From: Morten Reistad <mrr@reistad.priv.no>
Newsgroups: alt.folklore.computers,comp.arch,comp.sys.super
Subject: Re: Electrical power outlets
Date: Tue, 7 Oct 2003 10:17:20 +0200
Message-ID: <gqstlb.5cm.ln@via.reistad.priv.no>

In article <ihc4ov8r5a2bbba9g9hikaqiqj7ii5eo1k@4ax.com>,
Brian Inglis  <Brian.Inglis@SystematicSw.ab.ca> wrote:
>On Tue, 7 Oct 2003 02:31:05 -0000 in alt.folklore.computers,
>"Dennis Ritchie" <dmr@bell-labs.com> wrote:
>
>>
>>"Sam Yorko" <JOATnospamMON@computer.org> wrote in message news:3F81F7EF.ED132B3D@computer.org...
>> ...
>>
>>>
>>> From what I understand, they're switching to optically amplified
>>> amplifiers on undersea optical cables:
>>>
>>> http://searchnetworking.techtarget.com/sDefinition/0,,sid7_gci214539,00.html
>>>
>>> Cool technology.
>>
>>It is cool, but the lasers that pump the erbium fiber
>>are electrically powered.
>
>That answers the question I was going to ask about where the
>energy for amplification comes from. I could see pulse reshaping
>possibly being done purely optically, but amplification requires
>additional energy input. Has anyone looked at sending power for
>amplification through as beams at non-signal wavelengths? Could
>say half the bandwidth on long links be used purely for optical
>energy transmission to power amplifiers with the other half used
>for optical signal transmission.

The problem with this is that you get into problems with energy
densities.

The fibers for long haul connections depends on being single mode;
this means that the photons are trapped in a grating where they
self-resonance; and they only have one free dimension; the direction
along the wire. This means the width of the optical path is a few
light wavelengths in diameter. The actual width varies a little, as
different materials give resonance at different frequencies and
wavelengths. There has been extensive work to widen the frequency
path where the cables have photon resonance; and this has been a great
success. Modern cables give good resonance from 1300-1500 nm and
from 1600-1750 nm; giving a vast potential bandwidth. The normal
witdth of these optical paths is between 7 and 9 nanometers.

The upside of giving the photons a one-dimensional universe
to live in is that they have no dispersion whatsoever. They
will carry a signal "forever". 160 km is standard technology,
300 km advanced, but well commercialized, and 800 is pushing it.
This means that some short runs like Japan-Korea, cross-baltic
and mediterranean links don't need repeaters at all. For
the North Sea you can have the repeaters on dry land on platforms.

DWDM depends on sending lots and lots of different frequency light
down such a path; and let the erbium boosters amplify all of them.

The practical lower limit per frequency is around one watt for really
long hauls. You would want to send 80+ of these; so you want 80+ watts
sent into this 9 nanometer wide path. In practice you send more than
this.

So, into this ~6.3 E -17 meter cross-section you send an effect
of 8E1 watts, for an energy density of ~1.3 E18 watts/m2.
This is sufficiently close to some fundamental constants to
be problematic in itself, and transporting energy this way
is not a winning proposal.

Any other method of transport through fiber will have dispersion
loss that disintegrates signals and energy in just a few
kilometers.

So, we will be using DC feeds through sea cables for a long time
to come.

-- mrr

From: Terje Mathisen <terje.mathisen@hda.hydro.com>
Newsgroups: alt.folklore.computers,comp.arch,comp.sys.super
Subject: Re: Electrical power outlets
Date: Thu, 09 Oct 2003 11:40:40 +0200
Message-ID: <bm3aep$p9e$1@osl016lin.hda.hydro.com>

Jan C. Vorbrüggen wrote:

>>You would want to send 80+ of these; so you want 80+ watts
>>sent into this 9 nanometer wide path.
>
>
> Shouldn't that be micrometer, instead?`

It is indeed micrometer, not nm.

I did my thesis 22 years ago working on the then-quite-new single-mode
optical fibers, some of which were manufactured at the university.

Afair the core diameters were in the 2-7 um range.

The wider core diameters would of course require a smaller optical
density step between the core and the cladding, so as to preserve the
single-mode optical path.

