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From: Phil Hobbs
Subject: Re: laser diode operating range
Date: Thu, 21 Nov 2002 11:20:02 -0500
Organization: IBM T. J. Watson Research Center
References: <3DD9B41E.A6D85CCB@webaccess.net> <3DDA6E5D.EAB572C6@SpamMeSenseless.us.ibm.com> <3DDAECF1.F1412F2F@webaccess.net> <3DDBFA1F.CF5FB980@SpamMeSenseless.us.ibm.com> <3DDC2F6D.5D54F1@webaccess.net>
NNTP-Posting-Date: 21 Nov 2002 16:20:04 GMT
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Chuck Simmons wrote:
> 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
When you say "very narrow", we're talking about a maximum line width in
the terahertz. One nanometer in the visible corresponds to about 1 THz
line width. Self-pulsating lasers (or those with UHF quenching) used in
DVDs are about 1 THz wide. This leads to a temporal coherence time of
about a picosecond.
A garden-variety single mode diode laser has a line width of 10 MHz if
you don't do anything special, a coherence time around 100 ns, so
"narrow" can mean different things to different people.
> 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).
Right, which proves the point that you need high spatial coherence for
the dominant consumer applications.
> > 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.
Not even slightly true.
1. A distant star is almost perfectly spatially coherent, but just about
totally temporally incoherent--its coherence time is about one
femtosecond--because it's a quasi-blackbody source. The same is true of
an unresolved pinhole with blackbody illumination. (Full spatial, ~zero
2. A cold interstellar molecular cloud. Its 21-cm hydrogen line has a
natural linewidth much less than 1 Hz, so a sufficiently cold and quiet
cloud can have a fractional linewidth of well under 10**-9, narrower
than a diode laser's. Yet this is a thermal source, with no spatial
coherence except that contributed by small angular size (as in (1)).
Near or inside the cloud, this emission will have virtually no spatial
This is getting _way_ off-topic for this group, so let's continue in
sci.optics or just let it go.
Advanced Optical Interconnect
IBM T. J. Watson Research Center
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