Nonlinear interferometric bending sensor

Abstract

The present invention provides a nonlinear interference curvature sensor, including pump light, seed light, fiber parametric amplifier (including two coarse wavelength division multiplexers and a dispersion-shifted fiber), curvature sensing element, single-mode fiber, photodetector and oscilloscope, etc. The fiber parametric amplifier is characterized in that the pump light wavelength (1550nm) is located in the anomalous dispersion region of the dispersion-shifted fiber, which can meet the phase matching condition, so the four-wave mixing effect in the fiber can occur. When the curvature of the single-mode fiber at the idle light output end of the first fiber parametric amplifier changes, part of the idle light intensity escapes into the cladding, thereby changing the interference fringe contrast at the output end of the second fiber parametric amplifier, and then the corresponding relationship between the maximum light intensity and the interference fringe contrast and the curvature can be obtained, and finally a curvature sensing with higher sensitivity is realized.

Description

Technical Field

[0001] The invention provides a nonlinear interference type curvature sensor, belonging to the technical field of optical fiber sensing. Background Art

[0002] Since the measurement sensitivity of traditional linear fiber interferometers is limited by the shot noise limit, in order to achieve higher sensitivity sensing, it is necessary to resort to quantum technology, that is, to use nonlinear interferometers. The biggest difference between it and linear interferometers is that it uses nonlinear media such as fiber parametric amplifiers (FOPA) instead of traditional linear beam splitters, and can achieve a significant increase in sensitivity by inputting coherent states.

[0003] The present invention utilizes the four-wave mixing effect in optical fiber, which is a parametric amplification process caused by the optical third-order nonlinear effect. Its process can be described as follows: if two light beams with different frequency components propagate in the same direction in the optical fiber, assuming that the two input frequency components are v1 and v2, two new frequency components v3 and v4 will be generated under phase matching, and the energy conservation relationship between them is satisfied: v3+v4=v1+v2.

[0004] Nonlinear interferometers can be applied to multiple sensing fields such as curvature sensing. Curvature sensing is a technology that uses the fact that when an optical fiber is bent, part of the core energy will escape into the cladding, resulting in the loss of its transmitted light intensity. The change in core energy leads to changes in some parameters (such as the maximum light intensity and the contrast of interference fringes) to measure curvature. Since it is relatively simple in technical implementation, it is widely used in practical applications. Summary of the invention

[0005] The purpose of the present invention is to solve the problem that the measurement sensitivity of the existing linear interferometer is limited by the shot noise limit, and to provide a nonlinear interference type curvature sensor.

[0006] The technical solution adopted by the present invention is as follows:

[0007] A nonlinear interference curvature sensor comprises two beams of pump light, a seed light, two optical fiber parametric amplifiers, a curvature sensing element, a single-mode optical fiber, a photodetector and an oscilloscope. The connection method is as follows: the pump light 1 and the seed light are injected into the first optical fiber parametric amplifier, the single-mode optical fiber of the idler light channel output by the first optical fiber parametric amplifier is bent to form the curvature sensing element, the signal light output by the first optical fiber parametric amplifier, the idler light after passing through the curvature sensing element and the pump light 2 are injected into the second optical fiber parametric amplifier, the signal light and the idler light at the output end of the second optical fiber parametric amplifier are connected to the oscilloscope through the photodetector, and the curvature detection can be realized by analyzing the maximum light intensity displayed on the oscilloscope and the change of the contrast of the interference fringes.

[0008] In the above technical solution, the fiber parametric amplifier is composed of two coarse wavelength division multiplexers and a dispersion-shifted fiber located in the middle. Because the wavelength of the pump light 1550nm is located in the anomalous dispersion region of the dispersion-shifted fiber, it can meet the phase matching conditions of four-wave mixing.

[0009] The beneficial effects of the present invention compared with the prior art are:

[0010] 1. When the maximum output light intensity is used to realize curvature sensing, it also has higher sensitivity than the linear interference curvature sensor, within 0-500m -1 The sensitivity was improved by 5.855-5.928 times.

[0011] 2. The sensor uses two beams of light (signal light and idler light) and combines them organically to achieve a high sensitivity within the range of 0-194.232m -1 When using the signal light at the output end of FOPA2, at 194.232-500m -1 By utilizing the idler light at the output of FOPA2, it is possible to accurately measure a wide range of curvatures. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] In order to more clearly illustrate the implementation examples or technical solutions of the present invention, the present invention is further described below in conjunction with the accompanying drawings.

[0013] Figure 1 The schematic diagram of the implementation and application system of the present invention is shown in FIG. 1: pump light 1; 2: seed light; 3, 5, 8, 10: coarse wavelength division multiplexer; 4, 9: dispersion-shifted optical fiber; 6: bending sensor element; 7: pump light 2; 11, 12: photodetectors; 13: oscilloscope.

[0014] Figure 2 The schematic diagram of the principle analysis of the sensor of the present invention, wherein: (a) FOPA1 and FOPA2 form a nonlinear interferometer. (b) The schematic diagram of the bending sensing element, the chord length of the bending part is 2L, and the chord height is d.

