CN108168729B - Two-point seawater temperature sensor based on cascade connection of fine core optical fiber and standard single mode optical fiber - Google Patents
Two-point seawater temperature sensor based on cascade connection of fine core optical fiber and standard single mode optical fiber Download PDFInfo
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01K—MEASURING TEMPERATURE; MEASURING QUANTITY OF HEAT; THERMALLY-SENSITIVE ELEMENTS NOT OTHERWISE PROVIDED FOR
- G01K11/00—Measuring temperature based upon physical or chemical changes not covered by groups G01K3/00, G01K5/00, G01K7/00 or G01K9/00
- G01K11/32—Measuring temperature based upon physical or chemical changes not covered by groups G01K3/00, G01K5/00, G01K7/00 or G01K9/00 using changes in transmittance, scattering or luminescence in optical fibres
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Abstract
The invention discloses a two-point seawater temperature sensor based on cascade connection of a fine core optical fiber and a standard single mode fiber, which comprises a first single mode fiber, a first fine core single mode fiber and a second single mode fiber which are sequentially connected, wherein the front end of the first fine core single mode fiber and the rear end of the first single mode fiber are welded together in a staggered manner, and the rear end of the first fine core single mode fiber and the front end of the second single mode fiber are welded together in a non-staggered manner. The rear end of the second single-mode optical fiber is sequentially welded with a second fine core single-mode optical fiber and a third single-mode optical fiber without dislocation. The invention can measure the sea water temperature at two continuous points, has high measurement accuracy and sensitivity and good repeatability, and can adjust the distance between two temperature measuring points within a certain range.
Description
Technical Field
The invention belongs to the technical field of strain detection, and particularly relates to a quasi-continuous two-point seawater temperature sensor based on cascade connection of a fine core optical fiber and a standard single-mode optical fiber.
Background
The temperature of sea water is typically a physical quantity that is responsive to the cold and hot properties of the marine climate and is also an important feature of sea water. Therefore, research on temperature change of seawater plays a very important role in research of ocean science. The research on the temperature change of the seawater is not only an important research content of the oceanography, but also has very important significance on the development of the disciplines such as climate change, ocean science, ocean fishery, hydroacoustic science and the like. The prior researches focus on the process of large spatial scale, and the spatial resolution requirement on ocean parameters is not high. However, with intensive investigation of the ocean, it was found that many parameters varied over a small spatial range, which may be closely related to large-scale marine phenomena, such as small-scale temperature profiles and variations thereof reflecting heat exchange and mass circulation occurring in sea-gas interactions and turbulent mixing processes. In addition, the temperature salinity of the delamination phenomenon in the internal wave at the layer interface is also greatly different. Thus, measuring the temperature at two or more quasi-continuous points in seawater will likely reveal the mechanism by which heat exchange and material circulation occurs in the sea interaction, providing a valuable reference for fine ocean research, whereas achieving quasi-continuous two-point temperature measurement often requires detection methods with spatial resolution on the order of less than centimeters and the ability to measure synchronously at multiple points.
At present, the most commonly used methods for measuring the temperature of the seawater mainly comprise a thermistor, a thermocouple sensor for measuring the temperature, a satellite remote sensing temperature measuring method and the like. However, these measurement methods have certain limitations: the temperature measurement of the thermistor sensor needs to be cleaned regularly to ensure the measurement accuracy; the thermocouple measurement accuracy can only reach the measurement accuracy of the reference joint temperature, and is generally within 1-2 ℃; the satellite remote sensing temperature measurement needs to depend on a satellite, the requirements on the receiving technology and hardware and software are large, electromagnetic waves cannot penetrate the sea surface, and the measurement accuracy is not high.
