A pathway allocating two-way data or communication exchange.
In optical digital communication, bits can be transmitted using intensity (e.g., non return to zero (NRZ), return to zero (RZ)), or phase modulation (e.g., DPSK). These modulations use two states to represent numeric values (0; 1). In the case of non return to zero (NRZ) modulation, the '1s' are represented by the presence of signal and the '0s' by the absence of signal. In DPSK modulation, the '1s' and '0s' are represented by a 180° phase difference. This numeral system is called binary.
In telecommunications lexicon, a binder is a grouping of wires inside a common sheath. The common two-pair telephone cable you can buy at hardware stores for household wiring jobs (black, yellow, green, and red wires) is a two-pair binder. Binders can hold almost any number of wires. Thick telephone company trunk binders may hold 250 pairs. Neighborhood streets generally have 20, 50 or even 100 pair telephone binders supplying “dialtone” to the neighborhood. Within a house, two-pair binders are very common. Modern office building often funnel 4, 6, or 8 pair cables to each desktop to provide telephone and computer network connections.
Having both positive and negative polarity.[Arr11]
A type of direct current signal in which consecutive marks are of opposite polarity and a space is represented by zero voltage.[Arr11]
Also known as double refraction, birefringence is the decomposition of a ray of light into two rays when it passes through certain anisotropic materials, such as crystals of calcite or boron nitride. The effect was first described by the Danish scientist Rasmus Bartholin in 1669, who saw it in calcite. The effect is now known to also occur in certain plastics, magnetic materials, various noncrystalline materials, and liquid crystals. [Wik1111] Crystalline materials may have different indices of refraction associated with different crystallographic directions. A common situation with mineral crystals is that there are two distinct indices of refraction, and they are called birefringent materials. If the y- and z- directions are equivalent in terms of the crystalline forces, then the x-axis is unique and is called the optic axis of the material. The propagation of light along the optic axis would be independent of its polarization; its electric field is everywhere perpendicular to the optic axis and it is called the ordinary- or o-wave. The light wave with E-field parallel to the optic axis is called the extraordinary- or e-wave. Birefringent materials are used widely in optics to produce polarizing prisms and retarder plates such as the quarter-wave plate. Putting a birefringent material between crossed polarizers can give rise to interference colors.
A widely used birefringent material is calcite. Its birefringence is extremely large, with indices of refraction for the o- and e-rays of 1.6584 and 1.4864 respectively. [Hyp11] Liquid crystals are found to be birefringent, due to their anisotropic nature. That is, they demonstrate double refraction (having two indices of refraction). Light polarized parallel to the director has a different index of refraction (that is to say it travels at a different velocity) than light polarized perpendicular to the director. In the following diagram, the blue lines represent the director field and the arrows show the polarization vector. Thus, when light enters a birefringent material, such as a nematic liquid crystal sample, the process is modeled in terms of the light being broken up into the fast (called the ordinary ray) and slow (called the extraordinary ray) components. Because the two components travel at different velocities, the waves get out of phase. When the rays are recombined as they exit the birefringent material, the polarization state has changed because of this phase difference. [cwr11]
Erasmus Bartholin, Experimenta crystalli islandici disdiaclastici quibus mira & infolita refractio detegitur [Experiments on birefringent Icelandic crystal through which is detected a remarkable and unique refraction] (Copenhagen, Denmark: Daniel Paulli, 1669). See also: Erasmus Bartholin (January 1, 1670) "An account of sundry experiments made and communicated by that learn'd mathematician, Dr. Erasmus Bartholin, upon a chrystal-like body, sent to him out of Island," Philosophical Transactions of the Royal Society of London, vol. 5, pages 2039-2048.
^ The Science of Color, by Steven K. Shevell, Optical Society of America. Published 2003. ISBN 0444512519
Birefringence Diagram courtesy of CWRU, http://plc.cwru.edu/tutorial/enhanced/files/lc/biref/biref.htm
A device that modulates the polarization of light at a high frequency, like a modulated quarter wave plate. The peak phase shift, or retardation amplitude, can be adjusted to any desired value. In high sensitivity applications it is often advantageous to work with ac modulated light signals. Lock-in amplification at the modulation frequency greatly reduces signal noise. For ellipsometry, modulation of the elliptical polarisation of the incident light beam leads to simple relationships between the ac signals and the real and imaginary parts of the complex reflectivity ratio. [Bea01]
Birefringence Modulator Illustration courtesy of Beaglehole Instruments, http://www.beaglehole.com/modsys/modulator/modulator_main.html