Polymer Optical Fiber Sensors in Healthcare Applications: A Comprehensive Review
However, ultraweak backscatter signals within optical fibers constitute an inevitable problem for DOFS, thereby increasing the burden on the entire system in terms of limited spatial resolution, low measurement speed, high system complexity, or high cost. We propose a novel resonance frequency mapping for a real-time quasi-distributed fiber optic sensor based on identical weak fiber Bragg gratings FBG , which has stronger reflection signals and high sensitivity to multiple sensing parameters.
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The resonance configuration, which amplifies optical signals during multiple round-trip propagations, can simply and efficiently address the intrinsic problems in conventional single round-trip measurements for identical weak FBG sensors, such as crosstalk and optical power depletion. Moreover, it is technically feasible to perform individual measurements for a large number of quasi-distributed identical weak FBGs with relatively high signal-to-noise ratio SNR , low crosstalk, and low optical power depletion.
By mapping the resonance frequency spectrum, the dynamic response of each identical weak FBG is rapidly acquired in the order of kilohertz, and direct interrogation in real time is possible without time-consuming computation, such as fast Fourier transformation FFT. Over the last two decades, fiber optic sensors have emerged as one of the fastest growing and most researched areas among modern monitoring technologies. In particular, distributed fiber optic sensing techniques, based on Raman 1 , Brillouin 2 , 3 , and Rayleigh 4 scattering within the optical fibers, have been successfully adopted in a wide range of strain- or temperature-sensing applications owing to their advantages of a large number of sensing points and a long sensing range 5.
To overcome such issues, novel technical solutions, such as optical correlation-domain scanning 3 and optical frequency-domain scanning based on optical interference 4 , have been proposed. These techniques provide a higher spatial resolution in the sub-millimeter range ; however, they suffer from some drawbacks, such as increased cost, high system complexity, and reduced sensing range 6. On the other hand, fiber Bragg grating FBG sensors have stronger reflection signals within an optical fiber, which can be used to acquire multiple physical and chemical parameters from discretized local points of a few millimeters along a single optical fiber 7 , 8 , 9.
In general, conventional FBG sensors can be simultaneously multiplexed at high measurement speeds of the order of kilohertz 10 , However, the FBG should be designed to guarantee a measurable wavelength range of the interrogator and non-overlapping spectra between the Bragg wavelengths in the wavelength domain 8. The maximum available number of FBG sensors is limited to a few tens or less, which is a major drawback of FBG interrogation systems based on wavelength-division multiplexing WDM 10 , 11 , 12 , Another critical disadvantage of conventional FBG sensors is that the writing of periodic grating, to ensure high reflectivity at different Bragg wavelengths, entails high labor costs, which limits mass production and cost-effective manufacturing processes 14 , Recently, the limits of multiplexing capability and mass productivity have been overcome by identical-wavelength weak-reflectivity FBG array sensors 16 , 17 , 18 , 19 , 20 , 21 , 22 , This fabrication process involves on-line ultraviolet UV exposure during the optical fiber drawing process 24 , 25 , 26 , or femtosecond-laser writing on a single optical fiber 27 , 28 , Because identical FBGs can be immediately manufactured without applying conventional stripping and recoating processes to the optical fiber, the production time can be reduced, and the pristine mechanical strength of the optical fiber is maintained 14 , Weak FBGs have a shorter grating length and require lower UV exposure time compared to highly reflective FBGs, which is convenient for alignment and reduces the processing time 9.
In addition, with single manufacturing systems of an identical weak FBG array, the additional fiber-splicing process of individual FBGs can be avoided, which not only reduces the splicing loss significantly but also improves the sensitivity and capability of the interrogation system Currently, an identical weak FBG array is typically interrogated by two types of methods: time-division multiplexing TDM 16 , 17 , 18 , 19 and optical frequency-domain reflectometry OFDR 20 , 21 , The quantitative strain or temperature response of individual FBGs have to be measured by spectroscopic instruments, such as a tunable laser 16 , 17 , 18 or an optical spectrum analyzer OSA Thus, the measurement speed is limited to a few tens of hertz owing to the slow response time of the spectroscopic instruments and the individual scanning of all the FBGs.
To overcome the drawback of low measurement speed of TDM methods, the dispersive Fourier transformation technique 30 , 31 , 32 , 33 is adopted to rapidly obtain strain information at up to 20 kHz However, as with other TDM methods, it is difficult to widely apply this technique to various fields because it requires high-performance instruments, such as a high-speed oscilloscope, fast and highly sensitive photodetectors, and optical amplifiers.
