A

B

C

D

DATA RATE
Bits of information (data) transmitted per second depending on the transmission code used, for ex-ample, RZ (Return-to-Zero), NRZ (Non-Return-to-Zero). The data rate is affected by the data for-mat. RZ delivers a lower data rate than NRZ. If a system's signaling rate is limited to a high of 10 Bd, a higher data rate may be achieved by dividing the data stream among additional channels. Data Rate (Mb/s) = Signaling Rate (Mbaud) x Efficiency of Transmission Code See also Baud. dBc Decibels referenced to the signal carrier level. dBm Decibels referenced to 1 mW. The standard unit of power level used in microwave work. dBm mW O 1 10 10 20 100 See also dB.

DECIBEL (dB)
A unit of gain equal to ten times the common (base 10) logarithm of the ratio of two power levels or 20 times the common logarithm of the ratio of two voltage or current levels.

DETECTOR, PHOTO
A device that reacts to light by producing or modulating an electrical signal. See also Photodiode, Pin Diode and Avalanche Photodiode.

DISPERSION
The increase in rise/fall time of the optical signal at the far end of long fiber-optic cables. It is ex-pressed in nanoseconds per kilometer. Dispersion is linearly related to the inverse of the bandwidth-times-length constant. It results from either modal or material effects. See also Modal Dispersion and Material Dispersion.

DIGITAL
Pertaining to data in the form of discrete quantities or digits. See also Analog.

DOPING
Adding impurities to a substance to change its electrical properties.

E

EMITTER, OPTICAL
A device that generates optical power. See also ILD, LED, and Light Source.

EXTINCTION RATIO
The ratio of the output power of a digital transmitter for a logic 1 state compared to a logic 0 state. This ratio is important in system design because it affects the recovered signal level at the receive end. Laser transmitters are usually designed with a compromise between switching speed and extinction ratio performance.

F

FIBER OPTICS
The technique of conveying light through a particular configuration of glass or plastic fibers.

FIBER STRENGTH
(See Tensile Strength)

FLUX BUDGET
(See Optical Power Budget)

FREQUENCY (F)
The repetition rate of a periodic signal, expressed in cycles per second. The units of frequency are Hertz (Hz). Common multipliers include: kHz (kilohertz): 103 Hz MHz (megahertz): 106 Hz GHz (gigahertz): 109 Hz

FREQUENCY RANGE
The minimum and maximum frequencies between which a particular component will meet all guaranteed specifications. Alternatively, the minimum and max imum frequencies between which a particular component will have "useful" performance.

FULL-WIDTH HALF-MAXIMUM (FWHM)
(see Spectral Width)

G

GAIN
The amplification available from a component expressed as the ratio of the output power to the input power. Gain is usually measured in decibels (dB).

GRADED-INDEX FIBER
An optical fiber whose index of refraction decreases as a function of the distance from the center of the core, until it reaches the cladding index of refraction at the core radius. Gradedindex fibers offer multi-mode (large NA) characteristics along with low dispersion and are used in all fiber applications. It is one of the two most popular fibers, the other being Step-lndex Fibers. See also Index Profile.

H

HERTZ
A unit of frequency equal to one cycle per second.

HIGH-ORDER MODE
(see Modes of Propagation)

I

INDEX GRADING COEFFICIENT (G)
Describes the variation of refractive index within the fiber core as a function of radial distance, r, from the center of the core, relative to the core radius, a.

INDEX PROFILE
The index of refraction through the fiber core as a function of the distance from the center of the core. A sharp reduction of the index at the boundary between core and cladding results in a step-index fiber. A gradual lowering of the index in the core areas approaching the cladding results in a graded-index fiber. (See also Index Grading Coefficient)

INDEX OF REFRACTION (N)
The ratio of the velocity of light in a vacuum (such as the fiber core, n, or cladding, n) to the velocity of light in a given medium. (See also Snell's Law)

INFRARED (IR)
Wavelengths that extend beyond 770 nanometers.

