Fiber Optic Loopback Adapters: A Troubleshooting Tool

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Things can go wrong even when a fiber optic network is installed properly without exceeding loss budgets, cabling bend radius, or pull tension. Problems that creep up in an existing fiber optic network can be related to the cabling and connectivity infrastructure, such as contaminated connector end faces or damaged cable. However, before going down the rabbit hole of hiring a technician to check the infrastructure with an optical time domain reflectometer (OTDR) or inspect connector end faces for contamination with an optical inspection scope, it makes more sense first to check the functionality of the active network equipment. Thankfully, a simple, inexpensive option for quickly carrying out this initial step is the fiber optic loopback adapter.

What is a fiber optic loopback?

A fiber optic loopback test is a common way to check the functionality of your network transmission equipment. It can be performed internally via network management software, known as a soft loopback, or externally via a physical loopback adapter, known as a hard loopback.

Most network switches today have built-in software functionality to perform a soft loopback. This is especially handy because you can perform the test remotely via the management software. However, this soft loopback is a very basic test performed logically through the equipment's internal physical interface card. It does not test the integrity of the physical transmit and receive ports.

In contrast, a hard loopback test is performed on-site using a fiber optic loopback adapter. These simple little devices consist of a fiber optic connector that can be easily plugged into a switch port, essentially creating a closed loop where a switch port sends a data signal directly back to itself rather than to another device on the network.

Types of fiber optic loopback adapters

As with any fiber test equipment, a fiber optic loopback adapter must match the fiber and connector type of the switch. Loopback adapters are available in multimode or singlemode fiber and in various connector interfaces.

The loopback adapter typically uses an LC or SC connector type for duplex applications. Duplex loopback adapters are also available with newer very small form-factor (VSFF) duplex connectors such as the CS or SN connectors from Senko. For parallel-optic applications that transmit and receive over multiple fibers, the loopback adapter will have an MPO/MTP connector. The connector could be an 8-, 12-, 16- 24-, or even 32-fiber MPO/MTP depending on the application.

One of the key differences between the MPO/MTP and duplex loopback adapters is the polarity, which is essential for ensuring that the transmit signal corresponds correctly to the receive signal. In a duplex application, polarity is not a concern as there is always one fiber for the transmit signal and one for the receive signal. However, polarity becomes a bit more complex in an MPO/MTP parallel optics application since multiple transmit fibers must align correctly with multiple receive fibers. To complicate matters, there are three types of MPO/MTP polarity&#;A, B, and C.

The two most common polarity methods are MPO Type A and MPO Type B. Type A components feature a straight-through connection, meaning that the fiber in the first position at one end lines up with the first position at the other end. In a 12-fiber Type A MPO/MTP loopback adapter, fiber 1 (transmit) corresponds to fiber 12 (receive), fiber 2 (transmit) corresponds to fiber 11 (receive), and so on.

Type B components are reversed, meaning that the fiber in the first position at one end lines up with the last position at the other end. In a 12-fiber Type B MPO/MTP loopback adapter, fiber 1 (transmit) corresponds to fiber 7 (receive), fiber 2 (transmit) corresponds to fiber 8 (receive) and so on.

Type C components that transmit and receive in pairs (1-2, 3-4, 5-6, 7-8, etc.), are not used in parallel optics applications but may be used to form multiple duplex channels. Selecting a fiber loopback adapter that matches the polarity type of your network is essential.

How to test a fiber optic cable with a hard loopback

Before performing a hard loopback test, ensure the loopback adapter fiber, connector, and polartiy type are compatible with your active equipment and application. You may also want to have a light source on hand, such as a visual fault locator (VFL).

• Step 1: Physically connect the loopback adapter to the transceiver port at the near end of a fiber link.
• Step 2: Power on the equipment and observe the port's receiver activity by checking for the indicator light.
• Step 3: If there is activity, you can then check the port status via the switch&#;s management software, which will indicate the speed of the connection.
• Step 4: Repeat steps 1-3 at the far end of the link.
• Step 5: (optional): Use a light source to verify continuity through the loopback cable to confirm functionality.

A loopback test confirming a fully functioning switch port at both ends of a link indicates that the problem lies within the cabling infrastructure. In this scenario, you will then need to troubleshoot the fiber cabling and connectivity using other test tools. However, suppose the loopback test confirms that the switch port at one or both ends of the link is not properly functioning. In that case, you&#;ve succeeded in eliminating the cabling infrastructure as the source of the problem and the time involved in troubleshooting the infrastructure. You may then need to replace the switch transceiver for the non-functioning switch port or work with your switch manufacturer to troubleshoot the problem further.

