can't get sharp focus from DVD laser

[Alaskan]

OK, I was wondering if you had a read-write DVD which also has 405 nm blue, my bad. Theoretically, you should be able to get down to 1/2 a wavelength with the right optics which for 650 nm is much smaller than 10 um, of course.

Edit; went looking for CD spot sizes for the different wavelengths, found this:

https://commons.wikimedia.org/wiki/File:Comparison_CD_DVD_HDDVD_BD.svg

Of course, this information provides the spot sizes produced on the CD, not necessarily how small you can get the spot to be for another application.

 

[manu de hanoi]

I removed the laser diode from the DVD optic, and focused it using a microscope objective (with diaphragm, cause it's not a "plan" objective). Then I ve got enough focal length to take a picture of the spot, and upon zooming on the pic *ahem*, *cough*, I got a single spot.
It would appear the problem is in the optics of....my eyes, even wearing glasses i've some astigmatism that remains.:yh:
Thanks a lot for your guidance, taking the picture was the key. Sorry for inconvenience !

 

[paul1598419]

A CD or DVD optic has a grating in it to have beams on the sides of the track for..........wait for it........tracking. Without these beams the laser can't track the digital information line. There are photo detectors in the CD or DVD to "see" these tracking beams and keep the main beam on track.

 

[manu de hanoi]

A CD or DVD optic has a grating in it to have beams on the sides of the track for..........wait for it........tracking. Without these beams the laser can't track the digital information line. There are photo detectors in the CD or DVD to "see" these tracking beams and keep the main beam on track.

Thanks, I think I saw the grating, it's the transparent thing right on the laser output, when unfocused the light has a series of lines.

How far do you think the side beams are from the central beam on the focus point ?

 

[paul1598419]

It depends on the device. For CDs it is about 1000nm, give or take. The further away from the disc, the wider they get, but the focus would be off so you might not see them.