Research On The Characteristics Of Off-axis Rotary Connector Of Doublet Lens And Thermally Expanded Core Fibe

With the rapid development of optical fiber technology,Fiber Optic Rotary Joint(FORJ),as a transmission device for rotating optical signals,has the advantages of simple structure,small volume and high coupling efficiency,and has a very good application prospect in the field of optical fiber communication and transmission.However,in special transmission environments,the center rotary position not only to transmit optical signals,but also air pressure,oil,data cable lines,etc.,when the center rotary is occupied,it is necessary to apply Off-Axis Fiber Optic Rotary Joint for transmission.Increasingly,Fiber Bragg Grating(FBG)sensing also involves off-axis transmission of fiber optic signals.However,most of the current direct-alignment Off-Axis Fiber Optic Rotary Joints use Grin lens collimator,with small outgoing spot,high coupling loss and short effective alignment time,which is not conducive to the combination with FBG sensing technology.For this reason,this paper focuses on direct alignment type Off-Axis Fiber Optic Rotary Joints.For direct alignment type off-axis fiber optic rotary joints in the grin lens collimator out of a small spot,fiber coupling loss and short effective alignment time problems.Firstly,the general design of the direct alignment type off-axis fiber optic rotary joint is proposed.The fiber collimator consists of a large spot Double Lens and a Thermally Diffusion Expanded Core Fiber(TECF).Based on the periodic variation law of the overlapping area during the relative rotation of the emitted spot,the coupling loss variation when the radial error,angular error and axial error exist between the double-combined lens collimators,respectively,is analyzed.The radial error coupling loss curves and angular error coupling loss curves of Single Mode Fibers(SMFs)with five mode field diameters are obtained by numerical simulation.The simulation results show that:with the increase of angular error deflection angle,the optical coupling loss increases the fastest,the radial error is the second,the normal working distance of the large spot double combined lens collimator is longer and the requirement of axial error is lower,so the influence brought by angular and radial direction needs to be strictly controlled.To meet the FBG sensing requirements,increase the effective alignment time and ensure that the fiber grating demodulator can collect more fiber grating signals within the rotational alignment time window,the threshold value of the off-axis rotary connector coupling loss is set to 3 dB.The theoretical effective alignment time for 4 Mode Field Diameters(MFDs)thermally expanded core fibers with a coupling loss of 3 dB was obtained based on the rotor ring parameters and the spot size of the fiber collimator outgoing.The coupling performance of single-mode fibers with five mode field diameters was tested in combination with a five-dimensional displacement stage.The experimental results showed that the larger the MFD of the single-mode fiber used in the fiber collimator,the lower the coupling loss,with a low coupling loss of 0.22 dB at a mode field diameter of 28 μm.Then the performance of off-axis fiber optic rotary joint is simulated in the presence of radial error,angular error and axial error.To verify the accuracy of the effective alignment time.The effective alignment time of four mode-field diameters of thermally expanded core fibers connected to a double lens collimator was obtained by photoelectric conversion and nonlinear multivariate function fitting by setting the rotational speed at 60 r/min.The experimental results show that the maximum effective alignment time is 7.05 ms for a mode field diameter of 17 μm,which is consistent with the theoretical value of 7.1672 ms.Finally,based on the results of coupling loss,three coupling errors and effective alignment time,a thermally expanded core fiber with a mode field diameter of 20 μm is optimally selected to make a fiber grating temperature sensor,which achieves bidirectional transmission of optical signals and monitoring of temperature.This design provides theoretical and technical support for the study of large spot off-axis fiber optic rotary joint and its application in the field of fiber optic sensing.It also has some guiding significance for fiber grating monitoring of rotary.