On-line tests of an optical fiber long-period grating subjected to gamma irradiation

The paper reports the outcomes of the tests we conducted as on-line measurements for the evaluation of an optical fiber long period grating (LPG) produced, by fusion technique, in F-doped radiation hardened optical fiber subjected to gamma irradiation. The dosimetry was performed with an Alanine-EPR dosimetry system and the maximum total irradiation dose reached was (45 +/- 0.8)kGy. Before the irradiation the temperature sensitivity of the LPG was 27.8 pm/°C, while the value of this parameter post irradiation was found to be 29.3 pm/°C. The spectral characteristics of the grating were measured (i) in the laboratory with an ANDO AQ6317C optical spectrum analyzer and (ii) on-line, for the first time, with a LUNA OBR 4600 backscatter reflectometer, operating in the frequency acquisition mode. Such on-line measurement enables the study of recovery effects during the irradiation. The central wavelength of the LPG shifted under gamma irradiation by 16 pm/kGy (for a total dose of 45 kGy). At room temperature the recovery of the irradiation induced shift of the wavelength dip was almost complete in about 120 h, at a rate of 6.7 pm/h. Post irradiation heating of the sensor produced the reversing of the recovery effect.