Pinhole for laser diode

Hello, I am an active participant since 3 months now.
I am building a set-up to reduce my beam size from 3mm to 1 micron. I used a large combinations of lenses but always had issues in size. So i decided to use a 1 micron pinhole. I want to ask for an expertise to know if it works.
Should i converge the beam a little and then put the pinhole?

Thanks in advance

Wha kind of power density do you need?
What kind of divergence?

The reason I ask is that you'll lose alot of power if you just attenuate the beam.

Lets assume you are using a 300mW beam with a diameter of 1mm. To simply shine that laser through a 1 micron hole would reduce your power by about factor of 1 milion (1 square millimeter is about 1 million times more area than 1 square micron). So you'd end up with a 300nW (nanowatt) beam.

Keep in mind that when you optically reduce beam size you also increase the divergnce of the beam because beam diameter and divergence are inversely proportional. A 1mm 0.5mrad beam (this is an excellent beam) entering a 1000X beam reducer will give a 1um exit beam but the divergence would be 500mrad. Every meter your micron beam travels, the diameter of the beam will diverge 500mm (terrible, like a flashlight). Also your lens contours would have to be crazy-accurate to produce beams around 1-3 wavelengths in diameter.

Also, you're starting to approch visible wavelength of light. 1 micron is 1000 nanomaters. As you approach the wavelength of your light you will begin to experience interesting phenominon with your light. Particularly, diffraction patterns will emmerge due to light displaying wave behavior as it passes throught apertures small relative to the wavelength of light.

I dont think divergence would be a problem because this is passing through a pin hole and not a beam reducer.

The set up he had used before with all the lenses would have definitely had that problem though.

You are right about the power, it will be greatly reduced.

What do you even do with a nanowatt range beam?

There's more excitation when I walk into a room full of women than there is involved with a nanowatt-scale laser... and I'm not a good looking guy.

jJack said:

I am building a set-up to reduce my beam size from 3mm to 1 micron.

Click to expand...

Why? See sig.

do you need the beam to have 1um on the full length, or do you just need a spot with 1um diameter? in the second case, you need just a colimating lens and some accurate adjustment mount which will allow you to set the smallest spot size.

A micron sized dot or beam waist?!

One micron is just under 2 wavelengths when working with 532 nm light, and will definitely give you interesting particle-wave duality effects, but not a micron-wide beam at all.

What actually would happen is this: The pinhole would act as a new point of origin for the beam coming out on the other side. This would be spread over almost a complete half sphere, with a gaussian distribution favouring the forward direction.

Optics may work to shrink a beam down to 10 microns or so at a given focal length, but beyond that things get complicated.

If you would state your goal with doing all this, perhaps we could provide some more insight.

Commercially there are no laser sources with 1 micron and less than 1mw power, which is the target. The application is correctly for low power application.
So I bought a laser diode module with 3mw (best i could get), 0.203mrad divergence, 650nm wavelength and around 4 mm waist diameter at the tip of diode. I thought I can reduce its size to 1micron using lenses.
I simulated the beam in a software, reducing the size step by step in 3 stages. Since i have to maintain the divergence angle ... the smallest spot i could get and almost collimated was 10 micron.

The application is for MEMS devices which are of the order of 100microns in size.
The beam hits the device, reflects and strikes a detector. So its like a beam falls on the mirror at an angle and gets reflected. The reflected beam is made to strike a light detector. The total length of light travel, starting from the laser diode is estimated to be within 200 -300mm.

I like to thank for all the replies. But I am seriously working on this project for 3 months now and searching for help.

1 micron is a stretch for a focal point. Making a beam that small is virtually impossible. If you only need it to be 1 micron at two points, you might be able to focus it twice instead.