WU Ni-shan，XIA Li

（School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China）

Abstract:Quasi-distributed fiber sensing systems play an important role in the fields of civil engineering,energy surveying,aerospace,national defense,chemicals,etc.Interrogation technology for quasi-distributed fiber sensing systems based on microwave photonics is widely used in high-speed and high-precision signal demodulation and sensor positioning in optical fiber multiplexing systems.Compared to conventional optical wavelength interrogation,this technology greatly improves system demodulation rate and compensates for the defects of traditional sensor positioning methods.This paper introduces the recent research progress of quasi-distributed fiber sensing interrogation technology based on microwave photonics;compares and analyzes the advantages and disadvantages of several existing microwave demodulation systems from the perspective of their fiber grating quasi-distributed sensing and fiber Fabry-Perot quasi-distributed sensing systems,respectively;and provides a summary of the prospective direction of future research in quasi-distributed fiber sensing interrogation technology based on microwave photonics.

Key words:radiofrequency photonics;quasi-distributed sensing;fiber sensing interrogation;fiber bragg gratings;fiber Fabry-Pérot

1 Introduction

With the advent of high-speed information era,the Internet of Things has gradually become an important scientific and technological basis for human beings to obtain external information,predict environmental changes,and improve the quality of production and life.As an indispensable key component of detection system,Optical Fiber Sensor(OFS)has been widely applied in civil engineering,biology,chemical industry,mechanics,electrics,aerospace and other fields owing to its advantages such as small size,light weight,high accuracy,corrosion resistance,resistance to electromagnetic interference,low cost,and good compatibility with the existing optical fiber communication system[1-2].

By networking,arranging and multiplexing discrete OFS units,the physical quantities such as temperature,strain and vibration can be sensed and measured in a long distance.Different from the fully distributed sensing system based on Rayleigh,Raman or Brillouin scattering effect,this quasi-distributed fiber sensing system does not need to stimulate the nonlinear effect in optical fiber,and can easily establish the quantitative relationship between the sensing optical signal and the parameter to be measured.It can flexibly design the system performance parameters such as covered distance and measurement accuracy in accordance with actual requirements,to help control and reduce the system construction cost.Meanwhile,it can detect more diverse changes in refractive index,curvature and other external factors,and can measure dynamic events.Therefore,quasi-distributed optical fiber sensing system has played an important role in oil well exploration,early fire warning,and monitoring the structural health of large buildings[3-6].

Depending on the type of multiplexed fiber sensor,quasi-distributed fiber sensing systems can be roughly divided into Fiber Bragg Grating(FBG)quasi-distributed sensing system and fiber Fabry-Pérot(FP)quasi-distributed sensing system.The FBG quasi-distributed sensing system takes the wavelength modulation sensor FBG as the sensing unit.By using the phase mask,UV irradiation and other methods,the refractive index of fiber core is periodically changed to reflect the light satisfying the Bragg wavelength.When the environmental change causes the thermo-optic or elastic-optic effect in the fiber,the central wavelength of the grating will change so that the parameter to be measured can be sensed[7-10].On the other hand,the fiber FP quasi-distributed sensing system takes the fiber phase modulation sensor FP structure as the sensing unit.At first,a reflector is formed in the fiber structure through welding,corrosion,film coating,laser micro-processing and other methods.Then,the phenomenon that the two-beam interference spectrum between reflectors is modulated by external physical quantities is utilized to detect the target parameter[11-12].

While the quasi-distributed fiber sensing system is widely used in many fields,the increasing application demand is raising higher requirements for the construction and maintenance cost,multiplexing capacity,coverage,measurement rate and localization ability of this system and urging researchers to ceaselessly explore new sensing demodulation techniques.In recent years,with the rapid development of microwave photonics,this new interdiscipline with the advantages of photonic and microwave techniques(such as large bandwidth,low loss,flexibility and reconfigurability[13-15])has been extensively researched in Radio over Fiber(ROF),space exploration,radar system and other fields[16-18],while showing attractive application potential in the field of fiber sensing demodulation.By modulating microwave signals to optical signals through microwave photonics technique and adopting the means of microwave measurement,the demodulation rate of the system can be improved,high measurement accuracy and strong localization ability can be secured,and the construction and maintenance cost of the system can be reduced to a certain extent.

Focusing on the application of microwave photonics in quasi-distributed fiber sensing demodulation,this paper presents the basic principle,experimental implementation and demodulation performance of microwave demodulation systems(including FBG quasi-distributed sensing system and fiber FP quasi-distributed sensing system),and finally analyzes and discusses the problems in the existing scheme and the future research direction.

2 Microwave photonic demodulation technique of FBG quasi-distributed sensing system

The basic principle of demodulating a FBG quasi-distributed sensing system with microwave photon technique is to convert the wavelength change of the sensing grating into the intensity or frequency change in the microwave domain through optical carrier modulation,to improve the demodulation precision and rate of the system by means of microwave detection,and to locate the sensing unit through time-frequency transformation.Based on the basic principle of demodulation system,the microwave demodulation solutions adopted in FBG quasi-distributed system can be broadly divided into Microwave Photonic Filter(MPF),microwave photon heterodyne,and optoelectronic oscillator(OEO).

2.1 Microwave demodulation of FBG quasi-distributed system based on MPF

The basic structure of the FBG quasi-distributed sensing demodulation system based on MPF is shown in Fig.1.The optical signal output by light source is firstly modulated with the input microwave signal through an Electronic Optic Modulator(EOM),and then is transmitted as a carrier in the optical fiber sensing system,and finally arrives at the photodetector(PD)after passing through the sensing area.At last,the output microwave signal carrying the information of parameter variation is obtained.

Fig.1 Diagram of FBG quasi-distributed sensing demodulation system based on MPF圖1 基于MPF 的光纖光柵準(zhǔn)分布式傳感解調(diào)系統(tǒng)示意圖

If the frequency of the input microwave signal isω,the total time for the optical signal traveling from the modulator through thei-th grating in the sensing area(where the signal is reflected)to the detector isti,the central wavelength of thei-th grating isλi,the power of the light reflected by thei-th grating isPi,and the total number of gratings in the sensing area isN,then the frequency response of the entire quasi-distributed grating sensing network can be expressed as:

Through the Inversed Fast Fourier Transform(IFFT)of the frequency response,the time-domain response of the system can be obtained:

As can be seen from the above equation,the signals reflected by different gratings present discrete impulse peaks in the time domain,and the intensity of impulse peaks contains the information on the power of the light reflected by gratings.Therefore,by detecting the microwave signal output from the sensing system,the grating sensing system can be demodulated and located.

As the input microwave signal modulated on optical carrier and the output signal obtained from photoelectric detector are required to be synchronously scanned according to the above principle in the actual demodulation process,the Vector Network Analyzer(VNA)commonly used in microwave system can fully meet the requirements of synchronous scanning.The MPF microwave demodulation system with VNA as the core is shown in Fig.2.This concept is simple in structure and easy to build,so it is widely used for the demodulation of quasi-distributed fiber sensing system[19-23].