Compared with the typical seawater temperature detection method, the optical fiber has the advantages of small volume and small size of the whole device, and is more suitable for solving the problem. The principle of temperature measurement of the all-fiber sensor is that according to the relation between the refractive index of the seawater and the temperature of the seawater, the temperature value of the seawater can be obtained by combining the thermo-optical coefficient of the fiber material and applying an empirical formula, so that the temperature value of the seawater is convenient to detect on line, and no secondary pollution is caused to the seawater, so that the temperature sensor is widely focused. The currently reported seawater temperature all-fiber sensors mainly comprise fiber gratings, tapered fibers, micro-fiber ring resonators and the like, all of which have special structures, require complicated preparation processes, have fragile sensor structures and have low sensitivity. The recently reported hollow multimode interference-based sensor has higher measurement sensitivity and can be applied to measurement of sea water temperature, but has higher cost, and the sensor preparation process is more complicated, so that the use of the sensor is limited.
The invention patent CN 106802190A discloses a high-sensitivity optical fiber torsion sensor without temperature cross interference, which comprises a first single-mode optical fiber, a first fine core optical fiber, a conical fine core optical fiber, a second fine core optical fiber and a second single-mode optical fiber; one end of the first single-mode fiber is connected with the first fine core fiber, a misplacement welding point between the first single-mode fiber and the first fine core fiber is used as a first welding point, and misplacement welding is used for coupling light transmitted in the first single-mode fiber into a fiber core and a cladding of the first fine core fiber more evenly; the other end is used for being externally connected with a wide source light source; the first single mode optical fiber is used for coupling the light emitted by the broadband light source into the fiber core of the first fine core optical fiber; the conical fine core optical fiber is arranged between the first fine core optical fiber and the second fine core optical fiber and is used for leaking a light part transmitted in the cladding of the first fine core optical fiber to the external environment; the other end of the second fine core optical fiber is connected with one end of the second single mode optical fiber, a misplacement welding point between the two is used as a second welding point, and misplacement welding is used for coupling light transmitted by a fiber core and a cladding in the second fine core optical fiber into the fiber core of the second single mode optical fiber more evenly; the other end of the second single-mode optical fiber is used as an output end to be externally connected with a spectrometer; the first welding point and the second welding point adopt a welding mode that the optical fiber axis direction is symmetrical and the section direction is misplaced, and form a Mach-Zehnder interference structure in an optical fiber line together with the first fine core optical fiber, the conical fine core optical fiber and the second fine core optical fiber, the axial direction symmetry of the first fine core optical fiber and the second fine core optical fiber and the axial direction symmetry of the first single mode optical fiber and the second single mode optical fiber enable the number of excited cladding modes in the fine core optical fiber to be relatively small, a purer interference pattern can be formed after the mode interference is formed with the fine core optical fiber, and the misplaced energy in the section direction perpendicular to the optical fiber axis enables the light intensity distributed to the fiber core and the cladding in the fine core optical fiber to be relatively average in the welding process. The sensor of the invention has the advantages of high sensitivity, large dynamic range, simple structure, low price, easy integration and the like. However, the sensor can only measure the temperature change of a single point, the structure of the sensor is not firm and is easily damaged by external disturbance, in addition, the thin core optical fibers adopted in the sensor are all manufactured by adopting a flame stretching method, and the manufacturing process is accidental and is not easy to repeat.
The all-fiber Mach-Zehnder interferometer (hereinafter referred to as MZI) has the advantages of simple and firm structure, small volume, simple manufacture, low cost, electromagnetic interference resistance and the like, and is widely applied to the fields of strain, temperature, refractive index, liquid level detection, biomedical treatment and the like. However, current MZI-based temperature sensing can only be used for one point or two points that are far apart (typically greater than 20cm apart). The existing Mach-Zehnder interferometer temperature sensor based on the cascade connection of the standard single mode fiber and the fine core fiber is characterized in that a section of fine core single mode fiber is connected between two sections of common single mode fibers, and light waves in a cladding mode are effectively excited through a structure with mismatched fiber cores and interfere with the fiber cores. For the sensor only connected with a section of fine core optical fiber, when the temperature change of the seawater is small, the problems that the change amount of the interference peak wavelength is small, the sensitivity is low, the spectral extinction is small, the peak shift amount is not easy to distinguish and the like exist in the transmission spectrum of the sensor no matter how much the core diameter and the length of the connected fine core optical fiber are, and the temperature of the seawater at a single point can be measured only. Even if two sensors are connected in series, the sensor can only be used for temperature sensing at two points far away, and the spectrum after the series connection is easily disordered due to direct superposition of two sets of interference signals, so that the demodulation and analysis are difficult. Therefore, an all-fiber MZI that can be used for seawater quasi-continuous two-point temperature sensing needs to be developed.
