beam expander logic

[Cyparagon]

it appears the beam is expanding greatly has it exits the front surface.

It isn't. I don't have prairies or massive hangars available to me, so you'll have to excuse the lack of long distance measurements.

 

[Benm]

With laser diodes you're not going to be able to use both divergence figures simultaneously because of the astigmatic beam, but I think you can use the data for a single axis for collimating with normal concave or convex lenses like we do with our Aixiz modules etc.. In order to balance the axes to get a symmetrical beam you'd need to collimate the fast axis first, making the optical train more complex.

It doesnt really matter in which -order- you do it. A typical setup would be to use a 'normal' lens first, adjusting the slow axiz to inifinity. With a diode like a 445 the fast axis will then have much bigger divergence (but a smaller initial beam size). This can be corrected to some degree using cylindrical lenses, since those only affect on of the axis if placed in the correct orientation.

 

[Jubathoph]

The main problem with 445 and Mitsubishi diode divergence is that these multimode diodes suffer from astigmatism; that is the fast axis and slow axis actually have slightly different focal lengths. Both axes cannot be in focus at the same time unless one is corrected; I.e. with cylindrical optics.

 

[ElektroFreak]

It doesnt really matter in which -order- you do it. A typical setup would be to use a 'normal' lens first, adjusting the slow axiz to inifinity. With a diode like a 445 the fast axis will then have much bigger divergence (but a smaller initial beam size). This can be corrected to some degree using cylindrical lenses, since those only affect on of the axis if placed in the correct orientation.

With most commercial high-powered multimode diodes the fast axis is corrected first via a fiber FAC lens. Often, the slow axis is then collimated by a cylinder lens or a pair of prisms. Once the beam profile is symmetrical, it can be collimated into a very low-divergence parallel beam by a fairly conventional lens.

You are right that the order of optics is not always crucial. I think the reason commercial builders use fiber lenses for FAC is because the fast axis is on a plane that makes this method fairly easy and efficient, but I could be wrong about that.

 

[Benm]

You are probably right about that, and its fine if you obtain a diode that has this fast axiz correction already included.

I was envisioning the scernario with 445s or 638s that have distinctly different fast and slow axes, but dont come with any corrective optics for that by default. I think the most practical method for those is to do the symmetrical lens first (as in an aixiz module), and use the cylindrical lens set (or prism pair) after that.

In terms of performance it doesnt matter much, but one advantage is that you keep the beam narrow enough to be handled by small optics.

 

[trussmonkey25]

I have a sparten laser..they haven't made an expander in over two years for this. Any ideas for a good beam expander for this.

 

[BShanahan14rulz]

In terms of performance it doesnt matter much, but one advantage is that you keep the beam narrow enough to be handled by small optics.

This.

This is why we have green modules the way we do. You could use a longer focal length lens, but it would have to be farther from the source light. Therefore, less light would hit the lens and be focused. A larger diameter lens would fix this, but adds bulk.

 

[Jubathoph]

Divergence is affected by Beam diameter and lens focal length, but is not actually affected by the diameter of the lens. Longer focal length = lower divergence.

You could have two lenses with different diameters and the same focal length, they would have identical divergence, but the smaller lens may not capture all the light. Conversely, you could have two lenses with identical diameters and different focal lengths; the longer focal length will have lower divergence.


The only place lens diameter factors in is in % of beam capture, and this is why you need larger diameter lens when you have longer focal length and why they tend to get confused with one another. In order to capture and collimate all the light from a diode you need to match or exceed the numeric aperture of the diode. You can think of this as the boundary of a cone of light. Numeric Aperture is the ratio of diameter to focal length for a lens. All the light outside the numeric aperture cone will be lost.

In summary, as you increase focal length in an attempt to lower divergence, you must also increase lens diameter in order to capture all the light, thus maintaining the same numeric aperture. This post is getting too long and I can't proof read it on my phone so I hope it makes sense.

 

[BShanahan14rulz]

Gotcha. Very well written, I understood your post just fine!

That makes more sense as to why large diameter lenses are used in those situations. You could use a small diameter lens with the same focal length and put it in the same spot, and you'd have the same divergence, but with less light. Therefore, "the longer the focal length, the lower your divergence" is the main thing to take away from all this, while lens diameter is about collecting more light from the source.

I guess in the flashlight world, loosing output for the sake of divergence is generally thought of as unacceptable. I will edit my post, that was a bad example.

 

[Bluefan]

Incorrect

The divergence is inverse proportional to the beam diameter, the larger the beam diameter the lower the divergence. A lens that clips the beam shrinks the diameter and thus increases the divergence. The focal length is indirectly related, changing the focal length in an otherwise fixed setup would change the eventual beam diameter which determiens the divergence.

But if both lenses in the beam expander would have twice the focal length the beam diameter magnification would still be the same and the divergence also, so the focal length is not the important figure. It's really the beam diameter, which is determined by the initial beam diameter and the magnification of the expander. Clipping the beam or misaligning the expander will only increase the divergence and/or mess things up.

