I do not recommend to use ultrasonic bath, because it is easy to damage or totally remove the coatings.

If standard cleaning procedure does not help, then , may be , your coating is damaged, but not dirty. Or substrate surface is damaged.

Could you explain your problem more detail ?

It is probably damaged more than dirty.

How often dirt deposition can damage a pellicle beam splitter?

You could call ThorLabs or check the product description to see if you can clean the beamsplitter using a solvent (ex. methanol). If you are permitted to clean the beamsplitter using solvent, apply a drop of solvent to high-quality optics cleaning tissue (http://www.newport.com/Optics-Cleaning-Tissue/141178/1033/info.aspx) and gently swipe the surface of the beamsplitter one time. Discard cleaning tissue and, after time has passed to make sure no residual solvent is on the beamsplitter, you can repeat this protocol until the "dirt"/dust is removed.

When you say that ThorLabs did not give you a positive answer, did they not tell you anything or did they say there is nothing you can do (i.e. damaged optics)? If the optics are damaged, you may be out of luck, and you may need to order a new beamsplitter. But, it also depends on the size of your spot. If only a fraction of the beamsplitter is damaged, you can re-position the beamsplitter so that the beam is passing through a non-damaged section (may sound obvious but I have encountered this exact problem before).

They have told me that they don't know how to clean it. And yes, I have ordered a new one, and we are using it as you described: avoiding the dirt.

Thanks Leopoldo! Thus the solution is the one I already implemented: buy a new one.

You may still want to see if you can figure a way to clean it. But expect to break a few.

It is highly recommendable not to act directly on the surface. Not to use any product or even ultrasonic bath. Only with a small jet of clean air, that's all.

Otherwise, it depends how extended is the damage. If you cannot change this element then try to operate in region out of the damage.

For anyone else who finds this, when searching for a way to get dirt off their pellicle beamsplitter:

"For some optics such as holographic gratings, ruled gratings, unprotected metallic mirrors, calcite polarizers, and pellicle beamsplitters, which can be damaged by physical contact, this (Blowing Off the Surface of an Optic with a canister of inert dusting gas or a blower bulb) is the only approved method for cleaning."

(Source: Thorlabs Tutorial)

However, I once got a tip for the gratings in our amplifier that they could be rinsed with deionized water - maybe that could be done with those pellicle BS?

It is made of nitrocellulose and even water dissolves it. I tried method of precise distilled water stream with 92:2 pellicle (scrap, fractured one) and it did not survived the cleaning. One can give a try with coated 45:55 one. The coating could isolate the membrane from water. I would limit the number of attempts to one (prolonged contact with water will peal off the coating from membrane). Please let me know if someone succeed with it. I would rule-out any solvents (ethanol, isopropanol etc.). Probably the method of solvent vapor also will not work out.

Using First Contact polymer will most likely break the membrane during pill-off (maybe someone can try apply thin layer of the polymer on coated version of the beamsplitter and pill-off - maybe I am wrong, )

If comes to CO2 snow method, hmm... one can try it.

Safe is an air puff cleaning from rubber bulb (any compressed air is very risky because if the stream will be too strong it will destroy the membrane). The problem is that in most cases the air will not clean tiny sticked dust (not mentioning about stains).

I was thinking about electrostatic cleaning - it can work for dust but for sure not for stains.

To summarize - 92:2 (uncoated) version of the pellicle is totally not clenable!

The coated one is extremely hard to clean, depend on type of dirt dust, maybe someone succeed aforementioned methods.

Only reliable method is protecting pellices from dust (I am putting plastic bags whenever a set-up is not in use).

It's sad but in most cases you need to buy a new one because it is the most troublesome optical element for cleaning that I have ever tried.

Good luck