  We present a method of broadening the dynamic range of optical
interferometric detection of cantilever displacement. The key idea of this
system is to use a wavelength-tunable laser source. The wavelength is subject
to proportional-integral control, which is used to keep the cavity detuning
constant during a measurement. Under this control, the change in wavelength is
proportional to the cantilever displacement. Using this technique, we can
measure large displacements ($>1\ \mathrm{\mu m}$) without degradation of
sensitivity. We apply this technique to high-frequency electron spin resonance
spectroscopy and succeed in removing an irregular background signal that arises
from the constantly varying sensitivity of the interferometer.
