can't get sharp focus from DVD laser

[manu de hanoi]

Hello
When I try focussing a regular cheap red laser or when I try with a DVD module, the minimum spot size I get is 0.1 mm (0.004") diameter.
On top of that I get several dots in both cases. The several dots could be transverse modes from what I read.

1)I dont understand why I cant get a smaller spot, especially from the DVD laser (in theory it should read much finer DVD tracks), and I dont understand why I get 2 lobes or more especially from the DVD laser, can someone please explain to me ?

2)I am looking for a cheap, low power (5mw is enough) laser diode in the visible range (blue is better but not mandatory) that could be focused extremely sharply ( below 10 um), suggestions are very appreciated.
Thanks

 

[Alaskan]

what optics are you using? i see from your name you must be in Hanoi?

 

[manu de hanoi]

yes im in hanoi
for focusing I used
-nothing : the (used) DVD module already has a lens
-microscope objective x10 achromat
-home made pinhole (about 0.25 mm)

They all give the same results, at least 2 lobes that can't be divided (by obstructing part of the light before the focusing optic)

 

[Alaskan]

any diode part number or specifications?

 

[manu de hanoi]

any diode part number or specifications?

no,
one is a 5mW 650 nm red pointer that costs 2.5 usd
the other is from Hewlett Packard rm465 DVD ROM drive

 

[diachi]

yes im in hanoi
for focusing I used
-nothing : the (used) DVD module already has a lens
-microscope objective x10 achromat
-home made pinhole (about 0.25 mm)

They all give the same results, at least 2 lobes that can't be divided (by obstructing part of the light before the focusing optic)

Why are you using a pinhole?

Not going to be transverse modes with a DVD diode unless there's something really wrong with it or you're pushing it way too hard, they're single mode diodes.

 

[manu de hanoi]

Why are you using a pinhole? .

i'm not sure anymore of why I use it. But originally it was for "spacial filtering", ie removing some artifacts from upstream optics.

 

[diachi]

i'm not sure anymore of why I use it. But originally it was for "spacial filtering", ie removing some artifacts from upstream optics.

Did you have those artifacts...? DVD diodes generally have clean, single mode beams, spacial filtering usually isn't required. Could be that you've adjusted the pinhole incorrectly or you're doing something else that's causing these artifacts.

What are you trying to achieve?

 

[Alaskan]

I was trying to substantiate this OP first before getting into those questions, but you are correct with all of your remarks diachi, same going through my mind too.

 

[manu de hanoi]

Did you have those artifacts...? DVD diodes generally have clean, single mode beams, spacial filtering usually isn't required. Could be that you've adjusted the pinhole incorrectly or you're doing something else that's causing these artifacts.

What are you trying to achieve?

i am trying to get a very small spot - for imaging purposes.
I tried with and without pinhole ( the configuration I stated above are the optics i've used, but I dont use all of them at once, it's just the things i've tried)

 

[Alaskan]

Can you please take a photo of your setup showing what you are doing?

 

[manu de hanoi]

Can you please take a photo of your setup showing what you are doing?

for now I just tried with another dvd drive, I just rip the optics part, power it with my bench supply (after guess work for the diode pins) and then project it on a piece of paper.

Same prob with the new (asus, 2nd hand, cant burn anymore) dvd I just tried.

Can't take a proper picture, because the focus is very close to the optics (as it should in a dvd player) and it takes a magnifying glass to see well the pattern on the focus dots.

If I take out the dvd diode (therefore not using the DVD lens) and shine it on the back of the microscope objective, I get the same pattern.

BTW has anybody ever seen up close what the focus point of a dvd or cd laser looks like up close? I cant find pics on google.

Are you sure the DVD diodes are single transverse mode ? From alibaba, there seem to be a price jump for the single transverse mode diodes, they often cost 15 usd at least.

 

[manu de hanoi]

if you dont look closely, i might seem to the naked eye that the beam is clean, but it really isnt if you look closely with magnification. And my target is 10 um, but what I see is more like 100 um (0.1mm) at best

 

[manu de hanoi]

as proof of my sincerity please find a pic of my bench with the 2 dvd modules and the microscope objective circled in red:
https://pasteboard.co/H7CZToY.jpg

 

[Alaskan]

Some members don't like repeating posts one after the other, better to just combine them into one, as an edit, but i understand why new members do that, they want to make sure the people they are communicating with see their changes. just editing doesn't guarantee that.

That is a busy bench - What diode are you using? Can you give us a manufacture and model number of the DVD unit you are taking the diode assembly out of? When looking with magnification, is it possible the spot is so bright you cannot see where most of the power is being delivered in the center? The spot size might be much smaller than it appears. Can you burn spots on heat sensitive thermal paper and then measure them, if brief enough they don't grow too fast? The beam has a waist size, it must be positioned the correct distance from the target or you won't be at the smallest waist portion of the beam, but i am guessing you are adjusting up and down to find it, so you are probably looking for that already. Have you looked at a DVD disk to measure the spot sizes on it? I don't know why this won't focus down small enough for you, it should be able to get down to 10 um easy if 405 nm but there are some unknowns, I don't know anything about the optics setup that DVD unit uses and am unsure of the diode too. I would try some other small lenses and see how they perform, maybe try a small laser pointer aspheric lens made for that wavelength.

your first post in the thread mentioned a red laser, what wavelength is the diode you are trying to focus? i assumed was 405 nm, just making sure.

 

[manu de hanoi]

-DVD lasers and cheap red laser pointers are 650 nm
-one drive was
Hewlett Packard rm465-h03 5
the other
asus DRW-24B3ST
-cant see the laser spot on a reflective surface (because it is reflected)