A wavelength customizable all-fiber MZI sensor and its manufacturing method

Abstract

The present invention discloses an all-fiber MZI sensor with customizable wavelength and a manufacturing method thereof. The all-fiber MZI sensor comprises: an optical fiber composed of a core and a cladding, and also comprises: a first straight waveguide and a second straight waveguide; wherein the first straight waveguide and the second straight waveguide are located on both sides of the junction of the core and the cladding; the first straight waveguide and the second straight waveguide are composed of a plurality of refractive index modulation lines; the refractive index modulation lines are formed by the laser radiating along the axial direction of the junction of the core and the cladding; the distance between the first straight waveguide and the core is fixed, and the distance between the second straight waveguide and the core is adjustable, and different high-order core modes are excited by changing the distance between the second straight waveguide and the core, so as to realize the tuning of the resonant wavelength. The present invention improves the interference contrast of the all-fiber MZI, improves the sensitivity of the intensity demodulated torsion sensor without increasing the structural size, and also improves the flexibility of selecting the light source band in actual torsion sensing applications.

Description

Technical Field

[0001] The present invention relates to the field of optical fiber sensing, and in particular to an all-optical fiber MZI sensor with customizable wavelength and a manufacturing method thereof. Background Art

[0002] All-fiber torsion sensors have become a hot topic of research at home and abroad due to their low cost, easy manufacturing and resistance to electromagnetic interference. They have broad application prospects in aerospace, industrial engineering detection and intelligent health monitoring.

[0003] Most fiber optic torsion sensors are based on fiber gratings and fiber interferometers. For fiber grating structures, such as fiber Bragg gratings and long-period fiber gratings, they are usually based on wavelength demodulation, requiring expensive and bulky spectrum analyzers to measure the drift of the spectrum. For fiber interferometers based on intensity demodulation, they have the advantages of fast response, low cost, and portable power meter monitoring, which are more suitable for sensing measurements and real-time monitoring in various scenarios. In recent years, more and more fiber optic torsion sensors based on intensity demodulation have been proposed, most of which are based on fiber Mach-Zehnder interferometers (MZIs), which are usually implemented by methods such as fiber taper, microcavity, offset splicing, and fusion of different types of optical fibers. However, the size of the above-mentioned fiber structures is relatively large and the robustness is poor. In addition, for intensity-demodulated fiber optic torsion sensors, the sensitivity of these structures is relatively low and the customization of the resonant wavelength is difficult, which makes it technically difficult to match the power meter with the relevant wavelength to monitor the change of the transmitted power, and further breakthroughs are needed. Summary of the invention

[0004] In view of the defects of the prior art, the purpose of the present invention is to provide a wavelength-tunable all-fiber Mach-Zehnder (MZI) sensor and a manufacturing method thereof, aiming to solve the problems that the existing interference wavelength cannot be customized and has poor structural robustness and low sensing sensitivity.

[0005] To achieve the above object, the present invention provides an all-fiber MZI sensor with customizable wavelength, comprising: an optical fiber consisting of a core and a cladding, and also comprising: a first straight waveguide and a second straight waveguide;

[0006] Wherein, the first straight waveguide and the second straight waveguide are located on both sides of the junction of the core and the cladding;

[0007] The first straight waveguide and the second straight waveguide are composed of a plurality of refractive index modulated lines;

[0008] The refractive index modulation line is formed by radiating the laser along the axial direction of the boundary between the core and the cladding;

[0009] The distance between the first straight waveguide and the fiber core is fixed, and the distance between the second straight waveguide and the fiber core is adjustable. By changing the distance between the second straight waveguide and the fiber core, different high-order core modes are excited to achieve tuning of the resonant wavelength.

[0010] Preferably, the optical fiber is a single-mode optical fiber; the laser is a femtosecond laser, the repetition frequency of the femtosecond laser is 750 kHz, and the pulse width is 262 femtoseconds.

