Fabry-Perot sensor cavity length demodulation system and Fabry-Perot sensor cavity length demodulation method

Abstract

A Fabry-Perot sensor cavity length demodulation system and a Fabry-Perot sensor cavity length demodulation method, the demodulation system comprising: a light source (1); a circulator (2) having a first port (21), a second port (22) and a third port (23), the circulator (2) being arranged to receive light emitted from the light source via the first port (21), and to transmit the light to a Fabry-Perot sensor (3) via the second port (22); the Fabry-Perot sensor (3) being arranged to receive light from the second port (22), so that the light is reflected at a first plane and a second plane of the Fabry-Perot sensor (3) respectively to generate multi-beam interference, and to return the interference light to the second port (22) of the circulator (2), and to transmit the interference light from the second port (22) to the Fabry-Perot sensor (3). The second port (22) transmits the interference light to the third port (23); a detector (4) is arranged to receive the interference light from the third port (23) to form a curve of the light intensity of the interference light relative to the cavity length of the Fabry-Perot sensor (3); a processor (5) is arranged to receive the curve and divide the curve into an integer wavelength part and a non-integer wavelength part. For the integer wavelength part, the processor (5) calculates the number of integer wavelengths contained in the integer wavelength part, and then calculates the first cavity length change. For the non-integer wavelength part, the processor (5) calculates the second cavity length change based on the function of the light intensity and the cavity length, and calculates the total cavity length change by adding the first cavity length change and the second cavity length change.

Description

Technical Field

[0001] The invention relates to a cavity length demodulation system and a cavity length demodulation method of a Fabry-Perot sensor. Background Art

[0002] Fiber Fabry-Perot (FP) sensors have the advantages of small size, high sensitivity, good stability, and immunity to electromagnetic interference. They are widely used in strain, temperature, pressure and other measurement fields. Fiber FP sensors sense the measured value by measuring the change of cavity length. The rate and accuracy of cavity length demodulation directly affect the rate and accuracy of measurement. Therefore, fast and accurate demodulation of the cavity length of fiber FP sensors is of great significance.

[0003] In the application of fiber optic sensing, the demodulation system is responsible for continuously sending optical signals to the fiber optic sensor and receiving the returned optical signals carrying the information to be measured. After photoelectric conversion, signal acquisition, and signal conditioning, the information we need is extracted. At present, the research on the demodulation system of Fabry-Perot cavity sensor is represented by FISO of Canada, Davidson of the United States, and Opsens of Canada. The first two companies use non-scanning correlation scanning technology, and Opsens uses white light polarization interference technology. Both technologies require well-made optical wedges and linear array CCDs, which are expensive. In addition, due to the need to collect spectral images, the demodulation rate cannot be improved, so it is not suitable for measuring rapidly changing signals, such as explosion pressure measurement. Intensity demodulation is the oldest and simplest method used by fiber optic Fabry-Perot sensors. It obtains the cavity length information of the fiber optic Fabry-Perot sensor by measuring the change in output light intensity. It has the characteristics of low cost and fast demodulation rate. In the experiment, a monochromatic light source is used, and the reflected light is directly received by a photodetector. As the cavity length changes, the output light intensity of the sensor changes accordingly. The relationship between the reflected light intensity and the cavity length is as follows:

[0004]

[0005] Among them I R is the reflected light intensity, and L is the cavity length.

[0006] With the cavity length as the horizontal coordinate and the reflected light intensity as the vertical coordinate, the reflected light intensity and the cavity length are in a sinusoidal relationship. Obviously, one output light intensity corresponds to multiple cavity lengths, that is, the output light intensity is a multi-valued function of the cavity length. Therefore, the cavity length cannot be directly calculated from the output light intensity. Summary of the invention

[0007] The above features and advantages and other features and advantages of the present teachings are readily apparent from the following detailed description of the best modes for carrying out the present teachings when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 FIG. 4 is a schematic diagram showing a cavity length demodulation system of a Fabry-Perot sensor according to a first embodiment of the present invention.

