Attenuation
A general term that refers to any reduction in the strength of a signal. This manual refers specifically to optical attenuation which is the decrease in signal strength or the reduction in optical power along a fiber optic waveguide caused by absorption and scattering of the signal. Attenuation is measured in decibels (dB).

Absolute attenuation is the attenuation relative to the source power.

Optical attenuation is based on the variable wave plate with liquid crystal cells and
birefringent crystals.

Relative attenuation is the attenuation measured relative to a reference. It is displayed
with the units (dBr).

REL(dBr)= ABS(dB)– REF(dB)

Ideal attenuation is the point of minimum loss, maximum power.

Attenuation-Limited Operation
The condition in a fiber optic link when operation is limited by the power of the received
signal rather than by bandwidth or distortion.

Attenuator
In optical systems, a passive device for reducing the amplitude of a signal without
appreciably distorting the waveform.

Bandwidth (BW)
The range of frequencies or bit rate within which a fiber optic waveguide or terminal
device can transmit data or information. Bandwidth is expressed in Hertz (Hz). In optical
fiber cabling, the bandwidth decreases with length.

Bit Error Rate (BER)
The fraction of bits transmitted that are received incorrectly. A bit error rate of 10-6
means that there is an average of one error per million bits. The bit error rate of a system can be estimated as follows:



• Where N0 = Noise power spectral density (A2/Hz)
• IMIN = Minimum effective signal amplitude (Amps)
• B = Bandwidth (Hz)
• Q(x) = Cumulative distribution function (Gaussian distribution)

Birefringent Crystal
A crystal that exhibits two different optical densities for a perpendicular polarization of
light. Fibers may exhibit birefringence, which causes lightwaves to travel at different
speeds, at right angles to each other, down the cable.

Decibel (dB)
A unit of measure of optical power as it propagates along a fiber. A logarithmic unit used
for expressing the loss or gain of signal strength.

Detector
An opto-electric transducer used to convert optical power to electrical current. Also
referred to as a photodiode.

Fall Time
The time required for the trailing edge of a pulse to fall from 90% to 10% of its amplitude; the time required for a component to produce such a result. Typically measured between
the 90% and 10% points or the 80% and 20% points. Also called the turn-off time.

Fiber
In fiber optics, a thin filament of glass or plastic that conducts an optical signal.

Frequency
Frequency is the number of identical cycles per second of a periodic wave. The standard
unit of frequency is hertz (Hz). If a current completes one cycle per second, then the
frequency is 1 Hz; 60 cycles per second equals 60 Hz (the standard alternating-current
utility frequency in some countries).

The frequency of a signal is mathematically related to the wavelength. The frequency (f)
is equal to the speed of the signal (v) divided by the wavelength of a signal (λ).

f = v/λ

Gaussian Beam
A beam pattern used to approximate the distribution of energy in a fiber core. It can also be used to describe emission patterns from surface-emitting LEDs. It is also recognized as the bell curve. The Gaussian beam is defined by the equation:
E(x) = E(0)e^-x2/w₀2

InGaAs
Abbreviation for the compound indium gallium arsenide. It is generally used to make
high-performance long-wavelength detectors.

Leading Phase
A wave that occurs "ahead" of another wave of the same frequency.

Lagging Phase
A wave that occurs "behind" another wave of the same frequency.

Loss
Attenuation of an optical signal, measured in dB.

Modulation
The process by which the characteristic of one wave (the carrier) modifies another wave
(the signal). Examples include amplitude modulation (AM), frequency modulation (FM),
and pulse-coded modulation (PCM).

Nanometer
One one-billionth of a meter. Used to measure wavelengths of light.

Phase
The position of a point in time (instant) on a waveform cycle. A complete cycle is defined
as 360 degrees of phase. Phase can also be an expression of relative displacement
between or among waves having the same frequency. Phase is sometimes expressed in
radians rather than in degrees. One radian of phase corresponds to approximately 57.3
degrees. Engineers and technicians generally use degrees; physicists generally use
radians.

Phase difference is a number greater than -180, and less than or equal to +180; Also
called phase angle.

Phase opposition refers to two waves that differ in phase by 180 degrees (-180 is
technically the same as +180).

Phase quadrature refers to two signals that differ in phase by -90 or +90 degrees.

Photodiode (PD)
A semiconductor device that converts light to electrical current. See also “Detector”.

Power (Amplitude)
The strength or volume of a signal, usually measured in decibels (dB).

Propagation
The movement or motion of a lightwave along an optic fiber.

Pulse
A current or voltage that changes abruptly from one value to another and back to the
original value in a finite length of time. Used to describe one particular variation in a
series of wave motions. The parts of the pulse include the rise time, fall time, pulse width, and pulse amplitude. The period of a pulse refers to the amount of time between pulses.

Relative Power Level
The relative power level (dB) is the ratio of input power and the reference. When power is
measured in linear units (mW, uW or nW), dB is calculated on a logarithmic scale using this formula:

Relative power (dB) = 10 log(power/reference)

The absolute power level display is significant when measuring system power levels or
determining a reference level. If you are measuring absolute power levels, the
measurement are generally referenced to 1 milliwatt (mW) is expressed as dBm. The
power level is displayed as a logarithmic value in dBm and the equation is:

ABS power (dBm) = 10 log(power/1 mW)

Thus 1 mW = 0 dBm, 1 uW = -30 dBm, 1 nW = -60 dBm and two equal powers
compared are 0dB (e.g. power being the same, there is no loss).

Retardance
A shift in phase.

Rise Time
The time taken to make a transition from one state to another, usually measured between the 10% and 90% completion points of the transition. Alternatively the rise time may be specified at the 20% and 80% amplitudes. Shorter or faster rise times require more bandwidth in a transmission channel.

Surface-Emitting Diode
A simple and inexpensive light-emitting diode (LED) that emits light from its flat surface
rather than its side with emission spread over a wide angle.

Transistor-to-Transistor Logic (TTL)
A digital logic design in which bipolar transistors act on direct-current pulses. Many TTL
logic gates are typically fabricated onto a single integrated circuit (IC). TTL ICs usually
have four-digit numbers beginning with 74 or 54.

A TTL device employs transistors with multiple emitters in gates having more than one
input. TTL is characterized by high switching speed (in some cases upwards of 125
MHz), and relative immunity to noise. Its principle drawback is the fact that circuits using
TTL draw more current than equivalent circuits using metal oxide semiconductor (MOS)
logic. Low-current TTL devices are available, but the reduced current demand comes at
the expense of operating speed.

Variable Optical Attenuator (VOA)
A device that can be adjusted to reduce the power level of an optical signal.

Waveguide
A waveguide is a material medium that confines and guides a propagating
electromagnetic or light wave. Used as a long transmission line consisting of a solid
dielectric filament (fiber) core, cladding protective sheath, etc. In integrated optical
circuits, an optical waveguide may consist of thin dielectric film.

Wavelength
The wavelength of a signal is the distance between points of corresponding phases of
two consecutive cycles of a wave, such as crest to crest. It is measured in nanometers
(nm).

The wavelength of a signal is mathematically related to the frequency. The wavelength of
a signal (λ) is equal to the speed of the signal (v) divided by the frequency of the signal
(f).

λ = v/f

The wavelength of a signal is reduced when it is propagated through a fiber, although the frequency remains the same. This reduction is due to absorption and dispersion
(attenuation).

Waveplate
An optical device containing a birefringent crystal with a precise thickness that shifts the
phase of the signal as it passes through it.

Sources:
http://www.tycoelectronics.com/customersupport/glossary.asp
http://www.fiber-optics.info/
http://www.wikipedia.org/