Disclosure of Invention
Aiming at the defects of the prior art, the invention provides a two-point seawater temperature sensor based on cascade connection of a fine core optical fiber and a standard single-mode optical fiber, which solves the technical problem of quasi-continuous two-point temperature sensing in seawater.
In order to achieve the above purpose, the technical scheme of the invention is that the two-point seawater temperature sensor based on cascade connection of a fine core optical fiber and a standard single mode fiber comprises a first single mode fiber, a first fine core single mode fiber and a second single mode fiber which are sequentially connected, wherein the front end of the first fine core single mode fiber and the rear end of the first single mode fiber are welded together in a staggered manner, and the rear end of the first fine core single mode fiber and the front end of the second single mode fiber are welded together in a non-staggered manner; the rear end of the second single-mode optical fiber is sequentially welded with a second fine core single-mode optical fiber and a third single-mode optical fiber without dislocation.
Preferably, each of the first single mode fiber, the second single mode fiber and the third single mode fiber comprises a fiber core and a cladding wrapped outside the fiber core, the diameter of the fiber core is 8.2 μm, and the total diameter of the fiber core and the cladding is 125 μm.
Preferably, the length of the first fine core single mode fiber is 0.85cm.
Preferably, the first fine core single mode fiber comprises a fiber core and a cladding layer wrapping the fiber core, the total diameter of the first fine core single mode fiber is 125 μm, and the fiber core diameter is 3.6 μm.
Preferably, the length of the second single mode fiber is 6.45cm.
Preferably, the length of the second fine core single mode fiber is 0.35cm.
Preferably, the second fine core single mode fiber comprises a fiber core and a cladding layer wrapping the fiber core, the total diameter of the second fine core single mode fiber is 125 μm, and the fiber core diameter is 4.4 μm.
Preferably, a metal tube is sleeved outside the dislocation welding region between the front end of the first fine core single mode fiber and the rear end of the first single mode fiber, and the metal tube is connected with the dislocation welding region through PDMS.
Further, the method for carrying out encapsulation and reinforcement by using PDMS comprises the following steps: after removing a part of side wall of the metal tube, sleeving the metal tube at the dislocation welding area, and using a PDMS main agent and a hardening agent according to the mass ratio of 10:1, floating bubbles in the PDMS mixed solution to the surface by a standing mode, cracking, injecting the mixed solution into a metal tube, and then placing the metal tube on a heating table at 120 ℃ for curing for 15 minutes.
Compared with the prior art, the invention has the beneficial effects that: the invention designs the composition structure of the MZI and the core diameter and length of each section of optical fiber by analyzing the working principle of the MZI; the interference spectrum after design and optimization is obviously divided into three parts, the short wave part of the spectrum mainly comes from the first fine core optical fiber, the long wave part mainly comes from the second single mode optical fiber, and the middle part is the transition part between the two parts; the invention also adopts dislocation fusion between the front-end single-mode optical fiber and the fine core optical fiber, and performs PDMS encapsulation and reinforcement on the dislocation fusion area. The invention has the advantages of simple and firm structure, accurate measurement, high sensitivity and good repeatability, can sense the temperature of the seawater at two continuous points in an aligned mode, and has flexible and adjustable distance between the two points in a certain range (the range is approximately one centimeter to twenty centimeters).
