KR100367297B1 - Fiber Fabry-Perot interferometric temperature measuring device - Google Patents

Abstract

The present invention relates to an optical fiber fabric interferometer type temperature measuring device which allows the measurement range to deviate from the limited range while maintaining the accuracy of the interferometer type temperature sensor. Sensing means having a primary sensor and an auxiliary sensor for generating; Optical means for inputting the modulated light to the main sensor and the auxiliary sensor, and converting the reflected light from the main sensor and the auxiliary sensor into an electrical signal, respectively; And signal processing means for converting each electric signal from the optical means into a phase and calculating the temperature of the measurement object based on the phase value, so as to detect a change in the optical phase corresponding to the temperature change of the measurement object. High accuracy fiber optic interferometer temperature for main sensor is obtained by receiving information about the position of the cycle to which the phase change of the main sensor belongs from the auxiliary sensor, which is relatively tens or hundreds of times slower, to maintain the accuracy of the main sensor stably. It extends the application area of the sensor.

Description

Fiber Fabry-Perot interferometric temperature measuring device

The present invention relates to an optical fiber Faberifero interferometer type temperature measuring apparatus, and more particularly to a high temperature fiber sensor using a high resolution optical fiber interferometer, such as a Fabrifero interferometer to a wide temperature range using a low-fiber optical interferometer It is related with an interferometer type temperature measuring device which is one of the fabrics which can be kept as it is.

Efforts to measure temperature simply and accurately have a long history.

Many attempts to measure temperature using fiber optic sensors have been reported, but there are not many methods currently commercialized in the proposed fiber optic temperature sensor to form a market.

Luxtron's products using the difference in the reduction time according to the temperature of the fluorescent materials and OptoMet's products that implement low-cost multi-channel sensors using low coherence light sources have a certain market. Custom or turn-key or OEM.

In Korea, we have been researching fiber optic distributed temperature sensors for many years at the Electric Research Institute, Standard Research Institute, National Defense Science Research Institute, LG Cable, and KAIST. In addition, Gwangju Institute of Science, Seoul National University, KAIST, KIST, etc., are actively researching fiber temperature sensors using Bragg grating, but research on high-precision interferometric fiber temperature sensors has been conducted by KAIST professor Kim Byung-yoon on a small scale. It is hard to find anything other than research activities on the external fiber Fabry Ferro (1 Reference (Han Young-geun et al., "Improvement of Temperature and Strain Sensors by Controlling Temperature Dependence of Long-period Gratings", 6th Optoelectronics Conference, p27-) , 1999), 2nd Reference (Song, Min-Ho et al., "Signal Processing of Optical Fiber Grating Pair Strain Sensors Using Mach-Zehnder Interferometers", Korean Journal of Optics and Photonics Vol.

In the United States, as part of a large-scale national project called 'Smart Structure and Smart Skin', research on optical fiber temperature sensors and optical fiber temperature sensors has made a lot of progress, but this is more suitable for multiplexing or distribution type than for unit products that emphasize precision. The main purpose is the development of optical fiber sensor. Therefore, the fiber Bragg grating sensor is taking the lead due to the mass production problem of the sensor having the same performance among the fiber Fabry Ferro sensor and the fiber Bragg grating sensor.

However, high-precision interferometric optical fiber sensor is composed of external fiber optic fabric sensor centered by Claus professor of VPI in the US and Taylor fiber of Texas AM University. Commercialization is under way, and research is being conducted at various angles, such as the direction in which the measurement area can be determined even if the precision is low, the direction to improve the precision in the limited measurement area, and the combination of the two above. (V. Arya et al., "Analysis of the effect of imperfect fiber endfaces on the perfor mance ....", Optical Eng. Vol. 35 No. 8, pp2262-2265, 1996), second (CE Lee et al., "Performance of a fiber-optic temperature sensor from -200 to 1050", Optics Letters, Vol. 13, p1038-1040, 1988).

In the UK, various models of interferometric optical fiber sensors are shown mainly in the laboratory of Jackson Professor Kent and Professor Meggitt of London College, but they all use additional complicated optics to overcome periodicity. It is quite difficult to secure (Y. Rao et al., "Improved synthesized source for white-light interferometry", Electronics Letters, Vol. 30, pp1440-1441, 1994), and second reference (DJ Webb et al., "Extended range interferometry using coherence tuned synthesized ...", Electronics Letters, Vol. 24, pp 1173-1175, 1988).

As described above, the high-precision optical fiber fabric measuring apparatus currently used is an interferometer type using a light source with high coherence.

1 is a block diagram of a conventional high-precision optical fiber fabric for a temperature specifying device, which is a laser diode 11 having a high coherence and a direct modulating light source; An optical isolator 12 which prevents the light from traveling backwards; A single mode optical fiber 14 which is an optical waveguide; Optical fiber couplers 13 for coupling and distributing optical signals; A high precision fiber fabric Fabry resonator 15 having a high speed of phase change with temperature; And a photoelectric converter 17 for converting the optical signal into an electrical signal; and a signal processing unit 18 for processing the analog optical signal modulated by the photoelectric converter 17 and converting the temperature into a signal.

The optical fiber coupler 13 divides the optical signal transmitted through the single mode optical fiber 14 into two single mode optical fibers 16 and 19, and the high-precision optical fiber Fabry Ferro resonator installed in the single mode optical fiber 16. The optical signal from (15) is transmitted to the photoelectric converter 17. At this time, the signal from the single-mode optical fiber 19 is made of a cross section through the end face of the optical fiber 19 or immersed in an index matching gel (index matching gel), the light is reflected from the end, That is, by preventing Fresnel reflections, the optical fiber coupler 13 does not return.

