[asac][almanews] ALMA Memo 463 Released

[Stacy Oliver]

Variation on the ALMA link design.
No Switch, maximum 18km link lengths and the use of an APD.

Roshene McCool
The University of Manchester

2003-07-07

Keywords: Fiber Optic, Data Transfer, Optical Switch, Photodiode

The link design of the ALMA data transfer system currently incorporates
an erbium doped fibre amplifier, an optical cross bar switch and will 
operate over a system specified link length of 25km. Over the course of
the ALMA project the optical cross bar switch has been removed from the
baseline plan and added to the contingency pool. In addition ongoing work
on the Chajnantor site indicates that the maximum fibre link length will
be 18km. This document illustrates that the inclusion, or not, of an optical
switch and the maximum link length specification are fundamental to the 
data transfer design. It will discuss how the removal of the cross bar
switch and reduced link length will eliminate the erbium doped fibre amplifier
from the link design. We show that a variant design using an avalanche 
photodiode, without an erbium doped fibre amplifier, will operate at the
required performance levels. However, whilst the design includes a 6dB 
start of life margin, there is no additional margin over and above this level.
The design is successful, only within strict limits of attenuation outlined
in the power budget. Until the fibre optic cable is installed from the
antenna pads to the AOS, a degree of uncertainty exists in the attenuation
figures of the power budget. Additional attenuation cannot be accommodated 
in this design, without the use of additional amplification, or a sacrifice
of system margin. The existing design, incorporating an erbium doped fibre
amplifier has additional margin over and above the 6dB start of life margin
and will work with the inclusion of an optical switch and longer link lengths.

View a pdf version of ALMA Memo #463.
http://www.alma.nrao.edu/memos/html-memos/alma463/memo463.pdf
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