Last Updated: April 2, 2026
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For coherent signals with wide optical spectrum, the traditional scanning method using an OSA or inband polarization method (EXFO) cannot correctly measure system OSNR. Therefore, use the integral method to measure OSNR of coherent signals.
Perform the following operations to measure OSNR using the integral method:
1.Position the central frequency of the wavelength under test in the middle of the screen of an OSA.
2.Select an appropriate bandwidth span for integration (for 40G/100G coherent signals, select 0.4 nm).
3.Read the sum of signal power and noise power within the specified bandwidth. On the OSA, enable the Trace Integ function and read the integral value. As shown in Figure 2, the integral optical power (P + N) is 9.68 uW.
4.Read the integral noise power within the specified bandwidth. Disable the related laser before testing the integral noise power. Obtain the integral noise power N within the signal bandwidth specified in step 2. The integral noise power (N) is 29.58 nW.
5.Calculate the integral noise power (n) within the reference noise bandwidth. Generally, the reference noise bandwidth is 0.1 nm. Read the integral power of central frequency within the bandwidth of 0.1 nm. In this example, the integral noise power within the reference noise bandwidth is 7.395 nW.
6.Calculate OSNR. OSNR = 10 x lg{[(P + N) – N]/n}
In this example, OSNR = 10 x log[(9.68 – 0.02958)/0.007395] = 31.156 dB
We follow integral method because Direct OSNR Scanning Cannot Ensure Accuracy because of the following reason:
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Optical Networking Engineer & Architect • Founder, MapYourTech
Optical networking engineer with nearly two decades of experience across DWDM, OTN, coherent optics, submarine systems, and cloud infrastructure. Founder of MapYourTech. Read full bio →
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