MEASURING SYSTEM SENSITIVE TO AMPLITUDE AND PHASE OF RF MAGNETIC FIELD USING LD AND OPTICAL FIBER.

A novel measuring system on the amplitude and phase of the RF magnetic field is proposed. It is composed of a laser diode (LD) linked with a small loop antenna, an optical fiber transmission line, and phase-sensitive detection instruments. The LD output light is directly intensity-modulated by the rf current induced in the loop antenna by the rf magnetic field under test. The instantaneous amplitude and phase of the rf magnetic field are preserved in the modulated light because of the quick response of LD and are detected by the remote detection system. The characteristics of the calibration, the sensitivity, the minimum detectable field, and the frequency response are analyzed. The system is highly sensitive, nonperturbing, and noninteractive. In addition, the excellent property for measuring the phase of the rf magnetic field is experimentally confirmed by field measurements. The minimum detectable rf magnetic field is obtained as 10 mu A/cm (38 nW/cm**2 in power density) remarkably lower than others.