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practical tips for optical interferometry

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johnf

Sat Aug 22 2015, 07:56PM

Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
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Dr Slack
you could try this

Conundrum

Sun Aug 23 2015, 07:32AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
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Yes, low power SM green, blue AND UV now exist (thanks to Nakamura's pioneering work) and these would obviously work much better for this.

Bored Chemist

Sun Aug 23 2015, 01:44PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
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You can cheat by bouncing the light to and fro between two mirrors, one fixed and the other moving.
If the light bounces between the two mirrors 10 times then the optical path length difference is 10 times the displacement of the moving .
mirror.
The alignment is a bit trickier (though not much, with a laser) but you get the equivalent of using a 10 fold shorter wavelength.
this sort of thing.

Shrad

Wed Sept 09 2015, 02:42PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
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use two corner cube mirrors and get the beam where it gets out, you'll get much more multipass without the hassle of alignment issues

Conundrum

Thu Sept 10 2015, 07:50AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
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Are these the same as the beam optics found in PS3 sleds?

Signification

Sun Oct 04 2015, 04:31PM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
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A quick comment that seems to relate...in the 70's for a science fair project on holography (only HeNe's back then), I attempted to make a double exposure of a small cut glass object under stresses to show interference fringes. Everything would blur the final image...until...I finally only poured a bit of lukewarm water into it, exposed it, then made the second exposure when the water reached room temperature--GREAT FRINGES--finally!

Shrad

Sun Oct 11 2015, 09:04PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
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this is a corner cube mirror :

those you find in PS3 sleds are polarizing beamsplitters which serve to separate beams of different polarities out of a combined beam, or combine two beams of different polarities into one combined beam

Conundrum

Sat Oct 17 2015, 07:07AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
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Ordered a 450nm PL450 diode, suitable as single mode.

Signification

Sun Dec 27 2015, 10:38AM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
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Posts: 367

Wastrel wrote ...

You can get low power SLM laser diodes that will do holography. 10m+ coherence length.

I'm not sure the application is right. If you can do fringes, then a sodium lamp is probably fine. You only need a coherence length of the order of 1um.

If you can use a lever to reduce the micrometer movement to 1 in 10 or lower, that might be the best way.

...never did get a good understanding of how such long coherence lengths (now I see >10m

) were obtained from laser diodes--compared to the explanation of how coherence length is determined by gas lasers...HeNe. Following a -well-written- book here--sold by Radio Shack--in the 80's... title: "UNDERSTANDING LASERS" IIRC the explanation was only a paragraph why a diode laser could get a CL of only mils.

Wastrel

Sun Dec 27 2015, 02:09PM

Registered Member #4095 Joined: Thu Sept 15 2011, 03:19PM
Location: England.
Posts: 122

I'm guessing the explanation of coherence length you've read put's it down to the length of the cavity. For the tiny cavity length, diode lasers have allowable lasing modes much further apart than gas lasers. Unfortunately they have a much much larger gain width, so lasing generally happens in multiple linear modes resulting in a short coherence length. A properly designed diode for SLM mode, or many others operating with carefully controlled current and temperature operate with the gain over a mode and the current too low to lase on any others. In this case the cavity is short but only one mode is lasing. A typical 30cm TEM00 (Single transverse mode) HeNe is normally lasing 3+ linear modes, so it's coherence length is limited by the cavity length (the length over which the modes interfere and cancel out).

SAM's laser FAQ has a section on frequency stabilisation of a HeNe that's extremely cunning and might flesh out these details for you.

TL/DR A laser with multiple linear modes has a coherence length limited by the cavity length.

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