Drawings
FIG. 1 is a schematic structural view of embodiment 1 of the present invention;
FIG. 2 is a plot of FSR versus SMF2 segment length for example 1;
FIG. 3 is a graph showing the spectrum contrast in example 1;
FIG. 4 is a schematic diagram of the sensor structure of comparative example 1;
FIG. 5 is a transmission spectrum of a 4.0cm section of Nufern 460-HP fiber welded between two sections of common single mode fiber in comparative example 1;
FIG. 6 is a transmission spectrum of a 2.8cm Nufern780-HP optical fiber welded between two sections of common single-mode optical fiber in comparative example 1;
FIG. 7 is a schematic diagram of the sensor structure of comparative example 2;
FIG. 8 is a transmission spectrum of comparative example 2;
FIG. 9 is the output spectrum 1 of the sensor structure at different temperatures in example 2;
FIG. 10 is an output spectrum 2 of the sensor structure at different temperatures in example 2;
FIG. 11 is the output spectrum of sensor II of example 2 at different temperatures;
FIG. 12 is an output spectrum of three typical positions B1, B2, B3 of sensor II in example 2 at different temperatures.
Detailed Description
The invention provides an all-fiber Mach-Zehnder interferometer formed by cascading multi-section single-mode fiber fine core fibers, which is used for quasi-continuous two-point temperature sensing in sea water. By analyzing the working principle of the MZI, the structure of the MZI and the core diameter and length of each section of optical fiber are designed, and a clear interference spectrum is obtained. By further optimizing, the interference spectrum is obviously divided into three parts, wherein the front part of the spectrum is from the first fine core optical fiber, the last part is mainly from the second single mode optical fiber, and the middle part is the transition part between the first fine core optical fiber and the second single mode optical fiber. In addition, in order to make the fusion splice between two optical fibers stronger, a practical and simple reinforcement method is introduced. Based on the fabricated MZI, a quasi-continuous two-point temperature sensing is demonstrated, sensitivity is estimated, and the present sensor measurement is compared to the thermometer measurement.
The experimental instrument and the consumable material of the invention are as follows:
super-continuous laser source welding machine
Spectrometer (AQ 6370C)
Heating table
Polydimethylsiloxane (PDMS)
Single mode fiber (SMF-28) core diameter 8.2 microns and overall diameter 125 microns
Fine core optical fiber
(1) Nufern 460-HP core diameter 2.5 microns, overall diameter 125 microns
(2) Nufern780-HP core diameter 4.4 microns, total diameter 125 microns
(3) Nufern 980-HP core diameter 3.6 microns, overall diameter 125 microns
The invention will be described in further detail with reference to the drawings and the detailed description.
Example 1 sensor Structure design
As shown in fig. 1, the two-point seawater temperature sensor based on cascade connection of the fine core optical fiber and the standard single mode optical fiber comprises a first single mode optical fiber (named as SMF 1), a first fine core single mode optical fiber (named as TCF 1) and a second single mode optical fiber (named as SMF 2) which are sequentially connected, wherein the front end of the TCF1 is welded with the rear end of the SMF1 in a staggered manner, and the rear end of the TCF1 is welded with the front end of the SMF2 in a non-staggered manner. The rear end of the SMF2 is sequentially welded with a second fine core single mode fiber (named TCF 2) and a third single mode fiber (named SMF 3) without dislocation.
Since the dislocated weld points are very fragile, the dislocated weld region between SMF1 and TCF1 needs to be encapsulated with PDMS (polydimethylsiloxane). PDMS (polydimethylsiloxane), which is an English abbreviation of polydimethylsiloxane, is an organosilicon, and is a polymer material widely applied to the fields of microfluidics and the like due to the characteristics of low cost, simple use, good adhesion with silicon chips, good chemical inertness and the like. The PDMS packaging mode is as follows: after removing a part of side wall of the metal tube, sleeving the metal tube at the dislocation welding area, and using a PDMS main agent and a hardening agent according to the mass ratio of 10:1, floating bubbles in the PDMS mixed solution to the surface by a standing mode, cracking, injecting the mixed solution into a metal tube, and then placing the metal tube on a heating table at 120 ℃ for curing for 15 minutes.
SMF1, SMF2 and SMF3 are standard single mode fibers comprising a core and a cladding layer surrounding the core, the total diameter of which is 125 μm and the diameter of which is 8.2. Mu.m.