As shown in FIG. 2, the high-precision optical fiber Fabry Ferro resonator 15 includes a single mode optical fiber 41 and two semi-transmissive mirrors 44a and 44b having a reflectance of about 10%. Reference numeral 42 is a cladding, 43 is a core, 46 is a diffused optical fiber termination, 47 is incident light, and 48 is reflected light.

According to the high-precision optical fiber Fabry Ferro resonator 15 configured as shown in FIG. 2, when the laser beam of short wavelength branched through the optical fiber coupler 13 is incident on the transflective mirror 44a, the transflective mirror 44a ) Reflects about 10% of the laser beam and transmits the remaining 90% of the laser beam in the direction of the transflective mirror 44b, while the transflective mirror 44b is about 10% of the transmitted 90% laser beam. Of the laser beam is reflected back to the transflective mirror 44a, and the remaining 90% of the laser beam is transmitted out of the resonator 15 with a high-fiber optical fiber fabric.

Accordingly, there is a laser beam that reciprocally reflects between the transflective mirror 44a and the transflective mirror 44b. Therefore, the laser beam returning back to the optical fiber coupler 13 generates interference due to the coupling of the laser beam with the optical path difference generated by reciprocally reflecting the inside of the resonator 15 with the high-fiber optical fiber fabric. Weakness of the laser beam occurs according to the phase peculiar to the interferometer.

In order to derive the transfer function of the resonator 15 with the high precision fiber fabric Fabry described above, assuming that the interferometer is an ideal Fabryfero resonator with no loss in a given fiber optic fabric and the bidirectional values of the reflection and transmission coefficients are equal, The electric field of reflected light due to multiple reflections of a ferro resonator is given by the following equation (3-1).

(3-1)

Where E _r and E _i are the electric fields for reflected and incident light, r ₁ and r ₂ are the reflection coefficients, t ₁ is the transmission coefficient, and φ is the incident light reciprocating in the resonator. This is the phase difference corresponding to the optical path difference that occurs.

Equation (3-1) represents the ratio of the reflected light to the incident light, and when the light power is applied instead of the electric field and the reflectance is applied instead of the reflection coefficient, the power returned to the incident light is expressed by the following equation (3-). It is expressed as reflectance, which is a normalized value as shown in 2).

(3-2)

Here, R ₁ and R ₂ are the reflectances of both reflectors of the resonator and the optical path difference generated when the light reciprocates in the resonator. The phase difference φ is given by the following equation (3-3).

(3-3)

Where n is the effective refractive index of the fiber core, L is the resonator length, and λ is the center wavelength of the laser light.

Since the reflectance relation becomes the transfer function of the sensor, it is necessary to adjust the range of the maximum minimum value of the sensitivity in all phase regions in order to operate the resonator with the optical fiber fabric.

FIG. 3 shows the transfer function characteristics for the various reflectances of the reflector in the conventional high precision fiber fabric Faberifer described above.

As shown in the figure, when the reflectance (R) is about 80%, the sensitivity is very excellent in a specific area, that is, every 2π, but most are null areas, so it is difficult to operate a quadrature point, which is not suitable as a general sensor.

However, if the reflectance (R) is as small as 5%, the overall sensitivity is inferior, but a relatively almost null area is generated in a narrow area around every?, So it is suitable as a general interferometric sensor.

In order to increase the operating range within one period by reducing the width of the null area while maintaining the overall sensitivity, the reflectances of both reflectors should be made to be the same value and designed to be within 10%. At this time, except for the null region, Equation (3-2) is simplified as in Equation (3-4) below.

(3-4)

Here, R ₀ is a value when R ₁ and R ₂ have the same reflectance, and the phase difference φ is given in the same manner as in the above formula (3-3).

4 is a graph showing the characteristic curve of the conventional high precision fiber fabric Fabry resonator 15 described above.

The temperature change ΔT in the optical fiber changes the optical waveguide conditions in the optical fiber by changing the physical properties inherent in the optical fiber such as the length change ΔL / ΔT due to thermal contraction expansion and the change in refractive index due to temperature Δn / ΔT .

This change in waveguide condition modulates the phase of the light passing through, and ignoring the axial change or other strain effects, and formulating it, a change can be obtained from equation (3-3) as shown in equation (3-5) below. (Ref. E. Udd, Fiber Optic Sensors: An Introduction for Engineers and Scientists, John Wiley Sons, 1991).

(3-5)

In the case of molten silica optical fiber, dL / (LdT) is 5x10 ^-7 / ℃ and dn / dT is 10 ^-5 / ℃, so considering the refractive index of the optical fiber, the first term is about 7-8% of the second term. . Therefore, ignoring the first term, equation (3-5) is expressed as the following equation (3-6).

(3-6)

Here, ΔΦ is the phase change with temperature change, L is the resonator length, λ is the center wavelength of the laser light, dn / dT is the temperature-refractive index constant for the silica optical fiber, and ΔT is the change in temperature.

The high accuracy fiber optic temperature sensor is theoretically very complicated because of the long interference length of the light source. However, the high accuracy fiber optic temperature sensor is interference type to the limit that can adjust the visibility of the interferometer and maintain the accuracy of the sensor within the operating range. To simplify, the transfer function of the interferometer actually observed in the phase domain is given by the sine wave equation as shown in equation (4).

