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Fiber Optic Cable Plant Link Loss Budget Analysis

July 7, 2010 Leave a comment

Loss budget analysis is the calculation and verification of a fiber optic system’s operating characteristics. This encompasses items such as routing, electronics, wavelengths, fiber type, and circuit length. Attenuation and bandwidth are the key parameters for budget loss analysis.

Analyze Fiber Optic Link Loss In The Design Stage
Prior to designing or installing a fiber optic system, a loss budget analysis is reccommended to make certain the system will work over the proposed link. Both the passive and active components of the circuit have to be included in the budget loss calculation. Passive loss is made up of fiber loss, connector loss, and splice loss. Don’t forget any couplers or splitters in the link. Active components are system gain, wavelength, transmitter power, receiver sensitivity, and dynamic range. Prior to system turn up, test the circuit with a source and FO power meter to ensure that it is within the loss budget.

The idea of a loss budget is to insure the network equipment will work over the installed fiber optic link. It is normal to be conservative over the specifications! Don’t use the best possible specs for fiber attenuation or connector loss – give yourself some margin!

The best way to illustrate calculating a loss budget is to show how it’s done for a 2 km multimode link with 5 connections (2 connectors at each end and 3 connections at patch panels in the link) and one splice in the middle. See the drawings below of the link layout and the instantaneous power in the link at any point along it’s length, scaled exactly to the link drawing above it.

fiber  optic cable

Fiber Optic Cable Plant Passive Component Loss

Step 1. Fiber loss at the operating wavelength

Cable Length 2.0 2.0
Fiber Type Multimode Singlemode
Wavelength (nm) 850 1300 1300 1550
Fiber Atten. dB/km 3 [3.5] 1 [1.5] 0.4 [1/0.5] 0.3 [1/0.5]
Total Fiber Loss 6.0 [7.0] 2.0 [3.0]

(All specs in brackets are maximum values per EIA/TIA 568 standard. For singlemode fiber, a higher loss is allowed for premises applications. )

Step 2. Connector Loss
Multimode connectors will have losses of 0.2-0.5 dB typically. Singlemode connectors, which are factory made and fusion spliced on will have losses of 0.1-0.2 dB. Field terminated singlemode connectors may have losses as high as 0.5-1.0 dB. Let’s calculate it at both typical and worst case values.

Connector Loss 0.3 dB (typical adhesive/polish conn) 0.75 dB (TIA-568 max acceptable)
Total # of Connectors 5 5
Total Connector Loss 1.5 dB 3.75 dB

(All connectors are allowed 0.75 max per EIA/TIA 568 standard)

Step 3. Splice Loss
Multimode splices are usually made with mechanical splices, although some fusion splicing is used. The larger core and multiple layers make fusion splicing abut the same loss as mechanical splicing, but fusion is more reliable in adverse environments. Figure 0.1-0.5 dB for multimode splices, 0.3 being a good average for an experienced installer. Fusion splicing of singlemode fiber will typically have less than 0.05 dB (that’s right, less than a tenth of a dB!)

Typical Splice Loss 0.3 dB
Total # splices 1
Total Splice Loss 0.3 dB

(All splices are allowed 0.3 max per EIA/TIA 568 standard)

Step 4. Total Passive System Attenuation
Add the fiber loss, connector and splice losses to get the link loss.

Best Case TIA 568 Max
850 nm 1300 nm 850 nm 1300 nm
Total Fiber Loss (dB) 6.0 2.0 7.0 3.0
Total Connector Loss (dB) 1.5 1.5 3.75 3.75
Total Splice Loss (dB) 0.3 0.3 0.3 0.3
Other (dB) 0 0 0 0
Total Link Loss (dB) 7.8 3.8 11.05 7.05

Remember these should be the criteria for testing. Allow +/- 0.2 -0.5 dB for measurement uncertainty and that becomes your pass/fail criterion.

Equipment Link Loss Budget Calculation: Link loss budget for network hardware depends on the dynamic range, the difference between the sensitivity of the receiver and the output of the source into the fiber. You need some margin for system degradation over time or environment, so subtract that margin (as much as 3dB) to get the loss budget for the link.

Step 5. Data From Manufacturer’s Specification for Active Components (Typical 100 Mb/s link)

Operating Wavelength (nm) 1300
Fiber Type MM
Receiver Sens. (dBm@ required BER) -31
Average Transmitter Output (dBm) -16
Dynamic Range (dB) 15
Recommended Excess Margin (dB) 3

Step 6. Loss Margin Calculation

Dynamic Range (dB) (above) 15 15
Cable Plant Link Loss (dB) 3.8 (Typ) 7.05 (TIA)
Link Loss Margin (dB) 11.2 7.95

As a general rule, the Link Loss Margin should be greater than approximately 3 dB to allow for link degradation over time. LEDs in the transmitter may age and lose power, connectors or splices may degrade or connectors may get dirty if opened for rerouting or testing. If cables are accidentally cut, excess margin will be needed to accommodate splices for restoration.

NOTE: Many techs forget when doing a loss budget that the connectors on the end of the cable plant must be included in the loss budget. When the cable plant is tested, the reference cables will mate with those connectors and their loss will be included in the measurements.

