Bold summary: The Agilent 8156A is a programmable optical attenuator with a 60 dB range, 0.001 dB step resolution, low polarization sensitivity, and (with Option 101) ≈45 dB return loss for single‑mode use — it supports remote control via GPIB and offers optional monitor and back‑reflector features for production and R&D test systems. 

Core specifications (OEM specsheet values)

• Attenuation range: 60 dB (full scale). 
• Resolution: 0.001 dB programmable step size. 
• Wavelength coverage: ≈1200 nm to 1650 nm (single‑filter design).
• Attenuation accuracy: typ. ±0.05 dB (model/option and wavelength dependent — see spec tables). 
• Polarization sensitivity (PDL): <0.02 dB p‑p (low PDL design). 
• Return loss by option: Opt.100 ≈35 dB (PC straight contact); Opt.101 ≈45 dB (high‑performance straight contact); Opt.201 ≈60 dB (angled contact/APC).
• Monitor tap option: Opt.121/221 provide a ~13 dB monitor tap for inline power monitoring. 
• Interfaces and control: HP‑IB / GPIB remote control; full programming and command set in the Operating and Programming Guide. 

Functional features and behavior

• Single‑filter, no‑ranging architecture minimizes sweep artifacts (no dark spots/overshoot during attenuation sweeps). 
• Back‑reflector mode and optional reference reflector kits support reflection‑sensitive tests (requires Option 203/back‑reflector accessories). 
• Connector interfaces: the mainframe requires two (or three for monitor variants) 81000xI series connector interface modules (FC, SC, DIN, ST, APC/PC variants) to match fiber type and connector style.