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Fiber Optics and Optical Communications|299 Article(s)

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)

Design and Fabrication of a Non-zero Dispersion Shifted Fiber with Low Dispersion Slope in S+C+L Band and Large Effective Area Based on Outside Vapor Deposition

Jianjiang ZHA, Yuanda WANG, Xuerong HE, Wei HOU, Jingsheng WANG, and Jianxiang WEN

Existing optical fibers struggle to support broadband dense wavelength division multiplexing and coarse wavelength division multiplexing transmissions, necessitating the development of optimized fibers with moderate dispersion, low dispersion slope, enlarged effective area, and low attenuation. Currently, None Zero Dispersion Shifted Fiber (NZDSF) fibers have a small effective area incompatible with conventional fibers, emphasizing the need for precise design control of the refractive index profile. Therefore, creating a S+C+L band NZDSF with a large effective area and low slope is crucial for meeting the escalating demand for bulk data transmission.The dispersion slope, a crucial parameter in optics, is determined by the interplay between waveguide and material properties. The effective area, a metric that signifies the fiber's optical performance, relies heavily on the refractive index profile and the chosen input wavelength. In order to find the appropriate dispersion slope and effective area, we need to find a suitable refractive index profile. In our research, we have employed a refined profile structure modal, featuring a triangular+ring core configuration embellished by a central depression and fabricated by a two-step process to prepare the core and cladding by using the Outside Vapor Deposition (OVD).Through experiments, adjustments were made to the doping levels in the core, thereby modifying the relative refractive index Δn1 and radius R1 of the first core layer. This enabled the formation of a triangular cross-sectional structure. Simultaneously, the relative refractive index Δn3 and thickness R3-R2 of the third core layer were also adjusted, resulting in distinct refractive index waveguide configurations. This approach strucks a balance between achieving low attenuation, a large effective area, a reduced dispersion slope, and an appropriate zero dispersion wavelength. After optimizing the preform preparation and drawing process, the optical fiber cross-section obtained has a high matching with the designed cross-section. The triangular structure of the first core layer has a relatively straight slope, and Δn1 is between 0.52% and 0.57%. In the third core layer, a slightly curved convex structure is formed due to the diffusion of GeO2, and Δn3 is between 0.13% and 0.17%. In line with the experimental findings, it has been observed that when the first core layer radius R1, the third core layer R3, and the second core layer's relative refractive index Δn2 remain relatively constant, an increase in Δn1 and a subsequent decrease in R2/R1 lead to a gradual reduction in the zero dispersion wavelength λ0 and a corresponding decline in the effective area Aeff. Our experimental target is to achieve a zero dispersion wavelength λ0 below 1460 nm, even approximating 1 420 nm, while maintaining a significant effective area Aeff. To balance these parameters, it is necessary to slightly reduce Δn1 to the range of 0.52% to 0.53% and adjust R2/R1 to approximately 2.6 to 2.7. By these adjustments, we can achieve a suitable equilibrium between the effective area Aeff and the zero dispersion wavelength λ0.The experimental fiber design achieved a mode field diameter of 9.35 μm and an effective area Aeff of 68 μm2. Additionally, the zero dispersion coefficient exceeding 1.5 ps·nm-1·km-1 at 1 460 nm, well-suited for S-band wavelength division multiplexing applications while effectively suppressing four-wave mixing in the S-band. Furthermore, the fiber exhibited a low dispersion slope of only 0.059 ps·nm-2·km-1, providing relatively suitable dispersion characteristics in the C and L bands. The fiber also exhibited superior attenuation coefficients of 0.276 dB·km-1 at 1 383 nm, effectively mitigating the impact of water absorption peaks. The attenuation coefficients at 1 550 nm and 1 625 nm were 0.195 dB·km-1 and 0.205 dB·km-1, respectively, facilitating extended transmission distances. Through comparison, it was confirmed that this S+C+L band NZDSF with low dispersion slope and large effective area is well-suited for high-speed, high-capacity, and long-distance optical communication systems. Existing optical fibers struggle to support broadband dense wavelength division multiplexing and coarse wavelength division multiplexing transmissions, necessitating the development of optimized fibers with moderate dispersion, low dispersion slope, enlarged effective area, and low attenuation. Currently, None Zero Dispersion Shifted Fiber (NZDSF) fibers have a small effective area incompatible with conventional fibers, emphasizing the need for precise design control of the refractive index profile. Therefore, creating a S+C+L band NZDSF with a large effective area and low slope is crucial for meeting the escalating demand for bulk data transmission.The dispersion slope, a crucial parameter in optics, is determined by the interplay between waveguide and material properties. The effective area, a metric that signifies the fiber's optical performance, relies heavily on the refractive index profile and the chosen input wavelength. In order to find the appropriate dispersion slope and effective area, we need to find a suitable refractive index profile. In our research, we have employed a refined profile structure modal, featuring a triangular+ring core configuration embellished by a central depression and fabricated by a two-step process to prepare the core and cladding by using the Outside Vapor Deposition (OVD).Through experiments, adjustments were made to the doping levels in the core, thereby modifying the relative refractive index Δn1 and radius R1 of the first core layer. This enabled the formation of a triangular cross-sectional structure. Simultaneously, the relative refractive index Δn3 and thickness R3-R2 of the third core layer were also adjusted, resulting in distinct refractive index waveguide configurations. This approach strucks a balance between achieving low attenuation, a large effective area, a reduced dispersion slope, and an appropriate zero dispersion wavelength. After optimizing the preform preparation and drawing process, the optical fiber cross-section obtained has a high matching with the designed cross-section. The triangular structure of the first core layer has a relatively straight slope, and Δn1 is between 0.52% and 0.57%. In the third core layer, a slightly curved convex structure is formed due to the diffusion of GeO2, and Δn3 is between 0.13% and 0.17%. In line with the experimental findings, it has been observed that when the first core layer radius R1, the third core layer R3, and the second core layer's relative refractive index Δn2 remain relatively constant, an increase in Δn1 and a subsequent decrease in R2/R1 lead to a gradual reduction in the zero dispersion wavelength λ0 and a corresponding decline in the effective area Aeff. Our experimental target is to achieve a zero dispersion wavelength λ0 below 1460 nm, even approximating 1 420 nm, while maintaining a significant effective area Aeff. To balance these parameters, it is necessary to slightly reduce Δn1 to the range of 0.52% to 0.53% and adjust R2/R1 to approximately 2.6 to 2.7. By these adjustments, we can achieve a suitable equilibrium between the effective area Aeff and the zero dispersion wavelength λ0.The experimental fiber design achieved a mode field diameter of 9.35 μm and an effective area Aeff of 68 μm2. Additionally, the zero dispersion coefficient exceeding 1.5 ps·nm-1·km-1 at 1 460 nm, well-suited for S-band wavelength division multiplexing applications while effectively suppressing four-wave mixing in the S-band. Furthermore, the fiber exhibited a low dispersion slope of only 0.059 ps·nm-2·km-1, providing relatively suitable dispersion characteristics in the C and L bands. The fiber also exhibited superior attenuation coefficients of 0.276 dB·km-1 at 1 383 nm, effectively mitigating the impact of water absorption peaks. The attenuation coefficients at 1 550 nm and 1 625 nm were 0.195 dB·km-1 and 0.205 dB·km-1, respectively, facilitating extended transmission distances. Through comparison, it was confirmed that this S+C+L band NZDSF with low dispersion slope and large effective area is well-suited for high-speed, high-capacity, and long-distance optical communication systems.