(4)

Here, I is the light output change according to the phase change, I ₀ is the maximum light output intensity, φ is the phase change value.

In particular, in the case of the optical fiber Faberifero interferometer, as shown in FIG. 3, the distribution of clarity varies depending on the reflectance R of the reflector, but the periodicity according to the fringe is the same.

Here, the phase of the abscissa shows some nonlinearity according to the sensitivity of the interferometer, as shown in the calibration curve of FIG. 4, but has a one-to-one correspondence with the measured physical quantity (ie, temperature here). The value of the measured temperature is easily determined.

However, since the value observed in real time through the photoelectric converter 17 when the temperature changes in the high-precision optical fiber Fabry-Peero temperature measuring apparatus of FIG. 1 is not a phase change value but an optical output of an interferometer whose output intensity changes according to the phase change, When the optical power is measured and the actual phase value 53 corresponding thereto is obtained from FIG. 3, the actual phase value 53 must be found from the plurality of candidate phase values 52 corresponding to the measured optical power 51. A problem arises.

The problem is that if the phase movement is continuously observed, the fringe (period) to which the current phase belongs is immediately known. Otherwise, it becomes unclear which fringe the current phase belongs to. . In addition, even when the phase movement is continuously observed, when the temperature change is fast, that is, when the sampling rate seen in the phase region is relatively slow, there is not enough information on the fringe movement, which may blur the overall number of fringes. a lot of works.

Therefore, even if this problem is solved by the ultra-fast sampling that can cope with a rapid change in temperature by improving the performance of the signal processing unit 18, the chirping is inevitably generated due to the characteristics of the semiconductor laser used as the light source of the optical fiber sensor. It is difficult to find the reference phase when there is a burst in the phase region due to unstability in semiconductor laser frequency or when a power-on reset is required.

Thus, although the relative value of the phase change with respect to the initial value of phase can be read correctly, it is difficult to set the initial phase value. That is, the high-precision optical fiber Fabry-Pelo temperature sensor, which uses a single mode optical fiber and a coherent laser light as a light source, has a transfer function characteristic as shown in Equation (4) or FIG. 3, wherein the short periodicity of the transfer function Therefore, when reading the phase value in the phase region from the measured reflectance intensity (reflective intensity), there is a problem that often misjudge the fringe to which it belongs.

To overcome this, various signal processing techniques have been proposed. Among them, the method commercialized as a temperature sensor by Yeh et al. (Reference; Y. Yeh et al., "Fiber optic sensor for substrate temperature monitoring", J. Vac. Soc. Technol. A 8, pp3247-3250, 1990 R. Sadkowski et al. (Ref. R. Sadkowski et. al., "Multiplexed interferometric fiberoptic sensors with digital signal ....", Applied Optics, Vol. 34, No.25, pp5861-5866, 1995), have a weaker price-per-channel ratio due to their excessive dependence on laser performance. .

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide an interferometer type temperature measuring device, which is an optical fiber fabric, which allows the measurement range to deviate from a limited range while maintaining the accuracy of the interferometer type temperature sensor. have.

1 is a block diagram of a conventional high-precision optical fiber fabric temperature measurement device,

FIG. 2 is a detailed view of the high precision optical fiber Fabry Ferro resonator shown in FIG. 1;

3 is a graph showing a transfer function of the high-precision fiber fabric Faberifer resonator shown in FIG.

4 is a graph showing a characteristic curve of the high-precision optical fiber Fabry-Pero resonator shown in FIG.

5 is a block diagram of an optical fiber Fabry Ferro interferometer type temperature measuring apparatus according to the present invention,

FIG. 6 is a detailed view of the low-precision asymmetric optical fiber Fabry-Pero resonator (low-precision dielectric thin-film optical fiber Faberipher resonator) shown in FIG. 5;

FIG. 7 is a graph showing a transfer function of the low-precision asymmetric optical fiber Fabry Ferro resonator (low-precision dielectric thin film optical fiber Fabry resonator) shown in FIG. 6;

FIG. 8 is a graph showing characteristic curves of the low-precision asymmetric optical fiber Fabry Ferro resonator shown in FIG. 6;

FIG. 9 is a graph comparing the transfer functions of the optical fiber Fabry Ferro resonator (main sensor) and the low precision asymmetric optical fiber Fabry Ferro resonator (auxiliary sensor) shown in FIG. 5;

10 is an internal configuration diagram of the signal processing means shown in FIG. 5;

11 is a configuration diagram when the optical fiber Fabry Ferro interferometer type temperature measuring device according to the present invention is applied to a measurement object,

FIG. 12 is a graph comparing the transfer functions of the high precision optical fiber Fabry Ferro resonator (main sensor) and the low precision asymmetric optical fiber Fabry Ferro resonator (auxiliary sensor) shown in FIG. 11.

