directly collimating a 532nm crystal?

[Krutz]

hello everyone,

i plan to build a matched set of red, green and violet pointers. the green bare module (o-like) is too long. so i try to shorten it by changing the optics. i removed the frontal collimating lens, with replacing it with a shorter focusing one i could gain a few mm. not enough..
i removed the widening lens directly on the kpt chip, so now i have the bare lasing crystal without any optics afterwards. the beam is surprisingly good! at the crystal, its a fraction of a mm, perhaps a tenth. after two meters, its "only" a few cm, perhaps two (or one inch). round and with no spill. so far nothing unexpected..

i tried to collimate the beam with a single lens. no matter what i try, the dot gets much larger than without anything. a 0.8mm short-focus lens (objective lens from a sled), a medium-length focus aixiz lens, a long-focus (several cm) telescope lens, they all dont focus at all but widen the dot after two meters to ten or more cm. no matter how close or far i put the lens (unprecisely with my fingers).

i would have thought that i simply put a lens at its focal length to the chip (or, worst case, at the distance to the back end of the chip), and voila, whatever the size of the beam hitting the lens is, will be a somewhat collimated beam? just like with regular diodes? i dont get it!

the module with the lenses removed is less than 2 cm (1 inch), so a lens a few mm away from it would really shorten it! originally its more than twice that length!

any ideas? any way to collimate it with a single element?
or is there a reason why they use two elements.. *smile*


manuel

 

[chipdouglas]

the easiest way i can explain, is to get three matching larger hosts (i am not making a joke.)

 

[dr-ebert]

I don't have a green laser yet, so I'm guessing. With a non-DPSS LD, you have a light emitting region that's about 1um squared, that's easy to collimate into a parallel beam. How's that with a DPSS laser - I would assume the crystal has a size of some 100um squared? Is all of that area emitting light? (and how did you measure a beam diameter of 1/10 mm?) Then there'd be a limit to how much you could collimate that with just a single lens. The lens you removed, directly on the crystal - what shape did it have?

 

[HIMNL9]

As far as i've understood, he removed the expander ..... it's, usually, a little lens, negative, with a very short focus (usually it's a bi-concave, around a pair of millimeters of diameter, or few more)

Am i right ?

Anyway, it's an expander, for increase divergence like a diode, so it can then focused with a single front lens, same as a diode.

Probably you can focus the bare crystal using a long focal distance lens (low magnification), like some of these lenses that you find inside the sleds, before the moving one, or also less magnification (trying to guess, cause you said nothing about the actual divergence of the beam, sorry, anyway if you say that after 2 meters is like 2 cm diameter, can be 1 degree, also less, so maybe is enough a lens like, 1,5X, or similar)

any more strong lens, focus it at very short distance, then expand it again.

 

[Krutz]

the emitting area of the crystal: i see a faint glow in one of the crystal's corners. the crystal is around 1x1mm, the glowing dot is around a tenth of the crystal's width. i didnt even bother to try measure the beams diameter at the crystal, lets say its small ;-) (or 0.1mm).

yes, the two optical elements after the crystal form a "telescope". directly on the crystal, glued on, is a negative lens, widening the beam from the crystal. its not as wide as from an uncollimated diode, but pretty wide, compared to the bare crystal. the second lens is a regular lens, with long focal length, twice the length of an aixiz. both elements are acrylic and uncoated.

i didnt figure out yet how to collimate the crystal's beam with just one element, yet. when i leave the widening (negative) lens on the crystal, i can use any lens to make a parallel beam. an aixiz acrylic is shorter to the module than the original lens, so the beam is tighter, but still collimated. an old cd-drive (?) lens would need to be glued directly onto the negative lens. even shorter-focus lenses (dvd, blu-ray) cant be placed close enough to the negative lens to collimate at all.

with an aixiz-lens, i gain several mm in length, with a nice dot. that wasnt enough, anyway..

