Optical module link budget calculation method and margin points

How to look at the key parameters of optical modules: luminous power, receiving sensitivity and link budget

Published: 2025-06-28 EB-LINK Views: 1589 Link budget, receiving sensitivity, luminous power, DDM

After replacing the module, the link is intermittent. After checking for a long time, the optical power budget is often insufficient. There are many parameters in the specification book. What really determines "whether this link can survive stably" are three numbers: luminous power, receiving sensitivity, and overload optical power. Combined with the link budget to calculate the entire loss, most hidden dangers can be eliminated before placing an order.

First understand the three power parameters

Luminous power (TX Power) refers to the light intensity emitted by the module, and the specification gives a range value, such as -9dBm to -3dBm; receiving sensitivity (RX Sensitivity) is the minimum light intensity that can be decoded correctly, the lower the stronger, the common Gigabit single-mode is around -20dBm; the overload optical power is the upper limit of the receiving end without being saturated. If the light is too strong, it will cause bit errors. The relationship between the three is: the actual optical power at the receiving end must fall between the sensitivity and the overload point, and the link budget is built around this pair of windows.

link budget algorithm

Subtract the sensitivity from the minimum transmit power to get the total allowable loss in the entire process, and then deduct each loss one by one:

Optical fiber attenuation: 1310nm single mode is estimated at 0.35dB/km, 1550nm is estimated at 0.22dB/km, 40kmSFP+ 10G single-mode dual-fiber 40km optical moduleOptical path loss is about 14dB;
Movable connector: Each pair of flanges/adapters is calculated as 0.3dB to 0.5dB, and two ODF jumps account for about 1dB;
Welding points: 0.05dB to 0.1dB each, more than a dozen joints also need to be budgeted;
Engineering margin: It is generally recommended to leave at least 3dB to cover aging, cleaning and temperature drift.

Use DDM to bring parameters on paper to the field

The module that supports digital diagnosis can directly read the current transmitting and receiving optical power. After installing the link, you can take a look to see if the RX Power falls within the specification window, which is more reliable than guessing with a multimeter. If the received light is close to the lower limit of sensitivity, first check the connector contamination and bending loss, and then consider upgrading to a longer distance model or adding an optical amplifier. For long-distance scenarios with strong interference such as electricity, please refer toCommunication transformation plan for electric power and new energy stationssegmented design. When choosing a module, calculate your budget first and then negotiate the price. EB-LINK technical support can assist in checking the parameters based on the actual link. Tel: 0755-83179002.