※ Explanation of code for main part of drawing

11, 31, 101: laser diode

12, 32, 102: optical isolator

13, 33, 103: fiber coupler

14, 16, 19, 34, 104: single mode fiber

15: high precision fiber fabric Fabrero resonator

17, 39, 109: photoelectric converters

18: signal processing unit

35, 105: main sensor (high-precision fiber fabric resonator)

37, 107: optical signal retarder

38, 108: auxiliary sensor (low-precision asymmetric fiber fabric resonator)

111: protection sleeve 112: optical fiber phase controller

113: electric heater 121: pretreatment unit

122: pulse generator 123: pre-addition storage unit

124: microcontroller system 125: personal computer

130, 160: sensing means 140, 170: optical means

150, 180: signal processing means

In order to achieve the above object, an optical fiber fabric interferometer type temperature measuring apparatus according to a preferred embodiment of the present invention, the sensing having a primary sensor and an auxiliary sensor to differentially cause the optical phase change according to the temperature change of the measurement object Way; Optical means for inputting the modulated light to the main sensor and the auxiliary sensor, and converting the reflected light from the main sensor and the auxiliary sensor into an electrical signal, respectively; And signal processing means for converting each electric signal from the optical means into a phase and calculating the temperature of the measurement object based on the phase value.

The change rate of the optical phase according to the temperature change of the main sensor is faster than that of the auxiliary sensor, and the output signal of the auxiliary sensor is used as information for indicating the position of a cycle to which the light phase of the main sensor belongs.

Hereinafter, with reference to the accompanying drawings for the optical fiber Fabry ferro interferometer type temperature measuring apparatus according to an embodiment of the present invention will be described in detail.

5 is a block diagram of an optical fiber fabric interferometer type temperature measuring apparatus according to the present invention, the sensing means for causing a change in the optical phase according to the temperature change of the measurement object; Optical means (140) for modulating the intensity of incident light to the sensing means (130) and detecting the reflected light from the sensing means (130); And signal processing means 150 for calculating a phase of the reflected light detected by the optical means 140 and calculating the temperature of the measurement object based on the calculated phase.

The sensing means 130 is composed of a high precision main sensor 35 and a low precision auxiliary sensor 38.

The main sensor 35 is a high-fiber optical fiber fabric with a length of 1 to 5 cm and is composed of a resonator, and a speed of phase change with temperature is faster than that of the auxiliary sensor 38. The auxiliary sensor 38 is a low precision asymmetric fiber fabric, which is composed of a resonator.

The low asymmetric optical fiber Fabry resonator cleanly cuts the end of the single-mode optical fiber using an optical fiber cleaver, and then titanium oxide (TiO ₂ ), tin oxide (SnO ₂ ), zirconium oxide (ZrO ₂ ) or magnesium oxide (MgO). The dielectric is coated by a method such as sputtering so as to have a thickness of between 1 μm and 50 μm, and the coating is stabilized by heating for 3 to 24 hours at a constant temperature in the range of 400 ° C. to 750 ° C. In addition, the fringe resolution of the low asymmetric fiber Fabry Ferro resonator is 0.0005 fringe, but has a low resolution ranging from 1 ° C. to 5 ° C. with respect to temperature, and the phase change due to temperature change is 100 times higher than that of the main sensor 35. Here, the auxiliary sensor 38, the low-precision asymmetric fiber fabric Fabrero resonator is asymmetrical, unlike the symmetrical pattern of the typical Fabry resonator in terms of the optical signal transmission function. In the embodiment, the dielectric thin film is coated so that the reflective form of the light has an asymmetric characteristic. Accordingly, the auxiliary sensor 38 may be referred to as a "low dielectric film thin film fiber fabric resonator" in terms of its components, while in the functional aspect of light reflection may be referred to as a "low profile asymmetric fiber fabric Fabry resonator," In the description of the terminology, the term “lowly asymmetric fiber fabric resonator” is referred to for the sake of unity.

The main sensor 35 and the auxiliary sensor 38 are installed at the same position in space to be equally affected by the temperature.

The auxiliary sensor 38 indicates the position of the fringe to which the phase of the main sensor 35 belongs in a wide range of temperature ranging from 0 ° C to 1000 ° C by using a characteristic insensitive to temperature, and the main sensor 35 Is manufactured to have a resolution of about 0.0005 fringes, and the auxiliary sensor 38 obtains a more accurate phase with a resolution of 0.0005 fringes within the fringe whose current position is determined. Accordingly, there is a complementary relationship so that the final precision of the entire system in signal processing follows the precision of the main sensor 35 throughout the measurement temperature range.

The optical means 140 includes a laser diode 31 capable of direct modulation while having high coherence as a light source; An optical isolator 32 which prevents the light from traveling backwards; A single mode optical fiber 34 which is an optical waveguide; An optical fiber coupler 33 for coupling and distributing optical signals; An optical signal retarder 37 for separating the signal of the main sensor 35 and the signal of the auxiliary sensor 38 in the time domain; And a photoelectric converter 39 for converting the optical signal into an electrical signal.

The optical fiber coupler 33 divides the optical signal incident through the single-mode optical fiber 34 into a high-fiber optical fiber fabric resonator 35 for the primary sensor and a low-precision asymmetric optical fiber fabric resonator 38 for the auxiliary sensor. And transmits the reflected light (optical signal) from the high accuracy optical fiber fabric ferro resonator 35 for the primary sensor and the low asymmetric optical fiber fabric ferro resonator 38 for the auxiliary sensor to the photoelectric converter 39.

Figure 6 is a jeojeongdo asymmetric fiber optic Fabry-Perot resonator shown in Fig. 5; and also a detail of (38 dielectric thin optical fiber Fabry-Perot resonator), the titanium oxide used in the optical system (TiO _2), tin oxide (SnO _2), zirconium oxide ( ZrO ₂ ) or common materials for dielectric thin films such as magnesium oxide (MgO) are described here, but only titanium oxide (TiO ₂ ), which is somewhat easy to handle and is most often used as an optical coating material, is described.

