Signal fidelity from fiber optic crosstalk
Does modern fiber in homes (or businesses) send enough physical signal from cross-talk to be usable in some way?


Fiber to the home (FTTH) /to the premises (FTTP) is typically a passive optical network (PON) – i.e. you and all your neighbors get every neighbor's downlink through passive splitters (if you remember the ethernet hubs: like that).

On the uplink, it's a slotted time-division multiplex scheme, so only one neighbor transmits at a time, and all others have to be silent for that time. I don't actually know how directive (i.e. only letting the user uplink through to the optical link terminal, not giving the other users much) splitters are, but I'd presume well, but not perfectly.

So, it's fair to assume that at least in the good (O)SNR case, a neighbor doesn't only get every neighbor's downlink, but at least a somewhat OK uplink of a few neighbors that are especially strong in splitter crosstalk / scattering further up.

However, don't underestimate the hardness of decoding that – you're losing a lot of information through the splitter side channel.


  That seems...not great.


Why? That's a broadcast channel by physical constraints, yes, but that's why we have cryptography. See, for example, the commonly used GPON Standard, G.984.3's Transmission convergence layer
specification, chapter 12, "Security", and especially "12.1 Basic Threat Model" on page 90 (emphasis mine):


  The basic concern in PON is that the downstream data is broadcast to all ONUs attached to the
  PON. If a malicious user were to re-programme his ONU, then the malicious user could listen to all
  the downstream data of all the users. It is this 'eavesdropping threat' that the PON security system is
  intended to counter. Other, more exotic threats are not considered practically important because, in
  order to attempt these attacks, the user would have to expend more resources than it would be
  worth.
  
  Furthermore, the PON itself has the unique property in that it is highly directional. So any ONU
  cannot observe the upstream traffic from the other ONUs on the PON. This allows privileged
  information (such as security keys) to be passed upstream in the clear. While there are threats that
  could jeopardize this situation, such as an attacker tapping the common fibres of the PON, these
  again are not considered realistic, since the attacker would have to do so in public spaces, and
  would probably impair the very PON being tapped.


All in all, from your neighbors, your downlink should cryptographically be pretty secure, your uplink pretty safe optically; as usual in security, you really need to come up with a threat model. Will someone with a 100 kilodollar device hook up to the next OLT box and try to log what you're sending? Would that someone not much more likely spend that money on getting onto your premises and bugging your meeting room? Or would that someone most likely be a secret service and simply force your provider to give them direct access to the data?

All in all: this is 2019. Internet traffic that's not end-to-end encrypted should be considered compromised, anyways. Even large providers are not immune to inserting their own ads into unencrypted http traffic, so I think you're worrying at the wrong end.

Use TLS.