CN108332947A - A kind of fiber grating refractive index sensing demodulation method based on cutoff wavelength - Google Patents

Abstract

The invention discloses a fiber grating refractive index sensing and demodulation method based on the cut-off wavelength. The method uses the transmission spectrum when the refractive index of the environment where the inclined fiber grating is located is greater than or equal to the cladding refractive index as a reference spectrum, and uses different refractive index The transmission spectrum of the down-tilted fiber grating is the sensing measurement spectrum minus the reference spectrum to obtain the demodulation spectrum. The shortest wavelength when the relative transmission intensity in the demodulation spectrum is equal to a certain set value is used as the measurement standard. According to the wavelength and the refractive index function The relation demodulates the refractive index. Compared with the method of performing refractive index sensing and demodulating the offsets of different refractive indices through the cladding mode and the method of performing refractive index sensing and demodulating the offsets of different refractive indices through the cut-off mode of the cladding mode, the method, It can provide higher precision demodulation; and it is easy to automatically perform demodulation method with computer programs and algorithms. The method of the invention can be applied to the field of optical fiber grating refractive index sensing.

Description

A Fiber Bragg Grating Refractive Index Sensing Demodulation Method Based on Cutoff Wavelength

technical field

The invention belongs to the technical field of optical fiber grating sensing, and relates to a demodulation method suitable for sensing the refractive index of an inclined optical fiber grating.

Background technique

In fiber Bragg grating refractive index sensing, at present, it is mainly used to measure the shift of the cladding mode wavelength at different refractive indices in the transmission spectrum of the tilted fiber grating and measure the shift of the cutoff mode wavelength of the cladding mode at different refractive indices. A method for refractive index sensing demodulation. The method of demodulating different offsets of different refractive indices through the cladding mode is mainly to first measure the offset of the cladding mode wavelength in the transmission spectrum of the tilted fiber grating under different refractive index environments, and further according to the offset and The functional relationship between the refractive indices is demodulated to obtain the refractive index of the medium to be measured. The method of demodulating different offsets of different refractive indices by the cut-off mode of the cladding mode is mainly to first measure the offset of the cut-off mode wavelength of the cladding mode in the transmission spectrum of the inclined fiber grating under different refractive index environments, The refractive index of the medium to be measured is further demodulated according to the functional relationship between the offset and the refractive index.

Refractive index sensing and demodulation of different refractive index offsets through the cladding mode is simple to implement, but the sensitivity is low (less than 30nm/RIU) and the resolution is low (under the condition of a spectrometer resolution of 0.02nm, the resolution is 10 ^-4 RIU magnitude). The demodulation method through the cut-off mode of the cladding mode has high refractive index sensitivity, which can reach 574nm/RIU. However, since the cladding modes are discrete, it is difficult to measure when the wavelength change of the cladding cut-off mode caused by the refractive index is in the middle of the two cladding modes, so the resolution of the refractive index change that can be measured is limited. In addition, when the refractive index sensing is performed on the offsets of different refractive indices, no specific criteria for judging the cut-off mode are given, and there are errors in judging the cut-off mode.

Contents of the invention

The purpose of the present invention is to overcome the shortcoming of above-mentioned prior art, provide a kind of fiber grating refractive index sensing demodulation method based on cut-off wavelength, when it adopts refractive index sensing, the transmission spectrum of tilted fiber grating minus the cladding mode is suppressed The difference spectrum is obtained from the smooth transmission spectrum of the smooth transmission spectrum, which is used as the demodulation spectrum, and the threshold value is set in the demodulation spectrum to determine the cut-off wavelength, and then the refractive index is demodulated according to the functional relationship between the cut-off wavelength and the refractive index of the medium. This method not only has high demodulation accuracy, but also the cut-off wavelength can be easily obtained by computer or other algorithms.

