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From: sam@colossus.stdavids.picker.com (Sam Goldwasser)
Subject: Re: Broken CD Rom drives
In-Reply-To: rstevew@armory.com's message of Thu, 29 Sep 1994 17:07:09 GMT
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Date: Fri, 30 Sep 1994 01:59:54 GMT
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In article <CwwI80.Gso@armory.com> rstevew@armory.com (Richard Steven Walz) writes:

   The solid state lasers are so poorly collimated that they barely work in
   CD's!!! Without the plastic lens which focuses them on the CD at the
   aluminum layer, they are usually so spread out that 3mW of anything
   couldn't hurt you at any reasonable range, and by the way, they are this
   way because they are not very coherent either or monochromatic! They have

Not the reason.  Example: a HeNe laser, which is inherently very well
collimated would also produce such a cone shaped beam if put through a short
focal length lens.  However, another lens will recollimate it just fine.

   lines in the visible as well as the IR, and when the CD focusing lens is
   over them, they cone all their output to about a two foot circle at a one
   foot distance!!! 

--- deleted ---

Just because the beam coming out of a bare laser diode is wedge shaped does
not imply that it is either incoherent or non-monochromatic.  It is true that
these characteristics of solid state laser diodes do not compare to other
types of lasers such as HeNe.  However, in order to read the CD, the optics
do generate a nearly defraction limited spot on the disc's aluminized layer.
(The track pitch is 1.6 um and the pit width is on the order of .5 um.)
A bit more than a glorified LED.

>If you do the math, you will find that it would be damned
>   hard to hurt anything with them! The W/m^2 is simply minute!! Even the best
>   solid state laser diodes issue light in a wedge shaped output from the
>   substrate junction that is hard pressed to have much of it reasonably focus
>   by lens into anything like a parallel beam!

Was your professor an optics expert?

Have you ever seen a pocket laser pointer?  Seems fairly collimated.
And, quite efficient, too.  Of course, they are visible so it is obvious.
The beam from the diode in a laser pointer is wedge shaped also before
it is collimated.

> I would love to see how little
>   these actually put out, but I bet they are below all safety limits and not
>   sufficiently concentrated without their conizing lens to harm tissues even
>   on the retina!!! Any solid state laser people to confirm this? This is what

At any reasonable distance, true enough.  I believe this has been posted
before.  The power output printed on the CD laser assemblies typically
are somewhere between .3 and 1 mW.  This is the power level specified
at the output of the optical pickup - not the diode.

>   I learned in a good solid state device course! In other words, mere
>   amateurs are ruling on the safety of basically glorified LEDs!!!!!
>   -Steve Walz   rstevew@armory.com

>   P.S., And yes, I DO know that they CAN and DO make solid state lasers that
>   can be harmful. My contention is that these are NOT THEM!!!! Unless someone

Yes, you can buy more powerful diodes, visible and IR, from the national
electronics distributers - about $20 for a 5 mW (output) diode.

>   had a special focal length lens made to focus this spread out light precisely
>   on their retina, harm to the eye from them would be impossible!!! But this
>   is as dumb as warning college students not to look at a 100W bulb with a

You can see the light from a 150 watt bulb and therefore take the appropriate
precaultions!

Also, as the popularity of magneto-optical and other read/write
optical drives increase, laser assemblies from these devices will become
available surplus.  The power levels are often much higher than your common
CD since they need to heat the disk material during the writing process.
You cannot tell the difference in power by looking at an invisible IR beam!!!

>   magifying glass!!!! Of course they shouldn't! But in this case they would
>   nearly have to grind their own special lens!

And someone posted to this very group about using some grab bag lenses 
from Edmund Scientific to collimate the beam quite effectively.

OK, I am not suggesting that CD lasers are so dangerous that a casual glance
will cause any kind of damage.  However, as pointed out in previous
postings, safety around any kind of laser is important based on awareness of
the potential for harm with the particular type of laser and associated
optics.  Many people reading these postings are the type of experimenters
who will modify the optics to build rangefinders or illuminators.

The point about IR lasers is that there is almost no visible indication
of whether the beam is on or off, or how powerful it really is.

--- sam

Samuel M. Goldwasser
Technical Director, Visualization, Computed Tomography
Picker International, St. Davids, PA 19087
sam@stdavids.picker.com