Acta Photonica Sinica

Publication Date: Feb. 25, 2024
Vol. 53, Issue 2, 0206003 (2024)

Error Performance Analysis of Serial-relayed Underwater Wireless Optical Communication Systems over Generalized Gamma Distribution Weak Turbulence Environment with Pointing Error

Yixu WANG, Yueheng LI, Ping HUANG, and Meiyan JU

The channel fading generated by seawater absorption and scattering, oceanic turbulence, and pointing error will severely reduce the communication quality of an Underwater Wireless Optical Communication (UWOC) system. Many scholars directly transplanted the weak atmospheric turbulence model to depict the oceanic turbulence statistics, and this had been proven to be incorrect by a series of laboratory measurements and associated data-fitting tests. So, it is obviously of great significance to study and evaluate the effects of a composite fading channel on the key performance of the UWOC system, especially with a proper weak oceanic turbulence model.In this paper, the Generalized Gamma Distribution (GGD) verified by a series of experimental tests was selected to characterize the weak oceanic turbulence. Then, a new hybrid fading channel model was proposed to more reasonably simulate the communication environment in the ocean, which had integrated the GGD weak turbulence, the zero/nonzero boresight pointing error, the implicit path loss and multipath propagation effect characterized by Fading Free Impulse Response (FFIR). Next, the mathematical expressions of the Probability Density Function (PDF) considering GGD weak turbulence and zero/nonzero boresight pointing errors were derived using higher transcendental Meijer-G and Whittaker functions. Subsequently, based on this, the closed-form expressions of the average Bit Error Rate (BER) were derived for the serial-relayed UWOC systems with both zero and nonzero boresight pointing errors, respectively. Finally, the accuracy and rationality of the derived closed-form formulas for the average bit error rate of the relaying UWOC system derived above were verified by some Monte Carlo numerical simulations; meanwhile, the influences of different key parameters on the system BERs were also investigated.The results show that the introduction of serial relaying nodes can effectively improve the end-to-end BER performance of the UWOC systems in a long-distance communication environment. With the increase of the relaying nodes number, the system's BER decreases rapidly at the same transmission power, indicating that the serial-relayed scheme dramatically improves the performance. For instance, if the end-to-end distance is fixed as 45 m and the target BER is set to be 10-3, under the zero boresight pointing error condition, the required node transmission power will be reduced to 24 dBm, 12 dBm, and 6 dBm, respectively, when the relaying node number is assumed to be 1 to 3 individually. Similar to the working situation of zero boresight pointing error, when there is a non-zero boresight pointing error, the serial relaying node still effectively improves the system's BER performance. Unfortunately, the delayed spread expansion of the FFIR caused by the increase of the initial divergence angle of light source, that is, the so-called Inter-symbol Interference (ISI), will seriously degrade this performance improvement. For example, when the initial divergence angle is increased from 0.01° to 3°, the transmission power needs to be increased in general by about 10~15 dBm to obtain the same BER compared with the original transmission one. In addition, the jitter standard deviation and the initial boresight displacement associated with the pointing error will also significantly impact the BER performance of the relaying UWOC systems. For instance, if the jitter standard deviation is enhanced from 10 cm to 20 cm, the node transmission power needs to be increased by nearly 20~25 dBm to achieve the same BER value; meanwhile, when the initial boresight displacement increases from 5 cm to 10 cm, the transmission power needs to be added by about 5 dBm to reach the same BER target. Therefore, when one considers the BER evaluation of the serial-relayed UWOC systems, it is necessary to consider the impacts of the initial divergence angle, the jitter standard deviation, and the initial boresight displacement on performance thoroughly.The analytical results of this paper can provide calculation support for analyzing the BER performance of the relaying UWOC systems. The channel fading generated by seawater absorption and scattering, oceanic turbulence, and pointing error will severely reduce the communication quality of an Underwater Wireless Optical Communication (UWOC) system. Many scholars directly transplanted the weak atmospheric turbulence model to depict the oceanic turbulence statistics, and this had been proven to be incorrect by a series of laboratory measurements and associated data-fitting tests. So, it is obviously of great significance to study and evaluate the effects of a composite fading channel on the key performance of the UWOC system, especially with a proper weak oceanic turbulence model.In this paper, the Generalized Gamma Distribution (GGD) verified by a series of experimental tests was selected to characterize the weak oceanic turbulence. Then, a new hybrid fading channel model was proposed to more reasonably simulate the communication environment in the ocean, which had integrated the GGD weak turbulence, the zero/nonzero boresight pointing error, the implicit path loss and multipath propagation effect characterized by Fading Free Impulse Response (FFIR). Next, the mathematical expressions of the Probability Density Function (PDF) considering GGD weak turbulence and zero/nonzero boresight pointing errors were derived using higher transcendental Meijer-G and Whittaker functions. Subsequently, based on this, the closed-form expressions of the average Bit Error Rate (BER) were derived for the serial-relayed UWOC systems with both zero and nonzero boresight pointing errors, respectively. Finally, the accuracy and rationality of the derived closed-form formulas for the average bit error rate of the relaying UWOC system derived above were verified by some Monte Carlo numerical simulations; meanwhile, the influences of different key parameters on the system BERs were also investigated.The results show that the introduction of serial relaying nodes can effectively improve the end-to-end BER performance of the UWOC systems in a long-distance communication environment. With the increase of the relaying nodes number, the system's BER decreases rapidly at the same transmission power, indicating that the serial-relayed scheme dramatically improves the performance. For instance, if the end-to-end distance is fixed as 45 m and the target BER is set to be 10-3, under the zero boresight pointing error condition, the required node transmission power will be reduced to 24 dBm, 12 dBm, and 6 dBm, respectively, when the relaying node number is assumed to be 1 to 3 individually. Similar to the working situation of zero boresight pointing error, when there is a non-zero boresight pointing error, the serial relaying node still effectively improves the system's BER performance. Unfortunately, the delayed spread expansion of the FFIR caused by the increase of the initial divergence angle of light source, that is, the so-called Inter-symbol Interference (ISI), will seriously degrade this performance improvement. For example, when the initial divergence angle is increased from 0.01° to 3°, the transmission power needs to be increased in general by about 10~15 dBm to obtain the same BER compared with the original transmission one. In addition, the jitter standard deviation and the initial boresight displacement associated with the pointing error will also significantly impact the BER performance of the relaying UWOC systems. For instance, if the jitter standard deviation is enhanced from 10 cm to 20 cm, the node transmission power needs to be increased by nearly 20~25 dBm to achieve the same BER value; meanwhile, when the initial boresight displacement increases from 5 cm to 10 cm, the transmission power needs to be added by about 5 dBm to reach the same BER target. Therefore, when one considers the BER evaluation of the serial-relayed UWOC systems, it is necessary to consider the impacts of the initial divergence angle, the jitter standard deviation, and the initial boresight displacement on performance thoroughly.The analytical results of this paper can provide calculation support for analyzing the BER performance of the relaying UWOC systems.

