About getting a gaussian beam...

Does anyone here actually have a spatial filter set up that provides them with a more or less clean gaussian beam? Seeing 'noisy' beams really irks me lol. I'd really like to have a set up that produces relatively clean gaussian beams so that it'd stay a circle even at distance.

Any tips?

good and clean lenses, that's all

Hallucynogenyc said:

good and clean lenses, that's all

Click to expand...

No, you need a spatial filter set up. ie focussing the beam to a point and passing it through a pinhole slightly smaller than the size of the focused point.

Anyone else?

Limecat said:

Does anyone here actually have a spatial filter set up that provides them with a more or less clean gaussian beam? Seeing 'noisy' beams really irks me lol. I'd really like to have a set up that produces relatively clean gaussian beams so that it'd stay a circle even at distance.

Any tips?

Click to expand...

Very few hobbyists use spatial filters due to the expense of quality filters and the expertise required when building complex laser systems (not that spatial filters are complex, but they are generally incorporated into more complex laser systems).

First off, are you referring to diodes or DPSS?

If diodes, spatial filtering is one way to get a near diffraction limited beam, but another simpler way is to pass the diode output through a short piece of single-mode fiber prior to collimation.

With DPSS the beam quality is typically quite a good approximation of TEM00, so spatial filtering is normally not needed.

ElektroFreak said:

Very few hobbyists use spatial filters due to the expense of quality filters and the expertise required when building complex laser systems (not that spatial filters are complex, but they are generally incorporated into more complex laser systems).

First off, are you referring to diodes or DPSS?

If diodes, spatial filtering is one way to get a near diffraction limited beam, but another simpler way is to pass the diode output through a short piece of single-mode fiber prior to collimation.

With DPSS the beam quality is typically quite a good approximation of TEM00, so spatial filtering is normally not needed.

Click to expand...

You know, I've noticed my green laser pointers produce close to clean gaussian outputs. But no I'm referring to diodes actually. Single mode fiber? What's that?

There are two types of optical fiber, mutimode and singlemode. Multimode has a group of fiber strands or 'cores' inside of a larger piece of fiber, which will result in a multi-lobed output profile. Singlemode fiber has only a single strand of fiber, which gives the cleanest output profile. When light enters a fiber, it bounces along the reflective inside walls of the fiber, which is how fiber transmits light with such little loss over long distances, but it has another useful side effect. During the course of bouncing along the fiber, any power inconsistencies in the beam profile are smoothed out. The input can be any kind of mess, but the output will always be a smooth, round flat-top profile that closely approximates a gaussian beam. Of course, this is assuming quality fiber is used, and that the fiber has been cut properly so that the cut surfaces are smooth with minimal aberrations.

Using fiber has the advantage of very little power loss compared to spatial filtering.

ElektroFreak said:

There are two types of optical fiber, mutimode and singlemode. Multimode has a group of fiber strands or 'cores' inside of a larger piece of fiber, which will result in a multi-lobed output profile. Singlemode fiber has only a single strand of fiber, which gives the cleanest output profile. When light enters a fiber, it bounces along the reflective inside walls of the fiber, which is how fiber transmits light with such little loss over long distances, but it has another useful side effect. During the course of bouncing along the fiber, any power inconsistencies in the beam profile are smoothed out. The input can be any kind of mess, but the output will always be a smooth, round flat-top profile that closely approximates a gaussian beam. Of course, this is assuming quality fiber is used, and that the fiber has been cut properly so that the cut surfaces are smooth with minimal aberrations.

Using fiber has the advantage of very little power loss compared to spatial filtering.

Click to expand...

Ah very cool. So where would one get a short piece of single mode fiber and how would one connect it to an aixis module securely? :s
I bet this would cost a lot lols.

It could be done cheaply with a little creativity. The housing of the system is up to you, but the laser diode must be focused into the end of the fiber. You can do this with just an ordinary lens in an aixiz module, but holding everything tightly in place will be challenging. basically you just need to position the cut fiber face (which is exceedingly tiny) so that the focal point of the diode's output is just past the surface.

Obviously this is the simplest approach. A professional would carefully calculate the lenses required for maximum power transfer. Given the numerical aperture of the fiber, there are angles at which the light transfer is greatest. When the focal point is close to the lens, the edges of the beam as it enters the fiber are at a steeper angle than if the focal point was further away from the lens. Below is a crappy paint diagram I drew that describes what I mean:



Somewhere between the two focal length extremes there is an optimum angle at which any losses are minimized.

For more information on numerical aperture, see:
Encyclopedia of Laser Physics and Technology - numerical aperture, NA, optical fiber, lens, objective
Numerical aperture - Wikipedia, the free encyclopedia

and for focal length see:
Encyclopedia of Laser Physics and Technology - focal length, dioptric power, curved mirror, lens equation, microscope, photographic objective, focus, beam radius
Focal length - Wikipedia, the free encyclopedia

I see. But from what you're suggesting, that's about as easy to do as spatially filtering the beam anyway since both methods require precision and sturdy mounts.

Could a singlemode fiber strand be used along with a 405nm diode and a hi-power glass lens, to clean up the splash produced from the glass lenses? And also is there a power limit of this fiber strand?

Pete

Drivinfast247 said:

Could a singlemode fiber strand be used along with a 405nm diode and a hi-power glass lens, to clean up the splash produced from the glass lenses? And also is there a power limit of this fiber strand?

Pete

Click to expand...

That's what spatial filtering is supposed to do, clean up the unwanted stuff from the beam. It seems that's what the single mode fiber method is supposed to do as well.

Limecat said:

That's what spatial filtering is supposed to do, clean up the unwanted stuff from the beam. It seems that's what the single mode fiber method is supposed to do as well.

Click to expand...

Hmmmm.... Very interesting. Im going to keep an eye on this thread.

Pete

Limecat said:

I see. But from what you're suggesting, that's about as easy to do as spatially filtering the beam anyway since both methods require precision and sturdy mounts.

Click to expand...

A proper pinhole filter and the mounts will cost more than a strand of fiber, some epoxy and a little ingenuity.. Also size concerns favor the fiber.

@drivinfast: Fiber can handle some very big power. I don't know what the upper limit is, but it should handle blu-ray diodes no problem.

The problem with both methods is that securing the fibre or the pinhole securely costs a lot. A set up for spatial filtering (not DIY) costs a lot if bought from professional optics equipment suppliers and I don't know where else one would find the high precision xyz thing (don't feel like looking it up right this moment) second hand.

ElektroFreak said:

A proper pinhole filter and the mounts will cost more than a strand of fiber, some epoxy and a little ingenuity..

Click to expand...

Or how about a pinhole and some tape?

^if that does a good enough job for your tastes, go for it.. The fiber gives exceedingly clean output, that and the other benefits are why I suggested it. All you would need to set things up is an adjustable driver for the diode, your aixiz module w/ lens, a jeweler's loupe, some epoxy and some creativity. The mechanics of getting truly clean output from a spatial filter aren't just as simple as a pinhole. The hole must be a specific size, and it is usually all but invisible to the unaided eye. Working with fiber is working on a larger scale than a true spatial filter.