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Fiber Optics and Optical Communications|306 Article(s)
Design of Self-collimating Fiber FP Cavity and Localization of Sound Source Based on Silver Film
Yilin GUO, Yihao LI, Binbin LUO, Xue ZOU, Decao WU, Gongye LI, Sheng CAO, Shenghui SHI, and Mingfu ZHAO
Sound source localization is a pivotal research area within the field of acoustics, finding extensive applications in domains such as Unmanned Aerial Vehicle (UAV) navigation, intelligent traffic systems, medical imaging, and structural health monitoring. Traditional sound source localization methods typically rely on arrays of multiple microphones or sensor networks. Nevertheless, these conventional approaches are beset by challenges related to complex installation, intricate data processing, and poor resistance to interference. In recent years, there has been considerable attention directed towards the emerging field of optical fiber-based acoustic localization, within which most optical fiber-based detection systems have employed Fiber Bragg Grating (FBG) sensor arrays due to their wavelength-based multiplexing capabilities. However, FBG sensors exhibit limitations in sensitivity. In contrast, optical fiber Extrinsic Fabry-Perot Interferometer (EFPI) sensors, with their probe-like structure and advantages in terms of high sensitivity and structural simplicity, are better suited for sound source localization. In this research endeavor, we have introduced optical fiber collimators within EFPI sensor arrays to develop a self-collimating optical fiber-based EFPI acoustic sensor array. The primary objective is to augment the sound pressure sensitivity and detection range of the sensor array. The designed sensor array exhibits elevated acoustic sensitivity and an expanded spatial detection range, thus holding immense potential for applications in sound source localization and the detection of partial discharge phenomena.Firstly, the optical field distribution of a quarter-pitch-length gradient multimode optical fiber was verified using Rsoft software. Subsequently, an EFPI (Extrinsic Fabry-Perot Interferometer) acoustic sensor with a self-collimating optical fiber was designed. To assess whether the proposed sensor exhibits enhanced sensitivity to sound pressure, it was compared to an EFPI acoustic sensor without the self-collimating feature. Next, three EFPI acoustic sensors with identical structures and self-collimating optical fibers were fabricated for sound source localization experiments. Prior to conducting the localization experiments, the consistency of sound pressure sensitivity and sound source directionality among the three sensors with self-collimating optical fibers was verified. Subsequent to these preparations, time-delay signals were acquired using an intensity demodulation technique and recorded on an oscilloscope. The time-delay signals were processed using conventional cross-correlation algorithms to calculate the time delays between pairs of sensors. Finally, based on the geometric positions of the sensor array, an estimation of the approximate sound source location was determined.The experimental results show that the interference spectrum FSR of EFPI sensor with collimator is 5.25 nm, and the maximum fringe visibility is 14.96 dB. The EFPI sensor without a collimator has a FSR of 5.18 nm and a maximum fringe visibility of 9 dB. The FSR of them is almost the same, but the interference spectral intensity of the former is increased by about 6 dB. In addition, the EFPI spectral slopes were 6.5 dB/nm and 10.2 dB/nm, respectively, without and with collimators, and the spectral slope of the latter increased nearly twice as much as the former. In the response characteristic experiment for the single sensor, EFPI acoustic sensor with collimator is superior to EFPI acoustic sensor without collimator in sound pressure response waveform and sound pressure sensitivity test. EFPI acoustic sensor with collimator has sound pressure sensitivity of 185 mV/Pa. The minimum detectable sound pressure is 52.7 μPa/Hz1/2@500 Hz, and the signal-to-noise ratio reaches 62 dB. In the experiment of sound source directionality, the designed sensor showed good performance under different sound pressure directionality. When the sound source was placed directly in front of the sensor, its sound pressure sensitivity reached 185 mV/Pa. When the sound source was set on the side of the sensor (90°, 270°), its sound pressure sensitivity could still reach 177 mV/Pa. This indicates the ability of the sensor array to achieve sound source localization within a wide-angle range. In the two-dimensional plane sound source location experiment, the signal delay in the time domain signal is extracted by the correlation algorithm, and the two-dimensional plane sound source location within the range of 200 cm×200 cm is finally realized. The theoretical spatial resolution is 0.71 cm, and the maximum positioning error of the system is no more than 2.8 cm. Finally, the performance comparison with other EFPI acoustic arrays shows that the system has the advantages of high sensitivity, low production cost, simple demodulation system and large detection range. Sound source localization is a pivotal research area within the field of acoustics, finding extensive applications in domains such as Unmanned Aerial Vehicle (UAV) navigation, intelligent traffic systems, medical imaging, and structural health monitoring. Traditional sound source localization methods typically rely on arrays of multiple microphones or sensor networks. Nevertheless, these conventional approaches are beset by challenges related to complex installation, intricate data processing, and poor resistance to interference. In recent years, there has been considerable attention directed towards the emerging field of optical fiber-based acoustic localization, within which most optical fiber-based detection systems have employed Fiber Bragg Grating (FBG) sensor arrays due to their wavelength-based multiplexing capabilities. However, FBG sensors exhibit limitations in sensitivity. In contrast, optical fiber Extrinsic Fabry-Perot Interferometer (EFPI) sensors, with their probe-like structure and advantages in terms of high sensitivity and structural simplicity, are better suited for sound source localization. In this research endeavor, we have introduced optical fiber collimators within EFPI sensor arrays to develop a self-collimating optical fiber-based EFPI acoustic sensor array. The primary objective is to augment the sound pressure sensitivity and detection range of the sensor array. The designed sensor array exhibits elevated acoustic sensitivity and an expanded spatial detection range, thus holding immense potential for applications in sound source localization and the detection of partial discharge phenomena.Firstly, the optical field distribution of a quarter-pitch-length gradient multimode optical fiber was verified using Rsoft software. Subsequently, an EFPI (Extrinsic Fabry-Perot Interferometer) acoustic sensor with a self-collimating optical fiber was designed. To assess whether the proposed sensor exhibits enhanced sensitivity to sound pressure, it was compared to an EFPI acoustic sensor without the self-collimating feature. Next, three EFPI acoustic sensors with identical structures and self-collimating optical fibers were fabricated for sound source localization experiments. Prior to conducting the localization experiments, the consistency of sound pressure sensitivity and sound source directionality among the three sensors with self-collimating optical fibers was verified. Subsequent to these preparations, time-delay signals were acquired using an intensity demodulation technique and recorded on an oscilloscope. The time-delay signals were processed using conventional cross-correlation algorithms to calculate the time delays between pairs of sensors. Finally, based on the geometric positions of the sensor array, an estimation of the approximate sound source location was determined.The experimental results show that the interference spectrum FSR of EFPI sensor with collimator is 5.25 nm, and the maximum fringe visibility is 14.96 dB. The EFPI sensor without a collimator has a FSR of 5.18 nm and a maximum fringe visibility of 9 dB. The FSR of them is almost the same, but the interference spectral intensity of the former is increased by about 6 dB. In addition, the EFPI spectral slopes were 6.5 dB/nm and 10.2 dB/nm, respectively, without and with collimators, and the spectral slope of the latter increased nearly twice as much as the former. In the response characteristic experiment for the single sensor, EFPI acoustic sensor with collimator is superior to EFPI acoustic sensor without collimator in sound pressure response waveform and sound pressure sensitivity test. EFPI acoustic sensor with collimator has sound pressure sensitivity of 185 mV/Pa. The minimum detectable sound pressure is 52.7 μPa/Hz1/2@500 Hz, and the signal-to-noise ratio reaches 62 dB. In the experiment of sound source directionality, the designed sensor showed good performance under different sound pressure directionality. When the sound source was placed directly in front of the sensor, its sound pressure sensitivity reached 185 mV/Pa. When the sound source was set on the side of the sensor (90°, 270°), its sound pressure sensitivity could still reach 177 mV/Pa. This indicates the ability of the sensor array to achieve sound source localization within a wide-angle range. In the two-dimensional plane sound source location experiment, the signal delay in the time domain signal is extracted by the correlation algorithm, and the two-dimensional plane sound source location within the range of 200 cm×200 cm is finally realized. The theoretical spatial resolution is 0.71 cm, and the maximum positioning error of the system is no more than 2.8 cm. Finally, the performance comparison with other EFPI acoustic arrays shows that the system has the advantages of high sensitivity, low production cost, simple demodulation system and large detection range.
