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What are we supposed to do?

276 views · Published 17 February 2011 · 0:48 · Indexed 5 October 2026

Channel: Faissal aka Md. Faisal Halim · 2011 · People & Blogs

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(Continuum PY61C Laser)
The AOM still needs to be raised:

I tried optimizing the laser cavity (first, just the back mirror, then both, the back mirror as well as the aperture) but could not get the lasing threshold below 1.52 kV, which is absolutely ridiculous, given that I am used to an operating voltage of 1.2 kV when I really want it high.

So, I removed the aperture, and realigned the back mirror (it turned out that at 1.7 kV -- the voltage the oscillator was set to when aligning the back mirror -- the laser will lase, regardless of the position of the back mirror, so that was a bad position to start with).

After getting the free running lasing threshold (without aperture) to be below the scale of the oscillator power unit (which is what I expect) I, once again, found that the free running beam flies above the AOM window height, so I am back to square one.

This video demonstrates the problem.

Any suggestions, please?

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Response from the Professor:

"The spiking pulse you see on the oscilloscope while you are moving the AOM around does not constitute the proper lasing mode in the cavity - remember there are many longitudinal and transverse modes in a cavity. It takes time and patience to align the laser cavity properly so that you are selecting the optimum mode.  There is no reason to believe that the height of the optic axis of the cavity has changed so much that it does not run through the center of  AOM opening with its based firmly mounted to the laser table. 

"What you can do is remove the aperture and AOM. Run the a reference beam through the entire cavity. Check using a gauze like material (tissue) over the output of the laser head and a mirror that the reference beam is running down the center of the laser rod. Be careful not to look directly into the diode laser light unless it is properly scattered.

"Check the back reflection of the reference beam from the back mirror (mirror on your left when facing the laser) using a pinhole in a white business card or small piece of paper. Because the back window of the dye cell is a wedge you will see two reflections (one reflection from the front surface of the window and the second from the back surface). 

"Choose one of the back reflections and adjust the back mirror so that the reflected beam overlaps with the on coming beam through the pinhole. Put the aperture back with the reference beam going through it. Turn on the laser and set the oscillator voltage high (1.6 KV) until the laser begins to lase. Adjust the pinhole position, the back mirror and voltage until you get a large pulse at a low voltage (around 0.9 KV). If this does not work. You may need to remove the aperture again and choose the other back reflection and start the process over."

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