What is "packaging stress" for fiber optic patch cords, fiber optic sensors, and FBG sensors?

OFSCN Global Technical Center OFSCN Technical Knowledge Base
1

Will excessive sheath extrusion affect the sensing accuracy of optical fibers?

2

Yes, excessive jacketing extrusion (i.e., excessive encapsulation stress or radial/lateral stress) can severely impact the sensing accuracy of optical fibers, and may even lead to complete sensor failure or fiber damage.

In optical engineering and fiber optic sensing, excessive extrusion on optical fibers by the jacket or encapsulation structure primarily interferes with sensing accuracy through the following physical mechanisms:

1. Stress-induced Birefringence

Under no-stress conditions, the core of a single-mode optical fiber has rotational symmetry (is an isotropic medium). When the outer jacket is squeezed too tightly, especially with uneven radial force, transverse shear stress is generated within the fiber, breaking the symmetry and introducing stress-induced birefringence.

  • Impact on FBG (Fiber Bragg Grating) Sensors: Birefringence causes the originally single reflection peak of an FBG to split into two polarization-dependent reflection peaks (peak splitting phenomenon). The fiber grating demodulator will experience wavelength drift or confusion when locking the wavelength, leading to measured wavelength jumps and thus severely reducing the measurement accuracy of temperature or strain.
  • Impact on Distributed Fiber Sensing (e.g., OFDR, BOTDA): Significant changes in polarization state increase polarization-dependent noise, reducing the signal-to-noise ratio (SNR) of the demodulated signal, thereby decreasing spatial resolution and the accuracy of physical quantity measurements.

2. Microbending Loss

Excessive jacket extrusion, the presence of microscopic particles within the jacketing, or the shrinkage of the jacket material with temperature can introduce microscopic, non-uniform bends (microbends) along the fiber axis.

  • Microbends can cause guided modes in the fiber core to couple into cladding modes, resulting in significant optical power attenuation.
  • Once the optical loss becomes too large, the intensity of the signal light returning to the demodulator weakens. In weak signal conditions, the noise of the demodulation system increases, and the stability and accuracy of measurements rapidly deteriorate.

3. Hysteresis and Non-linearity of Stress Transfer

In strain or stress sensors, sensing accuracy depends on the