CN107402054B - Optical fiber water level sensing device and method based on Mach-Zehnder interference - Google Patents

Abstract

The application relates to an optical fiber water level sensing device and method based on Mach-Zehnder interference, wherein the device comprises a laser, a first coupler, a first collimator, a second collimator, a sensitive structure, a second coupler, a photoelectric detector and an upper computer demodulation system; the output of the laser is connected with the input end of the first coupler; the output end of the first coupler is divided into two paths, one path is connected with the first collimator and the second collimator and then is connected with the input end of the second coupler to form a detection arm, the sensitive structure is used for fixing the first collimator and the second collimator and sensing water level change, and the other path is connected with the other input end of the second coupler to form a reference arm; the output end of the second coupler is sequentially connected with the photoelectric detector and the upper computer demodulation system, and Mach-Zehnder interference is formed by structures that the first coupler reaches the second coupler through the detection arm and the reference arm respectively. The application effectively improves the measuring precision and measuring range of the water level.

Description

An optical fiber water level sensing device and method based on Mach-Zehnder interference

technical field

The invention belongs to the technical field of optical fiber sensing, in particular to an optical fiber water level sensing device and method based on Mach-Zehnder interference.

Background technique

In some complex environments, real-time monitoring of water levels is essential. For example, groundwater resources are complex and unpredictable, and groundwater pollution and land subsidence caused by overexploitation of groundwater are slow-changing. Once accumulated to a certain extent, irreversible damage will be caused; the seabed is complicated. It will bring huge economic and property losses, and even threaten life safety, so it is very important to monitor water level changes in real time and effectively. In order to improve the measurement accuracy and realize the non-contact absolute measurement, the optical interference technology is adopted. In order to simplify the optical path structure and improve the stability of the optical path, combined with the optical fiber sensing technology, a fiber optic interference water level sensor is proposed.

There are many different types of water level sensors. In the published Chinese patent No. CN205049204U, a piezoresistive water level gauge is proposed. The water level gauge uses a current-type piezoresistive sensor. Through the action of a constant current source current, the output changes The voltage signal of the water level gauge is sent to the data acquisition device after amplification, analog-to-digital conversion, and signal processing. The disadvantage of the water level gauge is that the circuit structure is complex and is seriously affected by electromagnetic interference; An ultrasonic water level gauge is developed. The water level gauge includes a transmitting module, a receiving module, etc., and uses the principle of ultrasonic transmitting and receiving to measure the liquid level. However, the ultrasonic transmitting and receiving has relatively high environmental requirements and cannot be applied in complex and harsh water environments; In the published Chinese patent No. CN204461547U, an optical fiber interference water level sensor is proposed, using optical fiber sensing technology, constructing a Michelson interference structure, and realizing water level measurement through phase demodulation, but this structure winds the optical fiber on a silica gel column In general, when the water level changes, the optical fiber is stretched. When the optical fiber reaches the stretching limit, the rubber cylinder can also undergo elastic changes. Therefore, when the optical fiber is stretched a lot, it cannot accurately reflect the change of the water level.

To sum up, there is still no effective solution on how to solve the problems of low measurement accuracy and small range of the water level sensor in the prior art.

Contents of the invention

In order to solve the above problems, the present invention overcomes the problems of low measurement accuracy and small range of water level sensors in the prior art. The first purpose of the present invention is to provide a fiber optic water level sensor device based on Mach-Zehnder interference.

In order to achieve the above object, the present invention adopts the following technical scheme:

A fiber optic water level sensing device based on Mach-Zehnder interference, the device comprising:

Laser (1), first coupler (2), first collimator (3), second collimator (4), sensitive structure (5), second coupler (6), photodetector (7 ) and host computer demodulation system (8);

The output of the laser (1) is connected to the input of the first coupler (2);

The output end of the first coupler (2) is divided into two paths, one path is connected to the first collimator (3) and the second collimator (4) and then connected to the second coupler (6 ) to form the detection arm, the sensitive structure (5) is used to fix the first collimator (3) and the second collimator (4) and sense the water level change, the other way is connected to the The other input end of the second coupler (6) constitutes a reference arm;

The output end of the second coupler (6) is connected to the photodetector (7) and the host computer demodulation system (8) in sequence, and reaches from the first coupler (2) through the detection arm and the reference arm respectively. The structure of the second coupler (6) constitutes Mach-Zehnder interference.

