Technical Dictionary

» 0dB Reference Level
» 0.5dB Bandwidth (Bandwidth @ 0.5dB) (nm)
» 1dB Bandwidth (Bandwidth @ 1dB) (nm)
» 3dB Bandwidth (Bandwidth @ 3dB) (nm)

» Adjacent Crosstalk
» Arrayed Waveguide Gratings (AWGs)
» Artificial Intelligence (AI)
» Athermal

» Bands (S-, C-, and L)

» Center Wavelength (nm)
» Center Wavelength Offset (pm)
» Channel
» Channel Pass Bandwidth (nm)
» Channel Spacing (GHz)
» Channel Uniformity (dB)

» Dense Wavelength Division Demultiplexer (DWDM DEMUX)
» Dense Wavelength Division Multiplexer (DWDM MUX)
» Diplexer
» Duplexer

» Flat-Top Passband (nm)
» FTTx

» Gaussian Passband (nm)
» Genetic Algorithm

» Insertion Loss (dB)
» Insertion Loss Uniformity
» ITU Grid

» Modulation bandwidth

» Non-adjacent Crosstalk
» Non-uniformity (dB)

» OADM (Optical Add/Drop Multiplexer)

» Passband Uniformity
» Passive Optical Networks (PONs)
» PIN photodetectors
» Photonic Crystals (PhCs)
» Planar Lightwave Circuits (PLC)
» PLC Splitter
» Polarization Dependent Loss (PDL) (dB)

» Quantum efficiency

» Responsivity, spectral responsivity (A/W)
» Ripple (dB)

» Tapers
» TE (Transverse Electric) Polarization
» TM (Transverse Magnetic) Polarization
» Transceiver
» Triplexer

» Vertical Cavity Surface Emitting Laser (VCSEL)

» Wavelength Division Multiplexing (WDM)
» Wavelength Range (nm)

0dB Reference Level refers to the would-be power level if there were no losses or, in other words, if there was no device inserted and the signals could move straight through the fiber. Parameters are often defined according to the 0-dB reference.

0.5dB Bandwidth (Bandwidth @ 0.5dB) (nm) is the width of the output signal when measured at a point 0.5 dB lower than the highest point of the passband shape for any given channel. Bandwidth equals upper frequency minus lower frequency (taking into account the Polarization Dependent Wavelength).

1dB Bandwidth (Bandwidth @ 1dB) (nm) is the bandwidth when measured at 1 dB lower than the highest point (taking into account the PDW).

3dB Bandwidth (Bandwidth @ 3dB) (nm) is the bandwidth measured at 3 dB below the highest point (taking into account the PDW).

Adjacent Crosstalk refers to the unwanted power from the output signal under test in the passbands of adjacent output signals. It can be defined as the difference between the maximum insertion loss of the signal under test measured within the passband and the maximum contribution from that output signal in the passband of the next adjacent channel (it is the worst case for both neighboring channels and takes into account both polarization states).

Arrayed Waveguide Gratings (AWGs) are a type of DWDM MUX and DWDM DEMUX. These are also known as PHASARs (phased-array based devices), because an array of waveguides is used for the multiplexing and demultiplexing of optical signals carrying transmission data with various wavelengths. The differences in length between the arrayed waveguides are critical for the necessary focusing and dispersion (whereby the signals are focused into the different output waveguides according to their wavelength).

Artificial Intelligence (AI) is a subfield of computer science concerned with concepts and methods of symbolic inference by computer. AI programs try to model aspects of human thought on computers.

Athermal is used to describe a device which is not temperature dependent, or in other words, thermally stable. Certain performance parameters at operating temperature indicate whether a device can be called athermal. For example, the thermal wavelength stability – how far a wavelength drifts – should not exceed 1.0 pm/°C and its temperature-dependent insertion loss should be less than 0.015 dB/°C.

Bands (S-, C-, and L) (see ITU Grid) are given ranges of wavelengths used for data transmission. The S-Band (short band), as the name implies, covers the shortest wavelengths from about 1450 nm to 1530 nm. The C-band (conventional band) extends from about 1525 nm to 1565 nm, and corresponds to the range in which the erbium-doped fiber amplifiers function best. The L-band (long band) covers wavelengths from about 1568 nm to 1610 nm.

Center Wavelength (nm) is the wavelength around which a given channel is centered. It can also be expressed as a frequency (THz) and hence is also referred to as the Channel Center Frequency.

Permitted optical channel frequencies have been set down by the International Telecommunications Union (ITU) in the so-called ITU grid. The grid specifications, given as both frequency and wavelength, are based on 100 GHz spacing (Channel Spacing). The channel center wavelengths used by AWG DWDM devices must be wavelengths corresponding to the ITU Grid.

However it should be noted that a chosen wavelength may drift (Center Wavelength Offset), which means the actual measured center position of a passband may then be different.

