柔性多芯超长光纤布拉格光栅的大曲率三维位姿重构方法

Optical fiber sensors are very suitable for three-dimensional pose reconstruction because they are small and easy to embed. For applications such as shape tracking of minimally invasive surgical instruments, the sensor needs to be able to achieve precise distributed sensing of large strain when it is bent with extremely large curvature. Even though investigated extensively, the conventional silica-based optical fibers are not flexible enough for the aforementioned applications, due to silica’s high Young’s modulus. Therefore, we propose to use a chalcogenide optical fiber with low Young’s modulus to fulfill the requirement on the flexibility of the sensor. Because of the difference in the thermal expansion coefficient of the core and the cladding, the thermally induced force produced by the thermal expansion of the cladding compensates the thermal expansion and thermal-optic effect of the core. Consequently, the effective refractive index of the fiber core is temperature low-sensitive, and the fiber has a temperature self-compensation effect. Due to the high photosensitivity of chalcogenide, ultra-long fiber Bragg grating can be fabricated on the flexible multi-core fiber using interference standing wave method. By investigating the sensing characteristics of the grating, algorithms compensating the fiber torsion and enhancing the demodulation resolution are proposed , so that strain sensing can be achieved with high spatial resolution under large curvature. With such a distributed sensing network based on the FBG sensor, precise three-dimensional pose reconstruction can be facilitated, which provides a new solution to shape tracking of minimally invasive surgical instruments.

光纤传感器由于尺寸小、易埋入而非常适用于三维位姿重构。对于微创手术器械的形状跟踪等领域，需传感器具有在大曲率弯折时的精确分布式大应变检测功能。常规石英光纤由于具有的高杨氏模量材料属性，致其柔韧性不足而难以应用于上述环境的三维位姿重构。因此，本项目拟设计一种具有低杨氏模量的多芯硫系光纤，以满足该应用环境中传感器所需要的高柔性。提出一种由包层与纤芯热膨胀系数差形成热诱导力来平衡纤芯热膨胀效应和热光效应的方法，可实现一种纤芯有效折射率对温度低敏感、即带有温度自补偿特性的结构光纤。基于纤芯的高光敏特性，采用干涉驻波法完成多芯超长布拉格光纤光栅的制备。通过研究该多芯布拉格光栅的传感特性，提出扭转补偿算法以及解调分辨率优化算法，构建基于该光栅的分布式传感网络，实现大曲率条件下高空间分辨率的应变检测，进而获取一种高精度的三维位姿重构方法。该方法有望在微创手术器械形状跟踪领域获取一种新的形状跟踪方法。

针对微创手术器械的设备跟踪领域，需进行在空间小的复杂环境内进行三维位姿重构，难点是在大曲率弯折应用条件下进行轴向的高空间分辨率应变分布检测，并不受温度影响。本项目通过研究高空间分辨率的多芯光纤光栅分布式传感技术，基于硫系光纤所具有的高光敏性、低杨氏模量等特性制备传感光纤，构建高精度三维位姿还原方法。通过理论分析、模型仿真和试验研究相结合的方式实现了①多芯光纤的温度应变传感机理研究。结合耦合模理论，利用有限元分析法，通过分析多芯光纤芯间模式耦合原理，构建多芯光纤的超模模型，基于多芯光纤超模干涉原理设计多芯传感光纤材料及结构；②一种纤芯有效折射率对温度低敏感、具有温度自补偿效应的多芯硫系特种光纤。在温度应变传感模型的基础上选择聚甲基丙烯酸甲酯（Polymethyl Methacrylate, PMMA）为包层、硫系光纤为纤芯组成特种光纤，利用两种材料在热膨胀系数存在数量级差异的特性，温度改变时产生热诱导力平衡热膨胀效应和热光效应，实现温度自补偿。③干涉驻波法完成多芯超长布拉格光纤光栅的制备。基于特种光纤纤芯的高光敏特性，采用中心对称且几何结构具有两个不同的对称轴的双芯光纤，通过选取较小的芯径和芯间距，实现超模干涉以及高耦合效率，诱导光纤纤芯的布拉格光栅形成。④高精度三位位姿还原算法。基于特种光纤的多芯布拉格光栅传感特性，通过检测中心波长漂移量获取光纤应变变化值，进而得到传感单元处曲率信息，根据空间曲线性质结合运动坐标系转换方法，构建分布式曲、挠率的三维形状还原算法及扭转角补偿算法。实验采用栅长3mm、间距2mm、长度423mm的密集型多芯光纤光栅阵列传感器，结合还原算法验证了曲率、弯曲方向角补偿方法的有效性和基于密集型多芯光纤光栅的三维位姿还原方法的可行性，该方法有望在微创手术器械形状跟踪领域获取一种新的形状跟踪方法。进一步的，有望应用于传感光纤植入式的碳纤维复合材料原位检测中，解决碳纤维复合材料内部无法进行固化过程及成型后结构监测的瓶颈难题。