Acta Photonica Sinica
- Publication Date: Apr. 25, 2024
- Vol. 53, Issue 4, 0406007 (2024)
Real-time Spectrum Analysis of Wideband RF Signals Based on Fractional Temporal Talbot Effect
Bo YANG, Lei ZHAO, Shuna YANG, and Hao CHI
To enhance the bandwidth and real-time capabilities of Radio Frequency (RF) spectrum analysis, the optical real-time Fourier transform method has been proposed. The optical real-time Fourier transform method based on the temporal Talbot effect has the advantage of a simple structure by using optical pulse sampling and dispersion delay structure. However, its frequency measurement bandwidth is limited by the optical pulse's repetition rate. Addressing this limitation, a real-time spectrum analysis scheme of wideband RF signals based on the fractional temporal Talbot effect is proposed and demonstrated. Based on the sampling and dispersion structure, the scheme realizes the mapping of the RF signal frequency to the optical pulse time interval. At the same time, the repetition rate of the optical pulse before sampling is multiplied by passing through a dispersive element satisfying the fractional Talbot distance in advance. The frequency measurement bandwidth of the system can be significantly improved by using the fractional Talbot effect. A proof-of-concept experiment is carried out to test the performance of the proposed scheme. The repetition period of the optical pulse is set to 151.5 ps, that is, the repetition frequency is 6.6 GHz. Each pulse has a Gaussian shape and the full width at half maximum of the pulse is approximately 30 ps. Dispersion compensation fiber is used to provide dispersion for the system. The total dispersion value of the two sections of dispersion is about 3 650 ps2, which is about 0.1% different from the theoretical result. The RF signal to be measured is generated by a RF signal generator. The output optical signal is converted into an electrical signal by a 40 GHz bandwidth photodetector and recorded by a sampling oscilloscope with a bandwidth of 50 GHz. Comparing the experimental results under integer-order, 3rd-order fractional, and 9th-order fractional temporal Talbot conditions, it is verified that the measurement frequency bandwidth of the system increases with the order of the temporal Talbot effect. Real-time spectral analysis of single-tone and two-tone RF signals within a 29.7 GHz bandwidth is achieved using the 9th-order fractional temporal Talbot effect. Numerical simulation is carried out to achieve time-frequency analysis of a large-bandwidth linear chirp signal. Based on the 3rd-order fractional temporal Talbot effect, a linear chirp signal with a frequency range of 2~13 GHz and a chirp rate of 2.2 GHz/ns is successfully identified. Numerical simulation results further verify that this scheme can effectively analyze frequency transient signals. The main causes of frequency measurement errors include the time jitter of the input optical pulse train, the limited bandwidth of the pulse detection system, the deviation between the dispersion value and the theoretical value, high-order dispersion terms, etc. In the experiment, the time jitter root mean square value of the optical pulse is approximately 1 ps, which is 0.066% of the period of the optical pulse train. When the frequency measurement bandwidth is 29.7 GHz, the frequency measurement error caused by the time jitter is about 200 MHz. In order to improve frequency measurement accuracy, methods such as reducing optical pulse jitter, increasing the bandwidth of the pulse detection system, and compensating for high-order dispersion can be used. It should be noted that the increase of frequency measurement bandwidth will sacrifice the frequency resolution of the system. In practical applications, the requirements of system bandwidth and frequency resolution should be fully considered to select the order of the fractional temporal Talbot effect. With its advantages of simple structure, large bandwidth, and real-time processing, this scheme has potential application value in the fields of broadband radar, cognitive radio, and other fields. To enhance the bandwidth and real-time capabilities of Radio Frequency (RF) spectrum analysis, the optical real-time Fourier transform method has been proposed. The optical real-time Fourier transform method based on the temporal Talbot effect has the advantage of a simple structure by using optical pulse sampling and dispersion delay structure. However, its frequency measurement bandwidth is limited by the optical pulse's repetition rate. Addressing this limitation, a real-time spectrum analysis scheme of wideband RF signals based on the fractional temporal Talbot effect is proposed and demonstrated. Based on the sampling and dispersion structure, the scheme realizes the mapping of the RF signal frequency to the optical pulse time interval. At the same time, the repetition rate of the optical pulse before sampling is multiplied by passing through a dispersive element satisfying the fractional Talbot distance in advance. The frequency measurement bandwidth of the system can be significantly improved by using the fractional Talbot effect. A proof-of-concept experiment is carried out to test the performance of the proposed scheme. The repetition period of the optical pulse is set to 151.5 ps, that is, the repetition frequency is 6.6 GHz. Each pulse has a Gaussian shape and the full width at half maximum of the pulse is approximately 30 ps. Dispersion compensation fiber is used to provide dispersion for the system. The total dispersion value of the two sections of dispersion is about 3 650 ps2, which is about 0.1% different from the theoretical result. The RF signal to be measured is generated by a RF signal generator. The output optical signal is converted into an electrical signal by a 40 GHz bandwidth photodetector and recorded by a sampling oscilloscope with a bandwidth of 50 GHz. Comparing the experimental results under integer-order, 3rd-order fractional, and 9th-order fractional temporal Talbot conditions, it is verified that the measurement frequency bandwidth of the system increases with the order of the temporal Talbot effect. Real-time spectral analysis of single-tone and two-tone RF signals within a 29.7 GHz bandwidth is achieved using the 9th-order fractional temporal Talbot effect. Numerical simulation is carried out to achieve time-frequency analysis of a large-bandwidth linear chirp signal. Based on the 3rd-order fractional temporal Talbot effect, a linear chirp signal with a frequency range of 2~13 GHz and a chirp rate of 2.2 GHz/ns is successfully identified. Numerical simulation results further verify that this scheme can effectively analyze frequency transient signals. The main causes of frequency measurement errors include the time jitter of the input optical pulse train, the limited bandwidth of the pulse detection system, the deviation between the dispersion value and the theoretical value, high-order dispersion terms, etc. In the experiment, the time jitter root mean square value of the optical pulse is approximately 1 ps, which is 0.066% of the period of the optical pulse train. When the frequency measurement bandwidth is 29.7 GHz, the frequency measurement error caused by the time jitter is about 200 MHz. In order to improve frequency measurement accuracy, methods such as reducing optical pulse jitter, increasing the bandwidth of the pulse detection system, and compensating for high-order dispersion can be used. It should be noted that the increase of frequency measurement bandwidth will sacrifice the frequency resolution of the system. In practical applications, the requirements of system bandwidth and frequency resolution should be fully considered to select the order of the fractional temporal Talbot effect. With its advantages of simple structure, large bandwidth, and real-time processing, this scheme has potential application value in the fields of broadband radar, cognitive radio, and other fields.