TCF1 has a length of 0.85cm, TCF1 is a fine core single mode optical fiber comprising a core and a cladding layer surrounding the core, and TCF1 employs Nufern 980-HP having a total diameter of 125 μm and a core diameter of 3.6 μm. TCF2 is a fine core single mode fiber comprising a core and a cladding surrounding the core, TCF2 is Nufern780-HP having a total diameter of 125 μm and a core diameter of 4.4 μm.
In the embodiment, the SMF1-TCF1-SMF2-TCF2-SMF3 are sequentially connected, and the obtained sensing structure can be applied to quasi-continuous two-point temperature measurement in seawater. The principle is as follows:
the light wave propagates in the form of a fundamental mode in an input single mode fiber, with a substantial portion of the light energy being bound within the core. At the 1 st junction (where SMF1 interfaces with TCF 1), due to mismatch and dislocation fusion of the core diameters, part of the light is injected into the cladding of TCF1, thereby exciting the cladding modes to propagate in the cladding, the effective refractive index of which is related to the refractive index of the external liquid. Another portion of the light is coupled into the core of TCF1 and propagates within the core in the form of a core die, the effective index of which is not affected by the index of refraction of the external fluid. Due to the phase difference, the cladding mode and the core mode interfere. This constitutes a first mode interference unit (hereinafter indicated by IMI-a).
After passing through TCF1, the two parts of the light will be re-coupled in SMF 2. However, at the TCF1-SMF2 junction, a portion of the cladding modes transmitted in the cladding of TCF1 will propagate a short distance into the cladding of SMF2 as cladding modes, and this cladding mode will interfere with the core mode to form a second mode interference element (hereinafter IMI-B). However, if SMF2 is too long, the cladding mode will be degraded, and in order to maintain the propagation of the cladding mode in SMF2, the invention cuts SMF2 to about 6.45cm and then welds it with a very short section of TCF2 (Nufern 780-HP) of 0.35cm. Since the TCF2 is very short in length, the free spectral range generated by the length of fiber itself is very large, so that excessive modulation and interference of the entire spectrum can be avoided.
To further verify that interference of IMI-B portions occurs mainly in the SMF2 portion, the present invention measures the transmission spectrum of SMF2 having different lengths based on the following theory (the structural composition of the other portions is the same as fig. 1 except for the length of the SMF2 portion), see fig. 2.
Wherein Δn eff Is the difference between the effective refractive indices of the core and cladding modes, m is the interference order of the MZI, L is the length of the region where interference occurs, Δλ dip，m Is the wavelength separation (i.e., free spectral range, hereinafter FSR) between two adjacent interference minima.
As shown in fig. 2, the FSR gradually decreases as the SMF2 length increases. This experimental result is in agreement with the theoretical formula (1) described above, thereby verifying that the interference phenomenon does occur in the SMF2 section. Furthermore, as can be seen from FIG. 2, if SMF2 is too long (approximately greater than 20 cm), the cladding modes therein decay, and to maintain propagation of cladding modes in SMF2, we cut SMF2 to 6.45cm and then weld it with a very short 0.35cm TCF2 (Nufern 780-HP). Since TCF2 is very short in length, its free spectral range is very large, so that it can avoid excessive modulation and interference of the whole spectrum.
Finally, SMF3 is connected with TCF2 and used as signal output connected with a spectrometer, and the transmission spectrum collected by the spectrometer is shown in figure 3. As shown in FIG. 3, the interference effect of the first interferometer IMI-A is dominant in short wavelength band, the interference effect of the second interferometer IMI-B is dominant in long wavelength band, for comparison, the solid line in FIG. 3 is the transmission spectrum of a Nufern 980-HP fine core fiber (TCF 1) with a length of 0.85cm, which is fusion spliced in a dislocation manner between two standard single mode fibers, and the dotted line is the transmission spectrum of the entire MZI structure according to the present invention with SMF2 and TCF2 added. It can be seen that the transmission spectrum in fig. 3 includes the interference information of the IMI-a interference unit and the interference information of the IMI-B interference unit, and the two sets of interference are relatively independent in the spectrum, namely, a dominant short wave region and a dominant long wave region, so that the method is not only suitable for measuring the sea water temperature of two points at the same time, but also is convenient for spectral analysis.
