KR101172965B1 - Temperature sensor using MMF interferometer and temperature sensing thereof - Google Patents

Abstract

Provided are a temperature sensor using an MMF interferometer and a method for measuring the temperature thereof. The temperature sensor is configured to distribute the light emitted from a first MMF interferometer located at a temperature measurement target, a second MMF interferometer located at a location other than a temperature measurement target, and a light source to a first MMF interferometer and a second MMF interferometer. An optical coupler for coupling the light reflected by the interferometer and the second MMF interferometer and a calculation unit for calculating the temperature of the temperature measurement target based on the light coupled and output from the optical coupler. As a result, a temperature sensor having the same function can be manufactured using an MMF interferometer without using an FBG, thereby reducing the manufacturing cost of the temperature sensor.

Description

Temperature sensor using MMF interferometer and temperature sensing

The present invention relates to a temperature sensor and a temperature measuring method thereof, and more particularly, to a temperature sensor using a fiber optic interferometer and a temperature measuring method thereof.

Existing fiber optic sensors for precise sensing are quite expensive due to their complex configuration. For example, an optical fiber sensor using FBG (Fiber Bragg Grating) is typical.

FBG is an optical fiber device that irradiates UV in the optical fiber to give a periodic change in the refractive index in the longitudinal direction to reflect at a specific wavelength and transmit the remaining wavelengths. At this time, the phenomenon that the center wavelength is shifted by the temperature and tension occurs, it is possible to implement the sensor using the FBG.

However, expensive equipment must be used to obtain information about the wavelength directly or to translate it into a change in light intensity. In addition, the cost of manufacturing FBG is also formidable.

The present invention has been made to solve the above problems, an object of the present invention, to reduce the manufacturing cost, to provide a temperature sensor capable of accurate temperature measurement without using the FBG.

According to the present invention for achieving the above object, the temperature sensor, a light source; A first MMF (Multi Mode Fiber) interferometer positioned at the temperature measurement object; A second MMF interferometer located outside the temperature measurement object; An optical coupler for distributing light emitted from the light source and transferring the light emitted from the light source to the first MMF interferometer and the second MMF interferometer, and to combine the light reflected from the first MMF interferometer and the second MMF interferometer; And a calculator configured to calculate a temperature of the temperature measurement target based on the light coupled and output from the optocoupler.

The calculation unit may be configured to analyze frequency components of light coupled and output from the optocoupler, convert the analysis result into a time domain, and calculate a temperature of the temperature measurement target based on a peak position of the light in the time domain. desirable.

In addition, it is preferable that the peak position of the light changes due to the change in the optical refractive index generated in the first MMF interferometer when the temperature of the temperature measurement target changes.

In addition, as the position of the peak of the light is far from "0", the temperature of the temperature measurement target is calculated higher, and as the position of the peak of the light is closer to "0", the temperature of the temperature measurement target may be calculated lower.

In addition, the length of the first MMF interferometer is preferably different from the length of the second MMF interferometer.

The MMF of the first MMF interferometer and the second MMF interferometer may be TMF (Two Mode Fiber).

In addition, the TMF may be boron doped to the optical fiber core.

On the other hand, according to the present invention, the temperature sensor, the light source; At least one MMF (Multi Mode Fiber) interferometer; An optical coupler for distributing the light emitted from the light source and transferring the light to the at least one MMF interferometer and combining the light reflected from the at least one MMF interferometer; And an operation unit configured to calculate a temperature of a temperature measurement target at which any one of the at least one MMF interferometer is located, based on the light coupled and output from the optocoupler.

Meanwhile, the temperature measuring method according to the present invention distributes light emitted from a light source and transmits the light to a 'first MMF interferometer located at a temperature measurement target' and a 'second MMF interferometer located at a position other than the temperature measurement target'. step; Combining the lights reflected from the first MMF interferometer and the second MMF interferometer; And calculating a temperature of the temperature measuring object based on the light output by being combined in the combining step.

