CN106940300A - A kind of liquid refractivity characteristic research method - Google Patents

Abstract

A kind of liquid refractivity characteristic research method based on Mach once moral filtering and LPFG, it is characterised in that including step 1：The system of processing that infrared femtosecond laser makes LPFG has been built using devices such as femto-second laser, three-D displacement platform, wideband light source and spectroanalysis instruments；Step 2：Making of the resonance wavelength for 1548.5nm LPFG is completed using the femtosecond laser system of processing built；Step 3：The making of fiber Mach -Zehnder interferometer is completed by way of fibre core dislocation welding using optical fiber splicer；Step 4：Mach-Zehnder interferometer is linked together with LPFG, LPFG transmitted spectrum is filtered；Step 5：The system constituted using Mach-Zehnder interferometer and LPFG is completed to liquid refractivity characteristic research.

Description

A Research Method of Liquid Refractive Index Characteristics

technical field

The invention relates to the field of optical fiber sensors, in particular to a research method for the direction of liquid refractive index characteristics based on Mach-Zehnder filtering and long-period optical fiber gratings.

Background technique

Optical fiber sensors have the advantages of light weight, small size, high temperature resistance, corrosion resistance, electromagnetic interference resistance and high sensitivity. At present, there are Fabry-Perot interferometry [2], Mach-Zehnder interferometry (MZI), Michelson interferometry and long period fiber grating (Long period fiber grating, LPFG) methods to measure the refractive index of liquids using optical fiber sensing. Wait. Traditional long-period fiber grating fabrication methods, such as ultraviolet exposure mask method, high-frequency CO2 laser point-by-point writing method, and etching groove, etc., have varying degrees of limitations in terms of hydrogen-carrying fiber processing, high-temperature stability, mechanical strength, and sensitivity. defect. The LPFG made by femtosecond laser has the advantages of high sensitivity and high temperature resistance.

The fabrication of LPFG and all-fiber MZI by using femtosecond laser is a research hotspot at home and abroad in recent years. In 2011, Li B et al. used the ultraviolet femtosecond laser point-by-point writing method to realize the preparation of LPFG with a resonance wavelength of 1476nm and a gate length of 25mm, and used LPFG for liquid refractive index sensing tests. increase, the transmission wavelength of LPFG is blue-shifted. In 2012, Miao Fei et al. used a femtosecond laser to fabricate an LPFG with a gate length of 40 mm and studied the high-temperature sensing characteristics of the LPFG. In 2013, Zhang Qi et al. used single-mode fiber and long-period fiber grating to make a cone fiber-long-period fiber grating-cone fiber type all-fiber MZI; in 2014, Yang Kai et al. used single-mode fiber to expand the fusion splicing technology with cone waist Made a coaxial liquid seal MZI. In 2015, Ahmed F et al. used a femtosecond laser to fabricate an LPFG with a grating length of 26.97mm in an undoped pure silicon fiber for the first time, and analyzed the high temperature and refractive index characteristics of the LPFG, and found that the resonance increases with the increase of the refractive index. The wavelength is blue shifted. In 2016, Bian Jicheng et al. fabricated an MZI temperature sensor in a polarization-maintaining fiber by means of arc discharge and fusion splicing, with a temperature sensitivity of 0.0998nm/°C. In summary, it is of great significance to study the fabrication of MZI by core dislocation fusion splicing and femtosecond fabrication of LPFG with high refractive index sensitivity and short gate region.

Contents of the invention

The present invention proposes a method for researching liquid refractive index characteristics based on Mach-Zehnder filter and long-period fiber grating, which is characterized in that it includes step 1: using devices such as femtosecond lasers, three-dimensional displacement platforms, broadband light sources, and spectrum analyzers A processing system for making long-period fiber gratings with an infrared femtosecond laser was built; Step 2: Use the built femtosecond laser processing system to complete the production of long-period fiber gratings with a resonance wavelength of 1548.5nm; Step 3: Use a fiber fusion splicer to pass through the fiber core The production of the optical fiber Mach-Zehnder interferometer is completed by dislocation fusion; step 4: connect the Mach-Zehnder interferometer with the long-period fiber grating, and filter the transmission spectrum of the LPFG; step 5: use the Mach-Zehnder interferometer and The system composed of long-period fiber gratings completes the research on the refractive index characteristics of liquids.