Acta Photonica Sinica
- Publication Date: Apr. 25, 2024
- Vol. 53, Issue 4, 0406006 (2024)
Small Torque Detection of Bolt Connection Based on Suspended-FBG
Chunfang RAO, Peng CHEN, Youde HU, Xuefeng ZHAN, Ziwei JIANG, Yuexiang WANG, and Wenxin YU
Loosing of the bolt connection structure affects its working and operation safety. The main reasons for the loosing coming from loading, vibration, and friction. Consequently, loosing is inevitable and its monitoring is important for its application. At present, the theory and technology of testing the tightness state of bolt connection are still not mature. The detection of small torque is a technical difficulty in this field. In this study, for the structure to be tested on the uneven surface in the narrow space, the Bragg Fiber Grating (FBG) was used as the sensor to identify the small torque of the bolt connection. In the testing, a periodic vibration in the tested structure with bolt tightness information was excited and employed for the identification. One tail of the suspended- FBG was sticked on the tested structure, and the vibration yielded periodic strains in the tail of the FBG, which acted as the source of the elastic longitudinal wave propagating along the optical fiber with a FBG written in it. The edge-filter method was used to demodulated the signals in the FBG sensor to satisfy the high frequency signals. The information coming from the FBG was used to be identified. Firstly, the Empirical Mode Decomposition (EMD) method was used to decompose the original signal, basing on which, we removed the unstable components and noise by calculating the correlation function of each component and the original signal. Then the signals were restructured for later identification. The dimensional features (standard deviation, residuals, peak- peak value, and energy) and dimensionless features (skewness, kurtosis, waveform factor, amplitude factor, impact factor and margin factor) of the signals were exacted, and were inputted to the recognition system based on the Support Vector Machine (SVM) finally, where we used the ten-fold cross-validation algorithm and Gaussian kernel function SVM for higher accuracy. Results show that the recognition accuracy reaches to 97.2% and the torque recognition ability is on the order of N·cm. This study proves that optical fiber is a good acoustic waveguide, and the installation technique of suspended FBG effectively mitigates spectral distortion resulting from uneven stress due to direct adhesion, thereby decreasing the complexity associated with sensor installation. At the same time, because the optical fiber as an acoustic waveguide does not sense the torsional displacement, the bending stress wave cannot form an effective transmission in the optical fiber, the FBG only senses the vibration displacement along the optical fiber axis that causes the longitudinal wave. Therefore, the signal deviation caused by the excitation and sensor setting in the actual test process is relatively small and limited, which can reduce the difficulty of the signal processing. On the other hand, the study identifies that the signal processing and identification method are suitable for the non-linear, non-stationary and small sample test data in this study. This study presents a new detection method for the bolted state, especially for the detection of small torques in small mass structures on the uneven surface in the narrow space. Loosing of the bolt connection structure affects its working and operation safety. The main reasons for the loosing coming from loading, vibration, and friction. Consequently, loosing is inevitable and its monitoring is important for its application. At present, the theory and technology of testing the tightness state of bolt connection are still not mature. The detection of small torque is a technical difficulty in this field. In this study, for the structure to be tested on the uneven surface in the narrow space, the Bragg Fiber Grating (FBG) was used as the sensor to identify the small torque of the bolt connection. In the testing, a periodic vibration in the tested structure with bolt tightness information was excited and employed for the identification. One tail of the suspended- FBG was sticked on the tested structure, and the vibration yielded periodic strains in the tail of the FBG, which acted as the source of the elastic longitudinal wave propagating along the optical fiber with a FBG written in it. The edge-filter method was used to demodulated the signals in the FBG sensor to satisfy the high frequency signals. The information coming from the FBG was used to be identified. Firstly, the Empirical Mode Decomposition (EMD) method was used to decompose the original signal, basing on which, we removed the unstable components and noise by calculating the correlation function of each component and the original signal. Then the signals were restructured for later identification. The dimensional features (standard deviation, residuals, peak- peak value, and energy) and dimensionless features (skewness, kurtosis, waveform factor, amplitude factor, impact factor and margin factor) of the signals were exacted, and were inputted to the recognition system based on the Support Vector Machine (SVM) finally, where we used the ten-fold cross-validation algorithm and Gaussian kernel function SVM for higher accuracy. Results show that the recognition accuracy reaches to 97.2% and the torque recognition ability is on the order of N·cm. This study proves that optical fiber is a good acoustic waveguide, and the installation technique of suspended FBG effectively mitigates spectral distortion resulting from uneven stress due to direct adhesion, thereby decreasing the complexity associated with sensor installation. At the same time, because the optical fiber as an acoustic waveguide does not sense the torsional displacement, the bending stress wave cannot form an effective transmission in the optical fiber, the FBG only senses the vibration displacement along the optical fiber axis that causes the longitudinal wave. Therefore, the signal deviation caused by the excitation and sensor setting in the actual test process is relatively small and limited, which can reduce the difficulty of the signal processing. On the other hand, the study identifies that the signal processing and identification method are suitable for the non-linear, non-stationary and small sample test data in this study. This study presents a new detection method for the bolted state, especially for the detection of small torques in small mass structures on the uneven surface in the narrow space.
Acta Photonica Sinica
- Publication Date: Apr. 25, 2024
- Vol. 53, Issue 4, 0406005 (2024)
Fiber Optic Temperature Sensor Based on Harmonic Vernier Effect of Sagnac Interferometer
Yuqiang YANG, Yuying ZHANG, Yuting LI, Jiale GAO, Xiaoguang MU, and Lei BI
Fiber optic interferometer has the advantages of small size, light weight, anti-corrosion, anti-electromagnetic interference, high sensitivity, etc., and is widely used in the measurement of temperature, humidity, magnetic field and other parameters. In recent years, researchers have dramatically improved the measurement sensitivity of interferometric fiber-optic sensors by cascading or paralleling two fiber-optic interferometers to produce an optical Vernier effect. When the free spectral ranges of the two fiber optic interferometers are close but not equal, the resulting Vernier effect is called the normal Vernier effect, when the free spectral range of one fiber optic interferometer is about an integer multiple of the other fiber optic interferometer, the resulting Vernier effect is called the harmonic Vernier effect.In this paper, a parallel optical fiber temperature sensor based on two Sagnac interferometers is proposed, where the interferometers SI1 and SI2 are connected to the two outputs of the fiber-optic coupler C3, where SI1 is a reference interferometer and SI2 is a sensing interferometer. When the length of the Panda fiber in the SI2 interferometer is approximately i+1 of the length of the Panda fiber in the SI1 interferometer (i is 1,2,3 …) times, the two interferometers will produce an i-order harmonic Vernier effect. When i is 0, it produces an normal Vernier effect, at which time there will be a single envelope in the interference spectrum. When i is 1, it produces a first-order harmonic Vernier effect, at which time there will be a double envelope in the interference spectrum. When i is 2, it produces a second-order harmonic Vernier effect, at which time there will be a triple envelope in the interference spectrum. In other cases, the order is analogous.We have numerically simulated the theoretical analysis and the free spectral range of the interferometric spectrum of the length interferometer SI1 with a Panda fiber of 520 mm at constant temperature is 9.13 nm. The SI2 interferometers with lengths of 572 mm, 953 mm, and 1 430 mm of the Panda fiber have free spectral ranges of 8.30 nm, 4.98 nm, and 3.32 nm, respectively. When the temperature is increased from T0 ℃ to T0+1 ℃, the interference spectra of SI2 interferometers with different Panda fiber lengths are all shifted to the short-wave direction, and the shifts are all about 1.89 nm, which is consistent with the theoretical analysis. The parallel interference spectra of interferometer SI1 and SI2 with Panda fiber lengths of 572 mm, 953 mm and 1 430 mm show single, double and triple envelopes respectively, indicating that the two interferometers produce the normal Vernier effect, first-order and second-order harmonic Vernier effects, respectively, and from the theoretical calculations. It can be seen that the amplification of the normal Vernier effect, first-order and second-order harmonic Vernier effects are all 11 times. When the temperature increases from T0 ℃ to T0+1 ℃, the single envelope moves in the short-wave direction, while the double and triple envelopes both move in the long-wave direction, which is opposite to that of the single SI2. In addition, the shifts of the single, double and triple envelopes are all about 20.7 nm due to the fact that the Vernier magnification is the same for the normal Vernier effect, first-order and second-order harmonic Vernier effects.It is experimentally concluded that the interference spectra of SI2 are blueshifted in the temperature range from 40 ℃ to 50 ℃, and the shifts are all about 1.89 nm , which is consistent with the theoretical analysis and simulation results. The temperature sensitivity of the sensor corresponding to the normal Vernier effect is -20.67 nm/℃, the temperature sensitivity of the sensor corresponding to the first-order harmonic Vernier effect is 21.34 nm/℃, the temperature sensitivity of the sensor corresponding to the second-order harmonic Vernier effect is 21.18 nm/℃, and the fiber optic sensors corresponding to the harmonic Vernier effect and the normal Vernier effect have almost the same temperature sensitivities, which are both about 21 nm/℃. This results are consistent with the theoretical analysis and simulation results. The above experimental results show that the temperature sensitivity of the SI2 interferometer is independent of the length of the Panda fiber, although the magnification is the same, the harmonic Vernier effect and the normal Vernier effect correspond to the detuning of the length of the Panda fiber are obviously different, the detuning corresponding to the normal Vernier is 52 mm, and the detuning corresponding to the first-order and second-order harmonics is -87 mm and -130 mm, respectively. This shows that the higher the order, the larger the detuning amount, which is approximately a multiple increase. The above experimental results are consistent with the theoretical analysis. Since the larger the detuning amount, the easier the Vernier magnification can be controlled and realised, the harmonic Vernier effect is obviously superior to the normal Vernier effect from the preparation point of view. This study can provide an important reference for the subsequent study of optical Vernier effect. Fiber optic interferometer has the advantages of small size, light weight, anti-corrosion, anti-electromagnetic interference, high sensitivity, etc., and is widely used in the measurement of temperature, humidity, magnetic field and other parameters. In recent years, researchers have dramatically improved the measurement sensitivity of interferometric fiber-optic sensors by cascading or paralleling two fiber-optic interferometers to produce an optical Vernier effect. When the free spectral ranges of the two fiber optic interferometers are close but not equal, the resulting Vernier effect is called the normal Vernier effect, when the free spectral range of one fiber optic interferometer is about an integer multiple of the other fiber optic interferometer, the resulting Vernier effect is called the harmonic Vernier effect.In this paper, a parallel optical fiber temperature sensor based on two Sagnac interferometers is proposed, where the interferometers SI1 and SI2 are connected to