[0011] The present invention also provides a method for manufacturing an all-fiber MZI sensor with customizable wavelength. The method is used to manufacture the above-mentioned all-fiber MZI sensor, and the steps include:

[0012] S1. placing the optical fiber on an electrically controlled three-dimensional displacement platform so that the axial direction of the optical fiber is perpendicular to the incident direction of the laser beam;

[0013] S2. adjusting the longitudinal position of the optical fiber, focusing the laser beam into the fiber core by focusing the objective lens;

[0014] S3. Adjust the electrically controlled three-dimensional displacement platform so that the laser is focused on the core-cladding junction on one side of the horizontal plane where the core center is located, turn on the laser beam, and move the electrically controlled three-dimensional displacement platform along the axial direction of the optical fiber at a preset rate;

[0015] S4. Repeat step S3 to form a plurality of refractive index modulation lines to obtain the first straight waveguide;

[0016] S5. Adjust the electrically controlled three-dimensional displacement platform to focus the laser on the core-cladding junction on the other side of the horizontal plane where the core center is located, adjust the distance between the laser focus and the core, and form the refractive index modulation line in the same way to obtain the second straight waveguide.

[0017] Preferably, during laser beam exposure, the refractive index modulation line is formed by controlling the pulse energy, repetition frequency and focal position of the laser beam.

[0018] Preferably, when the distance between the second straight waveguide and the fiber core is changed, the resonance wavelength tuning is expressed as follows:

[0019]

[0020] Among them, λ dip represents the resonant wavelength of the MZI sensor; L represents the length of the MZI sensor; m represents the interference order; Δn eff Represents the effective refractive index difference between the fundamental mode and the higher-order core modes.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] (1) In a wavelength-adjustable all-fiber MZI sensor provided by the present invention, compared with only writing one straight waveguide, adding a second straight waveguide can significantly improve the contrast of the interference spectrum of the all-fiber MZI, and can further improve the sensing sensitivity of the intensity-demodulated torsion sensor without increasing the structural size. Compared with the torsion sensors made of offset fusion fiber and polarization-maintaining fiber in the prior art, the all-fiber MZI sensor proposed by the present invention has the advantages of compact structure, small size, strong robustness and significantly improved sensitivity. And by controlling the focal position of the laser and changing the distance between the second straight waveguide and the fiber core, the position of the resonant wavelength with the highest interference contrast in the transmission spectrum can be accurately adjusted, and the wavelength tuning range can reach nearly 200 nanometers, which improves the flexibility of light source band selection in actual torsion sensing applications.

[0023] (2) The manufacturing method of the all-fiber MZI sensor provided by the present invention adopts a femtosecond laser to directly write on the optical fiber. The manufacturing process is simple, the processing cycle is short, no secondary processing is required, the device has high repeatability, and the manufacturing efficiency of the device is greatly improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the technical solution of the present invention, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0025] Figure 1 A structural diagram of a wavelength-tunable all-fiber MZI sensor according to an embodiment of the present invention;

[0026] Figure 2 A schematic diagram of a device for manufacturing and processing an optical fiber sensor using a laser according to an embodiment of the present invention;

[0027] Figure 3 Spectral transmission diagrams of all-fiber MZI sensors with distances between the fiber core and the second straight waveguide of 4 μm, 5 μm, 6 μm and 7 μm respectively, which are processed according to the preparation method of an embodiment of the present invention;

[0028] Figure 4 It is an evolution curve of the resonance wavelength with the highest interference contrast in the transmission spectrum of the all-fiber MZI sensor processed by the preparation method according to the embodiment of the present invention as the distance between the fiber core and the second straight waveguide changes;

[0029] Figure 5 The all-fiber MZI sensor with a distance of 6 μm between the fiber core and the second straight waveguide processed by the preparation method according to an embodiment of the present invention has a spectral response with a twist angle of 0° to 180°.

[0030] Explanation of the accompanying drawings: 11. single-mode optical fiber; 12. first straight waveguide; 13. second straight waveguide; 21. femtosecond laser; 22. laser beam; 23. focusing objective lens; 24. electrically controlled three-dimensional displacement platform. DETAILED DESCRIPTION

[0031] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the 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.

