FW: modal noise, PMD power penalty
-----Original Message-----
From: Gair Brown [mailto:gdbrown@xxxxxxxxxxxxx]
Sent: Friday, November 03, 2000 11:04 AM
To: Vipul Bhatt
Subject: Re: FW: modal noise, PMD power penalty
Vipul,
I suspect there will be some issue here. We have preliminarily
wrestled
with this in TIA 2.3 in developing the modal noise test procedure.
The
issue of filter frequencies arose. The test procedure initially
utilized a receiver with a DC capability. Most folks really used or
wanted to use AC coupled recievers for the test. At that time, it was
felt that the bulk of the modal noise was above 1 Mhz.
This can be understood as follows. Some of the forcing functions are
slow (temperature, vibration), but others are not (MPN). Both are
expected in VCSEL implementations. Moreover, even those low frequency
forcing functions can lead to much higher frequency observables. This
occurs because one cycle of the forcing function may cause many
multiple
cycles in the observable.
There is good news here though. I don't think that we should expect
to
see great variations in the shape of the impulse response with this
noise. We are talking about a few tenths of a dB modal noise or less
with typical VSCELS. This arises due to a change in the power
weighting
function across the fiber face. The effects of the change in power
weighting function should not be great in the shape of the impulse
response. This would correlate to changes in the weighting of the
DMD,
not drastic changes in the DMD.
Gair
Naval Surface Warfare Center
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