Linear structure closed-loop polarization optical fiber temperature sensor

Abstract

The present invention discloses a closed-loop polarization-protected optical fiber temperature sensor with a linear structure, belonging to the field of optical fiber temperature sensors. The sensor comprises: transmitting two orthogonal polarized lights through the fast and slow axes, modulating the two orthogonal polarized lights with a preset modulation signal, rotating the two orthogonal polarized lights by 45 degrees, and transmitting the two orthogonal polarized lights, spliced ​​together through a 45-degree optical fiber connector, at different speeds through the optical fiber fast and slow axes. The difference in propagation constants of the two orthogonal polarized lights within the PM sensing fiber is proportional to the temperature. The two orthogonal polarized lights are reflected and returned along their original paths, and the orthogonal polarized lights output by the fast and slow axes are exchanged to generate an output optical signal, which is then demodulated and calculated to obtain the current temperature. Thus, the sensor utilizes a reciprocal structure to suppress noise and interference in the optical path, overcoming the defect that the system's cosine response is insensitive to small temperature changes. Accurate measurements can be provided in microwave, radio frequency, and electromagnetic interference environments.

Description

Technical Field

[0001] The present invention relates to the technical field of optical fiber temperature sensing, and in particular to a closed-loop polarization optical fiber temperature sensor with a linear structure. Background Art

[0002] Fiber optic temperature sensors (FOTS) offer advantages such as passivity, fast response, high sensitivity, and resistance to electromagnetic interference, making them ideal solutions for harsh environments such as microwave, radio frequency, and strong electromagnetic interference. Remarkably, the optical properties (phase, polarization, wavelength, etc.) of functional FOTS change with temperature, while the optical fiber itself, as a sensing medium, offers a wider measurement range, longer transmission distance, and higher resolution. Typical examples include fiber Bragg grating temperature sensors and Raman / Brillouin scattering distribution FOTS. Polarization-maintaining (PM) fiber temperature sensors are a novel functional FOTS solution. Interference between the PM fiber core and cladding modes is a common method, which can be achieved through special processing methods such as dislocation splicing, tapered stretching, and embedding fiber Bragg gratings, or by inserting the PM fiber into a Sagnac ring or Mach-Zehnder interferometer.

[0003] Compared with the transmission-type FOTS, the functional FOTS has the characteristics of high sensitivity and multi-parameter measurement, but it generally has the problems of complex structure and high cost. Its sensitivity and resolution are not enough to meet the needs of high-precision temperature measurement. How to distinguish between temperature and strain and effectively resist vibration interference is also a difficult problem to solve.

[0004] Compared to the common approaches mentioned above, polarization interferometry is a simple and effective temperature sensing method. It can be achieved by measuring the phase difference between two orthogonal polarization modes in PM fiber. However, this method is dependent on optical power, and variations in light intensity caused by optical path loss or light source power variations can affect accuracy. Summary of the Invention

[0005] The present invention provides a closed-circuit polarization fiber temperature sensor (CPMF-TS) with a linear structure. The CPMF-TS can suppress noise and interference in the optical path, overcome the defect that the system cosine response is insensitive to small temperature changes, improve linearity and measurement range, and has the advantages of high resolution and high sensitivity, so that it can provide accurate measurements in microwave, radio frequency and electromagnetic interference environments.

[0006] A first embodiment of the present invention provides a linear structured closed-loop polarization optical fiber temperature sensor, comprising:

[0007] Fiber laser light source, used to emit laser;

[0008] a circulator, wherein a first end of the circulator is connected to the fiber laser light source and is used for directionally transmitting the laser;

[0009] an optical fiber polarizer, a first end of which is connected to the second end of the circulator, and is used to convert the laser light into polarized light;

[0010] a first 45° optical fiber connecting line, configured to split the polarized light into a first orthogonal polarized light and a second orthogonal polarized light, so that the first orthogonal polarized light and the second orthogonal polarized light are transmitted along a fast axis and a slow axis, respectively;

