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Terminology
Additional definitions can be found at these
links:
Athermal
A term used to describe performance parameter insensitivity relative
to temperature changes. In the past, many grating-based DWDM
Mux/DeMux devices needed temperature controls to maintain optimal
optical performance within typical telecom operating temperatures
(0-70°C). Wasatch Photonics designs and manufactures athermalized,
hermetically-sealed gratings that can be used as building blocks
for current and next generation athermal Mux/DeMux devices.
Bandpass
See Passband
Bandwidth
The range of wavelengths or frequencies over which a particular
device is designed to function within specified limits.
Bellcore
Requirements
See Telcordia
Requirements
C, L, S,
& Hybrid - Bands
Infrared wavelength ranges typically used in DWDM devices.
See ITU.
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C-Band:
"Conventional" Band is about 1525 nm - 1565 nm
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S-Band:
"Short" Band is about 1450 nm - 1530 nm
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L-Band:
"Long" Band is about 1565 nm - 1610 nm
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Hybrid
Bands: For instance a hybrid C + L Band could be 1520-1620nm.
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Channel
/ ITU Grid
One signal channel has a frequency band with a finite pass bandwidth
and is centered at a given frequency specified by the ITU
Grid. Each DWDM channel corresponds to
one particular wavelength and carries an individual stream of data.
Typically high channel count devices contain 32, 40, 80, and 160
channels spaced at 50, 100, and 200 GHz.
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Channel
Center Wavelength:
the wavelength, in nm, at which a particular signal channel
is centered.
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Channel
Isolation:
the ratio, in dB, of the light intensity at the undesired port
to the light intensity at the desired port.
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Channel
Spacing:
the frequency difference, in GHz, between two adjacent channel
center frequencies in DWDM components or modules. Many Mux/DeMux
devices have channel spacings of 50, 100 and 200 GHz (e.g. the
wavelengths are spaced apart 0.4nm, 0.8nm, and 1.6nm respectively).
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Channel
Uniformity:
the maximum difference of insertion loss, in dB, over all signal
channels.
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Dense Wavelength
Division Multiplexing (DWDM)
A fiberoptics communication system transmitting and receiving
multiple signals over a glass fiber network. Each signal,
containing separate data streams, is assigned to a different wavelength
of laser light. This leverages the existing fiberoptic infrastructure
by multiplying the amount of information that can be sent thereby
providing a migration path for telecom companies. Many DWDM
devices and subsystems can use gratings as their optical platform.
See diagram below.
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DWDM
Multiplexer (Mux):
A transmitter that combines (multiplexes) many laser light signals
of different wavelengths coming from individual glass fibers
and inputs them into an optical fiber transmission line.
This can also be performed through free space, without fiberoptic
lines.
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DWDM Demultiplexer (DeMux): A receiver that separates
(demultiplexes) many laser light signals of different wavelengths
coming from an optical fiber transmission line and outputs them
into individual glass fibers. This can also be performed
through free space, without fiberoptic lines.
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Dynamic
Gain Equalizer (DGE): DGEs correct the power imbalance
in multiple channels, through user-selectable criteria, and
without bit-stream interruption. To ensure low data-error
rates, it is critical that all DWDM channels have approximately
the same power level.
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Optical
Channel Monitor (OCM): A spectrometer module for identifying
DWDM channels along with measurements of wavelength, power,
and OSNR (Optical Signal to Noise Ratio).
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Optical
Add/Drop Multiplexer (OADM): A switching/routing device
that adds and drops multiple wavelengths at various locations
along an optical network.
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Dickson
Grating® & Dickson Grism®
A specialized,
patented form of a Volume Phase Holographic Grating (VPHG) combining
high dispersion, high S & P efficiencies, and low PDL.
Diffraction
Efficiency
The ratio of diffracted flux to incident flux.
Diffraction
Grating
Also see Volume Phase Grating (VPG) and Volume Phase Holographic
Grating (VPHG). This "Grating
Tutorial" link contains more detailed information.
