Online Tests of an Optical Fiber Long-Period Grating Subjected to Gamma Irradiation

This paper reports the outcomes of the tests that we conducted as online measurements for the evaluation of one optical fiber long-period grating produced by a fusion technique in a single-mode radiation-hardened optical fiber, and subjected to gamma irradiation. During the irradiation, the grating temperature was monitored. Before the irradiation, the temperature sensitivity of the grating was 27.7 $\hbox{pm}/^{\circ}\hbox{C}$ , while the value of this parameter postirradiation was found to be 29.3 $\hbox{pm}/^{\circ}\hbox{C}$ . The spectral characteristics of the grating were measured (i) in the laboratory with an ANDO AQ6317C optical spectrum analyzer and (ii) online, for the first time , with a LUNA OBR 4600 backscatter reflectometer, operating in the frequency acquisition mode. Such online measurement enables the study of recovery effects during the irradiation. The wavelength dip of the grating shifted under gamma irradiation, with 16 pm/kGy for the maximum 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. Postirradiation heating of the sensor produced the reversing of the recovery effect. The investigation indicated that, up to 45 kGy, the grating is more sensitive to radiation than other optical fiber sensors.