The low-asymmetric fiber Fabry Ferro resonator used as the auxiliary sensor 38 replaces a dielectric film such as titanium oxide (TiO ₂ ) of about 20 μm with the resonator itself, and lambda = 1.3 μm from Equation (3-6). , L = 20㎛, dn / dT ~ 5.0x10 ^-5 / ℃, the phase change for temperature change 1000 ℃ is about 0.6π (about 1/4 fringe) radian and the clarity during operation of quadrature point Since the visibility drops to 50% of the half cycle, the resolution is about 2 ° C but a very wide operating range.

FIG. 6 shows a structure diagram of a microfiber fabric fabric which is made by thin-filming titanium oxide (TiO ₂ ), which is a typical material for optical coating, on an optical fiber terminal.

The reflectance by the single-layer thin film reflector in units of microns is given by the following equation (4-1) for the structure shown in FIG.

(4-1)

Where α = (n ₁ ² + n ² ) (n ² + n ₀ ² )

β = 4n ₁ n ² n ₀

γ = (n ₁ ² -n ² ) (n ² -n ₀ ² )

The refractive indices n ₁ , n, n ₀ are _values for the optical fiber core 73, the dielectric thin film 74, and the air 75, respectively.

At this time, the phase difference φ is given as in Equation (3-3), and the refractive index n and the resonator length L in Equation (3-3) are values for the dielectric thin film 74. Reference numeral 71 denotes a single mode optical fiber, 72 is cladding, 76 is incident light, 77 is reflected light.

Substantially, the dielectric thin film optical fiber fabric has difficulty in using it as an interferometer because the reflectivity of both ends of the dielectric thin film, which is a substitute for the resonator, is difficult, but the above-mentioned formula (4-1) is a commonly used method to roughly estimate the reflectance of the optical fiber coating. As a result of approaching the predictive model, the role as an auxiliary sensor proved to be sufficient.

FIG. 7 shows a characteristic curve of reflectance according to a phase change when assuming that a change in temperature or other physical quantity brings about a slight change in refractive index n or thickness L of the dielectric thin film portion in the expression of the reflectance represented by Equation (4-1). Indicates.

In this case, it can be seen that the transfer function pattern is a function of a rather complicated form, not a typical optical fiber Faberifero transfer function close to the sine wave of equation (4-1), as predicted by equation (4-1). That is, since it has an asymmetric sensitivity around a quadrature point, reproducibility is not good and thus it is not suitable as a sensor.

However, since the purpose of the auxiliary sensor 38, that is, the low asymmetric optical fiber fabric resonator (low dielectric film thin film fabric resonator) needs only to specify a specific phase region, the auxiliary sensor of the proposed sensor system has sufficient performance.

Among the oxides, titanium oxide (TiO ₂ ) has good strength and chemical resistance, and transmits visible light and near-infrared light. Therefore, there have been many studies on the optical properties of titanium oxide (TiO ₂ ) for use in optical fiber coating. The characteristic curve of the low precision asymmetric optical fiber Fabry Ferro resonator (low dielectric thin film Fabry Ferro resonator) used as the auxiliary sensor 38 is different depending on the method of coating titanium oxide (TiO ₂ ). It is well known that it is nonlinear. The refractive index n of the titanium oxide (TiO ₂ ) film deposited on the optical fiber cross section can be expressed by the following equation (4-2) considering only the temperature change.

(4-2)

Here, n _s is the reference refractive index of titanium oxide (TiO ₂ ) at room temperature converted to 0 ℃ temperature, Δn (T) is the experimental characteristics of the refractive index change characteristics according to the temperature of titanium oxide (TiO ₂ ) experimentally at a wavelength of 1300 nm It is obtained as shown in the following equation (4-3) within an absolute error of 0.0001.

(4-3)

FIG. 8 is a graph showing the characteristic curve of the low-precision asymmetric optical fiber Fabry-Pero resonator (low-precision dielectric thin film Fabry-Fero resonator).

FIG. 9 is a graph comparing the transfer functions of the main sensor 35 and the auxiliary sensor 38 shown in FIG. 5, since the auxiliary sensor 38 passes only half a cycle while the main sensor 35 passes ten cycles. It shows that the fringe to which the phase of the main sensor 35 belongs can be easily recognized from the information of the auxiliary sensor 38.

FIG. 10 is an internal configuration diagram of the signal processing means shown in FIG. 5, which is a pre-processing unit for converting and filtering the modulated electric signals (analog optical signals for the main sensor and the auxiliary sensor) from the photoelectric converter 39 into digital signals. (121); A pulse generator 122 for injecting a pulse modulated current into the laser diode 31 according to a predetermined sequence; A pre-addition storage unit 123 for leveling a predetermined number or more (at least 64 times) in order to reduce random noise of the digital signals for the main sensor and the auxiliary sensor output from the preprocessor 121; By converting the intensity of the two leveled reflected light, that is, the reflected light for the main sensor and the reflected light for the auxiliary sensor to the phase according to the programmed logic, and estimated the fringe position of the main sensor 35 from the phase information of the auxiliary sensor 38 A microcontroller system (124) for obtaining an accurate absolute phase value of the main sensor (35); And a personal computer for restoring a phase to a temperature, storing and displaying it in a memory with reference to a built-in phase-temperature characteristic curve as the exact absolute phase value of the main sensor 35 is transmitted from the microcontroller system 124. 125 is provided.