A
diffraction grating is an optical device that has a periodic variation
of some property of the device such that it imposes on an incident
wave a periodic variation of amplitude or phase or both. The property
of the device that is periodically varied may be its transmittance,
its reflectance, its thickness or its refractive index. If the transmittance
or the reflectance of the device is periodically varied, the resultant
diffraction grating is said to be an amplitude grating, since
the amplitude of the transmitted or reflected wave is periodically
varied by the grating. If the thickness or the refractive
index of the device is periodically varied, the resultant diffraction
grating is said to be a phase grating, since the phase of
the transmitted or reflected wave is periodically varied by the
grating.
When
a monochromatic optical wave is incident on a diffraction grating,
a portion of the transmitted or reflected light will be deflected
(diffracted) by an angle that will be directly dependent on the
wavelength of the incident wave and on the spatial frequency of
the periodic variation of the grating properties (transmittance,
reflectance, thickness, refractive index). The spatial frequency
of a diffraction grating is normally expressed in lines per mm (lpmm).
The relative portion of the wave that is diffracted will be dependent
on the diffraction efficiency of the grating (See Diffraction
Efficiency above).
If
the optical wave incident on the diffraction grating consists of
multiple wavelengths (i.e., the wave is not monochromatic), the
grating will angularly disperse the diffracted wave into its component
wavelengths in accordance with the grating equation (See Grating
Equation).
The
two most common forms of diffraction gratings are Surface Relief
(SR) gratings, in which the thickness of the device is periodically
varied while the refractive index of the medium is fixed, and Volume
Phase Gratings (VPG) in which the refractive index of the device
is periodically varied while the thickness of the medium is fixed.
Both are forms of phase gratings. Commercial SR gratings are generally
reflection gratings while commercial VPGs are generally transmission
gratings.
VPGs
can be further classified as thick or thin, depending on the properties
of the grating (thickness, spatial frequency, refractive index)
and the wavelength of the incident optical wave. In general, thick
VPGs are to be preferred over thin VPGs because thick VPGs will
have higher diffraction efficiency and will diffract the light into
a single diffracted wave. That is, there will be a single diffracted
order.
All
Wasatch Photonics Volume Phase Gratings (VPGs)
are thick VPGs. They are designed and manufactured to provide high
dispersion, extremely high diffraction efficiencies, and polarization
insensitivity making them ideal for DWDM and most other applications.
Also see Hologram.
Dispersion
Dispersion
for a given diffraction grating is defined as the rate of change
of the angle of diffraction with wavelength for a fixed angle of
incidence, or
. (For more detail, see the "Grating
Tutorial")
Free-Space
Optical Interconnect
A type of internal photonic connection in an integrated circuit
in which a holographic grating is used to focus light at points
on a silicon chip, maximizing the speed of signal propagation.
Grating
See Diffraction Grating. Also see Volume Phase
Grating (VPG) and Volume Phase Holographic Grating (VPHG).
This "Grating Tutorial"
link contains more detailed information.
Grating
Equation
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where:
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= free-space wavelength of the incident wavefront
= spatial frequency of the grating, generally expressed
in lines/mm (lpmm)
= angle of incidence (measured CCW from the surface normal)
= angle of diffraction (measured CW from the surface normal)
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(In
general, the first term is
, where
m
is the order of diffraction. However, in telecom applications,
m
is generally 1.)
Grating Substrate
The substrate upon which a diffraction grating is coated or
ruled. It must be dimensionally stable, and the surface must
be polished to an accurate flat or spherical form as required by
the grating. For a reflection grating the substrate need not
be transparent, but for a transmission grating it must be transparent.
Head-Up Display
(HUD)
An optical system that superimposes a synthetic display providing
navigational or weapon-aiming information on a pilot's or driver's
field of view. The system includes a cathode ray tube, collimating
optics, and a combiner that projects the image in front of the window.
The combiner may be made using holographic means in the form of
a narrow-band "mirror".
Hologram
An interference pattern recorded on a high-resolution recording
medium using two or more beams from a laser. Holograms can
reproduce three dimensional images.
Holographic
Diffraction Grating
See Diffraction Grating. Also see Volume Phase Grating
(VPG) also referred to as a Volume Phase Holographic Grating
(VPHG). This "Grating Tutorial"
link contains more detailed information.
Insertion
Loss (IL) in dB
The total optical power loss caused by the insertion of an optical
component such as a connector, splice, grating, into a fiber optic
system.