Further, the sensitive structure (5) includes a frame (10), an external joint (9), a pressing block (11), two welded thin tubes (12), a connection fixing piece (13), a locking screw ( 14), hose (15), clamping piece (16), two transition welding pieces (17), the external joint (9) is fixed by the frame (10);

The compression block (11) compresses the water belt (15), two transition welding pieces (17) are installed on the compression block (11), and two welding thin tubes (12) are respectively connected to the The first collimator (3), the second collimator (4) and two transition welding pieces (17), one end of the connecting fixed piece (13) is fixed to the described On the frame (10), the other end is welded together with the pressing block (11), and the clamping piece (16) clamps the hose (15) to realize sealing.

Further, the external joint (9) is connected to the water belt (15), and when the water level changes, water enters the water belt (15) through the external joint (9).

Further, the pressing block (11) includes a first pressing block and a second pressing block, and the first pressing block and the second pressing block are respectively located on both sides of the hose.

Further, the gap between the first compaction block and the second compaction block is variable, and changes according to the water volume in the water belt (15).

Further, two transition soldering pieces (17) form a first transitional welding piece group, and the sensitive structure further includes a second transitional welding piece group, and the second transitional welding piece group is installed on the pressing block (11) The symmetrical side of the above-mentioned first transition pad group.

Further, two welded thin tubes (12) form a first welded thin tube group, and the sensitive structure also includes a second welded thin tube group, and the second welded thin tube group is respectively connected to the first collimator ( 3), the second collimator (4) and two transition pads in the second transition pad group.

Further, the clamping piece (16) includes a first clamping piece and a second clamping piece, the first clamping piece and the second clamping piece are respectively located on both sides of the hose, the Both the first clamping piece and the second clamping piece are connected with the frame (10).

Further, the laser (1) is a frequency-swept laser.

Further, the distance between the first coupler (2) to the first collimator (3) and the distance from the second collimator (4) to the second coupler (6) and are the distances from the first coupler (2) directly to the second coupler (6) through the optical fiber.

The working principle of the device is:

The external joint (9) of the sensitive structure (5) is directly in contact with water, and when the water level changes, water enters the water belt (15) through the external joint (9), and the water belt (15) ) becomes more water, the gap between the pressing blocks (11) increases, and the distance between the first collimator (3) and the second collimator (4) also increases, resulting in interference The length of the probe arm changes in the structure, which in turn causes the phase of the interference signal to change.

In the present invention, two collimator structures are adopted, and when the water level changes, the distance between the two collimators is changed without being limited by the stretching amount of the optical fiber, and the measurement accuracy and measurement range of the water level can be improved at the same time. It effectively avoids the problem that the traditional water level gauge winds the optical fiber on the variable cylinder. When the water level changes, the optical fiber is stretched. When the optical fiber reaches the stretching limit, the rubber cylinder can also undergo elastic changes, which affects the accuracy of water level measurement.

In the present invention, the outer joint of the sensitive structure is directly in contact with the water. When the water level changes, the water enters the hose through the outer joint, and the water in the hose becomes more, which increases the gap between the pressing blocks. The two collimators The distance between them also increases, resulting in a change in the length of the detection arm in the interferometric structure, which in turn causes a change in the phase of the interferometric signal. The optical fiber water level measurement based on Mach-Zehnder interference is realized, and the accuracy of water level measurement is improved.

The second object of the present invention is to provide an optical fiber water level sensing method based on Mach-Zehnder interference.