Fig.2 Diagram of MPF FBG quasi-distributed sensing demodulation system based on VNA圖2 基于矢量網(wǎng)絡(luò)分析儀的MPF 光纖光柵準(zhǔn)分布式傳感解調(diào)系統(tǒng)示意圖

In 2013,Ricchiutiet al.from the Polytechnic University of Valencia in Spain used VNA to demodulate 500 cascaded long-period FBG quasi-distributed systems[20]for the first time on the basis of the previous demodulation experiment of a single long-period FBG sensing system[19].However,since long-period grating was a transmission-type OFS,a reflection facet and a reference tap should be added to the cascade system to improve the light reflectivity in the sensing area.As a result,the complexity and difficulty of the system were increased.

To adapt microwave demodulation system to the requirements of actual FBG sensing network better,HUST Professor Xia Li’s research group proposed a new concept(shown in Fig.3)applicable to large-scale long-distance sensing in 2015[21].In this system,identical weak inverse gratings with the same central wavelength were used as the sensing unit to improve the multiplexing capability of the system.An Optical BandPass Filter(OBPF)was used for matched filtering to convert the variation in the central wavelength of gratings into the variation in reflected light intensity so as to demodulate the specific information of each grating.When an external stress is applied to a grating,the amplitude of IFFT peak corresponding to the grating will change.The demodulation result obtained by this concept shows good linearity and good compatibility with the existing FBG sensing network.

Fig.3 Weak-reflection FBG quasi-distributed sensing demodulation system based on MPF[21]圖3 基于MPF 的弱反光纖光柵準(zhǔn)分布式傳感解調(diào)系統(tǒng)[21]

As the MPF-based quasi-distributed grating microwave demodulation system is based on intensity demodulation,the fluctuation in the output power of light source or the losses(such as bending and winding)in the fiber sensing link will have a negative impact on the accuracy of demodulation results.To solve this problem,Chenget al.proposed a demodulation concept for quasi-distributed ultrashort FBG that combined differential filtering with microwave network[24].As shown in Fig.4,a pair of Gaussian filters with the central-wavelength difference of only 0.4 nm are set up on the upper and lower circuits of Mach-Zehnder interferometer,and the optical path difference between the two arms is used to convert the optical signal reflected by a grating into a pair of adjacent impulse signals.By calculating the intensity ratio of the two adjacent impulse peaks,the wavelength drift of the grating can be obtained.The demodulation result of this concept is immune to not only the power fluctuations of the light source but also the bending loss in the transmission fiber.

Fig.4 Basic structure and principle of ultra-short-FBG differential demodulation system based on multi-tap MPF[24]圖4 基于多抽頭MPF 的超短光柵差分解調(diào)系統(tǒng)及其基本原理示意圖[24]

Although the above-mentioned microwave demodulation concepts have strong localization ability,high measurement accuracy and other advantages,they are only applicable to identical FBG network rather than a Wavelength Division Multiplexing(WDM)system composed of the gratings with different central wavelengths.To solve this problem,the researchers firstly proposed a solution of wavelength-RF delay mapping,introducing different time delays to the reflected light with different wavelengths through fiber dispersion to finally achieve the goal of distinguishing the sensing information of different gratings[25-26].However,the system raised the requirements for the length and type of the fiber to which the time delay was introduced and for the sweep range and sampling number of VNA.Moreover,the system was susceptible to the cross sensitivity of sensors.

To minimize the error of demodulation result caused by the cross sensitivity of sensors and by the optical power fluctuation and transmission loss in optical fiber link,Wuet al.proposed the use of dual Sagnac loop to realize the simultaneous differential demodulation of WDM grating multiplexing system in 2019[27].As shown in Fig.5,a Sagnac filter based on“single-mode-polarization-maintaining-singlemode”is set up on either arm of the Mach-Zehnder interferometer,and the spectra of the two filters are misaligned by 0.48 nm.Since Sagnac loop is a wide-spectrum filter with Gaussian-like transmission spectrum in each channel,the simultaneous independent linear demodulation of multiple different gratings can be achieved through the one-to-one correspondence between the central wavelengths of different FBGs and the different channels of Sagnac transmission spectra.

Fig.5 WDM quasi-distributed sensing microwave demodulation system based on double Sagnac loops and differential filtering.(a)System structure diagram;(b)Spectra of double Sagnac loops;(c)Time-domain response spectrum obtained from frequency response IFFT[27]圖5 基于雙Sagnac 環(huán)和差分濾波的WDM 準(zhǔn)分布式傳感微波解調(diào)系統(tǒng)。(a)系統(tǒng)結(jié)構(gòu)示意圖；(b)雙Sagnac 環(huán)光譜圖；(c)由頻域響應(yīng)逆傅里葉變換得到的時(shí)域響應(yīng)譜[27]。

2.2 Microwave demodulation of FBG quasi-distributed system based on microwave photonic heterodyne

The FBG quasi-distributed microwave demodulation system based on microwave photon heterodyne also adopts the method of external modulation.That is to say,the microwave signal at high sweep frequency is loaded into the incoherent broadband light source.By detecting the heterodyne beat signal between grating reflection signal and original optical signal,the change of external environment applied to the grating is demodulated[28].In this process,the frequency change of beat signal is proportional to the change of grating wavelength and the total intra-fiber dispersion.

Based on the above principle,the researchers proposed diverse FBG microwave demodulation concepts,and obtained excellent demodulation accuracy and response rate[29-31].In 2018,Li Zhengying,a Professor in Wuhan University of Technology,and his research group combined microwave photonics with dispersion to build a quasi-distributed FBG microwave demodulation system based on microwave photon heterodyne(see the Fig.6),and realized the high-speed demodulation of weakreflection quasi-distributed FBG system[32].In this system,a 14.7 km Dispersion Compensation Fiber(DCF)was used to provide high dispersion for wavelength-frequency conversion.Eventually,high demodulation accuracy(8 pm)was achieved,and the demodulation result was highly consistent with the direct measurement result given by spectrometer.In addition,the system demonstrated an excellent dynamic sensing capability,achieving a demodulation rate of 40 kHz in high-speed vibration experiment.

Fig.6 Microwave photon heterodyne quasi-distributed grating demodulation system based on DCF[32]圖6 基于DCF 的微波光子外差準(zhǔn)分布式光柵解調(diào)系統(tǒng)[32]

Although the above system has the advantages of high demodulation rate and high measurement accuracy,the thermo-optic effect and high-temperature ductility of dispersion compensation fiber at the varying environment temperature will seriously affect the beat frequency of the two optical signals,introduce an error to the demodulation result of grating wavelength and reduce the credibility of demodulation result during large-scale multiplexing.To solve this problem,Lianget al.in the research group proposed an improved concept based on DCF-SMF(Dispersion Compensation Fiber-Single Mode Fiber)dual channel in 2019[33].In the new concept,another 15.2 km SMF channel was added to the aforementioned system to eliminate the measurement error caused by environmental temperature drift and correct the demodulation wavelength value.While inheriting the advantages of singlechannel system such as high speed and low cost,the dual-channel system improves the accuracy and linearity of wavelength measurement results and eliminates the negative effect of temperature cross-sensitivity.

