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For our F-SV fiber, for which V = 2, the Gaussian width is approximately 28% larger than the core diameter, so the light should be focused to a spot size 1.28 times the core diameter at the fiber surface. For a Gaussian laser beam, the required beam diameter D incident upon focusing lens of focal length f to produce a focused spot of diameter w is D = 4λf/( πw). Given the laser beam waist and divergence, it's easy to determine the distance needed between the focusing lens and the laser to expand the beam to the required diameter.

Some anti-reflective lenses have surface treatments that are both hydrophobic and "oleophobic" (also called lipophobic), which means they repel both water and oil. These combination treatments typically contain fluorinated materials that give the lenses properties that are very similar to those of nonstick cookware.

Many multimode fiber experiments are sensitive to the distribution of power among the fiber's modes. This is determined by the launching optics, fiber perturbations, and the fiber's length. Mode scrambling is a technique that distributes the optical power in a fiber among all the guided modes. Mode filtering simulates the effects of kilometer lengths of fiber by attenuating higher-order fiber modes.

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By eliminating reflections, AR coating also makes your eyeglass lenses look nearly invisible so people can see your eyes and facial expressions more clearly.

For greater environmental protection, fibers are commonly incorporated into cables. Typical cables have a polyethylene sheath that encases the fiber within a strength member such as steel or Kevlar strands.

Anti-reflective coating also is a good idea for sunglasses, because it eliminates glare from sunlight reflecting into your eyes from the back surface of tinted lenses when the sun is behind you. (Generally, AR coating is applied only to the back surface of sunglass lenses because there are no cosmetic or visual benefits to eliminating reflections from the front surface of dark-tinted lenses.)

For example, regular plastic lenses reflect roughly 8 percent of light hitting the lenses, so only 92 percent of available light enters the eye for vision. High index plastic lenses can reflect up to 50 percent more light than regular plastic lenses (approximately 12 percent of available light), so even less light is available to the eye for vision.

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These benefits are due to the ability of AR coating to virtually eliminate reflections from the front and back surfaces of your eyeglass lenses. With reflections gone, more light passes through your lenses to optimize visual acuity with fewer distractions (especially at night), and the lenses look nearly invisible — which enhances your appearance by drawing more attention to your eyes and helping you make better "eye contact" with others.

The Numerical Aperture (NA) of a fiber is defined as the sine of the largest angle an incident ray can have for total internal reflectance in the core. Rays launched outside the angle specified by a fiber's NA will excite radiation modes of the fiber. A higher core index, with respect to the cladding, means larger NA. However, increasing NA causes higher scattering loss from greater concentrations of dopant. A fiber's NA can be determined by measuring the divergence angle of the light cone it emits when all its modes are excited.

Most premium AR lenses include a surface treatment that seals the anti-reflective layers and makes the lenses easier to clean. "Hydrophobic" surface treatments repel water, preventing the formation of water spots.

The Normalized Frequency Parameter of a fiber, also called the V number, is a useful specification. Many fiber parameters can be expressed in terms of V, such as: the number of modes at a given wavelength, mode cut off conditions, and propagation constants. For example, the number of guided modes in a step index multimode fiber is given by V2/2, and a step index fiber becomes single-mode for a given wavelength when V<2.405. Mathematically, V=2 π·NA·a/λ where “a” is the fiber core radius.

There are two broad classifications of modes: radiation modes and guided modes. Radiation modes carry energy out of the core; the energy is quickly dissipated. Guided modes are confined to the core, and propagate energy along the fiber, transporting information and power. If the fiber core is large enough, it can support many simultaneous guided modes. Each guided mode has its own distinct velocity and can be further decomposed into orthogonal linearly polarized components. Any field distribution within the fiber can be expressed as a combination of the modes. The two lowest-order guided modes of a circularly symmetrical fiber — designated LP01 and LP11 — are illustrated in Figure 1.

This principle implies that a pulse with a wider FWHM will spread more than a pulse with a narrower FWHM. Dispersion limits both the bandwidth and the distance that information can be supported. This is why for long communications links it is desirable to use a laser with a very narrow line width. Distributed Feedback (DFB) lasers are popular for communications because they have a single longitudinal mode with a very narrow line width.

When cleaning AR-coated lenses, use only products that your optician recommends. Lens cleaners with harsh chemicals may damage the anti-reflective coating.

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End surface quality is one of the most important factors affecting fiber connector and splice losses. Quality endfaces can be obtained by polishing or by using a fiber cleaver. Polishing is employed in connector terminations when the fiber is secured in a ferrule by epoxy. The following describes the popular connectors and their endface preparation styles.

