Re: self interference with half wave plate

On Mar 18, 8:04 am, Thomas Gutzler <tgutz...@xxxxxxxxxxxxxxxxxxxxxx>
wrote:
jk wrote:
On Wed, 18 Mar 2009 21:10:22 +0900, Thomas Gutzler
<tgutz...@xxxxxxxxxxxxxxxxxxxxxx> wrote:

Hi,

I have built a telescopic system to generate a large diameter plane wave
from a standard He-Ne laser beam.
[...]

On the CCD sensor I observe an interference pattern consisting of
linear, diagonal, parallel fringes. There are about 30 across the
sensor. I found that removing the half wave plate also removes the
fringes. Tilting the half wave plate to any direction does not affect
the pattern. [...]

Can anyone explain where this interference pattern comes from and maybe
suggest how to get rid of it without having to take the half wave plate out?

Dono, but: tilt and rotate ?

You rotate the halfwave plate. But tilt as I understand it can both
mean that reflections from the two surfaces of the plate does not make
interence for the beam, and further if it is not a zero order plate
make the thickness match a half wave period.

It is a zero order plate and I did try to tilt it to see if it made a
difference to the interference pattern - unfortunately, it didn't.

Tom- Hide quoted text -

- Show quoted text -

Is the plate antireflection coated or not coated for the appropriate
wavelength? Do you have any indications that the half wave plate is a
little off of half wave, like the null isn't as low as you expect?

If the plate isn't coated, you could be getting fringes from the
reflections off of each surface of the plate.you could get it coated
or you could contact or use light machine oil or glycerine to contact
a piece of AR coated glass to one side of the half wave plate getting
rid of one set of reflections and destroying the interference. When
quartz waveplates are being manufactured this is done when the
retardance of the waveplate is checked at the end of the polishing
process and before coating - if this isn't done the measured
retardance is off because of the multiple bounces that the reflections
make inside the plate.

Is this a true zero order plate (very thin) or is it 2 quartz plates
contacted or cemented together with their axes 90 degrees apart (the
optical axis of the plate is usually indicated by a small flat spot on
the diameter) or does it have an air gap with some kind of spacer
between the 2 plates, or a thin layer between 2 pieces of glass (a
polymer retarder rather than quartz)?

.

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