In some studies, identical weak FBGs have been interrogated using the OFDR method, which is suitable for densely distributed measurement applications owing to its high spatial resolution of the order of millimeters and limited sensing length of the order of meters 20 , 21 , The maximum sensing length and measurement speed are determined by the coherence length and sweep rate of the tunable laser source 20 , 21 , Reflection distribution along an optical fiber requires high-performance data computing capabilities, which reduce the feasibility of real-time measurement.
In addition, such interrogation methods based on single round-trip measurement for the identical weak FBG array suffer from the intrinsic crosstalk problem owing to multiple reflections between each identical FBG Hence, the identical FBGs in the sensing array must have ultra-low reflectance less than 0. Moreover, when an optical signal propagates across an identical FBG array, the signal power decreases exponentially In this study, we demonstrate a real-time quasi-distributed fiber optic sensor system based on resonance frequency mapping for simultaneous multiplexing and strain measurement of an identical weak FBG array.
Resonance configurations based on a ring cavity can simply and efficiently address the intrinsic problems of identical weak FBG sensors, such as crosstalk and optical power depletion, thereby reducing the cost and complexity of conventional single round-trip measurement systems.
When a modulation frequency fed into the gain of the laser cavity is synchronized with the resonance frequency of the cavity formed by each individual FBG in the sensing array, a synchronized FBG can be selected and isolated from the other FBGs. Thus, identical weak FBGs can be separately interrogated with relatively high SNR, low crosstalk, and low optical power depletion.
In addition, this technique is based on an active mode-locked fiber laser cavity, enclosing a chirped FBG CFBG for chromatic dispersion. The highly dispersive CFBG induces a significant change in the total cavity length when a strain or temperature variation is applied to each FBG. Therefore, strain or temperature variation of the FBG can be measured by detecting the shift in the resonance frequency resulting from the change in the Bragg wavelength. The proposed quasi-distributed fiber optic sensor based on resonance frequency mapping is not only more compact and cost-effective but also delivers better performance than other identical weak FBG interrogation systems.
The improved sensing performance of the identical weak FBG array was demonstrated experimentally. The principle of identical weak FBG interrogation based on resonance frequency mapping is shown in Fig. The configuration of the resonance frequency mapping system consists of an active mode-locked fiber laser cavity with multiple laser cavities, employing an optical gain where the driving current is externally modulated at a modulation frequency f m between a sensing head and a CFBG. Each identical weak FBG on the sensing head has a different resonance frequency induced by the total cavity length, which is determined by its position along the sensing head.
Principle of identical weak FBG interrogation based on resonance frequency mapping. Each identical weak FBG has a different resonance frequency induced by a total cavity length determined by its position along the sensing head. The modulation frequency f m , repetition rate of the pulse signal was continuously swept through the programmed frequency range.
During the sweeping of the modulation frequency from the lowest to the highest values, strong output lasing powers peak signals were detected at each specific resonance frequency f R,i. This frequency was a positive-integer multiple of the order N of the resonance of the free spectral range FSR , defining the reciprocal of the round-trip time of the laser cavity configured by each FBG. The experimental setup of a real-time quasi-distributed fiber optic sensor system based on resonance frequency mapping for multiplexing and measuring an identical weak FBG array is shown in Fig.
The main component of the interrogation system is an all-fiber ring cavity laser consisting of two different semiconductor optical amplifiers SOA , an optical isolator ISO , two optical circulators CIR , 31 identical weak FBGs for the sensing head, and a optical coupler OC for the laser output coupling. All the optical fiber components have low polarization sensitivity less than 1. One of the SOAs amplifies the optical signals in the laser cavity, while the other modulates the optical intensity by a driving signal applied electrically.
The in-line SOA was inserted to increase the low net gain of the cavity owing to the high loss and low reflectivity from the sensing head. A m delayed optical fiber L d was inserted into the main cavity to prevent signal overlap between the first- and second-order resonance signals. Schematic of real-time identical weak FBG interrogation based on resonance frequency mapping. The resonance frequency spectrum was repeatedly obtained as the frequency of the gain modulation was linearly swept in time. The sensing head consists of an array of 31 identical weak FBGs spaced at approximately 1-m intervals along the optical fiber.
The main peaks of the 31 identical weak FBGs are clearly obtained at each resonance frequency, indicating that the multi-reflection crosstalk effect is significantly suppressed even though FBGs with much higher reflectivities are employed compared to other identical weak FBG interrogations 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , The measured average power was not constantly maintained along the modulation frequency, because the repetition rate of the output pulse varied during the average power measurements using a low-speed PD.
Therefore, we defined the power density as the calculated intensity of a single pulse, obtained by dividing the measured output power by its modulation frequency. As the modulation frequency fed into the switch SOA was linearly swept from 1. From Eq. This value corresponds to the sum of the main cavity length Similarly, the peak signal at the resonance frequency of 1. The modulation frequency x-axis in Fig. As shown in Fig. As the modulation frequency is inversely proportional to the total cavity length according to Eq.