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FC — the FC has become the connector of choice for single-mode fibers and is mainly used in fiber-optic instruments, SM fiber optic components, and in high-speed fiber optic communication links. This high-precision, ceramic ferrule connector is equipped with an anti-rotation key, reducing fiber endface damage and rotational alignment sensitivity of the fiber. The key is also used for repeatable alignment of fibers in the optimal, minimal-loss position. Multimode versions of this connector are also available. The typical insertion loss of the FC connector is around 0.3 dB. Drilled-out, metallic FC connectors, having insertion losses of >1 dB, are being used with Newport's large-core (>140 µm) fibers.

AR coating is especially beneficial when used on high-index lenses, which reflect more light than regular plastic lenses. Generally, the higher the index of refraction of the lens material, the more light that will be reflected from the surface of the lenses.

Good coupling efficiency requires precise positioning of the fiber to center the core in the focused laser beam. For multimode fibers, with their large cores, optical fiber positioners can achieve good coupling efficiency. Single-mode fibers require more elaborate couplers with submicron positioning resolution, like the ULTRAlign and 562F stainless steel positioners F-915 and F-1015 fiber optic couplers. These are also useful with Multi-mode fibers when maximum coupling efficiency is required.

SC — the SC connector is becoming increasingly popular in single-mode fiber optic telecom and analog CATV, field deployed links. The high-precision, ceramic ferrule construction is optimal for aligning single-mode optical fibers. The connectors' outer square profile combined with its push-pull coupling mechanism, allow for greater connector packaging density in instruments and patch panels. The keyed outer body prevents rotational sensitivity and fiber endface damage. Multimode versions of this connector are also available. The typical insertion loss of the SC connector is around 0.3 dB.

Some eyeglass lenses have factory-applied AR coating on both lens surfaces. Other lenses, particularly progressive lenses and other multifocal lenses (bifocals and trifocals), have the coating applied after the lenses have been customized to your eyeglass prescription by an optical lab.

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Polarization Mode Dispersion (PMD) is actually another form of material dispersion. Single-mode fiber supports a mode, which consists of two orthogonal polarization modes. Ideally, the core of an optical fiber is perfectly circular. However, the fact that in reality, the core is not perfectly circular, and mechanical stresses such as bending introduce birefringency in the fiber, causes one of the orthogonal polarization-modes to travel faster than the other, hence causing dispersion of the optical pulse.

Since each layer affects different wavelengths of light, the more layers there are, the more reflections that are neutralized. Some high-quality AR coatings have up to seven layers.

The characteristics of the focused beam must match the fiber parameters for good coupling efficiency. For multimode fibers this is straightforward. General guidelines are:

Anti-reflective coating (also called "AR coating" or "anti-glare coating") improves vision, reduces eye strain and makes your eyeglasses look more attractive.

To maximize coupling into a single-mode fiber, you must match the incident field distribution to that of the fiber mode. For example, the mode profile of the HE11 mode of a step index fiber can be approximated by a Gaussian distribution with a 1/e width w given by:

Each AR coating manufacturer has its own proprietary formula, but generally all anti-reflective coatings consist of multiple microscopic layers of metallic oxides of alternating high and low index of refraction.

Port Configuration: Number of input ports x number of output ports. e.g. 2 x 2 Coupling Ratio: The ratio of the power at an output port to the launched power expressed in dB. e.g. -10log (P2/P1). Isolation: The ratio of the power at an output port in the transmitted wavelength band to that in the extinguished wavelength band, expressed in dB. Directivity: The ratio of the power returned to any other input port to the launched power, expressed in dB. e.g.-10log (P4/P1). Bandwidth: The range of operating wavelengths over which performance parameters are specified. Excess Loss: The ratio of the total power at all output ports to the launched power, expressed in dB. e.g. -10log [(P2+P3)/P1]. Uniformity: The difference between maximum and minimum insertion losses. Extinction Ratio: The ratio of the residual power in an extinguished polarization state to the transmitted power, expressed in dB. Return Loss: The ratio of the power returned to the input port to the launched power, expressed in dB. e.g.-10log (P5/P1). Polarization-Dependent Loss (PDL): The maximum (peak-to-peak) variation in insertion loss as the input polarization varies, expressed in dB.

Depending on your lifestyle, your optician might suggest a specific brand of anti-reflective coating. If you spend a lot of time working at a computer, you might benefit from an AR coating that filters out blue light.

When light is launched into a fiber, the modes are excited to varying degrees depending on the conditions of the launch — input cone angle, spot size, axial centration and the like. The distribution of energy among the modes evolves with distance as energy is exchanged between them. In particular, energy can be coupled from guided to radiation modes by perturbations such as microbending and twisting of the fiber — increasing the attenuation.

