Bi-Directional OTDR Testing: Traditional and Automated

Bi-Directional OTDR Testing: First I would like to discuss about the technique of bi-directional, actually bi-directional analysis is the technique and this bi-directional technique is used to minimize the effect of back scattering coefficient differences along a optical fiber link, back scattering coefficient can turn to generate the wrong OTDR splice measurements. Bi-directional is used when accurate baseline data is required or during the acceptance testing of splice measurement.

By using bi-directional OTDR technique a complete commission of network can be accomplished. Without using bi-directional OTDR technique cannot be consider as a complete commission network and it cannot be accomplished, considering the difference in performance of the fiber section (main diameter, back scattering coefficient, etc.) on link. Bi-directional OTDR tests require two technicians, one at each end of the fiber.

Measurement process for bi-directional OTDR test

Bi-directional OTDR Measurements

What is traditional bi-directional OTDR Process

For optical fiber characterization, two technicians are required with two OTDR units, one at each end of the optical fiber link. Two connectors used during the bi-directional OTDR test, must to be measure the both connectors. Actually bi-directional OTDR test consist of two measurements on the same optical fiber, it will be possible by launching light into opposite ends of the optical fiber.

One technician at each end of the optical fiber and both end technician measure and store the result of all optical fibers. Then, the results of the two OTDRs have to compile in order to generate the acceptance report of traditional bi-directional OTDR. In the event of problems in traditional bi-directional OTDR, errors or incorrect results are usually only apparent by analyzing the averaged readings. If there is such a mistake in the compiling of acceptance report of traditional bi-directional OTDR test process, technicians go back and re-perform the OTDR data acquisition process. So it is not better for good performance.

Drawbacks of traditional bi-directional OTDR Process

➥It may happen incorrect or different OTDR settings on one of the two units.
➥Optical fiber continuity cannot check, two different optical fibers can be measure by technicians and generating incorrect bi-directional testing report.
➥A mismatch can be happen between the optical fiber number and the stored data, resulting from a file management error. This all drawbacks can be happen by the operators of OTDR.

Other drawbacks are Inefficient use of time during the process of traditional bi-directional OTDR. Meeting of both end point technicians is necessary in order to exchange the data. Cannot affix the real-time problem detection. The technicians must complete the test process at this date and they can return from the test sites the following day, may be additional costs required due to travel expenses and test setup. Lack of reliability of the result, inefficiency, and like this traditional bi-directional OTDR process can increase in the overall cost of the job.

Automated bi-directional OTDR Test

Automated bi-directional OTDR solves the all issues of traditional bi-directional OTDR analysis. Testing times depend upon the number of optical fiber. Number of optical fibers tested within a given period of time. The main and important point is that automated bi-directional OTDR testers having fully automatic tools.

Automated bi-directional OTDR test provides error-free operation by exchanging the master unit’s OTDR test configuration if it differs from the slave unit’s OTDR test configuration.

Automated bi-directional OTDR test to ensure the check of optical fiber continuity and both units are testing the same optical fiber. Data acquisition perform on the slave unit and trace result transfers the  to the master unit and also data acquisition on the master unit and transfers the trace to the slave unit.

Automated bi-directional OTDR test procedure is fully automated and all of the test results are immediately accessible on both units. Both ends perform automated acquisition and trace storage. Reducing the amount of time,required to a few seconds for the file transfer through the optical fiber. It create a bi-directional OTDR analysis report.

During the automated bi-directional OTDR test if two units are different a warning indicator may appear in this case platform unit transfer and applies the master configuration to the remote platform unit.

How to check fiber continuity by Automated bi-directional OTDR test ?

It is very simple that if master platform unit successfully detect the far end remote remote platform unit it means two units are connected to the same optical fiber and OTDR acquisition can be starts.

Acquisition Process by Automated bi-directional OTDR test

Data Acquisition

Bi-directional OTDR data acquisition

In automated bi-directional OTDR test file transfer is very easy and no need to meet to perform file transfer. It is very easy and simple,technician simply selects the files and transfer them through the optical fiber to the far end test unit. There are no need of modem and LAN connections.

For the file transfer optical fiber allow the permanent communication media for the technicians. Technicians can immediately reply about the test result if any incorrect measurement analysis occur in the report. By this way technician can save the time to re-visit the test site later.

After the completion of measurement the trace is transfer to the remote end & remote end trace transfer to the master unit. By this easy ways technicians can complete the bi-directional measurement. 

What trying to measure and Why do it

I have already explain above the attenuation measurement of the optical fiber link requires two connections along with the measurement of cabling under the test. A and B two points of the link. Kindly also measure the attenuation of both connectors which is used during the cable under test.

If you want to do a proper bi-directional OTDR test,if required only OTDR to be moved to the far end,never be removed connectors from the cable under the test. Move only OTDR.The OTDR test is done from both ends of the cabling, when it contains splices or connectors. Due to the change in backscattering coefficient any difference of component attenuation can be canceled by the average of the component attenuation measure taken from both ends of the system.

To accurately measure the first and last connection of the link using a two-directional average, the launch and tail cords are made in their initial measurement positions. It will ensure that the same optical fiber should be carried to the cabling under testing so that the effect of mode field mismatch between test cords and cabling can be average out . An individual attenuation is defined as half some of attenuation recorded from each end

When an OTDR sends a pulse of light from its optical port in the fiber under the test, the reflected light is obtained on the same port. Reflected light may be Rayleigh backscattering which defines the fiber attenuation of OTDR trace and / or Fresnel reflections, for example, the connector returns loss.

Rayleigh Backscattering is fundamental for OTDR operations, and OTDRs are used to measure end-to-end losses, and discrete losses on Splice and connectors. Backscatter coefficient is a measurement of the amount of backscatter when an OTDR emits one 1 Nano second pulse. This parameter is defined by all optical fiber vendors for their fiber specification. To make accurate measurements, the OTDR backscatter coefficient (such as -79 dB) should be set in OTDR. One things keep in your mind backscatter coefficients vary along a fiber and from one supplier to the other.

Conclusion

Bi-directional OTDR test provide the speed improvement of the fiber optic test and reliable test report. Traditional bi-directional OTDR analysis completely forgotten to take into consideration because technicians spend in their vans travelling from one end of a cable to the other and then back again to exchange data. Automated bi-directional fast OTDR acquisition and fast report generation by bringing speed-of-light data transfer through the fiber in order to fully automate the bi-directional OTDR acquisition process.