In conclusion, the fine core optical fiber adopted in the invention is a commercial finished product in terms of structure, no accident exists, and PDMS packaging protection is adopted at the dislocation welding part, so that the structure is stable and not easy to damage. In terms of the generated spectrum, the output spectrum comprises two sets of interference spectrums, the two sets of spectrums are relatively independent of the modulation of the whole spectrum, and one dominant shortwave region and one dominant long wave region, so that simultaneous measurement of two-point seawater temperature can be realized theoretically, the analysis of the spectrums can be conveniently carried out by utilizing a Fourier analysis method, and a reference is provided for the improvement of the subsequent sensing sensitivity.
Comparative example 1
Unlike example 1, both ends of the fine core optical fiber were fusion-spliced with standard single mode optical fibers (SMF-28 e) by a common optical fiber fusion splicing apparatus, respectively, without decentration. The transmission spectrum of the sensing system was measured using an AQ6370C spectrometer (up to 20pm resolution) using a supercontinuum light source incident on the sensor structure as shown in fig. 4. The liquid used in the experiment is a seawater solution with a salt concentration of 33 per mill.
The transmission spectrum of the fine core fiber Nufern 460-HP is shown in FIG. 5 when the length is 4.0 cm. And selecting the trough of the spectral line as a characteristic peak. As shown in fig. 5, it can be seen that the spectrum curve significantly moves in the long wavelength direction as the temperature increases by changing the sea water temperature by the heating stage. The temperature sensing sensitivity is 13 pm/. Degree.C.
When Nufern780-HP with a length of 2.8cm and a core diameter of 4 μm was used for the fine core fiber, the transmission spectrum was shown in fig. 6. As shown in fig. 6, it can be seen that the spectrum curve significantly moves in the long wavelength direction as the temperature increases by changing the sea water temperature by the heating stage. The temperature sensitivity was 32 pm/. Degree.C.
As can be seen from a comparison of fig. 5 and 6, the FSR of the welded Nufern 460-HP structure is much larger than that of the welded Nufern780-HP structure, and the dynamic range is larger, but the sensitivity is lower. And only the sensor with the Nufern780-HP structure is welded, as can be clearly seen from fig. 6, the interference extinction ratio of the spectrum is shallow, the observation of obvious characteristic peaks and subsequent peak shifts is not easy to observe, and the two structures are only suitable for sea water temperature measurement of a single point.
Comparative example 2
Unlike comparative example 1, in order to improve the sensitivity and to produce a more pronounced mode-drying effect, a dislocated fusion splice structure as shown in FIG. 7 was designed, in which a length of Nufern 980-HP thin-core fiber of 0.85cm was fusion spliced in two sections of single-mode fibers, and the resulting spectrum was as shown in solid lines in FIG. 8 (a) (before encapsulation with PDMS). However, because the dislocation fusion is not firm, the dislocation connection region selects the PDMS encapsulation mode as described in example 1 to stabilize the structure, and the transmission spectrum of the encapsulated structure is shown by the broken line in fig. 8 (a) (fig. 8b is a temperature spectrum of a structure in which TCF1 is fusion-bonded alone and SMF2 and TCF2 are not joined). As can be seen from FIG. 8 (b), as the temperature increases, the spectral curve significantly shifts toward the long wavelength direction, with a temperature sensitivity of 46.3 pm/. Degree.C. It can be seen that this encapsulation does not cause too much change in spectral lines, which may be caused by the large difference in refractive index of PDMS (refractive index 1.406) from that of air (refractive index 1), and also by the stress of PDMS on the fiber after the encapsulation is cured. However, the sensor system of comparative example 2 can also be used only to measure a single point sea water temperature, and is not applicable to measurement of two point sea water temperatures.
In summary, the dislocation-free fusion in comparative example 1 may have problems of low sensitivity, low extinction ratio, etc. in single-point temperature measurement, whereas the structure of SMF-TCF-SMF in comparative example 2 is only suitable for single-point temperature measurement because of only one set of interference signals, and is not suitable for quasi-continuous two-point sea water temperature sensing. Even if the two sensors in comparative example 2 are connected in series, the interference signal (the signal is concentrated in the 1150nm to 1270nm wave band) with high extinction ratio of the two sensors is directly overlapped in the same wave band, so that the transmission spectrum is very chaotic and cannot be recognized.