As described above, according to the present invention, it is possible to manufacture a temperature sensor having the same function by using an MMF interferometer such as a TMF without using an FBG, thereby lowering the manufacturing cost of the temperature sensor.

1 is an effective refractive index for the V-number for each mode,
2 is a view provided to explain the optical fiber base material manufacturing method,
3 is a view showing the structure of an optical fiber extraction tower;
4 is a view showing a refractive index analysis result of the Boron deposition efficiency analysis method and the prepared sample according to the flow rate of SiCl4;
5 is a view showing the results of EPMA quantitative analysis of the prepared sample,
Figure 6 is a view showing the deposition efficiency investigation analysis results of Boron according to the deposition temperature change,
7 is a view showing a mode analysis result of the TMF produced through the above process,
8 illustrates a temperature sensor using a TMF interferometer according to an embodiment of the present invention;
9A and 9B are photographs of the results of actual fabrication of the temperature sensor shown in FIG. 8;
FIG. 10 is a diagram illustrating a frequency component analysis result by a PC and a result of converting the same into a time domain; FIG.
11A and 11B are graphs showing the change in peak position generated when the temperature of the oven is increased from 30 ° C. to 70 ° C., and
12 is a diagram illustrating a temperature sensor according to another embodiment of the present invention.

Hereinafter, with reference to the drawings will be described the present invention in more detail.

In the present invention, the present invention proposes a TMF (Two Mode Fiber) sensitive to change in refractive index and a temperature sensor implemented by applying a TMF interferometer to an Optical Frequency-Domain Reflectometry (OFDR).

One. TMF

1-1. mode  Analysis and Fiber Optic Structure Analysis

Mode analysis and fiber structure analysis are essential to fabricate MMF (Multi Mode Fiber) with multimode.

In general, in the case of SMF (Single Mode Fiber), the fiber core radius is about 4-5 μm and the refractive index difference between the core and the cladding is about 0.3%.

In order to have multimode, the fiber core must be made larger or the refractive index must be increased. How much the size should be adjusted was confirmed through the mode analysis of the optical fiber structure. Roughly calculating the conditions to have a single mode at 1550nm wavelength (λ) is as follows. The single mode cutoff condition can be calculated from the V-number. Equation 1 shows V-number.

Here, when the radius (a) of the core is kept the same at 4μm, the range of the V-number that can have two modes by increasing the refractive index of the core is 2.405 to 3.8, which can be confirmed through FIG. 1. By calculating the range of refractive index values from this, 1.4645 to 1.4757 (when the refractive index of the cladding is 1.457) can be obtained. In FIG. 1, it may be assumed that the TE01 mode and the HE21 mode correspond to one mode corresponding to the LP11 mode as a mode group.

1-2. Design optimization criteria

The optical fiber has a step-index structure, the radius of the core is 4μm, the refractive index is preferably such that the relative refractive index (refractive index difference) for the cladding has a value between 0.0075 ~ 0.019.

In addition, it is preferable to dope Boron together to increase the temperature sensitivity of the optical fiber core. In addition, since it is different from the general method of doping only Ge, it is necessary to set a process optimization condition for Boron doping which reduces refractive index.

1-3. Optical Fiber Fabrication and Characterization

1-3-1. Fiber Optic Fabrication Method

1-3-1-1. Fiber optic Base material  making

2 shows an MCVD process chart. Halogen gas of silicon and refractive index dopants, which are the main material of the optical fiber, is mixed with oxygen (O2) and fed into a quartz glass tube which is bitten by a shelf.

A gas burner is mounted on the conveyer of the shelf to reciprocate and to heat the quartz glass tube from the outside so that the soot of the optical fiber quartz glass and the dopant oxide is deposited on the inner wall of the tube. At this time, each material gas is passed through concentrated sulfuric acid to remove water (H 2 O) in the gas.

