Re: Is there any heat dissipation along a fiber optic cable?

Mark, this is an insightful and interesting question, pulling together some related facts and trying to draw some 
conclusions.   Let me try to clear things up a bit:

- yes, infrared light is typically used to pass information through fiber optic cables.  Depending on the application,
the infrared light might be 850 nm in wavelength (just outside the red region of the spectrum), or may have a higher
wavelength, around 1300 nm or 1550 nm or so.

- yes, in some situations, infrared light is referred to as heat radiation.  However, infrared light - or microwaves or 
visible light - only provides heat if it is absorbed.  When you wear dark clothes, they will absorb all sorts of 
wavelengths of light and infrared, and the absorbed energy will be transformed into heat.

- fiber optic cable is virtually transparent at the wavelengths of infrared light that is used - that is how the
wavelength of laser or LED and the fiber optic material is chosen, to allow the infrared light carrying the information
to be transmitted over long distances. 

- the fiber optic cable is not perfectly transparent to the infrared light, so some is absorbed.   Typical total losses for 
an optical fiber will be about 2 dB per kilometer at 850 nm wavelength, <.5 dB / kilometer at 1300 nm wavelength,
and <.2 db/kilometer at 1550 nm wavelength (Handbook of Fiber Optic Data Communication, chapter 1, p 18, Academic 
Press 1998). These losses are converted to heat, but the heat dissipation is so small it might not be detectable.
To put it into perspective, the 2 dB per kilometer at 850 nm wavelength means that about a third of the power
has been dissipated over a kilometer distance.  Let's imagine that the input signal was 100 mW (milliWatts);
then, about 30 to 35 mW would be lost over a kilometer.   This means that, if you transmitted a continuous
signal for an hour through a kilometer of the fiber, the fiber would lose or generate heat with  about 
.03 Calories (kcal is the same as a Calorie in terms of Calories in food) of energy distributed over that 
1 kilometer of fiber.

- Some of the mechanisms of losses along the fiber are exactly what you described - geometrical nonuniformity
at core-cladding boundary, and imperfections at connection or splicing between fibers are referred to as "structural 
imperfections". Other mechanisms include absorption by impurities (transition metals in the fiber, molecular vibration
of OH bonds), or intrinsic loss mechanisms (tail of infrared absorption by Si-O coupling, tail of ultraviolet absorption
due to defects, Rayleigh scattering due to fluctuations in the refractive index of the fiber).

I hope this helps!