- #1
fevemo
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- TL;DR Summary
- Is a SHG process (936 nm to 468 nm) similar in efficiency to a SFG process (850 nm+1040 nm to 468 nm) assuming same properties of the two lasers (both fs pulsed, same focal spot, same polarization, etc.?
I am trying to align two ~100-140 fs pulsed lasers, one at 1040 nm and one between 750-950 nm through a working custom 2p microscope. The end goal is to do stimulated Raman (CARS and SRS) which requires the two beams to be coincident and the pulses to be perfectly synchronized in time, however for now I am testing using a crystal that generates second harmonics signal.
To find the temporal alignment I look at the blue channel (~475 nm) signal from the microscope and maximize the SFG signal coming from the crystals when both the laser beams are enabled (1040 nm + 850 nm) by moving a delay line for the 1040 nm beam. This procedure works well and I can consistently find a small (200-300 fs) window where the SFG signal is clearly visible. This signal scales linearly with the power of each laser which reassures me it is in fact SFG.
Now the problem I am facing is that once I try to get some CARS/SRS signal there is practically none, even with samples that should generate very large responses. Going back to SFG I noticed that the efficiency is significantly lower than just setting the tunable beam to 936 nm. To get a similar intensity from SFG I need almost 10x more power on the sample than when using SHG with a single 936 nm beam (68 mW vs 7.4 mW). Obviously, I would expect for a single beam to be more efficient than two superimposed beams of different wavelengths, but 10x more power feels like too much. Is this an indicator that the focal points of the two beams are barely overlapping or is the 10x lower efficiency within reason for a properly aligned setup?
P.S.: I am aware CARS and SRS are usually done with ps pulsed lasers because of the spectral resolution required, I'm not concerned about resolution at the moment and fs-based CARS and SRS have been shown to work and even generate a stronger signal in some cases. Also, to add to the mystery, a previous version of this setup worked and gave me a decent SRS signal.
To find the temporal alignment I look at the blue channel (~475 nm) signal from the microscope and maximize the SFG signal coming from the crystals when both the laser beams are enabled (1040 nm + 850 nm) by moving a delay line for the 1040 nm beam. This procedure works well and I can consistently find a small (200-300 fs) window where the SFG signal is clearly visible. This signal scales linearly with the power of each laser which reassures me it is in fact SFG.
Now the problem I am facing is that once I try to get some CARS/SRS signal there is practically none, even with samples that should generate very large responses. Going back to SFG I noticed that the efficiency is significantly lower than just setting the tunable beam to 936 nm. To get a similar intensity from SFG I need almost 10x more power on the sample than when using SHG with a single 936 nm beam (68 mW vs 7.4 mW). Obviously, I would expect for a single beam to be more efficient than two superimposed beams of different wavelengths, but 10x more power feels like too much. Is this an indicator that the focal points of the two beams are barely overlapping or is the 10x lower efficiency within reason for a properly aligned setup?
P.S.: I am aware CARS and SRS are usually done with ps pulsed lasers because of the spectral resolution required, I'm not concerned about resolution at the moment and fs-based CARS and SRS have been shown to work and even generate a stronger signal in some cases. Also, to add to the mystery, a previous version of this setup worked and gave me a decent SRS signal.