Quantitative in situ measurement of optical force along a strand of cleaved silica optical fiber induced by the light guided therewithin
Mikko Partanen, Hyeonwoo Lee, and Kyunghwan Oh
We propose an optomechanical system to quantify the net force on a strand of cleaved silica optical fiber in situ as the laser light is being guided through it. Four strands of the fiber were bonded to both sides of a macroscopic oscillator, whose movements were accurately monitored by a Michelson interferometer. The laser light was propagating with variable optical powers and frequency modulations. Experimentally, we discovered that the driving force for the oscillator consisted of not only the optical force of the light exiting from the cleaved facets but also the tension along the fiber induced by the light guided therewithin. The net driving force was determined only by the optical power, refractive index of the fiber, and the speed of light, which pinpoints its fundamental origin.
  • Sep. 17, 2021
  • Photonics Research
  • Vol.9 Issue, 10 10002016 (2021)
  • DOI:10.1364/PRJ.433995
Optical Fiber High and Low Temperature Mechanical and Thermal Multi-Parameter Sensing System Based on Tunable Laser
Guo Hairuo, Liu Kun, Jiang Junfeng, Xu Tianhua, Wang Shuang, Sun Zhenshi, Zhou Zichun, Xue Kang, Huang Yuelang, and Liu Tiegen
Objective Spatial, oceanic and geological exploration technologies have increasingly become important directions in our country development. In order to meet the demand of accurate detections, it is necessary to obtain basic physical information (e.g. temperature and strain). However, the requirement for sensors in these complex environments is extremely stringent. Compared with traditional electromagnetic sensors, optical fiber sensors have the characteristics of small size, light weight, corrosion resistance, anti-electromagnetic interference, etc. For optical fiber mechanical and thermal sensing, there are two common methods. One is using the fiber-Bragg-grating-based sensing structure which obtains the mechanical and thermal parameters through the spectral information of the reflected light. The other is using a combination of different sensing structures such as the combination of long-period fiber gratings (LPFG) with photonic crystal fiber (PCF). The above two methods both have shortcomings such as small measurement range, large measurement error, complex structure, and high cost. In response to the above problems, a fiber Bragg grating for temperature and strain multi-parameter sensing system based on a tunable laser is theoretically proposed and environmentally tested.Methods In this study, the system consists of a tunable laser, a Fabry-Perot etalon, a driving circuit, a beam splitter, fiber Bragg grating temperature and strain sensors, photodetectors and a data acquisition card. After passing through the coupler, the light output from the tunable laser is divided into two paths, which respectively enter the multi-channel sensor and the etalon. The light entering the multi-channel sensor passes through the beam splitter and enters the 16 sensing channels. The light reflected by the sensor and the light passed through F-P etalon are transmitted to the photodetector and converted into electrical signals. The driving circuit generates a square wave signal synchronized with the triangular wave, which is used as the trigger signal of data acquisition. It controls the acquisition card and the field programmable gate array (FPGA) data processing module to process the electrical signal and demodulate the temperature and strain value according to the relationship between the wavelength and the mechanical and thermal parameters. In the aspect of sensor, we utilize a special packaging structure. The fiber optic temperature sensor is packaged with a ceramic tube. The grating coated with a high temperature resistant polyimide is used as the sensing element. The fiber grating is bonded with the outer layer of an alumina ceramic tube by using low melting point glass. This method avoids the problem of adhesive aging at low temperatures, so the temperature sensor can be applied to a wider temperature range, and the stability of the sensor is improved. The fiber optic strain sensor is composed of a metal substrate, a fiber Bragg grating and a sleeve. A desensitized substrate is used to protect the strain sensing grating. In addition, the 316L stainless steel is used as the base material of the strain sensor. The material has corrosion resistance. Besides, the thermal expansion coefficient of the 316L stainless steel is close to that of the measured structure in engineering applications.These characteristics can further improve the accuracy of strain measurements. In the manufacturing process, the metal substrate should be polished and wiped with ethanol to remove foreign matters on the surface. The fiber Bragg grating is welded with the metal substrate by using low melting point glass in the pre-stretched state. After the substrate is cooled, the epoxy adhesive with good temperature adaptability is used for further fixation. This method improves the temperature adaptability of the sensor while ensuring the accuracy of strain measurements.Results and Discussions It can be seen that in the range of 0-200 ℃, the relationship between temperature and wavelength has a good linear relationship (Fig. 6). The sensitivity of the temperature sensor is about 11.60 pm/℃. The wavelength resolution of the optical fiber mechanical and thermal sensing instrument is 1 pm, so the demodulation sensitivity of the corresponding temperature sensing system is about 0.09 ℃. In the range of temperature below zero, temperature and wavelength quadratic polynomial fitting can improve the demodulation accuracy of the temperature sensing system. At each temperature node, the measurement error of the temperature sensing system is less than ±0.8 ℃ (Fig. 8). For the strain sensing, the FBG strain sensor can still work normally in the whole temperature range of -252.75--200.94 ℃. The sensitivity of the strain sensor is 1.66 pm/με, and the average measurement error is less than 2.9 με (Fig. 7). The experimental results show that the proposed sensor system has good accuracy and stability.Conclusions This study describes a wide range and high-precision optical fiber temperature and strain sensing system based on a tunable laser. Besides, the fabrication method of an optical fiber temperature and strain sensor is improved to enhance the temperature adaptation range. Finally, the instrument development is carried out, and the performances in high and low temperature environments are tested and analyzed. The experimental results show that the system can realize accurate temperature and strain measurements in the temperature range of -252.75--200.94 ℃. The temperature measurement accuracy is less than ±0.80 ℃ and the strain measurement accuracy is less than ±2.90 με. In addition, the system can realize a multi-channel and multi-parameter measurement at the same time, which is suitable for an engineering application in special environments with high and low temperatures.
