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Conference 7.286::fddi

Title:FDDI - The Next Generation
Moderator:NETCAD::STEFANI
Created:Thu Apr 27 1989
Last Modified:Thu Jun 05 1997
Last Successful Update:Fri Jun 06 1997
Number of topics:2259
Total number of notes:8590

964.0. "BACK REFLECTION for SMF, what do I order???" by KYOA::KOCH (It never hurts to ask...) Thu May 20 1993 14:06

    I have a question. I'm trying to get SMF patch cables. My vendor is
    asking me what BACK REFLECTION characteristics I want. He has 35 - 45
    db for me. However, I don't know what to tell him.
    
    How do I determine what cable to order?
T.RTitleUserPersonal
Name		DateLines
964.120 dB is a minimum NPSS::KIRKWed Nov 08 1995 09:1131
    This question seems to have come up again. At least I received a call
    on this issue from a field source. 
    
    Back reflections in SMF systems need to be minimized, because optical
    power reflected back to the laser can cause deterioration of the normal
    operation of the laser.  The FDDI PMD document uses the term Optical
    Return Loss (ORL), and defines it as:
    
    	"the ratio (expressed in units of dB) of optical power reflected by
    	a component or an assembly to the optical power incident on a
    	component port when that component or assembly is introduced into
    	a link or system."
    
    Section 7.3.1 of the SMF FDDI PMD states:
    
    	"the optical return loss of SMF-PMD optical connectors, splices, 
    	bypass switches, and attenuators shall be 20 dB or greater when
    	tested according to FOTP-107"
    
    
    My experience is than 20 dB is pretty easy for fiber component vendors
    to achieve.  If a site has any plans to utilize the SMF cable plant for
    higher data rate systems in the future, it would be wise to obtain
    components with ORLs of 30 dB or better.  Higher speed lasers systems
    have need for better ORL components.
    
    
    Dick Kirk
    Network Product Support
    (Fiber Optics a Specialty)
    DTN 226-7048
964.2Higher is better?CHOWDA::FAHEYAre we having 'FUN' yet?Mon Nov 13 1995 08:405
    I had not heard of this issue before. Just so I understand... The
    higher number the better.(?) (ie 30db is better than 20db)
    
    Thanks
    Jim
964.3Bigger is better, and why.NPSS::KIRKMon Nov 13 1995 09:0227
    Right - the higher magnitude number is better. 30 is better than 20.
    
    The reason behind this is that the calculation is done with logarithms.
    
    		ORL = -10*Log(Pr/Pi)      from FOTP-107
    
    Since the reflected power, Pr is always less than the incident power, Pi, 
    then the ratio of Pr/Pi is always between 0.0 and  1.0. If the
    reflected power is one tenth of the incident power the the ORL is:
    
    		ORL = -10*Log(0.10)
    		ORL = -10*(-1.0)
    		ORL = 10 dB
    
    For a reflected power that is one twentieth of the incident power:
    
    		ORL = -10*Log(0.05)
    		ORL = -10*(-1.30)
    		ORL = 13 dB 
    
    
    As a further point of information, ORLs of 20 and 30 db correspond to
    reflected power ratios of one-hundredth (1 percent), and one-thousandth
    (0.1 percent) of the incident powers, respectively.
    
    Dick Kirk
    Network Product Support