[0032] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0033] Embodiment 1

[0034] like Figure 1 As shown, it is a schematic diagram of a wavelength-customizable all-fiber MZI sensor of this embodiment, including: an optical fiber composed of a core and a cladding, and also including: a first straight waveguide 12 and a second straight waveguide 13; wherein the first straight waveguide 12 and the second straight waveguide 13 are located on both sides of the junction of the core and the cladding; the first straight waveguide 12 and the second straight waveguide 13 are composed of a plurality of refractive index modulation lines, and the refractive index modulation lines are formed by the laser radiating along the axial direction of the junction of the core and the cladding. Specifically, the refractive index modulation lines are formed by multiple writing modulations, and the writing process is specifically: the refractive index inside the optical fiber is changed by the action of the laser. The distance between the first straight waveguide 12 and the core is fixed, and the distance between the second straight waveguide 13 and the core is adjustable. By changing the distance between the second straight waveguide 13 and the core, different high-order core modes are excited to achieve the tuning of the resonant wavelength.

[0035] In this embodiment, the selected optical fiber is a single-mode optical fiber 11 with a core diameter of about 8.2 μm, and the selected laser is a femtosecond laser 21 with a central wavelength of 517 nm, a repetition frequency of 750 kHz (kilohertz), and a pulse width of 262 fs (femtoseconds).

[0036] Embodiment 2

[0037] like Figure 2 As shown, this embodiment also provides a method for manufacturing an all-fiber MZI sensor with customizable wavelength, the steps comprising:

[0038] S1. The optical fiber is placed on the electrically controlled three-dimensional displacement platform 24 so that the axial direction of the optical fiber is perpendicular to the incident direction of the laser beam 22;

[0039] S2. Adjust the longitudinal position of the optical fiber, and focus the laser beam 22 on the fiber core by focusing the objective lens 23;

[0040] S3. Adjust the electrically controlled three-dimensional displacement platform 24 so that the laser is focused on the core-cladding junction on one side of the horizontal plane where the core center is located, turn on the laser beam 22, and move the electrically controlled three-dimensional displacement platform 24 along the axial direction of the optical fiber at a preset rate;

[0041] S4 repeats step S3 to form a plurality of refractive index modulation lines to obtain a first straight waveguide 12;

[0042] S5. Adjust the electrically controlled three-dimensional displacement platform 24 to focus the laser on the core-cladding junction on the other side of the horizontal plane where the core center is located, adjust the distance between the laser focus and the core, and form a refractive index modulation line in the same way to obtain a second straight waveguide 13.

[0043] Embodiment 3

[0044] The advantages of the present invention compared with the prior art will be described in detail below in conjunction with this embodiment.

[0045] For the convenience of explanation, the fixed distance between the first straight waveguide 12 and the fiber core in the all-fiber MZI sensor of this embodiment is set to 4 μm; the distance between the second straight waveguide 13 and the fiber core is adjusted to 4 μm, 5 μm, 6 μm, and 7 μm, respectively. The transmission spectrum is shown in FIG. Figure 3 shown.

[0046] Figure 3 It can be seen that when the distance between the second straight waveguide 13 and the fiber core gradually increases, the resonant wavelength position with the highest interference contrast gradually redshifts; this is because when the distance between the second straight waveguide 13 and the fiber core is changed, different high-order core modes will be excited, so that the effective refractive index difference between the fiber core fundamental mode and different high-order core modes changes. According to the resonant wavelength formula:

[0047]

[0048] Among them, λ dip represents the resonant wavelength of the MZI sensor; L represents the length of the MZI sensor; m represents the interference order, usually m=0,1,2,…; Δn eff Represents the effective refractive index difference between the fundamental mode and the higher-order core modes.

[0049] Figure 4 The resonance wavelength with the highest contrast in the spectral transmission diagram of the all-fiber MZI sensor responds to the change in the distance between the fiber core and the second straight waveguide 13. When the distance changes by only 3 μm, the wavelength tuning range can reach 195 nm.

[0050] Figure 5The all-fiber MZI sensor prepared according to the preparation method of the present invention has a first straight waveguide 12 with a distance of 4 μm from the fiber core, and a second straight waveguide 13 with a distance of 4 μm from the fiber core, and its spectral response with twisting.