  • Sep. 16, 2021
  • Chinese Journal of Lasers
  • Vol.48 Issue, 19 1906003 (2021)
  • DOI:10.3788/CJL202148.1906003
Realizing Optical Absorption Properties of Polymer Waveguides using LED Pump Source
Xue Jiabi, Lai Shouqiang, Liu Xin, and Zhang Dan
Objective The amplification function that describes an optical signal can be realized in rare-earth-doped polymer optical waveguide amplifiers based on the stimulated radiation of rare-earth ions when they experience excitation at the pump source. As an active device, polymer optical waveguide amplifier can be integrated with multiplexer/demultiplexer, beam splitter, optical switch, and other devices to compensate for various losses in the optical field that may occur during device transmission. To fabricate optical waveguide amplifiers, we typically use an SU-8 photoresist polymer and polymethyl methacrylate (PMMA) as the doping matrices for rare-earth ions. Further, to ensure population inversion of the produced rare-earth ions, pump sources are usually required. A majority of the research spanning the past three decades has focused on selecting semiconductor lasers as the pumping sources. Compared with the end-coupled pumping method using semiconductor lasers as its pumping source, the use of a low-power and low-cost light emitting diode (LED) is a new development trend that can effectively solve the problems of up-conversion problems and polymer waveguide damage caused by high-power semiconductor laser pumping (200--400 mW) sources. Additionally, this development greatly reduces the commercialization costs involved in fabricating these devices and is expected to replace the traditional pumping method of semiconductor lasers. The absorption of the pump source by the polymer matrix material directly affects the gain performance of the rare-earth-doped polymer optical waveguide amplifier. However, SU-8 and PMMA materials are seldom reported to negatively impact absorption performance during the excitation of ultraviolet (UV)-visible LEDs. Based on this point, we utilized SU-8 and PMMA materials in a core layer and fabricated these materials via lithography and reactive-ion etching processes to form passive polymer optical waveguides. We demonstrated the absorption characteristics of polymer optical waveguides with pump sources derived from four different wavelengths of LEDs.Methods Using a one-step lithography process, a rectangular SU-8 polymer waveguide and a Mach-Zehnder waveguide with a cross-section of 5 μm×5 μm were fabricated. A rectangular PMMA waveguide as core material was prepared via lithography and reactive-ion etching. Next, the morphology of these waveguides was characterized using scanning electron microscopy. Using a vertical top pumping mode, the absorbabilities of the SU-8 and PMMA polymer waveguides were measured at 1064, 980, and 635 nm wavelengths under the excitation wavelength of 310, 365, 405, and 525 nm for the LED-based approach as well as an excitation wavelength of 808 nm using the vertical top pumping mode.Results and Discussions For the polymer SU-8 waveguide with a cross-section of 5 μm×5 μm and a length of 20 mm, the optical intensity attenuation reached ~91.7%, 48.3%, and 26.7% at 1064-nm wavelength laser under the LED with excitation wavelength of 310, 365, and 405 nm and 50-mW pump power. The optical intensity could remain stable under the excitation wavelength of 525 and 808 nm using LED and laser, respectively [Fig. 5 (a)]. The optical intensity attenuation reached ~70.8%, 41.1%, and 24.2% [Fig. 6(a)] at 980-nm wavelength laser under the LED with excitation wavelength of 310, 365, and 405 nm, respectively. There was no obvious attenuation of the optical intensity under laser pumping at 635-nm wavelength [Fig. 6(b)]. For an SU-8 polymer waveguide with a length of 20 mm, a thickness of 5 μm, and the widths of 4, 6, and 8 μm, the optical intensity attenuations of approximately 53.1%, 65.1%, and 70.6%, respectively, were obtained at laser with wavelength of 1064 nm for LED with an excitation of 310 nm and 80-mW pump power (Fig. 8). Turning to the SU-8 polymer Mach-Zehnder waveguide, using a 50-mW LED-based pump source, the optical intensity attenuations of approximately 98.9%, 38.1%, and 24.1% were obtained at a 1064-nm wavelength laser for LED with excitation wavelength of 310, 365, and 405 nm, respectively. The optical intensity remained stable for an excitation wavelength of 525 nm using the LED-based approach [Fig. 10(a)]. There was no obvious optical