Absorption
In optics, a source of loss or attenuation that is due to material properties of an optical fiber. Absorption is quite often wavelength dependent.

Acceptance angle
The maximum cone half-angle for which incident light is captured by and will travel through the optical fiber. If the acceptance angle is q then the acceptance cone is defined by a solid cone of 2q. See NA for more details.

Acrylate
A polymer material used in optical fibers as a buffer coating or cladding or in capillary as a coating.

Adsorption
In chemistry, the taking up by the surface of a solid or liquid (adsorbent) of the atoms, ions, or molecules of a gas or other liquid (adsorbate). Porous or finely divided solids can hold more adsorbate because of the relatively large surface area exposed.

Attenuation
The amount of light loss experienced in an optical fiber or optical media as a function of length. For optical fiber it is usually expressed in dB (decibels) per kilometer (km). See Transmission.

Bandwidth
The range of frequencies (or wavelength) which are useful for a device or system. In optical fiber, it is a measure of information carrying capacity. The frequency bandwidth is usually described as the frequency where the optical power is one-half the power at zero frequency. The wavelength bandwidth is usually expressed in terms of wavelength-dependent attenuation or transmission, and is not necessarily related to the signal bandwidth.

Bending loss
Loss in an optical fiber caused by bending of the fiber. This loss is usually due to internal light paths exceeding the critical angle for TIR. Both micro-bending and macro-bending are characteristics of optical fibers.

Bend Radius
The radius of a drum or mandrel around which an optical fiber or cable is wrapped or wound. The radius of the center of the fiber or cable is the bend radius plus one-half the fiber or cable diameter.

Buffer
The buffer is an outer coating on an optical fiber. Typically a plastic material, it protects the fiber from external stresses or abrasion. In some applications, it can serve an optical function as a second reflection layer or as an absorption layer.

Cable, fiber optic
A package or assembly for an optical fiber or fibers that may include buffering, strength members and/or an outer jacket.

Capillary tubing
Quartz or glass tubing which has internal diameters from less than 2mm to more than 2000mm. The outside diameter can range from 90mm to greater than 3500mm, depending on the application requirements. An outside buffer coating of polyimide, silicone, acrylate, aluminum or fluoropolymer can be added.

Capillary Electrophoresis (CE)
Capillary electrophoresis is a separation method based on the differential electrophoretic migration rate (endo-osmotic flow) of sample components in a capillary when a voltage is applied. The detection method is usually “on-column” using UV spectrometric or fluorescence analysis through a window in the capillary. Performing electrophoresis in small-diameter capillaries allows the use of very high electric fields because the small capillaries efficiently dissipate the heat that is produced. Increasing the electric fields produces very efficient separations and reduces separation times. CE detection includes absorbance, fluorescence, electrochemical, and mass spectrometry.

Chromatic dispersion
The separation of a beam into its various wavelength components. In an optical fiber, dispersion occurs because of the differing wavelengths propagating at different speeds. This causes pulse spreading or broadening. See Dispersion.

Chromatography
In chemistry, analytical technique used for the chemical separation of mixtures and substances. The technique depends on the differential distributon of solute between the mobile and stationary phases.

Chromophore
A naturally occurring pigment that selectively absorbs light at certain wavelengths. In tissue, it can be used to aid in targeting a laser surgery beam. See also Raman Spectroscopy.

Cladding
A low refractive index optical material that surrounds the core of an optical fiber. It is used to cause reflection of the core light while preventing surface contact from scattering or the generation of frustrated internal reflection. In all-glass fibers, the cladding is glass. In plastic-clad silica fibers, the plastic cladding also may serve as the coating.¹

Color
The attribute of visual experience that can be described as having quantitatively specifiable dimensions of hue, saturation, and brightness or lightness. It does not include aspects of extent (e.g., size, shape, texture, etc.) and duration (e.g., movement, flicker, etc.). A color that is seen can be a single wavelength or a combination of many. Most colors are a very complex combinations of many wavelengths of various amplitudes.

For detailed information see Color Science: concepts and methods, quantitative data and formulas, G. Wyszecki & W.S. Stiles, John Wiley & Sons, Inc., 1967.

Components
Polymicro Technologies’ products that include assemblies made with optical fibers and/or quartz capillaries. Components and assemblies also include laser machined and formed fiber end tips and couplings. See Micro-components.

