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using DPSS optics to correct a multi-mode beam?

ixfd64

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This is probably a dumb idea that won't work, but here goes.

It is a well-known fact that DPSS lasers have beam specs that are very different than that of their pump diode. This is because the diode is merely pumping the crystal, which generates a well-collimated laser beam.

I'm wondering if it's possible to use a similar setup to get a single-mode beam out of a multi-mode diode by using DPSS optics (pump focusing, expanding and collimating lenses) and some sort of crystal. Of course, the crystal would have to be able to output a beam whose wavelength and optical power is similar to the original diode.

Has anyone ever tried something like this?
 





HIMNL9

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Sorry, but no.

You can use DPSS optics (or, better, "matched" optics), for build easily a beam expander, but not for change the geometry and shape of a multimode beam ..... for this, you need or a cylindrical lens, or a prism couple.
 
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Not possible. You have to understand the workings of a DPSS laser to understand why.

There are two types of crystals in most common DPSS crystals- the gain medium, and the non-linear crystal.

The gain medium is the one that's pumped by the diode. This crystal then lases at a different wavelength (higher than the pump wavelength).

This is why it's called a DPSS laser. A laser diode pumps a solid state gain medium.

The non-linear crystal is responsible for generating the second (or even third, in some cases) harmonic, commonly known as frequency doubling. This crystal will not emit when pumped directly with the pump diode, they require polarisation and phase matching (this is done using the gain medium).

You can see in this picture, borrowed from Sam's Laser FAQ:

dpss1asm.gif


In this case, the YVO4 is the gain medium, and the KTP is the non-linear crystal.

If you want better beam specs, you need correctional optics.

These optics include anamorphic prism and cylindrical lenses for fast axis collimation, and other lenses for correcting the astigmatism of laser diodes.

These optics are used in higher-end DPSS lasers to couple the pump diode to the gain medium.

With good optics, DPSS-like beam quality can be obtained from laser diodes. There is no need to use a crystal in-between, especially as one crystal won't work.

Not to mention the fact I haven't come across any lasing medium which lases within 20nm of the pump wavelength. (maybe some dyes, but that's opening another can of worms).

On a slightly-relavent side note, the beam emerging from the KTP in most handheld DPSS lasers is on the order of 17~mRad. A collimating lens is then required to collimate this.
 
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The prism set from 808 assemblies works fine, but you get something like a 30% loss which is unacceptable for most people.
 

JLSE

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Common bonded crystal sets will lase as high as 836nm, just very poorly.
I have tried this first hand and got approx 5mW or so with a 500mW pump.

Also the beam leaving the gain medium is already perfectly round
and meets with two separate optics.. First the expander then the collimating
lens...

I have also tried FAC from an 808 with a 445nm LD' but there is no noticeable
difference.. If any it may lower the divergence, but as far as beam shaping goes, forget it...

As already stated, a prism pair or cylindrical optics are the only way to
fix up the 445's beam..
 
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Common bonded crystal sets will lase as high as 836nm, just very poorly.
I have tried this first hand and got approx 5mW or so with a 500mW pump.

Also the beam leaving the gain medium is already perfectly round
and meets with two separate optics.. First the expander then the collimating
lens...

I have also tried FAC from an 808 with a 445nm LD' but there is no noticeable
difference.. If any it may lower the divergence, but as far as beam shaping goes, forget it...

As already stated, a prism pair or cylindrical optics are the only way to
fix up the 445's beam..

That's one of the benefits of using YVO4, it's a broadband absorber. The pump can be crappy (to a reasonable extent)

YAG, however... :undecided:

Also, are anamorphic prisms for beam profile correction or fast-axis correction? I've never been able to get a clear answer.:yabbmad:
 
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if you are interested in getting good beam specs for multi mode 445, i will post a link that andycon gave me. there is a guy on pl that sells what the call prism pairs. there is low loss and good beam correction. basical 2 pyrimid optics that the 445 runs though at different angles.


edit, i just dug out andycons link Anamorphic prisms are go
michael

Don't forget about Dr. Lava's cylindrical lenses as well.
 
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^^^^ GOOD point. but i ponder if i can forget about something i never knew about LoL..
can you post a link. i'm interested.

michael
 




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