阅读说明：本技术 一种单模多模光纤级联结构的长周期光纤光栅制备方法 (Preparation method of long-period fiber grating of single-mode multimode fiber cascade structure ) 是由 滕传新 朱永洁 苑立波 于 2020-12-08 设计创作，主要内容包括：本发明提供了一种单模多模光纤级联的长周期光纤光栅的制备方法。包括：将单模光纤和多模光纤熔接在一起,然后将其水平放置在带有高精度切割刀和位移控制的移动平台上,通过使用显微镜,使光纤在计算机上呈现出清晰的图像。然后利用移动精度为1μm的千分尺找到单模和多模光纤的熔接点,并保证熔接点与切割刀的位置刚好重合,记录此时千分尺的读数,旋转千分尺使熔接点的位置向单模光纤方向移动,并移至所需的多模光纤长度位置处,然后使用切割刀对光纤进行切割,之后将该段多模光纤与另一根单模光纤进行熔接,重复上述步骤,就可以制备得到单模多模光纤级联的长周期光纤光栅。该长周期光纤光栅具有结构和制备工艺简单、成本低廉,并且对温度、应变、弯曲等物理量的变化具有较高的灵敏度。(The invention provides a preparation method of a long-period fiber grating cascaded by single-mode multimode fibers. The method comprises the following steps: the single mode fiber and the multimode fiber are welded together and then horizontally placed on a moving platform with a high precision cutter and displacement control, and the fiber is made to present a clear image on a computer by using a microscope. Then finding the welding point of the single mode fiber and the multimode fiber by using a micrometer with the moving precision of 1 micrometer, ensuring that the welding point is exactly superposed with the position of the cutting knife, recording the reading of the micrometer at the moment, rotating the micrometer to enable the position of the welding point to move towards the direction of the single mode fiber, moving the welding point to the position of the required length of the multimode fiber, cutting the fiber by using the cutting knife, then welding the section of multimode fiber with another single mode fiber, and repeating the steps to prepare the single mode and multimode fiber cascaded long period fiber grating. The long-period fiber grating has the advantages of simple structure and preparation process, low cost and high sensitivity to the change of physical quantities such as temperature, strain, bending and the like.)

1. A method for preparing a single-mode multimode fiber cascade long-period fiber grating is characterized by comprising the following steps: the optical fiber is formed by cascade fusion of a single mode optical fiber and a multimode optical fiber, and the preparation steps are as follows:

the end faces of the single-mode optical fiber and the multimode optical fiber are cut well, the single-mode optical fiber and the multimode optical fiber are welded together through an optical fiber welding machine, then the single-mode optical fiber and the multimode optical fiber are horizontally placed on a device with a high-precision cutter, the single-mode optical fiber end and the multimode optical fiber end are respectively fixed through a clamp, and then the optical fiber presents a clear image on a computer through a microscope. The cutting knife and the two optical fiber clamps are arranged on the displacement table, and the optical fibers on the cutting knife and the two optical fiber clamps can be in a horizontal state. Then, a micrometer with the moving precision of 1 micrometer is used for finding the welding point of the single-mode optical fiber and the multimode optical fiber, the position of the welding point is ensured to be exactly superposed with that of the cutting knife, and the reading of the micrometer at the moment is recorded. And rotating the micrometer to enable the position of the welding point to move towards the single-mode optical fiber direction, obtaining the required length of the multimode optical fiber by reading the reading of the micrometer, cutting the multimode optical fiber by using a high-precision cutting knife when the micrometer reaches the reading of the multimode optical fiber to be intercepted, welding the section of multimode optical fiber with another single-mode optical fiber, and repeating the steps to prepare the single-mode multimode fiber grating with the long period cascaded with the multimode optical fiber.

2. The single mode, multimode fiber-welded long period fiber grating of claim 1, wherein: the outer diameters of the single-mode and multimode fibers were 125 μm, the diameter range of the single-mode core was 9 μm, and the diameter range of the multimode core was 65 μm.

3. The single mode, multimode fiber-welded long period fiber grating of claim 1, wherein: the length of the single-mode fiber in the long-period fiber grating is 400 micrometers, the length of the multimode fiber is 300 micrometers, the total 4 periods of the long-period fiber grating are 4, and the length of the gate area is 2.8 mm.

Technical Field

The invention belongs to the technical field of optical fiber sensing, and particularly relates to a preparation method of a long-period fiber grating of a single-mode multimode fiber cascade structure.

Background

The long-period fiber grating (LPFG) is a grating with the period size of tens of microns to hundreds of microns, the mode coupling is the coupling between a core fundamental mode and a cladding mode which are propagated in the same direction, a resonance peak can be formed in a specific wavelength range, and the LPFG is a transmission type all-fiber band-stop filter. The resonance wavelength and amplitude of the LPFG can change along with the change of parameters such as temperature, strain, refractive index and the like of the external environment, and the LPFG is more sensitive to the change of external physical quantity than the optical fiber Bragg grating. Therefore, LPFG has attracted a great deal of attention.