Terje

--
- <Terje.Mathisen@hda.hydro.com>
"almost all programming can be viewed as an exercise in caching"

From: mrr@via.reistad.priv.no (Morten Reistad)
Newsgroups: alt.folklore.computers,comp.arch,comp.sys.super
Subject: Re: Electrical power outlets
Date: Thu, 9 Oct 2003 16:26:21 +0200
Message-ID: <d6r3mb.5ci1.ln@via.reistad.priv.no>

In article <bm3aep$p9e$1@osl016lin.hda.hydro.com>,
Terje Mathisen  <terje.mathisen@hda.hydro.com> wrote:
>Jan C. Vorbrüggen wrote:
>
>>>You would want to send 80+ of these; so you want 80+ watts
>>>sent into this 9 nanometer wide path.
>>
>>
>> Shouldn't that be micrometer, instead?`
>
>It is indeed micrometer, not nm.
>
>I did my thesis 22 years ago working on the then-quite-new single-mode
>optical fibers, some of which were manufactured at the university.
>
>Afair the core diameters were in the 2-7 um range.
>
>The wider core diameters would of course require a smaller optical
>density step between the core and the cladding, so as to preserve the
>single-mode optical path.

You are correct indeed. All my documents insist on using Ångstrøm as a
unit; and I blinked when converting. It proves that I get rusty very
fast after not working with this for a few years.

This means the energy density is ~2E12 watts per m2, but this is still
very high. This large energy density makes it close to impossible
to avoid the electrical power feed. It also makes a very dangerous
laser weapon out of a broken fiber strand.

The light delay in such a cable is around 6E6 m / (3E8m/s x ~0.6) = 33ms.

Also, it is amusing that the glass that carries a signal across the
Atlantic is actually ~9E-6 m x ~9E-6 m x ~6E+6 m = 246E-6 m3 ~= 2.5E-4 m3,
in volume; and would have a mass of ~ 6E6 g/m3 x 2.5E-4 m3 = 15E2 = 1500 grams.

It would not work without the cladding though, and that destroys such
amusing trivia.

-- mrr

From: Terje Mathisen <terje.mathisen@hda.hydro.com>
Newsgroups: alt.folklore.computers,comp.arch
Subject: Re: Electrical power outlets
Date: Thu, 09 Oct 2003 21:43:07 +0200
Message-ID: <bm4doc$g06$1@osl016lin.hda.hydro.com>

Del Cecchi wrote:

> "Terje Mathisen" <terje.mathisen@hda.hydro.com> wrote in message
>>Afair the core diameters were in the 2-7 um range.
>>
>>The wider core diameters would of course require a smaller optical
>>density step between the core and the cladding, so as to preserve the
>>single-mode optical path.
>>
>
> The long distance high frequency stuff seems to be using longer wavelength
> lasers, which makes probably makes larger fibers work.  And larger fibers
> let you couple in more power.....

AFAIR, there are a couple of interesting points on the
frequency/wavelength scale for near-visible light passing through a
single-mode glass fiber:

You get a minimum dispersion point, where the speed as a function of
frequency is nearly constant, and you also have a minimum loss point.

Both of these occur in the 1300-1500 nm wavelength range, which is a key
reason for the immense bandwidth achievable on fiber.

OTOH, when I worked on this, semiconductor lasers working in this range
were still very much theoretical. :-(

Terje

--
- <Terje.Mathisen@hda.hydro.com>
"almost all programming can be viewed as an exercise in caching"

From: Morten Reistad <mrr@reistad.priv.no>
Newsgroups: alt.folklore.computers,comp.arch,comp.sys.super
Subject: Re: Electrical power outlets
Date: Fri, 10 Oct 2003 10:15:08 +0200
Message-ID: <cqp5mb.g0p1.ln@via.reistad.priv.no>

In article <iu1cov8nfr1b4l1l28qsq96lslc3lk8kae@4ax.com>,
David Powell  <ddotpowell@icuknet.co.uk> wrote:
>In article <d6r3mb.5ci1.ln@via.reistad.priv.no>,
> mrr@via.reistad.priv.no (Morten Reistad)  in alt.folklore.computers
>wrote:
>
> <snip>
>
>>This means the energy density is ~2E12 watts per m2, but this is still
>>very high. This large energy density makes it close to impossible
>>to avoid the electrical power feed.
>
>Earlier, you posted one conductor and sea / earth return. Does this
>conductor actually cost anything? Expect plastics sheath, steel wire
>armour, plastics oversheath over the fibre,  - I'd guess that you'd
>need some additional negative buoyancy and tensile strength, teeth
>breaking steel for gnawing sharks, and there's always those yachtsmen
>with dragging anchors. Might as well put some amps down the armour as
>well.

Long distance fiber optic cables are amazing devices. I was able
to observe the laying of the Eurorings from KPNQwest when they
overexpanded and went bankrupt; and also the TAT14. I was the
IP guy, and their "customer" so to speak.

Long distance sea cables are amazing devices.

