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From: Chuck Simmons
Organization: You jest.
X-Mailer: Mozilla 4.61 [en] (X11; U; Linux 2.0.33 i586)
Subject: Re: laser diode operating range
References: <3DD9B41E.A6D85CCB@webaccess.net> <3DDA6E5D.EAB572C6@SpamMeSenseless.us.ibm.com> <3DDAECF1.F1412F2F@webaccess.net> <3DDBFA1F.CF5FB980@SpamMeSenseless.us.ibm.com>
Date: Thu, 21 Nov 2002 00:57:35 GMT
NNTP-Posting-Date: Wed, 20 Nov 2002 16:57:35 PST
Phil Hobbs wrote:
> Chuck Simmons wrote:
> > >
> > Aside from communications, you are pushing way outside the usual range
> > of use of diode lasers the bulk of which seem to be consumed in consumer
> > products. Communications does not precisely depend on coherence and that
> > is rather good because laser diodes are pretty poor as coherent light
> > sources comparatively. The common consumer products that use laser
> > diodes need a much narrower line than can be had with a bright LED
> > source but they tolerate mode hopping which is a disease of lasers in
> > general.
> "Outside the usual range of diode lasers"? I invite you to try building
> a CDROM drive or a printer using a tungsten source. Do you really think
> that laser pointers are the largest application of diode lasers?
CD-ROMs require a laser or, at least, a very narrow line source. This is
because the laser and the OPIC are combined. There is a hologram which
is really a diffraction grating to diffract the return beam to the OPIC
in the package. Focus in these is done by splitting the hologram into
two gratings producing a knife edge. In addition, these devices use
non-achromatic optics so focusing to the required spot size (very near
the difraction limit) could not be accomplished without a narrow line
source. DVDs have the same trouble as more and more use the hologram
method of combining the laser with the OPIC and the higher NA objective
(0.6 to 0.65 as compared to CD with 0.4 to 0.45) is even more sensitive
to chromatic aberation.
I have a hunch that CD players and drives and DVD players and drives are
the biggest application of lasers with maybe 30 or 40 million lasers per
year as a low estimate. A typical DVD player, for example has two
lasers, one for CD and one for DVD (the wavelengths must be different).
I don't know anything about the innards of printers. I never worked in a
printer design group.
> I don't think you understand the distinction between temporal and
> spatial coherence. The two are almost completely unrelated.
Except for a point source where one implies the other.
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons email@example.com
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