International
Telecommunication Union (ITU) Grid
The ITU, headquartered in Geneva, Switzerland is an international
organization within the United Nations System where governments
and the private sector coordinate global telecom networks and services.
The ITU telecom grid specifies
the standard C-Band, S-Band, and L-Band channels, wavelengths, and
frequencies.
Multiplexer/Demultiplexer
(Mux/DeMux)
See Dense Wavelength Division Multiplexing (DWDM)
Optical Spectrum
Analyzer
Optical version of a Spectrum Analyzer.
Passband
Band of wavelengths or frequencies that will pass through a device.
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Gaussian
Passband
in nm: Mux/DeMux devices whose spectrum profiles within
the pass band are essentially Gaussian.
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Flat-Top
Passband
in nm: Mux/DeMux devices whose spectrum profiles within
the pass band are relatively flat by comparison with the Gaussian
profile. A flat-top spectrum profile may be super-Gaussian or
ideally box-like.
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Polarization
Dependent Loss (PDL)
in dB
The insertion loss difference between two orthogonal polarization
states. For gratings, PDL is the difference between
the losses for the highest diffraction efficiency polarization state
and the lowest diffraction efficiency polarization state.
This is typically the orthogonal Te(S) and Tm(P) polarizations.
Raman Spectroscopy
The branch of science dealing with Raman Spectra. It is used
to study ground-level vibrational and/or rotational energy
changes in molecules caused by inelastic light scattering, typically
from a laser source. Raman Spectroscopy devices normally employ
a diffraction grating for spectral analysis.
Spectroscopy
The branch of science dealing with the theory and interpretation
of spectra. Spectroscopy devices normally employ a grating.
Spectrum
Analyzer
A scanning device used to cyclically tune through a given frequency
range to determine the amplitude-frequency distribution of the signals
present. Display is usually on a CRT. Spectrum Analyzers
normally employ a grating.
Telcordia
Requirements
These were formerly known as Bellcore requirements. GR-1221
(Passive Optical Components) and GR-1209 (Fiber Optic Branching
Components) are "Generic Requirements" used in telecom/fiberoptics
industry. Specifications relate to such items as temperature
and humidity performance.
Volume Phase
Grating (VPG) & Volume Phase Holographic Grating (VPHG)
VPHGs, the technology employed by Wasatch Photonics, use field-proven
technologies and well understood physical principles. This
"Grating Tutorial"
link contains more detailed information.
The hologram - the core component of VPHG - is produced by exposing
films to laser irradiation. The refractive index of the VPHG is
periodically modulated at some predetermined spatial frequency,
which results in the diffraction grating structure. This periodic
structure (the grating) diffracts each wavelength at a slightly
different angle, as illustrated in the following diagram:
Holograms & VPHGs have distinctive qualities that make them
outstanding candidates for DWDM and many other applications. These
qualities include:
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Parallel
Wavelength Separation: VPHGs separate wavelengths in parallel,
eliminating the need to cascade filtering elements. As a result,
DWDM high channel counts with close channel spacings are very
efficient, while remaining simple and cost-effective.
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High
Efficiencies: VPHGs provide efficiencies approaching 95%,
resulting in very low insertion loss.
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Polarization
Insensitive: Certain types of VPHGs are inherently polarization
insensitive; see Dickson Grating®
term above and the "Grating
Tutorial". Such VPHGs produce low PDL.
This eliminates the need for costly optical "compensation"
schemes that are used to overcome polarization effects in DWDM
and other devices.
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Wider/Flatter
Usable Bandwidths: Because VPHGs are extremely efficient,
they can operate over a larger range of wavelengths and still
have diffraction efficiencies suitable for the applications.
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High
Isolation: VPHGs provide good separation, resulting in very
low cross-talk for adjacent and non-adjacent DWDM channels.
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Simple
Component Design: VPHGs can be designed to minimize optical
elements and simplify optical design, manufacturing, and alignment.
Because they are inherently efficient and enhance the overall
device efficiency, they work particularly well in low intensity
light applications such as Raman spectroscopy.
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Wavelength
Division Multiplexing (WDM)
See Dense Wavelength Division Multiplexing (DWDM).
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