Sony looks to be making steady progress with its attempts at holographic recording, recently announcing that it's managed to bump up its previous single-layer Micro-Reflector recording technique to a full four-layers. As with the previous system, Sony used a standard blue-violet semiconductor laser diode to write on the 250 μm-thick photopolymer recording layer, with nothing more than a few optical parts added to change the focal point depth in order to write on multiple layers. They've apparently still got quite a bit more work to do, however, as they've reportedly discovered that the reproduction signal gets significantly weaker by the time it reaches the fourth layer (about half that of the first layer), with the data transfer rate also suffering as a result. Sony seems confident that it'll be able to overcome that problem soon enough though, even going so far as to boast that it'll one day have a 500GB disc made up of twenty layers packing 25GB apiece.