The laser diode 31 is a multimode laser or a single mode laser which is a communication semiconductor laser having a wavelength of 1.3 mu m or 1.55 mu m, and the photoelectric converter 39 uses a PIN type.

The injection current modulation pulse width of the laser diode 31 coming out of the pulse generator 122 is set to 200 mV-1500 mV in consideration of the frequency chirping effect and the system response time, and is emitted from the pulse generator 122. The frequency of the injection current modulating pulse of the laser diode 31 is set to 1 kHz to 200 kHz. In addition, the injection current of the laser diode 31 exiting the pulse generator 122 uses a stable square wave pulse in which vibration at initial rise is suppressed within several Hz.

The pre-addition storage unit 123 is installed at the front end of the microcontroller system 124 to realize at least 64 fast moving averages.

The frequency of the pulse output from the pulse generator 122 is set to sufficiently track the temperature change according to the application, and is determined in consideration of the number of measurement channels and the optical signal retarder 37.

The optical signal retarder 37 loosely winds the single-mode optical fiber 10 cm or more in diameter to minimize the polarization change and bending loss, and the main sensor 35 and the auxiliary sensor 38. The length is set from 50 m to 1000 m so that the time difference between the reflected light signals is 0.5 m to 10 m.

11 is a configuration diagram when the optical fiber fabric interferometer type temperature measuring apparatus according to the present invention is applied to the measurement object, the sensing means 160 for causing a change in the optical phase according to the temperature change of the measurement object; Optical means (170) for modulating the intensity of incident light to the sensing means (160) and detecting the reflected light from the sensing means (160); And signal processing means 180 for calculating a phase of the reflected light detected by the optical means 170 and calculating a temperature of the measurement target based on the calculated phase.

The sensing means 160 is composed of a high precision optical fiber fabric ferro resonator 105 for the primary sensor and a low precision asymmetric fabric ferro resonator (low dielectric dielectric thin film fabric ferro resonator) 108 for the auxiliary sensor as in the sensing means described with reference to FIG. 5. It is located near the electric heater 113 to be measured while being protected by the protective sleeve 111.

The signal processing means 18 has an internal configuration (preprocessor, preaddition storage, pulse generator, microcontroller system, personal computer) described in FIG.

The optical means 170 is a laser diode 101, optical isolator 102, optical fiber coupler 103, single-mode optical fiber 104, optical signal retarder 107, and the same as the optical means described in FIG. A photoelectric converter is provided.

In addition, the optical means 170 is further provided with an optical fiber phase controller 112 using a strain effect for maximum sensitivity point control.

Indeed, since the clarity of the high-fiber optical fiber fabric resonator 105 for the main sensor and the low asymmetric optical fiber fabric resonator 108 for the auxiliary sensor are different from each other, there is some trade-off in designing the amplifier. Since noise factors vary according to temperature changes, sufficient consideration is required when preparing a look-up table for temperature conversion. In addition, since the operation of setting the actual main use area near the maximum sensitivity point (quadrature point) of the auxiliary sensor is not easy, the optical fiber phase controller 112 using the strain effect is used.

In other words, the optical fiber phase controller 112 shifts the phase of the initial incident light to partially compensate for the resolution of the resonator 108 with the low asymmetric fiber Fabry Faber. Set to the current operating point.

In other words, a certain length of optical fiber is wound on a cylindrical piezo-electric transducer and a constant voltage is applied to the cylindrical piezoelectric transducer to shift the phase of the incident light near the maximum sensitivity point of the resonator 108 with the low asymmetric optical fiber fabric. After the voltage is maintained as it is, the resolution of the resonator 108 is locally increased by the low asymmetric optical fiber fabric.

Applying the resonator length L = 10 mm, the center wavelength of the semiconductor laser, λ = 1.3 μm, and the constant dn / dT = 10 ^-5 / ° C. to obtain the temperature resolution of the resonator 105 with the high-fiber optical fiber fabric, the equation (3) -6), the phase change (ΔΦ) with respect to the temperature change ( ΔT ) of 1000 ° C is given as about 300π radians, and it can be easily seen that the half fringe in the phase region corresponds to the temperature change of 3.3 ° C.

Therefore, the temperature resolution of the low precision asymmetric optical fiber fabric for the auxiliary sensor in the present invention resonator 108 may be better than 3.3 ° C. The phase change (ΔΦ) of the auxiliary sensor with a temperature change ( ΔT ) of 1000 ° C is less than or equal to π radians (possibly less than or equal to 0.5π radians) to operate around the maximum sensitivity point, avoiding the upper and lower parts with lower sensitivity as shown in FIG. Since the length of the low precision asymmetric optical fiber fabric for the auxiliary sensor resonator 108 is about 30 μm or less.

