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ALMA MEMO #463

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.



Last modified: 2003-07-15

alma-memos@nrao.edu