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Fresnel lenses were widely used in lighthouses until the mid-20th century, when they were replaced with beacons. However, the most widespread use of Fresnelʼs invention, at least for a period of time, was in automobile headlamps! The Fresnel system is still used for auto taillights. Flexible plastic sheet-type Fresnel lenses are used for reading small print in books and may be purchased on Amazon!

You should always match the test light source wavelength to the network requirements. While one refers to 850, 1300, or 1550nm routinely, actual source wavelength may vary. One long runs of fiber typical of WANs or long-distance networks, the difference becomes important.

Fresnel lenses are normally ranked in seven orders of power. The weakest (Sixth Order) was suitable for lakes and harbors, while the largest (First Order) was favored for fogbound seacoasts. A First Oder lens could weigh up to three tons! A lens ranked 3.5 was developed for use primarily on the Great Lakes.

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Lower cost field optical power meters usually have a resolution of 0.1dB, laboratory meters display 0.01dB, and a resolution of 0.001dB is available on a few specialized fiber optic power meters.

The Fresnel lens used in a lighthouse looks like a big glass beehive. The larger lenses could be up to 12 feet tall. Concentric rings of glass prisms — steeper at the edges, flatter toward the center — are arranged above and below a central panel of bull’s-eye magnifying glasses. These are supported by a brass framework. The prisms bend the light into a narrow beam which captures up to 83% of the light, and the bulls eye panel throws the light out toward the horizon. The lensʼ efficiency allows the beam to be cast 20 miles or more.

Optical power meters, like Digital Multimeters, come in a variety of types. The measurement uncertainty of practically all fiber optic power meters is the same, limited by the physical constrains of transferring standards with optical connectors. Most meters have an uncertainty of +/-5% or approximately 0.2dB, no matter what the resolution of the display may be.

The fiber attenuation coefficient is a function of wavelength, so source spectral characteristics can be important. When testing long lengths of fiber, you may need to make corrections to nominal source wavelength losses.

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The fiber optic power meter is a special light meter that measures how much light is coming out of the end of the fiber optic cable. The power meter needs to be able to measure the light at the proper wavelength and over the appropriate power range. Most power meters used in datacom networks are designed to work at 850nm and 1300nn. Power levels are modest, in the range of –15 to –35dBm for multimode links, 0 to –40dBm for single mode links. Power meters generally can be adapted to a variety of connector styles such as SC, ST, FC, SMA, LC, MU, etc.

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French engineer and physicist Augustin-Jean Fresnel was born at Broglie, in northern France, on May 10, 1788. As a child he appeared to be a slow learner. Indeed, at age 8 he still could not read. But before long his scientific brilliance became apparent.

Presque Isleʼs Third Order Fresnel Lens was removed from the New Lighthouse tower in 2003 and held in storage for nearly a decade. In 2012, at considerable cost, the lens was restored and placed on display in the entrance to the New Lightʼs Gift Shop, located in the 1870 Keeperʼs House. The display is similar to a lighthouseʼs lantern room and gives you (literally!) a birdʼs-eye view of this remarkable artifact. A small light burning inside the lens makes clear how successfully this lens concentrated and “threw” its light.

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About 1814 he began work in optics and helped develop the wave theory of light. In 1822 he invented the lens which bears his name — widely regarded as the most significant breakthrough in lighthouse illumination since Pharos, the famous lighthouse at Alexandria.

It was a brilliant technological advance and, in time, made Fresnelʼs name almost synonymous with lighthouses. Unfortunately, the inventor did not live to see his lens adopted across the globe. Fresnel died of tuberculosis on July 14, 1827, at Ville-dʼAvray, France. He was just 39 years old.

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For a lighthouse to be useful, it has to throw its light over a distance of several miles. Single lights, as well as lights with reflectors, proved inefficient because so much of their light was lost. A double convex lens (similar to a magnifying glass) could produce an appropriate focal length (the distance between the lens and the point at which the light converges), but the size and weight of such a lens made it impractical for lighthouse use. Fresnel reasoned that it was the curvature of the lens which provided the focusing power. He reproduced the curvature in segments, maintaining the desired focal length with just a fraction of the weight.

Optical power is usually measured in dBm, or decibels referenced to one milliwatt. This log scale is used because of the large dynamic range of fiber optic links, a range of 1000 or more. Some power levels may be given in microwatts, which many meters measure directly.

The appropriate resolution for a measurement should be chosen according to the test. Laboratory measurements of low-loss patch cables, connectors, and fiber splices can be made to 0.01dB resolution and an uncertainty of 0.05dB or less if great care is used in controlling the test conditions.

Fresnel lenses were initially regarded with suspicion in America. Stephen Pleasonton is justly remembered for saving the Declaration of Independence from being burned by the British during the War of 1812. In 1820 he was placed in charge of the Treasury Departmentʼs Lighthouse Establishment. He was a sober administrator, always reluctant to spend the publicʼs money. Pleasonton considered the cost of the new Fresnel lenses prohibitive and refused to order them. However, mariners who experienced Fresnel-equipped lighthouses in Europe came home to complain about the weak lights displayed by U.S. lighthouses.

The test source is a portable version of the source that is used in the communication network attached to the fiber. It simulates the signal in the fiber for loss testing with a power meter, so the test source should match the wavelength and source type of the system source.

This is a must-see stop for visitors to Presque Isle. The display is open on the same schedule as the New Lighthouse. For days and hours when the New Light is open to  visitors, click here.

Field measurements of absolute power are no better than the absolute calibration uncertainty, but relative power measurements can be made to 0.1dB. Cable plant loss is affected by not only the meter uncertainty, but also the characteristics of the test source, and may have an uncertainty of 0.5dB or more.

Power meters measure average optical power, not peak power, so they are sensitive to the duty cycle of the data being transmitted. It is important to specify the test conditions for measuring the optical power of a transmitter or at a receiver in terms of the data transmitted. Most networks have a diagnostic test signal for just this purpose.

In 1838 Congress launched an investigation and imported a few of these new lenses for experimental use. The first was installed in 1841 at the Navesink Lighthouse overlooking the approach to New York Harbor. In 1852 the Lighthouse Establishment was dissolved, ending Pleasontonʼs reign over U.S. lighthouses. The new United States Lighthouse Board approved use of Fresnel lenses, cost notwithstanding. By the Civil War nearly all American lighthouses had been equipped with them.

Fresnel was educated at the Ecole polytechnique near Paris, and then studied at the Corps des Ponts et Chaussees, the world’s oldest civil engineering school. As a supporter of the Bourbon monarchy, he temporarily lost his engineering post during the Emperor Napoleon’s return from Elba in 1814. When the monarchy was restored he obtained an engineering position at Paris and held that job for the rest of his life.

Fresnel received little recognition during his lifetime for his contributions to science. However, six decades after his death he was included in the list of 72 French scientists, engineers, and mathematicians whose names were engraved on the Eiffel Tower as an enduring tribute to their work.

The Fresnel (pronounced Fra-NELL, or Fray-NELL) Lens was invented by Augustin-Jean Fresnel in 1822. An engineering marvel, it uses an array of glass prisms and a bullʼs-eye lens to focus light into a narrow beam visible at a distance of several miles. By the Civil War his lens had become the standard illumination device in nearly all American lighthouses.

Generally, multimode fiber is tested with LEDs at both 850nm and 1300nm and single mode fiber is tested with lasers at 1310nm and 1550nm. The test source will typically be a LED for multimode fiber unless the fiber is being used for Gigabit Ethernet or other high-speed networks that use laser sources. LEDs can be used to test single mode fibers less than 5000 meters long, while a laser should be used for long single mode fibers.