OFDR optical fiber sensing system based on 3*3 coupler

Abstract

The invention relates to an OFDR optical fiber sensing system based on a 3 * 3 coupler. The OFDR optical fiber sensing system comprises a tunable laser source, a 50: 50 optical fiber coupler, a 1: 99 optical fiber coupler, an optical fiber delayer, an optical fiber circulator, a sensing optical fiber, a 3 * 3 optical fiber coupler, a photoelectric detector, a data acquisition card and a computer. Light from a self-tunable laser source is uniformly divided into two arms by using a 50: 50 optical fiber coupler, one path passes through an arm A, passes through a 1: 99 optical fiber coupler serving as a main interferometer and a 3 * 3 optical fiber coupler and reaches a photoelectric detector, and the other path passes through an arm B and reaches a 3 * 3 optical fiber coupler; the 1: 99 optical fiber coupler splits light and reaches the 3 * 3 optical fiber coupler through the common reference arm C, the two paths of light interfere in the 3 * 3 optical fiber coupler to form an auxiliary interferometer beat frequency signal for correcting the nonlinear tuning effect of the laser source, an auxiliary interferometer does not need to be additionally built, and the total cost of the system is reduced.

Description

Technical Field

[0001] The present invention relates to the field of optical fiber sensing technology, and in particular to an OFDR optical fiber sensing system based on a 3×3 coupler. Background Art

[0002] Optical frequency domain reflectometry (OFDR) is a distributed fiber optic sensor based on Rayleigh scattering. In recent years, with the continuous improvement of OFDR system performance, it is no longer limited to the measurement of strain and temperature, and has gradually developed the ability to measure other parameters, such as shape, distortion, vibration, etc., thus deriving many relatively novel applications, such as structural health monitoring, discharge source location, magnetic field detection, radiation monitoring and medical fields.

[0003] In early OFDR models, it is generally believed that the frequency conversion process of the tunable laser is linear, that is, the beat signal generated by the reverse Rayleigh scattered light of the measurement arm and the reference arm ignores the phase jitter; however, in practical applications, the frequency modulation process of the adjustable light source is not completely linear. The tuning speed will change with time during operation, so the instantaneous optical frequency output no longer changes linearly with time; in this case, although the time intervals of the OFDR beat signals are equal, the optical frequency intervals are not equal; at the same time, the signal required for Fourier transform in the subsequent signal processing is a discrete signal with equal optical frequency intervals. If it is directly transformed to the distance domain through Fourier transform, the amplitude of the reflection peak signal in the distance domain will be reduced, that is, energy diffusion, and ultimately reduce the spatial resolution of the OFDR system. Therefore, it is very important to compensate for the nonlinear tuning effect of the light source for the OFDR system.

[0004] At present, the main way to solve the nonlinear tuning effect of OFDR system is to build an auxiliary interferometer to output a beat signal as a trigger reference for the acquisition card, so that the beat signal of the main interferometer is sampled at equal optical frequency intervals, thereby compensating for the nonlinear tuning effect of the light source or estimating the nonlinear phase of the local oscillator reference light through the beat signal of the auxiliary interferometer, and then using a de-skew filter to compensate for the nonlinear phase in the beat signal of the main interferometer; however, these nonlinear tuning compensation methods in OFDR systems require additional auxiliary interferometers, which makes the system more complex and expensive. Summary of the invention

[0005] In view of this, the present invention provides an OFDR optical fiber sensing system based on a 3×3 coupler, which effectively solves the nonlinear tuning effect generated by the OFDR system in the prior art, reduces the overall system cost, and simplifies the system configuration.

[0006] To achieve the above object, the present invention provides an OFDR optical fiber sensing system based on a 3×3 coupler, comprising a tunable laser source, a 50:50 optical fiber coupler, a 1:99 optical fiber coupler, an optical fiber delay device, an optical fiber circulator, a sensing optical fiber, a 3×3 optical fiber coupler, a photodetector, a data acquisition card and a computer, and comprising the following steps:

[0007] Step 1, obtaining frequency-sweeping nonlinear continuous light through the tunable laser source;

[0008] Step 2, using the 50:50 fiber coupler to evenly split the light from the tunable laser source into two arms, one way is emitted through arm A to the 1:99 fiber coupler, and the other way is emitted through arm B to the 3×3 fiber coupler;

[0009] Step 3, using the 1:99 fiber coupler to split the light into two paths, one path is transmitted to the 3×3 fiber coupler through arm C, and the other path is transmitted to the fiber delay device through arm D;

[0010] Step 4, adjusting the main beat frequency signal and the reference beat frequency signal successively generated by the optical fiber delay device in the 3×3 optical fiber coupler;

[0011] Step 5: Process the two beat frequency signals through the photoelectric detector, data acquisition card and computer.

