The two point method.

Two measurement points on the graph: at (x1, Y1) and (X1, Y2)

 

Two point method

The splice loss A (s) is calculated according to :

A(s)=Y1 – Y2

Different types of OTDR measurement.

Only simply descriptions of the different types of measurement that can be carried out with an OTDR are given in the following. For more detailed information refer to the instrument meanufacturer’s manual.

1** Distance measurement.

Using an OTDR, the length of an optical fiber can be determined with a high degree of accuracy.

L=              vt/2=ct/2n

V =            velocity of light in the optical fiber

T =             propagation time (s)

L =             length of the optical fiber (m)

E =            velocity of light in a vacuum (m)

N =            refractive index of the core

2**Optical power loss (attenuation

Because power is shown on the vertical axis, the power less can be osciculated by subtracting one power level from another.

A(a)=p₁-p₂/2

Optical power loss per unit of length (Lu)

A(a) = p_1-p₂/2L

OTDR Curve and its use in attenuation measurement

3**Splice loss measurement.

When an OTDR is used for measurement, the graph obtained shows the attenuation in a cable section. If the section contains splices (either mechanical or fusion splices) these are normally shown as knees on the graph. The loss in each splice can be determined by measuring in these points – either according to the two point method or the five point method.

 

How the OTDR works

As its name suggests, the OTDR’s function is based on the propagation of light in an optical fiber during a specific length of time. Figure is a schematic illustration of an OTDR instrument.

 In a pulse generator, short (e.g 0.1, 1 or 10 us) electric pulses with specific time intervals are generated. The electric pulses are transformed into light pulses in the laser diode. A light pulse passes a directional coupler before it is sent out to the fiber. Because the fiber is not completely transparent a certain amount of light will be returned into the instrument due to Raleigh scattering. The returned light is detected by an avalanche photodiode (ADP) .The signal from the APD is amplified by an amplifier and processed by a microprocessor. The Optical Signal is weak and the process is therefore repeated several times to provide a basis for an average value.

The signal strength is represented by the vertical axis of a coordinate system and time (often transformed into distance) is represented by the horizontal axis. The coordinate system is shown together with other useful information on a video screen. The optical signal from the fiber is shown as a curve sloping from left to right. Normally the video screen image is printed out directly on a built in printer. This printout is then saved for the final documentation of the installation.

 schematic illustrations of the main parts of an OTDR.

 

simplified graph from an OTDR.

Usability (OTDR)

OTDR can be used for a number of different types of measurement.

·        Distance measurement (fault tracing)

·        Attenuation measurement in fibers, fiber optic links and splices.

·        Splice loss after fusion splicing.

·        Connector loss.

·        Detection of local attenuation variations (stepwise or gradual increases ).

·        Reflection at connectors or the end reflection pulse (end pulse).

Optical Time Domain Reflectometer (OTDR)

The OTDR is used for most measurements and is more or less the universal fiber optic measuring instrument. The OTDR is used for quality control of cable on drums and after laying and for determining splice loss, verification of an installation and fault tracing.

 Modern OTDR instruments allow easy change of the laser diode module with a flick of a switch, which facilitates measurements at different wave lengths.

 The different fiber parameters should be verified of the same wavelength as that of the system to be connected to the installation. For single mode fiber cable, 1550 nm is the most common wavelength but fibers should be tested at 1310 nm.

 As a rule, the instruments have built in printers that provide a hard copy of measurement results. An external video plotter can also be connected to an OTDR.

 An alternative to a complete OTDR is an OTDR printed board assembly, which can be installed in most PCs. Measurements are carried out in the same manner as with an OTDR instrument but the results are stored in the PC for print out or storage on a diskette.

FIBER OPTIC MEASURING INSTRUMENTS

OTDR

1. INTRODUCTION

 The business of fiber optic measurement instruments in flourishing: new instruments offering better performance and facilities are being developed all the time. This chapter on measuring instruments therefore deals with general principles.

  Fiber Optic Measuring Instruments.

Generally, three types of measuring instrument are used when optical fiber cable is installed:

·        Optical Time Domain Reflectometer (OTDR)

·        Stabilized light source (laser diode module)

·        Optical power meter with optical sensor.

OPENING A SPLICE

Secure the fibers of the Light bridge splice to be opened in the installation tool’s fiber clamps.

 With the operating lever in the up position, insert the Light bridge splice to be opened into the splice retainer.

 Rotate the lever all the way toward you. This opens the Light bridge splice.

 To fine tune the splice, you can now rotate the fibers. If necessary, you can also remove the fibers, recleave them and reinsert them into the Light bridge splice.

 

Before closing the splice be sure the fibers are touching. Reclose the splice and eject the Light bridge splice by rotating the lever all the way to the back and down.

Move the splice to the organizer tray and place the splice into the splice holder pad.

Move the operating lever to the up position in preparation for the next splice.