What is OTDR?
OTDR stands for Optical Time-Domain Reflectometer. It is an optoelectronic instrument that is used to specify the character of an optical fiber. A series of light pulses are injected into the optical fiber under test and light is extracted from the same end of the optical fiber. This light is either scattered (Rayleigh backscattered) or reflected along with the fiber. The characterize of the optical fiber is defined by the scattered or reflected light. Electronic time-domain meters measure reflections in the same way due to changes in cable impedance. The return light pulse strength is determined and integrated over time, then plotted as the fibre length.
OTDR Working Principle
Compared to sources and power meters that directly measure fiber loss, OTDRs work indirectly. By replicating the transmitter and receiver of a fiber optic transmission line, the source and meter provide a good correlation between measurements and actual system losses. However, OTDRs use a unique optical phenomenon called “light backscatter” to measure with reflected light.
The instrument pulses a stronger laser or fiber optic light source from one end of the fiber into the fiber during OTDR testing. A cable with an OTDR port to receive the returned information. As light pulses are sent through the fiber, some scattered reflections are returned to the OTDR.
The Working Characteristics of OTDR
To measure the characteristics of fibers, OTDR employs Rayleigh scattering and Fresnel reflections. Rayleigh scattering is the irregular scattering that occurs when optical signals travel through a fiber. Only the scattered light back on the OTDR port is measured by OTDR. To demonstrate the power of backscatter reduction, the backscatter signal represents the degree of attenuation (loss/distance) of the optical fiber and will be displayed as a declining curve.
The power of Rayleigh scattering is proportional to the wavelength of the transmitted signal: the shorter the wavelength, the greater the power, so the backscatter loss generated by the 1310nm signal’s trajectory will be greater than that of 1550nm signals.
At higher wavelengths (above 1500 nm), Rayleigh scattering continues to decrease, while another phenomenon called infrared attenuation (or absorption) increases, possibly increasing the overall attenuation value. Therefore, the 1550 nm wavelength shows the lowest attenuation, which explains why it is a long-distance communication wavelength.
Fresnel reflections are discrete reflections that occur at individual points throughout the optical fiber. These points are the result of changes in mutual factors such as glass and voids. A strong backlight is reflected at this point. As a result, OTDR makes use of Fresnel reflection data to determine connection sites, fiber terminals, and breakpoints.
OTDR Testing
With the fast improvements in the fiber optic era and new fiber community deployments, OTDR testing techniques have to end up integral for building, certifying, preserving, and troubleshooting fiber optic structures.
An OTDR (Optical Time-domain Reflectometer) is a device that is designed to produce the route of fiber optic cable in digital format. The analyzed information can provide statistics on the situation and overall performance of the fibers, as well as any passive optical additives alongside the cable direction like connectors, splices, splitters, and multiplexers.
once this data has been captured, analyzed, and saved, it may be recalled as needed to examine the equal cable through the years.
Troubleshoot Fiber Optic Cable Failures with OTDRs
Only OTDR can troubleshoot fiber optic cable faults by calculating the fault distance and determining the type and cause of faults such as bends, breaks, bad connectors, and excessive insertion loss “events”.This method uses Rayleigh OTDR measurements and can perform unidirectional (unidirectional) testing, but bidirectional OTDR testing improves test accuracy.
Hand-held OTDR
Rack-mounted OTDR
OTDR Form Factors
For continuous monitoring of the network, we can place the OTDR device either portable or rack-mounted and it alerts automatically if any damages happen to the fiber.
Predictive OTDR Measurements
In addition to the Rayleigh scattering method used to characterize fiber optic links, Raman and Brillouin OTDR technologies can also be used for fault prediction, fiber health monitoring, and translation loss prevention. These three techniques form a powerful combination for managing fiber optic networks or using fiber for distributed fiber optic sensing.
Originally designed for long-distance fiber-optic networks, next-generation OTDR tools can also be used to diagnose much shorter cables such as aircraft interior cables and enterprise objects such as structured cables. OTDR multi-pulse data acquisition technology was developed to test more complex configurations, including PON and Fiber to Home (FTTH).
When to use an OTDR?
The OTDR is a piece of test equipment that is very essential to get the most accurate as well as complete end-to-end link validation. The OTDR can find various potential faults such as macro bends, breaks that affect the performance of a network by using a light source and power test method.
OTDR testing terminology
Dynamic Range: From the backscattering level at the OTDR port down to a given noise level, an OTDR’s maximum optical loss can be calculated.
Event Dead Zone: The minimum distance after a Fresnel bounce at which the OTDR can detect another event. That is, the minimum fiber length required between two reflections.
Pulse Width: The time the laser was turned on. Since time is converted to distance, pulse width has length.
What to look for in an OTDR?
Reliability and accuracy: The investment in OTDR is very important for ensuring the reliability of fiber optic networks. Make sure the fiber foundation can support the latest technologies such as 5G and IoT services. The use of low-grade OTDR may lead to higher risk as well as high costs eventually.
Easy to use: Take a look at our automated testing process and result from analysis, given the complexity of OTDR testing. Intelligent diagnostics and user-friendly visual displays can assist field personnel to enhance efficiency while requiring little training.
Ruggedness and compact form-factor: As fiber density grows, OTDR usage will increase in the field, data centers, and even near client residences. Look for ODTRs that are both functional and long-lasting.
Testing a Fiber Optic Cable
This test will obtain a trace of a single-mode or multimode fiber optic cable plant, including the loss of all-fiber, splices, and connectors.
Equipment needed to perform this test
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OTDR of fiber to be tested (multimode: 850 and/or 1300 nm, single-mode: 1310, 1550 and/or 1625 nm)
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Use the same fiber type and size as the cable plant for launch and reference and need connectors compatible with the reference cables
Notes:
a. You only need a launch reference cable if you’re merely checking for length. The receive cable allows you to determine the loss of the cable’s end connector.
b. Need a long enough reference connection to allow the OTDR’s initial test pulse to return to the base.
c. Launch and receive cable connectors must be in good condition (low loss) in order to adequately test connectors on the cable under test.
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Mating adapters that are compatible with connectors
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Cleaning materials
Test Procedure
Step 1:- Start the OTDR and give it some time to warm up.
Step 2:- Thoroughly clean all connectors and adapters.
Step 3:- Connect the launch cable to the OTDR. Connect the receiving cable (if utilized) to the far end of the cable.
**Step 4:-**Configure the test parameters on the OTDR.
Step 5:- Connect the wire to test to the end of the launch cable. Connect the receiving cable (if utilized) to the far end of the cable.
Step 6:- Obtain a trace.
OTDR Manufacturer and Model No.s
| Manufacturers | Models |
|---|---|
| Anritsu | MT9085, MT1000A, MW90010A |
| Yokogawa Electric | AQ1000, AQ1210, AQ7280, AQ7933, AQ7922,AQ7277B |
| Tarluz | OT600, OT700, OTDR EXFO AXS-110, OFR300 |
| GAOTek | OTDR 116,OTDR 102, OTDR 114 |
| Techwin | TW2100E |
| ORIENTEK | K720, K70, K330 mini |
| Xingcheng | AOR-500 |