[0011] a phase modulator, wherein a first end of the phase modulator is connected to a second end of the optical fiber polarizer via the 45° optical fiber connecting line, and is configured to modulate the first orthogonal polarized light and the second orthogonal polarized light using a preset modulation signal to superimpose a 90-degree non-reciprocal phase shift in the optical path;

[0012] a Faraday rotator, wherein a first end of the Faraday rotator is connected to the second end of the phase modulator, and is configured to rotate the first orthogonal polarized light and the second orthogonal polarized light by 45 degrees;

[0013] an optical fiber delay line, wherein a first end of the optical fiber delay line is connected to a second end of the Faraday rotator;

[0014] a PM sensing fiber, wherein the PM sensing fiber is connected to the optical fiber delay line via a second 45° optical fiber connector, and is configured to transmit the first orthogonal polarized light and the second orthogonal polarized light spliced ​​by the second 45° optical fiber connector at different speeds through the optical fiber fast axis and slow axis, wherein the difference in propagation constants between the first orthogonal polarized light and the second orthogonal polarized light in the PM sensing fiber is proportional to the temperature;

[0015] An SM fiber reflector is configured to reflect the first orthogonal polarized light and the second orthogonal polarized light, and reverse the direction through the PM sensing fiber so that the first orthogonal polarized light and the second orthogonal polarized light are rotated 45° after passing through the Faraday rotator again. The orthogonal polarized light outputted from the fast axis and the slow axis is exchanged, and then outputted through the fast axis and the slow axis before reaching the phase modulation for remodulation. After passing through the first 45° optical fiber connecting line and the optical fiber polarizer, an output optical signal is generated and reaches the circulator.

[0016] A processing module is connected to the third end of the circulator and is used to demodulate the output optical signal to obtain an output signal, and calculate the output signal to obtain the current temperature.

[0017] Optionally, in one embodiment of the present invention, the fiber laser light source is a superluminescent laser diode.

[0018] Optionally, in one embodiment of the present invention, the delay time of the optical fiber delay line is τ / 2, where τ is the time required for a signal to pass through the optical fiber delay line.

[0019] Optionally, in one embodiment of the present invention, the relationship between the propagation constant difference between the first orthogonal polarized light and the second orthogonal polarized light in the PM sensing fiber and the temperature is:

[0020] δ x -δ y =Δβ0L0(1+C(T-T0))

[0021] Among them, δ x and δ y are the phase delays of the fast and slow axes of the sensing fiber, Δβ0 is the propagation constant difference at temperature T0, L0 is the length of the sensing fiber, C is the temperature coefficient, and T is the temperature.

[0022] Optionally, in one embodiment of the present invention, the preset modulation signal is the sum of a square wave modulation phase shift and a step wave feedback phase shift.

[0023] Optionally, in one embodiment of the present invention, the method further comprises: a digital accumulator, wherein the digital accumulator is used to calculate the feedback phase shift of the step wave.

[0024] Optionally, in one embodiment of the present invention, the processing module includes:

[0025] a photodetector connected to the third terminal of the circulator;

[0026] an analog-to-digital converter, wherein a first terminal of the analog-to-digital converter is connected to the photodetector;

[0027] a filter connected to the second terminal of the analog-to-digital converter;

[0028] a demodulator, wherein a first terminal of the demodulator is connected to the second terminal of the filter, and a second terminal of the demodulator is connected to the first terminal of the digital accumulator;

[0029] An output port is connected to the second end of the digital accumulator.

[0030] The linear, closed-loop polarization-protected fiber optic temperature sensor of the present invention utilizes a reciprocal structure to suppress noise and interference in the optical path. To overcome the system's insensitivity to small temperature changes in its cosine response, a square-wave modulated signal is superimposed on the optical path via a phase modulator, resulting in a 90-degree non-reciprocal phase shift. This adjusts the system's operating point to the most sensitive region of the temperature response. To improve linearity and measurement range, a closed-loop demodulation scheme is employed, using a step wave to generate a compensating phase shift to offset the nonreciprocal phase shift caused by temperature changes. Furthermore, the CPMF-TS exhibits excellent linearity and high processing tolerances, accommodating sensing fibers of varying lengths. Consequently, the CPMF-TS's high resolution and sensitivity enable it to provide precise measurements in microwave, radio frequency, and electromagnetic interference environments.

