Research On Three-dimensional Shape Restoration Method Of Dense Multi-core Fiber Gratings

Minimally invasive surgery is mainly performed through an endoscope,a catheter or a robot guided by a doctor into the patient’s body.Among them,tracking the shape and path of the minimally invasive surgical instrument in the human body is vital to the surgery’s success.Currently,the traditional shape sensing technology has shortcomings such as being susceptible to electromagnetic interference,large size,unclear imaging,and have side effects on patients,which cannot meet the needs of modern medical care.As a passive sensor,fiber grating has the advantages of anti-electromagnetic interference,small size,and high stability.It is suitable for medical environments with limited scene volume.However,the existing optical fiber shape sensing methods have the disadvantages of insufficient sensor density and large torsional error of flexible structures,cannot meet the high-accuracy shape sensing of catheters in minimally invasive surgery.In order to solve the above problems,this paper based on the dense multi-core fiber grating array,combined with the Optical Frequency Domain Reflectometer(OFDR)technology,realizes the distributed curvature and torsion detection with high accuracy and high spatial resolution.At the same time,a 3D restoration algorithm based on the initial twist angle is studied,which effectively compensates the restoration error caused by the twist of the optical fiber,and realizes the high-accuracy shape sensing.The main research work is as follows:(1)Fabrication of dense multi-core fiber grating array sensor.For the fabrication of dense three-core fiber gratings array,the related factors affecting the writing of three-core fiber gratings were analyzed with the coupling mode equation.Combined with 3D reconstruction algorithm for sensor density design,we research on automatic writing method of multi-core fiber grating with high consistency.Aiming to obtain signals from different cores of multi-core fibers,a multi-core fiber fan-in and fan-out device based on the taper-inserted is designed.The parameters of the fan-in and fan-out device structure are simulated and the optimal design structure is analyzed.Finally,manufactured the three-core fiber fan-in and fan-out devices based on simulation.(2)Research on three-dimensional pose restoration algorithm of multi-core fiber grating array.Based on the fiber bragg grating sensing principle,the strain change value of the fiber is obtained by detecting the center wavelength shift,and then the curvature information of sensor is obtained.Combined with the designed multi-core fiber sensor,the functional relationship between the three-dimensional curve curvature,the bending direction angle and the center wavelength drift is constructed.According to the properties of the space curve and motion coordinate transfer,a three-dimensional shape restoration algorithm based on distributed curvature and torsion is studied.At the same time,a torsion compensation method based on the initial torsion angle is proposed for the existing torsion error problem of flexible structures.(3)3D pose restoration experiment verification and analysis.A dense multi-core fiber grating array sensor with a grating length of 3 mm,a spacing of 2 mm and a length of 423 mm was fabricated,and a 3D shape sensing system based on optical frequency domain reflectometry was built.In this paper,a mold with known curvature is used to conduct curvature and torsion and the curvature sensitivity of the sensor is calibrated.Simultaneously,the curvature coefficient and the initial torsion angle are measured,and the 3D restoration accuracy before and after the correction of the curvature coefficient and the initial torsion angle is compared.The effectiveness of compensation method and Feasibility of shape restoration method based on dense multi-core FBG is verified.