Into the quartz glass tube, a mixture of gaseous silicon halogen compound (SiCl4) and dopants such as germanium, phosphorus, and boron such as halogen compounds (GeCl4, POCl3, BBr3, etc.) is blown, and the length of the quartz glass tube is transferred by the shelf of the shelf. When the outside of the tube is heated to more than 1800 degrees with the hydrogen burner reciprocating in the direction, the bonding force of oxygen is stronger than that of the halogen element, so these halogen oxides release chlorine molecules and combine with oxygen to form an oxide soot It is deposited on the inner wall of the quartz glass tube.

The soot powder is sintered by the hydrogen burner flame heating, which forms a clear glass and forms a glass on the inner wall of the quartz tube, which gradually grows into a thick layer of glass inside the tube.

The deposition / sintering process initially forms a cladding layer. Once the cladding layer is grown to its designed thickness and completed, the core portion is then formed by increasing the concentration of the dopant, which increases the refractive index.

When forming the core portion, the dopant concentration is adjusted according to the refractive index distribution design so that the refractive index distribution of the optical fiber base material matches the refractive index distribution of the optical fiber design.

1-3-1-2. Fiber optic extraction Draw tower )

3 shows the structure of the optical fiber extraction tower. The optical fiber base material is put into a furnace and heated to 2000 degrees. When the end of the semi-melted base material falls down by gravity, the fiber is pulled out to pull out the optical fiber.

The speed of extracting the optical fiber is about 10m / sec ~ 20m / sec. In order to keep the optical fiber outer diameter exactly as designed, if the optical fiber outer diameter measured by the optical fiber outer diameter measuring device is different from the design value, adjust the fiber extraction speed to match the outer diameter of the optical fiber. Faster withdrawal narrows the outer diameter of the fiber and slower withdrawal results in a thicker outer diameter.

Since the optical fiber is broken when it is bent or subjected to mechanical impact, it is coated with an acrylic resin on its outer surface. The coated acrylic resin is cured quickly and treated with ultraviolet light to have a moderate elasticity. Through this process, the optical fiber is finally manufactured.

1-3-2. Investigate Doping Concentration Optimization Conditions

The optical fiber fabrication process basically uses SiCl4, which is responsible for SiO2 deposition only, without changing the refractive index, GeCl4, POCl3, which increases the refractive index, and CF4, BCl3, which reduces the refractive index. The conditions of deposition efficiency and refractive index of each chemical sample of the optical fiber were investigated. The deposition degree of the sample was quantitatively analyzed by EPMA method and the refractive index was analyzed by using a refractive index measuring device.

1-3-2-1. BCl3 While keeping it constant SiCl4 Deposition analysis according to the change of

The deposition temperature was kept constant at 1980 degrees. As shown on the left side of FIG. 4, SiCl 4, BCl 3 and POCl 3, CF 4, and SiCl 4 were alternately deposited in a donut shape around the SiO 2 substrate. When the amount of SiCl 4 is reduced while maintaining the BCl 3 constant, the relative deposition rate of Boron is expected to increase, but as shown on the right side of FIG. 4, it can be seen that there is no significant change.

On the other hand, Figure 5 is a view showing the results of EPMA quantitative analysis of the prepared sample. As shown in FIG. 5, it can be seen that the amount of Boron doping does not depend greatly on the SiCl4 flow amount change of 60 to 20 SCCM when the BCl3 flow is kept constant at 20 SCCM.

1-3-2-2. Depending on deposition temperature Boron Investigation of Deposition Efficiency

It was confirmed that the increase in flow rate of SiCl4 did not significantly affect the deposition efficiency of Boron. This time, we investigated Boron's deposition efficiency according to the deposition temperature. The sample was prepared in the same manner as the previous experiment while increasing the temperature by 1 degree from 1680 to 1980 degrees.

FIG. 6 is a diagram illustrating an analysis result of investigating the deposition efficiency of Boron according to a change in deposition temperature. As shown in FIG. 6, it can be seen from the refractive index measurement results that the higher the temperature, the lower the refractive index. This means that Boron's deposition efficiency is high at high temperatures. Therefore, in this experiment, it was decided to experiment at 1980 degrees.

