DWDM transmission is a core technology in an optical transport network. The essential components of DWDM can
be classified by their place in the system as follows:
- On the transmit side, lasers with precise, stable wavelengths.
- On the fiber link, optical fiber that exhibits low loss and transmission performance in the relevant wavelength spectra, in addition to flat-gain optical amplifiers to boost the signal on longer spans.
- On the receive side, photodetectors and optical demultiplexers using thin film filters or diffractive elements.
- Optical add/drop multiplexers and optical cross-connect components
In this article, we are talking the DWDM multiplexer.
- Function of DWDM Multiplexers and Demultiplexers
Because DWDM systems send signals from several sources over a single fiber, they must include some means to combine the incoming signals. This is done with a multiplexer, which takes optical wavelengths from multiple fibers and converges them into one beam. At the receiving end the system must be able to separate out the components of the light so that they can be discreetly detected.
Demultiplexers perform this function by separating the received beam into its wavelength components and coupling them to individual fibers. Demultiplexing must be done before the light is detected, because photodetectors are inherently broadband devices that cannot selectively detect a single wavelength.
In a unidirectional system (see Figure 1), there is a multiplexer at the sending end and a demultiplexer
at the receiving end. Two system would be required at each end for bidirectional communication, and
two separate fibers would be needed.
Figure 1, Multiplexing and Demultiplexing in a Unidirectional System
In a bidirectional system, there is a multiplexer/demultiplexer at each end (see Figure 2) and communication is over a single fiber pair.
Figure 2, Multiplexing and Demultiplexing in a Bidirectional System
Multiplexers and demultiplexers can be either passive or active in design. Passive designs are based on prisms, diffraction gratings, or filters, while active designs combine passive devices with tunable filters. The primary challenges in these devices is to minimize cross-talk and maximize channel separation. Cross-talk is a measure of how well the channels are separated, while channel separation refers to the ability to distinguish each wavelength.
- Techniques for DWDM Multiplexers
1), 3-port TFF DWDM
The 3-port FWDM is based on thin film filter (TFF) technology. It consists of a tubular device with one input and two outputs (one for the transmitted light and another for the reflected light). A TFF is sandwiched between a pair of collimating gradient index (GRIN) lenses, so that the light impinging the filter is collimated and transmitted or reflected by the TFF in an efficient way. The TFF wavelength in each tubular device will define the DWDM channel wavelengths.
A key benefit of the 3-port DWDM device is very low insertion loss, around 0.6 dB per filter. This makes the technology a good candidate for use in cascading devices.
In addition, 3-port DWDM is also characterized by a wide operating wavelength range and excellent channel isolation. An important application for the technology, therefore, has been the delivery of triple-play services in fiber cable networks.
The main drawback of the 3-port DWDM technology is the arduous manual assembly it requires. A 12-channel DWDM device requires twelve 3-port tubular devices. Each must be correctly and sequentially cascaded via 11 successful fiber splices. The number of components in a cascaded device scales with the number of channels, which can be as high as 40 channels, and, consequently, insertion losses, footprint and costs scale as well.
2), AAWG, Arrayed waveguide gratings (AWGs) are based on diffraction principles. An AWG device, sometimes called an optical waveguide router or waveguide grating router, consists of an array of curved-channel waveguides with a fixed difference in the path length between adjacent channels (see Figure3). The waveguides are connected to cavities at the input and output. When the light enters the input cavity, it is diffracted and enters the waveguide array. There the optical length difference of each waveguide introduces phase delays in the output cavity, where an array of fibers is coupled. The process results in different wavelengths having maximal interference at different locations, which correspond to the output ports.
Figure 4, Arrayed Waveguide Grating
3), CDWDM, the free-space compact dense DWDM (CDWDM) technology, uses interference filters in devices called thin film filters or multilayer interference filters. By positioning filters, consisting of thin films, in the optical path, wavelengths can be sorted out (demultiplexed). The property of each filter is such that it transmits one wavelength while reflecting others. By cascading these devices, many wavelengths can be demultiplexed (see Figure 4).
The optical losses of a free-space CDWDM device are typically less than 2.5 dB for a 12-channel device. Most importantly, the optical losses do not scale severely as the number of ports increases, giving an advantage over
the fusion-spliced cascaded FWDM for some applications. The footprint also is extremely compact (about 5x3 cm), enabling it to be easily integrated into a small subassembly.
Figure 5, Multilayer Interference Filters in CDWDM
Of these designs, the AWG and thin film interference filters are gaining prominence. Filters offer good stability and isolation between channels at moderate cost, but with a high insertion loss. AWGs are polarization-dependent (which can be compensated), and they exhibit a flat spectral response and low insertion loss. A potential drawback is that they are temperature sensitive such that they may not be practical in all environments. Their big advantage is that they can be designed to perform multiplexing and demultiplexing operations simultaneously. AWGs are also better for large channel counts, where the use of cascaded thin film filters is impractical.
Technology comparison
The following table gives a comparative overview of the technologies explained above:
Table 1, Comparison of 3-port TFF, CDWDM and AAWG DWDM technologies
For more information, visit Etern Optoelectronics website: https://www.szetern.com, or email to: sales@szetern.com