Preferably, the titanium sapphire femtosecond laser with a center wavelength of 800nm, a pulse width of 120fs, and a repetition rate of 1kHz in the step 1; the magnification of the microscopic objective lens used is 100 times, and the numerical aperture is 0.70; the work of the spectrum analyzer The wavelength range is 1200nm～2400nm, and the minimum resolution accuracy is 0.02nm.

Preferably, the period of the LPFG in step 2 is 400, the duty cycle is 0.5, the resonance strength is 7dB, and the half maximum width is 6.3nm.

Preferably, the discharge time of the fiber fusion splicer in step 3 is 2500ms, and the pre-melting time is 180ms; the interference spectrum contrast of the Mach-Zehnder interferometer is 10dB.

Preferably, the FWHM of the resonant wave filtered by the Mach-Zehnder interference filter of the long-period fiber grating in step 4 is 5.6 nm.

Preferably, the measurement system in step 5 tests the refractive index properties of alcohol, sucrose and NaCl solutions respectively, and the refractive index sensitivities of the three solutions are 01.78nm/RIU, 132.67nm/RIU and 138.80nm/RIU respectively.

It should be understood that both the foregoing general description and the following detailed description are exemplary illustrations and explanations, and should not be used as limitations on the claimed content of the present invention.

Description of drawings

With reference to the accompanying drawings, more objects, functions and advantages of the present invention will be clarified through the following description of the embodiments of the present invention, wherein:

Fig. 1 shows the structural representation of the MZI made based on the optical fiber core dislocation fusion splicing method according to the present invention;

Fig. 2 shows femtosecond laser to make LPFG processing system diagram;

Fig. 3 shows the structure diagram of long period fiber grating; (a) Schematic diagram of writing track of LPFG grid area; (b) microstructure diagram of LPFG grid area;

Fig. 4 shows the LPFG transmission spectrogram made based on femtosecond laser;

Figure 5 shows the MZI prepared based on core dislocation fusion; (a) micrograph of fiber 1 and fiber 2 core dislocation fusion; (b) micrograph of fiber 2 and fiber 3 core dislocation fusion; (c) MZI Transmission interference spectrum;

Fig. 6 shows the LPFG measurement experimental system diagram of refractive index based on MZI filtering;

Figure 7 shows the transmission spectrum after the MZI is connected with the LPFG; (a) the spectrograms before and after the MZI is connected to the LPFG; (b) the transmission spectrum near the resonance wavelength of the LPFG;

Fig. 8 shows the alcohol solution refractive index characteristic curve of LPFG; (a) the transmission spectrum of the LPFG under different refractive indices; (b) the fitting curve of the resonant wavelength and the refractive index of LPFG;

Fig. 9 shows the NaCl solution refractive index characteristic curve of LPFG; (a) the transmission spectrum of LPFG under different refractive indices; (b) the fitting curve of the resonant wavelength and the refractive index of LPFG;

Fig. 10 shows the characteristic curve of the refractive index of LPFG in sucrose solution; (a) the transmission spectrum of LPFG under different refractive indices; (b) the fitting curve of the resonant wavelength and the refractive index of LPFG.

detailed description

The objects and functions of the present invention and methods for achieving the objects and functions will be clarified by referring to the exemplary embodiments. However, the present invention is not limited to the exemplary embodiments disclosed below; it can be implemented in various forms. The essence of the description is only to help those skilled in the relevant art comprehensively understand the specific details of the present invention.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In the drawings, the same reference numerals represent the same or similar components, or the same or similar steps.

The present invention designs and builds the processing system based on 800nm infrared femtosecond laser to make LPFG, and makes the LPFG with the gate area length of 3.2mm; makes the MZI of all-fiber structure through the dislocation welding of the fiber core, and the long-period fiber grating The connection filters the resonant wavelength of the LPFG. The refractive index sensing characteristics of alcohol solution, NaCl solution and sucrose solution were analyzed by LPFG based on MZI filter. Experiments have found that with the increase of the refractive index, the transmission wavelength of the long-period fiber grating moves to the long-wave direction, which has a higher refractive index sensitivity.

