Fiber Optics Illustrated Dictionary away at full speed.) 2. The tendency to continue a sig- nal, echo, electrical charge, or data transmission af- ter the actual communication has ceased or the mes- sage part has been received. 3.In aphosphor display system, the tendency of the phosphors to continue to fluoresce after the stimulus has stopped. This may be an undesired property, causing smear, or may be a de- sired property, enabling the image to remain view- able while the rest of the frame is being imaged. persistence of vision A phrase that describes the way in which human visual perception "holds" an image for a brief moment, about a tenth ofa second, even if the objects in the visual field have changed or moved. Thus, humans can only scan or perceive still images up to a speed of about 24 to 60 frames per second. Faster than that and they are no longer seen as still images, but as a series of moving or related images, especially if the forms in the images are closely re- lated to the previous ones. Researchers Muensterberg and Wertheimer demonstrated in the early 1900s that this was a property of brain processing and percep- tion more than a physical property of the retina. These characteristics of visual perception have greatly in- fluenced the design and development of moving vi- sual communications technologies. See frame, scan lines. Personal Communication Network PCN. See Glo- bal System for Mobile Communications for the back- ground and technology base for PCN. PCN was de- veloped, starting in the late 1 980s, as a modified form ofGSM operating in the 1800-MHz frequency band (GSM is 900 MHz). It has smaller cell sizes, requires lower power, and is optirnized to handle higher den- sity traffic than GSM, but otherwise is essentially the same. The PCN standard was finalized in 1991. It is primarily used in the United Kingdom. See Global System for Mobile Communications. Personal Communications Service. PCS. A low- power, higher frequency, standards-based, wireless mobile communications system, operating in the 1800- and I900-MHz range, implemented in the mid- 1990s. Most PCS systems are 100% digital. In con- trast to cellular, which is limited to A and B carriers, PCS operates across six (A to F) carriers. In other words, cellular can be thought of as a subset of PCS in its broadest sense. Three operational categories of PCS have been de- fined by the Federal Communication Commission (FCC) as shown in the PCS Categories chart. In PCS, particular channels are assigned to specific cells, with provision for reuse. A channel is associ- ated with one uplink and one downlink frequency. A specific number of channels is assigned to an operator's authorized frequency block. PCS service can be installed as a centralized or distributed archi- tecture, and supports both time and code division multiple access (TDMA, CDMA). Designed to broaden market distribution ofwireless services, the system may have more limited range than traditional cellular, but the cheaper connect times and handsets may be appealing to consumers. Industry watchers are predicting steady growth in mobile communications. 732 In Japan alone, there were more than 20 million In- ternet-capable PCS system subscribers by 200 I. See AMPS, cellular phone, DAMPS, DCS, GSM, Per- sonal HandyPhone Service. PCS Categories Category Notes narrowband PCS PCS operating in limited bandwidth in the 900-MHz spectrum and not suited to high speed data communications, although low-bandwidth short text messages would work. Best suited to in-building and near outside-premises use, pagers, and cordless phones. broadband PCS PCS in the 1.9-GHz spectrum range for better quality voice communications and higher duplex-mode data communications. unlicensed PCS PCS in the 1910- to 1930- MHz range, suitable for in- house and in-company systems, and small independent service providers. Limited to low-power signals. personal computer Pc. A compact, relatively low- cost computer system designed for home, school, small business, and prosumer (high-end consumer) use. The first fully assembled, affordable PC with a keyboard and CRT monitor was probably the SPHERE computer released in 1975, but it didn't sell well. Subsequently, the Radio Shack TRS-80 series, followed closely by the Apple computers and the Commodore PET were all commercially successful. At the time of the introduction ofpersonal comput- ers in the mid- and late-I 970s, the cost ofa worksta- tion-level computer was typically $40,000 and more, so the price tag of about $2000 to $6000 for a per- sonal computer with useful peripherals (printer, mo- dem, etc.) was revolutionary in terms of availability to individuals. In the early 1980s, when networks that could interconnect individual PCs began to prolifer- ate and CPUs became more powerful, the distinction between personal computers and higher end systems began to blur - a progression that continues to this day, with personal computers of the 1990s being more powerful than minicomputers a decade earlier and laptop computers of the 2000s being more powerful than mid-range institutional computing systems of the late 1980s. The development of PC networks also opened up hybrid systems, with PCs sharing the com- puting power of mainframes and mainframes using PCs as 1/0 devices. © 2003 by CRC Press LLC The tenn PC has been generically applied to systems used by individuals for personal, educational, and business purposes, and so does not fit the tenn "per- sonal" in its strictest sense. Some people use PC to refer only to ffiM-compatibles, which is not really a correct l1se of the tenn and has probably proliferated because "ffiM-compatible" is such a mouthful. The distinction between a PC and a workstation is not as cut-and-dried as many people think. By the time you add a graphics card, sound card, CD-ROM drive, more memory, and network interface card to a per- sonal computer, its cost is comparable to many off- the-shelf workstation-level computers. See Amiga, Atari, Intel, Macintosh, TRS-80, workstation. Personal Digital Assistant The Palm Personal DigitalAssistant (PDA) provides handheld mobile computing through a colorgraphics display resolution better than early desktop comput- ers. Full point-and-click Web browsing capabilities (right) are provided by the SojtSource/Catarra display client/proxyserverprograms communicating through a wireless radio link to the Internet. Personal Digital Assistant PDA. A handheld com- puterized wireless device optimized for common time-scheduling and note-taking