Currently, there are several methods for fabricating LPFGs, the most commonly used are the following: UV exposure method, CO₂Laser irradiation method, fused biconical taper method, etc. The method of the ultraviolet exposure method has good stability, but is influenced by the external temperature, when the temperature is higher than 500 ℃, the grating can be erased, the cost is high, the manufacturing period is long, and the like. From CO₂The long-period fiber grating prepared by the radiation point-by-point writing method has better stability, high flexibility and convenient and quick manufacturing process, and omits the fussy preparation process, but the method needs high cost. The fused biconical taper method uses the computer to control the moving speed, distance, stretching length of the clamping device and other parameters of the heater to manufacture the long-period fiber grating with different performances, and the fused biconical taper grating is connected with other structures in series to carry out multi-parameter measurement. However, the long-period fiber grating manufactured by the method has complex process and high cost. In addition, there are some reports of LPFG manufacturing methods, such as patent application No.: 201510267320.8 patent of the invention "Long period fiber Grating and preparation method" provides a method for reaming an optical fiber to prepare LPFG. However, the process of the manufacturing method is complex and difficult to control. Therefore, it is important to develop an LPFG with simple structure and fabrication process and low cost.

Disclosure of Invention

The invention aims to provide an LPFG preparation method of a single-mode multimode fiber cascade structure, which has the advantages of simple structure, simple preparation process and low cost.

To solve the above problems, a first aspect of the present invention provides a method for preparing an LPFG of a single-mode multimode fiber cascade structure, including: the end faces of the single-mode optical fiber and the multimode optical fiber are cut well, the single-mode optical fiber and the multimode optical fiber are welded together through an optical fiber welding machine, then the single-mode optical fiber and the multimode optical fiber are horizontally placed on a device with a high-precision cutter, the single-mode optical fiber end and the multimode optical fiber end are respectively fixed through a clamp, and then the optical fiber presents a clear image on a computer through a microscope. The cutting knife and the two optical fiber clamps are arranged on the displacement table, and the optical fibers on the cutting knife and the two optical fiber clamps can be in a horizontal state. Then, a micrometer with the moving precision of 1 micrometer is used for finding the welding point of the single mode and the multimode optical fiber, the welding point is ensured to be exactly coincided with the position of the cutting knife, the reading of the micrometer is recorded at the moment, the micrometer is rotated to enable the position of the welding point to move towards the direction of the single mode optical fiber, the required length of the multimode optical fiber is obtained by reading the reading of the micrometer, when the micrometer reaches the reading of the multimode optical fiber to be intercepted, the cutting knife with high precision is used for cutting, then the section of multimode optical fiber is welded with another single mode optical fiber, and the steps are repeated, so that the long-period fiber bragg grating of the single mode multimode optical fiber cascade structure can be prepared.

The present invention may further comprise:

1. the interception of the lengths of the single-mode optical fiber and the multi-mode optical fiber in the LPFG structure is controlled by a micrometer with the precision of 1 micrometer; the preparation process of the long-period fiber grating is carried out under a microscope system;

2. the length of a single-mode optical fiber in the LPFG is 200-400 mu m, the length of a multi-mode optical fiber is 100-300 mu m, the total period of the LPFG is 3-6, the period length is 300-1000 mu m, and the gate region length is 1-10 mm.

3. The outer diameters of single-mode and multi-mode fibers in the long-period fiber grating are 125 micrometers, the diameter range of a single-mode fiber core is 8-9 micrometers, and the diameter range of a multi-mode fiber core is 50-65 micrometers.

The invention provides a preparation method of LPFG, which has the advantages of simple structure, simple preparation process, low cost, controllable length and period of fiber bragg grating and the like.

The working principle of the invention is as follows: the light emitted by the light source firstly enters the single-mode fiber for transmission, and when the light is transmitted to a first single-mode-multimode interface, a fundamental mode transmitted in a fiber core of the single-mode fiber is incident into the multimode fiber and is converted into a high-order mode; when light continues to be transmitted in the multimode fiber through the multimode-single mode interface, because the diameters of the fiber cores of the single mode fiber and the multimode fiber are not matched, one part of light returns to the fiber core of the single mode fiber and is converted into the fundamental mode in the fiber core of the single mode fiber, and the other part of light can enter the cladding of the single mode fiber and is converted into the cladding mode which is easy to be lost by the coating layer. Because of the short length of single mode fiber in such LPFGs, the cladding modes are not completely lost as they travel to the next multimode fiber, and some of the energy is re-coupled back into the core and interferes with the fundamental mode in the core. Therefore, when the single-mode-multimode optical fiber is arranged in a periodic structure, fundamental mode energy is periodically coupled into a high-order mode and then coupled back to the fiber core, thereby forming the fiber grating. When the light wave with specific wavelength meets the phase matching condition, the interference effect of the fiber core fundamental mode and the specific cladding mode is strongest, and thus a loss peak appears on the output spectrum. When the refractive index, temperature and strain of the environment around the LPFG change, the phase matching condition of light of a specific wavelength changes, which may cause a shift of loss peak. Therefore, the change of the measured substance can be detected from the positional shift of the monitoring loss peak.

