  We present modeling and design of singly-resonant optical parametric
oscillator (SR-OPO) with an intracavity idler absorber to enhance the
conversion efficiency for the signal, by suppressing the back conversion of the
signal and idler to the pump. Following plane wave analysis, we arrive at the
optimum parameters of the OPO to achieve high conversion efficiency for the
signal. For a given pump intensity, we have analyzed the effect of position and
number of absorbers required for optimum performance of the device. The model
is also extended to the case in which the signal is absorbed, yielding higher
conversion efficiency for the idler (in mid-IR region). The magnitude of
absorption and the effect of inter-crystal phase shift on the conversion
efficiency are also discussed. We also present an analytical solution for
twin-crystal SR-OPO with an absorber in between; taking into account the
variation of signal amplitude inside the cavity, we re-affirm that the often
used 'constant signal-wave approximation' is valid if the reflectivity of the
output coupler is high for the signal.