activities that many business and personal users particularly desire. These include calendars, account keepers, note-takers, cal- culators, alarm signals, modem connections, data- bases, etc. Some PDAs support handwriting recog- nition through a penlike interface, others have small text keypad input screens, and some have both. The more recent PDAs have color graphics displays and the capability of full Internet browsing without the HTML and security certificate restrictions of WAP-based limited-resource instruction sets. PDAs were introduced in the late 1980s, with pen- recognition PDAs coming out in the early 1990s. Most PDAs work on batteries or AC power with a converter. Some work only with batteries. Battery life ranges from 2 to 5 hours on most systems, depend- ing upon usage. Apple ClockWorker is an interesting evolution in PDA technology. This little 300-MHz RISC chip with 30-MBytes of RAM and 70-Mbyte memory chip out- runs many full-sized desktop computers. Even more surprising is that it is powered by a clockwork mecha- nism developed in the U.K. Twelve turns of the AppleKey are said to provide up to 3 hours of con- tinuous use. The idea is not entirely new; analog wound watches have existed for decades, but this is an interesting adaptation to computer technology be- ing tested in full-sized notebook computers. See PDA macrobrowser, PDA microbrowser, SoftSource, Wireless Application Protocol. Personal Digital Cellular PDC. Fonnerly called Ja- pan Digital Cellular, this is a time division multiple access (TDMA) digital cellular phone system used in Japan and, to a small extent, in the Asia-Pacific re- gion. PDC seIVices operate in the 800- and 1500-MHz radio frequency bands. It is an important standard due to the large number of subscribers (over 50 million) using PDC-based services. See Personal HandyPhone Service. Personal HandyPhone Service PHP. A commercial 32 Kbps mobile data Personal Communications Ser- vice (PCS) popular in Japan. PHP was established in 1995 and began providing services to sub~cribers in 1997. In 1998, 64 Kbps services were introduced in some areas. The PHS network can be accessed by subscribers through various Personal Digital Assis- tants (PDAs) and notebook computers. The PHS net- work is separate from or totally independent of the public switched telephone network (PSTN). Personal Identification Number PIN. A system of alphanumeric characters, usually numerals, which identifies aparticular user or holder of an identifica- tion card. PINs are commonly used for credit cards, bank cards, ill cards, calling cards, and other fonns of wallet-sized identification to access security doors, ATMs, phones, and vending machines. PersonalJava applications environment See Java. Personal Wireless Telecommunications PWT. An in-building wireless telecommunications transmis- sion standard in North America (U.S., Canada, Puerto Rico) developed in the mid-1990s. It is similar to the Digital European Cordless Telecommunications (DECT) standard in Europe. It is intended for short distance, high-bit-rate, packet-based communica- tions. PWT uses unlicensed Personal Communications Sys- tem (PCS) spectrum in the 1.9-GHz radio frequency band. Standards for the use of Frame Relay for mobile PWT-compliant devices (Project 4247) and for ex- panded PWT in the 1850 to 1910 and 1930 to 1990 MHz frequency bands were initiated within the TIA and EIA. Enhanced PWT uses licensed PCS spec- trum. peta- P. A prefix for an SI unit quantity of lOIS, or 1,000,000,000,000,000 - a really huge quantity. See exa-, femto petticoat insulator A historic utility pole electrical line insulator that still has practical use. Many histo- rians have suggested they were developed around 1910, but it was certainly much earlier, as glass or porcelain petticoat insulators were already listed as a requirement for outside wiring in the National Elec- trical Code of 1899. The earliest fonns were single petticoats, with double-petticoats developed later. 733 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary The name refers to the outer underskirt-like shape of the insulator, which has flare for channeling moisture away from electrical wires, a shape practical for both glass and non-glass insulators. See insulator, utility pole. PGP See Pretty Good Privacy. PGP Inc. A company jointly established by Philip Zimmermann, the developer ofPretty Good Privacy, and Jonathan Seybold. See Pretty Good Privacy; Zimmermann, Philip. PGPIMIME Pretty Good Privacy/Multipurpose In- ternet Mail Extensions. An IETF working group In- ternet messaging standard for the transmission of se- cure network communications. Avariety of content types have been provided for MIME, and more con- tinue to be added. Unlike SIMIME, PGPIMIME does not use public keys distributed through X.509 digital certificates. PGP can generate ASCII armor (required) or binary output for the encryption of data. The trend is for the signed portion of the message and the mes- sage body to be treated separately. PGP/MIME can support 128-bit encryption, although not all imple- mentations will use the full 128 bits. See S/MIME, RFC 1847,RFC 1848,RFC 2015. phantom circuit In telephony, a means ofdevising an additional circuit by utilizing resources from ex- isting circuits on either side. Thus, three circuits can be configured to prevent crosstalk and used simulta- neously with only four line conductors. The use of phantom circuits has, for the most part, been super- seded by a variety of multiplexing techniques. See Carty, John 1. phantom group In telephony, aphantom circuit and the balanced circuits that flank it and from which it draws some of its circuitry. phase alternate line PAL. Acolor television broad- cast and display standard widely used in the United Kingdom and a number of European, South Ameri- can, and Asian countries. The name originates from the fact that the color signal phase is inverted on al- ternate lines. The format was introduced in the early 1960s. It displays at 25 frames per second and can support up to 625 scan lines (not all are seen on the screen; some at the bottom may be obscured). It pro- vides a better picture than the NTSC format preva- lent in North America and is not compatible with NTSC or SECAM. PAL-M is a variation on PAL which supports 525 lines. phase change rewritable PCR. A type of high-ca- pacity optical storage technology, developed gradu- ally over the period from the early 1980s to the mid- 1990s. During the 1980s, Matsushita developed a number ofPCR WORM drives, and