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How to choose the optical module in the case of known link attenuation

Time: 2021-11-09
1. Concept of Fiber Optic Link Attenuation
        Fiber optic link attenuation consists of fiber attenuation, connector attenuation, and splice attenuation. All these factors must be considered when calculating the total attenuation of a fiber optic link. In addition, the system margin needs to be factored in—this covers fiber bending loss and other unpredictable optical attenuation, and a minimum margin of 3dB should be reserved.
        Link Attenuation Testing Method: Connect a light source to one end of the fiber under test and an optical power meter to the other end. Let the optical power of the light source be P0; after transmission through the fiber, the optical power measured by the meter is P1. The link attenuation of the tested fiber is calculated as (P0 - P1).
Figure 1: Schematic Diagram of Link Attenuation Test (Loss = P0 - P1)
2. How to Select an Optical Module with Known Fiber Link Attenuation
        The selected optical module must meet the following conditions (for bidirectional transmission):
    • RX2 Sensitivity < TX1 Output Power - Link Attenuation - 3dB (System Margin) < RX2 Receive Optical Overload Power
    • RX1 Sensitivity < TX2 Output Power - Link Attenuation - 3dB (System Margin) < RX1 Receive Optical Overload Power
Figure 2: Schematic Diagram of Module Application
        Example Calculation
        Suppose we use our SFP10-10G-ZR module in an environment with a link attenuation of 15dB (using a 1550nm light source). The actual parameters of the SFP10-10G-ZR module are as follows:
Table 1: Actual Parameters of SFP10-10G-ZR Module
 
Sample Transmitter Optical Power (dBm)
Sample receiving end sensitivity (dBm)
Receiving end overload optical power (dBm)
Sample 1 (TX1/RX1）
1.3
-24.5
-7
Sample 2 (TX2/RX2）
2.1
-25.2
-7
 
        Calculations:
    1. TX1 Output Power - Link Attenuation - 3dB = 1.3 - 15 - 3 = -16.7dBm
    2. TX2 Output Power - Link Attenuation - 3dB = 2.1 - 15 - 3 = -15.9dBm
        Validation:
    • RX2 Sensitivity (-25.2dBm) < -16.7dBm < RX2 Overload Power (-7dBm) — Meets the condition
    • RX1 Sensitivity (-24.5dBm) < -15.9dBm < RX1 Overload Power (-7dBm) — Meets the condition
        Therefore, our SFP10-10G-ZR module is applicable in environments with 15dB link attenuation (1550nm light source).
        Moduletek provides the products mentioned in this application guide. Welcome to place your orders!
        If you have any questions about the above content, please contact us via email: sales@moduletek.com
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