The Mysterious dB of Fiber Optics

In my last quiz, I challenged your knowledge on fiber optic testing. Fiber optic test results are usually expressed in decibels (dB), a term that can be very confusing. Do you understand decibels, and do you know how to interpret test data expressed in decibels?

1. The term “Bell” was chosen in recognition of ________.

Correct Answer: Alexander Graham Bell

2. When making measurements, dB is used in measurements of ________.

Correct Answer: All of the above

3. Decibels are used for measuring other systems besides fiber optics, such as electrical, wireless/radio and acoustics/sound.

Correct Answer: True

4. Changes in optical power, for example when measuring loss of a fiber optic cable plant, are expressed in ________.

Correct Answer: dB

5. When measuring the actual optical power output from a fiber optic transmitter or the input power at a receiver, the reading is expressed in ________.

Correct Answer: dBm

6. Since dB is a logarithmic term based on a factor of 10, a change of power of 10 times is represented by a change of 10 dB and a factor of 100 times change in power is ________.

Correct Answer: 20 dB

7. “0 dBm” is a typical power output of a laser transmitter, which has an output of ________.

Correct Answer: 1 milliwatt

8. If a “0 dBm” laser transmitter is used on a fiber optic link with 10 dB loss, the receiver power measured in a fiber optic power meter will be ________ dBm.

Correct Answer: -10

9. A typical multimode fiber has an attenuation of 3 dB/km. At the end of 1 km, the power transmitted into the fiber will be reduced by ________%.

Correct Answer: 50

10. When you calculate or measure loss with a light source and power meter, the result in decibels is a negative number, but most optical loss test sets and optical time-domain reflectometers display a positive number when they measure loss.

Correct Answer: True