Principles Of an OTDR (Optical Time Domain Reflectometer)

Principles Of an OTDR: OTDR, the full name of which is "Optical Time Domain Reflectometer". The OTDR depends on two types of optical phenomena: Rayleigh Backscattering and Fresnel Reflections: 1.Rayleigh scattering is intrinsic to the fiber material itself and is present along the entire length of the fiber. 2.Fresnel reflections are "point" events and occur only where the fiber comes in contact with air or another media such as at a mechanical connection/splice or joint. OTDR tester are widely used for optical cable maintenance and construction, and it can be used for the evaluating the fiber cable length, measuring optical transmission and connection attenuation, detecting the fault location of the fiber links, etc.

An OTDR is a fiber optic tester characterizing fibers and optical Networks.The aim of this instrument is to detect,locate and measure events at any location in the fiber optic link.An OTDR can test a fiber from only one end,that is it operates as a one dimensional Radar System.The OTDR technique produces geographic information with regard to localized loss and reflective events providing a pictorial and permanent record which may be used as a permanent baseline.The OTDR’s ability to characterize a fiber is based on detecting small signals returned to OTDR in response to the injection of a large signal.

OTDR Image

Principles Of an OTDR

Principles Of an OTDR (Optical Time Domain Reflectometer)

During the process of OTDR testing, the instrument inject a higher power laser or fiber optic light source pulse into a fiber from one end of the fiber cable, at the OTDR port to receive the return information.

OTDR depends on two types of Optical Phenomena:

1. Rayleigh Backscattering: Rayleigh scattering is intrinsic to the fiber material itself and is present all along the length of fiber.Discontinuities in the back scatter light can be used to identify anomalies in transmission along the length of fiber.

2. Fresnel Reflections: Fresnel reflections are only point events.Fresnel reflections occur only where the light comes in contact with air or any other media with different refractive index.

OTDR use Rayleigh scatting and Fresnel reflection to characterize fibers’ characteristics. Rayleigh scattering refers to the irregular scattering generated when the optical signals transmitting in the fiber. OTDR only measure the scattered light back on the OTDR port. The backscatter signal show the attenuation degree (loss/distance) of the optical fiber, and will be tracked as a downward curve, illustrating the power of backscatter is decreasing, this is because that both transmission signal and backscatter loss are attenuated.

Fresnel reflection is discrete reflection, which is caused by the individual point of the whole fibers. These points are caused by a change in reverse coefficient elements such as glass and the air gap. At these points, there will be a strong backscattering light reflected back. Therefore, OTDR is using the information of Fresnel reflection to locate the connection point, fiber optic terminal or breakpoints.

When the optical pulse is transmitted through the fiber, due to the nature of the fiber itself, the connector, the engagement points, bending or other similar event, there will be a scattered reflection. Part of the scattering and reflection will return to the OTDR. Useful information returned will be measured by the OTDR detector, and act as the time or curve segments of fibers at different positions. By recording the time used of the signals from transmission to returning, the transmission speed of the light in the glass fibers, the distance can be calculated.

OTDR testing has some limitation when it come to the applications for measuring the outside able plant loss. The OTDR tester will not be always sufficiently for testing. The OTDR will not work well with short cables in a building or LAN environment. The source and power meter should be used for these tasks as a result of the OTDR is not equipped to show actual cable plant loss.

OTDR Block Diagram

Block diagram

Block Diagram of OTDR

Light from the source is coupled to the fiber using a coupling device Light from the source is coupled to the fiber using a coupling device.Light signal back reflected from all points along fiber length is coupled to the photodiode using a coupler. A pulse generator controls the LASER DIODE which sends powerful light pulses to the fiber. These pulses can have a width in the order of 2ns upto 20msec and a reoccurrence of some KHz. The duration of the pulse (pulse width) can be selected by the operator for different measuring conditions. The repetition rate of the pulses is limited to the rate at which the pulse return is completed, before another pulse is launched. The light goes through the coupler/splitter and into the fiber under test.

The OTDR measures the time difference between the outgoing pulse and the incoming backscattered pulses hence the word "time domain". The power level of the backscattered signal and the reflected signal is sampled over time. Each measured sample is called an "acquisition point" and these points can be plotted on an amplitude scale with respect to time relative to timing of the launch pulse. It then converts this time domain information into distance based on the user entered index of refraction of the fiber. The index of refraction entered by the user is inversely proportional to the velocity of propagation of light in the fiber. The OTDR uses this data to convert time to distance on the OTDR display and divide this value by two to take the round trip (or two way) into account. If the user entered refractive index is incorrect or inaccurate, the resulting distances displayed by the OTDR can be in error.

Laser diodes: Laser diodes are selected according to the wavelength of the test.

Pulse generator with laser diode: A pulse generator controls a laser diode which sends powerful light pulses (from 10 mW to 1 Watt) into the fiber.

Photodiode: OTDR photodiodes are especially designed to measure the extremely low levels of backscattered light, at 0.0001% of what is sent by the laser diode.

Time base and control unit: The control unit is the brain of the OTDR. It takes all the acquisition points, performs the averaging, plots them as a log. function of time and then displays the resulting trace on the OTDR screen. The time base controls the pulse width, the spacing between subsequent pulses and the signal sampling.

Please check the details of OTDR trace.

OTDR Trace

OTDR Traces details

OTDR Specifications

Dynamic range:The dynamic range is one of the most important characteristics of an OTDR, since it determines the maximum observable length of a fiber and therefore the OTDR suitability for analyzing any particular network.
Dead Zone:The length of fiber which is not fully characterized during the recovery period after an event is termed the dead zone.
Resolution
Accuracy
Wavelength

Conclusion

With the help of OTDR transmission testing perform End-to-end optical link loss. Rate of attenuation per unit length. Attenuation contribution to splices, connectors, couplers (events). Length of fiber or distance to an event. Linearity of fiber loss per unit length (attenuation discontinuities). Reflectance or optical return loss.