Calculating Optical Power Budgets

This Technical Tip is designed to assist you in calculating your optical power budget in networks requiring multi-mode fiber. The suggestions below can be used in Ethernet, Token Ring and FDDI installations. This is only a guideline, the numbers below can vary widely for different manufacturers and grades of cable. Consult the manufacturer's specifications for the exact specifications of your fiber cables.

Optical Power Budget

The difference between transmitter power and receiver sensitivity is defined as the optical power budget (or margin). Power loss in the link must not exceed this value.

Transmitter power is defined as the minimum peak optical power injected into the fiber by the optical transmitter

Receiver sensitivity is defined as the minimum received peak optical power that will produce a recovered electrical signal with timing distortion less than + 4 nanoseconds. Allowing for receiver sensitivity is extremely important in environments where single mode to multi mode converters are used.

Allocating the Optical Power Budget

The optical power budget available for each fiber optic link must be allocated into three categories:

Aging Margin
Fiber Attenuation
Connector Loss

Aging Margin

To protect against deterioration of fiber or component performance, 3 dB of the power budget for each link is reserved as an aging margin.

Fiber Attenuation

62.5/125 �m fiber typically has an attenuation of 3.75 dB/km or less.
50/125 �m fiber typically has an attenuation of 3.5 dB/km or less.

Connector Loss

ST connectors have a maximum loss of 1.0 dB for each fiber-to-fiber connection. This connector loss must be allowed for when calculating your optical power budget.

The example below shows a 62.5/125 �m fiber connection using two multi mode transceivers with ST connectors:

When calculating your optical power budget, the goal is to create a configuration where the launch calculation minus the receive calculation is the maximum db loss possible. This will create a network with strong signal integrity.

The picture above can be calculated as follows:

        Launch:          -12 db (+/- 2 db)
        Receive: minus   -29 db
                          -----
                 equals   17 db

Because of the +/- 2 db factor on Launch power a worse case scenario will make the power budget 15 db instead of 17 db.

The calculation of connector loss, aging timer and attenuation must be less than 15 db.

        Connector loss:   4 db (1 db per connector)
        Aging Timer loss: 3 db
        Attenuation loss: 4 db
                         -----
                         11 db total

This design should work, but the following must also be considered when configuring your fiber optic network.

Cable Length Limitations

To accurately determined your optical power budget the cable length is also a consideration. The length of a fiber optic cable connection is limited by the need for the receiver to recover the data signal accurately.

The maximum length permitted depends on transmitter power, receiver sensitivity, the amount of light captured by the fiber, the attenuation in the fiber link, and the connector loss.

The maximum length will also depend on distance limitations associated with Ethernet or Token Ring network diameters.

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