Dear Maryam Mokari , a lens shouldn´t be necessary at all. What you are sending to the SLM should be a phase plate encoding Bessel beam you want. That means that you shouldn´t see any image, the SLM just define an arrangement of refractive index variation, and you only can see the effect when send a light beam and observing how it is modified by the SLM.

If the encoded information of phase sent to the SLM doesn´t produce a Bessel beam, and it just shows a focal spot at a defined distance, that means you are encoding a diffractive element (probably a Gaussian beam) and not a Bessel beam.

Probably one of the best known elements to obtain a Bessel beam would be the axicon, basically a cone shaped "lens" which has a focal line instead of a focal spot. That focal line is a Bessel beam. A Gaussian beam can be transformed into a Bessel beam when the Gaussian beam passed through the axicon.

https://en.wikipedia.org/wiki/Axicon

The basic geometry of the axicon can be combined with some other funtions while retain the non-diffractive properties. For instance the light sword optical element we developed some years ago:

Article Imaging with extended focal depth by means of the refractive...

Article Assessment of imaging with extended depth-of-field by means ...

Diffractive flat lenses with the right encoding also produce Bessel beams, please look for instance the following preprint by Banerji et al :

https://arxiv.org/ftp/arxiv/papers/1910/1910.07928.pdf

Spiral diffraction grattings also can generate Bessel beams, as shown by the work by Jiménez et al:

Article Formation of high-order acoustic Bessel beams by spiral diff...

I hope it helps.

Manuel Gómez

Thank you so much

It helped a lot.

Hi Maryam Mokari , glad to know :). You are welcome.

Good luck with your research.

Regards,

Manuel