[0009] Figure 2 FIG. 1 shows a curve obtained by the cavity length demodulation system of the Fabry-Perot sensor according to the first embodiment of the present invention.

[0010] Figure 3 FIG. 4 is a schematic diagram showing a cavity length demodulation system of a Fabry-Perot sensor according to a second embodiment of the present invention.

[0011] Figure 4a and 4b The first curve and the second curve obtained by the cavity length demodulation system of the Fabry-Perot sensor according to the second embodiment of the present invention are respectively shown.

[0012] Figure 5 FIG. 1 shows a third curve obtained by the cavity length demodulation system of the Fabry-Perot sensor according to the second embodiment of the present invention.

[0013] In different drawings, the same elements are denoted by the same reference numerals. DETAILED DESCRIPTION

[0014] In order to make the purpose, technical solution and advantages of the technical solution of the present invention clearer, the technical solution of the embodiment of the present invention will be clearly and completely described in conjunction with the drawings of specific embodiments of the present invention. The same figure marks in the drawings represent the same parts. It should be noted that the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the described embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0015] Unless otherwise defined, the technical terms or scientific terms used herein shall have the usual meanings understood by persons of ordinary skill in the field to which the present invention belongs. The words "first", "second" and similar words used in the patent application specification and claims of the present invention do not indicate any order, quantity or importance, but are only used to distinguish different components. Similarly, words such as "one" or "one" do not necessarily indicate a quantity limitation. Words such as "include" or "comprise" mean that the elements or objects appearing before the word include the elements or objects listed after the word and their equivalents, without excluding other elements or objects. Words such as "connect" or "connected" are not limited to physical or mechanical connections, but may include electrical connections, whether direct or indirect. "Up", "down", "left", "right" and the like are only used to indicate relative positional relationships. When the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0016] Hereinafter, the cavity length demodulation system of the Fabry-Perot sensor of the present invention will be described in detail with reference to the accompanying drawings.

[0017] <First Embodiment>

[0018] Figure 1 FIG. 4 is a schematic diagram showing a cavity length demodulation system of a Fabry-Perot sensor according to a first embodiment of the present invention. Figure 2 FIG. 1 shows a curve obtained by the cavity length demodulation system of the Fabry-Perot sensor according to the first embodiment of the present invention.

[0019] like Figure 1 As shown, the cavity length demodulation system of the Fabry-Perot sensor includes a light source 1 (e.g., a semiconductor laser), the wavelength of the light emitted by the light source 1 is preferably in the range of 640nm to 660nm, more preferably 650nm. The light emitted by the light source 1 is transmitted to the first port 21 of the circulator 2, and the circulator also has a second port 22 and a third port 23. The light is transmitted from the first port 21 to the second port 22, and then transmitted to the Fabry-Perot sensor 3. The Fabry-Perot sensor has a cavity and a diaphragm. The light received by the Fabry-Perot sensor is reflected at the bottom of the cavity and the diaphragm, respectively. Therefore, the bottom of the cavity and the diaphragm are respectively called the first plane and the second plane. At the Fabry-Perot sensor, the light is reflected at the first plane and the second plane of the Fabry-Perot sensor, respectively, to cause multi-beam interference. The interference light returns to the second port of the circulator, and then is transmitted to the third port. The interference light emitted from the third port is transmitted to the detector 4 (e.g., a photodetector), which receives the interference light and forms a curve of the light intensity of the interference light relative to the cavity length of the Fabry-Perot sensor, as shown in FIG. Figure 2 The processor 5 receives the curve and divides the curve into an integer wavelength portion and a non-integer wavelength portion. For the integer wavelength portion, the processor calculates the number of integer wavelengths contained in the integer wavelength portion, and then calculates the first cavity length change. For the non-integer wavelength portion, the processor calculates the second cavity length change based on the function of light intensity and cavity length, and calculates the total cavity length change by adding the first cavity length change and the second cavity length change. For example, Figure 2 In the equation, the part from point A to point B is the whole wavelength part, recorded as n, then the first cavity length change is nλ / 2, the part from point B to point C is the non-whole wavelength part, the second cavity length change is calculated based on the function of light intensity and cavity length, and finally the first cavity length change and the second cavity length change are added to obtain the total cavity length change. The method of calculating the second cavity length change based on the function of light intensity and cavity length is well known to those skilled in the art, for example, the cavity length is the horizontal coordinate, the reflected light intensity is the vertical coordinate, the reflected light intensity and the cavity length are in a sinusoidal relationship, and the cavity length change can be obtained from the light intensity from the sinusoidal relationship, and at this time, one light intensity corresponds to one cavity length.