the two outputs of the fiber-optic coupler C3, where SI1 is a reference interferometer and SI2 is a sensing interferometer. When the length of the Panda fiber in the SI2 interferometer is approximately i+1 of the length of the Panda fiber in the SI1 interferometer (i is 1,2,3 …) times, the two interferometers will produce an i-order harmonic Vernier effect. When i is 0, it produces an normal Vernier effect, at which time there will be a single envelope in the interference spectrum. When i is 1, it produces a first-order harmonic Vernier effect, at which time there will be a double envelope in the interference spectrum. When i is 2, it produces a second-order harmonic Vernier effect, at which time there will be a triple envelope in the interference spectrum. In other cases, the order is analogous.We have numerically simulated the theoretical analysis and the free spectral range of the interferometric spectrum of the length interferometer SI1 with a Panda fiber of 520 mm at constant temperature is 9.13 nm. The SI2 interferometers with lengths of 572 mm, 953 mm, and 1 430 mm of the Panda fiber have free spectral ranges of 8.30 nm, 4.98 nm, and 3.32 nm, respectively. When the temperature is increased from T0 ℃ to T0+1 ℃, the interference spectra of SI2 interferometers with different Panda fiber lengths are all shifted to the short-wave direction, and the shifts are all about 1.89 nm, which is consistent with the theoretical analysis. The parallel interference spectra of interferometer SI1 and SI2 with Panda fiber lengths of 572 mm, 953 mm and 1 430 mm show single, double and triple envelopes respectively, indicating that the two interferometers produce the normal Vernier effect, first-order and second-order harmonic Vernier effects, respectively, and from the theoretical calculations. It can be seen that the amplification of the normal Vernier effect, first-order and second-order harmonic Vernier effects are all 11 times. When the temperature increases from T0 ℃ to T0+1 ℃, the single envelope moves in the short-wave direction, while the double and triple envelopes both move in the long-wave direction, which is opposite to that of the single SI2. In addition, the shifts of the single, double and triple envelopes are all about 20.7 nm due to the fact that the Vernier magnification is the same for the normal Vernier effect, first-order and second-order harmonic Vernier effects.It is experimentally concluded that the interference spectra of SI2 are blueshifted in the temperature range from 40 ℃ to 50 ℃, and the shifts are all about 1.89 nm , which is consistent with the theoretical analysis and simulation results. The temperature sensitivity of the sensor corresponding to the normal Vernier effect is -20.67 nm/℃, the temperature sensitivity of the sensor corresponding to the first-order harmonic Vernier effect is 21.34 nm/℃, the temperature sensitivity of the sensor corresponding to the second-order harmonic Vernier effect is 21.18 nm/℃, and the fiber optic sensors corresponding to the harmonic Vernier effect and the normal Vernier effect have almost the same temperature sensitivities, which are both about 21 nm/℃. This results are consistent with the theoretical analysis and simulation results. The above experimental results show that the temperature sensitivity of the SI2 interferometer is independent of the length of the Panda fiber, although the magnification is the same, the harmonic Vernier effect and the normal Vernier effect correspond to the detuning of the length of the Panda fiber are obviously different, the detuning corresponding to the normal Vernier is 52 mm, and the detuning corresponding to the first-order and second-order harmonics is -87 mm and -130 mm, respectively. This shows that the higher the order, the larger the detuning amount, which is approximately a multiple increase. The above experimental results are consistent with the theoretical analysis. Since the larger the detuning amount, the easier the Vernier magnification can be controlled and realised, the harmonic Vernier effect is obviously superior to the normal Vernier effect from the preparation point of view. This study can provide an important reference for the subsequent study of optical Vernier effect.
Acta Photonica Sinica
- Publication Date: Apr. 25, 2024
- Vol. 53, Issue 4, 0406004 (2024)
Temperature Measurement Error and Its Influencing Factors of FBG Sensor under Rotor Whirling Conditions Based on Space-Coupled Transmission Method
Sitong CHEN, Junbin HUANG, Hongcan GU, Gaofei YAO, Dan XU, and Zheyu LI
The increased power of the motor directly results in a higher temperature rise effect on the rotor. High temperatures can cause turn-to-turn short circuits or permanent demagnetization of the motor rotor, which can seriously affect the reliability of the generator operation and the stability of the combat system. Therefore, the research of online rotor temperature measurement technology is of great significance. Compared to electronic sensors, Fiber Bragg Gratings (FBGs) are resistant to electromagnetic interference, small in size, require no power supply, and can be used for quasi-distributed measurements. This is a great advantage for monitoring the motor rotor temperature. However, research on FBG rotor temperature monitoring systems is still relatively rare. The state of motion of the motor rotor is an important factor affecting the accuracy of the system. The effect of rotor whirling on measurement accuracy due to unbalanced mass is even more difficult to ignore and has not been studied.In this paper, a model of FBG scanning spectra under rotor whrling conditions is developed by combining the transmission matrix theory of FBG and the coupled transmission theory of self-focusing lenses. The scanning error of the center wavelength and its influencing factors have been investigated in conjunction with relevant experiments. The results show that the whirling of the rotor leads to aberrations in the scanning spectra of the FBG, which are mainly manifested in the offset of the reflection peaks and the reduction of the 3 dB bandwidth. The main factors affecting the temperature measurement error of the system are the rotor whirling frequency, the demodulator scanning frequency, the radial displacement of the rotor at the end face, the axial ratio of the axial trajectory and the deflection angle of the axis. As the ratio of the coupling loss period to the spectral scan time (q-value) increases, the maximum center-wavelength scan error and the peak-seeking error both show a rapid decrease, followed by a slow decrease to stability. The key to ensuring a low level of temperature measurement error in the system is to ensure that q>10. The peak-finding error of the Gaussian curve fitting method is reduced to the level of the centroid method when q>40. When the rotor radial vibration amplitude is 200 μm, the axis ratio of the axial trajectory is 3, and the axis deflection angle is 0.1°, for a typical demodulator operating bandwidth of 40 nm and FBG bandwidth of 0.3 nm, if it is hoped that the measurement error of the polyimide-coated FBG to be less than 0.5 ℃, it should be ensured that the q value reaches 115 or more. The corresponding scanning frequency must be approximately 1.74 times higher than the whirling frequency. When using the centroid method or the Gaussian curve fitting method for peak finding, it is only necessary to make the scanning frequency about 0.21 and 0.44 times higher than the whirling frequency, respectively. The centroid method is more advantageous than the Gaussian curve fitting method for rotors with strong whirling.In addition, a study was conducted to investigate the effect of the intensity of the rotor whirling on the maximum scanning error at the center wavelength of the FBG and the system temperature measurement error. The results show that the maximum scanning error at the center wavelength increases slowly and then linearly as the rotor radial displacement amplitude or axis declination amplitude increases. The peak-finding error of the centroid and Gaussian curve fitting methods increases slowly and then dramatically. As the axial ratio of the rotor axis trajectory increases, the maximum scanning error and the peak detection error at the center wavelength both increase dramatically and then slowly increase to a steady state. When the amplitude of radial displacement is less than 200 μm and the axial deflection angle is less than 0.167°, the maximum temperature measurement error caused by whirling motion is 2.9 ℃, which is reflected by 15 sampling spectra under the condition of q=10 and the axial ratio of n=3. When peak detection is performed using the centroid method or the Gaussian curve fitting method, the temperature measurement error of the system is reduced to 0.6 ℃ and 1.1 ℃, respectively. The increased power of the motor directly results in a higher temperature rise effect on the rotor. High temperatures can cause turn-to-turn short circuits or permanent demagnetization of the motor rotor, which can seriously affect the reliability of the generator operation and the stability of the combat system. Therefore, the research of online rotor temperature measurement technology is of great significance. Compared to electronic sensors, Fiber Bragg Gratings (FBGs) are resistant to electromagnetic interference, small in size, require no power supply, and can be used for quasi-distributed measurements. This is a great advantage for monitoring the motor rotor temperature. However, research on FBG rotor temperature monitoring systems is still relatively rare. The state of motion of the motor rotor is an important factor affecting the accuracy of the system. The effect of rotor whirling on measurement accuracy due to unbalanced mass is even more difficult to ignore and has not been studied.In this paper, a model of FBG scanning spectra under rotor whrling conditions is developed by combining the transmission matrix theory of FBG and the coupled transmission theory of self-focusing lenses. The scanning error of the center wavelength and its influencing factors have been investigated in conjunction with relevant experiments. The results show that the whirling of the rotor leads to aberrations in the scanning spectra of the FBG, which are mainly manifested in the offset of the reflection peaks and the reduction of the 3 dB bandwidth. The main factors affecting the temperature measurement error of the system are the rotor whirling frequency, the demodulator scanning frequency, the radial displacement of the rotor at the end face, the axial ratio of the axial trajectory and the deflection angle of the axis. As the ratio of the coupling loss period to the spectral scan time (q-value) increases, the maximum center-wavelength scan error and the peak-seeking error both show a rapid decrease, followed by a slow decrease to stability. The key to ensuring a low level of temperature measurement error in the system is to ensure that q>10. The peak-finding error of the Gaussian curve fitting method is reduced to the level of the centroid method when q>40. When the rotor radial vibration amplitude is 200 μm, the axis ratio of the axial trajectory is 3, and the axis deflection angle is 0.1°, for a typical demodulator operating bandwidth of 40 nm and FBG bandwidth of 0.3 nm, if it is hoped that the measurement error of the polyimide-coated FBG to be less than 0.5 ℃, it should be ensured that the q value reaches 115 or more. The corresponding scanning frequency must be approximately 1.74 times higher than the whirling frequency. When using the centroid method or the Gaussian curve fitting method for peak finding, it is only necessary to make the scanning frequency about 0.21 and 0.44 times higher than the whirling frequency, respectively. The centroid method is more advantageous than the Gaussian curve fitting method for rotors with strong whirling.In addition, a study was conducted to investigate the effect of the intensity of the rotor whirling on the maximum scanning error at the center wavelength of the FBG and the system temperature measurement error. The results show that the maximum scanning error at the center wavelength increases slowly and then linearly as the rotor radial displacement amplitude or axis declination amplitude increases. The peak-finding error of the centroid and Gaussian curve fitting methods increases slowly and then dramatically. As the axial ratio of the rotor axis trajectory increases, the maximum scanning error and the peak detection error at the center wavelength both increase dramatically and then slowly increase to a steady state. When the amplitude of radial displacement is less than 200 μm and the axial deflection angle is less than 0.167°, the maximum temperature measurement error caused by whirling motion is 2.9 ℃, which is reflected by 15 sampling spectra under the condition of q=10 and the axial ratio of n=3. When peak detection is performed using the centroid method or the Gaussian curve fitting method, the temperature measurement error of the system is reduced to 0.6 ℃ and 1.1 ℃, respectively.