[0015] Figure 3 Schematic diagram of the principle of a linear interferometric bending sensor (Beamsplitter (BS) + Beamsplitter (BS) structure).

[0016] Figure 4Figure 3 is the dependence of the maximum light intensity (signal light and idler light) at the output of FOPA2, the maximum light intensity at the output of the linear interferometer and the loss on the curvature when G1=4, G2=3.5, where: the dotted line, the dash-dotted line and the solid line (yellow) are the dependence of the maximum light intensity of the signal light at the output of FOPA2, the idler light at the output of FOPA2 and the linear interferometer on the curvature, respectively; the dotted line is the dependence of the loss on the curvature; the solid line (black) is the ratio of the maximum light intensity of the idler light to the maximum light intensity at the output of the linear interferometer.

[0017] Figure 5 : is the dependence of the first-order derivative of the maximum light intensity (signal light and idler light) at the output of FOPA2 and the maximum light intensity at the output of the linear interferometer on the curvature when G1=4, G2=3.5, where: the dotted line, the dashed line and the solid line are the dependence of the first-order derivative of the maximum light intensity of the signal light at the output of FOPA2, the maximum light intensity of the idler light at the output of FOPA2 and the maximum light intensity at the output of the linear interferometer on the curvature, respectively.

[0018] Figure 6 Figure 2 shows the dependence of the interference fringe contrast and loss on curvature at the output of FOPA2 (signal light and idler light) and the output of the linear interferometer under G1=4, G2=3.5, where: (a): The dashed line, dot-dash line and solid line are the dependence of the interference fringe contrast on curvature of the signal light at the output of FOPA2, the idler light at the output of FOPA2 and the output of the linear interferometer, respectively, and the dotted line is the dependence of the loss on curvature. (b): A partial enlarged view of (a).

[0019] Figure 7 Figure 2 shows the dependence of the first-order derivative of the interference fringe contrast on the curvature at the output of FOPA2 (signal light and idler light) and the output of the linear interferometer under G1=4, G2=3.5, where: (a): The dashed line, solid line and dash-dotted line are the dependence of the first-order derivative of the interference fringe contrast on the curvature of the signal light at the output of FOPA2, the linear interferometer output and the idler light at the output of FOPA2, respectively. (b): A partial enlarged view of (a). DETAILED DESCRIPTION

[0020] The present invention will be further described below in conjunction with specific examples and accompanying drawings:

[0021] like Figure 1As shown, it is a specific device of the nonlinear interference type curvature sensor of the present invention. The following only uses this example to illustrate and explain the principle of the present invention, but this example does not limit the scope of the present invention. In this example, the sensor includes: pump light 1; seed light 2; coarse wavelength division multiplexers 3, 5, 8, 10; dispersion shifted optical fibers 4, 9; curvature sensing element 6; pump light 7; photodetectors 11, 12; oscilloscope 13. Among them, coarse wavelength division multiplexer 3, dispersion shifted optical fiber 4, coarse wavelength division multiplexer 5 constitute parametric amplifier FOPA1; coarse wavelength division multiplexer 8, dispersion shifted optical fiber 9, coarse wavelength division multiplexer 10 constitute parametric amplifier FOPA2; the curvature sensing element is composed of two optical fiber clamps and a single-mode optical fiber, and the change of the curvature of the single-mode optical fiber is achieved by changing the distance between the two optical fiber clamps. The connection method is as follows: pump light 1 and seed light 2 are injected into FOPA1, the idle light output by FOPA1 passes through the curvature sensor element 6 and the signal light output by FOPA1 and pump light 2 7 are injected into FOPA2, and the signal light and idle light at the output end of FOPA2 are connected to the oscilloscope 13 through photodetectors 11 and 12 for maximum light intensity and interference fringe contrast analysis.

[0022] Principle analysis:

[0023] The overall structural principle of the present invention is exemplified as follows: Figure 2 By placing the seeds and pump light p1 are injected into FOPA1, and two output lights (signal light Idle light ), which can be described as

[0024]

[0025] Let the FOPA1 idler light pass through the bend sensing element (bending loss is S), then After passing through FOPA2, two output lights (signal light Idle light ), which can be described as

[0026]

[0027] Among them, FOPA1 is non-phase sensitive, while FOPA2 is phase sensitive, so the phase factor exp(iθ) is considered here.

[0028] The particle number calculation formula have to To inject seed light, the light intensity is high.

[0029]

[0030]

[0031] From the interference fringe contrast formula, we can get:

[0032]

[0033]

[0034] In order to demonstrate that the nonlinear interference type bending sensor provided by the present invention has higher sensitivity than the linear interference type bending sensor, Figure 2 The nonlinear interference type bending sensor and the linear interference type bending sensor (BS+BS) structure (such as Figure 3 ) for comparison.

