Common Ways to Test Optical Fiber Cable

As the popularity of bandwidth-intensive applications has increased continuously, demand for fiber optic installations and infrastructures has accelerated parallelly. Optical fiber cable thus has possessed a rather essential position in telecom industry. However, the testing of optical fiber cable is often considered to be one of the most confusing and misunderstood phases of installing a fiber optic system, meanwhile it is also one of the final and most important procedures in installing optical network. Then how to deliver valid optical fiber cable testing? Here, we introduce you three most common methods.

Why Optical Fiber Cable Testing Matters

Let’s start by talking about the importance of optical fiber cable testing. Proper testing of optical fiber cable increases the system’s longevity, minimizes system downtime, reduces maintenance needs, and supports system upgrades and reconfigurations. And all these contribute significantly to your network performance, reliability and manageability in the long run.

Optical Fiber Cable Testing Methods

Fiber optic cable is tested to ensure continuity and attenuation. Basically, there are three test methods commonly performed for optical fiber: visible light source, power meter and light source (one jumper method), and optical time domain reflectometer (OTDR).

Visible Light Source Testing

Visible light source tests optical fiber continuity. Optical fiber communication systems operate in the infrared region of the electromagnetic spectrum which is invisible to the human eye. However, (red) visible light sources are available for testing and troubleshooting optical fiber systems. They are also referred to as visual fault locators and visual fault finders.

When testing optical fiber cable with a visible light source, you could follow the suggested procedures:

Step 1. Connect the optical fiber flashlight to one end of a fiber strand (with most units, the fiber must be terminated).

Step 2. Look at the opposite end. (Notice: Be careful not to look directly at active optical fiber strands. Laser light sources can cause serious eye damage.)

Step 3. If the light is not visible at the opposite end, a break or other problem is likely present somewhere along the length of the fiber. In many instances, the fault location will glow red from the light of the visible light source.

Step 4. Document the test result information.

Power Meter and Light Source Testing

Power meter and light source testing, also known as the one jumper method, is the most accurate way to measure end-to-end signal loss of the fiber, referred to as attenuation. Listed below are TIA/EIA- 568 insertion loss limits for the various components. Specific installations or protocols may impose stricter limits.

Loss budget (TIA/EIA specification limits)

Element	Insertion Loss
Splice	< 0.3 dB at all wavelengths
Connector Pair	< 0.75 dB at all wavelengths

Test results should be compared to the link attenuation allowance calculated as follows:

Link Attenuation Allowance (dB) = Cable Attenuation Allowance (dB) + Connector Insertion Loss Allowance (dB) + Splice Insertion Loss Allowance (dB)

When testing optical fiber cable with power meter and light source, perform the following steps.

Step 1. Disconnect active equipment.

Step 2. Acquire suitable light source for the single mode (generally 1310 nm or 1550 nm), multimode (850 nm or 1300 nm), and power meter.

Step 3. Verify proper wavelength to set source and meter. (Note: Calibration of the equipment is required before each test. Follow the equipment manufacturer’s procedures.)

Step 4. Acquire accurate test jumpers and couplers, which should be part of the light source and power meter kit.

Step 5. Connect the jumper (containing the same fiber size as the system fiber) to the optical source and the optical power meter. Turn unit on. Record the reference power reading (Pref), displayed in dBm.

Step 6. By applying an adapter, insert a second jumper (Test jumper 2) between the jumper used in Step 5 and the optical power meter. Verify the attenuation added by the second jumper is not greater than 0.75 dB: Pref-Pcheck ≤ 0.75 dB.

Step 7. Attach the jumpers to the optical source and optical power meter. Disconnect the two jumpers at the adapter. Connect the optical source/Test jumper 1 to one end of the system fiber to be tested. Connect the optical power meter/Test jumper 2 to the other end of the system fiber. Document the test power (Ptest). Subtract the test power (Ptest) from the reference power (Pref), recorded in Step 5, to conclude the end-to-end attenuation: Attenuation (dB) = Pref-Ptest.

Step 8. Document the test results.

Optical Time Domain Reflectometer (OTDR) Testing

Optical time domain reflectometer (OTDR) measures the fiber cable length, attenuation, and “events” along the length of the fiber. Here, the events can be splices, breaks, or stress points that cause excessive attenuation. The OTDR does this by sending light pulses down the cable and measuring the timing and power of light reflected back to the OTDR by the events and the fiber itself. It uses this information to display a “trace”, which is a graph of power versus distance.