1-3-2-3. Two-mode fiber

Through the above experiment, the deposition rate of the flow rate of BCl3 was investigated, and TMF was obtained from the information of the existing GeCl4.

2. of optical fiber mode  analysis

Figure 7 shows the results of the mode analysis of the TMF produced through the above process. For mode analysis, a 1550nm gain-switching pulse laser and a 40Gbps high-speed sampling oscilloscope were used. The length of the optical fiber was about 100 m. As shown in FIG. 7, it can be seen that the manufactured TMF has two modes, and the second mode is a higher order mode. In the higher order mode, it can be easily identified because it is removed by bending the fiber. In addition, it can be seen that a delay time difference of about 20 ps / m occurs between modes.

3. TMF  Temperature sensor using interferometer

8 illustrates a temperature sensor using a two mode fiber (TMF) interferometer according to an embodiment of the present invention. As shown in FIG. 8, the temperature sensor according to the present embodiment includes a tunable laser source (TLS) 110, an optical isolator (OI) 120, an optical circulator (OC) 130, and an optocoupler 140. And a first TMF interferometer 150-1, a second TMF interferometer 150-2, a photo diode (PD) 160, a DAQ 170, and an operation unit 180.

The TLS 110 functions as a light source that continuously emits a laser whose wavelength is variable.

The OI 120 is an optical device that allows light to travel in only one direction, and the light incident from the TLS 110 proceeds only to the OC 130, while the light emitted from the OC 130 is directed to the TLS 110. Do not proceed.

The OC 130 transmits the light incident from the TLS 110 through the OI 120 to the optocoupler 140, while the transmitted light is reflected by the TMF interferometers 150-1 and 2 so that the optocoupler 140 When it is incident through), it is transmitted to the PD (160).

The optocoupler 140 distributes the light incident from the TLS 110 through the OI 120 and the OC 130 and transmits the light incident to the first TMF interferometer 150-1 and the second TMF interferometer 150-2. . In addition, the optocoupler 140 combines the light reflected from the first TMF interferometer 150-1 and the second TMF interferometer 150-2 and transmits the combined light to the OC 130.

The first TMF interferometer 150-1 may be implemented using a TMF manufactured by the above-described method. The light incident on the first TMF interferometer 150-1 is separated into two modes, which are reflected at the ends and then combined again. At this time, interference between all occurs in the first TMF interferometer 150-1, which is referred to as Internal Modal Interference (IMI).

The first TMF interferometer 150-1 is positioned at a temperature measurement object. The refractive index of the first TMF interferometer 150-1 varies with temperature, and the change in refractive index causes a change in the peak position of the light emitted from the first TMF interferometer 150-1.

The second TMF interferometer 150-2 may also be implemented using a TMF manufactured by the method described above. Light incident on the second TMF interferometer 150-2 is also split into two modes, which are reflected at the ends and then coupled again. At this time, interference between all occurs in the second TMF interferometer 150-2, which is referred to as IMI.

On the other hand, the second TMF interferometer 150-1 is located in the atmosphere, not where the temperature measurement target is located. The second TMF interferometer 150-1 functions as an auxiliary interferometer for removing and compensating for noise added when the temperature is measured by the first TMF interferometer 150-1.

As shown in FIG. 8, the length of the first TMF interferometer 150-1 and the length of the second TMF interferometer 150-2 are different. The inter mode delay in the first TMF interferometer 150-1 is τ 1, and the inter mode delay in the second TMF interferometer 150-2 is τ 2.

The light reflected by the first TMF interferometer 150-1 and the light reflected by the second TMF interferometer 150-2 are coupled at the optocoupler 140. In this process, interference between the light reflected by the first TMF interferometer 150-1 and the light reflected by the second TMF interferometer 150-2 occurs, which is called an external modal interference (EMI).