Specific steps are as follows:

1 Theoretical analysis

Fig. 1 is a schematic structural diagram of an MZI fabricated based on optical fiber core dislocation fusion splicing. The MZI uses a fusion splicer to make two core mismatch attenuators through the method of optical fiber core dislocation fusion. The light in the fiber 1 core enters the fiber 2 after passing through the first core mismatch attenuator, and part of it is in the The fiber propagates in the core, and the other part enters the cladding for transmission. After the light transmitted in the core and cladding of the optical fiber 2 passes through the second optical fiber core mismatch attenuator, part of the light will be coupled into the core of the optical fiber 3 for transmission. Due to the phase difference between the core mode and the cladding mode of the optical fiber 2, different interference light intensities are generated, corresponding to different peaks on the transmission spectrum, forming an interference spectrum.

The transmitted light intensity of the Mach-Zehnder interferometer is:

In the formula: I ₁ and I ₂ respectively represent the light intensity of the core mode and cladding mode of fiber 2 coupled to the core of fiber 3; λ is the wavelength of light transmitted in the fiber; L represents the length of fiber 2; with is the effective refractive index of the fiber 2 core mode and the jth cladding mode. The phase difference between the core mode and the j-order cladding mode is:

Among them, Δn _eff is the effective refractive index difference between the core mode and the jth cladding mode.

When the phase difference satisfies When , the interference light intensity is the weakest, where k is an integer, and the wavelength corresponding to the trough in the transmission spectrum is:

According to the coupled-mode theory, LPFG is the coupling between the forward-propagating core mode and the forward-propagating cladding mode in the fiber, and the resonant wavelength in its transmission spectrum satisfies the following phase matching conditions:

Among them, λ _LP is the resonant wavelength of LPFG transmission spectrum, with are the effective refractive indices of the core fundamental mode and the j-order cladding mode, respectively, and Λ is the grating period. Since the change of the external refractive index will lead to the change of the effective refractive index, which will cause the shift of the resonant wavelength on the transmission spectrum of the long-period fiber grating, the LPFG can be used for the liquid refractive index sensing test.

2 Fabrication of long period fiber grating and Mach-Zehnder interferometer

2.1 Fabrication of long period fiber gratings

The composition of the system for fabricating long-period fiber gratings based on the femtosecond laser direct writing method is shown in Figure 2. The writing system uses a titanium sapphire femtosecond laser with a center wavelength of 800nm, a pulse width of 120fs, and a repetition rate of 1kHz. The infrared laser emitted by the laser first passes through a half-wave plate, a polarizer, an attenuation plate and a high-reflection mirror, and then focuses the light spot on the HI-1060 optical fiber fixed by the optical fiber clamp through a 100-fold microscope objective lens. The optical fiber clamp is fixed on the high Precision 3D motion platform. The Broadband light source (BBS) produced by JDSU Company was used as the test light source, and the output wavelength range of the light source was 1530nm-1600nm. The optical spectrum analyzer is AQ6375 optical spectrum analyzer (OSA) from Japan YOKOGAWA Company, the working wavelength range is 1200nm-2400nm, and the minimum resolution accuracy is 0.02nm. In the process of processing, a spectrum analyzer is used to observe the transmission spectrum of the fiber grating in real time, and the movement of the three-dimensional motion platform and the closing of the shutter are controlled by software to complete the production of the long-period fiber grating.

The femtosecond laser processes the optical fiber by writing line by line, and its writing track is shown in Figure 3(a). Set the processing speed of the femtosecond laser to 10 μm/s, the power to 50 μw, the grating period Λ=400 μm, the interval between dots and dashes a=40 μm, the writing length b=200 μm in a single period, and the duty cycle to be 0.5, as shown in Figure 3( b) Micrograph of the gate structure of LPFG.

In the experiment, 8 cycles were written, and the transmission spectrum of the LPFG is shown in Figure 4. The depth of the resonance peak of the LPFG at 1548.5nm is 7dB, and the full width at half maximum (FWHM) is 6.3mm. The length of the optical fiber grating region is less than about 3.2 mm, and the writing time is less than 3 minutes.