The core fiber is built and extensively tested as a unit. All of it.
The current length limit to this is around 6500 km; so Pacific
cables must be spliced; but the US-Euro ones is actually one
ready-made cable as it is deployed.

The core has some supporting materials to shield the glass, and then
there is a very solid steel mantle. Then come the conductor, and
a protective shielding of kevlar or some other material like it.
This core just fits into an 11cm wide conduit. The tail ends
usually go underground for some kilometers in such conduit when
it is on land.

But in the sea the rest can vary. Along the beaches and in shallow
water it is embedded in concrete. Further down it goes in other
materials.

And it is laid out in an exact road, with a tolerance of less than
50 meters down there on the sea bottom. They even build bridges
down there for the cable. The bottom of the North Sea has several
of these where cables cross pipelines. Also, there are some of these
where there are big gorges along the sea bottom; especially near
the Atlantic Ridge.

These cables are laid with a pretty exact tension, and thay avoid
having in steeper inclines than 4 degrees. This means it must
be placed very gently on the bottom too.

Repeaters have to be included. They need power. All of this power
is DC; and there is only one connector. The loss of having the
return through water is much smaller than the loss would be
if there were two connectors in the same cable.

AC power would have even larger losses because of the sea
water that would participate in the magnetic side of the
circuit.

All of these cables take on distinct personalities. The pressure,
the repeaters, the long stretches all make us work pretty close
to the edge of what is possible.

If you can build a cable without repeaters you do so, even if that
means bringing it up on land. CANTAT-3 which goes Europe-UK-Scotland-
Various islands-Iceland-Greenland-Newfoundland-Canada-US Border
is a huge success just because it does NOT do it in one hop,
and has been resqued from failure a number of times because of
this.

In such cables components fail all the time, so the capacity
slowly degrades. In normal cases it takes 15 or so years before
it is unusable, but by then the new cables are so much faster
there is no point in running them any longer. Such cables
are normally turned over to academia as science devices.

They make _excellent_ devices for measuring some of the
more esotheric physics.

>>It also makes a very dangerous
>>laser weapon out of a broken fiber strand.
>
>If the shark gnaws through, it gets blinded. Probably won't do it
>again.

Such a beam can cut ANYTHING in two. But, it seals the
wound nicely.

-- mrr

From: Morten Reistad <mrr@reistad.priv.no>
Newsgroups: alt.folklore.computers,comp.arch,comp.sys.super
Subject: Re: Electrical power outlets
Date: Fri, 10 Oct 2003 22:09:32 +0200
Message-ID: <sl37mb.0rq1.ln@via.reistad.priv.no>

In article <bm71pg$34q$1@osl016lin.hda.hydro.com>,
Terje Mathisen  <terje.mathisen@hda.hydro.com> wrote:
>Del Cecchi wrote:
>> "Terje Mathisen" <terje.mathisen@hda.hydro.com> wrote in message
>> news:bm6ter$bv$1@osl016lin.hda.hydro.com...
>>
>>>Steve O'Hara-Smith wrote:
>>>
>>>
>>>>On Fri, 10 Oct 2003 10:15:08 +0200
>>>>Morten Reistad <mrr@reistad.priv.no> wrote:
>>>>
>>>>MR> Such a beam can cut ANYTHING in two. But, it seals the
>>>>MR> wound nicely.
>>>>
>>>>2E12 Watts per square meter ?? That's only about four suns (open
>>>>space or clear sky desert), fairly warm but nothing lethal.
>>>This has to be wrong by a few orders of magnitude:


I think the notation is confusing. With 2E12 i mean 2000000000000, not
2 to the twelfth power, or 4096.

>>>1 W will heat one gram of water 1 degree C in one second, right?
>>
>> Programmers.  Gotta love em.
>>
>> The unit you are thinking of is a calorie.  A watt second (Joule) is 4
>> calories.
>
>Oops, you're absolutly correct.
>
>However, that only adds another factor of about 4.1 or so.
>
>> The power density is 2*10**12, but the diameter is like 5 microns, or
>> 25*10**-12 square meters so the total power is 50 watts.  Not going to boil
>> many oceans with 50 watts.
>
>Surely not, however the argument was about power density, compared the
>the usual 1kW/square meter from Sol.

In direct contact with skin, flesh or even teeth it will do significant
damage. A few meters or so of water, or a few tens of meters with dense fog
is enough to dissipate it. It is invisible, and you don't feel the damage
before it is too late. A sheet of paper will stop it for a second or so
until the paper starts to burn; so you can use paper as a detector.
Special sunglasses are used, but they are only el-cheapo plastic ones
that have been measured.

Or, this was what we learned in the "safety with lasers" course.

