A novel intrinsic multimode fiber-optic sensor for the detection or monitoring of microwave power

A graded-index silica multimode fiber with an acrylate primary coating has been shown to act as an intrinsic sensor of microwave power. When under the influence of a microwave field, the fiber absorbs a small fraction of microwave energy and this alters the propagation condition for transmitted laser light (632.8 nm). This alteration is manifested in the speckle pattern which is projected, after transmission, from the cut end of the fiber, and is recorded either by an optical detector or by visual observation. The rate of change of the speckle pattern is the important feature. The sensitivity of the effect increases as the microwave power increases, as the fiber length increases, and when the fiber is coated with conducting silver paint. Microwave powers, transmitted at 2.45 GHz in a rectangular waveguide is in the range 0.07-700 W were detected with an accuracy of better than 2 dB. These power levels correspond to electric field levels of
2.8 \times 10^{2}
V . m
^-1
and
2.8 \times 10^{4}
V . m
^-1
at the position of the fiber. The results are briefly compared with recent similar observations using a silica single-mode fiber as the sensor and an interferometer as detector. Discussions relate to use of the speckle pattern for detection and to possible applications.