The purpose of the present invention is solved by the following technical solutions:

A kind of fiber grating refractive index sensing demodulation method based on the cut-off wavelength provided by the present invention comprises the following steps:

(1) measure the transmission spectrum of the tilted fiber grating when the refractive index of the external environment is greater than or equal to the refractive index of the cladding of the tilted fiber grating, and use it as a reference spectrum;

(2) Measure the transmission spectrum of the tilted fiber grating in the environment where the refractive index changes that require sensing and demodulation, and use it as the sensing measurement spectrum;

(3) subtract the reference spectrum obtained in step (1) from the sensing measurement spectrum obtained in step (2), to obtain the demodulated spectrum;

(4) According to the cladding mode transmission spectrum intensity in the demodulation spectrum, set a threshold value, start to calculate from short wavelength to long wavelength, and the first wavelength that reaches the set threshold value is recorded as the cut-off wavelength;

(5) Measure the function curve between the cut-off wavelength and the refractive index, and calibrate the response coefficient of the cut-off wavelength of the inclined fiber Bragg grating under different refractive indices;

(6) According to the function curve relationship between the cut-off wavelength and the refractive index measured in step (5) and the transmission spectrum difference spectrum under the environment to be measured, the refractive index of the environment where the refractive index change needs to be sensed and demodulated to demodulate.

Among the above technical solutions, there are further limited solutions.

Further, in step (1), the refractive index of the external environment is greater than or equal to the refractive index of the cladding, which means that the refractive index in the environment is greater than or equal to the refractive index of the cladding of the tilted fiber grating, so that in the tilted fiber grating The spectral modulation produced by the cladding mode response disappears, and only the cladding mode causes the slowly-varying transmission loss in the transmission spectrum.

Further, in step (4), the modulation intensity of the cladding mode transmission spectrum gradually decreases from short wavelengths with the increase of the environmental refractive index, and the wavelength position at which the modulation intensity begins to decrease decreases with the increase of the environmental refractive index. Large and shifted to the long wavelength direction.

Further, in step (4), the threshold k can be selected as n<k<m, where m is the maximum value of the cladding mode intensity in the demodulated spectrum, and n is the minimum cladding mode intensity in the demodulated spectrum value.

Further, in step (4), the cut-off wavelength is determined by computer: first set the error range, and then interpolate the demodulated spectrum, so that the discrete interval of the demodulated spectrum is smaller than the error value, from the short wavelength to the long wavelength direction , so that the difference between the intensity of the cladding mode in the demodulated spectrum minus the threshold k is less than the error value, and the wavelength corresponding to the intensity is the cut-off wavelength.

Further, the tilted fiber grating is prepared based on femtosecond laser

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

The cut-off wavelength-based fiber grating refractive index sensing and demodulation method provided by the present invention uses the transmission spectrum when the refractive index of the environment where the tilted fiber grating is located is greater than or equal to the cladding refractive index as the reference spectrum, and uses the sensor to measure the spectrum during demodulation Subtract the reference spectrum of the tilted fiber grating to obtain the difference spectrum, as the demodulation spectrum, set the threshold in the demodulation spectrum to determine the cut-off wavelength, the shortest wavelength when the relative transmission intensity in the demodulation spectrum is equal to the threshold is recorded as the cut-off wavelength, and then The refractive index is demodulated according to the functional relationship between the cut-off wavelength and the refractive index of the medium. Compared with the prior art, the method has the advantages of simple and accurate recognition of the cut-off wavelength, easy realization of computer automatic recognition, and high demodulation precision of the refractive index sensor.

Further, the present invention solves the problem of low sensitivity (less than 30nm/RIU) and low resolution (under the condition that the resolution of the spectrometer is 0.02nm, the resolution of the refractive index sensing and demodulation of different refractive index offsets is low by using the cladding mode). rate of 10 ^-4 RIU level); the cut-off wavelength-based fiber grating refractive index sensing and demodulation method used in the present invention can achieve a precision as high as 949.9nm/RIU.