Acta Photonica Sinica

Publication Date: Feb. 25, 2024
Vol. 53, Issue 2, 0206002 (2024)

Surface Reconstruction and Error Compensation Method Based on NSGA-II Sensing Position Optimization

Qiufeng SHANG, and Xiaoxu ZHANG

In order to improve the accuracy of shape sensing, this paper optimizes the sensing position based on the Non-dominated Sorting Genetic Algorithm-II (NSGA-II), and uses the Radial Basis Function-Particle Swarm Optimization (PSO-RBF) neural network algorithm to improve the accuracy of structural reconstruction. In this study, the goal was to reconstruct the shape of a 150 mm×150 mm×0.5 mm nitinol version. Firstly, the finite element model of the nitinol version was established by using ANSYS workbench software. After a series of operations such as meshing, adding constraints, adding materials, and modal analysis, the surface strain modal matrix and displacement modal matrix of the model were extracted. According to the modal analysis results and the principle of modal reconstruction, 8 sensing points can be selected to realize the shape reconstruction of the model. The strain mode matrix is used as the input matrix of the NSGA-II algorithm. According to the modal confidence criterion, the conditional number criterion and the modal mode shape similarity criterion, three objective functions were obtained. The NSGA-II multi-objective optimization algorithm, which introduces fast non-dominance sequences, business strategies and congestion operators, was used to select the best sensing location. It not only reduces the computational complexity of the algorithm, but also better retains the excellent individuals. Then, the wavelength of the center of the Fiber Bragg Grating (FBG) was demodulated by the SM125 interrogator, and the linear relationship between the wavelength change and curvature of the eight FBG centers was obtained by linear fitting. Since epoxy resin has a high strain transfer rate, the FBG was glued to the selected optimal sensing position. The nitinol plate was bent into different arcs to obtain FBG strain data. The displacement and shape of the nitinol plate at this time were recorded. The strain-mode mode shape, displacement mode mode and FBG strain data were input into the reconstruction algorithm. According to the modal reconstruction algorithm, the shape reconstruction was preliminarily realized, and the best sensing position point reconstruction results obtained by the K-means++ algorithm were compared. Finally, the PSO-RBF neural network algorithm was used to fit the nonlinear relationship between the reconstruction error and the reconstruction displacement. The PSO-RBF neural network algorithm has strong nonlinear fitting ability, which can avoid falling into local optimum. The ratio of the training, validation, and test sets is 6∶2∶2. In this way, the prediction of the reconstruction error can be realized, and the accuracy of the shape reconstruction can be improved. The NSGA-II algorithm was used to optimize the sensing position, and the FBG strain information was collected to reconstruct the structure shape, and the reconstruction effect was better than that of the K-means++ algorithm, and the root mean square error was reduced by 30% and the maximum error was reduced by 15% compared with the K-means++ algorithm. After fitting the nonlinear relationship between the reconstruction error and the reconstruction displacement by PSO-RBF, the root mean square error and the maximum error are reduced by 90% and 70% respectively compared with the non-error compensation, and the reconstruction shape is almost the same as the structural shape, which can achieve high-precision reconstruction of the structural shape. This paper successfully realizes the high-precision shape reconstruction of the nitinol version. By optimizing the optimal sensing position, the root mean square errors are 0.500 mm, 0.561 mm and 0.636 mm, and the maximum errors are 2.102 mm, 2.315 mm and 2.561 mm, respectively, when the bending curvature radius of the nitinol plate is 200 mm, 180 mm and 160 mm, respectively. When the bending curvature radius is 180 mm and 160 mm, the root mean square error is 0.038 mm and 0.046 mm, and the maximum error is 0.686 mm and 0.778 mm, respectively. In order to improve the accuracy of shape sensing, this paper optimizes the sensing position based on the Non-dominated Sorting Genetic Algorithm-II (NSGA-II), and uses the Radial Basis Function-Particle Swarm Optimization (PSO-RBF) neural network algorithm to improve the accuracy of structural reconstruction. In this study, the goal was to reconstruct the shape of a 150 mm×150 mm×0.5 mm nitinol version. Firstly, the finite element model of the nitinol version was established by using ANSYS workbench software. After a series of operations such as meshing, adding constraints, adding materials, and modal analysis, the surface strain modal matrix and displacement modal matrix of the model were extracted. According to the modal analysis results and the principle of modal reconstruction, 8 sensing points can be selected to realize the shape reconstruction of the model. The strain mode matrix is used as the input matrix of the NSGA-II algorithm. According to the modal confidence criterion, the conditional number criterion and the modal mode shape similarity criterion, three objective functions were obtained. The NSGA-II multi-objective optimization algorithm, which introduces fast non-dominance sequences, business strategies and congestion operators, was used to select the best sensing location. It not only reduces the computational complexity of the algorithm, but also better retains the excellent individuals. Then, the wavelength of the center of the Fiber Bragg Grating (FBG) was demodulated by the SM125 interrogator, and the linear relationship between the wavelength change and curvature of the eight FBG centers was obtained by linear fitting. Since epoxy resin has a high strain transfer rate, the FBG was glued to the selected optimal sensing position. The nitinol plate was bent into different arcs to obtain FBG strain data. The displacement and shape of the nitinol plate at this time were recorded. The strain-mode mode shape, displacement mode mode and FBG strain data were input into the reconstruction algorithm. According to the modal reconstruction algorithm, the shape reconstruction was preliminarily realized, and the best sensing position point reconstruction results obtained by the K-means++ algorithm were compared. Finally, the PSO-RBF neural network algorithm was used to fit the nonlinear relationship between the reconstruction error and the reconstruction displacement. The PSO-RBF neural network algorithm has strong nonlinear fitting ability, which can avoid falling into local optimum. The ratio of the training, validation, and test sets is 6∶2∶2. In this way, the prediction of the reconstruction error can be realized, and the accuracy of the shape reconstruction can be improved. The NSGA-II algorithm was used to optimize the sensing position, and the FBG strain information was collected to reconstruct the structure shape, and the reconstruction effect was better than that of the K-means++ algorithm, and the root mean square error was reduced by 30% and the maximum error was reduced by 15% compared with the K-means++ algorithm. After fitting the nonlinear relationship between the reconstruction error and the reconstruction displacement by PSO-RBF, the root mean square error and the maximum error are reduced by 90% and 70% respectively compared with the non-error compensation, and the reconstruction shape is almost the same as the structural shape, which can achieve high-precision reconstruction of the structural shape. This paper successfully realizes the high-precision shape reconstruction of the nitinol version. By optimizing the optimal sensing position, the root mean square errors are 0.500 mm, 0.561 mm and 0.636 mm, and the maximum errors are 2.102 mm, 2.315 mm and 2.561 mm, respectively, when the bending curvature radius of the nitinol plate is 200 mm, 180 mm and 160 mm, respectively. When the bending curvature radius is 180 mm and 160 mm, the root mean square error is 0.038 mm and 0.046 mm, and the maximum error is 0.686 mm and 0.778 mm, respectively.