In addition to setting the reference link and utilizing the dispersion characteristics of DCF to realize the demodulation based on microwave photon heterodyne,the reflection technique of incoherent optical frequency domain based on chaotic source can also be used to realize the beat demodulation of quasi-distributed grating system[34-35].As shown in Fig.7,the incoherent light of the system is produced by DFB(distributed-feedback)laser,which generates chaotic oscillations under the action of optical feedback.This chaotic light source has the advantages of large line width,short coherence length and low interference noise,which can help improve the overall multiplexing capacity of the sensing demodulation network.This system experiment realized the localization and wavelength demodulation of 3 640 weak-reflection gratings,demonstrating an excellent demodulation capability of the IOFDR(Incoherent Optical Fourier-Domain Reflectometry)technique based on chaotic source in large-scale intensive multiplexing scenes.However,when demodulating the grating wavelength,the DFB laser needs to be thermo-tuned to change the wavelength of its laser beam.As a result,the demodulation speed of the system is significantly reduced and its application potential in dynamic parameter sensing is limited.

Fig.7 Schematic diagram of ultra-weak FBG quasi-distributed sensing demodulation system based on chaos source IOFDR[35]圖7 基于混沌源IOFDR 的超弱光纖光柵準(zhǔn)分布式傳感解調(diào)系統(tǒng)示意圖[35]

2.3 Microwave demodulation of FBG quasi-distributed system based on OEO

The FBG quasi-distributed microwave demodulation system based on optoelectronic oscillator(OEO)is built on the photoelectric-hybrid feedback loop structure shown in Fig.8.The modulated input optical signal is optically amplified and transmitted into the optical delay line and then converted by detector into an electrical signal.Most of the electrically amplified signals are input as feedback microwave signals to the modulator port,while the rest are output as the microwave signals for spectrum measurement.The OEO system will oscillate only when the closed-loop gain is large enough.As the oscillation frequency of OEO system mainly depends on the loop length,the key parameters such as the length of fiber ring cavity can be determined by measuring the output microwave spectrum[36-38].

Fig.8 Basic structure of OEO system圖8 OEO 系統(tǒng)基本結(jié)構(gòu)示意圖

In 2020,Wanget al.from Nanjing University proposed a quasi-distributed FBG microwave sensing demodulation system based on the principle of OEO system[39].In this concept,the cascaded grating array is connected to the OEO oscillation loop through a circulator.By using the reflection characteristics of a grating,each grating in the multiplexing system is corresponding to a different loop length.By measuring and analyzing the oscillation peaks with different Free Spectral Ranges(FSRs),the information on the wavelength and spatial position of each grating can be demodulated.As a digital discrete location coding system,this system is strongly immune to interference.However,the theoretical multiplexing capacity of this concept is only 62,which still needs to be further improved in order to meet practical application requirements.

The above three different FBG quasi-distributed microwave demodulation concepts are compared in Table 1.It can be seen that the microwave demodulation concept based on MPF is more advantageous in high-precision spatial positioning.In addition,through the optimization methods such as wavelength-frequency delay mapping and differential filtering demodulation,the influence of power fluctuation on the accuracy of demodulation results can be eliminated and the demodulation requirements of WDM system can be met.However,due to the limitation of VNA scanning rate,the demodulation speed in the first concept still needs to be improved.In contrast,the demodulation concept based on microwave photon heterodyne has a much faster demodulation rate that can meet the requirements of high-speed demodulation and dynamic sensing.Its dual-channel balanced detection structure can also solve the problem of cross-sensitivity very well.However,its spatial resolution is sacrificed to some extent.The OEO-based demodulation concept has an excellent signal-to-noise ratio and stability,but its system capacity is relatively limited.Therefore,it is not suitable for the demodulation of a large-scale grating multiplexing system for the time being.

Tab.1 Comparison of different microwave demodulation concepts for FBG quasi-distributed system表 1 針對光柵光纖準(zhǔn)分布式系統(tǒng)的不同微波解調(diào)方案比較

3 Microwave photonic demodulation technique of fiber FP quasi-distributed sensing system

The fiber interference structure represented by FP is another common sensing unit in the quasi-distributed fiber sensing network.Similar to the principle of FBG microwave demodulation,the optical carrier based microwave signal can also be used for auxiliary frequency domain measurement in the quasi-distributed sensing system based on fiber FP interferometer.Depending on the specific demodulation method,the microwave demodulation concepts of fiber FP quasi-distributed systems can be divided into Optical Carrier based Microwave Interferometry(OCMI)and Coherent Microwave-Photonic Interferometry(CMPI).

3.1 Microwave demodulation of fiber FP quasidistributed system based on OCMI

OCMI microwave demodulation technique is mainly based on low-coherence light source.When the light source in the system is incoherent,its coherence length will be short and its bandwidth will be far less than the optical path difference between any two reflection points.At this time,the cross product term of the responses in the complex frequency domain of the system is zero.The selfproduct term can be used to reconstruct the interference spectrum in the microwave domain,where the microwave phase shift can be used to calculate the change of optical path difference[40,44].

The above fiber quasi-distributed demodulation method using low coherence light and microwave photonics has been widely concerned by researchers since it was proposed[40-43].In 2013,the fiber FP quasi-distributed sensing demodulation concept based on OCMI was first experimentally implemented by the Xiao Research Group of Clemson University in the United States[40].As shown in Fig.9,the all-fiber FP cavity formed by single mode fiber and capillary tube through fusion is used as the cascaded sensing unit.By adjusting the central wavelength of tunable filter,the intensity of IFFT impulse signal in time domain under different wavelengths can be determined,and then the spectral information of each fiber FP can be recovered.The experimental results show that the spectrum obtained by microwave reconstruction is consistent with the direct measurement result on spectrometer.Therefore,the system has not only high accuracy but also the ability of sensor localization.

Fig.9 Fiber FP quasi-distributed sensing demodulation system based on OCMI technique[38]圖9 基于OCMI 技術(shù)的光纖FP 準(zhǔn)分布式傳感解調(diào)系統(tǒng)[38]

The OCMI technology can demodulate the fiber FP sensing system with spatial continuity,in addition to the quasi-distributed sensing link composed of cascaded discrete fiber FP sensors.By means of hot fusion[39]or femtosecond laser inscription[41-42],a series of reflection points can be formed in the sensing fiber.Two adjacent weak-reflection points can be regarded as a pair of fiber FP structures.This FP quasi-distributed system has no dark measurement zone and applies to the measurement and monitoring of mechanical-structure surface deformation and other scenes.In addition,the OCMI technique can be applied to other types of waveguides or free-space interferometer systems,as well as the quasi-distributed measurement in a variety of physical,chemical and biological fields.

