OPTICAL FIBRE : TESTS AND MEASUREMENTS .

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

2. Main Features and Benefits of Optical Fiber Cables FEATURESBENEFITS * 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.

3. System Composition Electrical Signal Electrical Signal Optical Signal Transmitter E/O Converter O/E Converter Receiver DDF DDF FDF FDF Application area of Measuring Instruments In Optical Fiber Communication system Data In Data Out

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

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

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

7. OPTICAL POWER METER • 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 Detector’s power to current conversion which changes with Ageing.

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

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

10. 0% Dark Light Receiver Motion Fiber Light Source Fiber Light Source 100% Dark (VARIABLE ATTENUATOR)

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

12. OPERAING PRINCIPLES • 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.

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

14. FRESNEL REFLECTION Break FIBER CORE • 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 IN FIBER

15. Fresnel Reflection n2=1.5 n1=1.0 (n2-n1)2(1.5-1.0)2 (n2+n1)2 (1.5+1.0)2 = 0.04 = 4% = - 14dB =

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

17. 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=1,2,3-------n. • It scans the entire signal. Larger the No. of Samples (n), the smaller the Mean Square Noise Current:- i2noise = Constant /n

18. Explanation of the Z/2 uncertainty of the OTDR Signal BACKSCATTERING z T= t1 backscattering from pulse front z from pulse front  t= t1+ t from pulse tail direction of pulse propagation Z/2 Z-Z/2

19. Calculation of Pulse Length in Fiber For 100ns Pulse width Z = Pulse Width (W) x Group Velocity = W x Speed of Light/Refractive Index. = 100x 10-9 x 3 x108/1.5 = 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 10-9 x 3 x 108/1.5 = 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. = 4000x10-9x3x108/1.5 = 800m.  Z/2 = 400m i.e. ± 200m

20. 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 OTDR pulse Increasing the pulse width increases the backscatter level.

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

22. Typical Display on CRT of OTDR 2.0 km/DIV 4.0 db/DIV DR=36km Start point of Measurement Shifted distance 0.000 km 0 0.000 Starting point LOSS----- (LSA) Total loss =4.00 db Distance = 4.000 km Loss/km=1.00 db/km 10.000 km --End point of Measurement Wavelength= 1.31, SM – Type of fibre under test PW=100ns –Pulse setting for transmission REF= 1.5000 – Refractive Index of Core under test Gain= 5.0db– Gain of Amplifier inside OTDR

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

24. Reason for Dead Zone Y Dead Zone X

25. Dead Zone depends on Pulse Width 100ns 1s

26. 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.

27. 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. Splice Point L1 x1 Splice Loss x2 L2 x3 x4

28. 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).

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

30. Two Point Approximation Method(2PA) This method draws the line linking the two measured data points at the two markers. X1 X2

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

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

33. Loss Errors in OTDR Measurements 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.

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

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

36. Thank You Any Questions & Suggestions, please.