Acta Photonica Sinica
- Publication Date: Apr. 25, 2024
- Vol. 53, Issue 4, 0406003 (2024)
Shape Sensing Based on Brillouin Optical Time Domain Analysis
Zijuan LIU, Jiaqi WU, Lixin ZHANG, Yongqian LI, Jianjian WANG, and Kuan WANG
In recent years, optical fiber shape sensing technology has been widely studied in various fields, and has been widely used in robot, medical, aerospace, industrial equipment structure monitoring and submarine cables. With the change of application scenarios and the gradual improvement of measurement performance requirements, the research needs of optical fiber shape sensing technology are becoming increasingly urgent. At present, the research on fiber shape sensing is mainly divided into two directions. One is the shape sensing technology based on FBG, which takes advantage of the wavelength drift of FBG under strain and realizes shape measurement by writing FBG on multi-core fiber, which has the advantages of high precision and simple data processing. In this direction, some scholars have done more in-depth research, but this technology is limited by the number and interval of FBG writing, and cannot achieve long-distance distributed shape measurement. The other direction is the shape sensing based on the distributed optical fiber measurement system. As a medium of shape sensing technology, optical fiber is small in size, light in weight, and has strong electromagnetic interference resistance and corrosion resistance. It can be either a transmission medium or a sensing medium. When the light wave is transmitted in the optical fiber, the optical intensity, phase, frequency and other parameters of the optical fiber will change with the change of environmental parameters such as strain and temperature. The data processing equipment is used to demodulate the modulated light, and then the information of strain and temperature of the optical fiber is obtained. In this paper, the Brillouin scattering in the fiber is used to reconstruct the shape of the fiber or the measured object in contact with it, and the strain change values of more than two fiber cores in the shape sensor are measured at the same time. Then the shape reconstruction algorithm is used to reconstruct the shape of the sensor or the measured object. In this paper, the BOTDA system is built with a spatial resolution of 1 m. A homogenous low-crosstalk seven-core fiber from Changfei Company is selected as the distributed shape sensor. The total length of the fiber is 300 m, the core diameter is 8 μm, the cladding diameter is 150 μm, and the protective layer diameter is 245 μm. The remaining six cores are located at a distance of 42 μm from the middle core and are symmetrically distributed around each other at 60°. At the same time, the seven pigtails of the multi-core fiber are labeled and separated by a fan-in fan-out coupler. By using the BOTDA system, the Brillouin gain spectra of the intermediate core and the off-core are measured, and it is verified that the intermediate core is not affected by bending, and the strain values of each two symmetric off-core are negative to each other. Three unsymmetrical cores with 120° distribution were selected, and the intermediate cores were used as temperature compensation to demodulate the induced variables of each core at different curvature radii. Finally, parallel transmission frame shape reconstruction algorithm is used to reconstruct the shape of seven-core fiber when the curvature diameter is 0.112 m and 0.052 m. When the curvature diameter is 0.112 m, the curvature reconstruction error is 0.375%, which is mainly due to the low spatial resolution of the construction system and the torsion problem in the winding process. Distributed fiber shape sensing technology has a very large application prospect, but there are still many technical difficulties that need to be overcome by researchers. The work in this paper has laid the research foundation for the subsequent distributed fiber shape sensing, and has certain practical significance. In recent years, optical fiber shape sensing technology has been widely studied in various fields, and has been widely used in robot, medical, aerospace, industrial equipment structure monitoring and submarine cables. With the change of application scenarios and the gradual improvement of measurement performance requirements, the research needs of optical fiber shape sensing technology are becoming increasingly urgent. At present, the research on fiber shape sensing is mainly divided into two directions. One is the shape sensing technology based on FBG, which takes advantage of the wavelength drift of FBG under strain and realizes shape measurement by writing FBG on multi-core fiber, which has the advantages of high precision and simple data processing. In this direction, some scholars have done more in-depth research, but this technology is limited by the number and interval of FBG writing, and cannot achieve long-distance distributed shape measurement. The other direction is the shape sensing based on the distributed optical fiber measurement system. As a medium of shape sensing technology, optical fiber is small in size, light in weight, and has strong electromagnetic interference resistance and corrosion resistance. It can be either a transmission medium or a sensing medium. When the light wave is transmitted in the optical fiber, the optical intensity, phase, frequency and other parameters of the optical fiber will change with the change of environmental parameters such as strain and temperature. The data processing equipment is used to demodulate the modulated light, and then the information of strain and temperature of the optical fiber is obtained. In this paper, the Brillouin scattering in the fiber is used to reconstruct the shape of the fiber or the measured object in contact with it, and the strain change values of more than two fiber cores in the shape sensor are measured at the same time. Then the shape reconstruction algorithm is used to reconstruct the shape of the sensor or the measured object. In this paper, the BOTDA system is built with a spatial resolution of 1 m. A homogenous low-crosstalk seven-core fiber from Changfei Company is selected as the distributed shape sensor. The total length of the fiber is 300 m, the core diameter is 8 μm, the cladding diameter is 150 μm, and the protective layer diameter is 245 μm. The remaining six cores are located at a distance of 42 μm from the middle core and are symmetrically distributed around each other at 60°. At the same time, the seven pigtails of the multi-core fiber are labeled and separated by a fan-in fan-out coupler. By using the BOTDA system, the Brillouin gain spectra of the intermediate core and the off-core are measured, and it is verified that the intermediate core is not affected by bending, and the strain values of each two symmetric off-core are negative to each other. Three unsymmetrical cores with 120° distribution were selected, and the intermediate cores were used as temperature compensation to demodulate the induced variables of each core at different curvature radii. Finally, parallel transmission frame shape reconstruction algorithm is used to reconstruct the shape of seven-core fiber when the curvature diameter is 0.112 m and 0.052 m. When the curvature diameter is 0.112 m, the curvature reconstruction error is 0.375%, which is mainly due to the low spatial resolution of the construction system and the torsion problem in the winding process. Distributed fiber shape sensing technology has a very large application prospect, but there are still many technical difficulties that need to be overcome by researchers. The work in this paper has laid the research foundation for the subsequent distributed fiber shape sensing, and has certain practical significance.