3.2 Microwave demodulation of fiber FP quasidistributed system based on CMPI

Different from OCMI technique,CMPI microwave demodulation technique is based on coherent light source.For a fiber withNweak-reflection points,when the light source is coherent light,the intensity of reflected light at the corresponding reflection points will be closely related to the change of optical path difference between reflection points due to the interference between adjacent reflection points.Therefore,the CMPI system can effectively convert the change of optical path difference between reflection points into the change of the intensity of time-domain pulses corresponding to specific reflection points.In 2017,Huaet al.built the CMPI microwave sensing demodulation system as shown in Fig.10,and verified the conclusions of their theoretical analysis through experiments[44].The experimental results showed that the system achieved a strain resolution of 0.6μεfor the fiber FPI with a cavity length of 1.5 cm.When the cavity length of FPI was increased to more than 1 m,the strain resolution could be further improved to the nε level.

Fig.10 Fiber FP quasi-distributed sensing demodulation system based on CMPI technique[44]圖10 基于CMPI 技術(shù)的光纖FP 準(zhǔn)分布式傳感解調(diào)系統(tǒng)示意圖[44]

Due to the sensitivity of CMPI technique to the optical path difference of reflection points,this demodulation method also shows excellent performance in the measurement of dynamic parameters.For example,in the multi-point vibration sensing experiment,the researchers realized the vibration demodulation and vibration point location at the frequency up to 2.3 kHz with the help of a CMPI microwave demodulation system where the cascaded fiber FP was used as vibration sensing unit[45].The experimental results show that the CMPI-based fiber FP quasi-distributed sensing system is expected to be applied in the measurement of more complex physical quantities such as pressure wave.

4 Summary and outlook

Compared with the traditional fiber demodulation technique based on wavelength demodulation,the quasi-distributed fiber sensing demodulation technique based on microwave photonics has a high demodulation rate,high measurement accuracy,strong localization ability and other merits,which can help the system reduce the construction cost and improve the overall performance.With a long-distance high-precision sensing ability,this technique has a remarkable application potential in civil engineering,mechanics,aerospace and other fields.According to the type of sensing unit and the principle of demodulation,this paper reviews the research progress of domestic and overseas quasi-distributed microwave demodulation techniques used for FBG quasi-distributed sensing system and fiber FP quasidistributed sensing system,and compares the demodulation concepts such as microwave photonic filter,microwave photonic heterodyne and optoelectronic oscillator in terms of spatial positioning,demodulation rate,multiplexing capacity,system complexity and stability.

Although the microwave demodulation technique for quasi-distributed fiber sensing system has made great achievements in theoretical and experimental research,there are still some problems to be solved and broken through in the engineering approach of this technique.Firstly,limited by the response characteristics of sensor,the existing quasidistributed fiber-sensing microwave demodulation systems are mostly applied to the measurement of a single physical quantity(such as temperature or strain),and can’t cope with more and more diversified practical application scenarios.One of the future research directions of microwave demodulation technique is to realize the simultaneous accurate demodulation of different parameters in microwave system and to solve the problem of cross sensitivity among the parameters to be measured.To expand the types of sensed physical quantities,traditional FBG and fiber FP structures can be combined with special optical fiber[26],surface plasma effect[46]and other new technologies to improve the sensitivity of a multiplexing sensor to refractive index,air pressure,electric current,magnetic field and other parameters and to realize its application in special scenarios.To eliminate the cross-sensitivity between the parameters to be measured,the accurate and stable measurement of physical quantities can be achieved by optimizing the sensor package,adopting the double-channel differential demodulation[24,27],using Mach-Zehnder interferometer and other interference structures for heterodyne detection[32-33],and setting up a control link[33].

Secondly,due to the limitation of demodulation concept and device indicators(such as low sensitivity of sensor structure to high frequency parameters,low VNA scanning rate of core devices,contradiction between VNA frequency scanning accuracy and scanning range,and long wavelength tuning time for DFB laser source),most of the currently reported microwave demodulation techniques can achieve high accuracy in sensing static parameters or low frequency parameters but low accuracy in the measurement of high frequency parameters.To solve this problem,the OFS structures[47]for sensing dynamic parameters(such as sound wave)should be optimized.Meanwhile,the microwave photon technique without light source tuning(such as CMPI)[44-45]is adopted,and the vector network analyzer[31-33]is replaced by signal generator,photoelectric detector or high-speed acquisition card.In addition,Chirp-Z,Hanning window and other subsequent processing algorithms[32]are combined with fiber quasi-distributed sensing system to improve the system’s perception,response,measurement and demodulation towards the rapidly changing signals.

In addition,all the existing microwave demodulation systems are provided with discrete experimental devices.This increases the overall system volume and signal transmission loss,reduces the system stability,and adversely affects the application of this technique in the actual fiber quasidistributed system.Therefore,through the combination with silicon photonics and other emerging frontier domains,the miniaturization and integration of photoelectric and microwave devices has become an important research direction to promote the development of microwave demodulation technique towards low cost and high practicability.

——中文對照版——

1 引言

隨著高速信息時(shí)代的到來，物聯(lián)網(wǎng)技術(shù)已經(jīng)逐漸成為人類獲取外界信息、預(yù)測環(huán)境變化、提高生產(chǎn)生活質(zhì)量的重要科技基礎(chǔ)。其中，作為探測環(huán)節(jié)中不可或缺的關(guān)鍵組成部分，光纖傳感器因其具有體積小、質(zhì)量輕、精度高、耐腐蝕、抗電磁干擾、成本低廉、能夠與現(xiàn)有光纖通信系統(tǒng)良好兼容等優(yōu)勢[1-2]，已經(jīng)在土木工程、生物、化工、機(jī)械、電氣、航天等領(lǐng)域得到了廣泛應(yīng)用。

通過對分立的光纖傳感器單元進(jìn)行組網(wǎng)、排列和復(fù)用，可以在長距離范圍內(nèi)對溫度、應(yīng)變、振動(dòng)等物理量進(jìn)行感知和測量。有別于基于光纖瑞利、拉曼或布里淵散射效應(yīng)的全分布式傳感系統(tǒng)，這種準(zhǔn)分布式光纖傳感系統(tǒng)不需要激發(fā)光纖內(nèi)的非線性效應(yīng)，且易于建立傳感光信號(hào)與待測參量之間的定量關(guān)系；可以按照實(shí)際需求靈活設(shè)計(jì)覆蓋距離、測量精度等系統(tǒng)性能參數(shù)，有助于控制和降低系統(tǒng)搭建成本；能夠?qū)φ凵渎省⑶实雀鼮樨S富多樣的外界變化進(jìn)行探測，并具備測量動(dòng)態(tài)事件的能力。因此，目前準(zhǔn)分布式光纖傳感系統(tǒng)已經(jīng)在油井勘探、火災(zāi)預(yù)警、大型建筑物結(jié)構(gòu)健康監(jiān)測等方面發(fā)揮了重要作用[3-6]。