Further, compared with the prior art, the sensitivity of the demodulation method of the cut-off mode of the cladding mode is up to 574nm/RIU. The cladding mode is discrete, and the wavelength change of the cladding cut-off mode caused by the refractive index is in two cladding layers. It is difficult to use this method to measure in the middle of the mode, so the resolution of the refractive index change that can be measured is limited; in addition, because the cladding mode not only changes the modulation degree, but also shifts the wavelength position of the cladding mode, the discrimination of the cut-off mode The algorithm is more complicated. The demodulation method of the present invention can automatically identify the cut-off wavelength through a computer program or algorithm by setting a threshold when performing refractive index sensing on the offsets of different refractive indices, and the determination method is simple and easy to perform quantitative determination .

Description of drawings

Figure 1 is the transmission spectrum of the tilted fiber grating when the ambient refractive index is 1.4578;

Fig. 2 is the transmission spectrum diagram of the tilted fiber grating when the refractive index of the environment is 1.4401;

Fig. 3 is the demodulation spectrogram when selecting threshold to be 0.1dB;

Figure 4 is the corresponding relationship between the ambient refractive index and the cut-off wavelength when the threshold is selected as 0.1dB;

Fig. 5 is the demodulation spectrogram when selecting threshold to be 0.3dB;

Figure 6 is the corresponding relationship between the ambient refractive index and the cut-off wavelength when the threshold is selected as 0.3dB;

Fig. 7 is the demodulation spectrogram when selecting threshold to be 0.9dB;

Figure 8 shows the corresponding relationship between the ambient refractive index and the cut-off wavelength when the threshold is selected as 0.9dB.

Detailed ways

The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

In order to more clearly illustrate the purpose, technical solutions and advantages of the present invention, the present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described here are only used to explain the present invention, not to limit the present invention.

The fiber grating refractive index sensing demodulation method based on the cut-off wavelength of the present invention is specifically carried out according to the following steps:

(1) Place the tilted fiber Bragg grating refractive index sensor for refractive index sensing in an environment with variable refractive index, and measure the transmission spectrum of the tilted fiber Bragg grating at the same time, by changing the refractive index of the environment to be greater than or equal to the refractive index of the fiber cladding, The spectral modulation caused by the response of the cladding mode in the tilted fiber grating disappears, and only the cladding mode in the transmission spectrum causes the slow-varying transmission loss. The transmission spectrum is displayed as a smooth curve, and the transmission spectrum at this time is saved as a reference spectrum.

The above-mentioned tilted fiber grating is prepared based on femtosecond laser.

(2) In the medium to be measured that needs to be sensed and demodulated, measure the transmission spectrum of the tilted fiber grating, and use it as the sensing measurement spectrum.

(3) During demodulation, subtract the reference spectrum saved in step (1) from the sensing measurement spectrum obtained in step (2) to obtain the demodulation spectrum.

(4) According to the amplitude of the cladding mode in the demodulated spectrum, a threshold k is set, and the threshold k can be selected as n<k<m, where m is the maximum value of the cladding mode intensity in the demodulated spectrum, and n is The minimum value of the cladding mode intensity in the demodulated spectrum may be, for example, 0.1dB, 0.3dB, 0.9dB or the like. The modulation intensity of the cladding mode transmission spectrum decreases gradually with the increase of the ambient refractive index from the short wavelength, and the wavelength position where the modulation intensity begins to decrease shifts to the long wavelength direction with the increase of the ambient refractive index.

Draw a dotted line based on the threshold value, from the short wavelength to the long wavelength direction, the first wavelength corresponding to the cladding mode that intersects with the dotted line is recorded as the cut-off wavelength, and different cut-off wavelengths correspond to different refractive indices in the environment, which can be Perform refractive index demodulation.

The cut-off wavelength is determined by computer or manually: first set the error range, and then interpolate the demodulated spectrum, so that the discrete interval of the demodulated spectrum is smaller than the error value, from the short wavelength to the long wavelength direction, so that the cladding mode in the demodulated spectrum If the difference between the intensity minus the threshold k is less than the error value, the wavelength corresponding to the intensity is the cut-off wavelength.

(5) Measure the function curve between the cut-off wavelength and the refractive index, and calibrate the response coefficient of the cut-off wavelength of the inclined fiber Bragg grating under different refractive indices.