In order to achieve the above object, the present invention adopts the following technical scheme:

An optical fiber water level sensing method based on Mach-Zender interference, the method is based on the above-mentioned optical fiber water level sensing device based on Mach-Zender interference, the method comprises the following steps:

(1) Obtain the initial frequency, scanning step frequency and working time of the laser (1), calculate the scanning frequency, and calculate the output fields of the first coupler (2) outputting two optical signals through the detection arm and the reference arm respectively powerful;

(2) Calculate the output light intensity of the detection arm and the output light intensity of the reference arm according to the output field strength of the detection arm and the output field strength of the reference arm, and calculate the detection arm according to the output light intensity of the detection arm and the output light intensity of the reference arm and the interference light intensity of the reference arm, thereby calculating the phase difference of the two optical signals output by the first coupler (2) after passing through the detection arm and the reference arm; the phase difference is differentiated to time, and the first collimation is calculated The distance between the device (3) and the second collimator (4);

(3) according to the distance between the first collimator (3) and the second collimator (4), combine the pressure formula of the water level and the pressure of the water level with the first collimator (3) and the second collimator ( 4) The relationship between the distance between them, and finally calculate the current water level value.

Further, the output field strength _E of the detection arm in the step (1) is:

Among them, E ₀ is a constant, f ₀ is the initial frequency of the laser (1), k is the scanning step frequency of the laser (1), t is the sweep time of the laser (1), n is the refractive index, and C is the speed of light , is the initial phase of the output optical signal of the first coupler (2), L ₁ is the distance between the first coupler and the first collimator, L ₂ is the distance from the second collimator to the second coupler, ΔL is The distance between the two collimators, ΔL is the variable to be calculated.

Further, the output field strength _E of the reference arm in the step (1) is:

Among them, E ₀ is a constant, f ₀ is the initial frequency of the laser (1), k is the scanning step frequency of the laser (1), t is the sweep time of the laser (1), n is the refractive index, and C is the speed of light , is the initial phase of the optical signal output by the first coupler (2), and _L3 is the distance from the first coupler (2) directly to the second coupler (6) through the optical fiber.

Further, in the step (2), the specific steps for calculating the phase difference of the two optical signals output by the first coupler (2) after passing through the detection arm and the reference arm are as follows:

The interference light intensity I of the detection arm and the reference arm is:

Among them, I ₁ is the output light intensity of the detection arm, I ₂ is the output light intensity of the reference arm, Outputting the phase difference between the two optical signals passing through the detection arm and the reference arm for the first coupler (2);

According to the interference light intensity I of the detection arm and the reference arm, the AC term signal I′ of the interference light intensity I of the detection arm and the reference arm is obtained as:

Hilbert transform is performed on the AC term signal I′ of the interference light intensity I of the discretized detection arm and the reference arm, and the phase difference between the two optical signals output by the first coupler (2) after passing through the detection arm and the reference arm is obtained. for:

Further, the distance between the first collimator (3) and the second collimator (4) in the step (2) is ΔL:

in, The first coupler (2) outputs the phase difference of the two optical signals after passing through the detection arm and the reference arm /> Differentiation with respect to sweep time t.

Further, in the step (2), the first coupler (2) outputs the phase difference of the two optical signals after passing through the detection arm and the reference arm Differentiation with respect to sweep time t /> The calculation method is:

According to the phase difference of the two optical signals output by the first coupler (2) after passing through the detection arm and the reference arm Do phase unwrapping and calculate the phase difference /> The relationship with the sweep time t;

phase difference Do curve fitting with the frequency sweep time t, and get the slope of the fitting curve according to the fitted curve, which is />

Further, the specific steps in the step (3) are:

The formula for water level pressure is:

P = ρgh

Among them, ρ is the density of water, g is the acceleration of gravity, and h is the current water level;

The relationship between the pressure of the water level and the distance between the first collimator (3) and the second collimator (4) is:

P＝C ₁ ×ΔL

Among them, C ₁ is a constant determined by the mechanical structure;

Combining the pressure formula of the water level and the relationship between the pressure of the water level and the distance between the first collimator (3) and the second collimator (4), the current water level value h is finally calculated as:

Further, the measurement range of the current water level value h is determined by the phase difference The slope of the fitting curve after curve fitting with the sweep time t and scan step frequency k to determine:

The maximum amount of change per unit time is 2π, then the maximum distance ΔL between the first collimator (3) and the second collimator (4) is /> The minimum amount of change per unit time is 2π/N, where N is the number of scanning steps, then the minimum distance ΔL between the first collimator (3) and the second collimator (4) is /> Therefore the measurement range of the water level h is />

Theoretically, the number of frequency steps N can be infinitely large, and the value of the scanning step frequency k of the laser (1) can be infinitely small. Therefore, the present invention can further increase the measurement range of the water level.

Further, the measurement accuracy of the current water level value h Determined by the number of scanning steps N of the laser (1):

Beneficial effects of the present invention:

(1) A fiber optic water level sensing device based on Mach-Zehnder interference of the present invention adopts two collimator structures, and when the water level changes, the distance between the two collimators is changed without being limited by the stretching amount of the optical fiber , can simultaneously improve the measurement accuracy and measurement range of the water level. The invention effectively avoids the problem that the traditional water level gauge winds the optical fiber on the variable cylinder. When the water level changes, the optical fiber is stretched. However, when the optical fiber reaches the stretching limit, the rubber cylinder can also undergo elastic changes, which affects the accuracy of water level measurement. .

(2) A kind of optical fiber water level sensing device based on Mach-Zehnder interference of the present invention, the external joint of sensitive structure directly contacts with water, when water level changes, water enters water belt through external joint, and the water in the water belt changes If there are too many, the gap between the pressing blocks will increase, and the distance between the two collimators will also increase, resulting in a change in the length of the detection arm in the interference structure, which in turn leads to a change in the phase of the interference signal. The optical fiber water level measurement based on Mach-Zehnder interference is realized, and the accuracy of water level measurement is improved.

(3) A kind of optical fiber water level sensing method based on Mach-Zender interference of the present invention, according to a kind of measuring range of the current water level value calculated by the optical fiber water level sensing device based on Mach-Zender interference by phase fitting slope after and scan step frequency k decision; /> The maximum change in unit time is 2π, then the maximum value of ΔL is /> The minimum change in unit time is 2π/N, then the minimum value of ΔL is /> Theoretically, the value of the frequency step frequency N can be infinite, and the value of the scanning step frequency k can be infinitely small, and the present invention can further increase the measurement range of the water level.

Description of drawings

Fig. 1 is the structural diagram of a kind of optical fiber water level sensing device based on Mach-Zehnder interference of the present invention;

Fig. 2 is a system block diagram of an optical fiber water level sensing device based on Mach-Zender interference of the present invention;

Fig. 3 is a structural diagram of a sensitive structure in an optical fiber water level sensing device based on Mach-Zehnder interference of the present invention;

Fig. 4 is a sectional view of a sensitive structure in an optical fiber water level sensing device based on Mach-Zehnder interference of the present invention;

Fig. 5 is a flow chart of an optical fiber water level sensing method based on Mach-Zehnder interference of the present invention.

Among them, 1. Laser, 2. First coupler, 3. First collimator, 4. Second collimator, 5. Sensitive structure, 6. Second coupler, 7. Photodetector, 8. Upper Machine demodulation system, 9. Outer joint, 10. Frame, 11. Compression block, 12. Welded thin tube, 13. Connection fixing piece, 14. Locking screw, 15. Water belt, 16. Clamping piece, 17. Transition solder tabs.

Detailed ways:

It should be pointed out that the following detailed description is exemplary and intended to provide further explanation to the present application. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs.

