  We experimentally demonstrate that both a converted and an unconverted light
pulses after wavelength conversion with various conversion efficiencies
preserve phase information of an input light at a single-photon level. In our
experiment, we converted temporally-separated two coherent light pulses with
average photon numbers of $\sim$ 0.1 at 780 nm to light pulses at 1522 nm by
using difference-frequency generation in a periodically-poled lithium niobate
waveguide. We observed a single-photon interference between
temporally-separated two modes for both the converted and the unconverted light
pulses at various values of the conversion efficiency. We observed interference
visibilities greater than 0.88 without suppressing the background noises for
any value of the conversion efficiency the wavelength converter achieves. At a
conversion efficiency of $\sim$ 0.5, the observed visibilities are 0.98 for the
unconverted light and 0.99 for the converted light.