Acta Photonica Sinica

Publication Date: Feb. 25, 2024
Vol. 53, Issue 2, 0206001 (2024)

A Helical Multicore Optical Fiber Design for Coherent Imaging

Jinhu ZHENG, Bingshen XU, Henan SHEN, Fei YU, and Jian CHEN

Multicore Fiber (MCF)/imaging fiber bundle is a key device of flexible optical endoscope. In imaging applications, MCFs are widely used in the non-coherent imaging which transmits the intensity distribution only. The bending sensitive distortion of phase plane and inter-core crosstalk bring challenges in the coherent imaging application. In this paper, a Helical-Core MCF (HC-MCF) is designed for the application of coherent imaging.Due to the intricate nature of HC-MCF, neither semi-analytical models nor empirical methods can fully resolve the modal properties. Consequently, a full-vector finite-element method is employed for numerical simulation of HC-MCF. By utilizing the optical transformation technique, HC-MCF in the natural space is equivalently represented in the helical coordinate maintaining the translation invariance. The original isotropic permeability and dielectric constant (both scalars) of the optical fiber material are adjusted to equivalent dielectric constant and equivalent permeability values. This simplification can effectively reduce the computational complexity of the field distribution and equivalent effective refractive index of fundamental mode in HC-MCF. By using this method, the inter-core group delay differences of HC-MCF is simulated for optimization of fiber design.Then, an optimized design of HC-MCF is proposed. In order to minimize the distortion of phase front after transmission in HC-MCF, each core of HC-MCF should have a similar optical path. An appropriate core spacing should be selected to balance between the spatial sampling density and crosstalk among fiber cores. The helix period is preferred smaller than the critical bend radius in the application. Our final design of HC-MCF are arranged in a densely stacked hexagonal configuration, comprising 6 layers with a total of 91 cores. The radii of the fiber cores are 4 μm, 3.3 μm, 3.1 μm, 3 μm, 2.9 μm and 2.8 μm from the inside to the outside, with a core pitch of 20 μm and helical pitch of 10 cm. The inter-core group delay difference per unit length of straight HC-MCF is calculated and the maximum is found to be 6 fs/m. When the bending radius is significantly larger than the wavelength, the change in mode equivalent refractive index caused by bending is disregarded, and the variation in group delay difference resulting from core bending is determined solely by changes in core geometry length. The trajectory equation of the bent core is derived to obtain its geometry length, enabling determination of the corresponding change in group delay. Calculations are performed for two different bending radii (0.5 m and 0.05 m) to assess variations in group delay difference per unit length for helical fibers under these conditions. Remarkably, similar changes are observed under both bending scenarios, indicating that alterations in bend state do not induce significant phase modifications within transmitted light fields. By carefully designing the structure of HC-MCF, excellent bend performance can be achieved.At last, the bend induced inter-core crosstalk of HC-MCF is calculated. The crosstalks between cores of adjacent layers for HC-MCF with a total length of 100 m, torsion rate of 20 π/m, and core spacing of 20 μm are calculated and compared. Due to slight variations in mode phase mismatch between different layers during bending, there exists a maximum crosstalk value when phase matching conditions are satisfied. When the bending radius is smaller than that at which phase matching occurs, inter-layer core crosstalk becomes insensitive to bending radius and maintains a consistently low level. In this design, featuring slightly varied core sizes and a helical structure within each layer, and an exceptionally low level of crosstalk (-550 dB/100 m) is achieved. This remarkably reduced crosstalk could significantly enhance the imaging quality.Due to the helical core design of the designed helical MCF, the complex random disturbance of the optical field phase transmitted by the multi-core fiber under the bending condition is reduced, and the group delay difference caused by the bending between the cores is effectively suppressed. HC-MCF can help to reduce the complexity of the coherent image restoration, which finds useful applications in fiber optic micro-imaging, ultrafast laser imaging and other fields. Multicore Fiber (MCF)/imaging fiber bundle is a key device of flexible optical endoscope. In imaging applications, MCFs are widely used in the non-coherent imaging which transmits the intensity distribution only. The bending sensitive distortion of phase plane and inter-core crosstalk bring challenges in the coherent imaging application. In this paper, a Helical-Core MCF (HC-MCF) is designed for the application of coherent imaging.Due to the intricate nature of HC-MCF, neither semi-analytical models nor empirical methods can fully resolve the modal properties. Consequently, a full-vector finite-element method is employed for numerical simulation of HC-MCF. By utilizing the optical transformation technique, HC-MCF in the natural space is equivalently represented in the helical coordinate maintaining the translation invariance. The original isotropic permeability and dielectric constant (both scalars) of the optical fiber material are adjusted to equivalent dielectric constant and equivalent permeability values. This simplification can effectively reduce the computational complexity of the field distribution and equivalent effective refractive index of fundamental mode in HC-MCF. By using this method, the inter-core group delay differences of HC-MCF is simulated for optimization of fiber design.Then, an optimized design of HC-MCF is proposed. In order to minimize the distortion of phase front after transmission in HC-MCF, each core of HC-MCF should have a similar optical path. An appropriate core spacing should be selected to balance between the spatial sampling density and crosstalk among fiber cores. The helix period is preferred smaller than the critical bend radius in the application. Our final design of HC-MCF are arranged in a densely stacked hexagonal configuration, comprising 6 layers with a total of 91 cores. The radii of the fiber cores are 4 μm, 3.3 μm, 3.1 μm, 3 μm, 2.9 μm and 2.8 μm from the inside to the outside, with a core pitch of 20 μm and helical pitch of 10 cm. The inter-core group delay difference per unit length of straight HC-MCF is calculated and the maximum is found to be 6 fs/m. When the bending radius is significantly larger than the wavelength, the change in mode equivalent refractive index caused by bending is disregarded, and the variation in group delay difference resulting from core bending is determined solely by changes in core geometry length. The trajectory equation of the bent core is derived to obtain its geometry length, enabling determination of the corresponding change in group delay. Calculations are performed for two different bending radii (0.5 m and 0.05 m) to assess variations in group delay difference per unit length for helical fibers under these conditions. Remarkably, similar changes are observed under both bending scenarios, indicating that alterations in bend state do not induce significant phase modifications within transmitted light fields. By carefully designing the structure of HC-MCF, excellent bend performance can be achieved.At last, the bend induced inter-core crosstalk of HC-MCF is calculated. The crosstalks between cores of adjacent layers for HC-MCF with a total length of 100 m, torsion rate of 20 π/m, and core spacing of 20 μm are calculated and compared. Due to slight variations in mode phase mismatch between different layers during bending, there exists a maximum crosstalk value when phase matching conditions are satisfied. When the bending radius is smaller than that at which phase matching occurs, inter-layer core crosstalk becomes insensitive to bending radius and maintains a consistently low level. In this design, featuring slightly varied core sizes and a helical structure within each layer, and an exceptionally low level of crosstalk (-550 dB/100 m) is achieved. This remarkably reduced crosstalk could significantly enhance the imaging quality.Due to the helical core design of the designed helical MCF, the complex random disturbance of the optical field phase transmitted by the multi-core fiber under the bending condition is reduced, and the group delay difference caused by the bending between the cores is effectively suppressed. HC-MCF can help to reduce the complexity of the coherent image restoration, which finds useful applications in fiber optic micro-imaging, ultrafast laser imaging and other fields.

Acta Photonica Sinica

Publication Date: Jan. 25, 2024
Vol. 53, Issue 1, 0106001 (2024)

In-situ Monitoring of the Internal Status of Lithium Batteries Based on Fiber Bragg Grating Sensors