released a read! write drive in 1991. PCR enables multiple rewrites on the same cartridge. Using a pulsed laser diode at a higher power level, the recording surface of a disc can be changed be- tween low reflectivity amorphous states to crystalline states, enabling data to be erased and written/rewrit- ten. The data can be written in one pass rather than the two passes required for a number of magneto-op- tical technologies. Once the technology appeared commercially promising, Matsushita developed a 734 combination PCR/CD drive, announced in 1994, and Toshiba led a development group to adapt phase change technology for creating rewritable Digital Versatile Discs (DVDs). At first, industry adoption and standardization efforts were not broadly sup- ported. phase conjugation A phenomenon discovered in the 1 960s, phase conjugation is now a general concept used to describe a number ofnonlinear optical phas- ing processes. Phase conjugation involves the precise reversing of the direction of the phase and propaga- tion of a wave such that it travels back through the same path through which it originally arrived. Thus, optical phase conjugation is the precise reflection of a light beam back through its original path. Phase conjugation has many applications. It can be used in the development of tracking systems, lens- less imaging technologies (e.g., holograms), and de- fect detection systems. It can also be used to filter a signal or to regenerate a signal that has degraded en route, which would be a boon to many types of com- munication transmissions. NASAlJPL is using the concept to propose designs for very fine fiber optic- based probes for imaging in tightly confined spaces. See phase conjugation mirror. phase conjugation mirror PCM. A reflecting mir- ror that may be used in conjunction with other mir- rors in laser light beam directing systems, for ex- ample, but which is distinguished by its capability of precisely reversing the direction ofa wave hitting the mirror. Contrast this with conventional mirrors, in which the direction of the reflected wave is related to the angle at which the wave hits the mirror. In ad- dition, in a conventional mirror, only the sign of the wave vector component is changed, while in a PCM, the entire propagated beam reverses direction and the phase of the beam is conjugated or joined together. The phase conjugation process can be enhanced, de- pending upon the environment in which the process is carried out. Freon has potential as a stable medium. In the early I 990s, photorefractive polymers were developed in IBM laboratories. Since then, layered versions have increased their usefulness for industrial purposes. New polymer-based photorefractive com- pounds may replace crystals for some types of PCM applications as their technology improves and the cost dramatically drops. See phase conjugation, photore- fraction. phase drive PD. A type ofoptical data storage drive based upon phase-change recording such that the op- tical medium can be rewritten. See change rewritable. phase jitter Aparticular type ofundesirable aberra- tion in which analog signals are abnormally shortened or lengthened. See jitter. phase-shift keying PSK. A type of modulation scheme which distin~ishes between a binary"I" (one) and a binary "0' (zero), by changing the phase of the transmitted signal 180° if the next input unit is a binary "0" (zero). If it is binary "I" (one), then a phase shift is not executed. See frequency modula- tion, frequency shift keying, on/off keying, quadra- ture phase-shift keying. © 2003 by CRC Press LLC Phase-Shift Keying irJ, '" '. ( \ ( \ ~ , I \ I v J 1\ 1, \ ) I j \ (:)I~e L "e ',," ." I 1\ (\ I \ 1 , "J, \ I \ J !_~__ \ j \ J ·l ,,+- .Cycle· ·I ~'¥",,~"'II4I ~+- ',' I' '.' ".;-,~,~, +, nl-~ A wave period is one segment from the repeating sinusoidal cycles of the wave taken over time from a reference point on the wave. The period varies with the wave - longer wavelengths have longer periods. In A, the period of the wave begins at zero (0). In B, the wave has been shifted by a quarter of its period such that it is referencedfrom the highest point in the wave cycle rather than the point at which it crosses the X axis. The length of the wave period hasn t changed, only the time point in the phase at which it is referenced, relative to thefirst wave. If the two differentphases in the wavewereplottedon top of oneanother, they would undulate with the same period length, shape, and am- plitude - only the phase has been shifted. By creating a series of shifts in the waves, relative to the preceding wave, it is possible to use each indi- vidual wave to represent a binary value. Thus a half- period shift in a four-phase system changes a 2-bit binary valuefrom 00 to 10 andfrom 10 to 00. phase-locked loop PLL. A technology used in sili- con-based integrated circuits (ICs), a PLL circuit con- trols an oscillator at a constant phase angle relative to a reference signal. The three basic aspects ofa digi- tal PLL are a controllable oscillator, a filter, and a phase detector/comparator combined within a closed- loop frequency feedback system. PLLs are useful for signal processing and synchronization applications such as controlling automatic phase adjustments in a signal. The signal can be referenced by the PLL in various ways; it can be based upon acarrier signal or linear or nonlinear baseband references. PLL was traditionally analog, but there are now also digital versions and both are suitable for various types of applications. PLL has been around for several de- cades; it is commonly used to synch a reference broadcast signal to the horizontal oscillator ofa tele- vision receiver, for example. Because it is a basic tim- ing technology, it is found in components ranging from voltmeters and spectrometers to cell phones and space-based tracking and synchronization systems. In communications devices, newer PLL circuits sup- port products with higher data transfer rates, higher frequencies, and smaller footprints. Commercial dual phase-locked loop-based ICs are small, low-power- consumption components that can offer frequencies up to 2.5 GHz (in some cases, up to 4.8 GHz), mak- ing them suitable for radio transceivers for a variety of types ofproducts, including cellular phones and pcs. PLL ICs can also be used as secondary circuits for providing intermediate frequency radio waves that are commonly used in cell phone receivers. PLL circuits can be readily modeled in software for educational and design purposes. Java-based