It should be noted that the terminology used here is only for describing specific implementations, and is not intended to limit the exemplary implementations according to the present application. As used herein, unless the context clearly dictates otherwise, the singular is intended to include the plural, and it should also be understood that when the terms "comprising" and/or "comprising" are used in this specification, they mean There are features, steps, operations, means, components and/or combinations thereof.

In the case of no conflict, the embodiments in the present application and the features in the embodiments can be combined with each other. The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

Example 1:

As introduced in the background technology, in order to solve the above problems, the present invention overcomes the problems of low measurement accuracy and small range of water level sensors in the prior art. The first purpose of the present invention is to provide an optical fiber based on Mach-Zehnder interference Water level sensing device.

In order to achieve the above object, the present invention adopts the following technical scheme:

As shown in Figure 1,

A fiber optic water level sensing device based on Mach-Zehnder interference, the device comprising:

Laser (1), first coupler (2), first collimator (3), second collimator (4), sensitive structure (5), second coupler (6), photodetector (7 ), PC demodulation system (8).

Among them, the sensitive structure (5) is used to fix the two collimators, as shown in Figure 3 and Figure 4, including the external joint (9), the frame (10), the pressing block (11), two welded thin tubes (12), connecting the fixed piece (13), the locking screw (14), the hose (15), the clamping piece (16), and two transition welding pieces (17). The outer joint (9) is connected with other parts through the frame (10), the compression block (11) compresses the hose (15), and the two welding thin tubes (12) are respectively connected to two collimators and two transition welding piece (17), one end connected to the fixed piece (13) is fixed by a locking screw (14), the other end is welded together with the pressing block (11), and the clamping piece (16) clamps the hose (15) to realize For sealing, the transition welding piece (17) is installed on the compression block (11).

The external joint (9) is connected to the water belt (15), and when the water level changes, water enters the water belt (15) through the external joint (9).

The pressing block (11) includes a first pressing block and a second pressing block, and the first pressing block and the second pressing block are respectively located on two sides of the hose.

The gap between the first compacting block and the second compacting block is variable and varies according to the water volume in the hose (15).

Two transition soldering pieces (17) form a first transitional welding piece group, and the sensitive structure also includes a second transitional welding piece group, and the second transitional welding piece group is installed on the pressing block (11). The symmetrical side of the first transition pad set.

Two welded thin tubes (12) form a first welded thin tube group, and the sensitive structure also includes a second welded thin tube group, and the second welded thin tube group is respectively connected to the first collimator (3), The second collimator (4) and the two transition pads in the second transition pad set.

The clamping piece (16) includes a first clamping piece and a second clamping piece, the first clamping piece and the second clamping piece are respectively located on both sides of the hose, and the first clamping piece Both the tight piece and the second clamping piece are connected with the frame (10).

The outer joint (9) of the sensitive structure (5) is directly in contact with the water, and when the water level changes, the water enters the hose (15) through the outer joint (9), and the water in the hose (15) becomes more, causing the pressing block to As the gap of (11) increases, the distance between the two collimators also increases, resulting in a change in the length of the detection arm in the interference structure, which in turn leads to a change in the phase of the interference signal.

As shown in Figures 1 and 2, the output of the laser (1) is connected to the input of the first coupler (2), and the output of the first coupler (2) is divided into two paths, one of which is connected to the first collimator (3) and the second collimator (4) are connected to the input end of the second coupler (6) to form a detection arm. The sensitive structure (5) is used to fix the two collimators and can feel the water level change. The other input end of the second coupler (6) is directly connected to form a reference arm, and the output end of the second coupler (6) is connected to the photodetector (7), and the first coupler reaches through the detection arm and the reference arm respectively. The structure of the second coupler constitutes Mach-Zehnder interference, and the photodetector (7) is connected to the host computer demodulation system (8).

Example 2:

The second object of the present invention is to provide an optical fiber water level sensing method based on Mach-Zehnder interference.

