I was designing a spectrometer with a dispersion prism and got a question about incidence angle and optical coating. In the design, the collimated beam will enter the prism with some incidence angle. To secure the dispersion, the incidence angle should be higher than 49 degrees or more. The prism surface will be AR coated.
Are there any disadvantages of having a ‘high’ incidence angle? I guess there could be polarization error with a high incidence angle or some energy loss? But have not much experience in this topic.
Any thoughts will be appreciated.
I know woefully little about thin films (despite Dr. Falco’s best attempts), but I am pretty confident that you will start to get undesirable effects at a high incident angle. I know there are some absolute experts on here so hopefully one of them replies to your question!
At higher angles of incidence, it can be difficult (but not necessarily impossible) to simultaneously antireflect both S- and P-polarizations. I assume for your application your AR coating will be very broadband, which will limit the minimum reflection achievable. In general, the greater the bandwidth that the AR coating needs to operate over, the higher the minimum reflectance over that band. I believe one way of avoiding this issue is to use a grating rather than a dispersive prism if total throughput is critical.
@hkang Can you share the bandwidth of the design?
and do you care about maintaining equal power in both polarizations or does just the total throughput matter?
@craigament Thank you! I also considered grating, but as we want the dispersed beam to go back to its original position and due to very limited space, the prism was more suitable for our purpose.
@Isaac from 0.9 um ~ 2.4 um, and we didn’t care polarization.
@hkang I took a very quick stab at designing an AR coating on Fused silica, 49 degree AOI. See attached photo, the p-polarization is always going to be the lower one, and it tends to be the S-polarization that picks up. Hopefully that gives you a bit of an idea of what is achievable. If you need a zemax file with that coating prescription I can send it to you. Hope that helps.
Wow, this graph looks awesome. It helped me to get a better understanding of coating… Thank you!
By the way, @craigament is the owner of Arizona Thin Films, so you could always work with him in a more official capacity to hammer out the details of the coating!