PLL modelers are available on the Web. Phelps, George M. (1820-1895) An American ma- chinist and inventor best known for his telegraphic key and printer inventions, although he also designed stock tickers (a type of specialized telegraph) and early telephone equipment. As a youth, Phelps was apprenticed as a machinist to his uncle, Jonas H. Phelps, to build scientific instruments. The Phelps and Gurley surveying instruments company evolved into Gurley Precision Instruments, which is still in business. George Phelps set up shop in 1850, in Troy, New York, and began designing and patenting a wide va- riety ofprecision electromechanical devices, includ- ing telegraph keys (e.g., a camelback key). He was known for elegance of design and superior workman- ship. When approached about improving upon the popular but complex telegraphic instrument ofR.E. House, Phelps joined with Jarius Dickerman to form Phelps and Dickerman and House's Printing Tele- graph Instrument Manufacturer, located in Ferry Street in Troy. Thus, Phelps built House instruments for several years. The American Telegraph Company purchased the Phelps and Dickerman holdings, retaining Phelps as a superintendent. After the American Civil War, American Telegraph was purchased by Western Union, again retaining Phelps for his knowledge and experience in the field. Western Union also acquired 735 ,xgP , l f .' , '~f © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary the patent rights to Phelps' printing telegraph. Phelps was assigned to work on a"harmonic telegraph," the forerunner to the telephone, adevice first patented in the u.s. by A. Graham Bell. Phelps was an associate ofThomas Edison and cre- ated some of the patent models for Edison's early in- ventions. Phelps became the superintendent of West- em Union Telegraph in New York and remained as a staff inventor in his later career. He may also have been associated with the Field brothers, who were instrumental in laying the first successful transatlan- tic telegraph cable. See Phelps Combination Printer. PHIGS Programmer's Hierarchical Interactive Graphics System. An official standard for 3D graph- ics from the late 1980s. The PHIGS+ extension added sophisticated rendering ofrealistic looking objects on raster displays. Simple PRIGS (SPRIGS) is a pow- erful, display-independent subset of PRIGS which in- corporates some PRIGS+ features. Phillips code A shorthand telegraphic code as- sembled/revised from existing systems by Walter Polk Phillips, published in 1879. Originally an American Telegraph messenger, Phillips became an accom- plished press telegrapher (2731 wph) and his code was widely used for decades. See 73 in Numerals chapter. phoneme A unit of speech, considered to be the small- est distinguishable unit, which may vary from lan- guage to language and among dialects of aparticular language. Phonemes are of interest to programmers for speech recognition and speech generation appli- cations. See speech recognition. Photo CD Kodak Digital Science Photo CD System. An image storage and retrieval format developed by Kodak and introduced in 1992. PhotoCD is a means to store digitized still images in various resolutions on a compact disc so it can be read back from CD- ROM drives. It is used by many stock photo suppli- ers and graphic design professionals. Conventional35mm film shot with a traditional cam- era can be taken to photofinishers supporting PhotoCD and developed into both pictures and digi- tal images. At the lab, the file is scanned with a high resolution drum scanner and saved onto Photo CD discs. If there is room, additional pictures can be added to the disc later, and read back with a multi- session CD-ROM XA drive and an appropriate soft- ware driver (including Apple QuickTime Photo CD extension, SOl's IRIX, Sun's Solaris, IBM's OS2/WARP, AmigaOS 3.1, IBM AIX, etc.). A Photo CD disc can hold about 100 images, that is, about three or four rolls of film. The images are stored in Photo YCC color encoding, with multiple resolu- tion levels. Pixel resolutions include: 2048 x 3072, 1024 x 1536,512 x 768, 256 x 384, 128 x 192. The Photo CD Pro format also includes 4096 x 6144. See compact disc. Photocopy Machine - Original Invention C. F. CARLSON ~L£CfllOrIiOTOORArIlT Pll April -I. lt311 Oct. 6, 1942. INVENTOR 2,297,691 OeL 6, 1942. C, P. C""LSOH 1;l.CCfllOfltOTOGurtty 'Ilel! AprIl 'i. II;» 2,297.691 The 1942 Carlson patentshows the various basic parts of a photocopier (right), with a detail of the drum mecha- nism (left). Large companies were not willing to purchase the new technology. A small company calledXerox did! 736 © 2003 by CRC Press LLC photocopyA dry transfer replication process from an optically imaged source, sometimes also called a xerograph, after Xerox, the company that popularized the technology. C.F. Carlson was awarded a patent for a photocopy invention in 1942 and failed to sell it to some of the larger business-oriented companies. But a small company called Xerox took a chance on the technology. See the Carlson patent diagram. photodetector PD. A component or biological sys- tem that responds to stimulation by light. Plants have photosensitive structures and mechanisms that enable them to detect sunlight and orient themselves towards it and certain natural and synthetic materials have photodetecting properties that can be incorporated into industrial device assemblies. Since light has a number of wave-like and particle-like properties and emits heat at different levels depending upon loca- tion and time of day, the definition ofphotodetector is somewhat broad, reflecting the capability of react- ing to the presence oflight without necessarily speci- fying what aspect of light is causing the reaction. In general, photodetectors are subclassified as thermal detectors and photon detectors. Simple photodetector components may respond only to the presence (or absence) of light within certain pa- rameters and some may be sensitive to light without discriminating its intensity or character. More sophis- ticated photo detectors may be "tuned" to detect spe- cific wavelengths or regions of wavelengths and some are also sensitive to the magnitude ofa light stimu- lus. Even at its most basic level, however, photode- tection is an important capability at the heart of many systems. Photodetectors are widely used in imaging devices, security systems, robotic vision, and signal- ing and transmission systems. In practical applications, the response of a photodetecting substance is often very