The simple answer is that there isn't one, but what exactly are you trying
You would need to connect some form of A/D Converter in the field to convert
the 4-20mA signal to a digital signal and then send this across the fibre.
This is usually in the form of a Remote I/O module or RTU connected over the
fibre to your control system.
I had a quick look at the link below. Looks to me that the black box
described is used to convert electrical serial data (digital) to and from
fiber for long runs. Apart from good old rs-232, some ancient systems used a
five bit code sent over a current loop (not 4 - 20)
RJB was looking to send an analogue 4 - 20 mA signal over fiber.
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