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I came across "we used Nd:YAG laser achromats" in a paper, and I was wondering what exactly is the purpose of achromats here. The paper in question is "Dynamics of laser-induced cavitation bubbles near an elastic boundary" by Brujan et. al., and the relevant part of the text is:

...a Nd:YAG laser achromat (f = 200 mm) to allow a large focusing angle together with a large distance between focus and cuvette walls. A second Nd:YAG laser achromat (f = 125 mm) focused the laser pulses into the glass cuvette. Achromats were used for beam collimation and focusing to minimize spherical aberrations...

I understand the main point behind achromats for general optics - to reduce or eliminate chromatic aberrations for different wavelengths (for visible light typically red and blue, so the green and the rest of visible spectrum ends up mostly fine too).

However, what is the point behind using these elements for laser light? This light is essentially monochromatic anyway. I also found http://catalog.rossoptical.com/catalog/doublets-and-triplets/laser-achromats/ that says their laser achromats "have been corrected for spherical aberration, coma and astigmatism.". No emphasis on chromatic aberrations for laser light either.

So, what is the purpose of "laser achromat" in this context? Is the point reducing that bit of chromatic aberration for mostly monochromatic light, or are they somehow inherently better for those other aberrations than other lenses?

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  • $\begingroup$ Not all lasers are monochromatic. Where have you seen these used? $\endgroup$ – Emilio Pisanty Dec 10 '18 at 10:43
  • $\begingroup$ @EmilioPisanty There was a paper regarding use of few ns pulsed Nd-YAG laser (lambda = 1064 nm) to make bubbles in water, where they stated they used some achromats for focusing and I am wondering what was the purpose of achromats vs ordinary lens (good lens regarding other aberrations but without emphasis on chromatic aberration). I know the wavelength isn't a single exact number, but I guess it should be still narrow for both the Nd-YAG and the mentioned IR diode laser so I believe chromatic aberrations are a minor concern - is achromat simply stand-in for good lens in these setups? $\endgroup$ – Zizy Archer Dec 10 '18 at 11:33
  • $\begingroup$ Which paper? Always provide a full reference to the thing that you're looking at - you never know when there might be additional context in the text that you're not seeing but that answerers might require. $\endgroup$ – Emilio Pisanty Dec 10 '18 at 12:43
  • $\begingroup$ @EmilioPisanty I have edited the question now. Hopefully it is clear(er) now. $\endgroup$ – Zizy Archer Dec 10 '18 at 15:03
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    $\begingroup$ Achromatic lens should have nothing to do with reducing spherical aberrations. It may be that achromats are commonly aspherized as well (see www.edmundoptics.com/resources/application-notes/optics/why-use-an-achromatic-lens/). Spectral width of the nanosecond pulses used in the paper should be well within 100 MHz range, hence chromatic aberrations seem to be a non-issue. On the other hand, they say they used a He-Ne laser for aiming, and in this case achromats would indeed come in handy. $\endgroup$ – Arturs C. Dec 10 '18 at 20:38

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