| As a new technology for acquiring the information of external environment and physicals,optical fiber sensors play an important role in the development of the Internet of Things.Compared with traditional electrical sensors,optical fiber sensing systems have the advantages of high sensitivity,light structure,corrosion resistance,electromagnetic interference resistance,and easy realization of long-distance and largescale measurements.The optical fiber sensing system is mainly composed of two parts:the sensing unit and demodulation module.The sensing unit is used to modulate the physical quantity to be measured to a certain characteristic parameter of the optical signal.To obtain the physical information to be measured,a variety of demodulation methods have been developed to convert the optical sensing signal into the intensity,phase and time delay of the electrical signal,so as to facilitate subsequent signal processing.However,these methods suffer from slow response and poor accuracy.Optoelectronic oscillator(OEO),as one of the research hotspots in the field of microwave photonics(MWP)technology,is a hybrid resonator consisting of an optical link and an electrical link.Its special structure provides a new method for realizing optical fiber sensing demodulation: to map the optical sensing signal into the frequency change of the oscillating microwave signal.In this way,a new dimension of electrical signal(frequency)is added for the mapping mechanism of optical fiber sensing demodulation.Compared with other characteristic parameters,the demodulation scheme by mapping the physical quantities to be measured into the frequency variation of the oscillating signal has the advantages of high sensitivity and strong signal-to-noise ratio.Combined with the research hotspots and development needs of OEO in recent years,the novel OEOs with new structures and the optical fiber sensing demodulation technology based on OEO are studied in this paper.The main results obtained by the research are as follows.(1)A rational harmonic mode-locking OEO based on an electro-optical intensity modulator is proposed to generate microwave frequency combs.By setting the modulation frequency so that there is a frequency detuning(1 / p)f0 between it and the oscillating mode of the OEO,where f0 is the fundamental oscillating frequency of the OEO,the comb spacing of the generated microwave frequency comb can be increased by a factor of p.In the experiment,the modulation frequencies of 564 k Hz,940 k Hz and1.253 MHz were used to generate microwave frequency combs with comb spacings of1.128 MHz,1.88 MHz and 3.76 MHz,respectively.Compared with other active modelocking OEO structures,the proposed rational harmonic mode-locking OEO can generate microwave frequency combs with a larger comb interval even at a lower modulation frequency,which greatly alleviates the demand for the bandwidth of the mode-locking device.(2)Based on the phase condition of OEO,an OEO with an embedded Sagnac interferometer is proposed to realize high-sensitivity demodulation of angular velocity measurement.For the proposed scheme,optical carrier and first-order sideband signals with different frequencies and orthogonal polarizations travel in opposite directions along the Sagnac loop.With this design,the rotation rate of the Sagnac loop will change the phase of the OEO resonant cavity without affecting its cavity length,which will then lead to a variation on the oscillating frequency of the OEO.In the experiment,a 91-m long Sagnac fiber loop was used,and the scale factor of the oscillating frequency of OEO as a function of angular velocity was measured to be 51.8 k Hz/(rad/s).For the proposed demodulation scheme,its sensing sensitivity is proportional to the frequency of the optical carrier(in the order of 100 THz),and has nothing to do with the initial oscillating frequency of the OEO,thus the OEO can be operated in a low frequency state,thereby reducing the cost of the system and facilitate subsequent data collection and processing.(3)On the basis of the above research,a demodulation scheme for angular velocity sensing based on the cascaded structure of Sagnac loop and OEO resonator is proposed.The proposed scheme makes the cavity length of OEO independent of that of the Sagnac loop,so the measrement sensitivity of angular velocity can be further improved by increasing the length of Sagnac loop or reducing the OEO cavity length.When the length of Sagnac loop was increased to 560 m and the cavity length of OEO was reduced to 53 m,the scale factor of angular velocity sensing was measured to be 742 k Hz/(rad/s)in the experiment.In addition,compared with the demodulation scheme mentioned above,since the two signals travelling along the Sagnac loop in opposite directions have the same frequency and orthogonal polarization states in this scheme,the reciprocity of the Sagnac loop is improved,thereby reducing the impact of the external environment.(4)Based on the amplitude condition of OEO,a microwave photonic filter is designed for demodulation of angular velocity measurement,which is then used to select the oscillating modes of OEO.Through this scheme,the mapping relationship between the angular velocity-the center frequency of the microwave photonic filter-the oscillating frequency of the OEO is realized.Firstly,the angular velocity measurement based on the microwave photonic filter is experimentally demonstrated.The results show that the scale factors measured by the frequency change of the three notches in the frequency response of the filter were 1.45 GHz/(rad/s),0.80 GHz/(rad/s)and 0.61 GHz/(rad/s);and when the external modulation frequency is set to 4 GHz and 8 GHz,the measured slopes of the peak power(unit: m W)as a function of angular velocity were 0.016 m W/(rad/s)and0.0038 m W/(rad/s),respectively.Then the angular velocity measurement based on the wideband-tunable OEO is verified,and the experimental results show that when the Sagnac loop rotated at 1 rad/s,the frequency variation of OEO was 700 MHz.In addition,the maximum measurement error of the system was 0.06 rad/s.(5)A large-stain measurement based on a chirped fiber grating and an OEO is proposed.For the proposed scheme,the large-bandwidth chirped fiber grating acts as both a sensing unit and a spectral shaping filter for a broadband light source.When there is a small change for external strain,the wavelength drift of the grating is not enough to cause a change of oscillating frequency of the OEO;but as a large strain is introduced,the oscillating frequency of OEO will change with the wavelength drift of the chiped grating.The experimental results demonstrate that a strain sensing can be achieved with a sensitivity of-11 MHz/ mε based on the variation of the oscillating frequency of OEO. |
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