This part of IEC 60793 defines test methods for both the phase beat length, and the group beat length. These two parameters are defined differently, and will give different results depending on the type of polarization-maintaining (PM) fibre. The phase beat length is the relevant parameter for the fibres ability to maintain a high extinction ratio. This is described in more details in Annexes A and B.dition 1.0 2017-02 INTERNATIONAL STANDARD colour inside Optical fibres – Part 1-60: Measurement methods and test procedures – Beat length IEC 60793-1-60:2017-02(en) THIS PUBLICATION IS COPYRIGHT PROTECTED Copyright ？ 2017 IEC, Geneva, Switzerland All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means, electronic or me