The PD 160 detects the light coupled from the optocoupler 140 and transmits the detection result to the DAQ 170. Then, the DAQ 170 collects the detection result by the PD 160 and transfers it to the operation unit 180.

The calculator 180 calculates the temperature of the temperature measurement target through the detection result of the PD 160 collected by the DAQ 170.

Specifically, the calculation unit 180 is reflected by the first TMF interferometer 150-1 and the second TMF interferometer 150-2 through the detection result of the PD 160, and then coupled to the optocoupler 140. Analyze the frequency components of the light.

The calculation unit 180 converts the analysis result into the time domain and calculates the temperature of the temperature measurement target based on the peak position of the light in the time domain. This is because a change in refractive index is caused in the first MMF interferometer 150-1 when the temperature of the temperature measurement target is changed.

The farther the peak position of light is from "0", the higher the temperature of the temperature measurement object is calculated, and the closer the position of the peak of light is "0", the lower the temperature of the temperature measurement object is calculated.

Hereinafter, a process of measuring the temperature of the temperature measurement target in which the first TMF interferometer 150-1 is located in the temperature sensor illustrated in FIG. 8 will be described in detail.

First, the light emitted from the TLS 110 is transmitted to the optocoupler 140 through the OI 120 and the OC 130. Light incident on the optical coupler 140 is distributed to and incident on the first TMF interferometer 150-1 and the second TMF interferometer 150-2.

Light incident on the first TMF interferometer 150-1 and the second TMF interferometer 150-2 is separated into two modes, which are reflected at the ends and then combined again.

On the other hand, since the first TMF interferometer 150-1 is positioned at the temperature measurement target, the refractive index changes according to the temperature change of the temperature measurement target and the atmosphere. On the other hand, since the second TMF interferometer 150-2 is positioned in the atmosphere, the refractive index changes only in accordance with the change in the temperature of the atmosphere.

The light reflected from the first TMF interferometer 150-1 and the second TMF interferometer 150-2 is combined by the optocoupler 140, detected by the PD 160, collected by the DAQ 170, and then collected by the DAQ 170. Is passed to 180.

Then, the calculation unit 180 converts the result of analyzing the frequency component of the light into the time domain, and calculates the temperature of the temperature measurement target based on the peak position of the light in the time domain.

4. TMF  Result of Temperature Sensor Room Using Interferometer

FIG. 9A is a photograph of a result of actually fabricating the temperature sensor shown in FIG. 8. In FIG. 9A, the equipment designated as "TLS" is the TLS 110 shown in FIG. 8, and the optical fiber labeled as "interferometer" and "secondary interferometer" in FIG. 9A is the first TMF interferometer 150-1 shown in FIG. ) And the second TMF interferometer 150-2, respectively. 9A corresponds to the operation unit 180 illustrated in FIG. 8.

In addition, the object indicated by "oven" in FIG. 9A is a temperature measurement object, and the object indicated by "precision thermometer" in FIG. 9A is a thermometer which directly measures the temperature of an oven for performance analysis of a manufactured temperature sensor.

As shown in Figure 9b, after installing the TMF portion of the interferometer and the probe of the precision thermometer in the oven, the temperature was measured using a temperature sensor and a precision thermometer, respectively.

The frequency component analysis result of the light incident on the PC is shown in the upper part of FIG. 10, and the result of converting the light into the time domain is shown in the lower part of FIG. 10.

As shown in the lower part of FIG. 10, it can be seen that the peak position of the light is delayed by a predetermined time in the time domain. In this case, the degree of delay, that is, the peak position of the light, may be referred to as information indicating the temperature of the oven.

That is, it can be said that the farther the peak position of light is from "0sec", the higher the temperature of the oven. The closer the peak position of light is to "0sec", the lower the temperature of the oven.

FIG. 11A graphically shows the peak position change, ie, the delay change, which occurs when the temperature of the oven is increased from 30 ° C. to 70 ° C. FIG. As shown in FIG. 11A, the change in temperature of the oven causes a change in the peak position. Specifically, as the temperature increases, the peak position moves away from “0 sec”.

