| Microwave photonics is a field where microwaves and optical waves cross each other.It uses light to generate,distribute,process,control,and analyze microwave signals.Wide-area and multi-point remote sensing technology plays an important role in the fields of disaster monitoring and locating.The microwave photonic sensing system implemented in the optical domain has many advantages not available in other sensing technologies such as large bandwidth,high speed,antiinterference and low loss.The basic principle is to convert the sensing information into microwaves in the form of optical signals.It can improve the recognition speed and accuracy,is more stable and easier to control,and has good repeatable measurement characteristics.Based on the optoelectronic oscillator(OEO)established by the uniform fiber Bragg grating(UFBG),this paper proposes several new quasi-distributed fiber sensor arrays and fiber sensor demultiplexing technologies.In addition,a scheme of single-passband and multi-passband microwave photon filter(MPF)based on external light injection-locked FP lasers is also proposed,which can be used in MWP sensor.The main content of this paper is:?.We demonstrate a demultiplexing method for quasi-distributed identical strong fiber Bragg grating(FBG)sensors based on optoelectronic oscillator(OEO).When the FBG sensor is affected by environmental changes,the wavelength of the FBG will shift.An oscillation will be stimulated in the OEO since the laser power is reflected by the FBG.The demultiplexing system utilizes the oscillating frequency of the OEO to encode the location information of grating series.Compared with other schemes,the proposed system has three superior advantages.Firstly,as a digital discrete position encoding system,it has a strong anti-interference ability and does not require continuous sweeping wavelength and rapid digital signal processing,which can achieve high speed and high stability.Second,it can demodulate identical FBGs with strong reflectivity,thus ensuring long-distance and large capacity.Third,it can work in a low-frequency range without using high-frequency devices,which has the potential to be a low-cost interrogator in practical applications.The theoretical capacity of the system is 62 multiplexed points in one line and the experimental results show that the sensing area can range from 1 m?1 km with the response time of ?0.61 s and the oscillating frequency stability of < 28 k Hz.The linear relationship between the wavelength of the laser and the alarm strain/temperature,corresponding to 1.3 ??/pm or 0.1 °C/pm,can be used to set alarm threshold values.?.We propose a novel demodulation method of FBG sensor array based on wavelength-to-time(WTT)mapping and multiloop OEO,which can locate each FBG position and linearly detect strain and temperature with ultra-high sensitivity.In our theory,the measurable variation which causes the Bragg wavelength shift,is mapped to the time delay change of the loop,thereby affecting the oscillating frequency.There are some advantages:(1)the multiloop OEO based on multiple FBGs realizes the sensor array with high-speed,large-distance,low phase-noise,and large free spectral range(FSR);(2)to achieve large-scale and multi-point demodulation,wideband laser source and strong FBGs with different Bragg wavelengths have been used;(3)the ultra-high sensitivity is achieved by using the high-order harmonic frequency and the element with a large negative dispersion coefficient.The ultra-high sensitivity of strain and temperature measurements is about 40.2 Hz/?? and 146.5 k Hz/°C,respectively.The frequency offset caused by disturbances and noise is within 1 k Hz,and the slight variation of peak power ensures the average SMSR over 45 d B.?.We propose and experimentally demonstrate another fiber Bragg grating(FBG)sensor interrogation technique based on WTT and OEO to improve the second scheme.The main components are also an lectro-optic modulator(EOM),a section of dispersive element,an electric filter and a photodiode(PD).Due to the dispersive element in the loop,the wavelength change of the FBG can be converted into the oscillating frequency shift of the OEO.The main improvement point is that the FBG sensor array is placed in front of the EOM.After reducing the optical link length,it has the following advantages:(1)increase the FSR to increase the measurement range;(2)the sensitivity of each FBG sensor is the same;(3)increase the sensitivity of the sensor array.The experimental results show that the frequency of the OEO generated microwave signals have a good linear relationship with the axial strain applied to the FBG.The sensitivity is about 58 Hz/?? when using dispersion compensation fiber(DCF)with dispersion of-165.8 ps/nm as the dispersive medium and the filter with central frequency near 2056.4 MHz.?.We propose a tunable single-passband or multiple-passband microwave photonic filter(MPF)that is incorporated with an injection-locked Fabry-Perot(FP)laser.In the proposed MPF,multiple passbands can be easily generated based on the frequency-selection effects of the laser structure in the case of multiple light waves injection.The novelty here is that the obtained multiple-passband MPF can achieve either a dual-passband or a single-passband by using merely one experimental scheme.Moreover,since the laser injection ratio of the proposed scheme is high,the central frequency of each passband has a large tunable range.More tunable passbands can be generated by employing more external wavelengths.By finedetuning the injection parameters,the frequency tuning range of 17 GHz and the outof-band rejection ratio of 24.1 d B are achieved for the dual-passband MPF,and the out-of-band rejection ratio of 22 d B and the 3-d B bandwidth of 360 MHz are achieved for the single-passband MPF.In addition,the attained peak power and bandwidth of the proposed MPF are investigated with respect to the injection parameters,including detuning frequency,injection ratio,and bias current of FP laser.The stability and dynamic range of the MPF are also evaluated through experiments. |
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