Wenhui SHI, Hao WANG, Hui CAO, Yixin LIU, Jianyu LI, Jiajin ZHENG, and Wei WEI

As one of the most important energy storage technologies today, the safety and reliability of lithium batteries have always been of great concern. The thermal stability and pressure stability of lithium batteries are important parameters that affect their safe and reliable operation. The internal electrochemical reactions will cause changes in temperature and stress during operation. Abuse of lithium batteries can cause rapid increases in temperature and stress of electrodes, leading to degradation of battery performance and even safety accidents such as combustion or explosion. Therefore, real-time monitoring of internal temperature and stress changes in lithium batteries is crucial for the long-term safe and stable operation of lithium batteries. However, current monitoring methods used for temperature and stress inside lithium batteries just focus on single parameters or external measurements, which have problems such as poor resolution and limited accuracy, making it difficult to monitor the changes inside the battery. In order to improve the healthy level of lithium-ion batteries monitoring, this paper proposes to use fiber Bragg grating sensing technology to monitor the changes. The gratings are implanted to collect real-time temperature and stress changes of the battery anode, realizing an optical channel for in-situ monitoring of the lithium-ion battery anode. Furthermore, combined with the battery test system, the connection between electrical and optical sensing signals is established. In the system, the temperature sensitivity of the FBG sensors is 9.3 pm/℃, and the stress sensitivity is 1.2 pm/με. The FBG sensors are mounted in different ways to achieve accurate measurement of dual parameters. Both ends of FBG1 are fixed for strain measurement. FBG2 fixes single end to monitor temperature and functions as temperature compensation for FBG1 at the same time. FBG3 is outside the battery, which is used to measure the external temperature of working environment. The experimental results show that the FBG sensors can remain good sensing performance at 400 ℃. The implantation has no effect on pouch cell performance, nor does it affect the sensing performance of FBG sensors. During the working cycles of lithium batteries, the detachment and embedding of lithium ions can cause temperature changes, resulting in a sensor wavelength shift of 100 pm, which means temperature increases by 11.1 ℃. The coefficient of thermal expansion of anode is 25.5 με/℃. After temperature compensation, the stress change of anode can be observed, indicating that the change in stress is influenced by current. In other words, the hop of current can cause the anode to contract and the resulting stress will result in a wavelength drift of 21.96 pm at most, which is approximately 18.3 με. According to our research, different charge and discharge rates have different effects. The faster the rate, the greater the variations in temperature and stress. The temperature change is 2.8 times and the stress is 4.4 times higher at 10 mA than at 2.5 mA. If the rate of charge or discharge further increases to 50 mA, the operating temperature will exceed 45 ℃. After 300 cycles at 45 ℃, the volume expansion rate of battery is about 10%, and the battery is likely to malfunction. The implantable grating monitoring system in this paper can not only measure the temperature and stress changes caused by electrochemical reaction with high precision, but also has fast demodulation speed, which is conducive to real-time and accurate monitoring of the thermal runaway and deformation bulge failure of lithium batteries. The research results are conducive to quantifying and evaluating the possible thermal runaway and volume expansion problems in the electrochemical process, which is expected to provide an effective experimental reference for the safe use of lithium batteries. As one of the most important energy storage technologies today, the safety and reliability of lithium batteries have always been of great concern. The thermal stability and pressure stability of lithium batteries are important parameters that affect their safe and reliable operation. The internal electrochemical reactions will cause changes in temperature and stress during operation. Abuse of lithium batteries can cause rapid increases in temperature and stress of electrodes, leading to degradation of battery performance and even safety accidents such as combustion or explosion. Therefore, real-time monitoring of internal temperature and stress changes in lithium batteries is crucial for the long-term safe and stable operation of lithium batteries. However, current monitoring methods used for temperature and stress inside lithium batteries just focus on single parameters or external measurements, which have problems such as poor resolution and limited accuracy, making it difficult to monitor the changes inside the battery. In order to improve the healthy level of lithium-ion batteries monitoring, this paper proposes to use fiber Bragg grating sensing technology to monitor the changes. The gratings are implanted to collect real-time temperature and stress changes of the battery anode, realizing an optical channel for in-situ monitoring of the lithium-ion battery anode. Furthermore, combined with the battery test system, the connection between electrical and optical sensing signals is established. In the system, the temperature sensitivity of the FBG sensors is 9.3 pm/℃, and the stress sensitivity is 1.2 pm/με. The FBG sensors are mounted in different ways to achieve accurate measurement of dual parameters. Both ends of FBG1 are fixed for strain measurement. FBG2 fixes single end to monitor temperature and functions as temperature compensation for FBG1 at the same time. FBG3 is outside the battery, which is used to measure the external temperature of working environment. The experimental results show that the FBG sensors can remain good sensing performance at 400 ℃. The implantation has no effect on pouch cell performance, nor does it affect the sensing performance of FBG sensors. During the working cycles of lithium batteries, the detachment and embedding of lithium ions can cause temperature changes, resulting in a sensor wavelength shift of 100 pm, which means temperature increases by 11.1 ℃. The coefficient of thermal expansion of anode is 25.5 με/℃. After temperature compensation, the stress change of anode can be observed, indicating that the change in stress is influenced by current. In other words, the hop of current can cause the anode to contract and the resulting stress will result in a wavelength drift of 21.96 pm at most, which is approximately 18.3 με. According to our research, different charge and discharge rates have different effects. The faster the rate, the greater the variations in temperature and stress. The temperature change is 2.8 times and the stress is 4.4 times higher at 10 mA than at 2.5 mA. If the rate of charge or discharge further increases to 50 mA, the operating temperature will exceed 45 ℃. After 300 cycles at 45 ℃, the volume expansion rate of battery is about 10%, and the battery is likely to malfunction. The implantable grating monitoring system in this paper can not only measure the temperature and stress changes caused by electrochemical reaction with high precision, but also has fast demodulation speed, which is conducive to real-time and accurate monitoring of the thermal runaway and deformation bulge failure of lithium batteries. The research results are conducive to quantifying and evaluating the possible thermal runaway and volume expansion problems in the electrochemical process, which is expected to provide an effective experimental reference for the safe use of lithium batteries.

Acta Photonica Sinica

Publication Date: Sep. 25, 2023
Vol. 52, Issue 9, 0906002 (2023)

Absolute Phase of the Radio Frequency Transfer over Optical Fiber with Phase Self-calibration

