This paper describes the use of optics to enable collective communications between clusters of processors, within a `loosely synchronous' model of computation. In this model, phases of internal computation inside processors, alternate with phases of communication among processors. Collective communication, as in the new message passing interface (MPI) standard, means concerted action in a single command, e.g. shift of data from each processor to its right neighbor. Unlike previous work on general purpose optical implementation of given interconnects topology, that was concerned with providing only point-to-point communication, our work tries to provide services that improve programmability by the end- user. We are implementing a prototype with 3 clusters of 4 processors each, with an intra- cluster electronic crossbar. Inter-cluster communication is done via lasers transmitting message beams, which are suitably deflected in free-space to other clusters by means of acousto-optic devices. A typical collective communication comand is (multicast(source,Targetlist)), where the individual processors of the TargetList may reside in different clusters. Cluster addresses are encoded as acoustic wave input. Processor addresses are transmitted in the optical message headers, decoded at the detectors and redirected within the crossbar. The prototype itself and preliminary experiments are discussed.