# Lock-in Amplifier

I am a physics student currently trying to make measurements of Semiconductor saturable absorber samples. In my setup laser light reflected from the structure falls on a Thorlabs DET100A/M - Si Detector, then it is processed via Lock-in Amplifier and through NI USB-6218 BNC it goes to the computer.

I am getting strange "absolute value - like" effect. The x axis is in femtoseconds and y axis in volts. (sorry for the quality, I left measured data at the lab and have only photos)

Blue curve is measurement I got and the orange one is after I inverted left and right side of the curve at the point that they "reflect" from the level of about -4 V. The orange curve looks exactly how it should look like physically and I am pretty sure that this effect is not optical nor a feature of the particular sample.

Is it possible that Lock-in Amplifier is causing this effect and how could it be avoided?

• I don't understand "I inverted left and right". To me it looks like you inverted up and down not left and right.
– Curd
Jul 9 '16 at 15:45
• I meant that I left the center part not inverted. I inverted (by which I meant reflected through the -4 V line) parts of the graph from -3000 fs to near 0 fs and from near 3000 fs to the end. Jul 9 '16 at 15:52
• Ok, now I understand.
– Curd
Jul 9 '16 at 15:58

I'm not sure I understand your measurement. How are you measuring a 10pS long event with a SR830 lock-in which has a minimum time constant of 10uS? Still, there is a common mistake with lockins which might cause what you see.

A lock in amplifier makes a phase sensitive measurement of an AC signal. It measures in-phase and quadrature signals, which are usually referred to as X and Y respectively. The stanford lock-in you linked also calculates R and θ from X and Y, where R is the magnitude of X+jY theta is the argument. If you have a signal which passes through zero, then R will look like the absolute value of the signal.

The SR830 can be configured to provide X, Y, R or θ on the BNC outputs by pressing the one of the buttons under the LCD displays. Probably you have connected the left hand output to the computer, and accidentally selected R instead of X.

As an aside, the SR830 is a digital instrument. It digitises your signal, then does the actual detection and filtering in the digital domain. It then uses a DAC to convert back to analog, and you're using the NI ADC to go back to digital. You'll get better accuracy cutting out the second round of analog and reading straight from the SR830 using serial or GPIB.

Update I have only a passing familiarity with pump-probe, but here are a few tips for using lockins in general and the SR830 in particular:

• A lock-in is a really powerful tool, but it's complicated and it's quite possible to shoot yourself in the foot with it. If you can spare the time to really understand the theory of how it works, that will pay off. But if you don't then at least get familiar with what X, Y, R, θ really mean.
• A lock-in measures two things. That can be X and Y, or R and θ, but always record both of them. It makes debugging much easier. If I'm right, then you'll see an abrupt 180 degree change in θ where your curve gets reflected, which is a clear indicator something's wrong.
• I don't know the details of your experiment, and I don't want to contradict your supervisor, but R is rarely what you want to measure. People often measure R because they aren't sure how to best configure the phase reference. You'll probably find that measuring X instead avoids the funny reflection feature and gains you a factor of 1.5 in signal to noise.
• The SR830 has the ability to scale and offset it's outputs. Handy if that's what you want, but confusing if it's turned on when you don't. That could be responsible for the 4V offset. Check there are no "expand" or "offset" lamps on the leftmost LCD or just below.
• You need a stable reference. In your comment you note that you might not have one, if so it's hurting your signal to noise and may cause all sorts of weird behaviour. The square wave from the chopper should go into the "ref in" BNC in the bottom right, and either "pos edge" or "neg edge" not "sine" should be selected. There should be no red "unlock" lamp and the frequency displayed above should be completely stable.
• If you're going to try to use serial (and I suggest you do, if practical) you can use a standard USB-RS232 converter, and a 9-pin to 25-pin serial cable. You'll need to set the visa read terminator to '\r' and send the string 'OUTX 0' to the instrument at the start of each session.
• Measurement process is generally termed pump-probe in ultrafast optics and I just wanted to keep my post as short as possible and so I didn't explain the optical details. In reality I am not making single experiment in which I have that fast decay but I make many experiments probing the sample at different time delays. As for your explanation, thanks very much! It may be really the problem here, because we were measuring it with R selected, but it was intentional because my supervisors told me to make it so. I will speak with them on that matter. Jul 9 '16 at 16:10
• Thanks for the last paragraph advice too! (by serial you meant RS232?) I had problems with measuring those samples for a month so I hope that your help will finally end all problems with them. Now I see I had the same problem from the start but it was masked because of thermal effects (we were heating the sample too much with laser pulses) and only that week we finally noticed and eliminated them so that absolute value effect was clearly visible. Jul 9 '16 at 16:24
• I was thinking about that part that my signal goes through zero. On the photodiode are falling 230fs long pulses, repetition rate from the laser is 80MHz but the signal that I give as a reference for lock-in is square wave 200-300Hz modulation via optical chopper. The amount of light that reflects off of the sample changes based on the second pulse that excites it and that is what I want to measure how that reflectivity changes. But how photodiode would make negative signal out of it? The frequency that lock-in gets from the chopper is a bit unstable, could it be responsible for that problem? Jul 9 '16 at 16:44
• I'm afraid I'm still not completely clear on how your measurement works. It can be very hard to debug this sort of thing over the internet. I'll add some general tips to my answer. Jul 9 '16 at 17:58
• Thank you very much for additional tips. Is there some kind of lock-in reference you would suggest reading so I could understand better how it works? I am pretty sure it is not last time I have to use such equipment and it will certainly pay off understanding it. From what I can say right now there was "sine" selected (I have photos of settings on which measurements were taken), so it is another cause of failure probably. As for stability of the frequency I probably can't help much here because of optical reasons. Jul 9 '16 at 21:00