The principle of OTDR is the same as TDR used for copper telephone cables. In TDR a pulse is sent which is reflected back. The receiver will measure the power level of the reflected pulse with time. When we used to implement coaxial cable projects connecting towns and countries, we used to test the quality of the work especially cable joints by TDRs. OTDRs contain lasers with output pulse power of about 0 to -10dBm. The width of the pulse is usually between 10 to 5,000 ns.The wavelengths used are usually of 1310, 1490, 1550, 1625 and 1650 with dynamic range of 36, 37, 40, 41, 43, 45 dB. Some times during installation the cable is stressed physically or bent too much, the OTDR will show that. During installation, the main reason to test the fibers by OTDRs is see whether the joints loss are within about 0.04 dB or less at 1550 nm and there is no bending.

In addition to what Mahmud has stated above, and thank you for this explication, the time delay and the dispersion chromatic can also be calculated thanks to the lasers with different wavelengths (4 to 8 wavelengths). For example, TD and D of the dispersion-shifted single-mode fiber are:

Time Delay = Aλ2 + Bλ−2 + C + Dλ-4 + Eλ4

Dispersion = 2(Aλ - Bλ-3 - 2Dλ-5 + 2Eλ3)

You can also watch this video, the EXFO company explains by animation the OTDR equipments :

http://www.youtube.com/watch?v=k10fMtcVBus

Thank you for the explanation, help me a lot..

But, I want to ask again :

1. About the reflection, can someone explain to me with a picture, how can the light reflected back if there are two or more connector? Because as I know, the reflected pulse only will occur if the pulse hit a connector. And if there are two or more connector, the reflected pulse from the second connector will hit the first connector again and it will cause a reflected pulse back to the second connector. It will cause losses at the reflected pulse from the second connector which will affect the receiving power at the OTDR.

2. Is there any formula to calculate bending loss?

Suppose there are 10 splices on the way, the reflected signal from the first splice or joint is shown by the OTGR, the reflected signal from the second splice is also shown by the OTDR, but the reflected signal from the second splice in passing through the first connector is much weaker such that if the OTDR is very sensitive, it can be shown but because the signal is very weak, so you see that very clearly with no ambiguity. If an optical fiber of 150 m long is made out of three pieces each of say 50 m, the OTDR will show that very clearly.

Bending loss depends on the radius of the bend. Each fiber has a certain minimum radius, less than that, the fiber will have a high loss. The loss is due to skew rays

which go out of the core of the fiber. I suggest to go through the following book

Fiber-Optic Communication Systems, Fourth Edition

by Govind P. Agrawal

Copyright © 2010 John Wiley & Sons, Inc.

page 27.

I see that in the Fiber-Optic Communication Systems, Fourth Edition by Govind P. Agrawal, the bending loss is micro bending loss. How about the macro bending loss?

And how can OTDR know the End of Fiber (at OTDR's trace, the bottom left side)?

OTDR knows the end of fiber from the latest back reflecting pulse. If the fiber is terminated with a regular flat surface connector such as ST, SC, FC / PC, then there will be a good amount of back reflection from this farthest discontinuity in the fiber. This back reflection is mostly due to index mismatch between fiber and air. Mind you, if this end is matched as in the 50 ohm loading for RF transmission systems you will not get any back reflection from this point. Matching is done by a) connecting an isolator, b) terminating fiber with angled connector to minimize back reflection, c) dipping the connector end in index matching gel, which looks like liquid soap. But a high quality OTDR can still discern this kind of discontinuity, because from this point forward there will not be any Rayleigh back reflections, which is a major contributor to the fiber attenuation. That is; as light travels in smf there will be a small but continuous back reflection even for an infinite length of fiber.

Also if I may add on the multiple events in fiber; there will be many - in theory infinite number- of back and forth reflections between multiple splices, stresses, connectors etc. which will appear as attenuating farther and farther events. Again a good OTDR distinguishes these ghost events from the real ones.

I need to check my notes bu I remember micro bending loss as the loss due to slight perturbations for a flat lying fiber, and the macro loss being due to coiling the fiber in tighter than a 20-30 cm diameter winding.

Presentation of Asaf Sahin, is really very good. For your information connectors loss whether they are ST, SC , FC is about 2-3 dB while a splicing loss is about 0.04 dB.

In long haul optical fiber networks, connectors are not used. They are only used in access networks.

Thank you so much for your answer Asaf Sahin and Mahmud Wasfi.. It helps me a lot.. ^^

Just want to ask again about the Fresnel Reflection that used by OTDR..

How to calculate the loss caused by Fresnel Reflection? Because in Fiber Optics Communications, Second Edition by Joseph C. Palais, the Reflection Coefficient is based by the refractive index of the core fiber and the air. And since the gap between 1st_fiber-1st_connector-2nd_connector-2nd_fiber is air, we'll get the same result as the terminating point.

May I add more information: Bending loss is proportional to:

exp(-R/Rc) where R is the radius of fiber curvature and

Rc=a (n1 to power of 2 - n2 to power 2). For example for single mode fibers

Rc= 0.2 to 0.4.

I agree with you stevan about Fresnel reflection, the reflection ratio is R=((n1-n2)/(n1+n2))^2, then we can calculate the reflection loss by L(dB)=-10*log10(1-R). This equation is used when we talk about the fiber connections. When we talk about the termination point, you can select this point or not in your OTDR screen for calculating the fiber loss.

About the connectors loss, the loss are less than 0.2dB per connector when we talk about FC/PC, FC/APC, E2000,... I agree with Mahmud, we use the connectors only in access networks when we talk about optical telecommunications because 0.2dB is not so much because we use a weak power in access networks, and also because we use the switchers.

@Mahmud : What is the unit for R and Rc,, because as I know, the radius of a singlemode FO is about 8-10 um.

@Ibrahim : I'm planning to make a simulation about how an OTDR works and how can it measures the loss that happen in FO. So that I need to know the difference between the connector point and the terminating point.

And as I know, the connector is used to join the FO to the device and the splice is used to join the FO with another FO.

Thank you.

Just for information, the connector is the same as the splice, we use the connector because it easier to switch between the equipements, but the loss is higher than splice. In the experience, we use the connectors for connecting FO with FO but if we need to optimise the measurements we use the splice of FO. Generally "the splice" or "the connector" is used for connecting two pieces of FO.

After the last point or the terminating point , you haven't any propagation (that mean, you haven't any collision between the laser and silica, any reflection after this point), and the loss is higher. But when we talk about the connections, you have n1(FO), n2(air) and n3(FO) in this case you have the propagation of the ligth in the second piece of FO.

Why is there a big difference of loss between using a splice and using a connector?

Losses in a fiber connection fall into two categories:

1. Intrinsic losses are caused by different numerical apertures, diameters or index profiles of the fibers, Fresnel reflection losses.

At each glass-air interface 4 percent of the optical power is reflected, which results in a loss of 0.14 dB. Intrinsic loss can be avoided by matching parameters of the fibers.

2. External losses are caused by the mechanical misalignment of fibers with identical parameters; these losses are minimized by aligning the end-faces precisely

You can see this link for understanding connector loss mechanisms

http://www.thefoa.org/tech/ref/basic/term.html

@Volodymyr Fesenko : I'm still don't get the reason why is there a big difference between using a splice and a connector..