Jinbo ZHANG, Liang HU, Qi LI, Jianping CHEN, and Guiling WU

As basic physical quantities, time and frequency are the research basis of many applied fields, such as verification of basic physics, clock-based geodetic survey, positioning and navigation. Optical fiber is an ideal medium for high stability time-frequency transmission due to its advantages of low loss, large bandwidth and strong anti-electromagnetic interference. At present, mainstream frequency transmission schemes can be roughly divided into optical frequency, optical frequency comb and radio frequency transmission. However, most of the existing time-frequency transmission systems can only guarantee a constant phase difference during the operation of the system, and ignore the phase difference change that may occur after relocking when the system is restarted or the system link length is changed. This situation cannot meet the needs of some coherent applications, such as distributed phased array radar, radio telescope arrays. These applications not only require stable phase difference during operation, but also require that the value of phase difference does not change after multiple restarts, which can provide reference signals of the same frequency and phase between different sites to achieve more effective coherent processing.In this paper, an absolute phase transfer scheme for optical fiber radio frequency based on adjustable optical delay line is proposed. The scheme makes full use of the round-trip transmission delay of the time signal to determine the integer period of the phase of frequency signal. Combined with the high precision phase measurement of microwave frequency, a microwave frequency signal with a fixed phase difference between the remote signal and the local signal can be obtained when the system is shut down and restarted for many times. In this scheme, 1PPS signal is not directly used as a time reference signal. First, RC differential circuit and avalanche triple laser are used to convert 1PPS signal into narrow pulse signal, and then surface acoustic wave filter is used to filter the narrow pulse signal to obtain the narrow band time reference signal. By means of frequency division multiplexing, time signal and microwave frequency signal are coupled in the same wavelength channel for transmission so as to avoid the delay difference introduced in different wavelength transmission. The system uses the way of wavelength division multiplexing to transmit. The return time frequency signal is obtained by a loop method and compared with the local reference signal to obtain the round-trip link delay and link cumulative phase. The cumulative phase of the link is used as an error signal to control the optical delay line to stabilize the link, and then the absolute phase transmission is realized by using the calculated link delay to calibrate the integer period of frequency signal. Compared with other time-frequency co-transmission modes, the system of this scheme is simpler. It does not need to adopt multiple modulation modes, nor does it directly modulate 1PPS signal. Narrow-band filter can be used to separate the time signal from the microwave frequency signal so that they do not affect each other.The experiment was carried out on a 60 km optical fiber link. After transmission, the system obtained a frequency stability of better than 4×10-14@1 s, 5×10-17@10 000 s. After the link is stabilized, the measured stability of time transfer (TDEV) is 10 ps@1 s and 0.3 ps@10 000 s. The results show that this scheme has good link compensation effect. When the system is restarted several times, the maximum inconsistency of the average phase difference is 0.008 rad, corresponding to 0.15% of the whole cycle, which can ensure the realization of good phase consistency. As basic physical quantities, time and frequency are the research basis of many applied fields, such as verification of basic physics, clock-based geodetic survey, positioning and navigation. Optical fiber is an ideal medium for high stability time-frequency transmission due to its advantages of low loss, large bandwidth and strong anti-electromagnetic interference. At present, mainstream frequency transmission schemes can be roughly divided into optical frequency, optical frequency comb and radio frequency transmission. However, most of the existing time-frequency transmission systems can only guarantee a constant phase difference during the operation of the system, and ignore the phase difference change that may occur after relocking when the system is restarted or the system link length is changed. This situation cannot meet the needs of some coherent applications, such as distributed phased array radar, radio telescope arrays. These applications not only require stable phase difference during operation, but also require that the value of phase difference does not change after multiple restarts, which can provide reference signals of the same frequency and phase between different sites to achieve more effective coherent processing.In this paper, an absolute phase transfer scheme for optical fiber radio frequency based on adjustable optical delay line is proposed. The scheme makes full use of the round-trip transmission delay of the time signal to determine the integer period of the phase of frequency signal. Combined with the high precision phase measurement of microwave frequency, a microwave frequency signal with a fixed phase difference between the remote signal and the local signal can be obtained when the system is shut down and restarted for many times. In this scheme, 1PPS signal is not directly used as a time reference signal. First, RC differential circuit and avalanche triple laser are used to convert 1PPS signal into narrow pulse signal, and then surface acoustic wave filter is used to filter the narrow pulse signal to obtain the narrow band time reference signal. By means of frequency division multiplexing, time signal and microwave frequency signal are coupled in the same wavelength channel for transmission so as to avoid the delay difference introduced in different wavelength transmission. The system uses the way of wavelength division multiplexing to transmit. The return time frequency signal is obtained by a loop method and compared with the local reference signal to obtain the round-trip link delay and link cumulative phase. The cumulative phase of the link is used as an error signal to control the optical delay line to stabilize the link, and then the absolute phase transmission is realized by using the calculated link delay to calibrate the integer period of frequency signal. Compared with other time-frequency co-transmission modes, the system of this scheme is simpler. It does not need to adopt multiple modulation modes, nor does it directly modulate 1PPS signal. Narrow-band filter can be used to separate the time signal from the microwave frequency signal so that they do not affect each other.The experiment was carried out on a 60 km optical fiber link. After transmission, the system obtained a frequency stability of better than 4×10-14@1 s, 5×10-17@10 000 s. After the link is stabilized, the measured stability of time transfer (TDEV) is 10 ps@1 s and 0.3 ps@10 000 s. The results show that this scheme has good link compensation effect. When the system is restarted several times, the maximum inconsistency of the average phase difference is 0.008 rad, corresponding to 0.15% of the whole cycle, which can ensure the realization of good phase consistency.

Acta Photonica Sinica

Publication Date: Sep. 25, 2023
Vol. 52, Issue 9, 0906001 (2023)

Joint Compensation Scheme for Polarization Impairment Based on Density Matrix Formalism

