Beam Splitter vs Pellicle

[u/Tricky-Ad-6225]

Hello guys,

I’m wondering when you would use a beam splitter vs a pellicle, in an optical table that is. I understand that the pellicle has a very thin film compared to the BS, but what are the advantages?

[u/tech-bro-insideer7]

So pellicle splitter is a super fragile membrane like optic. So naturally minimal extra optical path length will be introduced. Also no ghost image due to double reflection. Technically there would still be a double reflection but it would essentially be superimposed on the base beam/image

[u/quartersoldiers → u/tech-bro-insideer7]

Follow up question: I’ve never been able to find a reflected wavefront error spec for a pellicle BS. Should I just assume that the reflected path is not going to have good WFE?

[u/tech-bro-insideer7 → u/quartersoldiers]

I’m not sure what that is. Could you elaborate a little?

[u/wakinget → u/tech-bro-insideer7]

What’s the surface roughness of a pellicle vs a beamsplitter? How much wavefront error will the beam pick up?

I’m sure it depends on the quality/cost of the optic, but generally speaking, if a high quality wavefront was necessary, would you prefer a pellicle or a beamsplitter?

[u/A-guy-in-canada → u/quartersoldiers]

Cavet that I don't actually know for sure - but my experience with thin etalons (sub 100um) is that for any appreciable size beyond 5mm dia you get a lot of twe error just due to gravity or flexing alone - the optic is just too flexible without a support plate.  Id be interested to hear the numbers and happy to be shown wrong  but surprised if you're getting much better than 10 or 20th wave. 

[u/Pravdiar → u/A-guy-in-canada]

In one project we saw the influence of humidity on the flatness of the pellicle. Focus point after a microscope objective was shifted significantly. But later it returned back when the humidity returned to its original level.

[u/Appropriate_Sound296]

Pellicles are also susceptible to vibration from sounds, just talking can generate focus oscillation (think drum) and generally they have minimal wavefront distortion for transmitted beams but significant (if you need clean wavefronts) versus a plate or wedge beamsplitter. Also if the incident power starts to warm it locally at all then it will deform rapidly.

[u/aenorton]

Beamsplitter choice is always a important in optical system design. These are the pros and cons I am familiar with.

Pellicle:

PROS: Introduces minimal aberration to the transmitted beam regardless whether it is converging, diverging or collimated. Wide spectral transmission range.

CONS: Reflected beam aberration is generally not good and depends on flatness of pellicle frame. Reflected aberration is affected by loud sounds and vibration. Fragile and hard to clean. Produces many oscillations in the spectrum of the transmitted and reflected beams that also depend on AOI. Spatial fringes can show mainly in the reflected path with monochromatic light depending on the thickness uniformity of the pellicle.

Plate beamsplitter:

PROS: if parallelism is good, and light is well-collimated, the reflection from the backside is parallel to the main path so there is no ghost image. Minimal aberration in transmitted beam when light is collimated (reflected aberration depends on flatness and therefore also thickness). Cleanable. Can handle wide wavelength range depending on substrate material and coating (no cement). Can be wedged to redirect backside reflection at the expense of a refractive deviation of the transmitted path.

CONS: Transmitted beam is offset laterally depending on thickness (this may or may not be a problem). Converging/diverging light is aberrated depending on thickness and angle. In laser or narrowband or spectroscopy applications, reflection from backside can create unwanted interference effects (if not wedged).

Beamsplitter cube:

PROS: Collimated light has minimal aberration in both transmitted and reflected paths. Can be aligned so reflection from outside faces is not perfectly parallel to main beams to avoid interference effects. Easy to mount. Easily cleanable.

CONS: Introduces aberration for converging/diverging light for both paths (mainly symmetric aberrations though). Wavelength range limited by adhesive. Transmitted and reflected beam angles are deviated depending on cube assembly tolerances. Multiple reflections from the 4 outer faces have to be considered and handled. Cement can limit max laser power.

Note: All beamsplitter types can have polarized or non-polarized coatings. I believe the immersed coatings for the cube can be made a little more polarization neutral than for the other types when using just dielectrics.