Networks - Attenuation Profile Testing

Attenuation is the total loss of intensity between the input and output of an optical fibre link. The attenuation profile is the attenuation as a function of wavelength.

Typically, the attenuation profile of an optical fibre is measured by launching broadband light in an optical fibre with an optical spectrum analyser (OSA) on the receiving end of the fibre. The OSA then compares the spectrum of the output signal with the spectrum of the input signal, and calculates the difference in intensity at each wavelength to provide the attenuation profile.

Greater transmission can then be achieved by sending data via wavelengths with the least attenuation.

Applications

Many fibres provide low attenuation, and therefore greater transmission, in a specific wavelength range (“transmission band”). Typical transmission bands in optical networks include 850 nm, O-band, C-band, and L-band.

• 850 nm is typically used for short-reach applications (approximately <2 km) as this wavelength range exhibits relatively high attenuation in standard optical fibre.
• The O-band (Original Band) runs from 1260 - 1360 nm and  is used for applications requiring an intermediate reach of up to approximately 20 km. Wavelengths in this range cannot be amplified using EDFAs, and transmission distances are limited. The O-band is also used for low cost WDM networks such as coarse wavelength division multiplexing (CWDM) and passive optical networks (PON) in fibre to the home applications (FTTH).
• The E-Band (Extended Band) runs from 1360-1460nm. Signals in the E-band are not compatible with erbium doped fibre amplifiers (EDFA) and are highly attenuated. As a result, wavelengths in this band tend to be used for short reach applications such as CWDM and PON/FTTH.
• The S-Band (Short Wavelength Band) runs from 1460nm – 1530nm
• The C-band (Conventional Band) runs from 1530 - 1565 nm and  is employed for almost all extended-reach applications (up to approximately 120 km) as this wavelength range has particularly low attenuation in standard optical fibre and  is compatible with erbium-doped fibre amplifiers (EDFA)
• The L-band (Long Wavelength Band) runs from 1565 - 1625 nm and is used to improve the transmission capacity of an optical fibre link by increasing the number of available wavelength channels. Optical amplifiers are avvailable for the L-Band (Thulium doped and Raman amplifiers) and provide similar transmission distances as the C-Band such as long reach DWDM.
• The U/XL-Band (Ultra/Extra long Wavelength Band) runs from 1625nm to 1675nm.
• Wavelength Division Multiplexing (WDM) is an application whereby several signals, each comprising different wavelengths are transmitted simultaneously down a single optical fibre. It enables increased data transmission across a single fibre for a  relatively insignificant additional investment.
• Dense Wavelength Division Multiplexing provides additional capacity for long haul back bone optical networks. As many as 160 wavelengths are multiplexed onto a single optical fibre. The wavelength separation of the individual optical channels is very small (0.2nm or less), and the performance demands on all of the components are high. As a result, DWDM tends only to be deployed for high data rate, long haul applications and wavelength availability is limited to the lowest loss part of the fibre optical spectrum, and where optical amplifiers are most effective, i.e. the C & mL bands.  
• Coarse Wavelength Division Multiplexing is a lower cost version of WDM. CWDM systems generally are not amplified and have limited range. They use lower specification components and larger spacing between the wavelengths of each channel is necessary (typically 20nm). CWDM is available across all of O, E, S, C & L bands.