The Tektronix ORR-24 Optical Reference Receiver is an optical/electrical converter plug-in for 11800 / CSA803 series scopes with a bandwidth from DC to 2 GHz and input wavelength range of 1100 nm to 1650 nm. The input has an FC/PC connector, the output is through an SMA connector to a sampling head in a neighboring plug-in slot.
From Tektronix' site:
The Synchronous Data Hierarchy (SDH) and the Synchronous Optical NETwork (SONET) standards are the cornerstones of today's high-speed digital telecommunications systems. The Tektronix ORR24 optical reference receiver provides an important interface for compliance testing of SDH/SONET optical transmission products or components.
The amplified design of the ORR24 converts the incoming optical light into a high gain, low noise electrical signal. This allows easy testing of optical designs as well as repeatable measurements. The ORR24 is DC coupled which is required for accurate extinction ratio measurements.