INJECTION-LASER DIODE (ILD)
A p-n junction semiconductor that converts direct forward bias electric input directly into coherent optical output power. The ILD provides a light source suitable for optical fibers. It can supply much higher optical power output in a smaller NA and smaller spectral width than LEDs, but is costly and temperature dependent.

INSERTION LOSS
A loss resulting from the insertion of a component in a system, the ratio of the power delivered to that part of the system following the component to the power delivered to that same part before insertion. Insertion loss is typically measured in dB.

INTERFACE
1. A surface forming a common boundary between adjacent regions. 2. A common boundary between adjacent components, or systems enabling the devices to yield and/or acquire information from one another.

INTERNATIONAL ELECTROTECHNICAL COMMITTEE REGULATION NO. 825 (IEC 825)
The standards body in Europe that sets general standards and safety regulations for various electrical products. IEC 825 sets the eye safety optical output power limits for laser- and LED-based devices. Similar in concept to the CHRH in the USA, the IEC has specific regulations that may cause different eye safety classification.

L

LIGHT-EMITTING DIODE (LED)
A p-n junction that emits light when biased in the forward direction. A semiconductor device that provides a spontaneous (non-coherent) light source through the process of electroluminescens. LEDs emit visible or invisible (infrared) radiation. LEDs are suitable for optical fibers when adapted to match the fiber's core area. This interface may be in the form of a pigtail fiber lead attached to the light-emitting slide.

LIGHT SOURCES
Injection Laser Diodes (ILDs) and Light Emitting Diodes (LEDs). ILDs are capable of coupling a few mW of light power into an optical cable; LEDs can couple only a few hundred IlW. (See also ILD and LED)

LUMEN
A basic unit of measure used for the total amount of light flux in any particular situation; for example, from a source, in a beam, through an aperture, and so forth.

M

MATERIAL DISPERSION
The separation of the light wave into its component frequencies over a specified time period as it travels through the fiber. This phenomenon limits bandwidth in fiber-optic cores. It is caused by two conditions: 1. The velocity of optical transmission in the core varies inversely as the index of refraction. 2. The index of refraction varies according to the wavelength spectrum. This means that although all portions of the optical signal may be launched simultaneously, they will not all arrive simultaneously. Due to differences in travel time, some portions of the spectrum will arrive at a different time (that is, be dispersed).

MEAN TIME BETWEEN FAILURES (MTBF)
A calculated figure representing the estimated average lifetime of a repairable device before it fails.

MEAN TIME TO FAILURE (MTTF, MTF)
A calculated figure representing the time at which 50% of a population of devices will have failed. MTTF is used to predict the reliability of nonrepairable devices such as individual semiconductors.

MODAL DISPERSION
The expansion of the rise and fall times of the signal as it travels along the fiber, expressed in ns/km. The time of arrival of simultaneously transmitted flux (optical radiation) depends on the path length traveled. Since reflections of the flux rays occur at different angles, path lengths will vary, causing dispersion. Also referred to as spreading. Modal dispersion is greatly reduced by use of fully graded fiber and reduced to zero in single-mode fiber. See also Material Dispersion.

MODES OF PROPAGATION
Optical rays may propagate at various angles with respect to the fiber axis. The smaller angles are called low-order modes and the larger angles are called highorder modes. Both classes of modes are totally reflected within the core.

MODE MIXING
Conversion of low-order modes to high-order modes due to bends in the fiber.

MMODULATION
Adding information to a carrier frequency by varying some combination of signal amplitude, signal phase, and or signal frequency. Modulation may be either analog (continuous) or digital (transmission of a finite number of discrete states). Common Fiber-Related Modulations Type Description AM Amplitude Modulation - information is passed by varying the signal amplitude FM Frequency Modulation - information is passed by varying the signal frequency PM Phase Modulation - information is passed by varying the phase of the signal.

