FIBRE OPTIC MEASUREMENT

 
Optical power loss (attenuation) in fiber access - types, values and sources
Light traveling in an optical fiber loses power over distance. The loss of power depends on the wavelength of the light and on the propagating material. For silica glass, the shorter wavelengths are attenuated the most (see Fig. 1). The lowest loss occurs at the 1550-nm wavelength, which is commonly used for long-distance transmissions.

Transmission of light by fibre optics is not 100% efficient. There are several reasons for this including absorption by the core and cladding (caused by the presence of impurities) and the leaking of light from of the cladding. When light reflects off the cladding /core interface it actually travels for a short distance within the cladding before being reflected back. This leads to attenuation (signal reduction) by up to 2db/Km for a multi-mode fibre. For example, with this level of attenuation, if light travelled over 10kM of cable only 10% of the signal would arrive at the following end.

The amount of attenuation for a given cable is also wavelength dependent. Figure 1 shows the attenuation profile for the two main types of fibre; multi-mode and single-mode cable (described in detail below). The absorption peak at 1000nm is caused by the peculiarities of single mode fibre while the peak at 1400nm is caused by traces of water remaining in the fibre as an impurity. Due to this water absorption peak there are two standard single-mode wavelengths in use, 1310nm and 1550nm. 1310nm has been a standard for many years, only now is there a trend towards using 1550nm brought about by the need to extend the distances between repeaters.

The loss of power in light in an optical fiber is measured in decibels (dB). Fiber optic cable specifications express cable loss as attenuation per 1-km length as dB/km. This value is multiplied by the total length of the optical fiber in kilometers to determine the fiber's total loss in dB.

Optical fiber light loss is caused by a number of factors that can be categorized into extrinsic and intrinsic losses:

• Extrinsic

• Bending loss

• Splice and connector loss

• Intrinsic

• Loss inherent to fiber

• Loss resulting from fiber fabrication

 

Figure 1. Optical fiber operating wavelengths.

• Fresnel reflection

Bend Loss. Bend loss occurs at fiber cable bends that are tighter than the cable's minimum bend radius. Bending loss can also occur on a smaller scale from such factors as:

• Sharp curves of the fiber core

• Displacements of a few millimeters or less, caused by buffer or jacket imperfections

• Poor installation practice

This light power loss, called microbending, can add up to a significant amount over a long distance.

Splice and Connector Loss. Splice loss occurs at all splice locations. Mechanical splices usually have the highest loss, commonly ranging from 0.2 to over 1.0 dB, depending on the type of splice. Fusion splices have lower losses, usually less than 0.1 dB. A loss of 0.05 dB or less is usually achieved with good equipment and an experienced splicing crew. High loss can be attributed to a number of factors, including:

• Poor cleave

• Misalignment of fiber cores

• An air gap

• Contamination

• Index-of-refraction mismatch

• Core diameter mismatch

to name just a few.

Losses at fiber optic connectors commonly range from 0.25 to over 1.5 dB and depend greatly on the type of connector used. Other factors that contribute to the connection loss include:

• Dirt or contaminants on the connector (very common)

• Improper connector installation

• A damaged connector face

• Poor scribe (cleave)

• Mismatched fiber cores

• Misaligned fiber cores

• Index-of-refraction mismatch

Loss Inherent to Fiber. Light loss in a fiber that cannot be eliminated during the fabrication process is due to impurities in the glass and the absorption

of light at the molecular level. Loss of light due to variations in optical density, composition, and molecular structure is called Rayleigh scattering. Rays of light encountering these variations and impurities are scattered in many directions and lost.

The absorption of light at the molecular level in a fiber is mainly due to contaminants in glass such as water molecules (OH-). The ingress of OUT molecules into an optical fiber is one of the main factors contributing to the fiber's increased attenuation in aging. Silica glass's (Si02) molecular resonance absorption also contributes to some light loss.

Figure 1 shows the net attenuation of a silica glass fiber and the three fiber operating windows at 850, 1310, and 1550 nm. For long-distance transmissions, 1310- or 1550-nm windows are used. The 1550-nm window has slightly less attenuation than 1310 nm. The 850-nm communication is common in shorter-distance, lower-cost installations.

Loss Resulting from Fiber Fabrication. Irregularities during the manufacturing process can result in the loss of light rays. For example, a 0.1 percent change in the core diameter can result in a 10-dB loss per kilometer. Precision tolerance must be maintained throughout the manufacturing of the fiber to minimize losses.

