Re: Faraday isolators

[Bob Sciamanda <trebor@VELOCITY.NET>]

Let me try to get this discussion going:

The Faraday isolator is effectively a pair of crossed polarizers for the
"blocked" radiation.
The (outside) radiation which enters the isolator, but does not exit, is
absorbed inside the isolator and raises the temperature of the isolator (and
the cavity walls of which it is a part).  ( As does the radiation trapped
between any pair of crossed polarizers.)

I think the physics of any  polarizing mechanism forbids you, in principle,
from ignoring such absorption (or equivalent energy re-direction) effects.

I must add that I didn't fully understand your F/S/F scheme.

-Bob

Bob Sciamanda
Physics, Edinboro Univ of PA (ret)
trebor@velocity.net
http://www.velocity.net/~trebor

-----Original Message-----
From: Carl E. Mungan <cmungan@UWF.EDU>
To: PHYS-L@LISTS.NAU.EDU <PHYS-L@LISTS.NAU.EDU>
Date: Wednesday, December 30, 1998 7:06 PM
Subject: Faraday isolators


> It bothers me that Faraday isolators don't obey optical reversibility. The
> fact that looking through one I could see you but you not see me is
> disquieting. In particular, I don't quite see how to save Kirchhoff's law
> (absorptivity = emissivity) with these gizmos.
>
> The arrangement I have in mind is simple. A black sample is inserted into a
> cavity having perfectly reflective walls. Then I make a small window in the
> walls into which I place a Faraday isolator. Now what prevents the sample
> from radiating away energy continuously and cooling down to 0 K? The basic
> idea is so simple I'm sure some of you have thought of it before.
>
> To dispense with a couple of objections off the bat:
>
> (1) a Faraday isolator only works at one wavelength - okay, put an ideal
> filter in front of it (i.e., one which perfectly transmits a narrow
> passband and perfectly reflects everything else)
>
> (2) a Faraday isolator only transmits one linear polarization - okay, how
> about the following arrangement:
>  F
> S / F
>
> where S is the sample, / is an ideal 45-degree cube polarizing beamsplitter
> which perfectly transmits say the p-polarization and perfectly reflects the
> s-polarization, and F are two isolators consisting of a magnetic rotator
> and an output linear polarizer (which perfectly reflects the rejected
> polarization from the outside and perfectly transmits the other
> polarization from both sides) oriented at 45 degrees (to pass the rotated
> light in the usual way). Radiation from the outside which passes through
> either F gets sent out through the open bottom channel of the cube. Notice
> that I have studiously avoided absorption in all components so there is
> nothing to heat up the sample.
>
> Carl Mungan <cmungan@uwf.edu> (who only reads the digest)