  We demonstrated the first-order interference between coherent light at 1580
nm and 795 nm by using frequency-domain Mach-Zehnder interferometer (MZI). The
MZI is implemented by two frequency-domain BSs based on a second-order
nonlinear optical effect in a periodically-poled lithium niobate waveguide with
a strong pump light. We achieved the visibility of over 0.99 at 50% conversion
efficiencies of the BSs. Toward photonic quantum information processing,
sufficiently small background photon rate is necessary. From the measurement
results with a superconducting single photon detector, we discuss the
feasibility of the frequency-domain MZI in a quantum regime. Our estimation
shows that single photon interference with the visibility above 0.9 is feasible
with practical settings.