 

[Jubathoph]

You have it backwards! Focal length drives beam diameter. Your example of clipping the beam increasing divergence is utterly wrong except in diffraction limited cases. Here's an example I am sure you are familiar with given your namesake: consider the 405 G1 lens vs the ubiquitous aixiz three element. It is well known that the aixiz clips the beam yet has better divergence. Why? Because it has a longer focal length, and our friendly 445 is not diffraction limited. Your above thought eggxperiment neglects this important qualification. Divergence is not only influenced by beam diameter, it is driven by focal length and aperture size which establish your ultimate beam diameter.

 

[Bluefan]

There happen to be 2 subjects in the thread, diode collimation and beam expanders. I assumed it was about beam expanders, but I'll consider both now.

You have it backwards!

Nope I'm pretty sure I'm not. Please be more specific where.

Focal length drives beam diameter.

There is a relation between the two, true.

Your example of clipping the beam increasing divergence is utterly wrong except in diffraction limited cases.

Clipping a beam definitely increases the divergence, lasers diodes are not always close to the diffraction limit but definately close enough to be significantly affected. Which is exactly my point.
In the beam expander for a DPSS laser case it's even more so as DPSS alsers are pretty close to the diffraction limit.

Here's an example I am sure you are familiar with given your namesake: consider the 405 G1 lens vs the ubiquitous aixiz three element. It is well known that the aixiz clips the beam yet has better divergence. Why? Because it has a longer focal length, and our friendly 445 is not diffraction limited.

The lenses have different apertures AND focal lengths AND abberations. You can't make such simple assumptions if 3 parameters are varied. The well known 445nm diodes aren't diffraction limited, but definitely diffraction affected! Clipping them will affect the divergence as it affects the M^2 value.

Your above thought eggxperiment neglects this important qualification. Divergence is not only influenced by beam diameter, it is driven by focal length and aperture size which establish your ultimate beam diameter.

My statement was that the beam diameter determines the divergence considering a fixed M^2 value, which is affected if you clip the beam.

 

[Sigurthr]

BlueFan, I'm terrible when it comes to optics, and have a hard time of picturing things in my head without a diagram; Is this correct:

For a beam expander: Two Biconvex or plano-convex lenses;
As long as the first lens's focal point is between the two lenses and the beam does not diverge so much after the first lens's focal point to be clipped by the second lens, it should lower the divergence of the beam. The degree of change in beam diameter is related to the distance between the two lenses and the focal lengths of the two lenses.

If so, I think I understand that you can change the diameter of the beam by changing either the focal lengths of the lenses or the distance between the lenses, but I am not clear on the relationship between the factors... Changing the distance between to biconvex lenses of equal FL changes the magnification how? Using two lenses of different FLs but spaced a fixed distance apart affects the beam how?

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And also, how do you determine the placement of and focal length of a lens(s) needed to collimate/converge a divergent beam?

--For example my 473nm labby has a divergence of nearly 9mRad, and it's beam is divergent from the nonlinear cavity's aperture with a diameter of about 0.5mm at the laser head's exterior aperture. There are no corrective lenses after the nonlinear cavity. How do I determine the focal length of the lens needed to correct the divergence of the beam to say 1mRad? I would need to expand the beam about 9 times to lower the divergence 9 times, right? But how do I determine what focal length of lens(s) I would need to do that?

I have a planoconvex lens with a 280mm FL, and if I place it 280mm from the nonlinear cavity's aperture (about 250mm from the head's aperture) it collimates and converges the beam nicely. The beam appears to reach a focal point about six feet out from the lens and has a divergence of about 1.5mRad (not accurate! just a guesstimate!) after my corrective lens.

What lens specs would I need to place the lens directly on the aperture (of either the nonlinear cavity, or the head)?

 

[Cyparagon]

This clipping concept is Interesting... it makes me wonder what kind of beam you would get with a 1m focal length collimating lens that is only 1mm wide. :undecided:

 

[Jubathoph]

Let's compare the following two scenarios:

1) light exits diode #1 at a 45 degree cone angle and 100% of its light is collimated to a 5mm beam.

2) light exits a diode #2 at 90 degree cone angle , but only 25% is captured and collimated to a 5mm beam. Identical lens, wavelength, and aperture as #1, but the #2 beam is cleanly clipped I.e. no stray reflections.

Question: compare the relative divergence? Answer: they are identical.

Does anyone disagree with this?

@Cyparagon, with a 1mm lens and 1mm fl I believe you would get an initially small beam that diverges Very rapidly.

 

[Cyparagon]

@Cyparagon, with a 1mm lens and 1mm fl I believe you would get an initially small beam that diverges Very rapidly.

nonono... 1m (one 'm' only - meter) focal length - as in clipping nearly all of the beam, yet having a very long focal length.