Weak Bragg Gratings Research Articles

Strength properties of femtosecond-induced defects and weak Bragg gratings for distributed optical fiber sensors

One of the possible ways to increasing the sensitivity of distributed fiber sensors is to create point defects or weak Bragg gratings in a sensitive optical fiber serving as artificial reflectors. Such reflectors can be created by means of femtosecond laser radiation. However, after the inscribing such structures, a noticeable decrease in the mechanical reliability of the sensor fiber can be observed, which can be critical for the fabrication and operation of sensor cables. The paper reports the results of a study of the point defects and weak fiber Bragg gratings strength characteristics created for distributed sensor systems, depending on the parameters of the laser radiation used to inscribe them.

Interrogation of weak Bragg grating sensors based on dual-wavelength differential detection.

It is shown that for weak Bragg gratings the logarithmic ratio of reflected intensities at any two wavelengths within the spectrum follows a linear relationship with the Bragg wavelength shift, with a slope proportional to their wavelength spacing. This finding is exploited to develop a flexible, efficient, and cheap interrogation solution of weak fiber Bragg grating (FBGs), especially ultra-short FBGs, in distributed sensing based on dual-wavelength differential detection. The concept is experimentally studied in both single and distributed sensing systems with ultra-short FBG sensors. The work may form the basis of new and promising FBG interrogation techniques based on detecting discrete rather than continuous spectra.

Vacuum-ultraviolet laser-induced refractive-index change and birefringence in standard optical fibers

Strong photosensitivity responses with the use of above-bandgap 157-nm F/sub 2/-laser radiation are reported for standard germanosilicate fiber. A large 1.3/spl times/10/sup -3/ effective index change at 1.55 /spl mu/m was inferred by trimming strong and weak Bragg gratings in hydrogen-free fiber. The F/sub 2/-laser fluence-processing window of 0.01 across a small 1-/spl mu/m penetration depth at the fiber core. The index asymmetry appears to underlie the large >5/spl times/10/sup -5/ value of laser-induced birefringence.