weak and may require further processing to make it useful. Ampli- fication of very subtle reactions to light has limits, due to noise that is introduced when a weak signal is amplified. Much of semiconductor technology is de- voted to improving the signal-to-noise ratio of am- plified signals. In addition, photodetectors are often environmentally sensitive. Light is ubiquitous and it is often challenging to detect only that light that is of interest. For example, athermal-sensitive detector in a hot environment such as a desert, may need to be cooled in order to detect other sources of light (e.g., a signal light). An astronomical photo detector (for studying light from celestial bodies) works more ef- fectively if placed in orbit around the Earth rather than in the observatory ofa university in the middle ofa large city, due to the interaction of ambient light sources. Depending upon the type of detector, commercial photo detectors are typically described in terms of re- sponsivity (the sensitivity and magnitude oftheir re- action to light), efficiency (how much signal is gen- erated per photon stimulus), response time, signal-to- noise ratios and types of noise (e.g., Johnson noise), and the linearity of the response. Figures of merit may also be used. Film photography is an example of directly harness- ing the selective photosensitivity of certain chemicals by embedding them in a film substrate and briefly exposing them to light. The image captured in film can then be transferred to paper by yet another pho- tosensitive process (with stray light excluded in a darkroom). Sometimes photodetection is only one step in a series of detection and conversion processes. F or example, a scintillating device that converts elec- tromagnetic energy outside the optical spectrum, such as X-rays, into optical wavelengths, may feed the signal to a photodetector. From there it may go to a photomultiplier that further converts the signal to electrical impulses. Thus, a photodetector assembly can indirectly detect wavelengths outside the optical spectrum. A complex light impulse can be characterized by us- ing a device in which multiple photo detectors are tuned to respond to different optical frequencies. The data derived from individual elements in the photo- detector array can be signal processed to produce a complex overall statistical picture of the light-emit- ting characteristics of sample specimens or light-car- rying transmissions media. The creation of semiconductor photo detectors is as much art as science and much of the fabrication is at the molecular level, crossing boundaries in geology, quantum physics, chemistry, and biology. Structures for photo detectors can be grown in molecular beam epitaxy (MBE) systems on semi-dielectric substrates. Such components are being developed for new high- speed photodetectors, giving them properties for meeting the greater bandwidth and distance demands of microwave fiber optic links. See photoelectric cell, phototube, thermopile, traveling-wave tube. photodiodeA semiconductor photodetector compo- nent for converting light energy into electrical energy. See photodetector. Sample Photodiodes / / / Photodiodes are semiconductorphotodetecting com- ponents. They come in a wide variety of shapes, sizes, andlevels of sensitivity to photonic energy. Illustrated here are common configurationsfor a gallium-arsenide diode (left) and an indium-gallium-arsenide diode. photodiode, avalanche APD. A semiconductor com- ponent commonly made of silicon (Si) or indium- 737 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary gallium-arsenide/indium-phosphide (InGaAs/InP). Silicon APDs are p-n junction solid-state detectors with high internal gain. They are reasonably immune to electric fields and sensitive enough to detect single photons at room temperature. APDs are used for optical detection for a variety of applications including fiber optic communication re- ceivers, fluorescence detectors, photon counters, time-of-flight ranging devices, and cryptography. Fi- ber optic receivers commonly use p-i-n photodiodes or APDs for detecting and converting an optical sig- nal into an electrical signal. New indium-gallium-arsenide/silicon (InGaAs/Si) APDs have been developed under a grant funded by AFRL/DARPA with separate absorption and multi- plication (SAM) regions for use in near-infrared fre- quencies. These offer faster, more sensitive photode- tection at wavelengths that were not previously prac- tical. See avalanche diode, Zener diode. photoelectric cell A type of electronic sensing device activated by light and widely used in security systems, automatic lighting systems (e.g., street lights), auto- matic doors, etc. A photoelectric cell can be made by coating cesium on one of the electrodes in a vacuum tube. This technology was used in early television cameras. See photo detector. photography The art and science of registering light from objects in a scene and storing them in the form of an image. Later it became possible to produce multiples of these images by a number of means. Most photography involves capturing three-dimensional imagery in a two-dimensional format. Light is usu- ally recorded from the visible spectrum, but there are cameras and films designed to record heat and infra- red radiation which show images in a form different from the way humans perceive them, and electron microscopes record the movement of a beam of electrons. Traditional photography was developed in the early 1800s by a number of inventors including Joseph Nicephone Niepce, a French inventor, who developed a process called heliography or sun drawing, on pa- per coated with silver chloride. Other pioneers in- cluded Daguerre (originator of the daguerreotype), Herschel, Talbot, and Archer. One of the earliest pho- tos was captured with silver chloride by Thomas Wedgewood in 1802. More than 150 years passed be- fore 3D photography, in the form of holographs, be- came practical. Newer digital cameras can immedi- ately relay an image to a computer network so the image can be viewed almost instantly at great dis- tances from the actual scene of the event. See Daguerre, Louis Jacques Mande; heliography. photometerAn instrument for determining the inten- sity of transmitted or reflected light, sometimes called an opticalpower meter. A photometer is a type of ra- diometer and photometers that measure the intensity of frequencies beyond the human visual range are sometimes termed radiometer/photometer devices. Photometers are used in scientific