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.
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.
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.