MODULATOR, OPTICAL
Light sources such as LEDs and ILDs can be modulated directly by varying the drive current (100 to 300 mA) at a data rate from dc to 150 Mb/s NRZ (for LEDs) and beyond (for ILDs).

MONOMODE FIBERS
(see Single-Mode Fibers)

MULTI-MODE FIBERS
Single fibers that can accommodate both high-order and low-order propagation modes. Such fibers have large NA.

MULTIPLEXER
A device that accepts a number of parallel inputs and serializes them for transmission to a remote demultiplexer where they are converted back to parallel signals.

MULTIPLEXING

The process of combining two or more channels of information for transmission on a single channel.

N

NOISE EQUIVALENT
An input signal that produces an output signal level that is equal in value to the noise level present when no signal is applied.

NUMERICAL APERTURE (NA)
A number that expresses the angular light-gathering characteristic of an optical fiber. It is ex-pressed by the sine of the acceptance angle, the maximum angle with respect to the fiber axis at which an entering ray will experience total internal reflection (TIR).

O

OPTICS
The scientific study of light and vision, chiefly the generation, propagation, and detection of electromagnetic radiation having wavelengths greater than X-rays and shorter than microwaves.

OPTICAL POWER RESPONSIVITY
The ratio of incremental photodiode current to the incremental optical power producing it. Usually expressed in amperes per watt (A/W).

OPTOELECTRONICS
Technology dealing with the coupling of functional electronic blocks by light beams. Circuitry in which solid-state emitters and detectors of light are involved.

P

PACKING FRACTION
The ratio of the active areas of a fiber-optic bundle to the total bundle area. Only the core areas are considered active; other areas of a cable bundle, including the cladding, do not contribute to coupling efficiency.

PEAK WAVELENGTH
The wavelength at which the output spectrum of an LED is at a maximum.

PHOTOMETRY
A branch of science which deals with the measurement of light intensity.

PHOTODIODE
A solid-state device similar to an ordinary diode with the exception that the light incident on the pn- junction causes the device to produce a current in the direction from cathode, n, to anode, p, rather than in the other direction.

PIN PHOTODIODE
In this semiconductor type, the p-doped and n-doped regions are separated by a lightly doped "intrinsic" region in which the absorbed photons generate an electric current. Hence the acronym PIN. PlNs have the advantage of broad spectral response, wide dynamic range, high speed and low noise, but no internal gain.

POINT SOURCE
A source of light which emits light in all directions as if from one point. Sometimes used to refer to very tiny light sources.

POLARIZED LIGHT
Light in which the electric field points in a known direction.

POWER BUDGET
The ratio (usually given in dB) by which the power provided by a transmitter exceeds the power required at a receiver. Power Budget describes the transmission loss ratio that is tolerable to the transmitter-receiver system.

POWER BUDGET, OPTICAL
Optical power (light rays) that radiates in a flux of energy packets called photons. These photons have no electrical charge and no magnetic field.

PRINTED CIRCUIT BOARD (PCB OR PC BOARD)
A circuit in which the interconnecting wires have been replaced by conductive strips printed or etched onto an insulating board.

PROPAGATION DELAY
The time difference between a logic transition at the output of a circuit (or device) and the input transition that caused it.

PULSE DISPERSION
(see Modal Dispersion, Material Dispersion and Dispersion)

R

RECEIVER (FIBER OPTIC)
A device that reacts to light signals, and converts them to electrical signals. See also Detector.

REFLEXION
When an optical ray encounters a change of refractive index, there is a reflection. If the ray is incident on the low-index side, the reflection is always partial. If it is incident on the high-index side, the reflection is partial only for incidence angles less than the critical angle. At incidence angles greater than critical, the reflection is total. The partial reflection is given by:

          (n1 - n2)
R = ------------------
        (n1 + n2)*n2

For n1 < n2, the critical angle = arcsin / n1

REFRACTION
The bending of an optical ray incident at a boundary where there is a change of refractive index of the optical material. A fraction of the ray will pass through if the angle of incidence is less than the critical angle. The fraction passing through emerges at an angle (called refraction angle or exit angle) that is related to the incidence angle by Snell's Law.

