During some special processing or under the special environment, usually the magnetic intensity must be tested with full
optics system. According to the light speed can be modulated in a photorefractive material in which written
volume-index grating by two laser beams in opposite direction, so as to the light signal may transmit with phase
modulating due to 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 measured
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 effective interference area is in the fiber. Comparing
with the light delayed signal that transmits through fiber that in or not in the magnetic field, the delaying value may be
corresponded to the average magnetic intensity. The output signal light could be processed by various smart way. This
research work was supported by the National Natural Science Foundation of China under grant No.60472023.