Tailor made glass optical slits

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Tailor made glass optical slits

Post by Wah » Mon Jul 27, 2015 6:18 am

I recently have tailor made several glass optical slits from mainland China.
The slits quality seems to be better than what I originally expected.
But the reflectivity is only about 62% which is lower than my expectation.

3 slits in one glass plate: 10, 20, 30 um
Glass plate 30X2mm
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Re: Tailor made glass optical slits

Post by DSobserver » Mon Jul 27, 2015 10:36 pm

now waiting for a shg test!

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Re: Tailor made glass optical slits

Post by Wah » Tue Jul 28, 2015 1:25 am

Here is a reflectivity comparison to another glass slit that I have.

http://adamslab.co.uk/Students%20Lab/Ph ... uct_id=816

I took some pictures before and after reflection.
The reflectivity of both slits are lower than my expectation, and the China made one is not a lot lower than the other.
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Re: Tailor made glass optical slits

Post by p_zetner » Tue Jul 28, 2015 4:16 am

Hi Wah.
Please let me know if I understand the "reflectivity" you're referring to. Your slits appear to be gaps in a reflective, metallic coating on a glass substrate, obtained either by etching or masking. By quoting a 62% reflectivity, are you saying that 38% of the incident light "leaks" through the metallic coating?
Thanks.
Peter.

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Re: Tailor made glass optical slits

Post by Wah » Tue Jul 28, 2015 4:52 am

Light pass through the coating is very little, I only could see very dim image of the light bulb.
So I guess 3X% of light is absorbed by the coating.
I will do some stress test later when I have time and the weather goes fine.

BTW, the manufacturer told me that such coating was designed to withstand high heat.

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Re: Tailor made glass optical slits

Post by p_zetner » Tue Jul 28, 2015 5:17 am

Sorry, then I don't understand why a reflectivity measurement is important to assessing the quality of the slit.
Are you talking about transmission through the slit?

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Re: Tailor made glass optical slits

Post by Wah » Tue Jul 28, 2015 5:31 am

Glass doesn't dissipate heat very well as metal does, I was expecting the reflectivity be higher to reflect more solar heat away.
For example, 95% reflectivity means only 5% of energy / heat is absorbed by the coating.
Now the reflectivity is not very high, that means the glass being exposed to the prim focus sun light may heat up very soon. If it cannot withstand the heat difference, it may break or even explode.

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Re: Tailor made glass optical slits

Post by p_zetner » Wed Jul 29, 2015 1:20 am

Ahhh .... thermal loading! Thanks, Wah.

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Re: Tailor made glass optical slits

Post by thesmiths » Tue Aug 11, 2015 12:53 am

Wah, your comments made me look at the Thorlabs metal slit that I have been using for SHG. These are stated to be: "302 Stainless Steel w/ Black Oxide Finish". I have actually not used it a huge amount but I notice that the "black oxide" has been completely burned off in an oval shaped pattern due to scanning the Sun over the slit. The oxide on the back side has also been partially removed. The exposed stainless steel metal looks slightly odd; perhaps the metal has recrystallized in some unusual way. The slit itself seems to be undamaged so far but I will have a closer look with a microscope at some point.

The Thorlabs slits were designed for laser work and are claimed to have "High Damage Threshold": 1 kW/mm2 CW at 10.6 µm. This is a much higher level than what I have used it for so I am slightly surprised. The stainless steel disk is 9.5 mm in diameter and 12.8 µm thick.

In any case, I agree with you that SHG slits are under very heavy thermal load. In fact, aluminium might even be evaporated off the glass eventually. I would probably recommend using a metal like chromium (much higher evaporation temperature). Ideally, it would be better to use sapphire rather than glass. Have you noticed any damaged to the slits you have used so far?

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Re: Tailor made glass optical slits

Post by Merlin66 » Tue Aug 11, 2015 1:58 am

I developed a chrome on glass multi-slit plate for the Spectra-L200. The minimum gap is only 20micron and the slit length 6mm - so not ideal for the SHG application.
I and others have successfully used this slit plate on the SHG and so far I haven't seen any deterioration.
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Re: Tailor made glass optical slits

Post by Wah » Tue Aug 11, 2015 5:39 am

Yes, my slit is using chromium coating.
I still don't have time to test it because I just had some time to capture the sunspots 2396 :lol:

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Re: Tailor made glass optical slits

Post by thesmiths » Tue Aug 11, 2015 11:48 am

If you have a chance to make another run of slits, I'd recommend getting a batch of 1 inch c-axis sapphire wafers. They are very commonly used in the semiconductor industry and are reasonably priced (maybe around $10 each these days?). As stated by Thorlabs: "sapphire provides exceptional surface hardness and a transmission range extending from the UV to the MIR. It can only be scratched by a few materials other than itself. The uncoated substrate is chemically inert and insoluble to water, common acids, or alkalis at temperatures up to ~1,000 °C." Another quote from Kyocera: "Sapphire is single-crystal alumina, a transparent material with excellent chemical stability and valuable mechanical, thermal and optical properties. Its material properties are vastly superior to glass, quartz and other mass-produced transparent materials."

Chromium should adhere to sapphire quite well. It has much higher thermal conductivity than glass. And even if you dropped a sapphire wafer on the floor, there is no chance it would break (unlike glass or quartz). Sapphire wafers also tend to be a lot flatter and smoother than glass wafers (I used to work in the semiconductor industry).

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Re: Tailor made glass optical slits

Post by Spectral Joe » Tue Aug 11, 2015 9:38 pm

Too much gut feeling here, and not enough science. Here are some facts and numbers to consider. First, we will use a 100mm aperture F/15 lens to collect our sunlight. 7.85 watts total falls through the aperture on a perfect day, forming an image 13.8mm in diameter. Most of that power (except for lens losses) will fall on the slit. A properly made slit consisting of chromium on glass will reflect 95% of that power, absorbing 0.393 watts. If it was aluminum, it would absorb 0.785 watts. Glass is opaque in the far IR, and conversely, it radiates very well there, meaning it sheds much of the absorbed energy as far IR radiation (from the uncoated backside). It absorbed very little anyway, and you would be hard pressed to detect the temperature rise. Far, far from evaporating the metal, exploding, or whatever. Blackened stainless jaws, on the other hand, will absorb almost all of the energy, and the coating would be bleached by both heat and UV exposure. This is a major reason that slits on solar instruments are always polished, I've used polished tool steel slits for a very long time, no issues with heat or otherwise. As for sapphire, consider that the refractive index of sapphire is much higher than glass, leading to a reflection of 7.7% per surface as opposed to 4% for glass. Having a slit deposited on a transparent material (as opposed to an air gap) causes some loss of contrast due to reflections bouncing between the glass surface and the metal surface. The effect is small with glass, but would be almost twice as large with sapphire, in addition to the reduced overall transmission. My recommendation is still for air slits in this application, but if the chrome on glass slit is made as specified it should cause no problem. If it's to spec.

Edit:
I re-read the first post and you say the reflectivity is only 62%. That's going to get warm, but might still survive. Why the low reflectivity? Evaporated chrome should be 90%. Make regular air slits, you'll be happier, just make sure they're shiny.
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Re: Tailor made glass optical slits

Post by Wah » Wed Aug 12, 2015 4:46 am

Regular slits can easily be dusty and hard to be cleaned, for me.
If the blades touch each other, it also can be damaged.
Both can cause serious banding problem to the SHG images.
That's why I would like to try glass slit instead.

For the reflectivity, I can only test visible light based on my camera images, it doesn't mean the slits would have same low reflectivity in IR.

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