From: schlegel@cwis.unomaha.edu (Mark Schlegel)
Subject: Re: Help on Fiber-optics
Organization: University of Nebraska at Omaha
Date: Tue, 15 Sep 1992 01:25:44 GMT

lth@nevada.edu  writes:


>Hello Everyone,

>I am looking for information and/or references on how fiber-optic network
>works.  If you do have any source(s), please reply to my account.  Thanks
>you very much in advance.
>						Lam.
>					

Lam,

I read some of the other answers to your post and here are some new points,
the previous followups only discussed multi-mode fibers, these are no longer
the best in use, there are generally three fiber types (increasing in 
sophistication and capacity as you go down the list).

1) multi-mode---- the width of the fiber is much larger than the wavelength
                transmitted so that many modes (angles of reflection) are 
                allowed.  The shortest path down the fiber is straight down
                the axis and the largest are the ones with large angle of 
                incidence to the cladding of the fiber, these pathes are 
                susceptible to attenuation due to transmittion through the
                cladding.
                
                Advantages-- cheap, can carry large amounts of light (useful
                  for illuminating hard to reach places), are very tough due to
                  thickness.
                Disadvantages-- the many allowed light paths results in 
                        time dispersion of digital pulses as they travel down
                        the fiber, this is due to different paths having 
                        different path lengths.  The photons travelling 
                        down the fiber axis arrive first and then the other
                        more heavily reflected photons arrive later.  This 
                        gets worse proportional to the length of the fiber and
                        limits the maximum data rate down the fiber because the
                        pulses begin to widen and mix together until they can't
                        be distinguished.
                        
Graded-Index ---The above problems with dispersion due to different path 
                lengths in the multi-mode fiber is reduced in the graded 
                fiber by making the fiber with a gradual decrease in 
                refractive index as you move outward from the axis.  This
                makes the light paths bend gradually and in the outer regions
                of the fiber the light moves faster (light velocity v in a 
                refractive material with index n is  v = c/n where c=speed of
                light in a vacuum).  This results in a much reduced time 
                difference for different paths since the photons that travel
                farther also travel *faster*.  Basically there is no sudden 
                boundary between the cladding and the fiber.
                
                Advantage-- less time dispersion than the multi-mode fibers
                 
                Disadvantages-- thinner and more difficult to manufacture and
                        splice together, still has dispersion and probably leaks
                        more light out of the cladding (anyone out there know
                        about if this is so?)
                        
Single Mode---  These are the best, the fiber is so thin (on the order of the
                wavelength it carries) that the only allowed mode is straight 
                down the fiber.  No reflection in the classical sense occurs off
                the jacket and for pure, monochromatic light there is no 
                dispersion.  In real life there still is a dispersion due to
                the laser not being totally monochromatic but single mode fibers
                are designed to put the local minimum of the dispersion curve
                versus wavelength right at the wavelength of the laser.  These
                fibers usually use light in the near infrared because of 
                superior transparency at these wavelengths (a few microns, I
                think).  There is even research being conducted on solid state
                optical amplifiers based on erbium that will be incorporated 
                right into the fiber with no optical to electronic to optical
                stage, all amplification will be done in the optical regime to
                give superior reliability and speed.  The erbium doped region
                of the fiber will be pumped with light in the visible range 
                that is sent down the fiber with the infrared light carrying the
                data.  The erbium will coherently emit with the passing IR 
                signal amplifying it.  Very clever data compression techniques
                and the simultaneous use of many phases and polarizations to 
                give many channels at a time will allow data rates in the
                thousand Gigabit per second range (this is shaky, I'm estimating
                but you can believe it'll be HUGE) eventually.
                
Mark S.
Univ. of Nebr. at Omaha
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