Optical spectrum analyzer related

[chisuma]

TL;DR Summary

Can anyone explain why optical spectrum analyzer plots are in negative dB

Can anyone explain why optical spectrum analyzer plots are in negative dB

 

[berkeman]

Summary:: Can anyone explain why optical spectrum analyzer plots are in negative dB

Can anyone explain why optical spectrum analyzer plots are in negative dB

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What optical power would correspond to 0dB?

Also, which optical spectrum analyzer are you using, and are you using it to measure attenuation in optical fiber?

 

[chisuma]

Thanks Berkeman. Appreciate your response. 0 db translates to 1 mW of power. We are using Yokogawa spectrum analyzer. We are measuring power out of the fiber. Please correct me if I am wrong - I think attenuation in the fiber can be calculated by the delta in power between free space power and power out of fiber. Regarding my question as to why there are negative values in OSA spectrum, is it a relative measurement with respect to 1mW power (0 dB).

 

[Twigg]

Often (but not always), you'll see dB with respect to 1mW written as units of dBm instead of dB. Its only a valid unit for absolute power. If you're not sure if the dB your spectrum analyzer reports are with respect to 1mW, check if the units say "dBm". If they just say "dB", you may want to test the spectrum analyzer with a signal of known power (like a visual fault indicator, the laser pointer thingy for aligning fibers). Check the signal against a power meter and the spectrum analyzer. (Note: lots of people don't distinguish between dB and dBm so just because your y-axis doesn't say dBm doesn't mean its not with respect to 1mW.)
And yes, the attenuation is typically given as the difference in input and output power on a dB scale (not a linear scale!) divided by the length of the fiber in km.