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Optical Time Domain Reflectometer

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OTDR- animated glossary of Fiber Optics

OTDR Setup

What you need to know about OTDR Trace Analysis and Interpretation

Using An OTDR – Lecture by FOA

The optical time-domain reflectometer (OTDR) is the most informative tool for evaluating fiber-optic cables and links. It provides insight into the loss and reflectance of interconnections and splices, determines the attenuation rate of optical fibers, and helps locate faults. There are two types of OTDRs: the long-haul OTDR, designed for use with cable lengths in telecom systems (50m to several kilometers), and the high-resolution OTDR, designed for short cable segments like those found in aircraft or ships and for longer telecom applications. The high-resolution OTDR also evaluates optical sensors. The use of high-resolution optical reflectometry to study and map the optical link is being developed by SAE International’s AS-3 Fiber Optics and Applied Photonics Committee as described in AIR6552/2. However, the OTDR is a complex device with many variables that must be correctly programmed before testing. Proper OTDR setup and cable preparation guarantee accurate test results.

Working Principle of the OTDR
An Optical Time-Domain Reflectometer (OTDR) is a device used to measure the performance of fiber-optic links or cables. Unlike other testing equipment, the OTDR offers a graphical representation of what’s happening in the optical fiber being tested. It works by sending a pulse of light into one end of the fiber and then measuring the amount of light energy that is reflected back.

The reflection of light in an optical fiber is caused by changes in the refractive index, which result in Fresnel reflections. Backscatter, also known as Rayleigh scattering, is caused by variations in the density and composition of the optical fiber. The OTDR is able to show how light passes through each segment of the fiber-optic link.

OTDRs come in various sizes and shapes, ranging from pocket-sized models to larger ones that require a shoulder strap. Some OTDRs are equipped with a screen while others are controlled through a PC via USB. They are typically battery-powered because they are often used in locations where there is no electrical power available. Some OTDRs can test both multimode and single-mode optical fibers and can hold up to three light source modules, with some even featuring an integrated Visual Fault Locator (VFL).

The basic components of an OTDR include a directional coupler, laser, timing circuit, single-board computer, digital signal processor (DSP), analog-to-digital converter, sample-and-hold circuit, and avalanche photodiode. The avalanche photodiode converts light energy into electrical energy, which is then processed by the DSP and sent to the single-board computer for storage and display. The timing circuit is responsible for coordinating this process during a single test.

To ensure accurate results, the refractive index of the optical fiber being tested must be known and entered into the OTDR. This information can be obtained from the manufacturer. The OTDR samples light energy from the optical fiber at specific intervals, and each sample represents the round-trip time for light energy in the optical fiber.

OTDR Set up
etting up the OTDR correctly is crucial for obtaining accurate test results. The process involves selecting the appropriate fiber type, wavelength(s), range and resolution, pulse width, averages, refractive index, thresholds, and backscatter coefficient. This setup may take a few minutes, but it guarantees the most accurate results. There are various OTDR models available in the market, and the setup process may vary for each. This article focuses on general setup parameters for both long-haul and high-resolution OTDRs. Note that some models may have extra parameters or lack some of the parameters discussed.

Fiber Type
Long-haul and high-resolution OTDRs come with light source modules designed for specific fiber types. It’s important to make sure that the OTDR you are using has the correct module for the fiber you want to test. For example, a multimode module shouldn’t be used to test a single-mode fiber, and vice versa.

The wavelengths that a long-haul or high-resolution OTDR can test depend on the light source module(s) it comes with. Some modules have a single laser, while others come with two different wavelength lasers. Having two lasers enables testing of the fiber at two wavelengths without the need to disconnect the cable being tested, thus simplifying the testing process and reducing testing time.

Range and Resolution
The distance range and the distance between data points on an unzoomed trace displayed on an OTDR is determined by the range and resolution parameters. As a general guideline, the OTDR range should be set to 1.5 times the length of the fiber-optic link. Setting the range too short may result in unpredictable results and an incomplete display of the link. On the other hand, if the range is set too long, the trace will occupy only a small portion of the display area. Increasing the range will increase the distance between data points, reducing the resolution. It’s best to choose the first range value that exceeds the length of your fiber-optic link. For example, selecting a 2km range on a longhaul OTDR for a 1.3km link will produce more accurate results than selecting a 20km range.

Pulse Width
The pulse width determines the size of the “dead zone” and the maximum length of optical fiber that can be tested. A short pulse width creates a small dead zone but limits the length of fiber that can be tested. The pulse width should be set such that the trace doesn’t disappear into the noise floor. If set correctly, the trace should remain smooth until the end of the fiber-optic link. If the pulse width is too wide, events may be lost in the dead zone.

When setting the averages parameter on an OTDR, choose the number of averages that result in a smooth trace in the shortest amount of time. Taking too few averages will result in a noisy trace due to a high noise floor. On the other hand, taking too many averages will produce a smooth trace but waste valuable testing time.

Refractive Index
The manufacturer should specify the refractive index or group index of refraction for the optical fiber. The refractive index of similar optical fibers doesn’t vary much between manufacturers, with most values being within 1% of each other.

Thresholds can be adjusted for various factors such as end of fiber, event loss, and reflectance. Most OTDRs come with preset default values, but these may not be appropriate for every testing scenario. It is important to set the thresholds carefully, as this will impact the accuracy of the event table generated by the OTDR. To ensure that a majority of events are captured, it is recommended to set the thresholds to a sensitive setting.

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The Huihongfiber Optical Time-Domain Reflectometer is an all-in-one device that features automatic OTDR, expert OTDR, event mapping, an optical power meter, a visual fault locator, a power-adjustable stable light source, optical fiber end face detection, optical loss testing, network line length/sequence testing, network line searching, and more. With its double touch screen and button design, this device is user-friendly and offers a simple, straightforward operation, making it the perfect assistant for optical cable construction, installation, maintenance, project acceptance, and on-site repairs. Additionally, Huihongfiber also offers a mini and cost-effective OTDR that is capable of quickly analyzing optical fiber links and faults, accurately detecting the location and type of faults in optical fibers and cables. This device is widely used in the engineering construction, maintenance testing, and emergency repair of optical fiber communication systems, as well as in the development and production testing of optical fibers and cables.