(6) According to the function curve relationship between the cut-off wavelength and the refractive index measured in step (5) and the transmission spectrum difference spectrum under the environment to be measured, the refractive index of the environment where the refractive index change needs to be sensed and demodulated to demodulate.

The present invention will be described in further detail below by several specific embodiments:

The present invention is applicable to sensing demodulation of various inclined fiber grating refractive index. In this embodiment, the inclined angle of femtosecond laser processing is selected as 3.8° inclined fiber grating for experiments, and the cut-off wavelength is selected with different thresholds in the demodulation spectrum.

Example 1

In this embodiment, an experiment is carried out with an inclined fiber grating whose inclination angle is 3.8° as an example, and the cut-off wavelength is selected with 0.1 dB as the threshold in the demodulation spectrum, as follows:

Raw materials: Femtosecond laser fabricated tilted fiber gratings with a tilt angle of 3.8°.

The detailed implementation of the refractive index sensing demodulation step is described as follows:

(1) Place the tilted fiber Bragg grating refractive index sensor for refractive index sensing in an environment with variable refractive index, and measure the transmission spectrum of the tilted fiber Bragg grating at the same time. By changing the refractive index of the environment to 1.4578, the The spectral modulation produced by the cladding mode response disappears, and only the cladding mode in the transmission spectrum causes the slow-varying transmission loss, and the transmission spectrum is displayed as a smooth curve, as shown in Figure 1, and the transmission spectrum is saved as a reference spectrum;

(2) When the refractive index of the environment is changed to 1.4401, the transmission spectrum of the tilted fiber grating refractive index sensor is measured, and it is used as the sensing measurement spectrum, as shown in Figure 2;

(3) subtract the difference spectrum (Fig. 1) that obtains from the reference spectrum (Fig. 1) preserved in the step (1) from the sensing measurement spectrum (Fig. 2) obtained in the step (2), and record it as the demodulation spectrum, see Fig. 3;

(4) According to the relative transmission intensity of the cladding mode of the demodulation spectrum (Fig. 3), setting a threshold k is 0.1dB, see 1 in Fig. 3, the size of the threshold is between the cladding mode maximum amplitude and the minimum amplitude , draw a dotted line with the threshold value as the standard, from the short wavelength to the long wavelength direction, the wavelength corresponding to the first transmission intensity that reaches the set threshold value is recorded as the cut-off wavelength under the refractive index, see 2 in Figure 3, the The cut-off wavelength corresponds to the environment with a refractive index of 1.4401;

(5) Change the refractive index of the medium to 1.4125RIU, 1.416RIU, 1.4205RIU, 1.4251RIU, 1.4324RIU, 1.4346RIU, 1.4392RIU, 1.4435RIU, 1.4487RIU, 1.4532RIU, 1.4552RIU, and measure the The cutoff wavelength is calibrated. See Figure 4 for the relationship between the refractive index and the cut-off wavelength at a threshold of 0.1dB.

(6) According to the function curve relation between the cut-off wavelength measured in step (5) and the refractive index is: Y=423.675-281.185X (the external environment refractive index range is 1.4125～1.4435), Y=1389.557-949.878X ( The refractive index of the external environment ranges from 1.4435 to 1.4552), where Y is the offset of the cut-off wavelength, and X is the refractive index of the unknown environment to be measured. The demodulation accuracy can reach 949.878nm/RIU.

Example 2

In this embodiment, an experiment is carried out with an inclined fiber grating whose inclination angle is 3.8° as an example. In the demodulation spectrum, 0.3 dB is used as the threshold to select the cut-off wavelength, as follows:

Raw materials: Femtosecond laser fabricated tilted fiber gratings with a tilt angle of 3.8°.