In order to achieve the above object, the present invention adopts the following technical scheme:

As shown in Figure 5,

An optical fiber water level sensing method based on Mach-Zender interference, the method is based on the above-mentioned optical fiber water level sensing device based on Mach-Zender interference, the method comprises the following steps:

(1) Obtain the initial frequency, scanning step frequency and working time of the laser (1), calculate the scanning frequency, and calculate the output fields of the first coupler (2) outputting two optical signals through the detection arm and the reference arm respectively powerful;

(2) Calculate the output light intensity of the detection arm and the output light intensity of the reference arm according to the output field strength of the detection arm and the output field strength of the reference arm, and calculate the detection arm according to the output light intensity of the detection arm and the output light intensity of the reference arm and the interference light intensity of the reference arm, thereby calculating the phase difference of the two optical signals output by the first coupler (2) after passing through the detection arm and the reference arm; the phase difference is differentiated to time, and the first collimation is calculated The distance between the device (3) and the second collimator (4);

(3) according to the distance between the first collimator (3) and the second collimator (4), combine the pressure formula of the water level and the pressure of the water level with the first collimator (3) and the second collimator ( 4) The relationship between the distance between them, and finally calculate the current water level value.

In this embodiment, the specific method steps are:

The laser (1) in an optical fiber water level sensing device based on Mach-Zehnder interference adopts a frequency-sweeping laser, the starting frequency of the laser (1) is denoted as f ₀ , the scanning frequency of the laser (1) is denoted as f, and :

f＝f ₀ +kt

Where k is the scanning step frequency of the laser (1), and t is the frequency scanning time of the laser (1).

In the present embodiment, the starting frequency f of the laser (1) is _191200GHz , the scanning step frequency k of the laser (1) is 1GHz, and the scanning frequency f of the laser (1) is:

f=191200GHz+1GHz×t

The distance between the first coupler (2) and the first collimator (3) is L ₁ , the distance between the two collimators is the distance between the first collimator (3) and the second collimator (4) The distance is ΔL, the distance from the second collimator (4) to the second coupler (6) is L ₂ , and the distance from the first coupler (2) directly to the second coupler (6) through the optical fiber is L ₃ , and L ₁ +L ₂ =L ₃ .

The first coupler (2) outputs two optical signals, which pass through the detection arm and the reference arm respectively, and their field strength expressions are respectively:

where _E1 is the output field strength of the detection arm, _E2 is the output field strength of the reference arm, _E0 is a constant, n is the refractive index, C is the speed of light, The initial phase of the output optical signal for the first coupler (2).

In the present embodiment, the starting frequency _f0 of the laser (1) is 191200GHz, and the scanning step frequency k of the laser (1) is 1GHz and substituted into the output field strength _E1 of the detection arm and the output field strength _E2 of the reference arm The formula for calculating:

The interference light intensity of the detection arm and the reference arm is:

Where I ₁ is the output light intensity of the detection arm, I ₂ is the output light intensity of the reference arm, The first coupler (2) outputs the phase difference of the two optical signals after passing through the detection arm and the reference arm.

The AC term signal of interference light intensity is:

Hilbert transform is performed on the AC term signal I′ of the interference light intensity I of the discretized detection arm and the reference arm, and the phase difference between the two optical signals output by the first coupler (2) after passing through the detection arm and the reference arm is obtained. for:

In the present embodiment, the starting frequency _f of the laser (1) is 191200GHz, and the scanning step frequency k of the laser (1) is 1GHz and is substituted into the above formula:

The first coupler (2) outputs the phase difference of the two optical signals after passing through the detection arm and the reference arm As the sweep time t changes, we can differentiate the sweep time t to get:

In this example,

So the distance between the two collimators is:

In this example,

in, The calculation method is as follows:

First, do a Hilbert transform on the discretized I′, and get the phase value Since the phase jumps at π, in order to obtain the real phase change, do phase unwrapping and calculate the relationship between phase and sweep time, and then do curve fitting on the phase value and sweep time to get the slope of the fitting curve , which is />

The formula for water level pressure is:

P = ρgh

Where ρ is the density of water, g is the acceleration of gravity, and h is the current water level.

