Fiber Optics Illustrated Dictionary - Part 75 potx

phase conjugationAphenomenon discovered in the 1960s, phase conjugation is now a general concept used to describe a number of nonlinear optical phas-ing processes.. Thus, optical phase c

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 a phosphor 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 Aphrase that describes the way

in which human visual perception "holds" an image

for a brief moment, about a tenth of a 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 1980s, as a modified form

ofGSM operating in the 1800-MHz frequency band

(GSM is 900 MHz).Ithas 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 bothtimeandcode division

multiple access (TDMA, CDMA) Designed to

broaden market distribution of wireless services, the

system may have more limited range than traditional

cellular, but the cheaper connect times and handsets

may be appealingtoconsumers Industry watchers are

predicting steady growth in mobile communications

In Japan alone, there were more than 20 million In-ternet-capable PCS system subscribers by200 I See

AMPS, cellular phone, DAMPS, DCS, GSM, Per-sonal HandyPhone Service

PCS Categories

narrowband PCS PCS operating in limited

bandwidthinthe 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 of personal comput-ers in the mid- and late-I 970s, the cost of a 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 ofthe 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

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 ofthe 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 color graphics

display resolution better than early desktop

comput-ers Full point-and-click Web browsing capabilities

(right) areprovided by the SojtSource/Catarra display

client/proxy serverprograms communicating through

a wireless radio link to the Internet.

Personal Digital AssistantPDA 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 ofRAM and 70-Mbyte memory chip

out-runs many full-sized desktop computers Even more

surprising is that it is powered by a clockwork

mecha-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 CellularPDC 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 ServicePHP A commercial

32 Kbps mobile data Personal Communications Ser-vice (PCS) popular in Japan PHP was established in

1995 and began providing services tosub~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 NumberPIN A system of alphanumeric characters, usually numerals, which identifies a particular user or holder of an identifica-tion card PINs are commonly used for credit cards, bank cards,illcards, calling cards, and other fonns ofwallet-sized identification to access security doors, ATMs, phones, and vending machines

PersonalJava applications environmentSee Java Personal Wireless TelecommunicationsPWT.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.Itis 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 ofFrame 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 anSI unit quantity oflOIS,or 1,000,000,000,000,000 - a really huge quantity See exa-, femto-

petticoat insulatorA 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

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

PGPSee Pretty Good Privacy

PGP Inc.A company jointly established by Philip

Zimmermann, the developer of Pretty Good Privacy,

and Jonathan Seybold See Pretty Good Privacy;

Zimmermann, Philip

PGPIMIMEPretty Good Privacy/Multipurpose

ternet Mail Extensions An IETF working group

In-ternet messaging standard for the transmission of

se-cure network communications A variety 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 circuitIntelephony, a means of devising

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 groupIntelephony, a phantom circuit and

the balanced circuits that flank it and from which it

draws some of its circuitry

phase alternate linePAL A color 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 rewritablePCR 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

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 conjugationAphenomenon discovered in the 1960s, phase conjugation is now a general concept used to describe a number of nonlinear 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 mirrorPCM A reflecting 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 of a 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

Inthe early I990s, 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 drivePD A type of optical data storage drive based upon phase-change recording such that the op-tical medium can be rewritten See change rewritable

phase jitterA particular type of undesirable 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

Phase-Shift Keying

irJ, '"

'.

1,

(:)I~e L "e

',,"

."

I

-!_~ \ j \ J

nl-~

A waveperiodis 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 ofthe 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 ofthe wave period hasntchanged, only

the time point in the phase at which it is referenced,

relative to thefirst wave. Ifthe two different phases in

the wave were plotted on top ofone another, they would

undulate with the same period length, shape, and

am-plitude - only the phase has been shifted.

By creating a series ofshifts 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.

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 a carrier 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 of a tele-vision receiver, for example Because it is a basic tim-ing technology, it is found in components rangtim-ing 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 of products, 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., acamelback key) He was

known for elegance ofdesign 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

, x g P

,l

f ' ,

'~f

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, a device first patented in

theu.s.byA.Graham Bell

Phelps was an associate of Thomas Edison and

cre-ated some of the patent models for Edison's early

in-ventions Phelps became the superintendent

ofWest-em Union Telegraph in New York and rofWest-emained 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

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 ofPRIGS which

in-corporates some PRIGS+ features

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

small-est distinguishable unit, which may vary from

lan-guage to lanlan-guage and among dialects of a particular

language Phonemes are of interesttoprogrammers for speech recognition and speech generation appli-cations See speech recognition

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 PhotoCD 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 resoluresolu-tions 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 patent shows the various basic parts ofa photocopier (right), with a detail ofthe drum mecha-nism (left) Large companies were not willing to purchase the new technology A small company called Xerox did!

