Re: fiber laser amplifiers

[John Denker <jsd@MONMOUTH.COM>]

At 09:42 AM 1/29/00 -0500, Ludwik Kowalski wrote:

> Can somebody describe these optical amplifiers (doped
> fibers) for us at the level appropriate for an introductory
> physics course?

Quick background:  Recall LASER stands for Light Amplification by
Stimulated Emission of Radiation.  Note the word "amplification".

There are two broad classes of objects that use the the laser
principle:  amplifiers and oscillators.  If you grab a typical so-called
laser off the shelf you are getting a laser oscillator, not an amplifier.

To underline the distinction, I suppose a laser oscillator ought to be
called Light Oscillation by Stimulated Emission of Radiation -- but for
some reason the corresponding acronym has never caught on.

A laser oscillator is made by sticking a laser amplifier inside a resonant
cavity.

The basic three-level laser amplifier operates on a principle that is easy
enough to explain:

Step 1:  You choose a system having a ground state A, an excited state, B,
and an even high excited state C.  You also need to have a reasonably fast
one-photon transition from A up to C, and a nice fast spontaneous
transition from C down to B, but a very much slower transition from B to
A.  If you pump the A/C transition, you can create a big-enough population
in the C level.  Some of these decay down to the B level, where they
accumulate.

In order to have gain, you need to have a population inversion, that is,
more population in the excited state B than in the ground state A.

Step 2:  A photon from outside the system waltzes in and causes (with some
probability) stimulated emission from B to A.  You get (with some
probability) two photons out for the one that went in.

That's all there is to it, at this level of detail.

> I remember reading about them about two
> years ago.  Are they already in use?

Yes.