[0031] Additional aspects and advantages of the present invention will be set forth in part in the description which follows and, in part, will be obvious from the description which follows, or may be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the following description of the embodiments with reference to the accompanying drawings, in which:

[0033] Figure 1 The figure is a schematic structural diagram of a linear closed-loop polarization optical fiber temperature sensor provided according to an embodiment of the present invention.

[0034] Figure 2 The figure is a schematic structural diagram of a closed-loop polarization optical fiber temperature sensor with a specific ground-line structure according to an embodiment of the present invention.

[0035] Figure 3 Schematic diagram of square wave and step wave signals provided according to an embodiment of the present invention. DETAILED DESCRIPTION

[0036] The following describes embodiments of the present invention in detail, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present invention, and are not to be construed as limiting the present invention.

[0037] Figure 1 The figure is a schematic structural diagram of a linear closed-loop polarization optical fiber temperature sensor provided according to an embodiment of the present invention.

[0038] like Figure 1 As shown, the linear structure of the closed-loop polarization optical fiber temperature sensor 10 includes:

[0039] Fiber laser light source, used to emit laser;

[0040] A circulator, wherein a first end of the circulator is connected to a fiber laser light source for directionally transmitting the laser;

[0041] a fiber polarizer, wherein a first end of the fiber polarizer is connected to the second end of the circulator and is used to convert the laser light into polarized light;

[0042] A first 45° optical fiber connecting line is used to split the polarized light into a first orthogonal polarized light and a second orthogonal polarized light, so that the first orthogonal polarized light and the second orthogonal polarized light are transmitted through a fast axis and a slow axis respectively;

[0043] A phase modulator, wherein the first end of the phase modulator is connected to the second end of the fiber polarizer through a 45° fiber optic connecting line, and is used to modulate the first orthogonal polarized light and the second orthogonal polarized light by a preset modulation signal to superimpose a 90-degree non-reciprocal phase shift in the optical path.

[0044] As a possible implementation method, the preset modulation signal is the sum of the square wave modulation phase shift and the step wave feedback phase shift. The square wave modulation signal is used to superimpose a 90-degree non-reciprocal phase shift in the optical path through the phase modulator, adjusting the operating point of the system to the most sensitive area of ​​temperature response. The step wave feedback signal is used to generate a compensating phase shift to offset the non-reciprocal phase shift caused by temperature.

[0045] a Faraday rotator, wherein a first end of the Faraday rotator is connected to the second end of the phase modulator, and is configured to rotate the first orthogonal polarized light and the second orthogonal polarized light by 45 degrees each;

[0046] an optical fiber delay line, wherein a first end of the optical fiber delay line is connected to a second end of the Faraday rotator;

[0047] The PM sensing fiber is connected to the optical fiber delay line via a second 45° optical fiber connector, and is used to transmit the first orthogonal polarized light and the second orthogonal polarized light spliced ​​through the second 45° optical fiber connector at different speeds through the optical fiber fast axis and slow axis, wherein the difference in propagation constants between the first orthogonal polarized light and the second orthogonal polarized light in the PM sensing fiber is proportional to the temperature.

[0048] The SM fiber reflector is used to reflect the first orthogonal polarized light and the second orthogonal polarized light, and reverse the direction through the PM sensing fiber, so that the first orthogonal polarized light and the second orthogonal polarized light pass through the Faraday rotator again, and the orthogonal mode is rotated 45 degrees. The orthogonal polarized light output from the fast axis and the slow axis is exchanged, and then output through the fast axis and the slow axis to reach the phase modulation for re-modulation. After passing through the first 45° optical fiber connecting line and the optical fiber polarizer, an output optical signal is generated and reaches the circulator.

