OPTICAL FIBRE : TESTS AND MEASUREMENTS. BY TX-I FACULTY A.L.T.T.C; GHAZIABAD

FEATURES BENEFITS *Low TX Loss. *Long repeater Spacing or Repeater less N/W. * Wide Bandwidth.* Larger Chl. Capacity * Non-inductive. * No damage to Eqpt. due to surge voltage. * Immunity from * No shielding to Eqpt. Electro-magnetic no X-talk or Signal interference. leakage. *Small size, * Easy to install, bending radius and reduction in space light weight. needed. *Difficult to tap. * High Security and Copper resource savings. Main Features and Benefits of Optical Fiber Cables

System Composition Transmitter E/O Converter O/E Converter Receiver Application area of Measuring Instruments In Optical Fiber Communication system Electrical Signal Optical Signal Electrical Signal Data InData Out DDFDDF DDFDDF FDFFDF FDFFDF

Cable Loss. Splice Loss. Connector Loss. Fibre Length. Continuity of Fiber. Fault Localizations/Break Fault. MAIN TESTS ON OPTICAL FIBRE CABLES

Calibrated Light Source. Optical Power Meter. Optical Attenuator. Optical Time Domain Reflectometer (OTDR). INSTRUMENTS REQUIRED

Generates Light signals of known power and wavelength (LED or LASER). Wavelength variations to match Fiber's Wavelength. CALIBRATED LIGHT SOURCE

Measures Optical Power over wide range (Typically 1 nW to 2mW/-60dBm to + 3dBm) It is never measured directly, but measured through Electrical conversion using Photo Electric conversion. It is known as OPTICAL SENSOR of known Wavelength. The accuracy of the Optical Power meter depends upon the stability of the Detectors power to current conversion which changes with Ageing. OPTICAL POWER METER

TYPES:- –F–Fixed Attenuators. –V–Variable Attenuators. APPLICATIONS:- –T–To Simulate the Regenerator Hop Loss at the FDF. –T–To Provide Local Loop Back for Testing. –T–To measure the Bit Error Rate by varying the Optical Signal at the Receiver Input. (RECEIVER SENSITIVITY) OPTICAL ATTENUATORS

REQUIREMENTS OF ATTENUATORS Attenuation Range. Lowest Insertion Loss. Independent of Wavelength. Type of Connectors at the Input and Output.

Fiber Light Source 100% Dark Light Receiver Fiber Motion 0% Dark (VARIABLE ATTENUATOR)

Used for measuring –Fiber Loss. –Splice Loss. –Connector Loss. –Fiber Length. –Continuity of Fiber. –Fault Localization. OPTICAL TIME DOMAIN REFLECTOMETER (OTDR)

One Port Operation. Works on the Principle of Back Scattering (Raleigh Scattering, see Figure ). –Scattering is the main cause of Fiber Loss –Scattering Coefficient=1/ 4 –An Optical Pulse is launched into one End of Fiber and Back Scattered Signals are detected. –These Signals are approximately 50 dB below the Transmitted level. Measuring conditions and Results are displayed. OPERAING PRINCIPLES

Scattering in an Optical Fiber Light is scattered in all directions including back towards the Source in the Fiber.

FRESNEL REFLECTION It happens when there is a great change of Refractive Index:- –Break Fault. –Connecter Loss. –Free Fiber-End. Received reflected signal depends on surface conditions. It is normally 14 db below Transmitted signals. Break FIBER CORE BREAK IN FIBER

n 2 =1.5n 1 =1.0 (n 2 -n 1 ) 2 ( ) 2 (n 2 + n 1 ) 2 ( ) 2 = = 0.04 = 4% = - 14dB Fresnel Reflection

OTDR INSTRUMENT PRINCIPLE Fiber APD Signal Oscilloscope Amplifier Trigger Pulse Generator Laser

BOX CAR AVERAGER AMPLIFIER It is provided to improve S/N of the RX. Signal in OTDR It is done by sampling the signal at each point in Time, starting at time, t=0. An Arithmetic Average is generated by a Low Pass Filter (LPF). Then a variable delay is used to move to the next point in Time t=1,2, n. It scans the entire signal. Larger the No. of Samples (n), the smaller the Mean Square Noise Current:- i 2 noise = Constant /n

z t= t 1 + t from pulse front from pulse tail direction of pulse propagation BACKSCATTERING z backscattering from pulse front T= t 1 Explanation of the Z/2 uncertainty of the OTDR Signal Z/2 Z- Z/2

