  A conceptual design work and a basic experimental study of a new beam profile
monitor have been performed. The monitor will be used to measure emittance of
an electron beam in the ATF damping ring at KEK, in which the transverse beam
size of about 10 micron is expected. It utilizes a CW laser and an optical
cavity, instead of a material wire, to minimize interference with an electron
beam. A laser beam with a very thin waist is realized by employing the cavity
of nearly concentric mirror configuration while the intensity is amplified by
adjusting the cavity length to a Fabry-Perot resonance condition. We built a
test cavity to establish a method to measure important parameters such as a
laser beam waist and a power enhancement factor. Three independent methods were
examined for the measurement of the beam waist. It was found that the cavity
realized the beam waist of 20 micron with the power enhancement factor of 50.