2.2 Fabrication of all-fiber Mach-Zehnder interferometer

In the experiment, the MZI was prepared by dislocation fusion splicing of optical fiber cores. The schematic diagram of the structure is shown in Figure 1. Among them, MZI is composed of three HI1060 optical fibers. The production process is as follows: remove the coating layer from the optical fiber 2, cut the end face flat with an optical fiber cutter, and then use an optical fiber fusion splicer (Fujikura FSM-80s) to splice the optical fiber 1 and the optical fiber 3 in dislocation respectively. In order to achieve dislocation fusion of optical fiber cores, the parameters of the optical fiber fusion splicer are: the core alignment method is manual alignment, the pre-melting time is 180ms, the pre-melting power is standard +10, and the discharge time is 2500ms. During the fusion splicing process, the dislocation distance of the fiber is manually controlled. Since the core diameter of the HI1060 fiber is 5.3, in order to ensure that the MZI interference spectrum has a high contrast and a high transmitted light intensity, the dislocation distance of the fiber core is manually set to about 2 to complete the MZI. make. Fig. 5(a) is a micrograph of dislocation welding of the left and right cores. Connect one end of the completed MZI to a broadband light source with a spectral range of 1530nm to 1600nm produced by JDSU Company, and connect the output end to the AQ6375 spectrum analyzer of Japan YOKOGAWA Company. The transmission spectrum of MZI is shown in Figure 5(b), which can be seen in the figure It is shown that the interference fringe contrast of the MZI is 10dB.

3 Liquid Refractive Index Sensing Experiment

In the experiment, the MZI fabricated based on dislocation welding was connected with LPFG, and the transmission spectrum of LPFG was filtered. LPFG was used to measure the three solutions of alcohol, NaCl and sucrose with different refractive indices. The experimental system is shown in Figure 6. The system consists of broadband light source, spectrum analyzer, MZI, LPFG and NaCl solution, alcohol solution, sucrose solution and slide glass and other devices. Fix LPFG on a glass slide, drop solutions with different refractive indices on LPFG, and observe its transmission spectrum in real time with a spectrometer. In order to avoid the influence of residual liquid on the LPFG on the experiment when each measurement is completed, wipe the LPFG with absolute ethanol after each measurement, and wait for the resonant wavelength of the LPFG to be consistent with the original resonant wavelength in the air before proceeding to the next set Refractive index experiment. The whole experimental process was completed in a clean room with a temperature of 20°C, which avoided the influence of temperature changes on the transmission spectrum.

The LPFG made by the infrared femtosecond laser is connected with the MZI made by the core dislocation fusion method, and its transmission spectrum in air is observed by a spectrometer. Figure 7(a) is a comparison of the transmission spectrum before and after the MZI is connected to the LPFG. It can be seen from the spectrogram in the dotted box in the figure that after connecting the LPFG, the transmission spectrum of the MZI changes significantly at the resonance wavelength of the LPFG. 7(b) is the spectrogram of the wavelength near the resonance peak of LPFG when MZI is connected with LPFG. It can be seen from the figure that the resonance intensity of the resonance peak of LPFG is 9dB, and the full width at half maximum is 5.6nm. Compared with Figure 4, the quality of the resonance peak has been significantly improved.

3.1 Refractive index sensing experiment of LPFG in alcohol solution

Alcohol solutions with concentrations of 5%, 10%, 15%, 20% and 25% were prepared with distilled water and absolute ethanol, and the distilled water and the above five concentrations of alcohol solutions were dropped on the LPFG respectively, and the refraction of several liquids The change range of the rate is 1.3331～1.3414, and the change of the transmission spectrum is observed with a spectrometer. Figure 8(a) is the transmission spectrum of the LPFG in alcohol solutions with different refractive indices. It can be seen that as the refractive index of the alcohol solution increases, the resonance wavelength of the LPFG moves to the long-wave direction, and the resonance peak red shifts. Figure 8(b) is a fitting curve of the relationship between the refractive index of the alcohol solution and the resonance wavelength of the LPFG. The response sensitivity of LPFG resonance wavelength to the refractive index of alcohol solution is 301.78nm/RIU, and the linearity of the fitting curve is 0.9917.

3.2 Refractive index sensing experiment of LPFG in NaCl solution

Add distilled water and NaCl solutions with concentrations of 3%, 6%, 9%, 12% and 15% to the LPFG on the glass slide with a rubber dropper, and the refractive index of the liquid ranges from 1.3331 to 1.3609 , the transmission spectra of LPFG in NaCl solutions with different refractive indices were observed by a spectrum analyzer. The transmission spectra of LPFG under different refractive indices of NaCl solution are shown in Fig. 9(a). As the refractive index of NaCl solution increases, the resonance wavelength of LPFG shifts to the long-wave direction. Fig. 9(b) is a fitting curve of the relationship between the refractive index of NaCl solution and the resonance wavelength of LPFG. It can be seen that the refractive index response sensitivity of LPFG based on MZI filtering is 138.80nm/RIU, and the linearity of the fitting curve is 0.9915.