[0020] A polarizer 6 may be further arranged between the light source and the circulator to convert the light from the light source into linearly polarized light, which is transmitted to the circulator. In this case, the circulator is a polarization-maintaining circulator.

[0021] The light source in the present application emits light of 650nm wavelength. When the cavity length changes the same, the number of interference fringes formed by the short-wavelength light source is greater, so that the demodulation is more accurate. In addition, the present application adopts the principle of interference superposition of linearly polarized light, which avoids the incoherent superposition between non-polarized light, is conducive to improving the contrast of interference fringes, thereby improving the sensitivity of the optical fiber FP demodulation system. The Fabry-Perot sensor used in the present application preferably has a large cavity diameter and a thin diaphragm. According to the Fabry-Perot cavity sensitivity calculation formula (1), the Fabry-Perot cavity has a high sensitivity.

[0022]

[0023] Where S is the sensitivity, a is the diaphragm diameter, μ is the Poisson's ratio, E is the Young's modulus, and h is the diaphragm.

[0024] <Second Embodiment>

[0025] Figure 3 FIG. 4 is a schematic diagram showing a cavity length demodulation system of a Fabry-Perot sensor according to a second embodiment of the present invention. Figure 4a and 4b The first curve S1 and the second curve S2 obtained by the cavity length demodulation system of the Fabry-Perot sensor according to the second embodiment of the present invention are respectively shown. Figure 5 A third curve S3 obtained by the cavity length demodulation system of the Fabry-Perot sensor according to the second embodiment of the present invention is shown.

[0026] The difference between the second embodiment and the first embodiment is that a quarter wave plate 7 is arranged between the polarizer 6 and the polarization-maintaining circulator 2, a polarization beam splitter prism 8 is arranged downstream of the third port 23 of the polarization-maintaining circulator, and the detector includes a first detector 41 and a second detector 42. After the light emitted by the light source 1 passes through the polarizer, linear polarized light is formed. The direction of the polarizer and the fast and slow axes of the quarter wave plate are at an angle of 45 degrees, so that the linear polarized light will become circularly polarized light after passing through the quarter wave plate. The circularly polarized light enters the Fabry-Perot sensor through the polarization-maintaining circulator, and multi-beam interference occurs between the two parallel surfaces of the Fabry-Perot sensor. The interference light is transmitted to the polarization beam splitter prism 8 through the third port of the polarization-maintaining circulator, and the circularly polarized light is divided into two beams of linear polarized light, namely, the first beam L1 and the second beam L2 after passing through the polarization beam splitter prism. The phase difference between the first beam and the second beam is π / 2, and the first beam and the second beam are received by the first detector and the second detector respectively, forming a first curve and a second curve of light intensity relative to the cavity length, and the first curve of light intensity relative to the cavity length formed by the first beam is proportional to sin(wt+φ1), as shown in Figure 4aAs shown, the second curve of the light intensity formed by the second beam relative to the cavity length is proportional to cos(wt+φ1), as Figure 4b The first curve and the second curve are input to the processor 5, which performs a division operation to obtain a third curve proportional to tan(wt+φ1), as shown in FIG. Figure 5 The third curve is steeper than the first curve and the second curve, which is beneficial to improving the accuracy of demodulation.

[0027] Next, the processor divides the third curve into an integer wavelength portion and a non-integer wavelength portion. For the integer wavelength portion, the processor calculates the number of integer wavelengths contained in the integer wavelength portion (for example, Figure 5 For the non-integer wavelength portion (point A to point B and point C to point D), the processor calculates the second cavity length change based on the function of light intensity and cavity length, and calculates the total cavity length change by adding the first cavity length change and the second cavity length change.