Cone
In micro-components, a tapered optical fiber or component that comes almost to a point. It can be used for concentrating laser or other light to a point at or very near the apex of the cone.

Core
The light-conducting portion of an optical fiber having a higher refractive index than the cladding. Usually made of a pure synthetic silica, but can be a doped material to provide special fiber characteristics.

Cut-off wavelength
1. In detector technology, the long wavelength at which detector response falls to a set percentage (usually 20 or 50 percent). 2. In fiber optics, the wavelength below which a waveguide operates in multimode and not singlemode. 3. In optical filter terminology, that wavelength where the filter transmission falls below 5 percent.

Decibel
The standard unit used to express gain or loss and relative power levels. The decibel (dB) = -10 log (P_o/P_i), where P_o is the power-out and P_i is the power-in.

Diffuse
The type of reflection/transmission from many powders (e.g., phosphors, M_gO₂ or BaSO₄), or matte surfaces and transmitting materials such as ground quartz, flashed opal glass or Teflon® (PTFE). Flat white paint is an example of a nearly Lambertian, diffuse coating. Diffusers are often used to remove imaging characteristics from an optical beam. See also Specular reflection, Spread and Lambertian.

Dispersion
The separation of a beam into its various wavelength components. In an optical fiber, dispersion occurs because the differing wavelengths propagate at differing speeds. In fibers, it is the cumulative effect of three types: chromatic, modal and waveguide dispersion.

Deuterium discharge lamp
A discharge lamp filled with deuterium to produce high-intensity ultraviolet radiation for use in spectroscopic analysis.

Doped (synthetic) fused silica
See Fused silica.

Fiber optics
A branch of optics that deals with the transmission of light through fibers, tubes or thin rods of a transparent material. If light is injected into one end of an optical fiber or rod, it can travel through it with very little loss, even if the fiber is curved. How far the light goes, i.e., how much is not lost, depends on the color of light (wavelength), the optical fiber design, the materials used, and the manufacturing configuration and process controls.

Fiber optic assemblies
An assembly of optical fibers, usually used to change the light flux format or distribution or to route light to multiple locations. For example, one end can be a cluster of optical fibers in a hexagonal array and the other end could be a linear array of the same fibers or multiple branches of individual fibers.

Flexible tubing
In fused silica or quartz capillary tubing, used to refer to tubing less than 1mm in diameter, although larger diameters are somewhat flexible.

Fluorescence
The emission of light or other electromagnetic radiation of longer wavelengths by a substance as a result of the absorption of some other radiation of shorter wavelengths, provided the emission continues only as long as the stimulus producing it is maintained. In other words, fluorescence is the luminescence that persists for less than about 10ns after excitation.

Focal ratio degradation (FRD)
The reduction of relative f-number (or f-ratio) in an optical fiber due to the characteristics of the fiber. Basically, the emittance light cone is always greater than or equal to the incident light cone in an optical fiber. The effect of FRD results in the output cone of light being larger than the input cone of light. Because of this effect, insertion of optical fiber in an f-number matched optical train (e.g., a astronomical fiber optic spectrometer) will cause a signal loss greater than that expected from the normal loss per unit length values. Failure to account for FRD in transmission measurements, e.g., measuring light output with an integrating sphere, will give optimistically high transmission values for higher f-number systems.

Fresnel loss
The portion of light reflected (lost) at the surface of an ordinary dielectric material such as glass. It is given by:

where i and r are the angles of incidence and refraction. The first term is the reflection of light polarized in the plane of incidence, and the second the reflection for the other plane of polarization. At normal incidence the equation reduces to:

For a single air-glass interface, non-polarized light and a glass of index 1.523, the Fresnel loss (reflection) is 4.3% at 0° rising rapidly to 10% at 60° and 40% at 80°.  In decibels (dB), the value can be calculated from:

Fused quartz
See Fused silica.

