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RE: modal noise, PMD power penalty




Vipul,
To question 1:
This response may be just a clarification, not a disagreement. I believe
that strictly speaking, the two phrases are related but not the same. Modal
noise is caused by the time variance of the channel (which can be observed
as changes to the speckle pattern at the end of the fiber) passing thru a
mode-selective loss. Without the loss, there is no modal noise generated
even though there is time variance. 
Paul

	----------
	From:  Vipul Bhatt [SMTP:vipul.bhatt@xxxxxxxxxxx]
	Sent:  Thursday, November 02, 2000 12:58 PM
	To:  Equalization Ad-Hoc Reflector
	Subject:  modal noise, PMD power penalty
	Importance:  High


	For those who won't be able to attend the Sunday meeting, but feel
	qualified to answer the following two questions... please attempt a
	reply before Saturday evening. These two questions are unrelated to
	each other.

	1. I don't see any difference between the phrases "time variance of
a
	multimode fiber as a channel" and "modal noise". Do you agree? I am
	assuming that the effect of laser is implicitly accounted for. If
you
	agree, isn't the wheel already invented - don't we already know that
	modal noise is a slow variation phenomenon? [If there is no mode
	partition noise - and this is where the answer may be different for
	10G VCSEL and DFB lasers - refractive index changes due to
mechanical
	stress and local temperature variations will occur on a time scale
	representative of the strain mechanisms, i.e., on the order of
	milliseconds or longer. Paraphrased from Lasky, Osterberg and
	Stigliani, Optoelectronics for Data Communication. ISBN
	0-12-437160-4.]

	2. Is it true that PMD Power Penalty, if accounted for in the link
	power budget, will make it unnecessary to account for horizontal eye
	closure? The converse may not be true - compared to a zero-PMD link,
a
	link with PMD will see the receiver eye suffer from both vertical
and
	horizontal eye closure. But if more power is thrown at the link, the
	eye will open up both vertically and horizontally. Do you agree? [I
	don't want to recommend something to P802.3ae that falsely includes
a
	double penalty.]

	Thanks,
	Vipul

	vipul.bhatt@xxxxxxxxxxx
	(408)542-4113