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Fiber loopback adapters are handy, inexpensive devices to keep on hand for quick switch port troubleshooting in the case of a non-functioning fiber link. The good news is that CablesPlus USA offers a wide range of loopbacks in multiple fiber and connector types for testing 10 to 800 Gig&#;from duplex multimode and singlemode LC, SC, CS, and SN connectors to multiple MPO/MTP configurations. And if you find that the problem lies within the cabling infrastructure, we&#;ve got a wide range of fiber optic test equipment to help you troubleshoot that too. Contact us for all your fiber network needs.

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Visual Inspection

Visual Tracing

Continuity checking makes certain the fibers are not broken and to trace a path of a fiber from one end to another through many connections. Use a visible light "fiber optic tracer" or "pocket visual fault locator". It looks like a flashlight or a pen-like instrument with a lightbulb or LED soure that mates to a fiber optic connector. Attach a cable to test to the visual tracer and look at the other end to see the light transmitted through the core of the fiber. If there is no light at the end, go back to intermediate connections to find the bad section of the cable.

A good example of how it can save time and money is testing fiber on a reel before you pull it to make sure it hasn't been damaged during shipment. Look for visible signs of damage (like cracked or broken reels, kinks in the cable, etc.) . For testing, visual tracers help also identify the next fiber to be tested for loss with the test kit. When connecting cables at patch panels, use the visual tracer to make sure each connection is the right two fibers! And to make certain the proper fibers are connected to the transmitter and receiver, use the visual tracer in place of the transmitter and your eye instead of the receiver (remember that fiber optic links work in the infrared so you can't see anything anyway.)

Visual Fault Location

Do you know what a visual fault locator is?

Also known as a VFL, this is designed for finding breaks in your fiber. It works by shooting a laser down the core of the glass. If you have light at the other end, then you know you have continuity in your connection.

A higher power version of the tracer uses a laser that can also find faults. The red laser light is powerful enough to show breaks in fibers or high loss connectors. You can actually see the loss of the bright red light even through many yellow or orange simplex cable jackets except black or gray jackets. You can also use this gadget to optimize mechanical splices or prepolished-splice type fiber optic connectors. In fact- don't even think of doing one of those connectors without one ­ no other method will assure you of high yield with them.

You could also use a basic LAN tester to make sure all of your pins are properly lined when testing continuity. It uses an LED display to show the condition of patch cables and installed cabling.

It tests for faults such as open wires, shorts, reversed pairs, crossed pairs, split pairs and mis-wiring in seconds. With a push of the button, the cable tester will automatically scan all wires and pairs to determine any existent faults. 

It also tests the Shield connection on shielded (STP) cabling. The network cable tester includes one master and one remote unit. 

Visual Connector Inspection

Fiber optic microscopes are used to inspect connectors to check the quality of the termination procedure and diagnose problems. A well made connector will have a smooth , polished, scratch free finish and the fiber will not show any signs of cracks, chips or areas where the fiber is either protruding from the end of the ferrule or pulling back into it.

Fiber optic microscopes are used to inspect connectors to check the quality of the termination procedure and diagnose problems. A well made connector will have a smooth , polished, scratch free finish and the fiber will not show any signs of cracks, chips or areas where the fiber is either protruding from the end of the ferrule or pulling back into it.

The magnification for viewing connectors can be 30 to 400 power but it is best to use a medium magnification. The best microscopes allow you to inspect the connector from several angles, either by tilting the connector or having angle illumination to get the best picture of what's going on. Check to make sure the microscope has an easy-to-use adapter to attach the connectors of interest to the microscope.

And remember to check that no power is present in the cable before you look at it in a microscope ­ protect your eyes!

Optical Power - Power or Loss? ("Absolute" vs. "Relative")

Practically every measurement in fiber optics refers to optical power. The power output of a transmitter or the input to receiver are "absolute" optical power measurements, that is, you measure the actual value of the power. Loss is a "relative" power measurement, the difference between the power coupled into a component like a cable or a connector and the power that is transmitted through it. This difference is what we call optical loss and defines the performance of a cable, connector, splice, etc.

Practically every measurement in fiber optics refers to optical power. The power output of a transmitter or the input to receiver are "absolute" optical power measurements, that is, you measure the actual value of the power. Loss is a "relative" power measurement, the difference between the power coupled into a component like a cable or a connector and the power that is transmitted through it. This difference is what we call optical loss and defines the performance of a cable, connector, splice, etc.

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