Why Is My Nd:YAG Laser Beam Quality Deteriorating?

[shamrock08]

I am working with a Nd:YAG pulsed laser system produced by Continuum back in the early 2000s called the Precision II 8000. We just received the laser from another department at the University and have been setting it up to work unseeded at 532nm.

The previous lab team had adjusted the output coupler (OC) which is usually a big no but with the help of a very patient tech, we were able to get the OC realigned(ish) and then went on to realign the oscillator. We began routing the beam out of the cavity into a test area for ensuring beam quality.

We quickly realized that the beam was contracting significantly despite using 532nm laserline mirrors and the optical path only being a little more than 11'. The beam's contraction was noted using burns as reference. To better explain the contraction, the major axis reduced from 7mm to 6mm and the minor axis reduced from 7mm to 5mm.

Along with contracting, burns taken show that the beam quality is slightly elliptical and jagged around the edges, which is a far cry from the ideal circular shape we would want. As for what I've done so far: adjusted the telescoping optics prior to the amplifier, adjusted the rear mirror in the oscillator, aligned the amplifier and SHG crystal for proper power output, checked all optics for clipping and damage, changed the polarization of the beam using a waveplate and polarizer, and pleaded with the laser gods for guidance. I have not replaced the YAG rods yet.

If you have any questions, I will answer to the best of my ability, and any suggestions, I will take into consideration. Thanks for any assistance you can provide!

 

[trurle]

As for what I've done so far: adjusted the telescoping optics prior to the amplifier, adjusted the rear mirror in the oscillator, aligned the amplifier and SHG crystal for proper power output

This is already 9 degrees of freedom. Most likely, you are getting wrong modes in oscillator due rear mirror not positioned at bull`s eye. When i was a student, similar problem have happened in lab i attended. It took weeks and flesh on one palm charred to bone to tune properly. Because lab junior finally realized the (paper) targets are not selective enough to catch narrow resonances, and used his own hand in beam path. With other hand slowly rotating micrometer wheel. Not the practice i would really recommend though...
Well, in your case it would be easier - your laser at least visible light output.

 

[Dr_Nate]

Photos of the lenses and/or the setup might be informative.

Some thoughts:
Are these low quality lenses?
Is there dust on your lenses? Did you clean the lenses with the proper solution and lens tissue?
Have you filtered out the pump wavelength?

 

[Andy Resnick]

Along with contracting, burns taken show that the beam quality is slightly elliptical and jagged around the edges, which is a far cry from the ideal circular shape we would want.

This is a tricky problem, indeed! When you mention that the beam is not circular, I think 'astigmatism', which can result from optical components being decentered and/or tilted with respect to each other. Similarly, slight focusing of the beam could result from optical components not being correctly spaced.

Is there a way to shoot a low-power beam through the optics for alignment checking?

 

[shamrock08]

This is a tricky problem, indeed! When you mention that the beam is not circular, I think 'astigmatism', which can result from optical components being decentered and/or tilted with respect to each other. Similarly, slight focusing of the beam could result from optical components not being correctly spaced.

Is there a way to shoot a low-power beam through the optics for alignment checking?

We are currently operating at a lower energy to work on alignment. We are also enabling and disabling the amplifier flashlamp to ensure that the beam isn't clipping in the head while we proceed adjusting the other optics.

 

[shamrock08]

Photos of the lenses and/or the setup might be informative.

Some thoughts:
Are these low quality lenses?
Is there dust on your lenses? Did you clean the lenses with the proper solution and lens tissue?
Have you filtered out the pump wavelength?

I will update the post with a current layout.
To your questions: 1) the lenses are high quality and do not have any marks/burns on their faces. 2) Any dust is swept first with a blower tool and then the optic is cleaned with a methanol/acetone solution and lens cleaning tissues. 3) The 1064nm wavelength is filtered out to <1% of the 532nm beam outside of the cavity. We have not yet used a 532nm filter to check if intensity goes to ~0 however.

 

[shamrock08]

This is already 9 degrees of freedom. Most likely, you are getting wrong modes in oscillator due rear mirror not positioned at bull`s eye. When i was a student, similar problem have happened in lab i attended. It took weeks and flesh on one palm charred to bone to tune properly. Because lab junior finally realized the (paper) targets are not selective enough to catch narrow resonances, and used his own hand in beam path. With other hand slowly rotating micrometer wheel. Not the practice i would really recommend though...
Well, in your case it would be easier - your laser at least visible light output.

Although I will likely not use my hand, improving oscillator alignment is definitely something I will continue to work on.