Center Wavelength Offset (pm) refers to the shift in position of the actual (measured) center wavelength relative to the standard ITU grid. A number of factors could account for the wavelength’s drifting, including the alignment or the design of the optical system, or exposure to temperature changes within the operating temperature range of the device.

Channel is defined as a specified frequency band for the transmission of a signal. The frequency band has a finite width (Channel Pass Bandwidth) and is centered around a given wavelength (Channel Center Wavelength). In the DWDM process, one channel is needed for each center wavelength transporting its own individual data stream.

AWG DWDM devices with different numbers of channels are used for multiplexing or demultiplexing the corresponding number of center wavelengths i.e. some DWDMs have 8 channels, some 16, some 40 and so on.

Channel Pass Bandwidth (nm), (also known as the Channel Passband or ITU Passband), is the range of wavelengths surrounding the center wavelength for any given channel. It usually equates to 25% of the respective channel spacing.

Channel Spacing (GHz) indicates how far apart two neighboring channels in DWDM components are positioned i.e. 200 GHz, 100 GHz etc. It can be defined as the distance between the center frequencies of two adjacent channels and can also be expressed in terms of nanometers (100 GHz is equal to 0.8 nm).

Channel Uniformity (dB) see Insertion Loss Uniformity

Dense Wavelength Division Demultiplexer (DWDM DEMUX) is the device used at the other (receiving) end of the process. As soon as the multiplexed signal has been received, the demultiplexer then separates it back into the many individual signals at different wavelengths. These individual signals are then guided into output waveguides.

Dense Wavelength Division Multiplexer (DWDM MUX) is the device used to combine the numerous signals before they are transported. Each coming from a separate input fiber, the signals with their different wavelengths enter just the one fiber, on which the "multiplexed" signal is then transmitted.

Diplexer is a passive device that implements frequency domain multiplexing. Two ports L and H are multiplexed into third port S. The signals L and H do not occupy the same frequency band, so they can coexist on port S without interfering with each other.

Duplexer is a bidirectional optical communication device that receives and sends signals over a medium. It combines two signals into a common channel or medium to increase its transmission efficiency.

Flat-Top Passband (nm) refers to a passband with a flat shape. Although a flat-top passband type usually results in higher losses, it makes network design easier.

FTTx Fiber-To-The-(H=Home, B=Business, C=Curb, Cab=Cabinet) is a new class of network systems, which bring the digital optical signals closer to the business premises or to the home. By continuous replacement of copper cable lines by optical fibers, a considerably larger bandwidth and subsequently higher data rates are obtained which satisfies the requirements for present and future telecommunication systems.

Gaussian Passband (nm) describes a passband with a Gaussian shape. A Gaussian shape may range from the classic bell-shape to almost pointy. The advantage of this shape is lower loss than with flat-top shapes.

Genetic Algorithm refers to a search algorithm based on the mechanics of natural selection and natural genetics. It is an iterative procedure maintaining a population of structures that are candidate solutions to specific domain challenges. During each generation, the structures in the current population are rated for their effectiveness as solutions and, on the basis of these evaluations, a new population of candidate structures is formed using specific 'genetic operators' such as reproduction, crossover, and mutation.

Insertion Loss (dB) denotes the loss in power each optical signal suffers as it moves through an optical device. It is defined as the maximum loss within the channel passband for all polarization states and calculated as the difference (in dB) between the reference level (0dB) and the measured output signal‘s point of lowest power within the passband.

Insertion Loss Uniformity (also known as Non-uniformity) represents the difference between the maximum and minimum insertion losses taken over all channels. The result indicates how evenly power is distributed at output.

ITU Grid (International Telecommunications Union): The ITU-T G.692/G.694-1 defines the frequency grids for DWDM applications. These specifications are given as both frequencies and wavelengths and are based on 100 GHz channel spacing.

Modulation bandwidth is the frequency or wavelength range, measured at the 3 dB decay of the "close-to-DC" response to the modulation signal. In optical commucation devices the term "modulation bandwidth" means the data rate (e.g. in Gbit/s) that can be achieved in an optical communication system.

Non-adjacent Crosstalk refers to the unwanted power from the output signal under test in the non-adjacent channels.

Non-uniformity (dB) see Insertion Loss Uniformity

OADM (Optical Add/Drop Multiplexer) is the name for a multiplexer used in optical networks that can add and drop wavelengths to and from an optical signal without converting them back to electrical form.

Passband Uniformity (also known as Ripple) is the difference in insertion loss within just one peak, that is within the passband of just one channel.

Passive Optical Networks (PONs) are broadband fibre optic access networks that employ a single, shared fibre to the home (FTTx), thereby eliminating the need to run individual fiber optic lines from an exchange point or central office to the subscriber’s home. The term 'Passive' suggests that the optical transmission has no power requirements or active electronic components.

