Re: how does light cause interference phenomena?
From: Tom Roberts (tjroberts_at_lucent.com)
Date: 09/25/04
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Date: Sat, 25 Sep 2004 00:11:44 GMT
John Kennaugh wrote:
> I am under the impression that very early radio transmitters, which used
> sparks were in effect EM noise generators.
Yes. As are modern radio transmitters, and electronic signal generators
of all sort. What differs is the bandwidth of the output signal -- very
early radio transmitters used a spark gap to generate a wideband noise
signal, and this was filtered with one or more LC circuits to limit the
bandwidth to a somewhat narrow region of the spectrum. A modern radio
transmitter uses a high-gain amplifier in a feedback loop to generate a
signal with an enormously-smaller bandwidth (and stability, etc.).
> It was not until the
> thermionic valve oscillator was invented that sinusoidal EM radio waves
> could be generated.
Hmmm. Strictly speaking, of course, they are not perfect sinewaves, but
have finite bandwidth. It's just that for most modern oscillators the
output bandwidth is smaller than the resolution of common test
equipment. Here "modern" merely means an oscillator based on a high-gain
positive feedback loop, using vacuum tubes or transistors.
> My understanding is that something like a filament
> light bulb generates light 'noise' and that it was not until the
> invention of the laser that light joined radio in producing a pure
> waveform.
The exact same remarks above apply -- the laser merely has a greatly
reduced output bandwidth (and stability, etc.).
Note that all common He-Ne lasers are multimode, and their output
consists of several (typically 5-8) narrow peaks separated by typically
1 part per million or so -- they are not at all a "pure waveform". This
is for a common He-Ne laser, and semiconductor lasers are different
(much wider output bandwidth with individual modes indistinguishable);
ditto for Nd:YAG lasers. Laboratory He-Ne lasers can achieve singlemode
output, but that takes refined technique and exquisite care in setting
it up. A multimode laser typically has an output bandwidth of perhaps
5-10 parts per million of its frequency (quite narrow by most
standards); a singlemode laser can be a thousand times narrower than that.
BTW the narrow bandwidth of a tube or transistor oscillator comes
primarily from its high gain. The narrow bandwidth of a multimode laser
comes from the sharpness of the atomic transition involved. The narrow
bandwidth of a singlemode laser comes from the sharpness of the
Fabry-Perot interferometer formed by its mirrors. This last is
significantly narrower than the others.
> If I have that right, and it seems logical that a filament should
> produce light via a random process then it is the em equivalent of band
> limited noise.
Yes, a filament produces light via a random process: thermionic
emission. But all sources of any type of signal are "band limited noise"
-- the differences are in the bandwidth, not the principle.
> Scott Murray suggested (from theory underlying the laser) that when a
> photon is randomly emitted it acts like a catalyst and stimulates the
> emission of a mass of other photons.
This only happens if there is a "population inversion" present in an
amplification material. And even then the gain is very low unless you
arrange for multiple passes throught the medium, as in a laser
(typically the light bounces back-and-forth ~20-100 times in a
well-built laser).
> This would suggest that light from
> a filament is not in fact random but in short bursts of coherent light
> with no fixed phase relationship between one bust and the next but a
> fixed relationship within the burst.
That is indeed the case. But individual "bursts" are basically a single
photon.
The coherence length of a common He-Ne laser is usually a few
centimeters, which means you can construct a hologram over a volume of
about that size. A singlemode laser can have a coherence length of many
meters. IIRC a lightbulb has a typical coherence length of a few microns
-- long enough to generate Newton's rings in soap bubbles and oil films,
but not any larger-scale interference phenomena.
Tom Roberts tjroberts@lucent.com
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