Digital Signal Transmission


Introduction: all electromagnetic waves in a vacuum travel at the same speed, equivalent to the universal constant c, with an approximate value of c = 2.988 x 108 m/s = 29.98 cm/ns (≈ 30 cm/ns)

In part 1 of this news article, we saw that the speed of electromagnetic waves from electrical signals is reduced to 80-90% when transmitted through a good metallic conductor. The same happens with light when moving through different media:

V_vacuum = c ≈ 2,998 x 108 m/s

V_air ≈ 2,997 x 108 m/s

V_water ≈ 2,250 x 108 m/s

V_glass ≈ 1,970 x 108 m/s

V_quartz ≈ 1,940 x 108 m/s

V_silicon ≈ 0.748 x 108 m/s

Optical fiber is a flexible, transparent thread made of glass (silicon oxide, commonly known as silica) or plastic with a diameter on the order of a human hair

The main difference between optical fiber and normal glass is their attenuation, which in normal glasses is in the range 100-1000 dB/km, while current technology for the manufacture of optical fiber allows to achieve attenuation values ​​as low as 0.16 dB/km.

For comparison, you could look through a kilometer-thick fiber optic window as clearly as you would through standard hair-thick glass.

The other great advantage of this technology is that the transmission of the signal is by means of laser that being a “coherent” light beam, it is carried in a single frequency, so it does not present a limiting phenomenon such as phase or amplitude distortion that occurs in the transmission of electrical pulses or in open space.

Therefore, its transmission frequency (or channel bandwidth) is much higher. With current technology, it is possible to generate pulses of laser light with a transmission period of up to a few femtoseconds (T=10-15 s, f = 1015 Hz) which is equivalent to 0.000001 nanoseconds, or the time it takes for light to travel 0.3 micrometers in vacuum.

Due to the thinness of its interior, a fiber optic is a waveguide (1) for optical frequencies. For lengths greater than 1000 meters, single-mode optical fiber (SM) is usually used and for shorter lengths multimode (MM).

In 2021 a joint NICT team, Nokia Bell Labs and Prysmian Group managed to achieve a record transmission of 1 Petabit/s = 1000 Terabits/s = 106 Gigabits/s = 1015 bits/sec through a single-core fifteen-modal 0.125mm- optical fiber.

(1) A waveguide is any physical structure that guides electromagnetic waves. In the waveguide, the electric and magnetic fields are confined in its interior so that the losses and interferences with other signals are minimal.

The number of frequencies and signal shapes that can propagate in a waveguide is limited to the propagation modes, which are the solutions of the wave equations derived from Maxwell’s equations, which satisfy the boundary conditions.

The transmission of signals in modes of propagation by waveguides is also used in PLC communications (Power Line Communications) and to feed microwave signals (a practice that is gradually being phased out).

References: Wikipedia  Signals, The Science of Telecommunications, John R. Pierce and A. Michael Noll, Editorial Reverté, S.A., 1995