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.
Experiment: Beam profile Vs laser power
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 and also not good for the stiffness determination. The motivation behind doing this experiment is to answer some questions. The laser temperature was kept constant around 85o Fahrenheit.
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?
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 related note is linked here:User:Pranav_Rathi/Notebook/OT/2010/04/01#High_power_beam_profiling
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?
- 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:
1. 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.
2. 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.
3. Q: How much does it change at 4.1W in comparison to the point at which it starts changing?
4. 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.
Experiment: Beam profile Vs laser temperature
The purpose of this experiment is to establish a relationship between laser temperature and beam profile. In my tests sofar, i have seen a correlation between temperate and laser going to higher modes at some power. I want to explore the possibility of using higher powers at lower temperature without laser being tripped into higher modes.
The beam profiling is done at several laser temperature with constant laser power output of 4W. The 2nd order diffraction beam is chosen for profiling at around 110mW. The temperature of the laser was modulated using TEC peltier. The optical setup and procedure is same.
Answer the following questions.
- At what temperature beam starts getting out of single mode?
- What is the safest highest output power at the lowest laser temperature?
Well according to the pictures the beam profile does not look good at any temperature. At and above 70o the profile has distinctive higher order modes. Profile at 85o for 2W and 4W look completely different.
Answers to the question:
1. Q:At what temperature beam starts getting out of single mode?
A: At no temperature beam looks good.
2. Q:What is the safest highest output power at the lowest laser temperature?
A:Since at no temperature the beam looks good at 4W output power so i doubt it that we can use power higher then 2W at any temperature.
Looks like temperature makes only a tiny difference, so i am not comfortable using the laser above 2W even at the lowest achievable temperature.