The Tektronix 11F10 is an optical-input plug-in introduced in 1989 for 11000-series scopes.
The 11F10 has DC to 1 GHz bandwidth and appeared only in the 1989 catalog, without price indication. Like the 7F10A, which was also announced in the 1989 catalog, it was not released to the market, most likely because external O/E converters like P6701 or SD-46 provided more flexibility.
The 11F10 is specified for the 700 nm to 1550 nm range of optical wavelengths. It uses a germanium avalanche photodiode as the O/E transducer.