DWDM

Wavelength Division Multiplexing (WDM) is the process by which a few wavelengths of individual light signals, each of which carries a separate data stream, are assembled on a single optical fiber at the transmitting end, and then the multiplexed signal is separated into their respective channels at the receiving end.

Dense Wavelength Division Multiplexing (DWDM) is applied to the WDM process when multiple wavelengths of light signals are involved. For example, 40 channels with a frequency spacing of 100 GHz are assembled, each of which is centered at ITU Grid.

Dense Wavelength Division Multiplexer (DWDM MUX) is a device that combines input multiple light signals of a plurality of wavelengths into one composite signal for transmission. It is also called DWDM by convention, more precisely DWDM device.

Dense Wavelength Division Demultiplexer (DWDM DEMUX) is a device that separates the received composite signal into multiple light signals of a plurality of wavelengths, which are directed to several fibers for output.

S-, C-, and L-Band: S-Band (short band) is not well defined. Usually it is the spectral window from about 1450 nm to 1530 nm. C-band (conventional band) is the spectral window from about 1525 nm to 1565 nm corresponding to the strong amplifying range of the erbium-doped fiber amplifiers. L-band (long band) is the spectral window from about 1568 nm to 1610 nm.

0-dB Reference Level is defined as the straight-through intensity level when the devices to be tested are removed.

Wavelength Range (nm) is the spectral region over which the device is operated. All signal channels are confined within this wavelength range.

Channel: A single signal channel consists of a frequency band that has a finite pass bandwidth and is centered at a given frequency such as one specified by the ITU Grid. In DWDM, each channel corresponds to one particular wavelength and carries an individual data stream. For example, BaySpec produces 40 Channel 100 GHz DWDM MUX/DEMUX. Each channel has its pass bandwidth of 0.2 nm at 0.5 dB down.

Channel Center Wavelength (nm) is the wavelength at which a particular signal channel is centered. The International Telecommunications Union (ITU) has defined the standard optical frequency grid (channel center frequency) with 100 GHz spacing based on the reference frequency of 193.10 THz (1552.52 nm), the so-called ITU Grid. Channel center wavelengths are chosen at the wavelengths corresponding to the ITU Grid.

Channel Spacing (GHz) is the frequency difference between two neighboring channel center frequencies in DWDM components or modules. DWDM MUX/DEMUX devices in BaySpec have their channel spacing of 50, 100 and 200 GHz.

Center Wavelength Offset (pm) is a relative drift of the actual central wavelength of a particular channel with respect to the standard ITU Grid. The wavelength drift may result from inappropriate alignment and the design of the optical system.

Channel Pass Bandwidth (nm) is defined as a maximum wavelength (or frequency) range around the corresponding center wavelength (or frequency) at a given power level. Now, the industry well accepts the definition at 0.5 dB down power level. Note that due to the center wavelength offset of a channel the operating channel pass bandwidth may be smaller than that when the center wavelength is accurately at the ITU Grid.

Thermal Wavelength Stability (pm/°C) specifies the maximum wavelength drift of the spectral center of a particular channel due to temperature variation with respect to the central wavelength value at the room temperature (23° C).

Gaussian Pass Band (nm) specifies a class of DWDM MUX/DEMUX devices whose spectrum profiles within the pass band are essentially Gaussian.

Flat-Top Pass Band (nm) specifies a class of DWDM 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. The BaySpec’s MUX/DEMUX devices feature both Gaussian and flat-top pass bands according to customers’ requirements.

Insertion Loss (dB) is the relative power level transmitted to the output end referenced to the 0-dB reference level when a device is inserted.

Ripple (dB) is the insertion loss variation within the pass band of a signal channel. It is often used for the thin-film-based devices. In the grating-based MUX/DEMUX, the spectrum profile is bell-like, and no ripples are specified.

Channel Uniformity (dB) is the maximum difference of insertion loss over all signal channels. Channel uniformity is a measure of how evenly power is distributed between the output ports of the devices.

Polarization Dependent Loss (PDL) (dB) is defined as insertion loss difference between two orthogonal polarization states.

Polarization Mode Dispersion (PMD) (ps) occurs when different planes of light inside a fiber travel at slightly different speeds, leading to spread of optical pulses. In a DWDM device, PMD measures the average time difference of two orthogonal polarization states elapsed when the two corresponding pulses pass through the device.

Return Loss (dB) is the relative power level reflected back to the input fiber in the backward direction referenced to the 0-dB reference level when a device is inserted.

Directivity (dB) is also called near-end crosstalk that is the ratio of the optical power launched into an input port to the optical power returning to any other input port. In DWDM, directivity is applied to MUX devices only.

Channel Isolation (dB) is also called far-end crosstalk at a given wavelength that is the ratio of the light intensity at the undesired port to the light intensity at the desired port. So it is a measure of how well different wavelengths are separated at the output of a dense wavelength division demultiplexer.

Non-adjacent Channel Isolation (Non-adjacent Channel Crosstalk) (dB) is the relative amount of unwanted power that occurs in a particular channel pass band from the non-adjacent channels. Commonly, only the two first non-adjacent channels (left- and right-hand sides) are accounted for.

Operating Temperature (°C) is the temperature range over which the device can be operated and maintain its specifications.

Storage Temperature (°C) is the temperature range over which the device can be stored without damage and can be operated over operating temperature according to its specifications.

Athermal is a term used to specify the thermal stability of the devices. If the performance parameters are well below some defined critical values over the operating temperature, the device is said to be athermal. Historically, the grating-based MUX/DEMUX required temperature control in order to maintain its optical performance. Now, the BaySpec’s MUX/DEMUX uses an athermalized design and advanced packaging technology, which make the devices fully passive – the thermal wavelength drift is less than 0.5 pm/°C and the temperature-dependent insertion loss is reduced below 0.01 dB/°C over the operating temperature range of 0-70°C.