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Under special environment, usually the magnetic intensity must be tested with full optical system. The purpose of the paper is to present theoretical analysis about testing magnetic parameter using light speed modulating and indicate the advantages to obtain new method of optical test. According to the light speed can be modulated in a photorefractive material in which written steady volume-index grating by two laser beams in opposite direction, so as to the light signal may transmit with phase modulating because of the intervening of another frequency shifting laser beam. In the experiment, the inner grating would be written in an optical fiber in which the suitable little Stannum has been doped. The periodic structure in the fiber can make the light signal reflect and the value of the phase shift is corresponding to the change of the periodic structure. With the quantum theory, the energy-band structure has a zero slope at the edges of the Brillouin zone and the group velocity of the photons at the band structure of a periodic structure is zero. According to the magneto-optical effect, the magnetic field may make the polarization plane change. After the two laser beams come into two ends of a fiber, the magnetic field can make their polarizations direction rotate and the interfering effect should be decreased simultaneously. During the testing period, a Sn-doped fiber is exposed to a magnetic field with linear distribution. Under this environment, the more increasing of magnetic intensity, the shorter length of effective steady grating in fiber is. Compared with light delayed signals transmitting through fiber in or not in the magnetic field, the delaying value may be correspondent to average magnetic intensity. This research work was supported by the National Natural Science Foundation of China under grant No.60472023.
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