How to correctly refocus a laser source?

[almosnow]

Hi,

I have this handheld laser hanging around, it burns stuff when is really near (around 10cms max), and then it basically becomes so diffuse that it doesnt burn anything.

Since I know almost nothing about optics I've tought this to be a good chance to learn stuff. What I'm trying to accomplish is (or I guess it should be) really simple, I want to refocus the laser back at different distances and see how does the power density behaves (basically, if it still burns things or not).

I've made the following diagram where I illustrate a few of the cases that I'll try to achieve.

As you can see, (1) represents the normal operation for my laser, I've calculated the beam diameter at different distances, and what I'd like to do is to put "something" in the way of the beam (I don't know if its a lens, or a magnifying glass or anything) so that the beam diverts differently and becomes refocused at an arbitrary distance (examples (2) and (3)).

Is that possible? Can anyone explain me the theory? Or point to some good tutorial or web resource?

Thanks and best.

 

[cheech226]

what kind of laser are you using? you probably want to keep any lenses close (like in the same unit) to the diode

 

[almosnow]

Its a handheld laser.

Yeah I went too far with those distances haha, could it be done at a shorter distance?

How do I do it?

 

[steve001]

Hi,

I have this handheld laser hanging around, it burns stuff when is really near (around 10cms max), and then it basically becomes so diffuse that it doesnt burn anything.

Since I know almost nothing about optics I've tought this to be a good chance to learn stuff. What I'm trying to accomplish is (or I guess it should be) really simple, I want to refocus the laser back at different distances and see how does the power density behaves (basically, if it still burns things or not).

I've made the following diagram where I illustrate a few of the cases that I'll try to achieve.

As you can see, (1) represents the normal operation for my laser, I've calculated the beam diameter at different distances, and what I'd like to do is to put "something" in the way of the beam (I don't know if its a lens, or a magnifying glass or anything) so that the beam diverts differently and becomes refocused at an arbitrary distance (examples (2) and (3)).

Is that possible? Can anyone explain me the theory? Or point to some good tutorial or web resource?

Thanks and best.

Putting the fancy illustrations aside have you attempted moving the positive focal length lens back and forth too observe what results it produces ?

 

[Cyparagon]

Just put a magnifying glass near the 8m point - like you'd do with the sun.

 

[Bionic-Badger]

Learn about ray tracing and lenses.

 

[almosnow]

Thanks everyone so far for your feedback.

"have you attempted moving the positive focal length lens back and forth too observe"

No, I haven't, because I don't know what that is. I'm sorry but I'm a complete noob in optics.

"Just put a magnifying glass near the 8m point - like you'd do with the sun."

I'm trying that tomorrow, do you know if there's any law or something to calculate the expected drop on power density?

And finally: "Learn about ray tracing and lenses."

Of course, that's why I've made this post.

 

[steve001]

Thanks everyone so far for your feedback.

"have you attempted moving the positive focal length lens back and forth too observe"

No, I haven't, because I don't know what that is. I'm sorry but I'm a complete noob in optics.

You are not that much of a noob unless you were born yesterday. You do know what a magnifying lens looks like then you know what other positive lenses look like. All positive lens have at least one outward curving face.