intensity attenuation for the LED-based approach at 635-nm wavelength [Fig. 10(b)], which aligns with results obtained for the rectangular straight waveguide. Finally, for the PMMA polymer waveguide, the optical intensity remained stable for the excitation wavelength of 310, 365, and 405 nm using the LED-based approach (Fig. 11).Conclusions In this study, we measure the optical absorption performance of SU-8 and PMMA polymer optical waveguides with excitation wavelength of 310, 365, 405, and 525 nm for an LED-based pump source, as well as an excitation wavelength of 808 nm for the traditional laser-based approach. Our experimental results show that when pumped by a blue-violet LED, the resulting optical intensity of the SU-8 polymer waveguide sharply decays at the wavelengths of 980 and 1064 nm. Additionally, we observe that the optical intensity attenuation weakens with a red shift of the center wavelength of the LED pump source and a decrease in the size of the polymer waveguide. For the SU-8 polymer waveguide with a cross-section of 5 μm × 5 μm and a length of 20 mm, using a 50-mW LED-based pump source, we attain an optical intensity attenuation of approximately 91.7%, 48.3%, and 26.7% at laser source with wavelength of 1064 nm for the LED with excitation wavelength of 310, 365, and 405 nm, respectively. Conversely, both the 525-nm LED-based approach and the 808-nm traditional laser-based approach, the resulting optical intensity of the SU-8 polymer waveguide remains stable. Further, for the PMMA polymer waveguide, no obvious optical intensity attenuation was observed under the excitation of LEDs. Therefore, we conclude that in rare-earth-doped SU-8 polymer optical waveguide amplifiers pumped by blue-violet LEDs, single-mode and small-size waveguides with a low-power LED pump source should be used to effectively avoid optical intensity attenuation. Here, we note that it is easier to achieve the optical gain using a PMMA polymer as the host for the rare-earth solution when pumped by LEDs.
  • Sep. 16, 2021
  • Chinese Journal of Lasers
  • Vol.48 Issue, 20 2006003 (2021)
  • DOI:10.3788/CJL202148.2006003
Location of Weak Faults in Optical Fiber Links of Direct Current Control and Protection System
Ruan Jun, Sun Hao, Zhu Zhijun, Tian Lin, Xu Wanli, Wu Baofeng, and Sun Xiaohan
Objective With the continuous development of power transmission systems from west to east, the advantages of ultrahigh voltage (UHV) direct current (DC) transmission have become increasingly evident. The optical fiber link used in the UHV DC control and protection system may fail, exposing the system to risks. Existing methods for inspecting the performance of the optical fiber link in such systems are deficient. Only a part of the optical fiber can be used to test the attenuation using a light source and an optical power meter, and the quality of the optical fiber link requires manual evaluation. Faults along the optical fiber link and decreasing transmission performance are difficult to determine during operation and maintenance, thus representing a security risk to the UHV DC control and protection system. Because the use of a light source and an optical power meter to detect link faults in the control and protection system requires cooperation on the two ends of the link, it is difficult to determine the fault location, which is not conducive to failure cause analysis and rectification. Considering the high security requirements of UHV DC transmission, phase-sensitive optical time-domain reflectometry (Φ-OTDR) is employed to detect and locate weak faults in the optical fiber link of the control and protection system.Methods The proposed Φ-OTDR system prepared under laboratory settings comprised an optical transmission module, optical fiber interferometer, sensing optical fiber, optical receiver module, simulated weak fault source, signal processing module, etc. The fault status and environmental information of the key components of the UHV DC control and protection system are simulated, and information from time-domain backscatter phase modulation at the bottom of the Φ-OTDR system is obtained and analyzed experimentally. The information entropy algorithms of segmentation, multiplication, and integration are introduced to determine the failure status of the key components of the control and protection system of the simulated UHV DC transmission project and environmental information. Furthermore, the location accuracy of weak faults is evaluated.Results and Discussions