Optical Power Budget and Power Margin for Fiber-Optic Links

The elements of an optical system including a CWDM, can be divided into 2 groups

  • elements which generate a signal in line
  • elements that reduce the signal power - they induce attenuation

The first group includes optical transceivers (SFP, XFP , etc.) and signal amplifiers. All other elements - connectors, multiplexers , OADM modules, splitters - causing loss of the signal power on the system.

It is evident that the optical system will operate only under the condition, that the optical power of the system is greater than the attenuation of all the elements therein. Accordingly, in order to assess the efficiency of the system , it is sufficient to calculate the difference between the optical power and the losses in the system.


The optical transceivers have two important parameters - the power of the laser (PT) and the receiver sensitivity (PR) . The difference between these values gives the optical power budget (PB) available in the system. In case of using optical amplifiers, the values of their transmission power levels must to be added.

In calculating the optical power margin (PM) of a system it should be considered the following sources of attenuation:

1. Signal attenuation in the optical fiber

The spread of the signal in the optical fiber is associated with a natural attenuation. The magnitude of this attenuation depends on many factors , but the calculations are typically taken into account only two of them - the line length and the wavelength of the signal. The average signal loss for single-mode fiber is about 0,25 dB / km at a wavelength of 1550 nm, and 0,35 dB / km for 1310 nm. As is known, CWDM systems are using wavelengths between 1270 and 1610 nm , but to calculate the fiber attenuation for these wavelengths it can be used the indicative values for the wavelengths 1310 nm and 1550 nm.Should to be noted that in CWDM systems , nowadays are available transceivers with different parameters and it is possible for waves with short lengths, transceivers with more powerful emitters or more sensitive receivers to be implemented and thus to achieve a balance of the system .

2. CWDM multiplexers

In the CWDM multiplexers, the losses are calculated for each carrier (wavelength ) and are measured between input / output port for a given wavelength and Com port. The average attenuation for each carrier is between 1.2 to 2,6 dB.

3. OADM modules

For the OADM modules, the losses are calculated for both drop / add carrier and for the line as a whole:

  • between Com and Express ports - average losses in the line are from 0.5 to 0,9 dB
  • between COM and Add ports - average losses for the carrier are from 0.5 to 0,9 dB
  • between Com and Drop ports - average losses for the carrier are from 0.3 to 0.9dB

It should be borne in mind, that in OADM modules for two-channel systems (point - point topology with branches ) there are 2 pairs of Com - Add and Com - Drop.

4. Connection points

For each Connector and Splice point - 0,5 dB approx.

5. Optical splitters

The splitter's attenuation calculation is necessary to be based on data in the technical specification of the specific splitter.

6. Power reservation

To ensure reliable operation of the optical system should take into account the possibility of increasing optical losses due to changing external factors and the deterioration of the characteristics of optical components caused by their aging. This is achieved by adding another 3 - 6 db to the other losses in the system.After calculating the losses in the line can be assessed the reliability of the system in the context of optical power margin (PM).

An optical system can be considered to be working when PM> 0

To calculate the power budget and power margin, you can use our calculator.

 

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