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.
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