按照復(fù)用的光纖傳感器類型，可以將準(zhǔn)分布式光纖傳感系統(tǒng)大致分為光纖布拉格光柵(Fiber Bragg Grating,FBG)準(zhǔn)分布式傳感系統(tǒng)和光纖法布里-珀羅(Fabry-Pérot,FP)準(zhǔn)分布式傳感系統(tǒng)兩大類。光纖光柵準(zhǔn)分布式傳感系統(tǒng)以波長調(diào)制傳感器光纖布拉格光柵為傳感單元，通過相位掩模、紫外照射等方法使光纖纖芯產(chǎn)生折射率周期性變化，反射滿足布拉格波長的光，當(dāng)環(huán)境變化引起光纖內(nèi)的熱光效應(yīng)或彈光效應(yīng)時(shí)，光柵中心波長發(fā)生變化，從而實(shí)現(xiàn)對待測量的傳感[7-10]。而光纖法布里-珀羅準(zhǔn)分布式傳感系統(tǒng)則以相位調(diào)制傳感器光纖FP 結(jié)構(gòu)為傳感單元，通過熔接、腐蝕、鍍膜、激光微加工等方法在光纖結(jié)構(gòu)中形成反射面，利用反射面間的雙光束干涉光譜受外界物理量調(diào)制的現(xiàn)象，實(shí)現(xiàn)對目標(biāo)參量的探測[11-12]。

準(zhǔn)分布式光纖傳感系統(tǒng)在眾多領(lǐng)域得到大規(guī)模應(yīng)用的同時(shí)，日益增長的應(yīng)用需求也對搭建系統(tǒng)和維護(hù)成本、復(fù)用容量、覆蓋范圍、測量速率、定位能力等都提出了更高的要求，促使著研究人員不斷探尋新的傳感解調(diào)方案。近年來，微波光子學(xué)這種融合了光子技術(shù)和微波技術(shù)各自優(yōu)勢的新型技術(shù)的快速發(fā)展，以及其大帶寬、低損耗、靈活可重構(gòu)等優(yōu)點(diǎn)[13-15]，不僅在光載無線通信、宇宙空間探測、雷達(dá)系統(tǒng)等領(lǐng)域得到了廣泛研究[16-18]，還在光纖傳感解調(diào)領(lǐng)域展現(xiàn)出引人注目的應(yīng)用潛力。通過微波光子技術(shù)將微波信號(hào)調(diào)制到光信號(hào)上，借助微波測量手段，能夠在提升系統(tǒng)解調(diào)速率的同時(shí)，獲得高測量精度和強(qiáng)定位能力，并在一定程度上降低了系統(tǒng)的搭建和維護(hù)成本。

本文將針對微波光子學(xué)在準(zhǔn)分布式光纖傳感系統(tǒng)解調(diào)中的應(yīng)用，從光纖光柵準(zhǔn)分布式傳感系統(tǒng)和光纖FP 準(zhǔn)分布式傳感系統(tǒng)兩大類系統(tǒng)出發(fā)，對微波解調(diào)系統(tǒng)的基本原理、實(shí)驗(yàn)實(shí)現(xiàn)及解調(diào)性能等進(jìn)行介紹，并在最后對現(xiàn)有方案存在的問題和未來研究發(fā)展方向進(jìn)行分析討論。

2 光纖光柵準(zhǔn)分布式傳感系統(tǒng)的微波光子解調(diào)技術(shù)

利用微波光子技術(shù)對光纖光柵準(zhǔn)分布式傳感系統(tǒng)進(jìn)行解調(diào)的基本原理，是通過光載波調(diào)制，將傳感光柵的光波長變化，轉(zhuǎn)化為微波域上的強(qiáng)度或頻率變化。借助微波檢測手段來提高系統(tǒng)的解調(diào)精度和速率，并通過時(shí)-頻域變換實(shí)現(xiàn)對傳感單元的定位。按照解調(diào)系統(tǒng)的基本原理，可以將光纖光柵準(zhǔn)分布式系統(tǒng)的微波解調(diào)方案大致分為微波光子濾波器(Microwave Photonic Filter,MPF)、微波光子外差和光電振蕩器(OptoElectronic Oscillator,OEO)3 種。

2.1 基于MPF 的光纖光柵準(zhǔn)分布式系統(tǒng)的微波解調(diào)

基于MPF 的光纖光柵準(zhǔn)分布式傳感解調(diào)系統(tǒng)的基本結(jié)構(gòu)示意圖如圖1 所示。由光源輸出的光信號(hào)首先通過電光調(diào)制器(Electronic Optic Modulator,EOM)與輸入微波信號(hào)進(jìn)行調(diào)制，之后作為載波在光纖傳感系統(tǒng)中傳輸，經(jīng)過傳感區(qū)到達(dá)光電探測器(photodetector,PD)，最終獲得攜帶了待測參量變化信息的輸出微波信號(hào)。

假設(shè)輸入微波信號(hào)的頻率為ω，光信號(hào)從調(diào)制器經(jīng)過傳感區(qū)域中第i個(gè)光柵反射到達(dá)探測器的總時(shí)間為ti，λi為第i個(gè)光柵的中心波長，Pi表示第i個(gè)光柵反射光功率的大小，N為傳感區(qū)中光柵的總數(shù)目，則整個(gè)準(zhǔn)分布式光柵傳感網(wǎng)絡(luò)的頻率響應(yīng)表示為：

對該系統(tǒng)的頻率響應(yīng)進(jìn)行逆快速傅立葉變換(Inversed Fast Fourier Transform,IFFT)，即可得到系統(tǒng)的時(shí)域響應(yīng)：

從式（2）可以看出，不同光柵反射回的信號(hào)在時(shí)域上呈現(xiàn)出分立的沖激峰，沖激峰的強(qiáng)度包含了光柵反射光的光功率信息。因此，通過檢測傳感系統(tǒng)的輸出微波信號(hào)，即可實(shí)現(xiàn)對光柵傳感系統(tǒng)的解調(diào)和定位。

基于上述原理，在實(shí)際的解調(diào)過程中需要對調(diào)制到光載波上的輸入微波信號(hào)和從光電探測器獲取的輸出進(jìn)行同步掃描，在微波系統(tǒng)中常用的矢量網(wǎng)絡(luò)分析儀(Vector Network Analyzer,VNA)能夠?qū)崿F(xiàn)同步掃描的要求。以VNA 為核心器件的MPF 微波解調(diào)系統(tǒng)的示意圖如圖2 所示，該方案結(jié)構(gòu)簡單，易于搭建，因此被廣泛應(yīng)用于準(zhǔn)分布式光纖傳感系統(tǒng)的解調(diào)[19-23]。

2013 年，西班牙巴侖西亞理工大學(xué)的Ricchiuti 等人在此前單長周期光纖光柵傳感解調(diào)實(shí)驗(yàn)的基礎(chǔ)上[19]，利用矢量網(wǎng)絡(luò)分析儀首次實(shí)現(xiàn)了對500 個(gè)串聯(lián)長周期光纖光柵準(zhǔn)分布式系統(tǒng)的解調(diào)[20]。但由于長周期光柵屬于透射式光纖傳感器，在級(jí)聯(lián)系統(tǒng)中需要通過增設(shè)反射端面和參考抽頭來提高傳感區(qū)的光反射能力，因此增大了系統(tǒng)的復(fù)雜程度和使用難度。

