This is an optical experiment. I have a SR830 lock-in amplifier connecting to a photodiode, which measures the reflection from a sample. The intensity of a laser beam is modulated by a chopper with a reference input to the lock-in amplifier. A phase is detected in the reflection signal both from the sample and control. In another part of my setup, the polarization of the laser beam is further modulated by a PEM. A second lock-in amplifier with a reference frequency from the PEM is used to measured the signal reflected from the sample. A relatively large phase is observed both from the sample and control.
My questions are as followed.
1) Should I press "auto-phase" in the second lock-in amplifier and maximize the detected signal in the in-phase term X?
2) As I want to see the difference between the target sample and control, should I first do "auto-phase" in the control and then measure the target sample? Sort of background subtraction.
3) Now, the properties of the sample is tuned by an application of an electric field / a magnetic field. This further increases the phase difference between the signal and reference, indicating a change in sample properties. Here I want to see how the reflection signal changes with an electric field / a magnetic field on the sample. I was wondering I should use the same phase shift measured in #2 (at zero electric field / magnetic field). However, it results in a very large phase shift when I measure my sample even at zero field. What is the general rule of thumb for the phase shift in intensity and polarization modulation optical experiment?
Dear Ting-Fung Chung,
if I were you, I would attempt to actually write down what happens in each of the modulators and, in a second step, what information you want to retrieve by measuring the modified modulated signal with a Lock-In amplifier. It might also be instructuve to actually take a oscilloscope and look at the time dependent signal in order to better understand what the Lock-Ins do for you (and whether that complies with the math you did).
This said, parts of what you ask for may depend on the specific steup. Let me assume that the comparison between sample and control is done by swapping the two samples (taking out one and inserting the other). You will probably have to make some (geometric) adjustments and the reflected beam might undergo some changes in its characteristics, e.g. due to different degrees of surface flatness, roughness etc. Nevertheless, for the intensity measurement I would expect the same main timing (i.e. the correlation between the chopper position and the phase of the corresponding reference signal) to be relevant. I'd therefore expect very small phase differenes to occur between different samples in the intensity measurement.
Is that the case?
[To get a feeling about it, do play with sample adjustment, detune slightly (e.g. with just the reference) and see what autophase detection tells you. Do also optimize the signal by hand to judge by yourself whether the autophase adjustment is doing a good job (it generally should with an SR830 :-). Again -- if you can -- use an oscilloscope to see what happens and whether the results you get from the SR830 make sense in the light of what you see. You can trigger the oscilloscope by the same signal you use as the reference input to the Lock-In to get reproducible timing.]
Lets go to phase modulation in a subsequent post i may require some more detail about the setup to give an adequate answer. (How are the modulation frequencies of chopper and PEM related, what frequency or frequencies do you detect the phase modulation at etc.)
- Badges
- Science topic
- Similar topics
- Filing