-- mrr
   Do not look in laser with remaining eye.

From: Morten Reistad <mrr@reistad.priv.no>
Newsgroups: alt.folklore.computers,comp.arch,comp.sys.super
Subject: Re: Electrical power outlets
Date: Fri, 10 Oct 2003 22:15:41 +0200
Message-ID: <d147mb.h4u1.ln@via.reistad.priv.no>

In article <45022fc8.0310101051.1d476167@posting.google.com>,
Iain McClatchie <iain-3@truecircuits.com> wrote:
>Great posting.  Fascinating stuff.  But this sounds odd:
>
>MRR> It also makes a very dangerous
>MRR> laser weapon out of a broken fiber strand.
>
>David>If the shark gnaws through, it gets blinded. Probably won't do it
>David>again.
>
>MRR> Such a beam can cut ANYTHING in two. But, it seals the
>MRR> wound nicely.
>
>Hmm.  I understand that the light is dangerous because it comes from
>a very small point and is coherent, so that it can be focussed, even
>accidentally (say, by your eye), down to very small points.
>
>But when the light leaves the rough end of a broken fiber, surely you
>would *need* auxilliary optics to focus the 80 watts of light down to
>a dangerously small area?  Otherwise, wouldn't the light just spread
>out, so that the danger zone (for cutting) is just within a cm or two
>of the tip?

The problem is with the small area, only <1/100th of a mm, and the material;
glass; that is likely to have a clean, straight surface at such small
scales.

And the light is "single mode"; i.e. dimensionally trapped photons.
That light is even more coherent than lasers can do alone.
You don't need to focus it. It IS already a beam of a few light
wavelengths diameter; close enough to have to be described by a
common wave function if you want to describe it in QED.

The "single mode" is the big trick to build long distance fibers.

-- mrr

From: Terje Mathisen <terje.mathisen@hda.hydro.com>
Newsgroups: alt.folklore.computers,comp.arch,comp.sys.super
Subject: Re: Electrical power outlets
Date: Fri, 10 Oct 2003 23:12:07 +0200
Message-ID: <bm77b8$6l4$1@osl016lin.hda.hydro.com>

Morten Reistad wrote:

>>But when the light leaves the rough end of a broken fiber, surely you
>>would *need* auxilliary optics to focus the 80 watts of light down to
>>a dangerously small area?  Otherwise, wouldn't the light just spread
>>out, so that the danger zone (for cutting) is just within a cm or two
>>of the tip?
>
>
> The problem is with the small area, only <1/100th of a mm, and the material;
> glass; that is likely to have a clean, straight surface at such small
> scales.

The easiest way to get a (nearly) perfectly perpendicular cut across a
fiber like this is to simply scratch the cladding, and then bend the
fiber until it breaks.

> And the light is "single mode"; i.e. dimensionally trapped photons.
> That light is even more coherent than lasers can do alone.
> You don't need to focus it. It IS already a beam of a few light
> wavelengths diameter; close enough to have to be described by a
> common wave function if you want to describe it in QED.

This was a crucial part of my thesis work: The power distribution
function inside a single-mode fiber can be assumed to follow a 2D Gauss
curve, with the tail (skirt?) actually residing outside the core.

If anyone's interested I'll describe how you can use a short length of
fiber as a very responsive temperature sensor, as long as you have a
single-mode laser to feed it with.

> The "single mode" is the big trick to build long distance fibers.

Actually, you can have long distance with multi-mode, you simply cannot
have any bandwidth worth mentioning. :-(

Terje

--
- <Terje.Mathisen@hda.hydro.com>
"almost all programming can be viewed as an exercise in caching"

From: mrr@via.reistad.priv.no (Morten Reistad)
Newsgroups: alt.folklore.computers,comp.arch,comp.sys.super
Subject: Re: Electrical power outlets
Date: Sat, 11 Oct 2003 09:36:21 +0200
Message-ID: <ltb8mb.37d.ln@via.reistad.priv.no>

In article <3f87225c$0$34183$a1866201@newsreader.visi.com>,
Craig A. Finseth  <news@finseth.com> wrote:
>In article <d147mb.h4u1.ln@via.reistad.priv.no>,
>Morten Reistad  <mrr@reistad.priv.no> wrote:
>	...
>>And the light is "single mode"; i.e. dimensionally trapped photons.
>>That light is even more coherent than lasers can do alone.
>>You don't need to focus it. It IS already a beam of a few light
>>wavelengths diameter; close enough to have to be described by a
>>common wave function if you want to describe it in QED.
>>
>>The "single mode" is the big trick to build long distance fibers.
>
>Yes, but neither air nor water are terribly condusive to maintaining
>the single mode.  (Hence the need for fibre optics in the first
>place.)
>
>Does anyone know how long it takes a single mode coherent beam to lose
>coherency in air or water?  I would imaging it would be at distances
>well under a centimeter.

