Relative intensity noise of laser

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Hi
Could anyone let me know how to measure relative intensity noise using
spectrum analyzer. I got two sets of data and i couldn't relate those
two for measuring RIN.

1) I used 2  Ghz photodetector and then i used oscilloscoope to measure
ac-component which was 8 mV and dc component was 500 mV. I used
spectrum analyzer and the power at that particular frequency id -50dBm.

2)I used 45  Ghz photodetector and then i used oscilloscoope to measure
ac-component which was 5 mV and dc component was 10mV. I used spectrum
analyzer and the power at that particular frequency id -55dBm.

My question is: while measuring RIN = delta P/ Pavg.  Is this delta P
is equal to -50 dBm or -55 dB or do i convert ac componet. Also, where
does frequency value come from in dB/Hz. Does it depend on
photodetector bandwidth or do i take 1Hz bandwidth.

 IS Pavg =(Vdc)^2/50 ?

Could anyone please help


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