LSRMDLTR: laser waist definition in manual

[pannek]

Dear all,

according to the elegant manual, the laser waist for the LSRMDLTR element should be specified as w_0 = sqrt(2)*sigma_xy. However, the referred paper in the manual (https://www.slac.stanford.edu/~achao/La ... cussed.pdf), analytical calculations, but also a look in the code indicate that it should actually be w_0 = 2*sigma_xy, which is also a common definition in laser physics.

The paper cited in the manual states that sigma(z)=w(z)/2 (page 4, start of chapter 3). In the text right below this equation, it is mentioned that sigma_0=w_0/sqrt(2), where w_0 = w(z=0), which in my view is not a correct conclusion, but is apparently used in the elegant manual.
(Edit: Here, sigma_0 actually refers to the rms focal spot size (radius), as mentioned in the paper, so that the statement in the paper is correct. In the elegant manual, however, it is (was) written that sigma_x=w_0/sqrt(2).)

Also the code itself seems to be based on the definition w_0=2*sigma_xy. This becomes particularly clear when you look at the code line for the Rayleigh length calculation, which is defined based on the laser waist of the LSRMDLTR element as:
lsrMdltr->ZRayleigh = lsrMdltr->k / 2 * sqr(lsrMdltr->laserW0);
This is a common definition and is actually based on w_0=2*sigma_xy, not w_0=sqrt(2)*sigma_xy as suggested by the manual.

Finally, analytical estimations for the imprinted energy modulation, as well as genesis simulations, only agree well with elegant results if the waist is specified as w_0=2*sigma_xy, otherwise a factor of about 2 less peak power is needed in elegant to achieve the same energy modulation, which is in agreement with the w_0 = sqrt(2)*sigma_xy vs w_0 = 2*sigma_xy issue, since the energy modulation scales as Delta Energy ~ sqrt(peak power)/waist.

This brings me to the conclusion that the correct definition in the manual should read w_0=2*sigma_xy, where sigma_xy and w_0 refer to the intensity distribution. The definition w_0 = sqrt(2)*sigma_xy would be reasonable only if sigma_xy refers to the field size, and w_0 to the intensity, but that's not further outlined in the manual.

Am I right about this? I hope I have not completely misunderstood something here and look forward to your opinion on this!

Best
Fabian

[michael_borland]

Fabian,

Yes, you are correct. In writing the manual, I confused the rms radius with the rms x or y value. These differ by sqrt(2).

Thanks for pointing out the error. The correction will appear in a future version (maybe not the next version since we are already preparing the release).

--Michael

[pannek]

Dear Michael,

thank you for the clarification.

Your reply also helped me to understand the term rms focal spot size (radius) mentioned in the paper above. I will edit/expand my original post so as not to confuse potential readers.

Thanks again! I am looking forward to future versions of elegant.

Fabian