I gather a light source somewhere on land gets the 1's and 0's on
their way through the cable.  That's clear enough.  But such signals
weaken with distance, I read, and require amplification every so
often. I found this bit at some site: "The reactive substance in a
conventional pump laser is gallium indium arsenide, which produces a
longer, 980-nanometer infrared beam. Used in an erbium-doped
amplifier, a pump laser fires its beam down a 100-meter optical fiber
that terminates in an impregnated segment. The beam excites the erbium
atoms, which then add their energy to optical data signals passing
through the fiber and boost the signals' power enough to carry them 50
miles or more before needing another boost." But my question is "HOW
do erbium-doped amplifiers or a "pump laser" get THEIR power - in the
middle of the ocean?"  So many sites are written explaining fiber
optics for people who know the answer to my question already.
already...but that ain't me!!!  I'm after the real final, bottom line,
nitty gritty, precise answer; the kind of answer a knowledgeable
scientist would give if you could ask in person.  Perhaps such is not
available on the net though.

---

Clarification of Question by wumply-ga on 12 May 2005 20:24 PDT

Re the first response (kottekoe-ga)to my initial question.
                                                                      
        This is more of a follow-up question than a clarification. 
But it just took me a week or so to realize I didn't fully understand
part of the first answer I got.

Can koettkoe-ga or someone explain in much more detail just how "the
current passing through Zener diodes regulates the voltage supply to
to power the lasers."  I'd be most appreciative.  I had the idea
(incorrect?) that a voltage going through a diode would be decreased. 
But the repeater's purpose is to amplify the voltage (increase the
light's energy, is it not?  How does it work?

Excellent question. The pump lasers are powered by electricty. A
current is passed through a conductor inside the cable by applying a
positive voltage on one continent and a negative voltage on the other.
In each repeater, the current passes through Zener diodes that are
used to regulate the voltage supply to power the lasers. Since the
voltage drop across all of the repeaters are in series electrically,
the terminal voltages on land can be quite large, in the thousands of
volts.

They can also use EDFAs or Ebrium Doped Fiber Amplifiers - like a
laser cavity, a "short" 50-300m length of fiber is doped with the rare
earth element ebrium.  It is then excited by a laser diode at 980nm,
this excited the electrons to higher states.  when a signal photon
passes through the cavity, the ebrium discharges a photon with the
same pahse modulation and frequency. These releases are usually int he
1500-1600nm bands, which is why all (ok most) transoceanic cables use
these frequncies.

These ADFAs or repeaters are only really needed for spans of 100km + though.
Ryan

Ryan,

Yes, the question was about how EDFA pump lasers get their energy,
which is what I tried to answer.

The reason the signal is near 1550 nm is not because that is where
EDFA's work, it is the other way around. Erbium is used so that the
fiber laser operates near 1550 because the attenuation of silica fiber
is minimized at that wavelength. The development of these fiber lasers
was a technological breakthrough in the late 1990's that enabled the
first submarine cables that used optical amplifiers instead of optical
detection, electrical amplification, and regeneration of the optical
signal.

JWTK

Oops! I meant to say the late 1980s, not 1990s.

Comment for kottekoe-ga (or anyone)
 
Hope you get to see my comment here.  It's OK for me to ask if a
'commenter' could provide further information and for him to
respond...I checked that out with Google, which it why it took me so
long to add this comment.  (I just can't ask for your address.) 
First, let me say thank you for your comment/answer of April 26.  It
was very satisfying to learn the answer to my question.

I wonder if you could answer a follow-up question which is: I've
always thought of a source of voltage as most frequently a physical
unit...a battery, an alternator or generator...with a - and +
component.  But your answer talked of a + voltage on 1 continent; a -
on the other. When one clearly does not have a physical unit on 1
shore only, how is the + voltage on one shore obtained and the - on
the other?

Another good question. Only voltage differences have any physical
meaning. In this case, I am using the local ground (or ocean) as the
reference electrode. Thus, when I said positive on one continent, I
meant that the cable conductor has a voltage that is positive with
respect to the ground (or ocean) as it reaches that continent. In a
normal electrical circuit, you would have two conductors, with equal
and opposite currents flowing through the two conductors and connected
to a power supply on one side. The extra conductor is unnecessary
because the high conductivity of the earth/ocean allows the use of the
earth as the return path. Thus, one builds a power supply on each
continent and connects one terminal to the cable and the other to the
ocean or to ground. The voltage drop across the ocean is non-zero and
not constant and, in fact, the first fiber optic transatlantic cable,
TAT-8, was used to measure the potential difference between Europe and
North America during the geomagnetic storms of 1989*. Voltage
fluctuations of 700 V were observed. This doesn't create a problem,
because the power supplies can be automatically adjusted to maintain
the desired current.

* See: http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1989GeoRL..16.1145M&db_key=AST

Thank you, Kottekoe-ga, for both your answers.  You certainly explain
things well and I am the beneficiary.  Methinks you are or could be a
teacher!  Or maybe you are an engineer.