Why does excessive reflected light interfere with the normal operation of an interrogator (demodulator)?
Excessive reflected light, or high optical return loss, significantly degrades the performance of an FBG interrogator (demodulator) due to the following technical reasons:
1. Signal-to-Noise Ratio (SNR) Degradation
An FBG interrogator works by detecting the specific wavelength reflected from an FBG sensor. If there is excessive parasitic reflection from connectors or damaged fiber ends (Fresnel reflections), this “noise” light overlaps with the sensor’s signal. This reduces the Signal-to-Noise Ratio, making it difficult for the interrogator to accurately identify the peak of the FBG reflection, leading to jitter or measurement errors.
2. Saturation of the Optical Receiver
Interrogators use highly sensitive photodetectors (such as InGaAs linear image sensors). If the returned light power is too high due to strong reflections, the detector may reach its saturation limit. Once saturated, the interrogator cannot distinguish the spectral characteristics of the FBG, resulting in a “flat” or “clamped” signal and a total loss of measurement data.
3. Interference and Laser Instability
Many interrogators use a Tunable Laser Source (TLS). While these lasers are often protected by internal isolators, extreme levels of reflected light can still cause “feedback” into the laser cavity. This feedback induces phase noise, wavelength instability, and power fluctuations in the laser, which directly impacts the precision of the wavelength scanning process.
4. Phantom Peaks (Ghost Signals)
In systems with multiple FBGs, high reflections can cause multiple-path interference. The light may bounce between a connector and an FBG multiple times, creating “phantom peaks” or “ghost signals” in the spectrum. These artifacts can be mistaken by the demodulation algorithm as real sensors, leading to incorrect data mapping.
DCYS (OFSCN) Solution:
To minimize these issues, we recommend using high-quality APC (Angled Physical Contact) connectors. For instance, our OFSCN® Fiber Bragg Grating (FBG) Interrogators are designed with high dynamic range and internal isolation to mitigate standard reflections, but maintaining a high Return Loss (RL) in your fiber network is always critical for precision.
Standard performance of a high-quality connector should look like this:
For more technical details on how to optimize your sensing network, you can refer to:
OFSCN® Fiber Bragg Grating Sensing Technology