  We proposed two entanglement concentration protocols (ECPs) to obtain
maximally entangled Greenberger-Horne-Zeilinger (GHZ)-type entangled coherent
state (ECS) from the corresponding partially entangled GHZ-type ECSs. We
obtained the first ECP using a partially entangled GHZ-type ECS assisted with a
superposition of single-mode coherent state, however the second ECP is designed
using two copies of partially entangled GHZ-type ECSs. The success
probabilities have also been calculated and discussed for both the ECPs. We
have further compared the success probabilities of our first ECP for 3-mode
GHZ-type ECS with an ECP of 3-mode W-type ECS and found that our ECP is more
efficient (maximal success probabilities) for larger value (\beta=0.7) of state
parameter. For the physical realization, two optical circuits (for two ECPs)
using linear optical elements, viz 50:50 beam splitter, phase shifter, and
photon detectors are provided, which support the future experimental
implementation possible with the present technology.
