Wavelength division multiplexing technique can greatly increase the bandwidth of all-optical networks without the reconstruction of physical infrastructure. Optical power splitters can reduce the number of required wavelengths for multicast requests by dividing the incoming optical signal into multiple outcoming signals with equal optical power. This paper considered the problem of how to reduce the number of splitters on a directed fiber tree and not to decrease the quality of service too much, compared against the case that each node on the tree is equipped with one splitter. The proposed splitter placing strategy showed satisfying performance in the experiments.