In the experimental system, different frequencies and weak displacement vibrations are simulated. First, disturbances are applied to collect 20 raw datapoints at 1.8 km of the optical fiber link. During the acquisition of the backscatter of the optical fiber link, the difference between the fault signal and background noise is not clear and the signal-to-noise ratio (SNR) is low (Fig. 3). Then, the information entropy algorithm is used to conduct a preliminary signal analysis. The segmented information entropy is proposed after identifying the shortcomings of the conventional information entropy algorithm. The amplitude of the fault signal is divided into k segments in the dynamic range, and the information entropy of the k-segment fault signal is calculated and the sum is used to represent the information entropy of the signal (Fig. 5). Finally, the information entropy location using segmentation and multiplication segmentation is proposed and our study shows that it is more suitable for the system. The improvement in the SNR using the signal processing of the information entropy location considering segmentation and multiplication segmentation is evaluated (Fig. 8). The optimal number of segments for the application of the system is determined (Fig. 9).Conclusions Herein, two algorithms for information entropy location using segmentation and multiplication segmentation are proposed and the results of processing multiple types of mixed signals are compared and analyzed. The information entropy algorithm obtained by multiplying integral segments can considerably improve the SNR of the system when two segments are considered. The location accuracy of weak fault can reach ±1.5 and ±2.0 m, respectively, for 100 and 40 sampling periods, respectively.
  • Sep. 16, 2021
  • Chinese Journal of Lasers
  • Vol.48 Issue, 20 2006002 (2021)
  • DOI:10.3788/CJL202148.2006002
Study on the Effect of New Inner Cladding Shape on Absorption Efficiency of Thulium-Doped Double-Clad Fiber
Wang Junjie, Wang Cheng, Yang Yanni, and Guo Yangyang
In order to improve the output power of thulium-doped double-clad fiber (DCF) lasers, starting from optimizing the absorption efficiency of DCF to the pump light, pump absorption characteristics of fibers with different core diameters and inner cladding shapes are numerically simulated. The simulation results show that the pump absorption efficiency tends to 100% when the reflection times increased for the new inner clad DCFs with different core diameters. For DCFs with different inner cladding shapes, the absorption efficiency of the new inner cladding DCF is the highest. In the experiment, regular hexagonal inner cladding thulium-doped DCF and new inner cladding thulium-doped DCF are selected as the working substances. The output power of the two thulium-doped DCF lasers is 13.2 W and 14.8 W, respectively. The slope efficiency of the two thulium-doped DCF lasers is 31% and 35%, respectively. The output power of the new inner cladding thulium-doped DCF laser is 12.1% higher than that of regular hexagon inner cladding thulium-doped DCF laser, which indicates that the new inner cladding DCF is more conducive to achieve efficient pump absorption.
  • Sep. 14, 2021
  • Laser & Optoelectronics Progress
  • Vol.58 Issue, 17 1706009 (2021)
  • DOI:10.3788/LOP202158.1706009
Indoor Visible Light Fingerprint Positioning Scheme Based on Convolution Neural Network
Xu Hao, Wang Xudong, and Wu Nan
This paper proposes a visible light fingerprint positioning scheme based on a convolutional neural network (CNN) to improve the performance of indoor visible light positioning systems. In the proposed scheme, optical intensity signals are employed as the features of the reference node LED, and receiver coordinates are employed as training labels to construct fingerprint database. In addition, a positioning model based on light intensity information is constructed, and a one-dimensional CNN learning model is adopted for training. CNN application solves the problems of low-positioning accuracy and poor stability of the fully-connected feedforward neural network method. In an indoor-positioning scene (size: 5 m×5 m×3 m), the proposed positioning scheme obtained high positioning accuracy with an average positioning error of 4.44 cm. In addition, the performance of several different indoor visible light positioning methods was compared and analyzed in simulation experiments, and the results verified the technical advantages of the proposed scheme.