Types of Fiber Optic Cables – Singlemode, Multimode

July 7, 2010 Leave a comment

Understanding the characteristics of different fiber types aides in understanding the applications for which they are used. Operating a fiber optic system properly relies on knowing what type of fiber is being used and why. There are two basic types of fiber: multimode fiber optic cable and single-mode fiber optic cable. Multimode fiber is best designed for short transmission distances, and is suited for use in LAN systems and video surveillance. Single-mode fiber is best designed for longer transmission distances, making it suitable for long-distance telephony and multichannel television broadcast systems.

Multimode Fiber
Multimode fiber, the first to be manufactured and commercialized, simply refers to the fact that numerous modes or light rays are carried simultaneously through the waveguide. Modes result from the fact that light will only propagate in the fiber core at discrete angles within the cone of acceptance. This fiber type has a much larger core diameter, compared to single-mode fiber, allowing for the larger number of modes, and multimode fiber is easier to couple than single-mode optical fiber. Multimode fiber may be categorized as step-index or graded-index fiber. Multimode Step-index Fiber Figure 2 shows how the principle of total internal reflection applies to multimode step-index fiber. Because the core’s index of refraction is higher than the cladding’s index of refraction, the light that enters at less than the critical angle is guided along the fiber.

fiber optic cable
Figure 2 – Total Internal Reflection in Multimode Step-index fiber

Three different lightwaves travel down the fiber. One mode travels straight down the center of the core. A second mode travels at a steep angle and bounces back and forth by total internal reflection. The third mode exceeds the critical angle and refracts into the cladding. Intuitively, it can be seen that the second mode travels a longer distance than the first mode, causing the two modes to arrive at separate times. This disparity between arrival times of the different light rays is known as dispersion, and the result is a muddied signal at the receiving end. For a more detailed discussion of dispersion, see “Dispersion in Fiber Optic Systems” however, it is important to note that high dispersion is an unavoidable characteristic of multimode step-index fiber. Multimode Graded-index Fiber Graded-index refers to the fact that the refractive index of the core gradually decreases farther from the center of the core. The increased refraction in the center of the core slows the speed of some light rays, allowing all the light rays to reach the receiving end at approximately the same time, reducing dispersion.Figure 3 shows the principle of multimode graded-index fiber. The core’s central refractive index, nA, is greater than that of the outer core’s refractive index, nB. As discussed earlier, the core’s refractive index is parabolic, being higher at the center. As Figure 3 shows, the light rays no longer follow straight lines; they follow a serpentine path being gradually bent back toward the center by the continuously declining refractive index. This reduces the arrival time disparity because all modes arrive at about the same time. The modes traveling in a straight line are in a higher refractive index, so they travel slower than the serpentine modes. These travel farther but move faster in the lower refractive index of the outer core region.

fiber optic cable
Figure 3 – Multimode Graded-index Fiber

Single-mode Fiber
Single-mode fiber allows for a higher capacity to transmit information because it can retain the fidelity of each light pulse over longer distances, and it exhibits no dispersion caused by multiple modes. Single-mode fiber also enjoys lower fiber attenuation than multimode fiber. Thus, more information can be transmitted per unit of time. Like multimode fiber, early single-mode fiber was generally characterized as step-index fiber meaning the refractive index of the fiber core is a step above that of the cladding rather than graduated as it is in graded-index fiber. Modern single-mode fibers have evolved into more complex designs such as matched clad, depressed clad and other exotic structures.

fiber optic cable
Figure 4

Single-mode fiber has disadvantages. The smaller core diameter makes coupling light into the core more difficult. The tolerances for single-mode connectors and splices are also much more demanding. Single-mode fiber has gone through a continuing evolution for several decades now. As a result, there are three basic classes of single-mode fiber used in modern telecommunications systems. The oldest and most widely deployed type is non dispersion-shifted fiber(NDSF). These fibers were initially intended for use near 1310 nm. Later, 1550 nm systems made NDSF fiber undesirable due to its very high dispersion at the 1550 nm wavelength. To address this shortcoming, fiber manufacturers developed, dispersion-shifted fiber(DSF), that moved the zero-dispersion point to the 1550 nm region. Years later, scientists would discover that while DSF worked extremely well with a single 1550 nm wavelength, it exhibits serious nonlinearities when multiple, closely-spaced wavelengths in the 1550 nm were transmitted in DWDM systems. Recently, to address the problem of nonlinearities, a new class of fibers were introduced. These are classified as non zero-dispersion-shifted fibers (NZ-DSF). The fiber is available in both positive and negative dispersion varieties and is rapidly becoming the fiber of choice in new fiber deployment. For more information on this loss mechanism, see the article “Fiber Dispersion.”

fiber optic cable
Figure 6 – Dispersion for Alternating 20 km Lengths of (+D) NZ-DSF and (-D) NZ-DSF Fiber

One additional important variety of single-mode fiber is polarization-maintaining (PM) fiber. All other single-mode fibers discussed so far have been capable of carrying randomly polarized light. PM fiber is designed to propagate only one polarization of the input light. This is important for components such as external modulators that require a polarized light input. Figure 7 shows the cross-section of a type of PM fiber. This fiber contains a feature not seen in other fiber types. Besides the core, there are two additional circles called stress rods. As their name implies, these stress rods create stress in the core of the fiber such that the transmission of only one polarization plane of light is favored. Single-mode fibers experience nonlinearities that can greatly affect system performance. For complete information, see “Fiber Nonlinearities.”

fiber optic cable
Figure 7

This artical is from: http://www.voscom.com/trainning/fiber-optic-cable.asp