The Visual Fault Finder is a visible laser light source used to check continuity, locate breaks, poor mechanical splices and damaged connectors in fiber optic cabling. It can be used to verify continuity, test and find breaks in fibre links, locate pinched fiber strands in termination cabinets, or anywhere fiber optic cables are terminated and subject to mechanical damage. It is often used as a stand-alone first-line basic troubleshooting tool. Typically with it the breaks in fiber optic cable will be seen as a conspicuously glowing or blinking red light area.

Visual fault finders are available from many sources and at different price ranges (typically from tens of dollars to hundreds of dollars). I found one very cheap ($12) visual fault locator to test: 1mW 5KM Red Light Pen Visual Fault Locator Fiber Optic Laser Cable Tester Meter.

1mW 5KM Red Light Pen Visual Fault Locator Fiber Optic Laser Cable Tester Meter puts out highly visible red light into a fiber optic cable. It has day light visible 650 nm laser diode which can operate either in continuous or flashing mode. It is fitted with 2.5 mm universal connector that accepts ST, SC and FC optical connectors. The laser power is said to be 1 mW, which should be useful over a distance of approximately 5km (3.1mi) into multi-mode fibre and single mode fiber. Aluminum alloy body, strong and durable.

Closer look at the durable aluminium body. The fiber end has cap that protects he fiber connector agains mechanical damage. Dust-proof end design keeps fiber connector clean when the device is in use and stops the laser beam if the device gets accidentally turned on while not in use.

This device is powered with two AA batteries

Continuous, off and flash are selected with switch.

There is proper power rating marking on the device.

Laser warning makes sense because laser can be dangerous. Fortunately this device is quite low power (1 mW) and beam spreads quite quickly. Be careful with laser, but I would consider this somewhat less dangerous than normal red laser pointer (they have typically 1-5 mW small beam).

Light output shows how the laser output spreads without fiber conneced.

The universal fiber connector worked well with single-mode fiber terminated with SC connector (expect to work also with ST and FC as they use same ferrule).

Bent fiber – breaks in fiber optic cable will be seen as a conspicuously glowing or blinking red light area.

This device was designed for glass fibers terminated to suitable connectors, but it also seems to works OK also with bare POF (Plastic Optical Fiber) cable.

One question on technical specifications, as the data says 1mW on the cover and on the device itself. but later on the page it says 30mW. I seem to get almost 1 mW to power meter on the other end of few meters of single mode fiber optic cable. So seems to give something in range of around 1 mW to connected fiber (meter shows 651 microwatts on the end of single mode fiber, measurement is not completely accurate because the laser 650 nm wavelengh does not match the power meter 850 nm setting – the closest setting I could find on the meter).

The end result is that this 1mW 5KM Red Light Pen Visual Fault Locator Fiber Optic Laser Cable Tester Meter seems to do well what it is designed to do. It fells well built. This device is cheap, but does not feel cheap in any way. At this price it does not make sense trying to hack yourself something for this task.

Here is a bonus that shows out how to use visual fault locator (different looking device but works the same):

FOA Lecture 21 Visual Fault Locator Demonstration

This is Lecture 21 in the FOA series of lectures on fiber optics. In this video, we will review the use of visual fault locators and demonstrate how they show stress loss in a typical patchcord.