core fiber, and this segment of fiber contains two spaced apart Bragg lights in the long axis direction The thumb region and the sensing region between the two Bragg light tree regions, wherein: the two Bragg grating regions are respectively formed with two Bragg gratings with different reflection wavelengths at their core centers; and the sensing region has A part of the outer covering layer is removed from the shrinking covering layer and the reaction medium layer attached to the outer surface of the shrinking covering layer; then the reflected light intensity ratio of the two Bragg gratings will substantially exclude external environmental disturbances and temperature effects, and Only affected by this reaction medium layer. 2. A series-strength fiber-optic sensor, comprising a plurality of series-connected intensity-type fiber-optic sensors as described in item 1 of the patent application scope. 3. A parallel intensity type optical fiber sensor, comprising a plurality of parallel intensity type optical fiber sensors as described in item 1 of the scope of patent application. 4. The optical fiber device according to item 1, 2, or 3 of the scope of patent application, wherein the reaction medium layer is a metal thin film layer. II Sensing surface ^ 5. The optical fiber sensor according to item 1, 2 or 3 of the patent application scope, wherein the reaction medium layer is a substance capable of reacting with a specific test compound. Page 17 499565 Page 18