research, photog- raphy, and many aspects of experimental and com- mercial optics. Human visual senses are quite good 738 at determining relative brightness, but photometric instruments are needed to make objective assessments of light intensity within and beyond the human vi- sual range. Photometers come in many shapes and sizes from simple photography or classroom models to high-end scientific research instruments. They may be used to measure power levels in laser beams, optical signals in modulated light beams, and solar radiation. Pho- tometers are used to measure the intensity of traffic lights (which may dim over time) to make sure they are bright enough to be seen clearly by motorists. Goniophotometers are common in the lighting indus- try. Photometers aid in assessing light propagation through different types of waveguides in the design and development of optical network technologies. The range of sensitivity ofa photometer is dependent upon its price and intended application. The spectral range within which it is sensitive also varies, but com- monly photometers measure visible and infrared fre- quencies. A basic classroom photometer may include several measurement scales with sensitivity to power levels ranging from about 20 microwatts to 20 milli- watts. Measurement scales may be linear or logarith- mic. Some industrial photometers have optional, in- terchangeable sensor heads for different applications. Simultaneous measurements of more than one wave- length are possible with some scientific models. The reading from a photometer may be output to a built- in LCD display or may be transmitted to other devices such as oscilloscopes, recorders, or computer periph- eral cards. Photometers designed for microscopes may have an adjustable iris to enable the sample to be viewed while the light is measured. A housing for filters may also be included. In astronomy, where light intensity provides informa- tion on the properties of celestial bodies, photometers are important research tools and may be integrated with spectrographs in telescopic systems. Sophisti- cated optical fiber-based photometers are now avail- able for studying fast variable astronomical phenom- ena. Multiple fibers enable reference images to be as- sessed in conjunction with the phenomena being ob- served. Fiber optics may be used to link individual telescopes in a telescopic array. The first known drawing of a photometer was by Peter Paul Rubens, who illustrated a book on optics by F. d' Aguilon, published in 1613. P. Bouguer de- scribed several simple photometers in a treatise pub- lished posthumously in 1760. This was an expansion of an earlier essay, published in 1729 and Bouguer is considered by many to be the inventor of the photom- eter. 1 0. Colladon developed a practical application of a photometer for his engineering proj ects in the mid-1800s. Prism-based spectrophotometers became available on the market after World War II but the technology remained relatively limited and expensive until the 1960s, when grating spectrophotometers became available. Since then advancements in elec- tronics have made photometers increasingly small and powerful. By the 1990s, built-in filters, exchangeable © 2003 by CRC Press LLC sensing heads, LCD displays, and computer interfaces were readily available. Fiber optics and lasers are now incorporated into a number of types of photometers. For example, in chemical photometry, a laser can be used as a light source for illuminating a sample to measure its pho- tometric characteristics. When the coherent light hits the obstacle (sample), the light is scattered and may be detected by a fine fiber filament that directs the light that enters the fiber to a photomultiplier, where it is passed on to a processing system and display. See Aguilon, Fran~ois de; Bouguer, Pierre; luminance; photopolarimeter; radiometer. photomultiplier PM. A light-sensitive component that emits electrons in response to stimulus by pho- tons (of sufficient energy levels). This is a very use- ful means to convert electromagnetic energy in the optical spectrum into electrical energy that can be used to activate and control other components. photomultiplier tube PMT. Typically, an evacuated glass component containing a photocathode that emits electrons when subjected to photonic energy suffi- cient to trigger aphotoelectric effect. The photocath- ode operates at a high negative voltage and the elec- trons emitted are accelerated towards a series (chain) of dynodes that are positioned along the electron path between the electron-emitting cathode and the elec- tron-attracting anode. The dynodes generate addi- tional electrons through secondary-emission multipli- cation. PMTs can be configured with multiple anodes, ar- ranged in linear (e.g., 1x 16) or grid patterns (e.g., 8x 8) for use with fiber faceplate scintillating appli- cations, for example. Photomultiplier tubes can respond to a wide range of wavelengths from ultraviolet to infrared, but respon- sivity and emission effectiveness are dependent, in part, upon the materials used. In general, PMTs are fast-response, low-noise components practical for a wide variety of applications, including laser technol- ogy, radiation measurement, spectroscopy, high en- ergy physics research, and others. Photomultipliers are sensitive enough to count pho- tons at very low light levels (down to one photon) and thus are highly efficient at distinquishing signal from noise. Thermal noise can be reduced by cooling and ambient light and magnetic interference can be re- duced with proper shielding. Commercial photomultiplier tubes commonly have 14, 20, or 21 pins. The primary connections are to the 10,12, or 14 dynodes, the anode, cathode, focus elec- trode, and shield. Simplified Drawing ofBasic Photomultiplier Components and Dynode Function photocathode anode photocathode anode This is a highly simplifieddraWing of a basicphotomultiplier tube usedto convert and amplify aphotonicsignal. The photocathode at the top converts electromagnetic energy in theform of photons into electron emissions which are attractedto the anode at the base of the tube. As the electrons travel towardthe anode, they encounter a series of dynodes in the middle 0/the tube, poweredwith voltages that are calibratedto one anotherto control the magnitude o/electron emissions. As an electron/rom the cathode (or the precedingdynode) hits a dynode, it is reflected along with secondary emissions governed by the voltage applied to the dynode to the next dynode in the chain, causing a cumulative amplification of the signal. When the electrons reach the anode, the signalisprocessed by a smallcircuit within the base and output through the contacts comingfrom the bottom of the base to inteiface with other compo- nents (21-pin sockets are common). A magnetic shield thatfits over the base can shield the electrical circuitsfrom external inteiference. As illustratedin the line diagrams, the voltages appliedto the reflective dynodes are relatedto the number of electrons emitted, with higher voltages (right) providing greater gain (within operating limits). Thus, very weaksignals, even as smallas onephoton, can be measured and manipulatedwithphotomultipliers to facilitate research in particle physics and to fabricate sensors, and scientific and industrial quality assurance, testing, and sampling instruments. 