PIN photodetectors are devices that detect light and convert it into current. Typically it is a photodiode with an intrinsic (i.e., undoped) region between the n- and p-doped regions. Most of the photons are absorbed in the intrinsic region, and carriers generated therein can efficiently contribute to the photocurrent.

A p-i-n photodiode has a well defined depletion region, which allows a more efficient collection of the carriers and thus a larger quantum efficiency. This leads to a lower capacitance and thus to higher detection bandwidth. The fastest p-i-n photodiodes have bandwidths of the order of tens of GHz. Some of them are available in fiber-coupled form and can be applied as receivers in optical fiber communications.

Photonic Crystals (PhCs) are materials exhibiting a periodic dielectric structure on a microscopic scale. This periodic structure implies a number of unique properties that makes photonic crystals especially attractive for integrated optics use. Photonic crystals are based on silicon-compatible materials and can be processed using technology that is already being employed on a daily basis by the semiconductor industry.

Planar Lightwave Circuits (PLC) are similar to microelectronic circuits, but use light signals to transfer data instead of electrical signals and therefore have optical waveguides instead of electrical conductors. Almost the same production processes can be used for PLCs as for microelectronic circuits.

PLC Splitter: an integrated optical device designed to split an optical input signal into N identical optical output signals.

Polarization Dependent Loss (PDL) (dB) is the variation in insertion loss within the defined passband, taking into account both polarization states. The PDL of a device is expressed in dB and is the worst case value for all channels.

Quantum efficiency is a measure of the light converting processes. It is defined as the percentage of the input photons which contribute to the desired effect. In a photodiode/photodetector, the quantum efficiency can be defined as the fraction of incident photons (or absorbed photons) producing the electron-hole pairs, which contribute to the external photocurrent. The quantum efficiency is a function of the photons' wavelength.

Responsivity, spectral responsivity (A/W ) shows how much current is obtained from the device for an incoming light beam of a given power. For a system that responds linearly to its input, there is a constant responsivity. For nonlinear systems, the responsivity is the local slope (derivative) of the curve for the “photocurrent-vs-optical power” characteristics.

In the specific case of a photodetector, the responsivity is given by the electrical output per optical input. Many common photodetectors respond linearly as a function of the incident power. The responsivity is a function of the wavelength of the incident radiation and of the sensor properties, such as the bandgap of the material, which the photodetector is made of.

Ripple (dB), see Passband Uniformity

Tapers are specially-shaped waveguides, whose modified geometry allows light of one intensity distribution to be converted into another intensity distribution.

TE (Transverse Electric) Polarization refers to linearly polarized light, where the electric field is perpendicular to the plane of propagation.

TM (Transverse Magnetic) Polarization refers to linearly polarized light, where the magnetic field is perpendicular to the plane of propagation.

Transceiver (the name is a combination of the words TRANSmitter and reCEIVER) is a bidirectional optical communication device that has both a transmitter and a receiver, which are combined and share common circuitry or a single housing. Two transceivers are usually connected by a fiber, and transmission from one side to the other, and vice-versa, can be performed either in ping-pong (half-duplex) mode or simultaneously (full-duplex mode).

Optical transceivers facilitate the use of fiber-to-the-premises (FTTP) services, in which optical fibers run from central hubs all the way to the end users. This can provide Internet access at very high data rates.

Triplexer is a bidirectional optical component for sending and receiving voice, data and video signals. As a compact sized and low-cost component, it is supposed to play a key role in the commercial implementation of FTTx networks.

Vertical Cavity Surface Emitting Laser (VCSEL ) is a type of semiconductor laser diode, typically realized with a cavity, that consists of of two so-called Bragg mirrors and an active region with one or more quantum wells. The emitted light leaves the device in a direction perpendicular to the top surface.

The common emission wavelengths are in the range of 750-980 nm (often around 850 nm) based on the GaAs/AlGaAs material system. Longer wavelengths like 1.3 µm or 1.55 µm can be obtained with dilute nitrides (GaInNAs quantum wells on GaAs) and with InAlGaAsP on InP.

VCSEL have a circular emission profile matching the geometry of the fiber, which ensures high coupling efficiency. Due to its surface-emission the device performance can be tested on the wafer immediately after processing. VCSELs are well-known for their excellent power efficiency and durability. Today VCSELs have replaced edge-emitting lasers in applications for short-range fiber optic communications.

Wavelength Division Multiplexing (WDM) denotes the process whereby up to 16 optical signals with different wavelengths are brought together to be transported on just one optical fiber. Each optical signal carries separate data. After the "multiplexed" signal (i.e. combined signals) has been transported, it is then divided back up into the individual signals (demultiplexed), each at its respective wavelength.

Wavelength Range (nm) denotes the range of wavelengths in which a device operates and corresponds to either the S-, C- or L-Band. All signal channels of the device are within this range.