OPTICAL TIME DOMAIN REFELECTOMETER (OTDR): PRINCIPLES

OPTICAL TIME DOMAIN REFELECTOMETER (OTDR): PRINCIPLES

Why Test Fiber with an OTDR? Single ended test that......characterizes fiber from end-to-end...locate and measure each event...provides a detailed map of fiber

Working Principles of OTDR Typical OTDR applications What can an OTDR tell you? How Does an OTDR Work? OTDR-Block Diagram Understanding OTDR Specs Typical OTDR Test OTDR Report

OTDR applications Fiber Installation Emergency Fault Locating Preventive Maintenance Monitoring of critical links (remote fiber test system - RFTS) Complete network database

What can an OTDR tell you? Total loss of a fiber Total reflection of the system (Optical Return Loss - ORL) Location and type of each event Loss and Reflection of each event Attenuation of each fiber sections

How Does an OTDR Work? For each pulse sent, up to 30 000 points are acquired Hundreds of pulses are sent in the fiber each second Each point is then averaged thousands of times The longer the acquisition time, the better the results!

How Does an OTDR Work? OTDR performance comes from a blend of technologies Quality, powerful optics Proper electronic design Efficient data acquisition and software processing Years of fine tuning... and engineering magic touch! Laser / Coupler / Photodetector Fresnel Reflections along the fiber are measured Rayleigh Backscatter along the fiber is measured

OTDR-Block Diagram

Function Diagrams OTDR launches a series of optical pulses into the fiber, and then measure the backward optical signal v.s. time taken for the light travels through the fiber The measurement trace, which is called fiber trace, is shown on the LCD panel. ISA bus interface RAM / Registers Microcontroller A/D converters APD Fiber Connector Fiber coupler Fiber under test Laser Distance Range d = ( t*c) / (2*IOR) d, displayed distance IOR, Index of Refraction t, time for round trip C, light speed in vacuum The distance range determines the time interval of successive pulse in OTDR

Understanding OTDR Specs Dynamic Range Dead Zones Loss Resolution Loss Threshold Splice Loss Accuracy Distance Accuracy Sampling Resolution Event Location Accuracy Reflectance Accuracy

Dynamic Range Used for many years, to indicate attainable distances SNR=1 The most popular method of calculating dynamic range method

Dynamic Range (SNR=1) Dynamic Range = 40 db Dynamic Range = 28 db SNR=1 SNR: Signal to Noise Ratio.

Dynamic range and Distance Distance covered depends of fiber tested Related to fiber attenuation, ex: 40 db at 0.2 db/km = 200 km max. 40 db at 0.25 db/km = 160 km max. Reduced by event losses, ex: 5 events of 1dB loss reduce distance range by 5 db at 0.2 db/km = 25 km

10 us pulse

20 us pulse

Dead Zone resulting from reflective event a temporary blinding of the OTDR detector Detector saturation due to high reflectance Two types of dead zones: Event and Attenuation dead zones

Event Dead Zone 1.5 db The ability to DETECT an event that closely follows a reflective event There is a Dead Zone for each reflective event Bigger reflectance (saturated) means larger Event Dead Zone Measured on the trace at 1.5 db from the peak of the reflective event

Attenuation Dead Zone 0.5 db The ability to MEASURE an event that closely follows a reflective event Extrapolate a line from the trace points following the reflective event (towards the event) Locate the point on the trace that is 0.5 db higher than the extrapolated line The attenuation dead zone is defined as the distance between the beginning of the event and the previously located point.

Dead Zone The longer the pulse, the longer the DZ Event dead zone examples: 2.5 meters with 10 ns (1m) pulse 1025 meters with 10 us (1km) pulse Specifications are based on best case scenario

Loss Resolution Minimal distance in db (y-axis) between two points It s a physical specification of the OTDR It has nothing to do with loss or splice loss accuracy. 0.001 db

Loss Threshold The minimal loss difference between two splice loss measurements: (0.01 db ) The OTDR can differentiate between a 1.05 and 1.06 db splice, but not 1.05 and 1.054 db Minimal splice loss that can be detected by the ToolBox analysis software is 0.03 db The OTDR can measure splices 0.03 db and greater with a resolution of 0.01 db

Splice Loss Resolution Q: Does this mean that the OTDR will detect every splice above 0.03 db? Answer : NO! It depends on the amount of noise! It depends on the pulse you are using!

Splice Loss Accuracy The ability for the analysis software to measure the loss of a splice precisely. Depends on many things... Noise Linearity Response to high reflectance Distance from the front end connector. Lower (± 0.1 db) at the limit of the measurement range Better (± 0.01 db) near the front end connector. Accurate Noisy data (less accurate) Linearity problems (less accurate)

Sampling Resolution Distance Range / Number of acquisition points More points = better distance & loss precision Example: 150 km / 30 000 points = 5 meters 5 m

Typical OTDR sampling resolutions and data points. RANGE(Km) Sampling Resolution Data Points.625km.08m (.0781m) 8,000 2.5km.15625m 16,000 10km.625m 16,000 20km 1.25m 16,000 40km 2.5m 16,000 80km 5m 16,000 150km 5m 30,000

Sampling Points & Distance Accuracy

Distance Accuracy Distance accuracy: physical limitation of the OTDR not software related It gives the expected error on the location of a point seen by the OTDR (in meters) with respect to its real physical location

Distance Accuracy Spec.: ± 1m ± 0.0025% of dist. ± Sampling res. On 150 km: ± 1m ± 3.75m ± 5m = ±9.75 m 1 m: Uncertainty of the distance zero location Fiber length in the unit, Rise Time Electronic Latency Time 0.0025%*Distance: Acquisition Clock Error Sampling resolution Fiber Index Uncertainty (IOR) can also induce distance reading variations

Event Location Accuracy Depends on......the ability for the analysis software to locate an event precisely...noise level, distance from the front end connector, and number of points. Don t forget! It s also dependant on these two factors... Helical factor Index of refraction

Reflectance Accuracy The ability for the analysis software to measure reflectance precisely Anritsu specifies ± 4 db.

Linearity The capability of the OTDR to produce a straight line for the entire trace. A trace that is curved would give a higher splice loss than what it is in reality. Anritsu specifies 0.05 db per db. 1 db Between 0.95 and 1.05 db

Using the OTDR Set the Index Of Refraction (IOR) Set the wavelength according to the system under test Set the pulse width Set the distance range Set the acquisition time Shoot the fiber.

Auto-Analysis using a Mini-OTDR Event table containing:(for each event) Type of fault Distance to fault Attenuation Optical return loss Splice loss.

OTDR Test Setup

Typical OTDR Test

Sample: OTDR Anritsu Report