為了使微波解調(diào)系統(tǒng)能夠更好地適應(yīng)實(shí)際光纖光柵傳感網(wǎng)絡(luò)的需求，2015 年，華中科技大學(xué)夏歷教授課題組針對大規(guī)模長距離傳感領(lǐng)域的應(yīng)用，提出了如圖3 所示的新方案[21]。該系統(tǒng)采用具有相同中心波長的全同弱反光柵作為傳感單元，以提高系統(tǒng)的復(fù)用能力，使用光帶通濾波器(Optical Bandpass Filter,OBPF)進(jìn)行匹配濾波，將光柵中心波長變化轉(zhuǎn)化為反射光強(qiáng)度變化，以解調(diào)出每個(gè)光柵的具體信息。當(dāng)有外部應(yīng)力作用在光柵上時(shí)，與該光柵對應(yīng)的IFFT 峰的幅度將發(fā)生變化。該方案獲得的解調(diào)結(jié)果具有良好的線性，且能夠與現(xiàn)有的光纖光柵傳感網(wǎng)絡(luò)良好兼容。

由于基于MPF 的準(zhǔn)分布式光柵微波解調(diào)系統(tǒng)均采用強(qiáng)度解調(diào)的方法，因此光源輸出功率波動(dòng)或光纖傳感鏈路中存在彎曲、盤繞等損耗變化都會(huì)對解調(diào)結(jié)果的準(zhǔn)確性產(chǎn)生負(fù)面影響。針對這一問題，Cheng 等人提出了一種結(jié)合了差分濾波和微波網(wǎng)絡(luò)的準(zhǔn)分布式超短光纖光柵的解調(diào)方案[24]。如圖4 所示，系統(tǒng)在馬赫-曾德干涉儀的上下兩路設(shè)置了一對中心波長相差僅0.4 nm 的高斯濾波器，利用兩臂的光程差將光柵反射的光信號(hào)轉(zhuǎn)化為一對相鄰的沖激信號(hào)。對兩相鄰沖激峰求取強(qiáng)度比，即可獲得該光柵的波長漂移情況。該方案不僅對光源的功率波動(dòng)免疫，還保證了解調(diào)結(jié)果不受傳輸光纖中彎曲損耗等的影響。

With the first breath of flame that swept over her when she ran with her friends Snowflake had melted away, and a little soft haze10 floating upwards11 was all that remained of her

前述的幾種微波解調(diào)方案雖然具有定位能力強(qiáng)、測量精度高等優(yōu)點(diǎn)，但是均只適用于全同光纖光柵網(wǎng)絡(luò)，對于由不同中心波長的光柵組成的波分復(fù)用(Wavelength Division Multiplexing,WDM)系統(tǒng)則不能發(fā)揮準(zhǔn)確定位和解調(diào)的功能。針對這一問題，研究人員提出了波長-射頻延遲映射的解決方案，通過光纖色散對不同波長的反射光引入不同時(shí)延，最終達(dá)到區(qū)分不同光柵傳感信息的目的[25-26]。但系統(tǒng)對引入時(shí)延的光纖長度和光纖種類，以及矢量網(wǎng)絡(luò)分析儀的掃頻范圍、采樣點(diǎn)數(shù)都提出了要求，并容易受到傳感器交叉敏感的影響。

為了盡可能降低傳感器交叉敏感以及光纖鏈路中光源功率波動(dòng)、傳輸損耗等給解調(diào)結(jié)果帶來的誤差，2019 年，Wu 等人提出使用雙Sagnac 環(huán)結(jié)構(gòu)實(shí)現(xiàn)對WDM 光柵復(fù)用系統(tǒng)的同時(shí)差分解調(diào)[27]。如圖5 所示，系統(tǒng)在馬赫-曾德干涉儀的兩臂分別設(shè)置了一個(gè)基于單模-保偏-單模結(jié)構(gòu)的Sagnac 濾波器，兩濾波器的光譜存在0.48 nm 的錯(cuò)位。由于Sagnac 環(huán)屬于寬譜濾波器，且其透射譜在每個(gè)通道下都具有類高斯的線型，因此只需不同光纖光柵的中心波長與Sagnac 透射譜的不同通道一一對應(yīng)，即可實(shí)現(xiàn)對多個(gè)不同光柵的同時(shí)、獨(dú)立、線性解調(diào)。

2.2 基于微波光子外差的光纖光柵準(zhǔn)分布式系統(tǒng)的微波解調(diào)

基于微波光子外差的光纖光柵準(zhǔn)分布式微波解調(diào)系統(tǒng)同樣采用外調(diào)制的方法，將高速掃頻的微波信號(hào)加載到作為光源的非相干寬帶光上，通過檢測光柵反射信號(hào)與原始光信號(hào)之間的外差拍頻信號(hào)信息，實(shí)現(xiàn)對施加在光柵上的外界環(huán)境變化進(jìn)行解調(diào)[28]，其中拍頻信號(hào)的頻率變化與光柵波長變化情況、纖內(nèi)總色散量成正比。

基于上述原理，研究人員已經(jīng)提出了形式多樣的光纖光柵微波解調(diào)方案，并獲得了出色的解調(diào)精度和響應(yīng)速率[29-31]。2018 年，武漢理工大學(xué)李政穎教授課題組將微波光子技術(shù)與色散相結(jié)合，搭建了如圖6 所示的基于微波光子外差的準(zhǔn)分布式光纖光柵微波解調(diào)系統(tǒng)，實(shí)現(xiàn)了對弱反光纖光柵準(zhǔn)分布式系統(tǒng)的高速解調(diào)[32]。其中，系統(tǒng)采用14.7 km 的色散補(bǔ)償光纖(Dispersion Compensation Fiber,DCF)為波長-頻率轉(zhuǎn)換提供高色散值，最終獲得8 pm 的高解調(diào)精度，且解調(diào)結(jié)果與光譜儀的直接測量結(jié)果具有高度一致性。此外，該系統(tǒng)還展現(xiàn)出優(yōu)秀的動(dòng)態(tài)傳感能力，在高速振動(dòng)實(shí)驗(yàn)中實(shí)現(xiàn)了40 kHz 的解調(diào)速率。

雖然上述系統(tǒng)具有高解調(diào)速率和高測量精度的優(yōu)點(diǎn)，但當(dāng)環(huán)境溫度發(fā)生變化時(shí)，色散補(bǔ)償光纖的熱光效應(yīng)和熱延展效應(yīng)會(huì)對兩路光信號(hào)的拍頻造成嚴(yán)重影響，給光柵波長解調(diào)結(jié)果帶來誤差，降低在大規(guī)模復(fù)用情況下解調(diào)結(jié)果的可信度。為了解決這一問題，2019 年，該課題組的Liang 等人又提出了基于DCF-SMF 雙通道的改進(jìn)方案[33]。新方案在前述系統(tǒng)的基礎(chǔ)上增設(shè)了另一路15.2 km SMF 通道，用以消除環(huán)境溫度漂移帶來的測量誤差，達(dá)到修正解調(diào)波長值的目的。雙通道系統(tǒng)在保留了單通道系統(tǒng)的高速率、低成本優(yōu)點(diǎn)的同時(shí)，提高了波長測量結(jié)果的準(zhǔn)確性和線性度，消除了溫度交叉敏感帶來的負(fù)面影響。

