The two most confusing units in the optical communications field are dB and dBm. One represents the ratio and the other represents the absolute power. If you write a wrong decimal point, the entire link judgment will be off. This article uses the actual numbers on the optical module label to clarify the relationship between the two.
dBm is the absolute value, dB is the difference
dBm is referenced to 1 milliwatt, 0 dBm is equal to 1 mW, -10 dBm is equal to 0.1 mW, and -20 dBm is equal to 10 microwatts. dB describes the multiple relationship between two quantities, with power approximately halving for every 3 dB decrease and approximately tenfold for every 10 dB increase. So 3 dB attenuation and -3 dBm power are completely different things. The former refers to loss and the latter refers to reading.
The three numbers on the label determine the available distance
Using SFP+ 10G single-mode dual-fiber 10km optical moduleTaking this type of common model as an example, the specifications usually give three items: the minimum transmit power is about -9 dBm, the receiving sensitivity is about -11 dBm, and the overload point is about -1 dBm. The difference between emission and sensitivity is the total loss allowed for this link, which is called the power budget in engineering. For models with longer nominal distance, the difference is generally larger, such asSFP 100M single mode dual fiber 20KM optical moduleThe available loss headroom is typically several dB wider than the ten kilometer model.
Self-checking with an optical power meter only takes three steps
- Clean the end face first, unplug the jumper from the device side and connect it to the power meter. Compare the reading with the value reported by DDM. If the deviation exceeds 2 dB, check the jumper and adapter.
- Then measure the actual incident light power at the receiving end and compare it with the sensitivity. The difference is the current margin.
- When the margin is less than 3 dB, it is listed as pending rectification. Priority is given to checking the welding points and bends. There is no need to replace the module first.
When reading, the wavelength range of the power meter must be consistent with the module window. Selecting the wrong window will introduce calibration errors. EB-LINK tests and retains data one by one before shipment. If you need to compare the self-test results, please call 0755-83179002.