The use of Draw Tower Gratings in Multi Core Fibers (MCF-DTG^®s) offers a clean and elegant way of tracking shape along the length of an optical fiber with a high spatial resolution. Not only the size of the curvature can be deduced but also its direction, resulting in a reconstruction of the fiber shape in 3D. In combination with the small size, the high sensitivity and the MRI compatibility of the optical fiber, this so-called ‘shape sensing’ ability opens a whole new area of medical and industrial applications. The emergence of the MCF-DTG^® for shape sensing is therefore a paradigm shift and will for many applications be a preferred solution over conventional methods. The technology will enable totally new applications in the field of robotic and non-robotic minimal invasive surgery such as position tracking, instrument and catheter navigation, force and bending detection and deformation monitoring.

Draw-Tower Gratings in Multicore Fiber: A Paradigm Shift in Curvature Sensing

Fiber-optic shape sensing has great potential for a number of medical and industrial applications because alternative solutions are practically nonexistent, for similar small sizes. FBGS has shown the feasibility of performing shape sensing using a newly developed multicore optical fiber (MCF) inscribed with Draw Tower Gratings (DTG^®). The resulting MCF-DTG^® exhibits the advantageous properties of a standard single-core DTG like high mechanical robustness and higher sensor density, including the option for system designers and integrators to take advantage of the densely configured arrays of DTGs. The emergence of the MCF-DTG is destined to present a paradigm shift in curvature, shape, and deflection sensing and will for many applications be the preferred solution over conventional camera or xray methods.

Multicore Fiber DTGs
MCFs are specially configured optical fibers with multiple single-mode cores sharing the same cladding. The cores can all be addressed individually. The DTG fabrication process has successfully been adopted for writing fiber Bragg gratings (FBGs) into the MCFs. Hence, one can simultaneously produce DTGs of specific configurations up to 7 cores at the same exact axial location and with the same wavelength. This precision in the inscription of these high density DTGs in the MCF represents a major milestone.

Signal Interrogation Schemes
Two well-established detection schemes — wavelength division multiplexing (WDM) and optical frequency domain reflectometry (OFDR) — are well suited for monitoring the wavelength shift associated with the induced strains of the MCF-DTG sensors. The signals from the individual cores are routed via a specially configured fiber optic fan-out device to separate optical fibers, which can be easily connected to the different channels on the interrogator. These techniques are designed to provide detection speeds up to 100 Hz for OFDR and several kilohertz for WDM.

Test Data
The WDM technique can identify the exact locations of the DTGs along the length of the optical fiber through the uniquely defined Bragg wavelength for each grating position. The measured data closely represents the actual curvatures of the fiber along it’s lenght. With these values the shape and the fiber position can be calculate using advanced in house developed algorithmus. With the right fiber and system design it’s possible to make a precise prediction of the fiber shape.

Related Design Issues
Depending on the application requirements, sensors must meet certain physical properties to fit and navigate their landscape. Current MCF-DTGs with a cladding diameter equal or smaller than 125 μm present a favorable design feature. In addition to its good sensitivity to curvature, it also withstands the stress of tight curvatures (R > 3 mm). The customization flexibility of the MCF-DTG configuration (number, size, spacing, and layout of cores) and parameters (number, wavelength, length, density, and spacing of FBGs) provides a powerful design feature for optimizing system performance.

A growing interest from the healthcare sector is the development of a more reliable and innovative interventional (diagnostic and therapeutic) tools that can provide better efficacy at a lower cost. This trend is driving the medtech industry to pursue novel and smarter sensors that can be integrated within the minimally invasive gear. The MCF-DTG are well suited for this purpose because of their small size. Their size makes them ideal for integration within introducers, catheters, needles, laparoscopes, endoscopes, and robotic arms with rigid or flexible shafts, especially when disposability or limited reusability is part of the utility projections. These devices are common in applications in which the MCF-DTG sensors can aid the radio-frequency, microwave, laser, or cryogenic ablation procedures. They are also a great option for navigating biopsy and brachytherapy needles under MRI imaging, curvature sensing of endoscopic devices, and delivering medicinal compounds to delicate and confined spaces in vivo, etc.