From: Tomalin, IR (Ian) 
Sent: 20 December 2002 18:00
To: Coughlan, JA (John) ; Pearson, MR (Matthew) ; Williams, JH (Joolz) ;
Halsall, RNJ (Rob) ; Salisbury, J (James) 
Cc: Tomalin, IR (Ian) ; Costas Foudas (E-mail); Bell, KW (Ken) ; Karl
Gill (E-mail)
Subject: Optical fibre safety

Dear all,
  After rereading Karl Gill's CMS opto-link safety stuff http://cms-tk-opto.web.cern.ch/cms-tk-opto/tk/QA/lasersafety.html , and a subsequent exchange of Emails with him (see below), I conclude that:

  The single-way fibres (both those carrying the TTC information and those carrying the analogue data) produce at most 3.25 mW each. This should be compared with the rules for a class 1 hazard fibre-optic system. For single fibres and 12-way CMS Tracker ribbons, Karl shows that the maximum power limits for Class 1 are 15.6 mW/fibre and 11.7 mW/fibre respectively. (See p42 of his talk). (N.B. The ribbon is not 12 times worse than the fibre, because it is an extended source). In consequence, both fibres and ribbons are Class 1. (N.B. With the old laser classification scheme, the ribbons would have been rated higher, so we are fortunate that the new classification scheme is now in place). The only draw-back of the new scheme is that one is not supposed to stare at the laser for more than 10 seconds, whereas with the old scheme it used to be 100 seconds. However, as we're a factor over 3 within the safety limit, I guess one can stare for slightly longer than this, although Karl recommends that we don't. 

  I therefore believe that the only safety requirements are use of laser safety trained staff, and some stickers on the equipment saying "Class 1 Laser Product - Invisible Laser Radiation". In principle, since the hazard is not Class 1M, we can even look at the fibres through a magnifying glass (but not a microscope), though it's presumably best not to do so.

                              Cheers,
                                Ian
=================================================================
Hi Ian,

I'm in a rush to get home to strat my Christmas holiday, so I'll try and give you a quick but clear answer - (starting on slide 22 of the
presentation) 

The 8.6mW/fibre you mentioned I think is only the Accessible Emission Limit for Class 3A under the "total power condition", not the power emitted from the fibre from our laser. 
In any case, for CLass 3A the more stringent condition is the 5.0mW/fibre "irradiance condition". 

Now since there is at most 3.2mW/fibre then under the old classification, then the 12-way ribbon connector was 3A under the old classification.  

However, now things have changed a lot, and there is a big relaxation (correction actually) to the minimum angular subtence limit at this wavelength, which increases the relaxation for a linear shaped source of laser light such as a ribbon connector. PLus the exposure time was limited to a more realistic 10s, instead of 100s.

page 42 - the new AEL that I calculated for the ribbon is 11.7mW/fibre for Class 1, therefore the ribbons are in this class. Strictly speaking they are 'hazard level 1', since they are not lasers and the laser source is can be far remote from the connector.

Hope that's clear - wishing you a Happy Christmas and New Year,

Karl.