The detailed implementation of the refractive index sensing demodulation step is described as follows:

(1) Place the tilted fiber Bragg grating refractive index sensor for refractive index sensing in an environment with variable refractive index, and measure the transmission spectrum of the tilted fiber Bragg grating at the same time. By changing the refractive index of the environment to 1.4578, the The spectral modulation produced by the cladding mode response disappears, and only the cladding mode in the transmission spectrum causes the slow-varying transmission loss, and the transmission spectrum is displayed as a smooth curve, as shown in Figure 1, and the transmission spectrum is saved as a reference spectrum;

(2) When the refractive index of the environment is changed to 1.4401, the transmission spectrum of the tilted fiber grating refractive index sensor is measured, and it is used as the sensing measurement spectrum, as shown in Figure 2;

(3) subtract the difference spectrum (Fig. 1) that obtains from the reference spectrum (Fig. 1) preserved in the step (1) from the sensing measurement spectrum (Fig. 2) obtained in the step (2), and record it as the demodulation spectrum, see Fig. 5;

(4) According to the relative transmission intensity of the cladding mode of the demodulation spectrum (Fig. 5), a threshold is set to be 0.3dB, see 3 in Fig. 5, the size of the threshold is between the cladding mode maximum amplitude and the minimum amplitude , draw a dotted line with the threshold value as the standard, from the short wavelength to the long wavelength direction, the wavelength corresponding to the first transmission intensity reaching the set threshold value is recorded as the cut-off wavelength under the refractive index, see 4 in Figure 5, the cut-off wavelength The wavelength corresponds to the environment with a refractive index of 1.4401;

(5) Change the refractive index of the medium to 1.4125RIU, 1.416RIU, 1.4205RIU, 1.4251RIU, 1.4324RIU, 1.4346RIU, 1.4392RIU, 1.4435RIU, 1.4487RIU, 1.4532RIU, 1.4552RIU, and measure the The cutoff wavelength is calibrated. See Figure 6 for the corresponding relationship between the refractive index and the cut-off wavelength at a threshold of 0.3dB.

(6) According to the function curve relation between the cut-off wavelength measured in step (5) and the refractive index is: Y=479.764-321.845X (the external environment refractive index range is 1.416～1.4435), Y=1374.859-941.715X ( The refractive index of the external environment ranges from 1.4435 to 1.4552), where Y is the offset of the cut-off wavelength, and X is the refractive index of the unknown environment to be measured. The demodulation accuracy can reach 941.715nm/RIU.

Example 3

In this embodiment, an experiment is carried out with an inclined fiber grating whose inclination angle is 3.8° as an example, and the cut-off wavelength is selected with 0.9 dB as the threshold in the demodulation spectrum, as follows:

Raw materials: Femtosecond laser fabricated tilted fiber gratings with a tilt angle of 3.8°.

The detailed implementation of the refractive index sensing demodulation step is described as follows:

(1) Place the tilted fiber Bragg grating refractive index sensor for refractive index sensing in an environment with variable refractive index, and measure the transmission spectrum of the tilted fiber Bragg grating at the same time. By changing the refractive index of the environment to 1.4578, the The spectral modulation produced by the cladding mode response disappears, and only the cladding mode in the transmission spectrum causes the slow-varying transmission loss, and the transmission spectrum is displayed as a smooth curve, as shown in Figure 1, and the transmission spectrum is saved as a reference spectrum;

(2) When the refractive index of the environment is changed to 1.4401, the transmission spectrum of the tilted fiber grating refractive index sensor is measured, and it is used as the sensing measurement spectrum, as shown in Figure 2;

(3) subtract the difference spectrum (Fig. 1) that obtains from the reference spectrum (Fig. 1) preserved in the step (1) with the sensing measurement spectrum (Fig. 2) obtained in the step (2), and record it as the demodulation spectrum, see Fig. 7;

(4) According to the relative transmission intensity of the cladding mode of the demodulation spectrum (Fig. 7), a threshold is set to be 0.9dB, see 5 in Fig. 7, the size of the threshold is between the cladding mode maximum amplitude and the minimum amplitude , draw a dotted line with the threshold value as the standard, from the short wavelength to the long wavelength direction, the wavelength corresponding to the first transmission intensity reaching the set threshold value is recorded as the cut-off wavelength under the refractive index, see 6 in Figure 7, the cut-off wavelength The wavelength corresponds to the environment with a refractive index of 1.4401;

(5) Change the refractive index of the medium to 1.4125RIU, 1.416RIU, 1.4205RIU, 1.4251RIU, 1.4324RIU, 1.4346RIU, 1.4392RIU, 1.4435RIU, 1.4487RIU, and measure the cut-off wavelengths under different refractive indices for calibration. The corresponding relationship between the refractive index and the cut-off wavelength at the threshold of 0.9dB is shown in Figure 8.