Compared with the prior art, the invention has the following advantages:

1. the LPFG of the invention does not need expensive grating writing equipment, and has simple manufacturing process and low cost.

2. The preparation method of the LPFG is flexible, and the grating period can be adjusted by controlling the length of the welded single-mode or multi-mode optical fiber.

3. The LPFG has higher sensitivity to physical quantities such as temperature, strain, bending and the like, and has important application value in the field of sensing.

Drawings

FIG. 1 is a schematic LPFG of a single mode multimode fiber cascade structure of the present invention;

FIG. 2 is a schematic view of a single mode, multimode fiber cleaving apparatus of the present invention;

FIG. 3 is a transmission spectrum of a long period fiber grating according to the present invention.

Detailed Description

Specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings.

Referring to fig. 1, the structure of the long period fiber grating of the single mode multimode fiber cascade of the present invention is schematically shown. The working principle is that when the light energy in the optical fiber is transmitted to the first single-mode-multimode interface surface, the fundamental mode in the fiber core of the single-mode optical fiber 1 is incident into the multimode optical fiber 2 and is changed into a high-order mode. When the light energy in the multimode fiber passes through the multimode-single mode interface, because the diameters of fiber cores of the single mode fiber and the multimode fiber are not matched, one part of the light energy returns to the fiber core of the single mode fiber to become a basic mode in the fiber core, and the other part of the light energy enters a cladding layer of the single mode fiber to become a cladding mode which is easy to be lost by a coating layer. Because the length of the long-period fiber grating single-mode fiber is short, the cladding mode is not completely lost when being transmitted to the next multimode fiber, and part of energy is recoupled to the fiber core and generates interference with the fundamental mode in the fiber core. When the single-mode-multimode fiber structure is periodically arranged, fundamental mode energy is periodically coupled into higher-order modes which are then coupled back into the fiber core to form the fiber grating. When the light wave with a specific wavelength meets the phase matching condition, the interference effect of the fiber core fundamental mode and the specific cladding mode is strongest, and thus a loss peak appears on the output spectrum.

Referring to fig. 2, the schematic diagram of the apparatus for preparing the long-period fiber grating of the single-mode and multi-mode fiber cascade structure of the present invention comprises a microscope system 3, a high precision cutter 4, two clamps 5, a moving platform 6 and a micrometer 7. The preparation method comprises the following steps: before cutting, calibration is needed firstly, the process needs to be carried out by observing and adjusting the microscope 3 through a computer, and when the position of the optical fiber cutting knife 4 can generate a bright light on a display screen, the calibration is finished. Then, the operation of fusion splicing and cutting of the optical fiber is carried out, specifically: the end faces of the single-mode optical fiber and the multimode optical fiber are cut well, the single-mode optical fiber and the multimode optical fiber are welded together through an optical fiber welding machine, then the single-mode optical fiber and the multimode optical fiber are horizontally placed on a device with a high-precision cutter, a single-mode optical fiber end and a multimode optical fiber end are respectively fixed through a clamp 5, and then the optical fiber presents a clear image on a computer through a microscope 3. The cutting knife and the two optical fiber clamps are arranged on the displacement table 6, and the optical fibers on the cutting knife and the two optical fiber clamps can be in a horizontal state. Then, a micrometer 7 with the moving precision of 1 mu m is used for finding the welding point of the single-mode optical fiber and the multimode optical fiber, ensuring that the position of the welding point is exactly coincided with that of the cutting knife, and recording the reading of the micrometer at the moment. The method comprises the steps of rotating a micrometer to enable the position of a welding point to move towards the direction of a single-mode optical fiber, obtaining the length of the needed multimode optical fiber by reading the reading of the micrometer, cutting the multimode optical fiber by using a high-precision cutting knife when the micrometer reaches the reading of the multimode optical fiber to be intercepted, welding the section of multimode optical fiber with another single-mode optical fiber, and repeating the steps to prepare the long-period fiber grating of the single-mode multimode optical fiber cascade structure.

Referring to fig. 3, a transmission spectrum of the long period fiber grating of the present invention. The phase matching conditions of the long-period fiber grating are as follows:

n_co-n_cl＝λ/Λ

wherein n is_co、n_clThe effective refractive indexes of the fiber core and the cladding of the optical fiber respectively, lambda is the grating period, and lambda is the wavelength of the characteristic peak. When the light wave with specific wavelength meets the phase matching condition, the interference effect of the fiber core fundamental mode and the specific cladding mode is strongest, and a characteristic peak appears on the output spectrum. When the effective refractive index of the optical fiber cladding or the grating period changes due to environmental changes, the position of the characteristic peak changes, and therefore sensing of related parameters can be performed by monitoring the position change of the characteristic peak. As can be seen from FIG. 3, the LPFG of the prepared single-mode multimode cascade structure has an obvious characteristic peak at about 1563nm, and the duty ratio of the LPFG is-35 dB, so that the application in the aspect of sensing can be met.