OPTICAL TIME DOMAIN REFLECTOMETRY (OTDR) Dr. BC Choudhary Professor, NITTTR, Chandigarh
WHAT IS AN OTDR? A measurement technique which provides the loss characteristics of an optical link down its entire length giving information on the length dependence loss. Also allows splice and connector losses to be evaluated as well as location of any faults on the link. Also called backscatter measurement method. Relies upon the measurement & analysis of the fraction of light which is reflected back within the fiber s numerical aperture due to Rayleigh scattering within the fiber. A small proportion of the scattered power is collected by the fiber in backward direction and returns to the transmitter, where it is measured by a photodiode.
Back Scattering of Light intensity Incident light wavelength Rayleigh scattering Brillouin scattering Raman scattering (Anti-stokes) Raman scattering (stokes) Depending on temperature Wavelength Depending on strain and temperature
In operation, an OTDR launches pulses of light into the line fiber of an optical network and monitors the backscatter signal as a function of time relative to the launch time. As the pulse propagates down the fiber it becomes weaker with increasing distance due to power loss, and the measured backscatter signal decreases accordingly. The rate of signal decrease for a continuous section of fiber represents the fiber loss and any abrupt drops correspond to losses from the presence of components, terminations or faults which can be readily identified. OTDR An industry standard for measuring the loss characteristics of a link or network, monitoring the network status, locating faults and degrading components.
OTDR Models
Principle of OTDR Incident light (Pulse) Transmitted light Fiber core scattering light Back scattering light z Laser Detector Distance z = t.v /2 t : two-way propagation delay time V : velocity of light in the fiber
The received backscattered optical power as a function of time t down an uninterrupted fiber is given by: P Ra (t) 1 2 P.S. i R.w 0.v g.exp( v g t) where; P i - optical power launched into the fiber. S - fraction of captured optical power. R - Rayleigh scattering coefficient. w o - input optical pulse width. v g - group velocity in fiber. - attenuation coefficient per unit length for the fiber. Generally, OTDR output is expressed in db relative to the launched power, and the directly measured loss is then halved electronically before plotting the output trace.
OTDR Display
OTDR Testing to locate a fault point
Events in OTDR Traces In addition to decaying signal associated with the fiber losses; Abrupt drops in the backscatter signal on the trace: Losses due to the presence of non reflective elements such as fused coupler components, tight bends or splices. Presence of large return pulses - arise from Fresnel reflections- followed by a drop in the background signal: Fiber interruptions at connectors, non-fiber components, termination or breaks. Such readily identified features on the OTDR signal Events Their location and loss associated with them may be obtained directly from the trace.
Dead Zones and Ghosts Large Fresnel reflection signals can cause problems for the detection system- they lead to transient but strong saturation of the front end receiver which requires time to recover. Usually detection of strong Fresnel reflected pulses from the fiber interruptions or termination drive the receiver into deep saturation. The length of the fiber masked in terms of event detection by this way is known as a Dead Zone The length of which is determined by the pulse width and, for reflection events, by the amplitude of the reflected pulse. Dead zones arising from large Fresnel reflection signals from the fiber input and output(s) near and far end dead zones respectively.
Full OTDR trace of fiber reel at 1550 nm. Fiber attenuation = 0.185 db/km Fiber length = 2.014 km
Expanded trace to show near end dead zone (NEDZ). Fluctuations in the trace after the NEDZ are due to the detection electronics (receiver). Strong Fresnel reflections can give rise to dead zones of the order of hundreds of meters corresponding to detector recovery periods of many tens of receiver time constant, whereas splices result in dead zones of only a few tens of meters.
Many OTDRs incorporate a dead zone masking feature, which can be set up, to selectively attenuate large incoming reflected signal pulses just for the period over which they arrive prevent deep saturation and hence minimum dead zone. Near end dead zone and event dead zones present greater problems in shorter networks. Large Fresnel reflected pulses from the terminations of shorter branches of a network are reflected again from the fiber input face to repeat the journey to the termination and back to the detector. If the signal strength is sufficiently high, these multiply reflected pulses will be detected and will appear on the OTDR trace as what are referred to as Ghosts. Ghosts are much smaller than the detected pulses from primary Fresnel reflections and appear at exact integer multiple distances relative to a primary reflected pulse.
Trace with a ghost after Far end dead zone Ghost signal is displayed at twice the fiber length. Can be confusing if appear within the maximum length of a network, mostly appear in the noise region.
