Will future sensors be as thin as a strand of hair?
In fact, in the field of fiber optic sensing technology, fiber optic sensors in their bare fiber state have already reached “hair-like” physical fineness.
I. Physical Size Comparison: From Hair Strands to Fiber Optic Sensing
The average diameter of human hair is typically between 60\ \mu\text{m} and 100\ \mu\text{m} (i.e., 0.06\ \text{mm} to 0.10\ \text{mm}).
In optical engineering, the overall outer diameter of the core and cladding of standard single-mode quartz optical fiber is 125\ \mu\text{m}. To protect the fiber from scratches, a layer of polymer material is applied to the fiber surface:
- Polyacrylate-coated Fiber：The outer diameter after coating is approximately 255\ \mu\text{m} (about 0.255\ \text{mm}).
- Polyimide-coated Fiber：The coating layer is extremely thin and high-temperature resistant, with an outer diameter of only 155\ \mu\text{m} (0.155\ \text{mm}) after coating.
- Thin-diameter Optical Fiber：The outer diameter of the cladding can be reduced to 80\ \mu\text{m} or even lower, matching the physical outer diameter of thick hair strands.
Therefore, fiber Bragg grating (FBG) sensors written on bare or thin-diameter optical fibers have sensing units that are as thick as hair strands.
II. Technical Drivers for the Miniaturization Trend in Fiber Optic Sensing
The rapid development of fiber optic sensing towards hair-like miniaturization is primarily driven by the following engineering demands:
- Structural Health Monitoring and Non-destructive Implantation：Miniaturized sensors can be directly embedded within carbon fiber composites, metal 3D printed structures, or building components, enabling “in-material sensing” without compromising the mechanical precision of the base material.
- Biomedical Applications and Minimally Invasive Surgery：In the medical field, micro-fiber optic sensors are embedded in puncture needles, catheters, or 3D tactile sensing tips for precise force measurement and body temperature monitoring.
- Advanced Processing Techniques (Femtosecond Laser Point-by-Point Writing)：Traditional UV lithography requires stripping the fiber coating, making the fiber prone to breakage. In contrast, femtosecond laser point-by-point writing technology can inscribe gratings directly into the fiber core through the protective coating, preserving the extremely small size and significantly enhancing mechanical tensile strength.
III. OFSCN® Typical Miniaturized Fiber Optic Sensor Products
1. Bare/Thin-Diameter Fiber Bragg Gratings (Hair-like Scale)
- OFSCN® High-Strength Fiber Bragg Gratings / FBG Strings (Bare)：Utilizing femtosecond laser point-by-point writing technology that does not damage the fiber coating, the overall outer diameter is only 155\ \mu\text{m}. It can withstand large strains of \ge 25000\mu\varepsilon, making it ideal for embedding and miniaturized integration.
- OFSCN® Thin-Diameter Fiber Bragg Gratings / FBG Strings (Bare)：Manufactured using thin-diameter optical fibers, featuring a smaller bending radius and occupying less physical space.
2. Sub-millimeter Metal/Polymer Encapsulated Sensors
In harsh industrial environments, protective metal or polymer layers are typically added to prevent fiber damage. Even with protection, modern packaging technologies have reached sub-millimeter levels:
- OFSCN® 800°C Fiber Bragg Grating Temperature Sensor：Encased in seamless steel tubing, it provides industrial-grade mechanical protection and high-temperature resistance up to 800^\circ\text{C}, with a default outer diameter not exceeding 1.1\ \text{mm}, customizable to as small as 0.5\ \text{mm}.
- OFSCN® Polymer-encapsulated Fiber Bragg Grating Strain Sensor (0.7mm/1.2mm diameter)：Miniaturized strain sensors encapsulated in polymer materials, with default product outer diameters of only \le 0.7\ \text{mm} (with seamless steel pipe protection, the outer diameter becomes 1.2\ \text{mm}).
Summary
Future sensors will not only be as thin as hair strands; in the field of fiber optic sensing, hair-like miniaturization has already been achieved with bare fiber sensors. For industrial sensors with anti-corrosion and anti-crush housings, the encapsulated outer diameter is continuously evolving towards sub-millimeter levels of 0.5\ \text{mm} to 0.7\ \text{mm}.