wrong results of interferometry test ?

On: Thu, Dec 20, 01 05:15:44 PM

T. Klein wrote:

Hello, recently, I did a interferometry test with my 7mW He-Ne and I measured the coherence length. Surprisingly it wasnīt that ridiculous length of 10 cm but the fringes just didnīt disappear so far I could move the mirror - at least 2 metres. That sounds funny you might claim but itīs not at all. Due to this experience I placed the subject( a assembly of several subjects also containing a glass item ) to be taken the wavefront of in a distance of about 30 cm relative to the film and was just able to see the reflection spots on the glass but nothing else. Do you know whats happening and how to solve the problem ?
RESPONSES

Jonathan - Fri, Dec 21, 01 03:26:04 AM

It may be that your interferometer test was a bit misleading. Sometimes a quite strong interference pattern can derive simply from one of the (main) beams only. This can happen if the beamsplitter glass is fairly thin. What you may be seeing is two beams interfering with each other due to secondary reflection (from the other side of the glass). You could try using a thick piece of glass to mount your beamsplitter (with optical glue to assure index matching). Or simply do the coherence length test again and make sure the main fringes are well defined. If you need to remount the BS you'll find it improves your holograms considerably. Hope this helps. 209.90.160.166

Tom B. - Fri, Dec 21, 01 04:18:08 AM

My guess is that at that distance, you're not getting enough reflected light from the non-shiny objects. Diffusely reflected light spreads out quickly, and the plate is only catching a small part of it. You need to either increase the amount of light on the object to compensate for the distance or reduce the reference beam brightness (and expose for a longer time). Adjusting relative object/reference brightness is easiest to do in a split beam setup, but is also semi-possible in a single-beam setup: If the object is far from the plate, you can put a filter halfway into the spread beam so that the object is fully lit, but the plate is lit by the filtered part of the beam. If you use a polarizing filter (with a polarized laser) you can adjust the reference brightness by rotating the filter. 24.67.253.203

T. Klein respectively Thilo K. is better - Fri, Dec 21, 01 07:33:01 AM

To Jonathan: youīre right in most cases. But this time there is no interference any more if I block one of the reflected beams. Thus fringes only occur when both beams overlap. By the way: what did you think of when writing "main fringes are well defined" ? To John: You wrote: "and expose for a longer time". What time do you suggest for a 4 x 5 inch PFG-01, the laser mentioned and what development time in the known developer containing metol(Sorry i canīt remember the missing chemical items). 145.254.228.194

Thilo K. - Fri, Dec 21, 01 07:36:01 AM

some corrections: To Jonathan: youīre right in most cases. But this time there is no interference any more if I block one of the reflected beams. Thus fringes only occur when both beams overlap. By the way: what did you think of when writing "main fringes are well defined" ? To Tom B.: You wrote: "and expose for a longer time". What time do you suggest for a 4 x 5 inch PFG-01, the laser mentioned and what development time in the known developer containing metol(Sorry i canīt remember the missing chemical items). 145.254.228.194

Jonathan - Fri, Dec 21, 01 01:46:26 PM

Thilo, by "main fringes are well defined" I was thinking that if you are getting an interference pattern throughout 2 metres of mirror movement then it's important to know the source of the fringes. As far as I'm aware there are only two ways to explain your results - 1) your He-Ne has an etalon, or 2) the fringes are not from overlapping beams that were split and then recombined, or the mirror you moved is before the split. Interesting puzzle. 209.90.160.136

Thilo K. - Fri, Dec 21, 01 04:31:57 PM

Jonathan, i think i should give you a small picture to explain what setup Iīm referring to when measuring the coherence length: /<Mirror> \ <Lens>() <BS>|<< / <<<<<Laser> \<Mirror being moved to BS> i hope you can recognize it. it should be as a michelson one, which i found in 'practical holography' from saxby. 145.254.232.87

- Fri, Dec 21, 01 04:52:16 PM

another try +---------+ / -+ | | | | | / Mirror *** -- | | | | | | / * ** --- | | | | | | /-- Lens --- | | | | | | / ---- * -* | | | | | | / --- | **-- ** | | | | | | / ---- | - -** * Fringes --- | --- ***** +---------+ --+--- ** --|--- -- | - -- -- | --- Laser -- | ---- -- BS --- \ -- ---- \ -- -- \ -- \-- \ \ Mirror to be moved \ \ 145.254.232.87

Jonathan - Fri, Dec 21, 01 10:54:01 PM

Thilo, do you mean to have the BS in that position? Try it again but turn it 90 degrees. Suggestion: since remounting your BS is a hassle and will take time, just find a thick piece of glass (at least 1/4", preferably more). It'll work just as well, since the ratio of the beam splitter when used this way is irrelevant. 209.90.160.103

Tom B. - Sat, Dec 22, 01 04:29:27 AM

Exposure time for 4X5, 7 mW HeNe - could try 10 seconds to start. Development time in unknown developer containing metol - 5 minutes at 20 Celsius. Adjust exposure time up or down to get desired density after full development time - density 2.5 for relection holograms (0.3 percent of light gets through), for transmission holograms use density 1.5 = 3 percent. And yes, the beamsplitter needs to rotate 90 degrees from what's shown in your drawing. In "Homemade Holograms" figs. 2-3 and 2-5 the same mistake is made. "Holography Handbook" gets it right. If you mentally trace the beam's path, dimming it by 50% each time it bounces off or passes through the splitter and compare path lengths, you'll see why one arrangement works and the other only sort of works, producing weak fringes at best. 24.67.253.203

Thilo K. - Sat, Dec 22, 01 05:18:12 AM

both: youīre right the bs should be drawn rotated 90 degrees from its current position. Indeed it was just a wrong drawing - my BS has always been standing in the right position. So to summarize: 1. I should use a thicker glass for beam splitting purposes 2. I should change the ratio 3. Exposure time ought to be 10 secs - also using PFG-01 4. Development time 5 Min á 20°C 5. Film density 2.5 to 1.5 Do you have any further suggestions ? 145.254.230.161

Colin - Sat, Dec 22, 01 06:46:13 AM

The thicker beam splitter will make it easier to card off the secondary reflection off the back side of the beam splitter. It seems to me like you had interference between the two paths seperated by the thickness of the beam splitter. 64.167.148.235

Jonathan - Sat, Dec 22, 01 12:29:33 PM

Thilo, regarding your summary - your number 2 needs comment. You do not need to "change the ratio". The point is that the ratio of the beam splitter in a Michelson interferometer doesn't matter. Any ratio will produce virtually the same result. This is because each of the two recombined beams that end up producing the fringes will have been transmitted once and reflected once by the beamsplitter, but in different order. The symmetry of the components on the table results in the more or less equalization of the two recombined beams, whether you use a 50:50 splitter or a piece of glass, which is about 90:10. 209.90.160.119

Thilo K. - Sat, Dec 22, 01 01:07:51 PM

Jonathan, first of all: thanx for your fast answers ! to your correction: naturally iīm referring to Tomīs answer: "[...] You need to either increase the amount of light on the object to compensate for the distance or reduce the reference beam brightness (and expose for a longer time). Adjusting relative object/reference brightness is easiest to do in a split beam setup [...]" - i summarized the answers of you as well as Tomīs... 145.254.236.190

Use this form to add your comments to the current forum entry above
Your Name:
The Text...

in
HTML, or
Plain text
Action or
| Main Index | Reload | Help? |