i did some more experiments (with the three 5mw modules from o-like).
i opened the modules pump-end. after cutting through a lot of brass, i found a 5.6mm closed-can (windowless) pumpdiode. it can be rotated to match the crystal for efficient pumping, but i expect this to be near optimum already when unaltered. anyway, there isnt more than two mm to be gained there.

so i followed chipdouglas' suggestion and ordered a few 18650 hosts. with the regular diode lasers (red, bluray, orange) i will use a 18650 cell, with the green one a cr123a.
i will surely have to shorten the green module anyway to fut, or at least do something about this awfully long driver. i will post pics about all this when i'm on it then.

if someone wishes, i can measure the divergences and diameters of each beam, with different optics in front of the crystal.. i will when i do the larger post about the shortening and host anyway.

so.. noone any idea about how to collimate the crystal directly with one lens? google was no help..


cheers,

manuel

 

[HIMNL9]

just as curiosity, how long is the driver ? ..... any pics ?

 

[Krutz]

the driver is 19x8mm, without the spring. its case-positive of course.. 5.6mm diode solders directly onto it.

manuel

 

[HIMNL9]

Oh, ok, then my idea can't work.

was wondering to cut part of the module and re-wire it "folded" on the main part, but if you still need the spring and the button, it's not realizable, sorry.

 

[Benm]

the emitting area of the crystal: i see a faint glow in one of the crystal's corners. the crystal is around 1x1mm, the glowing dot is around a tenth of the crystal's width. i didnt even bother to try measure the beams diameter at the crystal, lets say its small ;-) (or 0.1mm).

Thats still a whopping lot bigger than the emitting area from a diode: Single mode LD's have emitting areas in the order of the wavelength, no more than half a micrometer. The active area in your crystal is about 200 times as wide.

You cannot collimate the output from the crystal to a paralel beam with one lens: it already IS a paralel beam, but with rather large divergence due to its small initial size. This makes it look like a divergent beam coming from a single point, but it defnitely is not.

The optics in the DPSS form a beam expander, resizing the original ~100 um wide paralel beam to about 2 mm or so. The divergence improves a factor of 20 for the 20x increase in beamwidth. It goes from, say, 100 um with 40 mrad divergence to 2mm with 2 mrad... this yields a smaller dot at a distance.

So.. if you want to make the optics shorter, you would need a stronger expanding lens and a secondary focus lens to match. You could attempt to stack 2 expanding and focussing lenses on top of eachother, though i doubt it will work well.

 

[Krutz]

interesting! yes, makes sense. i already wondered why the beam isnt collimated (better) right off the crystal? hmm, then i have to widen the beam, and therefore no other way than using two elements. will play around with the few negative lenses i have, to replace both of the elements.
..would never have thought the beam was parallel to begin with, with such a high "divergence"!

thank you, ben!

manuel

 

[Benm]

Post some pictures of your setups

I'm fairly sure it is doable. Looking at lasers like these DealExtreme: $21.34 Green Laser Keychain 5mW , i'd think the optics in them are shorter than in your average green module. I have never owned one of them, but judging by the pics, the whole optics can't be much longer than a CR2 battery.

You could be able to swap the optics, though those bottle pointers seem a bit expensive to order just for that experiment

Alternatively, you could just order a '50 mW' unit DealExtreme: $28.32 True Green Laser Keychain (50mW) - though i have no idea of what module is in there nor of the overall product quality.

 

[HIMNL9]

About "shortening" the modules, i have just checked the 50 mW green ones that i got from O-like, for a work, and look the pics ..... the first is the module, usual 35,5 mm lenght of the brass body ..... the second is the same module, seen from the front ..... 4,5 mm of bare, unused, empty threaded tube, that, using careful, can be cutted away ..... :eg:

Edit: reloaded the pics, they disappeared ..... don't know why :thinking:

 

[Krutz]

"setup"? there is no setup.. just slaughtered modules and fingers crudely holding random optics in front of half-dead modules ;-)

when my longer hosts come i will finish this and make pics of it as well as from the poor smaller (output) modules..

manuel