Phase damping is the main decoherence in the evolution of quantum states. In
most of the known quantum computing and quantum information processing,
devices have amplitude damping times that are around 1-2 orders of magnitude
larger than the corresponding dephasing times. For lossy optical fibre as a
quantum channel, the quantum capacity is well known. The channel is
degradable. We know that dephasing channel is also degradable, thus it can
be anticipate that the channel capacity is available. We calculate the
capacity with various methods in this paper.