Your question is OSA vs ESA with Optical Head. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
Describe the difference between an Optical Spectrum Analyzer (OSA) and an Electrical Spectrum Analyzer (ESA) with an optical head, and outline the relevant block diagrams.