An OTDR only requires access to one end of a fiber for testing. Because an OTDR is an indirect measurement method, it is not as accurate as a light source and power meter for measuring attenuation. However, due to its ability to display a graph of the fiber, it is particularly useful in troubleshooting. Like a power meter and light source, an OTDR tests at specific wavelengths (generally 1310 nm and/or 1550 nm for single mode and 850 nm or 1300 nm for multimode).

Conclusion

Appropriate understanding of the testing methods and referencing procedures plays a critical role in testing accuracy for both legacy and future systems. Among these three methods mentioned in the article, which one to choose actually depends on your specific needs and real circumstances. Hope what we discussed above could fix your problems and assist you to deliver better optical fiber testing.

First published: http://www.fiber-optic-solutions.com/common-ways-test-optical-fiber-cable.html

Visually Locating Fiber Loss With VFL

As fiber links support higher speed network bandwidths with increasingly stringent requirements, it is becoming all the more important to ensure that your backbone links meet tightening loss standards. As network applications grow and expand, the need for higher data transmission capacity continues to grow as well. To guarantee reliable and efficient network connectivity and data transfer, the testing tool holds a key position in reducing the time spent identifying and locating the fiber loss. In this article, we will introduce an useful tool—visual fault locator (VFL) to achieve quicker, easier and more efficient fiber loss identification.

Introduction to VFL

VFL, also known as visual fault locator, serves as a fiber optic testing device that is widely used to locate the breakpoint, bending or cracking of the fiber glass. It can also locate the fault of OTDR dead zone and make fiber identification from one end to the other end. Designed with FC, SC and ST universal adapter, this fiber fault locator can be used without any other additional fiber adapters to locate fault up to 10 km in fiber cable. Featured by compact size, low weight and red laser output, the VFL is widely adopted to visually locate loss locations on fiber links and can also be employed to confirm fiber continuity.

Generally, there exist two types of commonly used VFLs : pen shape visual fault locator and hand-held visual fault locator, the following picture shows the outlook of them.

How Does VFL Work?

Since the light involved in transmitting signals over fiber optic is usually at 1300nm to 1650nm wavelength which is invisible to human, we can barely see it with our naked eyes. However, by injecting powerful visible light at 360nm to 670nm wavelength to the fiber, VFL helps to visually and directly locate the faults in fiber optic cable. This visible light keeps traveling along the core until it reaches a fault, then it leaks out, which can be seen through plastic coating and jackets. Thus, we can visually locate loss locations be it a macrobend, faulty connectors or a poor splice. In addition, VFL also helps to cover the range where optical time-domain reflectometers (OTDRs) are not useful because of the dead zone of the OTDR.

Application of VFL

VFL is an ideal tool for locating defects that occur at connection and around fiber cabinets which are hidden in an OTDR “blind-spot” or “dead-zone”. Fiber breaks, faulty connector, sharp bends, bad splices and similar faults can be visually located by VFL. Visual fault locator can boost productivity in the field by providing fast detection, precise fault location, distance, loss, and ORL measurements.

Reference Guide to Use VFL

Network environments sometimes are too complicated for technicians to find the fault location, which makes visual fault locator a vital and indispensable tool for fixing the problem timely and precisely. The VFL is also used for conducting continuous tests and performing fiber identification. With visual fault locator, you can easily isolate high losses and faults in optical fiber cables. Here we offer you step-by-step procedures on how to use a VFL.

• Step 1: Remove the plastic connector covers from both ends of the test fiber cable.
• Step 2: Connect the fiber optic visual fault locator to one end of the fiber. Press the tester button and observe that light emanates from the other end of the fiber. This gives a simple indication of the continuity of the fiber link.
• Step 3: Repeat with several other fibers. Check for light that can be seen leaking from a faulty splice. This may illustrate an easy way of carrying out visual fault finding on bad splices or joints.
• Step 4: Disconnect all equipment, put the plastic covers back on the connector ends and return everything to the state it was before you started the practical, so the next group can carry out the practical in full.

What should be addressed is that during the testing process, you should never look into the output of VFL directly. After finishing the whole procedure, and remember to cover the VFL’s output with the dust cap.

Conclusion

Visual fault locator provides us a simple and convenient way to quickly locate faults in fiber optic cable, which is proved to be time saving and economical as well. Besides, it also alleviates the problems and pressures when dealing with massive fiber optic cabling system. Hope this article would help you get a better understanding of visual fault locator.

First published: http://www.china-cable-suppliers.com/visually-locating-fiber-loss-vfl.html