However, dBc/Hz is the power referenced to the carrier and I'm not sure what that is in this case.
I suspect the carrier in this case is the average optical power, which they may be thinking of as a many-terahertz carrier.
some authors present system noise floor measurements in units of dBc/Hz. Is this wrong since in this case there's no carrier?
It's not clear to me why somebody would choose those units for a noise floor. It may be wrong, but I'd want to see the context where you read it to say for sure.
I find that the trace on the RF spectrum analyzer shows harmonics as a series of peaks. The levels between peaks is at the same level as the background level (i.e. when there is no signal input). Can we therefore infer that the RIN at these points (i.e. if we integrate from 10 Hz, say, up to the 1st harmonic) is equal to or less than the system RIN?
In the RIN measurements I've seen, there are no measurable harmonics, just a single peak related to the laser's intrinsic relaxation oscillation frequency. Are you testing with a modulation signal applied to the laser? Most RIN measurement's I've seen were done with the laser operated CW, and I'd think the results are easier to interpret for a CW optical signal.
In general spectrum analyzers have a noise floor, but I wouldn't call it "RIN", because it is not "relative intensity" --- it doesn't change in proportion to the optical power. The measurement system noise is a fixed "floor" and you can't measure power spectral density below that floor. So whenever the trace is down at the noise floor, you're not measuring anything about the device you are testing, just the capabilities of the analyzer.
General comment
The RIN measurement is fairly difficult to do. Unless the laser has very bad performance you need a very low-noise detector, very low-noise preamplifier, and a very sensitive spectrum analyzer (with a low noise floor). You will want to test the noise floor of your whole receiver system (detector, preamplifier, spectrum analyzer) before measuring your laser to be sure you know when you're measuring the laser behavior and when you're just seeing instrument noise.
Edit
To follow up your questions in comments:
Sorry I'm not familiar with RIN measurements on pulsed lasers. But the units of dBc/Hz make a lot more sense now --- they're just talking about the fundamental of the pulse signal as the carrier.
The measurements I'm familiar with, you're most interested in the peak frequency in the RIN spectrum. I don't think you could do this with a pulsed laser because you'd have to pulse at a higher frequency than the RIN peak, which would also be beyond the modulation capabilities of the laser. But maybe there are tricks I'm not aware of.
I will suggest that for a pulsed RIN measurement, you don't need the bias tee, though you might want a blocking capacitor for the sake of your SA input. The peak of the fundamental of the pulse signal gives you the laser signal power that you'd be measuring the noise relative to.
is it fair to say then that the laser has equal or better noise performance?
I'd say it this way: if the laser noise is too small to measure on your detector/SA system, then the measurement system is not adequate to measure the noise of that laser.
how would you recommend characterising the system noise floor?
Typically, you turn on the photodetector and pre-amp, but don't apply any laser signal. Then take a sweep on the spectrum analyzer, using the exact settings you'll use for your measurement. This gives the combined floor for the detector plus the SA.
You should be able to display this for comparison to your laser RIN measurements by just using the save-trace features of the SA, without any need for calculations.
Best Answer
The first thing to consider is the Johnson noise of the resistor which cannot be eliminated. The higher the resistance the greater the noise. Reducing bandwidth will also reduce Johnson noise. So if your scope has bandwidth settings, and if you don't need high bandwidth for your signal, you can get cleaner results using the reduced bandwidth modes.
The second thing to consider is noise which couples in to the oscilloscope, particularly if it couples through the probe wiring arrangement by way of a magnetic field. The time-varying magnetic field will induce a current in the probe. The termination resistance inside the oscilloscope will convert that current to a voltage. If the termination resistor is 50 Ohms, that will lead to a much smaller voltage than if it is 1M Ohm.
In general, lower impedance termination is more resistant to noise. This is a very important concept when you encounter situations where noise immunity is required. Usually any noise coupling path will have some series resistance or fundamental power limiting just by its nature. So the lower your termination resistance, the lower the voltage due to noise coupling. Sometimes a 20 pF capacitor on a digital input can make the difference between a flaky and totally unreliable piece of junk and a rock solid product.
Often if I need to put the oscilloscope on a shunt resistor, I will use the 50 Ohm termination feature of the oscilloscope. This greatly reduces noise, and since the shunt resistance is much less than 50 Ohms (for the shunts I deal with) there is no worry of excessive current flowing into the oscilloscope, even if the shunt current may be high.
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