INTRODUCTION

 

As it relates to fiber optic communications, light has properties and behaviors, both of which properties determine the capabilities and limitations of fiber transmission.

 

 

Light can be described by at least four different concepts:

• Rays
• Particles
• Waves
• Energy fields

Light is described as a ‘ray’, since it travels in a straight line. Light is described as a particle, since light reflects in a manner similar to that in which a particle bounces. Light is described as a wave in that it exhibits a periodic nature like that of ocean waves.

 

 

Waves carry energy, as we all know from being in the water during a windy day. This energy exists within a volume. These concepts of light as a wave and as an energy field become important in understanding the behavior of light in singlemode fibers.

MAIN PROPERTIES OF LIGHT 

 

 

 

 

In next articles of our blog we wil examine eight properties of light. These properties are;

 

 

• Wavelength
• Spectral width
• Speed
• Power
• Volume
• Pulse width
• Polarization
• Phase

WAVELENGTH

 

When we think about light, we use the term ‘color¹. We translate ‘color’ into the technical term, which is ‘wavelength. A stone dropped in a pond creates ripples with a circular form. The distance from peak to peak, or from trough to trough, is the ‘wavelength’ of the ripples.

 

 

 

Similarly, light exhibits a periodic wave-like nature. The wavelength of light, X, is the measure, in nanometers (nm), of the distance between successive peaks, or between successive troughs.

 

 

 

The wavelength of determines the light behavior in a fiber. Specifically, the wavelength of light determines the two fiber behaviors of dispersion and attenuation. In addition, the wavelength determines the mode of transmission.

Common wavelengths range from 780 nm to 1625 nm (Table 1).

 

 

 

 

	Multi-	mode	Single-	mode
Data	850 780	1300	1310
CATV			1310	1550
Telephone			1310	1550
DWDM CWDM			1310-	1550 1625
WDM	850	1300	1310	1550
FTTH/PON			1310	1490 1550

 

Table 1: Common Communication Wavelengths

 

While some multimode Fiber Channel links operate at 780 nm, this wavelength is not common in other applications.

 

 

1625 nm is used as a testing wavelength and for out-of-band status monitoring.

 

 

As a practical matter, common transmission wavelengths range from 850 nm to above 1550 nm.

 

 

For example Fiber Optical Media Converters, for a distances up to 40 km, usually have 1310 nm lasers, and for 60 km and more – 1550 nm lasers installed.

 

 

Because wavelength determines behavior, the installer must know the wavelength at which the link will operate. The installer will test at this wavelength so that testing simulates, as closely as possible, the operation of the optoelectronics on the cable ends. In the telephone industry, wavelength ranges are referred to by the term ‘band’ (Table 2).

 

 

 

Band	Descriptions	Wavelength Ranqe, nm
O	Original	1260-1360
E	Extended	1360-1460
S	Short	1460-1530
C	Conventional	1530-1565
L	Long	1565-1625
U	Ultra long	1625-1675

Table 2: Telephone Wavelength Bands