In FIG. 11, the length of the resonator 108 is 20 μm with the low asymmetric optical fiber fabric for the auxiliary sensor, wherein the phase change of the auxiliary sensor 108 with respect to the temperature change ΔT 1000 ° C. from Equation (3-6). (ΔΦ) becomes about 0.9π radians, so that the temperature range of 1000 ° C to be measured can be sufficiently converted into a phase change within a half cycle. For example, when the lengths of the resonators 105 and 108 of the resonator are 10 mm and 20 μm, respectively, as shown in FIG. 11 and the optical fiber fabric for the auxiliary sensor is operated near the maximum sensitivity point, On the average, a temperature change of 3.3 ° C. corresponding to a half period of the main sensor 105 changes the phase of the auxiliary sensor 108 by about 0.002 pi radians (or 0.001 fringes). Since this is a phase value sufficiently detectable by the signal processing means 110, the absolute position of the current fringe is simply determined from the output signal of the main sensor 105 (Ref .; Kim, Kwang-soo et al., "An optical fiber fabric with interferometric reflector interferometer type. Performance Improvement of Temperature Sensor ", Journal of the Korean Institute of Electrical Engineers, July, 2000).

FIG. 12 is a graph comparing the transfer functions of the high precision fiber fabric Fabreferro resonator 105 for the primary sensor and the low precision asymmetric fiber Fabry ferro resonator 108 for the auxiliary sensor shown in FIG. Fig. 1 shows the transfer function of the resonator 105 in the optical fiber fabric, and reference numeral 210 denotes the transfer function of the resonator 108 in the low precision asymmetric optical fiber fabric for the auxiliary sensor.

Next, the operation of the optical fiber Fabry ferro interferometer type temperature measuring apparatus according to an embodiment of the present invention will be described with reference to FIG. 11.

The optical signal uses a highly coherent laser light as a carrier, and travels only along the single-mode optical fiber 104, which is an optical waveguide, and only light that is exposed to air is scattered at the end of the optical fiber.

When pulse-modulated current is injected into the laser diode 101 according to a predetermined sequence in the pulse generator 122 in the signal processing means 180, 50 units are provided via the optical isolator 102 which prevents the reverse direction of the optical signal. It is sent to a 2x2 3dB fiber coupler 103 which distributes evenly to 50.

The optical fiber coupler 103 equalizes the incident light into a high-fiber optical fiber fabric for the main sensor resonator 105 and a low-precision asymmetric fiber fabric for the auxiliary sensor. The two resonators 105 and 108 are installed at the same spatial position to undergo the same perturbation, that is, the refractive index change in the resonator according to the temperature change, but in the high-precision optical fiber fabric ferro resonator 105 It shows a fast phase change and a relatively slow phase change in the low asymmetric optical fiber Fabry resonator 108, and the ratio of the phase change rate is almost proportional to the ratio of the resonator length by equation (3-6).

In the optical signal retarder 107 composed of a predetermined length of optical fiber installed in front of the low asymmetric optical fiber Fabry Ferro resonator 108, the interference signals of the two interferometers are separated in the time domain by the time required by the signal processing means 180. At this time, the delay time τ is given by the following equation (5) in consideration of the round trip of the optical signal.

(5)

Here, n is the refractive index of the fiber core, c is the speed of light in vacuum, l is the length of the optical fiber for the delay circuit. For example, the optical signal delay time for the optical fiber length of 100 m is about 1 kHz from the refractive index of 1.47 of the silica-based optical fiber core.

The change of the refractive index in the resonator according to the temperature change of the electric heater 113 which is the measurement target modulates the phase of the reflected light as in Equation (3-6).

The reflected light having the modulated phase passes through the optical fiber coupler 103 in the opposite direction to the incident light with a time difference of 1 ms from the main sensor 105 and the auxiliary sensor 108 and enters the photoelectric converter 109 as an electrical signal. The converted and converted electrical signals are conditioned by the preprocessor 121 and converted into digital data.

The converted digital signals are branched into two (for primary sensor and secondary sensor) from the preaddition storage unit 124 and leveled at least a predetermined number (at least 64 times) to reduce random noise, respectively. ).

The microcontroller system 124 converts the intensity of the two equalized reflected light into phase according to the programmed logic, and estimates the fringe position of the main sensor 105 from the phase information of the auxiliary sensor 108. The exact absolute phase value of the sensor 105 is obtained. For example, sampling of at least two analog-to-digital conversions is performed with a phase difference of π / 2 within a pulse width from the pulse-shaped electrical signal, and sampling of at least three analog-to-digital conversions with a phase difference of π / 3 is performed. At least four analog-to-digital conversion samplings are performed at a phase difference of π / 4 so that the digital data converted by the above process is applied to each of the sampling and to each of the main sensor 105 and the auxiliary sensor 108. The moving average of at least 64 times is obtained using the pre-addition storage unit 123, and the absolute value of the main sensor 105 is calculated based on a preset light output-phase conversion table (look-up tabled table). Calculate the phase value.

Given the exact absolute phase value of the main sensor 105, the microcontroller system 124 restores the phase to temperature with reference to a built-in phase-temperature characteristic curve (look-up tabled table) and displays it as a display. Or stored in the memory through the personal computer 125.

As described in detail above, according to the present invention, when detecting a change in the optical phase corresponding to the temperature change of the measurement object, information about the position of the period to which the phase change of the main sensor belongs is relatively tens to hundreds of times. By taking in a slow auxiliary sensor and keeping the accuracy of the main sensor stable, the area of application of the high accuracy fiber optic interferometer type temperature sensor for the main sensor is expanded. In other words, by solving the difficulty of signal processing due to periodicity, which is a chronic problem of the interferometer type temperature sensor, by constructing inexpensively with a simple structure of an auxiliary sensor, the application area of the temperature sensor can be extended with an optical fiber fabric which is limited to a specific field. have.

