  We present a theoretical analysis of the properties of an unseeded optical
parametic amplifier (OPA) used as the source of entangled photons for
applications in quantum lithography. We first study the dependence of the
excitation rate of a two-photon absorber on the intensity of the light leaving
the OPA. We find that the rate depends linearly on intensity only for output
beams so weak that they contain fewer than one photon per mode. We also study
the use of an N-photon absorber for arbitrary N as the recording medium to be
used with such a light source. We find that the contrast of the interference
pattern and the sharpness of the fringe maxima tend to increase with increasing
values of N, but that the density of fringes and thus the limiting resolution
does not increase with N. We conclude that the output of an unseeded OPA
exciting an N-photon absorber provides an attractive system in which to perform
quantum lithography.