[0028] The operation steps of the cavity length demodulation method of the Fabry-Perot sensor according to the present invention are as follows: guiding the light emitted by the light source to a circulator, the circulator having a first port, a second port and a third port, the light entering the circulator from the first port and being transmitted to the Fabry-Perot sensor via the second port; causing the light to be reflected at the first plane and the second plane of the Fabry-Perot sensor respectively to cause multi-beam interference to form interference light, the interference light returning to the second port of the circulator and being transmitted from the second port to the third port; using a detector to receive the interference light from the third port of the circulator to form a curve of the light intensity of the interference light relative to the cavity length of the Fabry-Perot sensor; using a processor to receive the curve and divide the curve into an integer wavelength part and a non-integer wavelength part, for the integer wavelength part, the processor calculates the number of integer wavelengths contained in the integer wavelength part, and then calculates the first cavity length change, for the non-integer wavelength part, the processor calculates the second cavity length change based on a function of light intensity and cavity length, and calculates the total cavity length change by adding the first cavity length change to the second cavity length change.

[0029] Additionally, the demodulation method further comprises the steps of arranging a polarizer between the light source and the circulator, arranging a quarter wave plate between the polarizer and the polarization-maintaining circulator, and arranging a polarization beam splitter prism downstream of the third port 23 of the polarization-maintaining circulator. In the case of arranging a polarizer, the circulator is a polarization-maintaining circulator. Furthermore, the detector is arranged as a first detector and a second detector to respectively receive the first light beam and the second light beam separated by the polarization beam splitter prism.

[0030] The present application divides the curve of light intensity relative to cavity length into an integer wave part and a non-integer wave part, calculates the number of integer wavelengths in the integer wave part, obtains the first cavity length change, and calculates the second cavity length change in the non-integer wave part. In this way, the demodulation speed is guaranteed and the demodulation accuracy is improved, thus realizing high-speed and accurate demodulation of the Fabry-Perot sensor.

[0031] Although the best modes for implementing many aspects of the present teachings have been described in detail, those skilled in the art will appreciate that, without departing from the concept of the present invention, various modifications and variations may be made to the above-mentioned specific embodiments, and various technical features and structures proposed in the present invention may be combined in various ways without exceeding the scope of protection of the present invention.

Claims

1. A Fabry-Perot sensor cavity length demodulation system, characterized in that: The demodulation system comprises: light source; a circulator having a first port, a second port and a third port, the circulator being arranged to receive light emitted from the light source via the first port and to transmit the light to the Fabry-Perot sensor via the second port; a Fabry-Perot sensor arranged to receive light from the second port, so that the light is reflected at the first plane and the second plane of the Fabry-Perot sensor respectively to cause multi-beam interference, and the interference light is returned to the second port of the circulator and transmitted from the second port to the third port; a detector arranged to receive the interference light from the third port and form a curve of the intensity of the interference light relative to the cavity length of the Fabry-Perot sensor; A processor is arranged to receive the curve and divide the curve into an integer wavelength part and a non-integer wavelength part. For the integer wavelength part, the processor calculates the number of integer wavelengths contained in the integer wavelength part, and then calculates a first cavity length change. For the non-integer wavelength part, the processor calculates a second cavity length change based on a function of light intensity and cavity length, and calculates a total cavity length change by adding the first cavity length change and the second cavity length change.

2. The demodulation system according to claim 1, characterized in that The demodulation system also includes: The polarizer is arranged to receive light from the light source, form linearly polarized light, and transmit the linearly polarized light to the circulator, which is a polarization-maintaining circulator.

3. The demodulation system according to claim 2, characterized in that The demodulation system also includes: A quarter wave plate is arranged to receive linearly polarized light from the polarizer, convert the linearly polarized light to circularly polarized light, and transmit the circularly polarized light to the polarization maintaining circulator.