[0012] Preferably, on arm A, one path of light split from the 50:50 fiber coupler reaches the photodetector through a 1:99 fiber coupler and a 3×3 fiber coupler, and the other path of light is used as a reference signal of the auxiliary interferometer through arm B, which can be expressed as:

[0013]

[0014] Among them, E D represents the electric field amplitude, represents the time-varying phase component;

[0015] The common reference arm C is obtained by splitting the light through the 1:99 optical fiber coupler. The electric field expression of the common reference arm C is:

[0016]

[0017] Among them, E R represents the electric field amplitude, τ represents the time delay;

[0018] The two paths of light passing through the arm B and the arm C interfere in the 3×3 fiber coupler to form an auxiliary interferometer beat frequency signal I(t) A :

[0019]

[0020] Among them, I R and I D Denotes the light intensity. Using Taylor expansion and ignoring high-order terms, the auxiliary interferometer beat frequency signal can be expressed as:

[0021]

[0022] Among them, represents the time delay, represents the nonlinear tuning distortion term of the laser;

[0023] Preferably, a vibration signal is added at 15 m of the sensing optical fiber, and the generated backward Rayleigh scattered light signal interferes in the 3×3 optical fiber coupler to form a main beat frequency signal.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] 1. The present invention utilizes an optical fiber delay device to separate the auxiliary beat frequency signal from the main beat frequency signal in the 3×3 optical fiber coupler in time to avoid cross interference, which is beneficial to subsequent signal processing;

[0026] 2. The energy allocated to the common reference arm of the 1:99 optical fiber coupler used in the present invention is greater, which is conducive to forming a leading role in two different interference effects and generating the required main beat frequency signal and auxiliary beat frequency signal;

[0027] 3. The OFDR optical fiber sensing system provided by the present invention eliminates the need for additional detection equipment and acquisition channels, reduces the total cost of the system, reduces data acquisition time and computing load, and makes the system configuration simpler. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 A schematic diagram of an OFDR optical fiber sensing system based on a 3×3 coupler of the present invention;

[0029] Figure 2 This is a diagram of experimental results after compensation in an embodiment of the present invention. DETAILED DESCRIPTION

[0030] In order to further explain the technical means and effects adopted by the present invention to achieve the predetermined invention purpose, the specific implementation mode, structure, characteristics and effects of the present invention are described in detail below in combination with the accompanying drawings and preferred embodiments.

[0031] The present invention provides the following technical solution: an OFDR optical fiber sensing system based on a 3×3 coupler, comprising a tunable laser source (1), a 50:50 optical fiber coupler (2), a 1:99 optical fiber coupler (3), an optical fiber delay device (4), an optical fiber circulator (5), a sensing optical fiber (6), a 3×3 optical fiber coupler (7), a photoelectric detector (8), a data acquisition card (9) and a computer (10);

[0032] The tunable laser source (1) is a nonlinear continuous laser with an output power of 8 mW, a wavelength tuning range of 20 nm (1540 nm-1560 nm) and a frequency sweep speed of 100 nm / s;

[0033] The optical signal is evenly divided into two paths by the 50:50 fiber coupler (2). One path is used for subsequent sensing and is transmitted to the 1:99 fiber coupler (3) through arm A. The other path is used to generate the auxiliary beat signal. One of the signals is transmitted to the third input end of the 3×3 fiber coupler (7) through arm B. It can be expressed as:

[0034]

[0035] Among them, E D represents the electric field amplitude, represents the time-varying phase component;

[0036] The 1:99 fiber coupler (3) then splits the light into two paths. One path is emitted through the common arm C to the 3×3 fiber coupler (7), and the other path is emitted as sensing light through the arm D to the fiber delay (4), and then passes through the fiber circulator (5) to reach the 30m sensing fiber (6). The electric field expression of the common reference arm C is:

[0037]

[0038] Among them, E R represents the electric field amplitude, τ represents the time delay;

[0039] The optical fiber delay device (4) is used to distinguish the auxiliary beat signal from the main beat signal in time to avoid cross interference;