Fused Silica
Fused silica is silicon dioxide (SiO₂) in its amorphous (glassy) state. Silica is silicon dioxide (SiO₂). Synthetic fused silica is amorphous silicon dioxide that has been produced through chemical deposition rather than refinement of natural ore. This synthetic material is of much higher purity and quality as compared to fused quartz made from natural minerals. Doped (synthetic) fused silica is amorphous silicon dioxide that has been produced through chemical deposition. It has been intentionally doped with trace elements such as germanium, boron, phosphorous, titanium, fluorine or other element to adjust the optical properties of the glass. Quartz is a natural grade of crystalline silicon dioxide (SiO₂). This is the most common phase of SiO₂. This is also referred to as “rock crystal.” Fused quartz is a natural grade of amorphous SiO₂. Typically produced from the melting (fusing) of crystalline quartz and refined such that an amorphous (glass) is formed.

Gas chromatography (GC)
Gas chromatography is a method for separating substances in a mixture and measuring the relative quantities of substances. It is a useful technique for substances that do not decompose at high temperatures and when a very small quantity of sample (micrograms) is available.

In this type of chromatography, a sample is rapidly heated and vaporized, and then a stream of gas carries it along a column that contains a stationary phase. The sample becomes distributed between the mobile gas phase and the stationary phase. The higher a substance’s affinity for the stationary phase, the more slowly it comes off of the column. The substances are detected as peaks on a chart recorder.

Glass
The word “Glass“ refers to the solid phase of a material with no long-range molecular order. It is used almost interchangeably with “amorphous,” “non-crystalline,” and “vitreous.” Glass is a disordered structure, as compared to a crystalline material that exhibits a symmetrical, ordered structure. The most common glasses are oxide based, such as silicates (SiO₂), borates (B₂O₃), germanates (GeO₂) or mixtures of these.

Graded index
1. Descriptive of an optical fiber having a core refractive index that decreases almost parabolically and radially outward toward the cladding. This type of fiber combines high bandwidth with moderately high coupling efficiency. Sometimes called graduated-index. 2. Glass material is now being manufactured with an axial graded index rather than the radial graded index more common in optical fiber. Since this material has an axial change in index with thickness, cutting a spherical surface on the face can generate a similar optical function to an aspheric surface.

High-performance liquid chromatography (HPLC)
High-performance liquid chromatography (HPLC) is a form of liquid chromatography used to separate compounds that are dissolved in solution. HPLC instruments consist of a reservoir of mobile phase, a pump, an injector, a separation column, and a detector. Compounds are separated by injecting a plug of the sample mixture onto the column. The different components in the mixture pass through the column at different rates due to differences in their partitioning behavior between the mobile liquid phase and the stationary phase. Solvents must be degassed to eliminate formation of bubbles. The pumps provide a steady high pressure with no pulsating, and can be programmed to vary the composition of the solvent during the course of the separation. Typical detectors rely on a change in refractive index, UV-VIS absorption, or fluorescence after excitation with a suitable wavelength.

Hollow waveguide (HSW)
A flexible hollow capillary with an internal surface coating which has a very high reflection at the wavelength of light to be transported. Usually used where there are no transmissive materials that can be formed into a flexible optical fiber at the wavelength of interest. Most commonly used in the IR.

Index of refraction (n)
The ratio of the velocity of light in a vacuum to the velocity of light in a refractive material at a given wavelength. See Refractive index.

Isotropic
Invariant with respect to direction. The property of an optical material that allows the velocity of propagation of electro-magnetic radiation to be the same for all directions.

Lambert
A unit of luminance (brightness) equal to 1/p candela per square centimeter.

Lambertian
A relation between the illumination on to a surface and the light flux from a surface versus angle. Lambert’s Cosine Law states that the light flux, E_f, at some angle, f, is equal to the illumination perpendicular to the surface, E, times the cosine of the angle.

A perfectly Lambertian surface has a uniform brightness (luminance, radiance) from any viewing angle.²

Light
1. Electromagnetic radiation that may be perceived by the human eye. 2. The sensation of perceiving light; brightness. 3. a. A source of light, especially a lamp or electric fixture. b. The illumination derived from such a source.³

Liquid chromatography (LC)
Liquid chromatography (LC) is an analytical chromatographic technique that is useful for separating ions or molecules that are dissolved in a solvent. If the sample solution is in contact with a second solid or liquid phase, the different solutes will interact with the other phase to differing degrees due to differences in adsorption, ion-exchange, partitioning, or size. These differences allow the mixture components to be separated from each other by using these differences to determine the transit time of the solutes through a column.