For identifying the resonance frequency from the Gaussian fitting of the peak signal, the standard deviation increases with decreasing modulation frequency owing to the relatively low data density. The initial resonance frequency, f R,1 , without strain on FBG 1 was measured at 1. This result is in good agreement with the theoretical sensitivity, calculated as 8. The dynamic range D. The dynamic range of the resonance frequency mapping system is proportional to the distance between adjacent FBGs L inter , and it is inversely proportional to the total dispersion. The strain response was obtained by converting the measured resonance frequency using the measured sensitivity see Fig.
The inset of Fig. Four identical weak FBGs were attached to the centers of the third, fourth, fifth, and sixth strings of a guitar. The four strings strings 6—3 were vibrated by manually plucking them in sequence at approximately 2-s intervals. The measured temporal strain variations of the sensing FBGs are shown in Fig.
These plots illustrate the vibration characteristics frequency and amplitude of each guitar string. In the frequency analysis, the vibration frequencies including fundamental frequency first order and harmonic frequencies of guitar strings were successfully measured by the FBGs placed on each string. The results closely matched the ideal fundamental frequencies of For comparison, the sounds of the vibrated guitar strings were recorded by the microphone of a mobile phone, and similar frequency spectra were obtained see Supplementary Fig.
Real-time vibration detection of guitar strings: a schematic of vibration detection of guitar strings; b photograph showing the positions of the attached FBGs green brackets ; c dynamic strain responses of the attached FBGs after oscillating the open guitar strings. We proposed a real-time quasi-distributed fiber optic sensor with identical weak FBGs, based on resonance frequency mapping.
The proposed system can successfully and efficiently interrogate up to 31 identical weak FBGs by detecting the resonance frequencies of the FBGs.
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By mapping the resonance frequency spectrum, the dynamic response of each identical weak FBG is rapidly acquired in the order of kilohertz, and it is directly interrogated in real time without time-consuming computation, such as fast Fourier transformation FFT.
The resonance frequency mapping approach offers several advantages. The multiplexing capacity in the current study is limited to several tens of identical weak FBGs, because we employed an identical weak FBG array with relatively high reflectivity in order to suppress signal depletion and crosstalk from multiple reflections among identical FBGs. The multiplexing capacity can be increased considerably to over by decreasing the reflectivity of each FBG below 0.efyhepegut.tk
Real-time quasi-distributed fiber optic sensor based on resonance frequency mapping
Moreover, when additional identical weak FBG arrays with slightly different center Bragg wavelengths are connected in series by splicing them to the ends of the original identical weak FBG array , the multiplexing capacity can be significantly enhanced to tens of hundreds of sensing FBGs by using a single multiplexing unit see Supplementary Fig. The sweeping range of the modulation frequency can be expanded to detect the extended resonance frequencies of additional FBGs in extremely long cavities without serious technical difficulties.
Moreover, to prevent the degradation of the standard deviation resulting from the decreased data density at longer total cavity lengths see Fig. This modification can acquire data at equal intervals on the length axis and solve the problem of uneven data density.
In conclusion, our novel resonance frequency mapping system can simultaneously interrogate an identical weak FBG array. This technique can potentially enable us to sense various physical parameters on the basis of multiple identical FBG sensors, which is not possible with currently available distributed sensing techniques. From a mass-production perspective, this method offers two distinct advantages, i.
Therefore, in the future, we expect identical weak FBG sensors based on resonance frequency mapping to become more competitive and to be widely adopted in various fields. To measure the static strain response, the acrylate coating was stripped from the optical fiber of FBG 1 , and each end of FBG 1 was attached to a separate micro-linear stage MX-M, Newport using an instant adhesive Loctite Static strain was applied to FBG 1 by moving the micro-linear stage from 0 to 2.
As the strain on FBG 1 incrementally increased, the power density spectra were obtained by a personal computer. The resonance frequencies were identified by Gaussian-peak fitting. Each FBG was fixed at the center of a different guitar string strings six, five, four, and three. Tanner, M. High-resolution single-mode fiber-optic distributed Raman sensor for absolute temperature measurement using superconducting nanowire single-photon detectors.
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Zhou, D. Light Sci. Each of the aforementioned POFs can be characterized using this method, which show the differences in the responses of each polymer material as well as in their different microstructures [ , , ]. Typically, the polymers present responses to oscillatory loads in DMA similar to the ones depicted in Figure 5 for temperature, frequency, and creep responses obtained in PMMA analysis [ ] and these responses with respect to each parameter can be used to compensate or eliminate some unwanted behaviors of the POF sensors such as hysteresis, creep responses, and temperature cross-sensitivity.
Typical response curves from POFs. The knowledge of the POF material features enables the development and performance enhancement of the presented different POF sensors.
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