[0049] The processing module is connected to the third end of the circulator and is used to demodulate the output optical signal to obtain an output signal, and calculate the output signal to obtain the current temperature.

[0050] like Figure 2 As shown, in one embodiment of the present invention, the processing module includes:

[0051] a photodetector connected to the third terminal of the circulator;

[0052] an analog-to-digital converter, wherein a first terminal of the analog-to-digital converter is connected to the photodetector;

[0053] a filter connected to the second terminal of the analog-to-digital converter;

[0054] a demodulator, wherein a first terminal of the demodulator is connected to the second terminal of the filter, and a second terminal of the demodulator is connected to the first terminal of the digital accumulator;

[0055] The output port is connected to the second terminal of the digital accumulator.

[0056] In one embodiment of the present invention, the fiber laser light source may be a superluminescent laser diode, and the delay time of the fiber delay line is τ / 2, where τ is the time required for the signal to pass through the fiber delay line. The relationship between the propagation constant difference between the first orthogonal polarized light and the second orthogonal polarized light in the PM sensing fiber and the temperature is:

[0057] δ x -δ y =Δβ0L0(1+C(T-T0))

[0058] Among them, δ x and δ y are the phase delays of the fast and slow axes of the sensing fiber, Δβ0 is the propagation constant difference at temperature T0, L0 is the length of the sensing fiber, C is the temperature coefficient, and T is the temperature.

[0059] In one specific embodiment, light from a 1310 nm superluminescent laser diode (SLD) is directed to a fiber polarizer via a circulator. The resulting polarized light is evenly split into the fast and slow axes of the PM fiber via a 45° connection between the polarizer and the phase modulator. These orthogonal polarization modes are modulated by a LiNbO3 integrated optical phase modulator and then coupled into a Faraday rotator, where each mode is rotated 45° before entering a PM fiber delay line with a delay time of τ / 2. The two waves then enter the PM sensing fiber via a 45° splice. The two beams, traveling along the fast and slow axes of the sensing fiber, pass through the sensing region at different speeds, where the propagation constant difference (Δβ) is proportional to the temperature (T) and can be expressed as:

[0060] δ x -δy =ΔβL=Δβ0L0(1+C(T-T0))

[0061] Among them, δ x and δ y are the phase delays of the fast axis and slow axis of the sensing fiber, respectively, L is the length of the sensing fiber, Δβ=Δβ0, L=L0, at a certain temperature T0, C is the temperature coefficient.

[0062] After reflecting from the SM fiber reflector, the two waves are reversed through the sensing fiber, so the temperature-induced birefringence phase shift is two times and three times that of the Sagnac interferometer version, which can improve the sensitivity.

[0063] After passing through the Faraday rotator again, the orthogonal modes rotate another 45 times and then exchange, meaning that the outgoing wave on the fast axis returns along the slow axis, and the outgoing wave on the slow axis returns along the fast axis. Therefore, this reciprocal structure can suppress noise and interference in the optical path. At the phase modulator, the two polarized waves undergo different modulation again, are then spliced ​​by 45 and interfered with by the polarizer, and brought back together before being coupled to a photodetector (PD).

[0064] According to the above description, the polarization evolution of the light propagation path can be described by the Jones matrix as follows:

[0065]

[0066] here and are the Jones vectors of the input and output light, L M The Jones matrices of the polarizer, 45° splicing, phase modulator, Faraday rotator, PM sensing fiber, and SM fiber reflector can be expressed as:

[0067]

[0068]

[0069]

[0070]

[0071]

[0072]

[0073] Therefore, the detected intensity Iout is:

[0074]

[0075] Among them, Iin is the input intensity, δ(equal to δ x -δ y ) is the temperature-induced phase difference, ψ(t) (equal to ) is the modulation signal provided by the integrated phase modulator.

