CD Transport: Laser Diode Guru?? HELP!!!

[bear]

Ok, this ought to be fairly simple, or not.

Does anyone know their way around the actual laser diodes that are typically used (by the millions apparently) in CD transports?

These are the little gold plated things... not the entire assembly.
ALTHOUGH if you are in a place that has access to the entire assembly, I'm interested in that too...

What I want to know is their typical operating parameters, AND what output levels they are, beamwidth too! Why? I want to replace the laser diodes in a few very good transports that have become "blind". I have dissected one and it is very much removable. Without knowing this, there are too many variants to pick a good replacement.

I see that they come in 4 pin and three pin.
I see many on the market, even in Digi-Key and Mouser.
BUT, which one is going to be a good replacement, how to choose??

So, any CD transport Laser Diode Guru's??

_-_-bear

_-_-bear

[musicomputer]

Originally posted by bear
Ok, this ought to be fairly simple, or not.

Sorry to disappoint you, but read this.

"Even replacement with an identical laser diode would prove challenging without the optical alignment jigs and specialized test equipment."

[bear]

Don't believe it for a second. I've looked at the optical path and it doesn't look very sensitive, and the diode is locked in place.

I'll take my chances if I can find the right specs - not a thing to lose except for a few bux on a laser diode!

_-_-bear

[scott wurcer]

This is a thankless task. You are better off replacing the whole assembly which is generally available as a separate part. Also there is a sensitivity trim, that little tiny pot on one of the boards, that needs to be adjusted using a proceedure that you probably can't find documented anywhere.

[sidiy]

Originally posted by bear

....
BUT, which one is going to be a good replacement, how to choose??

_-_-bear

I imagine that you were not lucky in finding the whole optical pickup as a spare part.

Anyway, concerning the laser diode, first it is best to try to determine what is the original model used, and do your best to find it. In case it is obsolete at least you have a datasheet and a starting point to find a compatible replacement. Laser diodes aren't really 'plug and play' but you can expect similar characteristics for this application from several manufacturers. Obviously you have to exclude from your search diodes for CD burners or DVD, DVD writers... Only those for CD or Video CD are well suited

What to look for:
- same emission point from reference plane
- same package type and polarity
- obviously same wavelength and power. Max. power can be higher, no problem, but it needs to have:
- same threshold current which actually can be a bit lower than original (but not higher)
- same or very close differential efficiency
- preferably the same photodiode monitoring current

Luckily the last three conditions will ensure about the same optical power, but there's no guarantee - fine adjustments might still be needed. At least you're in the same ballpark as the original and you don't need to change the value of the little pot...

[mhifi508]

Think of replacing the laser pickup before but drop it when I can find the assembly.

Before you replace it:

1. did you confirm there is no laser coming out from the len by using your mobile phone camera to check the beam?
2. did you try to adjust the tinny trim pot on the assembly. It is for the receiver gain, that might help a little.

Good luck.

[bear]

sidiy,

Thanks!

right, and if I could figure out a way to ID the laser diode part, then it might be relatively easy. That's the issue. Iirc, no visible part number - and if there was one, not sure how to find the mfr.



I suspect strongly that there is very very little difference in the output lumens between most diodes in most players of similar vintage and construction...

However:

- same emission point from reference plane

A function of the physical internal construction and the package?

- same package type and polarity

Well, yes, perhaps this can be determined by testing in situ?
(polarity, at least)

- obviously same wavelength and power.

I would think they are all pretty much dead bang the same.


Max. power can be higher, no problem, but it needs to have:
- same threshold current which actually can be a bit lower than original (but not higher)

Mmmm... how to determine empirically using a unit that has now low output?? Or can one pick amongst a given mfr's offerings for one with the lowest threshold current?

- same or very close differential efficiency

Please explain this spec? I am unfamilliar with it.

- preferably the same photodiode monitoring current

Oh? I has its own photodiode internal for feedback??

Regards,

_-_-bear

PS. can you ID any Laser Diode that is/was known to be used in any CD player - especially of vintage mid/late 90s?

[scott wurcer]

bear, here's a picture of the characteristics of one of those little lasers in a laser mouse. It gets across the idea. There is a lasing threshold and a slope of optical power out vs current which is pretty linear. The diode voltage is on the high side like an LED. You need to match the slope to get the same modulation depth, IIRC that is what the SMT pot is for. I don't remember when the power monitoring diode is built in or when there is a beam splitter and a separate detector.

If you're working with a CD writer I shouldn't have to tell you that they are NOT eyesafe.

BTW I don't think adjusting the pot while the CD is playing is an option, during mfg the whole thing is put in a test jig. They really are assembled to optical tolerances that you would not believe.

Here's an info link too http://members.misty.com/don/laserdio.htm#diocss4

[sidiy]

Here is a typical datasheet of a laser diode used in CD-R(W) systems. It has much higher power than a normal LD for playback only.
It answers to some of your questions, I'll try to get back later with some comments. Note the tolerances used for optical alignment (+/- 0.08mm).

I forgot to add a disclaimer: I'm not an LD guru, still ... I had some experience in the past.

[sidiy]

And here is the whole assembly as used in mechanisms like CD-Pro2M (LF). Again the mounting tol. on all three axes has to be better than 80um.