the lens rant educating everyone.

[Kenom]

OK, I have to do this.. It's been on my mind forever and it's a simple fact of lenses that we all seem to have overlooked when comparing all of the assorted lenses available to us to date.

Now, this is not taking into consideration coatings. Coatings are an aspect that just aren't relevant to this discussion. If we were to compare two different lenses with the exact same coatings, the information discussed in this posting would not vary at all because they are the same coatings.

What I am going to discuss is how a lens WORKS. This is a simple thing that we've all pretty much overlooked in how it relates to our lasers. We see a beam coming from our lasers and notice that it can affect the way that our lasers focus and so forth, but there is an aspect that is widely forgotten in lenses and lasers.

Ok on to the discussion. /rant

Lenses bend light. Change the path. However lenses do not change the paths so completely that you change the light to create something other than the original light.. an example

The bulb on the left is the source of the light. After passing through the convex lens, the image gets inverted and it reduces teh size of the image. On the other side (the right) our bulb is now upside down and considerably smaller. However, the bulb can clearly be see AS A BULB. The lens does not distort the image of teh bulb other than make it smaller. If it does distort the image, then it's not a very good lens. we're going to switch to laser sources now to continue our representations. A laser diode shines into a plano convex lens. This just means it's flatish on one side instead of curved on both. These are the lenses used in 99% or our lasers. The image of the laser diode is still the same with this lens. If however we get splatter or stuff in our dots we have to look at is the source the lens or the diode. The wings we all see on our diodes are pretty clearly a result of an enhancement of the bleed over from the junctions in a laser diode that is then getting projected through the lens. (there is a better image of this in a macro on a LOC barely lasing, but I can never find it anymore. Maybe someone can point it out to me and we can end this debate on the source of the wings once and for all)

Lets go back to this other splatter though. When seen big as in the example of a 405-g-1 lens, (hope you don't mind me using your image jay)

this splatter is NOT caused by the diode. We know this because other lenses create a nice clean dot. However, when you focus this mess down to a small spot, the dot does not clean up. Instead it just gets smaller. However no matter how small we get this messy image, it will still be a messy image. All of that light is not focused in a small .(dot) it is spread over a smaller area yes, but certainly not like our nice clean dot. If we were to focus that nice small dot, all that energy would be focused in one place. We have to imagine looking at that image through a microscope. It's still that messy image. Yes, the 405-g-1 as our example passes more light thanks to it's coatings, but we're not discussing coatings. We are discussing true raw power. In our perfect world without the need for coatings, we would then see the messy image not capable of burning as well as the nice clean dot since more energy is now focused into a cleaner small image, than a messy one.

In conclusion, what we need to see is not a messy image, but a clean one with the ability to pass as much light as the messy lenses. (the 405-g-1 is not the only one incapable of creating a good image).
Consider yourselves edumacated.

/rant

 

[ElektroFreak]

Lets go back to this other splatter though. When seen big as in the example of a 405-g-1 lens, (hope you don't mind me using your image jay)

this splatter is NOT caused by the diode. We know this because other lenses create a nice clean dot. However, when you focus this mess down to a small spot, the dot does not clean up. Instead it just gets smaller. However no matter how small we get this messy image, it will still be a messy image. All of that light is not focused in a small .(dot) it is spread over a smaller area yes, but certainly not like our nice clean dot. If we were to focus that nice small dot, all that energy would be focused in one place. We have to imagine looking at that image through a microscope. It's still that messy image. Yes, the 405-g-1 as our example passes more light thanks to it's coatings, but we're not discussing coatings. We are discussing true raw power. In our perfect world without the need for coatings, we would then see the messy image not capable of burning as well as the nice clean dot since more energy is now focused into a cleaner small image, than a messy one.

Nice ken! One thing I'd like to point out is that the outline of the squareish halo above is actually produced by the diode. The splatter haze inside the halo and everywhere else is from the lens.