4. The demodulation system according to claim 3, characterized in that: The demodulation system also includes: a polarization beam splitter prism arranged to receive the interference light from the third port and split the interference light into a first light beam and a second light beam, The detector includes a first detector and a second detector, which are respectively arranged to receive the first light beam and the second light beam, and form a first curve of the first light beam and a second curve of the second light beam. The processor receives the first curve and the second curve, and performs a division operation on the first curve and the second curve to obtain a third curve. The processor divides the third curve into an integer wavelength part and a non-integer wavelength part. For the integer wavelength part, the processor calculates the number of integer wavelengths contained in the integer wavelength part, and then calculates the first cavity length change. For the non-integer wavelength part, the processor calculates the second cavity length change based on a function of light intensity and cavity length, and calculates the total cavity length change by adding the first cavity length change and the second cavity length change.

5. The demodulation system according to claim 4, characterized in that The first curve is a sine curve, the second curve is a cosine curve, and the third curve is a tangent curve.

6. The demodulation system according to claim 1, characterized in that: The wavelength of the light emitted by the light source is in the range of 640nm to 660nm.

7. The demodulation system according to claim 6, characterized in that: The wavelength of the light emitted by the light source is 650nm.

8. A method for demodulating the cavity length of a Fabry-Perot sensor, characterized in that: The demodulation method comprises the following steps: guiding light emitted from a light source to a circulator, wherein the circulator has a first port, a second port, and a third port, wherein the light enters the circulator from the first port and is transmitted to the Fabry-Perot sensor via the second port; The light is reflected at the first plane and the second plane of the Fabry-Perot sensor respectively to generate multi-beam interference to form interference light, the interference light returns to the second port of the circulator, and is transmitted from the second port to the third port; Using a detector to receive interference light from the third port of the circulator, a curve of the intensity of the interference light relative to the cavity length of the Fabry-Perot sensor is formed; The curve is received by a processor and divided into an integer wavelength portion and a non-integer wavelength portion. For the integer wavelength portion, the processor calculates the number of integer wavelengths contained in the integer wavelength portion, and further calculates a first cavity length change. For the non-integer wavelength portion, the processor calculates a second cavity length change based on a function of light intensity and cavity length, and calculates a total cavity length change by adding the first cavity length change and the second cavity length change.

9. The demodulation method according to claim 8, characterized in that: The demodulation method further includes: arranging a polarizer between the light source and the circulator, the polarizer receiving light from the light source to form linearly polarized light, and transmitting the linearly polarized light to the circulator, wherein the circulator is a polarization-maintaining circulator.

10. The demodulation method according to claim 9, characterized in that: The demodulation method also includes arranging a quarter wave plate between the polarizer and the polarization-maintaining circulator, which receives linearly polarized light from the polarizer, is designed to convert the linearly polarized light into circularly polarized light, and transmits the circularly polarized light to the polarization-maintaining circulator.

11. The demodulation method according to claim 10, characterized in that: The demodulation method further includes arranging a polarization beam splitter prism downstream of the third port of the polarization-maintaining circulator, which receives interference light from the third port of the polarization-maintaining circulator and splits the interference light into a first light beam and a second light beam; The detector includes a first detector and a second detector, which respectively receive the first light beam and the second light beam and form a first curve of the first light beam and a second curve of the second light beam. wherein the processor is used to receive the first curve and the second curve, and to perform a division operation on the first curve and the second curve to obtain a third curve, The processor is used to divide the third curve into an integer wavelength part and a non-integer wavelength part. For the integer wavelength part, the processor calculates the number of integer wavelengths contained in the integer wavelength part, and then calculates the first cavity length change. For the non-integer wavelength part, the processor calculates the second cavity length change based on a function of light intensity and cavity length, and calculates the total cavity length change by adding the first cavity length change and the second cavity length change.

12. The demodulation method according to claim 11, characterized in that: The first curve is a sine curve, the second curve is a cosine curve, and the third curve is a tangent curve.

13. The demodulation method according to claim 8, characterized in that: The wavelength of the light emitted by the light source is in the range of 640nm to 660nm.

14. The demodulation method according to claim 13, characterized in that: The wavelength of the light emitted by the light source is 650nm.

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