Coherency for signal purposes is lost at a few millimeters distance;
but this is due to the loss of the "single mode" conduit. The photons
will go multimode the instant they escape from the fiber.

But the beam will be dangerous for a lot longer. A meter or so in sea
water, a lot longer in pure fresh water. Several hundred in open, dry
air. Only a few tens of meters with humid air or fog.

Or at least this was what they told us on the "safety with
fibers" course.

-- mrr

Newsgroups: alt.folklore.computers,comp.arch,comp.sys.super
Subject: Re: Electrical power outlets
From: Morten Reistad <mrr@reistad.priv.no>
Message-ID: <8jb8mb.37d.ln@via.reistad.priv.no>
Date: Sat, 11 Oct 2003 08:15:03 GMT

In article <bm77b8$6l4$1@osl016lin.hda.hydro.com>,
Terje Mathisen  <terje.mathisen@hda.hydro.com> wrote:
>Morten Reistad wrote:
>
>>>But when the light leaves the rough end of a broken fiber, surely you
>>>would *need* auxilliary optics to focus the 80 watts of light down to
>>>a dangerously small area?  Otherwise, wouldn't the light just spread
>>>out, so that the danger zone (for cutting) is just within a cm or two
>>>of the tip?
>>
>>
>> The problem is with the small area, only <1/100th of a mm, and the material;
>> glass; that is likely to have a clean, straight surface at such small
>> scales.
>
>The easiest way to get a (nearly) perfectly perpendicular cut across a
>fiber like this is to simply scratch the cladding, and then bend the
>fiber until it breaks.

You can get a pretty decorative display out of such a fiber, without
cladding, that you bend just to the point before breaking, and then
you light it with some visible light, the "safe" limit of 30mW will
do.  It is so thin that you cannot see it at a distance of a few
meters, but the light is very much there.  Dont try this at home
etc. Danger of broken glass, etc.

>> And the light is "single mode"; i.e. dimensionally trapped photons.
>> That light is even more coherent than lasers can do alone.
>> You don't need to focus it. It IS already a beam of a few light
>> wavelengths diameter; close enough to have to be described by a
>> common wave function if you want to describe it in QED.
>
>This was a crucial part of my thesis work: The power distribution
>function inside a single-mode fiber can be assumed to follow a 2D Gauss
>curve, with the tail (skirt?) actually residing outside the core.

Did you describe it with a wave function or with classical physics?

>If anyone's interested I'll describe how you can use a short length of
>fiber as a very responsive temperature sensor, as long as you have a
>single-mode laser to feed it with.

It is a very good sensor for a lot of things. Making a set of
standing light waves at different frequencies is the trick for
tectonics and ocean sensoring. This is what old sea cables are
used for.

>> The "single mode" is the big trick to build long distance fibers.
>
>Actually, you can have long distance with multi-mode, you simply cannot
>have any bandwidth worth mentioning. :-(

Here we are on the Internet (well, usenet, actually). We must feed
this bandwidth monster.

-- mrr

From: Terje Mathisen <terje.mathisen@hda.hydro.com>
Newsgroups: alt.folklore.computers,comp.arch,comp.sys.super
Subject: Re: Electrical power outlets
Date: Sat, 11 Oct 2003 19:51:30 +0200
Message-ID: <bm9fv3$e6j$1@osl016lin.hda.hydro.com>

Craig A. Finseth wrote:
> Does anyone know how long it takes a single mode coherent beam to lose
> coherency in air or water?  I would imaging it would be at distances
> well under a centimeter.

If you want to send a single-mode wave from one fiber into another, over
an air gap, that gap is limited to a few um or so, i.e. similar to the
core diameter.

Beyond that distance the loss is starting to become significant.

Terje

--
- <Terje.Mathisen@hda.hydro.com>
"almost all programming can be viewed as an exercise in caching"

From: Terje Mathisen <terje.mathisen@hda.hydro.com>
Newsgroups: alt.folklore.computers,comp.arch,comp.sys.super
Subject: Re: Electrical power outlets
Date: Sat, 11 Oct 2003 20:13:52 +0200
Message-ID: <bm9h91$etk$1@osl016lin.hda.hydro.com>

Morten Reistad wrote:
> Terje Mathisen  <terje.mathisen@hda.hydro.com> wrote:
>>This was a crucial part of my thesis work: The power distribution
>>function inside a single-mode fiber can be assumed to follow a 2D Gauss
>>curve, with the tail (skirt?) actually residing outside the core.
>
> Did you describe it with a wave function or with classical physics?

