User:Pranav Rathi/Notebook/OT/2010/04/13/Mode profiling at high power
Power Step-Up Beam Profile experiment (BPE)
Laser under investigation: 1064 nm 4W coherent compass laser.
Correct beam profile is very important for optical trap; in our case its (0, 0) Gaussian mode. It looks like that at high powers our laser moves to higher modes, which introduces some extra difficulties in focusing and getting diffraction limited spot. It is also not good for the stiffness calibration. The motivation behind doing this experiment is to answer some questions.
Answer the following questions.
- At what output power the beam profile starts changing?
- Is this change slow with power?
- How much does it change at 4.1W in comparison to the point at which it starts changing?
- What is the appropriate power range for tweezing?
The profiling is done from .2W to 4.1W in .3W steps at 85 deg F, laser temperature. The beam was passed through AOM (AOM at CW mode) and 2nd order diffracted beam was picked for profiling. The profiling is done through a regular CCD camera with following set-up:
After AOM, beam was passed through an IR mirror (at 45o mirror gives maximum reflectance, so to control the transmission the angle can be used); reflected beam forwards to beam stop. After mirror it goes through a regular aperture just sized (not small enough to introduce diffraction) to fit the beam (aperture was used to save the filters from over exposure). Than a 3ND filter, a Plano-convex lean and a 2ND filter were used in series to project the beam on the CCD.
A series of profile pictures were taken starting at .2W with .3W step. The pictures were analyzed through Image-j. The analyzed image is 90 deg right rotated image of the original (this is done to enhance the surface plot profile), so the horizontal axis in the set-up plane is vertical axis in the image plane.
Note: Every element which is added to a optical path tempers the beam profile, so keeping that in mind this technique is not absolutely accurate. But the results are beyond such tempering in this case, it has to be laser.
Results are presented in the referred link: 
- Steve Koch 01:11, 14 April 2010 (EDT): To me looks like between 1.7 and 2 Watts is when it gets bad?
Up to 1.1W (pic: 1.1w) the profile is very smooth and Gaussian all the way. At 1.4W the very first time an additional pointy-peak appears at the peak of Gaussian peak, but the profile remains the same. At 1.7 and 2.0 W the peak of Gaussian profile gets sharper and sharper with an additional short pointy-peak at the top. The overall profile and its width do not change. The most significant change comes at 2.6W; the top of the profile starts getting FLAT. The profile is partially single mode Gaussian now. I think between 2 and 2.6W especially after 2.3W the higher order mode/modes start kick in.
These modes now share power with the (0.0) mode, so besides increasing the power the (0, 0) mode is not that strong. As we can see in the surface, horizontal and vertical plots that besides increasing the power over 2.3W the gray value does not change as quickly as it did in between .5W to 2.0W. The main cause behind that might be:
- The power is shared by higher order modes.
- The lens cannot focus the higher order modes to the diffraction limit, so the intensity at the focal spot is much spread out. And that is also the main cause that profile at 4.1W is much spread out. In profiles from 3.2W on wards, higher order modes can be seen with any doubt.
- CCD camera is not calibrated so the mode spread at high powers may be due to the higher number of photons received by the pixels.
Answers to the questions:
- Q: At what output power the beam profile starts changing?
A: At 2W the profile starts changing and the change is significant above 2.3W.
- Q: Is the change slow with power?
A: The change is slow but the higher order mode profile can only be seen after 3.2W.
- Q: How much does it change at 4.1W in comparison to the point at which it starts changing?
A: At 4.1W it looks completely different. (0, 1) mode profile can clearly seen with (0, 0) and possibly other higher order mode profiles in the back ground.
- Q: What is the appropriate power range for tweezing?
A: By looking the profiles, their looks like that up to 1.7/2.0W. But the exact boundary line can only be decided after doing the SUM Signal Experiment; the next task.
The results suggest that the laser certainly have higher order modes above 2.0W at 85 deg F. The temperature is certainly one of the factors; it can change the cavity length, which can change the resonance of the laser. In the case of laser like we have, the higher order mode resonance is only 10s of hertz away which corresponds to extremely small cavity length changes, can be caused by the temperature. Another way temperature can affect the resonance is by affecting the gain medium. By changing the temperature the refractive index of the gain medium is changed, which changes the group velocity and path length [(n=no+ni*t)*d].
I have also done a study of beam/mode profile Vs laser temperature.