[0040] A vibration signal is manually added at 15 m of the sensing optical fiber (6), and the generated backward Rayleigh scattering signal is input to the first input end of the 3×3 optical fiber coupler (7) through the optical fiber circulator (5) as one of the main beat frequency signals for characterizing external disturbance information;

[0041] The other optical signal of the 1:99 optical fiber coupler (3) with a higher splitting ratio is input as a common arm C to the second input end of the 3×3 optical fiber coupler (7) to dominate the two interference effects of generating the main beat frequency and the auxiliary beat frequency signal, forming the auxiliary interferometer beat frequency signal I(t) A :

[0042]

[0043] Among them, I R and I D Denotes the light intensity. Using Taylor expansion and ignoring high-order terms, the auxiliary interferometer beat frequency signal can be expressed as:

[0044]

[0045] Among them, represents the time delay, represents the nonlinear tuning distortion term of the laser, and the instantaneous frequency exhibits the same nonlinear pattern as the laser tuning frequency, so I(t) can be used A The expression is then used to extract phase information to correct the nonlinear tuning effect, without the need to build an additional auxiliary interferometer.

[0046] The two obtained beat frequency signals are finally subjected to oblique filtering processing through a photoelectric detector (8), a data acquisition card (9) and a computer (10) to compensate for the nonlinear tuning effect and obtain sensing information with high spatial resolution.

[0047] like Figure 2 As shown, after the vibration sensing of the present invention is implemented, the spatial resolution of the vibration collected at a sensing optical fiber distance of 30m is 0.5cm, indicating that the sub-streamlined structure can also obtain a cm-level spatial resolution.

[0048] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. Although the present invention has been disclosed as a preferred embodiment as above, it is not used to limit the present invention. Any technical personnel in this field can make some changes or modify the technical contents disclosed above into equivalent embodiments without departing from the scope of the technical solution of the present invention. However, any brief modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention are still within the scope of the technical solution of the present invention.

Claims

1. An OFDR fiber optic sensing system based on a 3×3 coupler, comprising a tunable laser source, a 50:50 fiber optic coupler, a 1:99 fiber optic coupler, a fiber optic delay device, a fiber optic circulator, a sensing fiber, a 3×3 fiber optic coupler, a photodetector, a data acquisition card and a computer, characterized in that: The following steps are involved: Step 1, obtaining frequency-sweeping nonlinear continuous light through the tunable laser source; Step 2: Use the 50:50 fiber coupler to evenly split the light from the tunable laser source into two paths, one path is transmitted to the 1:99 fiber coupler through arm A, and the other path is transmitted to the 3×3 fiber coupler through arm B; Step 3, using the 1:99 fiber coupler to split the light into two paths, one path is transmitted to the 3×3 fiber coupler through arm C, and the other path is transmitted to the fiber delay device through arm D, and then passes through the fiber circulator to reach the sensing fiber; Step 4, adjusting the main beat frequency signal and the reference beat frequency signal successively generated by the optical fiber delay device in the 3×3 optical fiber coupler; Step 5: Process the two beat frequency signals through the photoelectric detector, data acquisition card and computer.

2. The OFDR optical fiber sensing system based on 3×3 coupler according to claim 1, characterized in that: On arm A, one path of light split from the 50:50 fiber coupler reaches the photodetector through a 1:99 fiber coupler and a 3×3 fiber coupler, and the other path of light is used as a reference signal of the auxiliary interferometer through arm B, which can be expressed as: Among them, E D represents the electric field amplitude, represents the time-varying phase component; The common reference arm C is obtained by splitting the light through the 1:99 optical fiber coupler. The electric field expression of the common reference arm C is: Among them, E R represents the electric field amplitude, τ represents the time delay; The two paths of light passing through the arm B and the arm C interfere in the 3×3 fiber coupler to form an auxiliary interferometer beat frequency signal I(t) A : Among them, I R and I D Denotes the light intensity. Using Taylor expansion and ignoring high-order terms, the auxiliary interferometer beat frequency signal can be expressed as: Among them, represents the time delay and represents the nonlinear tuning distortion term of the laser.

3. The OFDR optical fiber sensing system based on 3×3 coupler according to claim 2, characterized in that: A vibration signal is added at 15 m of the sensing optical fiber, and the generated backward Rayleigh scattered light signal interferes in the 3×3 optical fiber coupler to form a main beat frequency signal.

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