除了通過設(shè)置參照鏈路、利用DCF 的色散特性實(shí)現(xiàn)微波光子外差法解調(diào)外，還可以利用基于混沌源的非相干光頻域反射技術(shù)來實(shí)現(xiàn)準(zhǔn)分布式光柵系統(tǒng)的拍頻解調(diào)[34-35]。如圖7 所示，系統(tǒng)的非相干光由在光反饋?zhàn)饔孟庐a(chǎn)生混沌振蕩的DFB(distributed-feedback)激光器產(chǎn)生，這種混沌光源具有線寬寬、相干長度短、干涉噪聲低的優(yōu)點(diǎn)，有助于提高傳感解調(diào)網(wǎng)絡(luò)的整體復(fù)用容量。該系統(tǒng)實(shí)驗(yàn)實(shí)現(xiàn)了對3 640 個(gè)弱反光柵的定位和波長解調(diào)，基于混沌源的IOFDR 技術(shù)在大規(guī)模密集復(fù)用場景下展現(xiàn)出了出色的解調(diào)能力。但該方案在進(jìn)行光柵的波長解調(diào)時(shí)，需要對DFB 激光進(jìn)行熱調(diào)諧以改變其激射光的波長，顯著降低了系統(tǒng)的解調(diào)速度，限制了其在動(dòng)態(tài)參量傳感方面的應(yīng)用潛力。

2.3 基于OEO 的光纖光柵準(zhǔn)分布式系統(tǒng)的微波解調(diào)

2020 年，南京大學(xué)的Wang 等人以O(shè)EO 系統(tǒng)原理為基礎(chǔ)，提出了一種準(zhǔn)分布式光纖光柵微波傳感解調(diào)系統(tǒng)[39]。在該方案中，級(jí)聯(lián)光柵陣列通過環(huán)行器連入到OEO 振蕩環(huán)路中，借助光柵的反射特性，使復(fù)用系統(tǒng)中的每個(gè)光柵都與不同的環(huán)路長度對應(yīng)。通過對具有不同F(xiàn)SR 的振蕩峰進(jìn)行測量和分析，即可解調(diào)出光柵的波長和空間位置信息。該系統(tǒng)作為一種數(shù)字離散位置編碼系統(tǒng)，具有很強(qiáng)的抗干擾能力。但上述方案的理論復(fù)用容量僅為62，對于實(shí)際應(yīng)用需求來說仍有待進(jìn)一步提高。

對上述3 種不同的光纖光柵準(zhǔn)分布式微波解調(diào)方案進(jìn)行比較，通過表1 可以看到，基于MPF的微波解調(diào)方案在實(shí)現(xiàn)高精度空間定位方面更具有優(yōu)勢，并且通過波長-頻率延遲映射、差分濾波解調(diào)等優(yōu)化方法，能夠消除功率波動(dòng)等對解調(diào)結(jié)果準(zhǔn)確性的影響，滿足WDM 系統(tǒng)的解調(diào)需求；但受到VNA 掃描速率的限制，其在解調(diào)速度方面仍有待提高?；谖⒉ü庾油獠罘ǖ慕庹{(diào)方案則在解調(diào)速率方面具有顯著優(yōu)勢，能夠滿足高速解調(diào)和動(dòng)態(tài)傳感的需要，且其雙通道平衡探測的結(jié)構(gòu)同樣可以很好地解決交叉敏感的問題，但該方案在空間分辨率方面有所犧牲。而基于OEO 的解調(diào)方案雖然具有優(yōu)秀的信號(hào)信噪比和穩(wěn)定性，但其系統(tǒng)容量較為有限，目前不適于大規(guī)模光柵復(fù)用系統(tǒng)的解調(diào)。

3 光纖FP 準(zhǔn)分布式傳感系統(tǒng)的微波光子解調(diào)技術(shù)

以光纖FP 為代表的光纖干涉結(jié)構(gòu)是準(zhǔn)分布式光纖傳感網(wǎng)絡(luò)中另一種常見的傳感單元。與光纖光柵微波解調(diào)原理類似，對于基于光纖FP 干涉儀的準(zhǔn)分布式傳感系統(tǒng)，同樣可以使用光載微波信號(hào)的方法進(jìn)行頻域輔助測量。根據(jù)具體解調(diào)方法的不同，可以將光纖FP 準(zhǔn)分布式系統(tǒng)的微波解調(diào)方案分為光載波微波干涉技術(shù)(Optical Carrier based Microwave Interferometry,OCMI)和相干微波光子干涉技術(shù)(Coherent Microwave-Photonic Interferometry,CMPI)兩種。

3.1 基于OCMI 的光纖FP 準(zhǔn)分布式系統(tǒng)的微波解調(diào)

OCMI 微波解調(diào)技術(shù)主要基于低相干光源實(shí)現(xiàn)。當(dāng)系統(tǒng)光源為非相干光時(shí)，光源的相干長度較短，其帶寬遠(yuǎn)小于任意兩個(gè)反射點(diǎn)之間的光程差。此時(shí)，系統(tǒng)復(fù)頻域響應(yīng)的交叉積項(xiàng)為零，可以利用其自積項(xiàng)重構(gòu)出微波域中的干涉譜，并通過微波相移求出光程差的變化[40,44]。

這種利用低相干光和微波光子技術(shù)的光纖準(zhǔn)分布式解調(diào)方法自提出以來就得到了研究人員們的廣泛關(guān)注[40-43]。2013 年，基于OCMI 技術(shù)的光纖FP 準(zhǔn)分布式傳感解調(diào)方案由美國克萊姆森大學(xué)Xiao 課題組率先實(shí)驗(yàn)實(shí)現(xiàn)[40]。系統(tǒng)結(jié)構(gòu)如圖9所示，系統(tǒng)使用單模光纖與毛細(xì)管(capillary tube)熔接而成的全光纖FP 腔作為級(jí)聯(lián)傳感單元，通過對可調(diào)諧濾波器的中心波長進(jìn)行調(diào)整，可以獲得不同波長下時(shí)域IFFT 沖激信號(hào)的強(qiáng)度值，進(jìn)而分別還原出各個(gè)光纖FP 的光譜信息。實(shí)驗(yàn)結(jié)果表明，微波重構(gòu)獲得的光譜與光譜儀直接測量的結(jié)果相吻合，在保證了高準(zhǔn)確性的同時(shí)，還兼具了傳感器定位的能力。

除了由分立的光纖FP 傳感器級(jí)聯(lián)組成的準(zhǔn)分布式傳感鏈路外，OCMI 技術(shù)還可以對空間連續(xù)型的光纖FP 傳感系統(tǒng)進(jìn)行解調(diào)。通過熱熔接[39]或者飛秒激光器刻寫[41-42]的方法，可以在傳感區(qū)光纖中得到一系列反射點(diǎn)，相鄰弱反射點(diǎn)可視為一對光纖FP 結(jié)構(gòu)。這種光纖FP 準(zhǔn)分布式系統(tǒng)不存在測量暗區(qū)(dark zone)，適用于機(jī)械結(jié)構(gòu)表面形變等場景的測量和監(jiān)控。此外，OCMI 技術(shù)還可以應(yīng)用于其他類型的波導(dǎo)或自由空間干涉儀系統(tǒng)中，以及各種物理、化學(xué)和生物領(lǐng)域的準(zhǔn)分布式測量。