Macro-bending
Bending of an optical fiber or fiber bundle at a radius comparatively larger than the fiber diameter. Macrobends are larger than microbends, being visible, and generally caused by bending the cable beyond the specified bend radius. Attenuation is increased due to light escaping at cladding interface at angles greater than the critical angle at these bends.

Micro-bending
In optical fiber, microscopic curvatures that create local axial displacements of a few micrometers and spatial wavelength displacements of a few millimeters. One frequent cause is longitudinal shrinking of the fiber buffer. But it can also result from poor drawing or cable manufacturing methods, installation, etc. Micro-bending causes transmission loss through a power-coupling effect from the guided modes to the radiation modes.

Micro-component
At Polymicro Technologies, an optical component fabricated from fused silica or quartz material, often directly from the end of an optical fiber.

Micron (mm)
Often used for micrometer (mm or 1x10^-6 meters).

Modal dispersion
Synonym for multimode distortion or modal distortion.

Modal distortion
Synonym for multimode distortion.   EIA* calls it erroneous.

Mode
1. The characteristic of the propagation of light through a waveguide or optical fiber that can be designated by a radiation pattern in a plane transverse to the direction of travel. 2. The state of an oscillating system, such as a laser, that corresponds to a particular field pattern and one of the possible resonant frequencies of the system.

Multi-Lumen
A capillary configuration which has multiple channels.

Multimode distortion
In an optical waveguide, the distortion resulting from differential mode delay. (EIA*) Axial rays, with the shortest path length, will have the shortest transmission time, while rays entering the fiber at its maximum acceptance angle will travel further and require the maximum time.

Multimode fiber
An optical waveguide that will allow more than one mode to propagate (EIA*).

Multiple channel capillary tubing
A custom specialty of Polymicro Technologies, most commonly supplied in quartz material.

Nanometer (nm)
A unit of measure most often used for light in the visible (1x10^-9 meters). The peak of the human eye sensitivity is at a wavelength of 550 nm (green light). In the UV range where Angstroms are still sometimes used, 200 nm is 2000 Å. In the near infrared where microns (mm) are used, 1000 nm is 1 micron (mm).

Normalized frequency
A dimensionless quantity, which can be use to calculate or express the number of modes which can be propagated in a step index fiber. It is denoted by V, in the expression:

where a is waveguide core radius, l is wavelength in vacuum, and n₁ and n₂ are the maximum refractive index in the core and refractive index of the homogeneous cladding, respectively (EIA*).

Numerical aperture (NA)
In optical fiber, the sine of the maximum acceptance half-angle, q_max, times the refractive index of the core (assuming an air-to-core interface). The larger the NA, the greater the amount of light that is accepted into the fiber for propagation to the distal end.

,where n₁ = 1 for air.

The NA for a glass-core to glass-clad interface is often derived from calculations using the equation:

,where n_co is the core index and n_cl is the cladding index.

Values calculated in this manner do NOT take into account losses from Fresnel reflection or degradation of NA with fiber length. See Fresnel loss and FRD.

Optics
A branch of physical science dealing with the propagation and behavior of light. In a general sense, light is that part of the electromagnetic spectrum that extends from X-rays to microwaves and includes the radiant energy that produces the sensation of vision.⁴

Optical fiber
The transmission of light in an optical fiber depends on the principle of total internal reflection (TIR). When light traveling inside a fiber strikes the surface at an angle of incidence greater than the critical angle, the light is reflected back toward the center of the fiber with negligible loss. Thus, light can be transmitted over long distances by being reflected thousands of times, as long as the loss per reflection is extremely low. 

The most common optical fiber configuration is based on the use of two materials concentrically arranged as a center core and outer tubing (cladding). This arrangement avoids losses that would result from the scattering of light by impurities on the surface of the fiber. The optical fiber core is much higher in refractive index (n_CO), than the cladding index (n_CL); the reflections occur at the interface of the glass fiber core and the cladding. Sometimes a second outer layer (super-cladding) is added to the fiber to increase transmission, prevent lost light from getting into adjacent fibers, or to increase the strength or environmental durability of the fiber. See Fiber optics.

Packing fraction (P_f)
The fraction of the area of an optical fiber bundle surface that is actual core area. The actual number of fibers, N, which can be packed in a circle of diameter, D = (2m+1)×d_f , is given by the geometric summation:

where m is the number of rings around the central fiber in the pack.

