Interference effect - reflectance from two glass surfaces...

Reflection from two glass surfaces, 4% each is NOT 8% if light is monochromatic, spatially and temporally coherent. Depending of glass thickness and angle of incidence, it ranging from 0% - 16%. My Green DPSS is not the perfect option to show you that effect - it is nor perfectly coherent, nor has perfect narrow spectral bandwidth. Still, you may see some effect - changing brightness of the reflected laser beam onto screen.

Such effect is stronger close to 0° incidence. Above 15° it almost completely gone. With optical etalon, this effect may be evident even at wider angles. But, I have no any etalon at the present.

Category:

Science & Technology

Tags:

• laser
• green
• dpss
• constructive
• destructive
• interference
• interferences
• effect
• effects
• glass
• surface
• reflection
• reflectance
• spatial
• temporal
• coherence
• spectral
• bandwidth

[MilanKarakas]

@live4Cha

Oh, well... yes. I know. It just dim a little bit, but not completely disappears. Many reasons, from not so coherent laser - to not so perfect glass piece. It will be better to use thin glass instead thick one.

[live4Cha]

i can't see it disappears! tnx anyway

[MilanKarakas]

@MilanKarakas Oops. Not exactly 1064 nm for YVO4 crystal. But close to that.

And thank you, but this is not my observation - it is well known phenomena. I just wanted to point out to that possibility.

And, during rush to make this short film, I forgot to mention that it is not actually exactly from 0 - 16%. FIrst reflectance is say 4%, then second reflectance is 4% of 96%, and that is 3.84%, not 4% as I said above.

This is very important - 4-3.84=0.16%. And there are another reflections....

[MilanKarakas]

@magx1 This is hard to tell. "More speckles" may be due to very narrow beam hitting the target, or properly collimated or focused to the target - so that reflectance and it's interference nature is more pronounced.

Remember: green DPSS is actually IR laser at 1064 nm, just frequency doubled. There is problem with "dual wavelength" mirrors - green and IR. If one cavity mode is well matched (say IR), then other one is probably not. Not to mention thermal expansion, thermal lensing effect etc.

[magx1]

Very clever observation - I am impressed! Good work!

Are common "red" diode pointers more coherent than common DPSS "green" lasers? I'm just curious. Some of my red lasers seem to have more speckle than my green ones.