Fresnel Reflection. Fresnel reflection occurs at any medium boundary where the refractive index changes, causing a portion of the incident light ray to be reflected back into the first medium. The fiber end is a good example of this occurrence. Light, traveling from air to the fiber core, is refracted into the core. However, some of the light, about 4 percent, is reflected back into the air. The amount being reflected can be estimated using the following formula:

 

 

At a fiber connector, the light reflected back can easily be seen with an optical time domain reflectometer (OTDR) trace. It appears as a large upward spike in the trace. This reflected light can cause problems if a laser is used and should be kept to a minimum.

The reflected light power can be reduced by using better connectors. Connectors with the "PC" (Physical Contact) or "APC" (Angle Physical Contact) designations are designed to minimize this reflection.

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The fiber optic power meter and light source are used together to measure loss in a fiber or fiber optic device. The source launches the light into one end of the fiber, while the power meter is connected to the other end to measure the received optical power. The source can be an optical laser or light emitting diode (LED) designed as part of a test set, or alternately the lightwave communication equipment light source can be used.

Because optical fiber loss varies with light wavelength, power meter tests should be performed using the same wavelength as the one used by the lightwave communication equipment. If lightwave equipment operates at the 1310-nm wavelength, the power meter and light source should also be set to 1310-nm testing. If the lightwave equipment operates at 1550 nm, the power meter and source should also beset to 1550 nm. Special consideration should be made when testing through a WDM network. (dB)

(mW). However, a more convenient form of measurement used is called the decibel (dB).

THE DECIBEL

The power meter test is used to determine light power loss in a fiber optic link. The measured unit of light power is the milliwatt

The decibel is a common measurement used in the field of electronics to determine loss or gain in a system. It is the ratio, in logarithmic form, of power, voltage, or current levels between two points. One point is located at the beginning, or input, of the system to be measured, and the other point is located at the end, or output, of the system. The power formula for decibel gain is expressed as:

G(dB) = 10 X log (output power/input power)

 

When the output power is less than the input power, the value of this equation will always be negative. In most fiber optic applications, light power output from an optical fiber will always be less than the input light power into the optical fiber. Therefore, this value will always be negative. This negative gain can be referred to as a light loss, L(dB):

1.gif

sing the decibel power loss formula, the optical fiber loss can be calculated as follows:

2(1).jpg
unit is a logarithmic ratio of input and output levels and is therefore not absolute (i.e., has no units). An absolute measure of power in decibels can be made in the dBm form. The dBm unit is a logarithmic ratio of the measured power to 1 mW of reference power.

The formula is as follows:

3.jpg

4.jpg

where L(db) = 10 X log (input power/output power).

Light loss, L(dB), is a commonly used specification for fiber optic attenuation. For example, to determine the light loss of an optical fiber in a cable, a light source is connected to one end of the fiber cable (input). The light output power of the source is known to be 0.1 mW. When an optical power meter is connected to the opposite end of the fiber optic cable under test (output), the meter measures 0.05 mW. U

The light power loss of this optical fiber is 3 dB

The dB

The same result in loss can be achieved using the dBm. In the previous example, light power input by the source to the optical fiber is 0.1 mW, which is -10 dBm:

5.jpg

6.jpg

The light power received by the meter from the optical fiber's output is 0.05 mW, which is —13 dBm:

 

The light power loss in the fiber is equal to the light source power minus the received meter light power

7.jpg

Therefore, the light power lost by the optical fiber is 3 dB.

All measurements must be in either decibels or in milliwatts, but not both. Usually, all measurements are made in the decibel scale because it is easier to work with, ft is not necessary to convert between mW and dBm because most equipment specification data also use the decibel scale. The following table shows dBm equivalents for optical power in milliwatts:

Power in dBm

Power in

mW

+20

100

+ 10

/0

+3

2

0

1

-3

0.5

-10

0.1

-20

0.01

-30

0.001

-40

0.0001

-50

0.00001

 

It is helpful to remember that a loss of 3 dB is equivalent to a 50% loss in power. A loss of 10 dB is equivalent to a power loss of 90%. When you add or subtract a dB to or from a dBm, the result is in dBm. When you add or subtract two dB values, the result is always in dB. Decibel values are never multiplied together—they are always added or subtracted.

dB of a number of different sections of a fiber optic link, the total loss of all sections is equal to the sum in dB of each individual section.dB. Patch panel connection loss at each end is 0.8 dB. Pigtail loss is negligible. If a light source with optical power of — 10 dBm is connected to one end of the fiber link, what will the received light power be at the other end?

8.jpg

When measuring the loss in

Example:

First, the total link loss including patch panel connections is summed:

The optical power loss formula needs to be rearranged to equal the received optical power:

9.jpg

Therefore, the light power that would be measured by an optical power meter at the end is —15 dBm. It should be noted that two fiber optic connectors contribute to one connection loss

A fiber optic link with a 1-km cable has a loss of 3.4


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Thu, 05/21/2009 - 16:21

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