HP 81521B Opt 001 Ge Optical Head
The HP 81521B opt001 is an optical head that connects to the 81533A or 81533B which is a plug-in module that fits the 8153A and 8163 and 8166 Agilent/HP series mainframe. It also connects to the 8152A Optical Average Power meter. That model was specifically introduced to work in conjunction with the 8169A and the E5574A. The 81521B-001 optical head is made of a 5mm Ge (germanium) sensor element and covers the range of 900-1700nm wavelengths in a range of +3 to -64dBm. Various connector interfaces are available for that head.
Click the PDF Specification for further information.