739 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary A phototube is a simpler version of the photomulti- plier tube (without dynodes). See dynode, photo- sensor. photomultiplier tube base A mechanical and volt- age distribution/dividing component for coupling with a photomultiplier tube. The tube base may op- tionally include a magnetic shield to protect it from Earth- and equipment-originating magnetic fields. The shield may also protect the coupled photomulti- plier tube from ambient light and magnetic emissions. The photomultiplier tube typically connects to the base through 14 or 21 pins. Outputs from the base, such as connections to the anode or a specific dyn- ode, are typically through 50-ohm coaxial connec- tions. Some versions include low-noise preamplifi- ers incorporated into the base for use with scintilla- tion detectors. Photomultiplier tube bases have also been designed for use with multiple photomultiplier tubes (e.g, in arrays). Voltages for the tubes in the assembly may PhotoPhone - Bell and Tainter's Light-Based Communications Invention A. G, BELL" S. TAINTER. Photophone Transmitter. Patented Dec. 14. 1880. :J:i?~.t. No. 235,.496 • :, _ ~ , ~ _. ~ I Jl J The Photophone was based upon the concept of using light as amediumforthe transmission of sound. Sunlight was used to translate acoustic vibrations into light signals that were reflected to a receiver where they were converted to electrical signals through the use oflight-sensitive selenium (the same material usedfor early television inventions). By substituting aparabolic surface, Bellfound he could increase the intensity of the signals and was able to transmit signals over adistance of several hundredmeters on asunny day. Bell was very excitedabout thepotential of wireless communications and took out four patents on the Photophone with assistance from Sumner Tainter. {Library of Congress American Memory Collection and U.S. Patent Office (upper right).] 740 © 2003 by CRC Press LLC be controlled individually, in groups, or unison. Cockcroft- Walton voltage multipliers have been sug- gested in place of resistive voltage dividers for PMTs that are densely packed, in order to minimize dissi- pated power. See dynode, photomultiplier tube. photombltiplier tube chamber A housing for physi- cally protecting, electromagnetically shielding, and cooling photomultiplier tubes. Depending upon the temperatures required, the housing may include single- or double-paned windows to prevent conden- sation or icing. A variety of materials are available for the windows, including Plexiglas®, Pyrex™, or fused silica. Fused silica is effective over a broader spectrum of wavelengths. See photomultiplier tube. photonic crystal A photonic bandgap technology described and developed originally by E. Yablo- novitch, developed further by Ozbay at Ames Labo- ratory, Southampton Researchers, S. Kawakami and his collaborators in Japan, and a number of others. These photonic crystals have periodic dielectric struc- tures that exhibit large anisotropy, high dispersion, and photonic bandgap properties. The bandgap, which is similar in concept to gaps in semiconductor devices with a lattice-like structure and holes or "wells," makes it possible to selectively filter certain optical frequency ranges by means not available with con- ventionallenses or existing semiconductors. Varying the refractive index of the component or in- troducing point defects within an othelWise perfect dielectric structure have the potential for localizing light, essentially trapping it selectively. The size of the holes could further be controlled to manipulate energy levels. Yablonovitch et al. have further de- scribed how 3D circuit designs could extend the tech- nology into lower wavelengths. In 2002, Chen and Suzuki described an integrated fiber-photonic crys- tal system with a uniform bandgap and low insertion loss. This has potential for optical switches and rout- ers. Also in 2002, OFS Laboratories introduced a new fiber design incorporating a photonic bandgap for tuning the transmission through the fiber. There is much excitement surrounding photonic crys- tal technology. It has been suggested that highly-ef- ficient light reflectors for fiber optics transmission sources (e.g., LEOs) and computers operating in the hundreds of terahertz computing range could be de- signed with the technology. MIT has developed freely available software to model the dispersion relations in photonic crystals in order to visualize the band structures. It is available for download online as MIT PHotonic-Bands. See Kawakami, Sujiro; photonic crystal fiber. photonic crystal fiber PCF. A type of micro- structured optical fiber with low-index refractive materials fabricated within higher-index materials (e.g., silica). They may be categorized as low index (photonic bandgap) or high index guiding fibers that produce total internal reflection through a lower ef- fective index. PCFs were first demonstrated in the mid-l 990s and have made it easier to harness the properties inherent in optical transmissions through novel fiber fabrica- tion patterns. Dispersion properties, linearity, a broader range of numerical apertures, and other fac- tors can be utilized and better controlled through PCFs, increasing the practical range of optical com- ponents and telecommunications devices that can be devised. See photonic crystal. Photonic Information Processing Systems Labo- ratory PIPS. A research lab founded by N.A. Riza, a pioneering optical engineer, at the School of Optics and Center for Research and Education in Optics and Lasers (CREOL) at the University of Central Florida. The School of Optics offers