Detection of gamma irradiation with milligray resolution using a slow-light fiber Bragg grating

For many medical and safety applications, it is important to develop fiber sensors that can detect very low doses of gamma radiation (mGy) with integration times of 1 s or shorter. Here, we describe a sensor based on a new calorimetric technique that we believe is one of the most sensitive and compact reported to date. The fiber subjected to irradiation has a silica core doped with Ce-doped lutetium aluminum garnet nanocrystals selected to achieve a strong radiation-induced absorption (RIA). Light launched in the irradiated fiber is absorbed by RIA, the fiber heats up, and the temperature change is measured with a slow-light fiber Bragg grating (FBG) placed in physical contact with it. Thanks to the doped fiber's large RIA, and the excellent resolution (mK/√Hz) and low drift (a few mK/min) of the slow-light sensor, with 1.2 W of excitation power at 1040 nm, this sensor has a very low detection limit of ∼6 mGy/√Hz. Thanks to the use of a short FBG (7 mm), it is also extremely small. With straightforward improvements, the detection limit can be reduced to sub-mGy/√Hz. For in situ measurements, this technique can also be easily extended to use the slow-light FBG itself as the radiation sensor.