[0051] When the rotating fixture at one end of the all-fiber MZI sensor is rotated, the applied torsion angle changes, which in turn changes the polarization state of the linearly polarized light input to the MZI fiber sensor, causing the light intensity of each interference arm to change, resulting in a change in the spectral contrast. Figure 5 It can be seen that in the process of twist angle change from 0° to 180°, as the twist angle increases, the transmission intensity gradually decreases at the resonant wavelength position with the highest interference contrast (i.e. 1562.5nm); when the twist angle reaches 60°, the transmission intensity slowly decreases and the resonant wavelength redshifts; as the twist angle continues to increase, a new transmission peak appears at 1604.9nm, and the transmission intensity gradually increases. Experiments show that the all-fiber MZI sensor has a sensitive transmission intensity response to twisting; when the twist angle is in the range of 0° to 30°, the sensitivity can reach 9257.85dB / (rad / mm), which is twice the sensitivity of the fiber interferometers reported by other technical means, and the structural size is only a few millimeters.

[0052] In addition, this example only lists Figure 3 The sensing performance of an embodiment mentioned in the figure can achieve similar effects by performing a torsion test on the corresponding sensor when the fixed distance between the first straight waveguide 12 and the fiber core is changed and the distance between the second straight waveguide 13 and the fiber core is adjusted.

[0053] It should be noted that the processing parameters of the present invention can also be flexibly adjusted, and the laser parameters used in the above embodiments are not the only limitations.

[0054] The embodiments described above are only descriptions of the preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. An all-fiber MZI sensor with customizable wavelength, comprising: An optical fiber consisting of a core and a cladding, characterized in that it also includes: a first straight waveguide and a second straight waveguide; Wherein, the first straight waveguide and the second straight waveguide are located on both sides of the junction of the core and the cladding; The first straight waveguide and the second straight waveguide are composed of a plurality of refractive index modulated lines; The refractive index modulation line is formed by radiating the laser along the axial direction of the boundary between the core and the cladding; The distance between the first straight waveguide and the fiber core is fixed, and the distance between the second straight waveguide and the fiber core is adjustable. By changing the distance between the second straight waveguide and the fiber core, different high-order core modes are excited to achieve tuning of the resonant wavelength.

2. The all-fiber MZI sensor with customizable wavelength according to claim 1, characterized in that: The optical fiber is a single-mode optical fiber; the laser is a femtosecond laser, the repetition frequency of the femtosecond laser is 750 kHz, and the pulse width is 262 femtoseconds.

3. A method for manufacturing an all-fiber MZI sensor with customizable wavelength, the method being used to manufacture the all-fiber MZI sensor according to any one of claims 1 to 2, characterized in that the steps include: S1. placing the optical fiber on an electrically controlled three-dimensional displacement platform so that the axial direction of the optical fiber is perpendicular to the incident direction of the laser beam; S2. adjusting the longitudinal position of the optical fiber, focusing the laser beam into the fiber core by focusing the objective lens; S3. Adjust the electrically controlled three-dimensional displacement platform so that the laser is focused on the core-cladding junction on one side of the horizontal plane where the core center is located, turn on the laser beam, and move the electrically controlled three-dimensional displacement platform along the axial direction of the optical fiber at a preset rate; S4. Repeat step S3 to form a plurality of refractive index modulation lines to obtain the first straight waveguide; S5. Adjust the electrically controlled three-dimensional displacement platform to focus the laser on the core-cladding junction on the other side of the horizontal plane where the core center is located, adjust the distance between the laser focus and the core, and form the refractive index modulation line in the same way to obtain the second straight waveguide.

4. The method for manufacturing the all-fiber MZI sensor with customizable wavelength according to claim 3, characterized in that: During laser beam exposure, the refractive index modulation line is formed by controlling the pulse energy, repetition frequency and focus position of the laser beam.

5. The method for manufacturing the all-fiber MZI sensor with customizable wavelength according to claim 3, characterized in that: When the distance between the second straight waveguide and the core is changed, the resonance wavelength tuning is expressed as follows: Among them, λ dip represents the resonant wavelength of the MZI sensor; L represents the length of the MZI sensor; m represents the interference order; Δn eff Represents the effective refractive index difference between the fundamental mode and the higher-order core modes.

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