What is the minimum recommended length of single mode fiber for OTDR testing?

The OTDR sends a pulse of light energy (optical power), generated by a laser diode, into one end of an optical fiber. A photodiode measures the returning light energy or optical power (reflected and scattered back) over time and converts it into an electrical value that is sampled, amplified, and graphically displayed on a screen.

The location of each event and the overall cable length are calculated based on the round-trip time of the light pulse traveling through the fiber core. The insertion loss is calculated from the proportional amplitude change of the backscattered light.

Many modern OTDR tools automatically select the optimal acquisition parameters for a particular fiber by sending out test pulses in a process known as auto-configuration, auto setup, or auto test. Despite advanced technology that now allows many OTDR test systems to automatically determine the best settings for your test process, it is still important to understand what the underlying settings are and how they may impact your results

OTDR Testing Analogy

There are obvious comparisons between OTDR and the copper wire signal testing it has gradually replaced, as the communication networks have shifted to fiber optics. Another useful analogy can be found with Ultrasound technology. 

In medical imaging applications, high frequency (≥20KHz) inaudible sound waves are produced by the vibrating elements of an ultrasound transducer and reflected back to the source to create accurate images of bodily features. In much the same way, the reflected or scattered light waves from the OTDR allow the overall condition of the fiber core to be “seen”.

OTDR Test Terminology

Understanding the science behind OTDR begins with a few basic concepts that are important to the OTDR testing process.

OTDR Testing Process

Performing an OTDR test requires some basic setup, programming, test execution, and reporting processes to be followed.

• Power on the OTDR and verify the battery is charged and the test display is functioning.
• Clean and inspect the ends of all fibers under test, launch cables, connectors, and adapters.
• Carefully connect the launch cable to the output port of the OTDR at one end and the fiber under test at the opposite end.
• Select a pre-programmed test routine, based on the network type and test conditions, or set/adjust the test parameters appropriately. Manual OTDR test parameter settings typically include the following:
  • Range: Sets appropriate range (distance) based on the overall fiber length
  • Pulse Width: Sets the duration of each laser pulse emitted
  • Acquisition Time: Sets the time duration for averaging the measurements of reflected light
  • Refractive Index: Matches the index of the cable material being tested
  • Loss threshold settings for the system and individual elements or “events”
• Run the OTDR for the necessary time to acquire the test results and “trace”
• Store and/or upload test results as necessary
• Carefully disconnect all cables, connectors, and adapters

OTDR Best Practices

Before reference cables and the fiber under test are connected for measurement, fiber cleaning and inspection practices are of the upmost importance. Learn more about the VIAVI Inspect Before You Connect methodology on our Fiber Inspection page.

Mating connectors between launch cables, fiber under test, and OTDR must be compatible to minimize reflectance. Imagine a hose bib with a loose or crooked connection to the hose itself, causing water to leak and spray backwards from the junction. This is similar to the impact of an incorrect OTDR connection, when air gaps allow too much light to be reflected back and overload the photodiode.

The use of a receive cable at the far end of the fiber cabling run is another recommended best practice. These cables provide a “monument” that can help to accurately measure overall cable length and loss at the final connector of the run. Learn more about fiber characterization.

The best handheld OTDR test systems include features for one touch operation and interfaces tailored for different skill levels. The VIAVI SmartOTDR Handheld Fiber Tester delivers improved productivity with automated pass/fail test results.

Interpreting the OTDR Test Results

Once the OTDR test is completed, the system will display the OTDR results in both numeric and graphical formats. The graph, also called trace, will show where each connector/connection, splice, or break is located, along with the signal loss (in dB) and reflection characteristics of each element.

Advanced VIAVI OTDR test equipment with features like Smart Link Mapper (SLM) also translate this trace data into an iconic linear view where each element and event is represented as an easy-to-read icon, with pass/fail information visible immediately, and the name of each component/event clearly shown. This feature also provides customized workflows and icons for applications like FTTH, PON networks, or Fiber to the Antenna (FTTA).

The overall fiber length and link loss are displayed once a test run is completed. If loss thresholds were initially set, Pass or Fail will be indicated for each element of the cable run.

Types of OTDR Test Equipment

Although feature sets, size, and cost vary significantly, there are three main categories of OTDR test equipment available on the market today. 