Afair I (with a lot of help from my advisor) solved the wave function
directly, but I've forgotten how a long time ago. :-(

These days I'd use some numerical approximation.

>
>>If anyone's interested I'll describe how you can use a short length of
>>fiber as a very responsive temperature sensor, as long as you have a
>>single-mode laser to feed it with.
>
> It is a very good sensor for a lot of things. Making a set of
> standing light waves at different frequencies is the trick for
> tectonics and ocean sensoring. This is what old sea cables are
> used for.

Very nice!

When I built that single-mode fiber oscillator back in 1981, this had
never been done anywhere afaik. My advisor had calculated that it should
be possible, but his previous grad student had failed to make it work.

The guy before me tried to cut the fiber, polish the ends and then damp
aluminum mirrors onto them. This should have worked, but he either
didn't get the ends cut within 1 degree of perpendicular, or the mirror
surfaces didn't achieve the required 90% reflectance.

I did it by making (in the machine shop) a pair of external mirror
holders that were adjustable along the x, y and z axes, as well as phi
and theta angles.

Using a microscope I _very_ carefully adjusted these holders so that the
centers of the mirrors ended up about a um from the fiber ends, but
without ever touching since that would have scratched the mirrors.

About 70 days of building all the gear to make a total of six
measurement runs totalling less than 5 minutes. :-)

Now you're telling me that this has become SOP in the meantime. :-)

Terje

--
- <Terje.Mathisen@hda.hydro.com>
"almost all programming can be viewed as an exercise in caching"

Newsgroups: alt.folklore.computers,comp.arch,comp.sys.super
Subject: Re: Electrical power outlets
From: Morten Reistad <mrr@reistad.priv.no>
Message-ID: <dpn9mb.c0o1.ln@via.reistad.priv.no>
Date: Sat, 11 Oct 2003 22:05:01 +0200

In article <bm9h91$etk$1@osl016lin.hda.hydro.com>,
Terje Mathisen  <terje.mathisen@hda.hydro.com> wrote:
>Morten Reistad wrote:
>> Terje Mathisen  <terje.mathisen@hda.hydro.com> wrote:
>>>This was a crucial part of my thesis work: The power distribution
>>>function inside a single-mode fiber can be assumed to follow a 2D Gauss
>>>curve, with the tail (skirt?) actually residing outside the core.
>>
>> Did you describe it with a wave function or with classical physics?
>
>Afair I (with a lot of help from my advisor) solved the wave function
>directly, but I've forgotten how a long time ago. :-(
>
>These days I'd use some numerical approximation.

Numerical simulations lets you avoid to do solve the
_hard_ equations. :-)

>>>If anyone's interested I'll describe how you can use a short length of
>>>fiber as a very responsive temperature sensor, as long as you have a
>>>single-mode laser to feed it with.
>>
>> It is a very good sensor for a lot of things. Making a set of
>> standing light waves at different frequencies is the trick for
>> tectonics and ocean sensoring. This is what old sea cables are
>> used for.
>
>Very nice!
>
>When I built that single-mode fiber oscillator back in 1981, this had
>never been done anywhere afaik. My advisor had calculated that it should
>be possible, but his previous grad student had failed to make it work.
>
>The guy before me tried to cut the fiber, polish the ends and then damp
>aluminum mirrors onto them. This should have worked, but he either
>didn't get the ends cut within 1 degree of perpendicular, or the mirror
>surfaces didn't achieve the required 90% reflectance.

Doing this directly to the fiber has to be done just right; you
have to let the photons right back where they came from. If you are
to reflect, deflect or filter you either have to do it "single
dimension" style, or they will have to be "let out in 3d".
DWDM mux'es have the same problem multiplied.

>I did it by making (in the machine shop) a pair of external mirror
>holders that were adjustable along the x, y and z axes, as well as phi
>and theta angles.
>
>Using a microscope I _very_ carefully adjusted these holders so that the
>centers of the mirrors ended up about a um from the fiber ends, but
>without ever touching since that would have scratched the mirrors.

But still on the order of a wavelength; so the actual reflection
was not done while dimensionally trapped. A DWDM mux is a set of such
mirrors that each have resonance to a single color, so this will reflect,
while the rest pass through. A state of the art DWDM mux has
80 of these in less than 1/10th of a mm. The mirror block is made
layer by layer of atoms by substrate deposits. It is aligned to
fiber just as you describe.

Qwest was the first company to deploy this on a large scale.
This was what drove their price during the internet boom years.

