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Relationship Between Link Budget And Transmission Distance In Optical Modules

Time: 2023-06-26
1. Meaning of Link Budget
        Link budget refers to the calculation of all gains and attenuations in a communication system, including those from the transmitter, communication links, propagation environment, and receiver. It is typically used to estimate the maximum distance over which a signal can be successfully transmitted from the transmitter to the receiver.
2. Relationship Between Link Budget and Transmission Distance
        Under ideal conditions, the maximum transmission distance of an optical module is calculated by the following formula:
        Maximum Transmission Distance = Link Budget ÷ Attenuation Value of Fiber per Unit Length at the Module’s Emission Wavelength
        Where:
        Link Budget = Minimum Transmit Optical Power − Receive Sensitivity (Unit: dB)
Attenuation diagram of different wavelengths of light transmitted in the optical fiber
Figure 1 Attenuation Diagram of Different Wavelengths Transmitted in Optical Fiber
        As shown in the figure above, this diagram illustrates the attenuation of different wavelengths when transmitted in optical fiber. The vertical axis represents the attenuation value (in dB/km), and the horizontal axis represents the wavelength. It can be seen from the figure that there are four wavelength windows for optical communication. The most commonly used in single-mode fibers are the 1310nm and 1550nm windows:
        • At 1550nm (1.5μm), the fiber attenuation is approximately 0.2–0.3 dB/km;
        • At 1310nm (1.3μm), the fiber attenuation is approximately 0.3–0.4 dB/km.
Example Calculation
        Taking a 10G ER optical module with an emission wavelength of 1550nm (required transmission distance ≥ 40km) as an example:
        • Transmit optical power range of the module: -4 dBm ~ 4 dBm
        • Minimum receive sensitivity: -16 dBm
        • Emission wavelength: 1550nm (third window)
        • Fiber attenuation (median value): 0.25 dB/km
Calculation Process
        1. Link Budget = Minimum Transmit Power − Receive Sensitivity = -4 dBm − (-16 dBm) = 12 dB
        2. Theoretical Maximum Transmission Distance = 12 dB ÷ 0.25 dB/km = 48 km
        In practice, additional losses occur during transmission, such as bending loss and connector loss. Therefore, the actual transmission distance of the optical module will be shorter than the theoretical distance. This is why we specify the transmission distance of ER modules as 40km.
        Moduletek provides optical modules with different transmission distances. Welcome to purchase! For further inquiries about the above content, please contact us at: sales@moduletek.com
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