REFRACTIVE INDEX
(see Index of Refraction)

REPEATER, OPTICAL
A device that amplifies and reshapes the parame ters of an optical signal to agree with the original information and transmits the new signals through a fiber optic line to another repeater or to a receiving terminal.

RESPONSIVITY
The measure of a photodetector's performance. The unit of measure is signal current divided by optical power, A/W. See also Optical Power Responsivity.

ROLLOFF
A gradual decrease in signal strength.

S

SDH
Synchronous Digital Hierarchy, the European equivalent of ANSI SONET. (See also SONET)

SECONDARY OPTICS
Any reflector control or lens or both employed to redirect light emitted from an LED package.

SEMICONDUCTOR
A solid crystalline substance with electrical conductivity greater than an insulator but less than a good conductor.

SINGLE-MODE FIBERS
Fibers with core areas and numerical apertures so small that only one mode can be propagated through them. See also Modes of Propagation.

SNWLL'S LAW
At a boundary where there is a change of index of refraction light bends toward the normal when it enters an optically denser medium, and it bends away from the normal as it enters an optically less dense medium. nAsinOA = nBsinOB

SONET (SYNCHRONOUS OPTICAL NETWORK)
A set of standards that allow telecommunications equipment from various manufacturers to inter-operate. SONET specifies standard data rates of OC-1 (51 Mb/s), OC-3 (155 Mb/s), OC-12 (622 Mb/s), and OC48 (2.488 Gb/s). The data format, signaling and monitoring functions are also specified. In Europe SONET is known as SDH (Synchronous Digital Hierarchy), with STM-1 (OC-3) at 155 Mb/s, STM-4 (OC12) at 622 Mb/s, and STM-16 (OC-48) at 2.488 Gb/s.

SPECTRAL WIDTH
1. The incremental energy emitted by a light source as a function of wavelength. Because the Index of Refraction varies over the wavelength spectrum for a fiber, the travel time of different parts of a spectrum will vary, causing Dispersion. 2. The wavelength range over which the performance of emitters, detectors and other optical materials is regarded as being high enough to be useful. In most cases this is the wavelength difference between the half-peak levels, and is called FWHM (FullWidth Half-Maximum)

SPECTRUM/span>
A continuous range of electromagnetic radiation (power from a source is not the same at all wavelengths, and the response of a detector varies with wavelength).

SPREADING
(See Modal Dispersion)

STEP-INDEX FIBERS
Fibers whose index of refraction is reduced sharply at the boundary between the core and the cladding. They are useful for multi-mode operation over short distances up to one kilometer.

T

TENSILE STRENGTH
The total mechanical strength of a fiber, and the force in newtons or pounds at which breakage occurs. It is determined by the condition of its surface. Coating the glass fiber with a thin plastic film increases mechanical strength compared to the naked fiber.

TOTAL INTERNAL REFLECTION (TIR)
Reflections from rays incident on the high-index side of a boundary between materials of different refractive index, in which the incidence angle exceeds the critical angle. Optical fibers have a core surrounded by cladding with an index lower than that of the core. Optical rays that approach the wall of the core at inc idence angles greater than the critical angle are totally reflected back into the core until they reach the far end of the fiber. See also Reflection.

TRANSMISSION, OPTICAL
A method of transmitting information by means of modulated light waves through an optical system. The system may be composed of any combination of lens optics, mirror optics, or fiber optics.

TRANSMISSION LOSS
(See Attenuation)

TRANSMITTER, OPTICAL
A device that takes electrical signals and converts them to light signals. See also Light Source and Emitters.

W

WAVEGUIDE
A transmission line consisting of a plastic, liquid, or glass fiber or bundle of fibers through which light is propagated both axially and by internal reflections.

WAVELENGTH LENGTH
(usually in nanometers) of one cycle of a light wave.