When applied to photochromic lenses, AR coating enhances the clarity and comfort of these premium lenses in all light conditions without reducing their sun-reactive performance.

APC Polish — the Angled PC (APC) polish, adds an 8 degree angle to the connector endface. Back reflections of <-60 dB can routinely be accomplished with this polish.

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Depending on the AR coating formula, most lenses with anti-reflective coating have a very faint residual color, usually green or blue, that is characteristic of that particular brand of coating.

Today's modern anti-reflective coatings can virtually eliminate the reflection of light from eyeglass lenses, allowing 99.5 percent of available light to pass through the lenses and enter the eye for good vision.

Qualitatively, NA is a measure of the light gathering ability of a fiber. It also indicates how easy it is to couple light into a fiber.

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Since the core has a higher index of refraction than the cladding, light will be confined to the core if the angular condition for total internal reflectance is met. The fiber geometry and composition determine the discrete set of electromagnetic fields, or fiber modes, which can propagate in the fiber.

Some light is invariably launched into a fiber's cladding. Though cladding modes dissipate rapidly with fiber length, they can interfere with measurements. For example, the output of a single-mode fiber will not have a Gaussian distribution if light is propagating in the cladding. You can remove cladding modes by stripping a length of fiber coating and immersing the bare fiber in an index matching fluid such as glycerin.

SMA — due to its stainless steel structure and low-precision threaded fiber locking mechanism, this connector is used mainly in applications requiring the coupling of high-power laser beams into large-core multimode fibers. Typical applications include laser beam delivery systems in medical, bio-medical, and industrial applications. The typical insertion loss of an SMA connector is greater than 1 dB.

Intramodal Dispersion, sometimes called material dispersion, is a result of material properties of optical fiber and applies to both single-mode and multimode fibers. There are two distinct types of intramodal dispersion: chromatic dispersion and polarization-mode dispersion. As its name implies, intermodal dispersion is a phenomenon between different modes in an optical fiber. Therefore this category of dispersion only applies to mulitmode fiber. Since all the different propagating modes have different group velocities, the time it takes each mode to travel a fixed distance is also different. Therefore as an optical pulse travels down a multimode fiber, the pulses begin to spread, until they eventually spread into one another. This effect limits both the bandwidth of multimode fiber as well as the distance it can transport data.

In the near infrared and visible regions, the small absorption losses of pure silica are due to tails of absorption bands in the far infrared and ultraviolet. Impurities — notably water in the form of hydroxyl ions — are much more dominant causes of absorption in commercial fibers. Recent improvements in fiber purity have reduced attenuation losses. State-of-the-art systems can have attenuation on the order of 0.1 dB/km.

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Scattering can couple energy from guided to radiation modes, causing loss of energy from the fiber. There are unavoidable Rayleigh scattering losses from small-scale index fluctuations frozen into the fiber when it solidifies. This produces attenuation proportional to l/λ4. Irregularities in core diameter and geometry or changes in fiber axis direction also cause scattering. Any process that imposes dimensional irregularities — such as microbending — increases scattering and hence attenuation.

The lenses are then loaded into special metal racks with spring-loaded openings so the lenses are held securely but with virtually all lens surfaces exposed for the coating application. The racks are then loaded into the coating chamber. The door of the chamber is sealed, and the air is pumped out of the chamber to create a vacuum.

Once the optical fiber is terminated with a particular connector, the connector endface preparation will determine what the connector return loss, also known as back reflection, will be. The back reflection is the ratio between the light propagating through the connector in the forward direction and the light reflected back into the light source by the connector surface. Minimizing back reflection is of great importance in high-speed and analog fiber optic links, utilizing narrow line width sources such as DFB lasers, which are prone to mode hopping and fluctuations in their output.

When the coating materials are bombarded by electrons, they vaporize within the coating chamber and adhere to the surfaces of the lenses — creating a uniform, microscopically thin optical layer on the lens.

Applying anti-reflective coating to eyeglass lenses is a highly technical process involving vacuum deposition technology.

Bandwidth of an optical fiber determines the data rate. The mechanism that limits a fiber's bandwidth is known as dispersion. Dispersion is the spreading of the optical pulses as they travel down the fiber. The result is that pulses then begin to spread into one another and the symbols become indistinguishable. There are two main categories of dispersion, intermodal and intramodal.

PC Polish — the Physical Contact (PC) polish results in a slightly curved connector surface, forcing the fiber ends of mating connector pairs into physical contact with each other. This eliminates the fiber-to-air interface, there by resulting in back reflections of -30 to -40 dB. The PC polish is the most popular connector endface preparation, used in most applications.