• Benchtop
  This term typically describes the OTDR test equipment used in laboratories and production facilities. Benchtop devices can be placed on a laboratory workbench or in a production test bay, and usually have a larger display, more available expansion ports for applications like MPO testing, and a direct AC (outlet) power source. Benchtop OTDR test equipment may be used when a high level of accuracy, sensitivity or long-range measurement (with its inherent high power pulse intensity) is required.
• Hand-held OTDR
  As the name implies, hand-held OTDR test equipment is lightweight (less than 1kg), portable, typically battery-powered, and optimized for use in the field. The user interface is usually simple and straightforward so that technicians can be easily trained to operate the OTDR. Hand-held OTDR meters can also integrate additional tools needed for fiber certification and troubleshooting, such as Visual Fault Locators (VFL), Optical Power Meters (OPM), and compact fiber inspection microscopes. Connectivity options including Wi-Fi or Bluetooth can be used to communicate test results and work orders quickly.
• Embedded or Rack-Mounted OTDR
  Embedded OTDRs are designed and manufactured similar to many high-volume electronic products, with a small form factor that can be integrated easily within network monitoring equipment. Rack-mounted OTDRs are combined with an optical switch to automatically rotate across many fibers. A programmed test routine can prioritize critical fibers and important customers. These fiber monitoring applications can be used for either in-service or dark fiber monitoring.

OTDR Specifications

OTDR specifications are important to understand so one can choose the right OTDR for a dedicated application.

• Dynamic range
  Expressed in decibels (db), the dynamic range is defined as the difference between the initial power level reflected from the fiber when set to maximum pulse width and the upper level of the noise floor of the detector. The dynamic range determines the maximum observable length of a fiber.
• Event dead zone
  The event dead zone (EDZ) is the minimum distance that the OTDR can detect between two reflective events (typically two connections). In the case where the reflective events are more closely spaced than the EDZ, the OTDR will show them as one event.
• Attenuation dead zone
  The attenuation dead zone (ADZ) defined in the IEC 61745 standard is the minimum distance after a reflective (e.g. connector) or attenuation (e.g. splice) event, where a non-reflective event (splice) can be measured. Much like the EDZ, the ADZ is dependent on the pulse width.
• Wavelengths
  An OTDR sends a pulse of light based on to the wavelength(s) used for transmission when the fiber link is operational. The typical wavelengths are 850 nm and 1300 nm for multimode fiber and 1310 nm, 1550 nm and 1625 nm for single-mode fiber. Filtered 1625 nm or 1650 nm can be used for maintenance to avoid interference with the live traffic wavelength.

OTDR Manufacturers

By supporting the fastest-growing segment of the fiber testing market, OTDR equipment manufacturers have experienced ongoing expansion worldwide in all product categories, and this trend is expected to continue. 5G adoption is bringing new opportunities and challenges, with fiber monitoring, installation, and manufacturing operations all reacting to the ongoing demand. As an industry leading OTDR manufacturer, VIAVI is addressing the unprecedented customer requirements for OTDR safety, efficiency, and test product quality.

Calibrating OTDR Test Equipment

For all measurement equipment, periodic calibration is necessary to measure and correct equipment bias and reset functions based on reference standards. In industries where the accuracy of OTDR test results is essential, the IEC 61746 standard for calibration, as well as the TIA/EIA-455-226 standard (adopted from the IEC standard) are recognized.

The IEC standard includes specific practices for calibrating point-to-point accuracy, linearity, attenuation, power output and delay. Given the complexity, OTDR calibration is best left to OTDR equipment manufacturers or certified calibration labs.

The Future of OTDR Testing

Providing more functionality, accuracy, and resolution at a lower price point is an ongoing challenge. Improvement in OTDR auto test algorithms is continuing to lower the barrier of entry for technicians and increase acceptance. Improvements related to reflectance overload issues with short cable runs may help to expand the use of OTDR technology into new arenas.

Without technology such as OTDR testing, advanced application of fiber optics would not be feasible. The ability to “see” inside thousands of miles of optical fiber no thicker than a human hair has become both an incredible accomplishment and a practical necessity.

Over the next decade, new 5G networks carrying massive data loads, smart cities connected through communication networks, and the ongoing deployment of FTTH services will ratchet the industry demand for efficient, versatile, OTDR testing. With breakthrough OTDR innovations like Smart Link Mapper and Smart Acquisition making testing easier, more accurate, and more powerful, VIAVI is addressing the fiber installation and maintenance needs of the future.

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