I will try to link to this explanation for situations I come across in the future. It's very well argued.
For what it's worth to future readers, I went ahead and found examples of optical fiber rated for use up to 385C[0] (20km). There's research into fiber optic cables which may be able to withstand up to 1000C or more, but I couldn't find any products like that.
Maximum drilling temperatures currently top out at around 250C/500F[1].