An astigmatic collimator could be used, for example made up by two cylindrical lenses crossed orthogonally with each other. Of course out of a material that diffracts at the mid-IR wavelength you use - not my wavelength range, so I cannot help here. According to your graph, one cylinder lens should have about 100mm focal length, the other one 250mm.

Maybe the easiest is just putting a cylindrical lens with 155-170mm focal affecting the x axis and being neutral for the y axis.

Is the supplier of the mid-IR laser able to provide a collimation lens or perhaps they could recommend a supplier of a suitable lens? The use of a toroidal mirror might be another option.

Thanks guys

i ordered two cylindrical mirrors, since this is the simplest (and chipest) method i thought of.

nevertheless, mirror location and alignment is crucial, and i have no means to examine the collimation quality. maybe with knife edge?

Hi there,

There is another possible way that one may convert an astigmatic beam into a normal gaussian shape, which I used in the past with Ti:Sa laser. Then, use spherical lens to focus the beam.

Indeed, you might use a pair of prism together to turn the beam into a Gaussian beam or just simply order an anamorphic pair of prism available commercially for this purpose, see this link for instance:

http://www.thorlabs.com/NewGroupPage9.cfm?ObjectGroup_ID=149 .

Good Luck,

Thanks Ebrahim

thats sounds like a good idea, but in the mid-ir it will be very expensive, if the prisms are available at all.

BTW, the manufacturer of the source refuses to use the term astigmatic. they claim that it is "a beam with different focal locations and divergence angles for X and Y axes" but it is not astigmatic....

Another possibility, not elegant but efficient (mostly used for excimer lasers), is just tilting one of the spherical lenses in you setup, you add some astigmatism, and so you can also compensate some known astigmatism...

Again, not the best in the search for diffractive limit focus, but simply and cheap...

@ Ran,

Hi Ran, did you solve this issue with cylindrical lenses? what about beam quality after lens?. We have same issue with mid-IR thinking to use toroidal mirror? can you please add your input?

Hi.

as a matter of fact i wasnt free to deal with that issue. but feel free to contact me if you woulk like to think it over together.

Ran

Hi Ran,

We are thinking to use toroidal mirrors. Will update you if any.

You can use a cilindrical lens to change beam divergenge and correct the focal point in the X direction, for example

Yes. Cylindrical lens corrects astigmatism, but there will be a focus shift for broadband wavelength source (tunable laser). We also think of this option as toroidal mirror manufacturers say it a custom element and asking for 10 weeks lead time and expensive too.