Optical power meter

Optical power meters can measure very low powers like -50 dBm and -70 dBm, but require highly sensitive sensors and careful calibration.

Understanding Optical Power Levels

Optical power is often expressed in dBm, a logarithmic unit relative to 1 milliwatt. A reading of -50 dBm corresponds to 10 picowatts, and -70 dBm corresponds to 0. 1 picowatts.

These are extremely low power levels, typically encountered in long-haul fiber optic systems, low-power laser testing, or sensitive photonic experiments.

Sensor Requirements

To measure such low powers accurately, you need high-sensitivity photodiode sensors or low-noise detectors. Common sensor types include: InGaAs photodiodes: Suitable for near-infrared wavelengths (900–1700 nm) with high sensitivity and low noise, ideal for fiber optic communications. Silicon photodiodes: Used for visible wavelengths but less sensitive in the infrared range.

Thermopile sensors: Typically for higher power levels; not suitable for -50 dBm or -70 dBm. Low-power sensors often have femtowatt to picowatt resolution, enabling detection of signals in the -50 to -70 dBm range.

Meter Types

Benchtop meters: Provide the highest sensitivity and stability, often with multiple channels and advanced calibration options. They are ideal for laboratory or production testing where low-power measurement is critical. Handheld meters: Portable and convenient, but their sensitivity may be limited.

Some high-end handheld meters can still measure down to -70 dBm with appropriate sensors.

Calibration and Wavelength Considerations

Optical power meters are wavelength-dependent, meaning the sensor must be calibrated for the specific wavelength of the light source. Using the wrong wavelength setting can result in significant measurement errors, especially at very low powers. NIST-traceable calibration ensures accuracy and repeatability.

Practical Tips

Minimize ambient light and reflections to avoid noise interference. Use short, clean fiber connections to reduce insertion loss. Ensure the meter and sensor are thermally stable, as low-power measurements are sensitive to temperature fluctuations.

For dynamic measurements, choose meters with high sampling rates and low latency to capture transient signals.

Summary

Measuring -50 dBm and -70 dBm optical power is feasible with modern optical power meters, provided you use high-sensitivity sensors, proper wavelength calibration, and maintain a low-noise environment. Benchtop meters with InGaAs sensors are generally preferred for these ultra-low power levels, while some advanced handheld meters can also achieve these measurements in field applications.