Acta Photonica Sinica
- Publication Date: Apr. 25, 2024
- Vol. 53, Issue 4, 0406002 (2024)
Study on Scanning Velocity of Germanium Core Fibers with Different Outer Diameters Annealed by CO2 Laser
Yifan DU, Ziwen ZHAO, Shuangqi ZHONG, Zecheng MA, and Shaoye WANG
As the germanium (Ge) core is mostly in an amorphous or polycrystalline state after fabrication, laser annealing is an effective way to improve the properties of semiconductor core fiber. During the laser annealing process, the axial scanning velocity of the laser along the fiber is an important parameter that affects the properties of the annealed fiber. Therefore, it is of great significance to investigate the modification mechanism of laser annealing on the Ge core to improve the properties of annealed fibers.In this study, three sets of Ge core fibers with different outer diameters (OD) and the same inner diameter (ID) were annealed by CO2 laser at different scanning velocities. The laser annealing experiments were carried out on Ge core optical fibers with an ID of 41~43 μm and the ODs of 188 μm, 251 μm, and 270 μm, respectively. The Ge core fibers were annealed by the SK-3D30 CO2 laser. The laser spot is 1 mm in diameter, the output power is 0~30 W, and the laser wavelength is 10.6 μm. The scanning region along the fiber axis is 1 mm×50 mm, which can completely cover the Ge core fiber, and the laser scanning in this region was reciprocated along the fiber axis during the annealing time. After laser annealing, samples were analyzed by using the spectrometer. Raman experiments were carried out on the cross-section of the Ge core fiber to collect the Raman peak frequency information. The obtained data was processed into mapping by MATLAB software. The optical transmission loss of Ge core fiber was measured by the cutback method. The system consists of a laser, photodetector, and optical power meter. The samples were cut off at 5 mm each time and measured 3 times per fiber. All measurements were made at room temperature.Three sets of experiments were carried out, the laser frequency is 50 kHz, the laser power is 20%(6 W), and the laser scanning time is 20 s. 1) The Ge core fiber with an OD of about 188 μm was annealed by laser, and the scanning velocities were set at 8 mm·s-1, 10 mm·s-1, 12 mm·s-1, and 14 mm·s-1. The Raman frequency distribution and average value at 10 mm·s-1 laser scanning velocity closest to Ge bulk crystal and optical transmission loss values was 3.435 dB·cm-1. 2) The Ge core fiber with an OD of about 251 μm was annealed by laser, the scanning velocities were set at 10 mm·s-1, 12 mm·s-1, 14 mm·s-1, 16 mm·s-1, and 20 mm·s-1. The Raman frequency distribution and average value at 14 mm·s-1 laser scanning velocity closest to Ge bulk crystal and optical transmission loss values was 2.147 dB·cm-1. 3) The Ge core fiber with an OD of about 270 μm was annealed by laser, and the scanning velocities were set at 12 mm·s-1, 14 mm·s-1, 16 mm·s-1, and 18 mm·s-1. The Raman frequency distribution and average value at 16 mm·s-1 laser scanning velocity closest to Ge bulk crystal and optical transmission loss values was 3.578 dB·cm-1. The experimental results show that under the same OD conditions, the laser annealing effect becomes better first and then worse with the increase in laser scanning velocity, and the scanning velocity for obtaining the optimal annealing effect increases with the increase of the OD of the fiber. Temperature variation at fixed points on the surface of the Ge core on the laser-irradiated side during the annealing process was simulated by COMSOL Multiphysics. The simulation results indicated that under the same OD conditions, the faster scanning velocity leads to the formation of denser temperature pulses, so that the Ge core is in the relatively high-temperature region most of the time, and the strength of the modification effect of this temperature field structure on the Ge core is more enhanced.The experimental results and the simulation of temperature variation indicate that the laser scanning velocity is an important factor affecting the annealing effect of Ge core fiber. The annealing intensity of the laser-annealed Ge core fiber can be enhanced as the laser scanning velocity is increased. As the germanium (Ge) core is mostly in an amorphous or polycrystalline state after fabrication, laser annealing is an effective way to improve the properties of semiconductor core fiber. During the laser annealing process, the axial scanning velocity of the laser along the fiber is an important parameter that affects the properties of the annealed fiber. Therefore, it is of great significance to investigate the modification mechanism of laser annealing on the Ge core to improve the properties of annealed fibers.In this study, three sets of Ge core fibers with different outer diameters (OD) and the same inner diameter (ID) were annealed by CO2 laser at different scanning velocities. The laser annealing experiments were carried out on Ge core optical fibers with an ID of 41~43 μm and the ODs of 188 μm, 251 μm, and 270 μm, respectively. The Ge core fibers were annealed by the SK-3D30 CO2 laser. The laser spot is 1 mm in diameter, the output power is 0~30 W, and the laser wavelength is 10.6 μm. The scanning region along the fiber axis is 1 mm×50 mm, which can completely cover the Ge core fiber, and the laser scanning in this region was reciprocated along the fiber axis during the annealing time. After laser annealing, samples were analyzed by using the spectrometer. Raman experiments were carried out on the cross-section of the Ge core fiber to collect the Raman peak frequency information. The obtained data was processed into mapping by MATLAB software. The optical transmission loss of Ge core fiber was measured by the cutback method. The system consists of a laser, photodetector, and optical power meter. The samples were cut off at 5 mm each time and measured 3 times per fiber. All measurements were made at room temperature.Three sets of experiments were carried out, the laser frequency is 50 kHz, the laser power is 20%(6 W), and the laser scanning time is 20 s. 1) The Ge core fiber with an OD of about 188 μm was annealed by laser, and the scanning velocities were set at 8 mm·s-1, 10 mm·s-1, 12 mm·s-1, and 14 mm·s-1. The Raman frequency distribution and average value at 10 mm·s-1 laser scanning velocity closest to Ge bulk crystal and optical transmission loss values was 3.435 dB·cm-1. 2) The Ge core fiber with an OD of about 251 μm was annealed by laser, the scanning velocities were set at 10 mm·s-1, 12 mm·s-1, 14 mm·s-1, 16 mm·s-1, and 20 mm·s-1. The Raman frequency distribution and average value at 14 mm·s-1 laser scanning velocity closest to Ge bulk crystal and optical transmission loss values was 2.147 dB·cm-1. 3) The Ge core fiber with an OD of about 270 μm was annealed by laser, and the scanning velocities were set at 12 mm·s-1, 14 mm·s-1, 16 mm·s-1, and 18 mm·s-1. The Raman frequency distribution and average value at 16 mm·s-1 laser scanning velocity closest to Ge bulk crystal and optical transmission loss values was 3.578 dB·cm-1. The experimental results show that under the same OD conditions, the laser annealing effect becomes better first and then worse with the increase in laser scanning velocity, and the scanning velocity for obtaining the optimal annealing effect increases with the increase of the OD of the fiber. Temperature variation at fixed points on the surface of the Ge core on the laser-irradiated side during the annealing process was simulated by COMSOL Multiphysics. The simulation results indicated that under the same OD conditions, the faster scanning velocity leads to the formation of denser temperature pulses, so that the Ge core is in the relatively high-temperature region most of the time, and the strength of the modification effect of this temperature field structure on the Ge core is more enhanced.The experimental results and the simulation of temperature variation indicate that the laser scanning velocity is an important factor affecting the annealing effect of Ge core fiber. The annealing intensity of the laser-annealed Ge core fiber can be enhanced as the laser scanning velocity is increased.