3.3 Refractive index sensing experiment of LPFG in sucrose solution

In order to measure the characteristics of LPFG on the refractive index of sucrose solution, distilled water and sucrose solutions with concentrations of 3%, 6%, 9%, 12% and 15% were respectively dropped on LPFG, and the liquid refractive indices were 1.3331, 1.3386, 1.3441 , 1.3497, 1.3552 and 1.3607. A spectrometer was used to observe the change of its transmission spectrum in real time, and Fig. 10(a) is a transmission spectrum diagram of LPFG in sucrose solutions with different refractive indices. It can be seen from the figure that the transmission peak of LPFG red shifts with the increase of the refractive index. Fig. 10(b) is a fitting curve of the refractive index of the sucrose solution and the resonance peak wavelength of LPFG. The sensitivity of LPFG's response to the refractive index of sucrose solution was 132.67nm/RIU, and the linear fitting degree was 0.9938.

The beneficial effects of the present invention are:

A long-period fiber grating writing system was built using femtosecond lasers and three-dimensional displacement platforms, and an LPFG with a resonance wavelength of 1548.5nm, a resonance intensity of 7dB, and a gate length of 3.2mm was produced; the fiber core was dislocated and welded. The Mach-Zehnder interferometer with a contrast ratio of 10dB filters the transmission spectrum of the LPFG, reducing the full width at half maximum of the resonant peak of the LPFG from 6.3nm to 5.6nm. In the experiment, the refractive index sensitivities of LPFG based on MZI filtering in alcohol solution, NaCl solution and sucrose solution were 301.78nm/RIU, 132.80nm/RIU and 132.67nm/RIU, respectively, and the linear fitting degrees were 0.9917, 0.9915 and 0.9938, respectively. . Therefore, the designed optical fiber sensor based on Mach-Zehnder filter and long period fiber grating can be used for liquid refractive index measurement.

Other embodiments of the invention will be apparent to and understood by those skilled in the art from consideration of the specification and practice of the invention disclosed herein. The description and examples are considered exemplary only, with the true scope and spirit of the invention defined by the claims.

Claims (6)

1. A method for researching liquid refractive index characteristics based on Mach-Zehnder filter and long-period fiber grating, characterized in that, comprising Step 1: Using femtosecond laser, three-dimensional displacement platform, broadband light source and spectrum analyzer to build a processing system for making long-period fiber gratings with infrared femtosecond laser; Step 2: Use the built femtosecond laser processing system to complete the fabrication of long-period fiber gratings with a resonance wavelength of 1548.5nm; Step 3: Use the optical fiber fusion splicer to complete the production of the optical fiber Mach-Zehnder interferometer by dislocation fusion of the fiber core; Step 4: Connect the Mach-Zehnder interferometer with the long-period fiber grating to filter the transmission spectrum of the LPFG; Step 5: Use the system composed of the Mach-Zehnder interferometer and the long-period fiber grating to complete the research on the refractive index characteristics of the liquid. 2. The method according to claim 1, characterized in that: the titanium sapphire femtosecond laser of laser central wavelength 800nm, pulse width 120fs, repetition frequency 1kHz in the said step 1; the magnification of the microscopic objective lens that adopts is 100 times, the numerical aperture is 0.70; the working wavelength range of the spectrum analyzer is 1200nm-2400nm, and the minimum resolution accuracy is 0.02nm. 3. The method according to claim 1, characterized in that: the period of the long-period fiber grating in the step 2 is 400, the duty cycle is 0.5, the resonance strength is 7dB, and the full width at half maximum is 6.3nm. 4. The method according to claim 1, characterized in that: the discharge time of the optical fiber fusion splicer in the step 3 is 2500ms, and the pre-melting time is 180ms; the interference spectrum contrast of the Mach-Zehnder interferometer is 10dB. 5. The method according to claim 1, characterized in that: the FWHM of the resonant wave filtered by the Mach-Zehnder interference filter of the long-period fiber grating in the step 4 is 5.6 nm. 6. The method according to claim 1, characterized in that: the measuring system in the step 5 tests the refractive index properties of alcohol, sucrose and NaCl solutions respectively, and the refractive index sensitivities of the three solutions are respectively 01.78nm/ RIU, 132.67nm/RIU and 138.80nm/RIU.