11B is a graph showing a change in the peak position according to the temperature change of the oven, and as shown in FIG. 11B, it can be seen that the change in the peak position according to the temperature change is linear.

5. Variant

So far, the present invention has been described in detail with reference to a preferred embodiment of a temperature sensor using a TMF interferometer. In the above embodiment, the number of TMF interferometers is implemented as two, but this is merely an example for convenience of description.

It is possible to implement more than two TMF interferometers. At this time, it is preferable to implement the delay time between modes of the TMF interferometers differently. FIG. 12 shows a temperature sensor implemented with four TMF interferometers 150-1, 150-2, 150-3 and 150-4 having different inter-mode delay times.

The detailed description of the temperature sensor shown in FIG. 12 will be omitted since it can be inferred from the detailed description of the temperature sensor shown in FIG.

In addition, in the above embodiment, an interferometer using a TMF is assumed, but it is a matter of course that the temperature sensor is implemented by an interferometer using an MMF other than the TMF.

In addition, although the preferred embodiment of the present invention has been shown and described above, the present invention is not limited to the specific embodiments described above, but the technical field to which the invention belongs without departing from the spirit of the invention claimed in the claims. Of course, various modifications can be made by those skilled in the art, and these modifications should not be individually understood from the technical spirit or the prospect of the present invention.

110: TLS 120: OI
130: OC 140: Optocoupler
150-1, 150-2, 150-3 and 150-4: TMF Interferometer
160: PD 170: DAQ
180: calculation unit

Claims (5)

Light source;
The optical fiber doped with boron with a step-index structure and the core has a radius of 4 μm and the refractive index of the refractive index of the cladding is between 0.0075 and 0.019 has a first MMF ( Multi Mode Fiber) interferometer;
a fiber having a step-index structure and having a radius of 4 μm and a refractive index of between 0.0075 and 0.019 having a refractive index of the cladding and having a boron-doped optical fiber located outside the temperature measurement target 2 MMF interferometers;
An optical coupler for distributing light emitted from the light source and transferring the light emitted from the light source to the first MMF interferometer and the second MMF interferometer, and to combine the light reflected from the first MMF interferometer and the second MMF interferometer; And
And a calculator configured to calculate a temperature of the temperature measuring target based on the light coupled and output from the optocoupler. The method of claim 1,
The calculation unit,
And analyzing the frequency components of the light coupled and output from the optocoupler, converting the analysis result into the time domain, and calculating the temperature of the temperature measurement target based on the peak position of the light in the time domain. The method of claim 2,
The peak position of the light is,
The temperature sensor, characterized in that for changing the temperature of the temperature measurement object changes the refractive index generated by the first MMF interferometer due to the change. Light source;
At least one MMF (Multi Mode Fiber) composed of an optical fiber doped with boron with a step-index structure and a core radius of 4 µm and a refractive index of between 0.0075 and 0.019 with a relative index difference of cladding ) Interferometer;
An optical coupler for distributing the light emitted from the light source and transferring the light to the at least one MMF interferometer and combining the light reflected from the at least one MMF interferometer; And
And a calculation unit configured to calculate a temperature of a temperature measurement target at which any one of the at least one MMF interferometer is located, based on the light coupled and output from the optocoupler. By distributing the light emitted from the light source, it has a 'step-index structure', the core radius is 4㎛, the refractive index is the refractive index relative to the cladding (value of 0.0075 ~ 0.019) and doped boron together The first MMF interferometer located at the temperature measurement target and 'step-index' structure, the radius of the core is 4㎛, the refractive index has a value between 0.0075 ~ 0.019 relative refractive index (refractive index difference) for the cladding with boron Transferring the doped optical fiber to a second MMF interferometer located outside the temperature measurement object;
Combining the lights reflected from the first MMF interferometer and the second MMF interferometer; And
And calculating the temperature of the temperature measuring object based on the light output by being combined in the combining step.

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