Fiber optic light is primarily measured by its optical power, which is quantified using decibel-milliwatts (dBm).
Understanding dBm
- dBm Definition: dBm is a unit used to express the absolute power of a light signal. The "m" in dBm refers to a reference power of 1 milliwatt (mW).
- Reference Point: 0 dBm corresponds to exactly 1 mW of power.
- Logarithmic Scale: The dBm scale is logarithmic, meaning that equal steps in dBm don't represent equal steps in power. For example, a change of 3 dBm roughly doubles or halves the power.
- Calculating Power:
- A source with a power level of 0 dBm has a power of 1 milliwatt.
- A power level of -10 dBm indicates 0.1 milliwatts.
- A power level of +10 dBm indicates 10 milliwatts.
Why Use dBm?
The dBm scale is preferred in fiber optics because:
- Wide Range: It allows engineers to handle very large ranges of power levels, both very small and very large, using a manageable numerical scale.
- Loss Calculation: It simplifies the calculation of signal loss in fiber optic systems.
- Standardized Unit: dBm is a widely recognized standard in the telecommunications industry.
How is dBm Measured?
Optical power is measured using an optical power meter, which converts the incident light into an electrical signal, then calibrates and displays it in dBm:
- Photodiode: The light signal enters the power meter and falls upon a photodiode, which converts light into electrical current.
- Calibration: The current is then converted into a power level using a calibration curve specific to the power meter's photodiode.
- Display: Finally, the power is displayed in dBm.
Example of dBm Values
| dBm | Power (mW) |
|---|---|
| +3 dBm | 2 mW |
| 0 dBm | 1 mW |
| -3 dBm | 0.5 mW |
| -10 dBm | 0.1 mW |
| -20 dBm | 0.01 mW |
Other Optical Measurements
While dBm is the primary unit for power, other crucial measurements in fiber optics include:
- Wavelength: The color of the light, usually measured in nanometers (nm).
- Optical Loss: The reduction in signal power as light travels through the fiber, measured in decibels (dB).
- Dispersion: The spreading of light pulses as they travel through the fiber, which can limit data rates.