ST — the ST connector is used extensively both in the field and in indoor fiber optic LAN applications. Its high-precision, ceramic ferrule allows its use with both multimode and single-mode fibers. The bayonet style, keyed coupling mechanism featuring push and turn locking of the connector, prevents over tightening and damaging of the fiber end. The insertion loss of the ST connector is less than 0.5 dB, with typical values of 0.3 dB being routinely achieved. Drilled-out, metallic ST connectors, with insertion losses of >1 dB, are used with Newport's large-core (>140 µm) fibers.

One scrambling technique is to splice a length of graded-index fiber between two pieces of step-index fiber — this ensures that the downstream fiber's core is overfilled regardless of launch conditions. Mode filtering can be achieved by wrapping a fiber several times around a finger-sized mandrel; bending sheds the high-order modes.

Also, don't attempt to clean AR-coated lenses without wetting them first. Using a dry cloth on a dry lens can cause lens scratches. And because anti-reflective coating eliminates light reflections that can mask lens surface defects, fine scratches often are more visible on AR-coated lenses than on uncoated lenses.

The outer sheath of fiber cables can be removed using electrical cable stripping tools, and scissors or a razor blade can trim the Kevlar strength member. However, the fiber coating must be very carefully removed to avoid damaging the fiber — surface flaws and scratches are the cause of most fiber failures. The coating can be removed using our Fiber Optic Strippers.

Anti-reflective coatings are incredibly thin. The entire multilayer AR coating stack generally is only about 0.2 to 0.3 microns thick, or about 0.02 percent (two one-hundredths of 1 percent) of the thickness of a standard eyeglass lens.

The index of refraction varies depending upon wavelength. Therefore, different wavelengths will travel down an optical fiber at different velocities. This is known as Chromatic Dispersion.

Fiber Cleaving is the fastest way to achieve a mirror-flat fiber end — it takes only seconds. The basic principle involves placing the fiber under tension, scribing with a diamond or carbide blade perpendicular to the axis, and then pulling the fiber apart to produce a clean break. Our F-BK3 and FK11 fiber optic cleavers make the process especially quick and easy. It is wise to inspect fiber ends after polishing or cleaving.

The first step in the AR coating process is to meticulously clean the lenses and inspect them for visible and microscopic surface defects. Even a tiny smudge, piece of lint or hairline scratch on a lens during the coating process can cause a defective AR coating.

The mode field diameter is now given to provide easier matching of lens to optical fiber for a Gaussian beam. A high numerical aperture lens must collimate the diverging output beam of a laser diode. Newport's F-L Series Diode Laser Focusing Lenses, are AR-coated for high transmittance at popular laser diode wavelengths and — with numerical apertures up to 0.5 — are useful for collimating or focusing.

Typically, a production line includes multiple washing and rinsing baths, including ultrasonic cleaning to remove any traces of surface contaminants. This is followed by air drying and heating of the lenses in special ovens to further remove unwanted moisture and gases from the lens surface.

Optical fibers are circular dielectric wave-guides that can transport optical energy and information. They have a central core surrounded by a concentric cladding with slightly lower (by ≈ 1%) refractive index. Optical fibers are typically made of silica with index-modifying dopants such as GeO2. A protective coating of one or two layers of cushioning material (such as acrylate) is used to reduce cross talk between adjacent fibers and the loss-increasing microbending that occurs when fibers are pressed against rough surfaces.

While the lens racks are rotating in the coating chamber, a power source within the machine focuses a beam of electrons onto a small crucible that contains a series of metal oxides in separate compartments.

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SPC and UPC Polish — in the Super PC (SPC) and Ultra PC (UPC) polish, an extended polishing cycle enhances the surface quality of the connector, resulting in back reflections of -40 to -55 dB and < -55dB, respectively. These polish types are used in high-speed, digital fiber optic transmission systems.

Light power propagating in a fiber decays exponentially with length due to absorption and scattering losses. Attenuation is the single most important factor determining the cost of fiber optic telecommunication systems, as it determines spacing of repeaters needed to maintain acceptable signal levels.

The visual benefits of lenses with anti-reflective coating include sharper vision with less glare when driving at night and greater comfort during prolonged computer use (compared with wearing eyeglass lenses without AR coating).

One way to achieve both scrambling and filtering is to introduce microbending to cause rapid coupling between all fiber modes and attenuation of high-order modes. One approach is to place a stripped section of fiber in a box filled with lead shot. A more precise way is to use Newport'. FM-1 Mode Scrambler. This specially designed tool uses a calibrated mechanism to introduce microbending for mode scrambling and filtering.