Laser diode chip aperture, lens aperture & wavelength relationships to divergence?

[Alaskan]

Laser diode chip aperture, lens aperture & wavelength relationships to divergence?

From what I've been able to pull together over the last year of reading posts, and searching on google etc., is that the divergence of a laser is the product of:

1. Laser diode chip aperture (and if single mode?), the larger the laser chips aperture the lower the divergence?

2. Wavelength, i.e. 532nm, 405nm etc., I have searched for hours to find a graph which shows the relationship between wavelength and divergence but haven't been able to find one yet.

3. Clear aperture of the lens used to collimate the laser diode as well as wavelength being collimated.

Can someone please explain the relationships of the above in simple language from a big picture or top down understanding? I found some calculators and math to determine each, but haven't been able to pull it all together yet. If you look at my signature, you can find links to info as well as calculators about each of the above, but I don't have a full grasp of how the puzzle goes together yet to make the lowest divergence laser pointer possible from the available parts.

But here are my assumptions:

1. Single mode large aperture laser diode chip.
2. Shortest wavelength laser diode.
3. Largest diameter lens.

Perhaps using a large lens to reduce the divergence is simply the product of having an expanded beam? Please see this online lens divergence calculator: US-Lasers - Beam Divergence Calculator

Maybe I do understand all of the relationships well enough to buy what I need for a ultra low divergence laser pointer, if someone can confirm my above assumptions, I'd sure appreciate the help. Thanks!

 

[ARG]

Re: Laser diode aperature vs lens aperture vs wavelength... ???

Perhaps using a large lens to reduce the divergence is simply the product of having an expanded beam?

You've got it

That's what beam expanders do, they reduce divergence and increase beam size.

More on beam expanders:
http://assets.newport.com/webdocuments-en/images/how_to_build_a_beam_expander_5.pdf

The Wikipedia article explains this simply:
https://en.wikipedia.org/wiki/Beam_divergence

 

[ARG]

Re: Laser diode aperature vs lens aperture vs wavelength... ???

I think this article explains that/
https://en.wikipedia.org/wiki/Gaussian_beam#Beam_divergence

"Because the divergence is inversely proportional to the spot size, a Gaussian beam that is focused to a small spot spreads out rapidly as it propagates away from that spot. To keep a laser beam very well collimated, it must have a large diameter. This relationship between beam width and divergence is due to diffraction."

The larger the starting diameter of the beam, the better the divergence.

 

[Cyparagon]

Re: Laser diode aperature vs lens aperture vs wavelength... ???

I was so sure a smaller point source meant a lower divergence, now I'm confused.

Before, or after collimation?

A larger aperture will lower divergence at first glance, but the side-effect is that other transverse modes usually appear with anything much above a few microns. Of course, multi-mode beams diverge more rapidly than the same sized single-mode beam. A smaller emitter area is better for overall divergence (after collimation).

 

[kp4djt]

Re: Laser diode chip aperture, lens aperture & wavelength relationships to divergence

This thread is about 3 years old, I hope some of you are still tracking it, as I have a question.
I need to keep a beam as tight as possible. at least from about 15mm from the aperture to
about 170mm from the aperture. Beyond that distance I do not care.

The diode is a Osram 515 nm 10mw diode. Diode aperture is 5um. I would like to try to keep
the beam width as near to 5um as possible for the distance of 15 to 170mm from the lens
aperture.

Any recommendations?

 

[RedCowboy]

Re: Laser diode chip aperture, lens aperture & wavelength relationships to divergence

That's a single mode diode isn't it? You can start with a very short FL lens close to your diode to catch the output at as narrow a width as possible, but then you will trade beam width for divergence with a concave-convex beam expander set up, you could use a long telescopic reduction to get a tight beam for your working distance, what is your purpose, is there a limit to the device size/length ?

 

[kp4djt]

Re: Laser diode chip aperture, lens aperture & wavelength relationships to divergence

So far so good. I had been googling data, I am using the copper 12mm DTR diode modules, with the
Osram PLT-510. At this moment I have been using the lens that is sent with the module, I know it is
not ideal at all. There are several selections of lenses on DTR's site. some with 2 lenses and others
wtih 3. I recall using one of the 3 lens units some years ago and obtained quite low divergence for
well over 30cm from the aperture. BUT... I did not back that data up to a jump drive and when I left
the place where I was working I lost the data.

I understand that the divergence will never be 0, and in this particular case if I can find a nice medium
point at about the middle of the range and have it expand a bit as it goes from one end of the travel
to the other, we can live with it, At this moment with the present lens I get the following:
Aperture to Target --------------- Beam width
20mm ------------------------------- 2mm
50mm -------------------------------< 0.1 mm
80mm -------------------------------- 3mm

I am trying to find a M9X0.5 lens or lens set that will give me end values nearer to
the center point. Of course it would really be nice to be able to resolve it down to
the width of the diode aperture which is 5um, but I fear that this is a wish in one
and and spit in the other and see which fills fastest.

We are trying to monitor knife blades, the narrower I can keep the beam the better.
The ideal is of course something that just illuminates the cutting edge (see wishing
above) but if there is some overflow no big deal, but we have to be able to work in
the spacing from 20mm to 80mm from the beam aperture. I hope that one of the
lens sets that DTR carries will do the job or get near enough that we can get on with
the rest of it while we figure out what we need to have built in a M9X0.5 lens.

At this point I would be happy if I could get > 0.1mm width at the mid swing and
1mm on each end.

I just pulled down the document, it is a good one. NOTE, I have a line generator
lens after the output of the M9x0.5 lens. It does a good job of keeping the line
tracking to the divergence of the lens. I also have the slow axis in parallel with
the line generator lens which makes the resulting line much finer. So now the
trick is to find a M9X0.5 lense that will say bring the beam to tight focus at the
50mm point have have it spread a wee be at the 20 and 80mm points. The
nearer to parallel I can get it the better obviously.

Here is the spec sheet on the diode:
https://www.chips4light.com/assets/Uploads/PLT5-520-B1-2-3.pdf

 

[kp4djt]

Re: Laser diode chip aperture, lens aperture & wavelength relationships to divergence

It has to fit in a 12X30mm laser diode module. The one that accepts the TO-5 (5.6mm) diode. This thing goes mounted in the cassette chuck of a microtome so I do NOT have space for a HeNe. It is used to align the chuck to the knife blade and to check new blades or used blades for defects. The green lasers make the defects light up like stars on the cutting edge. This is a small device and we are trying to make it as bullet proof as possible as the users just need to be able to put the tool in the chuck power it up and align the line generated by the laser to the knife edge and check for dings on the blade.

I think that DTR has the lens I need, I ordered two of them, as I think they are the same lens that I am using on a 120mw 520nm laser. you can set that one to be a VERY narrow beam for quite some distance. But I cannot find my order for it as I ordered at the plant where I worked and for some reason do not have the info.

These devices normally cut at thicknesses of 0.5 to 10um (electron microscopes us either .5um or .6um thickness samples, regular optical ones normally use a 4um thick sample)

 

[kp4djt]

Re: Laser diode chip aperture, lens aperture & wavelength relationships to divergence

After I get the proper lenses I will make a video and put it out to see. I think that it is just a question of lenses at this point, I can always go tear apart my 120mw laser and pull that lens out but I remember that I ordered that lens from DTR and it was for the green lasers, there is still only one on his page, and I am pretty sure that was the lens.

 

[kp4djt]

Re: Laser diode chip aperture, lens aperture & wavelength relationships to divergence

Well I went and removed the lens from the 120mw module, got about the same results.