This paper proposes optimum power allocation strategies for a cooperative communications system consisting of multiple orthogonal links with amplify-and-forward (AF) relays between a source and a destination. We consider two optimization problems; In the first problem the relays have predetermined and identical powers, and the source has limited sum-power which must be shared between the whole links. So, the problem is to find optimal transmit powers over the links to maximize the achievable rate. In the second problem, it is assumed that the relays do not have dedicated powers and eceive energy from the source. Thus, the source must share its total power to charge the relays and still have enough power to transmit information over the AF relay links to maximize the achievable rate. We analytically derive the optimal power allocations for the two scenarios and finally present several numerical results to highlight the achievable rate improvement due to optimal power allocations.