In order to guarantee an optimal performance of a FBG sensor it is necessary to use interrogation techniques through which it is possible to obtain precise evaluation of the Bragg wavelength shift, correspondent to the variation of the physical parameters. the availability of measurement systems is limited by requirements like frequency, sensor multiplexibility, resolution, sample rate and ambient conditions.
The commonly used interrogation techniques are based on five different principles, and one of them is the resort to tunable laser.
This FOS concept does not resolve the sensor´s output spectrum. Therefore no filter or spectrometer is needed. Varying the center wavelength of the light source, e.g., with a tunable laser, the measured intensity at the photo detector changes.
It is maximal when the laser wavelength is equal to one of the Bragg wavelength of FBG, otherwise it is minimal. The smaller the laser bandwidth and the smaller the steps within the tuning range, the more accurately the peaks can be resolved. The measured spectrum after a single tuning cycle consists of convolutions of the theoretic sensor spectrum with the laser line spectrum.
The major advantage of this method is that ever scanned wavelength have almost the same intensity, and that the wavelength revolution is better that 1 pm.
Disadvantages of this concept are that the resolution depends mostly on the tuning step of the laser, and the availability of cheap tunable laser with a faster tuning speed is poor.
This might be different in the near future. But the main disadvantage for the LDS is that long coherence length of the Laser, what might affect interference effects on inner parts of the FOCS. In near future, perhaps it would be possible to reach a fast pulse rate, so that the coherence length of the laser is shortened artificially under a required length for interference on inner parts of the FOCS.
In the near future, perhaps it would be possible to reach a fast pulse rate, so that the coherence length of the laser is shortened artificially under a required length for interference on inner parts of the FOCS.