 

[Kenom]

I should have posted this in the lens section. I'll see if I can get a mod to move it for me

 

[Benm]

I don't think there is any debate on the cause of the wings - those are produced by the diode, no matter what lens is used.

That picture looks rather messy though - but i suppose this lens is engineered to pass maximum power and compromise on image quality. Personally i like the aixiz glass lenses best (at least for red), perhaps some power is lost in them, but the result is a very clear dot, apart from the unavoidable wings.

 

[FireMyLaser]

The best way to somewhat get rid of the wings is to put an aperture on the laser, some distance away from the lens. You can't completely remove it.

 

[Kenom]

well the wings question has come up many times... "where do the wings come from" "what causes it" "is my dot supposed to have these" "how do I get rid of them" As to the last firemylaser has indicated the only method of getting rid of them.

As to the source, it is my belief that the junction between the P material and N material leaves a micro gap that allows that light to then come out. That or it is like shining a laser into the side of a piece of glass or acryclic and having it come out there. Regardless of the reason behind the cause, this is what is causing them. Light from the laser diode does not just come from that one place. It comes from the whole junction and as such every diode I've seen has the wings and will have the wings in the future.

 

[Benm]

Yup, the wings are emitted from the diode. If you just shine the diode without any lens, they fall within the profile on the wall and hard hardly noticeable (perhaps as slightly brighter areas witin the round projection, above and below the main power oval).

You can get rid of them with an aperture, but that has to be pretty far from the lens to do any good. Near the lens the wings are still in the main beam, but they have bigger divergence. If you really must get rid of them an aperture will work, but it needs to be several inches from the lens to be effective... not practical for a pointer, but doable in a projector as you could place the aperture just before the scanner mirrors.

Personally i don't mind them that much though - its just what a diode laser looks like: a bright dot with a faint stripe through it. It can even be useful when aligning lasers for PBS combination as the line is at a fixed angle to the direction of polarization.

 

[Addonex]

Very nice post!

 

[JLSE]

It's still that messy image. Yes, the 405-g-1 as our example passes more light thanks to it's coatings, but we're not discussing coatings. We are discussing true raw power. In our perfect world without the need for coatings, we would then see the messy image not capable of burning as well as the nice clean dot since more energy is now focused into a cleaner small image, than a messy one.

In conclusion, what we need to see is not a messy image, but a clean one with the ability to pass as much light as the messy lenses. (the 405-g-1 is not the only one incapable of creating a good image).


/rant

Coatings assist the lens in absorption, and reduce reflection back into the cavity.
The primary purpose of anti reflective coatings with laser is to spare the diode
the stress of increased optical flux.

With the glass ashperics that are sold here, coatings actually play less a role as
far as power is concerned.

The higher NA with these lenses allow the lens to sit closer to the diode and in turn
passes all the light from the LD with no 'clipping' while producing a parallel beam.

Aixiz triple elements produce a nicer beam simply because there are 3 lenses
that shape it. If im not mistaken the first 2 lenses expand and the 3rd collimates,
resulting in a tighter beam with lower divergence.

The aixiz also sits 3-4mm further away from the LD to make a parallel beam
and clips the 'messy' edges of the of the LD's output..

The higher the NA rating the better for more power, but you cannot have
quality and power without sacrificing one or the other.


Some on it here, click 'collimation tutorial' > Thorlabs.com - Visible Laser Diodes: Center Wavelengths from 405 nm to 785 nm

 

[BShanahan14rulz]

Is the image 1:1? meaning, when you focus a laser, will the resulting "point" be the same size and shape as the actual emitting area on the facet of the diode?

Also, that picture is a great representation. You can clearly see the round halo caused by the lens, and the square ghosts caused by the diode structure and its substrate.

 

[lasersbee]

You can see the same square ghosts image without a lens..
holding the 445nm LD close to a white surface...even at
low output powers..

@ wannaburn... thanks for the Thorlab link...


Jerry