Example 2 measurement of two Point temperature
In order to demonstrate the application of the invention in quasi-continuous two-point sea water temperature sensing, the invention performs the following experiments: immersing the IMI-a part shown in fig. 1 into a first seawater container as a first temperature measuring point, and immersing the IMI-B part into a second seawater container as a second temperature measuring point; the distance between the two containers is about 1cm (considered as two points of quasi-continuous distribution); the temperature of the seawater at the two temperature measuring points is respectively changed through the heating table, and the thermocouple thermometer is utilized to measure and correct the temperature of the seawater at the two points.
First, under the condition that only the temperature of the first temperature measuring point at the IMI-A is changed, we obtain a transmission spectrum chart as shown in FIG. 9, and select two typical peaks Peak A and Peak B for analysis. As shown in fig. 9, as the temperature gradually increases, both peaks shift in the long wave direction. Since the seawater temperature at IMI-B in this part of the experiment was always kept at 12.8 ℃, the change in the output spectrum was caused by the temperature change at the first temperature measurement point only. By tracking the two peaks A, B we have obtained a relationship of the peak shift amount of the two peaks with temperature change, as shown in FIG. 9, the slope is the corresponding sensitivity, which is 42.7 pm/. Degree.C.and 9.7 pm/. Degree.C.respectively.
Similarly, where only the temperature at the second temperature measurement point at IMI-B is changed, the present invention yields a spectral diagram as shown in fig. 10, and it can be seen that as the temperature increases gradually, the two peaks also move to the right. The output spectrum is shown in fig. 10, and since the temperature of the seawater at the first temperature measurement point of the sample in this part of the experiment is always kept at 14.3 ℃, the change in the output spectrum is caused only by the temperature change at IMI-B. Fig. 10 shows that both peaks move in the long wave direction with increasing temperature. By tracking the two peaks of A, B, the relationship between their peak shift and temperature change was obtained, and as shown in FIG. 10, the slopes were the corresponding sensitivities of 8.4 pm/. Degree.C and 39.2 pm/. Degree.C, respectively.
In summary, the two-point seawater temperature sensor based on cascade connection of the fine core fiber and the standard single mode fiber obtained in example 1 and example 2 can measure the temperature change of two points of seawater. Although sensitivity below 15 pm/. Degree.C is generally considered to be temperature insensitive, strictly speaking, the present invention employs a matrix to correct for temperature cross sensitivity between two points, namely the two typical interferometric peak A and peak B shift amounts Δλ A 、Δλ B And the temperature change amount delta T of two points A 、ΔT B There is a relationship between:
the obtained matrix is suitable for the situation that the temperature of two points is changed arbitrarily. In order to test the matrix, the invention adopts two samples of test1 (t 1) and test2 (t 2) for testing, and the test and calculation results of the samples t1 and t2 are shown in table 1. The peak positions were substituted into the above formula to calculate the test1 and test2 sample temperatures, the temperature of the samples in the experiment was measured by a thermometer, and the test sample data are shown in the following table. The results show that the change in temperature of the seawater measured by the sensor of the present invention substantially matches the change measured by the thermocouple, indicating that the measurement of the structure has good accuracy.
