Agilent/Keysight 81520A – Optical Head

The Agilent/Keysight 81520A Optical Head is a silicon PIN photodiode sensor for precision optical power measurement across the visible and near-infrared spectrum. It integrates with Agilent/Keysight mainframe units (8153A, 8163, 8166 series) via interface modules (81533A/B) and the 8152A Optical Average Power Meter. The detector operates in photoconductive mode with a 5 mm sensor element, delivering 0.001 dB resolution and linearity of ±0.04 dB across its measurement range.

A one-stage Peltier cooler with NTC temperature sensing maintains stable operation at 20°C across the 0–55°C operating envelope.

Technical Specifications

• Detector: Silicon PIN diode, photoconductive mode


• Wavelength Range: 450 nm to 1020 nm


• Power Measurement Range: +10 dBm to −100 dBm


• Resolution: 0.001 dB


• Linearity: ±0.04 dB (+3 dBm to −80 dBm)


• Noise Floor: <0.5 pW (700–900 nm); well below −90 dBm


• Absolute Accuracy: ±2.2% (reference conditions)


• Total Uncertainty: ±4% ±50 pW (600–1020 nm)


• Wavelength Uncertainty: ±2.2% (600–1020 nm)


• Sensor Diameter: 5 mm


• Operating Temperature: 0–40°C (non-condensing)


• Temperature Stability: 0–55°C


• Analog Output: 0–2 V proportional to optical signal


• Physical Dimensions: 37.5 mm diameter × 140 mm length


• Weight: 0.45 kg

Key Features

• Hermetically sealed TO-8 housing protects detector from contamination and humidity


• Individual spectral responsivity calibration in 10 nm increments (450–1020 nm); values stored in head memory


• Accepts standard single-mode and multi-mode fibers up to 100 µm core, NA < 0.3


• Interchangeable optical connectors via 81000xI interface adapters (e.g., 81000FA for FC/PC)


• BNC analog output for direct signal monitoring

Typical Applications

• Optical communication system characterization


• Fiber optic power measurement


• Spectral power distribution assessment


• Telecommunications and test laboratory environments

Compatibility & Integration

• Mainframes: 8153A, 8163, 8166 series


• Interface Modules: 81533A, 81533B


• Power Meters: 8152A Optical Average Power Meter


• Fiber Connectors: Multiple adapter options available