Research On Marine Optical Sensor Based On SiO₂ Waveguide

In recent years,with the continuous advancement of the strategy of maritime powers,it is imperative to establish an all-round and complete monitoring system for all-round observation of the seabed.To achieve as much physical information as possible on a sufficiently large space and time scale,accurate and real-time measurements of marine physical parameters are required to ensure effective monitoring.Since the current commercial ocean temperature,salinity,depth and other sensors are mostly based on mature electronic sensing technology,they have the disadvantages of slow response time,large volume,large power consumption,large weight,high cost,non-environmental protection,etc.Moreover,the core technology are basically in the hands of foreign companies,and there are hidden dangers in national defense that cannot be ignored.Compared with the conventional electronic sensors,interferometric sensors based on optical waveguides have the advantages of small size,high accuracy,fast response speed,anti-electromagnetic interference,etc.In order to develop marine monitoring sensors with independent intellectual property rights to achieve portable,low-power,in-situ,real-time,fast,and accurate long-term marine monitoring,this paper responds to the research and development needs of high-end smart sensor chips and designs waveguide-type linear salinity sensor and high-pressure-resistant pressure(depth)sensor based on two optical structures.The results conform to the standard waveguide platform technology,can be used to achieve the goal of chip-level integration of marine physical,chemical and microbial sensors.The main work of this paper are:1.Based on the SiO2 waveguide with a refractive index difference of 0.75%,an arrayed waveguide grating(AWG)type linear salinity sensor is proposed.The salinity sensing is performed by using the characteristic that the center wavelength of the AWG drifts with the change of the effective refractive index.Based on the effective refractive index method,the salinity sensing mechanism of cladding etched SiO2 single-mode waveguide is analyzed.A SiO2 single-mode waveguide with a refractive index difference of 0.75%is designed,and the cladding etching structure is optimized to increase the sensitivity of the effective refractive index of the transmission mode E00y to seawater salinity.Combined with the empirical equation between the refractive index and salinity of seawater,the change of seawater salinity is converted into the change of the effective refractive index of the waveguide.Under this waveguide salinity sensing mechanism,the 1×1 AWG structure is used to convert the change in the effective refractive index of the waveguide into the drift of the output center wavelength.The structure of the AWG is designed,and a triangular cladding etching area is introduced into the array waveguides area as a filling area of the liquid to be measured.This area can amplify the change of the optical phase difference between adjacent array waveguides caused by the change of the liquid salinity,and improve the sensing sensitivity of the AWG type salinity sensor.The AWG type salinity sensor can avoid the wavelength drift caused by the aging of the light source.The sensing performance is simulated based on the beam propagation method(BPM)to verify the feasibility of using AWG for salinity sensing.The sensitivity of the AWG type salinity sensor with the triangular cladding etched area can be 60 times greater than that of the conventional AWG,the sensing sensitivity is 0.031nm/‰,and the resolution is 0.0319 ‰.2.Based on the SiO2 waveguide with a 2%refractive index difference,an asymmetric Mach-Zehnder interferometer(a-MZI)type pressure sensor is proposed.Pressure sensing is performed using the characteristic that the optical power in the linear region of the output curve of a-MZI changes as the effective refractive index changes.The photoelastic effect of SiO2 material has been studied.The pressure-induced birefringence in SiO2 material has been analyzed by using photoelastic matrix.The relationship between the refractive index of o/e light in SiO2 material with the pressure is obtained.The effective refractive index method is used to design the structure of the SiO2 single-mode waveguide with a 2%refractive index difference,and the relationship between the effective refractive index of the transmission modes Eo00y and E00y and the pressure is obtained.Using this sensing mechanism,the change of the pressure to be measured is converted into the change of the output optical power in the a-MZI linear region through the dual-arm sensing structure of a-MZI.The input and output couplers of a-MZI are designed based on the mechanism of symmetrical interference multimode interference(MMI),and the mode transmission analysis(MPA)method is used to analyze the multimode interference in the MMI.Through the beam propagation method(BPM),the beam splitting performance of MMI is simulated,and then the parameters of MMI are optimized and determined.Through numerical calculations,the ranges of arm length difference that can be achieved by the three arm design schemes(sine type,arc splicing type,and spiral type)are analyzed respectively,and the arm design scheme of a-MZI sensor under specific design requirement is given.Finally,based on the SiO2 waveguide with 2%refractive index difference,the prototype a-MZI pressure sensor with a arm length difference of 5mm is designed using a sine type arm,and its sensing performance is tested.Using comsol,the deviation between the experimental test results and theoretical simulation is analyzed.This prototype a-MZI pressure sensor exhibits an average half-cycle pressure of 4.83 MPa,the sensitivity in the linear region is 1.24 mW/MPa,and the pressure resolution is expected to be less than 0.08 MPa.The maximum pressure in the deep sea is about 110MPa.This a-MZI pressure sensor based on SiO2 waveguide does not require special treatment of the waveguide structure,such as etching.Its pressure tolerance can be much higher than the pressure sensor based on the waveguide structure on the thin film.Under the mechanical test platform,it can withstand a maximum pressure of about 40MPa,and it can show a higher pressure tolerance in a hydraulic environment.