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Author: allenmoos Two stars, 250 posts Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: of 2601  
Subject: Re: New DCM? Date: 1/9/2002 3:52 AM
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Hi corners,

Avanex does have two dispersion compensation technologies. The first is
the VIPA based product and the second is this "12 etalons" thing you read
about on the Gilder site.

You can make something called an all-pass filter using special etalons.
An etalon is simply 2 parallel mirrors spaced closely. In this case, one
mirror is 100% and the other is not. These are called Gires-Tournois
(G-T) etalons.

G-T cavities have a certain chromatic dispersion characteristic. It's
sort of a sine wave over wavelength. One cavity by itself is not
particularly helpful since the chromatic dispersion from optical fiber
is not periodic, but basically flat over wavelength. However, if you
cascade a whole bunch of different G-T cavities, the composite chromatic
dispersion can correct optical fiber dispersion.

The compensation by this cascaded set of G-T etalons will be periodic. It
will only work for some passband centered on the ITU grid. In the space
between channels, you'll get the wrong compensation. Fortunately, there
shouldn't be any signal in the space between channels.

Using this approach, you can not only correct fixed dispersion for all
channels, you can also correct dispersion slope over a set of channels.

This approach is mathematically identical to cascaded ring resonators in
planar lightwave circuits. Christi Madsen at Lucent (I think she's still
there) has published a number of papers and even a book on desiging
general filters using all-pass filters. Her empirical work has focused on
the ring resonators, but the theory works for G-T etalons as well.

-chris
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