[0076] The cosine relationship in the above formula shows that in the open loop state, I out It is insensitive to the change of δ, nonlinear to T, and has a small measurement range. Therefore, a closed-loop signal processing scheme combining square wave modulation with step wave feedback is designed, such as Figure 3 shown.

[0077] Let ψ(t)=φ s +φ f but:

[0078]

[0079] Among them, φ s and φ f represent the square wave modulation phase shift and the step wave feedback phase shift respectively. Therefore, according to φ(t)-φ(t-τ), φ s = ±π / 2, and φ f is equal to the step height of the step wave, then the following expression is obtained:

[0080]

[0081] I diff =I out- -I out+ =I in [sin(2δ+φ f )]

[0082] Among them, I out± Respectively represent when φ s = ±π / 2 when the output light intensity.

[0083] In one embodiment of the present invention, the closed-loop polarization optical fiber temperature sensor with a linear structure further includes a digital accumulator, which is used to calculate the feedback phase shift of the step wave.

[0084] In each square wave modulation cycle, the feedback phase shift is calculated by the digital accumulator to offset the non-reciprocal phase shift caused by temperature, that is, φ f =-2δ, making the system work in the most sensitive area with the largest slope, so as to improve the linearity and measurement range and enhance the detection capability of small differential phase shift.

[0085] φ f As the demodulation output and the step height of the step wave respectively. Obviously, this value is not affected by I inThe influence of changes can greatly reduce the sensitivity of the system to environmental disturbances and equipment aging. The digital accumulator overflows when it exceeds the range. Here, the range of the accumulator is designed to completely correspond to the 2π modulation voltage of the phase modulator. Therefore, automatic overflow can be used to achieve a 2π reset of the system, such as Figure 3 shown.

[0086] Compared with the existing technology, after comprehensive comparison between the CPMF-TS proposed in this invention and the open-loop solution, the output response error of the CPMF-TS before and after fiber re-splicing changes by about 0.1°C, which is much smaller than that of the open-loop system. The resolution of the CPMF-TS after Kalman filtering is about 0.038°C, which is less than the 0.06°C of 18B20.

[0087] The CPMF-TS has good linearity and high processing tolerance and can accommodate sensing fibers of different lengths. The nonlinear errors of 2.5 mm and 3.5 mm sensing fibers (1.038% and 1.047%) are both smaller than the corresponding values ​​of the open-loop system (2.984% and 1.648%) after fitting with third-order and fourth-order polynomials.

[0088] The high resolution and high sensitivity of the CPMF-TS enable it to provide accurate measurements in microwave, RF and electromagnetic interference environments.

[0089] The linear, closed-loop polarization-protected fiber optic temperature sensor proposed in an embodiment of the present invention swaps the fast and slow axes of orthogonally polarized light before transmission, utilizing a reciprocal structure to suppress noise and interference along the optical path. A square-wave modulated signal is superimposed on the optical path via a phase modulator, resulting in a 90-degree non-reciprocal phase shift. This adjusts the system's operating point to the most sensitive region of temperature response, overcoming the system's cosine response's insensitivity to small temperature changes. A closed-loop demodulation scheme utilizes a step wave to generate a compensating phase shift to offset the non-reciprocal phase shift caused by temperature changes, improving linearity and measurement range. The sensor offers advantages such as high resolution and sensitivity, enabling accurate measurements in microwave, radio frequency, and electromagnetic interference environments.

[0090] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or N embodiments or examples in a suitable manner. In addition, those skilled in the art can combine and combine different embodiments or examples described in this specification and the features of different embodiments or examples without contradiction.

[0091] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be understood to indicate or imply relative importance or implicitly specify the number of technical features indicated. Thus, a feature specified as "first" or "second" may explicitly or implicitly include at least one such feature. In the description of the present invention, "N" means at least two, such as two, three, etc., unless otherwise specifically defined.

