How to calculate fiber link budget: a simple guide for beginner

Initial Published: April 27, 2017

Introduction

The design of a fiber optic system is a balancing act. As with any system, you need to set performance criteria and 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. The fiber link budget is critical to a fiber optic system; it refers to the loss a fiber cable plant should have. This paper will explain how to determine the fiber link budget.

How do we test the fiber link budget?

There are many ways to tackle the problem of determining the link budget for a particular fiber optic link system. The easiest and most accurate way is to perform an Optical Time Domain Reflectometer (OTDR) trace of the fiber link. This will give you the actual loss values for all events (connectors, splices, and fiber loss) in the link. Without an actual OTDR trace, two alternatives can be used to estimate the link budget.

• Estimate the total link loss across an existing fiber optic link in the fiber length, and loss variables are known.
• Estimate the maximum fiber distance if the optical budget and loss variable are known.

How to calculate the fiber link budget?

A fiber optic system link budget is calculated based on a long list of elements. Following is a list of essential items used to determine general transmission system performance:

• Fiber Loss Factor – Fiber loss generally has the most significant impact on overall system performance. The fiber strand manufacturer provides a loss factor for dB per kilometer. A total fiber loss calculation is based on the distance x the loss factor. Distance, in this case, is 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 critical. Multimode fibers are used with LED transmitters, which generally don’t have enough power to travel more than 1km. Single mode fibers are used with LASER transmitters like DFB FP that come in various power outputs for “long reach” or “short reach” criteria.
• Transmitter – Two primary types of transmitters are used in fiber optic systems. LASER comes in three varieties: high, medium, and low (extended reach, medium reach, and short reach). Overall, the system design will determine which type is used. LED transmitters are used with multimode fibers. However, a “high power” LED 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 the required minimum level of received light, such as -27dB. A receiver is also referred to as a “detector”.
• Number and type of splices – There are two types of splices. Mechanical, which uses 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 at 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 essential factor. A system can’t be designed based on simply reaching a receiver with the minimum required light. The light power budget margin accounts for the aging of the fiber, aging of the transmitter and receiver components, the 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 dB to 10 dB.

The following table includes commonly accepted loss values in these calculations:

Fiber Type	Wavelength	Fiber attenuation / km	Connector Loss	Splice Loss
Multimode 50/150μm	850 nm 1310 nm	2.5 dB 0.8 dB	0.75 dB 0.75 dB	0.1 dB
Multimode 62.5/125μm	850 nm 1310 nm	3.0dB 0.7 dB	0.75 dB 0.75 dB	0.1 dB
Single Mode 9μm	1310 nm	0.35 dB	0.75 dB	0.1 dB
Single Mode 9μm	1550 nm	0.22 dB	0.75 dB	0.1 dB

Link Budget = [fiber length (km) × fiber attenuation per km] + [splice loss × # of splices]+[connector loss × # of connectors] + [safety margin]

For example, Assume a 10 km single mode fiber link at 1310nm with two connector pairs and two splices.
Link Budget= [10km × 0.35dB/km] + [0.1dB × 2] + [0.75dB × 2] + [3.0dB] = 8.2dB

In this example, an estimated 8.2dB of power would be required to transmit across this link. Of course, measuring and verifying the link loss values once the link is established is very important to identify any potential performance issues.

Estimate Fiber Distance

This calculation will estimate the maximum distance of a particular fiber optic link given the optical link budget and the number of connectors and splices contained in the link:

Fiber length = ([Optical budget] – [Link Budget]) / [fiber loss/km]
Fiber length = {[(min. TX PWR) – (RX sensitivity)]- [splice loss × # of splices]- [connector loss × # of connectors]- [safety margin]}÷ [fiber lost/km]

For example, assume a 1G SFP LX module (PN: OSP1250-3120DCR) link at 1310nm with two connector pairs and two splices:

Fiber length = {[(-9.0dB) - (-21.0dB)] - [0.1dB × 2] - [0.75dB × 2] - [3.0dB]} / [0.35dB/km] = 20.8km.

In this example, an estimated 20.8 km distance is possible before dissipating the optical power to a value below the Rx sensitivity. As always, measuring and verifying the actual link loss values once the link is established to identify potential performance issues is very important. Actual maximum distances will vary depending on the following:

• Actual optical fiber attenuation per km
• Optical fiber design and age
• Quality of connectors and actual loss per pair
• Quality of splices and actual loss per splice
• The number of splices and connectors in the link

How to reduce fiber optic link loss?

The above description shows that fiber optic link loss depends on many aspects. Some of them are difficult to change, such as absorption loss, atomic defect absorption loss, scattering loss, etc. 

However, further losses can be reduced by the following techniques to reduce the fiber optic link loss and give better link performance.

• Select high-quality optical fiber, and keep the end face of the optical fiber flat and clean.
• Ensure the technical level of construction personnel to prevent fiber optic cables from sheath abrasion and excessive bending, coiling, and stretching.
• Selection of high-quality optical transceivers because they can provide a more stable power budget and long lifetime
• Reduces the number of unnecessary fiber splices and fusion splice points while keeping connection losses at a shallow level. That helps to minimize the additional loss of fusion splices and splices at multiple points.

FAQs

What is fiber attenuation?

Attenuation is the value of signal strength or optical power loss when an optical pulse propagates through the optical fibers. This value is usually in dB or dB/km. The smaller the signal loss, the smaller the dB value.

Is there any fast method for fiber link loss measurement?

In general, if you have no OTDR. You can use an optical power meter and a light source to measure the fiber loss of a link. It connects a light source (LS) to one end of the fiber optic cable and an optical power meter (OPM) to the other. Then, disconnect the reference cable and connect the two ends directly to the cable to be measured. The difference between the two optical power values is the total loss of the line.

Reference

• http://www.thefoa.org/tech/lossbudg.htm
• http://ops.fhwa.dot.gov/publications/telecomm_handbook/chapter11.htm