Agilent/Keysight 81521B – Optical Head

The Agilent/Keysight 81521B Optical Head is a germanium-detector sensor module for precision optical power measurement in fiber optic systems. It connects to Agilent 8152A, 8153A, and 8163/8166 series mainframes via interface modules, delivering accurate measurements across the near-infrared spectrum. The 81521B serves telecommunications, fiber optics, and research applications requiring stable, repeatable power characterization.

Technical Specifications

• Detector Type: Germanium (Ge)


• Detector Element Size: 5 mm diameter


• Wavelength Range: 900 nm to 1700 nm


• Power Measurement Range: +3 dBm to −80 dBm


• Wavelength Accuracy: ±2.2% (1000 nm to 1650 nm)


• Linearity: ±0.04 dB (−60 dBm to 0 dBm)


• Fiber Repeatability: <0.02 dB with 81000BA/CA bare-fiber adapters


• Spot Diameter: 3 mm parallel-beam measurement capability


• Net Weight: 0.46 kg (1.1 lbs)

Key Features

• Supports standard single-mode and multimode fibers to 100 µm core diameter (NA < 0.3)


• Non-volatile memory for wavelength storage; enables custom wavelength entry beyond 1300 nm baseline


• Compatible with fiber pigtails via bare-fiber adapters and multiple connector interface types


• Compact detector geometry optimized for coupling efficiency in both guided-wave and free-space configurations

Typical Applications

• Optical power budgeting and loss measurements in long-haul and metro fiber networks


• Detector characterization and responsivity verification across the extended infrared band


• In-service power monitoring using standard telecom wavelengths and CWDM/DWDM bands

Compatibility & Integration

• Mainframes: Agilent 8152A, 8153A, 8163 and 8166 series


• Interface Modules: 81533A, 81533B, 81520A


• Adapters: 81000BA/CA (bare-fiber), 81000FA/SA/VA (connectors), 81000WA (Bicon), 81010BL (multimode lens), 81230FL (attenuating lens), 81001FF (10 dB filter) Note: This product is discontinued; limited support may be available through Keysight Service Centers and third-party providers.