  A high-speed 100 MHz strain monitor using a fiber Bragg grating, an optical
filter, and a mode-locked optical fiber laser has been devised, which has a
resolution of $\Delta L/L\sim10^{-4}$. The strain monitor is sufficiently fast
and robust for the magnetostriction measurements of magnetic materials under
ultrahigh magnetic fields generated with destructive pulse magnets, where the
sweep rate is in the range of 10-100 T/$\mu$s. As a working example, the
magnetostriction of LaCoO$_{3}$ was measured at room temperature, 115 K, and
7$\sim$4.2 K up to a maximum magnetic field of 150 T. The smooth $B^{2}$
dependence and the first-order transition were observed at 115 K and 7$\sim$4.2
K, respectively, reflecting the field-induced spin-state evolution.