For circular fibers packed in a tight hexagonal array, the number of fibers, N, is given approximately by:

Where d is the fiber core diameter and D is the fiber bundle diameter. The packing fraction is thus given by:

Where A = pD²/4, is the area of the bundle. See the Fiber Optic & Capillary Assemblies for additional information.

Preform
The starting form of glass or silica that is used to generate fiber or capillary by heating and drawing to produce the final smaller product size.

Polymer
A substance consisting of large molecules that are made of many small, repeating units of monomers. The number of repeating units in one large molecule is called the degree of polymerization. Materials with a very high degree of polymerization are called high polymers.

Polymers consisting of only one kind of repeating unit are called homopolymers. Copolymers are formed from several different repeating units.

Most of the organic substances found in living matter, such as protein, wood, chitin, rubber, and resins are polymers. Many synthetic materials, such as plastics, fibers, adhesives, glass, and porcelain are also to a large extent polymeric substances.⁵

Polyimide
Polyimide is an aromatic, linear polymer typically produced by condensation reaction, such as polymerizing aromatic dianhydride and aromatic diamine. It is not a true thermoplastic because it thermally degrades before its glass transition temperature. It is not a true thermoset because it is not cross-linked. The most notable properties are its solvent resistance, barrier properties, and performance at both high and low temperatures.

Proof testing or stressing
A means of testing the strength of optical fibers to ensure reliability by applying stress to the fiber. Flaws generated during manufacturing or handling will be exposed during the proof test spooling operation. The purpose is to expose any fiber or sections of fiber that might exhibit breakage during later use.

Raman spectroscopy
The field of spectroscopy that uses optical frequency shifts and intensity changes in the Raman chromophore(s) in test sample produced by monochromatic illumination, to determine the characteristics of the sample. Also, see UVRRS.

Ralyeigh scattering
Scattering by very small particles compared to the wavelength of the radiation being considered. A feature of Rayleigh scattering is that the scattered flux is inversely proportional to the fourth power of the wavelength. Thus in the visible region, blue light is scattered more strongly by the molecules of the air than longer wavelengths, accounting for the blue color of the sky.⁶

Refractive index
The ratio of the speed of light in a vacuum to the speed of light in the material. Also called the Index of Refraction, it is usually dependent on wavelength for optical materials.

Glass core material refractive indexes range from 1.35 up to 1.8 and cladding materials from 1(air) to 1.33(water) to around 1.56. Core materials include many amorphous glassy materials including synthetic fused silica, while the cladding can be air, plastics, polymers, epoxies, or other expansion compatible glassy materials.

Rod(s)
In optical fiber terminology, a rigid glass or quartz element, having the inherent properties of an optical fiber but of a much larger diameter. Rods are generally not flexible. Can also be used to describe the preform that is used to generate the optical fiber.

SI
Systeme Internationale d’Unites, the international metric system of units.

Silica
Silicon dioxide (SiO₂)

Silicone(s)
A class of polymer materials. In optical fiber, some are used as buffer materials and others for cladding. Not to be confused with silica (SiO₂) or silicon (the element, Si).

Singlemode fiber
An optical fiber in which only one mode, the fundamental mode, is transmitted. This mode travels straight through the fiber without reflection at the core-clad interface. Core diameters are typically 5-10mm, making the alignment very critical. Getting light into a singlemode fiber is sometimes difficult without wasting energy.

Snell’s law
The relationship between an incident ray at angle I in refractive index media n₁, and the refracted (or reflected ray) at angle R in refractive index media n₂ is:

Specular reflection
When light obeys the law of reflection, it is termed to be specular reflection. See also diffuse, Lambertian and spread.

Spectrum
In Optics, generally the electromagnetic spectrum within the wavelength region extending from the vacuum ultraviolet at 40nm to the far-infrared at 1mm. (EIA*)

Spectrum, Visible
The region of the electromagnetic spectrum to which the retina is sensitive and by which the eye sees. It covers the range from about 400 to 750nm in wavelength.

Spread
A type of reflection that is a combination of diffuse and specular reflections. This type of reflection has one or more dominant directional components.

Solarization
A change in material characteristics due to illumination of a material with ultraviolet (or solar) light. High intensities of UV illumination can cause photo-thermal damage in silica optical fibers, dramatically increasing the scattering and attenuation.

