OpNext 637nm diodes

[j4cbo]

Well, I've got one of each in hand now, and some mounts and lenses and whatnot showing up early next week, so... we'll see what happens!

[badger1666]

dumb question. whats the difference between single mode and multi mode then ??? i forgot

[steve-o]

dumb question. whats the difference between single mode and multi mode then ??? i forgot

As far as I know there is no simple explanation for single and multiple transverse modes. I just think of it as being (a) a clean TEM00 single beam , vs : (b) "dirty" ; multiple inter-cavity reflections / resonations resulting in a less than perfect Gaussian ellipse. Multimode laser diodes are associated with being able to achieve higher powers at the expense of divergence and beam quality.

[catalanjo]

dumb question. whats the difference between single mode and multi mode then ??? i forgot

Mode usually refers to TEM

TEM means Transverse Electromagnetic Mode...... look in http://www.microwaves101.com/encyclopedia/TEM.cfm = this addresses microwaves but is the same for light (just a bit more micro = tenths of micro).

If you move a small pinhole across the section of a single mode beam and note the power getting through the hole, in each position on a graph (one graph for horizontal and one for vertical).,the results looks like a smooth mountain shape, or camel hump. (Camel called Gauss)

With multimode they look like the Alps with peaks and valleys all over the place.

TEM01 looks like a volcano with a crater in the middle (doughnut mode) TEM02 has four peaks but valleys crossing the centre etc.

This is very important in interferometry and holography where beam has to pass through a spatial filter (small hole).

You get clean wavefronts with TEM00 instead of a choppy (gusty day at the seaside) type wavefront with multimode giving hugely ambiguous results in interferometry and a very misty object image in holography.

Diodes can never be totally TEM00 since they give elliptical not circular beam so at best they are quasi TEM00 but no one seems to give a shit so they call them TEM00 anyway if the ellipse remains constant shape for a "reasonable" distance (spatial coherence length).
Temporal coherence length is much the same but the measuring hole is in "time" rather than space if you catch my drift
This in holography gives rise to images in two and a half dimensions (slit holography) like on your credit card!
Moving head from side to side observer sees 3D but up and down gets change in colour.
If you illuminate with a flourescent tube instead of a point source you get "True?" 2&1/2 D in "black&white", although "black&white" has always been a dubious concept for me personally!


For projectors it doesn't matter much since you never really focus the beam to it's tightest possible spot size like you do for fibre injection etc. (although it does affect the divergence on VERY tight beams) and also SHOULD affect safety calculations for MPE since eyes DO focus a beam onto the retina.

I might have drifted off subject here and there, but shit it's a generalised "overview" !

Hope this helps!


Cheers

PS.
Arrays of diodes are sometimes referred to as multimode when they really mean multi-multi-mode or in plain English !

[laserdj]

http://en.wikipedia.org/wiki/Transve...de#Laser_modes

Check it out! Interesting... with pics...

My DHOM unit that is running under spec is clearly in TEM 10

Cheers!

[badger1666]

thanks for the explanation guys, so for general show scanning it does not matter if you use single mode or multi mode lasers then ?

[catalanjo]

Just tighter beams with single mode !

Cheers

[mccarrot]

I love your illustrations catalanjo

[catalanjo]

Not me mate... they are supplied by our illustrious janitor
just click on the [More] on right hand side of "advanced" posting screen and you get a page full.

King of smileys is "allthatwhichis"... I would dearly love to get my mitts on his library .. it would make posts slightly more laborious to write but much more entertaining to read !.

Cheers

PS. For some reason unknown to me my usual has been updated to which is a bummer !

[megaton]

Hi catalanjo

The library of Aaron is open to every one, he told that any one can use it.

Check it.

Cheers,
Ed
PS I hope not to screw this.
(Espero no meter la pata).