"Mark D. Zacharias" wrote in message
t...

wrote in message
ups.com...
It was my understanding that the IR laser in a cd player is just too
"fat" in terms of wavelength to resolve the tin

That was where firefox crashed. I was going to say something about
tinier pits on DVD's..

When you say emits from one side or the other, do you mean that both
ends of the crystal are used as emitters? Or are there multiple diodes
in a single package? I never heard of this; it sounds interesting.

I just assumed that it was a single laser, but I have definitely
observed that a much more powerful looking beam is used to read DVD's
than CD's.


I actually tore apart a dual-wave diode a couple years ago, but I don't
remember exactly how it was built. I do remember it was roughly
rectangular in shape with tiny wires going to it. I only "presume" it was
three connections total, either 3 wires or two wires with a center common
connection.

Mark Z.


I just had a quick look at a few scrap lasers which were hanging around the
workshop, and they have six connections to the laser diode unit. Looking at
a few schematics, these are one each for the anodes and cathodes of the
laser diodes ( one red, and one infra red for DVD and CD respectively ) and
one each for the feedback photo diode integrated with each laser diode. The
other connection of the feedback diode is commoned with one of the laser
diode connections in each case. The feedback diode is to allow for automatic
power control of the laser output. Laser diodes with only three connections
appear on the regular CD only laser heads. Here, the connections are again
anode and cathode of the laser diode, and one leg of the feedback diode, the
other leg again being common with one of the laser diode legs.

The general sequence of events is that the laser homes and then the red DVD
laser is switched on to burn. The focus sequence is then initiated and, once
focus has been obtained, the disc is spun up, and an attempt is made to read
data from the disc. Sometimes, the disc is spun up at random speed prior to
or during the focus search.

As Mark correctly says, the shorter wavelength red DVD laser is actually
able to read the longer pits on a CD, although not very efficiently. Thus,
once enough data has been gathered to evaluate whether the disc that's being
read is a DVD or a CD ( or even a VCD ), the system control processor will
make an evaluation as to whether the red DVD laser should be left burning,
or the infra red CD one, switched on instead. Other subtle changes are made
as well at this time, including altering servo time constants, and disc base
rotational speed etc. In the case of a DVD, the focus has to be adjusted to
ensure that it's reading layer zero at this time also ( assuming that it has
been informed by some of the early data that's collected, that it's a dual
layer disc that's in there. It's a complicated business, to be sure ...

Arfa