>About 70 days of building all the gear to make a total of six
>measurement runs totalling less than 5 minutes. :-)

DWDM equipment is still installed and tuned for weeks.

>Now you're telling me that this has become SOP in the meantime. :-)

Your setup is almost exactly what is used for measuring
ocean and tectonics stuff.

Indeed. And almost all the viewers of this message will see it
after is has passed thorugh a double-digit number of such setups.

-- mrr

From: Terje Mathisen <terje.mathisen@hda.hydro.com>
Newsgroups: alt.folklore.computers,comp.arch,comp.sys.super
Subject: Re: Electrical power outlets
Date: Sun, 12 Oct 2003 10:25:33 +0200
Message-ID: <bmb35u$a9m$1@osl016lin.hda.hydro.com>

Tony Nelson wrote:

> In article <bm77b8$6l4$1@osl016lin.hda.hydro.com>,
>  Terje Mathisen <terje.mathisen@hda.hydro.com> wrote:
>  ...
>
>>If anyone's interested I'll describe how you can use a short length of
>>fiber as a very responsive temperature sensor, as long as you have a
>>single-mode laser to feed it with.
>
>
> I see mentioned in the 3 Oct. _Science_ p.19 something about doing this
> with 50 km fibers, using backscattered light.  1 C sensitivity at the
> front end, 13 C at the back, 15 m resolution.  From /Opt. Lett./ 28,
> 1651 (2003)

That's quite a bit more advanced than what I did, but it's been 22
years. :-)

Anyway, using a proper oscillator setup instead of backscattering gives
enormous temperature resolution. I believe the reason for using two
different colors at the same time (in the experiments described by mrr)
must be that by using the beat between the pair, you can _reduce_ the
sensitivity enough to make it work over much longer distances.

When you heat a fiber you change the optical wavelength two ways: By
making the fiber stretch, and by changing the optical density. It turns
out that the second effect is far stronger than the first.

With 2-3 m of fiber, 0.01 C corresponded to a full (half) wavelength
change in optical distance.

Anyway, this is probably off-topic for all the crossposted newsgroups,
so I (we) should probably stop.

Terje

--
- <Terje.Mathisen@hda.hydro.com>
"almost all programming can be viewed as an exercise in caching"

From: Terje Mathisen <terje.mathisen@hda.hydro.com>
Newsgroups: comp.arch
Subject: Re: Cray to commercialize Red Storm
Date: Sat, 01 Nov 2003 11:36:00 +0100
Message-ID: <bo02ah$k9m$1@osl016lin.hda.hydro.com>

Thomas Womack wrote:
> Nick Maclaren <nmm1@cus.cam.ac.uk> wrote:
>>Fibre is a LOT more civilised than copper.
>
> Ah.  My immediate associations with fibre are "fragile, breaks if you
> try to run it round too tight a corner" and "don't you have to polish
> the ends and line them up to sub-micron tolerance under a microscope
> to make connections"; are these in fact no longer problems?  Is it

These problems are indeed a thing of the past: What you're describing is
exactly how it was back in 1981 when I did my monomode fiber optical
resonator.

Two years ago at a local telecomms exhibition I was amazed to see a tiny
little field-grade fiber splice unit:

It did all the required cutting, cleaning, alignment adjustment and
fusing with no operator input required.

It did have a little screen so you could see a microscope view of the
process as it took place.

The only problem was that it's a little bit more expensive than a RJ45
crimp tool. :-(

OTOH, the fact that the process can be totally automated so (relatively)
easily, means that it is possible to manufacture and sell premade fiber
patch cables quite cheaply, just like the 2, 3, 5, 10 and 25m lengths of
Ethernet cable I see in all local electronics stores.

Terje

--
- <Terje.Mathisen@hda.hydro.com>
"almost all programming can be viewed as an exercise in caching"

From: Terje Mathisen <terje.mathisen@hda.hydro.com>
Newsgroups: comp.arch
Subject: Re: point-to-point busses
Date: Thu, 20 Jan 2005 16:19:40 +0100
Message-ID: <csoi6d$kau$1@osl016lin.hda.hydro.com>

Del Cecchi wrote:

> Terje Mathisen wrote:
>> A graded index fiber usually means a big (~50 um?) multimode fiber
>> employing a near parabolic refractory index profile to minimize
>> multi-mode dispersion.
>>
>> However, even doing this leaves you with an order of magnitude less
>> bandwidth than a conceptually much simpler (step-index) single-mode
>> fiber afaik. (At least it did 24+ years ago when I wrote my thesis
>> about this kind of stuff!)
>>
>> Terje
>
> You prompted me to go do some review.  You are indeed correct, graded
> index is used to reduce dispersion in multimode fibres.  And single mode
> fibers are step index.  Well, sort of.  Apparently all sorts of stuff is
> now being done with the cladding to reduce dispersion type effects in
> single mode fibre as well.  But indeed I was wrong to refer to graded
> index single mode fiber.  Thanks for the correction
>
> And something I learned a while back that really surprised me is that
> single mode fibre is significantly cheaper per meter than multimode
> fiber is.