Acta Photonica Sinica
- Publication Date: Apr. 25, 2024
- Vol. 53, Issue 4, 0406001 (2024)
Optical Fibre Bragg Based Sliding-tactile Sensing and Classification Training Method for Material Recognition
Ruizhi PAN, Yan FENG, Hexiang LIU, Haoxiang WANG, Hongpu ZHANG, Yinxiang ZHANG, and Hua ZHANG
With the development of smart robots, intelligent tactile sensing is increasingly applied in industrial production, which can greatly improve efficiency and accuracy. Compared with traditional electrical sensors, optical fiber Bragg Grating (FBG) sensors have significant advantages, such as flexibility, electromagnetic immunity, and small size. They also demonstrate high sensitivity and rapid response in perceiving strain and pressure. Current researches on FBG-based tactile sensing mainly focus on strain, temperature, sliding positioning and contact force deduced from the Bragg wavelength shift of FBG. However, there are relatively few researches on combining feature extraction, machine learning, and other cutting-edge technologies to achieve more sophisticated intelligent perception, such as material recognition.In this work, we presented a FBG based sliding-tactile sensing and classification training method for online material recognition by the differential properties of contact surface materials, such as roughness and stick-slip phenomenon. We developed a horizontal two-layer silicone rubber covered FBG sensing unit and its sliding-tactile perception system. When sliding on the certain material, a continuous strain exerts to FBG through the silicone rubber sensing unit and FBG's response changes.To classify efficiently, this paper extracted the mean maximum difference λB(max)-λBˉ, extreme difference λB(max)-λB(min), and standard deviation Δλ(std) of the FBG's wavelengths as the three-dimensional feature for mapping the material properties. And the classification training of the Support Vector Machine (SVM) algorithm and its classification model was developed. The results show that the classification accuracy is 96.6% for rough cloth, PLA and 800-grit sandpaper under the mixed dataset of 5 cm/s, 10 cm/s and 15 cm/s sliding speeds. Compared with the direct wavelength and traditional mean/median feature classification methods, this three-dimensional feature-based method exhibits superior classification capability and adaptability.In order to achieve further intelligent applications, this paper also designs an interactive computer control system, including wavelength acquisition, speed control and material recognition result display. It can control the sliding speed and online material recognition as well. Utilizing the prediction function trainedModel.predictFcn(t_test), the corresponding predicted results were presented after extracting three-dimensional features. In 36 tests, 5~15 cm/s random sliding speed (3 types of materials×3 samples×4 times slip) were carried out, and the correct predictions were 34 tests, which verifies that this method is effective and accurate.This work indicates that the FBG sensor has great potential in the field of material recognition by slip-tactile sensing. The research results can provide a novel online material recognition method for intelligent sensing robots. With the development of smart robots, intelligent tactile sensing is increasingly applied in industrial production, which can greatly improve efficiency and accuracy. Compared with traditional electrical sensors, optical fiber Bragg Grating (FBG) sensors have significant advantages, such as flexibility, electromagnetic immunity, and small size. They also demonstrate high sensitivity and rapid response in perceiving strain and pressure. Current researches on FBG-based tactile sensing mainly focus on strain, temperature, sliding positioning and contact force deduced from the Bragg wavelength shift of FBG. However, there are relatively few researches on combining feature extraction, machine learning, and other cutting-edge technologies to achieve more sophisticated intelligent perception, such as material recognition.In this work, we presented a FBG based sliding-tactile sensing and classification training method for online material recognition by the differential properties of contact surface materials, such as roughness and stick-slip phenomenon. We developed a horizontal two-layer silicone rubber covered FBG sensing unit and its sliding-tactile perception system. When sliding on the certain material, a continuous strain exerts to FBG through the silicone rubber sensing unit and FBG's response changes.To classify efficiently, this paper extracted the mean maximum difference λB(max)-λBˉ, extreme difference λB(max)-λB(min), and standard deviation Δλ(std) of the FBG's wavelengths as the three-dimensional feature for mapping the material properties. And the classification training of the Support Vector Machine (SVM) algorithm and its classification model was developed. The results show that the classification accuracy is 96.6% for rough cloth, PLA and 800-grit sandpaper under the mixed dataset of 5 cm/s, 10 cm/s and 15 cm/s sliding speeds. Compared with the direct wavelength and traditional mean/median feature classification methods, this three-dimensional feature-based method exhibits superior classification capability and adaptability.In order to achieve further intelligent applications, this paper also designs an interactive computer control system, including wavelength acquisition, speed control and material recognition result display. It can control the sliding speed and online material recognition as well. Utilizing the prediction function trainedModel.predictFcn(t_test), the corresponding predicted results were presented after extracting three-dimensional features. In 36 tests, 5~15 cm/s random sliding speed (3 types of materials×3 samples×4 times slip) were carried out, and the correct predictions were 34 tests, which verifies that this method is effective and accurate.This work indicates that the FBG sensor has great potential in the field of material recognition by slip-tactile sensing. The research results can provide a novel online material recognition method for intelligent sensing robots.
Acta Photonica Sinica
- Publication Date: Feb. 25, 2024
- Vol. 53, Issue 2, 0206006 (2024)
A Vector Displacement Measurement Sensing Device Based on Fiber Bragg Grating and Its Experimental Study
Yong ZHENG, Jie YU, Hongkai CHEN, and Xing YI
In practical applications such as slope instability deformation, random cracking of concrete and collapse of wind turbines, position tracking always requires two-dimensional sensing. The fiber-optic displacement sensors have been widespread applied in civil engineering field due to their intrinsic advantages, including electromagnetic interference immunity, miniature size, electrically-passive operation, and multiplexing capability, however, they are not able to retrieve the displacement direction and amplitude simultaneously. In view of this reason, a vector displacement measurement sensing device based on Fiber Bragg Grating (FBG) with large range and simple structure is proposed to identify the displacement magnitude and direction of the monitored structure simultaneously. The sensing device is mainly constructed of four FBGs, a base, an upper free rotation rod, springs 1 and 2, self-made U-shaped structures 1 and 2. The pre-tensioned FBGs are respectively pasted on the inner and outer sides of center position of the U-shaped structure as the sensing unit. When the deformation occurs, the screw plays a connecting role, and the spring is not affected by the screw. The bottom spring of U-shaped structure 2 is connected with the monitored point, and the movement of the monitored point will cause axial tension of U-shaped structure 2. The force will be applied to the spring and the U-shaped structure. The internal and external sides of the U-shaped structure are subject to tension and compression, respectively. Additionally, the movement of U-shaped structure 2 causes the upper rod to rotate around the center point, which further makes the spring of U-shaped structure 1 elongate and produces tension on the U-shaped structure. FBGs are bonded to both the upper and lower surface of the stainless steel plate of the U-shaped structure for temperature compensation. Even if temperature change occurs in an FBG sensor unit, strain in the upper and lower two FBGs bonded to both surfaces of the stainless steel plate are equal, so thermal strain can be neglected. The sensing principle of determining the displacement direction and amplitude simultaneously is introduced, and its expression is also derived. Calibration experiments of six sets of auxiliary structures (i.e., core sensing elements) were conducted. The experimental results showed that the sensing element is characterized by a superb linearity, a measurement range of 0~140 mm, a sensitivity of 4.362 pm/mm, a hysteresis error of 3.25%, and a repeatability error of 6.62%, respectively. Additionally, an indoor accumulation slope model test was performed to verify the performance of the FBG displacement sensing device in monitoring the continuous sliding deformation process of soil. The displacement values calculated by FBG-based sensor is basically consistent with that measured by Particle Image Velocimetry (PIV) technology, with an average relative error of 5.63%. The maximum relative error of horizontal displacement is 10.83%, the minimum value is 0.11%, the maximum relative error of vertical displacement is 11.17%, the minimum value is 0.67%, which can meet the measurement requirement of the sensor in slope monitoring. The errors of some measuring points exceeding 10% may be due to the fact that the probe of FBG displacement sensing device was buried shallowly in the soil and not fully in accordance with the soil deformation. Meanwhile, the relative error of displacement azimuths calculated by two technologies is basically within 10%, with a maximum error of 10.31% and a minimum error of 0.09%, which is basically the same as the error analysis of above displacement. The error analysis of displacement azimuth calculated by two techniques also proves the reliability of the FBG displacement sensing device developed in this paper for monitoring vector deformation of soil slopes. This capability demonstrates its broad application prospects in the field of intelligent monitoring. In practical applications such as slope instability deformation, random cracking of concrete and collapse of wind turbines, position tracking always requires two-dimensional sensing. The fiber-optic displacement sensors have been widespread applied in civil engineering field due to their intrinsic advantages, including electromagnetic interference immunity, miniature size, electrically-passive operation, and multiplexing capability, however, they are not able to retrieve the displacement direction and amplitude simultaneously. In view of this reason, a vector displacement measurement sensing device based on Fiber Bragg Grating (FBG) with large range and simple structure is proposed to identify the displacement magnitude and direction of the monitored structure simultaneously. The sensing device is mainly constructed of four FBGs, a base, an upper free rotation rod, springs 1 and 2, self-made U-shaped structures 1 and 2. The pre-tensioned FBGs are respectively pasted on the inner and outer sides of center position of the U-shaped structure as the sensing unit. When the deformation occurs, the screw plays a connecting role, and the spring is not affected by the screw. The bottom spring of U-shaped structure 2 is connected with the monitored point, and the movement of the monitored point will cause axial tension of U-shaped structure 2. The force will be applied to the spring and the U-shaped structure. The internal and external sides of the U-shaped structure are subject to tension and compression, respectively. Additionally, the movement of U-shaped structure 2 causes the upper rod to rotate around the center point, which further makes the spring of U-shaped structure 1 elongate and produces tension on the U-shaped structure. FBGs are bonded to both the upper and lower surface of the stainless steel plate of the U-shaped structure for temperature compensation. Even if temperature change occurs in an FBG sensor unit, strain in the upper and lower two FBGs bonded to both surfaces of the stainless steel plate are equal, so thermal strain can be neglected. The sensing principle of determining the displacement direction and amplitude simultaneously is introduced, and its expression is also derived. Calibration experiments of six sets of auxiliary structures (i.e., core sensing elements) were conducted. The experimental results showed that the sensing element is characterized by a superb linearity, a measurement range of 0~140 mm, a sensitivity of 4.362 pm/mm, a hysteresis error of 3.25%, and a repeatability error of 6.62%, respectively. Additionally, an indoor accumulation slope model test was performed to verify the performance of the FBG displacement sensing device in monitoring the continuous sliding deformation process of soil. The displacement values calculated by FBG-based sensor is basically consistent with that measured by Particle Image Velocimetry (PIV) technology, with an average relative error of 5.63%. The maximum relative error of horizontal displacement is 10.83%, the minimum value is 0.11%, the maximum relative error of vertical displacement is 11.17%, the minimum value is 0.67%, which can meet the measurement requirement of the sensor in slope monitoring. The errors of some measuring points exceeding 10% may be due to the fact that the probe of FBG displacement sensing device was buried shallowly in the soil and not fully in accordance with the soil deformation. Meanwhile, the relative error of displacement azimuths calculated by two technologies is basically within 10%, with a maximum error of 10.31% and a minimum error of 0.09%, which is basically the same as the error analysis of above displacement. The error analysis of displacement azimuth calculated by two techniques also proves the reliability of the FBG displacement sensing device developed in this paper for monitoring vector deformation of soil slopes. This capability demonstrates its broad application prospects in the field of intelligent monitoring.