[0035] Figure 3 The process is described as follows: Inject light Output after passing through two beam splitters and the curvature sensor (bending loss is S) It can be described as:

[0036]

[0037] Therefore, the output light intensity can be described as:

[0038]

[0039] Among them, I L0 is the intensity of the injected seed light. From equation (1-8), it can be seen that interference will occur at each output end, and the contrast of the interference fringes is:

[0040]

[0041] In order to obtain the dependence of interference fringe contrast, maximum light intensity and loss on the curvature that characterizes the bending degree, some initial parameters of single-mode optical fiber are set, core refractive index n1 = 1.461, cladding refractive index n2 = 1.456, core radius a = 4um, so the cut-off wavelength can be obtained For the idle optical channel, λ = 1534nm, which meets the single-mode transmission condition. The chord length of the curved part L = 1mm. In a single-mode optical fiber, the loss formula caused by curvature is given by the following formula:

[0042]

[0043] in, ρ: curvature; is the radial normalized phase constant; W: radial normalized attenuation constant; k1 is the first-order modified Bessel function; δn: the difference between the core and cladding refractive index; V: normalized frequency;

[0044] In actual work, A cand U can be calculated by the following approximate formula:

[0045]

[0046]

[0047] And by Figure 2 (b) The arc length of the curved part is l = 2arcsin(Lρ) / ρ, so the loss S is:

[0048]

[0049] Considering that the sensitivity of the interferometer is related to the number of photons inside the interferometer, in order to make a fair comparison between the two interferometers, we adjust I0 and I L0 To ensure I 2' =I L2' Finally, under the condition of the same phase-sensitive field strength, I s1 ,I i1 ,I L22 The maximum value of and the relationship between the loss S and the curvature ρ are plotted on Figure 4 , and I s1 ,I i1 and I L22 The relationship between the first derivative of the maximum value and the curvature ρ is shown in Figure 5 It can be seen that the maximum intensity of the signal light and the idler light at the output end of the nonlinear interferometer FOPA2 are higher than the maximum intensity at the output end of the linear interferometer. The maximum intensity of the idler light is amplified by 5.714-5.856 times compared with the linear interferometer, and the sensitivity is improved by 5.855-5.928 times (when the absolute value of the first-order derivative of the maximum intensity to the curvature is larger, the sensitivity is higher).

[0050] In addition, V L22 、V s1 、V i1 The relationship between the loss S and the curvature ρ is plotted on Figure 6 , and V L22 、V s1 and V i1 The relationship between the first-order derivative of and the curvature ρ is plotted on Figure 7 .from Figure 6 Can be obtained in 0-500m -1 In the range of 0-107.827m, the contrast of interference fringes of signal light satisfies the one-to-one correspondence with curvature; however, the idler light does not satisfy this and cannot achieve -1 Accurate measurement within the range. Figure 7 Know at 0-194.232m -1 When the signal light is more sensitive than the idle light, the sensitivity is 194.232-500m -1When the idler light is used, the sensitivity is higher than that of the signal light, so it is necessary to combine the signal light and the idler light to achieve the 0-500m -1 The curvature can be accurately measured within the range of 0-194.232m -1 When using signal light to measure, at 194.232-500m -1 The idler light is used for measurement.

[0051] At the same time, by Figure 7 It can be seen that when the curvature is lower than 193.232m -1 When the linear interference bending sensor is less sensitive than the signal light, it is also less sensitive at 204.459-500m -1 The sensitivity is lower than that of idle light. -1 , the linear interferometric bend sensor has a higher sensitivity than the signal and idler light, but this is only for a small part of the bend range.

[0052] In summary, the nonlinear interference curvature sensor provided by the present invention can not only realize accurate measurement of curvature, but also has higher sensitivity than the linear interference curvature sensor.

[0053] The examples described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific case of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A nonlinear interferometric bending sensor, characterized in that: It includes two pump beams, seed beams, two fiber parametric amplifiers, bending sensing elements, single-mode optical fibers, photodetectors, and oscilloscopes; The connection method is as follows: pump light 1 and seed light are injected into the first fiber parametric amplifier, the single-mode optical fiber of the idle light channel output by the first fiber parametric amplifier is bent to form the curvature sensing element, the signal light output by the first fiber parametric amplifier, the idle light after passing through the curvature sensing element, and pump light 2 are injected into the second fiber parametric amplifier, the signal light and idle light at the output end of the second fiber parametric amplifier are connected to an oscilloscope through a photodetector, and curvature detection can be achieved by analyzing the changes in the maximum light intensity and the contrast of the interference fringes displayed on the oscilloscope.

2. The nonlinear interference type bending sensor according to claim 1, characterized in that: The fiber parametric amplifier is composed of two coarse wavelength division multiplexers and a dispersion-shifted fiber in the middle. The wavelength of the pump light, 1550nm, is located in the anomalous dispersion region of the dispersion-shifted fiber, satisfying the phase matching condition of four-wave mixing.

3. The nonlinear interferometric bending sensor according to claim 1, characterized in that: The sensor can detect bending in the range of 0-500m -1 , and the detection sensitivity is higher than the corresponding linear interference curvature sensor.

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