Not really surprising, since (a) you need much less high-quality raw
material (optical fiber is 3-4 orders of magnitude clearer than the best
glass used in expensive optical lenses), (b) you can make do with a
relatively coarse step between the core and the cladding.
>
> And both of them have way less loss and ISI than a copper wire.  :-(

The _only_ real cost of fiber is at the ends.

Terje

--
- <Terje.Mathisen@hda.hydro.com>
"almost all programming can be viewed as an exercise in caching"

Date: Sun, 04 Feb 2007 10:51:40 +0100
From: Terje Mathisen <terje.mathisen@hda.hydro.com>
Newsgroups: comp.arch
Subject: Re: Electrical gigabit transmission ?
Message-ID: <kumg94-ka9.ln1@osl016lin.hda.hydro.com>

Paul Repacholi wrote:
> Terje Mathisen <terje.mathisen@hda.hydro.com> writes:
>
>> Del Cecchi wrote:
>>> Terje Mathisen wrote:
>>>> AFAIR, 25 years ago, the very best mono-mode fiber was getting
>>>> close to a theoretical 0.01 dB/km limit, which is slightly (!)
>>>> better over the long haul, and you could still have a bend radius
>>>> in the sub-meter range.
>>>>
>>>> Terje
>>> and amazingly single mode is cheaper than multimode
>> Well, it does require less of that expensive optically _very_ pure
>> glass to make it! :-)
>
> But more to the point, they make/made it by the million M so people
> could throw it off boats. ;)

Indeed, except (my guess) at least 95 to 99% of the cost of a
trans-oceanic cable is in the cladding/packaging, repeaters &
terminating gear.

I.e. the glass itself is relatively cheap.

Besides, more seriously, a monomode fiber is optically _much_ simpler
than most multi-mode fibers: This is because a mono fiber can be
manufactured as a simple step index construction, with a cylindrical
core of relatively dense glass and a cladding layer of sufficiently
lighter (faster) glass to sustain complete reflection on the boundary.

(Afair, the transmission profile for a mono-mode fiber can be
approximated by a Gauss distribution, with the tails reaching quite a
bit into the cladding.)

Multi-mode fiber otoh can be constructed with a parabolic density
distribution, since this makes it possible to maintain nearly constant
propagation speed for most reflection modes.

A step index multi-mode fiber has quite limited distance*bandwidth product.

Terje
--
- <Terje.Mathisen@hda.hydro.com>
"almost all programming can be viewed as an exercise in caching"

Date: Wed, 13 Jan 2010 09:27:18 +0100
From: Terje Mathisen <"terje.mathisen at tmsw.no">
Newsgroups: comp.arch
Subject: Re: Unexpectedly, a slightly larger FFT
Message-ID: <9oc027-0oo2.ln1@ntp.tmsw.no>

Morten Reistad wrote:
> In article<aneu17-l6m2.ln1@ntp.tmsw.no>,
> Terje Mathisen<"terje.mathisen at tmsw.no">  wrote:
>> Not at all: My MS thesis needed a single-mode laser to work at all!
>
> It is not alone. Anyone who use the Internet to write their
> thesis can say the same.

At a _somewhat_ greater remove: :-)

My thesis was to build a working ~3 m long fiber optic oscillator, back
in 1981.

My professor had done the math and knew that it should be doable, so he
had persuaded several former students to do projects and thesis work on
trying to make practice match theory, but no go.

They had used ~90% mirrors deposited directly onto the ends of the
fiber, it turned out that it is _very_ hard to get a single-mode fiber
to break exactly perpendicular to the core, and even a single degree of
offset at one end would be enough to destroy the feedback loop.

I used external mirrors instead, which "reduced" the problem to simply
make sure that the mirror surfaces were both less than a um from the
fiber ends, while perfectly (0.1 degree or so) aligned, without ever
touching since that would destroy the mirror coating.

It took me 70 days of building gear from the bottom up, then it suddenly
worked extremely well: Pure comb filter output when I tuned the optical
fiber path length by heating it, resonance peaks about 300% stronger
than the background signal.

As I said, I definitely needed pure single-mode signals to get this to work.

Terje

--
- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"