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LFD-250B - live fiber detector

Introduces step-motor-activated bending and makes fixed-angle bending—and its drawbacks—a thing of the past.

Spec sheet (PDF) User manual (PDF)

Key features

  • Induces minimal loss: ≤1 dB
  • Fail-safe detection and results
  • Detects if a fiber is active or not prior to maintenance
  • Locates a particular dark fiber using tone recognition (270 Hz, 1 kHz, 2 kHz)
  • Identifies traffic direction on a live fiber
  • Displays the power transmitted through the fiber


  • Non-intrusive fiber identification and power measurement
  • Traffic direction verification


Breaking free of the limitations of traditional live fiber detectors

Traditional live fiber detectors (LFDs) use thumb-activated fiber bending at a fixed angle to enable the detector to read the power leaking from the jacket (see Figure 1). Since the bending is fixed and optimized for one wavelength and one fiber type, the bending often causes:

  • Excessive loss
  • Unreliable fiber detection (fiber activity is not detected)
  • Unreliable tone/traffic detection
  • Permanent damage to the fiber

In addition to being unreliable, the fact that they can cause excessive loss is the main reason why traditional LFDs are considered dangerous and are not used on high-data-rate routes and in long-haul-network applications.

EXFO’s LFD-250B Live Fiber Detector introduces step-motor-activated bending and makes fixed-angle bending—and the drawbacks stated above—a thing of the past.

EXFO’s LFD-250B: step-motor-activated bending, for guaranteed low loss

For all fiber types and all wavelengths, insertion loss is monitored as a function of the bending angle as the motor (and not human power) moves. Although the angles differ, the behavior remains the same. The adjacent graph shows that fixed-angle bending generates excessive loss in some cases, and leads to flawed identification in others.

The LFD-250B brings a unique approach: the power loss is monitored as the motor (and not human power) changes the angle. Therefore, the angle is automatically optimized for each fiber type and each singlemode wavelength. This results in clear-cut advantages:

  • Maximum loss of 1 dB guaranteed for any singlemode telecom fiber (most jacket types) and any wavelength
  • No damage to the fiber: bending is always minimal and the fiber is released when no power is detected*
  • Virtually 100 % reliability on traffic detection, direction identification and tone detection
  • Accurate in-line, non-disruptive power measurements
  • Safe to be used in long-haul applications and on high-payload fibers—contrary to traditional LFDs

Ambiant light offset

EXFO’s LFD-250B performs an ambient light offset prior to fiber bending, which makes it less sensitive to ambient light. A push-down cap can also be placed on the headend to block intense ambient light.

In-line power measurement

As stated, the LFD-250B controls the insertion loss within the fiber (IL) in dB. But the absolute value of the measured signal is in dBm, so knowing the loss in dB and the power level of this light exiting, power can be measured with a better accuracy than traditional fixed loss LFDs. Of course, coupling efficiency is a factor (3 mm jackets absorb more than 1.6 mm and 900 μm jackets). However, since loss is monitored as a basis for motor positioning, the unit knows what size of jacket is being tested (either 900 μm, 1.6 mm or 3 mm), so the LFD-250B automatically uses the proper coupling efficiency parameter and computes the power within any fiber, at any wavelength, with 1 dB repeatability.

Applications are numerous

  • FTTH deployment, where there is no protection fiber, making non-intrusiveness a key
  • Live network maintenance and troubleshooting
  • Link budget evaluation without having to disconnect

LFD-250B - live fiber detector

Interested in this product?

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Awards and reviews

Innovation and leadership have always been front and center at EXFO. Each year, we strive to develop solutions that will ensure superior customer experience in smarter, modern networks. Being recognized as such is always a great honor. Learn more about our awards:

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