Static fatigue
Degradation of the strength over time, of an optical fiber that is under stress. The stress can be due to a bend, tension, torsion or a combination thereof. Also, see tensile strength.

Step-index fiber
An optical fiber that has a uniform core index and a lower uniform cladding index, creating a step change in refractive index profile. Also, see graded index.

Super-cladding or secondary cladding
A secondary optical material surrounding the primary cladding which has a lower index than the cladding. Sometimes the buffer material can act as a super-cladding, giving rise to unexpected higher order modes. Some bonding materials as well as liquids can also act as super cladding.

Synthetic Fused Silica
See Fused Silica.

Taper
A section of optical fiber or a micro-component that has a continuously changing outer dimension, along its length, from one end to the other. It can be a separate component or an integral part of the optical fiber tip. Although most have a circular cross-section, they can be made in other shapes.

Tensile strength
The strength of an optical fiber when placed in tension. Usually given in units of kilo-pounds per square inch (kpsi). Also, see proof testing and kpsi.

Thick wall tubing
Quartz or silica tubing which has a large OD to ID ratio. It can have a very small inside diameter (e.g., <10mm) requiring a larger than normal outside diameter for handling and strength purposes.

Total Internal Reflection (TIR)
The condition resulting in total reflection from a glass/glass, glass/air or a glass/metal interface in which all of the light is reflected. In optical fiber, it is the condition defined as sin^-1 (n₂/n₁), where n₂ is the lower index media and n₁ is the higher index media. For optical fiber to work, the cladding is n₂ and the core is n₁. As the angle of incidence is increased from the normal to the surface, the critical angle is that angle where total internal reflection begins to take place.

Transmission
In optics it is often given as the percentage of light or energy getting through an optical system (light output divided by the light input). Transmission data may or may not include surface losses, such as fresnel reflections, but should.

Transmittance
The ratio of the radiant power transmitted by an object to the incident radiant power. This is normally considered a bulk property, and does not include external losses such as fresnel reflections.

Tubing
The raw material that is drawn down to the final dimension required in fused silica capillary production. Sometimes the capillary itself is referred to as tubing.

Ultraviolet (UV)
A wavelength range below the lower end of the visible spectrum. The UV most often refers to the range from 400nm down to 200nm. Below 200nm a vacuum system is used to get useful transmission. In addition to fused silica, only a few crystal materials transmit UV below about 350nm such as, magnesium fluoride, sapphire, calcium fluoride and lithium fluoride.

V number
The normalized frequency parameter or V-number gives the upper limit of the number of modes that can be transmitted in a multimode optical fiber. It depends on the core diameter, d_c, the NA, and the wave length, l. The V-number is given by:

where n₁ is the core index and n₂ is the cladding index.

Waveguide dispersion
For each mode in an optical waveguide, the term used to describe the process by which an electromagnetic signal is distorted by virtue of the dependence of the phase and group velocities on wavelength as a consequence of the geometric properties of the waveguide. (EIA*) Also, see dispersion.

Wavelength
The length of one wave cycle of a light wave. It is most commonly expressed in nanometers (nm) for the visible and UV, and “microns” (10nm/mm) in the IR spectral regions. The frequency (and also bandwidth) is inversely proportional to the wavelength.

Wave Number
The frequency of a wave divided by its velocity of propagation; the reciprocal of the wavelength.

WDM
Wavelength division multiplexing (WDM) is a term use in an optical fiber application where several discrete wavelengths are combined into one fiber. At the distal end, the process is reversed to separate the wavelengths. A diffraction grating and a focusing mirror are used to combine two or more optical fiber outputs (differing wavelengths) onto a single fiber. At the other end of the single fiber, the reverse process is used. To achieve low coupling loss, small fibers, thin cladding and tight, accurate spacings are required.

Xenon arc
The arc formed when the rare gas xenon is excited electrically and emits a brilliant white light. Xenon is used to fill electronic and stroboscopic flashlamps, and also large discharge tubes for lighting large areas, often used in place of a carbon arc for projection of motion picture films.

X-ray
A region of the electromagnetic spectrum at wavelengths below ultraviolet (shorter wavelengths). Customarily expressed in energy units rather than wavelength. Wavelengths range from <1nm to >0.01nm which is approximately 1.2keV to 120keV.