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From: rstevew@armory.com (Richard Steven Walz)
Subject: Re: Broken CD Rom drives
Organization: The Armory
Date: Thu, 29 Sep 1994 17:40:54 GMT
Message-ID: <CwwJs8.Hp4@armory.com>
References: <35sr1e$gco@handler.Eng.Sun.COM> <367uk4$jvp@hemp.imel.kyoto-u.ac.jp> <369oc0$t93@handler.eng.sun.com>
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In article <369oc0$t93@handler.eng.sun.com>,
Chuck McManis <cmcmanis@Sun.COM> wrote:
>Onat Ahmet (onat@turbine.kuee.kyoto-u.ac.jp) wrote:
>: I had the impression that laser diodes from CD players use red light. Up till
>: now, I took apart three of them, (audio) and all were emitting RED light. The
>: light was not  in a parallel beam, and there was only one point where it would
>: focus into a bright spot. 
>
>The Laser Diodes in the CD-ROM drives emit strongly in the IR spectrum. There
>appears to be a bit of red as well but I don't know enough yet about the 
>physics of the laser diode's construction to explain it. 
>
>Now in the case of the CD-ROM, the laser is co-packaged with a detector in
>an assembly mounted on a lead screw. This assembly has a lens on the top of
>it which is looking at a dichroic mirror. The mirror is mounted at a 45
>degree angle and reflects into a half silvered beam splitter. The beam
>splitter is pointing to the laser and the detector. Now powering up the
>laser in this configuration will get you a spot at the focal length of
>the lens, disassembling this and removing the laser diode (which in my
>case was brass) yields a package about the size of a pencil eraser with
>a clear end and three pins on the other. Powering up the removed laser
>diode results in just a raw beam coming out of the clear end. I happened
>to have a selection of small lenses that came in a kit from Edmund Scientific
>and using a couple I built a simple collimating setup. This produces a
>fairly small point (about 1mm) at a distance of 10'. Without the collimator
>the laser produced an approximately 6" diameter circle at 8'. Note these
>measurements were made with a steel ruler looking while watching a monitor
>(not the easiest way to measure things!)
>--Chuck McManis	     All opinions in this message/article are
>Sun Microsystems Inc.        those of the author, who may or may not
>Internet: cmcmanis@Eng.sun.COM       be who you think it is.
----------------------------------
Here is the confirmation I am looking for. Then for a 3mW laser diode, the
beam at 1" is spread over 1/96th of 6" diameter, which is 6.25 x 10^-2
inches, or .15875 cm and .0015875 m. Now the area of that circle is
1.98 x 10^-6 m^2. Thus the W/m^2 is  3 x 10^-3 W/ 1.98 x 10^-6 m^2 =
1515 W/m^2 or the solar flux level. Given that these lasers are about 10%
efficient, this becomes 150 W/m^2 or about like looking at a light bulb
while changing it with it on! And this is of course at the retina, an inch
back inside the eye. This seems unlikely to blind anyone anytime soon! It's
It's not even probably THAT concentrated, because the beam as it emerges
from a junction laser diode has width! The cone does not even come to a
point at the source! Thus I am being slightly conservative! Couple that
with not even presenting a large amount of heat to dissipate at the retina,
because it lacks size, and the retina can sink some heat to cooler adjacent
regions, and we find the equivalent of looking at perhaps a bright reading
lamp bulb at a several feet away. And THIS would require someone look
directly at this laser diode module's direct output with it virtually
touching the pupil!!! This does NOT present great danger from these
calculations! Anyone with more info????
-Steve Walz   rstevew@armory.com