  • Sep. 14, 2021
  • Laser & Optoelectronics Progress
  • Vol.58 Issue, 17 1706008 (2021)
  • DOI:10.3788/LOP202158.1706008
Optical Fiber Torsion Sensor Based on Pre-Twisted Polarization-Maintaining Fiber Sagnac Loop Mirror Structure
Song Zhuo, Li Yichun, Wei Jinpeng, and Hu Junhui
An optical fiber torsion sensor based on a pre-twisted polarization-maintaining fiber (PMF) Sagnac loop mirror structure is proposed and experimentally demonstrated for simultaneously measuring rotational angle and direction. The sensing part of the structure is composed of a 2-cm pre-twisted Panda-type PMF spliced between two segments of 2-mm length multimode fiber (MMF). Torsion direction discrimination and torsion angle measurement can be realized by measuring the wavelength and power of a resonant dip in transmission spectra. The experimental results show that the sensor has a high sensitivity, and the resonant dip shifts with changes in torsion angle and direction. The wavelength of the resonant dip blue-shifts as the torsion angle increases, with a maximum wavelength sensitivity of -455 pm·rad-1·m and a power sensitivity of -1.35 dB·rad-1·m for counterclockwise torsion. For clockwise torsion, the wavelength of the resonant dip red-shifts as the torsion angle increases, with a maximum wavelength sensitivity of 182 pm·rad-1·m and a power sensitivity of 2.20 dB·rad-1·m. The sensor has the advantages of simple structure, easy fabrication, low cost, and potential applications in the field of torsion or rotation measurement.
  • Sep. 14, 2021
  • Laser & Optoelectronics Progress
  • Vol.58 Issue, 17 1706007 (2021)
  • DOI:10.3788/LOP202158.1706007
Establishment and Simulation of Underwater Photon Spatio-Temporal Random Channel Model
Dai Weihui, Yan Qiurong, Wang Ming, and Yang Cheng
According to the complexity of photon motion under water caused by absorption and scattering attenuation, this paper establishes a photon spatio-temporal random channel model based on the tracked underwater single-photon motion state. Considering different types of water qualities, link distances, receiving apertures, launching angles, and field of view angles, the relevant information of the photons arrived at the receiving end was counted and the factors influencing optical receiving intensity and channel impulse response based on underwater single-photon communication system were studied. At the same time, comprehensively considering the photon emission, underwater photon movement process, detector characteristics, and synchronization signal extraction method, etc., a data demodulation scheme based on photon counting in the time slot was adopted and the system performance was analyzed. The simulation results show that the launching angle and receiving aperture are the main factors that affect the delay broadening. The larger the receiving aperture, the smaller the system bit error rate (BER); the larger the link distance, launching angle, and noise factor, the larger the system BER. The theoretical communication distance is about 185 m. The results well describe the characteristics of underwater photon scattering and pulse delay broadening.
  • Sep. 14, 2021
  • Laser & Optoelectronics Progress
  • Vol.58 Issue, 17 1706006 (2021)
  • DOI:10.3788/LOP202158.1706006
Performance Analysis of Hybrid Free Space Optical/Radio Frequency Communication System
Wu Yan, Mei Haiping, Dai Congming, Zhao Fengmei, and Wei Heli
Under the conditions of exponentiated Weibull turbulent channel and Nakagami-m fading channel, the performance of hybrid free space optical/radio frequency (FSO/RF) communication system based on the selective combination technology is studied in this paper. Considering the pointing errors, this paper deduces the average bit error rate and outage probability of the selective combination hybrid FSO/RF communication system which adopts subcarrier modulation and intensity modulation/direct detection scheme, and obtains their new expressions by Meijer-G function and extended generalized bivariate Meijer-G function. The bit error rate and outage probability performance of hybrid FSO/RF communication system and FSO system are investigated under different subcarrier modulation schemes, turbulence intensity, pointing errors and RF channel fading parameter m. The simulation results show that the performance of the hybrid FSO/RF communication system using coherent binary phase shift keying subcarrier modulation technology is significantly better than the other three modulation technologies, and the performance of hybrid FSO/RF communication system is better than that of FSO system.
  • Sep. 14, 2021
  • Laser & Optoelectronics Progress
  • Vol.58 Issue, 17 1706005 (2021)
  • DOI:10.3788/LOP202158.1706005
A Dynamic Programming Algorithm for Power Allocation Based on System Fairness and Blocking in Heterogeneous Networks
Fan Chao, Sun Changyin, and Jiang Fan
To solve the contradiction between resource utilization and resource waste caused by congestion in the dual-connection offloading process, a dynamic programming algorithm with congestion prediction is proposed. Considering the high intermittent situation of millimeter wave links and fairness between heterogeneous network links under the constraint of total power, taking system weighting and rate maximization as the optimization goal, the non-convex problem is modeled as a finite-horizon discrete-time domain Markov decision process. The proposed algorithm is used to solve the power allocation problem in parallel connection of microwave and millimeter wave. The simulation results show that the algorithm can significantly improve the system performance by learning the power allocation strategy.
  • Sep. 14, 2021
  • Laser & Optoelectronics Progress
  • Vol.58 Issue, 17 1706003 (2021)
  • DOI:10.3788/LOP202158.1706003