How to read the power of an optical power meter

Optical power meters measure light strength in fiber optics, displaying absolute power in dBm or relative loss in dB.

Units of Measurement

• dBm (decibels relative to 1 milliwatt): This is an absolute measurement of optical power. A reading in dBm indicates the actual power level of light exiting a fiber or source. Positive dBm values indicate power above 1 mW, while negative values indicate power below 1 mW. Typical laser outputs range from 0 to –10 dBm, and LEDs from –10 to –20 dBm, with high-power lasers reaching up to +20 dBm (100 mW) in specialized systems .
• dB (decibels): This is a relative measurement, often used to quantify loss. It compares the power before and after a connection, splice, or fiber segment. A negative dB value indicates a reduction in power, while a positive value indicates gain relative to the reference .

How Optical Power Meters Work

Optical power meters use a photodiode sensor (commonly InGaAs for telecom wavelengths or germanium for 850 nm multimode work) to convert incoming light into an electrical current. The meter then applies a calibration curve to display the optical power in dBm. The sensor typically detects light in the range of 1 nW to 1 mW, and the host unit amplifies and processes the signal for display .

Proper Measurement Procedure

1. Warm up the meter: Allow the device to stabilize for a minute or two before taking readings .
2. Set the correct wavelength: Match the meter to the source wavelength to ensure accurate readings .
3. Clean connectors: Dirt or dust on fiber end-faces can introduce significant errors, sometimes adding 1–5 dB of apparent loss .
4. Connect the fiber: Use a reference patchcord if measuring source power, or disconnect the cable from the receiver if measuring at the far end .
5. Read the display: The meter will show the optical power in dBm or relative loss in dB, depending on the mode .

Tips for Accurate Readings

• Calibration: Ensure the meter is calibrated according to the manufacturer's schedule to maintain accuracy .
• Reference setting: For loss measurements, set a zero reference before testing and check it periodically .
• Connector cleanliness: Always clean connectors before each measurement to avoid false loss readings .
• Understand ranges: Multimode fibers typically measure 0–30 dB loss, while singlemode fibers can range up to 50 dB for long-haul systems . By following these procedures and understanding the difference between dBm and dB, you can accurately interpret optical power meter readings and assess the performance of fiber optic networks.