I have a 10G XPF+ optical cable with market updates from a stock exchange. This cable goes into a switch, which then mirrors every packet to a couple of computers over two ports. The problem with using a switch for mirroring is that there is latency overhead, even with a pass-through switch (~200ns).
Are there "optical" solutions (I'm thinking of a beam splitter of some sort) which would allow for close to zero latency 10G mirroring?
No.
My old firm manufactures a port replicator that solves the problem you're having. It's a Layer 1 device that can take a market data feed and multiplex to a number of ports at <10ns latencies. This bypasses the Arista switch for one-way market data consumption.
Yes, port mirroring is possible using network taps, with latency an order of magnitude less than a using a switch. There is a video demonstration here.