For 100ns Pulse width Z = Pulse Width (W) x Group Velocity = W x Speed of Light/Refractive Index. = 100x x 3 x10 8 /1.5 = 20m. Z/2=10m i.e. ± 5m For 1000ns Pulse Width: Z = Pulse Width (W) x Group velocity = W x Speed of Light / Refractive Index. = 1000 x x 3 x 10 8 /1.5 = 200m. Z/2=100m i.e. ± 50m For 1000ns Pulse Width: Z = Pulse width (W) x Group velocity. = W x Speed of Light/Refractive Index. = 4000x10 -9 x3x10 8 /1.5 = 800m. Z/2 = 400m i.e. ± 200m Calculation of Pulse Length in Fiber

The amount of light scattered back to the OTDR is proportional to the backscatter of the fiber, peak power of the OTDR test pulse and the length of the pulse sent out. Length of OTDR Pulse in the fiber Increasing the pulse width increases the backscatter level. OTDR pulse

Reflections show OTDR Pulse Width and Resolution Connectors show both Loss and Reflections Splices are usually not Reflective. Splices Loss Slope of trace shows Fiber Attenuation Coefficient OTDR Trace Information

Typical Display on CRT of OTDR 2.0 km/DIV 4.0 db/DIV DR=36km Start point of Measurement Shifted distance km Starting point LOSS----- (LSA) Total loss =4.00 db Distance = km Loss/km=1.00 db/km km --End point of Measurement Wavelength= 1.31, SM – Type of fibre under test PW=100ns –Pulse setting for transmission REF= – Refractive Index of Core under test Gain= 5.0db– Gain of Amplifier inside OTDR

Backscattered Light Fresnel Reflection at connection Fresnel Reflection at near end connector Splice Fresnel Reflection at Far-end or fault Loss (dB) Distance (km) General Waveform Analysis

X Y Reason for Dead Zone Dead Zone

Dead Zone depends on Pulse Width 100ns 1 s

Splice Loss Measurement Principles The trace waveform at the Splice Point should be displayed as the dotted line in the figure below, but is actually displayed as the solid line. The waveform input to the OTDR shows a sharp falling edge at the splice point, so the circuit cannot respond correctly. The interval L gets longer as the pulse width becomes longer. Splice Point L Therefore, the Splice Loss can not be measured correctly in the Loss Mode.

In the Splice Loss mode, two markers are set on each side of the Splice Point and the lines L1 and L2 are drawn as shown below. The part of the straight line immediately after the splice point is the forward projection of the straight line, L2 The Splice Loss is found by dropping a vertical line from the Splice Point to this projection of L2, and measuring the level difference between the Splice Point and the intersection. x1 x2 x3 x4 L2 Splice Loss Splice Point L1

Approximation Methods At Loss Measurement and Splice Loss Measurement, the loss is found by drawing an imaginary line between two set markers. There are two methods for drawing the line. Least Square Approximation Method (LSA). Two Point Approximation Method (2PA).

In this method, the line is drawn by computing the least square of the distance from all the measured data between the two markers. LEAST SQUARE APPROXINATION METHOD (LSA) X1 X2

Two Point Approximation Method(2PA) This method draws the line linking the two measured data points at the two markers. X1X2

Measurement of Splice Loss by Least Squares Method Splice Loss Splice L1 L2 X2 X3 X4 * X1

Splice Loss Measurement by Two Point Approximation Measured Value Splice X1 True Value *

a. same fiber spliced actual loss error caused by fiber characteristics b. high loss fiber spliced to low loss fiber error caused by fiber characteristics actual loss c. low loss fiber spliced to high loss fiber can cause an apparent gain at a splice. Loss Errors in OTDR Measurements

Visible Light Source Visual Inspection:- Eye Light Source Optical Power Meter Continuity Test:- Optical Fibre Sensor

Receiver Sensitivity Test BER Test Set Transmitter DUT Receiver OF Patch Cords Variable Optical Attenuator Power Meter Optical Power Splitter

Thank You Any Questions & Suggestions, please.