orbital angular momentum data transfer controversy

[George Herold]

Using multiple modes is a perfectly fine way to get more capacity, if
you can keep them straight.  It just isn't what the twisty folks think
they're doing.

BTW optics folks have known about these modes for yonks--field
distributions with nulls in the centre and helical phase are called
"optical vortices".

I first encountered one in a microscope I designed in cooperation with
some guys from Sira Ltd in the UK, back in 1990 or so.  It was the first
silicon immersion microscope, based on contacting a silicon plano-convex
lens to the back of a silicon wafer.  (A technique that has become
widely used since then.)  It was to use a pressurized air bearing with
50 nm clearance to allow scanning and reduce surface damage.  That 50 nm
was enough to lose a lot of light by total internal reflection, but
interestingly it turned out to be possible to overcome the problem by
switching to tangential polarization, where the E field direction was
always perpendicular to the plane of incidence no matter where you were
on the lens.  This was accomplished using a segmented half wave plate,
shaped like an 8-petal daisy, which generated a very creditable
approximation of tangential polarization.

Wow, Nice story! Thanks

I'm going to have to ask for a pic of the 8-petal daisy, on some bar
napkin, when I buy you one.
(alternating segments of 1/2 wave plates?)

We were very proud of this idea, until we noticed that the resulting
focused spot would have a null at the centre, i.e. the spatial
resolution would be horrible.  (The different segments would be trying
to have it point in every direction, and none would win.)  That was an
example of an optical vortex, maybe the first one in a technological
example, I don't know.

We straightened it out by applying a coating of 1/8 wave to segment 1,
1/4 wave on segment 2, up to 1 wave on segment 8, so that the phase
advanced by one cycle as you went round the pupil.  That replaced the
central null with a circularly-polarized peak, and cleaned up the
resolution amazingly.

It keeps getting better, (sounds expensive?)

The design was all finished and ready for production by early 1992, at
which point IBM very nearly went bankrupt, and both our budget and our
customer went away.  I still have all the drawings, though.  If it were
built today, it would still hold the lateral resolution record in
silicon by about a factor of 2 over any current instrument.  (It ran at
a numerical aperture of 6.4.)

How many good designs are in drawers somewhere?

George H.

 

[George Herold]

I've been wanting to do that for ages, but every time I get what looks
like the right application, it turns out to be a force fit, and I have
to do it another way.   One reason is the huge chirp you get in a
cleaved-cavity laser as it turns on.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot nethttp://electrooptical.net- Hide quoted text -

- Show quoted text -

I'm reminded of laser intra cavity absorption, which is different, but
similar.

George H.

 

[Bill Sloman]

How many good designs are in drawers somewhere?

I can name at least two, though they'd take quite a lot of updating.
Twenty years is a long time in electronics.

 

[Jamie M]

Using multiple modes is a perfectly fine way to get more capacity, if
you can keep them straight. It just isn't what the twisty folks think
they're doing.

BTW optics folks have known about these modes for yonks--field
distributions with nulls in the centre and helical phase are called
"optical vortices".

I first encountered one in a microscope I designed in cooperation with
some guys from Sira Ltd in the UK, back in 1990 or so. It was the first
silicon immersion microscope, based on contacting a silicon plano-convex
lens to the back of a silicon wafer. (A technique that has become
widely used since then.) It was to use a pressurized air bearing with
50 nm clearance to allow scanning and reduce surface damage. That 50 nm
was enough to lose a lot of light by total internal reflection, but
interestingly it turned out to be possible to overcome the problem by
switching to tangential polarization, where the E field direction was
always perpendicular to the plane of incidence no matter where you were
on the lens. This was accomplished using a segmented half wave plate,
shaped like an 8-petal daisy, which generated a very creditable
approximation of tangential polarization.

We were very proud of this idea, until we noticed that the resulting
focused spot would have a null at the centre, i.e. the spatial
resolution would be horrible. (The different segments would be trying
to have it point in every direction, and none would win.) That was an
example of an optical vortex, maybe the first one in a technological
example, I don't know.

We straightened it out by applying a coating of 1/8 wave to segment 1,
1/4 wave on segment 2, up to 1 wave on segment 8, so that the phase
advanced by one cycle as you went round the pupil. That replaced the
central null with a circularly-polarized peak, and cleaned up the
resolution amazingly.

The design was all finished and ready for production by early 1992, at
which point IBM very nearly went bankrupt, and both our budget and our
customer went away. I still have all the drawings, though. If it were
built today, it would still hold the lateral resolution record in
silicon by about a factor of 2 over any current instrument. (It ran at
a numerical aperture of 6.4.)

Cheers

Phil Hobbs

Hi,

That is really neat! I guess the E field is radial to the beam on the
perimeter and then axial with the beam in the center of the beam? I'm
pretty sure I have no idea how it works, but thanks for sharing that,
something neat about the words "optical vortex"

cheers,
Jamie

 

[[email protected]]

Hi,

That is really neat!  I guess the E field is radial to the beam on the
perimeter and then axial with the beam in the center of the beam?  I'm
pretty sure I have no idea how it works, but thanks for sharing that,
something neat about the words "optical vortex"

Neat, indeed. I posted the stuff showing the linkage between optical
and RF EM vortices because I was disheartened by what I saw as a
combination of the Not Invented Here attitude and misinterpretation
that some people seem to have about physicists' "laboratory
curiosities". Nice to know people like Phil are doing some fairly
sophisticated physics while pursuing practical goals.

;>)


Mark L. Fergerson

 

[Bill Sloman]

  Neat, indeed. I posted the stuff showing the linkage between optical
and RF EM vortices because I was disheartened by what I saw as a
combination of the Not Invented Here attitude and misinterpretation
that some people seem to have about physicists' "laboratory
curiosities". Nice to know people like Phil are doing some fairly
sophisticated physics while pursuing practical goals.

It's also nice to see a physicist like Phil doing some fairly
sophisticated electronics while pursuing practical goals. Rev.Sci.
Instrum. publishes some fairly diabolical electronics from time to
time.

The U.K.'s "Measurement Science and Technology" isn't as bad, perhaps
because they publish more stuff reporting commercial instruments, and
seem to use those authors for refereeing - I've looked at the
occasional paper for them, though nothing much recently.

 

[Jamie M]

Typically, optical OAM beams are generated using fork holograms:

http://upload.wikimedia.org/wikiped..._generation.png/550px-Hologram_generation.png

For RF, one way is to take a dish reflector and cut it from center
to edge, then displace one edge in the direction of propagation and
the other edge "backwards" so to speak; IOW make it a sort of slice of
a corkscrew:

http://ej.iop.org/images/1367-2630/14/3/033001/Full/nj400111fA4_online.jpg

from:

http://iopscience.iop.org/1367-2630/14/3/033001/article


Mark L. Fergerson

Hi,

I have heard a similar idea about OAM in the past for explaining
what matter is made of. If you had enough OAM in a small enough space
for light, it will possibly have strong enough field strength to hold
itself together, and create matter. Not sure if that is already part
of the e=mc^2 but it probably is.

cheers,
Jamie