(6) According to the function curve relation between the cut-off wavelength measured in step (5) and the refractive index is: Y=704.062-479.631X (the external environment refractive index range is 1.4251～1.4487), Y is the cut-off wavelength in the formula Offset, X is the refractive index of the unknown environment to be measured. The demodulation accuracy can reach 479.631nm/RIU.

It can be seen from the above embodiments that the demodulation accuracy of the fiber grating refractive index sensing demodulation method based on the cut-off wavelength used in the present invention can reach up to 949.9nm/RIU.

Claims (6)

1. A fiber grating refractive index sensing demodulation method based on cut-off wavelength, is characterized in that, comprises the following steps: (1) measure the transmission spectrum of the tilted fiber grating when the refractive index of the external environment is greater than or equal to the refractive index of the cladding of the tilted fiber grating, and use it as a reference spectrum; (2) Measure the transmission spectrum of the tilted fiber grating in the environment where the refractive index changes that require sensing and demodulation, and use it as the sensing measurement spectrum; (3) subtract the reference spectrum obtained in step (1) from the sensing measurement spectrum obtained in step (2), to obtain the demodulated spectrum; (4) According to the cladding mode transmission spectrum intensity in the demodulation spectrum, set a threshold value, start to calculate from short wavelength to long wavelength, and the first wavelength that reaches the set threshold value is recorded as the cut-off wavelength; (5) Measure the function curve between the cut-off wavelength and the refractive index, and calibrate the response coefficient of the cut-off wavelength of the inclined fiber Bragg grating under different refractive indices; (6) According to the function curve relationship between the cut-off wavelength and the refractive index measured in step (5) and the transmission spectrum difference spectrum under the environment to be measured, the refractive index of the environment where the refractive index change needs to be sensed and demodulated to demodulate. 2. the fiber grating refractive index sensing demodulation method based on cut-off wavelength according to claim 1, is characterized in that, in step (1), described external environment refractive index is greater than or equal to the refractive index of cladding, refers to The refractive index in the environment is greater than or equal to the refractive index of the cladding of the tilted fiber grating, so that the spectral modulation generated by the response of the cladding mode in the tilted fiber grating disappears, and only the cladding mode in the transmission spectrum causes the slowly changing transmission loss. 3. the fiber grating refractive index sensing demodulation method based on cut-off wavelength according to claim 1, is characterized in that, in step (4), described cladding mode transmission spectrum modulation intensity increases with the environment refractive index It gradually decreases from the short wavelength, and the wavelength position where the modulation intensity begins to decrease shifts to the long wavelength direction with the increase of the ambient refractive index. 4. the fiber grating refractive index sensing demodulation method based on cut-off wavelength according to claim 1, is characterized in that, in step (4), described threshold value k can be chosen as n<k<m, and wherein m is is the maximum value of the cladding mode intensity in the demodulated spectrum, and n is the minimum value of the cladding mode intensity in the demodulated spectrum. 5. the fiber grating refractive index sensing demodulation method based on cut-off wavelength according to claim 1, is characterized in that, in step (4), described cut-off wavelength adopts computer or manual mode to determine: at first setting error range , and then interpolate the demodulated spectrum, so that the discrete interval of the demodulated spectrum is less than the error value, from the short wavelength to the long wavelength direction, so that the difference between the cladding mode intensity in the demodulated spectrum minus the threshold k is less than the error value, the intensity The corresponding wavelength is the cutoff wavelength. 6 . The method for sensing and demodulating the refractive index of fiber gratings based on cutoff wavelengths according to claim 1 , wherein the tilted fiber gratings are prepared based on femtosecond lasers. 7 .