In addition, since the whole system is composed of optical fibers, in addition to the advantages such as the unique characteristics of the optical fiber, such as small size, light weight, low signal attenuation ratio, etc., various unique characteristics can be used. Areas with high electromagnetic field induction due to the absence of electromagnetic induction, particularly in the field of temperature measurement of RF sputtering equipment, the measurement of the Curie point temperature of magnetic materials, the temperature measurement of power transformer cores with difficult insulation, and the specific parts inside gas insulated power equipment (optical It can be applied to the temperature measurement area around the sensor or the area with high corona noise.

In addition, since the measuring unit of the sensor is made of a material that is harmless to the human body, it is applicable to the temperature control field of the treatment area during the treatment such as hyperthermia and temperature control in the food processing field, especially snacks production.

In addition, since the signal transmission means of the sensor is an optical fiber, the multiplexing of the sensor is easy and can be used for intensive monitoring diagnosis through an optical fiber communication network.

On the other hand, the present invention is not limited to the above-described embodiment, but modifications and variations are possible without departing from the gist of the present invention, and such modifications and variations should be regarded as belonging to the following claims.

Claims (15)

Sensing means having a primary sensor and an auxiliary sensor that differentially induce an optical phase change according to a temperature change of a measurement object; Optical means for inputting the modulated light to the main sensor and the auxiliary sensor, and converting the reflected light from the main sensor and the auxiliary sensor into an electrical signal, respectively; And Signal processing means for converting each electric signal from the optical means into a phase and calculating the temperature of the measurement object based on the phase value; The optical phase change rate according to the temperature change of the main sensor is faster than the auxiliary sensor, and the output signal of the auxiliary sensor is used as information indicating a position of a period to which the optical phase of the main sensor belongs. Fabrifero interferometer type temperature measuring device. The method of claim 1, The main sensor is an optical fiber fabric interferometer type temperature measuring device, characterized in that consisting of a high-fiber fiber fabric resonator. The method of claim 2, The auxiliary sensor is an optical fiber fabric interferometer type temperature measuring device, characterized in that the low-symmetric optical fiber fabric is composed of a resonator. The method of claim 3, wherein The low asymmetric optical fiber fabric refractor is a fiber optic fabric interferometer type temperature measuring device characterized in that the end of the single-mode optical fiber is cut by using an optical fiber cleaver, the dielectric is coated by sputtering and then heated. The method of claim 4, wherein The dielectric material is any one of titanium oxide (TiO ₂ ), tin oxide (SnO ₂ ), zirconium oxide (ZrO ₂ ), magnesium oxide (MgO), the interferometer type temperature measuring device of the optical fiber fabric. The method of claim 5, And said dielectric material is coated within the range of 1 µm to 50 µm. The method of claim 5, The heating is optical fiber fabric interferometer type temperature measurement apparatus, characterized in that performed for a time range of 3 hours to 24 hours in the temperature range of 400 ℃ to 750 ℃. delete The method of claim 3, wherein The fringe resolution of the low asymmetric optical fiber Fabry Ferro resonator is 0.0005 fringe, the optical phase change of the optical phase Fabry Ferro interferometer type temperature measurement device, characterized in that the change in the optical phase is 100 to 2000 times slower than the main sensor. The method of claim 1, The optical means Laser diodes; An optical isolator which prevents reverse propagation of the light output from the laser diode; Optical waveguides of single-mode fiber; An optical fiber coupler for distributing an optical signal incident through the optical waveguide and coupling the optical signal from the main sensor and the auxiliary sensor; An optical signal delay unit for separating a signal of the main sensor and a signal of the auxiliary sensor in a time domain; And A photoelectric converter for converting an optical signal from the optical fiber coupler into an electrical signal, The optical fiber coupler divides the optical signal incident through the optical waveguide into the main sensor and the auxiliary sensor, and transmits the reflected light from the main sensor and the auxiliary sensor to the photoelectric converter. Temperature measuring device. The method of claim 10, The signal processing means A preprocessor converting and filtering the electric signals of the modulated main sensor and the auxiliary sensor from the photoelectric converter into digital signals, respectively; A pulse generator for providing a pulse modulated current to the laser diode according to a preset sequence; A pre-addition storage unit for equalizing the digital signals of the main sensor and the auxiliary sensor output from the preprocessor; A microcontroller system for converting the intensity of the reflected light of the leveled main sensor and the auxiliary sensor into a phase and estimating the periodic position of the main sensor from the phase information of the auxiliary sensor to obtain a phase value of the main sensor; And And a terminal for restoring and displaying the phase value as the temperature as the phase value of the main sensor is transmitted from the microcontroller system. The method of claim 11, The laser diode is an optical fiber fabric interferometer type temperature measuring device, characterized in that the multimode laser or single mode laser which is a semiconductor laser for communication of 1.3㎛ or 1.55㎛ wavelength. The method of claim 12, The injection current modulation pulse width of the laser diode coming out of the pulse generator is 200 Hz-1500 Hz, and the frequency of the injection current modulation pulse of the laser diode coming out of the pulse generator is 1 kHz to 200 kHz. Type temperature measuring device. The method of claim 10, The optical means further comprises an optical fiber phase controller for shifting the phase of the initial incident light to partially compensate for the resolution of the auxiliary sensor to set the maximum sensitivity point of the auxiliary sensor to the current operating point. Interferometer type temperature measuring device with optical fiber fabric. The method of claim 11, And the pre-addition storage unit equalizes the digital signals of the main sensor and the auxiliary sensor at least 64 times, respectively.