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

photodetectorPD 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 ofwave-like and particle-like properties and

emits heat at different levels depending upon

loca-tion and time of day, the definiloca-tion of photodetector

is somewhat broad, reflecting the capability

ofreact-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) oflight within certain

pa-rameters and some may be sensitive to light without

discriminating its intensity or character More

sophis-ticated photodetectors may be "tuned" to detect

spe-cific wavelengths or regions ofwavelengths and some

are also sensitive to the magnitude of a light

stimu-lus Even at its most basic level, however,

photode-tection is an important capability at the heart ofmany

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 ofsemiconductor technology is

de-voted to improving the signal-to-noise ratio of

am-plified signals.Inaddition, photodetectors are often

environmentally sensitive Light is ubiquitous and it

is often challenging to detect only that light that is of

interest For example, a thermal-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 photodetector (for

studying light from celestial bodies) works more

ef-fectively ifplaced in orbit around the Earth rather than

in the observatory of a university in the middle of a

large city, due to the interaction of ambient light

sources

Depending upon the type of detector, commercial

photodetectors are typically described in terms

ofsponsivity (the sensitivity and magnitude of their

re-action to light), efficiency (how much signal is

gen-erated per photon stimulus), response time,

signal-to-noise ratios and types of signal-to-noise (e.g., Johnson signal-to-noise),

and the linearity ofthe response Figures ofmerit may

also be used

ing the selective photosensitivity ofcertain 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 ofdetection and conversion processes For 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 photodetectors are tuned to respond to different optical frequencies The data derived from individual elementsinthe photo-detector array can be signal processed to produce a complex overall statistical picture of the light-emit-ting characteristics ofsample specimens or light-car-rying transmissions media

The creation of semiconductor photodetectors 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 photodetectors 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 ofmicrowave 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 ofshapes, sizes, and levels ofsensitivity to photonic energy Illustrated here are common configurationsfor a gallium-arsenide diode (left) and an indium-gallium-arsenide diode.

photodiode, avalancheAPD Asemiconductor com-ponent commonly made of silicon (Si) or

indium-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 sigsig-nal

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 Atype ofelectronic sensing device

activated by light and widely used in security systems,

automatic lighting systems (e.g., street lights),

auto-matic doors, etc Aphotoelectric 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 photodetector

photography Theartand science ofregistering light

from objects in a scene and storing them in the form

of an image Later it became possible to produce

multiples ofthese images by anumber ofmeans 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 ofthe earliest

pho-tos was captured with silver chloride by Thomas

Wedgewood in 1802 More than 150 years passed

fore 3D photography, in the form of holographs,

be-came practical Newer digital be-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

photometerAninstrument for determining the

inten-sity oftransmitted or reflected light, sometimes called

an opticalpower meter Aphotometer is a type

ofra-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

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 ofsensitivity 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 Abasic 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 ofmore 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

Inastronomy, where light intensity provides informa-tion on the properties ofcelestial 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 variavail-able 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 in1760.This was an expansion ofan earlier essay, published in 1729 and Bouguer is considered by many to be the inventor ofthe photom-eter.1.-0.Colladon developed a practical application

of a photometer for his engineering projects 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

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

photomultiplierPM 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 tubePMT Typically, an evacuated

glass component containing a photocathode that emits

electrons when subjected to photonic energy

suffi-cient to trigger a photoelectric effect The

photocath-ode operates at a high negative voltage and the

elec-trons emitted are accelerated towards a series (chain)

ofdynodes that are positioned along the electron path

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., 1x16) or grid patterns (e.g.,

8 x 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 ofapplications, 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 of Basic Photomultiplier Components and Dynode Function

photocathode

anode

photocathode

anode

This is a highly simplified draWing ofa basic photomultiplier tube used to convert and amplify a photonic signal The photocathode at the top converts electromagnetic energy in the form ofphotons into electron emissions which are attracted to the anode at the base ofthe tube As the electrons travel toward the anode, they encounter a series of

dynodes in the middle 0/ the tube, powered with voltages that are calibrated to one another to control the magnitude o/electron emissions As an electron/rom the cathode (or the preceding dynode) 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 ofthe signal When the electrons reach the anode, the signal is processed by a small circuit within the base and output through the contacts comingfrom the bottom ofthe base to inteiface with other compo-nents (21-pin sockets are common) A magnetic shield that fits over the base can shield the electrical circuits from external inteiference As illustrated in the line diagrams, the voltages applied to the reflective dynodes are related to the number ofelectrons emitted, with higher voltages (right) providing greatergain(within operating limits) Thus, very weak signals, even as small as one photon, can be measured and manipulated with photomultipliers to facilitate research in particle physics and to fabricate sensors, and scientific and industrial quality assurance, testing, and sampling instruments.

Fiber Optics Illustrated Dictionary

A phototube is a simpler version of the

photomulti-plier tube (without dynodes) See dynode,

photo-sensor

photomultiplier tube baseA 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 ofusing light as a medium for the transmission ofsound Sunlight was used to translate acoustic vibrations into light signals thatwerereflected 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 a parabolic surface, Bellfound he could increase the intensity ofthe signals and was able to transmit signals over a distance ofseveral hundred meters on a sunny day Bell was very excited about the potential ofwireless 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).]

Cockcroft-Walton voltage multipliers have been

sug-gested in place ofresistive voltage dividers for PMTs

that are densely packed, in order to minimize

dissi-pated power See dynode, photomultiplier tube

photombltiplier tube chamberAhousing for

physi-cally protecting, electromagnetiphysi-cally 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 ofwavelengths See photomultiplier tube

photonic crystalA 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 ofothers

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.Ithas 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.Itis 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-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-ratoryPIPS Aresearch 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 ofCentral 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

PhotophoneAhistoric device that transmitted voice

by means oflight 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

May 1878 In January 1879, he wrote a note describ-ing how he had worked out the idea as

" theartofcausing 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] ofsilenium 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 oflight 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 ofcausing 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 of a 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