  We present an observation of competition effect among multiple quantum paths
in a Raman-type Mach-Zehnder atom interferometer. By measuring the contrast of
interference fringes, the competition effect among multiple interference paths
is experimentally investigated. Due to the phase competition, the contrast
periodically oscillates when modulating either the phase or the interrogation
time between Raman pulses. The multiple quantum paths form because of the
imperfect population transfer efficiency in stimulated Raman transitions, and
are verified by modulating the duration of Raman pulses. The contrast could be
optimized by suppressing the phase competition.
