Beam quality problem ...

[smartcut]

Hello !

I am putting in service a new 100 W CO2 laser for ceramic products experiments. The tube is new.

I performed the first tests last week. The power delivered is the one
rated ... but the trouble is the beam shape (pseudo elliptic), due to higher
modes (probably due to a cavity astigmatism). So a bad "M" !

The higher modes are highly divergent so I will have some problem to focus the beam with my ZnSe lens.

I intend to reduce that defect in using a diaphragm (that will cut more
strongly the undesired modes than the TEM00).

My problem is : in what material to do it. I can use a metal sheet but my fear
is about the reflected beams that can return to the output coupler and
perhaps damage the laser at full power.

So what technical solution is practicaly used for that ?

Many thanks for any help !

All the best !

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[laserboy]

here is an option use what is called a phase shifter/ polarizer in right before focus lens. very common with co2 lasers with no shifted internal rear mirror in resonator.

[Laser Pro]

A spatial filter would help you significantly, essentially a collimator with a pinhole plate at the focal point of the collimator. This will help shave off the mulitorder fringe areas of your mode if used correctly. Edmunds and Newport both sell them to fit your power levels.

[smartcut]

Many thanks for the informations.

I already tried a very simple system that (roughly) works, before to purchase any optical (and expensive !) device.

I used a copper tube that acts as a wave guide and a mode mixer. The beam is quite cylindric at the output. And it insures that the beam will not burn the rubber seal of the lens : the tube limits the beam expension just before the lens.

Of course, this is just something to wait until a better and more serious system ! (to get a true TEM00 beam).

But in fact, my experiments deal with ceramic processing trials, so a high quality beam is not required. My most important problem was a good drive of the beam through the lens without any damage to the lens holder.

Nevertheless, I will get some informations about the optical systems described.

Thanks again !

[Laser Pro]

Many thanks for the informations.

I already tried a very simple system that (roughly) works, before to purchase any optical (and expensive !) device.

I used a copper tube that acts as a wave guide and a mode mixer. The beam is quite cylindric at the output. And it insures that the beam will not burn the rubber seal of the lens : the tube limits the beam expension just before the lens.

Of course, this is just something to wait until a better and more serious system ! (to get a true TEM00 beam).

But in fact, my experiments deal with ceramic processing trials, so a high quality beam is not required. My most important problem was a good drive of the beam through the lens without any damage to the lens holder.

Nevertheless, I will get some informations about the optical systems described.

Thanks again !

You are welcome, and I wish you the best of luck with your experiments.