Laser system inspection - Page 1
Forum section: electronics / beginners

author: jimon
author_rank: Regular Contributor
timestamp: January 23, 2015, 10:47:59 pm
content: I want to use cd drive head to read some information from some surface, my requirement is to focus laser beam to specific point radius (like 1-5 micrometers) and then read reflected beam. As I understand cd\dvd players use auto-focus technics to do this. Let's imagine that I will be able to implement auto-focus part, but I want be able to see what is happening (it's like developing electronics with or without oscilloscope). So how I can inspect the laser system ?I can buy microscope to do this, but then what ? I don't want to burn my eyesso should I search for some specific filters ? Or maybe I should buy a digital microscope and then just wear laser safety glasses ?PS. Total newbie in lasers and opticsKinda understand basics, but haven't played with them at all.

author: ConKbot
author_rank: Super Contributor
timestamp: January 24, 2015, 02:07:32 am
content: I'm not sure how the AF on a CD rom works as far as determining how to drive the suspension coil. Off the top of my head Id suspect that in a CD-rom application, you could look for the highest amplitude change between dark and light, and the steepest/sharpest transitions on the output from the photodiode as an indication that its in focus. I bet digging though patents may be helpful.Are you going to be examining a moving media like a CD rom drive is (Or scanning the read head) or just trying to measure the reflectivity of a static position?

author: SL4P
author_rank: Super Contributor
timestamp: January 24, 2015, 03:43:20 am
content: Sounds to me like you'd be better starting out with a low-power 1-3mWvisiblelaser in order to get your physics and control / reading concept knocked into shape. Then it's relatively trivial to convert to your preferred laser technology.If the target / symbol is specific to a particular wavelength, you could reproduce some sample targets into visible range - while you get part 1 done, then you know everything is working, and your logic works --- then it *only* the laser wavelength you have to re validate.

author: SeanB
author_rank: Super Contributor
timestamp: January 24, 2015, 05:18:08 am
content: IIRC CDROM focus works with a spherical lens and a 4 quadrant photodiode. As the beam comes into focus the 4 quadrants get equal lighting, as it goes out it either has one opposite pair or the other getting more light, and this error voltage is used to drive the focus coil to do height tracking. The 4 quadrant outputs are summed as the signal output as well.

author: jimon
author_rank: Regular Contributor
timestamp: January 24, 2015, 11:48:17 am
content: Quote from: ConKbot on January 24, 2015, 02:07:32 amIt's a moving media, vinyl record if to be precise, I'm investigating possibility to implement head from this patenthttp://www.freepatentsonline.com/4972344.pdfby using two CD drive heads and replacing main detector with analog position sensitive device (like thishttp://se.mouser.com/ProductDetail/First-Sensor/OD35-6SO8/?qs=sGAEpiMZZMtWNtIk7yMEscpTFN8GTr6jlRDQ9Lh8Pug%3dorhttp://www.hamamatsu.com/us/en/product/alpha/P/4010/4156/S2044/index.html,last one is a bit expensive though)So I need some way to directly inspect correctness of experimental jig setup. Otherwise it's like developing product while you are blindfolded.Quote from: SL4P on January 24, 2015, 03:43:20 amSure, but I think that even 1 mW can damage your eyesight if you gonna look on it through microscope.

author: ConKbot
author_rank: Super Contributor
timestamp: January 25, 2015, 05:30:52 pm
content: If you have an optical instrument in mind already(I.e. certain microscope setup ), just swap out an eye piece for a camera. A CCD is easier to replace, and can probably handle more power too since.Or one of those HD usb microscopes if you can make that work in your application.