NO SIGNAL

The title makes me feel like I work for SETI (the search for aliens). That is, except that the people at SETI expect to see no signal and I do. I have not been able to see an error signal associated with the lamb dip. The lamb dip is on the order of ~2 the natural linewidth of the transition and is shifted by ~90 MHz from the center of the absorption signal (AOM driven at 181 MHz). So we can see that the width is roughly on the same order as the shift which is a good check all around. I was looking for the signal using our homemade homodyne detector. Detector has two high pass filters in it: the first filter has a 3db point of ~40kHz and the second has a 3db point of 340kHz. Neither of which should pose a problem for a 1MHz signal. The mixer in the detector is a SYPD-1 from minicircuits whose response starts at 1 MHz, but should still be just fine.

To make sure that we are sweeping far enough around the 181 MHz AOM frequency I did some fourier transform plots in octave so that we have an idea from the spectrum analyzer how far we are modulating the 181 MHz signal at 1 MHz. We think that initially we were sweeping about .5 MHz. Now I’ve changed the signal so that we think we are sweeping around 2 MHz or so.

As an aside, I changed the board generating the the AOM signal. It consists of a 1 MHz signal that is added to DC voltage which goes to a voltage controlled oscillator (VCO). The output of the vco is then amplified and sent to the AOM. I used an extra op-amp on the board to buffer the DC voltage and then I changed the input and feedback resistors on the summing amplifier to values that were more reasonable to work with.

Watching the lamb dip on the raw signal it’s not hard to be believe that the dip is about 10 mV tall (maybe slightly bigger) coming out of the photodiode. So we want to know, what kind of signal should get generated in the mixer. Well, if we take the second derivative of the lorentzian and set it equal to zero we get the point at which the largest amplitude signal gets sent to the mixer. We can multiply this by the sweep we are imposing on the AOM signal and then by the gains in the homodyne detector. The output of the homodyne detector is cleaned up with a low pass filter that has an RC time constant of ~100 Hz I think but I will double check. In short, we are expecting to see something that is tens of millivolts.

Before I forget, now that I’m thinking about the signal that gets sent to the mixer… we actually get features out of the mixer that show up at places where the laser isn’t happy. So the laser intensity sharply decreases and then comes back in kind of an inverse “lamb dip” shape. For those features to show anything in the mixer they are much larger than our lamb dip and maybe give us some idea of how large the dip has to be in order to see something.

Since we didn’t see anything we decided to see what we could with some modular components. We took the output of the large area battery backed photodiode (this was maybe a mistake even though it should have been plenty fine for 1 MHz) and sent it through 2 amplifier each with a gain of 24 dB. This was then mixed with the original MHz signal in a ZAD-1 mixer from minicircuits. Did not observe anything.

We knew from the position of the lamb dip that we had the right offset from the AOM, but we decided to try and see if we could determine that we were actually sweeping the frequency around the main carrier frequency. So we aligned the pump and the probe beams on a small area battery backed photo diode (good to 500 MHz into 50 ohms), and put the output on the spectrum analyzer. We could not see anything that did not indicate we were simply amplifying RF pickup. The signal did not go away when blocking the beams and could not be seen unless both amplifiers were used. When it did show up it was about –90 dBm. This was very confusing because the 181 MHz difference between the 2 beams should have been KILLING the frequency spectrum and we would have to carefully align in order to see the sidebands well. Not sure what is going on, but this is getting ridiculous.