  High-speed 100 MHz strain monitor using fiber Bragg grating (FBG) and an
optical filter has been devised for the magnetostriction measurements under
ultrahigh magnetic fields. The longitudinal magnetostriction of LaCoO$_{3}$ has
been measured at room temperature, 115, 7 and 4.2 K up to the maximum magnetic
field of 150 T. The field-induced lattice elongations are observed, which are
attributed to the spin-state crossover from the low-spin ground state to
excited spin-states.
