+44 (0) 1793 784389
+1 (855) 4-VIALITE
sales@vialite.comOptical Multiplexing
This guide gives a top level understanding of Wavelength Division Multiplexing, Coarse Wavelength Division Multiplexing and Dense Wavelength Division Multiplexing.
Wavelength-Division Multiplexing (WDM)
WDM allows two or more signals to be combined (multiplexed) on a single fiber by using different wavelengths for each signal.
Understanding WDM:
- Uni or bi-directional
- Particularly useful when minimizing the fiber count is advantageous e.g. when the fiber is being leased
- Utilizes 1310 and 1550 nm lasers
- High isolation is achieved between channels (90 dB RF) to maintain excellent signal integrity
- Wavelengths used for optical multiplexing are specified in channel spacing grids issued by the International Telecommunication Union (ITU)
- Ideal for L-Band HTS and Reference or Tx/Rx in a single fiber, in satcom and diverse antennas within broadcast applications.
CWDM and DWDM multiplexing
The channel spacing between wavelengths determines the type of multiplexing. The narrower the channel spacing, the more signals that can be combined in a single fiber. A channel spacing of 20 nm is known as Coarse Wavelength-Division Multiplexing (CWDM). A channel spacing of 0.4 or 0.8 nm allows many more signals to be combined in the same optical bandwidth, which is known as Dense Wavelength-Division Multiplexing (DWDM).
Channel spacing versus laser performance
- The narrower the channel spacing, the higher the performance requirement for the laser. Note, this is not usually true for the receiver as it is usually just a wideband receiver that covers the whole frequency band.
- Temperature is a significant factor affecting the output of a laser. A very high degree of thermal stability is required to prevent drift.
- The design of a DWDM transmitter is significantly more complex than a single wavelength link. For complex RF over fiber systems incorporating DWDM products, we recommend the use of the ViaLite System Designer tool.
Differences between CWDM and DWDM multiplexing
| CWDM | DWDM | |
|---|---|---|
| Theoretical channel limit | 18 | 300 |
| Practical channel limit | 16 | 80 |
| Channel spacing | 20 nm | 0.4 / 0.8 nm |
| Wavelengths | 1270 nm to 1610 nm | 1492.25 nm to 1611.79 nm |
| Uni/bi-directional | Both | Both |
| RF isolation between channels | 100 dB, typ | 100 dB, typ |
| Number of fibers | Single strand for RF and data | Single strand for RF and data |
| Long distance link capability | Yes | Yes |
Both CWDM and DWDM multiplexing have theoretical maximum channels per fiber. For CWDM multiplexing, high attenuation caused by water peaks mean that not all of these channels are practical, therefore ViaLite equipment supports the combination of 2 to 16 signals (channels) on a single fiber – reducing the number of leased fibers required by up to 16 times.
For DWDM multiplexing, the limitation on power that can reasonably be sent down a fiber (roughly 13 dBm as a rule of thumb) without causing non-linearity and signals modulating each other, means ViaLite supports a practical limit of 80 channels per fiber. Sharing this limited power across multiple signals reduces the noise performance of each channel. Therefore there is a point at which the noise performance of an individual channel is no longer acceptable.
The ViaLite range of CWDM and DWDM products allow multiple channels, traveling in either direction, to be simultaneously combined over a single fiber. This means signals can be multiplexed into existing infrastructure or the number of dark fibers being leased can be reduced, thereby lowering costs. The products come with a 5-year warranty as standard.
For questions around optical multiplexing and supporting products, please contact us.
