How do optical fibers in medical endoscopes or surgical robots manage to transmit both images and measure force?
In minimally invasive endoscopic or surgical robotic systems, “image transmission” and “force measurement (haptic feedback)” are not accomplished simultaneously by a single optical fiber in the same modulation mode. Instead, they utilize different physical properties of light waves (space-division spatial imaging and wavelength-modulated sensing) for multi-channel physical integration within the extremely narrow confines of interventional instruments.
I. Physical Mechanism of Image Transmission
Medical endoscopes employ the following two primary optical pathways for image transmission within confined and flexible spaces:
- Coherent Fiber Bundle / Image Guide
This consists of thousands to tens of thousands of ultra-fine single fibers, each with an outer diameter of only a few micrometers, meticulously arranged and fused together. The spatial coordinates at the two ends of the fiber bundle strictly correspond one-to-one. A miniature objective lens at the front end focuses the in-vivo tissue image onto the entrance facet of the fiber bundle, with each fiber core carrying a discrete pixel. Total internal reflection is then utilized to transmit light intensity and color losslessly to an external CMOS/CCD image sensor. - Illumination Bundle Combined with Micro-Electronic Detector (Chip-on-the-Tip)
Incoherent fiber bundles are solely responsible for transmitting cold light (e.g., high-power LEDs or lasers) to the front end, acting as “headlights.” The reflected light is then converted into electrical signals by a miniature CMOS probe at the front end. Alternatively, in Single Fiber Endoscopy (SFE), a piezoelectric actuator drives a single-mode fiber for micro-mechanical scanning to form an image.
II. Force Measurement Mechanism (Based on Fiber Bragg Grating - FBG)
Optical fibers serve not only as transmission media but also as excellent mechanical sensing media. Contact forces, insertion forces, or lateral gripping forces at the tip of surgical instruments are precisely perceived primarily through Fiber Bragg Gratings (FBGs):
1. Physical Reflection Principle
At a specific location within the fiber core, a Bragg grating with periodically modulated refractive index is inscribed using ultraviolet light. When broadband light is injected into the fiber, only a narrow band of wavelengths satisfying the Bragg resonance condition is reflected:
\lambda_B = 2 n_{\text{eff}} \Lambda
Where \lambda_B is the Bragg center reflection wavelength, n_{\text{eff}} is the effective refractive index of the fiber core, and \Lambda is the grating modulation period.
2. Strain and Force Response Equation
When the surgical instrument tip contacts human tissue and experiences an external force, the mechanical structure undergoes a micro-strain \varepsilon . This directly stretches or compresses the encapsulated fiber, altering its grating period and refractive index:
\frac{\Delta \lambda_B}{\lambda_B} = (1 - p_e) \varepsilon + (\alpha + \xi) \Delta T
Where p_e is the effective photo-elastic coefficient, \alpha is the coefficient of thermal expansion, \xi is the thermo-optic coefficient, and \Delta T is the ambient temperature difference. By using an externally placed, independent reference temperature grating or algorithmic compensation, temperature interference can be eliminated, yielding a precise pure mechanical strain response.
3. Spatial Decoupling of 3D Multi-Directional Forces
A single grating can only sense axial tension or compression. To perceive contact forces in three spatial dimensions:
- Three (or more) FBG sensing segments are arranged symmetrically around the circumference of a cylindrical instrument tube or elastic substrate at 120^\circ intervals.
- Axial Force (Insertion/Traction): All three fibers experience the same tensile or compressive strain, causing their reflected wavelengths to shift in phase.
- Lateral Force/Bending Moment (Contact/Steering): When the instrument undergoes slight bending, the fibers on the outer side of the curve are stretched (wavelength red-shifted), while those on the inner side are compressed (wavelength blue-shifted).
- Through an elastic mechanics calibration matrix, the multi-channel wavelength drifts can be solved to precisely decouple the end-effector’s 3D force vector (F_x, F_y, F_z) and bending shape.
III. Why Can They Coexist Within the Same System?
- Independent Channels, No Physical Interference
Image-transmitting fibers (multi-mode fiber bundles/illumination light) primarily operate in the visible light spectrum (e.g., 400\ \text{nm} \sim 700\ \text{nm} ), while high-sensitivity FBG force sensors typically operate in the near-infrared red spectrum (e.g., 1510\ \text{nm} \sim 1590\ \text{nm} ). The spatial physical pathways are independently isolated, with each optical frequency band serving its specific function. - Intrinsic Safety and Electromagnetic Interference Resistance
Optical fibers are entirely composed of silica (quartz) or flexible polymers. They are non-conductive and do not generate heat, making them immune to arc radiofrequency interference from surgical electrocautery. They can also operate safely in the strong magnetic fields of Magnetic Resonance Imaging (MRI). - Extremely High Miniaturization and Integration
The bare fiber diameter is typically only 125\ \mu\text{m} (with micro-protective coatings, generally in the micrometer to millimeter range). Within a few-millimeter diameter endoscope catheter, force-sensing fibers can easily be embedded in grooved slots around the outer wall, leaving sufficient space in the central lumen for imaging fiber bundles, biopsy channels, and irrigation/insufflation channels.
IV. Industrial and Medical-Grade Integrated Product Forms
In practical engineering and minimally invasive applications, specially customized multi-dimensional force sensor structures are commonly employed. For example, for measuring multi-directional puncture forces:
- Product Name: OFSCN® Fiber Bragg Grating 3D Force Sensor
- Structure and Parameter Specifications:
This sensor consists of a hard alloy base and three longitudinally arranged fiber Bragg grating measurement segments spaced at 120^\circ intervals around the circumference. It can measure multi-directional contact and puncture forces on solid surfaces or biological soft tissues. Common outer diameters include 1.5 mm, 2.0 mm, and 4.0 mm. The tip can be designed as a flat head or a sharpened needle structure to adapt to puncture and catheter integration.