interdisciplinary gradu- ate programs in optics. See Riza, Nabeel. http://www.ucf.edu/ Photonics Components and Subsystems Newsletter PCSN. Global coverage of technology, applications and photonics markets. Published monthly by Infor- mation Gatekeepers, Inc. PhotophoneA historic device that transmitted voice by means of light waves, invented by A. Graham Bell in 1880. Charles Sumner Tainter, an experienced sci- entific instrument-maker, had a significant hand in the practical embodiment of the idea. Bell put great stock in the invention, filing for four patents for the device and its associated selenium cells. The concepts are still sound and the invention ahead of its time and worth mentioning in detail. At least as early as 1878, Bell was developing the idea for the Photophone. He described the possibility of "hearing a shadow" by the action of interrupting a ~:e~ ~:::~~~~~:it°~:~~~~u~ i~:e~C:~1:~ir~ • May 1878. In January 1879, he wrote a note describ- ing how he had worked out the idea as " the art of causing electrical signals and audible sounds in distant places by the action of light. It has been discovered that certain substances such as silenium [sic] have their electrical resistance af- fected by light. When a peice [sic] of silenium in a crystalline con- dition is placed upon the circuit with a telephone and voltaic battery a sound is audible from the tele- phone when a beam of light is allowed to fall upon the silenium. When a galvonometer is substituted for the tele- phone the needle is deflected indicating the in- crease of current, when the light falls upon the silenium thus showing that the electrical resistance of the silenium is diminished under the action of light. My invention consists in utilizing this property of silenium for the purpose of causing telegraphic sig- nals from a galvenometer [sic] or audible sounds from a telephone in distant places without the ne- cessity ofa conducting wire between the transmit- ting and receiving stations The transmitting instrument consists ofa power- ful ~ source of light and of an apparatus for interrupting or varying its intensity. The receiving instrument consists of a lens by means of which the distant light is focussed [sic] upon a peice of crystalline silenium, which is 741 © 2003 by CRC Press LLC . a gallium-arsenide diode (left) and an indium-gallium-arsenide diode. photodiode, avalanche APD. A semiconductor com- ponent commonly made of silicon (Si) or indium- 737 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary gallium-arsenide/indium-phosphide (InGaAs/InP). Silicon APDs are p-n junction solid-state detectors with high internal gain. They are reasonably immune to electric fields and sensitive enough to detect single photons at room temperature. APDs are used for optical detection for a variety of applications including fiber optic communication re- ceivers,. LLC Fiber Optics Illustrated Dictionary gallium-arsenide/indium-phosphide (InGaAs/InP). Silicon APDs are p-n junction solid-state detectors with high internal gain. They are reasonably immune to electric fields and sensitive enough to detect single photons at room temperature. APDs are used for optical detection for a variety of applications including fiber optic communication re- ceivers, fluorescence detectors, photon counters, time-of-flight ranging devices, and cryptography. Fi- ber optic receivers commonly use p-i-n photodiodes or APDs for detecting and converting an optical sig- nal into an electrical signal. New indium-gallium-arsenide/silicon (InGaAs/Si) APDs have been developed under a grant funded by AFRL/DARPA with separate absorption and multi- plication (SAM) regions for use in near-infrared fre- quencies. These offer faster, more sensitive photode- tection at wavelengths that were not previously prac- tical. See avalanche diode, Zener diode. photoelectric cell A type of electronic sensing device activated by light and widely used in security systems, automatic lighting systems (e.g., street lights), auto- matic doors, etc. A photoelectric cell can be made by coating cesium on one of the electrodes in a vacuum tube. This technology was used in early television cameras. See photo detector. photography The art and science of registering light from objects in a scene and storing them in the form of an image. Later it became possible to produce multiples of these images by a number of means. Most photography involves capturing three-dimensional imagery in a two-dimensional format. Light is usu- ally recorded from the visible spectrum, but there are cameras and films designed to record heat and infra- red radiation which show images in a form different from the way humans perceive them, and electron microscopes. of traffic lights (which may dim over time) to make sure they are bright enough to be seen clearly by motorists. Goniophotometers are common in the lighting indus- try. Photometers aid in assessing light propagation through different types of waveguides in the design and development of optical network technologies. The range of sensitivity ofa photometer is dependent upon its price and intended application. The spectral range within which it is sensitive also varies, but com- monly photometers measure visible and infrared fre- quencies. A basic classroom photometer may include several measurement scales with sensitivity to power levels ranging from about 20 microwatts to 20 milli- watts. Measurement scales may be linear or logarith- mic. Some industrial photometers have optional, in- terchangeable sensor heads for different applications. Simultaneous measurements of more than one wave- length are possible with some scientific models. The reading from a photometer may be output to a built- in LCD display or may be transmitted to other devices such as oscilloscopes, recorders, or computer periph- eral cards. Photometers designed for microscopes may have an adjustable iris to enable the sample to be viewed while the light is measured. A housing for filters may also be included. In astronomy, where light intensity provides informa- tion on the properties of celestial bodies, photometers are important research tools and may be integrated with spectrographs in telescopic systems. Sophisti- cated optical fiber- based photometers are now avail- able for studying fast variable astronomical phenom- ena. Multiple fibers enable reference images to be as- sessed in conjunction with the phenomena being ob- served. Fiber optics may be used to link individual telescopes in a telescopic array. The first