Yao GUO, Xia ZHANG, Qiuping DU, Zhenshan YANG, and Xiaoguang ZHANG

While effectively enhancing the transmission capacity of current commercial single-mode optical fiber communication systems, Polarization Division Multiplexing (PDM) technology also faces serious challenges from Polarization Mode Dispersion (PMD) and Rotation of State of Polarization (RSOP). PMD causes the broadening of optical pulses, resulting in crosstalk and distortion of the signals, which may significantly increase the Bit Error Rate (BER) of the system. Also, RSOP might cause rapid change in the polarization state of optical signals up to several hundred krad/s, preventing the two polarization signals from being correctly separated at the receiver. In practical optical fiber links, PMD and RSOP usually coexist and impose significant impairments on the system performance. The main work of this paper is to build an impairment model of PMD and RSOP and to conduct a joint compensation of the polarization-relevant impairments.Traditionally, the Stokes formalism is employed to analyze the PMD and RSOP in commercial single-mode fibers that actually support two orthogonal polarization modes. The corresponding Stokes vector is a 3-dimensional real vector with clear physical meaning, and can be intuitively represented in the geometrical Poincare sphere. However, the Stokes formalism requires 3 auxiliary 2×2 Pauli matrices, and when extended to the treatment of Modal Dispersion (MD) and Mode Coupling (MC) in an N-mode optical fiber (i.e., a modal space of dimension N), a number of N2-1 auxiliary N×N Gell-Mann matrices are required, which can drastically complicate the analysis of MD and MC effects as N increases. Recently, borrowing methodology from quantum mechanics, we proposed and developed the Density Matrix (DM) formalism for the MD and MC in a modal space of arbitrary dimension N≥2. Without requiring any auxiliary matrices, the DM formalism is simple and straightforward in formulation and application, and is thus particularly suitable for the study of modal properties and signal compensations in the optical communication system.In this paper, we apply the DM formalism in the PDM system to construct a joint polarization-impairment compensation scheme. We establish a polarization impairment model for coexisting PMD and RSOP based on traceless Hermitian impairment matrices. By tracking the corresponding independent parameters in the matrices, we achieve the joint impairment compensation of PMD and RSOP. To verify the proposed scheme, we build a 28 GBaud PDM quadrature phase-shift keying coherent optical communication transmission simulation system, and implement a joint impairment-compensation for Differential Group Delay (DGD) over a wide range of 30~170 ps and fast RSOP over 300 krad/s~2 Mrad/s. In simulations, with about 100 iterations, the tracking and compensation have already converged for the impairment of 100 ps Differential Group Delay (DGD), and with only 0.5 dB and 0.7 dB optical Signal-to-noise Ratio (OSNR) cost, joint compensation of RSOP can be achieved in typical (600 krad/s) and extreme conditions (2 Mrad/s), respectively, both in the presence of 100 ps DGD. When 170 ps DGD and 2 Mrad/s fast RSOP coexist, the BER is 3.22×10-3, which still meets the criterion for error-free transmission of signals. The fast convergence, the small OSNR cost, and the stable BER performance verify the validity and efficacy of our polarization impairment model and the corresponding joint compensation scheme in optical communication systems.Furthermore, taking advantage of the DM formalism, our joint compensation scheme can be readily generalized to modal spaces of arbitrary dimension N for the impairment analysis and signal compensation of MD and MC, simply by the extending the relevant matrices from 2×2 to N×N. Therefore, our work potentially provide a simple and effective theoretical approach for the impairment analysis and compensation of optical-signals in more general mode-division multiplexing communication systems. While effectively enhancing the transmission capacity of current commercial single-mode optical fiber communication systems, Polarization Division Multiplexing (PDM) technology also faces serious challenges from Polarization Mode Dispersion (PMD) and Rotation of State of Polarization (RSOP). PMD causes the broadening of optical pulses, resulting in crosstalk and distortion of the signals, which may significantly increase the Bit Error Rate (BER) of the system. Also, RSOP might cause rapid change in the polarization state of