Why does signal strength change when the fiber is rotated?
Rotating optical fiber causes changes in signal strength (optical power), a very classic physical phenomenon in fiber optics and fiber testing. The core mechanisms behind this mainly involve three steps: Introduction of stress-induced birefringence, random drift of light polarization state, and response to polarization-dependent loss (PDL) of optical path components.
Cause Analysis: Why Does Light Intensity Fluctuate?
1. Generation of Stress-Induced Birefringence
In an ideal state, the core of a single-mode optical fiber has a perfect cylindrical symmetric structure. When light propagates through it, light of any orthogonal polarization direction has the same refractive index.
However, in practical operations, when the fiber is manually rotated, bent, or twisted, asymmetric mechanical stress is generated within the fiber. This asymmetric physical compression destroys the isotropy of the fiber, thus producing stress-induced birefringence. This means that the refractive indices (n_x and n_y) along the two orthogonal polarization axes of the fiber (commonly known as the fast and slow axes) become unequal.
2. Random Drift of Polarization State (SOP)
When light propagates in a birefringent fiber, orthogonal polarization components accumulate different phase differences (phase delay).
When you rotate the fiber, the internal stress field distribution changes continuously, and the axial and phase delay of birefringence also change drastically. This causes the polarization state (SOP) of the light output from the end of the fiber to drift and rotate randomly and continuously.
3. Polarization-Dependent Loss (PDL) Converts It to Light Intensity Fluctuations
It is important to emphasize that merely changing the polarization state of light does not cause loss of optical energy (total light intensity).
The reason for signal strength variation is the presence of polarization-sensitive devices in the measurement optical path, i.e., polarization-dependent loss (PDL).
Many passive devices in an optical fiber link (such as fiber couplers, isolators, circulators, fiber gratings, wavelength division multiplexers, and even the surface chips of photoelectric detectors at the receiving end) have different transmittance or reflectance for light of different polarization directions. The mathematical definition of polarization-dependent loss is:
\text{PDL} = 10 \log_{10} \left( \frac{P_{\max}}{P_{\min}} \right)
Where P_{\max} and P_{\min} represent the maximum and minimum optical power output by the device when the input light polarization state rotates within 360^\circ.
When rotating the fiber causes the output polarization state to change, the projection components of the polarization direction onto the subsequent polarization-sensitive devices change accordingly, leading to fluctuations in the transmitted light intensity. This manifests as the signal strength varying significantly on the optical power meter.
How to Avoid Such Light Intensity Variations in Engineering or Experiments?
To eliminate or reduce light intensity instability caused by external physical disturbances (such as rotating or bending the fiber), the industry typically employs the following technical solutions:
1. Use Polarization-Maintaining Fiber (PM Fiber)
If the system has extremely high requirements for polarization stability and light intensity stability (e.g., high-precision coherent fiber sensing, high-precision fiber Bragg grating demodulation, fiber optic gyroscopes, etc.), ordinary single-mode fiber can be replaced with PM fiber.
PM fiber introduces a strong built-in stress region (such as a panda stress region), causing the fiber to have very large and constant birefringence. This strong “phase-locking” capability can overcome the stress drift caused by small external physical disturbances, thereby locking and maintaining the polarization state of light during transmission.
For PM requirements in extreme temperature and harsh environments, OFSCN® offers the following product support:
- Matching Product: OFSCN® 300℃ Polyimide Panda-type PM Optical Fiber utilizes a high-precision panda stress structure design, which can maintain an excellent polarization extinction ratio under severe temperature fluctuations and mechanical disturbances. The coating layer uses Polyimide material, supporting extreme temperature environments from -200\ ^\circ\text{C} to 350\ ^\circ\text{C} (or extremely -270\ ^\circ\text{C} to 350\ ^\circ\text{C} ), making it an ideal choice for ensuring polarization state and light intensity stability under extreme operating conditions.
2. Select Optical Devices with Low PDL
When setting up an optical path system, try to select passive devices with extremely low nominal polarization-dependent loss (e.g., couplers and isolators with a specification of \text{PDL} u 0.1\ \text{dB} or even lower) to reduce sensitivity to polarization state fluctuations.
3. Secure Fiber Layout
In daily testing and experiments, fiber patch cords or loose tubes should be fixed to stable laboratory benches using clamps as much as possible to prevent physical displacement and rotation of the fiber caused by wind, accidental touching, or equipment vibration.