Claims

1. A linear structure closed-loop polarization optical fiber temperature sensor, characterized in that: include: Fiber laser light source, used to emit laser; a circulator, wherein a first end of the circulator is connected to the fiber laser light source and is used for directionally transmitting the laser; an optical fiber polarizer, a first end of which is connected to the second end of the circulator, and is used to convert the laser light into polarized light; a first 45° optical fiber connecting line, configured to split the polarized light into a first orthogonal polarized light and a second orthogonal polarized light, so that the first orthogonal polarized light and the second orthogonal polarized light are transmitted along a fast axis and a slow axis, respectively; a phase modulator, wherein a first end of the phase modulator is connected to a second end of the optical fiber polarizer via the 45° optical fiber connecting line, and is configured to modulate the first orthogonal polarized light and the second orthogonal polarized light using a preset modulation signal to superimpose a 90-degree non-reciprocal phase shift in the optical path; a Faraday rotator, wherein a first end of the Faraday rotator is connected to the second end of the phase modulator, and is configured to rotate the first orthogonal polarized light and the second orthogonal polarized light by 45 degrees; an optical fiber delay line, wherein a first end of the optical fiber delay line is connected to a second end of the Faraday rotator; a PM sensing fiber, wherein the PM sensing fiber is connected to the optical fiber delay line via a second 45° optical fiber connector, and is configured to transmit the first orthogonal polarized light and the second orthogonal polarized light spliced ​​by the second 45° optical fiber connector at different speeds through the optical fiber fast axis and slow axis, wherein the difference in propagation constants between the first orthogonal polarized light and the second orthogonal polarized light in the PM sensing fiber is proportional to the temperature; An SM fiber reflector is configured to reflect the first orthogonal polarized light and the second orthogonal polarized light, and reverse the direction through the PM sensing fiber so that the first orthogonal polarized light and the second orthogonal polarized light are rotated 45° after passing through the Faraday rotator again. The orthogonal polarized light outputted from the fast axis and the slow axis is exchanged, and then outputted through the fast axis and the slow axis before reaching the phase modulation for remodulation. After passing through the first 45° optical fiber connecting line and the optical fiber polarizer, an output optical signal is generated and reaches the circulator. A processing module is connected to the third end of the circulator and is used to demodulate the output optical signal to obtain an output signal, and calculate the output signal to obtain the current temperature.

2. The linear structure closed-loop polarization optical fiber temperature sensor according to claim 1, characterized in that: The fiber laser light source is a superluminescent laser diode.

3. The linear structure closed-loop polarization optical fiber temperature sensor according to claim 1, characterized in that: The delay time of the optical fiber delay line is τ / 2, where τ is the time required for a signal to pass through the optical fiber delay line.

4. The linear structure closed-loop polarization optical fiber temperature sensor according to claim 1, characterized in that: The relationship between the propagation constant difference between the first orthogonal polarized light and the second orthogonal polarized light in the PM sensing fiber and the temperature is: d x -d y =Δβ0L0(1+C(T-T0)) Among them, δ x and δ y are the phase delays of the fast and slow axes of the sensing fiber, Δβ0 is the propagation constant difference at temperature T0, L0 is the length of the sensing fiber, C is the temperature coefficient, and T is the temperature.

5. The linear structure closed-loop polarization optical fiber temperature sensor according to claim 1, characterized in that: The preset modulation signal is the sum of a square wave modulation phase shift and a step wave feedback phase shift.

6. The linear structure closed-loop polarization optical fiber temperature sensor according to claim 5, characterized in that: Also includes: A digital accumulator is used to calculate the feedback phase shift of the step wave.

7. The linear structure closed-loop polarization optical fiber temperature sensor according to claim 6, characterized in that: The processing module includes: a photodetector connected to the third terminal of the circulator; an analog-to-digital converter, wherein a first terminal of the analog-to-digital converter is connected to the photodetector; a filter connected to the second terminal of the analog-to-digital converter; a demodulator, wherein a first terminal of the demodulator is connected to the second terminal of the filter, and a second terminal of the demodulator is connected to the first terminal of the digital accumulator; An output port is connected to the second end of the digital accumulator.

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