The relationship between the pressure of the water level and the distance ΔL between the two collimators is:

P＝C ₁ ×ΔL

Among them, C ₁ is a constant determined by the mechanical structure.

Then the current water level value h is:

The measurement range of water level is determined by the slope after phase fitting and scan step frequency k to determine:

The maximum change in unit time is 2π, then the maximum value of ΔL is /> The minimum amount of change per unit time is 2π/N, where N=5000 is the number of frequency steps, then the minimum value of ΔL is /> Therefore the measurement range of the water level h is />

Water level measurement accuracy Determined by the number of scan steps:

The above descriptions are only preferred embodiments of the present application, and are not intended to limit the present application. For those skilled in the art, there may be various modifications and changes in the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of this application shall be included within the protection scope of this application.

Claims (8)

1. An optical fiber water level sensing device based on Mach-Zehnder interference is characterized in that: the device comprises:

the system comprises a laser, a first coupler, a first collimator, a second collimator, a sensitive structure, a second coupler, a photoelectric detector and an upper computer demodulation system;

the output of the laser is connected with the input end of the first coupler;

the output end of the first coupler is divided into two paths, one path is connected with the first collimator and the second collimator and then is connected with the input end of the second coupler to form a detection arm, the sensitive structure is used for fixing the first collimator and the second collimator and sensing water level change, and the other path is connected with the other input end of the second coupler to form a reference arm;

the output end of the second coupler is sequentially connected with the photoelectric detector and the upper computer demodulation system, and Mach-Zehnder interference is formed by structures that the first coupler reaches the second coupler through the detection arm and the reference arm respectively;

the sensitive structure comprises a frame, an outer joint, a compression block, a welding tubule, a connecting fixing piece, a locking screw, a water belt, a clamping piece and a transition welding piece, wherein the outer joint is fixed through the frame;

the water hose is compressed by the compressing block, two transition welding sheets are arranged on the compressing block, two welding tubules are respectively connected with the first collimator, the second collimator and the two transition welding sheets, one end of the connecting fixing sheet is fixed through a locking screw, the other end of the connecting fixing sheet is welded with the compressing block, and the clamping sheet clamps the water hose to realize sealing;

the laser adopts a sweep frequency laser;

the sum of the distance from the first coupler to the first collimator and the distance from the second collimator to the second coupler is the distance from the first coupler to the second coupler through the optical fiber.

2. A method for sensing optical fiber water level based on mach-zehnder interference, which is based on the optical fiber water level sensing device based on mach-zehnder interference as set forth in claim 1, comprising the steps of:

(1) Acquiring the initial frequency, the scanning stepping frequency and the working time of a laser, calculating the scanning frequency, and respectively calculating the output field intensity of the first coupler for outputting two paths of optical signals through a detection arm and a reference arm;

(2) Calculating the output light intensity of the detection arm and the output light intensity of the reference arm respectively according to the output field intensity of the detection arm and the output field intensity of the reference arm, and calculating the interference light intensity of the detection arm and the reference arm according to the output light intensity of the detection arm and the output light intensity of the reference arm, thereby calculating the phase difference of the two paths of light signals output by the first coupler after passing through the detection arm and the reference arm; differentiating the phase difference with respect to time, and calculating to obtain the distance between the first collimator and the second collimator;

(3) And according to the distance between the first collimator and the second collimator, combining a pressure formula of the water level and the relation between the pressure of the water level and the distance between the first collimator and the second collimator, and finally calculating the current water level value.