optical signals up to several hundred krad/s, preventing the two polarization signals from being correctly separated at the receiver. In practical optical fiber links, PMD and RSOP usually coexist and impose significant impairments on the system performance. The main work of this paper is to build an impairment model of PMD and RSOP and to conduct a joint compensation of the polarization-relevant impairments.Traditionally, the Stokes formalism is employed to analyze the PMD and RSOP in commercial single-mode fibers that actually support two orthogonal polarization modes. The corresponding Stokes vector is a 3-dimensional real vector with clear physical meaning, and can be intuitively represented in the geometrical Poincare sphere. However, the Stokes formalism requires 3 auxiliary 2×2 Pauli matrices, and when extended to the treatment of Modal Dispersion (MD) and Mode Coupling (MC) in an N-mode optical fiber (i.e., a modal space of dimension N), a number of N2-1 auxiliary N×N Gell-Mann matrices are required, which can drastically complicate the analysis of MD and MC effects as N increases. Recently, borrowing methodology from quantum mechanics, we proposed and developed the Density Matrix (DM) formalism for the MD and MC in a modal space of arbitrary dimension N≥2. Without requiring any auxiliary matrices, the DM formalism is simple and straightforward in formulation and application, and is thus particularly suitable for the study of modal properties and signal compensations in the optical communication system.In this paper, we apply the DM formalism in the PDM system to construct a joint polarization-impairment compensation scheme. We establish a polarization impairment model for coexisting PMD and RSOP based on traceless Hermitian impairment matrices. By tracking the corresponding independent parameters in the matrices, we achieve the joint impairment compensation of PMD and RSOP. To verify the proposed scheme, we build a 28 GBaud PDM quadrature phase-shift keying coherent optical communication transmission simulation system, and implement a joint impairment-compensation for Differential Group Delay (DGD) over a wide range of 30~170 ps and fast RSOP over 300 krad/s~2 Mrad/s. In simulations, with about 100 iterations, the tracking and compensation have already converged for the impairment of 100 ps Differential Group Delay (DGD), and with only 0.5 dB and 0.7 dB optical Signal-to-noise Ratio (OSNR) cost, joint compensation of RSOP can be achieved in typical (600 krad/s) and extreme conditions (2 Mrad/s), respectively, both in the presence of 100 ps DGD. When 170 ps DGD and 2 Mrad/s fast RSOP coexist, the BER is 3.22×10-3, which still meets the criterion for error-free transmission of signals. The fast convergence, the small OSNR cost, and the stable BER performance verify the validity and efficacy of our polarization impairment model and the corresponding joint compensation scheme in optical communication systems.Furthermore, taking advantage of the DM formalism, our joint compensation scheme can be readily generalized to modal spaces of arbitrary dimension N for the impairment analysis and signal compensation of MD and MC, simply by the extending the relevant matrices from 2×2 to N×N. Therefore, our work potentially provide a simple and effective theoretical approach for the impairment analysis and compensation of optical-signals in more general mode-division multiplexing communication systems.

Acta Photonica Sinica

Publication Date: Aug. 25, 2023
Vol. 52, Issue 8, 0806003 (2023)

Topics

Atmospheric and Oceanic Optics

Atomic and Molecular Physics

Fiber Optics and Optical Communications

Image Processing

Imaging Systems

Instrumentation, Measurement and Metrology

Integrated Optics

Introduction for Special Issue

Lasers and Laser Optic

Lasers and Laser Optics

Medical Optics and Biotechnology

Nonlinear Optics

Optical Design and Fabrication

Optical Designand Fabrication

Optical Devices

Optics at Surfaces

Optoelectronics

Physical Optics

Remote Sensing and Sensors

Special Issue for Light Field Manipulation and Applications

Special Issue for Micro/Nanophotonics

Special Issue for the 60th Anniversary of XIOPM of CAS, and the 50th Anniversary of the Acta Photonica Sinica

Special Issue for the 60th Anniversary of XIOPM of CAS, and the 50th Anniversary of the Acta Photonica Sinica Ⅱ

Special Issue for Ultrafast Laser Precision Machining of Hard and Brittle Materials

Special Issue for Ultrafast Optics

Special Issue on Two-dimensional Optical Functional Materials and Devices

Ultrafast Optics