Criteria & Calculation Factors

Design of a fiber optic system is a balancing act. As with any system, you need to set criteria for performance and then determine how to meet those criteria. It’s important to remember that we are talking about a system that is the sum of its parts.

Calculation of a system’s capability to perform is based upon a long list of elements. Following is a list of basic items used to determine general transmission system performance:

  • Fiber Loss Factor – Fiber loss generally has the greatest impact on overall system performance. The fiber strand manufacturer provides a loss factor in terms of dB per kilometer. A total fiber loss calculation is made based on the distance x the loss factor. Distance in this case the total length of the fiber cable, not just the map distance.
  • Type of fiber – Most single mode fibers have a loss factor of between 0.25 (@ 1550nm) and 0.35 (@ 1310nm) dB/km. Multimode fibers have a loss factor of about 2.5 (@ 850nm) and 0.8 (@ 1300nm) dB/km. The type of fiber used is very important. Multimode fibers are used with L.E.D. transmitters which generally don’t have enough power to travel more than 1km. Single mode fibers are used with LASER transmitters that come in various power outputs for “long reach” or “short reach” criteria.
  • Transmitter – There are two basic type of transmitters used in a fiber optic systems. LASER which come in three varieties: high, medium, and low (long reach, medium reach and short reach). Overall system design will determine which type is used. L.E.D. transmitters are used with multimode fibers, however, there is a “high power” L.E.D. which can be used with Single mode fiber. Transmitters are rated in terms of light output at the connector, such as -5dB. A transmitter is typically referred to as an “emitter”.
  • Receiver Sensitivity – The ability of a fiber optic receiver to see a light source. A receiving device needs a certain minimum amount of received light to function within specification. Receivers are rated in terms of required minimum level of received light such as -28dB. A receiver is also referred to as a “detector”.
  • Number and type of splices – There are two types of splices. Mechanical, which use a set of connectors on the ends of the fibers, and fusion, which is a physical direct mating of the fiber ends. Mechanical splice loss is generally calculated in a range of 0.7 to 1.5 dB per connector. Fusion splices are calculated at between 0.1 and 0.5 dB per splice. Because of their limited loss factor, fusion splices are preferred.
  • Margin – This is an important factor. A system can’t be designed based on simply reaching a receiver with the minimum amount of required light. The light power budget margin accounts for aging of the fiber, aging of the transmitter and receiver components, addition of devices along the cable path, incidental twisting and bending of the fiber cable, additional splices to repair cable breaks, etc. Most system designers will add a loss budget margin of 3 to 10 dB

Calculating a “Loss Budget”

Let’s take a look at typical scenario where a fiber optic transmission system would be used. Two operation centers are located about 8 miles apart based on map distance. Assume that the primary communication device at each center is a wide area network capable router with fiber optic communication link modules, and that the centers are connected by a fiber optic cable. The actual measured distance based on walking the route, is a total measured length (including slack coils) of 9 miles. There are no additional devices installed along the cable path. Future planning provides for the inclusion of a freeway management system communication link within 5 years.

Note: All distance measurements must be converted to kilometers. Fiber cable is normally shipped with a maximum reel length of 15,000 feet (or 4.5km). 9 miles is about 46,000 feet or 14.5km. Assume that this system will have at least 4 mid-span fusion splices.

Table-1: Fiber Loss Budget Calculation

Fiber Loss 14.5 km × 35dB =5.075
Fusion splice Loss 4 × 0.2dB =0.8
Terminating Connectors 2 × 1.0dB =2.0
Margin 5.0
Total Fiber Loss 12.875

Table-2: The manufacturer of the router offers three transmitter/receiver options for single mode fiber

Reach Transmit Power Receiver Sensitivity
Short -3dBm -18dBm
Intermediate 0dBm -18dBm
Long +3dBm -28dBm

Table-3: To determine the correct power option add the transmit power to the fiber loss calculation.

Reach Transmit Power Fiber Loss Loss Budget
Short -3 -12.875 -15.875
Intermediate 0 -12.875 -12.875
Long +3 -12.875 -9.875

Table-4: Compare this to the receiver sensitivity specification

Reach Receiver Sensitivity Loss Budget Difference
Short -18 -15.875 +3.0
Intermediate -18 -12.875 +6.0
Long -28 -9.875 +19.0

Because a loss margin of 5.0dB was included in the fiber loss calculation, the short reach option will provide sufficient capability for this system. In fact, the total margin is 8.0db because the difference between the loss budget and receiver sensitivity is 3.0db.