3. The optical fiber water level sensing method based on Mach-Zehnder interference as claimed in claim 2, wherein: the output field intensity E1 of the detection arm in the step (1) is as follows:

wherein ,E₀ Is constant, f ₀ The initial frequency of the laser, k is the scanning stepping frequency of the laser, and t isThe sweep time of the laser, n is the refractive index, C is the speed of light,for outputting the initial phase of the optical signal, L ₁ L is the distance between the first coupler and the first collimator ₂ For the distance from the second collimator to the second coupler, Δl is the distance between the two collimators, Δl is the variable to be calculated;

the output field intensity E of the reference arm in the step (1) ₂ The method comprises the following steps:

wherein ,E₀ Is constant, f ₀ K is the initial frequency of the laser, t is the scanning stepping frequency of the laser, t is the scanning time of the laser, n is the refractive index, C is the light speed,for outputting the initial phase of the optical signal, L ₃ Is the distance from the first coupler through the fiber directly to the second coupler.

4. A mach-zehnder interferometer based optical fiber water level sensing method as defined in claim 3, wherein: the specific steps of calculating the phase difference of the two paths of optical signals output by the first coupler after passing through the detection arm and the reference arm in the step (2) are as follows:

the interference light intensity I of the detection arm and the reference arm is as follows:

wherein ,I₁ To detect the output light intensity of the arm I ₂ For the output light intensity of the reference arm,is a first couplerOutputting a phase difference of the two paths of optical signals after passing through the detection arm and the reference arm;

the alternating current term signal I' of the interference light intensity I of the detection arm and the reference arm is obtained according to the interference light intensity I of the detection arm and the reference arm, and is as follows:

performing Hilbert transformation on an alternating-current item signal I' of interference light intensity I of a discretized detection arm and an interference light intensity I of a discretized reference arm to obtain a phase difference of two paths of optical signals output by a first coupler after passing through the detection arm and the reference armThe method comprises the following steps:

5. the optical fiber water level sensing method based on Mach-Zehnder interference as defined in claim 4, wherein: the distance between the first collimator and the second collimator in the step (2) is Δl:

wherein ,the phase difference of the two optical signals after passing through the detecting arm and the reference arm is outputted for the first coupler>Differentiation of the sweep time t.

6. The optical fiber water level sensing method based on Mach-Zehnder interference as defined in claim 5, wherein: the first coupler in the step (2)Phase difference of two paths of optical signals after passing through the detection arm and the reference arm is outputDifferentiation of the sweep time t>The calculation method of (1) is as follows:

according to the phase difference of the two paths of optical signals after the first coupler outputs the two paths of optical signals to pass through the detection arm and the reference armPhase unwrapping is performed and the phase difference is calculated>Relationship with sweep time t;

for phase differenceFitting a curve with the sweep frequency time t, and obtaining the slope of the fitted curve according to the fitted curve, namely

7. The optical fiber water level sensing method based on Mach-Zehnder interference as defined in claim 6, wherein: the specific steps in the step (3) are as follows:

the pressure formula of the water level is:

P＝ρgh

wherein ρ is the density of water, g is the gravitational acceleration, h is the current water level value;

the relationship between the pressure of the water level and the distance between the first collimator and the second collimator is:

P＝C ₁ ×ΔL

wherein ,C₁ Constant determined for the mechanical structure;

combining a pressure formula of the water level and the relation between the pressure of the water level and the distance between the first collimator and the second collimator, and finally calculating the current water level value h as follows:

8. the optical fiber water level sensing method based on Mach-Zehnder interference as claimed in claim 7, wherein: the current water level h is measured in a range defined by the phase differenceFitting curve slope +.after curve fitting with sweep time t>And a step frequency k of scanning to determine:

the maximum change amount in unit time is 2 pi, and the distance DeltaLmax between the first collimator and the second collimator isThe minimum change amount in unit time is 2 pi/N, wherein N is the scanning step number, and the minimum value of the distance delta L between the first collimator and the second collimator is +.>

The measuring range of the water level h is

Accuracy of measurement of the current water level value hThe number of scanning steps N of the laser is determined by: