Practically every aspect of fiber optics, especially network design, testing and troubleshooting, depends on understanding optical power. Sometimes, something so basic gets overlooked, so let’s test your understanding of it. Correct answers and explanations are in red.

1. Optical power is the equivalent to ______ in electrical systems.
A. Voltage
B. Current
C. Resistance
D. Impedance

Fiber optic power is like voltage, and loss is like a voltage drop along a conductor.

2. Optical power is generally defined in units of watts but is usually measured in a logarithmic scale and expressed in ______.
A. Milliwatts
B. Microwatts
C. Decibels (dB)
D. Ohms

Since optical power in fiber optics can vary over a range of 1,000,0001 or more and dB is a more logical way to express loss, a log scale makes the measurements easier to understand.

3. A measurement in decibels is a relative power measurement, for example, when testing ______.
A. Transmitter power
B. Receiver power
C. Cable plant loss
D. Bandwidth

dB is a relative measurement, the ratio of two power levels as when measuring loss of a cable plan.

4. A measurement in dBm is an absolute power measurement, for example, when measuring ______.
A. Transmitter or receiver power
B. Connector loss
C. Cable plant loss
D. Bandwidth

Transmitter output power or receiver input power is expressed as an absolute power level in dBm. See question 5.

5. The “m” in dBm means the optical power is ______.
A. Measured by a “meter”
B. Referenced to another “measurement”
C. Referenced to “1 milliwatt”
D. A “mandatory” measurement

All measurements of absolute power in fiber optics are referenced to 1 milliwatt, the output of an average laser.

6. The difference between two measurements in dBm is expressed in dB, for example, in measuring loss.

Like question 3, the difference between two absolute power measurements (made in this case in dBm) is expressed in dB.

7. A 3-dB loss in the cable plant means the optical power has changed by a factor of ______, while 10 dB is a factor of ______.
A. 2, 10
B. 2, 100
C. 20, 1,000
D. 10, 100

3 dB is a factor of 2, 10 dB a factor of 10. 20 dB goes up to a factor of 100, 30 dB a factor of 1,000, since each 10 dB multiplies the ration by a factor of 10.

8. Fiber optic power meters are calibrated at different wavelengths because the sensitivity of their detectors varies with wavelength.

Fiber optic power meters use solid-state detectors which have varying sensitivity to different wavelengths of light and have to be calibrated at the wavelengths used in measurements.

9. Optical time-domain reflectometers (OTDRs) measure optical power on the vertical scale and ______ on the horizontal scale of a fiber trace.
A. Pulse width
B. Distance
C. Resolution
D. Reflectance

The OTDR measures power in dB on the vertical scale, looking at the decline of power over distance, the horizontal scale, so it can measure loss or attenuation coefficient of fibers.

10. The attenuation coefficient of a fiber as measured by an OTDR is calculated in ______.
A. dB
B. dBm
C. dB/km
D. dBx

The attenuation coefficient of fiber is defined by the loss in power, expressed in dB, over distance in km.

HAYES is a VDV writer and educator and the president of The Fiber Optic Association. Find him at