3.2 基于CMPI 的光纖FP 準(zhǔn)分布式系統(tǒng)的微波解調(diào)

有別于OCMI 技術(shù)，CMPI 微波解調(diào)技術(shù)是基于相干光源實(shí)現(xiàn)的。對于一個(gè)具有N個(gè)弱反射點(diǎn)的光纖來說，當(dāng)系統(tǒng)光源為相干光時(shí)，由于相鄰反射點(diǎn)間的干涉現(xiàn)象，使得反射點(diǎn)所對應(yīng)的反射光強(qiáng)度值與反射點(diǎn)間光程差的變化密切相關(guān)。因此，CMPI 系統(tǒng)能有效地將反射點(diǎn)之間的光程差變化轉(zhuǎn)化為與特定反射點(diǎn)對應(yīng)的時(shí)域脈沖的強(qiáng)度變化。2017 年，Hua 等人搭建了如圖10 所示的CMPI 微波傳感解調(diào)系統(tǒng)，利用實(shí)驗(yàn)對理論分析的結(jié)論進(jìn)行了驗(yàn)證[44]。實(shí)驗(yàn)結(jié)果表明，對于腔長為1.5 cm 的光纖FPI，該系統(tǒng)實(shí)現(xiàn)了0.6με 的應(yīng)變分辨率；當(dāng)FPI 的腔長增加到1 m 以上時(shí)，可以將應(yīng)變分辨率進(jìn)一步提高到nε 水平。

由于CMPI 技術(shù)對反射點(diǎn)光程差的靈敏性，該解調(diào)方法在動(dòng)態(tài)參量的測量中也展現(xiàn)出優(yōu)秀性能。例如在多點(diǎn)振動(dòng)傳感實(shí)驗(yàn)中，研究人員將級(jí)聯(lián)光纖FP 作為振動(dòng)傳感單元，借助CMPI 微波解調(diào)系統(tǒng)，實(shí)現(xiàn)了最高2.3 kHz 的振動(dòng)解調(diào)和振動(dòng)點(diǎn)定位[45]。該實(shí)驗(yàn)結(jié)果表明，基于CMPI 的光纖FP 準(zhǔn)分布式傳感系統(tǒng)有望在壓力波等更加復(fù)雜的物理量測量中得到應(yīng)用。

4 總結(jié)與展望

基于微波光子學(xué)的準(zhǔn)分布式光纖傳感解調(diào)技術(shù)，相較于傳統(tǒng)的基于波長解調(diào)的光纖解調(diào)技術(shù)，具有高解調(diào)速率、高測量精度、強(qiáng)定位能力等優(yōu)秀特性，有助于降低系統(tǒng)搭建成本、提高系統(tǒng)整體性能，在長距離、高精度傳感應(yīng)用場景中具有巨大的應(yīng)用潛力。本文根據(jù)傳感單元類型和解調(diào)原理，從光纖光柵準(zhǔn)分布式傳感系統(tǒng)和光纖FP準(zhǔn)分布式傳感系統(tǒng)兩大類系統(tǒng)出發(fā)，詳細(xì)介紹了國內(nèi)外準(zhǔn)分布式微波解調(diào)技術(shù)的研究進(jìn)展，對比分析了微波光子濾波器、微波光子外差、光電振蕩器等解調(diào)方案在空間定位、解調(diào)速率、復(fù)用容量、系統(tǒng)復(fù)雜度和穩(wěn)定性等方面的不同特點(diǎn)。

盡管目前針對準(zhǔn)分布式光纖傳感系統(tǒng)的微波解調(diào)技術(shù)在理論和實(shí)驗(yàn)室研究方面已經(jīng)取得了豐碩成果，但其在系統(tǒng)工程化方面仍存在著一些亟待解決和突破的問題。首先，受到傳感器自身響應(yīng)特性的限制，現(xiàn)有的準(zhǔn)分布式光纖傳感微波解調(diào)系統(tǒng)仍多限于單一物理量，如溫度、應(yīng)變的測量，難以滿足越來越多樣化的實(shí)際應(yīng)用場景。如何在微波系統(tǒng)中實(shí)現(xiàn)不同參量的同時(shí)準(zhǔn)確解調(diào)、以及待測參量間存在的交叉敏感問題，為未來微波解調(diào)技術(shù)的研究方向之一。在拓展傳感物理量類型方面，可以通過將傳統(tǒng)光纖光柵、光纖FP 結(jié)構(gòu)與特種光纖[26]、表面等離子體效應(yīng)[46]等新技術(shù)相結(jié)合，以提高復(fù)用傳感器在折射率、氣壓、電流、磁場等更多參量的靈敏度，實(shí)現(xiàn)其在特殊場景下的應(yīng)用。在消除待測參量間的交叉敏感方面，可以通過優(yōu)化傳感器件封裝、采用雙通道差分解調(diào)[24,27]、利用馬赫-曾德等干涉結(jié)構(gòu)進(jìn)行外差探測[32-33]、設(shè)置對照鏈路[33]等方法，實(shí)現(xiàn)物理量的準(zhǔn)確、穩(wěn)定測量。

其次，目前報(bào)道的微波解調(diào)技術(shù)受到解調(diào)方案、器件指標(biāo)等的限制，如傳感器結(jié)構(gòu)對高頻參量靈敏度不高、核心器件VNA 掃描速率較低、VNA頻率掃描精度與掃描范圍存在矛盾、DFB 激光光源波長調(diào)諧耗時(shí)較長等，大多僅能實(shí)現(xiàn)對靜態(tài)參量或低頻參量的高精度傳感，在高頻參量測量方面精度較低。針對這一問題，需要在優(yōu)化聲波等動(dòng)態(tài)參量光纖傳感器結(jié)構(gòu)[47]的同時(shí)，采用CMPI等無需進(jìn)行光源調(diào)諧的微波光子技術(shù)[44-45]，使用信號(hào)發(fā)生器、光電探測器、高速采集卡等器件替代矢量網(wǎng)絡(luò)分析儀[31,33]，配合Chirp-Z、Hanning窗等后續(xù)處理算法[32]，提高光纖準(zhǔn)分布式傳感系統(tǒng)對快速變化信號(hào)的感知、響應(yīng)、測量和解調(diào)能力。

另外，現(xiàn)有的微波解調(diào)系統(tǒng)均采用分立的實(shí)驗(yàn)器件，增加了系統(tǒng)的整體體積和信號(hào)傳輸損耗，降低了系統(tǒng)的穩(wěn)定性，不利于該技術(shù)在實(shí)際光纖準(zhǔn)分布式系統(tǒng)中的應(yīng)用。因此，通過與硅光子技術(shù)等新興前沿技術(shù)相結(jié)合，探索光電器件及微波器件的小型化、集成化成為促進(jìn)微波解調(diào)技術(shù)向低成本、高實(shí)用性發(fā)展的重要研究方向。