WDM Wave Lengths Multiplexing Technology: TFF (Thin-Film Filter) & AWG (Arrayed Waveguide Grating)
WDM (Wavelength Division Multiplexing) technology is a technique used to increase the bandwidth and improve the transmission capacity of optical fibers by transmitting multiple optical signals of different wavelengths. TFF (Thin-Film Filter) and AWG (Arrayed Waveguide Grating) are two commonly used WDM technologies.
TFF-Thin Film Filter（FWDM）Technology
TFF (Thin-Film Filter) technology, also known as thin-film filtering technology, is one of the commonly used technologies for WDM device, such as this CWDM mux demux . It utilizes certain optical properties of special thin-film materials to separate or multiplex optical signals of different wavelengths. Thin-film filters are usually composed of multiple layers of films with different thicknesses. These films have a specific distribution pattern and reflectivity, allowing specific wavelengths to be reflected while allowing other wavelengths to pass through the films. This enables the separation and multiplexing of signals. Compared other technologies, TFF technology has advantages such as simpler structure, smaller size, lower cost, and higher reliability.
Multilayer dielectric film filters are a type of high-reflection film with multiple layers, ranging from dozens to hundreds of layers. They are composed of two types of dielectric materials with different refractive indices, alternating between layers. The layers adjacent to the filter substrate and air have higher refractive indices. By combining several layers of different dielectric films, an interference filter with specific wavelength selectivity can be created, allowing for the separation or merging of different wavelengths.
The core of the TFF technology is the TFF filter, and the structure of the three-port WDM device shown in the figure clearly illustrates how the TFF filter is applied within the WDM device. The structure of the three-port WDM device based on TFF includes a dual-fiber coupler, a single-fiber coupler, and a TFF filter, with the TFF filter being attached to the end face of the collimating lens of the dual-fiber coupler.
In order to multiplex all wavelengths, multiple three-port devices need to be connected in series to form a WDM module, as shown in the figure below. Each TFF filter in each three-port device has a different transmission wavelength.
WDM modules based on three-port WDM devices have relatively large sizes (typical dimensions of a channel WDM module are 130x90x13mm3). However, in some special application fields, this size does not meet the requirements. To meet these requirements, compact WDM modules have been developed, such as compact DWDM (CDWDM) and CWDM modules (CCCWDM). All TFF filters are fixed on a glass substrate, and then aligned and fixed one by one with the input/output collimators. The typical size of a compact WDM module is 50x30x6mm3, which is much smaller than that of a conventional WDM module.
Compact WDM adopts free space cascading method, which uses an input lens to focus the light signals with wavelengths of in1, in2...An on the first filter. The light signal with wavelength M passes through the first filter and couples into the first output fiber through the first output lens, separating the light signal with wavelength M. The remaining light signals are reflected by the first prism to the next prism for further separation. This process continues until all signals are separated. The coupling between wavelength channels is achieved through the form of collimated light beams following a "Z" path.
With the increase of ports, the uniformity of the TFF-type DWDM mux and demux deteriorates. At the same time, the maximum loss generated at the last port is another factor restricting the number of ports. Therefore, DWDM mux demux based on TFF technology usually have no more than 16 channels. However, a typical DWDM system typically transmits 40 or 48 wavelengths in a single optical fiber, hence requiring DWDM mux demux with larger port counts. WDM mux demux with a serial structure accumulate too much power loss at the rear ports, so a parallel structure is needed to simultaneously multiplex/demultiplex dozens of wavelengths. Thus, an Arrayed Waveguide Grating (AWG) technology has been introduced.
AWG technology is also a commonly used WDM device technology. It is based on the optical waveguide and utilizes a planar wavefront beam splitter on the optical fiber. It is a technology that uses PLCQ (Planar Lightwave Circuit on Quartz) technology to fabricate an array waveguide grating on a chip substrate to multiplex and separate light signals of different wavelengths. AWG typically consists of a row of parallel waveguides with a specific distribution pattern and lattice on the optical waveguide. Each channel's wavelength is guided out by a specific waveguide, allowing for signal multiplexing and separation. Compared to TFF technology, AWG technology offers higher wavelength isolation, channel count, and bandwidth, making it suitable for higher-speed optical communication systems.
As shown in the diagram, AWG structure includes an input waveguide, an input star coupler (represented by the free propagation region FPR in the diagram), an array waveguide, an output star coupler, and multiple output waveguides.
The signal enters the input star coupler from the input waveguide and is then distributed to the array waveguides after free transmission. This distribution process is wavelength-independent, and all wavelengths are evenly distributed to the array waveguides. The array waveguides introduce phase differences to the multiple beams, with the phase of each beam forming an arithmetic progression, similar to traditional gratings. Different wavelengths are dispersed and focused at different positions in the output star coupler. Different wavelengths are received by different waveguides, enabling parallel demultiplexing of DWDM signals.
TFF typically consists of multiple layers of different thickness, with the most crucial and expensive being the thin film. To obtain a WDM mux and demux with a larger number of channels, the number of thin films needs to be increased, resulting in an increase in the price of TFF. With AWG, it is possible to obtain 40 channels simultaneously. However, one drawback is that you cannot select only one or two channels, meaning that the cost of a 10-channel system is the same as that of a 40-channel system. Therefore, in wave lengths multiplexing situations with a higher number of channels, AWG is more cost-effective than TFF. Many sources consider 16 channels as the turning point between these two technologies, with applications below 16 channels being suitable for TFF and applications above 16 channels being suitable for AWG.