Can a signal that is too strong blind the demodulator?
Yes, in the field of fiber optic sensing and photoelectric detection, excessive signal strength can indeed “blind” the demodulator. This phenomenon is known in academia and engineering as “Optical Power Saturation” or “Photodetector Overload”.
Here is a rigorous academic explanation from three perspectives: physical mechanism, resulting consequences, and engineering countermeasures:
I. Physical Mechanism of Optical Power Saturation
The core detection component of a Fiber Bragg Grating (FBG) demodulator is the Photodetector (typically a PIN or APD photodiode). Its fundamental physical working principle is:
Incident photons are absorbed within the semiconductor material, generating electron-hole pairs. Under the influence of a bias voltage, these pairs drift directionally, forming a photocurrent, thereby achieving “light-to-electricity” conversion.
Under normal operating conditions, the photocurrent has a strictly linear relationship with the incident optical power. However, this linear relationship has a physical limit, the upper bound of which is determined by the photodetector’s Saturation Power. When the incident optical power is excessively high:
- Carrier Screening Effect: High-density photogenerated carriers, during their drift, create an internal self-built electric field that opposes the external reverse bias voltage, thereby weakening the effective clearing field. This leads to increased carrier recombination, and the photocurrent no longer increases linearly with optical power, resulting in saturation.
- External Circuit Saturation: The transimpedance amplifier (TIA) and the analog-to-digital converter (ADC) at the detector’s backend also have full-scale limits for their operating voltages (e.g., 3.3\text{ V} or 5\text{ V} ). Once the current exceeds the range, the operational amplifier’s output will directly “clip” and flatten, failing to reflect the true light intensity.