13 Jun 2026
Measurement of Optical-Fibre Patch-Cord Loss in dB
Aim
To measure the insertion loss of an optical-fibre patch cord in decibels.
Apparatus
LED or laser fibre source, optical power meter, reference patch cord, test patch cord, fibre connectors, and cleaning kit.
Experimental arrangement
Theory
Light power is reduced as it travels through a fibre because some energy is absorbed, scattered, radiated at bends, or lost at a connector. The power meter is first connected directly to the source to establish a reference, and the test patch cord is then inserted without changing the source setting. Optical attenuation is therefore defined by comparing the reference power with the transmitted power. If $P_{ref}$ is the reference power and $P_{out}$ is the power after inserting the patch cord,
\[L=10\log_{10}\left(\frac{P_{ref}}{P_{out}}\right)\,\text{dB}.\]When both readings are displayed in dBm, the loss is simply $L=P_{ref}(\mathrm{dBm})-P_{out}(\mathrm{dBm})$. A clean connector and a gentle bend are essential because coupling and bend losses can be larger than the fibre attenuation itself.
Observations
| Patch cord | Reference power (dBm) | Output power (dBm) | Loss (dB) |
|---|---|---|---|
| 1 m multimode | -3.2 | -4.1 | 0.9 |
| 3 m multimode | -3.2 | -4.6 | 1.4 |
| 5 m multimode | -3.2 | -5.0 | 1.8 |
Calculation
For the 1 m patch cord, the reference and output readings are $-3.2$ dBm and $-4.1$ dBm. Therefore,
\[L=P_{ref}-P_{out}=(-3.2)-(-4.1)=0.9\,\text{dB}.\]For the 5 m cord,
\[L=(-3.2)-(-5.0)=1.8\,\text{dB}.\]The loss therefore increases by $0.9$ dB between these two trial cords.
Maxima Code
Download the optical-fibre loss calculation.
Result
The insertion loss increases with patch-cord length and connector number.
Viva Questions
- Why is a reference reading taken? To remove the source and meter baseline from the loss measurement.
- What is dBm? Optical power expressed relative to 1 mW.
- Why should connectors be cleaned? Dust produces additional coupling loss.
Discussion