Table 1 test of samples test1, test2 using the sensor of the present invention
In the above example 2, since the seawater container has a length of 3.5cm, the heated IMI-B portion has a length of 3.5cm and the second interferometer IMI-B (SMF 2) has a total length of 6.45cm, and thus we made another similar structure of sensor II (left-end infinite SMF1-0.85cm tcf1-6.3cm SMF2-0.4cm tcf 2-right-end infinite SMF 3) for which experiments on the relation of the length of heated SMF2 and the sensing sensitivity were performed, the structure of the sensor II is shown in fig. 12 (a). The transmission spectrum is shown in 11 (a), and we choose different heating lengths as typical lengths, 1.5cm, 3cm, 3.5cm and 5cm respectively. An interference peak around 1548nm is selected as a typical peak (represented by the B' peak). Accordingly, the peak B' is plotted as a function of temperature in FIGS. 11 (B) -11 (e), and the peak wavelengths at different temperatures are plotted in FIGS. 11 (f) -11 (i), with sensitivities of 27.75 pm/. Degree.C, 38.29 pm/. Degree.C, 39.62 pm/. Degree.C, 47.5 pm/. Degree.C, in that order. To further analyze the dependence of sensitivity on length, FIG. 11 (j) shows the sensitivity as a function of heating length, which shows that the sensitivity increases approximately linearly with increasing heating length. This also corresponds to the following theoretical formula for temperature sensitivity:
from the above equation (3), we can get that the position of the second temperature measurement point has no influence on the sensitivity, as long as the second temperature measurement point is located within the length range of the SMF 2. To verify this we have selected three different locations for the second temperature measurement point (B in FIG. 12 (a) 1 、B 2 And B 3 ) As three typical locations. The second interferometer IMI-B was fixed to a length of 3.0cm in the seawater-immersed portion. The same procedure is followed, peak B being plotted in FIGS. 12 (B), 12 (c) and 12 (d) 1 、B 2 And B 3 The sensitivity corresponding to the three positions is 39.41 pm/. Degree.C, 38.2 pm/. Degree.C and 37.10 pm/. Degree.C respectively according to the condition of temperature change, which shows that the temperature measuring points are not related to the sensitivity, in other words, the distance between the two temperature measuring points can be between 1cm and 6.45cm or even between the two temperature measuring pointsThe temperature measurement sensitivity of two points can not be influenced in a larger range (generally not more than 20 cm), and the temperature measurement device has certain flexibility and innovation.
While the invention has been described in detail in the foregoing general description and specific examples, it will be apparent to those skilled in the art that modifications and improvements can be made thereto. Accordingly, such modifications or improvements may be made without departing from the spirit of the invention and are intended to be within the scope of the invention as claimed.
Claims (1)
1. The two-point seawater temperature sensor based on cascading of the fine core optical fiber and the standard single mode fiber is characterized by comprising a first single mode fiber, a first fine core single mode fiber and a second single mode fiber which are sequentially connected, wherein the front end of the first fine core single mode fiber and the rear end of the first single mode fiber are welded together in a staggered manner, and the rear end of the first fine core single mode fiber and the front end of the second single mode fiber are welded together in a non-staggered manner; the rear end of the second single-mode optical fiber is sequentially welded with a second fine core single-mode optical fiber and a third single-mode optical fiber without dislocation;
the first single mode fiber, the second single mode fiber and the third single mode fiber comprise fiber cores and cladding layers wrapping the fiber cores, the diameter of the fiber cores is 8.2 mu m, and the total diameter of the fiber cores and the cladding layers is 125 mu m;
the length of the first fine core single mode fiber is 0.85cm, the first fine core single mode fiber comprises a fiber core and a cladding layer wrapping the fiber core, the total diameter of the first fine core single mode fiber is 125 mu m, and the fiber core diameter is 3.6 mu m;
the length of the second single mode optical fiber is 6.45cm,
the length of the second fine core single mode optical fiber is 0.35cm; comprises a fiber core and a cladding layer wrapping the outer part of the fiber core, wherein the total diameter of the second fine core single mode fiber is 125 mu m, and the diameter of the fiber core is 4.4 mu m;
the outer part of a dislocation welding area between the front end of the first fine core single mode fiber and the rear end of the first single mode fiber is sleeved with a metal tube, and the metal tube is connected with the dislocation welding area through PDMS;
the connecting is carried out by PDMS, namely, a metal tube is sleeved at the dislocation welding area after a part of side wall is removed, and a PDMS main agent and a hardening agent are used in a mass ratio of 10:1, floating bubbles in the PDMS mixed solution to the surface by a standing mode, cracking, injecting the mixed solution into a metal tube, and then placing the metal tube on a heating table at 120 ℃ for curing for 15 minutes.
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