Acta Photonica Sinica
- Publication Date: Feb. 25, 2024
- Vol. 53, Issue 2, 0206005 (2024)
Research on Routing Algorithms for Satellite Optical Networks under Space Debris Interference
Yang CAO, Wenjun XING, Xiaofeng PENG, and Chaoyuan BAO
With the advancement of the construction of space, space and ground integrated information networks, satellite communication systems nowhave a higher requirements for information transmission rate, satellite node storage capacity, satellite coverage and security. Traditional microwave communication methods are limited by bandwidth, speed, geographical location, spectrum, etc., and will be difficult to meet the ultra-high speed and ultra-large capacity communication requirements of multimedia broadband services for satellite networks. At the same time, laser communications are gradually becoming an important technical means for satellite communications due to its advantages of high transmission rate, high security and reliability, strong confidentiality, small terminal equipment, light weight and low power consumption. To achieve all-round coverage of communication signals, laser networking based on dynamic satellites and the establishment of high-speed, low-latency, high-reliability and large-capacity satellite communication systems will become the future development trend of satellite communication. In the future, space will inevitably gather a large number of products of human space activities, including rockets, satellites, and rocket ejections. As humans develop space, the increase in these space debris will also bring a series of hazards. Existing space debris research mainly focuses on how to avoid collisions with satellites and spacecraft in orbit. In addition, these space debris move randomly in space, which will block point-to-point laser communications. Therefore, more effective research on the reliability of satellite laser communication systems is needed.In order to solve the problem of inter-satellite link interruption that may be caused by space debris in low-orbit satellite laser communications, this paper proposes a Direction-enhanced Link State (DE-LS) routing algorithm. Firstly, the network topology of satellite communication is built. The polar orbit constellation model is selected. According to the orbital plane and the number of satellites, an initial and constant address is set for each satellite in the polar orbit constellation. Based on the changes in the satellite node addresses of the starting and ending points in different transmission tasks, the Direction Influencing Factor (DIF) is introduced. Then, based on the celestial motion patterns of satellites and space debris in polar orbit constellations. A joint simulation model of space debris and satellites is constructed to obtain the relative positions of satellites and debris at a given moment and to perform inter-satellite visibility analysis.. Based on the inter-satellite visibility data, a Direction Enhancement Index (DEI) is proposed corresponding to the four directions of each node. The direction impact factor and direction enhancement index are combined with the inter-satellite link distance and transmission delay to comprehensively represent the link cost. The cost is used as a measure to select the shortest path, and the shortest path is selected between each pair of satellite nodes in turn, and the number of routing hops is used as the evaluation index. The simulation experiment is carried out in the Walker constellation. Space debris and satellites are jointly modeled and simulated first. Then, in this environment, two situations are selected: the theoretical minimum number of hops in the same orbit is 4 hops and the theoretical minimum number in different orbits is 7 hops. Taking satellite communications No. 21 and No. 25 and satellite No. 21 and No. 55 as examples for routing selection, routing hop count and transmission delay are used as evaluation indicators, and compared with the Dijkstra routing algorithm, which also solves the shortest path. The simulation results show that the DE-LS algorithm can maintain the theoretical minimum number of hops when the link is interrupted. At the same time, it saves 14% of the hops and reduces the transmission delay by 17% compared with the Dijkstra algorithm, which reflects the effectiveness of DE-LS algorithm in avoiding faulty links. With the advancement of the construction of space, space and ground integrated information networks, satellite communication systems nowhave a higher requirements for information transmission rate, satellite node storage capacity, satellite coverage and security. Traditional microwave communication methods are limited by bandwidth, speed, geographical location, spectrum, etc., and will be difficult to meet the ultra-high speed and ultra-large capacity communication requirements of multimedia broadband services for satellite networks. At the same time, laser communications are gradually becoming an important technical means for satellite communications due to its advantages of high transmission rate, high security and reliability, strong confidentiality, small terminal equipment, light weight and low power consumption. To achieve all-round coverage of communication signals, laser networking based on dynamic satellites and the establishment of high-speed, low-latency, high-reliability and large-capacity satellite communication systems will become the future development trend of satellite communication. In the future, space will inevitably gather a large number of products of human space activities, including rockets, satellites, and rocket ejections. As humans develop space, the increase in these space debris will also bring a series of hazards. Existing space debris research mainly focuses on how to avoid collisions with satellites and spacecraft in orbit. In addition, these space debris move randomly in space, which will block point-to-point laser communications. Therefore, more effective research on the reliability of satellite laser communication systems is needed.In order to solve the problem of inter-satellite link interruption that may be caused by space debris in low-orbit satellite laser communications, this paper proposes a Direction-enhanced Link State (DE-LS) routing algorithm. Firstly, the network topology of satellite communication is built. The polar orbit constellation model is selected. According to the orbital plane and the number of satellites, an initial and constant address is set for each satellite in the polar orbit constellation. Based on the changes in the satellite node addresses of the starting and ending points in different transmission tasks, the Direction Influencing Factor (DIF) is introduced. Then, based on the celestial motion patterns of satellites and space debris in polar orbit constellations. A joint simulation model of space debris and satellites is constructed to obtain the relative positions of satellites and debris at a given moment and to perform inter-satellite visibility analysis.. Based on the inter-satellite visibility data, a Direction Enhancement Index (DEI) is proposed corresponding to the four directions of each node. The direction impact factor and direction enhancement index are combined with the inter-satellite link distance and transmission delay to comprehensively represent the link cost. The cost is used as a measure to select the shortest path, and the shortest path is selected between each pair of satellite nodes in turn, and the number of routing hops is used as the evaluation index. The simulation experiment is carried out in the Walker constellation. Space debris and satellites are jointly modeled and simulated first. Then, in this environment, two situations are selected: the theoretical minimum number of hops in the same orbit is 4 hops and the theoretical minimum number in different orbits is 7 hops. Taking satellite communications No. 21 and No. 25 and satellite No. 21 and No. 55 as examples for routing selection, routing hop count and transmission delay are used as evaluation indicators, and compared with the Dijkstra routing algorithm, which also solves the shortest path. The simulation results show that the DE-LS algorithm can maintain the theoretical minimum number of hops when the link is interrupted. At the same time, it saves 14% of the hops and reduces the transmission delay by 17% compared with the Dijkstra algorithm, which reflects the effectiveness of DE-LS algorithm in avoiding faulty links.
Acta Photonica Sinica
- Publication Date: Feb. 25, 2024
- Vol. 53, Issue 2, 0206004 (2024)
Topics
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