Resolution & Event Location Spatial Resolution: One of the key performance features of an OTDR depends upon receiver BW and input pulse width Minimum separation at which two events can be distinguished is determined by the pulse width. For good resolution of 10m or less, require a 50ns pulse. Length of fiber; L = v g t ; v g = c/n e (n e - effective fiber index) Typical, n e for fibers 1.45-1.47 With above data, a pulse travel approximately 1m in 5ns; means a pulse width of 5ns in time has a spatial width of 1m. By definition, two events may be distinguished if they are separated by ½ of the spatial pulse width Spatial resolution of the instrument - is defined as half the pulse width in time. For pulse width of 50ns, spatial resolution is 5m.
Time Resolution: The precision to which a feature can be located depends on the precision with which the OTDR can measure the arrival time of an event and the accuracy to which the propagation velocity is known. Uncertainty in the measurement of arrival time can be very much less than the pulse width and hence event location in principle may be achieved with a precision which is much better than the spatial resolution. Timing resolution of the instrument is simply the sample period used in the signal averaging scheme under a given set of circumstances, and this corresponds to a spatial sample in the fiber which can be taken as the precision to which event distances can be measured. Sampling period and its corresponding sample length in space (range resolution) vary enormously depending on the distance range addressed and the number of samples used.
Typically, however, time resolution is much less than the spatial resolution. For example, if the spatial resolution is 10m (for a 50ns pulse) on a range setting of 4km, the event distance (range) resolution may be 2m corresponding to 2048 samples being used to record the return signal from the 4km path. With the home-in feature on some instruments can examine a 1km section of 4km range using 4096 samples to achieve a range resolution of 0.25m for a spatial resolution of 5m.
Dynamic Range, Range & Range/Resolution trade-off Obviously, the greater the energy of the launched pulses, the better the SNR will be before and after averaging. The peak power from the laser is limited, but the pulse width may be increased to deliver greater energy into the fiber at the expense of resolution. With wide, high energy pulses and the levels of averaging, the peak SNR at the beginning of a trace, referred to as the signal dynamic range of the instrument, can be as high as 35dB. Range maximum length of the fiber which can be measuredis determined by the signal dynamic range and is the fiber length at which the signal has decayed to become equal to the noise.
Example: In a system based on the fiber with an attenuation coefficient of 0.2 db/km and in which the total loss of the components, splices and connectors is 10dB, A good OTDR with a 35dB dynamic range can probe to range of 125 km. Point to remember: As we decrease the pulse width (energy) to improve the spatial resolution, the dynamic range will also be reduced and the range of the instrument will be diminished; There is a range/resolution trade-off to be considered using these instruments.
Technical Specifications of an OTDR
OTDR Trace Analysis of Networks OTDR traces are analyzed to provide information about the loss of a network & whether or not the loss is increasing with time due to introduction of faults (sharp bend, breaks) or the degradation of components (splices, couplers, WDMs etc.). Schematic of a network, spices are shown as dots.
Testing port 1: Fibers with a single splice. OTDR trace of fiber link with one splice
Testing of a Coupler spliced to fiber OTDR measurement of coupler losses OTDR trace of single splice and coupler
Testing Three component network (WDM, 2 couplers) OTDR trace of 3 component network Events Distance Near end 0 WDM 265 Coupler 1 443 Short arm end 587 Coupler 2 804 Short arm end 941 Far end 1089
Fault Diagnosis with OTDR Link Loss Measurements: If loss is higher than its limit, then OTDR testing is required to check the link health OTDR Testing : It will give graphical presentation with loss table of each splice Bi-directional Testing : We have to take OTDR traces from both ends
Distance Based Analysis Distance between A and B is 10 km. OTDR distance up to cut point C from A is 6 A C B 6.001 Km OTDR distance from point B to check, if it is 4 km a single cut. If OTDR distance from point B is less than 4 km a possibility of multi cut.
Distributed Sensing Measurand field M(z,t) OTDR M(t) M(z,t) Fiber z
Quasi-Distributed Sensing Measurand field M(z,t) OTDR M(t) M(z j,t) z Fiber Sensitized regions FBG (Fiber Bragg Grating) Strain, Temperature
Optical Network Analysis with OTDR
Bibliography: The excerpts of this lecture are based on information taken from following reference. 1. Gerd Keiser, Optical Fiber Communica tion 3 rd edn., Mc Graw Hill, 2000. 2. WWW.google.co.in 3. Optical Network Analysis Educator Kit (ED-NET), Instructor Manual, Optosci Limited, Glasgow, Scotland. 4. Djafar K. Mynbaev, Lowell L. Scheiner, Fiber-Optic Communication Technology, LPE, Pearson Education Asia, 2002.
If you have any query, feel free to contact : Dr. BC Choudhary, Professor NITTTR, Sector-26, Chandigarh-160019. Phones: : 0172-2791349, 2791351 Ext. 356, Cell: 09417521382 E-mail: < bakhshish@yahoo.com>