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Optical Layer Monitoring
The light being monitored is first tapped off the main transmission line and fed to the hermetically sealed scanning device. While inside, it is first passed through an optical circulator that directs the light towards a broadband Fiber Bragg Grating reflecting only the optical spectrum of the communication channels (e.g. the C‐band) to be analyzed. This pass‐band filter thus rejects (transmits) any stray light and channels outside the communication band present on the transmission line. The reflected and filtered light, once again, passes through the circulator, this time directing the light towards the scanning filter.
The scanning filter is comprised of two main building blocks
- An acoustic‐optic actuator device
- A chirped Fiber Bragg Grating
An electronic signal generator drives the acoustic actuator that launches a short, longitudinal acoustic pulse into the core of the fiber. The pulse traverses the fiber at a great speed and passes over a long, chirped Fiber Bragg Grating. The chirped grating is characterized by being reflective for a certain wavelength at a well‐defined position along the grating.
As the acoustic pulse passes over the grating, a local and superimposed disturbance of the core’s refractive index is established. By generating a disturbance of well‐known character, a narrow transmission window with high sidemode suppression is created to allow a certain wavelength to pass through the grating. As the pulse moves over the chirped grating, the center wavelength of the transmission window is shifted and a scan is performed.