Shape at point of impact

[Feral]

Sorry for the newb question, but I'm finding it difficult to search for my answer... getting a lot of hits on irrelevant things--perhaps I'm not using the correct terminology.

I've had cheap red laser pointers for years. They always seem to project the shape of a small bar/rectangle. I figured this was normal and didn't much question it.
I got my first green laser in the mail today, a WL Core (hurray eBay!), which I heard written up very positively around here. The dot it projects is a "perfect" (to the naked eye), uniform circle.
Is this a function of green vs red? Is the Core just a higher quality laser pointer than I'm used to? If so, is there a reasonably priced <5mW red that is also well liked amongst the people here for its quality?
I noticed a lot of the people around here have interest in higher powered lasers, but I figure there is still some allure and fun to the lower power "no goggles needed" lasers, no?
Thanks for the read. If anyone can explain it, I'd love to know the science behind the shape of the laser dots I'm seeing.

 

[TTerbo]

well you should still have goggles. i dont and i am building a fully custom homemade bluray laser sooon yay. the dot on a cheap red laser is about a rectangle maybe because they use smd type flat laser diode. and when you ger good stuff you can ecpect a nice circle or oval.
cheers

 

[RA_pierce]

Cheap red pointers will usually have a "messy" dot.
Green laser pointers should have a nearly perfect circle.

The red pointers you have been using are diode lasers. That is, the beam is produced by applying current through a tiny semiconductor almost like an LED.
There are imperfections in the "chip" that cause the beam to look the way it does.

Green lasers are DPSS lasers (diode pumped solid state).
Searching terms like "Gaussian beam," "Transverse modes," "DPSS," should shed some light on this.

Low power Gas lasers and DPSS lasers have superb beam quality... much better than diode lasers. This is because of the different lasing mediums.
Optics can be used to correct the beams, but that is straying away from this topic.

 

[Emc2]

Sorry for the newb question, but I'm finding it difficult to search for my answer... getting a lot of hits on irrelevant things--perhaps I'm not using the correct terminology.

I've had cheap red laser pointers for years. They always seem to project the shape of a small bar/rectangle. I figured this was normal and didn't much question it.
I got my first green laser in the mail today, a WL Core (hurray eBay!), which I heard written up very positively around here. The dot it projects is a "perfect" (to the naked eye), uniform circle.
Is this a function of green vs red? Is the Core just a higher quality laser pointer than I'm used to? If so, is there a reasonably priced <5mW red that is also well liked amongst the people here for its quality?
I noticed a lot of the people around here have interest in higher powered lasers, but I figure there is still some allure and fun to the lower power "no goggles needed" lasers, no?
Thanks for the read. If anyone can explain it, I'd love to know the science behind the shape of the laser dots I'm seeing.

Well the red laser is a straight forward diode with the exit window in the shape of a rectangle, this gives the bar shaped spot you are seeing. Other red lasers like O-Like's 200mW new and old style red laser produces a round beam/spot because of the optics used. As for the green, well I can't give an expert reason for it's round beam, but the optics play a major role in how the exiting beam will look. Here is a good explanation on the red lasers:

Fundamentals of Lasers - Edmund Optics

Hope this helps

Todd

 

[pullbangdead]

The dot shape is a result of the shape of the optical mode, which is determined by the laser medium and the shape of the cavity. In a green laser, the lasing medium is the Nd:YAG or Nd:YVO4 rod, which is often lasing in a very good circular mode, hence a circular beam.

Edge-emitting diode lasers (which red and violet lasers are in sample pointers) do not generally allow for a true circular mode profile because of how the diodes are made and how they operate. Depending on how they operate, they could be an ellipse (aka almost an oval), which is most common around here as that is the shape you get from a single-more diode, and single mode diodes are used in optical media applications.

The rectangular shape can come from several things, like from a multimode output that is then corrected, or possible from an aperture of some sort. But the light coming out of a diode won't be a rectangle, it'll be the Fourier transform of the shape of the optical mode inside the laser, and the shape of the mode inside the laser will be some sort of an ellipse or a combination of multiple ellipses. The optical mode will approximate a Gaussian ellipse, and the Fourier transform of a Gaussian is a Gaussian, resulting in a Gaussian elliptical dot on the wall.

 

[Feral]

TTerbo: Thanks for the advice, I've got a pair of goggles for 650nm, and I just ordered some DIW lenses from OEM Laser Systems for the two DIY build's I'm waiting on parts for.

RA_pierce: Hrm... so if I eventually get my hands on a blue laser (473nm... I believe most of these are also DPSS?), it's will probably also have a nice perfect dot. You say gas lasers also have this characteristic. I can see how a cheap little greenie would be a "gateway" drug of sorts!

Emc2: Thanks for the link! I learned quite a bit of that from Sam's laser FAQ, but that's a nice short overview--answered a bit about questions I had concerning some of the terms used to describe the beam.

pullbangdead: Wow. That's what I was looking for! Though the thought of going back to fourier transforms from college gives me the chills--thankfully for my purposes here, knowing the output is a fourier transform of the internal mode is sufficient.

From this, I can expect the DIYs I'm accumulating parts (405nm , 650nm, both harvested from optical drives) will both have that elliptical dot. Its good to know this isn't a function of the quality of the diode, and instead an effect of the method used to produce the beam.

From the information you all gave me I started looking out further, armed with new terminology, and found the wikipedia article on laser diodes. The last portion of the theory of operation cleared up a bit of my confusion, maybe it'll help someone else down the line.

Thank you all for the responses.

Does everyone get into this hobby starting with "Oh, I just want a red and green pointer" then "Ooo... bluray..." followed by thoughts of nonstandard blue, yellow and orange lasers... and browsing through the gas laser section of the forums wondering if/when you'll get to that level.... so much for not needing any more expensive hobbies *sigh*